Fine fraction regolith chemistry from the East Wongatha area, Western Australia: tracing bedrock and mineralization through thick cover

Abstract

The fine fraction (<50 μm; ‘silt – clay’) of regolith from an area dominated by thick (up to 120 m) transported regolith is shown to be a viable sampling medium for detecting bedrock and bedrock-hosted mineralization. A partial digest of the fine fraction of regolith reveals that elements which have migrated vertically from bedrock have become sequestered at a shallow depth in regolith. Nickel, Cr, Li, Mn, V, Co and Rb are useful in tracing the extent of different bedrock types, with Ni, Cr, V and Co particularly useful for identifying more mafic bedrock. Higher concentrations of Au (maximum 29 ppb; median 2 ppb; n = 835) are spatially related to either Archean greenstones or unconformity-related uranium – base metal – gold mineralization. Although some samples with anomalous Au concentrations are carbonate-rich, there is no consistent relationship between elevated Au concentrations and the carbonate content of regolith. Deionized water digestion of the <50-μm fraction of 50 samples correlates with aqua regia data for several elements (particularly Au, Ni), suggesting that some mineralization- or bedrock-related elements in the fine fraction of regolith are either labile or microparticulate.