Gold Distribution and Genesis in Australian Volcanogenic Massive Sulfide Deposits and Their Significance for Gold Transport Models
Ross R. Large, David L. Huston, Peter J. McGoldrick, Peter A. Ruxton, Garry Mcarthur, 1989. "Gold Distribution and Genesis in Australian Volcanogenic Massive Sulfide Deposits and Their Significance for Gold Transport Models", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Volcanogenic massive sulfide deposits in Australia exhibit a range in average gold content from 0.2 to 4.75 ppm Au, with an overall mean of 1.6 ppm. The Mount Morgan Cu-Au deposit in eastern Queensland has been the major producer (237.5 metric tons of gold), followed by the deposits in the Mount Read Volcanics of western Tasmania (Rosebery, Hercules, Que River, Hellyer, and Mount Lyell) which together have a premining resource of 156.3 metric tons of gold.
Two distinct spatial and mineralogical associations of gold mineralization have been defined for the eastern Australian volcanogenic massive sulfide deposits: (1) a gold-zinc association (with lead, silver, and barite), which typically occurs throughout the massive and layered ores with gold and barite concentrated toward the stratigraphic hanging wall of the deposit (e.g., Rosebery, Que River, and Hellyer), and (2) a gold-copper association, which typically occurs in the footwall stringer and lower massive zones of some deposits, particularly those with a high Cu/Zn ratio (e.g., Mount Chalmers, Mount Morgan, and Mount Lyell). This biparite gold association observed in the eastern Australian deposits is also displayed in other volcanogenic massive sulfide provinces, such as the kuroko district (Japan) and the Canadian Archean.
Thermodynamic studies on the controls of gold transport and deposition indicate that the two gold associations described above may relate directly to the gold-transporting mechanism. The footwall gold-copper association reflects gold transport as the AuCl2 complex by high-temperature (>300°C), low pH (<4.5), moderate to high fO2, and high-salinity fluids (>seawater). The hanging-wall gold-zinc association reflects gold transport as the Au(HS)−2 complex by lower temperature (150°-300°C), moderate pH (4.5-6), and moderate fo2 fluids.
A process of gold refining where cooling hydrothermal solutions leach gold (plus zinc and lead) from the lower parts of the sulfide body and reprecipitate the gold at the top of the body, and which is associated with dropping temperature and increasing SO4/H2S ratio, is proposed as the mechanism which leads to gold enrichment at the top of zinc-rich deposits. This process is common in barite-rich Paleozoic deposits but less common in Archean deposits, due to lower SO4/H2S fluid ratios in the latter.