The Hydrothermal Chemistry of Gold and Its Implications for Ore Formation: Boiling and Conductive Cooling as Examples
T. M. Seward, 1989. "The Hydrothermal Chemistry of Gold and Its Implications for Ore Formation: Boiling and Conductive Cooling as Examples", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The solubility of gold has been calculated in the high-temperature (290°C) hydrothermal fluids of the Ohaaki-Broadlands geothermal system. If the dihydrosulfidogold(I) complex (Au(HS)−1) is assumed to account for the gold in solution, the calculated solubility is 11.1 μg kg−1 which is in reasonable agreement with the measured value of 1.5 μg kg−1. The concentration of gold in solution as AuCl2−1 is very small (1.2 x 10−7 μg kg−1) and this species is unimportant in the transport of gold in these ore-forming fluids.
If gold is present in solution as Au(HS)2-, single-step adiabatic flashing of the Ohaaki-Broadlands deep fluids (t = 290°C) leads initially to an increase in gold solubility, thus preventing gold precipitation until a temperature of 277°C is reached. If, however, the gas phase is removed (open system) at any intermediate temperature between 290° and 277°C, the gold solubility drops rapidly with further boiling and phase separation with gold deposition occurring within 5° to 10°C of the fractionation temperature. By contrast, simple conductive cooling causes immediate minor gold precipitation but does not lead to the dumping of essentially all the gold in solution over a small temperature interval.
It has also been shown that with fluid fluxes characteristic of active hydrothermal systems, 106 oz of gold may be transported into the boiling zone of an ore-depositing system over short periods of less than 1,000 years.
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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