Archean Carbon and Oxygen Reservoirs: Their Significance for Fluid Sources and Circulation Paths for Archean Mesothermal Gold Deposits of the Norseman-Wiluna Belt, Western Australia
S. D. Golding, N. J. McNaughton, M. E. Barley, D. I. Groves, S. E. Ho, N. M. S. Rock, J. V. Turner, 1989. "Archean Carbon and Oxygen Reservoirs: Their Significance for Fluid Sources and Circulation Paths for Archean Mesothermal Gold Deposits of the Norseman-Wiluna Belt, Western Australia", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Carbon and oxygen isotope studies of carbonate minerals from the two carbonate alteration styles that predate regional metamorphism and gold mineralization in the Norseman-Wiluna greenstone belt of Western Australia have significance for the sources of carbon and oxygen in ore fluids that produce Archean mesothermal gold deposits. The mantle-derived carbon reservoir in regional carbonation zones is the most likely source for auriferous ore fluids rather than the seawater-derived carbon reservoir in altered basalts. The wide range of δ13C values for gold-related carbonates reflects dissolution of different carbonate species and/or a variable input of organic and seawater-derived carbon.
The oxygen isotope compositions of auriferous ore fluids in the Norseman-Wiluna belt (δ18O = 4-9‰) indicate that these fluids (1) could have been in equilibrium with sea floor-altered basalts at 500° to 600°C, (2) derived from regional carbonation zones or interacted with them, or (3) derived from felsic porphyries or granitoids only if modified by subsequent fluid-rock interaction or changing P-T-X conditions.
There is a positive correlation between δ13C and δ18O for carbonates from gold-related alteration (both district and mine scale), with data for the two largest Archean mesothermal gold deposits in the world (Kalgoorlie, Western Australia; and Timmins, Ontario) plotting at the most positive end of this trend. Correlated δ13C and δ18O variations in gold-related carbonates on a district scale probably relate to source-conduit heterogeneities and host-rock compositional controls, whereas on a mine scale a model involving finite reservoir effects for carbon, temperature-controlled oxygen isotope fractionation, and possibly phase separation can account for the observed trends.
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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