ZINC AND CADMIUM AT SOUTH CROFTY MINE

Zinc and Cadmium at South Crofty Mine

N. LeBoutillier

BSc PhD MCSM CGeol EurGeol FGS

A report, for Baseresult Ltd, on the occurrence of zinc and cadmium within the

workings of South Crofty Mine, Pool, Cornwall.

April 2005.

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Contents Title Page………………………………………………………………………..…Page 1

Contents………………………………………………………………….………...

List of Figures in text……………………………………………………..……......Page 3

List of Plates in text…………………………………………………………..…....Page 4

List of Tables in text………………………………………...………………..……Page 5

Introduction…………………………….…………………...…………………….Page 6

Part 1 – The Cornubian Ore Province………………………………….……….Page 7

1.1 Introduction…………………………………………………………………….Page 7

1.2 History of research………………………………………………………..…..Page 11

1.3 Overview of mineralisation…………………………………………………..Page 16

1.3.1 Introduction……………………………………………………………..Page 16

1.3.2 Main-stage lode mineralisation……………………………..….……….Page 17

1.3.3 The nature of the mineralising fluids……………………..………...…..Page 26

Part 2 - The Mineralogy of South Crofty Mine……………………………….Page 28

2.1 Introduction……………………………………………………………..…….Page 28

2.2 Paragenesis: South Crofty Mine……………………………..………...…..…Page 29

2.3 Paragenesis: New Cook’s Kitchen Mine………………………..……..……..Page 40

Part 3 – Zinc and Cadmium at South Crofty Mine………………………..….Page 67

3.1 Introduction……………………………………………………………..…….Page 67

3.2 Distribution…………………………………………………..……….....……Page 68

Conclusions…………………...…………………………………..…………...…Page 72

References…………………………………………………………………..……Page 74

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List of Figures in text.

Part 1 – The Cornubian Ore Province Figure 1.1. A map of the orefield of South-west England……………………………….…….……….p.9

Figure 1.2. A comparison of the zonation models of Davison and Dines…………..……..……..…….p.12

Figure 1.3. The pattern of mineral zoning in the St Agnes District…………………..…………….…..p.13

Figure 1.4. A section through a typical Sn-Cu lode…………………………………..…………..…….p.18

Part 2 - The Mineralogy of South Crofty Mine Figure 2.1. A geological sketch map showing the location of South Crofty Mine………….……...…..p.29

Figure 2.2. A plan of the 340 fm level of South Crofty Mine,

showing the major lodes worked…………………………………………………………………..…....p.30

Figure 2.3. The paragenetic sequence seen in the lodes of

the deeper workings of South Crofty Mine………………………………………………...……..……..p.32

Figure 2.4. A sketch plan of the workings on North Tincroft Lode……………………………….……p.41

Figure 2.5. A section through North Tincroft Lode…………………………………………...….…….p.42

Figure 2.6. The development of the North Tincroft Lode……………………………………………...p.45

Figure 2.7. The paragenesis of phase 2 mineralisation in the North Tincroft Lode……………………p.54

Figure 2.8. The paragenesis of phase 3 mineralisation in the North Tincroft Lode……………………p.61

Part 3 – Zinc and Cadmium at South Crofty Mine Figure 3.1. A section through North Tincroft Lode…………………………………………...….……p.71

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List of Plates in text.

Part 1 – The Cornubian Ore Province Plate 1.1. The Dolcoath North Lode, South Crofty Mine………………………………………..……..p.21

Plate 1.2. The NPB2 Lode, South Crofty Mine……………………………………………..……….…p.22

Part 2 - The Mineralogy of South Crofty Mine Plate 2.1. The 3B Pegmatite Lode, 380 fathom level, South Crofty Mine……………………………..p.33

Plate 2.2. The No:10 Lode, 340 Fm level, South Crofty Mine…………………………………………p.35

Plate 2.3. An SEM backscatter micrograph of a lode sample from South Crofty Mine….………….....p.37

Plate 2.4. An SEM backscatter micrograph of a lode sample from South Crofty Mine……….…….....p.37

Plate 2.5 A stope on North Tincroft Lode, above deep adit level……………………………………....p.43

Plate 2.6. An SEM backscatter micrograph of a lode sample from North Tincroft Lode….………..….p.46

Plate 2.7. An SEM backscatter micrograph of a lode sample from North Tincroft Lode…………...….p.46

Plate 2.8. An SEM backscatter micrograph of a lode sample from North Tincroft Lode…………...….p.47

Plate 2.9. An SEM backscatter micrograph of a lode sample from North Tincroft Lode……………....p.47

Plate 2.10. An SEM backscatter micrograph of a lode sample from North Tincroft Lode….....…….....p.48

Plate 2.11. An SEM backscatter micrograph of a lode sample from North Tincroft Lode……..……....p.48

Plate 2.12. An SEM backscatter micrograph of a lode sample from North Tincroft Lode………..…....p.49

Plate 2.13. An SEM backscatter micrograph of a lode sample from North Tincroft Lode……..……....p.49

Plate 2.14. Phase 2 mineralisation, North Tincroft Lode………………………………………..…...…p.50

Plate 2.15. Phase 2 mineralisation, North Tincroft Lode………………………………………….....…p.51

Plate 2.16. Phase 2 mineralisation, North Tincroft Lode………………………………………..……...p.51

Plate 2.17. An SEM backscatter micrograph of a lode sample from North Tincroft Lode……..…...….p.57

Plate 2.18. Phase 3 mineralisation, North Tincroft Lode………………………………………..……...p.58

Plate 2.19. An SEM backscatter micrograph of a lode sample from North Tincroft Lode……….....….p.59

Plate 2.20. An SEM backscatter micrograph of a lode sample from North Tincroft Lode……….....….p.60

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List of Tables in text.

Part 1 – The Cornubian Ore Province Table 1.1. Estimated total mineral and metal production from South-west England………….….……p.10

Table 1.2. Hosking’s model of the paragenetic sequence within the Cornubian Orefield…………..…p.15

Part 2 - The Mineralogy of South Crofty Mine Table 2.1a. The results of XRF analysis of samples of lode material from

North Tincroft Lode of New Cook’s Kitchen Mine……………………………………………..…..….p.62

Table 2.1b. The results of XRF analysis of samples of lode material from

North Tincroft Lode of New Cook’s Kitchen Mine………………………………………………….....p.63

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Introduction This report describes the nature and distribution of zinc (and cadmium) mineralisation at

South Crofty Mine. Previously largely unreported, zinc mineralisation, as part of a

larger polymetallic assemblage, has been shown to be much more common than

previously thought. The zinc-rich stopes seen in the adit level workings on North

Tincroft Lode, within New Cook’s Kitchen Mine (within the South Crofty sett) have

their analogues in several of the surrounding mines, and similar assemblages have been

found across Cornwall.

These polymetallic assemblages occur close to surface and would have been opened to

mining as early as the 16th Century. At this time only the copper ores (and to a lesser

extent tin) would have been of any value, so zinc, arsenic and tungsten minerals (some

of these metals had yet to be isolated and were unknown) were dumped as waste

products. As mineralogy was in its infancy these details (even into the 19th Century)

went unrecorded and now only fragmentary information remains, recorded by a small

number of geologists (though they may not have thought of themselves as such), from

the late 18th to early 20th centuries, who were privileged to see Cornwall’s mines in their

heyday.

The ‘rediscovery’ of the polymetallic mineralisation on North Tincroft Lode in 1998 has

lead to a re-appraisal of the nature and distribution of this type of mineralisation and the

results are presented in this report, which is divided into 3 parts.

Part 1 gives an overview of the Cornubian Orefield and main-stage lode mineralisation;

Part 2 describes the mineralogy of South Crofty Mine and details the zinc-rich

mineralisation found within New Cook’s Kitchen Mine; and Part 3 discusses the

distribution of zinc and cadmium throughout South Crofty as a whole. A brief summary

is supplied in the conclusions and some recommendations made regarding measures to

combat potential zinc and cadmium transfer into the mine discharge, as and when the

mine reopens.

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Part 1: The Cornubian Ore Province. 1.1 Introduction.

The Cornubian Orefield is the most intensely mineralised belt in the British Isles and it

has been exploited continuously for over 3000 years (Penhallurick, 1986). Early legends

of visits by Phoenician traders remain unsubstantiated, but later Greek accounts of

trading for tin at the ‘White Hill of Ictis’ (St Michael’s Mount, which 2500 years ago

would have stood out in a flat-lying wooded plain close to the coast) in the

‘Cassiterides’ (Tin Isles) are generally accepted. Julius Caesar, writing in the first

century B.C, speaks of tin production in Britain and it is likely that the mineral wealth

of the island (and its strategic position on the Irish gold trade route) was an added

incentive for the Roman invasion in 44 A.D.

Most of this early tin production came from placer deposits (Penhallurick, 1986).

Surface exposures (particularly on the coast) of lodes were also worked, principally by

opencast methods or by driving on lode into cliffs. These ‘coffin’ workings are still

visible on the coast around St Just [SW370313], close to the workings of Geevor Mine

[SW375345] and at Botallack [SW363335] (Noall, 1993; 1999). Underground mining

seems to have started around the 12th century. The granting of royal charters in 1201

and 1305 (setting up the Stannary Parliament, with its independent taxation, legal and

control systems) was of major importance to the tin trade, granting miners special

privileges with regard to land access, prospecting and mineral extraction. The granting

of these rights saw in a major phase of prospecting across the south-west, initially from

the alluvial workings on the moorlands, out into the lowland valley floors and the

discovery of lode outcrops from steam exposures and exploratory trenches. From

perhaps the Roman period until the early 13th century, Dartmoor was the principal tin

producing area in the orefield (Scrivener, 1982), and during the latter part of the 12th

century it became the main source of the metal in Western Europe. During the early part

of the 13th century Cornwall took over as the major producer (Cornish tin production

rose to double that of Dartmoor, at around 500 tonnes per annum), a position it

maintained until the closure of South Crofty Mine [SW668412] in 1998. Dartmoor’s

production steadily declined (reaching a peak of 285 tonnes in 1515) until the mid 18th

century saw a revival of its fortunes (Scrivener, 1982).

Early workings were chiefly of alluvial and eluvial placer deposits, known as ‘tin

streaming’. The tin-bearing sands and gravels were dug out from the riverbed and banks

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and the heavy cassiterite separated by sluices and crude sediment traps. Underground or

opencast workings, prior to the introduction of gunpowder, were worked by a

combination of firesetting and manual extraction with picks and chisels. The cassiterite

concentrate or rough ore was then taken to a smelter (known as a ‘blowing house’) to be

further refined, in the case of the rough ore, by crushing (using water-powered sets of

stamps) and hydraulic separation, and was then smelted. The molten metal was often

poured into granite ingot moulds, some of which still survive (Penhallurick, 1986).

The introduction of gunpowder blasting by Bohemian miners (where they worked the

tin deposits of the Erzgebirge) during the reign of Elizabeth I (Penhallurick, 1986), saw

the rapid development of underground mining in Cornwall. Initially this was for tin, but

the manufacture of brass and the use of copper in the national coinage during the 18th

century saw this metal assume prime importance in the orefield. The discovery of large

deposits of copper in the Camborne-Redruth and Gwennap districts in the late 1600’s

spurred on a further phase of exploration throughout the county, the chief focus of

which was now the discovery of lodes, as the alluvial deposits were becoming

increasingly exhausted. As production increased, the main barrier to extending the mine

workings at depth was the position of the water table and the need to pump out excess

water to keep the workings dry. This was overcome by the development of horse-

driven, water-powered, and later, steam-powered pumping engines (Pryce, 1778). The

use of steam power (building on the work of Newcomen, Boulton and Watt and

Trevithick) revolutionised the mining industry and allowed deep mining to expand

rapidly during the 19th century (Buckley, 1997). Steam engines were used to pump

water, haul up ore (and, later, men), transport materials and drive sets of stamps.

Cornwall became not only a major mining centre, but also a test-bed of new industrial

and engineering ideas that were exported across the globe.

The 19th century was the heyday of Cornish mining. After a period of closure in the

1790’s (when cheap copper ore from Parys Mountain on Anglesey almost wiped out the

Cornish copper industry) the mines proliferated and during the century over 2500 mines

were operated in the orefield as a whole (Alderton, 1993). Copper and tin were the main

products of these mines, but considerable tonnages of other metals and minerals were

produced (see Table 1.1 and Figure 1.1), particularly iron, lead (Douch, 1964), arsenic

(Earl, 1993), manganese, zinc and tungsten. During the 1860’s copper mining reached

its peak (Dines, 1956) with production reaching 15,500 tons of metal; Britain supplied

8


Figure 1.1. A map of the orefield of South-west England (after Dunham et al., 1978).

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around 40% of world consumption (and was the largest producer). Production of tin

reached a peak in 1870 with a little over 10,000 tons of metal (Burt et al., 1987). A

ruinous fall in metal prices in 1866 (brought about by the discovery of new copper

deposits in Michigan (U.S.A) and Chile; and tin deposits in Malaya) saw the mining

industry go into a rapid decline with the closure of many mines and the emigration of

thousands of miners and their families to the opening mining fields of Australia, South

Africa, Mexico, North and South America and the Far East (Noall, 1999).

Table 1.1. EAlderton (19

World me

Devon bec

crash in pr

MINERAL OR METAL TONNES (approximate) Sn metal 2,770,000

Cu metal 2,000,000

Fe ore 2,000,000

Pb metal 250,000

As (as As2O3) 250,000

Pyrite 150,000

Mn ores 100,000

Zn metal 70,000

W (as WO3) 5,600

U (+Ra, At, Po) ore 2000

Ag ore 2000

Ag metal (from sulphide ore) 250

Co-Ni-Bi ores 500

Sb ores 300

Mo metal very small

Au metal very small

Barite 500,000

Fluorite 10,000

Ochre/umber 20,000

Kaolinite (china clay) 150,000,000

stimated total mineral and metal production from South-west England. After Dines (1956), 93) and South Crofty PLC (1988-1998).

tal prices became increasingly volatile and the industry in Cornwall and

ame caught in a cycle of ‘boom’ and ‘bust’ with fewer mines surviving each

ices (Morrison, 1980; 1983). Relatively few mines survived until 1900 (when

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tin production had fallen to 2000 tons metal per annum; and copper to around 50 tons

metal per annum; Burt et al., 1987) and after a brief rise in metal prices in the early

1900’s saw prospects improve, the First World War and the loss of labour brought many

of the surviving mines to the brink of ruin. Casualties in the immediate post-war years

included the famous Dolcoath [SW660401] in 1920, and Carn Brea and Tincroft mines

[SW667405] in 1921. By the Second World War only South Crofty Mine (Buckley,

1997) and East Pool Mine [SW673415] remained in the Camborne-Redruth District

with Geevor Mine at St Just, Castle-An-Dinas wolfram mine [SW946623] north of St

Austell and Cligga Mine [SW738538] at Perranporth.

East Pool Mine (Heffer, 1985) and Cligga Mine closed in 1945 and Castle-An-Dinas

closed in 1959 (Brooks, 2001). A rise in metal prices during the 1970’s (which saw tin

eventually reaching over £10,000 per tonne) saw renewed prospecting in the South-

West and the reopening of Wheal Jane Mine [SW771427] near Truro and the opening of

Wheal Concord [SW723458] at Blackwater and Wheal Pendarves [SW645383] near

Camborne. During this period the tin price was stabilised by the International Tin

Council (ITC), formed by the main tin-producing nations, buying and selling metal on

the London Metal Exchange to keep the price as high as possible. When Brazil and

China (non members) refused to be bound by any quota agreements and flooded the

market with tin metal in October 1985, the ITC Buffer Stock Manager was unable to

buy all the metal and ran out of money. Its trading was suspended on October 24th and

the price fell overnight from £8,140 per tonne to £3,300 per tonne (Down, 1986). Wheal

Concord and Wheal Pendarves closed in 1986. Geevor mine managed to survive, in a

much reduced form, until 1991 and also in that year Wheal Jane closed. This left South

Crofty Mine as the sole surviving mine in the Cornubian Orefield. It was hoped that the

tin price would rise, but it fluctuated between £2,900 and £4,300 per tonne (averaging

£3,400); with production costs of around £4000 per tonne the mine was continuously

losing money, despite every effort to minimise costs. The mine eventually closed on

March 6th 1998, bringing to an end some 3000 years of mining history. The mine was

purchased and unabandoned in 2001; at the time of writing (2005) mining has not yet

resumed, but plans to recommence production are proceeding.

1.2 History of Research.

The early works of Carew (1602), Borlase (1758), Pryce (1778), Phillips (1814),

Thomas (1819) and Carne (1822) are excellent first hand accounts of individual mines

11


and deposits. By the early part of the 19th century scientific descriptions of lodes and

deposits were beginning to be made. The works of Henwood (1843) and, later, Collins

(1912) provide excellent descriptions of lodes, relationships between lodes of different

orientations and the nature and variable mineralisation of later faults (crosscourses), in

many famous mines of the time that were inaccessible by the early 20th century.

MacAlister (1908) recognised that the granite intrusions and their accompanying

mineralisation took place in three stages: (1) intrusion of granitic magma with

accompanying thermal metamorphism of the country rocks close to the contact; (2)

intrusion of quartz porphyry dykes along fractures and cleavage planes in the

metasediments; (3) deposition of ores in both sedimentary and igneous host rocks. With

various refinements this statement remains valid today.

The 1920’s and 1930’s saw the Cornubian Orefield used to test new theories of ore

genesis and zonation (Halls et al., 1985). Dewey (1925) and Davison (1921, 1925a,

1927) developed a model of zonation (with tin/tungsten/arsenic mineralisation passing

out into copper, then lead/zinc, and finally antimony/manganese/iron mineralisation)

based on mono-ascendant single-pass hydrothermal fluids emanating from the

Cornubian Batholith, giving rise to a concentric arrangement of mineralisation radiating

out from the exposed granite cupolas (Figure 1.2).

Figure 1.2. A comparison of the zonation models of Davison and Dines (from Hosking, 1979).

Davison’s model suggested that the focal points of this mineralisation were grouped

around original high spots in the batholith roof and that the mineral zones formed a

series of ‘shells’ parallel to the granite contact with a copper zone overlying and

overlapping a central tin zone and itself overlain and overlapped by a lead/zinc zone.

Davison cited the concentric zonal pattern of mineralisation around St Agnes

[SW723507] as an example (though this is not as simple as it first appears, Sn lodes in

12


the district dip mostly to the north, while Cu lodes dip south and are of a later phase of

mineralisation), modified by denudation, of this original pattern (Figure 1.3).

Figure 1.3. The pattern of mineral zoning in the St Agnes District, used by Davison in the formulation of

his theory of zonation (after Bromley and Holl, 1986).

Dines (1934) provided field evidence against Davison’s model. He held the view that

the various zones were considerably flatter than the granite contact (Geevor mine

remains the classic example of this phenomenon) and that the higher the zone, the

greater its lateral extent. Dines also suggested that the appearance of certain minerals in

particular zones was temperature dependent and determined by the temperature gradient

between the granite and the surface. He also noted the irregular distribution (particularly

of tin) of mineralisation related to the position of cusps in the granite roof and coined

the term ‘emanative centres’ to describe these focal points of mineralisation. His model,

however, was still based on a ‘single pulse’ mono-ascendant premise, using the cooling

granite as the only source of heat.

In 1956 Dines’ exhaustive memoir The Metalliferous Mining Region Of South West

England was published, it remains perhaps the single most important account of the

geology of mining in South-West England published to date.

13


Hosking (1964, 1979) refined and expanded on Dines’ model and took into account

earlier phases of mineralisation that straddled the magmatic/hydrothermal boundary and

pre-dated the main stage lodes. He also saw localised temperature gradients and

wallrock interactions as more important than a regional temperature gradient related to

magmatic emplacement. Hosking recognised seven depth/temperature zones

characterised by distinctive assemblages of ore and gangue minerals and also noted

characteristic wallrock assemblages associated with each zone (Table 1.2). In applying a

temperature framework to his model he was greatly aided by the work of Sawkins

(1966) and Bradshaw and Stoyel (1968) who, through the study of fluid inclusions,

found that not only were there significant differences in the depositional ranges of tin,

copper and lead/zinc mineralisation, but that each mineral species has its own, fairly

restricted, temperature zone. He also tried to apply a time frame to the span of

mineralisation, envisaging a 200 million year protracted episode running from the

Permian to the Eocene.

While the work done by these authors was ground-breaking at the time, these

increasingly desk-based studies came to rely on a rapidly diminishing ‘pool’ of

exposures in the deeper sections of the working mines and a few, still accessible,

disused workings. Geologists were not employed in Cornish mines until the early 20th

Century and the early shallow workings of the mines received virtually nothing in the

way of scientific recording of the mineralogy and structure of the lodes they worked.

Later workers (Willis-Richards and Jackson, 1989; Willis-Richards, 1990) became

reliant on using mine production records to map metallogenic zones and ‘emanative

centres’ across the orefield. While this approach gave a ‘broad brush’ picture of the

orefield, it was fundamentally flawed. The production details for many mines are scant

or non-existent, and only show what the mines sold, not what the lodes actually

contained. Ores of metals thought to be of no value or application (which was several

until the mid-19th Century, including tungsten and zinc) were discarded, as were

components of complex ores that were unable to be recovered (or if they were, had a

detrimental effect on the recovery of other metals). Such components never appeared in

the production records, even though they may have made up a significant proportion of

the ore at the time of mining. The shallow workings of South Crofty are a case in point,

as is discussed in Part II.

14


Table 1.2. Hosking’s model of the paragenetic sequence within the Cornubian Orefield (from Hosking,

1964).

15


During the 1960’s structural studies at Geevor Mine (Garnett, 1962) and South Crofty

Mine (Taylor, 1965) made important contributions to our understanding of the

mechanisms of lode formation, as did a landmark paper by Moore (1975) on the origin

of the lode-bearing fracture systems across the Cornubian Orefield. Further studies at

Mount Wellington Mine (Cotton, 1972), Wheal Jane (Walters, 1988; Holl, 1990),

Wheal Pendarves (Alderton, 1976), the St Just District (Jackson, 1977), the Tavistock

District (Bull, 1982), Dartmoor (Scrivener, 1982), the Wadebridge District (Clayton,

1992) and South Crofty Mine (Farmer, 1991) looked at the individual deposits from

mineralogical, geochemical and structural perspectives.

Moore (1982) argued that the pattern of W-Sn-Cu-Zn zoning above emanative centres

could be explained in terms of a pattern of ‘hot spot’ geothermal circulation similar to

that seen at Wairaki in New Zealand. While this model explained some of the pervasive

alteration patterns seen in parts of the batholith (e.g., the St Austell Granite), it failed to

take into account the textural evidence used later by Halls (1987, 1994) in his

mechanistic approach to the formation of the lode system.

Complex models of polyascendant fluid phases and structural reactivation had now

replaced the earlier models of Davison and Dewey. The focus of research shifted to the

geochemistry of the ore fluids (Rankin and Alderton, 1983, 1985; Jackson et al., 1982)

and the building of a geochronological framework for the timing and duration of

mineralisation. The works of Halliday (1980), Darbyshire and Shepherd (1985, 1987,

1994) and Chesley et al. (1991,1993) favoured the view of a protracted history of

mineralisation, spanning over 200 million years, that was advanced in major reviews of

the orefield by Dunham et al. (1978), Bromley and Holl (1986), Bromley (1989),

Jackson et al. (1989) and Willis-Richards and Jackson (1989). Similar studies by Chen

et al. (1993) and Clark et al. (1993) point to a much narrower timeframe and a

diachronous pattern of mineralisation across the batholith, reflected in reviews by

Alderton (1993), and Scrivener and Shepherd (1998).

1.3 Overview of Mineralisation.

1.3.1 Introduction.

Most of the mineralisation present in the orefield can be directly linked to the granite

batholith in some way, although some deposits clearly pre-date the granite and a variety

of syngenetic sedimentary and SEDEX origins have been ascribed to these (Clayton et

16


al., 1990; Clayton, 1992). Deposits falling into this category include the manganese

deposits of East Cornwall and West Devon and the stratiform Pb-Sb-Cu deposits of the

Wadebridge district in North Cornwall. The Sn-W-As-Cu mineralisation for which the

region is famous occurs in a variety of forms, but principally in high-angle fissure veins

(lodes) in or close to the granites (Garnett, 1962; Farmer, 1991).

Mineralisation across the Cornubian Orefield can be divided into the following,

chronologically arranged, groups: (1) pre-granite orebodies of sedimentary/sedimentary-

exhalative type; (2) syn-granite intrusion orebodies – skarns and pegmatites; (3) early

post-granite intrusion orebodies – greisens and sheeted vein complexes; (4) main stage

polymetallic orebodies – Sn-Cu-As-Zn-Pb lodes and carbonas, etc; (5) late post-granite

mineralised (Zn-Pb-Ag-Co) and unmineralised fissure veins – crosscourses. At South

Crofty Mine only the main stage lodes were exploited (although these included

occasional pegmatitic bodies of a type not seen elsewhere) and these are reviewed in

more detail below.

1.3.2 Main-Stage Lode Mineralisation.

The typical lodes of the province are steeply dipping (most >70o) fracture-infill veins,

which are concentrated along the axis of the batholith and are closely associated with

elvan dykes (Jackson et al., 1989). The lode system as a whole has produced almost all

the metallic output of the orefield and has produced not only tin and copper, but a range

of metals including tungsten, iron, lead, zinc, silver, etc (see table 6.1). The origin of

these metals is still in debate; the tin (and tungsten) is likely to have been derived by

fractionation, from tin-rich sediments or protolith at the point of anatexis (Lehmann,

1987), though some authors point to the possibility of derivation from the mantle

(Hutchison and Chakraborty, 1979). Though this seems less likely than a crustal source,

Shail et al. (1998) have found traces of mantle helium in fluid inclusions from the

orefield, attesting to some mantle involvement in mineralisation. The origin of the Cu-

Zn-Pb mineralisation is thought to be due to a combination of xenolith assimilation and

hydrothermal leaching of basic rocks (and pelites); it has been calculated that the

volume of basic rocks and their copper content could easily supply the amount of

copper extracted in the province (Jackson, 1979).

Though on the scale of the orefield, the lode system is extremely complex; within

localised areas a number of fairly simple relationships can be established.

17


Mineralogically (as a rule) the lodes show decreasing complexity with depth. Close to

surface they are truly polymetallic and may show a mixed oxide/sulphide assemblage

that, in many cases, has been modified by supergene activity (see Figure 1.4).to give a

Figure 1.4. A section through a typical Sn-Cu lode, showing the relative position of the gossan,

supergene and primary sections and the zoning seen in some of the major structures in the Camborne-

Redruth District (from Hosking, 1988).

large potential list of secondary minerals. These include secondary sulphides,

hydroxides, oxides, sulphates, arsenates, carbonates and native metals; many of these

were first described in Cornwall and the area has been the focus of professional and

amateur mineral collectors for centuries (Embrey and Symes, 1987).

Within this near-surface zone the ores of tin, arsenic, copper and zinc were worked

(though often in separate stages, depending on the economics of the time); many mines

also produced minor amounts of lead and silver (Dolcoath) and occasional U, Fe, Bi,

Mn, Ni and Co (Wherry Mine [SW470294], Penzance) ores. In the Camborne-Redruth

and Gwennap areas (as elsewhere) the lodes close to surface were dominated by copper

mineralisation (Dines, 1956). In the supergene zone original simple sulphides

(chalcopyrite, pyrite) were replaced by malachite, azurite, tenorite, cuprite and native

copper. Rare secondaries, such as olivenite and liroconite, etc, are known from the

Gwennap area and Porthleven area (where Cu and Pb ores occur together). Below the

oxidised zone secondary chalcocite, bornite, enargite and covellite were deposited

18


before passing back into primary sulphides below the water table. These very shallow

(often < 50 metres) rich deposits were mined at an early date (1700 onwards) and the

high financial returns gained were responsible for the proliferation of mining activity

across the orefield during the 18th century.

With increasing depth the zinc and lead mineralisation died away leaving a zone of

simple sulphides dominated by copper and arsenic (Collins, 1912; Dines, 1956). As the

granite/killas contact was approached tungsten became locally important, reaching its

greatest development immediately below the contact (e.g. Rogers Lode, East Pool

Mine; Dines, 1956). Below the contact copper declined and tin became increasingly

important, and at depth (~500 metres from surface) cassiterite is often the sole ore

mineral present.

This change from a simple oxide-dominated assemblage at depth, passing into mixed

oxide/sulphide assemblages close to the contact and complex polymetallic assemblages

at surface was the foundation for the theories of hydrothermal zonation formulated

during the early 20th century. However, the relationship between the various phases was

not always as clear cut and within a single lode there is often evidence of a protracted

history of mineralisation, brecciation, shearing and further mineralisation, that negates

the idea of mono-ascendant fluids. Most lodes do not show a single continuum of

pressure/temperature-controlled mineralisation, they show a series of punctuated events,

with the later sections of the lodes showing lower temperature assemblages. In this way

some lodes initially worked for copper may later have had the walls of the existing

stopes reworked for their tin or tungsten content (e.g. the North Tincroft Lode of South

Crofty Mine; LeBoutillier et al., 2000a; 2000b; 2001).

The gangue minerals associated with the ores also vary with depth and are also

temperature dependent (see Table 1.2). At depth (associated with tin ore) the main

gangue minerals are tourmaline (fine-grained, powdery to flinty, Prussian blue to dark

blue) and quartz. At higher elevations, lower temperatures and in lower energy

environments (in areas reactivated by further fracturing) this gives way to a chlorite-

dominated (Taylor, 1965; Farmer, 1991) assemblage (though initially in places still

retaining a proportion of tourmaline) with quartz and fluorite. Lower temperature phases

are dominated by quartz, siderite, fluorite, marcasite and rare calcite. While this trend to

lower temperature mineral assemblages and lower energy environments over time and

19


proximity to the surface is broadly correct, recent studies have shown that some shallow

deposits can also be tourmaline-dominated and that some of the chlorite assemblages

record violent brecciation events with clasts transported considerable distances

(LeBoutillier et al., 2000a; 2000b; 2001). This again suggests a series of punctuated

mineralisation events, utilising fluids from a variety of sources and under a variety of

physiochemical conditions.

Many lodes show a complex interplay between tectonically-driven episodes of

mineralisation and remobilisation of constituents by convecting hydrothermal fluids.

This last particularly applies to copper and uranium mineralisation (pitchblende and

coffinite are themselves a late-stage infill in some lodes, e.g., No4 Lode at South Crofty

Mine; Cosgrove and Tidy, 1954), which, in many secondary phases, are highly mobile

and readily dissolved. An environment in which a cyclical system of

pressure/temperature changes occurs may see the deposition of the rare fibrous form of

cassiterite known as ‘wood tin’ (Hosking et al., 1987), if the fluids are supersaturated

with respect to tin.

Often in contrast to the mineralogy (particularly at depth) the structural history of many

lodes is complex and shows a series of brecciation and shearing events responsible for

depositing a variety of individual assemblages in the lode over time. Brecciation

textures are common in lodes in the deeper workings of many mines (Dines, 1956) and

occasionally at surface (Dunham et al., 1978; Goode and Taylor, 1980). In the deeper

workings of South Crofty Mine many lodes showed an early tourmaline (‘blue

peach’)/quartz ± cassiterite breccia with a cassiterite/quartz cement (Plates 1.1 and 1.2).

This was sometimes followed by other brecciation events, but was more often followed

by further lower energy dilational episodes, giving the lode a banded appearance (some

of these bands were, occasionally, microbreccias, emplaced within the lode),

particularly along the hangingwall. Some of the reactivation episodes lead to the

deposition of later chlorite-dominated assemblages, while other events lead to fine

fracturing across the lode and the deposition of low temperature chalcedony-marcasite-

siderite assemblages.

20


Plate 1.1. The Dolcoath North Lode, 380 fathom level, South Crofty Mine. The lode is predominantly

composed of brecciated blue peach and quartz with minor (<1%) cassiterite. Some later lode-parallel

shears carry minor fluorite and haematite. The wallrock adjacent to the lode contacts is irregularly

tourmalinised. The width of the lode is ~1 metre.

This brecciated texture is due to hydraulic fracturing and explosive decompression,

similar to the mechanism seen at Wheal Remfry (Halls, 1987; 1994; Halls and Allman-

Ward, 1986). The majority of lodes appear to be extensional faults and would have

communicated with areas of lower pressure, which were accessible during movement,

along their dip-length. With the loss in pressure boron-rich (or silico-stanniferous)

fluids, that had previously been building up pressure in the fluid reservoir, were

suddenly released and travelled upwards as wallrock pore fluid pressures caused

spalling off of fragments along the sides of the lode fracture. It is difficult to ascertain

the amount of transport that took place; lode textures are fine-grained and indicative of

very rapid nucleation and a number of clasts appear to have moved very little distance

before being arrested in the crystallising fluid. Occasionally some clasts show evidence

of entrainment (rounded, rolled clasts with ‘debris trails’ in their wake), but again the

distance travelled cannot be quantified.

21


Plate 1.2. The NPB2 Lode (east drive) 400 fathom level, South Crofty Mine. The blue peach lode carries

a series of internal quartz-filled shears. The lode cuts an elvan dyke and earlier quartz floors. Dip-slip

slickenlines and lode orientation suggest that the lode occupies a normal fault with downthrow to the

south (right); though in all likelihood the area has undergone a complex series of movements prior to the

formation of the final set of slickenlines. Hammer for scale (30 cm).

The same cannot be said of the ‘breccia lodes’ that outcrop (and subcrop), most

commonly, in the Gwinear District southwest of Camborne, along the line of a buried

granite ridge that appears to be an extension of the Carn Brea ridge between Camborne

and Redruth (Beer et al., 1975; Goode and Taylor, 1980; Taylor and Pollard, 1993).

These bodies consist of rounded pebbles and cobbles of a variety of rock types

(including granite, up to 1 metre in size, though the contact is some 750 metres below

surface) set in a matrix of comminuted rock fragments of various sizes. The bodies are

22


chaotic and disordered and in places the clasts (primarily slate) are so finely ground that

the rock appears similar to a sandstone in texture, indicative of violent, high-energy

emplacement (Clark, 1990). The walls of these fractures are often scoured and polished

by the passage of material and small breccia fragments are found forced into cracks in

the walls. Some of these breccia bodies carry (later, infilling) chlorite, cassiterite and

chalcopyrite in the matrix and were mined from surface as early as Tudor times (e.g.

Relistien Mine [SW601368] at Wall, near Gwinear). The lode at Trevaskis Mine

[SW607378], nearby, is in excess of 10 metres wide and lies between intensely

brecciated and silicified wallrocks of metadolerite. The ore consists of angular slate

clasts cemented by chlorite and fine rock fragments. Within this are veins of quartz

carrying a chlorite-arsenopyrite-chalcopyrite-chalcocite-cassiterite assemblage and also

a chalcopyrite-sphalerite-galena assemblage that may be later.

Both Relistien and Trevaskis breccias carry clasts of internally brecciated elvan and

other examples are also closely associated with elvan dykes. It appears that the elvans

and breccia bodies are roughly contemporaneous; clasts of elvan moulded around other

clasts found at Trevaskis (Goode and Taylor, 1980) suggest that still-plastic elvan was

utilising the same fracture pathways as the breccia lode at close to the same time. In

other examples host slates were brecciated before the intrusion of an elvan dyke. These

fluidised explosion breccias appear to have formed under conditions of very high

pressure where the system had instantaneous connection with areas much closer to

surface than other lodes in the district. Rapid boiling and the development of a gas-fluid

medium, similar to that seen in volcanic breccia pipes (e.g., Ardsheal Hill, near

Kentallen, Scotland) lead to a violent explosive reaction with material entrained and

blown up along the fracture. This appears to have been a largely barren episode, as in

almost every case the mineralisation that accompanies these structures post-dates their

emplacement. Some lodes preserve evidence that they originated as breccia lodes and

were later reactivated during later phases of mineralisation, with large-scale

replacement of original textures and materials. Rule (1865) notes the occurrence of

rounded stones in some of the lodes at Wheal South Frances, near Carnkie and Phillips

(1896) records a large block of killas found in the Main Lode of Dolcoath Mine, some

450 metres below the granite contact.

Lodes (or later assemblages in a pre-existing structure) emplaced in a lower-energy

environment typically show a banded appearance, due to repeated opening of the lode

23


fracture. Some of these lode sections may be mylonised by fault movements, while

others appear to be open-space dilational infillings with vugs and druses of crystals (and

occasionally, as at Dolcoath Mine, pockets of carbon dioxide in sealed vugs). They may

show a range of assemblages of various temperature/pressure characteristics, or may

have been crack-sealed in a single mineralising event.

A second, later, sequence of lodes occur in many districts, which cut across and displace

the earlier lodes. These later caunter lodes (Collins, 1912; Dines, 1956) generally carry

a lower temperature, mesothermal, assemblage (dominated by copper mineralisation)

and strike E-W (in the Camborne-Redruth District). Rotation of the stress field saw

these lodes emplaced in a fracture set offset from the dominant lode trend by ~30°.

Farmer (1991) saw their origin in terms of the opening of Riedel D shears during

conditions of dextral shear; field evidence from South Crofty Mine and other locations

around the northern margins of Carnmenellis show that while the main-stage lodes are

typically associated with dip-slip or oblique dip-slip movements, caunter-orientation

structures are associated with horizontal to sub-horizontal slickenlines formed by shear

movements..

Some of the 050°-060° (dominant lode trend) lodes were also reactivated during the

deposition of the caunter lodes. At South Crofty Mine segments of caunter lode

orientation were opened up within existing lode systems (e.g. Roskear D Lode) and

infilled with an assemblage dominated by low temperature quartz, earthy chlorite,

haematite, kaolinite, fluorite and chalcedony (LeBoutillier, 1996). These ‘caunter jogs’

were economically barren and were left as pillar areas along the strike of the lode.

Irregularly shaped sub-horizontal or pipe-like replacement bodies sometimes occur at

the junction of two, or more, lode structures. These are called carbonas and commonly

consist of a fine network of veins, in altered granite, and bunches of ore. They reach

their greatest development in the Lands End Granite and the Great Carbona (Collins,

1912; Dines, 1956; Noall, 1993) of St Ives Consols is arguably the most famous. This

body (10-20 metres thick by 230 metres long, dipping at 20°) carried cassiterite, copper

sulphides and fluorite (a mineral not found in the lodes of this mine) in

tourmalinised/chloritised/sericitised granite. The average grade of the ore was 1.5% Sn,

which compares very well with the average grade in the lodes; most Cornish mines

operated at grades of between 0.70% and 2%; at South Crofty Mine the average R.O.M

24


grade was 1.5% (Owen and LeBoutillier, 1998), but varied up to 2.5%, while individual

lode grades over short strike lengths could reach as high as 40% Sn.

The main-stage lodes throughout the province are accompanied by wallrock alteration

of varying type and intensity (Scrivener, 1982). The most common types of alteration

are tourmalinisation (the progressive replacement of chlorite, micas and feldspars by

tourmaline, which may lead to the development of a quartz-tourmaline rock; in pelites

replacement of phyllosilicates may be extensive and pervasive), chloritisation

(replacement of micas and feldspar by chlorite, due to the influx of Fe and Mg in

solution), haematisation (due to the breakdown of existing chlorite, although some

textures appear due to primary replacement of feldspars and micas) and sericitisation

(the replacement of feldspars by white mica). At South Crofty Mine wallrock alteration

haloes could extend in excess of 3 metres from the lode in either direction and

sometimes showed overprinting of one type of alteration (particularly haematisation

after chloritisation) on another. Such alteration was often barren, but it was not

uncommon for Sn grades in the wallrocks (these metasomatic haloes mirror the mobility

of Sn, seen in the country rocks around the granite) to exceed that in the lodes and the

alteration zone was often included in stoping patterns and formed a significant part of

the material extracted. The impregnation of granite with cassiterite is not uncommon in

the Camborne-Redruth District; the Great Flat Lode, south of Carn Brea, consisted

primarily of tourmalinised/chloritised granite (known to the miners as ‘capel’) carrying

cassiterite over widths of up to 5 metres (Foster, 1878) around a narrow quartz leader

vein.

The dating of the main-stage lodes has seen a revolution over the past decade. Halliday

(1980) obtained dates for main stage Sn-bearing lodes of between 279±4 and 269±4 Ma.

These dates were obtained from muscovite and orthoclase, using Rb-Sr methods,

associated with various lodes across the Lands End and Tregonning-Godolphin granites.

Using a previously published age of 295-300 Ma for the emplacement of the granite

batholith, he envisaged a 20 million year hiatus between the emplacement of the granite

and the onset of mineralisation. Between these two events, and intimately associated

with the mineralisation, he placed the intrusion of the elvan dykes. This model became

refined by later workers (Jackson et al., 1982; Thorne and Edwards, 1985; Bromley and

Holl, 1986; Bromley, 1989) with a ‘second magmatic event’ (the emplacement of the

elvan dykes) some 20 million years after granite emplacement, followed by main-stage

25


mineralisation over a protracted period. This model was reinforced by Chesley et al.

(1991), who obtained Nd-Sm dates of 259±7 and 266±3 Ma for fluorites from South

Crofty Mine and Wheal Jane respectively (though their samples were not

paragenetically constrained, and came from a late stage during mineralisation).

This model was radically altered by Chen et al. (1993), who, in dating the granites and

mineralisation, were able to show that each pluton of the Cornubian Batholith had its

own discrete history of magmatism and mineralisation (a view supported by Clark et al.,

1993 and Chesley et al., 1993). They were able to show that the batholith was made up

of discrete bodies, intruded between 293-274 Ma, and that mineralisation was

diachronous across the orefield, instead of being related to a series of pan-province

events; mineralisation and magmatism also overlapped, with mineralisation related to

one magmatic pulse occurring (e.g. Carnmenellis) prior to later renewed granite

magmatism.

Clark et al. (1993) give dates of 286 Ma for main stage lodes at South Crofty Mine,

272±4 Ma for the lodes of the St Just area and 278±6 Ma for the Sn lodes of central

Dartmoor. This data indicates that mineralisation started in the Carnmenellis area some

3 million years after the emplacement of the early granites (10 million years before the

emplacement of the fine-grained Boswyn granite in northern Carnmenellis) and was

almost complete before the intrusion of the oldest (Zennor Lobe) of the Lands End

granites at 274 Ma. The emplacement of the elvan dykes now appears to be related to

later pulses of granite being tapped by extensional fractures (rather than the

radiogenically-driven remelting of a single large intrusion), some of which were later

utilised by ascending hydrothermal fluids.

1.3.3 The Nature of the Mineralising Fluids.

The ore-bearing fluids responsible for the mineralisation across the Cornubian Orefield

were the product of a series of events involving the mixing and recycling of fluids from

a variety of sources: magmatic, metamorphic, meteoric, connate and basinal. The high

heat production (due to radiogenic decay of primary U and Th) of the granites (Willis-

Richards and Jackson, 1989; Jackson et al., 1989; Lucas and Willis-Richards, 1998)

generated a convective system around the granite plutons that scavenged metals from

the country rocks and redeposited them in the lode systems; in addition to metals

supplied directly from fluids of magmatic departure. As the convective system began to

26


migrate deeper and closer to the plutons (as overall temperatures began to fall), the

fluids were responsible for remobilising some of the mineralisation (particularly

copper), giving rise to complex overprinted paragenesis in some areas (Dines, 1956;

Seccombe and Barnes, 1990).

Circulating thermal brines are still encountered today and have been analysed at a

number of localities, including the Rosemanowes [SW736346] borehole and South

Crofty Mine. Alderton and Shepherd (1977) analysed a number of springs, finding a

range of temperatures from 16 – 52oC, with salinities up to 1.5% (much lower than that

recorded from the main stage of mineralisation, where salinities over 20% are frequent)

which is half that of modern sea water. The Br/Cl ratios are comparable to seawater and

they are typically of neutral pH. As well as Ca, Na and Cl, they carry substantial

amounts of Li, Mg, K and HCO3- and are of meteoric origin. There has been interest in

the alkali metal content of these brines in the past and they have received some attention

due to their lithium content (Beer et al., 1978) which may reach up to 118 ppm in some

springs.

The reason for the persistence of thermal springs so long after the magmatic event is

due to the high heat flow generated by radiogenic decay within the granite, which in

Carnmenellis runs at 3.9 x 10-3 Wm-3 (Burgess et al., 1982; Edmunds et al., 1984).

Geothermal gradients vary from 29.8oC/km in the Carnmenellis Granite, 20 – 50oC/km

in the surrounding sediments close to the contact, 39oC/km in South Crofty Mine and

45oC/km at Wheal Jane. Current heat flow values for the various plutons are broadly

similar with Land’s End and St Austell granites running some 15% higher than the other

plutons at 127mWm-2 (Lucas & Willis-Richards, 1998).

27


Part 2: The Mineralogy of South Crofty Mine. 2.1 Introduction.

South Crofty Mine [SW667413] lies in the centre of the Carn Brea District (Dines,

1956), which is a rectangular belt of ground, trending ENE, some 9 km long by 2 km

wide, spanning the area around Camborne [SW650400] and Redruth [SW700420], from

the line of the A30 trunk road in the north, to the ridge-line of the Carn Brea Granite in

the south. The country rocks are slates of the Mylor Formation, with greenstone sills

and ENE-trending elvan dykes, and granite of the Carn Brea pluton.

The lodes of the district trend ENE-WSW, while caunter lodes trend E-W and

crosscourses trend NNW-SSE. The ENE-trending lodes fall into two groups; those that

dip north are usually faulted by those that dip south, indicating two generations of lode

structures, separated by a change in stress conditions. The Carn Brea District is the most

productive in SW England (Dines, 1956); the mines of the district are chiefly copper

and tin producers, although considerable amounts of arsenic and tungsten have been

raised and small amounts of lead, nickel, bismuth, cobalt, silver and uranium. Zinc has

not been officially produced, but extensive dumps of sphalerite (containing many

thousands of tons) formerly existed on many of the old mine sites (where it was tipped

as a waste product, having no commercial value), or in the deep river valleys north of

Camborne and Redruth; testifying to its presence in many lodes of the district.

The most important mines of the district (Dines, 1956) were Dolcoath Mine

[SW660403] (80,000 tons of black tin and 350,000 tons of copper ore), Carn Brea and

Tincroft Mines [SW686406] (53,000 tons of black tin and 360,000 tons of copper ore),

East Pool and Agar Mine [SW674416] (46,000 tons of black tin and 91,000 tons of

copper ore) and South Crofty Mine [SW667413] (12,000 tons of black tin and 37,000

tons of copper ore; this is the historical production to 1919 and estimating production up

to 1998 puts the black tin tonnage up to ~88,000 tons – exceeding Dolcoath and making

South Crofty the most productive tin mine in the Cornubian Orefield).

28


2.2 Paragenesis: South Crofty Mine.

Figure 2.1. A geological sketch map showing the location of South Crofty Mine.

South Crofty Mine is situated at Pool (see Figure 2.1, above), mid-way between

Camborne and Redruth in the centre of the most intensely mineralised belt in the

Cornubian Orefield (Dines, 1956). The surface workings are situated predominantly on

slates (of the Mylor Slate Formation), with a large greenstone sill lying along the course

of the main Camborne-Redruth road, immediately to the north of the mine site. At a

depth of around 148 fathoms (271 m) below surface (in New Cook’s Kitchen Shaft),

these metasediments give way to the Carn Brea Granite, which outcrops a few hundred

metres to the south and forms the prominent hills of Carn Brea [SW683407], Carn

Arthen [SW669399] and Carn Entral [SW663397] close to the mine site.

The mine (see Figure 2.2, below) worked a series of sub-parallel lodes that trend ENE-

WSW, dip sub-vertically and are of a complex discontinuous nature. Formerly many of

these lodes were worked in the killas for copper and minor amounts of lead, zinc and

iron. This type of mineralisation was replaced progressively, as the granite contact was

approached, by higher temperature tin and tungsten mineralisation.

29


Figure 2.2. A plan of the 340 fm level of South Crofty Mine, showing the major lodes worked. Plan drawn

by South Crofty Mine Survey Dept.

30


The lodes occupy a series of fracture zones (complex normal and reverse faults), some

of which persist for 2 km or more along strike and for dip heights of over 600 m. The

deeper workings of the mine are hosted entirely within granite, with the contact being

observed at only a few locations at the northern and western extremities of the

workings. Very little modern working took place in the killas; that which did relates to

the sinking of, and driving from, Roskear Shaft [SW654410] (Davison, 1925b, 1929)

and New Tolgus shaft [SW686429] (Davey, 1925) during the 1920’s.

Within the deeper (below the 245 fathom level), granite-hosted, workings six main

phases (see Figure 2.3) of mineralisation (LeBoutillier, 1996) have been identified:-

A. Prior to the onset of the main hydrothermal phases of mineralisation, a very

early phase of quartz veins were generated. These stacked veins, forming in tensile

fracture zones, vary from a few cm to a metre thick. They occur in swarms, particularly

around the No.2, 3 and 4 lodes between Cook’s Shaft and Robinson’s Shaft and also in

the North Pool area. They are commonly discontinuous in extent and dip from 0° to 20°.

This flat orientation has lead to them being called ‘quartz floors’; many contain only

quartz, but wolframite, scheelite, arsenopyrite and feldspar are common associates,

along with minor chalcopyrite and stannite (Cu2FeSnS4); some contain minor

cassiterite, but this tends to be a later phase introduced by reactivation.

The floors occupy ENE-trending cylindrical stacks that extend for upwards of 100

metres through the workings. Taylor (1965) considered that they occupy marginal thrust

zones within the granite, which has lead to the development of the tensile stacked shears

that the floors infill. These zones are also unusual in that a number of minor lode

structures occur wholly within their confines (the 3ABC lode series), and die out on

approaching the margins of the zone (see Plate 2.1). Some of the main-stage lodes (e.g.,

No3 Lode), on approaching these areas, become disordered and dissipate, with only the

strongest structures being unaffected.

Taylor (1966) thought this due to the pegmatite zones acting as more brittle, competent,

zones during the formation of the main-stage lode fractures. These floors represent a

crossover from magmatic to hypothermal, hydrothermal processes and, though often

referred to as a pegmatite style of mineralisation, they are not true pegmatites as they

carry a hydrothermal assemblage (Farmer and Halls, 1993).

31


Figure 2.3. The paragenetic sequence seen in the lodes of the deeper workings of South Crofty Mine.

After LeBoutillier, 1996.

32


Plate 2.1. The 3B Pegmatite Lode, 380 fathom level, South Crofty Mine. This lode (composed of a quartz-

haematite hangingwall and a massive cassiterite-tourmaline-quartz footwall) persists only within the

confines of the ‘pegmatite zone’ occupied by the quartz floors and is one of a number of similar structures

that behave in this way. The width of the lode at this point is ~0.5 m.

Some of the lodes at South Crofty also show this paragenesis (Care’s Lode, Roskear

Complex, etc) and it may be that many lodes were originally of this type and were

largely replaced or overprinted by later phases. The North Pool section of the mine has a

number of steeply-dipping structures that show brecciated fragments of early quartz-

wolframite within blue peach lodes as well as an example where a hangingwall vein

preserved the early assemblage, while the main lode contains only fragments within a

tourmalinite breccia.

1. An early black tourmaline (schorl) phase, with thin stringers of schorl (1-3 mm

in width) emplaced in a joint/fracture system or ‘lode zone’, which may be tens of

metres across. These lode zones are oriented ENE-WSW, between 050° and 060° (many

veins are joint-controlled, while others occupy joint-parallel fractures) and formed the

zones of weakness later exploited by the main-stage tourmalinite lodes. The fractures

that the schorl veins infill have great persistence along strike and up-dip (during stoping

operations they were rockbolted as they would readily produce blocks, by wedge

failure, that would glide over the mirror-like fracture planes) and are often spaced less

than 1 metre apart. Along strike the veins would occasionally form anastomosing

33


networks (nested veins) with irregular areas of microbreccia. Slickenlines associated

with this phase vary from strike-slip to oblique-slip to dip-slip (the most common).

2.a) A blue tourmaline (‘blue peach’) phase. With the onset of the main phase of

mineralisation, the reactivated lode zones were host to boron-rich fluids, depositing fine

blue/black tourmaline (schorl) and quartz in the dominant fractures (trending 050° and

060°), giving rise to the blue peach lodes (see Plate 2.2) which form the main economic

structures in the deeper workings of the mine. This phase carries the majority of the

economic tin mineralisation in the form of microcrystalline cassiterite (Taylor, 1965,

1966; Farmer 1991; Farmer and Halls, 1993). Some of this is disseminated within the

blue peach veinstone, but there is evidence from a number of lodes that, after the initial

tourmaline deposition, a residual quartz/cassiterite phase was produced; this sub-phase

is seen commonly as a fine matrix in brecciated sections of the lode. The brecciated

textures in the lodes are evidence of explosive decompression with entrainment of

spalled wallrock and lode clasts (Halls and Allman-Ward, 1986; Halls 1987, 1994), and

very rapid injection and crystallisation of new fluids to crack-seal the lode. Some lodes

show several episodes of reopening and have complex textures.

The main stage lodes in the deeper levels of South Crofty Mine (445 fm to 290 fm) are

dominated by this ‘blue peach’ assemblage As the lodes are followed up dip the

character of the gangue mineralisation changes, with tourmaline becoming less

important and chlorite ± fluorite ± haematite assemblages becoming dominant (Taylor,

1965). Dines (1956) describing Main Lode (the down dip extension of North Tincroft

Lode) on the 175fm level records little or no tourmaline present with chlorite as the

principal gangue phase, associated with fluorite and comby quartz. Brecciation textures

are found infrequently at these elevations, with dilational layered infilling becoming the

norm, marking a shift to less energetic fracturing processes and lower fluid pressure

levels. Farmer (1991) also records the gradual decline of tourmaline away from the

deeper levels of the mine, until it becomes subsidiary to chlorite above the 180 fathom

level. During this phase, and subsequent phases, the 3ABC Pegmatite and North Pool

quartz floors were reactivated and impregnated with blue peach and cassiterite. Large-

scale replacement of the feldspars in the intervening granite between the floors by a

tourmaline/chlorite/cassiterite assemblage gave rise to the large replacement orebodies

in these areas.

34


Plate 2.2. The No:10 Lode, 340 Fm level, South Crofty Mine. Although narrow (0.50 m) and barren, the

morphology of this blue peach structure is typical of the worked lodes in the deeper levels of the mine.

The tourmaline associated with the lode systems shows a distinct geochemical evolution

with each successive phase (Farmer, 1991; Farmer et al., 1991; Farmer and Halls,

1993). The magmatic tourmaline in the granite has a large dravite component

(Williamson et al., 2000; London and Manning, 1995; Trumbull and Chaussidon,

1999), the succeeding black and blue tourmaline stages show a continuous trend

towards almost pure schorl, with a corresponding rise in the Fe/Ti ratio. Farmer (1991)

noted a second dark blue/green tourmaline cycle within this stage; this tourmaline

occurs as fine millimetre-scale veinlets and breccia matrix within the main blue-peach

lodes, and shows a wider range of compositions (although still predominantly within the

schorl field) with notably less Ti than previously.

35


Movements within the lode zones, and during later phases of mineralisation, have been

interpreted in terms of high-angle reverse movements, transpressive shear (Farmer,

1991; Farmer et al., 1991; Farmer and Halls, 1993) and reidel geometries, but there is

also considerable and widespread evidence (J Usoro, pers comm.; M. Lee, pers comm.;

N LeBoutillier, unpublished data) of true dip-slip and high-angle oblique-slip

movements, more consistent with the activation of extensional faults.

2.b) A chlorite (‘green peach’) phase. This phase is characterised by dark green

(often iron-rich) chlorite (daphnite-ripidolite) as the main gangue mineral, associated

with fluorite and quartz (Bull, 1982). Tourmaline, present in only minor amounts, is

often not visible with the naked eye. Tourmaline compositions in this stage tend toward

ferridravite and show a greater range than in the previous cycle; Farmer (1991)

suggested that this could be due to tapping a different fluid reservoir, although the

increased Ti in the tourmaline could be due to release via chloritisation of the wallrocks,

which would have added a number of components to the fluid.

This phase sharply cuts the previous phases, often forming a discreet layer on the

hangingwall of lodes, or replacing sections of the lode entirely. Textural evidence

suggests that several pulses of chlorite-dominated mineralisation took place over time;

the earlier examples are characterised by brecciated textures (sometimes with minor

blue present), while the later pulses are characterised by banded fault-infill textures and

are typically associated with quartz and minor amounts of kaolin (coating fractures in

the lode); occasionally lodes display the range of textures described above and trace the

decline in energy conditions over time. Farmer (1991), using geothermometry

techniques, placed phase 2b chlorite crystallisation in the range 400°C-285°C, citing

this as evidence of protracted crystallisation, but it is not clear what the textural setting

of his samples were.

This phase of mineralisation is often characterised by coarse cassiterite (crystals to 1 cm

have been found), which may form euhedral crystals of sparable type, and frequently

contains zones of highly elevated grades; this may be due to the tapping of a highly Sn-

rich fluid or, in some cases, may be due to remobilisation of cassiterite from earlier

phases of mineralisation. In the Pegmatite Zones (3ABC, etc) reactivation during this

phase was responsible for the bulk of the economic mineralisation with wide-scale

replacement and impregnation of the granite around existing structures.

36


Plate 2.3. An SEM Backscatter micrograph of a lode sample from South Crofty Mine. Cassiterite (Cst)

occurs as large subhedral grains in a chlorite (Chl) matrix with minor (later) pyrite (Py). A grain (REE)

consisting of Ce/La oxides (?) occurs towards the centre. 3ABC Pegmatite Zone, 360 fathom level.

Plate 2.4. An SEM Backscatter micrograph of a lode sample from South Crofty Mine. A large platey

crystal of a Ce/La hydrated oxycarbonate species (unidentified) occupies the centre of the photograph

(REE), sited within a chlorite (Chl) and quartz (Qtz) matrix. Cassiterite (Cst) is present, with minor

bismuthinite (BiS) and cobaltite (CoAsS). 3ABC Pegmatite Zone, 360 fathom level.

37


Ore samples from the pegmatite zone on 360 Fm level (see Plates 2.3 and 2.4, above)

were examined by SEM. These showed the presence of large euhedral to subhedral,

zoned, cassiterite crystals in a chlorite-quartz-fluorite gangue; no monazite was present,

but Ce and La (oxides?) were detected as inclusions (1-2 µm) in quartz, and a Ce/La

hydrated oxycarbonate species (unidentified) forming anhedral crystals up to 40 µm in

length was present in minor amounts from two localities. No anatase, rutile or ilmenite

were detected (Ti species are common at shallow levels), but one sample did contain Ti-

bearing magnetite. Also, in contrast to the material from Condurrow and elsewhere, the

360 fm samples contained a number of small anhedral grains of cobaltite (CoAsS).

Bismuthinite (Bi2S3) was also present, and biotite mica was found intergrown with

chlorite in the gangue. This shows that although broadly similar in appearance and

gangue mineralogy, the 360 fm and Condurrow/Tolcarne (etc) samples are

paragenetically unique and mark two distinct mineralising events.

3. A tin-barren fluorite phase. This phase occupies sections of the lodes with E-W

‘caunter orientation’. The rotation of the dominant stress field occurred some time after

the main phases of tin mineralisation, resulting in the locking of the 050° and 060°

segments of the lodes and the opening of fractures trending 090°. Farmer (1991), who

does not recognise this as a separate stage from 2a above, ascribes this change to the

development of conditions of sinistral transpressive stress throughout the Camborne-

Redruth District and the activation of reidel D shears to host the chlorite-fluorite

mineralisation. While this may be the case on some structures in the mine, the Roskear

D Lode (which best displays the development of these caunter segments, or ‘jogs’)

shows dextral offsets around the jogs (which carry horizontal slickenlines), which could

be ascribed to the ENE-WSW shortening event seen elsewhere in the local area

(Alexander, 1997).

Caunter jogs are typically infilled with a paragenesis of fluorite, earthy haematite and

chlorite (some of the haematite may have formed from the decomposition of chlorite)

with kaolin and occasional mineral pitch (which appear as later additions). On the

Roskear D Lode they represent low grade pillar areas between the normal-strike lode

segments. Very little cassiterite is present in these sections (which are essentially

barren), though occasionally some economic cassiterite concentrations (associated with

earlier phases of mineralisation) are present as disseminations in the wallrocks.

38


4. The caunter lode phase. These lodes occupy persistent fractures, having the

same orientation (090°) as the caunter jogs. They fault the earlier lodes, where they

cross them, and are typified by mesothermal/epithermal mineralisation.

Small structures of this type are found in the North Pool area of the mine, as is Reeve’s

Lode, which is a major structure in the district, running from Camborne to Redruth. At

depth, the lode consists mostly of fluorite, with subordinate quartz/haematite/chlorite

and marcasite. On 260 fm Level it was worked for tin (as the No.7 Lode) for a short

strike length, but is barren below this level. At shallower levels (surface to 180 fm), the

lode was a major copper producer and also carried minor amounts of lead, zinc and iron.

Fluorite from South Crofty Mine has been dated (Chesley et al., 1991) at 259 Ma. The

samples used were not paragenetically constrained and it is likely that they represent a

late-stage, low-temperature, infill into a pre-existing lode. Fluorite is associated with the

caunter jog and caunter lode phase in particular (although some crosscourses also carry

minor fluorite; Scrivener et al., 1986) and this date may reflect the closing stages of the

mineralisation cycle at the mine.

5. The crosscourse phase. Crosscourses are infilled NNW-SSE wrench faults; the

mineralisation of which post-dates phases 1-4, although the faults were present much

earlier and some (such as The Great Crosscourse) are likely to pre-date the granite. The

crosscourse faults have had a profound influence on the formation of the lodes at South

Crofty Mine (with lode segments forming between pairs of faults, in the same way that

they did at Geevor Mine; Garnett, 1961, 1962) and the control of tin grades within the

workings (Owen and LeBoutillier, 1998). Several lodes (No:2, No:4, No:8) show tin

enrichment against one side of a major crosscourse, while lodes on the other side are

barren in close proximity to the structure e.g. Roskear A Lode, Roskear D Lode). The

infilling of these faults took place by fluids moving in response to Permo-Trias wrench

faulting tapping into fluid reservoirs in (presently) offshore basins (Scrivener et al.,

1994).The faults carry an epithermal paragenesis of chalcedonic silica with earthy

chlorite, haematite and minor amounts of marcasite and occasional copper and bismuth

sulphides. Displacements along crosscourses vary from a few centimetres (they are

typically of the order of a metre) to over 100 m in the case of The Great Crosscourse.

Many lodes also show intralode shearing related to this phase and carry the same

paragenetic sequence as infilling/replacements within the lode, e.g. No.4 Lode.

39


2.3 Paragenesis: New Cook’s Kitchen Mine.

Investigation of the shallow workings of South Crofty Mine has centred around the

workings on North Tincroft Lode, accessed via Engine Shaft [SW664408] of New

Cook’s Kitchen Mine (part of South Crofty Mine). Despite forming one of the major

lode structures of the Camborne-Redruth Mining District comparatively little is known

of the mineralogy of the North Tincroft lode system. The shallow extensions of North

Tincroft Lode and its neighbours were worked from the mid-16th century onwards

(Buckley, 1997), initially for tin (occurring in the gossans) and later for copper as the

water table was approached. The accounts of the lode that do exist are sometimes

contradictory. MacLaren (1919) states that North Tincroft Lode was principally a

copper lode and did not produce tin until the granite contact was reached (though South

Crofty records refute this); Hill and MacAlister (1906) record that the top of the tin zone

in Cook’s Kitchen Mine was at 200 fathoms from surface (probably on Chapple’s

Lode), while in the neighbouring Tincroft Mine, tin was present continuously from

surface, occurring with Pb, Zn and Cu sulphides. This paragenesis is also recorded in

the shallow workings of the nearby North and South Roskear Mines (Carne, 1822).

Arsenopyrite was recorded as present in the shallow workings (Henwood, 1843) and

oral tradition at South Crofty spoke of the lode being divided into a copper-rich

hangingwall zone (removed at an early date) and a tin-rich footwall zone not worked

until the 1870’s.

Later accounts relate mainly to the down-dip extensions of North Tincroft Lode, such as

the workings on Great Lode (East Pool Mine; MacLaren, 1917) and Main Lode (South

Crofty Mine) and are supplemented by recent data from South Crofty Mine. The

shallow workings are briefly described by Dines (1956, p.314) in relation to Tincroft

Mine, but contain little data relating to the nature of the lode.

New Cook’s Kitchen Mine [SW 665406] was situated between Camborne and Pool The

mine (roughly 250 m N-S and 450 m E-W), lying between the setts of Dolcoath Mine

(to the west) and Tincroft Mine (to the east) was originally the northern part of Cook’s

Kitchen Mine. The New Cook’s Kitchen section was divided from the parent sett in

1872 as a separate company (Morrison, 1980) and was eventually abandoned in 1893.

The sett was acquired by South Crofty Mine in 1899 and in 1907 the vertical New

Cook’s Kitchen Shaft was begun on the sett’s northern boundary and continued as one

of the principal shafts until closure in 1998.

40


Figure 2.4. A sketch plan of the workings on North Tincroft Lode.

The principal lode of the New Cook’s Kitchen sett was North Tincroft Lode (known as

North Lode in the neighbouring Tincroft Mine). The lode was accessed via Engine

Shaft (almost central within the sett) and a number of shafts closer to the outcrop

position along the southern boundary, the most important of which was East Shaft (See

Figure 2.4, above). Engine Shaft was sunk vertically to the 13 fathom level (below adit)

then was driven on a crosscourse with a steep easterly dip until intersecting North

Tincroft lode at the 95 fathom level (Dines, 1956). From this point the shaft was driven

on the northerly dipping lode until the 195 fathom level (just below South Crofty’s 175

fathom level). New Cook’s Kitchen Mine was a financial failure, losing £53,471 of

shareholder’s capital during its 21 year lifespan; this was not a reflection of poor grades,

but of poor rates of development and the lack of a set of stamps and processing

equipment to produce black tin (Morrison, 1980). The stopes were reworked by South

Crofty until well into the 1930’s and formed a significant part of the mine’s reserves.

During the first decade of the 20th century, significant discoveries of ore were made in

the shallow workings around East Shaft, which, for a time, accounted for 33% of South

Crofty’s total production (South Crofty, 1909).

41


Figure 2.5. A section through North Tincroft Lode. The areas sampled during this study lie between the

deep and shallow adit levels. From South Crofty Geology section drawn by N. LeBoutillier.

The North Tincroft Lode (see Figure 2.5, above) strikes ENE-WSW and crops out along

the southern boundary of the sett, some 200 m south of Engine Shaft, where it dips to

the north at around 30o. It passes through shallow adit level (~28 m below surface) and

steepens slightly to ~35o by deep adit level (~45 m below surface), just below which it

steepens markedly to ~60-65o and continues with this inclination through the granite-

killas contact at the 100 fathom level. At around the 175 fathom level it is displaced by

the southerly dipping Pryce’s Lode and exhibits an extensional separation of ~40

fathoms. Below this intersection it is known as Main Lode and persists to South

Crofty’s 315 fathom level where it branches from the hangingwall of South Crofty’s

No. 1 Lode, which has been proved to the 400 fathom level. Main Lode was South

Crofty’s principal lode up to the 1950’s and has been correlated with East Pool Mine’s

Great Lode (MacLaren, 1917) and Barncoose Lode of Barncoose Mine. The North

Tincroft structure and its branches therefore persist vertically for ~800 m from surface

and can be traced along strike for distances in excess of 2 km.

North Tincroft lode was accessed via the vertical upper section of Engine Shaft, which

intersects the deep adit level 45 m below surface. From this point a drive runs south for

some 65 m before the lode drive is reached. The lode in this area is variably stoped (see

42


Plate 2.5) above the drive and also, in places, below, with sections of the drive floor

mined away.

Plate 2.5 A stope on North Tincroft Lode, above deep adit level. The stope is ~2.5 metres wide (floor to

back) and supported by rock pillars and timbers. Note the ubiquitous covering of limonite, from sulphide

decay, obscuring lode contacts and details.

On the Deep Adit Level a total of three stopes on separate lodes exist (refer back to

Figure 2.5), one on top of the next, separated by thin ribbons of supported rock. The

true North Tincroft lode is the most northerly of the group, hinging into an unnamed

lode a few metres above the level of the drive. The main adit level runs along a third

(unnamed) lode, which is also developed on the 10 fathom level below adit (but was not

pursued beyond this point) The lodes at this level are hosted in slates of the Mylor Slate

Formation (with thin greenstone sills in some of the drives), being north of the granite

contact on surface and some 100 m above the contact at Deep Adit Level. The slates are

dark grey and hornfelsed with occasional cordierite spotting.

The stopes and drive walls were coated with limonite and occasional blooms of

secondary copper sulphates, which generally obscure primary contacts. All of the

observed stopes had a gentle northerly dip of 30-35o with an average width (floor to

back) of 3 metres (Deakin et al., 1999); however, the largest stope, both in terms of

width and strike span, has a width in excess of 10 metres. The stope, measuring some 30

m along strike and in excess of 50 m up dip, is supported by a series of rock pillars (as

43


are all the other stopes) and timbers. At a distance of some 30 m up dip from Deep Adit

Level, the 17th Century Shallow Adit (which has been mined through) can be seen

penetrating some of the pillars. Above Shallow Adit Level the stope runs up dip,

coming to surface in the vicinity of East Shaft, where, until July 2002, it was covered by

timber and ~ 3 metres of gravel (since capped with concrete).

Within the workings, four main paragenetic assemblages can be recognised

(LeBoutillier et al., 2000a, 2000b, 2001), following the development of the lode system

over time (see Figure 2.6). The first phase of mineralisation (which may pre-date or

accompany the main phase 2 lode infill) involves the silicification of the wallrocks

around the margins of the lode structures. This silicified zone may extend for up to 0.50

m in places; the slates become siliceous and brittle, and are impregnated with

wolframite and cassiterite (both as anhedral masses up to 50 µm across, and as

millimetric fine vein infills), the greenstone sills are converted to chlorite and (sodium-

rich) actinolite and carry both quartz and orthoclase feldspar (evidence of alkali

metasomatism; Ball et al., 1998) with sphene, rare schorl and plagioclase; they are also

impregnated with danaite (cobalt-rich arsenopyrite) which forms subhedral crystals up

to 100µm across, wolframite and cassiterite (as anhedral crystals to 100 µm and as

veinlets in the host rock), and in addition also carry minor amounts of pyrite (subhedral,

to 350 µm in size and as veinlet infills) and sphalerite (as veinlets and anhedral masses

up to 250 µm across).

The first (and main) phase of vein infill is a complex hydrothermal breccia (Halls 1987,

1994) in which angular clasts of slate are cemented by a schorl-chlorite-quartz-fluorite

(± orthoclase) gangue, carrying sphalerite, chalcopyrite, wolframite, cassiterite and

arsenopyrite with minor lead and bismuth/silver sulphides. Lode contacts are very sharp

and regular, lying close or parallel to the S1 cleavage in the slates. The most important

gangue constituent of this phase is tourmaline (schorl) which occurs as dense masses,

but more commonly as open networks of euhedral crystals or as single euhedral crystals

scattered through the groundmass. Other phases such as chlorite and fluorite are also

very important, making up large areas of the groundmass in certain sections of the lode

(see Plates 2.6 to 2.13).

44


Figure 2.6. The development of the North Tincroft Lode. Four separate paragenetic/structural

associations can be seen within the lode. The earliest phase (which may be contemporaneous with phase

2) occurs as impregnations within the wallrocks; this is followed by the main stage of tourmaline-

sulphide infill under high energy and fluid pressure conditions. A later (phase 3) chlorite-cassiterite

dominated assemblage is followed by the addition and remobilisation of secondary minerals in phase 4.

45


Plate 2.6. An SEM Backscatter micrograph of a lode sample from North Tincroft Lode. Massive to

euhedral schorl (Tur) and quartz (Qtz) is intergrown with later anhedral arsenopyrite (Apy), sphalerite

(Sp) and fluorite (Cal).

Plate 2.7. An SEM Backscatter micrograph of a lode sample from North Tincroft Lode. A large subhedral

wolframite crystal (Wol) in a quartz matrix is surrounded by anhedral arsenopyrite (Apy) and sphalerite

(with anhedral inclusions (Cst) of cassiterite).

46


Plate 2.8. An SEM Backscatter micrograph of a lode sample from North Tincroft Lode. Anhedral

cassiterite (Cst) occurs as an inclusion in sphalerite (Sp). Anhedral wolframite (Wol) and subhedral

arsenopyrite (Apy) occur nearby in the orthoclase (Or) groundmass.

Plate 2.9. An SEM Backscatter micrograph of a lode sample from North Tincroft Lode. A large subhedral

cassiterite (Cst) crystal is surrounded by smaller scattered anhedral cassiterite, sphalerite (Sp) and

chalcopyrite (Ccp) in a quartz (Qtz), fluorite (Fl) matrix.

47



sphalerite (Sp), euhedral cassiterite (Cst) and quartz (Qtz), have grown around an open

network/aggregate of euhedral schorl (Tur) crystals.

Plate 2.11. An SEM Backscatter micrograph of a lode sample from North Tincroft Lode. A closer view of

the texture seen in the plate above. Cassiterite (Cst) and sphalerite (Sp) have rapidly crystallised around

originally free-floating euhedral schorl (Tur) crystals.

48



sphalerite (Sp) and cassiterite (Cst) in a fluorite (Fl)-quartz (Qtz)-schorl (Tur) matrix. The sphalerite,

cassiterite, fluorite and quartz are all crowded with euhedral schorl inclusions.


gustavite (Gus) intergrown with pyrite (Py), sphalerite (Sp), arsenopyrite and chlorite (Chl).

49


This phase of mineralisation displays multiple brecciation events that are succeeded by

extensive veining of the lode material by millimetric quartz-schorl veins that have been

jacked open by high fluid pressures (Halls et al., 2000), resulting in orthogonal crystal

growth from the fracture walls and the development of bridging textures (see Plates

2.14, 2.15 and 2.16). This transition from high-energy hydrothermal breccia conditions

(Halls and Allman-Ward, 1986), with complex percolation fluid flow (Cox et al., 2001;

Sibson, 2001), to a more stable regime, but with high fluid pressures (Halls, 1994; Halls

et al., 2000) still extant, post-dates the economic oxide-sulphide mineralisation at Deep

Adit Level, but may be related to further mineralisation events, deeper within the lode

system (which was effectively crack-sealed in the latter stages of this phase of

mineralisation, at this elevation).

The range of economic species present in the lode, in this phase, spans the hypothermal

to upper epithermal range (Hosking, 1964) and includes a number of rare and unusual

silver-bismuth species, some of which (if fully confirmed) are new to the British Isles.

Plate 2.14. Phase 2 mineralisation, North Tincroft Lode. The brecciated groundmass shows evidence of

several brecciation events during the early history of the lode. The latest was succeeded by a period of

relative stabilisation when continuing high fluid pressures saw the development of open-space (jacked

open) tensile veins with bridging textures indicating crystallisation under static conditions. ×4 PPL,

width of field ~2 mm.

50


Plate 2.15. Phase 2 mineralisation, North Tincroft Lode. A series of schorl-quartz veins cutting earlier

brecciated lode material. The veins show the orthogonal growth of delicate acicular schorl crystals from

the vein walls, evidence of a high fluid pressure regime in which crystallisation took place under stable,

static conditions. ×10 PPL.

Plate 2.16. Phase 2 mineralisation, North Tincroft Lode. As above, but a broader vein with the

development of coarser euhedral schorl crystals. ×10 PPL.

51


The most important phase within the gangue is tourmaline, the presence of which (and

quantity) was a surprise after seeing the decline of this phase in the middle workings of

South Crofty Mine, above the 260 Fm level. It is clear from the shallow workings that

hypothermal, boron-rich, fluids in fact reached well above the middle levels of the

mine; the textures indicating that the fluids moved at high speed and the hydraulic

fracturing showing that the host fractures communicated (in a punctuated manner) with

lower pressure regimes (possibly the surface?) above the current level (Halls, 1987).

The tourmaline is predominantly schorl, but ZAF analysis of some samples showed

strong enrichments in magnesium (sourced from the host rocks or directly from

magmatically-derived fluids?) or sodium to give a dravite or buergerite component,

respectively. Typically euhedral, tourmaline was one of the earliest phases to grow, the

fine nature of much of the material reflecting very rapid crystallisation related to

changes in temperature, pressure or fluid chemistry. The high levels of tourmaline

reflect the origin of the ore-bearing fluids, which one must assume are largely of

magmatic departure. The conditions necessary for tourmaline growth however, require

high iron contents and oxidising conditions (the latter required for cassiterite deposition

also; Taylor and Wall, 1993; Heinrich, 1990), that are typical of connate/meteoric fluids

(London and Manning, 1995). It seems likely therefore that the ascending magmatic

fluids intercepted a connate reservoir in this area, with rapid crystallisation taking place

as the plume of high-temperature, high-velocity, dense, high-salinity (Audetat et al.,

1998, 2000; Farmer, 1991, Bull, 1982; Scrivener et al., 1986; Alderton, 1976) brines

encountered and ‘punched through’ the cooler connate fluids.

High speed crystallisation, crack-sealing the lode, would have lead to a rapid pressure

rise as more fluids built up from below, eventually overcoming σ3 and the tensile

strength of the rock (Halls et al., 2000) to cause fracturing and explosive

decompression. Textural evidence shows this happened on a number of occasions

before the pressure fell far enough to only generate tensile veining without breaking

through the sealed main fracture. Each new pulse of mineralisation is associated with

the full paragenetic range of minerals; thus the schorl-cassiterite-sphalerite-fluorite-

quartz-chlorite, etc assemblage seen in Plate 2.10 forms clasts within a matrix of exactly

the same assemblage (though differing in texture). There is no separation between oxide

and sulphide mineralisation, as seen elsewhere in the local area (e.g. Pendarves Mine,

52


Alderton, 1976), within this phase, indicating a steady supply of (broadly)

compositionally uniform fluids to the site during this phase of mineralisation.

Other ‘magmatic’ phases sometimes present includes orthoclase (typically anhedral and

intergrown with other species) and rare biotite. This has largely been altered to chlorite

(2 biotite + 4HCl = chlorite + 3 quartz + MgCl + KCl), but whereas much biotite from

other locations is obviously in the form of relict breccia clasts, the material in the lode is

in optical continuity with the host chlorite and has regular contact faces with the

surrounding crystals, suggesting that it is of primary, hydrothermal origin.

The other main gangue phases (refer back to Plates 2.6 to 2.13) are chlorite, fluorite and

quartz. All are typically anhedral and are intergrown with other phases, although quartz

may form euhedral crystals which appear along side early tourmaline, suggesting two

stages of quartz growth (which, as quartz is often the latest phase to grow in any given

assemblage, is unusual). Fluorite often displays ductile deformation structures, that

formed in response to later brittle adjustments along the lodes, flowing around more

rigid crystals and appearing ‘streaked’ along the lode margins.

Of the metallic minerals present, galena (as rare globular inclusions, usually in

arsenopyrite or sphalerite, 1-10 µm, rarely to 80µm, in size), bismuthinite, wittichenite

(Cu3BiS3, also found at Botallack Mine; Collins, 1892) and gustavite (PbAgBi3S6) all

occur as early inclusions or rare scattered subhedral crystals to 50 µm in size. Other,

very rare, silver-bismuth sulphides also occur as anhedral crystals or inclusion, 10-20

µm in size. ZAF analysis of these grains, matched against mineralogical databases

suggests that two species are present, matildite (AgBiS2), previously described by Bull

(1982) from polymetallic lodes in the Tavistock district of Devon, and mackovickyite

(Ag1.5Bi5.5S9), which is unrecorded in the UK. The presence of so many bismuth-

bearing minerals may be partially explained by that elements high affinity for boron and

tourmaline (Ball et al., 1982) mineralisation.

Cassiterite is another early phase, along with (much rarer) wolframite. Both form

subhedral crystals and may reach well over 100 µm (500 µm in the case of cassiterite)

in size. Cassiterite may carry inclusions of tourmaline, but wolframite is free of

inclusions and may therefore be contemporaneous with early tourmaline formation. The

concentration of cassiterite is highly variable and XRF analysis (see Tables 2.1a and

53


2.1b) of 9 samples from this phase (over the three lodes exposed) shows values ranging

from 0.3% to 3.15% (with an average of ~1.5%), the majority of which would be

payable ore. Wolfram concentrations, on the other hand reach a maximum of only 318

ppm.

The sulphide phases appear to have largely crystallised simultaneously (with

arsenopyrite perhaps a little earlier), nucleating around earlier phases and carrying

numerous inclusions of tourmaline, cassiterite and lead-bismuth-silver sulphides (see

Figure 2.7). Typically anhedral, they form an interlocking network of crystals with the

later gangue phases. Although the ore is payable for tin (and given the lode widths, it

must have been a major orebody in its time), with some very good grades present, this

pales in comparison to the sulphide content. Arsenic (a valuable commodity until the

1950’s) varies from as little as 920 ppm to 16.5% of the ore, copper varies from 0.30%

to 3.17% and zinc varies from 9.3% to 35% of the ore, which is extremely rich. In hand

specimen this assemblage is usually black, or silver, reflecting the high concentrations

of sphalerite and arsenopyrite, respectively.

Figure 2.7. The paragenesis of phase 2 mineralisation in the North Tincroft Lode.

54


As a copper deposit it is, by the standards of the 1870’s, quite poor and may only have

been profitable by virtue of the huge tonnages of ore available for mining. Dines (1956)

says that the stopes were worked for copper on the hangingwall and, later, for tin on the

footwall, but apart from a highly-stanniferous, but impersistent, later phase (phase 3) of

mineralisation, the lodes are fairly uniform and contain both metals throughout. What is

surprising is not only the volume of sphalerite in the ore (twice as rich as that currently

mined in Ireland; M. Lee, pers. comm.), but its presence at all. South Crofty never

officially produced zinc ores and although sphalerite is present in small quantities in the

upper levels of local mines (Henwood, 1843; Carne 1822), it is not on anywhere near

the scale of the deposit on North Tincroft Lode.

Willis-Richards (1990) modelling metal distributions around the Cornubian Batholith

stated that there was strong contact control of Sn, W, As & Cu with Pb, Zn, Ag found in

embayments on the south side of the batholith; he also stated that Zn production began

at 200m from the contact. Both statements show the perils of only using production

data; the true picture of mineralisation within the orefield has been distorted by

economics and the change in use of metals over time; thus what would today be

regarded as principally a high-grade zinc deposit was, in the 1870’s, a problematic (in

terms of recovery) and low-grade copper deposit, later becoming a fairly rich tin-

deposit. Zinc mineralisation has largely gone unrecognised by virtue of its role as a

waste product for much of the 19th Century. Even when seen as a useful commodity,

zinc recovery may have detrimentally interfered with the recovery of more profitable

metals, and so was abandoned. Zinc mineralisation is far more widespread than

previously recognised and occurs not only in those areas traditionally seen as

mesothermal-dominated areas (such as the Porthtowan District), but occurs close to the

contact (New Cook’s Kitchen Mine), on the contact (Wheal Gorland) and within the

granite (Tresavean Mine); the last two formerly marked by large waste tips of almost

pure sphalerite.

Paragenesis 3 mineralisation, in North Tincroft Lode, cuts the earlier assemblages with

sharp contacts, occupying discontinuous segments on the hangingwall of the lodes and,

more rarely, on the footwall or internally within the lodes themselves. These fault-

controlled ‘bands’ pinch and swell along strike, but often reach 0.30 m in thickness

(sometimes 0.50 m). In rare instances the whole width of a lode may be replaced

(typically in narrow, 0.50-0.80 m, pinched sections) by this phase.

55


The paragenesis 3 assemblage is composed of cassiterite, chalcopyrite and arsenopyrite

in a matrix of dark green chlorite with minor, light green to pale green, fluorite and

quartz. The sulphides occur primarily as disseminations throughout the matrix

(particularly the arsenopyrite) but also as thin discontinuous vein fills, up to 5 mm wide,

and small vug infills. Some samples showed small sparable cassiterite crystals, up to 2

mm in length, occupying vugs, but these are not common. The chlorite is dense and

microcrystalline (though some crystals can be made out with a hand lens) with some

pale green earthy patches. The fluorite occurs as anhedral crystals in discontinuous

veins and vugs, sometimes singly, sometimes intimately intergrown with chlorite and

sulphides. A thin veneer of limonite and occasional streaks of chalcanthite

(CuSO4.5H2O) and connellite (Cu19Cl4(SO4)(OH)32.3H2O) occurs on joint surfaces. The

material is very dense; the chlorite (dominant), quartz and fluorite define a uniform

matrix in which there are a large number of vugs, typically ~ 2-4 mm in diameter.

Under the microscope chlorite occurs as a mass of interfering rosettes of varying sizes

(with rare spots of limonite). Individual subhedral to euhedral platey masses, up to 0.1

mm in diameter, usually vary in colour from deep to pale olive green, but are

occasionally pale yellow or brown; thin slivers appear almost colourless. The chlorite

species is confirmed by XRD as a ferroan member of the clinochlore family

((Mg,Fe)5Al(Si3Al)O10(OH)8), likely to be ripidolite. Zones of parallel chlorite plates

define “channels” up to 1 mm in width between the rosettes (which they do not cut);

they contain very little cassiterite and are poor in sulphides (present as much smaller

grains than elsewhere).

Fluorite infills irregular vugs and veins up to 50 mm in width and is typically anhedral

and inclusion-rich (chlorite aggregates and individual plates and quartz). The quartz is

typically subhedral to euhedral and unstrained. The fluorite is locally cut by chlorite

veinlets. Fluorite appears to be paragenetically late in the sequence.

Quartz is generally anhedral (although some amorphous masses occur) and under PPL

often shows undulose extinction and the occasional development of subgrains. Some

grains contain layers of minute opaque inclusions that are arranged parallel to crystal

faces and define hexagonal and prismatic outlines; some of these inclusions are thorite

(up to 1 µm). One inclusion was composed of a thorium phosphate and has yet to be

fully identified.

56


Within the matrix ilmenite crystals, up to 200 µm in length, appear to be paragenetically

very early, and occur as inclusions in all the other major phases. The crystals are lath-

like, euhedral to subhedral (see Plate 2.17) and show compositional zoning, both

internally and along the crystal margins.

Plate 2.17. An SEM Backscatter micrograph of a lode sample from North Tincroft Lode. Anatase (Ant)

and ilmenite (Ilm) crystals, with cassiterite (Cst) in arsenopyrite (Apy) and chlorite (Chl). The element

makers by some of the crystals mark metal enrichments (substitutions) within those crystals.

Several crystals depart from true ilmenite (FeTiO3), being either low in Fe (ilmenorutile

- (Fe,Ti)3O6), or enriched in Mn (pyrophanite - MnTiO3) or Zn (ecandrewsite -

(ZnFeMn)TiO3). Some crystals display pyrophanite cores with discrete ecandrewsite

rims (also low in Fe). Elemental mapping showed that crystals may also be zoned with

different sections of a single crystal being preferentially enriched in Mn or Zn. Anatase

occurs as subhedral laths up to 50µm in size, often closely associated with ilmenite. It

also displays Mn enrichment in several of the crystals analysed.

Cassiterite, as anhedral crystals (see Plate 2.18), is scattered through the lode material

and occasionally occurs as aggregates and veinlets of larger crystals, though these are,

individually, never above 0.1 mm in size. It is the dominant ore species and can

sometimes be seen lining vugs in the chlorite matrix; it is widespread and common in

57


the samples analysed. It also appears to be early in the paragenetic sequence, despite

also being present in the vugs. It occurs as subhedral to anhedral masses from <50 to

200 µm across in size, which are often irregular and intimately intergrown (or included

in) arsenopyrite The grains appear uniform and show no evidence of zoning and appear

to be largely free of inclusions.

Plate 2.18. Phase 3 mineralisation, North Tincroft Lode. Anhedral to subhedral cassiterite lies within the

chlorite matrix, adjacent to sulphide ores (opaque) and a minor quartz vein with fine acicular schorl. A

large anhedral monazite crystal is visible within the quartz vein, centre right. ×10 PPL.

The opaque sulphide species occur randomly through the matrix. They sometimes carry

inclusions of chlorite and occasionally are partially replaced by limonite. Several

intergrown sulphide phases occur. Arsenopyrite is ubiquitous and occurs as anhedral to

subhedral masses, as well as disseminated grains. It is often intergrown with cassiterite

and may contain inclusions of ilmenite, anatase, quartz, chlorite and monazite. Rare

inclusions of native bismuth (to 2×1 µm) were also discovered.

Chalcopyrite forms anhedral masses (up to 50 mm across), which are often intergrown

with sphalerite and arsenopyrite. Masses and grains are often rimmed with a thin layer

of bornite, which may also be seen lining cracks within grains, and appears to be

secondary. Sphalerite may occur intimately intergrown with arsenopyrite and

58


chalcopyrite or singly. It forms anhedral irregular masses or grains, which may exceed 1

mm in size. It carries occasional inclusions of galena up to 2 µm across.

Monazite is common and displays three main forms. It occurs as anhedral grains (see

Plate 2.19) up to 200 µm across, which are often highly irregular and may contain

inclusions of thorium silicate (possibly thorite, which is primary, or the dimorph

huttonite, which is a decay product from monazite) up to 1 µm in size; it may form

anhedral grains enclosed by chlorite, which display an ‘interdigitating’ texture with

individual chlorite plates (see Plate 2.20) or it may rarely form euhedral crystals

enclosed in sulphides.


monazite (Mnz) with chalcopyrite (Ccp) and chlorite (Chl).

Jeffries (1984) noted that monazite in the Carnmenellis Granite was often found in close

association with ilmenite and zircon and the same association (- zircon) is confirmed

here. Monazite crystals occur singly in the chlorite matrix and also intergrown (rarely)

with arsenopyrite. The forms that the monazite crystals take suggest that it continued to

crystallise from the end of the oxide phase (some crystals may be coeval with the early

cassiterite and Ti species), through the sulphide phase, with some crystals forming at the

59


end of chlorite crystallisation (shown by the anhedral ‘interdigitating’ crystals formed

between individual sets of chlorite plates).

Plate 2.20. An SEM Backscatter micrograph of a lode sample from North Tincroft Lode. Late monazite

(Mnz) crystals infilling spaces between chlorite (Chl) plates in the matrix, producing an ‘interdigitating’

texture. Here the texture is duplicated by late anhedral quartz (Qtz).

Although there is evidence of a sequence of crystallisation (see Figure 2.8) within this

assemblage (unlike the rapid, almost instantaneous, crystallisation seen in phase 2),

once again the admixture of different phases within the same paragenesis suggests that

this is also a telescoped deposit. After the crystallisation of the oxide phases, the

sulphide species appear to have crystallised in fairly rapid succession in the order

arsenopyrite-chalcopyrite-sphalerite (+ rare galena). Each sulphide occurs singly (with

their own characteristic inclusions) or in a combination of intergrowths. Chlorite, quartz

and fluorite appear to have crystallised last in the sequence. Fluorite and quartz are

anhedral and quartz, in particular, occurs infilling small spaces between chlorite plates

(in much the same manner as some of the monazite crystals). Chlorite (forming the bulk

of the gangue) generally encloses quartz and fluorite, but rare veinlets, <1 mm in width,

were observed cutting some of the fluorite grains.

60


Figure 2.8. The paragenesis of phase 3 mineralisation in the North Tincroft Lode.

XRF analysis of a sample of this material (see Tables 7.11a and 7.11b) shows that it is a

highly-enriched tin ore; the tin grade of the sample is 10.7%, while arsenic reaches

12.5%. In contrast with the previous phase, zinc reaches only 0.30% and copper 0.7%.

Tin (only) analysis of other samples recorded grades of between 10% and 12.5% Sn,

showing the ore to be consistently high grade. Although forming fairly narrow segments

of the main lode, this phase would have brought total tin values up to around 4% across

the full width of the lode in many areas (which is ~2.5 times the grade of South Crofty’s

average run of mine ore when working).

The cerium content of 2373 ppm is a reflection of the high concentration of monazite in

the ore; La and Nd are also highly enriched, recording values not seen in any other ore

sample from this study. Other elements, such as gallium and bismuth are highly

elevated, while vanadium and cobalt (both substituting in arsenopyrite) are only

moderately high in comparison.

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NCK Mine. North Tincroft Lode

Samples NT1-10 XRF Analysis

Output Results including and taking account of LOF%.

L.O.I.% Fe2O3% TiO2% CaO% K2O% Al2O3% MgO%

NT1 -38.87 28.67 0.00 3.38 0.00 5.12 0.26

NT2 -32.90 11.68 0.07 1.18 0.00 5.62 0.67

NT4 -38.29 17.54 0.13 2.93 2.99 7.53 0.30

NT5 -14.62 12.52 0.21 5.92 0.06 9.62 1.28

NT7 -32.80 14.16 0.07 3.75 0.19 6.36 0.65

NT8 -18.50 10.14 0.58 5.13 0.01 11.01 1.21

NT9 -45.92 18.13 0.00 6.86 0.04 3.06 0.70

NT10 -10.73 34.08 0.28 0.82 0.04 11.99 0.54

Na2O% MnO% BaO% S% P2O5% NT1 1.92 0.33 0.02 14.17 0.05 NT2 1.39 0.15 0.03 8.14 0.04 NT4 1.05 0.26 0.05 9.20 0.00 NT5 2.22 0.24 0.09 7.41 0.04 NT7 3.95 0.19 0.04 13.67 0.07 NT8 2.74 0.14 0.08 7.83 0.15 NT9 4.59 0.25 0.04 19.35 0.06 NT10 0.04 0.33 0.01 5.44 0.08

Table 2.1a. The results of XRF analysis (major oxides and sulphur) of samples of lode material from

North Tincroft Lode of New Cook’s Kitchen Mine. NT1-9 are paragenesis 2 lode samples. NT10 belongs

to paragenesis 3.

Phase 4 mineralisation within the North Tincroft Lode is related to the formation of

secondary species in vugs in the chlorite matrix. The vugs are often lined with an

overgrowth of large chlorite crystals and carry an extensive suite of minerals, which

often produce euhedral single crystals, twins and intergrowths. The most common

species are anatase (blue-colourless, transparent to opaque plates) and monazite

(transparent to opaque, pyramidal, honey coloured crystals, which are often twinned).

These are augmented by cassiterite (forming crystals of ‘sparable’ type), chalcocite,

chalcopyrite, bornite, cuprite, ilmenite, goethite, langite, brochantite, marcasite,

arsenopyrite, pyrrhotite, sphalerite and transparent apatite. The origin of the euhedral

crystals occupying the vugs in the lode material is open to debate, but the mobility of Sn

(Taylor, 1979; Heinrich, 1990) and Ti in lower temperature brines (<300oC) is low and

suggests that these at least are primary in origin.

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Traces ppm. Nb Zr Y Sr Rb Th Co

NT1 <10 <10 <10 98 <10 <10 345

NT2 <10 35 <10 88 <10 <10 268

NT4 <10 41 132 86 1139 <10 165

NT5 <10 27 <10 90 <10 <10 113

NT7 <10 20 12 101 20 <10 206

NT8 <10 72 <10 161 <10 <10 90

NT9 <10 30 13 38 <10 <10 311

NT10 69 67 21 130 <10 <10 556

V La Nd Ce Ga Pb Cu

NT1 49 27 66 871 <10 84 27606

NT2 216 10 10 58 <10 318 31701

NT4 115 40 38 955 <10 205 19600

NT5 105 <10 35 161 <10 293 7300

NT7 100 57 <10 17 <10 464 22221

NT8 174 <10 60 121 <10 257 3171

NT9 80 <10 <10 84 <10 268 24823

NT10 212 702 402 2373 65 967 7300

As Sn Zn Ni W Bi NT1 112000 31580 136000 45 <10 17 NT2 1400 2897 158000 307 318 <10 NT4 165700 14000 93400 65 26 15 NT5 4900 9200 147900 104 <10 <10 NT7 3000 17508 241000 101 34 <10 NT8 3900 28041 152000 62 <10 <10 NT9 920 3025 351000 186 <10 211 NT10 125100 107000 2803 100 <10 114

Table 2.1b. The results of XRF analysis (trace elements) of samples of lode material from North Tincroft

Lode of New Cook’s Kitchen Mine. NT1-9 are paragenesis 2 lode samples. NT10 belongs to paragenesis

3.

Hole et al. (1992) have shown that Ce may be transported at temperatures below 300°C

in fluoride- or carbonate-enriched aqueous fluids, which raises the possibility that some

of the monazite may be of a later date. Radiometric dating of internal and ‘external’

monazites would confirm the relationship between the two and give a date for this phase

of mineralisation. The remaining suite of minerals occupying the vugs spans a large

temperature/stability range and are secondary in nature, precipitated from percolating

fluids that dissolved primary sulphides and redeposited a new suite of minerals stable

under low temperature conditions (e.g., marcasite, pyrrhotite, cuprite, etc).

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The rapid crystallisation of the paragenesis 3 material may also explain the presence of

so many vugs in the ore. With rapid telescoping of conditions leading to crystallisation

across the full temperature range of the species present, residual pockets of

hydrothermal brines could become sealed off as isolated ‘fluid cells’ encased in chlorite.

If these brines still contained metal ions in halide complex form, it is logical to assume

that over time with changing P/T conditions, the dissolved phases would precipitate

forming euhedral crystals in the available free space. The remaining fluid would escape

over time along intergrain boundaries.

The most unusual feature of the ore is the high concentrations of Ti and Ce, in levels

above (in the case of Ce, far in excess of) the background found in the granites. Cerium

is most likely to have been partitioned into the fluids of magmatic departure as an

incompatible element. Monazite is not recorded in the deeper levels of South Crofty,

either in the tourmaline- or chlorite-dominated assemblages, but has been noted in this

type of assemblage at Wheal Jane Mine (Holl, 1990). Lister (1984) suggested that much

of the metal budget of the batholith had been obtained by xenolith assimilation; this was

refuted by Jeffries (1985) on the basis that the REE, Zr and Th contents of the granite

were not high enough to explain the amount of supposed assimilated material. From the

examination of shallow-level lode deposits an alternative explanation may be that the

REE, Zr and Th did not remain in the granite, but were partitioned into the fluids of

magmatic departure and incorporated in those assemblages that have a strong

magmatic-fluid affinity. Several sites examined in this study have produced ore samples

enriched in monazite and carrying zircon and thorium minerals and this may be the

‘missing link’ in Lister’s (1984) hypothesis.

The likely source of titanium is from the breakdown of biotite mica in the wall rocks

adjacent to the lode fissures. Tourmalinisation and chloritisation of the wall rocks would

result in the breakdown of feldspars and micas (Farmer, 1991; Taylor, 1965; Scrivener,

1982) releasing Fe, Mg, Al and Ti into the reacting fluids. Such reactions have been

estimated to take place at around 400oC (Alderton, 1993) initially, with fluids of

magmatic origin dominating in the early stages. Ti, most likely complexed as titanium

chloride, TiCl4, has an aggressive chemistry and is highly reducing; it would readily

form oxides and crystallise in the rising fluids. It is this behaviour that is responsible for

the appearance of ilmenite and anatase at the beginning of the paragenetic sequence.

64


The nature of the fluids responsible for this phase of mineralisation is open to question.

Fluid inclusion studies have shown that the deeper, tourmaline-dominated, assemblages

were deposited from a combination of magmatic and meteoric fluids at between 450oC-

250oC (Jackson et al., 1989; Scrivener and Shepherd, 1998; Alderton, 1993) with high

salinities (>20% NaCl equiv.). The Cu-Fe-As(±Sn)-chlorite assemblage (phase 2b

equivalent; LeBoutillier, 1996) typically shows Th values from 350oC-200oC and lower

salinities (5-20% NaCl equiv.) and is thought to represent a greater input from meteoric

water sources. O and H isotopic data for the hydrothermal fluids that deposited the main

stage lodes suggests an overwhelmingly meteoric origin for these fluids (Jackson et al.,

1989), but detailed studies of how mixing of magmatic and meteoric fluids influences

the isotopic ratios has still to be completed and the data on its own is inconclusive.

Sulphur isotope ratios (Jackson et al., 1989) suggest two distinct sources; one primary

magmatic and a second from sedimentary diagenetic pyrite.

Temperature data from fluorite samples taken from Pryce’s Lode of South Crofty Mine

(Bradshaw and Stoyel, 1968) shows a temperature range from 285oC to 250oC, with a

fall in temperature upwards of roughly 10oC/100 metres. Extrapolating this (for the

fluorite-rich Paragenesis 3) to the location where the current samples were taken, at

Deep Adit Level, would give a homogenisation temperature of 200oC; at the lower limit

of the Cu-Fe-As(±Sn)-chlorite assemblage temperature range. However, the fluorite

present in the lode at the sample depths (148-315 fathoms) is from a later paragenetic

stage (phase 3 of LeBoutillier, 1996) than that of the Tincroft assemblage, which

suggests temperatures of deposition for the Tincroft fluorite (of paragenesis 3) were

higher. There is no temperature data for the schorl-rich paragenesis 2, but in view of the

high proportion of hypothermal species, it seems likely that the ore-bearing fluids would

have had an original temperature of around 400°C (Bull, 1982).

In the case of the North Tincroft Lode, the lode samples suggest rapid crystallisation

from a hydrothermal fluid upon encountering a rapid change in P/T conditions, forming

a telescoped deposit. The reasons for this change in conditions are unclear and could be

due to a rapid drop in temperature away from the granite contact or interaction with

cooler meteoric and connate waters in the killas. The lode material (of parageneses 2

and 3) appears to have been deposited in two punctuated events and is inferred to form

part of a continuous depositional sequence with the ‘blue peach’ assemblage seen at

lower levels. Not only does the lode material not resemble the chlorite-dominated

65


assemblages seen at depth, but also the faulting by Pryce’s Lode would mitigate against

the North Tincroft section of the lode being reactivated during phase 2b deposition;

suggesting that the lode material sampled is from an early (phase 2a; LeBoutillier,

1996) event.

The close spatial and temporal association of tin and arsenic with copper, zinc and lead

points to a common origin and does not allow time for a protracted addition of these

chalcophile elements to the host fluids, nor is there any evidence that these species are

replacements of pre-existing minerals. The inference of a common origin suggests that

zinc, copper and lead were made available by the rapid addition of meteoric waters to

the ore-bearing fluids at depth, or were present in the residual fluids in the late stages of

the crystallisation of the granite and that they were supplied to the magma by

assimilation of the country rocks. The nature of the ore-bearing fluids remains unclear;

however, we can infer a hypothermal to mesothermal temperature of deposition and that

the fluids had a strong magmatic component (Audetat et al., 1998, 2000).

66


Part 3: Zinc and Cadmium at South Crofty Mine. 3.1 Introduction.

Zinc is the 30th element within the Periodic Table; metallic, it has a hexagonal crystal

structure and melts at 409°C. It forms a thin oxide coating on exposure to air, which

protects it from further reaction and this property allows it to be used as a weather-proof

coating, particularly for iron. Zinc is closely associated with cadmium, which occurs

directly below it (atomic number 48) in the Periodic Table. Cadmium was discovered

by Stromeyer in 1817 from an impurity in zinc carbonate. Cadmium most often occurs

in small quantities (usually making up less than 1%; www.Speclab.com) associated with

zinc ores, such as sphalerite (ZnS). Greenockite (CdS) is the only mineral of any

consequence bearing cadmium. Almost all cadmium is obtained as a by-product in the

treatment of zinc, copper, and lead ores. In addition, cadmium can occur as an impurity

in phosphate minerals. Some natural phosphate ores contain several hundred parts per

million (ppm) of cadmium, and are thus undesirable to use as fertilizers.

Cadmium is a soft, bluish-white metal which is easily cut with a knife. It is similar in

many respects to zinc. It is a component of some of the lowest melting alloys (it melts at

321°C); it is used in bearing alloys with low coefficients of friction and great resistance

to fatigue; it is used extensively in electroplating and in many types of solder, for

standard E.M.F. cells, for batteries, and as a barrier to control atomic fission. Because

cadmium is located between zinc and mercury in the Periodic Table, its physical and

chemical properties are rather similar to those of zinc, and to a lesser degree, mercury.

Cadmium and all its compounds are toxic.

Background levels of zinc in Cornwall have been established for both igneous and

sedimentary rocks (with cadmium). Analysis of 468 samples of unmineralised

metasedimentary rocks from across Cornwall (LeBoutillier et al. 2004) gave average

values of 106 ppm for zinc and 0.13 ppm for Cadmium. Analysis of unmineralised

granites in Cornwall (Hall, 1990; Stone and Exley, 1985) gave zinc values of 43-72

ppm; cadmium values associated with these are likely to be in the range of <1/400th of

the zinc concentration.

Cadmium is known to occur in a number of mineral species at a variety of

concentrations; details are given as follows:-Cadmium content in: sphalerite: 0.0001-

2%; greenockite: 77.8%; hawleyite: 77.8%; chalcopyrite: < 0.4-110 ppm; marcasite:

67


<0.3-<50 ppm; arsenopyrite: < 5 ppm; galena: < 10-3000 ppm; pyrite: < 0.06-42 ppm;

pyrrhotite: trace; tetrahedrite: 80-2000 ppm; magnetite: 0-0.31 ppm; cadmium oxide:

87.5%; limonite: <5-1000 ppm; wad and manganese oxides: <10-1000 ppm; anglesite:

120- >1000 ppm; barite: < 0.2 ppm; anhydrite and gypsum: < 0.2 ppm; calcite: < 1-23

ppm; smithsonite: 0.1-2.35%; otavite: 65.18%; pyromorphite: < 1-8 ppm; scorodite: <1-

5.8 ppm; beudantite: 100-1000 ppm; apatite: 0.14-0.15 ppm; bindheimite: 100-1000

ppm; silicates: 0.03-2.8 ppm.(www.Speclab.com). Of these minerals marcasite,

arsenopyrite, pyrite, limonite, apatite, calcite, chalcopyrite and gypsum occur in the

lower levels of South Crofty Mine (LeBoutillier, 1996), but are of extremely limited

distribution and account for only a tiny percentage of the mineralisation, which is

silicate/oxide dominated.

Zinc values recorded in silicate/oxide-dominated assemblages do not infer that cadmium

is also present; zinc substitutes in the lattices of several oxide species and may be

present in the tin ore cassiterite.

3.2 Distribution.

Zinc ores were not produced by South Crofty during its history from 1854 to 1998, but

patchy records show that zinc, as sphalerite, was present in the lodes of some of the

mines that were later incorporated into the lease area and that some small tonnages

(typically less than 10 tonnes) were raised. Dines (1956) records sphalerite as being

present in the lodes of North Roskear Mine, South Roskear Mine, Cherry Garden Mine,

North Wheal Crofty and Longlose Mine, where it occurs with copper and arsenic

sulphides. The references give no indication of the proportion of zinc present in the

lodes and are vague about its distribution, but they indicate that sphalerite was not

present below the 100 – 115 fathom levels in these mines (approximately 750 feet or

230 metres below surface). The deepest recorded occurrence of sphalerite within the

South Crofty lease area is within New Roskear Shaft at around 1340 feet (408 metres)

where a narrow copper/zinc lodes cuts across the shaft (Davison, 1929; Dines 1956), but

again there is no indication of the proportion of sphalerite present.

Zinc analyses of lode samples are sparse, but LeBoutillier (2003) reports the following

for a variety of paragenetic assemblages:- in east Cornwall, within chlorite-dominated

lode material, South Caradon Mine [SX272697] 4102 ppm, Craddock Moor Mine

[SX258702] 1914 ppm; in mid Cornwall in tourmaline-dominated lode material, South

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Terras Mine [SW935523] 4144 ppm. Closer to South Crofty, in the chlorite-dominated

shallow workings of South Tolcarne Mine [SW656384] and Great Condurrow Mine

[SW659393] values recorded were 575 ppm for South Tolcarne Mine and 701 ppm for

Great Condurrow Mine. In the adit level workings of Great Condurrow Mine lode

samples gave zinc values of 1627 and 2428 ppm. Wheal Roots [SW682315], near

Wendron has the closest lode assemblage to that of the deeper levels of South Crofty

(blue peach tourmaline + quartz, with cassiterite) and carries 53-108 ppm Zn.

Apart from the adit level samples from North Tincroft Lode at South Crofty, the highest

zinc values recorded were from North Pool Mine [SW675423] with 4.4% Zn (and 1.8%

Pb and 0.7% Sn; note: 1% = 10,000 ppm) and Wheal Gorland [SW731428] with 14-

18% Zn (and 4% Cu and 0.3-0.4% Sn). Considering that Wheal Gorland never

produced zinc, this is a massive component of the ore to put to waste. No record of its

existence is even registered by Dines (1956) and other workers, as no descriptions of the

lodes have existed until now. The copper values, low by the standards of the time

(where 17% was good ore, Dines, 1956), would have struggled to support the mine and

it is certain that the high sphalerite content would have caused problems in recovering

the copper, thus adding another layer of expense to the working of the mine. Wheal

Gorland never really made money and it is not difficult to understand why; however we

have no knowledge of the other lodes or changes in the mineralisation at depth, which

could see the sphalerite decline.

The polymetallic mineralisation seen at Wheal Gorland and within New Cook’s Kitchen

Mine are very similar to that seen at Wheal Jane (Holl, 1990) and it is becoming

increasingly apparent that this style of mineralisation is common in the near-surface

extensions of lodes across Cornwall. Zinc mineralisation, so long overlooked as to be

thought only a minor component of most assemblages, has been shown to be much

more widespread and present in much higher concentrations than previously thought.

Stockpiles of ‘waste’ sphalerite on several mine dumps across Cornwall (LeBoutillier,

unpublished data) are testament to its (and thereby cadmium) presence. Zinc and

cadmium values picked up in river water and thought to be pollution from deep mine

outfalls, are more likely to be largely derived from surface run-off (with percolating

rainwater picking up these elements where the lodes reach the surface) and shallow

workings.

69


It should not be assumed however that the very high zinc grades seen in North Tincroft

Lode and Wheal Gorland are the norm- they are not. Examination of the lodes of

Copper Tankard, Wheal Susan, Longclose, Wheal Vernon, Cherry Garden and North

Wheal Crofty (all now within the South Crofty lease area) at adit level (~140 feet below

surface) shows that the lodes are generally narrow and composed principally of quartz

with a scattering of sulphides. Sphalerite is present, but is very subordinate to

chalcopyrite, which itself is relatively scarce (hence the lodes are still in-situ and not

mined away). Therefore the circumstances leading to a bonanza zinc deposit have to be

specific.

As previously discussed in Part 2, it may have been that the type of mineralisation seen

in North Tincroft Lode was previously much more prevalent at this, and higher,

elevations, but reactivation and ‘flushing through’ of the lode system by later fluid

pulses (coupled with the removal of up to 4 km of overlying rock above the present

surface) has removed all trace of this. The fact that the upper section of North Tincroft

Lode was effectively sealed off by being faulted by Pryce’s Lode has been the key

factor in its survival to the present.

Descriptions of the lodes in the upper levels of South Crofty mine are scant, but it is

known that by 245 fm the tourmaline-dominated assemblage seen in the lower levels is

being replaced by one much richer in chlorite and fluorite and that sulphides of arsenic

and copper are becoming more common (Dines, 1956; LeBoutillier, unpublished data),

though are still well below economic concentrations. Only one stope, on Care’s Lode,

on 245 fm was worked for sulphides, being a massive arsenopyrite/feldspar lode

(LeBoutillier, unpublished data). Above this level many of the drives are sealed off, but

it is known that above 175 fm that copper becomes important and that tin declines

(Dines, 1956). A section of North Tincroft Lode, dating from the early 20th Century

(held at South Crofty) describes the lode as being rich in arsenic (as arsenopyrite) and

tungsten (see Figure 3.1); this is not surprising as the richest tungsten deposits in the

area around South Crofty and East Pool mines were found just below the granite contact

(Dines, 1956; Hill & MacAlister, 1906). This section appears to be the only data

available relating to North Tincroft Lode, apart from a brief description in Dines (1956),

of the lode at shallow levels, which again bears no resemblance to the lode assemblage

seen at adit level.

70


Figure 3.1. A section through North Tincroft Lode showing the mineralogy at known locations.

71


Conclusions The distribution of zinc and cadmium at South Crofty Mine can only be definitively

quantified in the workings above adit on North Tincroft Lode and the lodes exposed in

the adit system itself. Zinc values have been established for North Tincroft Lode, but

not cadmium; however this can be inferred at ~ 1/400th of the values for zinc itself,

though this could be better resolved by sampling and analysing specifically for the

element itself.

The high levels (up to 35%) of zinc seen in the North Tincroft Lode workings are

exceptional and do not reflect the values likely to be found (well below 1%) in the lodes

exposed within the adit system as a whole. Indications as to the prevalence of this type

of mineralisation at depth can only be inferred from the scant information that we have,

which suggest that zinc mineralisation within the district dies out in the 100-115 fm

range below adit. The deeper workings of South Crofty Mine carry only very minor

amounts of sulphide (typically marcasite with arsenopyrite restricted to small pegmatitic

bodies) and the minor amounts of zinc (~50-100 ppm) detected in assemblages at these

elevations is almost certainly incorporated within oxide species rather than being

present as a (potentially) cadmium-carrying sulphide.

Release of zinc and cadmium into the slightly acidic water within the flooded workings

at South Crofty appears to have stabilised (K. Williams, pers. comm), due to some

buffering of reactions within the fluid column. Figures from dipping at New Cook’s

Kitchen Shaft and at the adit portals suggest that the water column has stratified, with

fresh run-off now passing over the top of more dense, solid-charged, water below.

When the mine is drained this section of the water column will produce a high zinc

(+copper, cadmium, arsenic, lead) ‘spike’, but this will decline rapidly as the deeper

levels are drained.

Once exposed to the air sulphide-rich ores being to react and break down to sulphuric

acid and metal salts + iron oxide. Depending on the availability of water, oxygen and

heat (the reaction itself is exothermic) the ‘sulphide rot’ can penetrate rapidly and

deeply into the exposed sulphide, but the iron oxide by-product (limonite) effectively

acts as a sponge, retaining the breakdown products unless it is washed away. Under the

right conditions (similar to those seen above adit on North Tincroft Lode) this limonite

can form a hard, millimetre-thick, shellac-like carapace, precluding further reaction and

72


protecting the sulphides below. Even where the rot has penetrated deeply, water is

required to flush the resulting metal salts away, so for as long as the workings remain

essentially dry, little movement can take place. This precludes allowing them to flood

again once they have been standing in air for a long period of time.

It is recommended that the above adit level stopes on North Tincroft Lode and the main

adit system be sampled to ascertain zinc & cadmium levels and provide a mineralisation

datum similar to those for the granite and metasediments already established.

Any sulphide-bearing workings once drained be kept dry and/or strategically sealed off

by dams to remove potential metal transport in solution.

Any new sulphide-bearing lodes exposed during the extension of the Tuckingmill

Decline be kept dry and/or sealed with shotcrete or bitumen to prevent exposure to air

and water.

A full understanding of the nature and extent of zinc mineralisation (known to be

confined to the upper levels of the mine only) can only be made as the mine is drained

and old workings examined.

73


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