Sandstone-type uranium deposits in South Australia and North America: A comparison of their geophysical characteristics

Abstract

The sandstone-type uranium deposits in the Frome Embayment and Eucla Basin, South Australia, are characterised by uranium oxide mineralisation in the form of roll-fronts at redox boundaries. The host rocks are Tertiary clastic sediments deposited in palaeochannels, but now covered by up to 150 m of cover. None of the deposits are associated with surface radiometric anomalies. These deposits were discovered in the 1970s, but Government policy prevented exploitation at that time. With mining now permitted, there is renewed interest in these deposits and exploitation is underway, or imminent, and new resources are being sought. The comparatively low cost of drilling restricted the use of geophysical techniques during initial exploration, with downhole logging being the most extensively used method. Nevertheless, reconnaissance geophysical surveys were used to locate the prospective palaeochannels, with resistivity soundings proving most useful. Recent TEMPEST airborne EM surveys over the Billeroo/Curnamona and Yarramba palaeochannels, located in the Frome Embayment, have demonstrated that this method allows detailed mapping of the palaeochannels, despite the thickness of the cover. The electromagnetic response is attributed to the conductivity of contained saline groundwater. Once the palaeochannel sequence had been located, alteration zoning within the host sandstones, associated with characteristic responses on downhole logs, was used to locate the redox interface. An understanding of the channel stratigraphy and sedimentary environment, specifically the likely location of prospective permeable formations, was also largely based on downhole logs. Attempts to use electrical methods to detect areas of increased pyrite content, associated with the redox boundary, and to determine the redox state of the host sandstones, met with mixed success. A comparison of the use of geophysics in South Australia with exploration for similar deposits in the United States shows some aspects in common, in particular the use of downhole logging to map alteration. However, care must be taken when making comparisons, since variations in the type of deposit can significantly affect their geophysical signatures.