The Mt Bischoff mine was discovered by farmer and part-time prospector James “Philosopher” Smith in 1871, and for many years was one of the world’s richest tin mines (Groves et al., 1972). After 70 years of continuous production, the Mt Bischoff mine finally closed in 1947. Since then, mining ventures have only been spasmodic. It was the first major mineral resource developed in Tasmania, with a total production of around 62,000t of metallic tin. The discovery provided the impetus for exploration of other areas of the west coast and the subsequent discovery of other mining areas such as Mt Lyell and Renison.
Geology
Precambrian quartzite, shale and dolomite at Mt Bischoff have been intruded by a radial group of Devonian quartz-feldspar porphyry dykes and breccia dykes, related to the nearby Meredith granite. Tin-base metal sulphide mineralisation accompanies these dykes, and occurs as a replacement of dolomite, as greisenised dykes, and as veins and fracture linings (Groves et al., 1972; Wright & Kwak, 1989, Halley & Walshe, 1995; Sorrell, 1997). Tin mineralisation occurs within a radius of about 1km from the summit of Mt Bischoff, and several silver-lead-antimony deposits surround the mine. Supergene zones were minor.
Dolomite-Replacement Mineralisation
Dolomite-replacement mineralogy is dominated by sulphides (mostly pyrrhotite), Fe-Mg-Mn carbonates, fluorides (fluorite, sellaite), tourmalines and magnesium silicates (chondrodite, norbergite, clinohumite, phlogopite, talc and serpentine), in part as magnesian skarns and “wrigglite” (banded skarn), largely retrogressed. The highest tin grades were found within a quartz-pyrrhotite-topaz-fluorite assemblage (the most strongly altered dolomite replacement rocks) adjacent to the dykes, (now largely worked out). Dolomite-replacement mineralisation is found at the Greisen Face, where talc-pyrrhotite alteration occurs, and at the Slaughteryard Face, where quartz-pyrrhotite alteration and fluorite “nodules” occur.
Dyke Mineralisation
Greisenised dyke rocks are characterised by a white, fine-grained groundmass of quartz and topaz, with orthoclase phenocrysts variably pseudomorphed by siderite, pyrrhotite, quartz, topaz, pyrite, fluorite and cassiterite. One of the most prominent features of Mt Bischoff is the Western Dyke, a greisenised porphyry with cassiterite both disseminated and coating joint surfaces.
Veins and Fracture Linings
Fissure lodes are a late phase of mineralisation, cross-cutting dykes and country rocks. They contain variable proportions of quartz, siderite, tourmaline, topaz, fluorite, cassiterite, wolframite and sulphides (pyrite, pyrrhotite, galena, sphalerite, chalcopyrite, bismuthinite, As-Sb-Bi sulphosalts and stannite).
More than 100 mineral species have been recorded from Mt Bischoff,particularly because of the extensive alteration and large amounts of magnesium and fluorine in the system,producing uncommon minerals such as sellaite, chondrodite, norbergite, wagnerite and fluoborite.
References:
- Anon, 1970. Catalogue of the Minerals in Tasmania. Geol. Surv. Record No. 9, Tasmania Department of Mines.
- Groves, D.I., Martin, E.L., Murchie, H., & Wellington, H.K., 1972. A Century of Tin Mining at Mount Bischoff, 1871-1971. Geological Survey Bulletin No. 54. Tasmanian Department of Mines.
- Burrett, C.F. & Martin, E.L., eds., 1989. Geology and Mineral Resources of Tasmania., p. 383-398. Geological Society of Australia Inc., Special Publication 15.
- Dickens, G., 1995. Mining in Tasmania – a brief History. Tasmanian Geological Survey Record 1995/11.
- Halley, S. & Walshe, J.L., 1995, A re-examination of the Mt Bischoff cassiterite sulphide skarn, Western Tasmania. Economic Geology, 90, 1076-1683.
- Sorrell, S, 1997. Mt Bischoff – Mountain of Tin. Mineralogical Society of Tasmania, Newsletter, 20, 7-16.
- Wright, J.H. and Kwak, T.A.P., 1988. Endogreisen, brecciation and fluid activity at the Mount Bischoff Sn-deposit, North-west Tasmania, Australia. Journal of Metamorphic Geology 6, 629-650.
- Bottrill, R.S., in prep. Catalogue of Minerals of Tasmania. Mineral Resources Tasmania.
