Regolith characterization and landscape evolution for geochemical exploration of the covered Yamarna Terrane, Western Australia

Abstract

The easternmost Archaean Yamarna Terrane is the least explored terrane under transported cover in the Yilgarn Craton of Western Australia. Our aims are to understand the evolution of the paleolandscape, determine the weathering history and depositional processes, and the provenance, mineralogy and geochemistry of the Permian to Quaternary transported cover. This is a necessary step for selecting the appropriate sample media to vector towards buried mineral deposits in this terrane. The Permian cover was deposited in broad, structurally controlled valleys that were carved into and partly to completely truncate the previously weathered Archaean rocks. UPb dating of zircon grains indicates that the main provenance of the Permian cover on the Yamarna Terrane is the Albany-Fraser Orogen, with limited contribution from the Yilgarn Craton, Musgrave Province and Antarctica. The Permian cover of the paleovalleys wedges out laterally towards pre-existing, weathered basement paleohighs, capped by lateritic residuum. The Permian cover is deeply weathered, and consists of upper ferruginous (mottled, brecciated, pisolitic and nodular) and lower kaolinitic, poorly lithified sandstones and mudstones with a basal gravel at the bottom. Above the basal unconformity between the Permian and weathered Archaean, the Rb/Zr ratio decreases sharply and the Permian cover shows negative Eu anomalies (Eu/Eu*) compared to positive Eu/Eu* in the underlying weathered and fresh mafic Archaean rocks, suggesting distal felsic source rocks for the Permian cover. In places, where Au increases, the base of the Permian cover still shows positive Eu/Eu* anomalies, suggesting physical dispersion from proximal mineralized mafic rocks.The Post-Permian weathering and erosion processes produced an extensive layer of Au-bearing detrital ferruginous clasts derived from lateritic residuum and ferruginous pisoliths and nodules from the Permian cover. These ferruginous grains occur as loose or cemented by Fe oxides forming ferricrete. Ferricrete is not necessarily related to the underlying weathered dolerite compared to the lateritic residuum. This is shown, in some places, by a sharp decrease in the Ti/Zr ratio, high HFSE, negative Eu/Eu*, high LREE/HREE compared to the underlying weathered dolerite suggest an unconformity and physical dispersion from weathered felsic source rocks. The Au-enrichment in the ferricrete derived from the felsic rocks occurred after deposition as a result of hydromorphic dispersion by groundwater. The weathered Permian cover and Archaean rock are covered by aeolian sand within which Au is concentrated in authigenic pisoliths. These pisoliths were formed by periodical water saturation during seasonal water table fluctuation. The unconformities and the ferruginous regolith materials are relevant to Au exploration through cover in the Yamarna Terrane and are highly recommended for regional-scale systematic sampling.