Palaeochannel uranium signatures and exploration models, South Australia

Abstract

Sedimentary uranium mineralisation in South Australia is mostly in Cainozoic basinal palaeochannels developed on or proximal to Precambrian cratons. Precambrian basement of the Gawler and Curnamona cratons have uranium contents in the range 10-100 ppm, well above the crustal average of 2.8 ppm U. Deeply weathered basement rocks in these regions were incised during early Tertiary times and the sediments in these palaeodrainage networks now form several significant uranium provinces. The palaeochannel uranium deposits are typically hosted by medium to coarse-grained sandstone deposited in a continental fluvial, latchstring, alluvial, or marginal marine sedimentary environment. Impermeable clay units are intercalated in the sedimentary sequence and often occur immediately above and below the mineralised sandstone. Uranium mineralisation is associated with reduced conditions, and similar to elsewhere in the world, the host sediments to the Tertiary uranium mineralisation often contain pyrite and organic (plant) matter that is either disseminated or forms lignite seams. Tabular or roll-front shaped mineralised bodies formed along the contact with clay horizons and also along the palaeochannel margins. Tertiary continental sediments in SA are important favourable hosts because of the high organic content in channel sediments due to widespread colonisation by land plants during this time. The precise geometric definition of a palaeochannel is important in the selection of exploration targets for sandstone uranium. This often requires the integration of various geoscientific data sets in order to define targets and to improve the effectiveness of drilling. Refinements in remote sensing and geophysical techniques, data processing, sedimentology and computer-aided interpretations provide an effective, economic and efficient method of outlining the principal drainage patterns and channel dimensions. An improved understanding of sedimentary models and their relationship to uranium distribution will assist the effectiveness of exploration industry to explore for uranium in buried channel systems.