Gold Mineralization in Volcanogenic Massive Sulfides: Implications of Data from Active Hydrothermal Vents on the Modern Sea Floor
M. D. Hannington, S. D. Scott, 1989. "Gold Mineralization in Volcanogenic Massive Sulfides: Implications of Data from Active Hydrothermal Vents on the Modern Sea Floor", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The occurrence and distribution of gold in volcanogenic massive sulfides on the modern sea floor is a function of the physical and chemical characteristics of the hydrothermal fluids and theil ability to become saturated with respect to gold at a high concentration. Evidence from active hydrothermal vents at midocean ridges indicates that high-temperature (350°C) hydrothermal fluids may contain 0.1 to 0.2 μg/kg Au and could transport as much as 500 to 1,000 g Au/yr in a single deposit. However, in the absence of an effective precipitation mechanism, most of the gold in high-temperature vents may be lost to a diffuse hydrothermal plume and related metalliferous sediments. Associated high-temperature, Cu-Fe sulfides and subsea-floor stockwork mineralization are typically gold poor, containing ≤0.2 ppm Au. In contrast, low-temperature (<300°C) sulfides accumulating at or near the sea floor may contain up to 6.7 ppm Au. Gold-rich assemblages are formed at elevated sulfidation states and commonly contain high levels of Zn (>10%), Pb (>0.1%), Ag (>100 ppm), As (>200 ppm), and Sb (up to 500 ppm). High concentrations of H2S (up to 8.4 mmole/kg or 285 ppm) in the vent fluids stabilize Au(HS)2− complexes down to at least 200°C and account for the enrichment of gold in late-stage, low-temperature sulfides. Gold is precipitated from Au(HS)2− by oxidation during high-level mixing of the vent fluids with ambient seawater. The locus of mixing and the extent of sulfide-sulfate reactions are important controls on the site and temperature of gold mineralization. Precipitation of gold from AuCl2 occurs in one deposit where fluids have acquired significantly elevated salinities. Similar patterns of gold enrichment in examples from ophiolite-hosted Cu pyrite, Phanerozoic Zn-Cu-Pb, and Archean Cu-Zn deposits suggest similar controls on the occurrence and distribution of gold in ancient volcanogenic massive sulfide ores.
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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