Fluid Inclusion Geochemistry of Gold-Bearing Quartz Veins of the Juneau Gold Belt, Southeastern Alaska: Implications for Ore Genesis
Published:January 01, 1989
Richard J. Goldfarb, David L. Leach, Scott C. Rose, Gary P. Landis, 1989. "Fluid Inclusion Geochemistry of Gold-Bearing Quartz Veins of the Juneau Gold Belt, Southeastern Alaska: Implications for Ore Genesis", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Microthermometry, laser Raman spectroscopy, and mass spectrometry were used to study fluid inclusions in gold-bearing quartz veins from the mines of the Juneau gold belt, Unmixing of a CO2-rich parent fluid led to the contemporaneous trapping of H2O-dominant and CO2-dominant inclusions during gold deposition at the Alaska-Juneau, Reagan, and Ibex mines. Ore fluids at all other mines were trapped as homogeneous, H2O-dominant fluids, with less than 10 mole percent CO2. Both N2 and CH4 are present at the percent level within the volatile phases in all deposits; H2S makes up one-third of the volatile phase and 2 mole percent of the total ore fluid at the Sumdum Chief mine. The ore fluids contained less than 5 equiv wt percent NaCl. Gold deposition occurred at temperatures above 250°C and at depths of at least 5 km. The gold-forming fluids are believed to have been derived from devolatilization reactions associated with prograde metamorphism of dominantly pelitic, subducted crust.
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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