"Rock of the Month #152, posted for February 2014" ---

Wolframite

is a dense oxide ore mineral of the heavy metal tungsten. Strictly speaking, wolframite is a tungstate, an oxide of tungsten and other metals. It is also a solid solution series, between endmember molecules respectively rich in iron and manganese. These minerals are named ferberite (iron endmember, FeWO₄) and huebnerite or hubnerite (manganese endmember, MnWO₄). Since iron and manganese have similar atomic weights, the ideal tungsten content increases only very slightly from ferberite to hubnerite, from 60.54 to 60.72 weight percent.

These samples were collected as part of a thesis project (Wilson, 1980). The study concerned the abundance of lithium in tourmaline and quartz. Smith and Yardley (1996) examined boron isotopes in tourmaline from Cligga and related sites.

Cligga Head

is a famed mineral locality on the north coast of Cornwall (see, e.g., Embrey and Symes, 1987, pp.10-12). The granite here forms a small apophysis off the regional Cornish granite batholith. The Cligga granite is a small igneous body, at least at surface, in contrast to the principal granite massifs, such as Land's End, St. Austell and Dartmoor.

The peninsula of southwest England, jutting westwards into the Atlantic ocean, has more than 2,000 years of mining history, for tin, copper, lead, silver and other metals. Hugh Aldersey-Williams (2011, pp.199-212) has written an interesting essay on the mining allure of Cornwall, from Phoenician traders to modern times, with emphasis on the historically key product of the region, tin.

Cligga Head is located some 2 km W.S.W. of the centre of the coastal village of Perranporth. Like many places of interest, it has contrived to be at the corners of the relevant topographic map sheets (3 sheets to be precise, while to the northwest lies the Atlantic ocean). The site is shown, with Hanover Cove to the south, at the northwest corner of Ordnance Survey 1:50,000 First Series sheet 204, Truro & Falmouth. The area is 50 km, in a straight line, northeast of Land's End. The granite, greisen, host metasediments and mineralization are described by Scrivenor (1903). This paper is an early British reference to greisen. The word greisen is an old Saxon mining term for granitic rock with tin ore and virtually no feldspar, later referring to an altered granite consisting in the main of quartz and mica, first described by von Leonhard in 1823. Soon after Scrivenor's paper, he and other colleagues joined geologist Clement Reid, a prolific author, in describing the regional geology (Reid et al., 1906).

The mines of the area are described by Dines in his massive memoir (Dines, 1969, volume 1, pp.451-477 and especially pp.457-459). Rashed (1951) described the alteration of the granite, which retains some primary magmatic textures, such as a distinctive flow texture defined by alignment of K-feldspar phenocrysts. Rashed listed the local ore minerals as cassiterite, wolframite, bornite, covellite, arsenopyrite, and chalcopyrite, plus pyrite, malachite, azurite and hematite. The gangue (non-ore) minerals include the lithium-bearing minerals zinnwaldite (a mica) and spodumene, in addition to tourmaline, topaz and fluorite. The greisen veins contain abundant wolframite, cassiterite and stannite, plus rare molybdenite, and amounts of arsenopyrite, scorodite, pharmacosiderite, native copper, chalcopyrite, the tin oxide varlamoffite, olivenite, torbernite, topaz, micas and andalusite (Russell and Vincent, 1952). Russell (1924) reviewed 16 topaz localities in Cornwall, including Cligga Head where topaz occurs in greisen bordering the quartz- cassiterite- wolframite veins. Kingsbury (1958) reported the beryllium silicate euclase in quartz- tourmaline- topaz veins. Native gold has been noted at Cligga Head, in hematized metasediments (Camm, 1995, pp.52-53).

A variety of mineral species have been found at the Cligga locality, in addition to rock-forming minerals such as quartz, mica and feldspar. Tungsten and tin occur largely as oxide phases such as wolframite and cassiterite, but there are exceptions. One is ferrokesterite (Kissin and Owens, 1989), found in the greisen-hosted veins here, a rare sulphide of copper, iron, zinc and tin (previously referred to as zincian stannite). Associated sulphides are arsenopyrite, plus minor chalcopyrite, sphalerite and chalcocite. Moore and Howie (1984) also reported tin-bearing sulphides in the area. Arsenic is a relatively common element in much of the old mining areas, but arsenic minerals are relatively unimportant at some sites, such as Cligga Head. Background levels of arsenic in uncontaminated soils in the area of Hercynian granites may range from 1 to 50 ppm, averaging 5 ppm (Atkinson et al., 1990).

Figure 3. Another sample, an unusually arsenopyrite-rich quartz vein with wolframite. Left: strained vein quartz (dark grey) enclosing subhedral to euhedral white arsenopyrite and medium-grey wolframite. The ore minerals (especially the arsenopyrite) in the vein are associated with blue covellite, a copper sulphide probably formed by alteration of chalcopyrite or chalcocite. Nominal magnification 50X, in plane-polarized reflected light, long-axis field of view 1.7 mm. Right: Nearer the vein margin, quartz (grey, lower left) and brightly birefringent mica flakes envelope angular, euhedral crystals of arsenopyrite. The arsenopyrite here contains some 48.3 wt.% (35.3 atomic %) arsenic (average of six electron microprobe analyses), and occurs in association with cassiterite, tourmaline, wolframite and a Cu-Sn-(Fe,Zn) sulphide of the stannite group, either stannite or ferrokesterite (Wilson, 1988). Nominal magnification 50X, in crossed-polarized transmitted light, long-axis field of view 1.7 mm. Sample 973, collected at Cligga on 28 April 1987. Also noted in this thin section: chalcopyrite, chalcocite, a brownish tin sulphide of the stannite group, and cassiterite.

Figure 4. A white-light transmitted light image of almost the entire thin section of sample 973, showing an area circa 45x24 mm. On the right is a fine-grained foliated rock, evidently a screen or block of the killas (slate), perhaps preserved within the granite (this was a loose specimen). The killas has undergone strong alteration to quartz and brown prismatic tourmaline. The majority of the section samples a quartz vein. The quartz is colourless and essentially invisible here. The selvage of the vein against the killas is rich in pale mica, and dappled with the angular black axe- and rhomb-shaped outlines of opaque arsenopyrite crystals. On the left is the coarse interior of the vein which contains most of the ore minerals. The vein core is coarse quartz with a large mass of many small, angular subhedral arsenopyrite crystals, and a large, zoned brown cassiterite crystal with a colourless outer zone. The opaque triangular grain below the cassiterite is wolframite (see Figure 3). Wolframite is black in hand specimen, cassiterite brown, and arsenopyrite is silvery metallic grey-white. The intermediate zone of the vein is barren white "bull" quartz.

Geological Setting of the Mineralization

The Cornubian granite batholith was formed at the time of the Hercynian (Variscan) orogeny in Europe. The component stocks (discrete outcropping masses) of the batholith, from Dartmoor in Devon westwards to the Scilly Isles, have been dated to the early Permian, some 295 to 270 million years ago. Granites and mineral deposits of this age occur across much of western and central Europe, including parts of France, Germany and the Czech Republic. The storied mineral deposits are associated with the granites. The igneous rock itself is described as an S-type granitic batholith of the ilmenite series, with high heat production and abundant volatile elements, resulting in an abundance of minerals such as tourmaline. Geophysical data suggest that the total volume of the batholith is about 68,000 km³ (Willis-Richards and Jackson, 1989).

The Cornish mines were some of the first in which a concentric zonation of the composition of mineralization was observed from the interior of the granite out into the host rocks, which are commonly slaty sediments (`killas' in old miners' dialect). Thus most tin- tungsten- copper- arsenic production was located along the axial trace of the batholith, with the western end (including Cligga Head) enriched in Sn, Cu and Zn relative to the eastern end. In contrast, most iron, arsenic, antimony, fluorine, barium and china clay production has been situated further to the east. Lead and silver production has been concentrated in south-facing embayments on the flank of the batholith, whereas all significant wolframite is within the granite, or within 700 m of an inferred contact. It has been estimated (Willis-Richards and Jackson, 1989) that 90-100% of Sn- Cu- As production was within 1500 m of a contact, whereas 30% of Zn production and over 80% of Pb and Ag production were won >1500 m beyond a contact.

The Cligga Head granite, where least altered, is tourmaline -bearing, and notably rich in boron and fluorine. Greisen formation is marked by loss of Na and rise in other elements, namely Ca, Fe, B, F, Li, Mn, Rb, Sn and Zn (Hall, 1969). The U appears to be concentrated within the darker bands in the Sn oxide. Moore and Swart (1979) analysed ore minerals and found uranium at ppm levels in cassiterites and wolframites, but not in sulphides. The Hercynian granites are ¹⁸O-enriched, probably due to assimilation of pelitic host rocks at depth, with anatexis modelled at 860°C, 5.2 kbar, 3-5% H₂O (Thorne and Edwards, 1985).

Fluid inclusions in hydrothermal veins and in quartz overgrowths in granites and greisens indicate that both multistage high-T (200-400°C) and single/multistage low-T (70-150°C) hydrothermal events occurred in and around the stock. Three mineral assemblages are noted, namely:

1) Cassiterite- wolframite- stannite- arsenopyrite- pyrite- chalcopyrite with minor Bi, Zn and Mo phases, associated with greisens with quartz and white mica;
2) Pyrite and chalcopyrite with chlorite; and
3) Hematite and limonite.

In addition, extensive argillic hydrothermal alteration (quartz plus kaolinite) occurs in the core and the southern part of the stock. The earliest cassiterite- wolframite mineralization is associated with quartz showing inclusions with homogenization temperatures of 280-400°C (Jackson et al., 1977). Relatively rare pegmatites give fluid inclusion homogenization temperatures of 320-380°C (Moore, 1980). According to Moore, the Cligga Head area presents evidence for both high-T hydrothermal and late, supergene generation of kaolin. The most severe argillic alteration is supergene, associated with 2 major faults near the south contact. A range of sulphides were then deposited at 240-320°C: all this mineralization probably developed beneath at least 2 km of rock (Jackson et al., 1977). Primary fluid inclusions in minerals (Campbell and Panter, 1990) from Cligga homogenize at the following temperatures: cassiterite (352°C), wolframite (324°C) and quartz (295°C). The two main ore minerals may have been deposited earlier than the quartz, even though textural evidence is ambiguous.

The role of fracture sets in controlling the distribution of sheeted vein sets, greisens and mineralization is discussed by Moore and Jackson (1977). Moore (1977) noted the possibility of open-pit mining in this style of mineralization. Sheeted veins that host wolframite at Cligga Head have parallels elsewhere in the Hercynian of western Europe, e.g., St. Michael's Mount on the south coast of Cornwall, and three deposits in the Caceres and Salamanca districts of west-central Spain (Sanderson et al., 2008). A tabulation of basic characteristics of 36 such deposits of tungsten, hosted in siliceous rocks, includes Cligga Head, Castle an Dinas, and the older Caledonian deposit of Carrock Fell in the English Lake District (Wood and Samson, 2000). Greisens are widespread in the world, but are relatively rare in Britain beyond Cornwall: examples include Grainsgill and the Eskdale granite in the Lake District, and they occur also in the Mourne Mountains of Ireland.

Figure 5. Two photos of the polished thin section of sample 973, on a felt polishing pad. On the left, the section has a grey wash, a sub-micron thin film of evaporated carbon, which makes the glass slide and rock slice electrically conductive, to permit electron microprobe analysis. A carbon paint on one side was used to ensure a good contact to the sample holder in the instrument. At right, the slide is clean: the carbon paint was removed in alcohol, and the carbon film, which adheres strongly to the specimen, was gently polished off with water and a sub-micron alumina polishing powder on the pad, before being dried in alcohol.

Tungsten (W)

is element 74, a unique metal with numerous uses. It does not occur as a native metal, but chiefly as tungstates like wolframite and its calcium analogue, scheelite (CaWO₄, 63.85 wt% W). It is very dense, with a specific gravity of 19.25, very close to that of gold. Tungsten retains its strength at high temperatures, and thus has seen more than a century of service in the filaments of incandescent light bulbs (see Oliver Sacks' entertaining "Uncle Tungsten" in this context: Sacks, 2001). A very hard metal, it is used in many cutting tools. One modern high-speed machining steel for cutting metals contains about 18% W, 4.25% Cr, 1.1% V and 0.75% C (Street and Alexander, 1998, pp.218-219). Tungsten carbide is one of the hardest substances known, and is employed in rock drilling and tunnel boring. Tungsten has been used in shielding against x-ray and gamma radiations. Its density sees it used in counterweights, and to balance high-quality darts, for a game popular in Cornish pubs and elsewhere.

Figure 6. The greyish mineral just right of centre is identified as a tin-rich sulphide of the stannite family, associated with euhedral arsenopyrite crystals on the edge of a quartz vein. The blue phase is a very fine-grained covellite, perhaps formed from chalcopyrite replacing the arsenopyrite. Sample 973. Nominal magnification 50X, in plane-polarized reflected light, long-axis field of view 1.7 mm.

References

Aldersey-Williams,H (2011) Periodic Tales: a Cultural History of the Elements, from Arsenic to Zinc. HarperCollins, 428pp.

Atkinson,K, Edwards,RP, Mitchell,PB and Waller,C (1990) Roles of industrial minerals in reducing the impact of metalliferous mine waste in Cornwall. Trans.Inst.Min.Metall. A99, 158-172.

Camm,S (1995) Gold in the Counties of Cornwall & Devon. Cornish Hillside Publications, St. Austell, Cornwall, 116pp.

Campbell,AR and Panter,KS (1990) Composition of fluid inclusions in coexisting (cogenetic?) wolframite, cassiterite, and quartz from St. Michael's Mount and Cligga Head, Cornwall, England. Geochim.Cosmochim.Acta 54, 673-681.

Dines,HG (1969) The Metalliferous Mining Region of South-West England. Institute of Geological Sciences, 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.

Hall,A (1969) The geochemistry of the Cligga Head granite. Proc. Ussher Soc. 2, 136-140.

Jackson,NJ, Moore,JMcM and Rankin,AH (1977) Fluid inclusions and mineralization at Cligga Head, Cornwall, England. Quart.J.Geol.Soc. 134, 343-349.

Kingsbury,AWG (1958) Two Be minerals new to Britain: euclase and herderite. Mineral.Mag. 31, 815-817.

Kissin,SA and Owens,DR (1989) The relatives of stannite in the light of new data. Can.Mineral. 27, 673-688.

Moore,F (1980) The Geology and Mineralisation of St. Michael's Mount and Cligga Head Granites, Cornwall. PhD Thesis, King's College, London, 288pp.

Moore,F and Howie,RA (1984) Tin-bearing sulphides from St Michael's Mount and Cligga Head, Cornwall. Mineral.Mag. 48, 389-396.

Moore,F and Swart,PK (1979) The uranium content of ore minerals from St. Michael's Mount and Cligga Head, Cornwall. Proc. Ussher Soc. 4 part 3, 432-436.

Moore,JMcM (1977) Exploration prospects for stockwork tin-tungsten ores in S.W.England. Mining Magazine 136 no.2, 97-103, February.

Moore,JMcM and Jackson,N (1977) Structure and mineralization in the Cligga granite stock, Cornwall. Quart.J.Geol.Soc. 133, 467-480.

Rashed,AM (1951) The Greisenization of the Cligga Head Granite. PhD Thesis, Cambridge University, 134pp.

Reid,C, Scrivenor,JB, Flett,JS, Pollard,W and MacAlister,DA (1906) The Geology of the Country Near Newquay. Geol.Surv. G.B. Memoir 346, 131pp. + 6 plates.

Russell,A (1924) Topaz from Cornwall, with an account of its localities. Mineral.Mag. 20, 221-236.

Russell,A and Vincent,EA (1952) On the occurrence of varlamoffite (partially hydrated stannic oxide) in Cornwall. Mineral.Mag. 29, 817-826.

Sacks,OW (2001) Uncle Tungsten: Memories of a Chemical Boyhood. A.A. Knopf, 337pp.

Sanderson,DJ, Roberts,S, Gumiel,P and Greenfield,C (2008) Quantitative analysis of tin- and tungsten-bearing sheeted vein systems. Econ.Geol. 103, 1043-1056.

Scrivenor,JB (1903) The granite and greisen of Cligga Head (Western Cornwall). Quart.J.Geol.Soc. 59, 142-159.

Smith,MP and Yardley,BWD (1996) The boron isotopic composition of tourmaline as a guide to fluid processes in the southwestern England orefield: an ion microprobe study. Geochim.Cosmochim.Acta 60, 1415-1427.

Street,A and Alexander,W (1998) Metals in the Service of Man. Penguin Books, 11th edition, 300pp. plus 32 plates.

Thorne,MG and Edwards,RP (1985) Recent advances in concepts of ore genesis in south west England. Trans.Roy.Geol.Soc.Corn. 21 part 3, 113-152.

Willis-Richards,J and Jackson,NJ (1989) Evolution of the Cornubian ore field, Southwest England: Part I. Batholith modelling and ore distribution. Econ.Geol. 84 no.5 (Mineral Deposits of Europe issue), 1078-1100.

Wilson,GC (1980) Ion Microprobe Techniques, with Applications to Analysis of Lithium in Cornish Granites. PhD Thesis, University of Cambridge, 245pp.

Wilson,GC (1988) Some thoughts on the geology and mining history of the Perranporth area. Turnstone Geol.Serv.Ltd. Report, 11pp.

Wood,SA and Samson,IM (2000) The hydrothermal geochemistry of tungsten in granitoid environments: I. Relative solubilities of ferberite and scheelite as a function of T, P, pH, and mNaCl. Econ.Geol. 95, 143-182.

Graham Wilson, 02-10 January 2014

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