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Geology of the Beatons Creek conglomerate gold deposit, Pilbara, Western Australia: new constraints on Archean gold enrichment in sedimentary rocks
Galindo, Stephen
Galindo, Stephen
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2025
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The Mesoarchean Beaton’s Creek conglomerate-hosted gold deposit is located in the Pilbara region of Western Australia. Exploration in recent years identified an indicated and inferred resource of 265,000 ounces of Au. Mineralization is hosted by a succession of multiple sub-horizontal, narrow stacked conglomeritic horizons, interbedded with barren conglomerate, sandstones, and minor intercalations of shale, mudstone, siltstone, and tuffs. Auriferous conglomerates are composed of rounded to subrounded clasts of sandstone, siltstone, quartz and dromedary boulder with additional minor clasts of chert, and possible stromatolites. Auriferous strata are enriched in resistive clasts and rounded pyrites and are thought to represent fluvial channels or zones of marine reworking. Abrasion tests indicate that pyrite is not mechanically stable enough to withstand extended fluvial transport and thus the large, rounded pyrite clasts in the mineralized conglomerate were likely derived from a proximal source. The large spread in sulfur isotope values suggest that the pyrite could have formed as crusts at low-temperature conditions in a shallow marine environment, possibly aided by cyanobacterial activity, and as diagenetic growths in soft sediments. Gold is thought to be primarily introduced into the depositional environment by fluvial waters as sulfide complexes. Bacterial mats served as chemical and physical traps for gold particles. Due to wave action and/or flood events the gold-rich bacterial mats and pyrite crusts were broken up and reworked, forming placer-like enrichment zones. However, euhedral authigenic pyrite crystals and overgrowths on detrital pyrite formed paragenetically late from hydrothermal solutions during diagenesis or low-grade metamorphism. The same hydrothermal fluids also precipitated base metal sulfides, including galena, chalcopyrite and gersdorffite, and potentially caused minor local redistribution of gold. The model proposed could explain the formation of Witwatersrand-type gold deposits with abundant pyrite clasts in an otherwise mature sedimentary setting without the need for extended fluvial transport of unstable pyrite and erosion of unreasonably large gold deposits in the hinterland.
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