Geophysics and Uranium in the Curnamona and Billaroo Palaeochannels, Frome Embayment, South Australia.

Abstract

Tectonism is believed to have exerted a strong control on the current disposition of sedimentary hosted uranium mineral deposits in the Frome Embayment. However the extent of this control has not been well understood, nor documented. Here, we examine the combined use of regional and finer-scale TEMPEST AEM data, linked to a structural interpretation of magnetic and gravity data airborne magnetics and ground gravity, to extend our understanding of the evolution, geometry and variability of sediment packages associated with sedimentary uranium mineralisation in the Curnamona and Billaroo Palaeochannel systems. Through the analysis of both smooth and blocky model LEI inversions of these AEM data, we contend that structural control was critical in determining the initial orientation of the palaeovalleys and the location of basal sequences of the Eyre Formation, the host to known uranium mineralisation. The geophysical data suggest that NE orientated normal fault structures, and NW oriented transfer structures, initially developed in the Cambrian but reactivated with Cenozoic uplift caused by WNW ESE compression, were preferentially eroded by a series of braided stream systems flowing north. The inverted AEM data indicates that these streams cut wide channels, ranging from hundreds of metres to several kilometres wide in Cambrian Arrowie sediments before filling with Eyre Formation sediments which consist of sands with clay-rich interbeds sourced from the embryonic Flinders-Barrier ranges and Olary Spur during a period of tectonic upheaval and erosional. Understanding the variable geometry of this eroded channel system may assist in locating basal scours, bends, confluences or areas of channel-widening where uranium mineralisation is often located. It may also indicate sites which might have encouraged the accumulation of organic reductants (e.g. organic material) in the sediments, although the presence of reactivated basement faults in providing the loci for mobile reductants from underlying basins may also be significant.