Keeling and two co-authors reported in 2015 that unconformity-related uranium deposits currently produce ~20% of the world’s uranium (2013 data) and include resources with the highest uranium grades. This style of deposit is essentially restricted to geological settings where relatively undeformed Proterozoic clastic sedimentary basin deposits overlie crystalline basement. The uranium mineralisation is distributed either within faulted and altered crystalline basement rocks immediately below the unconformity contact with overlying basin sandstones, or extends across the contact, or is contained within the overlying clastic sediments.
Major resources are associated with the Athabasca Basin in Saskatchewan, Canada and the McArthur Basin in Northern Territory, Australia. In South Australia, the Mesoproterozoic Cariewerloo Basin, extending over 42,500 sq km of crystalline basement rocks of the eastern Gawler Craton, is an undeformed, clastic intracontinental basin with geological similarities to the Athabasca and McArthur basins sufficient to be regarded as prospective for unconformity-related uranium deposits (Dhu et al., 2010). This prospectivity is enhanced by the presence of uranium mineralisation in basement rocks on the eastern basin margin, and trace uranium (ave.7 ppm) in acid volcanic rocks that were the dominant westerly source of sediment into the basin (Wilson et al., 2010).
The Cariewerloo Basin is mostly covered by younger sediments and the extent of the basin is interpreted largely from drilling and regional geophysics. To assist with prospectivity analysis for uranium, the Geological Survey recently compiled existing data to produce a 3D model of the basin architecture, structural elements, and sediment distribution (Wilson et al., 2012). New data were collected on sediment properties, mineralogy and chemistry. These data include spectral logs for 95 cored drill holes, using the HyLogger™ spectral core scanner. Reflectance spectra were measured in the visible to near infrared (VNIR) and shortwave infrared (SWIR) wavelength range 380 – 2500 nm; a small number of scans, recorded late in the program, also included thermal IR measurements for wavelength range 6000 – 14500 nm. For those minerals with characteristic absorption features at these wavelengths, the data provide continuous mineralogy at ~1 cm interval through the basin sediments, collectively referred to as Pandurra Formation. Spectral interpretations were validated by X-ray diffraction analyses and electron microscopy, on selected samples. The Pandurra Formation is dominantly red-bed sandstones with siltstone and minor sandy mudstone interbeds.
Mineralogy is predominantly quartz as coarse to fine sand, typically thinly coated by hematite, with minor K-feldspar, detrital mica, lithic fragments and trace amounts of heavy minerals. Quartz shows no characteristic absorption in the SWIR, but this spectral region is particularly effective for mapping the distribution of clays and micas that result from diagenetic processes and occupy pore space and partially replace detrital grains. While the source of uranium in unconformity-related deposits is still debated, there is broad agreement that fluid flow within the basin plays a key role in mineral modification, and the transport and precipitation of uranium (Cuney, 2005). The distribution of authigenic minerals kaolin (predominantly dickite) and illite/muscovite show patterns that can be interpreted in terms of depth of burial and fluid circulation. General inferences drawn from these patterns are as follows:
• The maximum thickness of Pandurra Formation is inferred to be ~1500 m but the presence of blocky dickite crystals and growth of authigenic muscovite are consistent with burial depths of around 4,000 m (Lanson et al., 2002). This suggests either more than half the basin sediments were removed by subsequent erosion, or the thermal gradient across the basin was abnormally high. Fluid temperatures and salinity appear to have been sufficient to leach uranium from monazite and zircon grains within the sediment.
• In the basal sandstone (Member 1), the presence of K-feldspar remnants, and extensive infilling of pore space by 2M1 muscovite, are consistent with decreased permeability and restricted fluid circulation within the lower Pandurra, particularly for deeper parts of the basin. This was controlled, in part, by a persistent, thin siltstone/mudstone unit (Member 2), which effectively limited the interaction of circulating basin fluids with the underlying crystalline basement rocks.
• Active fluid flow was mainly in upper sandstones (Members 3 & 4). Insight into the flow regime of oxidised, saline and (weakly?) acidic fluids is provided by the distribution of illite/muscovite alteration, particularly where the alteration is characterized by a short wavelength spectral absorbance feature at ~2200 nm. This describes a pattern of flow that varies from vertical to horizontal and surrounds large patches of remnant authigenic dickite. This may be indicative of thermally-driven fluid circulation within the basin, but such interpretation needs to also take into account preliminary K-Ar dating of illite which indicates that at least some flow activity responsible for illite crystallization was episodic and probably related to tectonic activity.
• Fluid flow towards the basin margin appears more focussed and is characterized by intense illite/muscovite alteration with minor remnant dickite patches. In these zones, the fluid flux may have been too high to precipitate or accumulate uranium, even where reductants were present. Much of the western half of the Cariewerloo Basin is underlain by Gawler Range Volcanics, mostly of rhyodacite composition, with little potential for effective reductants to precipitate uranium. The exception is possibly along major faults acting as pathways for reduced fluids sourced from older basement rocks. Rock types beneath the eastern Cariewerloo Basin are more varied and include lithologies that are potentially reducing, particularly those containing biotite or amphibole that might be altered during interaction with oxidizing fluids to release of Fe2+ ions capable of reducing U6+ ions in solution, to form insoluble U4+ precipitates.
The pattern of diagenetic mineral alteration in Pandurra Formation sandstone, derived from spectral data, suggests an exploration strategy for unconformity uranium in the Cariewerloo Basin that focuses on the eastern portion of the basin. Most prospective regions are likely to be where fine-grained Member 2 is absent or where faulting has juxtaposed reactive basement rocks against upper units of Pandurra Formation (Members 3&4) and possibly also modified fluid circulation within the sandstone.
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