Genesis of Supergene Gold Deposits in the Lateritic Regolith of the Yilgarn Block, Western Australia
C. R. M. Butt, 1989. "Genesis of Supergene Gold Deposits in the Lateritic Regolith of the Yilgarn Block, 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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The supergene gold deposits occurring in the lateritic regolith of the Yilgarn block, Western Australia, are examples of a distinctive type of mineralization. The deposits are mostly small (<1.5 million metric tons) and of low grade (1.5-5.0 g/metric ton) but can be exploited because of the relative ease of mining and treatment. The lateritic gold deposits are more or less flat-lying zones contiguous with the ferruginous horizon and underlying mottled zone of the weathering profile. They contain residual, Ag-bearing, primary gold as fine-grained particles and rare nuggets, and Ag-poor secondary gold, commonly as very fine grained particles but with some euhedral crystals, typically associated with iron oxides. The deposits developed mainly during tropical, humid conditions in the early Tertiary by relative enrichment of primary gold and absolute enrichment of secondary gold. Gold mobilization was probably as organic complexes, reduced and precipitated by the oxidation of ferrous iron. The saprolitic gold deposits are enrichments deeper in the profile, either confined to the lode system or laterally dispersed into weathered wall rocks, commonly as one or more subhorizontal zones. They consist principally of secondary gold, with residual primary gold confined to the weathered lode system. The deposits formed after the change to more arid climates caused a gradual lowering of water tables and the development of salinity. Gold was probably mobilized as chloride complexes and reprecipitated following reduction by ferrous iron. Similar concentrations of gold in the regolith are known from other deeply weathered and lateritic terranes, particularly in Africa and South America. Regional differences between the types of deposit formed can be attributed mainly to differences in climatic history.
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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