Nature, Genesis, and Tectonic Setting of Mesothermal Gold Mineralization in the Yilgarn Block, Western Australia
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.