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GERMANIUM-BEARING COLUSITE FROM THE WATERLOO VOLCANIC-ROCK-HOSTED MASSIVE SULFIDE DEPOSIT, AUSTRALIA: CRYSTAL CHEMISTRY AND FORMATION OF COLUSITE-GROUP MINERALS

Thomas Wagner^1, and Thomas Monecke²

¹ Institut für Geowissenschaften, Universität Tübingen, Wilhelmstraße 56, D–72074 Tübingen, Germany
² Geological Survey of Canada, Natural Resources Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

E-mail address: th.wagner{at}uni-tuebingen.de

Bornite-bearing ores of the Waterloo volcanic-rock-hosted massive sulfide (VHMS) deposit, northern Queensland, Australia, host germanium-bearing colusite, ideally Cu_24+xV₂(As,Sb)_6–x(Sn,Ge)_xS₃₂, which has been studied by a combination of reflected-light microscopy, scanning electron microscopy and electron-microprobe (EMP) analysis. On the basis of textural criteria, colusite is associated with pyrite, chalcopyrite, sphalerite, tennantite and bornite. The EMP investigations showed that the colusite is chemically homogeneous, the concentrations of As, Sb, V, Sn and Ge being in the range of 3.42–4.08, 0.10–0.20, 1.86–2.19, 0.01–0.12 and 1.78–2.23 apfu, respectively. Data analysis demonstrates a statistically significant negative correlation between (As + Sb) and (Ge + Sn), in apfu. Moreover, the nominal number of vacancies per formula unit (pfu), calculated as 34 – cations apfu, was found to correlate negatively with (Cu + Fe + Zn). A compilation of literature data supports the existence of these two compositional trends in colusite-group minerals. However, the calculated nominal number of vacancies ranges between –2.0 and 2.5 pfu, indicating a significant deviation from the ideal crystal-chemical formulae proposed so far. The compositional range of the literature data can only be explained by the simultaneous operation of at least three different schemes of coupled substitution. Compositional discontinuities in the quaternary system As–Sb–Sn–Ge are interpreted to be indicative of the existence of miscibility gaps in the colusite group. Integration of the results of the present study and literature data with phase-equilibria constraints suggests that colusite-group minerals in VHMS deposits form under moderately oxidized conditions at distinctly high Cu:Fe activity ratios in the ore-forming hydrothermal fluids.

Keywords: colusite, germanium, solid solution, volcanic-rock-hosted massive sulfide deposits, Waterloo VHMS deposit, Queensland, Australia.

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