Are there any sandstone-hosted uranium systems in the Eromanga Basin?

As part of Geoscience Australia's Onshore Energy Security Program the authors have investigated whether there is any evidence that a sandstone hosted uranium system has operated in the Eromanga Basin and assessed the basin's potential to host significant uranium mineralisation. The Eromanga Basin is a Mesozoic, intracratonic, sag basin that covers ~1,000,000 km2 of central-eastern Australia and it onlaps several surrounding Proterozoic provinces that have elevated uranium contents. The Eromanga Basin has been subdivided into three major stratigraphic sequences: a lower non-marine sequence, a middle marine sequence and an upper non-marine sequence. Each sequence is characterised by basal sands that fine upwards into shales and silts, suggesting a gradual change from high- to low-energy sedimentary environments. The overall structural trend of the basin is oriented north-east, with a series of north-east striking troughs and ridges that locally controlled sediment deposition. Subsequent folding has produced broad, regional domes. To assess the uranium potential in the Eromanga Basin, the pmd*CRC five-questions framework was adopted. This framework attempts to understand the whole mineralising system by addressing (1) the geodynamic and pressure temperature histories; (2) the architecture of the system; (3) the fluid reservoirs; (4) the fluid flow drivers and pathways; and (5) the metal transport and deposition mechanisms. To answer these five questions, the following studies were made:

a literature review of the geodynamic setting for the Eromanga Basin and that of existing sandstone hosted uranium provinces. Data have been summarised in a series of space time plots construction of a 3D map from publicly available datasets to better understand the basin architecture and fluid flow pathways compilation of water chemistry to map potential fluid reservoirs and transport mechanisms whole rock and multi element geochemistry together with detailed petrology, to understand uranium deposition mechanisms.

A review of the geodynamic setting suggests that there are potentially three discrete mineralising events in the western portion the Eromanga Basin, which correspond to periods of known uplift and erosion. In the east there is potential for a single, long period, uranium mineralising event associated with the uplift of the Eastern Highlands. The 3D map of basin architecture shows that there are large, permeable sandstone packages that are bounded above and below by impermeable shales and that the sandstones are also exposed at the surface along the margins of the basin. This juxtaposition of lithologies is the most favourable architecture and fluid pathway for the formation of sandstone hosted uranium deposits, because fluids can enter the sandstone through outcropping aquifers and be contained above and below by aquitards. Groundwater hydrochemistry datasets show regional redox and pH fronts along the margins of the Eromanga Basin related to the movement of oxidised, acidic meteoric waters through porous sandstone layers. Uranium is commonly precipitated at redox gradients, particularly in areas close to a uranium source (i.e. sandstones with elevated background concentrations of uranium and/or uranium rich Proterozoic rocks exposed at the surface as in the case of Mt Isa and Mt Painter Inliers).