Oscillatory-Zoned As-Bearing Pyrite from Strata-Bound and Stratiform Gold Deposits: An Indicator of Ore Fluid Evolution
Michael E. Fleet, Peter J. MacLean, Jacques Barbier, 1989. "Oscillatory-Zoned As-Bearing Pyrite from Strata-Bound and Stratiform Gold Deposits: An Indicator of Ore Fluid Evolution", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The variation in As content of pyrite from three gold deposits (Agnico-Eagle, Quebec; Fairview and Pilgrim’s Rest, South Africa) has been studied by color-staining polished surfaces with a solution of KMnO4 in 1:1 H2SO4 followed by electron microprobe analysis. Pyrite grains from strata-bound and stratiform gold deposits are commonly oscillatory zoned in As content on a fine scale, with band widths on pyritohedral growth surfaces as narrow as 0.5 (im, Microprobe analyses with a surface resolution frequently overlapping several bands of alternating high and low As content give maximum average As contents of 4 to 8 wt percent (2-5 at. %). The As appears to be incorporated in pyrite as a metastable solid solution of the type Fe(S,As)2. These pyrite grains indicate that the As and S contents of the fluids responsible for the gold mineralization varied episodically. Many pyrite grains are corroded: low As cores have been dissolved away either to leave atoll-like structures or to be filled in and overgrown by later low As pyrite. As-bearing pyrite also has a TEM microstructure of (100) planar faults which under high resolution are seen to be thin coherent lamellae of marcasitelike structure. Idiomorphic growth zones in pyrite grains reveal that As (and presumably Au) mineralization was an early feature of strata-bound-stratiform gold deposits.
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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