Diagenesis of paleodrainages in Lake Way and Lake Maitland, Western Australia, and the role of authigenic Mg-clays and dolomite in the genesis of channel and playa uranium deposits

Abstract

This study integrates mineralogical and hydrogeochemical analysis of channel and playa uranium deposits to characterize aquifer evolution and the physico-chemical mechanisms that result in the accumulation of uranium into potentially economic deposits. This subset of surficial U deposits occur in Tertiary to Recent calcrete and dolomitic, clay-rich fluvial paleochannel and palustrine sediments, wherein uranium is largely bound in the potassium-uranyl-vanadate mineral carnotite [K2(UO2)2(VO4)2·3H2O]. Scanning electron microanalysis indicates that the carnotite mineralization is part of a late-diagenetic mineral assemblage that critically includes Mg-clays (sepiolite and stevensite), amorphous magnesium silicate, and synsedimentary dolomite. This authigenic mineral assemblage is observed concentrated in fractures and pores in groundwater calcrete and silty salt marsh “palustrine” sediments. Drill-hole gamma ray and conductivity data from the Centipede-Millipede uranium deposit indicate that the locus of uranium mineralization occurs near the present-day water table where oxidizing fresh-to-brackish groundwater interacts with playa brine, forming a hypopycnal groundwater estuary beneath the clay pan and salt marsh. It is interpreted that effective U fixing occurs in areas where groundwater, near-saturated with respect to carnotite, is hydrologically focused upward and into the zone of evaporation. The appreciable precipitation deficit in the Northern Yilgarn is interpreted to produce an evaporation-driven positive feedback mechanism that results in the co-precipitation of Mg-clays, dolomite, and carnotite. The presence of vanadium-rich Mn-oxide phases in high-grade U ore zones indicates that Mn-redox cycling may serve an important role in increasing the local activity of V, and thus carnotite saturation. Mineralogical comparison of other channel and playa uranium deposits throughout Western Australia and Namibia have identified a similar mineral association and paragenetic trend, suggesting that contemporaneous evaporative precipitation of Mg-clays and dolomite are integral in achieving carnotite saturation and precipitation.