Genesis of the Red Dome Gold Skarn Deposit, Northeast Queensland
G.R. Ewers, S.-S. Sun, 1989. "Genesis of the Red Dome Gold Skarn Deposit, Northeast Queensland", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Some of Australia’s most significant gold deposits are associated with late Paleozoic high-level felsic intrusions in northeast Queensland. The gold deposit at Red Dome near Chillagoe is hosted by a complex skarn related to Permo-Carboniferous rhyolites intruded into Siluro-Devonian limestone of the Chillagoe Formation. Native gold and minor electrum are associated with a wollastonite-bearing skarn phase related to an early rhyolite. Gold was also deposited during subsequent retrograde alteration and late veining and appears to have been locally reworked with the intrusion of a late rhyolite. Whole-rock geochemistry has indicated that during skarn formation, fluids introduced appreciable amounts of Fe, Mn, Cu, Mo, Sn, Zn, W, and As and leached Na from all lithologies.
Fluid inclusion studies yield pressure-corrected homogenization temperatures in the range 300° to 380°C for garnet with fluorite forming at 200° to 350°C in the skarns. The fluids have a very low CO2 content (<0.2 mole %), with salinities of 2 to 24 equiv wt percent NaCl in the skarns rising to 30 to 50 equiv wt percent NaCl in the rhyolites. Stable isotope data indicate the progressive interaction of a magmatic fluid with the country rocks during skarn formation and suggest a water/rock ratio much greater than 1, probably>10. Although the oxygen isotope data do not indicate the involvement of meteoric water with low δ18O values, it is possible that connate or surface water which has been extensively modified by interaction with the sedimentary rocks may have been involved in the later stages of skarn growth, thereby causing a decrease with time in the salinity of the skarn fluids. Sulfur isotope data suggest a mixing of two sulfur sources: a magmatic sulfur and a country-rock source with more positive δ34S values.
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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