Nature, Genesis, and Tectonic Setting of Mesothermal Gold Mineralization in the Yilgarn Block, Western Australia
Published:January 01, 1989
David I. Groves, Mark E. Barley, Susan E. Ho, 1989. "Nature, Genesis, and Tectonic Setting of Mesothermal Gold Mineralization in 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 Yilgarn block is a major metallogenic province, currently enjoying its highest ever annual gold production from greenstone-hosted Archean mesothermal gold deposits. Gold mineralization occurs in ca. 2.95 to 2.7-Ga greenstone belts throughout the block, with over 2,000 deposits known, but is best developed in the ca. 2.7-Ga greenstones of the Norseman-Wiluna belt. Most mineralization is sited in brittle-ductile structures, at or below the amphibolite-greenschist transition, commonly in rocks with high Fe/(Fe + Mg) ratios. Sulfidation, K-(± Na-) metasomatism and carbonation are important alteration styles associated with such mineralization in shear zones, quartz veins, and/or breccias. Gold occurs most commonly within Fe sulfides and mineralization has a typical element association of Au-Ag-As-W±Sb±Te±B with low Pb-Zn-Cu contents. Gold was deposited from reduced to slightly oxidized, near-neutral, moderate-density, low-salinity H2O-CO2 fluids at 250° to 350°C and 0.5 to 2 kbars in response to sulfidation and/or oxidation-reduction reactions, changes in pH, and pressure decrease over a limited temperature range.
On the regional scale, the distribution of gold deposits is controlled by kilometer-scale, oblique-slip, reverse or normal faults-shears linked to crustal-scale, largely strike-slip, shear zones that also appear to control the distribution of mantle-derived carbonation and the emplacement of I- and A-type granitoids, felsic porphyries, and/or calc-alkaline lamprophyres. These associations, combined with radiogenic and stable isotope data, suggest that gold mineralization was related to fluid flow on a crustal or even lithospheric scale, rather than simply being related to greenstone belt devolatilization or local magmatic intrusions; lamprophyres do, however, represent a potential gold donor to the metamorphic-hydrothermal systems. Despite this gross control, the provinciality of isotope data suggests that ore components were strongly influenced by upper crustal fluid pathways, probably controlled by transient fluid flow into brittle-ductile structures under the influence of fluid-pressure gradients.
The preferred model is that gold mineralization was the result of high lithospheric heat and fluid flux during compressional, oblique-slip deformation and mantle to crustal magmatism related to closure of the partly sialic-floored, marine basin now represented by the Norseman-Wiluna belt. As such, it shows similarities to Phanerozoic gold provinces in convergent-margin tectonic settings.
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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