Gold Distribution and Genesis in Australian Volcanogenic Massive Sulfide Deposits and Their Significance for Gold Transport Models
Published:January 01, 1989
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.
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The Geology of Gold Deposits: The Perspective in 1988
When the price of gold rose from about $200 (U.S.) an ounce in 1979 to nearly $700 an ounce by the end of the same year, the gold rush of the 1980s was under way. Gold production in the western world rose dramatically; from 1981 to 1986 production increased by 300 to 1,282 metric tons per year. Annual production may reach 1,500 to 1,600 metric tons by 1990 (Woodall, 1988). The major contributors to the increased stream of gold have been Australia, Canada, Brazil, and the United States together with other circum-Pacific countries. The increased price of gold and new methods of extraction have allowed many older deposits to be reopened, but the most important factor has been the high success level of exploration. This success has resulted in large part from the application of new genetic models and from the development of new exploration techniques.
There are hundreds of thousands of reported gold occurrences around the world. The majority are alluvial placers, but large numbers of bedrock occurrences have also been discovered. Most of these occurrences prove to be very small and are relatively unimportant in the overall world production level. Most mined gold has come from a small number of giant deposits, which were found by prospectors. It is becoming increasingly clear, however, that the discovery of giant deposits in the future will involve more than the sharp eyes and persistence of the old prospector. The use of sound geologic principles, and exploration programs based on those principles, is what the future holds. An example can be seen in the successful search for gold deposits in the South Pacific. There, exploration models have been based on principles developed in the study of modern geothermal systems. Giant deposits such as Lihir and Porgera have been the reward. Another example is the giant copper-gold-uranium deposit at Olympic Dam, South Australia, discovered beneath 300 m of cover using an exploration program based on models developed by Western Mining Corporation geologists for Zambian copper belt-type deposits.
Gold deposits are widely dispersed throughout many geologic settings and in virtually all kinds of rocks, but they do not seem to have formed at a uniform rate throughout geologic history. On the contrary, two very distinct metallogenic periods have been defined. The first is the Archean era, when most of the great deposits in greenstone belts were formed and the vast Witwatersrand basin deposits in