The Geology of Ore Deposits - Forgotten Books

ARN O LD’

S G EO LO G I CAL SER IES

Genera l E ditor ; DR . J . E . MARR,F.R . S .

THE

GEO LOGY OF O RE

DEPO SITS

H . H . THOMAS,M .A.

,B .Sc .

AND

D . A.M ACALISTER,Asso c . R.S.M .

ILLUSTRATED

LONDON

E DWA RD A RN O L D

I 909

!A l l rig/its reserved!

ED ITOR’

S PREFACE

THE economic aspect of geology i s yearly receivingmore attention in our great educational centres , andthe books of this series are designed in the fi rst placefor students of economic geology . I t i s bel ieved , however , that they will be found useful to the student ofgeneral geology, and also to miners , surveyors , andothers who are concerned with the practical applicationsof the science .

‘ The Geology of Coal and Coal-Min ing ,’ by Dr.

Walcot G ibson , the first of this series of works oneconomic geology undertaken by experienced geologists

,

has already appeared . The present work wil l shortlybe succeeded by others , dealing with the geo logy ofquarrying , water- supply , and precious stones.The Authors of this book have wide geological andmineralogical knowledge , a practical acquai ntance withmetall i ferous areas, and an exceptional knowledge ofthe very extensive literature of the subj ect . This hasenabled them to embody the important results obtai nedof recent years , which have added so largely to ourunderstanding of the ways in which ores occur

,an d

of the conditions under which they have been formed .

J . E . MARR .

236418

AUTHORS’

PREFACE

IN this work our chief aim has been to present aconcise account of the origin , mode of occurrence , andclassification of metalliferous deposits . To keep thevolume to a small and convenient size has been a taskfraught with much difficulty , fo r hardly any subj ecthas gathered round itself a greater mass of importantliterature than the Geology of O re Deposits .The choice of suitable i llustrative examples , drawnfrom many countries , has been made with considerabl ecare , and at the same time we have avoided in a greatmeasure minor commerc ial detai ls, which are often onlyof transitory interest . But while the geological featuresof the deposits form the main portion of the work , theirclose connection with the economic aspect of the subj ecthas throughout been kept in View.

The size of this work , together with the vast and

scattered literature with which we have had to deal ,has rendered it impossib le to make special reference toall the works and papers consulted , or to mention byname the numerous writers to whom we are indebted .

We take this opportunity, however , of expressing ourobligations to a great number of well -known authori ties ,and to the publications of several geological surveys and

learned societ ies .HERBERT H. THOMAS .

DONALD A. MACAL I STER .

LONDON ,1909 .

CO N TE N TS

CHAPTER I

INTRODUCT ION

CHAPTER I I

ORES DUE TO THE D IFFERENT IAT ION OF

IGNEOUS MAGMAS 2 1—7 1

Segregatio n o f N ative MetalsGo ldP latinumIro nN ickel

Segregatio n o f Metal lic O xidesSegregatio n o f Iro n Ores as O xidesIro n Ores from Gabbro s and Norites

Iro n Ores from Syenite Po rphyriesIro n Ores from N epheline Syenites

Segregatio n o f Chrome Iro n OreSegregatio n o f CassiteriteSegregatio n o f Alumina as Co rundum

Segregatio n o f Meta l lic SulphidesSegregatio ns o f N ickel and Co ba ltSegregatio ns o f Co pper as Pyrites

V11

CONTENTS

CHAPTER I I IPAGE S

PNEUMATOLYS IS 72— 1 15

Ores o f Pneumato lytic Origin connected with Granite and

its Al lied Ro cksNature o f the Vapo urs which Extracted and Depo sitedthe Minera ls o f Cassiterite Veins

N ature o f the Alteratio n in the Wa l ls o f Tin Lo desOrder o f Arrival o f the Minerals in Tin Lo desAsso ciatio n o f Tin and Co pper Ores with the Metamo rphic Ro cks surro undingthe Granite

Typical Examp les o f Depo sits o f Pneumato lytic Originco nnected with Acid Intrusive Ro cks

Ores o f Pneumato lytic Origin co nnected with Gabbro s and

Al lied Ro cks

CHAPTER IV

HYDATOGENES IS DEPOS ITS FORMED BY

AFTER ERU PT IVE ACT IONS WH ICH ARE

N OT PNEUMATOLYT I C 1 16— 236

Sulphidic Veins o f Hydatogenetic OriginPrimary Go ld Veins

Typica l E xamp les o f Go ld VeinsPrimary Co pperVeins

Typica l Examp les o f Primary Co pper Lo desPrimary Lead and Zinc Veins

Typica l Examp les o f Lead and Zinc VeinsPrimary Sil ver Ore Veins

Typica l Examp les o f Silver Ore VeinsVeins o f N ickel and Co ba lt Ores

Typical Examp les o f N ickel and Co ba lt VeinsBismuth OresAntimo ny VeinsArsenica l Ores

CONTENTS

Sulphidic Veins o f Hydatogenetic Origin— co ntinued .

Quicksilver OresTypical Examp les o f Quicksil ver Depo sits

O xidic Veins o f Hydatogenetic OriginOxidic Ores o f Iro n and ManganeseHydro silicate Nickel Ores

CHAPTER V

ixPAGES

OREs DUE TO M ETASOMAT I C REPLACEMENT 237—300

Metasomatic Iro n OresCo ntempo raneo us Metasomatic Iro n OresIron Ores due to Subsequent Rep lacement

Metasomatic Depo sits o f Alumina as BauxiteLead and Zinc Metasomatic Depo sits

Metasomatic Lead OresMetasomatic Zinc Ores

Metasomatic Cadmium Depo sitsMetasomatic Co pper Depo sits

The Su lphides, Carbo nates, etc .

N ative Co pperMetasomatic Antimo ny Depo sitsMetasomatic Manganese Depo sits

Metasomatic Go ld Depo sitsMetasomatic Tin Depo sitsMetasomatic Uranium and Vanadium Depo sits

CHAPTER V I

BEDDED ORES DUE TO PREC IP ITAT ION

Precipitatio n as OxidesIro n and ManganeseCo pperAlumina as B auxite

240

244

249

262

264

27 1

279

285

286

286

290

29 1

292

294

298

299

CONTENTS

Precipitatio n as CarbonatesIro n and ManganeseCo pper

Precipitatio n as SulphidesIro n as Iro n PyritesCo pper as Co pper PyritesSilver, Lead , and Zinc

Nickel and Co balt as Pyrites, etc .

Go ld in SintersPrecipitatio n as Silicates

CHAPTER V I I

METAMORPH IC ORE DEPOS ITS 331—358

The Ore Depo sits o f the Crystal line SchistsDepo sits o f Oxides (Haematite and Magnetite l

Depo sits o f Su lphidesThe Larger Ore B o dies o f Metamorphic Character

Depo sits o f O xides : Iro n Ores (Magnetite and Haematite)ManganeseCo rundum

Depo sits o f SulphidesMetamorphic Co ntact Depo sitsGo ld in Metamorphic Ro cks

CHAPTER V I I I

S ECONDARY CHANGES IN ORE DEPOS ITS APARTFROM METAMORPH ISM 359

—375

Seco ndary Changes by AscendingSo lutio ns 360

Seco ndary Changes by Weathering and Actio n o f MeteoricWaters

CONTENTS xi

CHAPTER I!

DETRITAL AND ALLUV IAL DEPOS ITS

Detrital and Al luvia l Go ldStannifero us Detrital Depo sits

Typica l E xamp les o f Stannifero us Detrita l Depo sitsCo rnwal l 399The Ma lay Peninsula Banka and B il lito n 40 1

The Stannifero us Al luvia o f Australia and Tasmania 403

The Stanmitero ns Al luvia o f Saxo ny and B o hemia 404Miscel laneo us Al luvial Tin Ores 405

P latinum and Al lied Meta ls 405The Rare Earths 407Iro n and the BaserMetals 408

INDE!

THE

GEO LOGY O F ORE DEPO S ITS

CHAPTER I

INTRODUCT ION

AN ore deposit may be defined as a body Of rock whichcontains metalli c compounds or native metals in su ffi

cient quantity and in such a form as to be of economicimportance— that is to say , from which One or moremetals can be profitably extracted .

The metals enter largely into the composition of theearth ’ s crust

,and i t i s conj ectured on good grounds

that , as a whole , they occur in increasing proportiontowards the interior ; for many of the richest oredeposits occur intimately connected with certain ancientigneous rocks and gneisses ; on which the oldest recognizable sediments were deposited . Again

,a great

number of ore deposits have thei r origin in masses O f

igneous rock which have been inj ected into the sol idcrust from the molten interior of the earth ; whileothers have been deposited from subterranean gasesand solutions .As a rule , i t is the intrusive igneous rocks and thedetrital or sedimentary rocks in their immediate neigh

I

g . !RI-

IE; GEOLOGY OF ORE DEPOSITS

bo urho o d that are the richest in the compounds of theheavier and more valuable metals but taking into consideration the average composition of the earth ’s crust

(see table below) , i t i s Clear that , in order to form anore deposit , any particular metal or group of metalsmust have undergone considerable concentration . Thefollowing table compiled by D r. F . W . Clarke gives anidea of the earth ’ s average composition , as far as can bej udged from surface Observations

Per Cent .

OxygenSiliconAluminium 8°

I3I ronCalciumMagnesiumPotassiumSodiumTitaniumHydrogen O

°

I 7

The percentages o f metals , other than those mentio ned above , are too small to be given , only amounting to thousandths or mill ionths of 1 per cent . Certainfresh igneous rocks from mining districts in the UnitedStates , have been examined by Professor Kemp , whoestimated that they contain the following metals insmall quant ities

Per Cent .

Copper 00 09Lead from 00 0 1 1 to 00 08

Z inc from O °OO48 to 00 09Silver from 0 0 0007 to

from 0 0 0002 to O °OOOO4

The two tables given above make it quite obviousthat every ore deposit must be due to some natural

INTRODUCT ION 3

process of concentration , by which the metall iferousmaterial has been collected and deposited within al imited area ; for no metal as originally distributed inany rock would exist in sufficient proportion to be ofeconomic value . I n this work we have endeavoured toexplain the occurrence of ore deposits by presenting asconcisely as possible the various processes Nature hasadopted to bring about the local concentration of metallifero us material .

The original source of all ore deposits lay in theprimitive igneous crust of the earth and in thoseigneous rocks which subsequently penetrated it fromthe interior. I t may thus be said that the source of allore deposits may ultimately be traced to rocks withigneous characteristics .The igneous rocks which carry, or are answerable for,the richest metall iferous masses are mainly those of thelarger intrusions . These are deep -seated or plutonic in

Character , by which is meant that they never reachedthe earth ’s surface in a fluid state , but were covered toa considerable depth at the time of thei r consolidation .

Generally speaking , the older igneous intrusions— thatis to say

,those which belong to the pre -Cambrian

and Palaeozoic periods— are richer in ore deposits thanthose which have been intruded at a later date .Deposits of considerable value are often associated

with igneous rocks , occurring as dykes and smalllaccol ites (hypabyssal) , but are seldom found to bedirectly dependent on volcanic or extrusive rocks . Thisdifference in the ore -bearing capacity of various igneousmasses does not so much depend on any variation in thecomposit ion of the rocks themselves , as on the greater

1— 2

4 TH E GEOLOGY OF ORE DEPOSITS

facil ity which the plutonic rocks O ffer for the co ncen

tratio n of metall i ferous compounds .O re -bearing igneous rocks

,whether they belong to

the most ancient fundamental complex of the earth ’ scrust or were intruded into sedimentary rocks of laterdate , generally exist in the form of large masses, towhich the name laccolites ’ i s given

,or as minor sheet

l ike intrusions with approximately parallel boundaries

(si l ls and dykes) . The ore deposits in the case oflaccol ites are often marginal - that is to say

,they fre

quently mark the boundary between the igneous massand the country rock into which it has been intruded ;and the same is true in the case of those associatedwith the minor intrusions , but occasionally the oremay exi st as rich strings or patches (schlieren) anywhere within the igneous rock , and not at its periphery .

The concent ration of ore material and the formationof ore deposi ts in general has been brought about by avariety of processes , both physical and chemical .I n the case of those deposits formed by segregation

from plutonic igneous rocks (p . the concentration ofore has been determined by such factors as the crystal

l izatio n of one mineral before another (fractional crystallizatio n) , or the separation Of a homogeneous fluid intotwo immiscible fluids during the fall of the temperaturetowards the solidifying -point of the whole . O re deposits formed in thi s manner are integral parts of therock masses in which they occur . All igneous rocks inthei r most fluid condition are regarded as being homo

geneo us, and in that state are known as magmas .

! Theseparation from a magma of any definite substance or of

two immiscible magmas is looked upon as part of the

process Of segregation or differentiation described later

INTRODUCTION 5

(p . The separation of any mineral from a rockmagma is analogous to the separation of a salt from itssolution

,and it i s a well -known fact that many fluids

which mix perfectly with each other at an elevatedtemperature

,separate from each other

,more or less

completely, on cooling .

Metal -bearing gases and solutions may be given offfrom the larger igneous masses during thei r so l idifica

tion . Metall i ferous solutions may also be furnished bywater charged with certain solvents passing upward ordownward through rocks i n which metal l ic ores arefinely disseminated . These gases and solutions maysubsequently deposit their metall i ferous contents infissures ; or in the country rock wi th or without Chemicalinterchange of material (pneumatolysi s , hydatogenesi s,and metasomasis) .

All large sheets of standing water receive di lutemetall i ferous solutions from the surrounding dry land ,and may either produce metasomatic Changes in therocks which form thei r beds

,or may

,on becoming

saturated by evaporation or other causes,deposit thei r

dissolved matter in the sol id form (precipitationsp . With such deposits are classed those which occurat the surface around Springs , caused by the evaporationof the solvent or its removal by some other naturalagency .

Since sedimentary rocks are detrital accumulationsformed from pre - existing rock masses , i t happens thatfinely divided stab l e metall ic compounds or nativemetals present i n the older rocks often undergo a concentration by natural agencies .

6 THE GEOLOGY OF ORE DEPOSITS

The above order of events— segregation from igneousrocks , pneumatolysis , hydatogenesis , precipitation , andsedimentation— seems to represent the most logical andscient ific manner in which to group original oredeposits , on account of this being more or less thesequence taken by the various processes in Nature .

Types of deposit formed in these various ways mayundergo secondary changes , in the description of whichin the following pages precedence has been given tometamorphosi s produced by thermal and dynamicagencies .

O re deposits generally may be roughly divided intotwo classes : fi rst

,those which have been formed con

temporaneously with the enclosing rock (syngenetic) ;and

,secondly

,those which have formed subsequently

(epigenetic)The fi rst class includes those deposits which formintegral parts of igneous rocks , and many of thoseeither chemically deposited from solutions or mechani

cally as detritus .The second class embraces the greatest variety , andincludes the ordinary veins and lodes , most metasomaticand pneumatolytic deposi ts , and all secondary enrich

ments .

The mode of occurrence of an ore mass is oftendependent on the folds and fractures resulting from themovements which have taken place in the earth ’s crust .

For in many cases these l ines of weakness and dislocation are connected with the intrusion of igneousmaterial

,and also permit the easy access O f mineralizing

solutions or vapours to the country rock . On the otherhand

,movements of the earth ’s crust have often taken

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such areas are intruded by igneous rocks , differingwidely in composition

,there may be corresponding

different types of ore deposits formed . Some regionsappear to have been disturbed several times , and oneach occasion invaded by igneous intrusion s , whichsuccessively gave ri se to types of ore deposi ts , differingwidely from one another .The lodes in many metall i ferous districts result fromthe infil ling Of fissures formed Shortly after the in

trusion and consol idation of the igneous rocks withwhich the ores are connected . The fissures , crushzones and faults are in many regions parallel with oneanother, and have some definite directional characterdetermined by the folding or cleavage of the rocks, andconsequently with the direction of the earth-movementswhich developed these structures . Generally, however ,such structures may be regarded as among the latereffects of these general disturbances

,and connected

with the final movements of adjustment in the rocks .

I n regions where the rocks have not been cleaved orhighly folded , as is the case in many districts composedof the younger format ions , the uniform directionalcharacter of lodes is not so noticeable . According toW . O . Crosby ’ s theory of j ointing , many lodes wereformed by earthquake Shocks , and , according to thiswriter

,whatever other agencies

,such as cooling or

desiccation,might have operated within the rocks , the

phenomena of parallel j oints or veins is best explainedby the assumption that they were created by earthShocks , and that , i f these take place slowly under bending or torsional strains

,the direction of the fissures will

be determined by the strain , while the‘ time and mode

of breaking ’ i s determined by the Shock . E xtensive

INTRODUCT ION 9

fissuring and brecciat ion of rock due to earthquakeShocks is found in many volcanic plugs . Such fissuri ng is therefore intimately connected with the disturbances produced during a period of volcanic activity .

I t i s probable that some fissures are formed by contraction due to cooling or desiccation of the rocks .Some igneous dykes are traversed by minute fissuresformed during the cooling of the rock . The calcitequartz veins in albite diorite dykes in the ReadyBullion M ine , Alaska , appear to have been formed inthis way. Some of the Cornish elvans (quartzporphyry dykes) are traversed at right angles by innumerable small veins

,varying in S ize from a thin film

to a quarter of an inch or more . Occasionally tin oreis found along lines of parting , the direction of whichwas determined by the flow- structures , as i n rhyolite inCornwall . F issures formed by contraction due to cooling also occur in quartz -po rphyry in the Thiiringerwald ,where the veins co ntain manganese ores . I n Victoria

(Austral ia) gold veins are found in contraction -fissures ingreenstone , and at the Hai le Mine , South Caroli na, inj oints traversing diorite dykes . Although thei r originhas been di sputed

,the numerous vein s traversing pro

pylite at Nagyag in Hungary may be mentioned inthis connection .

At Berezov, near Pyzhm insko ye, i n the Urals , greisenized gran ite veins

,known as ‘ beresite ,

’ varying from afew feet up to I 50 feet in width , are traversed at rightangles by numerous gold -quartz vein s

,giving a remark

able character to the auriferous mass .The occurrence of argenti ferous Copper ore veins

at Nasmark (Telemarken , Norway) traversing granitedykes at right angles has been noted by Vogt .


The remarkable occurrence of tin ore veins in pseudobedding planes in granite in the E rzgebirge has longbeen known . H ere there are horizontal fissuresparallel with the dome - l ike surface of the granite

,

and formed by contraction during cooling of themass . More i rregular contraction-fissures are found atAltenberg .

A particular form of fissuring connected with nickelifero us deposits i s formed by expansion of the parentrock during its serpentinization , but by far the greaternumber of lode-fissures were formed by external causesconnected with movements of adjustment of the earth ’ scrust , by which zones of Shearing and fault ing , as wellas S imple fracturing

,are produced .

Where a district has been subj ected to shearing , themetall i ferous solutions may have deposited thei r materialin a wide zone or belt of crushed rock . Such types ofdeposition are represented by some of the large cuprifero us- sulphide ore bodies . H ere

,however

,not only has

there been deposition in the region of the crushing , butgenerally much of the material of the rock itself hasbeen removed by solutions and replaced by new minerals .The structures of such lodes or ore bodies is co nse

quently varied but,although often containing very low

grade ore , the body as a whole , which is often lenticularin form

,may be Of great commercial value . L ike the

ordinary fissure lodes produced either by earthquakeShocks or movements of adjustment at the close ofregional disturbances

,these impregnation -zones are

also seen to be more or less parallel , the directionalcharacter being dependent partly on the direct ion o fearth - stresses

,and partly upon the structures of the

rocks .

INTRODUCT ION 1 1

The original structures of the crush or breccia—zonesare preserved in the veinstones which are formed whenthe mass i s mineralized .

The variety of structures which veinstones maypresent i s infinite , and depends partly on the composit ion of the rock in which the lode has been formed

,

partly on the nature of the crushed or broken materialwhich fi lled the fissures or lay between crush -planesbefore impregnation , and partly on intermittent deposition from the mineral izing solut ions . I n open fissuresthe comby , platy , or banded structures are seen tobe due to the successive deposi tion of m inerals arrivingin the fissure at different periods . Where the originalfracture contained brecciated material , the veinstonesconsi st of strings and masses of ore penetrating andenveloping the fragmentary material , and so giving ri seto exceedingly complex st ructures .In rocks which can be decomposed and replaced by

other substances , such as in the cases of the replacement - deposits of lead and fluo rspar in l imestone ,and of tin and tourmalin e in granite

,the veinstones

have a massive appearance,but frequently Show dimly

traces of the original structure of the rock .

I n some deposits , as of smithsonite in l imestone,

a wonderful variety of concret ionary structures areObserved , apparently abstrusely connected with theChange in volume which the rock suffered during itsreplacement .

I t has often been remarked that lodes are seldomequally productive throughout , and that the ore occursin patches , chutes , or bunches , as i f the deposition ofminerals had been favoured by certain places in the

1 2 THE GEOLOGY OF ORE DEPOS ITS

lodes . I t is thi s local concentration of the ores thatmakes mining such a Speculative enterprise , and rendersprospecting and thorough sampling imperative . Theconditions favouring the local concentration of theoriginal ores in lodes are many

,and include primarily

the proximity to the rocks from which the ores arederived . The country rock has an influence where itscomposition i s such that it can chemically react on themetall i ferous solutions which penetrate it . The textureof the rock and the degree of porosity are also powerful factors in modi fying the nature and form of deposits .In lodes originating under pneumatolytic conditions ,

the most favourable position for the principal ore bodiesis near the margin of the pluton ic igneous mass whichgave rise to them , either in the external portion of theigneous rock itself or in its metamorphic aureole . Thisapplies also to pegmatitic deposits . O f ores formedunder pneumatolytic conditions , some are complex ,and consi st of oxidic and sulphidic compounds . I nthis case , it . no t in frequently happens that the oxidiccompounds are deposited in one zone or horizon in thelode , while the sulphidic compounds , owing probab lyto thei r more solub le nature , are deposited at a greaterdi stance from the source .

In deposi ts of hydatogenetic and metasomatic origin ,the solutions have often t ravelled considerable distancesfrom the parent rock

,and the metall i ferous substances

have been deposited in favourable places determinedpartly by mechanical conditions , and partly by thenature O f the country rock . I n such cases , the form ofthe deposit is often influenced by the porosity andsolubil ity of the rocks

,which allows the solutions to

seep through and impregnate the mass with ore .

INTRODUCT ION 13

I t Should be remarked that the solutions whichpercolate lode -fissures are either of magmatic origin

(hydrothermal) , being directly derived from someigneous rock , or of meteoric origin . I n the latter casesurface water percolating the lodes oxidizes and dissolvescertain minerals near the surface

,carries them down

,

and causes them to be deposi ted at lower horizons ;while in the former case secondary actions are broughtabout by magmatic waters , as in instances where thenature of the solutions have changed and affectedminerals deposited previously . That thermal watersderived from deep -seated sources contain much di ssolved mineral matter o f various kinds, capable ofchemically acting upon the rocks they come in contactwith , is proved by the nature of the dissolved compounds o f thermal springs in well - known volcanicregions .

The alterations effected in this way have been classified into certain definite types , which may be referredto by such terms as silicificatio n , propyl itization , kaol inizatio n , tourmalinization , axinitizatio n ,

greisenization ,scapolitization , sericit ization , dolomitization , zeo litiza

t ion , etc . , according to the nature of the alterationeffected by the solutions or vapours on the rock . Manyof these alterations are complex , and others might beincluded in which thermal metamorphism has acted inconjunction with the solutions

,with the production of

such alteration types as garnet and epidote rock,or horn

b lende and pyroxene rock . I t i s unnecessary here todefine all these terms , as they are self-explanatory, butit may be mentioned that some of them are particularlycharacteristic of the rock in the vicin ity of certain kinds

14 THE GEOLOGY OF ORE DEPOS ITS

of ores , and consequently also of the rocks from whichthe ores were derived . P ractical ly all t in lodes, forinstance , are Characterized by a special kind of alterationof thei r walls

,known as ‘ greisenization .

’ The felspar ofgranite i s kaolin ized by removal of the si l icate of potashin solution . The same mineral is also converted intotopaz , lepidol ite , or gi lbertite , through the reactionsinvolved . Boron vapours have the effect of convertingfelspar to blue tourmaline , and biotite may be convertedto brown tourmaline or to chlorite .Where the lode traverses shale , the latter i s often

Silicified or to urmalinized . G rits are converted toquartzitic rocks

, and the tin ore IS found interstitiallyor in minute cracks t raversing the mass .I n calcareous rocks and greenstones the alterationseffected by tin lodes consist o f the development ofaxin ite , fluo rspar, garnet , and other minerals. Throughout the rock cassiterite or pyrit ic ores may be foundamong the alteration -minerals . All these actions aretrue metasomatic alterations

,but

,as in the maj ority of

cases the principal part of the ore i s found in or closeto the original fissures , tin lodes are not regardedas deposits of metasomatic origin , since the metasomasis i s on ly a subordinate phenomenon , and not theprincipal action

,as it i s in the case of many lead ore

and fluo rspar deposits traversing l imestones .

A large number of ore deposits are in some mannerchemically related to the rock in which they lie , whetherthey form integral parts of igneous masses or existas deposits from vapours or solut ions . This relation i smost conspicuous in deposits which occur as parts ofigneous masses , where similar metall ic compounds are


with gold ; tourmaline , topaz and lith ia-mica , withwolfram and tin ore , are only a few instances .

From a study of many examples, i t has been foundthat the minerals forming ore deposits have a more orless regular order of deposition , the formation of certainminerals regularly preceding or following others withwhich they are associated .

This is especially true in the case of segregations fromigneous rocks, in which the formation of metall icsulphides and oxides generally precedes that of thesil icates which form the bulk of the rocks .

Again , where similar compounds of two metals aresegregated in one deposit , one wil l precede the other,as, for instance , the case of the sulphides of nickel , whichare formed before those of cobalt .

Where an originally barren rock has been replacedby an ore deposit , i t i s again noticeable that the metall icminerals have a definite sequence . I n ores formedunder pneumatolytic or hydatogenetic conditions , thesequence of mineralization is generally readily madeout . The order in which the minerals were depositedin the Cornish tin -copper lodes has been referred toin the following pages

,and serves to illustrate this

point .

Amongst the metall ic minerals forming ore deposi ts ,and which occur as segregations from igneous rocks,are the native metals

,such as plat inum ; the oxides of

i ron with and without titanium , and the sulphides ofi ron

,nickel , cobalt , and copper .

The replacements include chiefly the sulphides ofiron

,lead and zinc ; the carbonates of the same metals ,

INTRODUCT ION 17

with manganese in addition ; the oxides of i ron andmanganese

,and , less frequently , native gold .

The metamorphic deposits are especial ly noted forthei r masses of magnetite and iron and Copper pyrites .

Precipitations consist ch iefly of oxides and carbonates ,as might be expected from their mode of deposition .

Bedded detrital deposi ts may contain gold , platinum ,

tinstone,i ron ores

,etc .

—in fact,any metal or compound

Of a metal which is sufficiently durable and insolubleto withstand the long -co ntInued action of meteoric

agents .

The changes which ore deposits may undergo afterthei r original formation may be divided into threeClasses dynamic , thermal , and chemical .

The dynamic Changes, often included under the headof metamorphosis,

’ consist chiefly in the deformation of pre - existing deposits , their disruption , brecciation , and displacement , and the setting up of certainstructures due to shearing stresses in the earth ’ s crust .The thermal changes produced by the heat of theearth ’s interior or the intrusion of igneous rocks includech iefly the production of new minerals or the recrystallizatio n of those which already existed , but , l ike thechanges connected with dynamic agencies, are independent o f the introduction of any new material fromoutside the affected area .

Among the secondary Changes of ores brought aboutby chemical actions, the most important i s that effectedby the oxidizing and dissolving action of surface waterswhich find their way into lodes and other kinds ofmetall ic deposits . By these waters meteoric waters

2


not only are certain minerals completely decomposedand fresh minerals formed

,but a secondary concentra

tion of the ores is effected by the reduction of metall ifero us substances brought down in solution to depthsin the lode beyond the reach of oxidizing influences .I n this way secondary enrichments are formed , which

in some districts are the only parts of the deposits worthworking . The metall i ferous minerals which are un

i nfluenced by the surface waters comprise t instone ,wolfram , scheelite , and a few other compounds . Othersare not affected unless the water already contains somesalt in solution , as , for instance , in the case of thesolution of metall ic gold by ferric sulphate and its repre

cipitatio n by ferrous sulphate . As a whole,then , the

chemical changes brought about in lodes by the actionof meteoric waters may be broadly described as thoseof oxidat ion and solution near the surface

,followed by

reduction at a variable depth in the lode , dependingupon the distribution of ground water

,which in turn is

largely dependent on the climate and structure of thecountry . The minerals most l iable to these secondarychemical changes are the sulphides , and the i ron andmanganese ores . The secondary minerals formed in this

way comprise sulphides, sulphates , si l icates, carbonates,and oxides . But some phosphates , vanadates , arsenates, and other compounds , are also formed , togetherwith native metals , such as copper, S i lver , and gold .

The distribution of ground water has been dealt within the following pages, but it should here be remarkedthat we do not accept in its enti rety the hypothesisthat most ores are concentrated by the action of ci r

cu latingwaters . According to some writers , the meteoricwaters are supposed to percolate through the rocks from

INTRODUCT ION 19

the surface , and to descend to great depths , collectingmetalli ferous minerals as they go . With increasing depththei r powers of solution are augmented by pressure andheat , and on reascending , the metall i ferous minerals areredeposited in fissures . In the newer and porous rocksa redistribution of certain minerals by such means doesnot appear improbable , but in the case of the Palaeozoicand Archaean rocks i t appears more likely that themeteoric waters have played only a subordinate part inthe formation of ore deposits , and in all cases the re

concentration of the ores by chemical actions is confined to shallow depths .

The various classes of ore deposits as set out in th iswork embrace most of the better-known occurrences.

At the same time it must be borne in mind that anyclassification of this kind can only be arb itrary, andthat there are many instances of an ore deposit beingequally well assignab le to two posi tions in the schemeof classification . This , i t i s true , often arises from themeagreness of the in formation obtainab le , but in othercases there is a distinct overlapping of two classes ,and thus it becomes a matter of some difficulty todecide the proper place that certain ore deposits shouldoccupy .

I t often happens,for instance , that a deposit may be

classed equally well with the metasomatic or metamo r

phio deposits . Many of the lodes formed under pneumato lytic conditions are characterized by metasomaticalterations in thei r walls but as this is only an attendant phenomenon

,and not the principal point of interest

in connection with the origin and nature of the deposi t ,this peculiarity i s subordinated in the Scheme of classi

2—2


ficatio n to certain types of ores . As in other cases, theform of the ore body was determined solely by theextent of the metasomatic replacement . A special groupof deposits i s treated under the separate heading ofMetasomatism .

I t has been the endeavour of the writers to presentthe main geological features of metall i ferous regions as

far as they bear on the origin and nature of the particu lar deposits described , and to classi fy them accordingto the leading types . For more detailed accounts ofthei r commercial value the reader is referred to standardtextbooks

,and in particular to A Treatise on O re De

posits,

’ by Phill ips and Louis ; Die E rzlagerstatten ,

’ byStelzner and B ergeat ;

‘ G i tes Métal liféres,’ by Fuchs

and De Launay ;‘ The Nature of Ore Deposits ,

’ byBeck (translated by Weed) The Copper Deposits ofthe World

,

’ by Weed ; and O re Deposits of the UnitedStates

,

’ by Kemp . O ther important sources of info rmation are the Memoirs of the Geological Survey ofthe United States of America ,

’

the Zeitschri ft fiir

Praktische Geologie ,’ the Transactions of the I nstitute

of Min ing and Metallurgy,’ and the ‘ Transactions of

the American Institute of Min ing E ngineers . ’

CHAPTER I I

ORE S DUE TO THE D IFFE RE NT IAT ION

OF IGN E OUS MAGMAS

A CONS IDE RAB LE number of ore deposits are intimately associated wi th masses of igneous rock

,in such

a manner as to prove without possibi l ity of doubt thatthey have been derived from , and formed part of, anigneous magma . These deposits must be regarded aspart of the igneous rock with which they are associated

,

for they stand petrographically related to the rockmasses, and there i s no evidence of the introduction ofmaterial from outside sources by solutions or pneumato lytic processes (p .

I t has been clearly demonstrated that many igneousmasses are far from homogeneous in character , and thatthe chemical composition varies considerably from onepart of the mass to another. This lack of uniformity,which is regarded as the result o f a process or series ofprocesses , to which the name

‘magmatic differentiation ’

has been appl ied,often takes place gradually, and is

chiefly noticeable in the fall ing Off of the amount ofS i l ica in the rock towards its margin . In many cases iti s safe to assume that a magma , prior to its intrusioninto the upper regions of the earth ’ s crust , was practi

cally homogeneous,but that , as soon as i t commenced

to cool , certain diffe rential processes started to operate2 1


within the fluid , resulting in a local concentration ofcertain mineral constituents .A study of any rock-mass of igneous origin , eitherextrusive or intrusive in character

,reveals the fact that

the variou s minerals of which it i s composed separatedout from the mother- l iquid at different t imes , andgenerally in a more or less definite sequence .

The earliest substances to separate out from a coolingigneous magma are

,generally Speaking , the native

metals , the metalli c oxides , and the metall ic sulphides ;these are usually termed accessory minerals ,

’ owing tothe fact that they form

,in most cases , an insignificant

part of the rock -mass as a whole . These are followedclosely by those sil icates rich in i ron and magnesia , suchas ol ivine

,the pyroxenes and amphiboles , leaving the

last portion to consolidate richest in sil ica and poorestin the heavier basic constituents . Normally the accessory minerals should be distributed sparingly

,and more

or less un i formly,through the rock-mass ; but we

observe that they have often undergone considerableconcentration

,i n many cases occurring in sufficient

quantity to const itute an ore .

The processeswhich brought about the concentrat ionof the accessory minerals are exceedingly complex , butthey were certain ly in itiated by various portions of thefluid magma being at different temperatures owing tothe cooling of the mass .

D i fferentiation is the outcome of either a fractionalcrystall ization of the various constituents of an igneousmagma

,or a separation of a magma into two or more

solutions which will not mix ; from which it followsthat the concentration of a mineral depends primari ly


gold , osmium , i ridium ,nickel

,cobalt

,i ron

,ti tanium ,

copper, etc . , o ccurring In the form of native metals ,oxides, or sulphides, as the case may be .To illustrate the result of the processes of differentiat ion in a basic igneous rock -magma

,we cannot do better

than take such a well -known E nglish example as thatof the gabbro of Carrock Fell in Cumberland

,described

by Mr. Harker .A large mass of gabbro was intruded into Lower

Palaeozoic sediments ; the intrusion has a more or lesssymmetrical outli ne

,and covers an area of about six

square miles . There can be l ittle doubt that the gabbrowas intruded as a homogeneous mass, but that , as itcame to rest and began to cool

,differentiation com

menced with crystall ization and concentration of themore basic minerals towards the margin , leaving theS i l icious material more prevalent towards the centre .The accompanying map

,based on the work of

Mr. Ha rker, shows most clearly how various portionsof the mass differ in chemical composition .

The margin of the gabbro consists of a rock very richin ilmenite

,and also richer in pyroxene than that

towards the centre ; it has a silica percentage of onlyand the i ron ores

,i lmenite and magnetite con

stitute 2 1 per cent . of the total rock .

The centre of the mass, however , i s a quartz -bearinggabbro , with a very small quantity of i ron ores , notmore than 1 per cent . , and the relatively high sil icapercentage of 535 .

These two extreme types pass through an intermediate variety

,which may be styled a normal gabbro

neither rich in iron ores nor contain ing free si lica in theform of quartz .

D IFFERENT IAT ION 2 5

The above is a most simple example of the differentiation of a rock -mass in place , by which it is meant thatthe differentiation took place in the area where itsresults can now be studied . I n a great number of cases

,

l g 15117 e

Qua rt: Ga bbra Gr a n o p hy r e

N o rma l Ga bbr o S k id d a wS/a tes etc .

l lm en ite Ga bbro

FIG . I .- IVIAP O F TH E CARROCK FELL D I STR ICT , CUMBERLAND ,

TO ILLUSTRATE THE D I FFERENT IAT ION O F A GABBRO-MASSAFTER INTRUS ION . (AFTER A. HARKER. )

however, i t i s found that the products of differentiationare not so symmetrically arranged as in the above case

,

but the more basic ore -bearing material occurs as patchesor strings (schlieren) i n a less basic part of the rock .


I t i s supposed that these basic patches and schlierenowe their origin to either convection currents or shearingstresses acting on the magma during the process ofconsolidation

,causing portions of the already segregated

and partially consol idated basic material to be carriedinto the still -fluid mass by which it failed to be com

pletely reabsorbed .

As has been stated before,there is good reason to

assume that in the majority of cases the differentiation

FIG. 2 .—VE IN OF NOR ITE

,TH IRTY TO SEV ENTY YARDS IN

WIDTH,W ITH SCHL IEREN OF T ITAN I FEROUS IRON O RE .

(AFTER J . H . L . VOGT .)

of a rock-mass is closely connected with,and probably

dependent on , the crystallization of i ts constituentminerals ; but certain banded ores , which , on accountof the extreme type of differentiation they exhib i t ,present marked affinities to the gnei sses , seem to havehad a different genesis .The ores i lmenite

,magnet i te

,etc .

— in rocks ofthis type occur generally as narrow ultrabasic streaks


separated by less basic material . I t is , moreover,evident that these ore -bearing layers and the less basicmaterial between them crystall ized simultaneously , andthat they existed together in the fluid state .I t i s probable that the various layers are the result of

the intrusion of a magma which had differentiated ,

Mica Sy en ite

Porphy ry

Ign e o us Vein sho wing

k margin a l s egrega tio n

rsanlifc

3

FIG. 3.—VE I N OF M ICA SYENITE -PORPHYRY

,SHOW ING MAR

GINAL SEGREGAT ION OF FERROMAGNE S IAN M I NERALS AN DI RON ORES . (AFTER J . H . L . VOGT . )

before intrusion , into two or more magmas which didnot intermingle . That is to say, that the originalmagma was in this case heterogeneous at the t ime ofi ts intrusion

,and that the differentiation was in a

great measure independent of the crystall ization ofthe individual minerals .With regard to the differentiation of si l ls and dykes

,

2 8 THE GEOLOGY OF ORE DEPOSITS

or the lesser intrusive bodies generally, the segregatedmineral s are identical with those from S imi lar magmasexisting in the larger masses, but the differentiation isnot often so well displayed .

I t has been noticed that occasionally S i l l s and dykesShow a concentration of basi c minerals on their lowersurfaces , and it is here supposed that the differentiationwas largely brought about by gravity acting on thepartially solidified magma , causing the early and heavierproducts of consol idation to S ink in the mother- l iquorand to form a basic substratum .

Rocks erupted at the surface as lavas have in generalcooled so rapidly that differentiation on anything likea complete scale was next to impossib le .

SO far we have been dealing chiefly with the differentiatio n of a magma in the position in which we arenow able to study it , but we can conceive many rockmasses existing in reservoirs far below the surface ofthe earth , which have already been differentiated intovarious rock types, and possibly with ore deposi tsmarking a zone .

Such a differentiated mass under the influence ofincreased temperature and earth movement m ight beintruded into higher and cooler regions of the earth ’ scrust .

The intrusion of thi s rock-mass would take place insuccessive stages , and each stage would be characterizedby a magma of different chemical composi tion .

From these magmas would originate a group of rocksdiffering widely from each other in type , but yet tiedtogether by some common mineralogical characteristic ,such as the presence of hypersthene

,for instance

FIG. 4.

— GENERAL GEOLOGICAL MAP OF TH E CHR I ST IAN IAREGION, SHOW ING THE D I STR IBUT ION OF VAR IOUS ROCKTYPE S DER IVED FROM A MAGMA DI FFERENT IATED BEFOREINTRUS ION . (AFTER W . C . BRO

'

GGER . )


(p . The rock-masses thus formed , although theyoccur in one district

,and are possibly in contact with

each other,will not graduate one into the other, but

will be separated by hard and fast l ines . Takentogether they will form what i s known as a petrologicalcomplex ,

’ or a group of cognate igneous rocks .A study of such a group of rocks will show that theywere intruded in a more or less definite order, and thatthis order was usually one of basic to acid in the caseof the larger plutonic masses

,and acid to basic in the

later minor intrusions .These rock-masses , although they are themselvesproducts of differentiation before intrusion into thei rpresent posit ion

,have in most cases undergone flirther

differentiation in place,and have often given rise to a

series of satell itic dyke rocks .I n the more basic rocks which have thus undergone

a double process of differentiation,marginal concentra

t ion of ore material i s O ften exceedingly well displayed .

E xamples of such cognate groups of igneous rocks areextremely numerous, but perhaps the best known arethose of E ssex County , Mass . , and of the Christianiadistrict of Norway , where the rocks vary from anO l ivine-gabbro -diabase , with a si lica percentage ofto a potash gran ite with 642 per cent . of si lica .

Pegmatites , which are often the carriers of many rareminerals , are highly S i l icious igneous rocks , generally thedirect result of the differentiat ion of the less basic plutonicmagmas, and represent the most acid and unconsolidatedpart of a differentiated mass squeezed out into the sur

rounding rocks . I n a petrographical complex they areusually seen to be the latest products of consolidation ,

32 THE GEOLOGY OF O RE DEPOSITS

trace the shades of difference in the various rock -massesto which they owe thei r origin .

S EGREGATIO N OF N ATIVE METALS .

There are many well -authenticated examples of nativemetals occurring in association with igneous rocks ina manner which proves thei r igneous origin , but veryl ittle is known concerning thei r segregation , or o f thestate in which they were carried by the igneous magma .

So far the chief metals known to occur native in igneousrocks are gold , platinum with osmium and iridium ,

n ickel , and iron , and these all seem to be present asoriginal constituents . In some few cases it can beproved that they have undergone a certain amount ofconcentration during the sol idification of the respectivemagmas by which they are carried .

Go ld .—That gold is an extensively distributed con

stituent of igneous rocks is proved over and over againby its widespread occurrence in alluvium of all agesformed from the waste of igneous rocks under the influence of denuding agencies . But apart from thisindirect proof, free gold has been detected in a greatvariety of rock types , amongst which may be mentioned

granites , pegmatites , diabase , diorite , and others of sti llmore basic character .From a Choice of many examples , i t may be statedthat free gold occurs in the granites of Brazi l and ofSonora in Mexico . The pegmatites and allied rockshave yielded this metal in the Dargo and Omeo districts of Victoria , Austral ia , in the Berezov districtof the Ural Mountains , in the Si lver Peak district ofN evada , and the Yukon district of Alaska . The diabases and diorites of Briti sh Guiana, the Appalachian

D IFFERENTIAT IO N 33

region,Queensland , and many other districts , have been

proved to be parent rocks of much gold now occurring in placers (p . Gold has been noted as anoriginal constituent of porphyritic syenites in BritishColumbia and other adjacent areas ; in pitchstone inChil i ; and in peridotite from Damaraland . I n theAustralian plutonic rocks it has only been found inassociation with pyrites . With regard to the occurrence of gold in N evada mentio ned

'

abo ve, a word ortwo will not be out of place concerning i ts mode oforigin .

I t i s contained in quartz - felspar rocks , to whichthe name a laskite has been given ; this rock graduatesinto pure quartz , which is looked upon as the ultimateproduct of differentiation of a granitic magma . TheSilver Peak district is one of abundant granitic rocks ,which are intrusive into the Palaeozoic strata . Thesemuscovite -bioti te granites pass into aplit ic and pegmatitic types consisting of quartz and alkal i- felspar.The alaskite , by the diminution of felspar, passes intopure quartz veins , which are said to have the samegenetic relation to the alaskite as it has to the gran ite .

These quartz veins , often of lenticular form ,are the

source Of much gold , and a figure showing their modeof occurrence i s given below .

By far the greater number of rich alluvial gold deposits have been derived from igneous rocks of an intermediate to basic character , and it can be proved thatthe gold in the placers of certain districts l ike Briti shGuiana is dependent on the distribution of diabase anddiorite . I n other cases of more rare occurrence

,alluvial

gold has been derived from rocks of granitic character,

including pegmatites and aplites . An example may be


cited from the B lago vyeschensk district , on the bordersof Siberia and Mongolia .

Free gold i s found associated with platinum in thechromite segregations from basic and ultrabasic rocks ,such as peridotite and other ol ivine - rich igneous masses

(p . The gold placers of the Go ro b lago dat regionin the Urals and of South -West O regon have hadthei r origin in rocks of this nature .

FIG. 5.—SECT ION To ILLUSTRATE THE MODE OF OCCURRENCEO F AUR I FEROUS QUARTZ LENSE S IN ALASK ITE .

P latinum.—As far as our knowledge goes

,native

plat inum , as an original rock-constituent , is restrictedto the most basic igneous rocks, such as peridotites andthei r altered equivalents , serpentines . With platinumare associated the two allied metals osmium andi ridium , which are usually comb ined with each other toform the alloys osmiridium and iridosmium . Platinum

,

D IFFERENT IAT ION 35

with the metals mentioned above and some gold , isextracted from the more basic parts of peridotite

,

dunite , and serpentine masses occurring in the UralMountains at N izhne Tagilsk , Mount S o lo vief, andother districts , and placers have been worked in thevalleys of the rivers I ssa , Wyja , Tura , and Njassma ,

which drain the peridotite and serpentine region .

Similar placers derived from S imilar rocks have beenworked in Northern California . The dunite of theTulameen River, British Columbia , contains plat inumassociated with Chromite . A single but unconfirmedrecord of platinum from the L izard Peninsula in Cornwall i s interesting o n account of the parent rock beingof the usual type which bears this metal .Plat inum is almost always associated with segre

gatio ns of chromite (p . and , though all chromitesegregations do not carry platinum , thi s mineral is animportant indicator of the rarer metals ; therefore thenecessity of examining all alluvial deposits drain ing aregion of basic igneous rocks with chromite segregationscannot be too strongly urged .

Iro n.

— The most striking occurrence of a nativemetal of true igneous origin is that of the i ron massesof Disco Island , on the west coast of G reenland .

Large blocks of almost pure i ron were fi rst detectedlying loose on the hil l -sides at Ovifak , and were originallysupposed to have a meteoric origin , thei r resemblance tometeoric masses being most marked both in appearanceand composition .

Subsequently this i ron -bearing rock was discoveredin place , and was found to exist as extremely basicpatches occurring in , and differentiated from , a basaltporphyrite .


The porphyrite consists of labradorite , augite , ol ivine ,and titaniferous magnetite , set in a glassy ground -mass ,while the basic patches consist chiefly of anorthite andnative i ron in grains and lumps . The ore has thefollowing composition

FeN iCo

A curious feature is the relatively high percentage ofcarbon

,which probab ly exists in the form of

graphite .I t has been suggested that the porphyrite was volcanic

or extrusive in character , but such an origin cannot beassigned to it without further evidence , especially as thecharacter of the rock and the extreme differentiationwhich has taken place point in the direct ion of in

trusion .

The nickel in the above ore probab ly exists as analloy with a small proportion of i ron , similar to thatdescribed below .

Many eruptive rocks, and particularly the basaltsI reland , Spain , America , Germany) , contain

minute grains of native iron , in some cases enclosed bymagnetite .

N ickel . —An alloy of nickel with iron,in which

nickel preponderates, seems to be a product of thedifferentiation of certain peridotites and olivine gabbros .The more usual alloy is that to which the nameawaruite has been given , after the district Awarua inN ew Zealand . I n chemical composition it comp aresvery closely with some nickel - i rons occurring asmeteorites, and contains per cent . of nickel ,


per cent . of cobalt , and per cent . of i ron , correSpo nding to the formula , 2N i+ Fe .

In N ew Zealand , on the west coast of the SouthIsland

,large masses of peridotites have been intruded

into the crystall ine schists of that region , forming the

Hope, O l ivine , and Redhill Ranges . They are some

what variable in character and partially serpentin ized ,

but usually contain enstatite . Th’

ey occupy an areaof 400 square miles

,the greatest length being twenty

five miles . Awaruite is the chief product of differentiat ion , but with it are segregated small quantities of theusual accessory minerals of peridotites

,chrome - i ron ore

(chromite) and chrome - spinel (picotite) . This alloywas first recognized in the alluvial deposits of theRiver George , which drains the peridotite -country ofthe O l ivine Range .

Amongst other local it ies,nickel alloyed with some

i ron is present in the alluvium of the R iver E lvo , i nPiedmont , I taly , where it has probably been derivedfrom the nickel -bearing serpentines of the SouthernAlps ; i t also presumably occurs in this state in theD i sco i ron -masses (p .

S EGREGATION OF METALL IC O ! IDES .

Oxidic segregation , or the local concentration ofmetall ic oxides in igneous rocks, i s one of the mostmarked phenomena connected with some types ofigneous intrusions .To this class of ore deposits belong the segregationsof the titaniferous i ron ores— existing as titan iferousmagnetite and ilmenite— chromite

,corundum

,and pos

sibly some cassiterite (tinstone) . All these segregations,with the exception of cassiterite

,are from basic igneous


magmas , and exist either as ultrabasic dykes and massesdue to di fferentiat ion of the magma before intrusion

,or

as ultrabasic margins to pluton ic masses and dykes ofbaSIc character differentiated in place .In the case of the t itan iferous i ron ores , most of the

ore deposits form sharply defined masses which do notgraduate into the country rock

,and were evidently in

truded as dykes or small bosses subsequently to theintrusion of the more acid members of the series towhich they belong and within which they occur .Dykes in which marginal concentration of metallic

oxides has taken place are not uncommon , and oxidicsegregations also occur in the parent rock as basicpatches

,strings

,and schlieren . The chief rock - types

yielding these ore deposits belong to the great gabbroand peridotite groups , and include such rocks as gabbroswith and without ol ivine

, Ophitic dolerites with andwithout ol ivine

,norites

,nephel in ites , peridotites, and

picrites .I t is particularly noteworthy that within certain

l imits the rocks which yield a segregation of one oxidediffer in character and composition from those segregating a different oxide ; thus , for instance , we findtitaniferous magnetite segregated by the olivine -gabbroof Taberg (p . i lmen ite by the hypersthene -gabbroand norites of the E kersund (p . and chromite and

corundum by peridotites .Together with these concentrations of oxides , as hasbeen pointed out before

,there has been a segregation

of the bisilicates, such as olivine , hypersthene , etc . , andoften of phosphorus in the form of apatite (chlorapatite) , so that it is seldom found possible to obtainthe ore unmixed with other minerals .

40 THE GEOLOGY O F ORE DEPOSITS

small quantities of magnesium and manganese . Theo rtho titanate i s generally titan iferous magnetite segregated by the olivine -gabbro magmas

,while the meta

titanate is ilmenite associated with the hypersthenebearing rocks .

The ilmenite segregations may be said to bridge thegap between segregations of o rtho titanates and segre

gatio ns of metallic sulphides , for in most of them wemeet with a fair percentage of sulphides . With thesegregations of o rtho titanates we most often get concentration of the metasil icate , an example of such acombination being displayed by the magnetite-olivin iteof Taberg while metatitanates occur with ortho

si l icates in the i lmenite-hypersthene rocks of the Eker

sund-Soggendal district .True magnetite deposits are seldom if ever met withas segregation s from igneous rocks . The ore-massesgiven rise to by the more acid magmas , Such as thoseof the nepheline syen ites (p . approximate morenearly to pure magnetites than any of the ones fromthe basic igneous masses . At the same time , however,the percentage of phosphoric acid is l iable to increase .Magnetite ores , without t itan ium and with a lowpercentage of phosphorus , are almost always associatedwith the crystall ine schists and other metamorphicrocks ; and it is more or less evident that they havebeen formed by the metamorphism of pre - existingi ron deposits of sedimentary or metasomatic origin

(PPIro n Ores from Gabbro s and N orites.

—In SouthernNorway and Sweden occur numerous masses of igneousrocks belonging to the gabbro family

,and from these

have been segregated some o f the most famous i ro n ores


of the world . They were studied by Vogt and others ,and , as regards the origin of the ores , are probab lybetter understood

,and have received more attention of

a scientific nature,than masses of similar rocks in any

other country . The district of E kersund -Soggendal , inSouthern Norway

,forms a well-defined petrographical

province,of which the various rock-members are

especially characterized by the occurrence of rhombicpyroxenes (hypersthene , and were evidently derivedfrom a common magma -basin .

This district furnishes us with a most typical exampleof the segregation of ilmenite from magmas of a noriticcharacter

,differentiated before their intrusion into thei r

present position . The rock - types occurring in thi sprovince range from a hypersthene -bearing rock , richin labradorite , through ilmenite -norite , which occursas dyke - l ike masses in the labradorite rock , into analmost pure ilmenite rock contain ing varying amountsof hypersthene , and perhaps a l ittle felspar . The purei lmenite deposits , when in the form of dykes , cut theore-bearing nori tes , and are therefore the last to beintruded . The labradorite rock compares very closelywith the anorthosite of South-E ast Canada and theAdirondack district (p . where the i ron ores havea S imilar origin .

The iron ores of the E kersund -Soggendal occur partlyas segregations from the norite dykes

,where they exist

as ilmenite -norites,and partly as veins , schlieren , or

basic patches, which occur in the labradorite rock ,having well -defined boundaries .The ore in the norite dykes

,such for instance , as that

at Sto rgangen , where the big ore occurs , consists ofan intimate mixture of ilmenite and hypersthene , with


a.

l ittle labradorite , and makes up 40 per cent . of thetotal rock .

In the basic strings and patches , such as at B laafjeld ,

i lmenite predominates to such an extent that the rockloses i ts norite character

,and the percentage of i ron

ore rises to 90 or 95 . The B laafjeld masses , however,are exceptionally pure , the more usual percentage forthe purer ores being 70 to 80 .

The magnetic i ron ore deposits of Taberg in Smaland , Sweden , consist of t itaniferous magnetite , anddiffer chiefly from ores of the Ekersund - S oggendal typein this respect , and in the fact that the parent rock is

FIG. 6.

— SECT ION TO ILLUSTRATE THE MODE OF OCCURRENCEOF IRON ORE S IN THE EKERSUND -SOGGENDAL D I STR ICT .

1,Labrado rite ro ck ; 2 , schlieren o f ilmenite ; 3, ilmenite-no rite dykes .

rich in ol ivine instead of hypersthene . The rocks forming the Taberg complex consi st of gneissose granites ,olivine gabbro

,which has locally given rise to amphib

o lites and hornblende - schists owing to the shearingstresses to which it has been subj ected , and a magnetiteO l ivin ite which is the ore -bearing mass . There is reasonto believe that they were all derived from the samemagma

,which probably differed but li ttle from the

O l ivine -gabbro in character .The gabbro

,or

,rather

,the chief outcrop of gabbro ,

forms a lens -Shaped mass about a mile and one -fifthlong

,and nearly half a mile in width . I t completely


surrounds the magnetite rock,which has a maximum

length of four-fifths of a mile , a width of 485 yards , andrises to about 400 feet (F ig .

e/s so s e Ga b bro d IranOreE Gram t e Ho rn b l end e Magne titeS chis ts Ol iw

'

m l‘e

FIG . 7 .

— MAP O F THE TABERG D I STR ICT .

(AFTER TORN EBOHM . )

Scale,235 inches to 1 mile .

The ore contains 43 to 45 per cent . of ferric oxide

(FegO 3) , with a relatively small percentage of titanium


oxide , being only 63 as compared with 44 per cent . insome of the E kersund-Soggendal ores . A most important feature of the Taberg ore i s the very smallpercentage of phosphoric acid

,which only reaches

although occasionally going up to 1 °

5 per cent . Between1 89 1 and 1 895 , 83 per cent . of the ore raised was magnetite , and 17 per cent . haematite .The Taberg ore contains a small quantity of vanadicacid , segregated with the titanium ,

and in this characteris similar to many other t itaniferous magnetite deposits,such , for instance , as those of Rhode I sland and theAdirondack region in the United States .I t was o rigInal ly thought that the Taberg complex

was the result of differentiation of an olivine gabbromagma in place , and that the basic material had segregated towards the centre

,in stead of , as is more usual ly

the case , to the margin of the intrusion . However, theextremely Sharp boundaries which exist between thevarious rock- types in this complex

,in almost every

instance , disposes of this idea for i f differentiation hadtaken place to any great extent after intrusion , themargins of the different rock-types would have been ill

defined , one rock passing more or less gradually into itsneighbours . We must regard the Taberg ore and thegabbro as separate intrusions , but drawn from the sameigneous reservoi r .O res similar in composition and origin to those of

Taberg and Ekersund -Soggendal occur atmany localitiesin Southern Scandinavia

,such as at LangO, GomO, etc . ,

and in Arctic regions similar occurrences are met with .

At Lofoten,in Northern Norway , and Valimaki , in

F inland,a basic magma has yielded segregations of

magnetite -olivin ite similar to that described above .

D IFFEREN 45

In South Africa,in the T i s a great

igneous complex known as the d PlutonicSeries. I t consists of a great group of igneous intrusions

,due to the differentiat ion of a magma before

intrusion,which range from serpentines (probab ly

altered peridotites) to granites ; ol ivine and hypersthenegabbros have given rise to rich segregations of very purei ron ore

,while the ultrabasic rocks are associated with

segregations of chromite (p . So far the study of

the relationship of these ores to the parent rock , andthe various rock- types to one another , i s far from com

plete , and most interesting results may be expected inthe future from a closer investigation of this series .

I n the N ew World,in the United States , i n N ew

Jersey , Minnesota , Wyoming , the Adirondacks , and inthe Norian areas of Quebec and Ontario in Canada ,occur masses of gabbro and allied rocks of the gabbroand norite families which have yielded segregations oftitaniferous i ron ores due to differentiation before orafter intrusion .

In the Adirondack region,the largest ore bodies are

met with in rocks,chiefly composed of labradorite

felspar with a l ittle hypersthene,olivine , or augite ,

which may be regarded as gabbros or norites,very poor

in the ferromagnesian constituents . This labradoriterock , to which the name anorthosite has been given ,bears a very close resemblance both in composit ion andmode of occurrence to the labradorite ore -bearing rocksof Southern Scandinavia .

Masses of a dark basic gabbro are most numerous inthis region , and have yielded segregations of ore . I t isprobable that the gabbros and the anorthosites werederived from separate magma basins, and are themselves


not the products of differentiation of the same magma ,although both are more than usually rich in calciumbearing si l icates . The anorthosite and gabbro massesare int ruded chiefly into gneisses and crystall ine l imestones ; the ores occur in a manner identical with thoseof Taberg , E kersund ,

and other areas in Sweden andNorway , either as basic patches and strings in theanorthosite

,as segregat ions from the dark basic gabbros

or norites,or as very rich basic dykes , l ike some of

those in Scandinavia , which were intruded subsequentlyto the gabbros .The olivine

,hypersthene , and augite in the labradorite

rock,and also in the gabbro , has almost always under

gone partial reabsorption , with the formation of a haloof garnet .The ores are titaniferous magnetite and ilmenite ,chiefly the former , the percentage of iron varyingfrom 60 to 30 , and the titanium dioxide from 5 to I 4.

O f course , some of the i ron exi sts as sil icate in the

hypersthene , etc . , and is not recoverable as metal .Sulphidic segregation (p . 61 ) i s generally not well displayed by these rocks, especially by the gabbro -masses ;and in this feature the ores are more l ike the Tabergsegregations than those of E kersund . Sulphur is , infact , hardly ever present in the ore in a greater quantitythan 1 per cent .Another point of resemblance between the Adirondackgabbro-ores and those of Taberg is that in almost everyinstance there has been a segregation of vanadium oxidewith the titanium

,the proportion of vanadic oxide

(V20 5 ) varying from O‘

I to O ‘

3 per cent .The best -known of numerous localities in thesemountains where gabbroic rocks occur are chiefly near


l iquor. When the ores had formed considerable aggregates the whole was intruded in higher regions of theearth 5 crust , and under these conditions the segregatedore bodies could take up any posit ion in the resultingrock . Almost the same result , however , could bebrought about by the magma separating out into twomagmas of extreme types , and this , perhaps , offers thebetter explanation of the ultrabasic ore -bearing dykes

(pIro n Ores from Syenite P o rphyries .

— A few casesare known in which valuable i ron ores have been segregated by rocks of this type . The ore i s usuallymagnetite , with little or no titanium ; but the percentage of phosphoric acid rises to a much higher figurein these rocks than in the gabbros and norites alreadydescribed

,and the amount of phosphorus is in some

instances su fficiently high to render the ore of verysmall value .Two important areas of i ron ores which are associatedwith porphyries are the Swedish province of N o rbo ttenand the U ral Mountains .In the province of N o rbo tten , which l ies about

67°

50’ north latitude , the chief ore deposits occur at

Kirunavaara and Luossavaara . They form two wellmarked parallel ridges , rising in a striking manner fromthe lower surrounding country, and running north andsouth , Kirunavaara being the more southerly of thetwo , and Luossavaara being a l ittle to the north andeast on the other side of the lake (Fig .

The ore bodies are associated with a quartz - freesyen ite porphyry

,in which they occur as dyke - l ike

masses , and are presumably the result of extremedifferentiation before intrusion into their present position .


The porphyry has been intruded into a variety of rockso f sedimentary origin , such as conglomerates andquartzites , which form the surrounding country .

The southern mass of ore is one and a half miles inlength

,and has an average thickness of 230 feet . The

Luossavaara mass , as far as is known , is smaller , beingthree -quarters of a mile long and 180 feet wide . Thetotal ore exposed above the level of the lake , representing about tons , consists

-

almost entirelv of

F IG . 8.

— MAP OF THE KIRUNAVAARA REG ION .

Sca le ,yards to 1 inch .

magnetite practically free from titanium much of i t i salso almost free from phosphoric acid and su lphur

,but

in some samples the percentage of phosphorus rises ashigh as 2 to 3 , and in extreme cases even 5 to 6 percent . We see , therefore , that in these masses there hasalso been an intense local concentration of the phos

pho ric acid , all of which exists in the form of apatite .

The percentage of magnetic oxide of iron (Fe3O 4)in these ores reaches as much as 96 2 5 per cent . , which ,

50 THE GEOLOGY OF'

O RE DEPOSITS

with per cent . of FegO 3 , represents 708 per cent . ofmetall ic i ron .

The two ore -masses described above are extremelyimportant they form two of the largest in Europe , andcompare very favourably with any in the United States ,both on account of their size and the general high valueof the ore they yield .

Another locality , also in Swedish territory , i s Ge l l ivarain North Sweden . The ores are similar to those ofKirunavaara

,occuring as magnetite with a varying

percentage of apati te ; they are evidently magmaticdykes

,and are associated with two other ro

'

ck - types— a

rock rich in soda which precedes them ,and a quartzose

soda-rich rock which follows them .

In the Ural Mountains there are several well -knowndistricts in which porphyritic rocks occur, and whichare famous for thei r i ron ore deposits . Amongst thesewe may mention the N izhne -Tagilsk district , includingtheWysso kaia Mountains and the Go roblago dat district .I n the Wysso kaia region there are many intrusionsof quartz- free porphyries consisting of orthoclase , plagioClase and augite , or augite converted into hornblende .

The segregations from these masses are largely magnetite , existing as i rregular patches and veins which passinsensibly into the surrounding rock . The ores therefore seem to have been formed S imultaneously with theigneous rock , and are not the result of a subsequentintrusion of the more basic part of a differentiatedmagma . They are distinguished by their purity andgood metallurgical qualities

,being practically free from

sulphur, and containing not too great a percentageof phosphoric acid .

’

The magnet ite of these ores hasgenerally undergone secondary changes , resulting in


the mineral martite,which is abundant all through the

Tagilsk district .The G o ro blago dat district offers very good examplesof iron ores existing as segregations from syenit icmagmas . The rocks of the Mount Blagodat regionare similar to those ofWysso kaia , but present a slightlygreater variety

,ranging from true augite syen ites to

fine compact orthoclase rocks . The figure below showsthe mode of occurrence of the ore bodies , and it will be

seen that they exist in several well -marked dyke - l ike

2 3 2 3 2

FIG . 9 .—D IAGRAMMAT IC SECT ION To ILLUSTRATE THE MODE

OF OCCURRENCE OF I RON ORE S IN THE MOUNT BLAGODATREGION .

1 , Epido te-

garnet ro ck ; 2 , po rphyry ; 3 , iro n o re .

layers , making a fai rly big angle with the horizontal .They are separated from each other by the syeniticrocks , and thei r margins are well defined. The rocksas a whole have undergone a good deal of m etamo r

phism , giving rise to such minerals as garnet , epidote ,etc . , and the district has been affected by many displacements subsequent to the formation of the ore .I n North America the i ron ores of I ron County

,

Utah , are partly due to segregation from igneous rocks ,and

fl partly to ,

metasomatic replacements (p . 240) and

4— 2


secondary enrichments in the surrounding rocks . Theore of igneous origin consi sts of magnetite with a lowpercentage of phosphoric acid , and occurs as nearlyvertical dykes in a hornblende porphyrite . I n mode o foccurrence these masses are similar to those of theG o ro blago dat region .

Iro n Ores from N epheline Syenites .-N epheline

syenites and thei r satellitic differentiat ion products area dominant featu re of some of the Continental coastal

ranges . But as regards segregations of i ron ore , thebest examples may be taken from Brazil . Two important areas of i ron -bearing rocks , known as Jacupirangaand Ipanema , occur in the district of Sao Paulo ; theyare separated from each other by the Serra do Mar, andare about ninety miles distant from each other. Theores owe thei r origin chiefly to the differentiat ion ofa magma before intrusion , but also in a measure to thefurther differentiation of some of the resulting rocks i nplace . The region of Sao Paulo i s o ne of clay S lates andphyll ites , cut by a great variety of intrusive rocks allrich in alkalies , and include such types as orthoclasepyroxene , orthoclase -nepheline , and plagioclase -nephel ine rocks , with teschen ites , vogesites, etc . From thefami ly resemblance presented by these intrusions andother Considerations , we are led to regard them as theproducts of differentiation of a magma allied to that ofa nephel ine syenite . The chief ore -bearing rock is apyroxenite

,occurring in bosses and dyke - l ike masses, and

consisting of a violet t itaniferous augite with magneticand titaniferous i ron ores . Locally the i ron oxides predominate to such an extent that the pyroxene practicallydisappears , and the rock becomes almost a pure ore .

Several of the ore-bearing pyroxenites contain nephe


l ine , especial ly that to which the name jacupirangitehas been given

,but with the prevalence of nepheline

magnetite becomes less common , while several otheraccessory minerals, such as perofskite and apatitebecome less rare .

A tantalo -niobate occurs associated with some of theores, and is interesting as being analogous to the segre

gatio n of the vanadinates in the Taberg and Adirondackgabbros .From a considerat ion of the above , i t will be seen

that the magnetite deposits of Sao Paulo and otherdistricts in Brazil have a somewhat different origin tothe i ron ores of other area s , and that the parent magma ,through differentiat ion , has given rise to a series ofrocks differing in a great measure from those associatedwith the i ron ores which we have previously dealt with .

A marked example of similar rocks yielding magnetitedeposi ts may, however , be cited from Aa

,in the

Gulf of Bothnia , where i ron ores have been segregatedby rocks having nepheline syenite affinities .

SEGREGATION OF CHROME IRO N ORE .

Chrome i ron ore , or chromite , in its mode of occurrence and general hab it i s closely all ied to magnetite

,

but,as far as is known , i s only found as a segregation

from basic and ultrabasic rocks . I t may be regarded asthe most characterist ic differentiation product of thehighly basi c ferromagnesian S i l icate magmas , such asthose of the peridotites , picrites , and highly basic gabb ros .All these ferromagnesian S i l icate rocks are most l iab l eto alterations , which result i n the formation of serpentines , and it is largely in masses of serpentine thatchromite is mined .


This mineral i s widely distributed , but it will sufficeto mention a few local ities. Perhaps the most important chromite deposits are those of the serpentinemasses of Asia Minor, especial ly those occurring in theneighbourhood of Daghardy. The ore exists as stockor dyke - l ike masses, schlieren , and ultrabasic patchesin serpentine formed by the alteration of peridotitesand picrites .Similar deposits exist i n the region of Kraubat, i nUpper Styria , where many peridotite masses occur ,as segregat ions in serpentine in N ew Caledonia

,in the

ultrabasic members of the Boschveld Pluton ic Seriesin South Africa , in fresh peridotites with in the NorthPolar Circle , i n Northern Norway, N orth Carolina , andmany other districts .An analysi s of the Kraubat ore gives the followingpercentages

S io2

MgOCao

FeOA1

203

Cr203

Although the Briti sh peridotites,picrites

,and ser

pentines, as well as other basic rocks , often containchromite , no deposits of economic value are known .

I n Africa some Southern Rhodesian serpentines haveyielded chromite and picotite

,with traces of platinum

and a small proportion of nickel and cobalt as sulphides .

SEGREGATION O F CASS ITERITE .

Cassiterite (tin dioxide) i s not usually met with inthe oxidic segregations from Igneous rocks , although it

56 TH E GEOLOGY O F ORE DEPOSITS

the gran ite , and consist of stanniferous quartzose andfelspathic varieties ; near them the granite containsfluo rspar. The copper lode is believed to be connectedwith the pegmatite -phase of intrusion , and consists ofquartz , felspar , muscovite , siderite , calcite , fluo rspar,and copper and i ron pyrites . The presence of speculari ron ore in small quantities in the lodes , and separatelyin quartz veins

,i s of interest . The extreme hydro

thermal phase i s supposed to be answerable for themetasomatic replacements of quartzite by tin ore

,de

scribed o n p . 298.

The granitoid rock of E tta Knob , in South Dakota ,which i s so rich in cassiterite , has been regarded asShowing marginal segregation of this mineral . Wecannot

,however, accept it as an example of true

segregation such as we have considered previously .

The outer zone of the granitoid mass differs considerably from the rest of the rock , i s highly alteredin Character, and contains abundant fluo rine-bearingminerals , such as Spodumene , etc . This deposit moreprobably results from pneumatolytic processes actingon the margin of the rock-mass during the later stagesof consol idation . A similar coarse pegmatite occurs inTexas (Llano County) , and contains many rare radioactive minerals

,such as gadolin ite , but , unlike the E tta

Knob pegmatite , i s non - stanniferous .

An interesting example of what appears to be a trueoxidic segregation of cassiterite has lately been described by the Geological Survey from Ross -shi re .

The ore occurs in a fol iated granite gneiss as streaksand veins 100 to 2 50 yards in length , and 1 0 to 1 5 yardsi n width ; i t consists chiefly of magnetite , with from

3 to 5 per cent. o f cassiterite , and , aswe should expect


in any true segregation of cassi terite , there i s a com

plete absence of tourmaline and other minerals ofpneumatolytic Origin .

The relat ion of the ore-masses to the rest of thegnei ss is Obscure on account of the shearing andfaulting which the district has undergone ; and althoughmagnetite occurs disseminated through the countryrock

,cassiterite has as yet not been detected .

Although the ore i s most probably a segregation , wemust not lose sight of the possibility that these veinsmight have belonged to the sediments of the MoineSeries , which as the result of profound metamorphismhave been converted into granulit ic gneisses .A somewhat similar occurrence appears to be thatof Nurunga , in Bengal , I ndia , where tinstone is associated with magnetite

,black mica , and felspar, in

lenticular beds in gneiss . The origin of the ore i suncertain , but the tin may possibly be connected withthe pegmatite vein s which traverse the gneiss.Cassiteri te occurs as an original constituent of afew granites and acid volcanic rocks Banka andBill iton I slands) , diffused more or less uniformlythrough the rock-masses

,but particularly as inclusions

in mica . I t seldom occurs in sufficiently large quantities to be of any direct economic value , but suchrocks may yield rich alluvial deposits . Tin -bearinggranites are usually characterized by the presence oftourmaline and lithia-bearing mica .

SEGREGATION O F ALUM INA AS CORUNDUM .

Corundum cannot truly be regarded as an ore , butthe mineral i s of such economic importance as anabrasive agent , and so well i llustrates the processes of

58 TH E GEOLOGY OF ORE DEPOS ITS

segregation and metamorphism , that it has been founddesirable to include it in th is work .

Although many of the corundum deposits of CentralE urope , India , and South Africa , are of metamorphicorigin (p . a certain number in other parts of theworld result di rectly from the differentiat ion in placeof basic igneous magmas

,and those of the United

States are especially instruct ive . True segregations ofcorundum occur in the Appalachian region of the UnitedStates , and are yielded in all cases by rocks rich inmagnesia and iron , and relat ively poor in si lica .

The greater number of corundum segregations arefrom peridotites and serpentines , a few from norites ,and a sti l l smaller number from amphibolites formedfrom the pyroxene-bearing rocks by the processes ofdynamic metamorphism . The corundum forms anintegral part of oval and lenticular intrusions of theserocks occurring in a metamorphic area largely composedof gneisses and schists . The peridotite masses of theUnited States

,i n which corundum has been found in

quantity, range from the central district of Alabamanorthwards to the Gaspe Pen insula on the Gulf ofSt . Lawrence . They are especially frequent in NorthCarolina and Georgia ; others occu r in Connecticutand Massachusetts

,and norites are mined in the neigh

bo urho o d of Peekskil l in the State of N ew York . I nevery case , the concentration of the ore has taken placeat the margin of the rock-mass in the cooler zone , andthus occupies a position analogous to that of some ofthe i ron ores described above

,and most of the nickel

bearing segregations of Norway and Canada , to bedescribed later . The figure given below shows thegeneral position occupied by the corundum ore

’

with

D IFFERENTIAT ION 59

reference to the parent peridotite mass , and is accom

panied by a section Showing the relations in depth .

FIG . Io .-MAP SHOWING THE OCCURRENCE O F CORUNDUM As

A D I FFERENT IAT ION PRODUCT OF PER IDOT ITE .

FIG. I I .— SECT ION ACROSS FIG. 10 .

1 , Perido tite ; 2 , co rundum ; 3, co untry ro ck .

The corundum ores of igneous origin differ in severalways from those formed by the thermal metamorphosis


of sediments,and should be readily dist inguished ; for

besides the marked differences in the mode of occurrence,

there i s usually a complete absence of true metamorphicminerals

,such as cyanite , which play such a prominent

part in the corundum rocks of Burma , Siam , and otherdistricts . I n the case

‘

of the peridotites , there i s l ittledoubt that the alumina was dissolved in the originalmagma at the t ime of its intrusion into the countryrock , and that it separated out together with other ratherin solub le oxides , such as chrom i te and magnetite , at anearly stage

,as the mass commenced to cool .

I t has been proved that alumina is soluble to someextent in molten magnesium silicate , and i f the magmahas no excess of magnesia , then all the alumina crystall izes as corundum . I f, on the other hand , there i s aS l ight excess of magnesia over the magnesium sil icates

,

some of the a lumina will combine with that,and

crystall ize as spinel , the remainder forming corundum .

I t is therefore not surprising to find spinel segregratedfrom basic magmas rich in magnesia , and occurring asa frequent associate of corundum .

I n Canada , supposed original segregations of co rundum occur in Ontario , and have been derived fromrocks ranging from a normal syen ite to a mica

,horn

blende , or nephel ine syenite . But thei r mode of originhas as yet not been sati sfactorily proved , and it seemsthat they present points of l ikeness to the deposits ofUpper Burma which are metamorphic in character .I t i s most probable that true segregations of corundumwill be found to be restricted to the more basic plutonicmasses representing the magmas rich in magnesiumsil icates .Corundum may be formed in some instances by the


absorption of aluminous material by an igneous magmaon its passage through a sedimentary series of rocks ,and the subsequent crystall ization of the excess ofalumina on cooling . The action between the igneousrock and sediment i s , however, generally quite local ,and takes place within narrow limits ; i t i s doubtfulwhether the formation of corundum in such cases shouldbe regarded as a true segregat ion , for i t seems morenatural to regard it as the outcome of a metamorphicprocess identical with that which gives rise to thecorundum in masses of sediment caught up and included ,without absorption

,in an igneous magma .

S EGREGATIO N O F METALL IC S ULPH IDES .

Segregations of sulphides of certain metals , as wellas small quantities of arsenides , are h ighly characteristicof several types of igneous rocks, and are clearly thedirect result of the differentiation of the magma eitherafter or before intrusion . By far the greater numberof sulphidic segregations are yielded by basic igneousrocks , rich in such ferromagnesian minerals as hypersthene and other orthorhombic pyroxenes , and with asil ica percentage of not more than 55 . The sulphidiccompounds are apparently soluble in fused sil icates athigh temperatures . The metall ic sulphides whichcharacterize this class of segregations are those ofn ickel , cobalt , Copper , and i ron ; there i s usually sometitanium , and occasional ly the percentage of oxidic i ronores may reach a considerable figure in segregationswhich form the connecting l inks between the oxidicand sulphidic types . Very occasionally some of therare metals , such as platinum , i ridium , rhodium , andpalladium , are found associated with the metallic


sulphides , but they exist only as arsenides, as far as isat present known . In the sulphidic segregations frombasic igneous rocks there i s generally an absence oflead , zinc , S i lver , antimony , bismuth , and t in , andarsenic is of rare occurrence . Molybdenum Is not anuncommon constituent of certain granites .Should several metals exist as sulphides in one andthe same magma , i t has been found that they have adefinite order of crystall ization , and therefore a varyingdegree of concentration for instance , suppose a magmato contain nickel , cobalt , i ron , copper in the form ofsulphides, and some ti tanium , on cooling , pyrites richin cobalt would be the fi rst mineral to crystall ize , andwould be segregated with the titanium . The pyriteswould be followed by nickel i ferous pyrrhotite , pyrrhotite ,and copper pyrites . Pyrrhotite appears to favour thoserocks rich in ferromagnesian minerals , while pyrites i snot so restricted .

The concentration of sulphides i s generally bestdisplayed by magmas which have differentiated in place

,

and the segregations are almost always marginal inCharacter , thus differing from so many oxidic segregations which occur as ult rabasic dykes and schlieren dueto differentiation of the magma before i ntrusion .

Another difference of importance i s that in the caseof sulphidic segregations there has been no complementary segregation of the ferromagnesian sil icates

,

as was such a conspicuous feature of many of thesegregations of oxides previously described .

Rocks with gabbro affinities may be divided roughlyinto two overlapping groups— the one olivine -bearing ,and the other containing some orthorhomb ic pyroxene ,such as hypersthene . The olivine -bearing rocks, as well


SEGREGATIONS O F N ICKEL AND CO BALT .

The nickel and cobalt ores that occu r as segregationsfrom norites and other hypersthene -bearing rocks, maybe divided into two groups :

(a) Sulphidic ores , consisting of the i ron -nickel sul

phides, pyrrhotite , and nickel iferous pyrites , and then ickel sulphides millerite and polydymite .

(b) Sil icates , consisting of several n ickel - bearingsil icates formed by the alteration of sulphides andarsenides subsequent to the segregation .

N ickel and cobalt invariably occur together, but theratio of the two metals i s a most inconstant quantity ,varying considerably according to the type of sulphidewhichc arries them , and being governed to a great extentby the rate of cooling of the magma . N ickel is segregated chiefly as nickeliferous pyrrhotite , a double sul

phide of iron and nickel , and occurs with titaniferousi ron ore and a fai r quantity of copper and iron in theform of pyrites (CuFeSz) .Pyrrhotite always contains 2 to 5 per cent . of nickeland cobalt taken together , and should a pyrites crystall ize out before the pyrrhotite , i t is generally richer incobalt than in nickel thus, while in the pyrrhotite theratio of nickel to cobalt is as 1 0 to 1 , in the pyrites it isreversed .

The iron sulphide segregations , containing a highpercentage of n ickel and cobalt ( 1 5 to 20 per donot carry these metals comb ined with pyrites , but asthe sulphides millerite and polydymite , contain ing 5 to1 0 per , cent . of nickel , intimately mingled with thepyrites .The average nickel i ferous ores contain about 1 per


cent . of Co to 6— 8 per cent . of N i , but in some casesthe ratio is as 1 to 3 or as 1 to I O or 1 5 . This variation is due

,as before stated , not so much to the original

content of the magma , as to the degree of concentrationdependent on the rate of cooling of the magma as awhole .Numerous examples of sulphidic segregations of n ickel

and cobalt from norites may be drawn from SouthernNorway and Sweden , P iedmont in

'

Italy, Canada , andthe United States of America ; and other nickel ores aremet with in Malaga, N ew Caledonia , O regon , and NorthCarolina , associated with serpentines .I n the E rteli district the ore -bearing rocks form a

transitional series from ordinary norites , which containmost of the ore , to an ol ivine gabbro without ores , andpresent a most instructive example of the results ofdifferentiation . The rock -mass varies gradually froman ordinary augite and olivine- free norite , through adiallage-bearing but O l ivine - free no rite , to a hypersthenegabbro ; which gives place to an olivine norite , andult imately to a hypersthene- free ol ivine gabbro . Theolivine is segregated on the one hand , and the rhombicpyroxene with the n ickel ores on the other . At Klefva

,

in Smaland , the rock-masses vary from a hypersthenegabbro to a pyrrhotite norite . and are identical inoriginal character to those described above , but subsequent changes have resulted in the transformation ofmany of the rocks from gabb ros to urali tes .Good examples of the marginal segregation of nickel

ifero us pyrites from norite and gabbro -magmas are alsofurnished by the following three local it ies— the Nystenand Bamle Mines

,the MeinkjaerMi nes , and the Skang

Mine .


The ore is magnetic pyrites with 5 to 6 per cent . Ofn ickel and cobalt

,and is met with at the j unction of

the gabbro -masses with the hornblende -schists , etc . ,

into which they are intruded .

Although the ores almost always exist at the peri

phery of the rock-masses

,veins and patches o ccur

‘

in

some cases . These patches are sometimes composed ofpyrrhotite norite

,sometimes of extremely pure pyrites ,

and occur well within the gabbro -masses .

Another well -known E uropean example is that ofVarallo in Piedmont . The mica schists and gneissesof the Monte Rosa neighbourhood are broken throughby masses of norite , which are sometimes withoutol ivine and augi te , but at other times contain these twom inerals .

The ore consi sts of a pyrrhotite norite , as -in the caseO f the Scandinavian masses ; i t contains 4 to 5 per cent .of nickel and cobalt , and occurs at the contact of theigneous rocks with the schists and gneisses , into whichthey have been intruded . Two well -known mines arethe Cevia and the Sella -Bassa .

The nickel ores of Canada and the United Statesoccur in three ways— either as masses fringing theintrusions , as impregnations through the rock -mass , oras veins fi lled by solutions subsequently to the intrusionof the magma . The last two types are not segregations

,but belong to a different category , and therefore

will not be discussed here , but in thei r place in asubsequent part of this volume . I n Canada the oresof Sudbury have received the most attention , andalthough several theories have been propounded toexplain thei r origin

,none seems SO satisfactory as

segregation .

D IFFERENT IAT ION

The ore-bearing rocks without exception are noritesor derivative diorites (ural ites) , and have been differen

tiated from ophitic quartz-bearing gabbros and pegmatites. These masses throw off occasional norite dykes

,

5—2


which yield nickel ores on thei r margins,and small

bosses of norite also occur . The most interest ing massi s the one shown in the accompanying map as rangingnorth -east and south -west of Windy Lake . H ere themarginal position of the ores is well seen , and the variat ion in the rock -mass may best be studied . The interiorof the igneous masses consists of a pegmatite with asi l ica percentage of 697 and a specific gravity offollowed towards the margin

,the si l ica falls to 49 9 , and

the specific gravity rises to 28 5 , as the rocks loose theirpegmatitic character and pass gradually into norites .I n the ore itself the si l ica percentage i s less than 10 .

Similar masses occur nearer to Sudbury itself,and

several n ickel-bearing norite dykes may be noticed,as

at Stobie and Worthington .

With the nickel i s segregated a fai r quanti ty of Copperin the form o f chalcopyrite , and some magnetite , so thatthe chief ores are chalcopyrite

,pyrrhotite , and pent

landite . The ratio of copper to nickel is most variable,

and,taken against 100 parts of copper, the nickel varies

from 2 15 to 1 70 .

I n consideration of the origin of these ores,i t i s

interesting to note that there i s almost a completeabsence of underground water in the mines , making itmore or less evident that solution has played no part

,

and this i s stil l further borne out by the fact that noneof the minerals Show traces of hydration .

Sudbury , besides furnishing us with some of the bestexamples of sulphidic segregations , is also characterizedby the comparatively rare segregations of arsenides , themost remarkable of which is sperryl ite , essentially aplatinum arsenide , having the theoretical compositionPtAsz.


A detailed analysis gives the following figures , andreveals the fact that other rare metals besides platinumare represented

Per Cent .

P t

RhPd traceAs 42 2 3Sb

Gold,si lver

,and some tin

,have also been detected in

the Sudbury ores,and platinum in the n ickel iferous

ores of Klefva . I n the United States o ne of the mostfamous nickel iferous districts i s that of Lancaster Gap ,in Pennsylvania . H ere we have a most beautifulexample of di fferentiation in place , for the intrusion ,which is almost symmetrical in form

,rapidly becomes

more basic towards the margin , along which the n ickelores form an almost continuous zone . The igneousmass consists of amphibolite , a uralit ized norite , intruded into mica schists , and the ores of pyrites andnickeliferous pyrrhotite . Analogous to the segregation of sulphides and arsenides from the Sudbury no ritesmentioned above are the segregations of nickel arsen ideconnected with the Malaga serpentines. These rocksvary from true peridotites , th rough picrites , to norites ,and the ores change from nickel si l icates near thesurface to nickel arsenide in depth . They are associatedwith a good deal of chromite

,which in the richer ores

i s seen to have been segregated at an earl ier period thanthe nickel arsen ide which fill s the interst ices betweenthe chromite crystals . The nickel S i l icate ores containfrom 1 to 20 per cent . of n ickel , and are evident lyderived from the arsenide

, which i s a true differentiationproduct , by the action of ci rculating waters .


D i stricts in N ew Caledonia , O regon , and NorthCarol ina , all yield n ickel sil icates associated with olivinerocks , such as peridotites , now converted into serpentines .

I t i s quite possible that these ores have been derivedfrom segregated sulphides and arsenides , but as yet theunaltered minerals have not been met with .

SEGREGATIONS O F CO PPER AS PYRITES .

Li ttle need be said regarding the segregation ofCopper ores from igneous magmas , for their mode oforigin is identical with that of nickel and cobalt

,and

they O ften accompany those metals in considerablequantity. Copper ores as sulphidic segregations arenot common

,however ; most of the rich deposit s have

a different origin , and exist either as precipi tationsfrom solutions or as pneumatolytic and metasomatic

deposits .I t has been previously stated that Copper in the

segregations almost always exists as the Copper- i ronsulphide chalcopyrite (CuFeSz) , and i s usually asso

ciated with basic pyroxenic igneous rocks . I n the

N amaqualand district of South Africa , sulphidic copperores occur associated with igneous rocks . The districti s one of granite , gneisses , and schists , capped locallyby sediments , and pierced by intrusions of basic igneousrocks of gabbro affinities . The ore deposits are irregularin form

,and occur both in the basic rocks and in the

granite . At N abatiep the ore occurs chiefly in thebasic rock as streaks and bunches , and seems to bea product of di fferentiat ion .

I n the nickel-bearing ores of Sudbury , in Canada , andother regions , the percentage of Copper i s occasionallylarge

,and , according to some authors , the rich copper

CHAPTER I I I

PN EUMATOLYS I S

A GREAT many ore deposits are now recognized to bedirectly connected with plutonic intrusive rocks

,and

to have been derived from them through the agencyo f magmatic gases dissolved in them at the time ofintrusion , and exist ing , therefore , at high pressure andabove their critical temperature .

The conditions of pneumatolysi s are those of thel iberation of magmatic vapours and steam during theconsolidation of the deep - seated intrusive rocks . Theaction is not thought to have ever occurred beforeconsol idation to any extent , while after complete consol idation the process may be regarded as havingal together ceased .

The vapours during the consol idation of the magmaplayed an important rOle in the extraction of the metalsdisseminated through it , so that in the absence of suchvapours

,pneumatolytic action , except in cases where

the metal itself is volatile , i s impossible , and ore depositsof this nature could not be formed . I n such a case anymetals present in the magma would remain in the rockon consol idation as accessory oxides , sulphides, orsil icates

,distributed through it l ike any other constituent

mineral ; or i f in large quantity would become essential

7 2

PNEUMATOLYS I S 73

constituents,or even massive differentiation products ,

forming deposits known as ‘ segregations . ’ The pneumato lytic action is therefore a process of extraction ofmetall iferous minerals by active superheated gases , inthe absence of which the sulphides or oxides of themetals would have remained in the rock as ordinaryconstituents .The assumption that superheated gases contain ingmetals as volatile compounds were the main agents inthe formation of certain types of ores necessarily implies that in the majority of cases the deposits occur not farfrom the plutonic rocks which gave ri se to them , and intypical regions this so far holds true that the principaldeposits are found in fissures and joints in the alteredrocks of the metamorphic zone surrounding the pluton icintrusion , or in the igneous rock itself.Although the general principles of pneumatolys iswere conceived nearly a century ago

,the most important

contributions to the subj ect were made by ProfessorVogt in a series of papers published between the years1894 and 18993

“ I t i s mainly upon Vogt ’ s work thatthe classificat ion is based .

In a general way, the mode of origin of the severalclasses of ores under this category is the same , yet theores are quite distinct

,and are characteristic products of

the type of rock giving ri se to them ; and it is now wellknown that the metals and minerals of lodes associatedwith basic rocks (gabbros) and acid rocks (gran ites)indicate to a considerable extent the nature of themagma from which they were extracted .

Broadly speaking,the genetic classification of ore

Z eit. f Preki. Geol . , 1894, 1895 , 1898, 1899 .


deposits formed under pneumatolytic conditions depends upon three things

1 . The nature of the rock giving rise to the ores.

2 . The particular metals of the ores .

3. The minerals associated with the ores,indicating

the nature of the gases which extracted the metals asvolatile compounds from the magma .

To these gases the terms ‘ carriers,mineral izers ,

’

and agents minera lisateurs have been variously applied .

There are two broad Classes of ores of pneumatolyticorigin— the sulphides and the oxides— one or both ofwhich may be secreted during the consolidation ofeither the acid or basic plutonic rocks ; but since eachmagma has its Characteristic metals and mineralizers

,

’

the deposit to which it gives ri se wi ll be oxidic or sul

phidic according to the affinities of the metals containedin the magma and to the presence or absence of sulphur .Thus , ti n practically always occurs in the form ofoxide , whether sulphur be present in the magma or not .I ron may occur either as oxide or sulphide , lead nearlyalways as sulphide .I n a fused condit ion , acid magmas , such as those

giving rise to granites , although intensely hot , are considered to have been very viscous or pasty , since onintrusion among the overlying rocks they have assumedmassive or bulky

,instead of sheet - l ike , forms . These

immense laccol ites are generally of deep - seated origin

(batholiths) , and consequently all the conditions of slowcooling were present .I n basic magmas

,on account of thei r lower con

sol idation points , the l iberation of magmatic gasestakes place at lower temperatures . Thei r fusibi l ityaffects to some extent the form of the intrusion .

PNEUMATOLYSI S 75

During the cool ing of the igneous masses the rockssurrounding them are thermally metamorphosed fordistances varying from a few hundred feet to a mile ormore

,so that they , too , are in a heated condition .

Although the metall i ferous minerals are very sparinglydistributed through the magma , they are neverthelessgradually withdrawn from it during crystall izat ion bymeans of active magmatic gases , and in time are moreor less concentrated in the not fully consol idated partsof the intrusion .

The next step in the process i s the escape of themetall i ferous gases , and concentration of the ores in theposit ions in which they are found . I t is only necessary

,

then , that the means of escape should be provided , andin deposits of this nature this i s invariably in fissuresformed by various causes in the solid parts of theigneous rock and in the older rocks beyond .

I n traversing fissures in the cooler rocks reactionstook place with the l iberation of metall ic oxides andsulphides , and the formation of characteristic mineralsthrough the action of the remaining gaseous materialon the country - rock .

Once the magma was consol idated , the condition s ofpneumatolysi s ceased , and , in stead of vapours , thermalmineral waters were given off. The change from theo ne condition to the other i s gradual , but it appearshighly probable that

,under the cooler conditions in

which liquid solutions occur,a set of substances may be

deposited in fissures,which , owing to thei r inabil ity to

form volati le compounds,were not carried off in the

earl ier gaseous phase,or

,at any rate , were only carried

Off in small quantity . At the same time , many substances which were readily volatil ized under pneuma


to lytic conditions became , under cooler conditions,stable compounds no longer capable of extraction .

This appl ies not only to the ores , but also to themineral izers, which l ikewise have to be taken intoaccount in treating of the origin of the deposits .According to BrOgger

’

s researches, the stage at whichthe pneumatolytic act ion is most intense i s that whichfollows the first possible phase of ore concentration

,

namely , that of magmatic segregation ; but it precedes , or i s in part contemporaneous with , the thirdphase at which zeolites may be formed through theaction of magmatic waters contain ing minerals in solution . In both th e pneumatolytic and zeol itic phases thealready consol idated minerals of the magma are l iabl eto be altered or decomposed .

Before discussing a few typical cases , i t should benoticed that o re deposits of pneumatolytic origin invariably occur as infil lings of pre - existing spaces and asimpregnations of the rock in their vicinity, so that theshape of the deposit i s largely determined by j oints

,

i rregular fissures,faults , frict ion- breccias , partings along

bedding°

planes or other spaces in the sedimentaryrocks of the metamorphic zone, or in the igneous rockwith which the deposits are genetically connected .

But although the form of the depos it i s of greatimportance to the miner

,the nature of the alte ration

which the vapours , once traversing the fissures , haveeffected i n the countryrock

,at the sides of the lode ,

i s of considerable interest . I n many cases the wall s ofthe lodes have been so entirely altered by the vapoursthat they are rendered unrecognizable as part of thecountry- rock ; and it i s by the nature of this alteration ,and by the minerals in the lodes , that the origin of the

PNEUMATOLYSI S 77

ores may be determined . I n all classes of ores ofpneumatolytic o rigin

,the rock in the vicinity of the

fissures,i n addition to other secondary minerals , in

variably contains some of the ore impregnating it tomore or less extent ; that i s to say , some of the minerals

of which the country-rock i s composed are replaced bythe ores, while the rest may not be al tered or affectedin any way .

Typical examples of pneumatolysis may be takenfrom the ore deposits connected with gabbros andgranites respectively

,and a comparison of one with the

other will show that,while the minerals taking part i n

the action in each case are different,the general mode

of origin i s the same .

ORES O F PNEUMATO LYTIC ORIGIN CO NNECTED WITHGRAN ITE AND ITS ALL IED RO CKS .

Normal granites are holocrystall ine rocks of hypidiomorphic structure , which consist of a ferromagnesianmineral , generally biotite (but occasionally hornblende) ,muscovite , felspar (orthoclase , and sometimes acidplagioclase) , and quartz . The percentage of free andcomb ined si l ica varies from 65 to 80 .

The principal all ied plutonic rock is syenite , which isof less sili ceous character .The family of fine-grained porphyritic rocks of similarchemical composition to normal granite , but occurringin the form of dykes or sheet - l ike intrusions, are notdirectly connected with ore deposi ts of pneumatolytico rIgIn, owing to their small bulk and thei r consequentinability to supply enough material for the productionof ore deposits , and to a certain extent to the conditions


of rapid cooling to which thei r fine-grained Characteris due . The principal amongst these are the quartzporphyries , or elvans . I ndi rectly , however , thei r connectio n with the granite on the one hand , and the oredeposits on the other, i s of great interest , as it is acommon feature in some of the typical regions for theprincipal metall i ferous deposits to occur in an areawhich had previously been a focus for dyke intrusions .The ores characterizing typical pneumatolytic depositsin granites are those of cassiterite

,and wolfram and

scheelite , any of which may be accompanied bysulphidic ores of copper

,i ron , and arsenic in large

quanti ty . I n less quantity,but not uncommonly ,

there may be ores contain ing the metals bismuth , oreven antimony, zinc , cobalt , occasional n ickel , manganese , molybdenum , uranium , and gold ; still rarer,mineral s contain ing tantalum and titanium . I n someinstances argenti ferous galena occurs with the tin ,tungsten , copper, arsenic , or other ores of undoubtedpneumatolytic origin , and its presence , although exceptio nal , may be regarded as one of the types connecting the filons stanniféres, or cassiterite veins , andthe filons plombiféres, or lead veins, since the conditions under which most lead veins are formed , althoughconnected with after-eruptive actions

,belong to the last

phases of igneous consol idation . I n Bolivia , however,the sulphides of lead and some other metals occur tothe exclusion of COpper ores , and may be contem

po raneo us with the tinstone .

I t should be remarked therefore that , according tothe metals originally contained in the eruptive magma ,the lodes may contain all or only one o f the aforementioned metals .


chlorine . The metals of this group of ore deposits existchiefly as oxides and sulphides .

NATURE OF THE ALTERATION IN THE WALLS O F TIN

LODES .

Hydro fluo ric and boric vapours , boro -S i l ico -fluo rides ,together with hydro fluo - sil icic acid and other gaseouscompounds , are able to decompose various constituentsof the countryrock ,with the production of newminerals .

While some rocks are readily altered in this way,there

are others which are practically unaffected .

Where the country - rock is not readi ly seen to betraversed by cracks , the microscope Shows that thealteration in the walls commenced in the first placealo ng

‘

m inute cracks or l ines of rifting , Cleavage cracks ,planes of bedding , and in interstit ial spaces . The corro sio n or alteration of the countryrock begun in thisway is readily carried on . The minerals which areparticularly attacked are felspars

,micas, and argil

laceo us and calcareous materials . The minerals whichare produced are topaz , axin ite , tourmaline (brown andb lue) , chlorite, kaolin , green and white mica , and fluo rspar ; and , probably, i ron pyrites, as a result of conversion o f oxide of i ron into the sulphide by sulphurvapours . F inally , silicificatio n of the countryrock isso common as to consti tute an important type ofalteration . In special cases garnet is also formed , butthis must generally be regarded as owing to a combination of contact and pneumatolytic actions .I t wil l be seen , therefore , that the type of alteration

was determined by the nature of the vapours or solutions which traversed the fissures , while the mineralsformed depended on the composition of the country-rock.

PNEUMATOLYS IS 81

Broadly speaking , the changes in the country- rockare brought about by greisen - action , silicificatio n ,

chloritization,kaol inizat ion , etc .

, and depend upon thenature of the vapours.I n granite the alteration is as follows : biot ite , by

loss of alumina , oxide of i ron , and magnesia , i s Changedto muscovite

,with occasionally some epidote and

oxide of i ron . I f boric acid be present , the mineral i s

FIG . I4.—D IAGRAM SHOW ING THE LODE STRUCTURE AND

NATURE O F ALTERAT ION O F THE COUNTRY-ROCK AT THE

BALLESWIDDEN M INE , ST. JUST , CORNWALL .

‘ Gry ,

’a tin-bearing j o int o r vein ; ‘ Hardwork,

’

greisen , consisting o fscho rl

,quartz , mica, and topaz ; ‘ Go ok

,

’

the so ft, kao linizedgranite beyo nd the hardwo rk.

altered to brown tourmaline . Chlorite may be formedfrom biotite by the action of sulphides (propylit ization) ,carbonic acid gas or carbonates , and frequently i sassociated with metall ic sulphides .The alteration of the felspar commences alongcleavage planes and cracks, with the productionof topaz (by fluorine) , muscovite (also gilbertite andzinnwaldite ), and quartz . Cassiterite frequently occurs

6


as pseudomorphs after the felspar of the ground massor of phenocrysts . Complete silicificatio n of the felSpar is not uncommon , while in the presence ofboric acid vapours blue tourmaline-needles are formed .

Although topaz is considered to be occasionally alteredto kaolin , the kaol inization of felspars is regarded asbeing mainly due to the action of carbonic acid gaswhich removes some of the potash and sil ica .

AI2 30 .K O .2 (3SiO 2) + 2H

2O + CO23

Orth20cllase

A12O .

3.2

K

SiO2

.2H2O + 4sio 2+ K2

CO2

.

The quartz of granite frequently remains unaffected ,with the exception of slight corrosion and the additionof secondary quartz grown in optic continuity.

M inerals such as apatite and zircon remain un

changed . Titanite and other t itanium minerals , including titaniferous biotite , are changed to ruti le .

I t should be observed, then , that the alterations areeffected by several kinds of vapours , of which fluorineinvariably belongs to the earliest emanations , whilecarbonic acid gas , hydrogen sulphide , and boric acid ,may belong to either the earlier or later emanations .The extent of alteration in the sedimentary rocks ofthe contact -zone depends partly on thei r composition .

In finely banded rocks consisting of alternat ions ofsandy and argil laceous films , i t i s the argillaceousmaterial which is principally affected by the vapours ,and in particular by boric acid and fluorine . The prin

cipal alteration when boric acid was present is tourmalinizatio n along the argi l laceous fi lms, for severalinches away from the vein (F ig . The altered rockis extremely hard, and , as all the previously formed

PNEUMATOLYSIS 83

folds or contortions in the rock are preserved,i t has a

remarkable appearance .I n some of the to urmal inized sediments from the

neighbourhood of Roche Rock , in Cornwall , the tourmaline of these mud fi lm s is of dirty brownish - greyor blue colour, with many impurities . With it therefrequently occurs cassiterite .

FIG . 1 5.- VE I N OF TIN -STONE , QuARTz, AN D TOURMAL INE

TRAVERS ING PALfEOZO IC SLATE S CONS IST ING OF ALTERNATEBANDS O F ARGILLACEOUS AN D S ILICEOUS MATER IALS

,

BELOWDA BEACON,CORNWALL . (NATURAL S I ZE .)

The argil laceo us bands are replaced fo r severa l inches o n either side

by to urmaline,while the siliceo us bands are unaffected .

In originally calcareous metamorphic rocks , such ascalcsil icate-hornfels

,the production of axin ite instead

of tourmaline takes place . This mineral occursin abundance in the altered calcareous bands of theLower Devonian rocks on the north of the St . Austellgranite mass

,in Cornwall , in which local ity tin veins

are also found .

84 TH E GEOLOGY OF O RE DEPOSITS

‘I n ther

o lder basic intrusive r’

ocks near the granitethe effects of pneumatolysis are also well marked . Inthe Corn ish greenstones axin ite is developed , and isassociated with veins of garnet and epidote . Theoccurrence of garnet veins appears to be due to contact alteration , with addition of si liceous materials bysolutions . On a large scale in Arizona garnet andcopper ore are found in association at the contact ofl imestone with d iori tic and granitic rocks , while similaroccurrences are known in Hungary and Transylvania ,Servia , the Urals, N ew South Wales, and Queensland .

I n the West of E ngland numerous similar occurrenceshave been noticed ; at Botallack , tinstone is foundin sheared garnetiferous green stone ; near Lostwithielcopper and i ron pyrites occur with massive garnet .On the northern margin of Dartmoor the ores areseen in connection with garnet at the Belstone ConsolsMine .The garnetizatio n of non -calcareous rocks has beennoticed in Idaho

,where at White Knob chimney- l ike

masses of garnet have been formed along cracks ingranite porphyry ; the garnet appears to have beendeveloped by the action of solutions contain ing i ronoxide and lime on the rock .

Silicificatio n of the walls of tin lodes is a commonphenomenon

,and the rock which results from this

alteration is quartz which encloses Chlorite or t ourmaline -needles . I t i s thi s variety of rock which is knownas peach in Cornwall .

ORDER O F ARRIVAL OF THE M INERALS IN TIN LODES .

I n tin lodes characterized by the presence of sul

phides i t is of interest to know at what (periods in the

PNEUMATOLYSIS 85

2M fi om M

Stray Park Engin e

S haf t

S ha f t

Go ss an Sha f t,

Hughb arrowWa t Sha f t

H'

tghbar row

Dw da n'

o MGrapp eb

'

8w 0 3

Eas t a p Sha f t,r

zi'l

B o un d cuy

D ownrcghb Shaf t

Mam E nguw 3mm.

.H’

ar v ey k Shaf t,

NB

HOI

M

SMOOO

3

N

IN

.LJ

OHO

N

.l

.


building up of the veins the di fferent minerals arrived ;or putting the question in another way

,what were

the different condit ions governing the emanation of theseveral vapours which brought the ores to thei r presentposit ion P

Where cassiterite and sulphidic ores occur in thesame lodes , as i n Cornwall , thei r association with oneanother is frequently so intimate as at once to suggesta contemporaneous origin for both classes of ores

,and

this V iew is supported in a sti l l more general way bythei r similar geological distribution . On the otherhand , from the frequent occurrence of copper sulphidedeposited on previously formed cassiterite , and fromthe occasional intersection of lodes principally contain ing tin by lodes °principally contain ing copper , i ti s quite certain that , while the ores were frequentlydeposited simultaneously, the copper. ores continued tobe deposited after the arrival of tin ore had ceased .

I t i s thus evident that vapours , such as fluorine , boron ,sulphur , carbonic acid , etc . , which conveyed the metals ,had a definite sequence of emanation .

There is one more fact , however , of particular interest in this connection , and that i s the remarkablezone - l ike arrangement of the ores of tin and copper inthe lodes , as showing a distinct tendency in each caseto a preference for one horizon over another . I nCamborne (Cornwall) the lodes in thei r upper parts wererich in copper

,but in depth are almost exclusively tin

bearing ; thus it may be inferred that much of the copperore was originally deposited from the vapours underconditions di ffering from those under which tin orewas readi ly formed . Where it happens , therefore , thatthe tin and copper ores are in intimate association ,


the last minerals to be deposited . Among these may bementioned nickel , antimony , manganese , and uranium ,

some of which , however, were deposited also in theearl ier phases of lode formation .

ASSOC IATION O F TIN AND CO PPER ORES WITH THE

METAMORPH IC RO CKS SURRO UND ING THE GRAN ITE .

The principal bodies of t in and copper ore nearlyalways occur somewhere about the peripheries of thegranite masses with which they are genetically connected .

The distribution of the ores as a whole i s , however ,not restricted to this position , and they may be foundat considerable distances from the granite - margins ,either in the heart of the granite -mass or near thel imits of the metamorphic aureole , and particularly inthe vicinity of elvans, or quartz porphyry dykes, whosepresence indicates a direct communication with theinterior parts of the granite (F ig . There can beno doubt that the conditions near the j unction of thegranite and the contact - rocks was more favourable tothe deposition of the metall iferous minerals than elsewhere ; while fissures

,faults , and crush -zones , of con

siderable size , affording Spaces for the building up of orebodies o f great commercial value , are also commonlyfound in such si tuations .

TYP ICAL E ! AMPLE S O F DEPO S ITS OF PNEUMATOLYTICORIG IN CONNECTED WITH AC ID INTRUS IVE RO CKS .

Cassiterite and Wo lfram .

— In Cornwall and Devonthe tin and Copper deposit s are intimately associatedwith five large granite batholiths , two o f which , i nWest Cornwall

,are intruded among sedim ents of

PNEUMATOLYSIS


O rdovician age , two in Central and E ast Cornwallamong the Devonian rocks , and one in Devonshireamong the Carboniferous and Devonian sediments.Here and there are smaller granite intrusions

,occur

ring as satell ites of the larger masses .Connected with the same eruptive centres

,and par

ticularly in the regions in which the most celebrated

mines occur Do lco ath, Cook’s Kitchen

,Carn

Brea , Wheal Basset , the Gwennap United Mines , etc .there are a number of elvans

,or quartz porphyry dykes

,

whose intrusion closely succeeded that of the granite,

but which preceded the last phase of eruptive activitynamely, the emanation of metalli ferous vapours fromthe consol idating granite magma .

The principal mineral centres are situated at one sideof the granite masses with which they are associated

,

and generally in the metamorphic aureole or in thegranite not far from the periphery .

This peculiar localization at one S ide of the granitei s i llustrated in the tin and copper districts of Camborneand St . Austell

,the latter of which i s shown in the

diagram (F ig . The explanation appears to bethat there was excessive disturbance in the sedimentaryrocks at one side of the mass during intrusion , andthis disturbed area subsequently became the centre forboth the intrusion of dykes and the formation of theore deposits . That this is so there can be but l ittledoubt since , i n the Camborne and St . Austell miningregions the lodes and elvans in each case have the samedirection

,while in the interior and on the Opposite sides

of the granite masses , respectively, the elvans are fewerand the lodes have a different direct ion .

There is also a marked difference in the structures


lying districts consist of clean cracks or j oints ofremarkable straightness , and, as already remarked

,

have a different di rection .

Consequently, although the veinstones from variousparts of either of these mining regions have such welldefined differences in structure , i t i s probable that thelodes all owe thei r origin to those disturbances whichmarked the close of the eruptive activity in the region ,so that the differences in bearing of the lodes

,as wel l

as the structures of the veinstones in different partsof the same region , must be regarded as having beendetermined by local peculiarities .I n no region in the world is the association of tin

and Copper so well exhibited as in the mines of Cornwall , where the evidence indicates that the t in andcopper ores , as well as such common mineral associatesas arsen ical pyrites

,wolfram

,and even small quantities

of zincblende , etc ., are of identical age .

E xamination of the lode materials , however, showsthat the stanniferous portions generally occur in whatare obviously the O lder parts of the veins, andappear to have been formed of the fi rst minerals deposited .

O ther cases Show such an intimate assemblage ofcassiterite and sulphide ores as to render the evidenceof successive deposition a matter for doubt .That the tin was probably deposited from such anactive compound as fluoride of tin points to the conc lusio n that cassiterite would frequently occur asreplacements and impregnat ions of the country-rock inpositions inaccessible to other ores , and so alwaysappear to have been the fi rst mineral to be deposited .

PNEUMATOLYSIS 93

Where the sulphides of copper do not occur alongsidethe cassi terite , they general ly occupy a higher horizon in

the lode,a peculiarity well exhib ited in the Camborne

FIG. 20 .

—TH E TIN D I STR ICTS IN THE E RZGEB IRGE .

(AFTER K. DALMER . )

mines,where

,excepting the outcrops , the lodes in

their upper parts yielded copper , and in depth tin ore .

This is part icularly the case with lodes traversing killas


i n the upper part and granite in depth . I t may beassumed , therefore , that the temperature of the rockshad much to do with the determination of the horizonat which the ores were deposited .

Tourmal ine and Chlorite accompany the ores . Theyoccur as alterations of the countryrock , or as slendercrystals embedded in quartz, and constitute the wellknown peach of the lodes . Topaz and white micaare also found as products of the alteration in therocks enclosing the lode .

I n the E rzgebirge , on the frontier of Saxony andBohemia

,the zonal distribution of the ores near the

granite intrusions with which they are associated is againwell exhib ited , as was shown by Dalmer (Fig . I nthat region the Palaeozoic sediments have been brokenthrough by successive acid igneous intrusions, the lastof which , i n post -Carboni ferous times , i s in the form ofa number of granite masses , or laccol ites , the largestbeing that of the Schellerhau in Saxony . I t is withthis and the smaller bosses of Obergraupen , Zinnwald ,Altenberg

,and Barenstein , that the several tin districts

are connected . I n the Zinnwald a proof of the closeconnection of the tin veins with the consol idation ofthe granite is cert ified by the occurrence of dykes of

aplite cutting the veins .I n other parts of the E rzgebirge , however , as at

Geyer,E hrenfriedersdorf, Sauberg , and in various places

in the E ibenstock granite -mass near Karlsbad , tin orehas been mined . The other minerals characteristical lypresent are scheelite and wolfram ,

fluo rspar, schorl ,arsenical compounds , uranium ores , with occasionallyoxide and carbonate of i ron , and sulphides of tin , leadand zinc , etc .


rence . They appear to be more or less connected withthe copper ores of the region .

At Long H i ll in Connecticut , wolfram ,

wolfram ochre,and scheel ite are associated with fluo r

spar,quartz

,topaz

,and white mica in such meta

One Fo o t

FIG. 2 1 .

—TIN LODE AT THE BUNNY M I NE , ST. AUSTELL ,CORNWALL .

The vein infil l ing is co arse caverno us quartz , having a distant

resemb lance to comby structure . It is a pegmatite vein , co ntain

ing tin -o re and wo l fram S ome felspar in the vein is kao linized ,

While the adjacent granIte is a l tered to greisen. A little fiuo rsparand to urmaline are present .

morphic rocks as quartz - zoisite - epidote - hornblenderock

,occurring at the junction of diorite-gneiss with

crystall ine limestone . To paz occurs in particular abundance in a separate lode containing wolfram and otherminerals . Some of the wolfram is pseudomorphous

PNEUMATOLYS I S 97

after scheeli te . The occurrence of pegmatites withminerals such as fluo rspar, topaz , etc . , indicates thatthe wolfram was formed under pneumatolytic condit ionsfrom the same magma as that which gave rise to thepegmatites (F ig .

One of the most remarkable deposits of tin ore , fromthe genetical standpoint , is that occurring at E tta

FIG . 2 2 .

—G EOLOGICAL STRUCTURE OF THE D I STR ICT AT TH E

TUN GSTEN M INE , NEAR LONG H ILL,CONNECTICUT .

(AFTER W. H . HOBBS . )

Mine , near Harneys Peak , in the Black H i l ls of SouthDakota . Here there i s an exceptional development ofpneumatolytic minerals .

O f a roughly oval form , measuring 1 50 feet by 200 feet ,the vein i s made up of layers of quartz and felspar

,

with stanniferous albite and mica ; then crystals ofspodumene of enormous size , varying from 10 to 50 feet


in length , surrounded by interstit ial albite and tinstone .Between this and the country rock is a complexaggregate of muscovite and bioti te ; tourmaline ; apatite .triphylite , autunite , and other phosphates ; columbiteone crystal of which weighed a ton ; tantalite andother rare minerals

,together with gold and silver ; lead ,

arsenic , and other sulphides . The large number ofrare minerals , considered in connection with the generalstructure of the mass , suggests that its origin is due tosegregation or intrusion . of a highly sil iceous residualmagma contain ing rare earths and ordinary pneumato lytic gases , such as boron , fluorine , etc . The deposi tmay be regarded as bridging over pure pneumatolysisand dynamic segregation (magmat ic segregation understress) .Wolfram has recently been di scovered in numerousstringers traversing mica schist at the village of Agargaon , twenty-five miles southeast of Nagpur in India .

The geological conditions of the tin lodes of Malay ,described by the Government geologist , Mr. Scrivenor,are analogous to those of Cornwall and the E rz

gebirge . There appears , however, to be a considerablenumber of deposits occurring in the form of pegmatites ,which have been exploited both by ordinary miningmethods (as at Bundi) , and by washing in the mannercommonly employed in the extraction of tin fromalluvium . I n Bol ivia and Southern Peru the conditionsare somewhat different

,as in that region many of the

tin ore deposits,found up to an altitude of feet ,

are closely associated with ores of silver , i n addition topyritic minerals ; the tin ore i s found in masses inthe gossan

,in the secondari ly enriched sulphides of

si lver , and . in the unaltered lode—ma terial in depth .


rule , principally developed near the margins of thegran ite masses . The slates are also invaded by quartzporphyry dykes , and all the rocks have been extensivelyaltered . I n th is alteration the felspar and mica havebeen replaced by topaz

,tourmaline , tinstone , and

secondary quartz . Fluorspar,pyrrhotite , mispickel ,

i ron pyri tes , and carbonate of iron , also occur as replacements .

FIG. 23.— D IAGRAM OF M ICRO -SLIDE , SHOW ING T INSTONE IN

BOL IVIAN QUARTZITE . (MAGNI F IED I4 D IAMETERS .)A

,Quartzite : the o riginal grains are enve lo ped in seco ndary quartzgrown in Optic co ntinuity ; B ,

tinstone,mica and seco ndary

quartz in bedding-

p lane .

I n the Pretoria district (Transvaal) tin ore has beenfound in brecciated veins , and as impregnations ingranite at Vlaklaagte , twenty - five miles from thecapital . Microscopic examinat ion shows that there wasmutual interference in the development of the cassiterite crystals and the secondary mica and quartz .A good account of the occurrence of cassiterite in

gran ite at Zaaiplaats Farm , Waterberg District (South

PNEUMATOLYSIS y4

,J OL

’

Africa) , has been given by Mr. Kynaston of the Transvaal Geological Survey . The farm is situated partlyon coarse Red G ranite and partly on a fine -grainedvariety

,which is separated from the former by pegma

tites. The tin deposit s occur in a zone a quarter of amile in width

,extending from Roodepoort to G roen

fontein . The ore -bodies are commonly in the form ofpipes

,about 2 feet in width . The inner portion of the

pipe is an altered gran ite impregnated with cassiterite .This is surrounded by a narrow zone of tourmalinerock and reddened granite , beyond which comes theordinary granite of the countryrock . The tin alsooccurs in coarse quartz veins along the j unction of thered and fine-grained granite . Sericite , fluo rspar, chlorite ,and occasionally calcite , with mispickel , copper pyritesand iron pyrites are associated minerals . Wolfram isrecorded from G roenvlei .I n Spain , however, near Malpartida de Caceres

(E stremadura) and other places , the stanni ferous lodesin granite are characterized by an unusually large proportion of apatite , sometimes accompanied by quartz ,calcium carbonate , i ron and manganese hydroxide , i ronand Copper pyrites , mispickel , copper carbonate , galenaand uranium phosphate . The walls are kaolin ized .

The apatite occurs in such quant ity as to make it aprofitable product of the mines . The Co stanaza veinis at least two and a half miles long , and where workedis from 1 0 to 20 feet in width . I t will be seen later thatthis type of vein may be regarded as t ransitional between normal tin veins on the one hand and apatiteveins on the other.The occurrence of ores in pegmatites appears to bemore intimately connected with the true differentiation

393253 ;THE‘

1GE011joGY OF O RE DEPOSITS

of igneous rocks than with after-eruptive actions , 50

that such‘

o ccurrences are referred to under segregations . I n many cases , however, particularly where thepegmatite or giant gran ite is rotted or altered , the tinore is of subsequent origin .

In the l imestone of Lower L iassi c age near Cam

piglia Marittima in Tuscany , veins of t instone areassociated with quantities o f brown haematite , whichoccurs in veins

,and also as metasomatic replacements

of the countryrock .

Whether there i s any actual genetic associat ion ofthe veins o ffitinsto ne with the b rown and red haematiteof metasomatic Origin or not , i t is certain that the tinstone was derived from the Tertiary acid eruptives ofthe region .

‘

Much of the iron ore with which the tinstone is associated is a secondary product from pyriticores . There i s a total absence of minerals contain ingl ith ium , tungsten , bismuth , molybdenum , fluorine , andboron ; but there i s a l ittle copper ore , principally asmalachite . At a distance of about two and a half kilometres north -east of Campigl ia a tourmal ine -bearinggranite of post-Jurassic age , or possibly of post -E oceneage , occurs , and i t i s probable that the ores are extraction products o f thi s mass .Among the tin and wolfram occurrences of Dakota

,

there i s one which i s comparable to that of tin in limestone near Campiglia— viz .

, the association of wolframand scheelite with beds of impure dolomite , grading intoquartzite

,of Cambrian age . This same dolomite which

has been largely silicified and impregnated with pyriticgold ore

,constituting the so -called refractory si l iceous

ore of that region . The wolfram occurs i n flat , hori

zo ntal , but somewhat i rregular masses , up to 2 feet in

1 9 4 TH E GEOLOGY OF ORE DEPOSITS

places in Mexico,granites rising through schists are

flanked by rhyolite,rhyolite tuff

,and obsidians of late

Tertiary age . The cassiterite occurs both in the graniteand in the rhyolite . I n the latter it is found as aggregates along parting -planes and faults in the rhyolit ictuffs , i n association with magnetite , haematite , quartz ,fluo rspar, topaz and kaolin , haematite and chalcedony ;in some districts with wolfram , as well as ores of lead ,molybdenum

,etc . I t i s quite certain that the metal

l ifero us mineral s of the rhyolites were derived from themagma that gave rise to the rhyolite itself, but thecondit ions of thei r emanation , and the pressure at

which the reactions took place were quite exceptional .I n Malacca a stanniferous siliceous sinter contains05 per cent . of SnO z. In the province of Satsuma inJapan , t in ore occurs in quartzose veins , t raversing softtuffs and Mesozoic sandstones, and shales with , here andthere , quartzite bands . Rhyolitic rocks are developedin the region . The tin ore is accompanied by galena

,

pyrites , and zincblende, of secondary origin .

O f the rarer metalli ferous and alkal ine- earth minerals,

it i s of part icular significance that while some of thedeposits appear to be of pneumatolytic origin , theyare all ied to the dynamic segregations already referredto . In particular may be mentioned the occurrence ofminerals contain ing fluorine

,chlorine , boron , zirconium ,

thorium , and tin , i n pegmatites differentiated fromthe augite and nepheline syenites of the igneouscomplex of South Norway ; in E astern Lapland (KolaPeninsula) near the bay of Kangerdluarsuak i n Greenland ; in Brazil , etc . These , l ike the pegmatites of E ttaMine , appear to bridge over the purely pneumatolyticWith the segregated types , and the minerals, such as

PNEUMATOLYSI S 1 05

lavenite , eucoli te , sodalite , melanocerite , tourmaline ,datol ite

,no rdenskio ldite , and zircon , bear the same

relation to the nephel ine and augite - syenite that thepegmatites containing topaz , beryl , and tinstone , do togranite .

In Greenland , t in ore , wolframite , sulphides of lead ,zinc

,and copper

,with mispickel , are accompanied by

rich deposits of fluo rine -bearing minerals in connectionwith gran ite masses . Cryolite , fluorspar, quartz, microCl ine , hydrated mica , carbonate of i ron , and columb ite ,are the principal vein associates ; while such mineralsas topaz

,tourmaline , and phosphates are absent .

Co pper‘

Su lphides .

— Sulphide ores , unaccompaniedby cassiterite , have in some districts been regarded asoriginating closely after the intrusion of the igneousrocks from which they were derived , and to have beenformed under conditions of pneumatolysis . The veryvaried conditions under which sulphidic ores may beconcentrated , owing to their solubil ity and wide occurrence in igneous rocks

,renders a classification of the

processes connected with the formations of pyriticnature a matter of di fficulty.

The copper lodes of Cornwall and their associationwith tin and tourmal ine have already been referred to

(p . 84) as‘ cassiterite veins characterized by presence

of copper ,’ but i t would be equally correct to speak of

them as copper veins characterized by the presence ofcassiterite . ’

There are , however , elsewhere copper lodes of pneumato lytic origin , i n which tin ore does not occur . I nNorway copper veins associated with greisen werederived during the consolidation of acid plutonic rocks .

Subordinate occurrences also exist in the Southern


Tyrol , where copper sulphide is found in veins , i naugite andesite , contain ing in addition iron pyrites ,tourmaline , scheel ite , apatite , felspar , and quartz , insuch intimate association as to indicate co ntempo rane

o us formation .

Tourmaline - copper veins also occur in O regon,

Mexico , and German South -West Africa . Copper vein scharacterized by tourmaline-rock and greisen are wellrepresented in Telemarken in Southern Norway

,where

they exist as bodies of various forms , in the gneisses ,schists , and gran ites . The other ores are pyrrhotite , i ronpyrites, tmolybdena , haematite , sulphides of arsenic , zinc ,l‘

ead , bi smuth , etc . ; fahlerz , uranium ores, magnetite ,with t itanite , etc . , also native gold and silver, inassociat ion with quartz , muscovite , calcspar and dolomite , fluo rspar, tourmaline , and occasionally beryl andapatite .In this local ity Vogt studied the mineralogical changeswhich took place in the country rock enclosing theveins . The granite consi sts of quartz , microcl ine ,orthoclase

,oligoclase , and a l ittle magnesia mica , with

accessory titanite,zircon magnetite , and apatite . There

i s practically no muscovite except where the rock hasbeen altered

,and there it consists mainly of muscovite

and quartz,with unaltered accessory minerals and a

little fluo rspar and epidote .

In N ew E ngland the copper deposits of Vermont ,South Strafford

,and other places , are connected with

gran ite . At Vermont the countryrock consists of

schists with calcareous bands , intruded by granite andpegmatite in which copper and iron pyrites are foundwith tourmaline . At E ly (near Copperfield) and the

E l iz abeth Mine ( South Strafford) the ores are similar ,


i s found near the walls of the veins , which are alteredto quartz and muscovite . The carbonate appears tohave been the last of the minerals to arrive in thelode -fissures.

Some cassiterite veins contain gold associated withmispickel , as at Vil ledo r, in the North of France ; whilethe cassiterite -tourmaline veins of Cornwall

,and of

E ibenstock in the E rzgeb i rge,are slightly auri ferous .

The pneumatolytic origin of some gold veins i s againproved by the occurrence of the metal with wolframand other minerals . I t i s found associated withwolfram in Colorado , and with scheel ite and sulphidesof i ron, copper , and arsen ic , in Idaho ; with scheel ite ,quartz

,sulphides of i ron , lead , and zinc , and carbonates

of l ime and magnesia , at the Val Toppa Mine , P iedmont ; in wolfram and scheelite at O tago (N ew Zealand) ,connected with granite . The veins are of the truecassiterite type , but are characterized by an unusualamount of gold .

I n Brazil , near Ouro Preto , an excessively sil iceousvein occurs between a hanging-wall of hornblende schists

(some of which are haematitic , and when decomposedproduce i tab i rite) and a footwall of micaschist andquartzite containing graphite , staurol ite , garnet , etc .The gold i s found principally in the vein , withtourmaline and sulphide of arsenic and i ron , but alsoin the quartz . A bismuth and gold alloy occurs here .

A younger generation of gold occurs with calcite .Sulphides exist in small quantity, while fine-grainedtourmaline

,felspar, biotite , etc . , are found as nests and

streaks in the vein -quartz and in the countryrock .

The b iot ite copper veins of Rossland (Britishby L indgren , appear to be of pneu

PNEUMATOLYS IS 109

mato lytic origin . The deposits yield both gold andS i lver values , particularly near the surface . The oresoccur either in single , or compound reticulated , fissures,in shear zones , or as i rregular impregnations in hornblendic intrusive rocks .

The ores consist of auri ferous pyrrhotite and mispickel , replacing the walls of fissures which are nowcomposed largely of b iotite , with hornblende , chlorite ,muscovite , calcite , quartz , and garnet . The breakdownof the hornblende and felspar of the country- rockresults in its replacement by b iotite and pyrrhotite .

T itanifero us Iro n Ore,M o lybdenite, etc —Accord

ing to Hussak , the origin of the gold deposits ofPassagem , in the province of Minas Geraes , near OuroPreto (Brazil) , is as follows : The vein i s an u ltrasi l iceous apophysis of a granite , and has been intrudedalong a series of schi sts , which it has thermallymetamorphosed . The intrusive nature of the vein i ssaid to be indicated by the presence in it of alb ite

,

zircon , monazite , xenotime , etc . But although this viewhas been combated , the gold and associated metall ifero us minerals (auriferous mispickel , pyrites , etc .

,

with quartz and tourmaline) may , at all events, beregarded as of pneumatolytic origin .

O f Special interest is the occurrence of titani ferousi ron oxide in veins in the G rossvenediger granite , andalso in the granit ic rocks of the Aarmassif (CentralSwitzerland) , where , although o f no economic importance as ores, they are of significance as constitutinga type of vein intermediate between tin veins ofg ranite and t itani ferous masses in gabbro . Althoughtitan i ferous minerals are not very prevalent in tin lodes ,there are some places, as at E hrenfriedersdorf, in the


Western E rzgeb i rge , where titanium is associatedwith the normal constituents . The above local ities ,however , are remarkable for containing it in unusualamount . I t generally occurs as rutile

,occasionally as

anatase and brookite and in titaniferous i ron ores,in

paragenetic association with quartz, fluo rspar, calcite ,apatite (fluo r-bearing) , tourmaline , chlorite , muscovite

(with and without fluorine) , albite , and rarely beryland molybdena . The alteration of the granite in theneighbourhood of the veins is as follows : Biotite hasdisappeared , and in the cavities so left an additionof secondary quartz and felspar (albite) has taken placeon that of the rock .

According toWeinschenk, the muscovite of the granitenear the contact with the slates is replaced by chlorite ,while in the schists and slates beyond zeolites occur ata moderate distance from the contact ; but near thecontact the place of the zeol ites

,i ncluding prehnite , is

taken by augite , epidote , asbestos, and occasionallygarnet . Along with these occur magnetite

,haematite ,

titaniferous i ron ore , and scheel ite . Other veins in thedistrict of the source of the R iver Aar occasionallycontain molybdena , leadhill ite and other plombifero usminerals , and born ite in association with the sulph idesand minerals already mentioned . From the occurrenceof small quantities of carbonates of the metals in theSwiss local ities , i t appears probable that there was asequence of deposition , and that the carbonates wereformed at the close of the true pneumatolytic phase .

The general mode of occurrence of molybdenumminerals suggests thei r pneumatolytic origin . Theprincipal ore , molybdenite , i s commonly found in quartzveins or as impregnations in pegmatites derived from

1 1 2 THE GEOLOGY OF ORE DEPOS ITS

as , for instance , in some gold veins— deposits may bedesignated by the name of a metal or mineral whichoccurs in them in quite subordinate ‘ amounts .But a well -marked type of deposit i s exemplified bythe occurrence of t itaniferous minerals and apatite withgabbro , which in Scandinavia (on the west of ChristianiaFj ord) has been well described by B rOgger and Vogt ,and in Canada by Sterry Hunt and others .The gabbros giving rise to these veins are olivinenorite (or ol ivine -hyperite) and olivine -gabbro , whichform two great groups

,in which the ol ivine and hyper

sthene appear to be interchangeable . This family ofrocks is somewhat inconstant as regards its structureand composition

,and while different names have been

given to the various deviations from the general type ,the rock as a whole may be described as a ho lo crystal

l ine plutonic intrusive mass consist ing of augite or otherpyroxene and a lime- soda -felspar . The constituentminerals are felspars

,ranging from labradorite to anor

thite ; augite , with accessory bronzite and hypersthene ;hornblende and brown b iotite as rare accessories ;olivine ; quartz occasionally ; and ilmenite , magnetiteand apatite as accessories .The principal agent in the extraction of t itanium andmost of the metall i ferous minerals of the lodes ischlorine

,which plays the same part in the gabbro

magma that fluorine does in granite , so that t itaniumis probab ly extracted as a chloride , and deposited ,according to the equation

TiC l4

2H2O Tio

g 4HC1;

and similarly i ron ,

Fe2CI

6 3H 2O Fe

2O36HC1,

PNEUMATOLYSIS 1 13

and other -minerals form soluble compounds which , inthe presence of steam , are capab le of extraction andsecretion . I n addition to these , however , Vogt bel ievesthat the phosphorus of the abundant phosphates in theveins was extracted according to the equation

PCI5 411 20 5HC1+ H 3

PO4

.

The phosphorus of the magma uniting with chlorineis extracted

,and in presence of steam is decomposed

and carried away as phosphoric acid . Fluorine andboron are seldom present in the basic rocks .In addition to elements extracted as chlorides

,there

are some metals which were possibly extracted atan early stage as sulphides , or were extracted aschlorides , and , subsequently reacting with sulphurettedhydrogen , changed to sulphides

Pbc121125 PbS 2HC1.

Carbon dioxide was probab ly evolved at a late stage .

The minerals of the titanium -apatite vein s are asfollows : Chlor-apatite , wagnerite (Mn lPO 4) , t itanite ,ruti le , t itaniferous iron , specular haematite , magnetite ,anatase , pyrrhotite , i ron and copper pyri tes , lead sul

phide, magnesia-mica , enstat ite , augite, actinolite ,asbestos , scapolite , orthoclase , alb ite , oligoclase , quartz ,tourmaline , epidote , prehnite , talc , chlorite , and calcite .

Fluorspar does not occur .The gabbro in the neighbourhood of the veins i schanged to scapol ite- rock . The hydrochloric acid ofthe fissures , together with sodium and calcium chloride

,

reacting upon labradorite,with the small addition of

chlorine up to 3 per cent . , and of lime , results in theformation of scapolite. This commences along twinning

8


planes and cracks,and at the margin s of the crystals .

The diallage goes to hornblende,while iron ore is partly

dissolved,and there i s also the formation of enstatite ,

magnesia-mica,and ruti le .

In Southern Norway two types of gabbro are represented

,and in the phosphate deposits of the Lau

rentian and Cambrian rocks of O ntario the conditions

FIG . 24.—T ITANIFEROUS APAT ITE VEIN IN GABBRO .

(AFTER J . H . L . VOGT .)

are S imilar , with the exception that the gabbro is probably more basic than the Norwegian types . Magnesia i snot so abundant

,and there is less rutile but more zi rcon .

In addition to this the apatite i s fluo r- bearing , so thatfluorine appears in the Canadian deposits to have re

placed the chlorine o f the Norwegian . I n Canadacrystals of apatite weighing 700 pounds have been found .

Mica i s extracted and sold . Sphene crystals 1 foot

CHAPTE R IV

HYDATOGE N E S I S— DE POS ITS FORME D BY

AFTE R -E RUPT IVE ACT ION S WHICH

ARE N OT PN EUMATOLYT IC

THE lodes formed under conditions less active thanthose of pneumatolysis , but nevertheless connected withthe intrusion of igneous rocks , may be divided into twoclasses , comprising the sulphidic and the oxidic ores .

O f these , by far the larger class is the sulphidic , including as i t does many copper, argenti ferous lead, zinc ,b ismuth and antimony, gold , and other ores.

Some of the deposits have close affinities with thoseof pneumatolytic origin in the nature of the minerals ,in thei r connection with plutonic igneous masses

,etc .

And others , although described under the separate heading of metasomasis (or metasomati sm are genetically connected with this group . Owing to the fact thatmany of the minerals in thi s group are decomposedwith comparat ive ease by underground waters , and undersuitable conditions reconstructed , i t is sometimes difficult to definitely assign the exact mode of origin to thembut

,fortunately, the general circumstances of thei r occur

rence assist in thi s determination ; the question whetherthese deposits are mainly original or secondary , orwhether

,as in cases of metasomatism , they are of direct

origin or result from the secondary action of ground1 16

HYDATOGENES I S 1 1 7

waters,i s usually readily disposed of . As a general

rule,however

,the deposits of this class are not confined

to the aureole of metamorphism of the igneous rockfrom which they are derived ; and if they happen to occurin the metamorph ic zone or in the igneous rock itself,the formation did not take place unti l the metamorphismwas complete and the Igneous rock consol idated .

Briefly,hydatogenesis i s generally post pneumato

lysis ° or i f i t happens to be contemporaneous , then itmust take place beyond the influence of the heatedigneous mass and outside its metamorphic aureole .

These condi tions may be diagrammatically i llustrated ,as in Fig . 2 5 .

I n some deposits of the hydatogenetic class , the conditio ns of deposition appear to have commenced withpneumatolysis and ended with hydatogenesis . This i sobvious where the greater part of the vein material wasdeposited in the final phases of lode - forming activity ;the pneumatolytic phase being indicated by certainalteration - types of the countryrock near the vein , or bypneumatolytic minerals occurring in subordinateamounts . I ndeed , i n the absence of minerals indicative of the mineral izers characterizing veins of pneumato lytic origin , the lodes may generally be consideredto belong to the hydatogenetic class . I t should be recollected , of course , that the solutions were at fi rstextremely hot , and minerals deposited from them aresaid to be of hydrothermal origin . From thi s point ofview the hydrothermal phase of deposition from underground water may be regarded as bridging over thepegmatitic and ordinary hydatogenetic types of oreformation .

There is a thi rd class with which some of these


DISTAN CE FROM IN TRUS IVE ROCK

Indica ting therma l cons equent/y b‘re

in tens ity of c on ta c t Metam orphl sm .

were

IgneousRoc/tSedimentary Rocks .


mineral - associations , with a reference to the parentmagma where this can be made . This method i sidentical in principle with the arrangement proposedby Breithaupt as far back as 1 852 , and , with the except ion that i t has undergone revision and amplification

,

i t holds good in its essential particulars to -day .

This mode of classification cannot be said to be strictlygenetic , and in itself is l iab le to fai l here and there ,owing to the numerous questionab le occurrences inwhich abnormal developments of one type of veinmay appear to have closer affinities with some othertype.I t must be observed , also , that there i s a tendency

in ordinary classifications to ignore the economicallyless important minerals . I t i s frequently the case withgold

,for instance , to designate a deposit as being

a gold ore,when it might more accurately have been

placed in the category of pyritic ores characterized bythe presence of gold or other minerals .With transitional or mixed types the case is oftenpecul iarly involved , and the genesis a matter for speculat ion only ; as several of the veinstone -accompani

ments of the metall i ferous minerals are capable ofbeing transported by different carriers or mineral izersin addition to water alone , the alterat ion of the countryrock does not generally supply the key to the origin ofthe deposits . Under these circumstances , therefore , thedescription of ore deposits of hydatogenetic origin undercertain heads i s somewhat arbit rary , and generallyconfined to a mere account of apparently independentand transitional types , without Special i

‘

eference to thei rgenetic connection with the igneous rocks .

HYDATOGENES IS 1 2 1

S ULPH ID IC VE INS O F HYDATOGENETIC ORIG IN .

In this group of mineral veins the modern writersrecognize certain classes connected by transitionaltypes

,in which the distinctions are in some cases

quantitat ive only, and in others are of doubtful value .

PRIMARY GO LD VE INS .

A large class of vein s which carry gold inimportant quantity is found in rocks of all ages , butprincipally in the O lder formations in proximity to acidand intermediate intrusive rocks . I n particular theseveins occur largely in gneisses and schists , beingfrequently associated with granite and its modifications .

In some cases the gold ‘occurs in what appears tobe the ultra-acid modifications of pegmatites , in whichthe various phases between true quartz vein s andtrue pegmatites are said to be traceable . Such veinsare regarded as differentiation products of igneousmagmas .Other igneous rocks which may give rise to goldveins appear to comprise practical ly the whole seriesof acid and basic intrusive and extrusive rocks

,but

among those to be particularly mentioned aregran i te ,diorite , augite -syenite , and other all ied pluto nics ; dykesof porphyry and lamprophyre ; and among the lavasandesite , t rachyte , and rhyolite .Gold has a wide distribution , occurring in appreci

ab le or minute quantities in most if not all sulphides ;i t i s generally found in S i lver compounds , and withthe rarer metals rhodium , palladium , etc . I t i s knownto occur widely in the form of compounds with silverand tellurium .

1 2 2 THE GEOLOGY OF ORE DEPOSITS

I t i s commonly present in i ron pyrites,and in work

abl e quantit ies i s frequently associated with other sul

phides, such as those of zinc , l ead , antimony, andco ppen

I t i s also frequently accompanied by zincblende,

galena , antimonite , and mispickel , and occasionallyby compounds of manganese , molybdenum ,

bismuth,

tungsten , and other metals .In California (San Andreas) uran ium ochre has beenfound characterizing some deposits of gold ; while inArizona (Yavapai and Yuma) vanadates of lead aredistinguishing features of argent i ferous gold ores . I nN evada and Arizona molybdate of lead in associationwith vanadates occurs in the Comstock and other goldmines .I t has been remarked that the gold obtained from

lodes in the Palaeozoic rocks of the Transvaal , Australia ,N ew Zealand

,California , and other parts of the world ,

i s high -grade,while that derived from lodes connected

with the latter eruptive rocks , such as andesite , isalloyed with a considerable proport ion of si lver .MOricke recognizes three types of lodes : ( 1 ) Those connected with syenites , granites , and gneisses , as inBohemia ; (2 ) those with trachyte and andesite ; and

(3) those connected with basic eruptives contain ing ,instead of quartz

, such minerals as calcite and barytes ,and characterized by high percentage of si lver . Afourth class may be added where , as in Queensland andthe U rals

,gold i s found in serpentinous rocks.

The principal non -metall i ferous vein -material i squartz, which may occur alone or with carbonates ofl ime , i ron , manganese , etc . Barytes and fluo rspar aresometimes distingui shing features , while tourmaline ,


no doubt , for the precipitation of many sulphide sand gold .

The original solutions which deposited pyritic goldores contained , in addition to the dissolved sulphides ,tellurides , carbon dioxide and sulphuretted hydrogen ,together with compounds of si l ica , l ime , i ron , manganese , etc .

The presence of quartz and of i ron pyrites in goldveins characterizes an important group . I n additionthere may be present sulphides of copper, zinc , andlead , sometimes occurring in such important amountsas to make it more proper to describe them as auriferous ,Copper, zinc , or lead lodes , as , indeed , they are so calledwhen the gold is not the most valuable part of the lode .

Compounds of arsenic are also frequent associates ofgold .

Pyrrhotite , molybdena , bismusthine , and tellurides arepresent in many gold-quartz veins . I n the typical goldquartz veins

,carbonates are present only in Small

amounts , and generally as alteration products of thecountry- rock . Barytes and fluo rspar are also rare , butin some districts the gold -quartz is accompanied bytourmaline

,felspar

,and white mica , indicating close

affinities with ores of pneumatolyt ic origin . These goldquartz veins when containing S i lver in appreciab l equantit ies fall into the group of gold -si lver veins , inwhich the S i lver i s one of the principal workableproducts . AS a group , however, the essential com

po nents are quartz, i ron pyrites , and gold .

The next important subgroup i s that containing

Copper in considerab l e quantity , and so may be desig


nated cupri ferous gold-quartz veins. Silver i s presentonly in subordinate amounts , but tourmaline and bismuth sulphide are typical accompaniments. L ike mostother groups of gold veins , this i s not very definite , asit has affinities with the tourmaline -copper-gold group ,the bismuth - gold group , and other subtypes . In someplaces the countryrock is greisenized . Pyrites , galena ,fahlerz , zincblende with arsen ical , manganiferous andother compounds , also occur in subordinate amounts .

The veinstones consist mainly of quartz , accompaniedby small quantities of tourmaline and calcite .

Auriferous arsenical pyrites characterizes another subgroup allied to the pyritic gold -quartz veins . I n additionto mispickel and quartz

,there are sulphides of copper,

i ron , lead , and zinc , in small quanti ties .

Subgroups, in which sulphide of antimony on theone hand , and of bismuth on the other, are characteristic constituents are recognized in some parts of theworld , but the distinction is somewhat arbitrary .

In the antimonial gold ore the gold occurs mainly inpyrites, but also occurs as free gold in quartz . I n thecase of bismuth -gold -quartz veinstone

,i ts specification

of a subgroup is scarcely warranted , as it has closerelationship with tourmaline - gold - copper veins

,t in

ores, etc . I n part icular,b i smuth sulphide and tel luride

is associated with gold quartz rich in copper .

An association of gold with a particular type ofalteration , fi rst noted by J . H . L . Vogt of Christiania

,

has recently been described by F . L . Ransome . I nN evada the gold ores occur partly in Tertiary rhyolites

,

andesites , and other lavas , which for the most part


overl ie Palaeozoic sediments and granite . The characteristic

’

alteratio ns of the intrusions of dacite are ofthree types , indicating that the auri ferous solutionsconsisted of acid and sulphidic compounds with carbondioxide . There i s no direct evidence of carbon dioxidehaving been present

,but its presence i s inferred . The

three types are ( 1 ) Silicificatio n (2 ) alteration to softrock consisting of quartz

,kaolin

,alunite

,and pyrites ;

(3) alteration to propylite , in which the rock consists

of quartz , calcite , epidote , and chlorite , with pyrites,but no alunite . In some of these changes it i s seenthat the i ron from magnetite and ferromagnesiansil icates has given rise to pyrites

,while alunite

and kaol in have been formed from the potash andalumina .

The foregoing types , as a whole , const itute a class inwhich the gold occurs with pyrites or as native gold inquartz associated with special minerals distinguishingthe subgroups .

The type described below has features which contrastit with the foregoing both in the manner of occurrenceof the gold -minerals and in thei r commercial t reatmentfor the extraction of the metal .The group referred to is that known as the ‘

goldtel luride group .

’ In the telluride ores the gold occurs

as a definite telluride of gold with si lver, l ead andantimony ; or it may also occur as native gold accom

panied by various tellurides . The veinstone is principallyquartz and fluo rspar, with which there may be somecarbonate ; while , as at Boulder County (Colorado)and Kalgoorl i e

,the vanadium -mica , roscoelite , i s found

in association with tellurides . The tellu ride ores in


and quartzose schists , slates and quartzites , known asthe Swaziland series ,

’ i s intruded by granite . Both ofthese are traversed by diorite .The lodes consist of quartz , with free gold , pyritesand

,rarely , tel lurides . O ther minerals

,such as pyr

rho tite , arsenic and copper sulphide , and rarely galenaand antimony

,are also found in small amounts . At

Lydenberg, farther north , the veins belong to the cupriferous gold-quartz group . They occur in sandstonesand in dolomite . The dolomite contains the most important deposits . N ear the veins the dolomite is silicified

,

while interbedded greenstone i s kaol in ized . The veinstones have been affected by secondary actions whichhave brought about a secondary enrichment of gold ;they consist of friable quartz with cavities containingSpecks of gold , secondary copper ores , and bismuthcompounds . (See p .

At Um Rus , in the eastern desert of E gypt , veins ofquartz occur in gran ite . The gran ite is intersected bydykes of greenstone and porphyry . The lodes inancient times were worked for gold .

I n Australia there is a wide distribution of gold invarious districts . I n Bendigo and Ballarat the lodesare very numerous , sometimes constituting stockworks .They occur principally in S ilurian sediments , and consi st of quartz in which gold can often be seen . Goldalso occurs in pyrites and in mispickel , while galena and

zincblende are also found . I n the Bendigo region thequartz veins traverse , at right angles , beds in whichthere are small persistent pyritous and bituminousseams

,known to the miners as ‘ indicators . ’ Where

the lodes cross the indicators they are generally rich .

The Bendigo district i s also noted for its saddle- reefs .

HYDATOGENESIS 1 2 9

The Silurian sandstones and shales in this district arethrown into a series of sharp anticl inal folds , extendingfor several miles ; at the apices of the folds there arebedded quartz veins in the form of saddles . I n somecases there is a succession of as many as ten of thesesaddles . There are three principal anticlinal axes .

The district i s traversed by dykes of monchiquite ,while a post -Silurian granite occurs in the south of

FIG . 26.—STRUCTURAL ARRANGEMENT OF THE S ILUR IAN

SLATE S AN D SANDSTONE S AT BENDIGO , IN WH ICH TH E

AUR I FEROUS SADDLE -REE FS ARE FOUND .

the region . Albite is occasionally found in intimateassociation with the gold-quartz of the saddle reefs , butlime-soda felspars are absent , and in this respect thelodes are genetical ly connected with the type representedby the Great Mother Lode of Cali fornia, the quartz veinsof Victoria , and the lodes of Douglas I sland (Alaska) .In Queensland the Carboniferous sandstones , coalbearing shales , and conglomerates , with intrusive greenstones, are traversed by quartz veins containing gold

9


principally where they occur In the sediments, andmore particularly where these are pyritous or carbo naceo us. (Hornblende granite is met with , and theall ied type known as tonalite . )At the Mount Morgan Mine the backs of the lodesconsist of secondary oxidized minerals . Below

,the lode

consists of quartz with auriferous pyrites . The pyriticquartz veins occur as a stockwork , in the neighbourhood o f which the countryrock is Silicified .

I n N ew South Wales the veins are of very variablewidth , occurring in schists and Slates , and also inaltered hornblende granite .

L ike most pyritic vein s , the upper parts Of the N ewSouth Wales veins are oxidized , and contain suchminerals as quartz , l imonite , gold , and secondary compounds of manganese

,copper , and lead . I n depth the

veins consist of quartz , with sulphides of i ron , zinc ,lead , copper , and arsen ic .

In the Hawkins H il l Mine , N ew South Wales ,situated between the Turon River and Green Valley,the gold is richest i n the lodes where they cross blackslates . The hill itself consists of S ilurian co nglom

erates, sandstones, and slates . The lodes are principallyquartz , which in places i s displaced in favour of muscovite . Calci te , zincblende, galena, and marcasite , are

also present as vein -minerals .

The Lucknow district in N ew South Wales issi tuated 2 00 miles from Sydney . There gold occurs incalcite veins in serpentin ized basic igneous rock . Thedeposits occur mainly at the junction of the serpen

tinized and unaltered rock .

I n the Northern Celebes (Malay Archipelago) the‘

gold veins are connected with highly felspathic diorite


The G reat Mother Lode of California in particularis a large belt of vein s of post -Jurassic age . I t is1 14 miles in length , stretching through several counties .I n width it varies from a few hundreds to many hundredsof feet , and consists of numberless ramifying veins andbrecciated masses . I n its neighbourhood the countryrock is extensively silicified and traversed by quartzve ins .

FIG. 2 7 .—ALTERED SERPENT INE , IDAHO M INE , GRASS VALLEY,

CAL I FORNIA . (MAGN I F IED I 5 D IAMETERS .) (AFTER W .

L INDGREN . )M , Magnetite ; Q , quartz ; S , serpentine ; P , pyrite s.

The quartz i s either pure white , banded , or a darksubtransparent variety containing sulphides . Thegreen ish potash mica called ‘ mariposi te ’ occurs withit . As might be expected in a series of lodes of suchextent , there i s great variation in the mineral contentsof the different parts . I n E l Dorado , fo r instance , theporphyrite dykes are traversed by albite veins containing gold . (Compare with the occurrence of gold atTreadwell , Alaska , p . I n Nevada the auri ferous

HYDATOGENESI S 133

pyritic quartz in the granite and diabase is associatedwith epidote and tourmaline .

At G rass Valley and N evada City the lodes are connected with tonalite

,diorite

,diabase , gabbro , and

serpentine . The vein material consists of quartz withoccasional calcite and potash mica . Opal and chalce

dony also occur .The metall i ferous contents

,i n addition to gold , are

sulphides of i ron,bismuth

, Copper, lead , zinc , arsenic ,

FIG . 28.—P INEH ILL GOLD DEPOS ITS, CALI FORN IA .

(AFTER W. LI NDGREN . )Scale , 2 inches ! 1 mile .

si lver, antimony, and mercury . Scheel ite i s found,and

molybdenite is also present .At P inehill (California) gold occurs in veins t raversingCretaceous rocks . The minerals appear to favour thosepositions where the sedimentary rocks are in contactwith intrusive rocks, but seldom occur in the diabaseand porphyrite . I n serpentine and gabbro on the eastof P inehi l l there are a few veins . H ere the gold occursin associat ion with quartz , sulphides of i ron , copper,

134 THE GEOLOGY O F ORE DEPOSITS

lead , and zinc , also with arsenical sulphides , and sometimes in combination with tellurides . Dolomite andcalcite are occasional accompaniments .At the top of P inehil l i s a mass of diabase porphyritemuch kaolin ized , and traversed by veins of barytes containing gold . N o sulphides are present , but S i lver isan invariab le accompaniment , and occurs separately ;the ratio of gold to si lver varies from 1 1 to 2 0 : 1 .

(Chloride of si lver i s found .) Lindgren bel ieves thatthe barytes acted as a carrier to the gold .

In the Archman schists of the Rainy Lake region , onthe west of Lake Superior, there are two types of veins.In the O lder gneiss and mica schist , gold occurssparingly in interbedded pyritous quartz lenses rangingup to 30 yards in length and 4 feet in width . I n similarlenticles in the newer sericit ic slates the gold is moreabundant . These quartz lenticles enclose fragments ofthe country rock . A second type of vein occurs ingran ite , and consists of quartz with auriferous sulphidesand free gold

,galena

,and blende in particular , the

granite on each side of the veins having been modifiedto a sericit ic rock .

I n Georgia auri ferous schists and gneissesoccur . The lodes proper are in hornblende gneiss ,which i s altered in some places to a rotten auriferousmaterial known as ‘

Saprol ite ,’ in which are found small

quartz veins contain ing pyrites , galena , copper pyrites ,and free gold .

I n Nova Scotia auri ferous quartz of a peculiar formoccurs in metamorphosed shales . The quartz Is In

the form of barrel - shaped masses , often l ike a series oflogs laid side by side . These masses are really connected together

,and it seems that they owe thei r form


calcite i s also found , but probably it i s a decompositionproduct from the igneous rocks . The veins sometimescut across the dyke - l ike intrusions from wall to wal l .There i s a series of these somewhat brecciated diorit icintrusions in Douglas I sland , occurring through a beltof ground three miles long and half a mile wide . N oore i s found in the gabbro of the region . The gold i sbel ieved to have been introduced by carbonated andmineral -bearing solutions , which attacked the diorite ,replacing ferromagnesian minerals by pyrites

,and the

more basic felspar by secondary albite .A well -known gold-region in the United States isthat of the eastern side of the Al leghany Mountains ,between North Carolina and Alabama , including SouthCarol ina and Georgia . The district geologically ismade up of crystall ine schists and gneiss of Archaeanage penetrated by dykes of diabase . These formationsare covered towards the coast by Triassic deposits , comprising sandstones and gold -bearing conglomerates ;while

,owing to the fact that the country here has not

been glaciated,it i s covered by a mantle of rotted rocks ,

the so -called ‘ frost -drift ’ or ‘ head ,’ comprising clays

and other materials in which gold is often found .

The principal gold deposits in this range are confinedto a belt in the Archaean rocks extending from Virginiato Alabama . The belt reaches a width of seventymiles in North Carol ina .

The gold occurs both free and in pyrites ; either ininterbedded segregation -veins of gold-quartz traversingthe metamorphic rocks associated with diabase dykes

,

or in ordinary veins, cutting slates , gneisses , etc . Thepyritous slates show some resemblance to the Norwegianfahlbands .

HYDATOGENESI S 137

In addition to this , the gold has been found in thepyrites contained in the diabase dykes , although thereare grounds for believing that these intrusives weremerely pyritized after intrusion , and assisted in someway in st imulating the lode-forming actions .In some parts of the gold belt the lodes are accom

panied by other sulphides , such as blende and galena°

and from the occurrence of tel lurium , b i smuth , tin ore ,molybdenum

,wolfram

,fluo rspar, epidote , etc . , in con

nectio n with slates and eruptive rocks , i t is .pro bable thatsome of the deposits belong to the pneumatolytic class .E l Callao . gold district of Venezuela was oncephenomenally rich in gold ores . The lode

,from a few

feet to 9 feet wide , i s situated in a hornblende rock , andconsists of pyrites

,quartz

,and free gold .

Although most of the ores of the desert of Atacama,

Chili , are secondarily deposited , reference may‘ be made

to the origin of the gold deposits at Guanaco . Thegold occurs in numerous cracks

,fissures

,and shattered

masses , in quartz - trachyte which has been much altered .

The b iotite i s changed to viridite and original oxides ofi ron have been converted to Sulphide which containsgold . Near the surface this sulphide has been oxidized ,with liberation of free gold

,and also formation of

atacamite from cupri ferous minerals . The deposits areS imilar in origin to those of Hungary (p .

The district north of Rio de Janei ro (Brazil) , nearD iamantina and Ouro Preto

,i s composed of Archman

gneisses and schists , with younger slates , quartzites ,i tabi rite (pp . 1 08

, and sandstones .The gold veins of hydatogenetic origin here consi stof quartz, pyrites , and mispickel . I n particu lar, theveins in the so -called j acutinga — a very friable ferru


gino us i tab i rite— are associated with impregnations inthe country rock for considerable di stances on eitherside of the actual veins . These have been worked inmany cases as open cuts .G reat Britain has been only a small producer of goldcompared with the colonial go ldfields, but a considerable output has been maintained from mines inMerioneth , such as the Clogau and Vigra mines, andBriti sh Gold -Min ing Company . The lodes are situatedin older Palaeozoic rocks (mainly Cambrian) , and consistof vertical quartz veins varying from a few inches to

9 feet in width . G ranular and coarsely crystall inecalcite , which i s sometimes auri ferous , pyrites , pyrrhotite , sulphides of tel luriumand bismuth , are also ofcommon occurrence as vein -minerals .The gold veins of Saxony

,Bohemia

,Silesia , and the

Tyrol , appear to be connected with granite . The classesof ores and thei r modes of occurrence vary ; the veinsare generally quartz containing pyrites

,with subordinate

and varying amounts of sulphides of copper , lead , zinc ,and arsenic . Calcite and carbonate of iron , with barytes ,occur in some places

,while some deposits are charac

terized by antimony, and others by scheelite , tourmaline , etc .The gold veins of Kootenay , British Columbia, areconnected with intrusions of diorite or monzonite , andwhile falling into the group of cupriferous gold -quartzveins

,they might possibly be regarded as metasomatic

deposits . They were formed after the consolidation ofthe intrusive rock

,but before the intrusion of a series of

dykes of lamprophyre (F ig . The conditions werethose accompanying dynamic metamorphism , underwhich head the veins might have been described .


to a greisen consisting mainly of quartz and muscovite ; it is impregnated with pyrites , and is known as‘berezite .

’ In the adjacent district of Pyzhminskoyethe quartz veins traverse diorite and serpentine in asimilar manner .The vein materials consist Of tourmaline and quartz

,

which appear to have been the first of the minerals toarrive in the veins , and may be ascribed to pneumato lytic action ; of calcite and zinc blende ; finally

,of

quartz, with sulphides of i ron , lead , antimony , andCopper, associated with the gold , and constituting thehydatogenetic phase . The outcrops of the lodes areoxidized , and contain minerals concentrated by secondaryreactions .At Ko tschkar, near Zlatoust , are deposits havingmuch the same features as those at Berezov, with theexception that the lodes of .Ko tschkar characterist ically contain thei r gold in associat ion with arsenicalpyrites . The district consists of metamorphic schists ,with intrusions of granite , pegmatite , etc . The goldveins occur exclusively in the granite , and are parallelwith the strike of the schists and crush -zones in thegranite . The veins contain sulphides of i ron , lead ,copper

,and antimony , while the granite near them con

tains such secondary minerals as biotite , chlorite , epidote ,quartz

,and carbonates . The lodes have good gossans ,

contain ing iodide , bromide , and chloride of si lver .At Tseliabinsk , 100 miles north of Zlatoust , aurif

ero us quartz occurs in kaolin ized masses of rock , withveins of berezite , in association with sulphides .Go ld with Arsenic .

— As an“

example of the occurrence of considerable amounts of arsenic among auriferous sulphides

,the Champion Reef in the Dharwar

HYDATOGENES I S 141

region in Mysore may be taken . The famous ChampionReef occurs in the Kolar Go ldfield , and is worked bythe gold mines of Champion Reef, Mysore , O o regum ,

Nandidrug, and Balaghat . I t has a north and southstrike governed by planes of o verthrusting in a seriesof basic lavas

,which have been dynamically m etam o r

pho sed into hornblende schists . On the eastern marginof these schists is a series of crush - conglomeratesderived from gran ite veins , while on the west is afringe of schistose and ferruginous sandstones andquartzites , beyond which lies a series of banded gran iticgneisses . Parallel with the Champion Reef, and S ituatedabout feet from it , i s the so -called O riental Reef

,

which also marks a l ine of thrusting , and , l ike theChampion Reef, consists of a number of i rregularlenticles Of dark bluish quartz, which on the Championhave been mined to over feet from surface . Themineral associates of the gold are sulphides of i ron ,COpper, arsen ic , zinc , and lead . Pyrrhotite also occurs,and some strings of tourmaline . These reefs occurringalong planes of o verthrusting are the most ancient ofthe gold deposits of the district and are intimatelyconnected with its dynamic metamorphism .

A later series of gold -quartz veins in the districts ofKolar

,Hutti

,and Garag are found traversing the

schists t ransversely, and are associated with dykes ofdiorite

,dolerite

,and diabase . They are observed to

cut and displace the Champion Reef.In the province of Ontario , not far from Ottawa ,

gold-bearing arsenical pyrites occurs in quartz veinstone , with a l ittle calcite , in connection with syen ite .

In the Lake of Woods district , in North-West Ontario ,gold is found in fissure- veins in Laurentian rocks and

142 THE GEOLOGY O F ORE DEPOS ITS

later granite , also in lenticles of quartz, and in fahlbands in Archaean schists . I n association with the goldare molybdenite , i ron and Copper pyrites , bornite , galena,and zincblende , rarely tetrahedrite and mispickel .Go ld with Antimo ny o r B ismuth — Gold veinscharacterized by considerable amounts of antimonite

,

with smaller quantities of other sulphides with arsenic,

occur in N ew South Wales (Arundale) and in theTransvaal (Murchi son Range) .The antimonial gold ore group i s , however , welltypified in Goldkronach

,near Bayreuth

,i n Bavaria

,

where veins of quartz with auri ferous antimonite ,native gold and antimony , and sulphides of lead , zinc,and copper , traverse the sericit ic Cambrian slates . I nsmaller quantities calcite and barytes are found , butalso more abundantly carbonate of i ron .

On the south of Prague , in Bohemia , there occursa number of lamprophyre dykes connected withgranite . The antimonial gold - quartz veins of thisregion traverse both the dykes and the granite . Themetal ifero u s associates are arsenical minerals andpyrites , with abundant antimonite containing appreci

able amounts o f gold and silver. Secondary enrichments of native gold associated with oxidized ores alsooccur in the gossan .

N ear Stavanger , in Kristiansand (Norway) , the islandof BOmmelO affords examples of gold-quartz veinscharacterized by the presence of b i smuth .

The veins trave rse the older diabase dykes anddiorite

,and also the younger acid dykes of quartz

porphyry.

I n the schists the gold occurs in lenticular quartzveins , of i rregular dimensions , which on the whole


The veins occur as fissure-zones varying from a footto 100 feet in width .

The infil lings of the veins consist of quartz with al ittle calcite and pyrites . The gold occurs native ini ron pyrites and as tellurides . There are also frequently sulphides of lead , zinc , b i smuth , and evencopper ; fahlerz and scheel ite , and occasionally tourmaline , are found . According to W . Lindgren , metasomatic replacements o f the walls of the lodes tookplace through the action of solutions containing carbo nic acid and sulphur, with the development ofcarbonates of l ime

,magnesia

,and iron , derived from

ferromagnesian minerals,and of sulphide of iron from

iron oxides . An amphibole -chlorite-zoisite- albite rockhas in this way been converted to a quartz- sericitealbite-carbonate rock ; gold , mercury, and telluriumwere deposited at the same time .At the outcrops of the veins the tellurides are notfound , and they appear to have been decomposed ; butin thei r place is sponge -gold

,or a fine yellow deposi t

of gold known as mustard-gold .

’

I n Colorado the well-known telluride ores of Cripple

Creek are connected with volcanic rocks , and dykesintrusive in the granite of the district ; the most pro

ductive ore i s restricted to a ci rcular area , about threemiles radius

,around Gold H il l .

There i s in this district a distinct sequence oferuption , showing changes from acid to basic rocks .

The oldest of the intrusives are porphyrites . Thesewere followed by phonolite

,which occurs as veins

throughout the district , and finally by basic rocks, suchas nephel ine-basalt

,l imburgite and tephrite . Volcanic

tuffs and breccias with interbedded lavas form the

HYDATOGENESIS 145

principal rocks in the neighbourho o d of CrippleCreek .

All the rocks are t raversed by narrow vein s, bothsimple and composit e

,in the neighbourhood of which

they are often brecciated and extensively altered tokaolin and micaceous minerals, and impregnated withopal and quartz , fluo rspar, dolomite , secondary potash

FIG . 3I .

—SMALL VE IN IN ANDE S IT IC BRECC IA,INDEPENDENCE

M INE,CR I PPLE CREEK , COLORADO . (MAGNIF IED

I I D IAMETERS .) (AFTER W . L INDGREN . )An , Andesitic breccia ; P , pyrites F , fluo rspar ; Q , quart2 °

V,valencianIte (o rtho c lase) .

felspar , pyrites , and tellurides (Fig . O f the lastthe principal is calaverite , which occurs as a coatingon walls of fissures . MO lybdenite , argenti ferous tetrahedrite , and stibnite , are present .Other sulphides are uncommon , but zincblende ,

galena , and chalcopyrite , have been noticed . Roscoelite , rhodochro site , celestine , and calcite , are seen inthe breccia . Molybdenum ores are also met with .

1 0


I n the granite,the narrow vein s are accompanied

by considerable alteration on either side , giving riseto replacement - deposits , i n which occur Chloritederived from mica

,fluo rSpar, and other minerals .

Porphyritic microclin e crystals remain unaltered , butthe microcl ine of the ground-mass

,oligoclase , quartz ,

and biotite , have been recrystall ized into a porous

FIG. 32 .

— ‘GRANITE O RE ,

’ INDEPENDENCE M INE , CRIPPLECREEK

,COLORADO . (MAGN IF IED 20 D IAMETER S . ) (AFTER

W. L INDGREN .)

Q . Quartz ; V, valencianite (seco ndary o rtho c lase ) ; 0,

o rigina lo rtho c lase ; B

,bio tite converted to valencianite and pyrites

P , pyrites .

mélange o f valencianite (secondary orthoclase) , quartz,fluorite

,pyrite , calaverite or sylvan ite , with occasional

zincblende and galena . The basalt dykes are traversedby minute fissures

,running parallel to the walls

,which

ordinarily contain calcite in thin strings .When the dyke coincides with a zone of fissuring,the fissures contain quartz, fluo rspar, and gold tel luride

(F ig . There i s this di fference , however, the ores


In South Dakota , the ores of the so - called S i l iceousgold-belt came originally through more or less verticalfi ssures , from which the much- fractured slates andsandstones were mineral ized along a zone over six mileslong and four miles wide . The mineral -contents ofthe impregnations are mainly quartz with a l ittle

fluo rspar, calcite , pyrites and tellurides . Sylvanite,

and occasionally thorium and uranium minerals arefound . The eruptive rocks— t rachyte and phonolite

are silicified .

I n Montana State (Judith Mountains) the gold oresare found in decomposed calcafeo us rocks which are incontact with porphyry . The vein-materials are mainlypyrites

,with fluo rspar and quartz . Below the gossan

the lodes contain gold-tellurides and a little si lver ore .

The eruptive rocks of the district comprise graniteand syen ite -porphyry , syenite , and diorite -porphyrite ,with associated minor intrusions of elmo lite syeniteand t inguaite

,all of which are comparatively young

in age .

Pyritous gold-quartz veins , with a l ittle copper andtel lurium

,occur in N evada . The eruptive rocks of the

district are granite porphyry and minette , but there issome doubt as to their connection with the ores , as

none of the veins occur in them .

Telluride gold ores are known from Eastern Brazil .Among the E uropean gold- telluride local ities , thewell- known veins of the Dacian go ldfield in Transylvania

,including those of Nagyag , Offenbanya , Veres

patak,etc . ,

have been described by several Continental

writers .The mountains near Nagyag consist of intrusiveandesitic and trachytic rocks of Miocene age (F ig .

HYDATOGENES IS 149

These rocks break through the Miocene sandstones,conglomerates

,and clays ; and in the mining area large

inclusions of sedimentary rocks are found in the igneousmaterial .The andesitic rocks comprise normal hornblende

andesite and quartz -andesi te or dacite with augite ,hornblende

,and biotite .

These rocks have been altered by various act ions .At the surface they are weathered and decomposed ,

Vo lcanic p lug o f dacite , ka o linizedand pro pylitized in the vicinityo f fissures co ntaining tel lurideso f go ld .

FIG. 34.—IDEALIZED SE CT ION OF VOLCANIC MOUNTA IN AT

NAGYAG,HUNGARY . (AFTE R VON IN KEY. )

while lower down they are propylit ized and kaolinized .

The kaolin ization is ext reme near the actual veins.

The ferromagnesian minerals are changed to Chloriteand carbonates, and magnetite is changed to pyrites .The veins in the dacite of Nagyag are formed ofnumberless small intersecting fissures

,with a general

parallel ism in a north and south di rection . These areintersected by a series of barren crush -zones of different


ages , containing clayey and sandy material , and brecciated fragments of the country-rock .

The vein -materials vary in different parts of thedistrict . I n one part they are characterized by tellurides

(nagyagite , associated with carbonates of l ime , i ronmagnesia , and manganese , with some tetrahedrite andpyrite (Moun t Szekeremb) in another, by tellurides ,quartz , with sylvanite , fahlerz , and a little free gold

(Mount Hatj o ) and in a thi rd part by sulphides of lead,zinc, and i ron , with carbonates and argentiferous tetrabedrite this may be regarded as an intermediate type .

In smaller quantit ies , gypsum ,arsenic

,and antimonial

and arsenical minerals,occur principally in the upper

parts of the veins . A study of the mineral paragenesisshows that the minerals arrived in a defini te order . Thequartz is the oldest , but i t reappears at later stages .The next infil l ing comprises most of the common sul

phides , after which come the tel lurides and nativegold , followed by carbonates , and finally by sulphidesof antimony and arsen ic , with barytes and gypsum .

The telluride -gold ores of the ancient mining districtof Verespatak occur in rocks consisting of dacite ,rhyolite , and volcanic breccias , i nterbedded with E oceneand Cretaceous sandstones .The veins contain quartz with free gold , furthercharacterized by the presence of carbonates of l ime andmanganese

,the sulph ides of lead , zinc , Copper, and

i ron , and tellurides .At O ffenbanya there are two modes of occurrence ofthe ores . I n one case the deposits exi st at the contactof crystal l ine l imestone bands , in the garneti ferous micaschist

,with Tertiary andesite and dacite , and in the

other case as veins .


O°

OO I to per cent . , and of the si lver from toper cent .

The lodes of the San Juan Mountains and otherplaces in Colorado (Ouray , Telluride , as well asnear Rosita , are connected with Tertiary eruptives . AtOuray the gold ores occur in veins traversing quartzite ,the richest portions being near overlying bituminousshales (60 feet thick) , which in turn are overlain byTertiary volcanic rocks .The gold occurs in clay in vein s containing quartz ,secondary oxides , sulphate and carbonate of lead , i ronpyrites , and a little barytes .N ear Telluride the famous Smuggler Vein is moreregular in character than the Comstock lode , and isin a fault t raversing augite andesite, and andesitic tuffsand breccias its width ranges up to 5 feet or more .The mineral paragenesis i s quartz , carbonate of manganese and l ime , dolomite and flu o rspar, wi th somebarytes. The ores are rich in silver

,and consist of

proustite , polybasite , with sulphides of i ron , copper ,lead , and zinc , and some native gold , the content of thelast being O '

OO 16 per cent .,and of silver per cent .

At Rosita the veins are in gneissose and graniticrocks, overlain by a series of Tertiary eruptive micahornblende andesites

,agglomerates

,and ashes , the

lodes occurring sometimes,as at the B assick mine , i n

volcanic plugs . The mineral association is S imilar tothat mentioned above .

For the auriferous silver-ores of P inehill , Califo rnIa ,see p . 133.

The Mexican examples are principally remarkablefor the connection of the rich secondary ores of si lverand gold in the colorados (or minerals of the

HYDATOGENES IS 153

upper parts of the lodes) with the Tertiary eruptiverocks .I n the State of Oajaca , for example , the vein s are inandesite

,and consist of quartz with various carbonates

and fluo rspar. I n the colorados occur chloride andbromide of si lver

,with other secondary ores of S i lver and

gold , while in depth , i n the negros’ or unaltered zone ,

are sulphides of zinc , lead , copper , and iron .

The veins of the famous Rosario mine in CentralAmerica (Honduras) are in connection with rhyolit icintrusions traversing Triassic sandstones and limestones ; the mineral paragenesis is similar to that mentio ned on the opposite page .

The well -known gold mines of the Thames district andCoromandel Coast in North Island (N ew Zealand) havepeculiarities similar to those of Comstock , for they areconnected with propyl itized andesites, dacites , andesitictuffs and conglomerates of Tertiary age . The district comprises the Hauraki gold region , Waihi and other mines .The Thames district i s intersected by two large

Pl iocene faults , which divide the region into threesections . The lodes consi st mainly of quartz , and theyare associated with numerous strings of the samemineral . The gold occurs in sugary quartz in the formof thread foil and grain gold . The other mineralsare mainly pyrites

,sulphides of Copper

,zinc

,arsenic

,

and antimony,while galena and tellurides of gold also

occur in subordinate quantity .

The E uropean examples of gold - S i lver veins havereceived much attention from Continental geologists ,and , as in the case of the sulphidic si lver- lead veins,i t i s on their work that the present classification ofthese types of ores i s largely based .


In thea stern Tyrol , near Gastein , narrow quartzveins or strings traverse gneisses and schists

,and con

tain , i n addition to gold and the common sulphidesof baser metals , st ibnite , molybdenite , and rich ores ofS i lver. Pyrites i s the commonest sulphide . I n addition

,

some localities show the presence of calcite and fluo rspar , and also zeolites .I n the Hungarian ore -mountains

,the deposits of

Schemnitz , Kremnitz, H o dritsch, and other places ,occur in veins cutting rocks of so recent a period as theMiocene . The region is mainly composed of Triassicl imestones and quartzites , overlain by E ocene shales .These rocks are penetrated by a series of igneousrocks , commencing with pyroxene andesites , followedby diorite , aplite, b iotite and hornblende andesite , andfinally by felsite , the last of which is the most abundant of the eruptives accompanying the building of theCarpathian Mountains . The andesites have been pro

pylitized , silic ifled and impregnated by pyrites . Thelodes , which are composite in form and ill-defined ,consist of quartz , calcite , dolomite, barytes , carbonateof i ron and manganese , and gypsum .

The metall i ferous minerals comprise various sulphidesof si lver (argentite , stephanite , pyrargyrite , polybasite) ,and native si lver and gold

,the veins being further

characterized by stibnite,fluo rspar, Cinnabar, copper

sulphide,pyrrhotite

,and felspar. Galena , zincblende ,

and auriferous pyrites,are important constituents .

I n the P i edmontese Alps auri ferous si lver- lead oreswith carbonates

,quartz

,and sulphides of the baser

metals, are found in lodes , and as interbedded veins , incrystall ine slates which are traversed by basic intrusions .In Chi -Ii Province (China) the gold veins are of the


i t include the rich copper ores formed in the upperparts of veins by decomposition and redeposition of theoriginal ores which stil l exist in depth .

The most important of the Copper ores is copperpyrites, but from its decomposition there arises— particu larly in the upper parts of lodes— a series of secondarily developed minerals comprising copper glance ,native Copper, oxide , carbonate , si l icate , sulphate , andphosphate of Copper

,and other compounds . The other

ores consist of compounds of copper with antimony ,arsenic , and selenium , forming complex minerals . Withthese ores there occur quartz

,carbonates of l ime , i ron ,

and sometimes manganese , and , more rarely , barytesand fluo rspar.

The subdivisions of the copper group depend solelyon the relative amounts of the copper-bearing minerals ,so that between one type and another there are transitio nal forms ; By far the most abundant of these subtypes is that characterized principally by the presence ofcopper pyrites, i ron pyrites, and quartz, with the subordinate presence of selen ium and antimonial compoundsof copper

,and carbonate of i ron . These lodes occur

under conditions which can be precisely paralleledwith those under which the ores of lead and zinc arefound , and this applies not only to the veins , but tometasomatic deposits referred to on p . 286.

When carbonates,fluo rspar, or barytes are in abun

dance , the vein - type belongs to one of the subgroupsof copper veins

,while with further additions of these

minerals or subordinate ores the transitional types toother groups are formed . Fahlerz with calcite andbarytes, or cobalt and nickel with siderite , are suchexamples . A copper ore of restricted occurrence i s

HYDATOGENESIS 1 57

that which has , owing to its obj ectionable nature , onlybeen worked extensively in recent years by an improved processes o f smelting . The mineral is known as

enargite and occurs in veins containing ,i n addition

,fahlerz and other Copper minerals.

The occurrence of native Copper as an original deposit

i n veins constitutes a very remarkable and exceptionaltype found in extensively developed deposits in theKeweenaw peninsula in the Lake Superior region .

N ative Co pper.

— In the Lake Superior region thedeposits belong mainly to a peculiar type of metaso

matism ,and are described under that heading (p . 290)

but in associat ion with them are true fissu re infil lingsof low-grade native Copper and other minerals . Broadlydescribed

,the central part of the Keweenaw peninsula

i s a thick series of conglomerates , sandstones , and bedsof lava (melaphyre , see p . which at the upper surfaces o f the flows are vesicular , and known as ashbeds . ’ As the vesicles contain native Copper and otherminerals, such as calcite , quartz, and zeol ites , theseportions of the lavas are called ‘ lodes ’ or ‘

amygda

loids . ’ The rest of the rock is considerably altered,

the ferromagnesian sil icates being chlo ritized,while

there is a development of epidote which is also foundamong the quartz porphyry pebbles forming the conglomerate . The native Copper also occurs between thepebbles .

In the Calumet and H ecla Mine , the ore i s principallywo rked in the conglomerate .In addition to the occurrence of native copper in the

conglomerate and the amygdaloid , the copper ores existin fissures , a few to many feet in width , which crossthe bedding from conglomerate to amygdaloid . Where


they cross the latter they are exceptio nately rich . Theore is found in ordinary j oints

,breccias, and stock

works, the principal vein material being native Copperand calcite . Various zeolites are among the mineralsassociated with the ores . The minerals present arelargely quartz

,prehnite , and native copper , from grains

up to masses several tons in weight ; a little native

A C LCA K A M A L A

FIG . 35.—SECT ION OF A COPPER VE IN IN THE LAKE SUPER IOR

D I STR ICT. (AFTER H . CREDN ER . )

A,Amygdalo id ; L , laumo ntite ; Q,

quartz ; E , epido te ; C,calcite

K,co pper ; P , prehnite ; M ,

apophyl lite .

si lver i s found , and orthoclase, analcime , datol ite ,laumontite

,apophyll ite , epidote , chlorite , and a little

copper sulphide (F ig .

The origin of the deposits is believed to be as followsThe ancient lava was much decomposed , and the o fesof copper were secreted from this rock itsel f. Thenative copper was precipitated from the solutions byoxide of i ron in the rock .

Similar amygdaloidal Copper deposits occur in the


consist of quartz with red copper ore , which in depthgives place to sulphide of copper .I n N ew South Wales the Great Cobar mines are the

most important Copper-producers of the region . Theseoccur in the districts of Cobar, Mount Hope , Nymagee ,and other places situated in a district poor in water .The rocks are slates and sandstones of S ilurian age ,and the deposits occur as interbedded veins . The ore ismainly cupri ferous pyrrhotite

,with 16 per cent . of si l ica

and up to 4 per cent . of Copper . There i s a l ittle Copperpyrites, Copper glance and magnetite , while bismuthoccurs in some quantity.

I n the Mount H o pe region , about 100 miles fromCobar, the ores are connected with diorite and andesite .The Copper deposits of California occur in fourgroups . Those of Shasta County are the most important

,while the other districts are those of the foot

hill s o n the west of the Sierra Madre,those on the

coastal range,and those i n the south- eastern desert .

The deposits of Shasta County are situated near thetown of Redding

,at the northern end of the Sacramento

Valley, where the ores occur largely in Triassic andPermo Carboniferous lavas and tuffs (rhyolite andandesite) , which are older than the gran ite intrusions ofthe region . Like the sedimentary series , the lavas havebeen much folded and sheared , and the ores occur asi rregular sulphidic deposits , varying from a few inchesto several hundred feet in width . The Copper depositsin the I ron Mountain belt are the most important inthis region

,and consist of Copper and iron pyrites ,

zincblende, a l ittle born ite and Copper glance . Theysometimes occur as impregnations of crush- breccias inrhyolite , etc .

HYDATOGENESIS 16 1

In the Bully H i l l district the ores are met with ina shear-zone following a basalt dyke ; they occur eitherin the dyke or at its contact with rhyolite . The principal ores are Copper and iron pyrites , zincblende , alittle quartz and barytes . N ear the surface secondaryores , with limonite and galena , are found in abundance .I n the foothill s of the Sierra Madre the lodes occurin metamorphic schists and slates , sometimes accom

panied by quartz porphyry . The o fes exist as a seriesof lenticular masses of Copper and i ron pyrites , with alittle zinc ore .In B eaverhead County, Montana , the copper ores

occur in crush -zones between granite and contactaltered l imestones

,i n association with much clay, and

consist of copper and iron pyrites and copper glance .

I n Alaska (Copper Mountain region) deposits ofCopper occur in metamorphosed garnetiferous limestones in the neighbourhood of greenstones and gran ite .

The ores exist as bands in the al tered l imestone inassociation with quartz, garnet , epidote , and magnetite ;they consi st of Copper and i ron pyrites , pyrrhotite andother sulphides . I n the région of Prince William Sound ,lenticular deposits of copper pyrites , i ron pyrites , andpyrrhotite with quartz and pieces of the countryrock ,occur in schistose rocks in the vicinity of gran ite

,

aplite , and schistose altered basic dykes and lava-flows

(F ig .

In the Copper R iver district near Mount Wrangell ,the ores of copper are found as disseminations ofbornite and as vein s of copper glance associated withmagnetite and pyrrhotite

,in masses of greenstone which

extend for nearly 300 miles in length . The greenstone, together with Permian l imestone which sur

1 1


rounds it , i s l i fted up , and forms part of the Alaskanrange .

The igneous rock , which i s intrusive , consists of felspar , pyroxene , chlorite , with a l ittle serpentine and

l —l -I

I 2 3 4Mil es

l im e s tone S chis t Gra nitet! Qua rtu te Gre ens t o ne

FIG . 36.—SKETCH-MAP OF THE COPPER MOUNTA I N D I STR ICT,

PR I NCE OF WALE S I SLAND , ALASKA . (AFTER F. E . AND

C . W . WR IGHT. )

magnetite . N ative copper and oxide of copper occurin shear-zones

,while disseminations and shoots of

born ite and black sulphide of Copper are met with atthe junction of the limestone with the greenstone . I n


granites,and are invaded by granite -porphyry . The

ores , consisting mainly of Copper glance , Copper pyritesand secondary basic sulphates of Copper

,occur as

contact deposits containing large amounts of oxidizedores

,as veins and impregnations i n rocks near or in

the porphyry , or as veins in dolerite dykes. The contact deposits in limestones are referred to on p . 354.

The ores of the vein s in addition to Copper pyritesconsists of zincblende , molybdenite , and iron pyrites .

These ores occur also as pyritic impregnations in thegranite -porphyry , and much of the ore in the upperlevels i s oxidized .

The Copper ores of Yavapai County consist of Copperpyrites and zincblende , with a good silver and goldcontent , in a carbonate veinstone . They are found insheared diorite near its contact with Algonkian schists .A variety Of secondary ores are also met with , including carbonates of Copper impregnating chert ;chrysocolla is also found , and Copper ores occasionallyform the matrix of l imestone -breccias .The copper deposits of Wyoming are mainly ofinterest on account of the forms taken by the deposits .The region consists of Cambrian quartzites , conglomerates, shales , and l imestones, with granite andhornblende schists intersected by dykes of diorite .

At the j unction of the schists with quartzites occurdeposits o f copper pyrites

,copper glance , covell ite ,

and other minerals . The quartzite is brecciated andtraversed by numberless intersecting j oints , so that ,while the ore has a well-defined wall against the schists ,it impregnates the quartzite in the form of interlacingstrings , and gives the ore body a most i rregularboundary (F ig .

HYDATOGENESIS 165

Chil i i s one of the important Copper-countries of theworld

,but mainly as regards secondary ores , which

are described later in Chapter VI I I . Briefly , the copperores of northern Chil i consist of sil iceous Copperpyrites veins cutting through syenite , porphyry, and

FIG . 37 .—FORM O F COPPER O RE LODE IN FERR I S HAGGERTY

M INE,WYOM ING. (AFTER SPENCER .)

basic intrusives . Copper pyrites i n the lower portionsof the veins is associated with bornite , which carriesa small proport ion of gold ; but in the upper, oxidized ,zones carbonates and sil icates of copper are the mostproductive ores , the sulphides being only low-grade .

In Central Chili the ores are mixed sulphides of


Copper, lead , and zinc , with veinstones o f j asper, andsometimes calcite , t raversing augite porphyry . Thesame general conditions as regards the secondarynature of the bulk of the ores hold good for theprincipal Peruvian mines (Cerro de Pasco) , which havealso been among the most famous of S i lver-producers .I n Peru , however , a type of copper deposit occurs inwhich i ron and copperpyrites are found

,in association

with manganiferous siderite,in lodes traversing schists ,

quartzites and limestones .The Ital ian copper deposits are confined to thedistrict of Tuscany and Liguria . The deposits of theMassa Marittima occur fi l l ing late-Tertiary fault fissuresof considerable width . The country-rock is silicified

or , in its calcareous portions , altered to pyroxeneepidote rock , and impregnated by Copper and i ronpyrites , galena , and zincblende ; the o re carries 3 percent . of Copper .The deposits appear to be connected with intrusionsof granite

,but the granite does not occur in the dis

trict i tself, the nearest mass being some miles away .

Gabbro and diabase form subordinate intrusions. Therocks in which the deposits are situated are dolomiticl imestones of Rhae tic , and Lower and Middle L iassicage ; also Upper L ias shales overlain by E ocene

(nummulitic ) l imestones , sandstones, shales and marls ;the remarkable veinstone is a di rect product ofthe alteration of the calcareou s beds . Metasomaticdeposits of argenti ferous galena , zincblende , S ilverbearing fahlore

,and copper pyrites , with calamine ,

quartz,calcite

,fluo rspar, and gypsum , have been worked

in the past in the Liassic limestones .The copper deposits of Monte Catin i occur in the


obtained from quartz veins in a mineral ized zone ,t raversing volcanic breccias interbedded with lavas .The ores consist of i ron pyrites and Copper pyrites ,frequently coated with covellite , i n a sil iceous veinstone .Other kinds of deposits occur in zones of fissuringand crushing in serpentine

,but thei r importance i s less

than those described above .

The island of Hayti in the West I ndies contains afew deposits of copper connected with basic igneousintrusions . The veins are in dykes of diorite and incontact -altered limestones near the dykes . Bornite andCopper pyrites , with some gold and si lver, constitutethe principal ores . E lsewhere in the island copperpyrites i s found in pockets , small masses , and im

pregnatio ns or disseminations in garnet- rock near andesitic and dioritic intrusions . The presence of Copperglance and copper carbonate in melaphyre , and of platinum and osmiridium in the ores of copper, makes thedeposits somewhat analogous to the covellite depositsof Wyoming .

At Kedabek,i n TranscaucasIa , deposits of Copper ore

are met with at an elevation of nearly feet in thecopper-mountain of Mio Dagh ; the ore is in the formof lenticles in a quartz porphyry, which i s cut bydiorit ic dykes .

,I n more immediate connection with

the deposits i s the rock known as kedabekite,’ a

garnetiferous felspar-pyroxene rock .

The ores are pyrit ic,but some copper glance and

covell ine occur,the ore bodies fading o ff at the sides

into the country - rock in the form of veins . Zincblende is also present , and very l ittle galena . Themost characteri stic veinstone mineral i s barytes , butquartz i s also represented .

HYDATOGENES IS 169

Lodes of copper and iron pyrites , tetrahedrite , born ite ,and native copper

,with galena and zinc sulphide , are

found between the Caspian and the Black Sea in andesite and diabase

,in the form of impregnations of crush

and brecciated zones .There are many large Copper-districts in the U rals

and Siberia . The principal Russian local ities are on thewestern side of the U rals

,in the neighbourhood of Perm

and N izhne -Tagilsk . The ores are mainly low-gradeoxides and carbonates, i n veins traversing Permian andDevonian slates and limestones . I n depth the ores arepyritic . On the eastern side of the U rals pyritic copperore , with calcite and quartz , i s found in lenticular massesat the junction of diorite with pyroxene -garnet rockand limestone .

I n Thuringia,near Saalfeld

,copper ores occur in

veins which fault the Permian limestone and the Carbonifero us rocks . The veinstones consist of barytes andsiderite , with a l ittle quartz , and are richest in thoseparts of the lodes which traverse the Kupferscheifer.

In the same region much fluo rspar i s found in similarveins , while haematite derived from the alteration ofSpath ic i ron ore has been worked .

The Spassky copper mine i s situated about 2 00 milessouth of the town of Smolensk

,i n the province of the

same name , and is one of the most important minesin Siberia . The ores are mainly born ite

,with glance

and grey copper ore . They occur in veins from whichthe sandstone of the country - rock has been muchimpregnated . The vein -material is quartz and barytes .The only rocks of igneous origin in the immediatedistrict are porphyrite and rhyolitic breccias .In North -Western Servia copper ores are met with in

1 70 THE GEOLOGY OF . ORE DEPOS ITS

connection with serpentines intruded into Mesozoicrocks . The copper ores are composed of copper and i ronpyrites , with calcite and serpentine . The ores originnated by lateral concentration during the serpentinizat ion of the rock , and are now found in brecciated massesand irregular veins.

The Copper ores of South Africa are largely obtainedfrom Namaland

,or Namaqualand , in the north -west

portion of Cape Colony . They are Copper pyrites andborn ite, and appear to be intimately connected withnorth -north -east diorit ic dykes traversing schists andgneiss . The ores are probably segregations. Similardeposits are found in Damaraland .

I n the southern part of the Congo Free State , about1 00 miles north of Rhodesia

, the deposits of Copper oresare stockworks and impregnations of Copper and i ronpyrites , occasionally with manganese , in lower Palaeozoic quartzites and limestone . The upper parts of thelodes contain oxides .The ores are very si l iceous , and occur largely as im

pregnatio ns of quartzite by malachite . Seams of thesame mineral also exist in the quartzite . The copperores are met with along a zone of fissuring whichextends in a north -westerly direction for 2 00 miles .

I n Rhodesia Copper ores have been worked for ages ,but not in a large way. I n the Sable Antelope minethe lodes traverse gneiss and vesicular basalt , and contain Copper pyrites , glance , and carbonate , also borni te ,with calcite and gypsum .

I n China Copper ores occur in the provinces ofYunnan and Kweicho u ,

in both of which the sulphidei s the principal ore

,and is found in sandstones near

masses of po rphyrite and melaphyre , and in l imestones .

1 72 TH E GEOLOGY OF ORE DEPOSITS

intruded into Palmo zo ic rocks . Situated near these deposits are other mines , connected with propylitizedandesites associated with Tertiary rocks. The oresare associated with specular i ron ore

,sulphides of lead,

zinc , and silver , with quartz, barytes , calcite , and diallogite .

In the South of Japan,in the more northerly of

the two islands, is the famous B esshi Mine , situatedin a series of quartz , chlorite , and graphite schists .The main body of the ore is embedded in the schists ,which near the ore contain the manganese epidote ,—piedmontite . The ore

,which is mainly copper su l

phide , occurs with i ron pyrites as bands and streaksin the schists , and yields 3 or 4 per cent . of copper .The whole mass of the ore forms a body over a milein length , and ranges from a few feet to 30 feet inwidth .

I n other parts of Japan the ores are found in vein snear granites and diorites , and also occur in brecciatedpropyl it ized andesites .Arsen ical Su lphidic Co pper Ores — Oi the type inwhich enargite is the dominant mineral , the most im

portant examples are those of Butte , Montana , whichcontains the richest Copper mines in the world . Thedistrict was originally worked

,up to 1892 , mainly for

si lver ores, but the fall in price of that metal causedmany of the S i lver mines to be closed down . TheCOpper now produced contains 84 ounces of si lver andO 3 ounce of gold to the ton of metall ic copper yielded .

The mineral region is enti rely made up of Tertiaryigneous rocks

,of which quartz monzonite , known locally

as the Butte granite,i s the most important , and covers

the largest area , being surrounded beyond the mining

HYDATO GENESIS 1 73

centre by Cretaceous metamorphosed limestones , etc .,

and partly covered by andesite and andesit ic breccias .The Butte granite i s cut by later aplit ic veins , andintrusions known as the Bluebird granite . ’ Sti ll laterintrusions

, as dykes , of rhyolite or quartz porphyry occurwith the lodes . The mineralization of the area com

1

Bu tte Blueb/rd Qua rtz Rhy o l ite LacustrineAl la vium Copp erGra n i te Gran i te -Po rp hy ry Lo des Lo d es

FIG . 39 .

— GEOLOGICAL SKETCH-MAP OF TH E BUTTE D I STR ICT,

MONTANA . (AFTER MESSRs. WEE D,EMMONS

,AND TOWER .)

menced after the intrusion of the quartz porphyry , andwas followed by intrusions and outpouring of rhyolite

(see F igs. 39 andThere are no outcrops visible , for the upper parts ofthe lodes have undergone extensive weathering , withsecondary concentrat ion of the ores . As pointed out in


describing the silver ores of Butte , the Copper and silverareas are dist inct

,although all the lodes belong to

the same period of mineralization . They are clearly

FIG . 4o .

—IDEAL IZED SECT ION OF A COPPER LODE AT BUTTE ,

MONTANA, SHOW ING TH E RE LAT ION O F QUARTZ PORPHYRYDYKE , LODE , AN D FAULT TO O N E ANOTHER . (AFTERH . V. W INCHELL .)

divisible into three groups . The oldest i s that uponwhich the famous Anaconda is situated , having an eastand west strike . This i s intersected by another richgroup of lodes, with a bearing north-west and south


cut bydykes of porphyry. The o re occurs in fissurestraversing the slates in every direction . The upperparts of the veins consisted of oxidized ores and carbo nates, below which were complex ores , such asenargite

,famatin ite

, eucarite , humangite (selenide ofcopper and silver) , and Copper glance

,but in depth

these diminished . Galena and zincblende occur insome quanti ty .

PRIMARY LEAD AND Z INC VE INS .

Deposits of galena and zincblende fall within important subgroups , and have such marked di fferences inform and mode of deposition that it i s convenient todeal with them under the heads of fissure or vein deposits and metasomatic deposits . I n this sect ion , onlythe veins will be described . The ores of lead and Z inchave an identical mode of occurrence , and they arecommonly associated with one another , the ores of leadsometimes giving place to ores of zinc and vice verso .

Although often characterized by a variety of otherminerals , their various vein-types are comparativelysimple , and are in marked contrast to the ores of goldand silver . In many respects , however , the lead veinsfrom the genetic standpoint are in separable from thoseof S ilver , as the difference is only that of the relativeproportions of these ores to one another .Galena usually contains a small percentage of silver

,

varying from per cent . up to 2 per cent . in thedifferent subgroups into which the veins are divi sible .

Other metall iferous minerals found in associat ionwith lead and zinc ores include sulphides of i ron andCopper, with compounds such as bournonite , fahlerz,j amesonite , antimonite , and very exceptionally also

HYDATOGENESIS 1 77

nickel , cobalt , and bismuth ores . Mi spickel i s occasionally present , and sometimes phosphates .

'

Cadmium iscommonly

,and indium and gallium are rarely , found

associated with ores of zinc .

Minerals which characterize ores of pneumatolyticorigin are absent , although microl ites of tinstone (cassiterite) have been detected in zincblende a t Freiberg , andwolfram in lead ore in the Hartz and in Cornwall .The veins of lead and zinc appear to be connectedwith acid intrusive rocks of all ages, but the ores notinfrequently Show a marked preference for certain kindsof rocks .I n the younger or Tertiary intrusives they almost

invariably contain appreciable amounts of gold andsilver, often in such quantity as to warrant thei r classificatio n with the gold and silver veins .The most important lead and zinc lodes are thosewhich occur in the Palaeozoic rocks.

The generally accepted classification of lead and zinclodes is dependent upon some dominant vein constituent, which leads to a threefold grouping .

The fi rst i s the pyri tes- lead -quartz group,which

includes such minerals as galena, zincblende, i ron and

copper pyrites , mispickel , and quartz . Subordinateamounts of carbonate of l ime , magnesia , and iron , withj asper and chlorite , may also be present . ° The amountof si lver present in the galena rarely exceeds 0 °

5 per cent .and is generally from O

°

I to O °

2 per cent . , while i t i s absentin the other minerals.

The second group comprises veins consisting of richargentiferous lead ores (galena) , with brown spar (dolomite) as the most important veinstone . The otherminerals present may be quartz , zincblende , i ron pyrites ,

1 2

1 78 TH E GEOLOGY OF ORE DEPOSITS

fahlerz, rich silver ores , carbonate of iron and manganese . The pyritic constituents may also be argentifero us, and there are sometimes argentite and rubysi lver ores . The amount of si lver in the galena variesfrom O 4 per cent . to 2 per cent . I n the zincblende

si lver in the form of sulphide may be present to theamount of per cent . , while the pyrites may containas much as O

°

2 per cent .

The third group is that of the barytic lead veins , inwhich barytes is the main vein const ituent , usuallyaccompanied by quartz , calcite , and fluo rspar of varioustints . The metall i ferous minerals are argenti ferousgalena

,sulphides of i ron , zinc , and Copper, with fahlerz

and rich S i lver ores . The zincblende frequently contain s inclusions of si lver sulphide , and the galenacontains from to per cent . of silver .All these three types of veins occur in the Freibergdistrict , where, in addit ion to the simple forms , thereare transition - types having the qualit ies of two or moregroups .Typica l E xamples o f Lead and Z inc Veins .

—Thelead and Zinc vein s of Cardigan and Montgomery occurin a belt twenty- two miles in length , extending fromthe south of Cardiganshire to Llan idloes , in Montgomery . No igneous rocks exist anywhere in thisdistrict ,

‘ so that the source of the ores , althoughprobably deep -seated , i s unknown .

There are six groups of lodes corresponding withundulations in the Palaeozoic shales , grits, and conglomerates . I n the west the lodes consi st of slightlyargenti ferous galena with zincblende , and occasionallysulphide of Copper. The rich district known as theWelsh Potosi yielded argenti ferous galena contain ing


hampton , B eeralsto n) , and in Cornwall (Calstock ,Call ington

,Menheniot , St . Minver, St . Teath , E ndell ion ,

N ewquay,Truro , Perranzabuloe , H elston) .

The ores appear to belong to the barytes-fluo rquartz type , although neither barytes nor fluor areuniversal . N ear Tavistock the ores consi st mainly ofsulphide of lead and zinc , with carbonate of i ron , oxideof i ron , and quartz . At B eeralsto n a siliceous veinstonecontains argentiferous galena .

The lodes of Wheal Mary Ann,in Cornwal l ,

Show a characteristic paragenesis . The veinstones arebanded

,and sometimes associated with brecciated killas

cemented by chalcedonic quartz . There are two periodsof infi ltrat ion of fluo rspar, with an intervening periodin which quartz and galena were deposited

,followed ,

finally , by deposition of quartz , i ron pyrites and calcite .Carbonate and phosphate of i ron and lead occur withnative si lver

,red and dark sulphide of si lver, some

copper sulphide , antimonial ore , bournonite , and barytes .Zinc sulphide is found at Hero dsfo o t . I n North Cornwall antimonial si lver and lead ores occur, but thei rconnection with the lead ores of E ast Cornwall and

D evon is doubtful .N ear Truro the veins consist of quartz, galena , and

zincblende, while near Helston carbonate of i ron , i ronpyrites

,and pyromorphite , exist in association with the

lead and Z inc sulphides .The lead deposi ts of Northumberland , Cumberland ,Durham

,and Derbyshire , are generally found in l ime

stones,which are metasomatically replaced by lead ores

near the lodes , so that they are usually metasomaticreplacements , but in some cases are infil lings of cavitiesformed by the solvent action of water. I n the Alston

HYDATOGENESI S 181

district (Northumberland) the lodes are found mainly inthe Carboniferous l imestone , but are particularly productive in certain beds . I n addition to fluctuations inproductiveness

,there are often marked changes in the

mineral contents of the lodes . In som e places thecharacteristic minerals are fluo rSpar and calcite , whilein others they are Spathic i ron ore or brown haematitescar limestone)In Derbyshire the variety of forms displayed by theore bodies has led to thei r classification under suchterms as rakes,

’ pipes , flats ’ and scrins .

’ The rakesare true fissures , often faults ; the scrims are mineralizedfissures crossing them ; the flats are mineral ized beddingplanes ; while pipes are i rregular pipe - l ike bodies of ore .At three horizons in the Derbyshire l imestone aresheets of basaltic rocks , known as toadstones .

’ Whenthe lodes pass from limestone into the Mi ll stone G rit ,shales , or toadstone , they become unproductive .

I n Colorado (the Aspen , R ico , E nterprise mines,etc .) the lodes have a north -east bearing

,and traverse

slates , sandstones , and limestones of Lower Carbonifero us age . They differ in character and vary invalue according to the rocks which they traverse .At the contact of a much - fractured limestone withoverlying shales the ore has spread out laterally

,and

by infi ltrat ion has acquired a banded structure . Belowthis horizon the lodes are barren ; they consist mainly ofbreccias ranging up to about a foot in thickness

,and

contrast strongly with the numerous strings in the l imestone . The vein materials are quartz and carbonate ofmanganese , with galena , zincblende , copper and ironpyrites, also the sulphides of si lver and antimony . Alit tle native si lver with gold occurs in someplaces .


At Plomo , Colorado , a granite-gneiss contains aurif

ero us pyrites along certain bands and zones . Theseappear to have been formed by impregnation and re

placement along crush -belts . Hornb lende and b iot iteare altered to chlorite . Secondary quartz and cubesof pyrites also occur

,and the altered rock passes in

sensibly into the gneiss . The mineral solutions traversing faults penetrated the crush -zones of the gneiss

,

and deposited auriferous pyrites and sil ica . At a laterdate renewed disturbance resulted in dyke - intrusionsof quartz porphyry and the formation of a system o f

fissures , through which a second access of mineral izingsolutions resulted in the formation of later lodes containing quartz

,galena

, Copper and Zinc sulphides , fluo rSpar , and micaceous i ron ore .

The lead deposits of the Coeur d ’

Alene district , i nNorthern Idaho

,between Spokane and the Bitter Moun

tains, belong to the carbonate type . The district i scomposed of Algonkian slates and quartzites pierced bya large syenitic intrusion . The lodes are later than thesyenite , but older than a series of intrusive basalticrocks which traverses it ; the Hecla vein , however, is

an exception,for it i s younger than a basic dyke with

which it is associated .

The veins,in addition to argenti ferous galena ( 10 per

contain considerable amounts of carbonate ofi ron

,quartz

,zincblende, and i ron pyrites , some copper

ore and barytes,and other minerals , including secondary

compounds of S i lver (F ig .

At Frisco,in U tah

,the H o rnsilver Mine i s on a lode

situated at the junction of l imestone with andesiticrocks . The l imestone is metamorphosed to garnet - rockby an intrusion of monzonite , which has also affected


lode traversing highly schistose rocks at right anglesto the fol iation . The associated minerals are , in addit ion to rose-coloured calcite-siderite , zincblende, pyrites ,and pyrrhotite ; ozokerite i s also found , but no quartz.I n N ew South Wales , the famous Broken H i l l MiiIe

i s another example of argent i ferous galena associawith quartz

,fluo rspar, and small quantities of calc

The country consists of metamorphic rocks penetrated by gran it ic masses and dykes of diorite . Theie

are two principal lodes , the larger of which is nearly1 00 feet wide in places . The actual rock traversed bythese lodes i s a garnetiferous biotite gneiss , which atBroken H i l l i s supposed to be bent in the form of asharp anticl inal fold , accompanied by crushing .

Crystall ine garnet . may be regarded as a true veinconstituent , for it contains inclusions of galena andblende . I t appears in the vein , however, to be dueto a reconstruction of brecciated garnetiferous countryrock

,since secondary garnet crystals are found grow

ing on garnetiferous fragments . I ntergrown with thegarnet i s sil icate of manganese .At Zeehan

,i n Western Tasmania , the much -folded

Lower Silurian conglomerates, sandstones, and clayslates , with altered basalt ic lavas and tuffs , are , in theneighbourhood of the lead veins , much altered bygranite . Gabbro and serpentine are also found in themineral district , but these are older than the graniticintrusions .The veins are connected with the granite , and threegroups, differentiated by thei r characteristic minerals ,have been recognized . The ores occur in fissures eitheras thin strings or as masses reaching 2 0 feet in width ;also as i rregular deposits from 60 to 200 feet wide .

HYDATOGENES IS 185

The first group contains sulphides of lead , zinc ,and iron .

The second contains argenti ferous galena , with a littleblende

,pyrites and carbonate of i ron .

The third is an insignificant series which belongs toa t ransition -type between tin and si lver- lead veins.

The ores consist of argenti ferous sulphide of tin , withwolfram

,copper and iron pyrites

,galena , sulphide of

b i smuth , and carbonate of i ron . The galena containsfrom 1 0 to 200 ounces of si lver to the ton ; the argen

tifero us pyrites , 146 ounces to the ton .

The veins , however, of the district generally belongto a mixed type

,representing the characters of the first

two groups ; they may also carry j amesonite , bournanite, and fahlerz .The upper parts of the lodes contain native silver andother secondary silver ores (chlorides and sulphides) ,oxide of manganese

,sulphate and carbonate of lead,

etc ., while the veinstone is generally quartz , with sub

ordinate amounts o f calcite , dolomite and carbonate ofi ron .

In the Dundas region,six miles east of Zeehan , the

deposits are similar , but in some cases Show a departurefrom the usual type , as , for instance , in the connectionof sulphidic minerals and carbonate of i ron withdolomite .In the Freiberg district (Saxony) the lodes are

situated mainly in gneiss,mica schists , quartzites , and

other metamorphic rocks . E clogites,garnetiferous

hornblende rocks , serpentine , and gabbro , also occur .These rocks are broken through by granite

,and by

quartz-porphyry and other dyke rocks . With the exceptio n of the rich silver-quartz veins of the district ,


the lodes are all younger than the quartz-porphyries,

the barytic lead veins being the youngest .The lodes may be classified according to their strike .

The pyrites - lead and part of the rich silver -quartz andrich lead types strike about north -north-east , while thebarytes- lead veins and the remainder of those abovementioned strike about north -west in common withnon -metall i ferous cross -courses .The pyritous lead ores also are well represented inthe mining - region of F reiberg

,where the infil ling of

the veins consists of quartz, i ron pyrites , pyrrhotite ,argentiferous galena

,zincblende , and sulphides of

arsenic , Copper , and other metals , with smaller quantit ies of the carbonates of i ron ,

manganese , calcium andmagnesium . Among other minerals may be mentio ned : barytes , fluo rspar, calcite , strontianite , pitchblende , and a variety of rarer minerals , together withsecondary ores . The zincblende contains small quantit ies of tin ore in the form of microl iths .I n this region a few of the lodes are characterized byan exceptionally high proportion of Copper ores.The barytes - silver- lead lodes , ranging from 3 to 1 2

and even 20 feet in width,occur in gneiss

,phyll ites

,and

mica-schist , and consist mainly of barytes, fluo rspar,quartz, calcite , and dolomite , with sulphide of lead ,copper, i ron , and zinc . Bournonite is also present withless common minerals , such as arsenical pyrites , ant imonial si lver-blende

,cerussite , anglesite, malachite , and

other secondary ores . Rarely they contain nickel andcobalt ores . They are later than the pyritous and highgrade lead veins, which they traverse l ike faults .At Schneeberg

,in Saxony, arsen ical pyrite occurs

i n large quantities,quite dominating the galena con


I n the .Clausthal region (Harz Mountains) the lodesoccur in Devonian and Lower Carboni ferous shales over

mics

DeVo n 1a n Cu Im 00k er Permia n J urass ic, Upp erGra n ite Tr/a s 8 Creta c eous

l owerCreta ceo us

FIG . 42 .— GEOLOG ICAL SKETCH -MAP OF THE Lo DEs OF THE

CLAUSTHAL REGION,U PPE R HARZ . (AFTER F . KLOCK

MANN )

an area ten miles long by five miles wide , and belongto the carbonate type . There are ten parallel series ofcomposite lodes with thei r numerous stringers , and

HYDATOGENES I S 189

each series i s traceable through practically the wholelength of the mineral area (F ig .

The average strike of these composite lodes is eithertowards the south -west , or i s east and west , and theyall have an underlie of from 1 0

° to 20° to the south .

The veins are later in age than the folding of the region,

and can be actually seen to traverse several zones ofbrecciated rock (the so -called ruscheln existing alongplanes of o verthrusting, and strikingtowards the northeast (F ig .

The contents of the lodes consi st mainly of quartz ,calcite , and in some cases barytes , with lesser amounts

II

ON E M IL E

FIG . 43.— PLAN O F THE FAULTS (RUSCHELN ) AND LODES AT

ST. ANDREASBERG , HARZ . (AFTER F. KLOCKMAN N . )

of siderite,strontian ite , and dolomite , with fragments of

crushed countryrock . The principal ores are argentifero us galena (O

°

2 to 03 per cent . of si lver) , zincblende ,Copper and i ron pyrites, tetrahedri te , and bournonite .

I n the State of Anhalt , the deposits of the HarzMountains belong to the high -grade lead ores, andconsi st of minerals of several generations associatedwith a vein of spathic i ron ore of considerable width .

The oldest vein minerals are quartz,pyrites

,and pyr

rho tite ; then comes fluo rspar with spathic i ron ore ,some zincblende , and barytes ; thi rdly comes galena


and Copper pyrites , with bournonite , fahlerz , and antimonial ores ; and finally spathic i ron and calcite .Wolfram and scheelite occur occasionally .

I n the Rhineland and Westphalian regions thereare several zinc and lead-fields i n the Palaeozoic rocks .The veins are the si liceous zinc - lead carbonate type

,

but are occasionally characterized by barytes.

Those of Cob lenz (Werlan) are of a type similar tothe Holzappel lodes .The other local it ies are near Arnsberg

,Gladbach ,

D iisseldo rf, etc .

In the Black Forest (Munsterthal ) the lodes areassociated with porphyritic dykes in b iotite gneiss

,and

consist of quartz , fluo rspar, calcite , siderite , and barytes .These occur as vein minerals with zincblende andgalena , with subordinate amounts of pyrites , si lver ,antimonial and arsen ical ores

,and secondary ores .

I n the Schapbach region the ores Show pseudomo rphs ofsaccharoidal quartz after barytes , with galena , fluo rspar,and both original and secondary copper ores . Calcite ,dolomite , and S iderite also occur . Bismuth , si lver , andother ores , are also found .

At Pribram , in Bohemia , lodes are found in a syn

cl ine of Cambrian grits , on the south - east of which are

slates contact -altered by the granite-mass which l iesstil l farther towards the south .

Between the grits and some shales lying to the norththere is a fault st riking north -east , which cuts Off thegrits against the shales . One of the principal miningcentres is situated close to thi s fault (known as the

L ettenkluft) , and is traversed obl iquely by the lodes ;these in the shales , however, become poor .

Parallel with the lodes are greenstone dykes in which


In the Anna Mine some of the ore i s highly sil iceous,

and contains but l i ttle carbonates , the metall i ferousminerals being argentifefo us galena , ruby and nativesi lver ore , fahlerz , ant imony sulphide , etc . , these beingyounger than those mentioned above .The lodes show a banded arrangement in which earlyore of the fi rst type occurs next to ore of the secondtype

,with a last vein -fil l ing Of

'

calcite .

There are several localities in Bohemia which aredistinguished as Zinc or lead producers .

To the west and south of Pi lsen , lead and zinc oresare found with quartz . I n one place the mineralparagenesis i s quartz with some barytes and fluo rspar,followed by sulphides o f lead (with per cent . ofS i lver) , Zinc , and iron . I n a neighbouring district thebarytes and fluo rspar are not present , but are replacedby calcite and dolomite ; the silver content of thegalena varies between and O °

2 per cent .South of these places (at Kuttenberg) , on the southeast of Prague , and at Budweis , in Southern Bohemia ,veins of zinc and lead sulphide , with quartz , calcite ,dolomite

,i ron pyrites , and sulphide of arsen ic , with

some native and red silver ores, occur in gneiss overlain byCretaceous rocks . The gneiss is invaded by an intrusionof granite

,and by veins of pegmatite and mica trap .

North of Graz,in Styria , in the Southern Tyrol , and

on the east of Trent , veins of Zinc and lead sulphidesare well known . North of G raz the lodes are incalcareous

,chlorit ic , and clay slates, and consist of

zincblende and galena (with about per cent . ofsi lver) , with sulphides of copper and i ron ; quartz ,barytes

, si l icate of zinc and carbonates of i ron , calciumand barium , are common associates .

HYDATOGENES I S 193

In some veins the order of mineralization was quartz,

albite felspar,sulphides of zinc and lead , fluo rspar,

and galena of a second generation with secondarycarbonates .At Pfundererberg the gneiss and slates are traversedby diorite veins

,and dykes of aplite and micropegmatite .

The lodes are brecciated,and contain sulphides of

lead, Copper , i ron , and zinc . The galena contains o °

3

to 06 per cent . of S i lver, while the copper and i ronsulphides are auri ferous .I n the district of Laibach , in Carniola , ores of lead

and zinc occur in fissure -b reccias and fractures inirregularly bedded Carboni ferous sandstones and shales .Three modifications of the veins are known . The fi rstconsists of sulphides of lead , zinc , and i ron , withbarytes . The second is haematite with barytes , galena ,and a litt le Cinnabar and Copper ore . The third isCinnabar in vein s with l imonite , barytes , aragonite ,witherite , etc .The Bosnian occurrences are associated with pro

pylitized dacite , slates , and quartzites. The lodes,

ranging up to 1 6 feet in width , consist of quartz, witha l ittle dolomite and siderite , sulphides of lead (O

°

2 percent . of silver) , zinc , and i ron , and small amounts offahlerz, antimonial copper , and secondary ores .Tuscany and Sicily have thei r lead mines . InTuscany the lodes are in Palaeozoic slates, and consistof quartz , a l ittle albite , carbonates of calcium andi ron , and sulphides o f lead (0

°

3 to 0°

5 per cent . of si lver) ,zinc , i ron , and Copper , with antimonial lead ores . Alittle tin , with cobalt and bismuth , i s recorded .

’ Some important lead and zinc deposits of metasomatic origin , as well as o f the nature of ordinary

I 3

1 94 TH E GEOLOGY OF O RE DEPOSITS

lodes, occur in Sardinia . The lodes occur in metamo rpho sed Silurian and Cambrian slates , sandstones,and limestones , and are connected with pre -Triassichornblende granite masses having syenitic m o difica

tions , also with later dykes of porphyry and microgranite .The nature o f the veins at Iglesias i s typical ofthe occurrences elsewhere in Southern Sardinia . Themetasomatic types connected with these veins are described under metasomasis . The veins of Montevecchio

,near Iglesias , occur as single or composite

lodes , varying from the thickness of a knife-edge upto 50 or I OO feet . The veinstones are quartz withbarytes

,and here and there fluo rspar, which in some

places shows a marked preference for the parts of thelodes situated in l imestone . Argentiferous galenato per . cent . of si lver) , sulphides of Copper andi ron , with carbonates of lead , i ron , and calcium , occurwith secondary, red and native , silver ores , ho rnsilver,and other secondary minerals .I n the same region sulphides of lead and zinc occur

in associat ion with sulphides of i ron and Copper asimpregnations in an intrusion o f . diabase. The oresare accompanied by hornblende and calcite

,which

appear to replace the felspar and augite o f the originalrock . Transitional types between the ores and thefresh rock are also recognized .

The deposits of the province of Murcia , in Spain ,occur in association with Tertiary dacites and augitehypersthene andesites , which are kaolin ized and part lyaltered to alumstone by the influence of sulphuricaction on the weathered rock .

The rich ores of si lver and lead contain sulphides


O°

2 to 0 8 per cent . of si lver , and are associated withzincblende , the sulphides of arsenic , i ron , copper, andantimony

!and argentiferous compounds ; carbonates

and barytes are uncommon . The lodes are in gneiss,schi st , and older Palaeozoic rocks , embracing dolomitesand l imestones , intruded by gabbro and soda granite ;the apophyses of the latter are often cut by the veins .

PRIMARY S ILVER ORE VE INS .Many of the important deposits of si lver occur assecondary concentrations in the gossans or upper partsof si lver- lead lodes , and do not come in the classof veins here considered . True silver - ore veins arecharacterized by the presence of original argenti ferousminerals

,comprising native si lver and alloys ; com

pounds of s ilver with the halogen s ; minerals contain ing

S i lver and Sulphur, tellurium , selen ium , antimony,arsenic

,b ismuth , Copper and lead , and complex com

pounds of these elements .Galena nearly always contain s a l ittle si lver, whileother sulphides , such as mispickel , zincblende , andi ron pyrites , are sometimes argenti ferous .I n the classification of the various si lver-ore types

,

the somewhat arbitrary mode of treatment which isused in the grouping of the gold ores must again beemployed

,since a true genetic treatment is rendered

di fficult by the lack of data which might enable theores to be l inked with the igneous rocks from which

they were derived .

One of the most important of the subgroups of silvervein s i s that in which quartz , often amethystine , i s atypical vein -constituent . The veins are further charac

terized by the presence of accessory minerals contain ing

HYDATOGENESIS 197

cobalt and nickel . Sulphides are generally present , allof which may be argenti ferous . I t i s to the presence ofthese sulphides

,in a fine state of division , that the dark

colour of much of the quartz is due .Among the metall i ferous minerals present are py

’

rites ,mispickel

,b lende

,galena

,and chalcopyrite . The

principal si lver minerals may be si lver glance , polybasite

,stephanite

,

: proustite,pyrargyrite , etc .

O f the non-metall i ferous minerals which occur asaccessory constituents , calcite with some carbonate ofmanganese

,and

,very occasionally , barytes , are the most

important .The principal local ities where this type of vein i srepresented show that it is particularly associatedwith the younger eruptive rocks , which are frequentlypropyl it ized , silicified or kaolin ized , and impregnatedwith pyrit ic minerals near the veins . I t will benoticed that this subgroup has a distinct geneticsimilarity to that of the gold - quartz veins , and , assome of the silver ores of this subgroup contain gold ,i t will be readily recognized that the subtypes maybe regarded as members of a gold- si lver- sulphide -quartzgroup .

A similar remark appl ies to the subgroup of si lverores which is distinguished by the presence in the lodesof an appreciable amount of calcium carbonate

,with

accessory metall ic sulphides and arsenides . The lodeshave modifications in which the metals , such as silver ,antimony, nickel , cobalt , and arsenic , occur native , andsometimes in association with zeol ites when developednear igneous rocks .

The presence of barytes in considerable quantity,i n

addition to sulphidic minerals , is characteristic of some


si lver veins , but of these a number are probably theenriched upper parts of lodes which in depth are rich insulphides of lead , Z inc , and copper .Silver ores which are accompanied by considerable

amO I'

Ints of cobalt and nickel compounds appear toform an indefinite subgroup

,in which bismuth ores

occur with sulphides of lead , Zinc , and copper . Inaddition there is O ften pitchblende

,arsenical pyrites

,

and antimonite . The veinstones are commonly quartzand carbonates , with accessory barytes and fluo rspar.

I n the Chapter dealing with deposits of pneumatolyticorigin , reference was made to the occurrence of certaintin ores in Bolivia associated with pyritic si lver ores ,and it is questionable whether they Should not be dealtwith here , seeing that they may be regarded either asdeposits of pyritic si lver ores characterized by thepresence of cassiterite

,or of cassiterite characterized by

the presence of S i lver ores and other sulphides . Thecomparative scarcity of minerals

,such as tourmal ine ,

topaz , and fluo rspar, j ustifies the View that these depositsconstitute an intermediate type between true tin veinsand sulphidic si lver veins

,but

,as they have already

been dealt with under pneumatolysis,they are only

briefly mentioned here .

Typica l E xamples o f S ilver Ore Veins - The Buttedistrict , Montana , has been one of the richest si lver andCopper producing regions in the world .

The lodes occur in a post -Cretaceous hornblendegranite

,known as quartz monzonite . I t i s the oldest

of the igneous rocks , and is cut by newer apliticor granulit ic intrusions . Both these intrusions weret raversed by dykes of quartz porphyry, at which stagein the eruptive history of the region the lodes were


The silver veins contain argentiferous fahlerz , rubysilver, and o ther si lver ores , with sulphides of lead ,zinc , arsenic , and i ron ; native gold and silver, withsecondary ores of lead and a l ittle occasional copper,occur in the gossan

,associated with quartz, fluo rspar,

and , rarely, barytes .Silicate and carbonate of manganese appear to existas original constituents of the lodes . The outcrops ofthe lodes are black , and , unlike the lodes of copper,show a well -marked banding of the ores .The si lver ores of N evada occur mainly in granite ,which in the district of Reese River is penetrated byfelsite and propyl it ized porphyrite . At Tonopah , however, the min ing area consists of Tertiary propylitizedhornblende andesite , dacite , rhyolite , basalt and tuffs ,in a region otherwise composed of Palaeozoic rocks .I n the Tonopah region the lodes occur in associationwith propylitized hornblende andesite , which i s theoldest of the Tertiary eruptives .

The N evada silver ores consist of stephanite , polybasite , red silver ores , and tetrahedrite , with a l ittleblende , galena , i ron pyrites , and chalcopyrite . Theprincipal vein -material i s quartz

,but this i s aecom

panied by carbonates of manganese and lime and byadularia felspar. Free gold occurs with the o res,

‘

and

antimony is occasionally present in important quantityat Tonopah . The gossans contain bromide and chlorideof silver .The alteration of the country-rock is mainly that of

silicificatio n and sericit ization , together with impregnation by iron pyrites .In the State of Colorado the si lver ores do not carrymuch gold , but are of a similar type to those described

HYDATOGENES IS 20 1

from the State of N evada ; but there is present somefluo rSpar and barytes . I n the south of Colorado thedeposits of Creede occur in Tertiary eruptives, whilenear Georgetown they are met with in granit ic andgneissose rocks .The recently discovered mining region of Cobalt inthe vicin ity of Lake Tem iscamang (Tem iscaminque) , onthe border of O ntario and Quebec

, _i s a good il lustrat ionof the association of si lver

,cobalt

,and nickel ores ;

these are in veins which vary from a knife-edge to a footor so in width . These traverse slates and co nglome

rates of Lower Huro nIan age , but in the southern partof the region occur in diabase and gabb ro of postMiddle Huronian age . The ores , which are probablyof hydrothermal origin

,consist of native si lver, argentite ,

and pyrargyri te , with antimonide of si lver and otherminerals . The deposits are marked by an abundance ofnickel and cobalt compounds

,such as smaltite , millerite ,

niccol ite , and chloanthite (with decomposition products,n ickel and cobalt bloom) . Compounds of bismuth ,arsenic and manganese

,also occur

,while

,i n small

quantities , are the commoner sulphides , blende , galena ,pyrites, and chalcopyrite . The principal veinstone iscalcite , with subordinate amounts of quartz . According to Messrs . Campbell and Knight

,there i s a distinct

order of mineralization,as follows : F i rst came arsenide

of cobalt , followed by arsen ide of n ickel and smallquantities of other ores . Subsequently there was aperiod of disturbance , resulting in brecciation of thevein materials , with the infiltration of calcite , and deposition of native si lver ; lastly, the formation of bismuthores.

Argentiferous deposits similar to those of Cobalt


occur on the Ontario shore o f Lake Superior in theSilver I slet district .Here the veins exist principally in a gabbro in

truded Into the Algonkian (pre -Cambrian) slates . Theprincipal ores are si lver and its sulphides, with galena ,b lende , and fahlerz . Compounds of nickel and cobaltare important constituents , while the principal veinmaterials are calcite with some fluo rspar, barytes , and al ittle quartz .

At Broken H i l l , N ew South Wales , antimonial si lverores occu r in a lode crossing the schists . The lodeis rich where it intersects bands of amphibol ite

,but

in the other rocks i t i s thin and poor . The ore consi stsof argenti ferous fahlerz and other antimonial si lvercompounds . Cobalt glance i s a characteristic accom

paniment. The argenti ferous Copper sulphide , strom eyerite , exists as a secondary mineral , while horn andnative si lver, with ruby silver ores, sulphides of lead ,copper, antimony, arsen ic , cobalt and nickel , iodideand carbonates of S i lver are also present

,accompanied

by carbonates of i ron and l ime .

O ne of the most celebrated mining countries of theworld is Mexico , where , i n addition to abundant depositsof si lver and gold , ores of Copper , lead , and othermetals , are worked .

The si lver ores are found at many places in thelength of country between Chihuahua in the northand Oaj aca in the south . The strike of the lodes isparallel with the l ines of upthrust and folding whichhave affected the rocks of the Sierra Madre .

The principal rocks of the area consist of micaschists and metamorphosed slates , with intrusivediorites and Mesozoic and Tertiary sandstones ,


and fluo rspar. N ative gold is commonly found in thequartz of the upper parts of the lodes .The lodes are sometimes characterized by thepresence of druses , or cavItIes, containing zeolites, suchas apophyllite

,while orthoclase also exists as a true

vein-mineral . The characterist ic types of alteration ofthe andesitic rocks, where traversed by the lodes , aresil icificatio n

,kaolin ization , and propylit ization .

E xceptionally , as in the State of San Luis Potosi ,the ores occur in l imestones as i rregular metasomaticdeposits connected with narrow fissures . The oresconsist of carbonates and arsenates of lead with nat iveand horn si lver.The deposits of si lver ore in Peru and in the neigh

bo uring country of Bolivia belong to that type of depositnow being described . I n particular should be mentio ned the deposits of Cerro de Pasco . The lodes areof post -Jurassic age , and consi st in depth of argentifero us grey copper ores, pyri tes , galena, and quartz .

The upper parts of the lodes are characterized bygossan , called locally the pacos

’ or ‘

cascajado s,’ i n

which , l ike the colorados of Mexico , rich secondaryores of si lver are found in quantity with a little gold .

I n subordinate amounts there occur here and therearsen ical compounds

,zincblende , and manganous com

pounds with carbonates,etc .

The lodes are connected with dacite (quartz-andesite) ,and occur either in this rock or in the Tertiary sandstones , slates , and limestones .The association of silver ores and cassiterite in Bol iviawas referred to in dealing with the tin veins on p . 99 .

At Cerro de Potosi the si lver ores occur in b leached ,kaolin ized and pyritized slates , and rhyolites they con

HYDATOGENE'

SIS 2 05

sist near surface of native and red - si lver ores, and indepth of sulphides of copper, zinc , and lead , withsmaller amounts of sulphides of t in and arsenic , andthus l ink the ores as a class with the sulphidic ores ofpneumatolytic origin .

The deposits of O ruro consist of i ron pyrites , sul

phides of si lver , antimony , and copper, with tin ore .Lead ore

,containing microl ites of tin oxide , i s found in

depth,with zincblende , arsenic , Wolfram , quartz, and

carbonate of i ron , etc . , so that the deposits constitutea mixed type . The district is formed of Palaeozoicslates traversed by dacite (quartz andesite) , with whichthe ores appear to be connected .

Various modifications of veins of this unusual type ,in which sulphidic si lver and lead compounds are associated with tinstone , occur in other parts of Bolivia , asat Colquiri

,where chloride of silver i s found with calcite

and carbonate of manganese ; also at Huayna Potosi ,Chorolque , and other local it ies , where minerals suchas tourmaline , topaz , and fluo rspar, are occasionallyObserved .

North -east of Serena , Chili , in the mining district ofwhich the San José i s the principal mine , the veins areconnected with intrusions of dacite . I n addition tonative si lver and the chloride

,sulphide

,telluride

,and

amalgam of silver, there is argenti ferous galena , polybasi te and cupriferous si lver sulphide , together withtraces of bismuth , cobalt , and nickel ores . The veinstone is barytes, calcite and quartz . At Coquimbo theUpper Jurassic l imestones and intrusive porphyries areconnected with veins of simi lar nature .N ear Chanarcillo , forty-eight miles south of Copiapo

(Chil i) , the veins traverse Jurassic l imestones cut by in


trusio ns of aug ite- porphyrite , in the vicin ity of whichthere is a development of calc-sil icates and an enrichment of the ores in the lode and countryrock the lodesvary from a few inches to 32 feet in width .

The original ores are mispickel , blende , and pyrites,which occur at a depth . Above these come nativesi lver and sulphide of silver , with ruby si lver Ores ;but near the surface the ores contain chloride andbromide of si lver, native si lver , with carbonates of l ime ,i ron

,and copper barytes also occurs . Clay and limonite

is the common gangue in the upper parts of the veins .The veins of Caracoles , in the desert of Atacama, areof similar nature , and are connected with dykes of quartzand augite porphyry intrusive in Jurassic l imestones andmarl s . The lodes are always richest when in or nearthe porphyry .

The cobalt - si lver ores of the E rzgebi rge , i n Saxonyand Bohemia, are typical examples of this class ofdeposit . They occur in the metamorphic rocks in thevicin ity of the post -Carboni ferous granites . The threeprincipal districts are those of Schneeberg , Schwarzenberg , Johanngeorgenstadt , and Joachimsthal Geyer andAnnaberg ; and Marienberg .

I n these regions the veins fall into four groups . Thetwo older groups comprise the tin veins and the pyriticlead and Copper veins , while the two younger groupscomprise the cobalt - S i lver, and i ron and manganese , lodes .I n the Joachimsthal group there are two series of

lodes,known respectively as the Morgenge

’

inge (strikingeast - north - east) and the Mittem o ektgéz

’

nge (strikinggenerally about north and south) . The principal veinminerals are quartz, calcite , carbonate of i ron , andmagnesite ; the lodes vary in width from a few inches


I n the neighbourhood o i Marienberg a number ofintersecting veins contain native arsenic and silver ,ores of si lver

,uranium

,cobalt , nickel , lead , copper

and iron pyrites,in associat ion with carbonate of i ron ,

barytes , fluo rspar, and quartz . These veins cut throughsyenitic and dioritic rocks .In E urope S i lver veins of the quartzose type occur

in Saxony (Freiberg) , Hungary (near Kaschau) , Baden

(Kinzig Valley , in the Black Forest) , and in othercountries .I n the Freiberg district the lodes occur in meta

morphic , and plutonic intrusive , rocks , and consist ofquartz with sulphides of si lver, red si lver ores , andnative si lver, together with argenti ferous sulphide ofi ron very rarely they carry sulphides of lead and zinc ,the latter being covered with fi lms of si lver glance .Thin section s of the sil i ceous veinstone Show thesilver minerals dispersed in small quantities throughout a granular mosaic of quartz.Some of the ores are complex compounds , such aspolybasite, freieslebenite , etc . , which are often met withl ining cavities .Later infiltratio ns also occur, the minerals com

prising carbonates of i ron and manganese , with calcite ,barytes , celestine , gypsum ,

and fluo rspar, and sulphidesof antimony

,arsenic , and copper .

I n the Kinzig Valley, Black Forest (Baden) , a seriesof si lver-ore veins of somewhat varying mineral composition and age exists in both the older and newerrocks of the district .The newer rocks consist of Triassic and Permiansandstones , which unconformably overlie the gneiss , ofwhich the principal part of the mineral area is com


posed. Serpentine and amphibol ite invade the gneissin restricted areas , while on the south there are intrusio ns of granite and syen ite . Numerous intrusivehypabyssal rocks , such as granophyre , are also met with .

The lodes occur principally in the older rocks,but

are also found in the f Bunter and Ro thliegende sandstones .The veins of the oldest vein -system in the districthave quartz as the principal veinstone , and are sim ilarin nature to those of the Freiberg region . O ther veinminerals are carbonates of i ron and lime , red silverores , silver glance and native silver , i ron pyrites , chalcopyrite , galena and sulphide of antimony ; a l ittle goldi s also present .The other series of silver lodes of the district arerespectively characterized by calcite , argenti ferous leadore , barytic Copper and lead ore , and cobalt -si lver ore .I n the Fro hnbachthal mineral -area , in the same region

(Baden) , quartz lodes in gneissose rocks contain argentifero us fahlerz , sulphide of lead , and a little copper,with carbonates of l ime and magnesia . I n a laterphase of the mineral-history of the veins there wasintroduced , i n addition to the foregoing , compounds ofnickel and arsenic . Barytes and dolomite , with sul

phides of lead , si lver and antimony, constitute a thi rdphase

,and

,finally, the youngest set of minerals com

prises native si lver , compounds of Copper and silver,

sulphides of Zinc , i ron , and antimony with fluo rspar,etc .Slight modifications of the above types are found in

Nassau and in Alsace .To the south of the Brocken Gran ite (Harz Mountains) and its metamorphic aureole , the mining dis

14


triet of St . Andreasberg is situated in theWiederschiefer(Lower D evonian) . The rocks consi st of slates withlimestone bands and intrusive sheets of diabase .The actual mineral -area is cut Off sharply from thesurrounding district by two large crush-zones or faults

(the so -called Ruscheln which converge westwards ,and enclose a wedge-shaped area 3 kilometres in lengthby 1 kilometre in width on the east (pp . 1 88,

The lodes are simple fractures varying in widthfrom an inch to 15 feet . The principal vein mineral iswell - crystall ized calcite , which exists in two generations . In the older generation there occur arsenicaland antimonial ores and sulphides with nickel iferous

and cobalti ferous minerals . I n addit ion there are Zincand lead sulphides with subordinate amounts of copperpyrites, i ron pyrites , and pyrrhotite . O f somewhatlater origin i s fahlerz

,which occurs with quartz

,the

latter replacing part of the calcite . After this comesulphides of copper, lead, and, sparingly , zinc withnative silver and millerite . A fourth stage in the historyis the conversion of si lver to stephanite and othersulphides , and the formation of arsenides , antimonidesand complex minerals . F inally

,native S i lver occurs

with realgar, calcspar and many zeoli tes ; and secondary carbonates with ho rnsilver, cobalt bloom , malachite , etc . Among the minerals present in small quanti tyare axin ite , epidote , garnet , and fluo rspar.

The barytic si lver ores of the Guadalaj ara district , inCentral Spain , are somewhat similar to those of thedistrict north of AlmerIa, in Andalusia (Southern Spain) .I n the former locality the lodes are in gneiss and micaschist , and consist of quartz, barytes, and carbonate ofi ron . Native silver with ruby and other S i lver ores , such


i n the fahlbands are sulphides of Copper, and cobalt andsilver ores

,in exceedingly small quantities . The lodes ,

of which there are a great number , often close togetherand forming a network , vary from a mere kni fe - edge to2 feet in width , and traverse the fahlbands obliquely ina north -westerly di rection . Only those parts of thelodes which traverse the fahlbands are rich enoughfor mining . The vein -minerals are calcium carbonate ,fluo rspar and quartz . Dolomite and barytes are rare .

Adularia and albite also occur with asbestos and asbestiform minerals . Carbonaceous minerals , such as anthracite and graphite , are occasionally met with in quantity.

Zeolites,such as apophyll ite , harmotome , stilb ite , etc .

,

are also found , and sometimes prehnite i s present .The principal ores are native si lver in various formsassociated with calcite , but quicksilver and gold alsoexist with mercurial compounds , ho rnsilver, red silvero res

,and complex compounds of i ron , cobalt , arsenic ,

antimony , Copper, si lver, and sulphur . Among theother metall iferous minerals we may mention mar

casite and other sulphides , such as those of Zi nc , Copper,lead

,arsenic , and iron , all of which are more or less

argentiferous . Pyrrhotite Is also present , and there i sa rarer occurrence of native arsenic and Copper wi thcobalt bloom and carbonate of i ron . As a whole , i tshould be remarked that the native si lver i s by far themost important mineral , and in comparison with its

sulphides occurs in the ratio of anything from 1 0 to 20

toThe reactions which brought about the formation of

native si lver are ,supposed to have been initiated by

the action of steam on sulphides and sulpharsenides

o f si lver, and . to _have produced native si lver, native

HYDATOGENESIS 2 13

arsenic,etc . ,

and a liberation of sulphuretted hydrogenand sulphur dioxide (see p . I t has been statedthat the silver was originally b rought to its presentposi tion in solution as the carbonate or b icarbonate ofsi lver , which was decomposed by carbonaceous matteror ferrous compounds . I t has also been Shown experi

mentally that a hot solution of ferric sulphate dissolvessi lver, but this action is reversed at lower temperatures ,as expressed in the following equat ion :

2Ag Fe2(SO2)3 é 2FeSO

4.

Veins similarly connected ‘with fahlbands have beenworked in Dauphine, France . The Hungarian silverore deposits , near Kaschau , were formerly worked forgold in the upper parts of the lodes , where it occurredwith antimony ochre . The primary ores consist ofargenti ferousantimonite , j amesonite , etc . ; and slightlyauri ferous pyrites , sulphides of zinc, Copper, and arsenic ,with quartz , a little calcite , and carbonate of i ron .

The silver ores of Japan (Akita , i sland of Sado , andnear Tokyo) belong to the sil iceous sulphidic group

,

They are connected With Tertiary eruptives,the inter

mediate types o f which are propylit ized . I n somelocalit ies compounds of manganese occur in abundance

,

but in others cupri ferous minerals enter largely intothe composition of the lodes . In addition to quartz ,calcite and other carbonates occur with sulphides ofsi lver

,lead , zinc , and i ron . Gold is of importance in

some of the veins .I n the province of Chi - l i (China) , ruby silver ores ,argentite , and chloride of si lver, are found in associationwith galena , blende , and arsenical ores which are connected with dykes of felsite .


The si lver ores ‘of Southern Sardinia occur in agranite and in the zone of metamorphosed Si lurianrocks into which it i s intruded . N ear the surface 1 thelodes consi st mainly of barytes and galena with quartz

,

calcite , and zincb lende . I n depth they are si l iceous,

but consi st largely of calcite with fluo rspar and zeolites ;the metall i ferous mineral s usually comprise native si lver ,argenti ferous galena and blende , ho rnsilver, and rubysilver ores ; marcasite , pyrrhotite , and sulphides ofCopper and arsenic are also present with cobalt andantimonial ores , carbonate of i ron , and other minerals .The final phase of lode fo rmation is represented bycarbonate , phosphate , and chromate of lead , with nickeland cobalt bloom , etc .

VE INS OF NICKEL AND COBALT ORE S .

The ores of nickel and cobalt are divi sible into twomain groups ; the first o f which includes the sulphidesof cobalt , nickel , and bismuth ; the second , however,comprises the hydrosil icate ores of nickel , which belongto the oxidic hydatogenetic types of veins , and whichare described under that head . The second group isthe more important commercially .

I n the sulphidic nickel and cobalt group the oresare further subdivided into two more or less indefinite types

,according to the dominant mineral con

stituents.

The ores of nickel and cobalt occur principallyas sulphides or arsenides , and comprise such mineral sas smaltine

,cobalt glance— double sulphide of nickel

and cobalt,erythrine

,asbolane (wad with oxide of

cobalt up to 40 per chloanthite , millerite andvarious sulphates

,arsenates , arsenides, and antimonides


found in faults or lodes traversing the Mansfeld copperdistrict , in the ThiiringerWald .

At Kamsdorf and other places in Thuringia (Saxony) ,lodes , occasionally brecciated , and traversing both theKupferschiefer and the underlying conglomerate of theZechstein series , contain asbolane , cobalt bloom , andsmaltite , with calcite and barytes . The cobalt lodes ofSpessart , near Baden , have a similar genesis .I n Styria , the hornblendic slates and gneiss contain

wide pyritic zones,or fahlbands

,i n which mispickel ,

pyrrhotite , and iron pyrites , form band - l ike masses .N ear the surface the sulphides are oxidized . Thecobalt and nickel ores are found in lodes t raversingthese fahlbands

,and are associated with calcite , quartz,

and the ores of lead,bismuth , arsensic , and copper .

I n the F ichtelgebi rge , the ores of cobalt and nickelare found with carbonate of iron

,bismuth , and barytes ;

while in the district of Siegen , Prussia , they are met within lodes contain ing carbonate of i ron and Copper ores .In the Schneeberg district , already referred to under

si lver ores (p . the cobalt- lodes are connected withthe gran ites which are intruded into schists and contactaltered slates ; they are younger than the tin lodes .

The cobalt- lodes are numerous , and the ores arecharacterized by quartz

,and nickel and bismuth o res ;

they are met with among decomposed crushed countryrock which fi ll s the vein s . Calcite , carbonate of iron ,and ankerite

,with occasional barytes , fluo rspar, and

ores of arsenic,si lver

,uran ium

,Copper, zinc , and lead ,

are met with . At the t ime of formation of the lodes ,the original minerals appear to have been mainlycarbonates with barytes ; these have been subsequentlyreplaced by si lica .

HYDATOGENESIS 2 17

I n the Wa l l i s Valley district , situ ated between thePennine Alps and the Bernese Oberland , lodes of copper,in chloritic gneiss

,contain cobalt and n ickel ores with

spathic veinstones ; they are characterized by thepresence of quartz

,copper sulphide , grey copper ore ,

galena , lead molybdate and other secondary ores of lead .

Cobalt and nickel ores occur in P iedmont , withquartz , calcite , and ores of copper .I n Nassau , lodes of quartz with spathic i ron ore ,ankerite

,copper and nickeli ferous pyrites, and b ismuth

ore , are found in ancient volcanic tuffs and picrites .I n Los Angelos County (Cali fornia) , argent iferous

cobalt ores are found in lodes having a baryt ic veinstone .

The occurrence o f cobalt and arsenical ores withgold and si lver in quartz lodes which t raverse talcoseslates has been noted in connection with acid intrusionsin Sierra Famatina

,Argent ine .

Cobalt ores are found in lodes traversing schists nearBalmoral (Transvaal ) . Here the lodes consist of chalcedo nic quartz and fibrous hornblende with smaltite ,cobalt bloom , and molybdena .

B ISMUTH ORES .

The most common bismuth -minerals are the nativemetal and it s sulphides . Among the subordinate oresmay be mentioned compounds of b i smuth with selenium , tellurium , si lver , gold , Copper, and lead ; whileoxides , carbonates , arsenates , and sil icates , are alsoknown in st il l smaller amounts as secondary products ,There i s no well-marked group of b i smuth veins , andin the natural classification minerals of b i smuth mustbe regarded

,not as the principal , but as subordinate


constituents, occurring occasionally in such quantit iesas to establ ish exceptional modifications of other veins .Thus , they sometimes occur

'

in appreciable quantit iesi n auri ferous quartz veins, as in the auriferous bismuthores , pyrites , and mispickel , of Queensland ; they areprobably connected with granite , and in N ew SouthWales , quartz, with native b ismuth , gold , and mo lyb

denum,occurs near the contact of granite with slates .

The most important deposits of bismuth are those ofBolivia (Chorolque and Tasna) , where they are met wi thin a . large number of vein s which have close affinitieswith stanniferous- si lver veins . The ores consi st largelyof bismuth intergrown with cassiterite , and seem to beconnected with dykes of dacite and quartz trachytetraversing altered clay slates ; wolfram also occurs . Inassociation with the ores are the usual accompanimentsof the silverftin lodes , such as pyrites and other ores ofi ron , sulphides of copper , lead , zinc , etc .

, with quartz ,siderite , barytes , and occasionally tourmaline .Bismuth ores occur also in Telemarken in associa

tion with tourmaline-copper veins contain ing sulphidesof i ron and lead with quartz and carbonates ; in manytin -fields, as in the Altenberg , in some parts of Cornwall (E ast Pool Mine) , and in Queensland .

I n association with Copper sulphide , bismuth oreoccurs in the Cobar Mi ne in N ew South Wales . I nAustral i a bismuth has been noted as occurring in anochreous deposit round a thermal spring ; the mineralspre sent were gold and bismuth carbonate , but on beingfollowed downwards the deposit was found to containthe telluride of bismuth .


characterized by the presence of quartz , fluo rspar,calcite , and chalybite .

I n York County , N ew Brunswick , ores of nativeantimony and stibn ite , contain ing a low percentage ofgold and silver, are found in association with calciteand quartz in vein s t raversing black slates . I n SonoraCounty, Mexico , important deposits of antimonialoxides are found (p .

In N ew South Wales and Victoria,ant imonial ores

have been mined . I n N ew South Wales they areassociated with quartz, while i n Victoria (as at Sunbury )the antimony i s found in gold quartz .O f E uropean local ities for antimony those of West

phalia , Saxony, and Bohemia are the mo st noted .

I n the Unterharz,the sulphide of antimony occurs

with various metall ic sulphides and arsenides , calcite ,strontian ite , ankerite , sideri te , barytes, gypsum ,

fluo r

spar , and quartz, i n breccias in Palaeozoic slates .At Mi leschau

,in Bohemia , veins containing ant imony

as radiating needles are found in connection withkersantite which traverses hornblende granite .I n Oporto (Portugal) antimony in association with

gold is found in a district contain ing , in addition to tinand wolfram veins , lodes of argentiferous galena .

I n France the departments of Cantal , Haute -Loire ,and Puy-de -Dome yield antimony ores , the lodes ofwhich occur in gneiss , mica -schist , and gran ite . Theveins consi st of antimonial ores with quartz and alittle zincblende and pyrites . Argenti ferous and plumbiferous veins with antimony also occur, while calciteand barytes are occasionally present as gangue minerals .Antimonial ores are also worked in Corsica andSardin ia . I n Corsica the antimonial ores are complex ,

HYDATOGENES I S 2 2 1

and consist of quartz with various sulph ides , includingcinnabar , native sulphur, and calcite . I n Sardin ia alsothe veins contain calcite .

The Tuscan antimony lodes contain sulphides ofarsenic and mercury with quartz , and occasionallysulphur in pockets and stringers . Between the Permianshales and the E ocene l imestones are sil iceous orebodies which yield antimonial ores .In Hungary the antimony lodes consist of quartz

and calcite,with stibnite and pyrites . The ores are

particularly rich where the lodes are in graphite orChlorite schists , and the latter become impregnatedwith sulphide of antimony

,pyrites , and a little cinnabar ,

for a distance of 1 2 feet on either side of the lodes .

Lodes contain ing antimonial ores with blende , aurifero us pyrites , quartz , and calcite , are also known .

Oxides of antimony occur in limestone of N eocomianage in the province of Constantine , Algeria .

I n Japan , K . Yamada has described antimonial oresin the island of Shikoku

,where lodes occur in sericitic

schists , which near the lodes are impregnated withpyrites . The veinstone is quartz with some calciteI n Sarawak , Northern Borneo , antimonial ores arefound in lodes t raversing limestones and slate . Theores consist of stibn ite , native antimony and oxide ofantimony

,in quartz .

ARSEN ICAL ORES .

The principal arsenical minerals are realgar andorpiment . Arsenic is , however, a very common associateof antimony, and occurs united with it in the complexores of other metals

, such as fahlerz , etc .

Native arsenic is also found with native antimony .

2 22 THE GEOLOGY OF’

OR'

E . DEPOS ITS

The occurrences of workable deposits of arsenical andantimonial ores are l imited

,but are known at Alchar,

near Salonica (Turkey ) , in N ew Caledonia , and otherplaces . I n comparison with the numerous antimonialveins , those containing similar compounds of arsenicare quite rare .

Realgar and orpiment are deposited from some hotSprings , and as sub l imates are connected with volcanicactivi ty . They are also met with in veins and asimpregnations .

I n Galicia realgar occurs with gypsum and quartz inargil laceous rocks

,i n Tuscany in marl

,i ntergrown with

gypsum and marcasite , and associated with sulphideof Copper, pyrrhotite , l imonite , sulphur, and fluo rspar.

I n the same province i t i s connected with cinnabar ,as has also been observed in Asturias (Spain) andat Huancavelica (Peru) .N ear Civita Vecchia veins traversing sandstonescontain calcite , realgar, and orpiment .N ear Kremnitz, in the Hungarian ore-mountains ,

Triassic dolomitic l imestones are traversed by irregularveins contain ing clay, in which are embedded lumpsof arsenic sulphide ; i t i s also found in pockets at theupper limit of the l imestones .

“While the arsen ic veins seem to fall into a group ofthen own , they do not constitute the main sourcesof arsenic .

The most important sources are those in which thearsenic i s a by-product of the working of other veins .

The principal ore i s the double sulphide of iron andarsenic (mispickel or arsenical pyri tes) ; in si lver andlead v eins i t occurs at St . Andreasberg , Pribram , andother places , and in the barytic si lver veins of Saxony .


and realgar ; i t i s sporadic in many sulphidic veinswhich carry gold , si lver , and the sulphides of Copper ,lead , and zinc ; the accompanying veinstones arequartz , chalcedony , opal , bitumen , calcite , dolomite ,brown spar, and , more rarely , barytes , fluo rspar,

gypsum , and sulphur .Antimony, as a mineral paragenetic with mercury,occurs in California , Mexico , Borneo , Russia (N ikito vka) , Corsica , Servia , Bosnia , and Hungary . Amineral having the composition Hn 4

S 7, is found inMexico , and in Hungary C innabar has , in some instances

,been deposited on antimonite . An abundance

of i ron pyrites in some places i s an indication of thepresence of mercury .

Mercury is freely soluble in hot strong sulphohydrateor carbonate solutions forming a double salt with

,

oneof the alkal ies— as , for instance , HgS4N azS ; fromthis i t may be precipitated , as a sulphide with a

l i ttle . free mercury, by dilution , or by the action ofsome bituminous matter with which it i s so frequentlyassociated

,as at Idria

,in Cali fornia , and in Russia .

As sodium sulphide also dissolves pyrites , blende ,gold

,etc .

,these minerals are found with C innabar .

When such solutions react on l imestone , the l imestonei s converted partly to gypsum and partly to moresoluble compounds , while any argillaceous materialwhich may have been present is impregnated with

C innabar and left in the place of the l imestone .Carbonaceous or bituminous material favours thedeposition of C innabar .The deposits appear to ~ have originated mainly bythe deposition of C innabar from such solutions asthese

,but , owing to the volati l ity of mercury and


mercurial compounds at comparatively low temperatures

,some think that they may be due to sublima

tion ; i t i s highly probable that steam was an importantfactor in many cases .The dark sulphide— metacinnabar— occurs as analteration of the red cinnabar , and is frequently presentin the upper parts of such deposits . Native mercurymay result from oxidation of the sulphide .

I n the deposi ts of Almaden , the C innabar occursimpregnating sandstone

,from which much of the

sil iceous material appears to have been dissolved .

Deposits of mercury are found in connection withthe following igneous rocks : G ranite (Corsica) diabase

(Almaden) ; i n greenstone -porphyry , melaphyre , andbasalt ; serpentine and olivine rock (Cali fornia) ; rhyol ite (N ew Almaden) trachyte and other lavas .Typical E xamples o f Quick silver Depo sits — Themost important deposits of mercury are those of Almaden ,i n the Sierra Morena (Southern Spain) . The rocksconsist of highly inclined Silurian and Devonian shales

,

and quartzites , broken through by diabase and mela

phyre intrusions .The Cinnabar occurs in three beds of quartzite asmore or less complete impregnations , or as stringsrunning through the beds . The impregnations die outgradually in the quartzites , but suddenly where theseabut against the shales .The Cinnabar i s associated with native mercury

,

pyrites, and copper pyrites . O ther minerals arepractically non -existant , but a l ittle barytes is presentwith some b ituminous material .Very little mercury occurs in the shales and limestone , while none i s found in the igneous rocks .

IS

2 2 6 THE GEOLOGY OF ORE DEPOSITS

In the province of Asturias, cinnabar, with arsenicalores contain ing up to 07

‘ per cent . of mercury, occursin breccias , and as impregnations of quartzites and co nglomerates in a Carboniferous l imestone- series .The celebrated quicksi lver deposits of Idria , in Carniola (Austria) , occur in a series of Triassic l imestones ,shales , tuffaceous sandstones , marls , dolomites , and conglomerates .The cinnabar exists as impregnations of dolomiteand dolomitic breccias in association with pyrites andnative mercury , with minerals such as quartz , secondarycalcite , dolomite , some fluor, and an appreciable amountof bitumen , which has probably acted as a precipitant .The richest of the mercury deposits occupied fissuresinconnection with the bodies of ore

,but they h ave all

been worked away .

There are'

two groups of mines in the region . Thoseof the south -eastern district are working on lodes about

3 feet wide , -si tuated in dolomitic breccias and conglomerates , which l ie between dolomitic beds above ,and calcareous slate and marly limestone

,below. The

cinnabar impregnates the overlying dolomites for CO Il -g

siderable distances , and also passes along the lessimportant but parallel zones of brecciation .

I n the north -west mine the rocks are much folded .)

The rocks overlying the conglomerate are highlybituminous shales impregnated with ore in the form ofi rregular bunches and pockets , ranging up to 60 feet ormore in width , and traversed by numerous veins. Theprocess of impregnation probably took place in E ocenetimes .The cinnabar deposits of the old volcan ic regions ofTuscany occur in zones of excessive fissuring in the


and other minerals , of which pyrites , galena , copperpyrites

,stibn ite , si lver , quartz , barytes , and bitumen ,

are the most important .

The Californian quicksilver deposits of Sulphur Bank,

N ew Almaden,N ew Idria , and G reat Western mining

districts are situated in the coastal mountains,which

consist of folded Cretaceous slates pierced by intrusionsof granite , quartz porphyry , andesites, rhyolites andbasaltic rocks ; the mercurial ores are most oftenassociated with andesi te and basalt . The deposits asa rule consist of i rregular lodes of cinnabar , withnumerous small veins and stockworks or impregnationsof the porous countryrock , but there i s no extensive

metasomatic replacement .At the G reat Western Mine the cinnabar is in association with pyrites

,quartz , and b i tumen , in a flat deposi t

at the junction of N eocomian sandstone with anopalized serpentine .

The N ew Almaden ores of cinnabar and native quicksilver occur as stockworks in the vicin ity of disturbedsandstones

,serpentines , and diabase . Associated with

them are Copper and iron pyrites in small quantities ,also quartz

,calcite , dolomite , and magnesite .

I n Brewster County , Texas , the ores are found inC retaceous shales and limestones , which are traversedby sheets

,dykes

,plugs , and laccoli tes, of rhyolite and

phonolite,and covered by lava - flows of a basaltic

character . The limestone is of Lower Cretaceous age ,and is represented in the Terligua District . H ere thelimestone is somewhat cavernous , and i s traversed byveins and strings of calcite , which contain cinnabar ,together with gypsum and the o xychlo rides of quicksilver . I n some cases the walls of cavities are lined

HYDATOGENES IS 2 29

with cinnabar,which is also met with in quantity on

the floors of such caverns . Friction -breccias cementedby calcite and gypsum

,with oxidized pyrites and

cinnabar , are not uncommon features .Cinnabar is also met with in the shales of theUpper Cretaceous . I n the Study Butte region theshales contain thin bands of l imestone , and the wholeis situated between beds of sandstone . The quicksilver ores are also found traversing the igneous rocksin the form of strings .The Peruvian deposits at Huancavel ica occur in slates ,l imestones , sandstones, and conglomerates , of Jurassicage , penet rated by trachytic rocks , with which theyappear to be connected . The accompanying mineralsare pyrites , mispickel , and sulphide of arsen ic .

At Huitzuco , i n the State of Guerrero in Mexico ,the mercurial ores consist o f cinnabar , metacinnabar ,livingsto nite , pyrites , gypsum , sulphur , calcite , andbitumen

’

. The occurrence of the antimonial sulphideof mercury , livingsto nite , is the most remarkable feature .I n the State of San Luis Potosi , the cinnabar o f

Guadalcazar is connected with massive l imestones intraded by granite and porphyry. Unimportant depositsof zinc and lead are also found . The cinnabar i s in thedisturbed limestone

,and in i rregular deposi ts in clay,

with calcite , fiuo rspar, gypsum ,and a little barytes .

I n the State of Santa Rosa , Mexico , cinnabar is foundin dolomitic l imestone , passing into white l imestone , inassociation with free mercury

,quartz

,and clay .

In China , cinnabar deposits associated with antimonyoccur in the province of Kweicho u ,

in quartzose vein st raversing dense magnesian l imestone . The depositsare found to be of several forms : ( 1 ) As impregnations

230 THE GEOLOGY OF '

ORE'

DE POS ITS

in well -defined beds ; (2 ) as impregnations along jointsand bedding planes ; (3) as i solated masses , pockets , andVugs , contain ing cinnabar , calcite , and q uartz ; (4)i rregular disseminations in a number of disturbedlimestone beds .

Om c VE INS OF HYDATOGENETIC ORIG IN .

This class of veins comprises ores which consist mainlyo f oxides , carbonates , and sil icates , which have beendeposited from solutions . Although not infrequentlyaccompanied by sulphides, such compounds are generally only present as accessory o r accidental minerals,except in the veins which are intermediate between thetrue oxidic veins and those carrying sulphidic ores withlarge amounts of carbonates .The most important minerals of thi s group are theoxides and ca rbonates of i ron and manganese , but thehydrosil icates of nickel are also included . With regardto the source of the ores of i ron and manganese , therei s much uncertainty, but in the case of the nickel thebo nnectionwith serpentine .i s undoubted . There i s atotal a bsencefo f ’minerals formed under pneumatolyticcondit ions .

-O ! ID IC ORE S O F‘

IRON AND MANGANESE .

I ron .and manganese ores have a similar mode ofoccurrence in N ature

,are not uncommonly assoc1ated

with one another,and have similar veinstone accompani

ments .

The comm o n ~o res of i ron are red and brown haematite and the spath ic Iron ore . The brown haematite

(l imo nIte, etc . )“

generally arises from a secondary alteration or hydration of red haematite or of spathic i ron


lodes of spathic iron ore have been worked with copperores, barytes , and occasionally a l ittle cinnabar .Spathic i ron ores have also been worked in Connecticut

where, in addition to spathic ores , sulphidesof various metals occur with quartz .

I n the red haematite ores the principal veinstoneaccompaniment is quartz or j asper

,with small

amounts of carbonates of l ime and iron,l imonite

,a

l ittle barytes , manganese , and fluo rspar. These deposits may be regarded as modifications of spathic i ronveins .

In Saxony the red and brown iron ores , in the districtnear Zwickau

,consist of red and brown haematite with

ferruginous quartz ; they exi st either as contact depositsbetween greenstone and S lates , or as i rregular si l iceousimpregnations of O rdovician quartzites, or as pocketsand ramifying veins in masses of O rdovician diabases .They are believed to have been formed by lateralsecretion , and possibly by decomposition of the diabase .In the E rzgebirge of Saxony, the veins occur in brec

ciated lodes in both granite and metamorphic rocks ,and contain haematite and limonite , with small amountsof bismuth and cobalt ores and sulphide of copper,wavell ite

,fluo rspar, carbonates of lime and iron , barytes ,

quartz,j asper

,chalcedony , clay , etc . Manganese ores

sometimes exist in considerable quantity ; they arealways among the later minerals of the lodes , for theyare found to traverse them in strings ; but occasionallythey occupy the whole width of the lode . Calc andCopper-uranite are also found .

The manganese lodes contain ores of manganese , inassociation with veinstone materials S imilar to thosecharacterizing the i ron lodes, the principal being quartz,

HYDATOGENES IS 233

calcite , and barytes . Like i ron ores it i s necessary , forcommercial purposes

,that the manganese ores Should

contain over 40 per cent . of the metal , and the numberof deposits of thi s type containing ore rich enough forworking i s somewhat l imited . The ores principallyused are secondari ly enriched bodies ; nine - tenths of thematerial wrought being used in the manufacture ofspiegeleisen and

“

manganese steel , while the rest isemployed in the preparation of chlorine , permaganate

of potash , and other substances of commercial value .The German manganese lodes have received con

siderable attention from Continental geologists . AtLangenberg , i n Saxony , there are large brecciated lodesand stockworks

,which at thei r outcrops are connected

with secondary deposi ts which occupy shallow , basin - l ikedepressions in schists

,and consist of i ron and man

ganese ores . I n some local ities,as at G raul , S imilar

deposits contain cobalt and manganese ores , withbismuth -ochre .In the E ibenstock granite

,and near Platten , in

Bohemia , the manganese ores form stockworks , andveins ranging up to 3 feet in width . At Johanngeo r

genstadt (Saxony) , the ores occur as bedded veins inschists and slates , the country - rock being impregnatedwith soft ores of i ron . Pseudomorphs of haematiteafter calcite , anhydrite , and pyrolusite , are found . Themanganese is mainly in the form of oxides.

In the Harz Mountains,at I l feld

,deposits of man

ganite , psi lomelane , braunite , hausmannite , pyrolusite ,and wad , i n association with barytes and carbonates ofi ron , magnesium and lime , are found in thin stringsand veins in a mass of weathered porphyrite whichis intrusive in the Rotl iegende (Lower Permian ) .

234 THE GEOLOGY O F'

IO RE’

DEPOS ITS

In theThuringerwald , veins of manganese ores traverse

porphyry, melaphyre , and melaphyre conglomerate .

In Styria (Veitsch) , the lodes‘ t raverse Si lurian l ime

s tones , and consist of rhodochrosite .

I n the departm ent of Saone -et-Loi re (France ) , lodesof psilomelane with quartz

,barytes

,fluo rspar, l imonite ,

and hydrated oxide of arsenic,are connected with

granite . The lodes are formed along l ines of overthrusting traversing Tertiary rocks .In the district of Huelva

,and in a zone of the Silurian

slates extending from Seville to Cape Sineo , on thePortuguese coast , are found lenticular masses of sil icatesof manganese , i ron , and lime , which have had thei rorigin in thermal springs . I n association with themthere are quartz ( frequently j asperoid) , carbonate ofmanganese , and pyrites . I n depth the deposits aremore pyritic: and i t i s only the dark weathered outcrop

.which is worked . At Santa Catalina,however , the ores

have been mined to a depth of 330 feet .

H'

YDRO S IL ICATE N ICKEL ORES .

These n ickel ores are connected with the serpentinization of peridotite and pyroxenite rocks ; they may beregarded ' as having been deposi ted in the fissures traversing masses of serpentine by the comb ined processesof lateral secretion and hydatogenesis

,acting during the

change of olivine -bearing rocks to serpentine , and of asubsequent concentration by weathering of the serpentine . Some of the ore deposits may possibly beregarded as all ied to segregations (see pp . 69 ,

The nickel "probably occurred in minute quantities in

the’ ferromagnesian sil icates of the original peridotites .

which . shortly after intrusion became changed to ser

2 36 THE GEOLOGY OF O RE DEPOSITS

The post-Cretaceous intrusions of harzburgite nearR iddles (O regon ) are connected with nickel ife rousveins . The rock has been to some extent serpentinized ,but contained original ly ol ivine and bronzite with chro

mite and magnetite . Serpentine vein s traverse themass , i n association with hydrous si l icate of n ickel . Atthe time of the separation of the nickel ore , numerousveins of quartz and i ron oxide resulted from the expansionwhich accompanied the chemical changes in the rock ;they consequently often radiate from centres .I n Silesia the deposits of nickel ore are connectedwith serpentine , formed from peridotite , in which theremains of olivine crystals occur , together with magnetite , tremol ite , and chromite .

The veins in this half-serpentinized rock consi st ofserpentine , talc , and clay , and are themselves traversedby narrow veins of garnierite and other nickel oresin association with quartz, Opal , and chrysoprase .The so -called ‘ nodular ores ’ are found in pocketsand stockworks .

CHAPTE R V

ORE S DUE TO METASOMAT ICRE PLACEME NT

BY metasomasis is meant a chemical process involvingthe replacement of one substance by another , notnecessari ly with the retention of the form , or evenvolume , of the original mineral . For instance , should apure l imestone be converted into a dolomite by the replacement o f part of its calcium by magnesium , such achange would be purely chemical , and would be knownas metasomatic replacement . ’

In all cases of true metasomasi s the replacing materialis introduced by circulating waters , frequently actingfrom above downwards , and the solvent action of thewater is increased by the presence in it of such compounds as alkaline carbonates

,sulphides , S i l icates , car

bo nic acid , or the humic acids, al l of which have specialactions on the country-rock .

Waters charged with mineral matter in solution ,acting on a set of strata of varying composition , suchas sandstones and limestones, have a selective capacity ,for it is not iced that the metall i ferous materials aredeposited in rocks of special composition . For instance

,

let u S imagine waters acting along the plane of contactof an igneous rock , fai rly rich in i ron , and a set of alternating grits and l imestones , as shown in the figure .

2 37


The iron of the igneous rock will pass partially intosolution during the process of weathering

,and a

chemical reaction will take place between this solutionand the l imestones , resulting in the deposition of ani ron ore , and the passage of an equivalent mass of l imestone into solution within the region through which thewater circulates .I t has been found convenient to restrict the use of

the term metasomatic to those chemical replacements

3

FIG . 45.— SECT ION TO ILLUSTRATE THE INFLUENCE OF ROCK

COMPOS IT ION ON THE POS IT ION OF METASOMA’

I‘

IC ORE

BODIE S .1 ,Igneo us ro ck ; 2 , grits ; 3, limesto ne . B lack patches indicate o re.

which only take place in rocks through the agency ofcirculating water . We thus exclude all those changes

brought about by high temperature , and aqueous andgaseous emanations from cooling igneous magmas,which

,on account of the attendant metamorphism ,

would be treated more appropriately under the head

ings of Pneumatolysi s ’

(p . or ‘ Metamorphism

(p 33 1 )MetasomatIc replacements are brought about primarily

by the selective influence of certain rocks for some


These two classes of ore deposits include under thei rrespective heads the greater number of ores of metaso

matic origin,the former embracing the most important

i ron and manganese deposits , the latter those of si lver ,lead , and zinc .

Metasomatic ores present certain features by whichthey may be recognized , and thus separated from thoseformed in other ways . There i s i n almost all cases anabsence of symmetrical banding or comb-structure inthe vein material . There is a notable absence ofbreccias cemented by vein -stuff, and a general lack ofdefinition between the countryrock and the ore .

We will now proceed to discuss the origin anddescribe the mode of occurrence of the various metaso

matic ore -bearing masses .

METASOMATIC IRON ORES .

The ores of i ron which owe thei r origin to metaso

matic replacement consist of the carbonate (PeCO 3 ) ;the anhydrous oxide , haematite (PezO 3) ; and the hydratedoxides , l imonite , etc . Occasionally si licates are metwith

,but by far the most prevalent are the carbonate

(chalybite) and oxide (haematite) . Many limonitic oresare due to the oxidation and hydration of the carbonateby waters

,which carry oxygen or alkalies in solution

,

percolating into the ore,and causing the carbonate to

decompose with the formation of ferric hydrate and thel iberation of carbonic acid gas .The source of the i ron which gives ri se to metaso

matic replacements i s generally in some land -area ofigneous rocks or pyritous sediments , which , during theprocesses of weathering , yield ferruginous solutions ofvarious characters

,dependent on the character of the

METASOMAS IS 24 1

rock,the climate

,the absence or presence of vegetation ,

and many other factors . Some idea of the immensequantity of iron leached out of rocks and carried awayin solution may be formed from the average compositionof river waters . The amount of ferric oxide containedin a cub ic mile of water is as much as thirteen tons , allderived from rocks existing above sea - level .A great number of i ron -bearing .minerals , including

many of the ferromagnesian sil icates , allow thei r contained i ron to pass readily into solution during theordinary process of weathering ; and pyrites is easi lyoxidized to the solub le ferrous and ferric su lphates .The more basic igneous rocks

,those richer in i ron

and magnesia,would furnish the more highly ferru

gino us solutions , and at the same time they are moreprone to decomposition than those rocks with a higherpercentage of si l ica .

I ron -bearing solutions may bring about metasomaticchanges in two ways— either by percolating into somestrata , differing in character from those originally yielding the i ron , which are capable of bringing about anexchange of material ; or they may find their way intoa Sheet of standing water , at the bottom of which issome porous deposi t which will extract the i ron by ametasomatic process .Calcium carbonate i s the compound which we findmost often suffers replacement under the influence ofsuch solutions , and the cause is not difficult to detect .Ferrous and ferric salts

,such as the sulphate and

chloride , are decomposed in the presence of calciumcarbonate , with the formation of the carbonate or theoxides of i ron , as the case may be ; and this i s thereason why so many iron ores occur associated with

,or

16


occupy the positions of,l imestones in the sedimentary

rocks .

I f a solution of ferrous sulphate (FeSO 4) acts oncalcium carbonate , the chemical reaction results in theformation of ferrous carbonate and calcium sulphate

,

which reaction is expressed in the equation

P eso,CaC0

3FeCO

3caso

,.

The iron carbonate will replace the calcium carbonateof a rock molecule for molecule

,and the calcium

Sulphate wil l either crystall ize as gypsum,or

,i f the

solution is sufficiently acid, be carried away .

A solution of ferric sulphate acting oncalcium carbonate will produce hydrated ferric oxide

,

calcium sulphate,and a liberation of carbonic acid gas

3CaCO 3 3H ,o 3CaSO 4 P e

, (o H )6 3co , .

Haematite may be produced by the direct action offerric chloride on calcium carbonate

FezCl6 3CaCO 3 PegOs

-I 3CaCl2 3CO z.

Ferrous carbonate under the influence of oxygenatedwater , or water contain ing alkaline carbonates in solution , is decomposed , with the formation of the hydratedsesquioxide of i ron

,and this i s the origin of most

l imonitic ores of metasomatic character

4P eCO 3O2 3H z

O Fe2(OH )6 Fe

2O3 4COz

.

O ther l imonitic ore deposits result from direct precipitatio n of the hydrate , and are not replacements ;these wil l be treated later under the head of Precipi

tation ’

(p .

The iron in the solutions which give rise to metaso

244 TH E GEOLOGY O F ORE DEPOS ITS

l imestone may be taken as an additional proof of theirmetasomatic origin .

The spathic i ron ores of the Coal -Measu res areprobably precipitat ions

,and not replacements , for it i s

unl ikely that l imestones recurred at such frequentintervals i n so argillaceous a series of freshwatercharacter . Certain l imestone bands which do occur inthese rocks are occasionally replaced in part by i ronores, but so far , no argillaceous i ronstone has beenproved to be a true metasomatic replacement Thealkal ine carbonates , such as those of sodium and

potassium , meeting with an i ron -bearing solution , willprecipitate the hydrous carbonate of i ron , and this isthe more probable origin of the Coal-Measure andmany other spathic ores .The iron ores of metasomatic origin may be dividedroughly into two classes

I . Contemporaneous, in which the replacement hastaken place during or immediately after the deposit ionof the original rock .

2 . Subsequent , i n which the replacement took placesome time after the deposition and consolidation of

the original rock .

The former class wil l include most of the beddedmetasomatic i ron ores occurring in sedimentary rocks,and the latter , those which exist as i rregular patchesand veins .

CONTEMPORANEOUS METASOMATIC IRON ORES .

Under the heading of contemporaneous metasomaticdeposits may be included most of the bedded iron oredeposits so frequent in the great Mesozoic sedimentaryseries .

METASOMASIS 245

In G reat Britain the ores occur interstratified withbeds belonging to the Lower , Middle , and Upper L ias ,the Inferior Oolites

,and the Lower Cretaceous rocks .

In other regions they occur largely in the Mesozoicdeposits , but several cases of metasomatic ores areknown in rocks of much greater antiquity .

In the Silurian system of America , above the MedinaSandstone , we have the great Clinton i ron -group exist ingin the States of New York

,Pennsylvania , Wisconsin ,

Virginia,and several others .

L iassic ores are worked extensively in Hanover . Rocksof Inferior O ol ite age yield ores i n Lorraine , Luxemburg , WI

'

irtemberg,Upper Silesia

,and Switzerland .

In E ngland the most important ores occur in theeastern counties

,especially in L incolnshi re and York

shire .At Frodingham

,in Lincolnshire

,a Lower L ias oolitic

l imestone belonging to the zone ofAmmonites (Am iocem s)

semicostatus has been replaced bv ferrous carbonate .

It has a maximum thickness of 2 5 feet , and a yield ofmetall ic i ron which varies from I I to 35 per cent .

The limestone has been only partial ly replaced , andthe rock is therefore sti l l rich in calcium carbonate ; atthe same time much of the ore

,especially near the out

crop , has been oxidized to l imonite .

The ore is generally a brown oolitic rock in whichthe greater part of the shell fragments and ooliticgrains have been replaced by ferruginous material . I toccurs below the horizon of the C leveland ores noticedbelow, and thins out with more or less rapidity whentraced in a south - easterly direction .

The Cleveland ore occurs in the upper part of theMiddle or Marlstone divi sion of the Lias in the zone of


Ammonites (Amaltheus) spinatus, a horizon characterizedby calcareous beds over the greater part of E ngland .

FIG . 46.— GEOLOGICAL MAP OF THE CLEVELAND H ILLS ,

SHOW ING THE OUTCROP OF THE CLEVELAND IRONSTONE .

Scale , 4 miles to I inch .

As will be seen from the accompanying map (F ig .

the outcrop of the ore i s most irregular in form , but


(H ildocems) Infl ows, the N eocomian ores of Claxby andTealby , i n Lincolnsh ire , and the Weald of Kent .In Continental E urope , similar ores occur in the Lias

of Hanover,in the Upper L ias and Inferior O olite of

Lorraine and Luxemburg , i n the zones of H arpocem s

opa linum and H . mumhisome.

The iron ores of Lorraine and Luxemburg , the

so -called ‘ minettes ,’ have a very variable composition

the i ron existing as carbonate,

si l icate , or oxides ,including magnetite . Although these ores are doubtless largely due to original deposition

,they are in a

measure contemporaneous replacements , and addit ionalferriferous material has been introduced by solutionspercolating downward from the pyritifero uS P osidoniaShales . There are five main seams of ore , covering anarea of about square miles

,averaging about 36

per cent . of i ron,and containing only 1 7 per cent . o f

pho spho rIc acid .

I n Upper Silesia , Switzerland , and especially in theneighbourhood of Aalen (Wurtemberg) , ores are foundwhich also belong to the H . muzchz

’

some Zone . I t wil lbe seen from the above , therefore , that most of thegreat bedded i ron ore deposits in E urope of metasomatic origin belong to well -defined horizons in theMesozoic sedimentary series .

I n America the famous Cl inton ores are associatedand interbedded with Silurian rocks (Llandovery) , andoccur above the Medina Sandstone . They cover avery great area , and have been worked in the WesternStates of N ew York , Pennsylvania , Wisconsin , Virginia ,Kentucky, Tennessee , and Alabama . They are oftenoolitic , but some of the deposits consi st largely ofcalcareous organisms , such as bryozoa , corals , etc .

,

METASOMAS IS 249

replaced by iron ore,and cemented by secondary

calcium carbonate in the form of calcite , in such amanner as to prove that much of the replacement ofcalcium carbonate by iron ores took place before thecementation of the rock as a whole . This character

has also been noticed in the case of the Frodinghamores described above .

IRON O RES DUE TO SUB SEQUENT REPLACEMENT.

We now pass on to consider another and mostimportant group of deposits of i ron oxides and car

bo nates. Like those described above , they owe thei rorigin to the replacement of limestone , and occasionallyof sil iceous rocks

,by ferruginous material , but differ in

the fact that they seldom occur as true interstratified

deposits, and also were introduced from some outsidesource long after the containing rock had consol idatedand taken on the characters which it now possesses .

These ores,which include the best -known hmmatite

deposits , occur in two ways , either replacing the wallsof a fault or j oint

,or at the j unction of two kinds of

strata of different porosity - in either case along somel ine that admits of the free passage of mineral -bearingsolutions .Such ore masses are of most wide distribution , andoccur all over the world

,chiefly associated with lime

stones .In Britain

,the hmmatite deposits of South Wales ,

Cumberland , and Lancashire , occur in the CarboniferousL imestones series , and are due either to the influence ofthe highly ferruginous Triassic and Permian seas , underwhich the Carboniferous rocks were locally submerged ,or to iron compounds leached out of the overlying


Triassic , . Permian , or Carboni ferous rocks , and re

deposited in the underlying limestones by metasomaticaction .

In a few other di stricts in the Brit i sh I sles,but

more especially on the Continent , deposits of l ikenature and origin occu r associated with limestones ofmany ages.

I n such districts as the Pyrenees , the Alps , and theCarpathians , a long and continued series of earth -movements has resulted in a succession of folds , faults , andunconformities ; l imestones have been thrown againstrocks of widely different character , and have beencovered at later periods by seas and sediments , con

tain ing much mineral matter. I t i s not surprising ,therefore , to find that in such regions as these thereplacement of limestones by i ron ores i s a mostmarked phenomenon . I n the Briti sh I sles the mostimportant ore deposi ts of this character are associatedwith the Carboniferous Limestone series

,and occur

chiefly in the districts of South Wales,Cumberland

,

and North Lancashire .

I n the district of South Wales , including the Forestof Dean , the ore occurs as haematite and limonite , forming irregular deposits either within the limestone or atits junction with some other rock . I n the case of oreswithin the limestone

,they lie more or less along the

planes of bedding and j ointing , and are connected byseveral small ore -bearing channels with the upper surface of the rock . Occasionally the ore has formed atthe junction of the l imestone with the overlying shalesof the Mi llstone G rit series , or with the Triassic rockswhich cover it uncomfo rmably.

I t i s worth remarking that a great number of im

2 52 THE GEOLOGY OF O RE DEPOS ITS

The metasomatic character of the South Welsh orei s Clearly demonstrated by thin sections , which bytransmitted l ight show crinoids, polyzoans , and otherorganic calcareous remains , replaced by the red oxideof iron .

I n the North of E ngland occur the st i l l better knownhaematite deposits of Cumberland and North Lancashire ;they are of identical character , and both are associatedwith Carboniferous L imestone . The Cumberland district comprises the Carboni ferous area of Cleator Moor

,

on the border of the Whitehaven co alfield ,while the

North Lancashire district l ies chiefly in the neighbourhood o f Furness . The ore deposits of these two districts occur in pockets , chiefly along fault - l ines andjoints, and evidently belong— as is the case in SouthWales— to a period long subsequent to the depositionof the l imestone . They yield , on an average , 50 percent . of metall ic i ron . Thei r metasomatic character i sproved beyond doubt by the occurrence of haematitepseudomorphous after scalenohedra of calcite , and bythe presence of brachiopod shells , crinoids, and otheroriginally calcareous organisms, replaced by iron ore .

At the same time the bedding of the l imestone mayoften be t raced through the ore , and thin beds of Shalemay be seen passing from the ore to the unalteredrock without interruption .

Associated with the lead and zinc veins of Denbighshire and F l intshi re , i n North Wales , are a few veinsand pockets of haematite , which are interesting onaccount of their contain ing a fai r quantity of manganeseas oxide , and some nickel and cobalt as pyrites . Theseores occur in the Carboni ferous L imestone series atCwm .

METASOMAS IS 2 53

In the case of the North of E ngland ores , i t has beensuggested that the i ron was introduced in the form of

cho ride, but the sulphate of i ron seems to us moreprobable

,especially as it i s not an uncommon feature

of the haematite veins to contain sulphates of the alkal ine earths among the gangue minerals .

In the Midland district of E ngland the Upper Coal

Measures occasionally c o ntaIn thin bands of marinel imestone (Spirorbis-Limestone) , which are often foundconverted into haematite by the action of ferruginouswaters .

FIG . 48.—SECTION To SHOW THE MODE OF OCCURRENCE OF

THE ANTR IM I RON ORES. (AFTER TATE AN D HOLDEN .)

1,Lower basalts ; 2 , piso litic iro n o re 3 , bo le ; 4, yel low o chre ;

5 ,lithomarge ; 6, upper basal ts .

In I reland , in County Antrim , i ron ores i n the formof haematite , l imonite , and the ochres, occur in connection with the basalts , which form such a prominentfeature of that district . The basalts themselves contain on an average 5 to 6 per cent . of the metal , andhave undoubtedly been the source of the i ron in mostinstances . The ores exist in the form of ferruginousclays , pisolit ic deposits , and ochres, lying between thesheets of basalt , and it is evident that they are due tothe weathering of the lower layer of basalt before the


upper one was deposited . The action giving rise tothem is partly physical , partly chemical , and is analogous to that which played a part in the formation ofcertain bauxites (p .

The above figure gives an idea of the mode ofoccurrence of these ores.

Similar deposits are found associated with the basaltsof the Faroe Islands and Iceland , which are of thesame period of eruption .

There are a great number of districts abroad which

FIG . 49 .—SECTION IN THE PYRENEES

,SHOWING A LIMESTON E

MASS FOLDED AMON G M ETAMORPH IC SCH ISTS , AND

PARTIALLY REPLACED BY O RE .

contain i ron ores similar to the above , and of identicalorigin .

I n Spain , especially in the E astern Pyrenees , and inthe districts of Torrente , Cartagena, Vizcaya (Biscaya) ,etc .

,haematite and limonite occur in a l imestone series

in a manner suggesting metasomatic replacement .The general mode of occurrence of the ores i s in

dicated in the figure above .

I n Greece , as at Laurium and H i larion , the orel ies at the summi t and base of a calcareous series

,

following the l ine of demarcation between it and theshales by which it is bounded on either side .


of metasomatic origin are numerous and well displayed— such , for instance , as in the Tintic district of Utah ,Where the ore has formed at the junction of an igneousrock (monzonite) with a limestone ; and the Salisbury

F! B IAN CA

!E Al l a wum

Lmes to ne 5 5 Ho tel Garnet Tourma l in e

Ol

a/om it:pg‘

chb olu s 2 60 8 15408 8Gran it e

FIG . 50 .— MAP OF THE SOUTH - EASTE RN PORTION OF

'

J HE

I SLAND OF E LBA, TO SHOW THE D I STR IBUT ION OF I RONO RE (HZEMATITE ) .

Scale , I mile to I inch .

district in Connecticut , where large l imonitic depositsexist in a l imestone series , deposited by waters descending from the overlying pyriti ferous Berkshire shales .

I n the i sland of Cuba, to the east of Santiago , i ron

METASOMAS IS ‘

2 57

compounds leached out of the basic igneous intrusionsof the Sierra Micaro have been deposited as haematitein Jurassic limestones at thei r contact with the igneousrocks . To the west of Santiago similar ores occur, butare less valuable owing to the high percentage of phos

pho ric acid they contain .

So far we have been dealing solely with the replacement of l imestones by i ron ores, but ferruginous solutions occasionally replace rocks of a different character .Many instances are known of haematite and other

i ron ores replacing diabase dykes and other basic orintermediate igneous rocks , but i t must be rememberedthat such rocks are rich in calcium - minerals

,which

weather with extreme ease under ordinary atmosphericconditions .Replacements of diabases by i ron ores may be cited

from the Vosges and Harz Mountains , and are knownto have taken place in a modified degree in the LakeSuperior region .

The ores of the Lake Superior region possess certaincharacteristics which cause them , so far as thei r origin isconcerned , to stand more or less alone . Although theyseem to be undoubted metasomatic deposits

,the con

ditio ns under which the replacement took place,as well

as the rock replaced , differ from any we have consideredbefore .The Lake Superior i ron-bearing strata form a belt ofrocks indicated on the map as occurring in the greatpre -Cambrian Huronian formation , and as lying betweenthe Keweenawan rocks and the great Archaean fundamental complex .

They consi st of cherty i ron carbonates , which are thechief orig inal sediments of this group of rocks ; the i ron

I 7

2 58 .THE GEOLOGY OF ORE DEPOS ITS

was directly precipitated from solution as carbonate ,after the manner of many spathic i ron o res

'

(p .

and was presumably derived from the more ancientigneous rocks of the Lake Superior region , many ofwhich were basic in character and proportionately richin i ron compounds.

Archa ea n .

FIG. 51 .—MAP OF THE LAKE SUPERIOR REGION .

Scale,appro ximately I 50 miles to I inch .

The cherty i ron carbonates , by metasomatic changes,pass insensibly into ferruginous quartz schi sts , j aspersand magnetite and haematite schists , and furnish amost striking example of the manner in which sediments may pass into crystall ine schists by a process

( 260 THE GEOLOGY OF ORE DEPOS ITS

that the metasomatic changes have been most intensethe igneous rock has been more or less completelychlo ritized , and the lower layers of the iron -bearingformation replaced by ores .I n the case of ores occurring within the formation

along the l ines of faults or dykes,i t i s noticed that the

ore gradually diminishes in richness as the distance fromthese l ines increases .A study of the various Lake Superior i ron -bearingdistricts has caused the ores of thi s region to be

FIG 5z._

—SECTION SHOW ING I RON ORES IN THE SOUDAN

FORMATION,VERM ILION D ISTRICT OF M INN ESOTA.

The o re replaces the ferrugino us jaspers at their junctio n with the

underlyingigneo us ro ck.

regarded as true replacement or metasomatic deposits .Furthermore

,i t has been proved that the replacement

took place after the district had been affected by earthmovements , and many of the cherty carbonates andj aspers brecciated . The cherty carbonates containedoriginally large quantit ies of ferrous carbonate . This

,

under the influence of carbonated waters , was dissolvedand carried downwards, O ften alongwell -defined channel snow marked by ore deposits . When two or more streamsconverged and met at the bottom of a synclinal trough

,

the metasomatic action was most marked .

METASOMAS IS 2 61

The igneous rocks underlying the district , on weathering under the influence of percolating waters , wouldyield a considerab le amount of alkal ine carbonates ,which would aid in the solution of the sil ica of thej aspers and cherts , and would help to bring about thei rreplacement by i ron ores .The Mesab i district differs not so much in the modeof o rIgIn of the ore , as in the fact that the i ron -bearingformation has a somewhat different character . I n thecherty rocks of the other districts the i ron existed asferrous carbonate , but in the Mesabi district it wasoriginally deposited as a si l icate , to which the namegreenal ite has been applied .

The average composi tion of the i ron ores of theVermil ion , Mesabi , Marquette , Menominee , and G o

gebic ranges is as follows : I ron from toper cent . ; phosphorus , 0

°

o 44 to per cent . ; sil ica ,to per cent . alumina , to per cent . ;

manganese , to per cent . ; l ime , to percent . magnesia , to per cent . sulphur

,trace

to per cent . ; organic matter and loss , to 107per cent .

Similar to the iron ore deposits of the Lake Superiorregion are those of Bechuanaland

,G riqualand West

and Rhodesia , in South Africa ; and in N ew Zealandthose at Parapara . The ores of Bechuanaland andGriqualand West are associated with a great calcareo us series forming a member of the TransvaalSystem . This l imestone is known as the Campbel lRand Limestone and Dolomite , and i s succeeded bythe Griqua Town G roup , a ferruginous series of j aspers ,cherty carbonates , and magnetite schists .The ores occur along lines of brecciated limestone


and dolomite , marking , i t i s supposed , old subterraneanwater channels . The replacement has in many instances been most complete , and the removal of muchcalcareous matter has often resulted in the lowering ofthe overlying Griqua Town series . The diagram givenbelow shows the general manner in which the ores

,

which are chiefly l imonite and haematite , occur .In the South I sland of N ew Zealand

,at Parapara

in the Cape Farewel l Peninsula , i s a group of stratatentatively classed as Si lurian

,and overlain by coal

bearing Tertiary sediments .

S ca le of Fac t0 300 600 m“ a

FIG. 53.— SECTION ACROSS RAMAGES KOP, BERKLY WEST,

SHOWING THE RELATION OF THE GRIQUA TOWN SER IES

To THE CAMPBELL RAND LIMESTON E . (AFTER A. WROGERS .)

The lower series consi sts of much folded hornblendeand felspathic schists , sideritic l imestones , and cherts .

The ores are metasomatic replacements of si l iceousrocks

,and occur in broad syncl inal areas where the

ferruginous solutions from above were concentrated in

a trough of some impervious stratum .

M ETASOMATIC DEPO S ITS O F ALUMINA AS BAU! ITE .

Alumina in its anhydrous form (A120 3 ) i s not a

Common mineral in Nature,although it occurs as

corundum in several districts (pp . 57 , The im

pure hydrated oxide,A120 3 .2H 2

0 , to whi ch the name

bauxite has been given , i s met with in g reater quantity .


somatically upon it,forming calcium sulphate , and

depositing the hydrated oxides of alumina and iron .

The bauxite deposits as worked in the GeorgiaAlabama district consi st of layers and masses Of oreenclosed in a residual clay

,the result of prolonged

weathering of the limestone series under somewhatspecial atmospheric conditions .Bauxite is also found embedded in clay at Oberhessenand Vogelsberg in Germany .

In the Puy-de-Dome country of Central France , theWesterwald , Vogelsberg , and the North of I reland ,bauxite occurs in association with basaltic rocks , andi s regarded as having been formed from these igneousmasses by an obscure metasomatic process .The deposits of Baux occur as i rregular masses inCretaceous l imestones

,and it has been suggested that

they were laid down by the agency of mineral springs .

Such an origin i s extremely likely,but at the same time

the action which brought about the precipitation wasalmost certainly metasomatic

,and dependent on the

chemical action of the l imestone .

LEAD AND Z INC METASOMATIC DEPO S ITS .

The ores of lead and zinc which owe thei r originto metasomatic replacements are most commonly thesulphides, galena (PbS) and zincblende (ZnS ) , and morerarely the carbonates

, sulphates , and S i l icates , cerussite

(PbCO a) , anglesite (PhSO 4 ) , calamine (ZnCO 3 ) , andsmithson ite (a SiO 4HzO ) . Lead and zinc ores arefrequently found in association with l imestones ; theygenerally occur together, and may include some silverand copper— the former existing as argenti ferous galena

,

METASOMAS IS 2 65

and the latter as pyrites (chalcopyrite) —also compounds of cadmium . Z incblende and galena seem inalmost all cases to be original minerals ; but the carbo nates and sulphates of lead and zinc are more O ftenformed in the upper part of the ore body or lode fromthe sulphides by the action of percolating meteoricwaters . Smithsonite , for example , may often be noticedto replace the other ores and gangue minerals ; i t hasbeen found pseudomorphous after fluor , calcite , andeven galena .

The metals lead and zinc have been mined in limestones of almost every age all over the world ; but inour own country by far the greater number of metasomatic deposits yielding ores of these metals occur i nthe great Mountain Limestone or Carboniferous Limestone Series , and in all cases thei r formation can beproved to have taken place long after the depositionof the l imestone , usually during some period of earthmovement and extensive unconformable conditions .The ores of lead and zinc often offer the clearestevidence of metasomatic replacement ; fossils of acalcareous substance have been replaced by the sul

phides or carbonates of these metals , the ore retain ingthei r external form , and sometimes even thei r Internalstructure . Such cases may be C i ted from Cumberland ,i n our own country ; from Monteponi , in Sardinia ; andfrom I serlohn

,i n Westphalia .

I n England and Wales,in Carniola

, Westphalia ,Baden , the Veille Montagne district , Monteponi andMalfatano districts of Sardinia

,Leadville and Silver

Peak in Colorado , Wythe County in Virgin ia , theE ureka district of N evada , and a great number of otherlocal ities of equal importance , ores of lead and zinc


occur associated with limes tones in a manner indicatingthei r metasomatic origin .

The metall ic minerals occupy fissures formed eitherby faults or j oints

,and extend more or less i rregularly,

as was the case with the i ron ores associated withl imestones (p . i nto the surrounding rock .

The ore -masses have been deposited from solution ,and are due primarily to a chemical selective influencewhich the l imestone exerted on the mineral -bearingsolution . They thus differ from true vein -deposits inwhich the countryrock played no essential part in theformation of the lode .

The mineral izing solutions in almost every case ofmetasomatic lead and zinc deposits came from abovedownwards

,thus differing from many of the solutions

which have given rise to true vein deposi ts .There are

,however

,good examples of metasomatic

replacements which have had a hydrothermal originthat i s to say

,have resulted from the action of heated

solutions rising from below.

We have already stated that metasomatic depositsof lead and zinc are generally restricted to some rock ,such as l imestone , which is capable of interacting withpercolating waters, and causing them to deposit thei rmetall i ferous contents ; this , therefore , supplies us withthe reason why so many lead and zinc deposits de

terio rate and ultimately fai l in depth .

I t i s obvious that the limestones did not originallycontain

'

the metals which are now collected in thevein s and fi ssures traversing them , and the source maygenerally be found in some series of igneous rocks orsulphide -bearing sediments

,either exposed at the surface

or subj ected to weathering influences below-ground .


are known to exist as sulphides held in solution byacids or by alkal ine sulphides .N either lead nor zinc sulphides are very insoluble ,their precipitat ion , as i s well known , being retarded orprevented by free acids .Assuming that these metals were carrIed i n solution

as sulphides , i t seems most reasonable , in point o f V iewof the other minerals present in the deposits , to suspectone or more of the alkal ine sulphides of being thesolvent in most cases , especially as we know that thesulphides of the alkaline earths , such as barium sul

phide , will dissolve several minerals including the redsi lver ores (pyrargyrite and prousti te) without change .

Let us take the case of barium sulphide . Acting ona limestone , i t would cause the formation of barytesand the solution of part of the calcareous matter ; atthe same time the metall ic sulphides would be deprivedof part of thei r solvent

,and deposited in the place of

the dissolved limestone .A point in favour of the metall ic sulph ides beingcarried as such in solution is the apparen t solution andredeposition of galena . I n some di stricts it has beenobserved that a surface enveloping the richest oredeposits bears , as regards posit ion , a more or less constant relation to the present surface of the ground andthe hydrostatic level . I f such be more than a merecoincidence , i t would point conclusively to the solutionof lead sulphide in the upper part of the enclosing rock

,

and its redeposition at a lower level .

F luorspar in these deposi ts clearly occurs as a metasomatic replacement resulting from the action of somedissolved fluoride on a l imestone

CaCO3RF

2CaF

2RCO

S.

METASOMAS IS 269

The more soluble carbonate would be carried away ,and its place taken by the less soluble fluoride ofcalcium

,which often occurs pseudomorphous after

calcite and dolomite .

Fluorspar i s not by any means a constant associateof either galena or zincblende

,but its occasional occur

rence with these ores suggests that the mineral izingsolut ions were not l ikely in such cases to have contained alkal ine carbonates , for these act on calciumfluoride with the formation of calcium carbonate ,thus

R2CO

3+ CaF2 CaCO3 + 2RF .

Fo r a similar reason we suggest that the fluoridewas not carried in combination with the alkal i metalsalone , for under those ci rcumstances a metasomaticreplacement of l imestone by fluo rspar would not bepossible .

Fluorspar may , however, be formed by the action ofthe alkaline fluorides on calcium si l icates , the moresoluble alkal ine si l icates passing away in solution .

In the case of the fluo rspar deposits associated withcalcareous beds , i t i s extremely difficult to imagine inwhat form the fluorine was introduced ; but i t must beremembered that there are several sparingly so luble

metallic fluorides, such as those of si lver and lead , andit is also possible that the double fluorides of the metalsplayed a part .I t i s also possible that the mineral izing solutions

were acid in character,and would thus not contain

free alkaline carbonates, such as is the case with mostalkal ine percolating waters .As has been stated previously

,the carbonates

,sul

phates, phosphates , and sil icates of lead and zinc are


usually due to alterations by percolat ing waters in theUpper parts o f lodes which carry sulphidic ores ; at thesame time they often exist as metasomatic replacements .I n the presence of l imestone the solutions of sulphates ,produced by the simple oxidation of the sulphides, willbe acted upon by the calcium carbonate , causing theprecipitation of the metals as carbonate or sulphate

,

according to the alkal inity or acidity of the solut ion .

The calcium sulphate f ormed in such cases wouldeither be retained in the vein or carried forward insolution .

Both lead and zinc sulphates under the further actionof percolat ing waters charged with carbonic acid maybe converted into carbonates , this being a frequentchange in the upper part of lead and zinc lodes .

Zinc sil icate can be formed from the sulphide bythe action of alkaline sil iceous waters ; the alkalinecarbonates are the most usual solvents for si l ica innatural waters .The phosphates of lead and zinc are general ly ofsecondary origin

,but there are instances where deposits

of such minerals are metasomatic in character , andhave had thei r origin in highly phosphatic beds traversedby mineral izing solutions . A striking example is thecase of a bone -breccia fil l ing a cave in the l imestonesof the Broken H il l di strict of Rhodesia , where thecalcium in combination with the phosphoric acid hasbeen replaced largely by zinc , giving rise to the zincphosphates hopeite

,tarbuttite , etc .

Although the metasomatic ores of lead and zinchave had presumably an identical origin , and are oftenassociated , i t will be well to divide the followingexamples i nto two , separating as far as possible those


veins , but contained less si lver. This probably points,

as does the presence of copper , cobalt , and nickel , tothe introduction of the metals at more than one period .

Secondary changes have resulted in solution and redeposition of the ores in caviti es , but most of them areof true metasomatic origin .

The Derbyshire lead ores were among the fi rst to beworked in Britain

,and they are noted for their asso

ciated gangue minerals , fluo rspar and barytes , twominerals highly characteristic of lead veins in limestones , and which have been profitably worked .

4

FIG . 54.— D IAGRAMMATIC SECTION ACROSS TALARGOCH

VE IN S. (AFTER STRAHAN .)

1,Grey and white limesto ne ; 2 , b lack limesto ne ; 3 , shales4, Talargo ch vein . The vein to the right carries hematite and

co pper o res.

The ores consist of galena with some zincblende , andin the upper part of the lodes which usually occur alongfault -fissures or joints , cerussite and calamine . SomeCopper pyrites also occurs .The lead ores of Derbyshire seem to be confinedmore or less to one horizon in the Limestone Series ,and to be governed , as regards depth , by certain sheetsof intrusive and extrusive O l ivine dolerite , locally knownas ‘ toadstones

,

’ through which and below which theores seldom pass .Some of the chief districts where galena has been

extensively ‘mined are those of Castleton , Bradwell ,

METASOMAS IS 2 75

Tideswell Moor , Longstone E dge , and Taddington , andmost of the deposits are metasomatic in character .Some of the lodes

,however, are brecciated , and others

show well -defined comb structure ; i t i s probable inthese cases that the ores are independent of m etaso

matic action,and are merely the infillings of cavities

and open fissures in the l imestones .

In E urope and the other continents, examples ofmetasomatic lead ore deposits are

“

far too numerousto mention in detail

,but the following are a few

selected on account of thei r importance and scientificinterest .Some of the most important deposi ts i n E urope arethose of Sala in Sweden

,of Raibl and Bleiberg in

Carinthia , and of Tunis in North Africa . I n Americathere are the famous deposits of Leadvi lle and Colorado ,and similar occurrences i n Wisconsin and Utah , as wellas the argenti ferous galenas of the E ureka district ofNevada .

The Sala deposits occur to the north Of Stockholmin Sweden

,and are intimately associated with an ancient

dolomitized limestone . The ore is chiefly galena , whichcarries a good percentage of si lver .These deposits have had many origins assigned tothem , but none seems so sati sfactory as that of metasomatic replacement .The famous districts of Raibl and Bleiberg in Carin

thia are situated in an area composed largely of UpperTriassic l imestones and dolomites . The Raibl regionis also noted for its zinc deposits , and presents theclearest evidence of metasomatic replacement of thelimestones by compounds of the metals . The Ore

bearing veins are steeply incl ined to the horizontal , and18


traverse a great thickness of strata ; a pecul iar featurei s that they contain large quantities of the mineralwu l fen i te , lead molybdate (PbMOO

4) , formed from theoriginal lead compounds by the action of later perco lating waters carrying molybdic acid or solublemolybdates .Pyromorphite , the chlo ro pho sphate o f lead found inthe upper portions of many lead lodes

,has had a similar

origin , being formed by the action of phosphoric acidsolutions on pre - existing lead compounds.

I n North Africa , associated with the Nummulit icL imestone in the neighbourhood of Tunis, occur severalimportant deposits of galena and cerussite mixed witha certain amount of zincblende and its alteration products . The galena , which is seldom argentiferous ,occurs in flat lodes at the unconformable j unction Of

the l imestones with a series of mica schists and quartzites . The secondary calamine occasionally shows themost remarkable concretionary structures . I t has beensuggested that the ores were introduced at some lateperiod in the Tertiary era .

In America the clearest proofs of metasomatic actionare displayed by the great lead ore deposits of Leadville , i n Colorado , and by those of Utah , Wi sconsin ,I ll inois, and several other States ; also important si lverbearing ores occur in the E ureka district of N evadaand in the Sierra Mojada district of Mexico .

The Leadville region is situated on the western S ideof the Mosquito Mountains , and is composed chiefly ofl imestone with porphyries and o ther igneous rocks .

The ores,which are mostly carbonates , exist in a blue

grey dolomitic l imestone of Carboniferous age ; theyinclude also the sulphides of iron and lead , together


The main ore bodies are due to the replacement of al imestone along its faulted plane of contact with im~

pervIo us quartzites .The ores in depth consist of the sulphide galena

,

associated with some i ron pyrites,but as the deposits

are followed to the surface they take the form of carbonate , oxide , molybdate , chlo ro pho sphate , and arsenate .An important feature of the E ureka lead deposi ts i sthat they contain gold in addition to a fai r quantity O fS i lver .

FIG. 55.— GEN ERALIZED SECTION To ILLUSTRATE

'

I HE MODE

OF OCCURRENCE OF THE LEAD ORES IN THE LEADVILLED ISTR ICT.

The o re lies between the l imesto ne below and the WhitePo rphyry abo ve .

The lodes are O ften brecciated by subsequent movement between the hard and soft rocks , and it can beclearly seen that mineral ization has taken place bothbefore and after the formation of the breccias .The accompanying figure depicts in section the usualposit ion of the ore-mass with reference to the l imestoneand quartzite .

The deposits of both Utah and Wi sconsin appear tobe associated with a series of dolomitic l imestones , andas far as deposition i s concerned to be independent ofany other rock-masses . They bear no relation to theigneous rocks of the district , but are occasionally overlain by recent superficial accumulations .

METASOMAS IS‘

2 77

The ore i s argenti ferous galena, and its late intro duc ~

tion i s suggested by the fact that certain caves in thel imestone have yielded bones impregnated with thi smineral .The deposit becomes rapidly poorer in depth , acharacter which is presented by a great number of themetasomatic ores of lead and zinc .

The ores of I ll inois,which occur in the Trenton Lime

stone o f O rdovician age , end off before the underlying

FIG . 56.

—SECTION TO ILLUSTRATE THE POSITION O CCUPIED BY

THE O RE B ODIE S IN THE EUREKA D ISTR ICT.

1 , M o unt Pro spect quartzite 2,limesto ne . The o re o ccurs between

the l imesto ne and the M o unt Pro spect quartzite .

Potsdam Sandstone is reached , j ust as the majority ofthe Derbyshi re ores do with respect to the toadstones .In the Cambrian of Dakota , the ore when unoxidized

consists of pyrite , argentiferous galena , and small quantities of zincblende . The pyrite occasionally carriesgold , and is of earl ier formation than the galena . Theore i s evidently a replacement of a dolomite bounded by


Shales , and i s accompanied by much silicification of thesurrounding rock , which recalls the refractory ores .

’

In the C arboniferous L imestones of the same regiondeposits of lead carbonate pass down into galena , andoccur as large i rregular masses in contact with a porphyryor partly fill ing crevices . The irregular masses are thericher in S i lver

,and the infil lings are accompanied by

the g reater amount of secondary S i l ica .

I n the Coeur d ’

Alene Mountains of Montana , fissurestraversing quartzites and quartzose shales of Algonkianage , carry galena , and other sulphides , in a gangue ofS iderite with quartz and barytes

,but no fluo rspar. The

S iderite shades off gradually into the surrounding rocks ,and is evidently a replacement . Some of the sulphidesappear to replace the quartzite .

I n the northern part of the State of Arkansas metasomatic depOsits of galena and zincb lende occur ' in theO rdovician and Mi ssi ssippi Limestones . They haveformed chiefly along l ines of fracture

,which are accom

panied by brecciation , and are controlled by beds ofshale . The ores as originally deposited consisted O f

galena and zincblende , and aszsuch are st i l l found

in depth ; but subsequent alteration by descendingalkal ine and sil iceous water has resulted in the solutionof the sulphides and their redeposition , together withmuch secondary si l ica

,at a lower level . The sil ica has

replaced the l imestone to a great extent , giving rise tocherts . AS already pointed out , the solution and ré

deposition o f sulphidic ores in mineral lodes i s not an

unusual phenomenon (p .

I n Mexico , lead ore exceptionally rich in silver also

occurs in limestone,and lies along , or near, the plane

of contact of the l imestone with a l imestone -breccia


blende in places ; although zincb lende i s a comparativelyrare mineral in the Mendip district , it probably occursin greater quantity in depth .

The zinc ores , blende and calamine , also occur asmetasomatic replacements associated with the galenaand haematite deposits of F l intshi re (p .

Compared with the zinc deposits of the Continent,

such as those of Sardinia and the Alps , the E nglish andWelsh occurrences sink into insignificance . The originand mode of occurrence of the Continental depositsseem to be identical with those we have already con

sidered ; we see the same intimate connection betweenthe zinc and lead ores , for they generally occur togetherin the same deposit . Such a relation between similarores of two metals , taken in conj unction with the factthat they are associated with the same gangue minerals ,can only po ifIt to an identical mode of introduction anddeposition .

Amongst the most important metasomatic zinc oredeposits of Continental areas are those of Sardin ia ,G reece , B elq m

, Westphalia , Silesia , the E astern Alps ,and Northern Spain

,in E urope ; Algeria and the

O tavi Mountains,in German West Africa ; and many

districts in the United States and Mexico , in NorthAmerica .

In Sardinia zinc and lead ores are mixed in severaldist ricts

,but only a part of the deposits can be claimed

to have had a metasomatic origin .

The metasomatic ores are associated with limestones and other beds of O rdovician age in the southwest port ion o f the i sland . They are met with eitherat the j unction of the limestones with a series of noncalcareous beds , or within the l imestones themselves .

METASOMAS IS‘ 2 81

The junction Of the l imesto ne with the other type ofsediment may be either a fault l ine or a normal planeof sedimentation

,the metall i ferous deposit following

the one or the other as the case may be .

The ores are mixed in character consist ing of galena ,calamine

,and smithson ite (zinc sil icate) , associated with

a good deal of i ron i n the form of oxide and hydrate .

The gangue contains barytes and fluo rspar, while thelimestone surrounding or bounding the ore body has

undergone extensive dolomitization .

The chief districts where zinc ores are raised arethose Of Montepon i and Malfatano , which have yieldedvast quantit ies of calamine .In ' the veins

,etc .

, O f Sardin ia , which are associatedwith l imestones , i t i s evident that zinc -charged watershave attacked and replaced the calcium carbonateparticle for particle . I t i s probable , however , that herecalamine is a secondary metasomatic deposit , result ing ,as in many other cases , from the alteration of zincblende ; for many of the Sardin ia deposits , when followeddownwards , are proved to contain a considerable quantity of this sulphide .The G recian ores of Attica , south -east of Athens

,i n

the neighbourhood of Laurium,are famous for thei r

extreme thickness , and for the very early date at whichthey were first exploited. The metal -bearing strataconsist of three distinct beds of l imestone of doubtfulage , separated from each other by non -calcareous rocks .The ores , as in many cases previously described ,occur chiefly at each contact of a l imestone band withthe adjoining rock , these j unctions representing theplanes of concentration of the percolating waters .The ores consist chiefly of zincblende and galena i n


a gangue of chalybi te , but oxidation has given riseto cerussite, calamine , haematite , and gypsum ,

all ofwhich are found in the upper parts of the deposits

.

The thickness Of the ore body diminishes in each bedOf l imestone when followed upwards ; for instance , inthe figure below, the zinc deposits in the lower bed

,

labelled I, will be pure , but those in 3 will have a con

siderable admixture of calcium carbonate.

From the above and some other reasons,it i s argued

that the Zinc - bearing solutions were hydrothermal,

came from below , and that they passed through small

FIG. 57 .

—D IAGRAMMATIC SECTION To ILLUSTRATE THE MODEOF OCCURRENCE OF ORE MASSES IN THE N E IGHBOURHOODOF LAUR IUM .

I and 3 , Limesto ne 2 , shales .

fissures in the non -calcareous rocksWithout depositingthei r metall i ferous contents . I n the l imestone , however

,the presence Of the calcareous material caused

them to deposit the ores along the junctions of thel imestones with the interven ing and resistant shalylayer . The solutions would pass on only after the workof replacement was complete .

The B elgIan deposits , l ike those of E ngland andWales, are confined almost entirely to the Carboni ferousL imestone Series

,and occur chiefly in the district of

Vieil le Montagne .

The limestones Show extensive'

dolomitization , a


the j unction Of the Schaumkalk , or metall i ferous dolomite , with the underlying S o lenkalk, into which the

ores pass .The minerals as originally deposited were sulphidesof the metals lead , zinc , and iron , for all the deposi tsappear to become highly sulphidic in depth ; so therefore , as in so many other instances , the presence ofcalamine , cerussite , and haematite , i s due to oxidation ofpre- existing sulphides .I t has been suggested that the original source of the

zinc and lead ores lay in the Schaumkalk dolomite,

where theywere precipitated at the time of the depositionof the l imestone , and that the present great ore deposi tsowe thei r origin to a metasomatic secondary enrichment .I n the Alpine region , the richest zinc -bearing regionis that of Carinthia , in which the Raibl dist rict i s thebest known: The ore , which , l ike those mentionedabove , occurs in the Triassic l imestones , i s chiefly calamine ; but a l ittle si l icate of zinc i s also met with , aswell as the minerals cerussite and l imonite .I t i s interest ing to note that the hydrous carbonate

of zinc occurs in some quantity , and is a mineral which ,as far as laboratory experiments teach us , would bereadily deposited by the influence of calcium carbonateon zinc solutions . Why the anhydrous carbonate i s inN ature generally much more common than the hydroussalt sti l l requires explanation .

The zinc ores of Raib l are intimately connected withthe faulting and j ointing of the beds in which theyoccur . They are usually surrounded by , or associatedwith

,secondary dolomites , but most of the common

gangue minerals,such as barytes , fluo r, etc . , are practi

cally absent . These deposit s present the most striking

METASOMAS IS .285

proofs of the replacement of a limestone , and even itsorganic remains

,by metall i ferous material ; for it is not

a rare thing to find fragments of limestone completelyreplaced by ore

,with the retention of thei r original

structures .The chief Spanish deposits occur in the province ofAsturias . At Santander they are associated with theH ippm/ite Limestone of Cretaceous age , but in the Picosde E uropa district the Carboniferous L imestone formsthe country - rock . O riginal b lende and secondary calamine with some sil icate are the usual ores in bothdistricts ; but in the Santander deposits the hydrouscarbonate of zinc occurs in addition .

I n Africa , in the L i ttle Atlas Range of Algeria , theTertiary Nummulitic Limestone contains considerablemetasomatic deposits of zincblende , which is locallyconverted into calamine .

The ores occur chiefly at the junction of the calcareo us with non - calcareous beds , as is the case inmost of the other districts already cited .

Lodes carrying zincblende associated with galena ,and having smithsonite

,calamine

,and copper carbonate

,

i n the gossans , occur in the Malmani dolomites of theTransvaal . They are interesting also on account oftheir contain ing the mineral cinnabar .In the United States and Mexico , zinc ores occur inmost of the lead -districts we have already mentioned

,

as , for instance , in the States of Colorado , Utah , Wisconsin , I llinois , Pennsylvania , etc .

METASOMATIC CADMIUM DEPOS ITS .

'

The metals cadmium and zinc are extremely similarin nature they are dissolved by the same solvents , and


are thrown out of solutions in almost all cases by thesame precipitant . I t i s , therefore , not surprising to findcadmium in almost all zinc ore deposits existing underconditions exactly similar to the zinc compounds, andhaving an identical origin . Analyses Of most samplesof zincblende Show the presence o f this metal in smallquanti ties ; i t probably replaces zinc , in combinationwith sulphur ; occasionally , however, the somewhatrare but pure sulphide , greenockite , i s met with , as inseveral districts in the United States .Cadmium also occurs in the deposits of zinc car

bonate and si l icate in almost all of the districts mentio ned above where such zinc ores are mined . I t i sespecial ly abundant in the calamine deposits of UpperS ilesia and of the Vieille Montagne district . Thecalamines o f Upper Si lesia have yielded as much as

5 per cent . Of cadmium .

METASOMATIC CO PPER DE PO S ITS .

The Sulphides, Carbo nates, etc .

— Reference hasalready been made to the occurrence o f copper ores inthe lead and zinc deposits of metasomatic origin ; andalthough they are general ly met with in small quantityonly

,they occasionally occur in sufficient force to give

a distinctive character to the deposit . The ores consistchiefly of chalcopyrite or copper pyrites, and the resultsof its reduction or oxidation in the form of nativecopper and oxides of copper ; while

'

phosphates , car

bo nates, and si licates , are not infrequent secondaryproducts .The mode of deposition of the sul phides in a certainnumber of deposits , l ike those formed in l imestone or at


precipitations l ining and fi ll ing fissures,thei r deposition

being in no way controlled by the nature or compositionof the countryrock . A few cases , however , of depositsof Copper ores which we are led to regard as metaso

matic will be cited below .

I n the district of N izhne-Tagilsk , i n the Ural Mountains, copper ores have been described as occurring atthe j unction of limestone with syenite , in a mannersuggesting metasomatic replacement of the l imestone .

The B lago sslowsk deposi ts, also in the Urals, and thoseof Banat in Hungary , were probably also of this nature ,but have been subsequently metamorphosed .

The ore i s largely chalcopyrite , but its alteration hasresulted in the formation of the carbonates , azurite andmalachite , the phosphates and sil icate of copper .In the United States the copper deposits of Bisbee ,

in Arizona , appear to be metasomatic replacements .The ores occur as masses in pockets in the Palmo zo icl imestones at thei r j unction with shales and igneous rocks ,more especially with the great porphyry mass of theSacramento district , which has produced l ittle or nometamorphism on the surrounding sediments . Thereseems little doubt that the metal was i ntroduced assulphide by solutions which worked along the plane ofcontact of the igneous rock with the sediments, andwhich brought about the replacement of the l imestone .

The ores have undergone great changes throughsubsequent alteration by oxygenated waters, chargedwith sulphates , which percolate from the surface ; thusthe minerals contained in the upper 200 to 600 feet ofthe lodes are qui te different from those found at agreater depth .

The earl iest process of ore formation was the depo

METASOMAS IS 289

sitio n of i ron and copper pyrites , zincblende, and somemolybdenite .The secondary changes include the formation o f

cuprite,malachite

,azurite

,chalco site , and many other

minerals,together with the elimination of much of the

zinc and sulphur.The original minerals below the weathered zone arechiefly iron and Copper pyrites , but some cerussite existsin the l imestone .

These ores are for the most part associated withmetamorphosed calcareous beds containing such metamorphic c alc - si licates as garnet , idocrase , diopside , andtremol ite . But the proof of thei r metasomatic originl ies in the fact that the ore bodies are not confined tothe zone of metamorphism , and seem to be more orless independent of it , occasionally stretching farbeyond the range of the metamorphic calc- sil icates .I n the district of the Hartville Uplift

,Wyoming

,

copper ores such as chalco site , malachite , etc .,replace

the ferruginous compounds , and even the matrix , O f thelower part of the Guernsey Sandstone Formation

, whichrests directly on an impervious floor of pre-Cambrianro cks .I n many of the lead and zinc deposits described

above , copper ores prove an important accessory constituent of the lodes . I n the Sierra Moj ada district ofMexico we are confronted with a series of interestingcomplex lodes of metasomatic origin contain ing ores ofCopper, S i lver , lead , zinc , and iron .

The high -grade copper ores of Monte Catin i andMonte Calvi , in N orthern Italy, are by some consideredto have had a metasomatic origin

,and it i s quite prob

able that Some of the chalcopyrite masses,such as those

I 9


of Nassau,found in altered diabases and andesites

( schalstein) , and associated with galena and zincblende ,

may be of a similar nature .

N ative Co pper.— Deposits O f native Copper present

many features of exceptional interest , for although muchO f the free metal has been formed in the lodes throughthe reduction of sulphides , carbonates , oxides , etc . , bythe action of organic compounds in solution , a certainnumber of deposits

,such as those of the Lake Superior

region,are claimed to be t rue metasomatic replacements .

Reference to the map on p . 2 58 shows that in theLake Superior region there occurs a great series ofrocks of pre -Cambrian age , the uppermost division ofwhich has been styled Keeweenawan ,

after the narrowpeninsula on the southern side of the lake , where iti s exceptionally well displayed. The KeeweenawanSeries is succeeded unconformab ly by the Lower Cambrian sandstones , and contains the chief copper depositsof the region . I t consi sts of a great mass of amygdalo idal lavas and sills , with some beds of sandstone .These igneous rocks and sediments are restricted to theLake Superior basin , and present remarkably constantfeatures over all the district . The igneous rocks areintermediate in character— that i s to say, neither richin sil ica nor in the ferromagnesian constituents ; theybelong to the augite-plagioclase family

,and hornblende

is seldom met with except as an alteration product .The native copper occurs as nodules , and even as amatrix of certain breccias , and fi ll ing the amygdaloidalcavities in the igneous rocks of the series .I n the Keeweenawan Formation it has been mostextensively met with in the State of Mi chigan , especiallyin the Keeweenaw Peninsula ; it also occurs in the


metasomatic in origin . We may mention those ofAlgeria and of the Sonora district of Mexico .

The Algerian deposits occur in the province o f Constantine , in the neighbourhood of Jebel Hammamet .The ore , which exists chiefly as oxide

,is found in

i rregular layers running parallel to the beds of blacklimestone O f Lower Carboniferous age with which it i sassociated . These ores were at one t ime thought tobe of simple sedimentary character, deposited contemporaneously with the enclosing rock . I t seems

,

however, that the idea of a metasomatic replacement ofthe l imestone more closely approximates to a trueexplanation of the facts Observed .

The Mexican deposits are similar to the above inmode of occurrence , and are associated with a limestoneseries of approximately the same age . The ore consistschiefly of tlie hydrated oxide , Hn ZO 5 .

Like the natural sulphides of i ron , Copper , etc thesulphide of antimony is soluble in solutions of thealkal ine sulphides, but the above deposits were probablyformed by the action of calcium carbonate on thesolution of some antimony-acid .

The other deposits of antimony ores occurring inassociat ion with l imestones, especial ly those of thesulphide stibnite , seem to be intimately connected with ,and dependent on , igneous intrusions , and it i s doubtfulwhether they should be referred to the metasomatic orto all ied hydatogenetic deposits .

METASOMATIC MANGANESE DEPOS ITS .

Manganese i s a metal very similar to iron in itschemical properties , and it occurs associated in varyingquantities with most i ron ores , either as carbonate ,

METASOMAS IS 2 93

or anhydrous and hydrated oxides . As regards formation

,the ores of manganese are identical with those of

iron,being deposited from similar solutions by the action

of the same reagents .I n some of the metasomatic i ron deposits mentionedpreviously

,manganese occurs as the carbonate dialogite ,

the oxide pyrolusite and its products of hydration ,according as the corresponding i ron compounds existas chalybite

,haemati te , or l imonite.

I n North ’ Wales , manganese ores of metasomaticorigin are associated with the chemically similarmetals i ron

,nickel

,and cobalt , i n the Fl intshi re lodes

(P 2 52 )On the Continent , in the Odenwald , manganesedeposits are met with in the Permian (Zechstein)dolomites ; and noted masses exist in the Pyrenees , inthe Cabesse district , associated with the Lower Carbonifero us L imestone Series .The Lower Carboni ferous L imestone , or the Marbre

G riotte ,’ contains several masses of carbonate of man

ganese , which occur as replacements . The ore bodiesin depth pass into grey manganese ore

,dialogite

,and

,

as is usual with metasomatic deposits,present no well

defined boundaries with the countryrock . O ne massmeasures 197 feet in length , 164 feet in width , and hasbeen worked to a depth of 230 feet .In Italy , deposi ts of manganiferous haematite occur

in the districts of O rbetel lo and Monte Argentaro ,

associated with Triassic or Permian limestones . Theores contain about 35 per cent . of i ron and 18 per cent .of manganese , with a general absence o f sulphur andphosphorus .In the vicin ity of Santiago , Cuba , deposits of man


ganite , pyrolusite , braunite , and wad , are associatedwith j aspers , the whole being regarded as a replacementof calcareous sediments by means of alkal ine

,sil iceous ,

and manganiferous solutions .

METASOMATIC GOLD DEPOS ITS .

Owing to the strong attraction of ferrous compoundsfor oxygen , and thei r consequent reducing action , theyare occasionally employed to precipitate the metalsgold and palladium from thei r solutions . That sucha reaction has taken place in Nature in beds rich inferrous i ron there can be l ittle doubt , and has occasionallygiven rise to bedded auri ferous deposits .Well -marked gold -bearing horizons have lately beendescribed from South Africa , and thei r mode of occurrence points conclusively to the metal having had ametasomatic origin .

I n the central portion of the Lydenburg district ofthe Transvaal , auriferous horizons are met with , chieflyin the dolomitic series , which forms the middle divisionof the Transvaal System

,and lies between the Pretoria

Series above and the Black Reef Series below ; butsimilar horizons also occur in the upper and lowerse ries (see F ig .

The auri ferous horizons follow,with the greatest

regularity, the strike of the associated beds , and haveparticipated equally in all the folding and faulting thatthe district has undergone . I t is therefore certain thatthe introduction of the metal was at a period previousto the faulting . Along the ore horizons there i s ampleindication of the former existence of i ron pyrites inquantity , where it probably existed as an original precipitatio n . Subsequent oxidation has caused the ore to have


It is probable that the S i l iceous gold-bearing solutions, acting on the pyrites below the permanent waterlevel , where there i s a deficiency of oxygen , caused apartial oxidation of the pyrites , and a consequent metasomatic deposition O f the gold . Subsequent oxidationof the remaining pyrites by the lowering of the groundwater level , or by reason of its exposure at the surfacefollowing a period of denudation , has left the ore depositin its present form .

Less important ore horizons of similar characteroccur in the Black Reef Series associated with interbedded secondary quartz veins ranging from 2 to18 inches in thickness .I n the Pretoria Series three interbedded ore horizons

have been detected , but the two important beds of thedolomitic series occur respectively 1 00 and 300 feetbelow the base of the Pretoria Series .I n the Lydenburg and Carolina districts , all the

original ore bodies , excepting those of true sedimen

tary character,belong to distinctly bedded sheets , and

are met with on well -defined and constant horizons .I n the United States of America , a number of deposits

known as refractory siliceous ores are associated withrocks of Cambrian and Algonkian age , i n the BlackH i ll region of South Dakota , and stretch as a broadbelt from Yellow Creek to Squaw Creek . The oresconsist of chalcedonic si l ica and quartz , with some

fluo rspar, pyrites , etc . , and form flat banded masses ,in which the banding follows the bedding planes of theenclosing rocks ; the bands vary from a few inches to

300 feet in thickness (see F ig .

These deposits do not partake of the nature of veinswith well - defined walls, but they are associated with

METASOMAS IS 2 97

beds of dolomite,into which they pass by insensible

gradations . They are evidently metasomatic in character

,the solutions forming them having been intro

duced down small and almost vert ical fissures , on eachside of which the rocks have suffered considerablesilicificatio n .

Cambrian Im erwoua Cambrian Porp hy ryha l os m th

Bas a l Dol om ite BedsCong l om e ra te

FIG . 59 . D IAGRAM ILLUSTRATIN G TH E REPLACEMENT OF

LIMESTON E BAN DS BY S ILICEOUS GO LD ORE IN THE BLACKH ILLS D ISTR ICT, SOUTH DAKOTA. (AFTER J . D . I RVIN G .)

The chief mineral of the ore-mass, other than sil ica ,is pyrites, which on decomposit ion has given rise tohaematite and limonite , and caused a solution of theadj oining l imestone . Arsenopyrite , stibn ite , fluo rspar,and gypsum , also occur, and there has been a formationof brown mica .

These mineral masses compare i n certain respectswith the ores of the Lydenburg district described above .


It i s Obvious that the mineralizing solution was alkalineand highly si l iceous . The gold

,which exists either

nat ive or as the telluride,has been introduced into

these deposits by the same set of fissures , and has beendeposited from its solution through the influence ofsome base in the country - rock .

I n the Carboniferous L imestone plateau of CrownH i ll , also in Dakota , gold and silver ores occur in asimilar manner

,associated with secondary silica and

fluo rspar, as metasomatic replacements of the l imestone .

The precious metals probably exist as tellurides , as wasthe case with the Camb rian ores mentioned above .

I n the Lydenburg district of South Africa , the statein which the gold occurs is not known , and it is quitepossible that it may be in the form of tel luride , althoughso far there has been no mention of tellurium havingbeen detectéd .

METASOMATIC TIN DEPO S ITS .

The tin -bearing brown iron ores of the Campigl iaMarittima , in the i sland of E lba , are considered by someto be of metasomatic origin . Their true nature , however ,i s st il l a matter for Speculation , and it is quite possiblethat thei r occurrence may be due to some process otherthan metasomasis .I nstances of the metasomatic replacement of quartzite by cassiterite and tourmal ine have been described from the Rooiberg district , Transvaal . Thereplacements are believed to have been effected byhydrothermal solutions connected with the pegmatitesdescribed on p . 55 , and are therefore of exceptional

interest . The sedimentary format ion in which thedeposits are found con sists of alternating beds o f


deposited in the sandsto e , which somehow acted asa precipitant

,or there Were original vanadium and

uranium minerals in the sandstone , which were subse

quently decomposed .

Although uranium is generally derived from acidand vanadium from the basic rocks , the importantmineral carnotite contains both these elements .

CHAPTE R VI

BE DDED ORE S DUE TO PREC IP ITATION

BY precipitat ion i s meant the process by which certainconstituents of a solut ion are thrown out of that solutionin a solid form and in all cases it i s dependent on thel iquid being in a state of supersaturation with regard tothe substance undergoing precipitation .

The precipitated ore deposits of this class may bedivided into two groups . The first embraces all thoseminerals which are precipitated from solutions in whichthey existed as the same , or approximately the same ,chemical compound .

The Second includes those minerals which are theresult of the addition of a substance which reacts withsome salt of a metal i n solution , forming a compoundin respect to which the solut ion is in a supersaturated

state .We know from laboratory experiments that precipitation from a solution can be produced in a variety ofways

,such as by lowering the temperature , by the

evaporat ion of the solvent or its removal by some othermeans

,and by the addit ion of some substance which

forms with the metal in solution a salt with a less degree

of solub i l ity .

The concentration of a solution , and the consequentprec ipitation of its contents by a fal l in temperature ,

30 1


does certainly occur in Nature in the case of risingthermal waters but , generally speaking , such a processgiving rise to bedded o re-masses is of rare occurrence .The deposition of the metall ic ores i s more Often dueto supersaturation caused by the evaporation of the

solvent or the reaction of the solvent with somesubstance

,either sol id or l iquid , with which it happens

to come in contact . Most common of all , however, i s

the precipitation brought about when salts of certainmetals are converted into different and less solub lecompounds, either by the action of some other salt insolution or by the absorption of oxygen .

To take examples,let us consider the precipitation of

certain carbonates, sulphides , and oxides .Several metall iferous carbonates , such as those of i ronand manganese , are held in solution by water chargedwith carbon dioxide

,thei r solub i l ity in pure water being

small . On the extraction of the carbon dioxide froman aqueous solution of such salts , ferrous and manganous carbonate will be precipitated i f oxidation isprevented . Such a reaction may give rise to beddeddeposits of these compounds , and at the same time isthe cause of the stalactitic and stalagmitic ores O f i ronand manganese , although these have most often undergone more or less complete oxidat ion . To take examplesof another type , the sulphides of copper and lead willbe precipitated from aqueous solutions of thei r sulphateson the reduction of these salts by organic matter or bythe action of sulphuretted hydrogen on their solublesalts in an acid solution . I n the case o f oxides , acommon form of precipitation is that of ferric oxideand ferric hydrate from solutions of ferrous carbonatewhich are absorb ing oxygen from the ai r . Magnetite i s


with the bedding of the enclosing rock,and thei r recur

rence at more or less frequent intervals on slightlydifferent horizons . The upper and lower l imits Of suchore -masses are most often clearly defined , and do notshade imperceptibly into the adjoining barren rock , asdo the metasomatic deposits previously described . I nthe case of the impregnations of unmetamorphosedshales and sandstones with

‘

minute patches of orematerial , i t i s Obvious that the ores could havebeen formed by no process other than precipitation , possibly helped subsequently by the process ofconcretion .

PREC IPITATION AS O ! IDES .

IRON AND MANGAN E SE .

The Chief deposi ts of oxide of i ron which owe thei rorigin to direct precipitation are those occurring as lakeor bog i ron -ore .The i ron is thrown out of solution as a mixture ofhydrated oxides by the agency of algae and bacteria ,which have the property of extracting i ron from solutions of its carbonate and sulphates .Ferric hydrate may, of course , be deposited from iron

solutions by the simple process of oxidation , as in thecase of the carbonate

4F6CO 3 02 3H ,

o F6, (OH )6 F6

203 4CO , ,

or by the action of some salt of an organic acid , such asammonium humate

,on a ferrous solution bicar

bonate of i ron) .Deposits of ferric oxides of sedimentary character arewell known in many districts , amongst which we maymention those of Lusace , Silesia , Banat , Jutland , etc .

PRECI PITATION 56 5

The lakes of Scandinavia,however , especially in the

provinces o f Smi land ,Vestrago ethland , and Dalarne ,

offer some of the best examples, and are most instructive , seeing that the actual formation of the ore can bewatched and studied .

I n the Scandinavian lakes the precipitation is broughtabout chiefly by fresh -water algae , to which the namesL apto thm

'

x o chm cea, Ga l lionel la fermginea ,

etc .,have

been given , and the iron is thrown down enti rely as

hydrated oxide .i

The alga Ga l lionél la flourishes at adistance of about 1 0 or 1 2 yards from the shore , and toa depth of 30 feet below the surface of the water . Theore deposit consists of a slimy mass of hydrated i ronoxides and gelatinous si l ica , and ranges from a fewinches up to I% feet in thickness .

After a time a concretionary process begins to actwithin the unconsolidated mass , with the formation ofool itic or pisol it ic granules— a feature which is mostinstructive and helpful when considering the origin O f

some of the pisolitic i ron ores associated with the

Mesozoic and Tertiary sedimentary rocks .The Swedish lake deposits are worked even whenonly a few inches thick , and it is interesting to note ,with regard to the rate of formation of these ores , thatthey renew themselves in a period of from fifteen tothirty years .I t has already been mentioned that magnetite is amineral rarely to be accounted for by original deposi tionfrom solution . There are , however , certain i ron orebeds which contain this mineral in some quantity, andin which i t was evidently deposited by the agency ofwater under normal condit ions . The occurrence ofsuch deposi ts as the magnetic ore of Rosedale , in York

2 0


shi re , and the magnetite in the hydrated ores of theRhine provinces

,i s a difficult thing to explain . I t must ,

however , be borne i n mind that a magnetic hydratedoxide of i ron , with the composition FeO ,

Fe20 3t O ,

can be made artificially by treating a ferrous saltwith a solub le ferric compound in the presence of analkali . Here

,however

,as in the case of the car

bonate of zinc,there seems a tendency in Nature to

form the anhydrous compound if possib le . I t may bethat the slowness with which the reaction takes placein Nature i s a controll ing factor, or i t may be that theloss of water is governed by the shrinkage of the massunder pressure . The value of the bog iron ores i sgreatly diminished by thei r relatively high percentage ofphosphoric acid .

Manganese deposits occur either alone or in association with iron compounds . We get bog -manganeseS imilar to bog - i ron ; and the bedded oxides , psilomelaneetc .

, have an identical origin with limonite and theother hydrated oxides of i ron .

With regard to the source of these metals, what wassaid with reference to the solutions which gave ri seto metasomatic replacements i s equally true for the i ronand manganese precipitations .The i ron was derived chiefly from the ferromagnesiansil icates of igneous rocks and from the pyrites of sediments

,decomposing under the influence of carbon

dioxide,alkal ine carbonates

,and vegetable acids , or by

partial oxidation . The source of the manganese , also ,lay chiefly in the ferromagnesian sil icates , many ofwhich contain this metal in some quanti ty. Manganesemost often seems to have been carried in acid solutionsrich in sil ica

,the precipitat ion of the oxides generally


tates may be cited from several districts and severalgeological horizons . I ron ores occur in the Ool ites ofRosedale in Yo rkshIre, i n the Corall ian of Westbury inVViltshire , and Abbotsbury , in Dorsetshire , and in theLower Cretaceous of the E ast of E ngland . The oresare mostly ool itic or pisolitic in character

,and consist

chiefly of hydrated oxides, with a small proportion ofcarbonate . The Rosedale deposit has the somewhatunusual feature of being magnetic , but in this respectis similar to some of the oolitic brown ores of Franceand the Rhine provinces , which also

’

o ccur in the MiddleJurassic strata .

I n E urope some of the ferri ferous deposits , l ike those

Of the Lias , Inferior Oolite , and Lower Cretaceous , seemto be due in part to original precipitation , and partlyto metasomatic replacement . The pisolitic ores especially seem to have the characters of original sediments .In the departments of Maine and Loire, haematite

occurs interbedded with the Lower Palaeozoic sediments .The deposits of Calvados occupy a position abovethe Armorican G rits , and below the Ca lymene- Shales Ofthe O rdovician System . They are about 6 feet inthickness

,and consist of haematite and limonite with

some carbonate .An oolitic i ron ore occurs in the syncl inal area of

Prague,in Bohemia , above the Przibram Quartzite . I t

contains some silica and a rather high percentage O f

phosphoric acid .

Similar ores,but stil l more impure , are found in

Andalusia , associated with Lower Palaeozoic sediments .Amongst the deposits of later date we may mention’

the oolit ic ores of the Wealden and N eocomian strata

PREC I P ITATION 36 9

of France,which are well displayed in the Bas -Bou

lo nais and the district Of Vassy respectively .

The ores in the Wealden deposits of Kent , associatedwith the Wadhurst Clay , seem to approximate mostclosely to a clay ironstone , and thus present characterswhich would group them with the bedded hydrous carbo nates of the Coal -Measures , to be subsequentlydescribed .

Brown iron ore of oolitic character forms a bed 30to 50 feet thick in the Tertiary deposits of the KetchPeninsula in Russia

,I t yields 38 to 42 per cent . o f

i ron , and 2 to 4 per cent . of manganese . At the sametime it has an except ionally low percentage of phos

pho ric acid .

Amongst recent deposits the fresh -water bog - i ronores are the most important . They occur , i n al l northtemperate regions , where ferruginous waters are moreor less stagnant in Shallow depressions of the surface ,and kept at a more or less constant level owing to theimpermeab i l ity of the subsoil .Such deposits are extremely prevalent in the lowlands o f North E urope . They are found in SouthernScandinavia , in the low- lying country of Holland , onthe drift plain of Northern Germany

,in Poland , and

Russia .

I n the British I sles,bog - i ron ores occur in several

districts in I reland in the central plain,and in the

Shetland Isles . They contain a variable quantity offerric oxide , ranging from 20 to 78 per cent . in differentgrades of ore . All these precipitated oxides of i ron andmanganese are associated with a good deal of phos

pho ric acid , sometimes as much as 9 or 10 per cent . , afeature which detracts seriously from thei r value .


The precipitates of oxide of manganese in the formof pyrolusite the anhydrous oxide

,psi lomelane the

hydroxide , manganite , and the earthy hydrated varietiesgenerally styled wad ,

’ are common,but not so widely

distributed as the equivalent ferruginous compounds .

Manganese in small quantit ies is associated with almostevery aqueous deposit Of i ron ore

,but its percentage

occasionally rises in excess of the i ron , giving rise totrue manganese deposits . Nodules of manganese oreare well known in recent deep - sea accumulations.

O n the Continent , some of the ores of Nassau andWestphalia are undoubtedly original sediments

,but

thei r character has been more or less completely changedby secondary local enrichments and by metasomaticreplacements of the countryrock .

As in the case of the i ron ores , i t i s in the oolit icdeposits that we have the clearest evidence of contem

po raneo us formation of manganese ores .I n Spain and in the Caucasus

,deposits of ool itic

character are well known . The Caucasus ores belong .

to the E ocene or Miocene ; they occur in the KvirilaValley in a series of beds of sand and sandstone resting on the Cretaceous limestones . The similar Miocenedeposits of Spain occur

,amongst other places , in the

Val de Pefias, in the province O f Ciudad Real .The chief ores of the United States of America arethose located along the border of the Appalachianregion , i n the States of Virginia , Georgia , and Arkansas .They occur above the Potsdam Sandstone of LowerPalaeozoic age

,and

,although now existing chiefly in

the form of oxides , were probab ly deposi ted in thefirst instance as carbonate . The manganese originallyexisted in a series of l imestones and shales , over


The beds are , for the most part , almost horizontal .Thei r ore-bearing layers are thin , ranging from a2 inches to 2 feet in thickness , and yielding a vaquantity of copper .I t i s quite possible that these oxides may be ori

precipitations ; but at the same time , chemically csidered , i t is more probab le that they have beenrived from the sulphides by a subsequent processoxidation .

The red and black oxides of Copper are oftenwith in the upper and oxidized part of lodes whichcarry Copper pyrites (p .

ALUMINA AS BAU! ITE .

Chemical deposits of bauxite which appear to beindependent of metasomatic action occur in severaldistricts i n E urope and the United States . They seemto have originated from igneous masses by the processof weathering , and to be derived largely from thefelspathic ingredients of these rocks .

The first stage is the formation of the aluminiumsil icate , which subsequently , under the influence ofalkaline carbonates in solution

,parts with its sil ica and

deposits hydrated aluminium oxide .The Chemical nature O f these deposits i s clearlyindicated by thei r pisolit ic and concretionary structure ,the spherules occasionally reaching 1 inch in diameter.The bauxite deposits of Arkansas , in N orth America ,have been well described

,and thei r mode of occurrence

is well known .

Arkansas is a district which may be roughly dividedinto two by a north -easterly and south -westerly lineseparating the O lde r Palaeozo ic rocks of the north-west

PREC I P ITATION 313

of the State from the Tertiary and Recent deposits of thesouth -east . The Palaeozoic rocks , consisting chiefly ofl imestones and shales, have been folded , and subse

quently to the folding intruded by masses of normaland elaeolite syenite .

The bauxite forms a more or less co'

ntinuous sheetover the surface of these igneous masses , and extendsbeyond them on the plane of denudation . I t is evidentthat they had thei r origin in a mass of syenitic detritus ,and were formed before the deposition of the Tertiarysediments .I t wil l be seen from the above that the bauxitedeposits occupy the position of a great unconformityin other words , occur on a surface which was undergoing denudat ion for a considerable period .

In South E urope,along the coastal country of the

M editerranean , pisolit ic bauxite deposits of considerable extent occur in a similar position . They restindiscriminately on beds ranging from the Rhaetic inHerault to the Gault at Reveset, and are overlain by theCenomanian H ippurite Limestone

,and by fresh-water

deposits of Danian age .

I n the Auvergne , bauxite occurs as a covering ofgneissose rocks

,and has probab ly had a similar origin

to the deposits of Arkansas.Certain deposits associated with basalts (p . 264) aresupposed by some to have had a metasomatic origin .

Their mode of formation , however, i s sti l l unsatis

facto rily explained , and they may quite possib ly proveto belong to the above class .Bauxite deposits of great thickness , ranging up to

1 2 feet , occur in the Wochein district of Austria ; andthey have also been worked in P iedmont .


PREC IPITATIONS AS CARBONATES .

IRON AND MANGANESE .

The carbonates of i ron and manganese are occasionally found associated with sediments in a manner thatpoints clearly to thei r having been deposited by chemicalmeans , contemporaneously with the enclosing rock .

They form well -defined layers parallel to the stratificatio n planes of the beds in which they occur ; andthere i s no evidence of metasomatic replacement of thecountry-rock , except , perhaps , i n certain secondary en

richments of the mass by subsequent solution andredeposit ion of the ores . I ron , manganese , and magnesium form S imilar carbonates in which any one ofthese metals may partial ly replace another .The carbonates of i ron and manganese are solubleto some extent in water containing an excess of carbondioxide , and where oxidation is prevented by thepresence of some reducing agent , such as decayingvegetable matter. Such solutions

,on undergoing a

certain amount of concentration and loss of carbondioxide , would deposit the metals as carbonates .Salts of i ron and manganese existing in the ferricand manganic states

, under the influence of decom

posing organic matter would be reduced to the correspo nding ferrous and manganous compounds , and theaddition of an alkaline carbonate would produce thespontaneous precipitation of the hydrous carbonates ofthe metals . I t i s most probable that the alkalinecarbonates have played a very important part in theprecipitation of the clay ironstones described below ,

but many of the manganese deposits are best explained

316 THE GEOLOGY OF O RE DEPOS ITS

The North Staffordshi re ores are dark brown to blackcompact masses

,varying from a few inches to 2 feet

in thickness . They invariably overl ie a seam of coal ,the th ickness of the ore varying inversely as the thickness of the latter . Compared with similar deposits ofother districts

,they are richer, the percentage of i ron

being given as— ferrous oxide , 40 to 46 per cent . ;ferric oxide

, 4 to I 5 per cent .

In all districts other than North Staffordsh ire it isextremely difficult to trace any connection between thebeds of i ronstone and the coal - seams , but that theyhad an identical mode of formation in all cases therecan be no doubt . They were evidently deposited asa ferruginous mud

,which has subsequently undergone

much dehydration and contraction . I n some of theconcretions (SphaerO - siderite) an organism , such as aportion of a plant or animal , has acted as a nucleus ,and the evidence of contraction is furnished by manycracks and cavit ies now fi lled with calcite or quartz .

Almost all the spathic ores contain some manganesewhich also exists as carbonate , and would have beenprecipitated simultaneously with the corresponding i roncompound .

In Coal-Measure - t imes it i s probable that the surfacewaters were richer in mineral matter than at almostany other period . The dense vegetat ion would giverise to humic acids in great quantity, and , as i s wellknown , these acids , from the ease with which theyoxidize , are some of the most powerful natural solvents.On the Continent , deposits of exactly similar nature

and mode of occurrence are associated with the UpperCa rboni ferous rocks of the Ruhr basin in Westphalia ,i n the neighbourhood of E ssen and HOrde ; in the

PREC I PITAT ION 317

lower part of the Coal -Measures in the basin of Saarbruch

,in Upper Silesia ; and in the basin of the Loire ,

in France .Identical clay ironstones occur in the productiveCoal -Measures of the United States of America, especial ly in the eastern State of Pennsylvania and in theAppalachian region . Spathic ores are also met with in

Nova Scotia .

In deposits of later date , ore-masses of this characterare occasionally met with— as

,for instance , in the

Keuper and Jura of Upper Silesia . I n E ngland anexample may be cited from the Lower Cretaceous bedsof the Weald

,i n Kent .

With regard to the deposits of carbonate of manganese , i t i s not so easy to put forward trustworthyexamples. In our own country , the best , perhaps , i sthat of Merionethshire , where , i n the neighbourhood ofBarmouth , manganese carbonate occurs on two distincthorizons : one just above , and the other j ust below , thegrits , which form a well -marked series in the lower partof the Cambrian System .

The ores have been extensively mined all along thel ine of outcrop in a series O f shallow workings . Theywere presumab ly deposited from solution as carbonates ,and as such they occur in depth ; but near the surfacethey have passed into various hydrated oxides in theprocess of weathering .

Manganese carbonate , as an original deposit , exists aslayers in the Devonian beds of the Pyrenees , in theO ligocene of Mahren , and at a few other localit ies inE urope .Besides the deposits of almost pure manganese car

bonate , such as those mentioned above , almost all the

318THE GEOLOGY OF ORE DEPOS ITS

- Mo elf’

re Grits.

- BlownSand .

- b’figfif17f7a§a - Harlu h Gri ts

Manganese beds ind ica ted by Hrich black line .

FIG . 60 .

—MAP SHOWING THE MAN GAN E SE -BEAR ING HOR IZON S

IN TH E CAMBR IAN ROCKS OF MERION ETHSH IRE , N ORTHWALES. (AFTER GOODCH ILD .)

Scale , 4 miles to I inch .

bedded masses of carbonate of i ron contain a certainamount of manganese in a similar state . I n these cases


rise to either by the action of sulphuretted hydrogenon metallic solutions or by the reduction of solublesulphates .The metals which most O ften exist as sulphides , occa

sio nally with some arsenic , are those O f i ron , Copper ,lead , and zinc , with sometimes nickel , cobalt , si lver,and antimony . The ore occurs either as impregnationsof some sedimentary rock , i n which it is finely disseminated as crystals or grains , or as more or less continuo us bands intercal lated between two less metall ifero us layers . Occasionally the place of a continuoussheet of ore is taken by a layer of concretionary nodules .The origin of the metall ic solutions has already beentouched upon , but the cause of the precipitation is notso Obvious . There seems

,however, to be two more or

less distinct ways in which the deposition may be produced .

As was pointed out in the case of the oxides , manyorganisms living in water are capable of decomposingsulphates , with the l iberation of sulphuretted hydrogen .

This gas , acting on the metall ic solutions , will accountin a measure for much of the pyrites disseminatedthroughout a variety of deposits

,such as the Oxford

and Kimmeridge clay . At the same time the pyrites ofmany of the shales , S lates , and clays , is evidently to beaccounted for by the direct reducing -action of decayingorganic matter

,either vegetable or animal , and as a

proof we find pyrites replacing the soft parts of fossi lsi n sedimentary deposits of almost every age . Pyritesreplaces graptolites in the O rdovician and SilurianSystems , plants in the Coal -Measures , mollusca in theMesozoic rocks , and in recent times impregnates thetimber of O ld mine levels, . all changes brought about


by the reducing agencies of organic compounds . Manyclays which now contain sulphides most Sparingly can beproved to have previously held them in much greaterquantity . The London and Kimmeridge clays , for instance

,are full of gypsum in large crystals, which have

been formed by the action of sulphuric acid , ari singfrom the oxidation of pyrites, on the calcareous testsof organisms . In a non -calcareo us _

c lay the oxidationof pyrites will take place with the formation of alum

shales .There are a few additional points in connection withthe precipitation of sulphides which are of interest . I nthe above l ist of metals it will be seen that there aretwo classes included— one contain ing the metals i ron ,nickel

,cobalt

,and zinc

'

,which are usually only pre

cipitated as sulphides from alkaline solutions , whilstthe others may be thrown down from acid solutions oftheir salts .I n the presence of organic acids , however , and underincreased pressure , i t must be remembered that thegeneral rules of precipitation have many exceptions

,

and that organic acids undoubtedly play a most important part in N ature both in the solution of the metalsin the first instance , and in governing their subsequentprecipitation .

To take examples , nickel and cobalt are precipitatedas sulphides by the action of sulphuretted hydrogen onthei r soluble salts of organic acids in the presence ofthese acids In a free state . Zinc sulphide

,on the other

hand , may be precipitated from a feebly acid solutionunder increased pressure .Manganese sulphide i s rarely found in N ature

,for i t

i s one of the most so luble compounds of this class ; i t2 I


is only precipitated in the presence of alkalies , andits precipitat ion is thus completely prevented by thepresence of free weak acids . Manganese as sulphidedoes occur , however, in combinat ion with the sulphidesof other metals .

Copper and lead form soluble , and at the same timestable , salts with hydroxy organic compounds , which ,although resi st ing the action of many of the usualprecipitants , are readily decomposed by sulphurettedhydrogen

,the metals separating out as sulphides . The

ease with which almost inso lub le metall ic salt s passi nto solution in the presence of certain organic compounds is exemplified by the case of lead sulphate ,which is practically insoluble in water

,but i s readily

dissolved by a solution of ammonium tartrate .Turning again to the natural deposits of sulphides ,and considering the metals which occur together , weare constantly met by a certain amount of confl ictingevidence , whether we consider the solution from whichthey are deposited to have been acid or alkaline i n reaction , and it i s almost impossible to form any definiteconclusion as to the true nature of the solution .

There i s no reason for supposing , however, that alldeposits of sulphides have been derived from one typeof solution , and it i s even possib le that at a considerabledepth the lowest stratum of a sheet of water, owing toS low diffusion , might have a different reaction from the

main mass .Taking into consideration a large number of cases , i tseems most reasonable to favour the idea that the precipitatio n of sulphides has chiefly taken place fromslightly acid or neutral solutions through the agency ofsulphuretted hydrogen l iberated by living organisms, or

324 THE GEOL OGY OF ORE DEPOS ITS

in the Tertiary deposits, while nodules of a concretionarycharacter are abundant in certain zones of the Chalk .

On the Continent , the most famous deposi ts arethose O f Rammelsberg , in the Northern Harz Mountains,occurring in rocks of Middle Devonian age . They formlayers up to 1 5 yards i n thickness , parallel to the beddingof the enclosi ng rocks ; but the whole sedimentary serieshas undergone extensive pl ication . The ore consistsof a mixture of sulphides , including pyrites , Copperpyrites

,zincblende, and galena , with a certain amount

of barytes . These ores,however , are by some regarded

as of secondary origin .

An almost equally important district is that of Meggen ,in Southern Westphalia

,where the ore i s also asso

ciated with deposits of Devonian age . I t occurs betweenthe lower shales of the Upper Devonian (Lenne Shales)and the Middle Devonian limestone -group .

As at Rammelsberg,i t contains other sulphides, such

as zincblende , etc ., and accessory barytes . Analyses

Show about 35 per cent . of i ron and 44 per cent . ofsulphur, some zinc , but little or no copper and lead .

I n both the cases cited above,although a metasomatic

origin has been claimed for the ores , thei r beddedcharacter, their association with bituminous material ,thei r oolit ic structure , as well as the presence of pyritesconcretions in the neighbouring beds , point more orless clearly to thei r having had a sedimentary origin .

I ron pyrites also occurs in abundance associated withthe Tertiary brown coals of Germany .

CO PPER AS CO PPER PYRITES .

Copper pyrites as a mineral of sedimentary origini s Of wide distribution , exist ing not only on many


geological horizons,but in widely separated districts .

One horizon,however

,more than all others In E urope ,

is famous for its copper deposits— namely, that of the

so -called Kupferschiefer . ’

The Kupferschiefer are bitumIno us shales and sandstones o f Permian age , and form a central memberof the Lower Zechstein , lying between the ZechsteinL imestone above and the Zechstein Conglomeratebelow

,which rests unconformably on older Palmo zo ic

rocks . The Kupferschiefer are remarkable , also , forthei r richness in animal remains , chiefly fish , and thei rconstant characters over wide areas ; thei r marine origini s clearly demonstrated by the presence of such fossilsas brachiopods and echinoids .The ore occurs as finely divided grains and crystalsof small size ; i t consi sts chiefly of copper pyrites , butit contains also some galena and o ther sulphides . Themost important min ing district i s that of Mansfeld

,in

the Lower Harz Mountains . The ore - layers are extremely thin —less than 2 feet in thickness— and thepercentage of ore seldom rises above 3 ; but , at thesame time , this district has had an annual yield ofabout tons of copper , as well as a fai r quantityof si lver .Although the Mansfeld district is now the onlyimportant area where the ores of the Zechstein areworked , the Kupferschiefer stretch into Hesse and

Westphalia , and at one time were extensively mined atRiechelsdo rf, Bieber , and near Saalfeld .

There is no doubt that these ores owe thei r origin tothe reduction of metall ic sulphates by means of decomposing animal and vegetable matter , of which there i s-such striking evidence .


In other countries similar deposits occur in NorthAmerica on a S imilar horizon . I n Texas, copperpyrites, locally altered by atmospheric agencies intomalachite and azurite , occurs associated with bituminousmaterial in the Wichita and Clear Fork beds , a seriesof deposits very like the Central E uropean Zechstein inorigin and appearance . There are three chief horizons— One i n the Wichita beds , and the others in the ClearFork group . Mention has already been made O f thecopper-bearing sandstones , such as those of Perm

,

Bohemia , the district between Aachen and Saralo uis,and of Corocoro in Bolivia , where it is supposed thatthe ores were originally deposited as sulphides , althoughthey now exist chiefly as oxides and carbonates . Theidea of precipitation as sulphides is strengthened bythe presence of b ituminous matter and large quantitiesof sulphates

,such as gypsum and barytes .

Yet another type of deposit , consist ing essentially ofthe sulphides of copper

,occurs associated with volcanic

tuffs and breccias of andesi tic composi t ion . The Boleodistrict of Lower Cali fornia O ffers us the best example .

The district of Boleo is an elevated plateau dissected bydeep valleys

,along the sides Of which outcrops a series

of almost horizontal volcanic rocks , largely of a fragmental character . These beds Of tuff and agglomerate ,which may be classed with the andesitic and trachyticgroups of volcanic rocks, contai n several Copper-bearinglayers varying from 2 to I O feet in thickness. Thereis no doubt that these beds were laid down under water,and it is more than probable that the ores are trueprecipitations . The metal exists as sulphide , but wealso meet with large amounts of secondary compounds ,such as the oxides

,chloride (atacamite) , si l icate (chryso


clays , there are no examples of this class . But wherethese two sulphides are met with

,they always occur

surrounding or replacing the soft parts of someorganism . As we have already seen , sulphides of thesemetals are associated with the copper ores of theKupferschiefer of Central E urope

,and were probably

present to a considerable extent in the copper-bearingsandstones of Bol ivia , St . Avold , and other districtsin fact

,in most areas from which copper ores have

been cited .

N ICKEL AND CO BALT AS PYR ITES , ETC .

N ickeli ferous and cobalti ferous pyrites,as well as the

sulphide millerite , are o ccasIo nal ly met with in connectio n with other sulphidic ores . In Britain theseminerals have been noticed associated with the pyritesand spathic i ron deposi ts of the Coal -Measures , whilen ickel and cobalt in an undetermined form have beenshown to exist in the precipitated i ron ores of otherhorizons .

GOLD IN S INTERS .

Gold in a native state occurs as precipitations i ncertain highly sili ceous accumulations formed at thesurface under hydrothermal condit ions . The gold isprobab ly carried in solution by sulphate of i ron , and onthe decomposition of this compound

,on reaching the

surface,i s deposited with the gelatinous sil ica and iron

oxides . A good example of such a deposit i s thatof Mount Morgan in Queensland , Austral ia , where asil iceous sinter impregnated with haematite yielded asmuch as 1 70 ounces of gold to the ton . The district ofMount Morgan is largely composed of dioritic rocksassociated with rhyolites . The former , as we have seen ,

PREC I P ITAT ION 329

are the chief original gold-carriers,and in this case also

were probably the source of the metal .

Similar deposits,but o f less economic value , occur in

N evada and in the Yel lowstone Park . I n both casesthe sinter has been deposited from highly alkaline andsil iceous water connected with rhyol itic rocks , whichform a feature of these districts .It must be born in mind that some of the so -called

quartz- reefs occurring amongst the older rocks might

have been given rise to by a similar process.

PREC I PITATION As S IL ICATES .

Occasionally the metals,i ron

,manganese , and zinc ,

are precipitated in the form of si l icates from aqueoussolutions , but thi s class Of deposit i s , as a rule , of nogreat importance . We have seen that many metall ifero us si l icates associated with metasomatic depositsare really of a secondary character

,and result from the

alteration of other metall ic compounds by alkal i nesil iceous waters . There are

,however

,cases where

silicates may exist as original precipitations . They areprobab ly derived directly from igneous rocks , passing intosolution during the process of weathering . They weredeposited , as was the case with the oxides and carbonates O f i ron and manganese

,as more or less spherical

grains or as continuous sheets , the precipitation beinglargely brought about by the loss of carbon dioxide fromthe solution and the neutral ization of the alkalinecarbonates .I ron in the form of sil icate has been precipitated in

the Huronian Series of the Lake Superior region,i n the

Mesabi district . The sil icate has been cal led greenal ite , and occurs as pale green grains , having an appear


ance and mode of origin similar to the glauconite of theMesozoic and other deposits .Manganese si l icate has been worked in the Louder

viel le and Genu districts O f the Pyrenees . I t occursinterstratified with beds of Devonian age , and on morethan one horizon . Locally i t has been converted intocarbonate and oxide by the action of carbonated andoxygenated waters .


subtraction O f some ingredient , such as sil ica,carbon

dioxide , or water, producing a considerable change inthe composition of the rock as a whole Such depositsas these are not regarded as truly metamorphic incharacter, and will be dealt with in the chapter onSecondary Changes ; we shall therefore restrict theterm ‘ metamorphism ’ to express those processes bywhich the changes of the second class are broughtabout , and Shall include under that heading thosereconstructions produced in a rock -mass by the influence of a high temperature

,or by shearing stresses

of sufficient intensity to generate heat .The general effect of raising a rock-mass to a con

siderable temperature above the normal i s to bringabout a decomposition and destruction of several ofits component minerals

,and to produce new compounds

by a process of exchange between the various acids andbases present In the rock ; and it i s therefore obviousthat such changes can take place without producingany marked alteration in the bulk-analysis of the rockmass

,except

,perhaps, the partial loss of water or some

other volati le constituent .I t follows that the new minerals formed during theprocess of metamorphism will be almost entirely dependent On the original Chemical composit ion of the rockand the temperature to which it is rai sed .

From a study of the metamorphic minerals present inany rock

,i t is possible to form a very accurate conclusion

as to the nature of the rock before metamorphism ;

whether,for instance , it was an almost pure l imestone ,

a sandy limestone,a shale , or a sandstone , for in each

of these cases the new minerals produced by the rise intemperature will have a distinctive character . At the

METAMORPH ISM 333

same time , many of these new minerals give us anindication of the relative temperature to which rockmasses have been raised .

The heating necessary to produce the rise in tempera

ture in a rock,and to bring about metamorphic effects ,

may arise in two ways— either by the intrusion of afluid igneous magma into the cooler regions of theearth ’s crust

,causing what i s known as an ‘ aureole of

contact metamorphism or it may arise from the internalheat of the earth , and affect large masses of rock Whichare buried at a great depth below the surface , producingmetamorphism of the regional type .

Thermal metamorphism is hardly ever a superficialphenomenon , but in the course of ages rocks whichhave been altered in this manner have been laid bareat the surface owing to the upheaval and denudation ofthe superincumbent strata .

To give an instance of the process of thermal metamorphism , let us consider the action of heat on a buriedmass of impure l imestone contain ing calcium andmagnesium carbonates , free silica in the form of quartz ,and alumina . At a higher temperature and underpressure , calcium and magnesium carbonate in thepresence of S il ica will give rise to calcium and magnesium sil icates such as forsterite

,calcium aluminum

si l icates such as garnet , idocrase , and diopside , andaluminum silicates such as cyanite , andalusite , andcordierite , according to the proportion O f each originalconstituent , and according to the temperature to whichthe rock -mass is raised .

Besides the formation of enti rely new minerals,

metamorphic act ion shows itself in the dehydrationof many hydrous compounds and hydroxides of the

334 THE GEOLOGY OF O RE . DE PO S ITS

metals, and by the simple recrystall ization of alreadyexisting stable minerals .The l imit of the migration of material during theprocess of metamorphism is exceedingly small

,and

therefore the interchange and comb ination of acids andbases in the original rock takes place only at very closerange .A rock such as a fai rly pure l imestone undergoingmetamorphism will s imply recrystallize after all itsS i l iceous and aluminous impu rities have acted on anequivalent quant ity of limestone , with the formation ofcalcium aluminium si l icates . The presence of forsteriteindicates that the original rock contained magnesia ,probably in the form of dolomite . Garnet , idocrase ,and diopside

,in al l cases indicate a somewhat impure

l imestone as the parent - rock ; while such minerals asandalusite , Cyanite , and cordierite , point with equalcertainty to argillaceous sediments .The ore deposits which existed in areas of sedimentary rocks previous to thei r metamorphism have

,

under the influence of heat , suffered distinct changes ;for instance

,the l imonites and the hydrated i ron ores ,

as a class , have been converted into magnetite by lossof water and recrystall ization . Pyrites may remainunchanged or be recrystall ized , but , i f originally mixedwith other sulphides , i t may comb ine with them toform the complex compound fahlore .The impure ores

,such as the earthy oxides of i ron

and manganese,which contain a large quantity of

si lica,may under certain condit ions give rise to the

si l icates of these metals .I t has been stated that the metamorphic minerals

present in any rock are a perfect indication of the


divided , according to thei r mode Of occurrence, into twosubclasses first , those of the crystall ine schists inwhich they occur as layers and lenticles, and whichinclude many important deposits— haematite , magnetite ,and metall ic sulphides . The second subclass includesthose larger i rregular ore -masses which probably occuras metamorphosed metasomatic deposits , and also manyof the so -called metasomatic contact-deposits and veinsin which the countryrock contains characterist ic metamorphic minerals .

THE ORE DEPO S ITS O F THE CRYSTALL INE SCH ISTS .

The ore deposits in the crystall ine schists, as we nowsee them

, are probably the result of metamorphic actionof a regional type on original sediments containingbands of metall iferous material deposited either bydirect precipitation or by metasomatic replacement .Their sedimentary origin is supported by their appearance of strat ification , and by the fact that a metall i ferousdeposit i s often confined to one horizon over a large

area .

DEPOS ITS OF O ! IDES (HE MA’

I‘

ITE AND MAGNETITE) .

These two minerals among the i ron ores, especiallythe latter

,are o f most frequent occurrence in meta

morphic rocks , haematite being produced by the dehydration of l imonite ; and magnetite by the alterationof ferrous carbonate by loss of carbon dioxide , or by thepartial reduction of hmmatite .

The derivation of magnetite from ferrous carbonatei s in most cases to be inferred from the mineralogicalcharacter of the rock which encloses it .

METAMORPH ISM ‘

337

The country -rock of magnetite deposits has almostinvariab ly been calcareous , and the metamorphic minerals which it contains are most often , therefore , thecalc -si l icates . Haematite , on the other hand , whichhas most O ften had its origin in l imonite , occurs in a

more sil iceous gangue .Haematite and magnetite are O ften interbanded

with the crystall ine schists and granular gneisses insufficient quantity to be of great economic importance ,and such deposits occur in almost every district whereArchaean rocks are exposed at the surface .Some of the most important are those of Scandinavia ,where the thin masses of haematite and magnetite ,associated with mica schists and similar rocks, arewell known and exploited— as

,for instance , those of

Arndal and the province of H elgeland , in Norway .

The North Ranen deposits i n the province of Helgelandoccur in ferriferous schists and gneissose rocks , as

layers of specular haematite and magnetite in varyingproportions . They hold an average of 13 per cent . ofmetall ic i ron , and , although most often in thin beds ,occasionally reach a considerab le thickness .

DEPOS ITS OF SULPH IDES .

The chief sulphide deposits O f the crystal l ine schistsare the so -called fahlbands

,

’ so well developed in Scandinavia . Although , perhaps, of no great importance inthemselves , they have been the source of the mineralmatter which has enriched the lodes crossing the district. These deposits contain a great variety of metall icsulphides , either free as Copper pyrites

,i ron pyrites

,

zincblende , or galena , or comb ined , together with some2 2


antimony and arsenic , to form the complex mineralfahlore or tetrahedrite .

One of the best examples of the fahlbands may bedrawn from the district of Kongsberg

,i n Norway

,

which includes the mining areas of Overberg andUnderberg . I t i s a region situated to the south -westof Christiania , and composed of gneisses , mica schists ,and hornblende schists , i ntruded by massive igneousrocks . The direction of the fahlbands and the fol iationof the district is north and south , and the dip is at asteep angle to the east . The mineral deposi ts occurin zones of impregnation , and the sulphidic material ,mostly pyrites

,is very finely divided . The Kongsberg

district also f urnishes a good example of the enriching influence of the fahlbands on fissure veins whichtraverse the country .

I n other parts of Norway and Sweden the fahlbandsyield valuab le ores of Copper, such as those of ROraasand Foldal

,and cobalt exists as sulphide in the de

posits o f Skutterud and Snarum , in Southern Norway .

I n all cases a variety of sulphides are present , butgenerally one or two metals are present in quantity ,and thus characterize any particular deposit .

THE LARGER ORE BO D IES OF METAMORPH IC

CHARACTER .

We will now pass on to consider those larger orebodies

,associated with the crystall ine metamorphic

rocks,which were mostly metasomatic in character

prior to thei r undergoing metamorphic changes of a

regional type .


pho sed calcareous rocks . I t builds masses occasionallyreaching 50 yards in length by 200 yards in depth , butthere is a gradual passage from the ore body into the

FIG . 6 I .

—MAP OF THE DISTR ICT OF PERSBERG , SHOWINGTHE ASSOC IATION O F TH E ORE WITH METAMORPHOSEDCALCAREOUS ROCKS . (AFTER SJORGREN .)

Scale,1

countryrock . The metamorphic minerals with whichi t is associat ed include garnets , pyroxene , amphiboles ,etc . but perhaps the most remarkable are knebelite , a

METAMORPH ISM 341

manganese -bearing olivine,and dannemorite , an actino

l ite rich in the same metal .Well -bedded deposit s of similar nature to thosedescribed above occur at Orebro , i n Sweden , and atDunderlandsdal and N aeverhaugen ,

in Northern N orway . They are associated with mica schists and limestones Of metamorphic character . The ore exists as aniron -bearing mica schist interbedde ’d with limestonesand hornblende schists . Its associates are the usualcalc - sil icates , but in addition a garnet exceptionallyrich in manganese .I n other part s of E urope similar ores are found , inSpain and Portugal with similar asso c Iatio ns, and , infact , i n most regions where the Archaean floor i s exposed at the surface .

The Malaga magnetite deposits are due to the metamorphism O f dolomites which had been partially re

placed by siderite or haematite .One of the best examples of a metamorphosedmetasomatic deposit i s furn ished by the magnetite andhaematite masses of Mokta el Hadid , i n the province ofConstantine , N orth Africa . Magneti te with haematiteoccurs i n several beds associated with pyroxene -garnetrocks and mica schists , the calc - si l icates representingin a measure the residual l imestone

,which had not

been replaced by i ron ores .The deposit as a whole is remarkably free fromimpurities in the form of sulphur and phosphorus— a

feature which , i n common with a large proportion ofmetamorphic i ron ores , greatly increases its value . Theiron was most probab ly deposited in the first instanceas carbonate , which was subsequently converted intoa mixture of magnetite and haematite .


In the United States Of America , in N ew Jersey andConnecticut , and in the regions of Lake Champlain andthe Adirondacks , magnetite masses occur in the O lderschists and gneisses , the axes of the ore bodies for themost part lying parallel to the foliation of the districts .

The ores have a similar mode of occurrence to thosealready described

,and are associated with an identical

set of metamorphic minerals .The iron ores of Prince of Wales Island , Alaska ,occur in masses associated with epidote , garnet , etc . ,

and l ie between parallel dykes of O l ivine dolerite .

Whether the manganiferous i ron -bearing series ofBrazi l , especially that of the Serra do E spinhaco ,Should be classed with the Lake Superior type of i ronores (p . or whether it should be treated here , isa matter of some doubt but on the whole the oresseem to be more closely all ied to metamorph ic deposits .I n addition to the lateritic i ron ores of the centralprovinces of India

,other masses of different origin

occur in the Jawadi H i l ls , in the Madras Presidency .

They exist in several bands associated with hornblendic ,garneti ferous

,and hypersthene -bearing gneisses . They

consist almost entirely of magnetite , and are clearly ofmetamorphic origin . Similar deposit s occur in the

Island of Madagascar .

MANGANESE .

Manganese in the metamorphic rocks usually occursin the form of hausmannite , braunite , and franklin ite .

Hausmannite is similar to magnetite in composition ,but the ferrous and ferric oxides of the latter are re

placed respectively by simi lar manganous and manganiccompounds. Frankl in ite may be regarded as a man


S i l icates , such as manganese epidote , manganese diopside , and manganese garnet .

CORUNDUM .

We have seen (p . 57) that this mineral may re

sult from the differentiation of basic igneous magmas ,but sedimentary aluminous rocks under considerablepressure , when subj ected to the heating influence ofthe earth ’ s interior or to that of some intruded igneousmass, will undergo a marked reconstruction , oftenaccompanied by the development of corundum . A rock ,such as an ordinary argillaceous sediment

,in which the

contained alumina is not in excess of the si l ica , willunder Such conditions give rise to the metamorphicalumin ium sil icates cyanite , andalusite , cordierite , etc .

But should the alumina be more than equivalent tothe sil ica , the excess of alumina wi ll crystallize ascorundum . I t i s to be expected , therefore , that themetamorphosi s of a highly aluminio us deposit wouldresult in the formation of an almost pure mass ofthi s mineral

,and it i s interest ing to find that the

changing of bauxite into corundum has been effectedin the laboratory by means of the electric furnace .

Corundum,either in the form of gem - stones , such

as ruby and sapphi re , or more commonly as emery ,occurs in a variety of rocks , but always in those inwhich there i s presumably an excess Of alumina oversi l ica .

I n metamorphosed impure dolomites and limestonesit is met with at Naxos , in G reece , and in the neighbo urho o d of Smyrna . !Vi th magnetite and aluminumsil icates it occurs as masses in the crystall ine schists ofmany districts . At Ochsenkopf, near Schwarzenberg ,

METAMORPH ISM 345

i n Saxony, a mica - talc schist contain s it in some

quantity,and in the Pyrenees i t has been developed

largely as a contact mineral at the junction of graniticmasses with Palaeozoic sediments .

Probably the best - known deposits are those ofNorthern India

,Tibet

,and China , as at Mysore ,

Canton,etc . ,

occurring in thermally metamorphosedaluminous sediments

,together with the aluminium

sil icates cyanite and andalusite .

DEPO S ITS OF SULPH IDE S .

Masses of i ron pyrites , pyrrhotite , and other sul

phides, such as copper pyrites , zincblende , and galena ,occur in a similar manner to those of magnetite ,already described , associated with the same types ofmetamorphic rocks

,including mica garnet horn

blende and zoisi te-hornblende -schists . These sulphidicdeposits may be roughly divided into two classes

(a) The Copper-bearing pyrrhotites and pyrites , whichoccur in rocks not particularly rich in sil ica , and fromwhich there is an absence of l imestones ; (b) sulphides ,including some zincblende and galena , occurring inmetamorphosed calcareous deposits .These sulphide masses are O ften auri ferous

,the gold

being sometimes original , but most O ften concentratedby the secondary metasomatic action of the pyrites

,

on descending auri ferous solutions .I t was pointed out , in the case of sulphidic depositsdue to precipitation , that they most often occurred inthe true argillaceous sediments , and were much lessfrequently found in association with arenaceous depositsor l imestones . Similarly, in the case of the metamorphic masses there is seldom any marked indicati on


of the original presence of much calcareous material,

or, i n other words , there is a smaller proportion of theusual calc -sil icates , such as were associated with themetamorphic magnetite deposits . The place of epidote

,

garnet , and diopside , is in such cases taken by morealuminous m inerals, such as the micas , chlorites , andthe aluminous amphiboles . At the same time , certainmasses of pyrites and other sulphides are occasionallyfound in association with calc - si l icates which could onlyhave had their origin in highly calcareous rocks .I t i s most probable that the sulphide masses of both

the above classes owe thei r present characters chieflyto a process of recrystal l ization

,together with a certain

amount of secondary enrichment and migration of orematerial .The greater number of su lphide masses fall into the

fi rst class mentioned above,and include most of those

which occur in association with mica schists , hornblende schists , and other metamorphosed aluminousrocks . E xamples may be drawn from I reland

,Norway ,

Hungary , the Carinthian and Styrian Alps , Tasmania ,and the United States of America . I n I reland , pyritesmasses occur in a series of talcose and hornblendicschists of metamorphic character in County Wicklow.

I n Norway,in the neighbourhood of ROraas, which is

the centre of an important min ing district , deposits O f

copper-bearing iron pyrites, pyrrhotite , and copperpyrites , are associated with the ancient talc , mica , andChlorite schists , with which they appear interstratified .

The masses reach as much as 6 yards in thickness , andextend laterally and vertically for considerable distances .

I t is supposed that they existed,in the first instance , as

precipitat ions in a sedimentary series, and have since ,


traversing faults . The position of the ore-masses as awhole 15 entirely dependent on the structural relations ofthe schists and conglomerates ; and it i s believed that thevarious masses were formed in crushed and fracturedschists at the points where

,during earth movements ,

they were forced against hard conglomerate masses .

The fact that the pyritic masses are traversed by laterfaults has no significance from the point of view of origin .

The same authority has suggested l ike origins for thepyritic masses of Rio Tinto and Tharsis, and of Rammelsberg and Ducktown .

The sulphide masses occurring with metamorphosedcalcareous sediments are characterized by the greatvariety of the sulphides they contain ; in fact , theyinclude all those Which occur in the metasomaticdeposits . As

.

examples we may take the ore-massesof many local ities In Sweden

,of Carinthia

,of Lamnitz

thal , and of several localities In the Tyrol and N orthernItaly .

The Lamnitzthal deposits occur at the j unction Of

hornblende schists with garneti ferous mica - schists ; andthe ore consists of pyrites with pyrrhotite , coppe rpyrites , galena, and zincblende .

Those of Panzendorf,in the Tyrol , fifteen miles south

of L ienz, are also associated with garnet iferous micaschists and hornblende schists

,and consi st of a great

variety of sulphides,including all those mentioned above

as occurring at Lamnitzthal . The deposits of Sterzing ,also in the Tyrol

,occur with hornblende schi sts and

metamorphosed dolomites .I n Northern Italy— at Monte Beth , in the district of

Pinerolo , south-west of Turin— sulphidic ores occur in acomplex of phyll ites

,schists

,and igneous rocks ; they

METAMORPH ISM 349

are associatedwith calc-phylli tes and hornblende schists .

There are several other S imilar occurrences in Piedmont .The famous Homestake gold ores in the NorthernBlack H i ll s of South Dakota occur as lenticular pyrit icimpregnations in zones in the Algonkian schists, in

association with intrusions of rhyolite porphyry . Theschists contain contact minerals such as b iotite , garnet ,term o lite , actinolite , t itanite , and graphite theseminerals are consequently found among the ore , whichis mainly composed of pyrites, mispickel , quartz ,dolomite , calcite , and gold . The pyrites occurs as infi l l ings along planes of schistosity , as i rregular massesin the schists and porphyry

,and in quartz veins . Some

parts of the deposit have been shattered by subsequentmovement .

METAMORPH IC CONTACT DEPOS ITS .

As regards their manner of formation and the contained minerals , contact deposits are in most ways similarto those which owe thei r present characters to metamorphism of the regional type

,but they differ widely

in thei r mode of occurrence . They are found only withinthe metamorphic aureole surrounding some plutonicintrusion , and result either from the local change insome pre-existing metasomatic or precipitated metallifero us deposit , or from a metasomatic change produced in the country-rock simultaneously with itsmetamorphism .

The changes brought about in a pre -existing metallifero us deposit , as we have already seen , will be of thenature of dehydration or recrystall izat ion ; but theintrusion of a large body of igneous material may resultin the introduction of fresh metall i ferous material by


means of the heated solutions given o ff from the magmaduring the last stages of its consol idation . The formation of ore deposits by such a process bridges the gapbetween true metasomasis and pneumatolysis .I n our own country there are several good examplesof metamorphic contact deposits

,the best , perhaps ,

being the two districts O f Hayto r i n Devonshire andGrampound in Cornwall .The Hayto r mine is situated on the eastern borderof Dartmoor ; the ore is in the form of thick beds ofmagnetite interstratified with altered shales and sandstones Of Carboni ferous age , and is associated with suchmetamorphic minerals as actinolite and garnet .There is l ittle doubt that the ferruginous material wasdeposited contemporaneously with the shales and sandstones i n which it occurs, and in all probab i l i ty resembledthe Cleveland i ronstone (p . 245) in many respects . Thisoriginal deposit was metamorphosed by the greatgranitic mass of Dartmoor , which b reaks across thebedding of the sedimentary rocks , and converted intomagnetite . The ores have been altered by surfacewaters for some d i stance down the lodes , with the formation of ochre and other secondary products .The Grampound lode (South Terras , Cornwall) carriesmagnetite with a l ittle haematite , and is associated withcalc ~silicates, such as green garnet .I n France beds of magnetite and haematite occur atseveral places in the department of Manche , especiallyat D iel lete , in the neighbourhood of Flamanvil le . Theores are held by Cambrian and Silurian rocks rich ingarnet , actinol ite , and epidote , the metamorphism beingproduced by intruded masses of granite . Althoughsuch examples are more usually found in connection


exist in a gangue of calcite , pyroxene , hornblende , andgarnet .Metasomatic i ron ores occurring in the Canton de

Vicdessos, in the department of Ariege , especially atRancie , have been metamorphosed into magneti te andhaematite , and are associated with masses of calcsi l icates . O riginally the metall i ferous deposit existedas a replacement of a L iassic l imestone , and wasprobably similar to the Cleveland ore of E ngland .

Metamorphic magnetite and haematite masses areknown in a great many more remote districts , suchas the Urals and F inland ; and with manganese in theneighbourhood of Santiago , in Cuba , in connection withmasses of porphyry .

Manganese ores in all cases have a similar originto those of i ron , and therefore it i s not surprising tofind compounds of this metal associated with the metamorphic i ron ores . I n Brazil the district of OuroPreto is noted for its rich manganese and i ron deposits . The rocks of this district are schists andgneisses of Archaean age , and a series of schistose quartzites , micaceous schists , and l imestones , known as theItacolumite Series ,

’ belonging to the Lower Palaeozoicdivision . At some later period than the Lower Palaeozoic ,intrusions of granitic

,dioritic

,and gabbroic rocks pene

trated the whole , and produced considerable contactmetamorphism .

The Itacolumite SerIes Is the most interesting , as i t is

the chief ore repository,and contains i ron , manganese ,

and some gold . The micaceous schists pass into palecoloured calcareous beds

,which in turn give place to

iron schists and beds of almost pure i ron ore , known asitabarite .

’

One of the principal districts is Serra do

METAMORPH ISM 353

E spinhaco , where the ores consist of the metam o r

pho sed residual deposits Of l imestone , which originallycontained carbonate of i ron and the carbonate or oxidesof manganese .The manganese ores consi st chiefly of the earthyhydrated oxides and pyrolusite

,but there i s l ittle doubt

but that they have undergone considerab le changessince thei r formation .

I n the Hautes Pyrenees , the schistose limestones ofSerre-d ’

Arrét are impregnated with a variety of manganese -bearing minerals of metamorphic origin .

SO far we have been deal ing almost exclusivelywith the oxidic contact deposits which are somewhatmonotonous in character

,but a much greater variety i s

displayed by the masses of sulphides wh ich occur in aS imilar manner .Sulphides of iron

,more or less cupriferous, occur ,

associated with igneous intrusions in metamorphicrocks of many ages and many districts . The mostfamous deposits are probably those of Rio Tinto , in theprovince of Huelva in Spain

,while S imilar ores occur

at Gavorrano , i n Tuscany . The sulphides of i ron arethemselves of no great value , but as they are usuallyassociated with some Copper and other sulphides , andoften contain the precious metals , they are frequentlyraised to great economic importance . All extensivedeposits of pyrItes Should be assayed for gold .

The Rio Tinto deposits occur as masses of i ron andCopper pyrites, stretching at intervals from Portugal onthe west to the borders of Sevi lle on the east . Theregion is one of intensely folded Palaeozoic sediments

,

i ntruded by masses of igneous rocks of types varying2 3


from micro -gran ites to doler’

i tes . I t i s with theseigneous intrusions that the pyrites masses are intimatelyconnected , the ore generally occurring at thei r junctionwith the sedimentary rocks which , as a rule , Show considerable S igns of metamorphism .

In the Ural Mountains , the district of B ogo slowskfurnishes a somewhat doubtful example of thi s class ofore deposit . I t i s said that Devonian limestones havebeen contact-altered by intrusive porphyrites , with theformation of masses of garnet , pyroxene , and idocrase ,and with the introduction of such metall iferous mineralsas Copper pyrites . A simi lar origin has been suggestedfor the ores of Go ro blago dat (p . 5 1 ) and other local ities , but i t does not seem conclusively proved that theyare not segregations from igneous magmas, which havesubsequently been affected by metamorphic action of theregional type . A contact metamorphic deposit formedby the introduction of materials in limestones , at themoment of thei r alteration by contact with an igneousrock , i s instanced in the Campiglia Marittima (Tuscany) ,where the Tempo rino vein consists of copper and i ronpyrites , a l ittle galena , zincblende , and mispickel , i n aveinstone of radiating manganiferous pyroxene needles ;the vein results from the contact - alteration of L iassicl imestone by augite porphyry.

I n the United States there are two important districtscontaining copper - bearing contact deposits—namely,those of Cli fton Morenci , and Bisbee in Arizona .

The Cli fton-Morenci deposits appear to be dependenton the intrusion of a mass of porphyry which hasproduced extensive metamorphism of the surroundingPalaeozoic limestones and Shales . The metamorphism ,

which takes the form of the production of new minerals


rocks with the Palaeozoic limestones , and have a gangueof metamorphic calc - si licates . Professor Lindgrenbelieves that the porphyry magma contained muchmoisture , holding metall ic salts in solution , which wasliberated at the time of the consolidation . Similardeposits may be cited in Vancouver I sland . InAustralia , in North Queensland , a good example of acontact Copper deposit may be taken from the Chillagoe district , where magnetite , chalcopyrite , and zincblende occur at the contact of igneous and calcareousrocks , in a gangue of garnet , wollastoni te , pyroxene ,and other calc -si l icates .I n Tennessee , cupri ferous deposits near Ducktown runparallel to the foliation of mica schists with gneissosebands of Lower Cambrian (Georgia Beds) age . Theores are mainly pyrrhotite , traversed by strings of copperand i ron pyrites with small amounts of sulphides oflead and zinc , in association with calcite , quartz, garnet ,epidote

,and actinolite . The pyrrhotite body is a massive

infil l ing, and contains an average of 2 per cent . ofco ppen

Zinc and lead sulphides occur with the other sul

phides in most instances , but the zincblende deposits ofAmmeberg

,and the galena masses of Sala, in Sweden ,

are both worthy of Special notice . The zincblende ofAmmeberg , to the north -east of Lake Wetter, existschiefly as lenticles in the Archaean schists and gneisses ,but it i s also found as a contact deposit in a zone ofhal leflintas which l ies between the younger graniticmasses and the schists .The galena deposits of Sala , north of Stockholm , areassociated with metamorphosed calcareous lenticles inthe crystall ine schists , and are clearly of metasomatic

METAMORPH ISM 357

character,the metasomasi s being accompanied by such

changes as the marmo rizatio n of the limestone , and ina less degree the production of new minerals .

GO LD IN METAMORPH IC RO CKS

Gold is O ften found in rocks which have undergone extensive thermal alteration , more especially in conjunctionwith sulphidic ore deposits of the metamorphic type ;but occasionally it occurs in metamorphic rocks independently of any

'

metall ic sulphides . I t usually owesits presence , in rocks of this nature , either to originalsedimentation as detrital gold

,or to deposition on

pyrites from auri ferous solutions by metasomatic action .

One of the best instances o f gold in metamorphic contact deposits i s that of the Deep Creek district ofUtah . The Productus Limestone , forming a chain ofhills , i s invaded by numerous veins and masses ofgranite and porphyry

,which have produced powerful

metamorphic effects . At the junction of these igneousmasses with the l imestone we find such metamorphicminerals as garnet and tremolite in abundance , whilefarther away the limestone has undergone recrystal lization . The gold occurs native in two ways either lyingfinely divided in the recrystall ized l imestone in theneighbourhood of sulphidic Copper ores

,or in fine

threads and strings in masses of tremolite .Gold also occurs in metamorphosed l imestones in

N evada ; and in the S ilver Peak district , solutions ,emanating from cooling granitic masses, have depositedthe metal in the marginal granite and in the surrounding metamorphic rocks .In the crystall ine schists , gold occurs in small quan

tities in many districts in America,E urope

,and Asia

,


and has given rise to some of the richest placer depositsof the world (p .

In the Alleghany region,in the Huronian rocks ,

pyritous and quartzose lenticles are met with in talc~ ,and chlorite-schists

,the gold being both in the lenticles

and finely disseminated through the schists . Similaroccurrences are known in North‘ and South Carol inaand the Black H ills of Dakota

,in association with

talc and hornb lende- schists also in Siberia , NorthernIndia , Japan , and the Guianas.


with the ordinary process of surface denudation . Wherethis denudation has proceeded very slowly there isO ften a large variety o f secondary minerals

, Showing amore or less orderly arrangement as regards their verticaldistribution ; the more completely oxidized of the oresare near the surface , but the reconcentrated, more sul

phidic types are below, at varying depths , and mergeinsensibly into the original sulphides . These Secondaryore zones are often the richest portions of lodes .I n some regions which have been extensively

glaciated , the lodes contain very few secondary ores ,owing to the removal by the ice- Sheet of the decomposed superficial countryrock , and with it the upperparts of the lodes . The iron and zinc lodes of Scandinavia and of North America may be referred to asexamples of this , while in Ontario , according to VolneyLewis, the gossans of the lodes have been removed bythe same agency .

S ECONDARY CHANGES BY ASCEND ING SOLUTIONS .

The secondary changes of minerals in lodes , effectedby waters derived from a depth , are not frequent . Theyare generally brought about by the solutions of a laterperiod of mineralization

,by which the O lder minerals

are largely replaced by newer .The ores of the Comstock lodes (N evada) appear to

have been largely accompanied by calcite , but thismineral is now met with only in small quantity , owingto its replacement by quartz .

The replacement of barytes and calcite in the cobaltores of the E rzgebi rge , and of the ores of lead andCopper in the Black Forest , i s connected with a later

period of mineral ization ; i n the latter c ase , .i ron and

SECONDARY CHANGES 36 1

manganese ores were deposited . To what precise periodin the history of some lodes the pseudomorphous replacement Of some lode minerals , such as fluo rspar,barytes

,cal cite , quartz , etc .

,by others , such as i ron

pyrites,chalyb ite

,l imonite

, Copper pyrites , etc . , should

be assigned is uncertain ; but there is l ittle doubtmany of these Changes should be ascribed to theordinary action of secondary concentration by surfacewaters .The possibil i ty of the secondary changes in mineralsbeing produced by deep - seated vapours or solutions issuggested by Professor Vogt , who points out thatsulphide of si lver may be acted upon by hot ai r orsuperheated steam , with the formation of capillarynative si lver and sulphur dioxide

,or sulphuretted

hydrogen , thus

Ag2S O2

2Ag SO2 ;

Ag2S HZO 2Ag H

zS O .

By reaction O f the sulphuretted hydrogen withsulphur dioxide , free sulphur or sulphuric acid may beformed , capable of further reactions , which in somecases involved regeneration of the si lver glance .Similar reactions may be effected on other si lver ores,or on copper ores , as follows

2Cu2S 60 2Cu

zO 2SO

Cq 2CuzO 6Cu SO 2

Ag3AsS3 3HzO 3Ag As 3H 2

S 30 .

The last of the foregoing reactions i s supposed toexplain the occurrence of the large amounts of nativesi lver in the Kongsberg dist rict , near Christiania .

I n the Ko ngsberg' veins calcite i s the dominant


mineral ; fluo rspar, quartz , anthracite and bitumen ,are also present , while barytes is found occasionally .

Axinite , soda and potash -felspar , chlorite , and zeolites ,are also met with . In addition to silver

,but in smaller

amounts , there are sulphides of iron ( in appreciablequantity) , zinc , lead , Copper and arsenic , and cobaltbloom . The order of mineral ization appears to havebeen Quartz ; ( 2 ) sulphide ores and silver glance ,with calcite and bituminous material ; (3) fluo rspar,barytes , with felspars (4) younger quartz veins , withaxinite , i ron sulphide , pyrrhotite , and zeolites . I t wasin stage 2 that the native S i lver was formed .

S ECO NDARY CHANGES BY WEATHERING AND ACTIO NOF METEORIC WATERS .

The reactions brought about in the upper parts oflodes by surface waters are ( 1 ) those of oxidation and

(2 ) those of reduction . Most sulphidic lodes Showthat , from thei r outcrops to a depth of from a few feetto hundreds of feet , the uppermost part consists of azone of oxidized minerals ; while the lower part consistsof secondari ly deposited minerals , O ften constitutingvery rich bodies of ore . An intermediate zone is O ftenpresent , which may be regarded as a transition zone , andthi s i s frequently at the ground-water level .Although there i s no reason for accepting the theoryin its enti rety , the view that the greater number Of oredeposits are the result of underground waters has foundfavour of late years . I t has been held that there aretwo types of underground water-circulation , knownrespectively as the shallow underground ,

’ or vadose ,’

and the deep - seated ci rculation . I t i s with the shallow

364 THE GEOLOGY OF ORE D EPOS ITS

the lode and redeposited by a second reaction . Thedissolved metall ic salts , or the free acids occasionallyl iberated in the general chemical changes, reacting onnon -metall iferous minerals in the lode or countryrock ,O ften form secondary non -metall i ferous minerals

, suchas gypsum . The reaction of sulphate Of zinc uponl imestone may be taken as an illustration .

In dry countries the minerals of the upper parts ofthe lodes are much more varied than those of moistcountries , owing to the more soluble compounds , suchas sulphates and some carbonates , being better ab le toexist in the solid state .Typical arid , regions are those Of the deserts ofArizona and of Atacama .

I n the upper parts of the cupriferous deposits ofChili , particularly near Copiapo , there i s a series ofcupri ferous and ferric sulphates and carbonates— bothneutral and basic— in various stages of hydration

,i llus

trative of the presence of soluble minerals in thegossans O f arid regions . Chlorides and o xychlo rides arealso typical compounds of such districts .The secondary enrichments of lodes brought aboutthrough these secondary reactions are often of greatimportance commercially . The gold concentration ofthe Mount Morgan Mine (Queensland) , the secondaryenrichment of argenti ferous ores in the gossan of BrokenH i l l (N ew South Wales) , the many secondary depositso f copper

,lead , and zinc , in the American continent ,

and those of our own country in Cornwall and otherlocalit ies , are all examples of concentration of metallifero us lode minerals by secondary processes .I n Guanajuato (Mexico) the secondary alterations

(colorados and negros , or pinta azul) extend to depths

SECONDARY CHANGES 365

of feet . From surface to a depth of 2 50 feet thelodes are poor , but from this level to in depththe gold is secondarily concentrated and free -mill ing .

Below this the gold occurs in ores of lead and antimony ,and consequently is not so easily separated .

Near the surface the veins sometimes contain blackoxide of manganese , and the material here goes by thename of que mazon .

’ At Catorce , ,where the depositsare in limestone , the colorados extend to a depth of

500 feet , and consist of oxide and sulphate of iron ,native si lver, chloride , bromide , and sulphide of si lver ,in a gangue of manganiferous calcite

,chrysocolla , cerus

site , and azurite .The secondarily deposited gold is found in ochreouscavities in a quartzose veinstone which once containedpyrites .When the veinstone was oxidized

,the i ron pyrites

was carried Off in the form of ferrous sulphate , leavingthe gold , which was originally disseminated throughthe pyrites , in the form of small segregated masses .Free gold is found in the gossans of many pyritic

lodes , as , for instance , in the Remedios district inColombia (South America) ; here the lodes containfilifo rm and grain gold

,i n an ochreous material extend

ing from the outcrop to a depth of 50 feet , belowwhich there occurs auriferous pyrites, with sulphides ofantimony , arsenic , lead , and Copper ; the molybdateof lead and native bismuth are also found .

The chemical reactions involved in the changes givingrise to the ores of the gossans

,and zones of secondary

deposit ion in lodes , depend on the relative affinities ofthe metals for the acid radicles , and this may be expressed by writing the metals in a certain order . Thus

,


the following series— mercury , S i lver, COpper, b i smuth ,cadmium , lead , zinc , nickel , cobalt , i ron , and manganese— i s so arranged that a solution of a salt of any o ne

of these metals will be decomposed by those sulphidesof the metals which follow it

,but wi ll be unaffected by

those which precede it .One of the commonest metall ic compounds of all

lodes is i ron pyrites , and as this comes at the end of theseries with manganese , i ts effect as a precipitant tosalts produced by oxidation of some of the other metallicsulphides must be very great .I n the zone of oxidation (in the gossan) i ron pyrites

i s decomposed by a series of reactions , which culminatein the formation of limonite

,a mineral extremely

common in the upper parts of lodes .The oxidation of i ron pyrites primarily results in theformation of ferrous sulphate and sulphuretted hydrogen ,thus

Ferrous sulphate i s easily oxidized to ferric sulphate ,which in the presence of some non -metall ic hydroxideis decomposed , with formation of limonite

Fe2(SO 4)3 6R .OH Fe

2033H z

O 3R2SO

4.

The carbonate of i ron is also liab le to destruction

4FeCO 3+ 3HzO 20 2Fe

2033H z

O 4COz.

While , i f the carbonate contains manganese , an oxideof manganese is formed at the same time .I t has been suggested that the formation of haematite

from the decomposition of i ron pyrites may be effectedby alkaline solutions, the action resulting in the formation of alkaline sulphides and thiosulphates . The effect


The copper pyrites of the second equation can beregenerated also in accordance with the followingreaction

Cuso4

2FeS2 O,CuFes

2P eso

,2SO

CuSO4

2FeS CuFeS2FeSO

4.

While intermediate compounds, such as bornite , maybe formed by similar reactions .A remarkable instance of the regeneration of copperpyrites is instanced in Vancouver island , where inBonanza Creek a layer Of Copper pyrites i s found at thesurface containing 10 per cent . of Copper ; thi s percentagediminishes with depth , unti l at 20 feet the ore ceases .I t is found as a secondary deposit in decomposedsurface- rocks , and was probably derived from cupriferoussolutions which were reduced by organic compounds .Native Copper i s Obtained by reduction of sul

phate of copper through the action O f ferrous salts,

which are converted to peroxide of iron or to ferricsulphate .

The reaction corresponds to that represented by theequation

CuSO4

2FeSO4Fe

2(SO 4) 3 Cu .

The relative ease with which the various cupriferoussulphides are decomposed by ferric sulphate i s indicatedby the following order : copper pyrites , bornite, andcopper glance .

Although the sulphidic Copper-ore deposits invariablyShow secondary alterations near the surface , there aresome exceptionally important deposits of secondaryorigin

,and among these may be mentioned ores

of the Mount Lyell and the Burra Burra Mines in


Australia,Rio Tinto ores

,those of Chili and Montana

(Butte) , etc . Deposits of secondary origin are found atMonte Catini

,and also in Tennessee .

Sulphate of copper is found in workable quantity inthe upper parts of lodes in N evada , and also in Chili ,associated with ores such as the carbonates and sil icateof Copper .I n the Urals oxidized copper ores , malachite , etc are

common . Impregnations of sandstones by the carbo nates of copper occur at Katanga (in Congo FreeState) . In Poland impure malachite i s worked out bythe use of sulphuric acid on the countryrock . Auriferousmalachite has been found in Guatemala at the contactof syenite with l imestone .The formation of the basic copper carbonate aboveground -water level i s effected by the action of somesoluble carbonate on the copper sulphate in a lode ,thus :

2CuSO4

2CaCO3HzO

CuO .HzO .CuCO

32CaSO

4CO

2.

The oxides of copper and the native metal areformed by reactions represented by the following equation

2CuSO4

2CaCO 3 2CuO 2CaSO4

2CO2

.

Cuprite (CuzO ) occurs in quant ity in the Cobre Mine

(Cuba) , and i s formed by the reduction of the cupricoxide to cuprous oxide , which by further reaction withferrous sulphate or free sulphuric acid gives ri se tonative copper

GuzO H

ZSO

4Cu CuSO

,HzO ;

3CuzO 6FeSO46Cu Fe

203

2Fe2(SO 4) 3

3Cu20 2FeSO4 4Cu 2CuSO

4Fe

zOs

.

24

376 THE GEOLOGY OF O RE DEPOS ITS

In Chil i , near Copiapo , secondary ores of copper ,such as Copper glance , atacamite , carbonate of copper ,cuprite , copper pyrites , and bournonite , are found infissures t raversing Jurassic sandstones and shales

,rhyo

l ite,porphyrite , melaphyre , and volcanic tuffs . I n the

district of Chuquicamata , quantities of atacamite arefound in shattered granite , forming a stockwork , to adepth of 300 feet , at an elevation of feet abovethe sea .

N ear Antofagasta the ore i s mainly atacamite,with

sulphate of copper, to 200 feet from the surface . Atthis level black copper ores are met with , and belowthese are the primary sulphides .

The gossans of the Western Argentine are auri ferous,

and even at moderate depths contain carbonates andoxides of cp pper.

The Peruvian lodes occur in Mesozoic rocks withmelaphyres , intruded by diorites and porphyrites . AtRecuay in that country, the ores of an east and westsystem of veins which contain much silver and someaccessory Copper , and of a north and south series whichcontains much Copper , are largely of secondary origin .

At Cerro de Pasco secondary ores are found in veins inaltered andesite ; to a depth of from 2 00 to 300 feetthe ores are secondary (colorados or pacos) ; belowthese come sulphides of i ron and copper, which areargentiferous (mulattos or pavanado s) . The originalores (negrillos) in depth are sulphides of i ron , zinc , andcopper

,with mispickel , tetrahedri te , and bornite , con

tain ing S i lver and gold .

I n Mexico the upper parts of the lodes Show thecharacteri stic secondary types of ores . In SonoraCounty (Cananea district) , oxides of copper and the


are associated with quartz, felspar, garnet) , i ron , copper ,and arsenical pyrites , l ead molybdate , and fluorSpar. Secondary incrustations of sooty sulphides arefound coating these ores . I n the zone affected bysecondary changes arsenical and antimonial sulphidesof si lver, stromeyerite , polybasite , and secondaryores of i ron , manganese , lead , and copper, are metwith .

I n the Virgi l ina district fragments of bornite arecoated by copper pyrites, the whole being cementedby oxide of i ron formed through the decomposition ofthe bornite .The secondary concentration of zinc and lead oresin l imestone by the action O f meteoric waters isanalogous to that of copper ores , but there appears tobe a closer connection between limestone and zincand lead ores than there i s between limestone and thecopper ores .The common lead ore cerussite may be formed indirectly from galena ; galena in the upper o parts oflead lodes i s converted into sulphate of lead , whichin turn becomes carbonate through the action of car

bo nic acid or carbonates in the solutions which ascendthe lode . From the carbonate or sulphate the lead maybe reconverted to sulphide by the action of sulphidesof either i ron or zinc

Pbco,Fes

,02PbS P eco

,SO

Phso,ZnS Pbs ZnSO

4.

The association of the zinc and lead ores with l imestones is explained by true metasomatic action . Thesulphides are converted to sulphates , which react onthe limestone and replace i t by the carbonates of the


metals ; whi le sulphates of l ime and magnesia formedduring the reaction are carried away in solution

PbSO4CaCO 3

PbCO3CaSO

ZnSO4CaCO 3

ZnCO 3CaSO

The secondary lead sulphides of Leadvi lle (Colorado) ,Broken H i l l (N ew South Wales) , and the various deposits o f lead ores in Nevada , Utah , and many otherplaces in America

,are good examples of this type of

action . Chloro-carbonates and sulphates of lead occurin Utah

,Leadville (Colorado) , Tarapaca (Chil i) , and

Sardinia (Monteponi) . Phosphate and Chloro-arsenateof lead

,and basic sulphate of lead and copper , occur

less commonly,while the mineral greenockite (CdS) i s

sometimes l iberated on the alteration of zincb lende

which contains cadmium .

The secondary concentration of the ores of si lver is

Of particular interest , since the richest argentiferousore bodies have been formed by secondary concentrat ions ; i n arid or steppe - l ike regions, as in the Desertof Atacama

,i n Northern Chil i , the secondary minerals

have a special character , owing to the unusual natureof the compounds formed .

The alkal ine salts (caliche , Chili saltpetre , or sodanitre) of the Desert of Atacama owe thei r origin to thenitrification of salts in the old muds and deposi ts ofdried -up lakes , which , owing to the aridity of theregion , have not been washed away . Solutions of thesesalts occasionally find thei r way into the lodes , and , asthey contain chlorides, iodides , bromides, carbonates ,and sulphates , a corresponding series of secondaryminerals are formed in the lodes . Chloride , bromide ,and iodide of si lver , with sulphates, carbonate , and


complex ores of silver and other metals,are found

together with polybasite and sulphides of silver. Ac

cording to C . R . Keyes, the pecul iar conditions o f aridregions favour the formation of chlorides in lodes byreason of the fact that saline substances are carriedby winds and deposited in the form of dust .Sulphide of si lver may be regenerated by the actionof sulphate Of S i lver on a sulphide . N ative si lver isformed also by the action of ferrous sulphate on silversulphate , the reaction resulting in the production ofnative si lver and ferric sulphate

,thus

An O4

2FeSO4

2Ag Fe2(SO 4)3.

I n Sonora County, Mexico , native silver formed in thisway is abundant in the upper parts of the lodes .That native si lver i s possib ly capable of solution bymeans of Cu SO

4or by Fe

2(SO 4) 3 , with formation ofsulphate of si lver , and sulphate of i ron or copper ,respect ively , is shown by H . N . Stokes . Under different conditions the reaction becomes a reversible one ,and the above equation represents the probable change .

Other secondary minerals found in the upper partsof lodes comprise phosphates ( formed by the action ofphosphoric acids or solutions of salts derived from thesoil penetrating the lodes) ; arsen ides and arsenates ;vanadates and chromates (from weathering of gabbrosand serpentines) ; molybdates ; ochres of bismuth andantimony ; nickel and cobalt bloom , and occasionallymetall i ferous compounds of organic acids , such as

pigo tite (from mudeso us acid) in Do lco ath Mine .

These secondary minerals occur either as incrustations

,as stalactites and stalagmites , or occasionally as im

pregnatio ns and replacements of the country- rocks near

CHAPTER I!

DETRITAL AND ALLUVIAL DE POS ITS

THE disintegration of rock-masses by natural agents ,such as rain , wind , waves , differences of temperature ,and frost , i s always taking place in all land areas ; i t ismore or less independent of latitude , but in temperateregions it i s perhaps most rapid .

A rock—mass is generally far from homogeneous,and

is composed of a mixture of minerals having unequaldensity , hardness, solubility, and co -efficients o f ex

pansio n . The greater the physical differences presented by the various constituents of a rock-mass , themore readily will it break down into its componentminerals . The material so formed collects at the footof rock - slopes and on level surfaces o f the rock undergoing disintegration , but ultimately , in t imes of increasedrainfall , finds its way into streams

,rivers

,and finally

the sea .

Under the influence of running water particles ofsol id matter wil l be sorted according to thei r size anddensity .

The transporting power of running water is directlyproportional to some power of its velocity, but as ageneral rule no stream travels throughout its course at

a uniform rate . I n its more rapid parts i t i s possiblycapable of carrying all the solid material poured into

376

DETR ITAL DEPOS ITS 377

it by lateral streams,and it can even erode the rocks

which form its bed . On the other hand, the wideningor bending of a stream , the confluence with a tributary,or the occurrence of bars, banks , l arge rocks , and otherobstacles

,will locally reduce the velocity, and inci

dentally the transporting power . The effect of re

ducing the velocity wil l therefore be a deposition O f

some or all of the suspended solid particles in the orderof thei r respect ive densit ies , those with the greaterdensity being deposited first . The greater the differencein density between the various minerals carr ied insuspension , the more complete will be the separationon deposition .

Sorting may also be accomplished by the simplewashing away of the l ighter material , thus leaving theheavier particles more or less in thei r original posi t ionthat is to say , they may remain as a detrital layer onthe rock in which they were originally held .

I n the case of the metall iferous deposits,the natural

concentrations o f the heavier detritus are generallytermed placers . ’

The process of concentration as exemplified by thelarger rivers may take place in several successive stages ,fo r such streams are constantly eating into and redepositing part of their own alluvium . The alluvia O f

many of the broader river—valleys are not confined toone level , and occur either as terraces one above theother, marking pauses in the excavation of the valley ,or at a considerable depth below the level o f thepresent drainage - system . The former are known asshallow placers ,

’ and the latter as deep leads . ’

The alluvium of any existing river-system is necessarily derived from the areas drained by the main


stream and its tributaries , and its character will be'

determined by the rock-types present within the basin .

I t often happens that a river- system receives detritalmatter from rocks o f many different types , and it i sthus quite common to find the alluvium of a tributarydiffering mineralogi cally from that o f the main streamabove the point of confluence . An example is furnishedby the River Po and its tributaries , which drain thecomplex region o f Northern I taly . Almost everyimportant tributary of the P0 is characterized by atleast one easi ly recognizable mineral which differs fromthat Of the other streams , and is an indication of therock - types o f its parti cular valley .

I t is possible,from an examination of the alluvium

o f any river,to form

,by means of the minerals observed ,

an accurate idea o f the rock- types present within thebasin , and also in many cases to track any desiredmineral to its source .All the minerals which are o f economic importancein alluvial and detrital deposi ts have high densities ,and are thus easily concentrated by natural agencies .Although running water i s undoubtedly the mostpowerful agent

,concentrat ion is also effected by the

action Of wind and waves .Wind acts largely on the detritus due to theweathering o f rocks in arid cl imates , and its action issimply that of winnowing

,but

,together with waves, i t

also brings about the concentration of heavy mineralsfrom the sands o f t idal flats and coastal regions . I nthe case of short rivers o f uniform gradient , there isl i ttle decrease in the velocity of the stream until i treaches the sea , where almost at once all the suspendedmaterial will be deposited

,giving rise to bars and banks .


but the ‘ deep leads,

’ although sometimes connectedwith the present systems o f drainage

,often belong to

so remote a period that there i s no traceable relationto the present surface -co nfiguratio n. They have oftenbeen overlain by more recent deposits of marine orfresh-water origin

,and have even in some instances

been obscured by coverings o f volcanic material whichhas been poured out over the old land-surface .Beach- placers are found in marine deposits of allages, and the sorting action o f wind has been detectedin rocks o f the pre -Cambrian

,Mesozoic

,Tertiary, and

Recent periods .As an illustration o f the influence of the countryrock on the character of shallow placers , a river systemis shown in the figure

,with streams drain ing a gran itic

area , a mass o f serpent ine , and crystall ine schists contain ing lenticles of metamorphic l imestone with ironores .The alluvium o f the stream A in its upper part willcontain only the minerals of the crystall ine schists,but lower will contain particles o f i ron ore togetherwith calc- silicates , such as garnet , etc .

The stream B will be characterized, perhaps, by theoccurrence of cassiterite

,derived from the granite and

its metamorphic aureole,associated with such pneu

mato lytic minerals as axinite , fluo rspar, and tourmaline .

The stream C might carry chromite with gold andplatinum , and the alluvium o f the united streams (D )would contain all the minerals of the tributaries .We see therefore how

,by starting low down a river

and going by the Character of the alluvium , i t ispossib le to trace any particular mineral to its source ,and in this way a great number of rich deposits of


precious metals have been discovered . The placerdeposits are always the first to be worked in anydistrict . I t O ften happens that the concentration whichhas given rise to them is very complete , and then theparent rock may contain the precious material in onlythe minutest traces .The number o f heavy minerals met with in theplacers is extremely large

,and includes the native

metals gold,platinum

,osmium

,i ridium , copper, si lver ,

nickel,and iron ; the sulphides cinnabar , copper and

iron pyrites,zincb lende and galena ; the oxides cassit

erite, thorite , chromite , corundum ,and zi rcon ; also the

spinels , the phosphate monazite , the titanite perovskite ,and the silicates axinite

,cyanite

,andalusite , topaz ,

garnet , tourmaline , hornblende , and a host of others , allindicative o f the rock - type in which they had their origin .

Of these minerals,only a small proportion are o f

direct economic value,but it has been found that

certain non -metall ic minerals , of no value in themselveswhen met with in alluvial deposits

,point more or less

conclusively to the presence of some ore or metal .For instance , chromite is an invariab le associate ofplat inum ; topaz , tourmaline , fluor , and axinite , o f cassiterite ; and cinnabar , wolfram ,

and hornblende , mightindicate gold .

We will now consider the detrital deposits respectivelycarrying gold , platinum , t in , copper , i ron ores, and therare earths , such as monazite , thorite , etc .

DETRITAL AND ALLUVIAL GOLD .

The disintegration of rocks containing auriferousveins , and the accumulation o f gold by the mechanicalagents mentioned above

,has given rise to deposits


which economically are the most important o f al lsources of the precious metal . Before proceeding toconsider the most prevalent alluvial deposits, we willmention a few o f detrital character which have had asl ightly different mode O f origin .

The chemical weathering of rocks under tropicaland subtropical conditions often gives rise to surfacedeposi ts of considerable thickness (laterite , Thesedeposits are due to the chemical changes taking placein the superficial portions o f the rock-masses, includingthe removal of certain constituents by solution , and theoxidation of others . I n character they are usuallycel lular , ferruginous clays which have suffered littleredistribution by running waters ; should they exist onrocks which are in any degree auri ferous

,they would

contain the metal in greater quantity than the rocksbe low, and it would be more readily extracted . Suchdeposits occur , amongst other local ities , in the Guianas ,Brazil , the Adirondacks , I ndia , Madagascar, andLydenburg in South Africa . I n French and BritishGuiana and Brazil , the lateritic deposits containing goldare more or less confined to the plateaux and lesserhill-slopes occupied by diorites , diabases, amphibolites ,epidiorites

,and hornblende schists— rocks which are

regarded as the most frequent carriers of primary gold .

It has been definitely proved in these districts that theacid gneisses , gran ites , and porphyries are not thesource o f the metal .I n the valleys occurs the usual type o f alluvialplacers. ’ There is l ittle doubt that much of the gold is

set free by the complete oxidation o f auriferous pyrites ,and such an origin has been claimed fo r the metal inthe detrital deposits of the Appalachians.


In the United States , the region surrounding theBlack H i ll s o f Dakota consists o f a series o f metamorphic rocks

,such as hornblende , chlorite and other

schists , overlain unconformably by the Potsdam Sandstone (Upper Cambrian) . The sandstones and conglomerates of the Potsdam Series are largely made upof detri tus from the underlying rocks . They containgold in a finely divided state derived from the metamorphic series

,which is traversed by auriferous quartz

reefs . The gold in the sandstones differs from that of

mgwo o o

Algonkian Po ts dam Beds Tra chy t o idS c h/s t Cambria n) Pho n o l it e p l a cers of

‘

(Pre Cambrian) withAurif ero us P l eis t o cene ageBa s a l Cangl o me ra te .

FIG . 64.— SECTION OF THE D ISTRICT N EAR LEAD IN THE

N ORTHERN BLACK H ILLS , SOUTH DAKOTA. (AFTER W . B .

D EVEREU! .)

the South African bankets in its non -crystall ine character and larger grain , but the concentration in bothcases has probably been affected by wave action .

Similar deposits o f supposed Silurian age occur inQueensland

,Australia . The Potsdam Sandstone itself

gives rise to shallow gold placers o f the usual type , inwhich are found garnet , tourmaline , cassiterite , andother minerals at Deadwood Gulch , BlacktailGulch , etc . (see Fig .

The chief gold-bearing conglomerates or bankets of


the Rand G o ldfield in South Africa occur on severalhorizons

,i n the Witwatersrand System . The origin o f

the gold is sti ll a matter of some uncertainty ; by someit i s claimed that it has been introduced subsequentlyto the formation o f the conglomerates , but the balanceof evidence is in favour o f i t being in a detrital statedeposited contemporaneously with the enclosing rock .

The Witwatersrand System in its upper auriferouspart is divided into the following

"

series in descendingorder

The E l sburg Series ,The Kimberley Series ,The Bird Series ,The Main Reef SerIeS

,

and has an aggregate thickness of about feet .The gold has been found in each series on o ne

somewhat variable horizon , marked by conglomerateseparated by quartz i te and shales which are not knownto contain the metals .

I n the Johannesburg district , the rocks of the Witwatersrand System form a synclinal basin which attainsto a length of 100 miles from east to west , and abreadth o f fi fty miles . They are bounded by graniticand schistose rocks belonging to the fundamental complex, and have been considered to belong to theDevonian System . I t i s quite possible , however , considering the evidence obtainable , that they are O lderthan the Devonian , and may even be pre -Cambrian .

They outcrop on the north side o f the syncline atJohannesburg , and on the south at Klerksdorp, Venterskroon

,Heidelberg

,and N igel .

Some conglomerates also occur at a higher horizonthan the Witwatersrand System , in the Black Reef

2 5


Series, at the base of the Transvaal System ,and above

the Ventersdorp or Witwatersrand amygdaloid (VaalSystem) .The banket of the Main Reef Series consi sts of threeor four seams of conglomerate , ranging from 2 to 20

i nches i n thickness ; but occasionally a seam will reacha maximum of about 3 feet .The gold of these deposits i s not visible to the unaidedeye , i s crystalline , and is , as far as can be j udged , onlyin the matrix o f the conglomerates . The pebbles ,which are mostly vein -quartz with a l ittle quartzite ,do not contain gold except when traversed by cracks ;the m atrix is highly silicified .

There are several good arguments against the theoryof placer-origin for these deposits

,and in favour of the

Introduction o f the gold by subsequent infi ltration ,such , for instance , as the absence o f nuggets ; but thefollowing facts point more or less conclusively to thedetrital character o f the metal .The gold values vary with the texture of the deposit ,the coarser conglomerates— those with more matrix inproportion— being the richer .The gold is restricted to the conglomerates , and isabsent from the other sediments associated with them .

I t was present in the conglomerates before they suffereda certain amount of contemporaneous erosion , with theformation of wash -outs . ’ I t is independent of dykesand faults , and also ‘

O f the distibutio n o f pyrites , whichmight have acted as a precipitant .I t i s assumed , therefore , at any rate fo r the present ,

that these deposits were in reality beach -placers—placersS imilar to those well known in N ew Zealand and theUnited States—and that the gold is no t the result o f


reefs , i t does not seem necessary to Claim fo r the placernuggets any other than a detrital origin . I t is

,how

ever , possible that the welding Of small particles of goldinto nuggets may take place naturally during theformation of alluvial deposits .The gold of placers is usually associated with some

si lver,which , when in considerable quanti ty , gives the

alloy an exceedingly pale colour . I t is also found ,associated with platinum and the allied metals

,in some

o f the gravels o f the Sierra N evada and Brazil,i n the

deep leads of Victoria in Australia,and in the rivers

o f Assam and Mysore in I ndia .

The richest alluvial deposits are always adj acent to ,i f not resting on

,the parentrock

,the coarser gold being

nearer the source .

I n E urope examples of alluvial gold deposits may bedrawn from

'

the British I sles,France

,Spain

,North

I taly , and Central E urope .

I n E ngland gold occurs in almost al l the alluvial tindeposits of Cornwall (p . associated with cassiteritemagnetite , wolfram , and less frequently molybdenite ,derived from the granites of the county, the metamorphic aureoles , and the veins in the killas .I n I reland it is met with in the gravels of the Gold

Mines R iver and other streams i n County Wicklow ,

with an assemblage of minerals similar to those ofCornwall .I n Scotland it has been washed from the alluvium of

the streams draining the Leadhills,and has had i ts origin

in auriferous pyrites . I n Wales it occurs in the alluviumo f the Co thi, in Carmarthenshire , and has been derivedmost probably from the numerous small quartz-veinswhich traverse the older rocks .


In Central E urope gold has been found in smallquantities in the alluvium of the Rhine and severalo f the larger rivers drain ing the Alps and Carpathians .The Rhine alluvium has been exploited between Rheinauand Dexland ,

about fi fty miles from Basle,where it con

tains,besides some gold , titaniferous iron ore and traces

o f platinum . Gold occurs in the rivers of NorthernItaly draining the Alps o f the Monte Rosa district , ando f France and Spain draining the igneous and metamorphic rocks of the Pyrenees .I n Asia the rivers of the Ural Mountains and Siberia

have long been famous for thei r al luvial gold andplatinum . I n the Ural district , t he noted field ofB ogo slowsk l ies to the north , and includes the minordistricts of the Rivers Lozva and S o ssva , tributaries ofthe Tavda . The parent - rocks are chiefly diorites andserpentines .Farther south lie the chlorite and talc-schist districto f E katerinburg , beyond which i s the rich field o fZlatoust .In the latter district the auriferous alluvium occurs

along the tributaries o f the Miask , which itsel f flows inthe granit ic and gneissose district between the I lmenMountains and the U rals .The Miask placers are some of the richest in theUrals

,and are famed for the large nuggets which from

time to time have been discovered . Thirty miles fromN izhne Tagilsk auriferous gravels l ie beneath peatmosses .

In Central Siberia the great district of the Yenise i isone of igneous and metamorphic rocks

,while to the

west lie the subsidiary drainage areas of the Vitim , theO lekma , tributaries o f the Lena , and of the upper part


o f the Amur . The deposits are often covered by peat ,varying from 2 feet up to 160 feet in thickness ; but asa rule the overburden does not exceed 1 2 feet .The auriferous deposits of the N orthern Yeniseiskdistrict al l contain bismuth

,magnetite , and occasionally

garnets ; but those of the southern area contain browniron ores , i ron and COpper pyrites, and occasionally

native copper .Near the head-waters of the Yenise i occur the

deposits o f the Atchinsk and Minussinsk districts , wherethe rocks are chiefly gran ite

,diorite

,syenite

,diabase

,

and basalt . I n China the province of Shantung hasbeen the main source o f Chinese gold fo r centuries .The ore occurs in river-al luvia , but the bed-rock i sdifficult o f access owing to large amounts of water .Alluvial gold has been found in India , Turkestan , andAfghanistan

,also in a great many districts in Tibet ,

China,and Burma

,and in the i slands O f Sumatra ,

Borneo , and Japan .

I n India most o f the streams o f Assam , Mysore ,Madras, Bombay , Hyderabad (Haidarabad) , ChotaN agpore (or Chutia Nagpur) , and Northern India ,which drain areas o f metamorphic rocks and theGondwana formation

,contain the metal .

The Fermo -Carboniferous rocks , as we have seen ,contain detrital gold . I t occurs also in the alluvium

o f the G o daveri, i n the neighbourhood of Go dalo re .

The streams of the H imalayas are practically barren .

The deposits of Sumatra occur chiefly on the western

coast,i n a region of metamorphic schists traversed by

granites . The ore , i n place , exists as quartz -veins withcopper and iron pyrites

,t raversing the schists, and in

association with diorites .

392 TH E GEOLOGY OF O RE DEPOS ITS

small quartz-veins and stringers which occur in themetamorphic schistose rocks . This is t rue in a measure

,

but there seems reason to believe that gold-values arealso controlled by basic igneous dykes

,epidiorites

,etc . ,

as in the Guianas and many other districts . The goldo f the Alaskan deposits has not travelled any greatdistance , and occurs either free or associated withpyrites . On the coast certain beach -placers are ex

tremely rich .

I n British Columb ia nearly every stream containsgold in i ts alluvium , but the richest placers are connected with the Rivers Anderson , Fraser , and Columb ia ,including between them the district o f Caribo o . Theworking o f these deposits , which consist largely ofre- sorted glacial dri ft , i s greatly impeded , as in Alaska ,by the Cl imate .

The parent - rocks are chiefly talcose and chlorit icslates and schists , which occasionally Show signs of

intense metamorphism .

The Caribo o district , which is probably one of themost important

,consists of a dissected plateau with an

average elevation of some feet above sea- levelThe deep valleys are l ined with an accumulation o f

glacial dri ft,O ften 1 50 feet in thickness , through which

the rivers make their way and carry on the processeso f sorting and concentration . I n these valleys theauriferous deposits O ften exist both as shallow anddeep placers

,the latter lying beneath a false bottom

o f consolidated and almost impervious dri ft .Farther south

,i n Vancouver I sland , similar rocks

have given rise to fai rly rich placers , especially in the

Leech R iver district .I n Western Canada valuable placers occur in connec

DETR ITAL DEPOS ITS

tion with the Whale R iver, and with the ChaudiereR iver in Quebec .

I n the northern States of the United States, such asOregon

,Washington

,Montana , Wyoming , and Dakota ,

auriferous al luvia are well known .

I n Central Washington the placers are similar tothose of Alaska in character, but the gold is O f a verypale colour

,owing to the fact that i t i s alloyed with

a certain amount O f si lver.The Wyoming deposits are o f interest in so far as

they contain platinum , and the Dakota shallow placershave already been mentioned in connection with thePotsdam Sandstone .I n Montana and Idaho placers have been met with in

the E lk City district at American H i ll and Tiernan ,those o f the latter place being between andfeet above sea- level . They vary in thickness from60 to 1 00 feet , but the gold is almost all containedin the lowest 20 feet . The bed - rock consists o f gneissesintruded by veins of pegmatite .

I n the western States , the alluvial deposits o f Califo rnia , Nevada , and Colorado with N ew Mexico , aresome of the most famous in the world .

The Cal i fornian placers occur on three distinct levels,and belong to three different ages . The oldest occupythe plateaux

,and rest direc tly on the parentrock they

are not connected in any way with the present drainagesystem , and are considered to be o f Tertiary age . O fmore recent date are the gravels o f the present valleys

,

which exist in at least two stages . The O lder valleygravels form terraces flanking the hi ll -sides they havebeen cut through at later periods by the river

,which has

deepened its valley and left the older gravels high above


the present drainage - level . O ccasionally they havebeen covered by flows o f l ava

,as in Tuolumne County

,

and are in many instancesworked as deep leads beneatha roof of volcanic rock . The youngest deposits consisto f the alluvium o f the rivers

,as now forming in the

river-bed and on the flo o d -plains .The chie f districts where such placers are developedare those contain ing the three Yubas and their tributaries , while plateau deposits occur between thevalleys .

The source o f the gold is i n part in the metamorphicand

’

igneous rocks o f the ancient complex , but thedistribution of the metal seems to be governed to someextent by quartz veins which traverse the Jurassic rocksof the S ierra Nevada

,and also by diabase intrusions and

lava -flows .

The gold in the veins of the Cripple Creek andBlack H i l ls districts exists in depth partly as telluride ,but at the surface i t i s most O ften native . I n fact ,i n such districts as N evada

,the San Juan region o f

Colorado , Cripple Creek , and Mexico , the placers arederived from propyl it ic gold-veins connected withaltered andesites

,from the fluo rspar-bearing telluride

veins,and from sericitic and calcitic masses which

also yield a variety of metallic sulphides .I ridium has been found in the placers of the Sierra

N evada .

I n Central America and the West Indies alluvial goldoccurs in Honduras and Hayti . I n Honduras theplacers are most common on the Atlantic slope, andhave had thei r origin in sulphidic quartz reefs whichcontain auriferous pyrites

,galena , zincblende, cinnabar,

and stibnite .


antimonial titanates of i ron and calcium (derbylite andlewisite) , together with tripuhyite arefound in gravels contain ing cinnabar .In Chi l i , where the rivers are almost al l short and

rapid , the placers are confined to the coastal regions,and are derived entirely from the volcanic and metamorphic rocks of the Andes .Austral ia has long been noted fo r the production oflarge quantit ies of gold

,and , as in most other cases, the

earlier mining was more or less confined to the Shallowplacers . Detrital gold deposits occur chiefly in Victoria

,

N ew South Wales , and Queensland ; in Character andage they correspond closely to the placers of California ,for they occur as plateau—deposits , deep leads occasio nal ly covered by lava-flows , and as shallow placers .The recent alluvium is often formed at the expense ofthe more ancient detritus.

I n Victoria deep leads are well developed in theMurray R iver district , while S imilar deposits are overlain by basalts, regarded as o f Pl iocene age , in theE astern P lateau and G ipp ’s Land .

I n the Ballarat district detrital deposits o f several agesare met with , and in G ipp

’s Land they are well developedi n the Valley of the Dargo , and are derived chiefly fromLower Palaeozoic rocks . Victorian placers , besides theusual minerals, are known to contain topaz, pleonaste ,sapphire , wolfram ,

and cassiterite (p . especially inthe districts o f Maryborough and Castlemaine . Theyhave been derived from a variety o f rocks, amongstwhich hornblendic rocks and granites play a prominentpart .I n N ew South Wales the capital l ies in the centre o f

the auriferous tract , but the placers of the interior are


difficult to work on account of the shortage of water .Gold occurs in the sands o f the River Clarence inassociation with cinnabar . The most valuable detritaldeposits are the marine placers, such as those ofMontreal ,and Corunna

,fourteen miles to the north , which are

b lack ferruginous sands, derived from basic igneousrocks

,and contain a good percentage of gold . Similar

deposits occur at Wagonga,and below the silt o f

Wallaga Lake .The alluvia o f the rivers of Queensland , on the seaward side of the coast range , all yield gold in smal lquanti ties ; but on the other side , flats and narrow stripsof alluvium are fairly rich . The gold has been set freeby the oxidation of auriferous pyrites in the countryrock . Some of it i s alloyed with silver ; i t has alsobeen found on the high - level plateaux .

In South Australia the River Torrens yields themetal in small quantities

,but fairly rich placers occur

in the districts of E chunga,J upiter Creek , and Barossa

thirty-five miles from Adelaide .The E chunga and Barossa deposits are shallowplacers and deep leads

,derived most probably from

auriferous quartz -reefs The deep leads in Victoriaare considered to be o f Pl iocene age , and vary from20 to 1 00 feet in thickness .I n Tasmania placers occur at L i sle , near MountArthur , and along the P ieman River , near the westcoast .

I n N ew Zealand the auriferous deposits are chieflyconfined to the southern island , along the westerncoast , west and south of the G reat Divide . They arebest developed in the districts o f Otago , Westland , andNelson , representing an area o f some square

398 THE GEOLOGY OF O’

RE DEPOS ITS

miles . They consist o f the ordinary shallow placers , ofa thick series o f gravels o f Tertiary age in the widervalleys

,and recent beach -placers . The Tertiary gravels ,

as developed in the O tago and Charlestown districts ,are partly concreted

,yield varying quantit ies o f gold ,

and rest usually on a denuded surface o f the parentrock .

STANN IFEROUS DETR ITAL DEPO S ITS .

Nearly all the important t in -fields o f the world contain , in addition to lodes , deposits O f detrital t in -orederived from the wearing away o f the granite and contact -metamorphic rocks in which the tin - lodes occur .The cassiterite i s as indestructible as most otherminerals o f which alluvial deposits may be formed , and ,although O ften found in grains o f m icroscopic size , isnevertheless capable o f being concentrated into workable deposits by reason o f its high specific gravity .

I n its concentration in true alluvia , the l ighterargil laceous , micaceous , or very fine sandy material ,derived from the degradation of the rocks during thel iberation o f the ore from the lodes

,i s washed away

from the heavier particles o f t in ore Which settle inlayers in the lower parts o f the deposits , and whichmay be subsequently covered by sands or gravelscontain ing no tin ore . This more part icularly happensnear the mouths o f stream -valleys which have beensubmerged .

As a shoad-material i t i s also found on gentle slopesbelow the outcrops of tin lodes

,mixed with ordinary

superficial broken and decomposed rock,or ‘ head ,

’ aformation which , although travell inggradually down thehill -slopes with more or less rapidity according to the


with in the inland detrital flats, or so -called moors .These moors or mosses are wide

,i rregular

,shallow

hollows, often united by low divides , infil led with coarsedetrital material and tinstone , and overlain by layerso f sand , gravel , and peat , of no great thickness . Theprincipal o f these moors , the Goss Moor and the RedMoor , etc .

,near St . Austell , are situated at a level o f

about 400 feet above the sea , on what appears to be therel ics o f an O ld marine shelf or platform , cleaned and

sharpened up in Pl iocene times . At various levels,either above , up to , or below 700 feet , less conspicuousdetrital flats occur

,which have also , i n

'

times past ,been worked for tin ore

,and more recently for wolfram .

These moors extend laterally in tributary - l ike armsup to altitudes of 700 , 800 ,

or even feet , and thedetrital material in them appears to merge insensiblyinto ‘ head ,

’ the pre-G lacial decomposed fragmentalrock which covers as a mantle the whole of the Westof E ngland . The flats , the widest of which is the GossMoor (one and a quarter miles) , have a comparativelyrapid slope towards the streams which d rain them , thegradient being that of torrential conditions , varyingfrom , but rarely exceeding , an angle o f thirty minutes .The tin -ground is mainly confined to channels at thebase o f the detritus

,and rarely exceeds a few feet in thick

ness . The material consists of coarse or fine subangularfragments o f schorl -rock

,greisen

,gran ite , hornfels , and

other igneous and metamorphic rocks , with quartz .

The streams which drain these upland flats ormosses are in comparatively narrow V shaped valleys ,the mouths o f which have been silted up at the coast ,owing to submergence of the land since their erosion .

I n the lower reaches,therefore

,the tin deposits o f the

DETR ITAL DEPOS ITS 46 1

streams are below the level of the sea , and buriedbeneath great thicknesses of beds of sand (both riverand sea), gravel , si lt , peat , and submerged forest , upto 90 feet . Such streams have had their alluvium turnedover fo r stream -tin from the coast to their source .The beach-sands of some of the coves round theCornish coast are worked in a small way occasionally.

The tin ore extracted from them occurs partly as anatural constituent o f the sands , or is partly derivedfrom the waste material flowing down the streams fromthe dressing floors o f the inland mines .The tinstone which occurs in the head ’ i s o f thenature of shoad -material . The ‘ head ’ i s a rel ic o fancient weathering under special cl imatic , probab lypartly steppe- l ike , conditions . As the districtwas neverglaciated , the superficial fragmental rock has beenallowed to accumulate , and under special conditionshas crept or been washed down the slopes into hollows ,from whence much of it has found its way into themoors .The working of stream tin in Cornwall i s veryancient , and has been mentioned by old classic writers ,who give accounts o f the early ti n -trade between Cornwall and the Mediterranean ports .An idea of the enormous yield o f tin ore from thealluvial deposi ts (which contained , generally , considerably under 1 per cent . even in the richer patches) maybe Obtained from the estimate by Mr. J . H . Collins

,

who states that between 1 20 1 and 1 800 A.D .

tons o f metall ic tin were yielded by the stream -tIn

works alone .The M a lay P eninsu la ; Bank a and Bil l ito n .

—Thedetrital tin deposits of Malay are the world ’

s most26


important supply of this metal . The principal deposits are contained in the States o f Selangor , Perak ,and Pahang. The l ine o f granite intrusions , formingthe higher lands of the Malay Peninsula , extend fromSiam to the islands o f Banka and Bill iton , and throughout are associated with t in ore .

The deposits consist o f the usual admixture ofmaterials derived from granite , and Palaeozoic shales ,l imestones

,and quartzites . The minerals found in

them,in addition to quartz, comprise magnetite ,

i lmenite,tourmaline , topaz , hornblende , monazite ,

sapphire, wolfram , and scheel ite .

As in Cornwall , the deposits consist of layers or bedso f various materials, and reach a thickness o f 100 feetor more

,but the tin -ground itsel f may average from 4

to 1 5 feet .' The Kinta Valley, which is the most im

portant of the alluvial tin -producers , i s a wide drainagearea , measuring about forty miles in length , thirtymiles in width at its southern end , and five miles atthe northern end . The material wrought containsfrom 3 to 1 per cent . of metall ic tin

,but in places as

much as 20 per cent . The so -called karang ’ is thetin -ground proper , and often occurs stained or firmlycemented by limonite

,produced by the oxidation of

carbonate of iron solutions percolating through thebed .

The so -called kong is a type of detritus consistingof a stanniferous mixture of very fine-

grained andcoarse material . According to Mr. Scrivenor , theState geologist , the term

‘ kong ’ i s used loosely formaterial having two modes o f origin . In one case it i smerely kaolinized or rotted pegmatite

,consisting of a

mixture o f kaolin and quartz,mistaken for detritus ;


those o f Vegetab le Creek , Stanthorpe , Cope’s Creek

(near I nverell) , and Watson’ s Creek .

I n the deep leads o f Vegetable C reek the Tertiaryvalleys have been fi lled i n by successive flows of basalts

,

and the total thickness o f material overlying the tingravels may in places amount to over 500 feet . Zircon ,garnet , topaz, tourmaline , gold , and wolfram , areassociated with the cassi terite .O f the Tasmanian deposits , those near Mount B is

cho ff are of the most importance . The tinstone occursin the decomposed fragmental rock which covers theslopes of the hills

,and is a true shoad - material

,

somewhat similar to the Cornish ‘ head .

’ I n part thedeposit i s l ike scree it extends to the tops o f the hills

,

and ranges in places up to 70 feet in thickness.

The hills themselves are composed o f Palaeozoicargillaceous sediments and l imestones , cut by quartzporphyry dykes , all o f which are traversed by tinveins . To the residual deposits the term ‘ face ’ i slocally applied . The so -called ‘ white face lies nearquartz -porphyry dykes , and consists o f pieces of stannifero us quartz-porphyry

,with finer fragmental material

and sands contain ing tin ore and other minerals .The ‘ brown face ’ i s situated in basins near quartzporphyry dykes , and consists o f fragments of highlyaltered stanniferous quartz - porphyry and S late ; itreaches nearly 300 feet in thickness

,and is cemented

by l imonite and other ferruginous oxides .The S tannifero us Al luvia o f S ax o ny and B o hemia .

— AS in the stanniferous head ’

o f Cornwall,tin ore i s

found in fragmental material covering the hill - slopesi n the tin -bearing regions of the E rzgebirge

,as at

Geyer and E ibenstock . The most important deposits


of this region are, however, those of the river-valleys ,which can be broadly grouped according to thei r age asRecent or Tertiary .

The modern valleys which drain the stanniferousareas

,l ike those of Cornwall , are fil led with alternating

layers of gravels,sands , clays , and peat . The coarser

detritus at the bottom of the valleys consists o f roundedfragments of granite , contact -altered killas , and piecesof veinstone . In addit ion to tin ore

,beryl

,apatite

,

topaz, fluo rspar, and carbonate of i ron , with some gold ,are found as minerals which are para

-

genetically related .

The Tertiary alluvial deposits are only represented inone place in the E rzgeb i rge , and that is near Abertham

(at Seifen ) . H ere there is an old river-bed of earlyO l igocene age , fi lled with stanniferous gravels alternating with tourmal ine and quartz -sands and clays

,

which are covered by flows Of basalt and phonolite .

The deposits are analogous to those of N ew SouthWales , and have yielded large amounts of tin ore .M iscel laneo us Alluvia l Tin Ores — At La Paz

,i n

Bolivia , a coarse detritus, consisting o f boulders andother débris , i s found in mo untaIn torrents , and workedfo r the detrital t in ore which it contains .I n the alluvia of the Embabaan district in Swazi

land , South Africa , tinstone is met with in associationwith monazite , corundum , magnetite , and other substances , o f which some are derived from stanniferouspegmatites .

PLATINUM AND ALL IED METALS .

The description of the detrital gold deposits givenabove applies also to those placers which containplatinum , except that platinum -placers are almost


always derived from one type O f rock , while gold maybe furnished by a greater variety .

I t was seen in an earl ier chapter (p . 34) that theoriginal source of almost all platinum , and the all iedmetals osmium and iridium , lay in ultrabasic and basicigneous rocks. I t is therefore not surprising to findthat all the platinum -placers are due to the breakingdown of rocks of this character .I n E urope and Asia , platinum , associated with osmium ,

i ridium,gold

,and the mineral chromite

,occurs in the

placers o f the Ural Mountains , in the districts o f

N izhne Tagilsk, G o ro blago dat, etc . The placers o f

the N izhne district are derived from the peridotitemasses of S o lo vsaia .

Although the Ural placers taken as a group are therichest in the world

,platinum has been discovered in

many other regions,such as in the auriferous gravels of

the Miask district,i n the alluvium of certain rivers in

Borneo derived from serpentines and gabbros , and inthe rivers of Assam .

I n New Zealand the placers o f the River Tayaka arevery similar to those of the Urals ; they contain osmiumand iridium as well as platinum , and are derived directlyfrom peridotites .At the Ruwe gold mine

,in the Tanganyika district o f

Africa,recent alluvial deposits and certain sandstones

carry gold,platinum

,and palladium , but so far only the

placers are worked .

I n North America plat inum occurs in Columb ia,

associated with gold , chromite , t itaniferous magnetite ,and magnetite , while some of the sands also containrhodium . P lacers containing platinum and gold havebeen met with in the Sierra N evada

,and in California


North and South Carolina, in Brazi l , at Sanarka in theUrals , in L iberia and N igeria ; but deposits of economicimportance are also known in the river-sands of BuenosAyres in the Argentine

,and in the gold-placers of Rio

Chico (Venezuela) , of Antioquia (Columbia) , and ofSiberia .

III the placers this mineral,which is usually asso

ciated with one or more of similar character , such asxenot ime

,fergusonite , jado linite, samarskite , uraninite ,

etc . , has undoubtedly been furnished by the crystallinerocks undergoing disintegration . I t has been co ncen

trated naturally by reason of its high specific gravity

(49 to 5 3) in the lower portions of the deposit , butthose of the river placers nearest the head-waters areinvariably the richer .The monazite sands Of Brazil , o f which those of

Prado are the most Important,are derived from granites

and gneisses . They seldom exceed hal f a yard in thickness, and occasionally yield 70 per cent . of the mineral .The North and South Carolina deposits were ex

tremely rich , but' are now largely exhausted .

I n the year 1893-

94 the United States alone yieldedpounds of monazite .

IRON AND THE BASER METALS .

I ron ores occur abundantly in certain beach -placersof the United States

,N ew Zealand

,and elsewhere ,

derived directly from basic igneous rocks, and generallyknown by the name of black sands.

’ They result froma process o f concentration carried on by the waves andtide between tidal l imits .In our own country , perhaps the best example is


the titani ferous iron sand o f Porth D inl leyn,i n North

Wales . Titani ferous iron oxide also occurs as a marinedeposit on the seashores of Pahang.

The United States deposits are rich in magnetite,

i lmenite , haematite , pyrite , chromite , and monazite ,with apat ite

,zircon , garnet , epidote , etc . Magnetite

and ilmenite are generally the most abundant heavyminerals .In N ew Zealand , on the shores of the Tasman Seanear N ew P lymouth , occur titani ferous and magnetici ron - sands

,which are derived directly from the horn

blende andesites and basic volcan ic rocks of the district .

Native copper and an alloy of the metals i ron and nickelare not uncommon in some of the fluviatile and marineshall ow placers . This iron -nickel alloy has already beenmentioned in connection with the segregation o f thenative metals (p .

Native copper occurs in some o f the black sandsmentioned above , and i t has also been met with in thealluvium of the White R iver in Alaska .

The compounds o f copper , and also those of i ronother than the oxides , are distinctly rare in detritaldeposits owing to the comparative ease with which theydecompose under humid conditions . I n Arizona , however , certain metasomatic copper veins have given riseto placers of some value , containing compounds o f

Copper such as chalcopyrite and born ite in associat ionwith several other metallic sulphides and wolfram .

412 INDE!

Co pper o re , 155 - 1 76 ; comp lex , 155157 ; detrita l , 409 ; Ox id ic , 155 ,156 ; su lphid IC ,

155 ,1 56 ,

159172 ; metasomatic , 286 - 29 1 ;

pneumato lytic , 105 - 107 ; pre

cipitatio n o f, 31 1 , 3 1 2 , 319 , 324

327 ; segregatio n o f, 70 ; with

arsenic,1 72

- 1 76

Co rundum , segregatio n o f, 57

metamo rphic , 344 , 345Crush-zo nes

, 7Crystal line schIstS

,o re depo sits

o f. 336

-

338

Comp lex , igneo us 30

‘ Deep leads , ’ 380 , 394, 396, 397 .

D epo sit . See Ore depo sits

D etrita l depo sIts ; 5 , 17 , 376-

409 ;minerals o f, 381 go ld , 381 -

398 ;

p latinum,etc 405

-

407 ; tin o re ,

398-

405 ; nickel o re, 409 ; iro n

o res , 408, 409 ; mo naz1te , etc

407 , 408 ; wo lfram, 400 , 409D iabase , iro n o re from

,257 ; go ld

In» 32 ) 33

D idymium , 407D ifferentiatio n

, 4, 2 I-

7 1 in p lace,24 28 ; sequence o f

,2 2 , 23 ;

o f gabbro magma , 24-26. See

a lso Segregatio nDio rIte

,c o ntractio n -fissures in, 9 ;

go ld In, 32 , 33

D o lomitizatio n,13 , 237

D unite, p latinum In , 35

E lvans, co ntractio n-fissures in , 9

Enargite , 172- 175

E nrichment , 18. See Seco ndarychanges

E pigenetic depo sits, 6Erbium , 407E xpansio n-fissures

,I o

Fahlbands, 337 , 338

Faulting, IO

Fissures , 7- 10 ; co ntractio n 9 , I O

Fluo r-apatite , 1 14Fractures

, 7- 10

Gabbro , Carro ck Fe l l , 24- 26 ;

sil Ica in,2 4 , 30 ; d ivine and

hypersthene 62 , 63 ; iro n o res

in,24 , 39 , 40

-

48 ; nickel In , 36 ,

37 ; pneumato lysis, 1 1 1 - 1 15Galhonel la ferrugmea , 305G arnet in tIn-veins

,84. See also

M etamo rphismG lauco nite

, 330

G o ld ,derivatio n o f

,123 ; relatio n

to igneo us ro cks , 1 2 1 , 1 2 2 ; de

trital, 381 398 metasomatic ,

2 94- 298 ; minera l asso ciates o f

,

1 2 1,1 2 2 ; mustard

,144 nuggets,

387 , 388 ; pneumato lytic,

107109 ; segregatio ns , 32

-

34 ; so lutio ns o f

,1 23 , 1 24 ; precipitatio n

o f, 1 23 : te l lurides, 1 26Go ld with a lunite

,1 26 ; with anti

m o ny,

1 25 , 142 , 143 ; witharsenic , 1 25 ,

140- 142 ; WIth

bismuth , 1 2 5 , 142 , 143 ; withco pper, 1 2 5 ; with mo lybdenum ,

1 2 2 ; with p latinum , 34 ; withro sco e lite

,1 26 ; with S i lver

,1 2 7 ,

15 1 - 1 55 ; w1th tel lurIum,

1 26,

143- 15 1 ; with vanadIum ,

1 26

Go ld in alaskite , 33 ; in diabaseand dio rite , 32 , 33 ; in granite,

32 ; in laterite , 382 ; in meta

mo rphic ro cks , 357 , 358 ; in

pegmatite , 32 ; in perido tite ,

33 , 34 ; in sinter, in

syenite , 33Go ld o re, gro ups ,

1 24—1 2 7 ; pyritic

o r su lphidic ,1 24 , 1 2 7 - 140

Go ld veins. 1 2 1 - 1 55 ; genera lacco unt

,1 2 1 - 1 2 7

G o ssans, 360 363

Granite,

co ntractio n-fissures in,

9 ,10 ; go ld in, 32 ; pneumato l~

ysis co nnected with , 77 -1 1 1

su lphidIc segregatio ns in ,62

Greenalite, 32 9GreisenizatIo n

,13, 14

‘ H ead,

’

stannifero us, 400 , 40 1

Hydatogenesis, 5 , 1 16-236 ; com

pared with pneumato lysis ,1 1 7

antimo ny o res,2 19

-2 2 1 arsen

ica l o res , 2 2 1 -2 23 ; bismu tho res

,2 17 , 2 18 ; co pper o res,

155 - 1 76 ; native Co pper, 15 7

INDE!

159 ; c o pper pyrites , 159- 1 72 ;

co pper with arsenic,

1 72 - 176 ;

go ld veIns, 1 2 1 - 1 55 ; go ld withantimo ny o r bismu th , 142 , 143

go ld with silver,1 5 1 -155 ; go ld

te l lurides,143

-15 1 hydrO SIl i

ca te n icke l o re,234

- 236 ; Iro n

and manganese o res,2 30

- 234 ;nicke l and co ba l t o res, 2 14o xidic veins

,230

-236 su lphid Ic

veins, 12 1 -2 30 ; lead and zinc

ve ins,176- 196 ; silver o re ,

1 96

2 14 ; su lph idic siliceo us go ldo re

,12 7

- 142 quicksi lver o res,

2 23-230

Hydatogenetic depo sits, c lassificatio n o f

, 120 re la tio n to igneo usro cks , 1 19 . See a lso Hydato

genesis

H ydro silicate nickel o re,2 34

-236

Hydro therma l phase o f o re depositio n , 31 , 55

Igneo us ro cks , commo n o res o f,

1 5 ; meta ls In, 2 ; o re depo sits

in , 3 5 See a lso D ifferentiatio n

,Pneumato lysis , H ydato

genesis, and Metam o rphismI lmenite . See Titanifero us iro n

o re

Iridium, 34 detrital

, 394 , 406

Iro n carbo nate , 3 14-

319Iro n o re, chrome

, 53, 54 detrita l ,408, 409 ; metamo rphic , 339

342 ; metasomatic,

240-244 ;

o o l Itic , 245 -249 ; segregatIo ns o fnative iro n

, 35 , 36 ; so urce o f,

240 , 241 titanifero us , 39 , 1091 1 1

Iro n o re in basa lt, 35 , 36, 2 53 in

carbo nifero us limesto ne,2 49

2 53 in co a l -measures, 2 53 , 3 14

317 in crystal lme schists, 336 ,

337 ; in diabase , 2 5 7 ; fromgabbro and no rite

, 40-

48 ; in

lias , 245-249 ; in Lower Creta

ceo us , 245 ; in nephelme syenite,5 2 ; In po rphyrIte , 35 , 36 : in

S ilurian , 245 , 248 ; in syenitepo rphyry , 48 ; in triassic lime

sto ne,255 in veins with man

ganese o re,230 -234

4 13

Iro n o xide,segregatio n o f, 39 -53 ,

precipitatio n o f, 304-

31 1

Iro n pyrites , precIpIta tIo n as , 323

324 . See a lso Segregatio ns

M etamo rphism ,etc .

I tabirite , 108 , 137 , 352

I taco lumite , 35 2

Jacupirangite , 53

Jo intIng, 8

Jurassic iro n o res, 245

- 249 . 2 57

Kao liniza tio n , 13 ,14 , 82

Karang,

’

402 , 403Ko ng,

’

402

Labrad o rite ro ck ,iro n o re in

, 4 1 ,

Lan thanum, 407

Laterite , go ld in , 382

Lead o re,metasomatic

,264

-2 79 ;

precipitatio ns o f, 32 7 , 328

Lead veins,176

- 196

Leptothrix o chmcea, 305

Lias,iro n o res in , 245 -249

L Imesto ne,iro n o res in

, 249 tin

o re in , 102 , 103 . See a lso Leado re , etc .

Lo des,a lteratio n o f wal ls o f, 13 ,

14 ; intermittent fo rmatio n o f,

16 ; o rIgin o f, 8 struc tures o f,

10 - 1 2 ; in fau lts and crushzo nes

,I o

Magma , igneo us, 4. See alsoD ifferentiatio n

Magnetite with cassiterite , 56 ;with tItanium , 39 . See a lsoIro n o re

M anganese o res,me tamo rphic,

342-

344 metasomatic , 2 92 -294 ;

precipItated , 304-

31 1

M ercury . See QuicksilverM eso zo ic ro cks , iro n o res in , 245

M eta les Co lo rado s,2 03

M etales N egro s, 2 03M etals

,in igneo us ro cks , 2 ; segre

gatio n o f natIve, 32

-

37M etamo rphism , 1 7 , 331

-336

M etamo rphIc aureo le , tin and

co pper in,88

4 14 INDE!

M etamo rph ic o re depo sits ,1 7 ,

33 1 -358 ; in crysta l line schists ,336-338 : co rundum

. 344 . 345 ;iro n o res

, 339-

341 manganese ,

342-

344 : § u1ph1des . 345-

349Metamo rphIc ro cks , go ld in , 357 ,

358

Metasomasis, 5 , 237 300 ; chemica lreactio ns in , 242 near tin lo des ,

79-84 ; selective influence o f

ro cks , 239Me tasomatIc a l teratio ns in lo des ,

14Metasomatic o re depo sits , anti

mo ny,29 1 , 292 ; bauxite , 262

264 ; cadmium ,co pper,

286- 29 1 native co pper, 2 90 ,

29 1 go ld , 294-298 iro n ,genera l ,

240-244 iro n,co ntempo raneo us,

244- 249 iro n , subsequent , 249

2 62 ; lead and zinc , 264- 285 ;

manganese , 2 92 -294 ; tin , 298 ,

299 ; uranium and vanadium ,

299-

300

Metasomatic replacements , 16 ,

1 7 . See a lso M etasomasis

Meteo ric waters , 1 7, 18 , 362 , 363M inera ls , sequence in tin lo des

,

84-88

M Inettes,’

248

Mitternachtgange, 206, 207M ixed o res

,15

M o lybdenite, 10 1 , 1 1 1

M o lybdenum , go ld with , 1 2 2M o nazite , detrital , 407 , 408M o rgengange , 206 , 207Mulatto s ,

’

370

Mustard-

go ld , 144

N ative meta ls, segregation o f , 32

co pper,1 55 , 157

- 159 metaso

matic co pper, 2 90 , 29 1 detrita lco pper, 409 ; iro n , segregatio n ,

35 . See a lso N ickel , P latInum ,

G o ld , etc .

N egril lo s , ’ 370N egro s,

’

364N epheline syenite ,

iro n o re in, 39 ,

5 2 , 53N ickel o res

,segregatio n o f

, 36 , 37 ,

64 , 69 ; hydatogenetic , 2 14-2 1 7

234-236 ; precipitated , 328

detrita l , 409 ; arsenide , 69 , 70

silicate , 69 , 70, 2 34 236 ; sul

phide , 328 . See a lso Segrega

tio n

N o rIte, co rundum in 58 ; iro n

o res in, 39

-

48 ; schl Ieren in ,26 ,

2 7 ; su lphides in , 63

O livine ro cks , go ld in, 34 ; nickelin

. 36. 37O o litic o res, iro n ,

245- 249 , 305 ,

308-

310 ; manganese , 310

Ore chutes , 1 1 , 12

O re depo sits ,asso ciatio n with

igneo us ro cks, 1 5 ; c lassificatio no f

,1 9 ; definitio n o f

,1 ; epi

genetic , 6 ; fo rm o f, 6-8 ; syn

genetic , 6

Ore -zo nes in lo des , 1 2

O smium , 34 ; detrita l , 405O xidatio n

,in o re depo sits, 13, 1 7 ,

359-

373 ; o f carbo nate o f man

ganese , 367 ; o f chalybite , 366 ;o f co pper o res , 369 ; o f iro n

pyrites, 366 ; zo ne o f, 362 , 363 ,

366-

375O xides , precipitatIo n o f , 304in schists

,segregatIo n

o f, 24 , 37-6 1 ; in veins , 230 236

‘ Paco s,

’

2 04 , 370

Pal ladium , 406

Pavanado s ,’

370

Peach ,

’

84Pegmatite

, 33 ; go ld in, 32 , 33 ;

tin o re in, 55

Perido tite , co rundum in , 58°

chromite In, 34 ; go ld In,

nicke l in, p latinum in

,

34 35Pho no lite , go ld tel luride in,

147Pinta azu l

,

’

364Pipes in D erbyshire , 181 ; tin in ,

10 1

Piso litic iro n o re , 2 53 , 305 , 308 ;bauxite

, 313Placer depo sits, 376-

409P latinum , detrita l , 405 ; segrega

tio n o f, 34 , 35 ; segregatio n as

arsenide , 68 , 69 ; in dunite , 35in perido tite and serpentine ,

34 . 35

4m

S o lutio ns,meta l lifero us , 5 ; ascending, 360

-

362 ; descending, 362

375,

SpathIc Iro n o res , 244, 315-

317Sperrylite ,

segregatio n o f, 68, 69Sphaero

-siderite , 316

Su lphides, metal l Ic , precipitatio n

o f, 319 -329 ; segregatio n o f , 6 1

7 1 ; in the crysta l line schists,

Su lphid ic o re depo sits ,hydato

genetic ,1 2 1 2 30 pneumato

l ytic,10 5- 107 ; segregated , 61

-

7 1

go ld veins , 1 24, 1 2 7- 140

Surface waters,Oxidatio n by

, 17 .

See a lso Meteo ric waters

Syenite , co rundum in , 60 ; chalcopyrite in , 7 1 go ld in , 33 , 135 ,

136 ; iro n o re in , 52 , 53Syenite po rphyry , iro n o re in , 48

Syngenetic o re depo sits , 6

Tanta lo -nio bates in iro n o re, 53Te l luride o f go ld , 1 2 6, 143Tho rium , 407Tin o re

,detrita l

, 398-

405 ; meta

somatic , 2 98 , 299 pneumato

lytic , 88- 105 ; segregated , 54 ;with Co pper o re , 88 ; with magnetite , 57 ; in metamo rph icaureo les

,88 with wo lfram

,88

105 in vo lcanic ro cks, 103 , 104Tin lo des , sequence o f minera lsin

, 84-88 a lteratio n o f co untry

ro ck near,80-84

Titanifero us iro n o re , see Iro no re ; iro n sand , 409

INDE!

Titanium in iro n o re , 39To adsto ne , D erbyshire

, 181

To urma l Ine-co pper veins , 106

To urma linizatio n,13 , I 4, 82 , 83

Triassic limesto ne,iro n o res in

2 55

U l trabasic ro cks , go ld in , 34 . See

a lso PlatinumUranium , metasomatic , 299 , 300

Vado se circu latio n , 362 , 363Vanadium ,

metasomatic , 2 99 , 300

segregatio n o f, 47 ; go ld with

,

Vapo urs in pneumato lysis, 79 ,80

VeInS ,Oxid ic, 230 - 236 ; pneumato

lytic , 75- 1 1 5 o f silver o re

,196

2 14Veinsto nes

,structure o f, 1 1

Wa l ls o f lo des,a lteratio n o f, 13 ,

14Weathering,

be l t o f, 363White Face 404Wo l fram , detrital , 400 , 409 withcassiterite , 88- 105

THE EN D

B ILLIN G AN D SON S , L IM ITED , PR INTE RS , GUILDFORD

Yttrium , 407

Z inc o res , metasomatic ,264

-2 7 1 ,

2 79 -285 precipitated , 32 7 , 328 ;

veins o f, 1 76-1 96

Z o ne o f o xidatio n , 362 , 363 , 366

Z o ne o f reductio n , 363, 364 , 367

3740

Z o na l dIstrIbutIo n o f o res , 1 2

Mr. Edward Arno ld’

s List of

Tec ical ScientificPublicationsEx tra c t fro m th e LIVERPO OL PO S T o f D e c . 4 , 19 07

“ During recent years Mr. E dward Arno ld has p laced in the hands o fengineers and o thers interested in app lied science a large number o f

vo lumes wh ich , independently a ltogether o f their intrinsic merits as

scientific works , are very fine ex amp les o f the printers’

and engravers ’

art, and from their appearance a lo ne wo u ld be an ornament to any scien

tific student’

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list, with the resu lt that the co ntents o f the vo lumes are a lmo st witho utex ception a s worthy o f perusa l and study as their appearance is

attractive.

The Dressing o f M inera ls .

BY HEN RY LOU I S , M .A.,

Pro fesso r o f M ining and Lecturer on Surveying, Armstro ngCo l lege , N ewcastle -o u -Tyne.

With abo ut 400 I l lustratio ns. Ro ya l 8vo . , 3o s . net.

The o bject o f this bo o k is to fil l a gap in techno logica l literature which existsbetween wo rks o n Mining and wo rks o n Metal lurgy . On the interme

diate pro cesses, by which the minerals unearthed by the miner are

prepared fo r the smelter and fo r their use in arts and manufactures ,no

E nglish text-bo o k has yet appeared The presentwo rk sho u ld ,therefo re ,

be very welcome to students, as wel l as to miners and metal lurgists.

ARNO LD ’S GEO LOGICAL S ER IES .

GEN ERAL EDITOR : DR . J . E . MARR ,F.R .S .

The economic aspect o f geo logy is yearly receiving mo re attentio n in o ur

great educatio nal centres , and the bo o ks o f this series are designed in thefirst place fo r students o f eco nomic geo logy . It is believed , h owever,that they wil l be fo und usefu l to the student o f general geo logy , and

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The Geo logy o f Co a l and Co a l-M ining.

BY WALCOT G IB SON , D .SC.,F.G .S .

352 pages. With 45 I l lustratio ns . Crown 8vo . , 7s. 6d . net (po st free 73 .

The Geo logy o f Ore Depo sits.

BY H . H . THOMAS AN D D . A. MACAL ISTER ,

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LONDON : EDWARD ARN OLD , 4 1 85 43 MADDO! STREET ,W.

2 M r. E dward A rno ld’

s L td of

E lectrica l Tractio n.

BY E RN E ST W I LSON,WH IT. SCH . ,

Pro fesso ro f E lectrica l Engineering in the Siemens Labo rato ry, King'

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A Tex t-B o o k Of E lectrica l E ngineering.

BY DR . ADO LF THOMALEN .

Translated by G . w. O . HOWE , M .Sc ., WH IT. SCH . ,

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Alternating Currents.

A Tex t-Bo ok for S tudents o f Engineering.

BY C . G . LAMB,M . .A

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