Here are two photomicrographs in offset-crossed-polarized reflected light of stibnite crystals in gold-antimony (Au-Sb) ore from an underground mine in the Murchison greenstone belt on the Archean Kaapvaal craton, south of the Limpopo river in the northeast corner of South Africa.
![stibnite 40x [50 kb]](stib40x.jpg)
![stibnite 160x [48 kb]](stib160x.jpg)
1. Left: Coarse stibnite, mount "A". Magnification 40x, long-axis field-of-view 2.8 mm.
2. Right: Stibnite, mount "C". Pressure twinning and twin gliding are characteristic, forming the "corrugation" lamellae and "undulate" extinction of Ramdohr (1980). Magnification 160x, long-axis field-of-view 0.7 mm.
"Rock of the Month # 10, posted for April 2002" --- sample 1750.
Stibnite is the major ore mineral of antimony (Sb). Antimony is a metal used to harden lead alloys for use in batteries and other applications. It is an ingredient of other speciality alloys, such as pewter, Babbitt metal and type-metal. Antimony oxides are used in paint pigments, as colouring agents in glass, and as fire retardents.
This gold-antimony ore contains an estimated 15 volume percent of the sulphide, much less than the hand- specimen appearance suggests, due to a tendency for the samples to break along stibnite-lined fracture surfaces. Minor pyrite and arsenopyrite also occur in the quartz-carbonate matrix.
Stibnite is a soft, orthorhombic mineral, steel-grey with a bright metallic lustre, specific gravity 4.63. In coarse-grained occurrences it is notable for forming spectacular, highly elongate, striated prisms, popular with mineral collectors. It is greyish-white in reflected light, with strong bireflectance and anisotropy and a marked tendency to undulose extinction. China has roughly 50% of world reserves of Sb and accounts for some 80% of world Sb production. The main producers (85% of the industry) are in Hunan province, but at least 12 regions of China have Sb deposits (Wu, 1993; Behling et al., 2002).
STIBNITE FROM THE MURCHISON BELT, SOUTH AFRICA
Local Au-Sb deposits are hosted in talc-carbonate and quartz- carbonate schists, formed at least in part by carbonatization of komatiites (Pearton, 1979; Pearton and Viljoen, 1986). Au-Sb mineralization is epigenetic and syntectonic, largely as quartz- carbonate- stibnite veins in syn-D1 brittle fractures in massive quartz-carbonate rocks (Boocock et al., 1988) related to shear zones, many of which host brittle-ductile vein systems (Nutt et al., 1988). Ore on the local "Antimony Line" is confined to a 10-25-m zone of massive quartz-carbonate rock in an envelope of schistose chloritic quartz-carbonate rock and talc-carbonate schist (Willson and Viljoen, 1986).The stibnite-rich sample was collected in November 1995 during a field trip of the Mineral Deposits Division of the Geological Association of Canada and the Society of Economic Geologists, organized by Dani Alldrick, with the assistance of geologist Colin Willson of the Consolidated Murchison Mine.
The mineralogical residence of gold in the ore is not apparent in this particular sample. No native gold was seen in the three polished mounts of the ore. If these mounts are representative, much of the gold may be in solid solution or submicroscopic inclusions in stibnite and arsenopyrite, consistent with the limited occurrence of native gold and aurostibite in the belt (Abbot et al., 1986). A brief energy-dispersive analysis using the proton microprobe facility at the University of Guelph (Ontario, Canada) revealed minimal impurities, with a nominal detection limit of 12 ppm for Au (Turnstone Reports 1996-100 and -101). Putting a very conservative upper bound of 50 ppm on the gold content of the stibnite, this sulphide may nevertheless contain a high proportion of the "invisible gold" in this ore.
Some common ore minerals such as chalcopyrite tend to be relatively pure, with impurities in most cases <0.1 wt.%, whereas others show more variability, e.g., pyrite in some geological environments contains low percent levels of arsenic, and sphalerite many contain 10 percent or more iron and several tenths of 1 percent cadmium substituting for the zinc.
References
ABBOT,JE, VAN VUUREN,CJJ and VILJOEN,MJ (1986) The Alpha-Gravelotte antimony ore body, Murchison greenstone belt. In `Mineral Deposits of Southern Africa' (Anhaeusser,CR and Maske,S editors), Geol.Soc.S.Africa, 2335pp., 321-332.
BEHLING,SC, LIU,G and WILSON,WE (2002) Stibnite from the Wuling antimony mine, Jiangxi province, China. Mineral.Record 33, 139-147.
BOOCOCK,CN, CHESHIRE,PE, KILLICK,AM, MAIDEN,KJ and VEARNCOMBE,JR (1988) Antimony-gold mineralization at Monarch mine, Murchison schist belt, Kaapvaal craton. In `Advances in Understanding Precambrian Gold Deposits, Volume II' (Ho,SE and Groves,DI editors), Geology Department and University Extension, University of Western Australia, Publ. No.12, 360pp., 81-97.
NUTT,THC, McCCOURT,S and VEARNCOMBE,JR (1988) Structure of some gold and antimony-gold deposits from the Kaapvaal and Zimbabwe cratons. Ibid., 63-80.
PEARTON,TN (1979) A geochemical investigation of the carbonate and associated rocks of the Monarch antimony mine, Murchison Range, North-Eastern Transvaal. Spec.Publ. Geol.Soc.S.Afr. 5, 159-166.
PEARTON,TN and VILJOEN,MJ (1986) Antimony mineralization in the Murchison greenstone belt - an overview. In `Mineral Deposits of Southern Africa' (Anhaeusser,CR and Maske,S editors), Geol.Soc.S.Afr., 2329pp., 293-320.
RAMDOHR,P (1980) The Ore Minerals and Their Intergrowths. Pergamon Press, 2nd edition in 2 vols., 1205pp. [see pp.705-709].
WILLSON,C and VILJOEN,MJ (1986) The Athens antimony ore body, Murchison greenstone belt. In `Mineral Deposits of Southern Africa' (Anhaeusser,CR and Maske,S editors), Geol.Soc.S.Afr., 2329pp., 333-338.
WU,J (1993) Antimony vein deposits of China. Ore Geology Reviews 8, 213-232.
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