Gold Mineralization in the Abitibi Greenstone Belt: End-Stage Result of Archean Collisional Tectonics?
C. Jay Hodgson, J. V. Hamilton, 1989. "Gold Mineralization in the Abitibi Greenstone Belt: End-Stage Result of Archean Collisional Tectonics?", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The Abitibi greenstone belt, the largest and best-preserved Archean granite-greenstone complex in the world, consists of five major lithologic assemblages which formed in four distinctive geotectonic environments. These comprise: (1) a tholeiitic and komatiitic assemblage formed in an oceanic extensional environment; (2) a calc-alkalic volcanic and volcanic-derived sedimentary rock assemblage formed in an island-arc environment; (3) an assemblage of craton-derived quartzose sedimentary rocks with interbedded komatiitic volcanic rocks formed on a passive continental margin; and (4) an assemblage of molasse-type sedimentary rocks, alkalic volcanic rocks, and felsic intrusions. The oceanic, arc, and continental margin assemblages were imbricated and tectonically stacked during the collision of a northward-moving continent and its attached marginal sediments with the arc, as a result of subduction of the intervening ocean crust. The molasse assemblage formed along the collisional suture zone. Interbedded alkalic volcanic rocks, syn- to postdeformational felsic intrusions, and the CO2-rich hydrothermal fluids responsible for widespread hydrothermal alteration and gold mineralization, all broadly part of this molasse assemblage, were generated by linked lower crust-mantle processes. The effect of the high density of the oceanic rocks on pressure-temperature conditions at the base of the tectonically thickened crust may have been instrumental in triggering the processes which resulted in auriferous fluids. Anatectic melts and gold ore-forming hydrothermal fluids were generated at depth as an end-stage manifestation of the collisional event. Rocks of the oceanic assemblage were the possible source of the gold. This model provides a rational and internally consistent explanation for the association of gold mineralization with greenstone belts, major faults, molasse-type sedimentary rock assemblages, felsic porphyry intrusions, and widespread hydrothermal alteration. Also, the model is consistent with the late timing of the mineralization in the geologic development of the Abitibi belt in particular, and both Precambrian and Phanerozoic greenstone belts in general.
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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