Magnetotellurics and Airborne Electromagnetics ? a combined method for assessing basin structure and exploring for unconformity related uranium

Abstract

Unconformity-type uranium deposits are characterised by high-grade and constitute over a third of the world's uranium resources. The Cariewerloo Basin, South Australia, is a region of high prospectivity for unconformity-related uranium as it contains many similarities to an Athabasca-style unconformity deposit. These include features such as Mesoproterozoic red-bed sediments, Paleoproterozoic reduced crystalline basement enriched in uranium (~15-20 ppm) and reactivated basement faults. An airborne electromagnetic (AEM) survey was flown in 2010 using the Fugro TEMPEST system to delineate the unconformity surface at the base of the Pandurra Formation. However highly conductive regolith attenuated the signal in the northern and eastern regions at a depth of 300 m or less, requiring the application of a deeper geophysical imaging methods. In 2012 a magnetotelluric (MT) survey was conducted along a 110 km transect of the north-south trending AEM line. Braodband (300 Hz - 300 s) MT data were collected at 29 stations and successfully imaged the depth to basement, furthermore providing evidence for deeper fluid pathways. The AEM data were integrated into the regularisation mesh as a-priori information generating an AEM constrained resistivty model, and also correcting for static shift. It was found that the combined techniques provided better structural information over depths of 0 - 5 km than could be obtained by inversion of each method alone.