The Association of Linear Orogenic Belts, Mantle-Crustal Magmatism, and Archean Gold Mineralization in the Eastern Yilgarn Block of Western Australia
Caroline S. Perring, Mark E. Barley, Kevin F. Cassidy, David I. Groves, Neal J. McNaughton, Nicholas M. S. Rock, Leigh F. Bettenay, Suzanne D. Golding, Jack A. Hallberg, 1989. "The Association of Linear Orogenic Belts, Mantle-Crustal Magmatism, and Archean Gold Mineralization in the Eastern Yilgarn Block of 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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Certain aspects of the genesis of Archean epigenetic gold deposits remain controversial, in particular the source of the auriferous fluids, which are arguably magmatic, metamorphic, or mantle derived. In an attempt to constrain the fluid source, it is essential to consider Archean gold mineralization in terms of the tectonic, magmatic, and metamorphic history of greenstone terranes. Asymmetries in the distribution of volcanic, sedimentary, and plutonic rock types, the pattern of deformation, and the rapid evolution of the greenstone sequences within the Norseman-Wiluna belt in the eastern Yilgarn block are akin to those of younger orogenic belts at obliquely convergent continental plate boundaries. Archean gold deposits show many similarities to younger, cordilleran-style gold deposits (e.g., the Mother Lode) which occur in a similar tectonic setting, particularly in terms of their strong dependence on structural controls and the composition of the ore fluids. In the eastern Yilgarn block there is a coincidence of lode gold mineralization, calc-alkaline porphyry, and lamprophyre dike swarms and craton-scale oblique-slip faults with their attendent mantle-derived carbonation. With no compelling evidence for direct derivation of ore fluids from felsic magmas, gold mineralization is best viewed as the upper crustal expression of a deep-seated tectono-thermal event with mantle-crustal outgassing, occurring in response to a deep mantle heat source, related to convergent tectonics. In all probability the ore fluid contained magmatic, metamorphic, and mantle components, but it is impossible at this stage to determine with which component the gold was predominantly associated.
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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