Introduction of Gold into Archean Crust, Superior Province, Ontario—Coupling between Mantle-Initiated Magmatism and Lower Crustal Thermal Maturation
J. A. Fyon, D. G. Troop, S. Marmont, A. J. Macdonald, 1989. "Introduction of Gold into Archean Crust, Superior Province, Ontario—Coupling between Mantle-Initiated Magmatism and Lower Crustal Thermal Maturation", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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In parts of the Superior province, anomalous concentrations of gold and CO2 were introduced into regionally extensive zones of deformation during late Archean magmatic, tectonic, and thermal activity leading to cratonization. A similar sequence of activity is recorded in all subprovinces, but the absolute timing of equivalent magmatic and metallogenic events may differ. In the Abitibi-Wawa subprovince, calc-alkaline volcanism and coeval intrusion of tonalite-trondhjemite plutons terminated at approximately 2,700 Ma. During a relatively short period (30 Ma) following 2,700 Ma, compositionally diverse plutons were emplaced into the crust. This period of late plutonism was coupled temporally and thermally with granulite and amphibolite facies metamorphism and partial melting of lower to middle crust. These crustal magmatic and thermal events may have been initiated by mantle diapirism which supplied magma, heat, and volatiles to existing crust. Gold mineralization and large volumes of CO2 were introduced into regionally extensive zones of deformation during this period of late magmatic and thermal activity. A scenario linking cratonization and gold metallogenesis with magmatic underplating is developed to account for the contemporaneity between the late tectonism and auriferous hydrothermal events.
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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