The Witwatersrand Gold Fields: Part II. An Origin for Witwatersrand Gold during Metamorphism and Associated Alteration
Published:January 01, 1989
G. N. Phillips, R. E. Myers, 1989. "The Witwatersrand Gold Fields: Part II. An Origin for Witwatersrand Gold during Metamorphism and Associated Alteration", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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An epigenetic replacement model is proposed for Witwatersrand gold mineralization. In this model, unconformities that developed during deposition of the Upper Witwatersrand sediments were sites of selective accumulation of iron pisolites-ferricretes and carbon seams. These Fe-C-bearing intervals are part of 1- to 2-m-thick reef packages that were preferred channelways for Au-S-bearing metamorphic fluids because of their lithologic heterogeneity and preferential accommodation of strain during deformation. It is suggested that the metamorphic fluids were associated with a high-strain, cleavage-forming event recorded in the Witwatersrand, Venters-dorp, and Transvaal successions.
Selective replacement of Fe-C-bearing phases by Au-S-bearing metamorphic fluids accounts for the secondary nature of the Witwatersrand mineralogy, apparent structural control on gold mineralization, the secondary texture of gold, and the presence of replacement textures involving pyrite and uraninite. Most importantly, the model also accounts for the close relationship of gold distribution to sedimentary features. If the present distribution of Fe and C approximates original sedimentary patterns, any epigenetic gold precipitated by interaction with Fe and/or C must also reflect sedimentary features.
This replacement model obviates the requirements imposed by the placer model for Witwatersrand gold for a more reducing Precambrian atmosphere, for a particularly enriched source terrane, or for an unusually efficient sorting mechanism to concentrate gold in a diverse range of rock types.
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The Geology of Gold Deposits: The Perspective in 1988
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