Geothermal signatures and uranium ore deposits on the Stuart Shelf of South Australia

Abstract

The Olympic Dam copper?uranium?gold deposit coincides with a major geothermal anomaly. Excess heat flow is expressed by elevated geothermal gradients in the flat-lying Cambrian and Late Proterozoic sediments which unconformably overlie the mid-Proterozoic basement (and the ore body). The Tregolana Shale within this sequence is assumed to act as an impermeable layer in which the lateral variation of thermal conductivity is negligible. Geothermal gradients in the Tregolana Shale range from 83.6°C/km at Roxby Downs to 52 ± 8°C/km elsewhere on the Stuart Shelf. Thermal conductivities were determined in representative core samples allowing estimates of heat flow ranging from 160 to 100 mW/m2 respectively. Most of the anomalous heat flow can be attributed to the radiogenic decay of uranium concentrated in the orebody. However minor anomalies in heat flow are observed at depths of 750 to 1000 metres. Anomalies of 65 mW/m2 at Roxby Downs can be explained assuming a one-dimensional model incorporating a 300 m thickness of 0.08% uranium oxide consistent with the published reserves.