Structural Controls on Mesothermal Gold Mineralization: Examples from the Archean Terranes of Southern Africa and Western Australia
J. R. Vearncombe, M. E. Barley, B. N. Eisenlohr, D. I. Groves, S. M. Houstoun, M. S. Skwarnecki, M. W. Grigson, G. A. Partington, 1989. "Structural Controls on Mesothermal Gold Mineralization: Examples from the Archean Terranes of Southern Africa and Western Australia", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Archean mesothermal gold mineralization is commonly located in brittle-ductile structures active during the late deformation events of greenstone belt evolution, and in several cases, reactivation of earlier, mechanically weak structures was a controlling factor. Intensely mineralized greenstone belts in both Western Australia and southern Africa are characterized by an early compressional to oblique-compressional event. Within these belts, zones of low-strain greenstones are bounded by narrow high-strain zones of up to hundreds of kilometers in strike length, which are in turn linked to the brittle-ductile structures hosting gold mineralization. This structural pattern ensured that there was strongly focused fluid flux and probably accounts for the generally high productivity of Archean greenstone belts. In Western Australia, the most mineralized greenstone belt (the Norseman-Wiluna belt in the Yilgarn block) is one in which deformation followed shortly after volcanism and sedimentation and probably resulted from the closure of an extensional basin. Other Yilgarn greenstone belts do, however, host large gold deposits, probably in structures synchronous with and related to those in the Norseman-Wiluna belt. In southern Africa, heterogeneously deformed greenstone belts with high-strain zones of reverse fault-thrust regimes related to externally imposed tectonics are highly prospective. Other important sites are reverse shear zones close to or at granitoid dome margins, which may be due to jostling of these rigid granitoids and differential movement of greenstones during externally imposed deformation.
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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