The Archean Superior Province of the Canadian Shield and Its Lode Gold Deposits*
K. D. Card, K. HOWARD Poulsen, Francois Robert, 1989. "The Archean Superior Province of the Canadian Shield and Its Lode Gold Deposits", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The Superior province, a major Archean craton of the Canadian Shield, was formed during middle and late Archean tectono-magmatic events from rocks of mantle or recycled juvenile crustal origin assembled by accretionary mechanisms in convergent tectonic settings. The Superior province consists of northern and southern high-grade gneiss terranes and a broad central region of alternating lower grade greenstone- and metasediment-rich subprovinces, all intruded by voluminous granitoid plutons. Subprovince boundaries are complex zones of facies, metamorphic, and structural transition, commonly telescoped by crustal-scale faults. Volcanism, plutonism, and sedimentation occurred at ca 3.1 to 2.8 Ga, notably in the north, and again at ca 2.75 to 2.7 Ga throughout the Superior province, in settings analogous to modern oceanic island arc-interarc basin-accretionary wedge systems. Polyphase deformation, metamorphism, and plutonism at ca 2.7 Ga, all products of subduction-driven accretion, resulted in ductile deformation and granulite facies metamorphism at deep crustal levels represented by the high-grade gneiss terranes and in ductile to brittle deformation and lower grade metamorphism at high crustal levels represented by the greenstone-granite subprovinces. Lode gold deposits formed during late ductile-brittle stages of deformation are related to major fault systems and to zones of rock alteration within greenstone-granite subprovinces. Granulite metamorphism at depth, involving dehydration and formation of HO2− and CO2-rich fluids, was synchronous with brittle deformation, rock alteration, and formation of lode gold deposits at high crustal levels. Although granulitization models probably best account for most genetic aspects of the Superior province lode gold deposits except their confinement to greenstone-granite terranes, degree of greenstone belt preservation is an important factor in their present distribution.
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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