The geological setting of the indium-rich Baal Gammon and Isabel Sn-Cu-Zn deposits in the Herberton Mineral Field, Queensland, Australia

Base metal mineralization at the Baal Gammon and Isabel deposits of the Herberton Mineral Field (HMF) is hosted in metamorphosed greywacke beds in the Hodgkinson Formation, which were intruded by granite, porphyry dykes and overlain by volcanic rocks of the Kennedy Igneous Association during the Carboniferous and Permian. The tin mineralization at the Baal Gammon deposit is hosted by a silicified, chlorite-altered, quartz-feldspar porphyry (UNA Porphyry). The tin mineralization at the Isabel deposit is in polymetallic veins hosting disseminated cassiterite. Polymetallic sulfides (Cu-Zn) and indium (In) mineralization at both deposits overprint the tin mineralization. Chalcopyrite, sphalerite, and stannite host indium in the polymetallic sulfide assemblage at both deposits. Based on overprinting relationships, the timing of tin mineralization is related to the magmatic activity at ca. 320 Ma, whereas the sulfide and indium mineralization are most likely associated with the emplacement of porphyry dykes at ca. 290 Ma. The overall magmatic activity in the HMF spreads between ca. 365 and 280 Ma, with peaks at ca. 337, 322, 305, and 285 Ma. The change from tin mineralization at ca. 320 Ma to sulfide and indium mineralization at ca. 290 Ma indicates a transition from a compressive to an extensional tectonic regime

Geochemistry of indium in magmatic-hydrothermal tin and sulfide deposits of the Herberton Mineral Field, Australia

The Herberton Mineral Field in Northeast Australia hosts world class magmatic-hydrothermal Sn–W polymetallic deposits that are enriched in In. The Baal Gammon and Isabel deposits from the Herberton Mineral Field contains early tin, as cassiterite, overprinted by sulfide mineralization as chalcopyrite, sphalerite, galena, pyrrhotite, and stannite. We investigated the distribution of In in the sulfide ores from these two deposits, calculated the temperature of formation via sphalerite-stannite geothermometer, and deduced the physicochemical conditions favorable for enriching In in this mineralizing environment. The Baal Gammon deposit is dominated by chalcopyrite, with In contained in chalcopyrite, sphalerite, and stannite. The average In concentrations measured by EPMA in chalcopyrite, sphalerite, and stannite are 0.10, 0.68, and 0.92 wt%, respectively. Chalcopyrite, pyrrhotite, and sphalerite textures indicate that In incorporation occurred during exsolution from an intermediate solid solution of cubanite composition. The Isabel deposit is dominated by sphalerite associated with galena and contains only minor amounts of chalcopyrite. The average concentration of In in sphalerite from the Isabel deposit is 0.11 wt%. The stannite-sphalerite geothermometer indicates mineralization temperatures of ~ 290 °C at the Baal Gammon deposit, and ~ 307 °C at the Isabel deposit. At these temperatures, the physicochemical modeling suggests that stable In chlorine complexes occur in acidic conditions (pH < 3). These results when combined with the Eh–pH phase model of the sulfide assemblage further constrain the redox conditions during mineralization