Chemical Geology, 99 ( 1992 ) 51-63 51 Elsevier Science Publishers B.V., Amsterdam

Role of organics and methane in sulfide ore formation, exemplified by Kupferschiefer Cu-Ag deposits, Poland

E. Craig Jowett Department of Earth Sciences, University of Waterloo, Waterloo, Ont. N2L 3G1, Canada

(Received December 5, 1990; revised and accepted May 20, 1991 )

ABSTRACT

Jowett, E.C., 1992. Role of organics and methane in sulfide ore formation, exemplified by Kupferschiefer Cu-Ag deposits, Poland. In: P.A. Meyers, L.M. Pratt and B. Nagy (Guest-Editors), Geochemistry of Metalliferous Black Shales. Chem. Geol., 99:51-63.

Organic material has been involved in the formation of the Kupferschiefer Cu-Ag-sulfide deposits in Poland in several different ways and at several stages in their development. During sedimentation, a mixture of continental humic material and marine algal material was concentrated in the euxinic, pert-marine Kupferschiefer black shale environment ( ~ 6% Cor~). Having the black shale directly on the continental redbeds below effected a strong redox gradient across which the ore is located. Early diagenetic pyrite-(marcasite) framboids and nodules were formed through bacteriogenic reduction of sulfate from seawater-derived pore waters. Metal zones and variations in organic composition indicate that upwelling oxidized ore fluids were reduced by organic material and Fe-sulfides in the shale, inducing the precipitation of Cu-sulfides. A network of natural hydrofractures, formed by generation of hydrocarbon-related gases in the shale, allowed the ore fluids to penetrate the otherwise impermeable caprock and concentrate sulfides. Natural gases also migrated up from Carbon- iferous coal source beds and accumulated in the Cor~-free eolian sandstone below the Kupferschiefer shale. Geologic and cathode luminescence evidence indicates that the CH4 reacted with anhydrite cement to form distinctive calcite cements and H2S which precipitated Cu-sulfide cements, although ~I3C- and ~34S-values are inconclusive. Quartz replacement concomitant with sulfide formation in the Kupferschiefer and Proterozoic Nonesuch ores (Michigan, U.S.A.), and in Athabasca oil sands (Canada), provides field support that silica-organic complexing is a mechanism that accelerates dissolution and increases solubility of quartz at low temperature and neutral pH.

1. Introduction

Organics play critical roles in concentrating metals in sediments and have been studied in a variety of sediment-hosted ore deposits (e.g., Macqueen and Powell, 1983). Recent organic geochemistry studies on stratiform Cu-Ag de- posits in black shales (e.g., Piittmann and Gossel, 1990) further document these roles. This paper will use the famous Kupferschiefer Cu-Ag deposits in Poland (Fig. 1 ) to show the

Correspondence to: E.C. Jowett, Department Earth Sci- ences, University of Waterloo, Waterloo, Ont. N2L, 3G1, Canada..

variety of ways that organic compounds di- rectly or indirectly affect the formation of ore- grade sulfide mineralization.

2. Geologic and geochemical setting

2.1. Black shale on rift redbeds

The Permian geology of southwestern Po- land is described in Jowett et al. (1987), and only a brief outline will be given here. In Early Permian time, rift-related volcanics were ex- truded onto the Carboniferous basins in the fo- reland of the Hercynian orogeny. Fault-con- trolled sedimentation of the continental

0009-2541/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

52 E.C. JOWETT

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T c • M T S . \'~.f...['-,..

,5*E 0, 50, I£0'7 E km

Fig. I. Map of southwest Poland showing tectonic zones and location of Kupferschiefer ore deposits. Mining districts: 1 = Lubin; 2 = Konrad, 3 = Nowy Kosciol and Lena. Section A - A ' is location of Fig. 3.

Rotliegendes siliciclastics formed a basin and range topography (Fig. 2 ) .The undulating sur- face of the Weissliegendes eolian dunes com- pletes the sequence• In contrast to the tectoni- cally active Rotliegendes, the Zechstein marine transgression and deposition of the euxinic Kupferschiefer black shale occurred during a tectonically quiescent period. Although the de- positional envi ronment is controversial, the Kupferschiefer may have been formed in a peri-marine, coal-swamp environment. Sev-

eral cycles of platform carbonates and thick evaporites complete the Zechstein sequence (Fig. 2).

This continental rift setting is ideal for stra- t iform Cu-Ag deposits and is common to the majority of its type around the world (Jowett, 1989). Of genetic importance are: ( 1 ) the ox- idized volcanic detritus as metal source; (2) the saline lakes for oxidized brines; and (3) basement highs to channel fluids. The black shale positioned directly on oxidized siliciclas-

ROLE OF ORGANICS AND METHANE IN SULFIDE ORE FORMATION 53

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E O'

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; • " • . " . • • • U. Rot l i egendes• . • ' . . " . . . . . .

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::::::::::::::::::::::::::::::::::::::::::::::

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/~2222" i" / , . . * • • * ° , • , • * •

. ° • • • • •

• ° • • ° • • ° • • • °

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/ / / / Fig. 2. Permian and Triassic basin across the Fore-Sudetic Monocl ine (looking N W ) showing the enclosed Rotl iegendes rift oxidized siliciclastics and volcanics capped by the reduced Kupferschiefer and Zechstein mar ine carbonate and evap- orite sequence. The Triassic Buntsands te in marks a second rift associated with the format ion of the Tethys ocean. Meth- ane deposits exist below the Kupferschiefer caprock, and metal zonat ion indicates upward and lateral movement of ore fluids from the Rotliegendes.

tic sediments effects strong geochemical and hydraulic conductivity gradients, hence an e f fective geochemical trap for upwelling ox i dized fluids

2.2. Indigenous organic matter

The diversity of original organic material in the Kupferschiefer and the equivalent Marl Slate in England is documented by Gregory (1930), Stoneley (1958), Blaszczyk et al. ( 1981 ), and Schaumberg ( 1981 ). Fossils in- clude continental plants, lingula brachiopods and reptiles, suggesting a nearshore, brackish environment; bivalves, echinoids and forami- nifera, suggesting a marine environment; and

abundant fish, which could be either freshwa- ter or marine in origin. Fossil land plants are commonplace in the Kupferschiefer and in- clude leafy, spore-bearing plants; a wide vari- ety of woody, seed-bearing plants; remains from a variety of conifer trees; and large branches of fossil wood (Stoneley, 1958 ). Fos- sils also include filamentous and branching non-calcareous algae. Because ( 1 ) the amount of continental plants and dolomite decreases upwards, (2) marine fossils increase upwards and (3) the Kupferschiefer is gradational into the overlying marine carbonate (e.g., Gregory, 1930; Tomaszewski, 1986; Oszczepalski and Rydzewski, 1987), the Kupferschiefer black shale may represent a transgressive sequence

54 E.C. JOWETT

from a peri-marine coal-swamp or lagoon en- vironment into a shallow-marine intertidal- subtidal environment.

Plentiful and diverse organic matter, on av- erage 6% Corg (S. Oszczepalski, pers. com- mun., 1990), presents the Kupferschiefer in sharp geochemical contrast to the oxidized, or- ganic-free redbeds immediately below (aver- age <0.1% Corg). As discussed below (p.56), the Corg in the Kupferschiefer shale acted as the major reductant to ascending metalliferous brines during diagenesis (Fig. 3 ).

2.3. Bacteriogenic iron sulfides

Pyrite (with minor marcasite) is the domi- nant "background" mineral in the Kupfer- schiefer, comprising 2-4% by volume outside the intensely mineralized zones (Rydzewski, 1969; Oszczepalski and Rydzewski, 1987; Roth, 1989). It occurs as disseminated sub- hedral grains, spheroids, framboids and lenses. Framboids consist of 9-30-/~m clusters of 0.5- 2.0-Ftm spheroids of FeS2 and are intimately associated with organic matter in the shale ma- trix (Rydzewski, 1969) and also occur in the marine carbonates immediately above the black shale (Fig. 4A). They are characteristic

A A" X Fore-Sudetic Monocline

• ~ Carbonate Ore zl CARBONATE Fore- Sudetic ~ Pyrite , \ ' , C " BSO"EEEO

:__ ____-- ____ __ ,

I Z ~ ............. --."1t ~ " Sandstone Ore

I/.,,f / / l . :

0 5 km Fig. 3. Successive metal zones cut across the Ca/carbon- ate, Kupferschiefer black shale, and Weissliegendes sand- stone (uppermost Rotliegendes) in the Lubin Cu-Ag mines (section A-A' in Fig, 1 ). The "'Rote F~iule" is the hematite-rich zone where upwelling ore fluids (arrows) have oxidized the indigenous pyrite in the basal Zechstein.

of pyrite formed early in sediments by sulfate- reducing bacteria, and are found in modern marine environments (Rydzewski, 1969; Ber- ner, 1984). Pyrite framboids are enclosed and replaced by later base..metal sulfides (Ryd- zewski, 1969; Oszczepalski, 1989; Roth, 1989) and are removed by ore-stage oxidation, indi- cating their indigenous nature. Pyrite- ( mar- casite ) also form elongate or irregularly shaped nodules ( 1-30 mm long) in the shale (Fig. 4B) and locally in the uppermost shaley sandstone, likely as a product of late diagenesis (Bonnell and Anderson, 1987 ).

Sulfur isotope and organic geochemical evi- dence show that some sulfur in the Kupfer- schiefer ore deposits is externally derived (Jowett et al., 1991a, b; Ptittmann and Gossel, 1990), but most comes, from oxidation or re- placement of FeS2 formed by reduction of sea- water SO]- during very early diagenesis. The indigenous organic matter therefore played an integral, though indirect, role in the ore-form- ing process by providing the appropriate eux- inic environment for sulfate-reducing bacteria to flourish.

3. Environment of ore formation

3.1. Rote Fdule and metal zoning

In central Germany and southwestern Po- land, the Kupferschiefer and adjacent litholo- gies are presently mined for Cu and Ag. Miner- alization crosscuts the stratigraphy at a very shallow angle from the overlying carbonate through the shale to the underlying sandstone (Figs. 2 and 3) (Rydzewski, 1978; Jowett et al., 1987). Areas of intense sulfide minerali- zation surround hematite-rich zones ("Rote F~iule" ) in which the original organics and py- rite of the Zechstein carbonate and Kupfer- schiefer shale have been oxidized and largely removed (Rentzsch, 1974; Piittmann et al., 1988; Oszczepalski, 1989).

Base metals are distinctly zoned laterally and vertically in successive belts of chalcocite, bor-

ROLE OF ORGANICS AND METHANE 1N SULFIDE ORE FORMATION 55

Fig. 4. A. Early diagenetic framboidal pyrite in basal Zechstein ( C a / ) from central Poland represents indigenous sulfide No. 1711/ B. Lenses of pyrite in the Kupferschiefer shale appear to be formed throughout the diagenetic history of the shale outside the ore zones No. 1750.

nite, chalcopyrite, galena and sphalerite around and above the Rote F~iule. This well-developed and regionally extensive zoning represents a permanent record of the paleo-flow directions of the metal-bearing and likely sulfur-bearing solution, i.e. upward to the Rote F~iule and then laterally basinward toward the pyrite zone (Figs. 2 and 3).

3.2. Hydro fracture permeability

Sulfide-calcite veinlets commonly occur in the Kupferschiefer shale and have been inter- preted as natural hydrofractures formed dur- ing primary hydrocarbon-related gas migra- tion out of the shale in a crack-seal mechanism (Jowett, 1987). When popped open to release the gases, the fractures were kept open by over- pressured fluids contained beneath the thick Zechstein halite-anhydrite beds. Distinct mineral bands parallel to the veinlets and sys- tematic isotopic variations away from the veinlets indicate that sulfur- and metal-bear- ing fluids migrated through the fractures and infiltrated the adjacent host rock (Fig. 5 ). The network of veinlets thus represent a fossilized fracture permeability system used by the fluids to penetrate the shale "seal". Similar fracture fillings occur in the Proterozoic Nonesuch

- . . - ' - +.. Cp e.+

.... .... C p . . • - + . . . . . . • . . . . ~ . . . . . . . . . . . . .

+ , . . . . . . . . . . .

.. ,+_ C p •

Cm fO _.

1

cp'PY

py'cp

p y •

6 ~4S -35 - 3 0

j .

t

1107

Fig. 5. Sulfides in veinlets and lenses in the Kupferschie- fer shale have consistently higher ~34S-values than dis- seminated sulfides, suggesting ,ore fluids migrated through these natural hydrofractures and penetrated the adjacent host rock to mix with the indigenous sulfide (Jowett et al., 1991a).

Shale in Michigan (Fig. 6), but in this case, the paleo-flow direction has been interpreted as vertically upward through the thick shale se-

56 E.C. JOWETT

:i~'~i~"~.: (discordant)

Cu2S ha

c m

~ C u 2 S veinlets ant)

o 1

L ¢n,l I

Fig. 6. Discordant and concordant CuzS veinlets occur within the Proterozoic Nonesuch Shale at White Pine Mine, U.S.A. These fractures have mineral haloes around them similar to the Kupferschiefer sulfide veinlet (Fig. 5 ), and may also represent natural hydrofractures.

quence with no lateral component (Brown, 1971). Mineralized veinlets in black shales may be a general indicator of fluid and gas ex- pulsion from hydrocarbon source rocks, or they may be the conduits used by fluids to traverse "impermeable" seals. The indigenous organic matter is necessary to form the fractures dur- ing thermal degradation and thus promote economic sulfide concentrations.

3.3. Thermochemical reduction of incoming ore fluids

3.1. I. Mass-balance evidence. The average Corg content of unoxidized Kupferschiefer (barren pyritic and ore zones) is ~ 6.1 wt% (range 2.4- 16 wt%), but in the oxidized zones (Rote F~iule) it drops rapidly to ~0.2 wt% (range 0.1-0.9 wt%) (Oszczepalski, 1989). Pyrite content similarly diminishes from ~ 3 wt% to

essentially all hematite in the Rote F~iule (Osz- czepalski and Rydzewski, 1991 ). By compar- ing geochemical analyses of the barren pyritic zones, the ore zones, and the barren Rote F~iule zones of the Kupferschiefer, Jowett et al. (1987) show that the economic metals (Cu, Ag, Pb, Zn) in the ore zones had to have been externally derived. The ore sulfides formed at the expense of indigenous pyrite and organic matter which were destroyed while acting as reductants for the incoming solutions.

3.3.2. Sulfur isotope gradients. The presence of high ~34S-values in sulfide veinlets within the Kupferschiefer indicate that the ore fluids used the network of fractures to penetrate the shale and introduce isotopically heavy sulfur from the Rotliegendes below to mix with indigenous pyrite sulfur. This external component is sup- ported by strong ~345 gradients within the dis- covery S-2 hole drilled in 1957 through what is now the Sieroszowice Mine (Fig. 7 ). The (~345- values of ~ - 3 2 . 5 % o in the basal ore zone (Cu2S only) decrease over mere centimeters to ~ - 40.0%0 in the upper ore zone (Cu2S only), demonstrating that incoming isotopically heavy sulfur has mixed with indigenous light sulfur. Incoming sulfur' from the Rotliegendes would be dominated by the oxidized SOl- species, and therefore reducing organic matter was integral to sulfide ore formation.

3.3.3. Organic geochemical gradient. Rentzsch (1974) demonstrated a carbonification trend of Kupferschiefer organics on a van Krevelen diagram. Organic matter in the oxidized Rote F~iule are of higher coal rank (lower H / C and O / C ) than in the Cu zones, which in turn are higher than the more distal Pb-Zn zones (Ta- ble 1 ). Ratios of C / N are less in the Rote F~iule (22-29) than in the sulfide zones (22-47). Rentzsch (1974) interpreted these variations as a more oxidizing environment during de- position of the Rote F~iule "facies" rather than the result of a diagenetic influx of ore fluids.

Evidence that indigenous matter was oxi-

ROLE OF ORGANICS AND METHANE IN SULFIDE ORE FORMATION 57

~345 (%0) -45 -40 -35 -30

ilili iii iiif • . . . . . . . . . . . . . - . - . . • . . - . . .

: : : : : : : : C h a l c o c i t e - (Cu2S)

Rote F~iule (Fe2 03)

I I I 1 I I I I I I ' l ~ Ji~ I JI •

". " i ". " . i"i "-J Rotliegendes i " i

-.s,?~,!o,, ~ I Fig. 7. Strong gradients of O34S-values of Cu2S in the his- toric S-2 drill hole decreasing away from the Rote F~iule hematite zone suggest isotopically heavier sulfur was in- troduced by upwelling ore fluids. Sieroszowice Mine, Poland.

TABLE 1

Organic maturity indicators (mean values) within Kup- ferschiefer metal zones determined by Rentzsch (1974)

Rote Fgule Cu Pb-Zn

H/C 0.584 0.799 0.960 O/C 0.104 0.118 0.126 C/N 22-29 22-47 22-47

dized by incoming fluids during late diagenesis is demonstrated by Piittmann et al. ( 1988 ). In a stratigraphic section from the Konrad Mine, Poland, the extractable bitumen is signifi- cantly oxidized within the Cu-Ag-sulfide zone immediately above the oxidized Rote F/iule. Compared to the overlying pyrite zone, bitu- men within the ore zone is dominated by un- substituted aromatic compounds (naphtha- lene, biphenyl, dibenzofuran, dibenzothio-

phene, phenanthrene) with relatively low amounts of related H-rich alkylated aromatic derivatives and saturated hydrocarbons (Ptitt- mann et al., 1988; Pfittmann and Gossel, 1990) (Fig. 8). Rather than degradation by micro- bial activity or meteoric: water, the removal of aliphatic hydrocarbons and aliphatic side- chains of aromatic systems is interpreted as the effect of incoming oxidizing ore solutions after partial diagenetic maturation of the organics (Pfittmann et al., 1988). The Cu-Ag-sulfides were precipitated by a redox-type reaction and the indigenous organic material and hydrocar- bons acted as reductants.

3.4. Methane-anhydrite reaction

Whereas there is sufficient indigenous re- ductant in the Kupferschiefer shale to reduce incoming ore fluids, the Weissliegendes eolian sandstone is exceptionally free of organics and pre-ore sulfides. This clean sandstone none- theless hosts half the economic sulfides in the Lubin district and therefore must have con- tained a strong yet ephemeral reductant at the time of ore formation. A likely ephemeral re- ductant is methane. The Weissliegendes plays host to natural gas deposits throughout Europe with the Zechstein evaporites as caprock; es- pecially significant is the giant Groningen res- ervoir in the North Sea with ~2-1012 m 3 re- coverable C H 4 (van Wijhe et al., 1980). Polish gases are mainly CH4 and N2 in composition and are often in the same locales as economic sulfide concentrations (Peryt, 1989). These deposits cannot be mined because of the gas but venting adjacent to Alpine-age faults al- lows mining in the Lubin district. The gas was generated in Carboniferous strata underlying the Permian Rotliegendes (Glogoczowski et al., 1977), and migrated up through the Rotlie- gendes basins, possibly coeval with the ore fluids (Fig. 2 ).

However convenient the presence of CH4 is now, direct evidence for the former existence of CH4 as a reductant is slim and consists of:

E . C . J O W E T T

% Cu I I I I

i-U 7-::: N _ Z : Z -

o ~ . ~

--I Ph/Y.MePh

58

ppm Ag

| i t t • i i L

0 10 0 4 0 100

Fig. 8. Gradients in organic composition (phenanthren/total methylphenanthrenes) away from the Rote F~iule hematite zone suggest that upwelling ore fluids oxidized the basal organics of the Zechstein rocks (Piittmann et al., 1987 ). Konrad Mine, Poland.

(1) significant quartz dissolution, discussed below (p.61), and (2) replacement of evapo- rites by secondary calcite and copper sulfides, discussed here. The Rudna mine, being far- thest from the zone of upwelling and oxida- tion, is hosted mainly by Weissliegendes sand- stone at the top of the Rotliegendes (Fig. 3). The paleotopography of the Weissliegendes sandstone formed a series of large NW-trend- ing sand dunes high enough to pinch out the Kupferschiefer shale (Blaszczyk, 1981 ). Within these dunes are isolated patches of an- hydrite cement formed during Zechstein-age sabkhas (Fig. 9). Impinging on these rema- nent patches and implying an initially far greater extent of anhydrite are cements ofchal- cocite (Cu2S) and calcite. Petrographic and cathode luminescence evidence shows that these cements formed at the expense of anhy- drite (Fig. 10A) and that calcite and sulfides are genetically associated, although not neces- sarily coeval.

Even though it cannot be duplicated in the laboratory, the non-biogenic reduction of SO4 z- by organic reductants at low tempera- tures ( < 100 ° C ) is generally accepted because

of the persuasive geologic evidence (e.g., An- derson and Garven, 1987). For the Weisslie- gendes situation, the following reaction to re- place anhydrite is appropriate:

C a S 0 4 + CH4 ~ CaCO:~ + H2 S + H2 0 ( 1 )

Copper chlorides are the dominant dissolved species when in equilibrium with the hematite, sulfate and calcite of the Rotliegendes, and would precipitate sulfides in the presence of reduced sulfur species:

2CuC1 + H2 S ~ C u 2 S -I- 2HC1 (2)

3.4.1. Sulfide precipitation. Calcite and H2S are produced in reaction ( 1 ) but when metal ions are present to precipitate sulfides at the site of H2S generation [reaction(2)] , calcite is not formed and may actually be dissolved (Ander- son and Garven, 1987) due to the formation of H2CO3 as in:

CaSO4 + CH4 + 2 C u C l - ,

C u 2 S + CaCl2 + H2 (703 + H 2 0 (3)

and

R O L E O F O R G A N I C S A N D M E T H A N E I N S U L F I D E O R E F O R M A T I O N 59

H~CO3 q-faCO3 ~ 2 H C O j - + C a 2+ (4)

3.4.2. Calcite precipitation. If metal ions are not present, calcite precipitates and the H2S from reaction ( 1 ) migrates away in solution, possi- bly precipitating sulfides "down stream" when it encounters metal ions. Replacement of cal- cite by Cu2S is common both in the sandstone and the shale hosts (Fig. 10B), suggesting sul- fide formation "down stream", even though some occurrences are close to anhydrite rema- nents. Replacement of sulfides by calcite is not seen.

3.4.3. Calcite-sulfide coprecipitation. The only locales in which ore sulfides and calcite are de- monstrably coeval and presumably in chemi- cal equil ibrium are in the black shale itself within certain antitaxial veinlets where calcite and ore sulfides are intimately intergrown (Jowett, 1987). The bui ldup and release of pore fluid pressure to form these natural hy- drofractures (discussed on p.55) would re- lease CH4 and CO2 generated from the humic material in the shale. The escape of CO2 out of

the hydrofractures would reverse reaction (4) causing calcite precipitation at the same time as sulfide precipitation.

3.4.4. Isotopic signatures. Reactions (1) and (2) explain the calcite and sulfide products formed from anhyrite and presumably CH 4 precursors. Calcite ~13C-values between ma- rine cements ( ~ - 3 to + 1%o) and Rotlie- gendes gases ( - 35 to - 24%o; Glogoczowski et al., 1977) and sulfide ~345-values between anhydrite ( + 10%o) and Kupferschiefer indig- enous pyrite ( - 50 to - 3 0 % o ) would confirm this reaction. However, isotope values to date from the Rudna dunes show no indication of light organic carbon in the calcite cements ( ( ~ 1 3 C a v g = - 1.4°/oo, s-- _+0.3) (E.C. Jowett, unpublished data) , nor of heavy sulfur in the sulfides (~34S~-40O/o0). Average values of Ol3C~ - 1.6°/oo, and ~34S= - 22.1°/oo, -31.3°/oo and -32 .5%o for chalcocite, bornite and chal- copyrite, respectively in calcite-sulfide vein- lets within the shale ore (Table 2 ) also do not support the strong geologic and petrographic evidence that the methane-anhydr i te reaction occurred. Further work is necessary to resolve

: " " :~r , ' ,~ ~ ' / ~ : " '

- . . ' . . . - . ' . . - : ' . . . - . ' . : ' . . . ' . . ' . . . . ..... -.'.'...'..........~..,~,~-r .,,-~-.'..:..................'~...'-~'--~:-~'..'...-: .'. " . : . . . . " . " . ' " ' . ' : ' . " . " ' , ' ' " : " . " . " • ". ' . ' ." -." . ' : " " . L - : ~ . , " ' ' " ' " " : . " . ' . . . . ' . . ' . - , ' " , . . . ' . . . ' . , . ' . . , . . z - - - ~ . _ . ' - ." ,

.":-. . . . : .'. ' .".:.:...'.'~';~.. -;:'.'.'.: :.:':.. ...... 10"1 " . " : : : . . . . ' : ' " " ' " " " S "'" " ' ~ ' ; " ' " " " " ...... " t

i..:. . : : i ' : i ~ e , . , e ~ ° ~ ~::~..:-.:.:.:.: : " : " ' I " . . , ~ e ' s S l ~ ~ . . . ~ . . . . ' : . ' . ' . . " / . . . . : . ~.,, I . . - , ~ : . " F - " ~ . ' . i

:: : - : ~ ~:~, '~.- ' te~ Oe° I m . ~ . - . - . %ao'U,'~--" I ' : . : . . . . " 0 L i

0 100

Fig. 9. Schematic cross-section of Weissliegendes dune with remnants of anhydrite cement surrounded and replaced by Cu2S and calcite cements (after Blaszcyk, 1981; J. Blasczyk, pers. commun., 1990). The petrographic and geologic evi- dence here in the Rudna Mine, Poland, suggests that CH4 and CaSO4 reacted at low temperatures ( < 105 °C) to produce H2S and CaCO3.

60 E.C. JOWETT

Fig. 10. A. Anhydrite cement a in Weissliegendes sandstone is replaced by calcite c No. 1226. B. Calcite and Cu2S (black grain) replace "insoluble" quartz grains q in the Weissliegendes sandstone ore. Cu2S replaces the calcite cement No. 26a. C. Native copper c, Cu2S and bitumen b replace quartz grains q in sandstone ore of the White Pine Mine, U.S.A. D. Mature bitumen b replaces quartz grains q at White Pine with classic "caries" texture. E and F. Athabasca tar sands quartz grains are highly corroded and embayed similar to (B)-.(D), suggesting that organic compounds facilitate quartz dissolution (Bennett et al., 1988) No. A4358.

ROLE OF ORGANICS AND METHANE IN SULFIDE ORE FORMATION 61

TABLE 2

.Average 834S-values of (%o vs. CDT) of Kupferschiefer sulfides of different textures (Roth, 1989; Jowett et al., 1991a, b)

Veinlets and lenses Disseminations and streaks

n m s n m s

Chalcocite 6 - 22.1 13.6 9 - 33.2 1.4 Bornite 19 -31.3 1.8 20 -32.4 1.5 Chalcopyrite 12 -32.5 2.0 14 -32.9 1.8 Galena 4 - 30.7 3.2 6 - 37.6 6.9 Pyrite 19 -36.4 10.6 14 -30.8 7.0

n=number of samples; m=mean value; s=standard deviation.

this contradiction, but it can be attributed to apparent "swamping" by carbon having a ~I3C marine carbonate signature of - 3 to + 1%o.

3.5. Enhanced quartz solubility

3.5.1. Kupferschiefer and White Pine deposits. In the Weissliegendes ore, calcite and chalco- cite commonly replace lithic fragments, sili- cate grains and quartz grains (Niskiewicz, 1980; Mayer and Piestrzynski, 1985) (Fig. 10B ). Because quartz solubility is very low ex- cept at unreasonably high pH ( > 10), replace- ment of quartz grains has been attributed to acid consumption associated with the reduc- tion of sulfate (Anderson and Garven, 1987), a reasonable scenario for the Kupferschiefer, but not the sole possibility.

A similar situation is seen in the White Pine Cu-Ag mine, Michigan, U.S.A., where chalco- cite and native copper replace quartz grains in the Copper Harbor Conglomerate sandstone below the Nonesuch Shale (Fig. 10C). Sur- prisingly, however, bituminous material also appears to replace quartz grains and over- growths (Fig. 10D ). Whereas the hydrocarbon (CH4) in the Polish Weissliegendes is ephem- eral and has left no trace in the ore deposits, the bitumen at White Pine has been, in effect, "caught in the act" of replacing quartz.

3.5.2. Athabasca oil sands. Oil-saturated sands from the Athabasca Formation in Canada were impregnated with epoxy and sectioned to see how widespread this phenomenon might be. Quartz grains with classic "caries" corrosion textures are commonly observed in the oil sands (Fig. 10E and F) in this case, filled with epoxy.

3.5.3. Silica-organic complexes. Silicate min- eral dissolution and corrosion are greatly in- creased in the presence of organic acids, espe- cially strongly complexing ones such as citric acid (Huang and Keller, 1972; see review in Jackson et al., 1978 ) although quartz was not considered. However, support for the petro- graphic observation of significant quartz dis- solution and replacement in this paper comes from recent experimental work by Bennett et al. (1988). They demonstrate that quartz sol- ubility is greatly enhanced in the presence of certain organic acids (especially citric and ox- alic) at low temperatures and at neutral pH. Silica-organic acid complexes were docu- mented in citrate, oxalate and pyruvate solu- tions by UV-difference spectroscopy (Bennett et al., 1988 ). Other field evidence suggests that the presence of oil contamination, peat and coal can all increase the rate and solubility of quartz (Bennett and Siegel, 1990) and with- out the need for geologic time.

The examples set out in this paper and in Bennett et al. (1988) suggest that quartz cor- rosion in association with natural hydrocar- bons may be commonplace and a contributor to secondary porosity in sandstone reservoirs and along migration pathways.

Acknowledgements

The author wishes to thank Phil Meyers and Lisa Pratt for the opportunity to present these ideas and the helpful reviewers of this journal. This research is funded by Natural Sciences and Engineering Research Council of Canada

62

grant ~URF0037724 and by the Centre for Groundwater Research.

Waterloo

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