"Rock of the Month # 19, posted January 2003" --- sample 1812 (ex: Paragon Minerals, Tempe, AZ).

This sample is from Lookout Pass, Tooele County, Utah, southwest U.S.A. It contains weissbergite as steel-grey, fine- grained metallic laths in a banded grey to massive black host. This mineral is a Tl-Sb sulphide, triclinic TlSbS₂, the Sb equivalent of lorandite, TlAsS₂. The latter was the first Tl mineral to be described, from its type locality of the prolific Allchar area of Macedonia (Rieck, 1993). The host rock seems to be a black, silicified, decalcified and brecciated limestone, with silica and Tl minerals in low-volume vuggy fractures between host-rock clasts. Cream, white, brown and sulphur-yellow secondary minerals include limonite and probably oxidized Tl-rich species. A similar sample (1843) displays silvery crystals in dull grey host rock. It is said to contain weissbergite, pink avicennite (Tl₂O₃) in voids, parapierrotite and pierrotite (Tl-Sb-As sulphides) and black vrbaite (a Hg-Tl-As-Sb-S phase). The crystals occur in a fine-grained silica-dominated host rock, darkened by organic matter. They are creamy-white in reflected light, bireflectant and intensely anisotropic in grey to bluish-grey tints. Anhedral, ragged-sided grains, soft, up to 2.8x1.0 mm in size. Near the extinction position, they may show a crinkly (?) pressure twinning, reminiscent of stibnite. The Tl-mineral content and grain size vary throughout the rock, apparently as a function of degree of silicification and the organic matter content.

Thallium minerals

Most Tl minerals, all rare, have been described from a few localities, such as Lengenbach, Switzerland; Allchar (a.k.a. `Alsar'), Macedonia; the Hautes-Alpes of France; sediment-hosted gold deposits in the U.S. Southwest (e.g., Carlin, Nevada; Mercur, Utah); the Noril'sk-Talnakh ore field, Siberia; Hemlo gold deposit (Ontario, Canada); and Ilimaussaq (Greenland).

Thallium minerals have been described from a number of mineral deposits in the American Southwest, notably gold deposits such as Carr Fork and Mercur in Utah (Cameron and Garmoe, 1987; Wilson et al., 1991, 1993; Foit et al., 1995). Most are sulphides and sulphosalts although, analogous to Sb, oxidized minerals do occur in supergene alteration zones. A table of 34 Tl minerals is provided. The abundant weissbergite in the sample is consistent with a tremendous enrichment of the rock in Tl, as much as 5 wt.% Tl, equivalent to the richest Tl ore described by Xiao (2001) in Guizhou province, China. Weissbergite was first found in grains up to 0.5 mm in size at the east pit of the Carlin gold deposit, Nevada, USA (Dickson and Radtke, 1978). The weissbergite occurs in silicified, brecciated dolomitic rocks, and microprobe work on the type samples indicated near-pure TlSbS₂ (although emission spectrography revealed minor arsenic, 300 ppm, and Fe, 200 ppm). Consistent with this finding, qualitative energy-dispersive analysis of the featured sample did not detect any elements besides Sb, Tl and S. A range of hydrothermal Tl minerals has been described at Carlin (e.g., Radtke et al., 1977) in mineralized silty argillaceous dolomites of the Roberts Mountains Formation. Other sites in Nevada and Utah, particularly gold deposits, are known to host Tl minerals. In addition to these rare, discrete phases with essential Tl, many ore minerals may contain appreciable Tl (Nowacki et al., 1982), e.g., up to 3.2% (stibnite), 1.56% (jordanite) and 0.2% (orpiment)

Thallium, the metal

The mean abundance of thallium in the bulk continental crust is estimated at 360 parts per billion by weight (ppb), compared to lead (Pb, 8,000 ppb), bismuth (Bi, 60 ppb), antimony (Sb, 200 ppb), silver (Ag, 80 ppb) and uranium (910 ppb: all values from Taylor and McLennan, 1995). Returning to the featured sample, weissbergite has an ideal composition of 52.37 wt.% Tl, 31.20% Sb, 16.43% S, and the rock in question contains roughly 100,000 times the crustal average content! This is a degree of "natural refining" 100 times more than that typically seen in modern ores of precious metals such as gold and platinum, which are often economic to recover at only 1,000 times or so their typical crustal abundances.

Thallium is one of a number of rare metals that can in principle be recovered via natural concentration by certain metal-tolerant plant species ("phytomining"). This neat harnessing of the power of plants to "hyperaccumulate" specific metals may provide a double reward: the economic value of the rare metal plus the "phytoremediation" of old mine sites, in which mine tailings and smelter wastes may contain chemically unstable forms of one or more potentially hazardous metals (Anderson et al., 1999). The converse possibility, that Tl may build up in the human food chain and cause illness and death, has been documented in detail in a small region of southern China by Xiao (2001) and Xiao et al. (2002,2004a,b,c). It is now well-established that Tl may present a threat via its presence in the food chain (e.g., concentrated into cabbage), in the water supply, and in fuel (Tl-enriched coal).

References

ANDERSON,CWN, BROOKS,RR, CHIARUCCI,A, LACOSTE,CJ, LEBLANC,M, ROBINSON,BH, SIMCOCK,R and STEWART,RB (1999) Phytomining for nickel, thallium and gold. J.Geochem.Explor. 67, 407-415.

CAMERON,DE and GARMOE,WJ (1987) Geology of skarn and high-grade gold in the Carr Fork Mine, Utah. Econ.Geol. 82, 1319-1333.

DICKSON,FW and RADTKE,AS (1978) Weissbergite, TiSbS₂, a new mineral from the Carlin gold deposit, Nevada. Amer.Mineral. 63, 720-724.

FOIT,FF, ROBINSON,PD and WILSON,JR (1995) The crystal structure of gillulyite, Tl₂(As,Sb)₈S₁₃, from the Mercur gold deposit, Tooele County, Utah, U.S.A. Amer.Mineral. 80, 394-399.

NOWACKI,W, EDENHARTER,A, ENGEL,P, GOSTOJIC,M and NAGL,A (1982) On the crystal chemistry of some thallium sulphides and sulphosalts. In `Ore Genesis, the State of the Art' (Amstutz,GC, El Goresy,A, Frenzel,G, Kluth,C, Moh,G, Wauschkuhn,A and Zimmermann,RA editors), Springer-Verlag, 804pp., 689-697.

POOL,R (1989) Superconductivity: is the party over? Science 244, 914-916, 26 May.

RADTKE,AS, DICKSON,FW, SLACK,JF and BROWN,KL (1977) Christite, a new thallium mineral from the Carlin gold deposit, Nevada. Amer.Mineral. 62, 421-425.

RIECK,B (1993) Famous mineral localities: Allchar, Macedonia. Mineral.Record 24, 437-449.

TAYLOR,SR and McLENNAN,SM (1995) The geochemical evolution of the continental crust. Reviews of Geophysics 33, 241-265.

TRUEB,L (1994) Controversy surrounds thallium production in Japan and elsewhere. Northern Miner 80 no.9, 12-13, 02 May.

WILSON,JR, ROBINSON,PD, WILSON,PN, STANGER,LW and SALMON,GL (1991) Gillulyite, Tl₂(As,Sb)₈S₁₃, a new thallium arsenic sulfosalt from the Mercur gold deposit, Utah. Amer.Mineral. 76, 653-656.

WILSON,JR, SEN GUPTA,PK, ROBINSON,PD and CRIDDLE,AJ (1993) Fangite, Tl₃AsS₄, a new thallium arsenic sulfosalt from the Mercur Au deposit, Utah, and revised optical data for gillulyite. Amer.Mineral. 78, 1096-1103.

XIAO,T (2001) Environmental Impact of Thallium Related to the Mercury-Thallium-Gold Mineralization in Southwest Guizhou Province, China. PhD Thesis, Université du Québec à Chicoutimi, 246pp.

XIAO,T, BOYLE,D, GUHA,J, ROULEAU,A, HONG,Y and ZHENG,B (2002) Groundwater-related thallium transfer processes and their impacts on the ecosystem: southwest Guizhou province, China. Applied Geochemistry 18, 675-691.

XIAO,T, GUHA,J, BOYLE,D, LIU,C and CHEN,J (2004a) Environmental concerns related to high thallium levels in soils and thallium uptake by plants in southwest Guizhou, China. Science of the Total Environment 318, 223-244.

XIAO,T, GUHA,J, BOYLE,D, LIU,C-Q, ZHENG,B, WILSON,GC, ROULEAU,A and CHEN,J (2004b) Naturally-occurring thallium: a hidden geoenvironmental health hazard? Environment International 30, 501-507.

XIAO,T, GUHA,J and BOYLE,DR (2004c) High thallium content in rocks associated with Au-As-Hg-Tl and coal mineralization and its adverse environmental potential in SW Guizhou, China. Geochemistry: Exploration, Environment, Analysis 4, 243-252.