Cassiterite from Cornwall, England

--- high-grade ore from an old mine

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Figures 1-2. Excellent sample of coarsely crystalline cassiterite with clear quartz from the Old Beam mine near Bugle, an area of the St. Austell granite mass more commonly associated with china clay, an important industrial mineral. The cassiterite, tin oxide (SnO2), is dark brown with a resinous lustre. The specimen labels display provenance: most recently from mineral dealer and ore-mineral specialist David K. Joyce.

Bugle lies 5 km north of the town of St. Austell. It lies 2 km northeast of Hensbarrow Beacon and 3 km southeast of Roche across Carbis Moor. The district is known for highly evolved granitic rocks and associated ores, and for the regionally intense alteration that produced the economically- important china clay deposits. Other "rocks of the month" from the area include the hydrothermal breccia from Wheal Remfry and the quartz-tourmaline rock which forms the towering outcrop of the Roche Rock.

"Rock of the Month #209, posted for November 2018" ---

CASSITERITE is the most abundant ore mineral of tin. The mines of Cornwall, especially the lodes of tin and copper, have a long history, traced back at least two millennia prior to the industrial-revolution heyday of the area in the 19th century, a development made possible by the invention of pumps powerful enough to drain the workings of groundwater. The Cornish deposits of tin, copper and other metals formed during the Hercynian (Variscan) orogeny, a mountain-building and magmatic episode that affected western Europe in Carboniferous and Permian time. Cassiterite is a tetragonal oxide, with specific gravity 6.8-7.1. Being so dense, the dark oxide grains, when eroded from primary mineral deposits, form placers (heavy mineral sands), and so can be panned or sluiced from well-situated alluvial deposits, in the same manner as placer gold or platinum. Agricola, in his De Re Metallica, described no less than eight ways by which black tin ore could be concentrated from sand, soil and rock (Hoover and Hoover, 1912, pp.336-349).

Dines (1956) describes the old mines of southwest England in great detail. This month's sample comes from an area of upland moors north of St. Austell, beneath which the kaolinized granite is cut by many quartz-tourmaline veins. The most important of the local mines were Old Beam, Rocks, and Bunny (ibid., vol. 2, pp.531-533). Tin in the form of cassiterite was the main product, though some tungsten occurs as wolframite. Lying in the centre of the St. Austell mass, the veins are relatively deep in the mineralizing system, hence the abundance of tourmaline rather than chlorite, and none of the mines extended more than 82 fathoms (492 feet, 150 metres) below adit level. Dines notes that the highly altered granite "has rendered mining difficult, necessitating much timbering". The veins of the Old Beam (Great Beam) mine are quartz with cassiterite, much tourmaline and some wolframite. Other minerals are melaconite (CuO), olivenite, pharmacosiderite, talc and wavellite. The mine, circa 800 metres southwest of Bugle, began as an open pit extracting china clay. Once some of the veins were exposed, these were exploited by 7 shafts dug from the pit floor, and 3 from ground level on the west side of the pit. The deepest level, at 82 fathoms, lay below the Engine Shaft.

The tin trade has been described in great detail, both in major reviews (Penhallurick, 1986; Gerrard, 2000) and in a wealth of mostly small-press booklets on the mining history of Cornwall (e.g., Williams, 1975; Atkinson, 1985), a treasure trove of old photographs and anecdotes. Based on these and other local experts, it seems that the old story of Phoenicians mining tin in Cornwall is an alluring myth, and that local miners traded tin into northern Europe and beyond (Boyle, 2017). Penhallurick (1986, pp. 123-131) refutes the widespread view that Phoenician traders went to Cornwall for tin (pp.123-131), and goes on to detail the history of tin mining in Cornwall (ibid., pp.148-172). In a later note, Penhallurick (1994) affirms this view, though tin was worked in Britain since the start of the Bronze Age, circa 2000 B.C.

The monograph by Dines (1956) remains the standard reference on the old mines. Mineral specimens of many species are featured in works by Collins (1892) and Embrey and Symes (1987). The element itself is described in many historical tracts (e.g., Pepper, 1866, pp.294-317), and the geology of tin deposits is reviewed by Taylor (1979).

Hall (1990) noted that the area of the Hercynian granites under southwest England is geochemically anomalous, with elements such as boron, lithium and tin enriched several times over mean abundances for other parts of the continental crust. In the mineral deposits, boron and tin are supplied by the granite, copper and sulphur from the host rocks.

A long-standing debate on local geology concerned the clay deposits, and whether they were older, and of hydrothermal origin, or geologically young, associated with deep weathering at low temperatures. Bray and Spooner (1983) analysed samples from the St. Austell granite mass (“cupola”) of the Cornubian batholiths. They determined that kaolinization was a single-stage hydrothermal process related to the formation of sheeted Sn- W bearing quartz- tourmaline veins. Mineralization, kaolinization and greisen formation occurred between time of granite crystallization (circa 301 Ma) and the time when the rocks cooled through the mica closure temperature (at which critical argon isotopes used in dating become "locked into" their host crystals, circa 250°C) around 273 Ma. These dates span end-Carboniferous through mid-Permian time.

Mineral specimens have been collected from Cornwall and sold by dealers for more than two hundred years. Some real curios have been found, such as cassiterite pseudomorphs after orthoclase feldspar (from Wheal Coates in the St. Agnes district, just southwest of Cligga Head: Cooper and Wilson, 2015). In this case, some of the material turned out to be early 19th century fakes (Cooper, 2006, pp.203-205), but the real thing does exist! More generally, cassiterite occurs both as macroscopic crystals (Figs.1-2) and as banded, colloform "wood tin", reminiscent both of growth bands in trees and of widespread compositionally-banded sphalerite (ZnS). Examples of both can be seen in Embrey and Symes (1987, pp.91-92).

Crystal chemistry and optical properties. Cassiterite, ideally pure tin oxide, may contain traces of Ta, Nb, W, Fe and Mn, which may all substitute for Sn in the crystal structure (Moller et al., 1988). Luminescence in cassiterite crystals is controlled by contents of Ti, Fe and W. The Ti and W act as activators, whereas Fe quenches luminescence. Variations in composition of the hydrothermal fluids are indicated by luminescence variations, and by zoning in associated tourmaline, as well as in cassiterite (Farmer et al., 1991).

References (n=18)

Atkinson,RL (1985) Tin and tin mining. Shire Publications Ltd, Princes Risborough, Bucks., U.K., 32pp.

Boyle,RW (2017-) A History of Geochemistry and Cosmochemistry. Illustrated and updated edition (Wilson,GC, Butt,CR and Garrett,RG, editors) in prep., 3 volumes, 38+706pp. (vol. 1, complete: vols. 2-3 in progress), volume 1, chapter 3C, pp.233-242,262-270.

Bray,CJ and Spooner,ETC (1983) Sheeted vein Sn- W mineralization and greisenization associated with economic kaolinization, Goonbarrow china clay pit, St. Austell, Cornwall, England: geologic relationships and geochronology. Econ.Geol. 78, 1064-1089.

Collins,JH (1892) A Handbook to the Mineralogy of Cornwall and Devon. D.Bradford Barton Ltd, 108pp. plus addenda and plates, reprint of 2nd edition in 1969.

Cooper,MP (2006) Robbing the Sparry Garniture. A 200-Year History of British Mineral Dealers. Mineralogical Record, Inc., Tucson, 358pp.

Cooper,MP and Wilson,WE (2015) Cassiterite pseudomorphs after orthoclase from Wheal Coates, St. Agnes, Cornwall, England. Mineral.Record 46, 239-246.

Dines,HG (1956) The Metalliferous Mining Region of South West England. IGS, 2 vols, 795pp., published in 1956 and amended in 1969.

Embrey,PG and Symes,RF (1987) Minerals of Cornwall and Devon. British Museum (Natural History) / Mineralogical Record Inc., 154pp.

Farmer,CB, Searl,A and Halls,C (1991) Cathodoluminescence and growth of cassiterite in the composite lodes at South Crofty Mine, Cornwall, England. Mineral.Mag. 55, 447-458.

Gerrard,S (2000) The Early British Tin Industry. Tempus Publishing Ltd., Stroud, Gloucestershire, 176pp.

Hall,A (1990) Geochemistry of the Cornubian tin province. Mineralium Deposita 25, 1-6.

Hoover,HC and Hoover,LH (1912) Translation of De Re Metallica, by Georgius Agricola (1556), 638pp. Reprinted by Dover Publications, Inc., New York, 1950.

Moller,P, Dulski,P, Szacki,W, Malow,G and Riedel,E (1988) Substitution of tin in cassiterite by tantalum, niobium, tungsten, iron and manganese. Geochim. Cosmochim. Acta 52, 1497-1503.

Penhallurick,RD (1986) Tin in Antiquity. Institute of Metals Book 325, reublished in paperback by Maney Publishing, Leeds, for the Institute of Materials, Minerals and Mining, 2008, 271pp.

Penhallurick,RD (1994) Cornwall. Mineral.Record 25 no.1, 67-68.

Pepper,JH (1866) The Playbook of Metals: including personal narratives of visits to coal, lead, copper, and tin mines: with a large number of interesting experiments relating to alchemy and the chemistry of the fifty metallic elements. George Routledge and Sons, London, new edition, 504pp.

Taylor,RG (1979) Geology of Tin Deposits. Developments in Economic Geology 11, Elsevier, 543pp.

Williams,HV (1975) Cornwall's Old Mines. Tor Mark Press, Truro, 46pp.

Graham Wilson, 24,28-31 October, 02,05 November 2018

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