Conglomerate-hosted Gold – Pilbara Region, WA
Recently discovered Au in boulder conglomerate between the Mesoarchean West Pilbara superterrane basement and the overlying volcano-sedimentary stratigraphy of the Neoarchean Fortescue Group in Western Australia has renewed comparisons with the Witwatersrand conglomerate Au deposits in South Africa. As such, this has reignited the question of the Pilbara and Kaapvaal cratons being linked as part of the postulated Vaalbara continent during the Archean. However, little is known about the origin of the Pilbara conglomerate Au and its host conglomerates, as they are hitherto unstudied, and their formation and/or source is uncertain.
In this project we conducted a detailed study on the textures, composition, and sedimentology of one newly discovered Pilbara conglomerate Au deposit at the base of the Neoarchean Fortescue Group in the northwestern Pilbara craton. We aim to understand the deposition and formation mechanisms of the conglomerate-hosted gold, what the potential source of the gold was, and if there are any links with the massive Witwatersrand deposits in South Africa.
The Pilbara conglomerate Au occurrences are characteristically Ag-bearing but Hg-poor polycrystalline discoid masses that are overgrown by Au-poor chloritic halos, which are further enveloped by a hydrothermal alteration halo of disseminated Au within chlorite. Both the discoids and the auriferous chlorite halo are Ag bearing, with up to ~9 wt % Ag, consistent with a hydrothermal (orogenic) origin. The discoids do not display any physical or chemical evidence for sedimentary transport; thus, their formation (placer versus hydrothermal) remains unclear. However, the position of the Au in the conglomerate, limited to the basal section of the conglomerate, is difficult to account for in a purely hydrothermal deposit model. We utilized world-class analytical facilities as part of this study including the Maia Mapper, state of the art electron backscatter diffraction, and Focused ion beam-time of flight-secondary ionization mass spectrometry.
We argue 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 (>~2.7 Ga) Venterspost Conglomerate Formation, which overlies the Witwatersrand Supergroup, inconsistencies remain, with different Au chemistries and tectonic, magmatic, sedimentary, and metamorphic-metallogenic histories of the Pilbara and Kaapvaal cratons prior to deposition of the >2.7 Ga conglomerate sequences. This collectively indicates the drivers of Au metallogenesis and ultimate Au deposition in conglomerate facies were fundamentally different in the Pilbara and Kaapvaal cratons.
Publications/reports:
Spinks, S.C., Pearce, M.A., Le Vaillant, M., Fox, D., Tyler, I.M., Godel, B., Stromberg, J., Mead, E., White, A.J., 2020. The Neoarchean Conglomerate-Hosted Gold of the West Pilbara Craton, Western Australia. Economic Geology. https://doi.org/10.5382/econgeo.4790.
Spinks, S., Pearce, M., Le Vaillant, M., Fox, D., Noble, R., Reid, N., Godel, B., 2019. Linking the Pilbara Archean Purdey’s Reward conglomerate gold and Quaternary 47K Patch colluvial gold occurrences. Perth: CSIRO EP192513.