Abstract

The Proterozoic sandstone-hosted Redtree and Junnagunna uranium deposits in the Westmoreland uranium field occur in the southern McArthur basin, Australia, at the contact between the Westmoreland Conglomerate and the Seigal Volcanics. Uranium mineralization consists of uraninite with hematite and illite and occurs within a zone of chlorite alteration that formed prior to the uraninite during peak diagenesis. Oxygen and hydrogen isotope ratios on synmineralization illite show that uranium was transported to the site of deposition by a basinal brine with δ18Ofluid and δDfluid values of 4 ± 3 and −33 ± 10 per mil, respectively. These values are consistent with evolved evaporated seawater but not with hot, oxidized fluids derived from underlying uraniferous granites or volcanic rocks as previously suggested. Illite crystallinity indicates that the uraninite-illite-hematite assemblage formed at 200° ± 50°C. The 40Ar/39Ar ages of illite and 207Pb/206Pb ages of uraninite indicate that mineralization occurred between 1655 ± 83 and 1606 ± 80 Ma, coincident with major tectonic events in northern Australia, and was later remobilized between ca. 1150 and 850 Ma. A U-Pb concordia age of 878 Ma is identical to the 870 Ma age recorded by previous workers on Westmoreland uranium deposits. The mineralogy, paragenesis, and geochemistry of the Redtree and Junnagunna uranium deposits are largely indistinguishable from the basement-hosted Nabarlek and Jabiluka uranium deposits in the northern McArthur basin, suggesting that the Westmoreland uranium field might also be prospective for basement-hosted uranium deposits where suitable structural and chemical traps can be found.

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