STABLE ISOTOPE CHARACTERIZATION OF THE THERMAL PROFILE AND SUBSURFACE BIOLOGICAL ACTIVITY DURING OXIDATION OF THE GREAT AUSTRALIA DEPOSIT, QUEENSLAND, AUSTRALIA

The application of stable isotope thermometry and phase relations was used to examine the thermal evolution of oxidation and secondary mineralization at the Great Australia mine, Cloncurry, Queensland, Australia. Results show that early secondary mineralization consisted of djurleite crystallization at temperatures <93ºC, followed by calcite mineralization at 66 to 75ºC. The secondary mineralization paragenesis continued with azurite that formed at 51ºC, followed by malachite crystallization at 41 to 38ºC, then malachite pseudomorphously after azurite at 34ºC. The final stage of secondary mineralization consists of calcite that formed at 25 to 30ºC, followed by goethite. The general trend observed is for early oxidation and supergene enrichment at apparent temperatures between 93 and 75 ºC, cooling to ambient surface temperatures late in the period of mineralization. A correlation is observed between decreasing isotope thermometry temperatures and increasing d13C values as the period of oxidation and supergene enrichment progressed. This is interpreted to result from exothermic sulfide oxidation and associated subsurface thermophilic bacterial oxidation within the oxidation front. Over the period of oxidation and supergene enrichment, temperatures decreased and d13C values increased, recording in the secondary minerals the passing of the oxidation front and a diminishing subsurface bacterial carbon source.