Domnick, and 5 co-authors, reported in 2020 that the Blackbush uranium prospect (~12,580 tonnes U at 85 ppm cut-off) is located on the Eyre Peninsula of South Australia. Blackbush was discovered in 2007 and is currently the single example of sediment-hosted uranium mineralisation investigated in any detail in the Gawler Craton. Uranium is hosted within Eocene sandstones of the Kanaka Beds and, subordinately, within a massive saprolite derived from the subjacent Hiltaba-aged (~1585 Ma) granites, affiliated with the Samphire Pluton.
Uranium is mainly present as coffinite in different lithologies, mineralisation styles and mineral associations. In the sandstone and saprolite, coffinite occurs intergrown with framboidal Fe-sulfides and lignite, as well as coatings around, and filling fractures within, grains of quartz. Microprobe U–Pb dating of coffinite hosted in sedimentary units yielded a narrow age range, with a weighted average of 16.98 ± 0.16 Ma (343 individual analyses), strongly indicating a single coffinite-forming event at that time. Coffinite in subjacent saprolite generated a broader age range from 28 Ma to 20 Ma.
Vein-hosted coffinite yielded similar ages (from 12 to 25 Ma), albeit with a greater range. Uraninite in the vein is distinctly older (42 to 38 Ma). The 17 ± 0.16 Ma age for sandstone-hosted mineralisation roughly coincides with tectonic movement as indicated by the presence of horst and graben structures in the Eocene sedimentary rocks hosting uranium mineralisation but not in stratigraphically younger sedimentary rocks. The new ages for hydrothermal minerals support a conceptual genetic model in which uranium was initially sourced from granite bedrock, then pre-concentrated into veins within that granite, and is subsequently dissolved and reprecipitated as coffinite in younger sediments as a result of low-temperature hydrothermal activity associated with tectonic events during the Tertiary.
The ages obtained here for uranium minerals within the different lithologies in the Blackbush prospect support a conceptual genetic model in which tectonic movement along the reactivated Roopena Fault, which triggered the flow of U-rich fluids into the cover sequence. The timing of mineralisation provides information that can help optimize exploration programs for analogous uranium resources within shallow buried sediments across the region. The model presented here can be predicted to apply to sediment-hosted U-mineralisation in cratons elsewhere.
The authors concluded that coffinite ages define an almost perfect normal distribution suggestive of a single coffinite-forming event at ~17 Ma, likely triggered by tectonic movement along a major lithospheric boundary. Uncertainties remain, however, about the proportion of U brought in the system during the event, compared to simple leaching of U from the basement by groundwater. Age constraints from coffinite in the saprolite (28–20 Ma) are based on fewer analyses and, in all probability, represent incomplete resetting of older uranothorite.
Uraninite in granite-hosted veins gives an age of 42–38 Ma. Uranium mobility would appear to predate sedimentation of the preserved cover sequence. The ages obtained here on hydrothermal minerals support a conceptual genetic model in which tectonic movement along the reactivated Roopena Fault triggered the flow of U-rich fluids into the cover sequence.
The new data will assist with the optimization of exploration strategies seeking to identify additional uranium resources in the region, even if some of the interpretations reached here may suggest that Blackbush may have formed where it is due to a combination of favorable factors that may not be widely repeated. The petrographic work detailed here on the nature of the uranium minerals represents valuable background information that may assist in the eventual exploitation of the Blackbush resource, e.g., by in situ leaching.
An earlier UraniumSA Samphire Project Update: https://www.asx.com.au/asxpdf/20130927/pdf/42jnqgsn2cqcgg.pdf
2007 News Release: https://www.asx.com.au/asxpdf/20071212/pdf/316fq84dlcgmsd.pdf
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