The Neoarchaean conglomerate-hosted gold of the West Pilbara craton, Western Australia.

Recently discovered Au in boulder conglomerate between the Mesoarchaean West Pilbara superterrane basement and the overlying volcano-sedimentary stratigraphy of the Neoarchaean Fortescue Group in Western Australia has renewed comparisons with the Witwatersrand conglomerate Au deposits in South Africa. This has reignited the question of the Pilbara and Kaapvaal cratons being linked as part of the postulated Vaalbara continent during the Archaean. However, little is known about the origin of the Pilbara conglomerate Au and its host conglomerates. A detailed study is presented here on the textures, composition, and sedimentology of one newly discovered Pilbara conglomerate Au deposit at the base of the Neoarchaean Fortescue Group in the northwestern Pilbara craton. The Pilbara conglomerate Au occurrences are characteristically Ag-bearing but Hg-poor polycrystalline discoid masses overgrown by Au-poor chloritic halos, which are further enveloped by a hydrothermal alteration halo of disseminated Au within chlorite. It is argued that the Pilbara conglomerate Au represents a modified placer deposit from a primary orogenic Au source, with surface evidence for sedimentation removed by partial dissolution during later hydrothermal alteration in the host conglomerate and the crystalline basement. While the basal Fortescue Group conglomerate Au shares commonalities with the time equivalent Venterspost Conglomerate Formation, which overlies the Witwatersrand Supergroup, inconsistencies remain prior to deposition of the conglomerate sequences older than 2.7 Ga. This collectively indicates the drivers of Au metallogenesis and ultimate Au deposition in conglomerate facies were fundamentally different in the Pilbara and Kaapvaal cratons.
Recently discovered Au in boulder conglomerate between the Mesoarchaean West Pilbara superterrane basement and the overlying volcano-sedimentary stratigraphy of the Neoarchaean Fortescue Group in Western Australia has renewed comparisons with the Witwatersrand conglomerate Au deposits in South Africa. This has reignited the question of the Pilbara and Kaapvaal cratons being linked as part of the postulated Vaalbara continent during the Archaean. However, little is known about the origin of the Pilbara conglomerate Au and its host conglomerates. A detailed study is presented here on the textures, composition, and sedimentology of one newly discovered Pilbara conglomerate Au deposit at the base of the Neoarchaean Fortescue Group in the northwestern Pilbara craton. The Pilbara conglomerate Au occurrences are characteristically Ag-bearing but Hg-poor polycrystalline discoid masses overgrown by Au-poor chloritic halos, which are further enveloped by a hydrothermal alteration halo of disseminated Au within chlorite. It is argued that the Pilbara conglomerate Au represents a modified placer deposit from a primary orogenic Au source, with surface evidence for sedimentation removed by partial dissolution during later hydrothermal alteration in the host conglomerate and the crystalline basement. While the basal Fortescue Group conglomerate Au shares commonalities with the time equivalent Venterspost Conglomerate Formation, which overlies the Witwatersrand Supergroup, inconsistencies remain prior to deposition of the conglomerate sequences older than 2.7 Ga. This collectively indicates the drivers of Au metallogenesis and ultimate Au deposition in conglomerate facies were fundamentally different in the Pilbara and Kaapvaal cratons.