Surface geochemical expression of bedrock beneath thick sediment cover, Curnamona Province, South Australia

ABSTRACT An increasing requirement for the discovery of new mineral deposits is the ability to detect mineralization through thick cover. Large areas of the southern Australian Curnamona Province are prospective for Pb–Zn and Cu–Au mineralization; however, much of the prospective basement is covered by 10–150 m of Cenozoic fluvial, alluvial and lacustrine sediments. Soil surveys were conducted at the Kalkaroo Cu–Au–Mo deposit and Polygonum multi-element prospect. The major challenge to exploration at both sites is the 40–150 m thickness of transported cover. The objective was to determine whether any surface geochemical method could be used to detect mineralization under such conditions. The range of methods examined included those where some success was indicated in previous surveys in the region. Soil samples were collected from 10–25 cm depth and from the zone of high soil moisture loss as indicated by the presence of secondary calcium carbonate and sulphate (60–300 cm). Samples were treated using aqua regia and the partial extractants weak cyanide, sodium hydroxide, magnesium chloride and the proprietary method MMI-M. Conductivity and pH measurements were made on all samples. A Chinese variant of the electrochemical CHIM technique was also tested over the Kalkaroo deposit. A consistent response to the mineralized zone at the Kalkaroo deposit is double-peak anomalies for Mo. Less regular double-peak anomalies are also present for U and Au, and for soil conductivity. The two survey lines with electrochemical CHIM results show mainly high-contrast, one-point anomalies over mineralization. The significance of these is still to be established. Soils collected over the Kalkaroo and Polygonum prospects contain Ag concentrations that vary coincidently with changes in underlying basement lithology. Relatively high Ag concentrations in soils over the Polygonum prospect show a spatial relationship to underlying mineralized zones. Although no single technique used in this study was identified as a reliable exploration tool, survey results add support to the contention that partial extraction geochemistry of soils in the region may reflect mineralization and/or underlying bedrock trace metal content through as much as 150 m of transported cover.