"Rock of the Month # 8, posted February 2002" --- Sample courtesy of Francis T. Manns.

Berthierite is a relatively uncommon iron-antimony sulphide, FeSb₂S₄. The photograph by Karyn Gorra displays a 10-cm hand specimen of an altered subvolcanic granitoid rock decorated with a nest of steel-grey, striated acicular prisms of the monoclinic sulphide. Qualitative electron microprobe analysis indicates a composition close to ideal, with a trace of copper. Berthierite occurs in antimony-(gold) and less commonly in massive sulphide ores (see, e.g., Dill et al., 1997; Ujer et al., 2000). Seldom abundant, it is widely distributed through deposits in Russia, Romania, Slovakia, France and Portugal, South Africa, Australia, Bolivia, the Hemlo and Timmins gold camps in Ontario, the Hazelton and Tagish Lake areas in British Columbia, and localities in New Brunswick and Washington state.

This sample is from the Bunut gold prospect in Kalimantan, Indonesian Borneo. This area has numerous gold showings and several epithermal gold deposits, including Mount Muro and Kelian (Simmons and Browne, 1990; Van Leeuwen et al., 1990; White et al., 1995). The Bunut locality lies at the head of the Mentebah river system, 50 km southwest of the town of Putussibau, in the Muller Mountains region. Miocene-age igneous rocks form upstanding inselbergs of granodiorite, diorite, dacite porphyry and andesite in a northeast-trending belt identified as a collision zone between two ancestral terranes in Borneo. These "Sintang intrusions" include porphyritic andesite, diorite and granodiorite, and display variable alteration, with abundant sheet silicates (clay minerals and sericite mica).

The mineral grains in the smaller photos are in a gold-bearing sulphide concentrate from Bunut. Pyrite is the most abundant mineral in the concentrate. Fine granular arsenopyrite lines planar replacements along fractures in the pyrite, and slightly coarser tabular arsenopyrite invests pyrite faces. Stibnite (Sb₂S₃) is also quite abundant. Native gold occurs liberated (loose) and also with the arsenopyrite veinlets in pyrite. The concentrate also contains secondary iron oxides (goethite), chalcopyrite and pyrrhotite. The mineral and chemical assemblage, enriched in arsenic, antimony, copper, gold and silver, is typical of many gold deposits.

There are numerous prospects and a number of hard-rock and placer gold mining operations in the region. One modest gold resource occurs at Busang, which was investigated by Australian companies in the 1980s (Anon, 1989). There is some "real" gold at Busang, but the amount was at that time judged quite insufficient to make a mine in such a remote setting. Busang nevertheless became infamous in 1997 as the scene for the Bre-X gold fraud (see Hutchinson, 1998, and at least four other books). After the Busang scandal made the headlines, funding for numerous junior resource companies active in Indonesia and elsewhere collapsed. One such company, Borneo Gold Corporation, nevertheless pursued its exploration at Bunut into the following year. The operation might be termed a technical success as there is evidently gold there, but limited drilling did not reveal evidence of an economically viable resource.

References

ANON (1989) Outlining Busang. Mining Mag. 161 no.1, 17, July.

DILL,HG, PERTOLD,Z and KILIBARDA C,R (1997) Sediment-hosted and volcanic-hosted Sb vein mineralization in the Potosi region, central Bolivia. Econ.Geol. 92, 623-632.

HUTCHINSON,B (1998) Fools' Gold: The Making of a Global Market Fraud. Alfred A. Knopf Canada, 267pp.

SIMMONS,SF and BROWNE,PRL (1990) Mineralogic, alteration and fluid-inclusion studies of epithermal gold-bearing veins at the Mt. Muro prospect, Central Kalimantan (Borneo), Indonesia. In `Epithermal Gold Mineralization of the Circum-Pacific; Geology, Geochemistry, Origin and Exploration, I' (Hedenquist,JW, White,NC and Siddeley,G, editors). J.Geochem.Explor. 35, 447pp., 63-103.

UJER,P, MICHAL,S and VITALOS,J (2000) The Pezinok antimony mine, Male Karpaty Mountains, Slovakia. Mineral.Record 31, 153-162.

VAN LEEUWEN,TM, LEACH,T, HAWKE,AA and HAWKE,MM (1990) The Kelian disseminated gold deposit, East Kalimantan, Indonesia. In `Epithermal Gold Mineralization of the Circum-Pacific; Geology, Geochemistry, Origin and Exploration, I' (Hedenquist,JW, White,NC and Siddeley,G, editors). J.Geochem.Explor. 35, 447pp., 1-61.

WHITE,NC, LEAKE,MJ, McCAUGHEY,SN and PARRIS,BW (1995) Epithermal gold deposits of the southwest Pacific. J.Geochem.Explor. 54, 87-136.