The geochemistry of Ga, Ge and In in magmatic–hydrothermal tin–polymetallic deposits of the Herberton Mineral Field, Australia

The geochemistry of In, Ga and Ge of muscovite and topaz was investigated from magmatic–hydrothermal systems related to tin and polymetallic vein mineralisation associated with Carboniferous–Permian granites from the Herberton Mineral Field (HMF) in northeast Queensland. Muscovite and topaz from the Baal Gammon and Isabel deposits were sampled and analysed. Each deposit shows two stages of hypogene mineralisation. The first stage is related to tin mineralisation associated with reduced intrusive rocks including the Jumna Granite, the Herberton Hill Granite and the UNA Porphyry. The second stage is related to Cu mineralisation associated with oxidised magmas that formed during the emplacement of the Slaughter Yard Creek Volcanics. Muscovite and topaz associated with the reduced intrusive rocks mainly occur in the greisen derived from the UNA Porphyry, whereas the muscovite associated with the oxidised phase occur with the massive sulfide mineral assemblage hosted by the Slaughter Yard Creek Volcanics and the Hodgkinson Formation. In general, the topaz grains have higher concentrations of Ga and Ge (average 196 ppm and 68 ppm, respectively) than the muscovite grains (average Ga = 8.4 ppm and Ge = 2 ppm). In contrast, muscovite grains have higher In concentrations (up to 60 ppm) than the topaz grains (below limit of detection). The normalised trace-element pattern of muscovite grains from different tin deposits indicates that Ga and Ge have a negative anomaly in oxidised granites and a positive anomaly in reduced granites. In contrast, the In anomaly is not sensitive to the oxidation state of the magmas. Geochemical modelling indicates that In forms stable complexes in acidic conditions, whereas Ga and Ge form stable complexes with oxide and hydroxide ions in near-neutral to weakly basic and reduced conditions. Thus, the geochemistry of In is decoupled from that of Ga and Ge owing to the redox conditions of the magmatic–hydrothermal systems.