A thermoluminescence study of the rôle of a middle proterozoic unconformity in controlling uranium mineralization, as shown at Eyre Peninsula, South Australia

Abstract

Thermoluminescence research over the last three years has reached the point that radiation effects can be recognized in natural minerals, among which quartz ideally suited because of its small variation in chemical composition. The major radiation-related effects of quartz are sensitization (increasing the TL efficiency of the test dose) and a high temperature shift of TL glow peaks with increasing radiation doses. Those two criteria have been used to assess the TL glow curves of quartz obtained from samples near a Carpentarian unconformity in Eyre Peninsula, South Australia. The TL results indicate that radiation affected a zone of narrow (50 m) width of Lower Proterozoic rocks occurring immediately under the unconformity sediments of Carpentarian age. Vertical drill holes cutting through the unconformity surface yielded sample material which demonstrated diminishing radiation effects away from the unconformity. Maximal radiation effects, which are compatible with an uranium content of 100 ppm residing in the zone over a period 1 400 Ma, are confined to the Carpentarian unconformity and its immediate vicinity. The results indicate that the mineralization took place under a Carpentarian cover, and was not sourced in the Lower Proterozoic metamorphics. From the homogeneous distribution of radiation effects it seems likely that the mineralization spread under the cover and only penetrated greater depths in shear zones. In areas where erosion has removed the Carpentarian cover and part of the metamorphic rocks under it, those shear zones are the only rocks affected by radiation.