Calcrete and plant inter-relationships for the expression of concealed mineralization at the Tunkillia gold prospect, central Gawler Craton, Australia

Abstract

Transported and typically weathered material that extends over much of Australia presents a challenge for the successful application of surficial geochemical exploration methods. The Tunkillia Au-prospect in the Gawler Craton, South Australia provides a case study where unconsolidated and potentially laterally transported aeolian and alluvial sediments overlie a deeply weathered profile with variable supergene and primary Au mineralization at depths typically exceeding 40 m. The Tunkillia Au prospect was discovered through a regional calcrete sampling program, but elevated Au concentrations in this material do not always express underlying mineralization, such is the case at Tomahawk, an area of elevated Au in calcrete results overlying limited, low grade mineralization. The distribution and sampling accessibility of calcrete is also not continuous across the landscape, where in this region young siliciclastic linear dune ridges bury older calcrete that are otherwise exposed or only at shallow depths in dune swales or across sheetwash plains. Vegetation cover is more continuous across this landscape and is dominated by several main species. Sampling of dominant plant species: Casuarina pauper, and Eucalyptus concinna, has been conducted across Tomahawk where some sites of elevated Au in calcrete have been drilled. Results show a limited spatial association between calcrete geochemistry and plant biogeochemistry. This demonstrates that plant sampling is able to express low grade, discrete areas of buried Au mineralization typically on the scale of tens of metres, whereas the expression provided from calcrete is more levelled, and typically representative of larger areas, especially when they have formed within transported detritus host materials. It is recommended that although plant biogeochemistry and calcrete geochemistry may have their strengths and weaknesses, such as scale constraints, in regional geochemical exploration programs when these methods are integrated they potentially provide a more comprehensive and widely available geochemical exploration assessment across a wider range of regolith-landform settings.