November 2006

Economic Geology

Economic geology of Cornish tin and copper deposits (Part 14)

By R. J. Cathro

 

Block diagram showing the arrangement of the Cornish ore zones relative to the contacts of the granite cupola (from Park and McDiarmid, 1970, after Hosking, 1951).


Cornish deposits were typically small, narrow, and erratic, which made them difficult to mine and marginally profitable. This undoubtedly contributed to the tenacious and thrifty character of its people. Production came from about 2,000 mines, many of them confined to single veins, which required some 2,400 kilometres of underground workings. The Great Country Adit, driven 12 metres above high tide, had a total length of almost 50 kilometre, including branches, and drained a large area. The deepest mines, which occurred at the west end of the 100- kilometre-long belt, reached depths of over 1,050 metres and encountered hot saline water. The largest mines, which were amalgamations of numerous small mines with operating histories extending over 200 years, had names such as Geevor, Botallack, Dolcoath, Levant, and South Crofty (the last one to close in 1998). According to Dixon (1979), total production from over 3,000 years of mining was about 2.25 million tonnes of tin metal and about the same amount of copper from ores that averaged about 1% Sn and 2% Cu.

Cornwall and western Devonshire are underlain by a large granitic body named the Cornubian batholith. The upper portions have been exposed by erosion and mapped as small plutons, referred to in this paper as cupolas, which are associated with all tin and copper mineralization (see map in Part 12, CIM Magazine, August 2006, p. 71).

Clark et al. (1993) have shown by means of recent geochronologic, isotopic, and fluid inclusion studies that the batholith was emplaced over a 20 million year (my) interval that straddles the boundary between the Pennsylvanian and Early Permian periods (293.1 to 274.5 Ma). The main part of the batholith was formed during the first half of the interval. The coarser grained monzogranite cupolas were emplaced later, but not all at the same time, although each had a similar cooling and mineralizing history regardless of its age. The cupolas are thought to have a genetic association with hybridized mafic magma that may be derived from the melting of a coeval volcanic suite.

The batholith represents the most westerly extent of activity throughout Europe during the progressive development of the Hercynian orogeny. In England, the orogeny consists of a fold-belt sequence composed of Devonian age slate, quartzite, and volcanic rocks (called killas) that separates Carboniferous turbidites and Permo-Triassic continental red beds to the north from mainly marine Hercynian facies to the south. The cupolas lie along an east-west trend that is interpreted to be a major structure parallel to the axis of the batholith. Each cupola is surrounded by a zone of contact metamorphism.

The cupolas are Li-rich granites that contain abundant, ubiquitous tourmaline. The tourmaline was formed in two phases, and occurs as a primary accessory mineral as well as a secondary variety in mineral veins. Two types of greisen alteration are also present. The first is a weak variety produced by late-magmatic fluids that mainly converted perthitic feldspar to pseudomorphs of white mica and quartz. The second variety is associated with veins containing quartz, tourmaline, topaz, fluorite, chlorite, muscovite, cassiterite, wolframite, arsenopyrite, chalcopyrite, molybdenite, and other minerals. A third type of alteration that is common in the Cornish cupolas is kaolinization, which is described later.

The mineral deposits consist of veins and alteration zones that strike roughly parallel to the trend of the cupolas, westerly at the eastern end and southwesterly at the western end. Individual lodes are generally steeply dipping and occupy fissures in the granite and country rock. Mineralization was introduced in two principal stages, with the first veins formed after the emplacement of a few pegmatite and aplite bodies. These consist of relatively small, greisen-bordered pods, veins or vein-swarms containing feldspar, quartz, wolframite, arsenopyrite, other sulphide minerals, and sometimes cassiterite and fluorite, that formed within four to six million years of the intrusion of individual cupolas.

The greisen-flanked veins of the first stage were followed about one to three million years later by the first true hydrothermal veins, which mostly occupied pre-existing fault zones. Although this more important stage is comprised mainly of narrow, persistent veins that average about one metre in thickness, movement resulted in shattering of the wall rocks and also led to the formation of complex orebodies such as stockworks and replacement zones that are bordered by greisen, sericite, and kaolin alteration. The initial hydrothermal assemblage consisted of quartz, cassiterite, and tourmaline, with some wolframite, arsenopyrite, iron-rich sphalerite, and stannite.

Later, as the chemistry of the fluids changed due to cooling, chalcopyrite was deposited with lesser amounts of stannite, scheelite, and sphalerite. Late-stage alteration includes silicification, sericitization, tourmalinization, hematization, chloritization, and sometimes the addition of orthoclase feldspar. In the parts of the mineralized zones that are more distal from the intrusions, the mineralization consists of galena, sphalerite, argentite, pitchblende, niccolite, cobaltite, and bismuthinite in a gangue that includes barite, dolomite, and chalcedony. An epithermal assemblage including tetrahedrite, bournonite, pyrargyrite, siderite, pyrite/marcasite, hematite, stibnite, and jamesonite is present still farther away from the cupolas.

The different types of mineralization tend to occur zonally, in the order described, around the main mineralized (heat) centres. That explains why tin, which is more proximal, predominated close to, or in, the cupolas near surface, but was found beneath the more distal copper zone farther away. Complex sequences of movement along veins and cross-faults have occurred in several parts of the belt throughout the period of mineralization. These strike roughly at right angles to the axis of the district and tend to be richer in lead, zinc, silver, and fluorite mineralization.

Although almost all the value of the Cornish lodes was derived from their tin and copper content, small amounts of silver, lead, zinc, tungsten, and arsenic were also produced. Information about ore grades and production of the minor metals is quite sparse but does show the relationship between silver and galena. The average silver content of the galena produced in Cornwall between 1851 and 1881 was about 1,433 g/t (41.8 oz/ton). Silver-lead mining started in Devonshire during Roman times, and galena from the Tamar Silver-Lead mine, which operated there between 1851 and 1880, averaged about 1,450 g/t (42.2 oz/ton) Ag (Phillips, 1884; Hosking, 1951, p. 334).

China clay

As the fortunes of the Cornish tin and copper mines entered their long decline, another mineral that was not subject to erratic changes in world price became important. In 1745, William Cookworthy, a Quaker apothecary-cum-potter, discovered a good clay deposit on Tregonning Hill. By 1768, he had patented a way to use his clay and developed his own porcelain factory in Plymouth. In 1775, Josiah Wedgwood began to ship Cornish clay to his Midland pottery, which opened in 1759 (Dixon, 1979) at the beginning of the famous English porcelain industry. By 1910, there were about 70 Cornish clay quarries in operation, producing nearly one million tonnes annually. Although porcelain is still an important market, around 80 per cent of the current annual production of about three million tonnes is used by the paper industry, and it is one of Cornwall’s principal exports (www.cornwall-calling.uk/mines/cla).

The most important deposits occur in the western half of the St. Austell pluton, the northern part of the Bodmin pluton, and the southern portion of the Dartmoor pluton. The chief mineral in china clay is kaolin, which is accompanied by finely divided white mica and quartz. The kaolin was formed by alteration of plagioclase feldspar in the cupola, with the alteration fluids travelling along the joint system. Most of the clay deposits are funnel-shaped and the amount and purity of the kaolinite is better at the top. The most intense alteration occurs in areas where channelways are well developed along sub-horizontal joints and even beneath caps of country rock. Although the kaolinization extends as deep as 300 metres into the granite, it appears to have irregular boundaries and tapers off at depth. The alteration also results in a decrease in the amount of primary mica and accessories such as fluorite, apatite, and tourmaline. Kaolinization of plagioclase is marked by the appearance of montmorillonite, which is subsequently replaced by kaolinite. Orthoclase, on the other hand, alters more slowly to secondary mica or illite (Floyd, 1982, p. 305-6).

Both the field relationships and isotopic evidence suggest that the kaolin deposits were formed by weathering. On the other hand, the association with vein-swarms at depth is universal. It seems probable that hydrothermal alteration produced zones of sericitized and kaolinized granite and that the purity of the kaolin was enhanced by Tertiary weathering (Dixon, 1979).

Comparison of the Cornwall and Erzgebirge tin districts

Stemprok (1995) has summarized the many similarities and fewer differences between the two major European tin districts, Cornwall (C) and Erzgebirge (E), both of which played vital roles in the history of economic geology and mining. Among the similarities are the age and size of the batholiths in the two districts (293 to 275 Ma and 6,000 square kilometres for C; and 330 to 290 Ma and 8,000 square kilometres for E). The composition of the granitic batholiths is also similar, although the Cornubian is more evolved. In both cases, a comparable mineral assemblage is associated with small cupolas that form the highest parts of the batholith and are composed of the youngest granitic phases. Another similarity is the level of erosion, which has extended deep enough to expose the cupolas and the upper parts of the lode systems but not far enough to erode them completely away.

The most obvious difference is the much larger amount of copper and the abundant tourmaline that is present in the Cornish granites, greisens, and veins. Although the metal assemblages are quite similar, including the unusual Bi-Co-Ni-As-(U) assemblage, the production differences between the two districts are quite pronounced. Cornwall produced far more tin (all quantities given as thousands of tonnes), either 2,250 (Dixon, 1979) or 1,500 (S˘temprok, 1995) compared to only 300. That is mainly because the Cornish tin zones persisted to greater depths, so many cassiterite veins occurred within the cupolas and the placer deposits were so extensive. The difference in copper production was even more extreme, either 2,250 (Dixon, 1979) or 2,000 (S˘temprok, 1995), respectively, from Cornwall, compared with only 25. The situation was reversed for tungsten and fluorite production, 5.6 and 10, respectively, from Cornwall compared with 27 and 2,000. Production comparisons of other metals from the two districts aren’t very meaningful because the Erzgebirge hosted important mines composed of uranium, silver, lead, and zinc but without significant tin or copper, such as Freiberg and Joachimsthal. Cornwall, on the other hand, was essentially a tin-copper district with the other metals present only in insignificant amounts.

The important concept of primary hydrothermal zoning of ore and alteration minerals was first recognized during the mining of the Cornish lodes. It was an important advance in economic geology that was exported around the world during the great migration of Cornish miners during the 19th century. Until recently, the zoning was thought to be related entirely to the amount of heat imparted by the cupola but that model failed to account for broad variations in the distribution of different metals within both of the European tin districts. That theory has now been abandoned by most specialists in favour of a model that combines multistage introduction of hydrothermal solutions into joints and fissures along with the influence of thermal gradients. Unlike the Erzgebirge, where much of the tin mineralization occurs in stockworks within zones of pervasive greisenization, that have been described as porphyry-like deposits, greisen alteration in Cornwall is mostly restricted to selvages a few centimetres wide along the borders of veins.


The information in this chapter is derived mainly from Hosking (1951), Dixon (1979), Floyd (1982), and Clark et al. (1993).

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