Abstract

A biogeochemical study was undertaken at Barns Gold Prospect, a Au-in-calcrete discovery in the Northern Eyre Peninsula (South Australia). The prospect is located in highly weathered Proterozoic rocks and is overlain by at least 1 m of aeolian quartz sand that thickens to 8 m as a longitudinal sand dune over part of the mineralization. The dune is well-vegetated, with Melaleuca shrubs and Eucalyptus trees up to 5 m high.

Over mineralization anomalous Au concentrations occur in plant organs, litter, soils and sand. The highest Au concentrations (9 ppb) occur in calcareous rhizomorphs high up within the dune. Luminescence dating shows that the dune took no longer than 27 000 years to form and mass balance calculations indicate that the Au anomaly in the dune has formed in less than 10 000 years. Mechanisms for the Au accumulation in the sand are postulated and it appears that a biological process, principally involving vegetation, is the most viable.

A 200-m sample spacing of vegetation appears to be adequate for exploration of this type of deposit. Below the sand, calcrete provides a robust sampling medium. At present, due to limited knowledge of exploration methods in this type of environment, the mineral explorer must either expend significant financial resources augering through areas of sand cover to collect buried calcrete samples, or have lower confidence that vegetation and surface soil samples will detect mineralization.

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