Sedimentary dolomite in Western Australia and the dolomite problem: Genesis of channel and playa uranium deposits

ABSTRACTQuaternary to Recent sedimentary dolomite in groundwater calcrete and saltmarsh sediments of Lake Way and Lake Maitland, Western Australia, provide new information about the formation of low‐temperature dolomite. Dolomite can form through numerous pathways depending on the depositional and diagenetic environment. Many pathways involve microbial processes and/or the presence of a nucleation substrate which help overcome kinetic barriers preventing precipitation in the laboratory. Petrographic, mineralogical, hydrogeochemical and stable isotopic data in this study reveal the importance of Mg‐clays as nucleation sites for dolomite precipitation in a range of aquifer environments. There is also a close association between authigenic Mg‐clays, dolomite and the potassium–uranyl–vanadate ore mineral carnotite in channel and playa uranium deposits. It is interpreted that evaporation‐driven precipitation of dolomite establishes a positive feedback loop promoting the dissociation of aqueous uranyl–carbonate complexes and concomitant increase in carnotite saturation. Critical to this model is the presence of authigenic Mg‐clays because they facilitate dolomite precipitation and promote carnotite nucleation by concentrating potassium ions on clay surfaces via adsorption. This Mg‐clay–dolomite–carnotite relationship is widespread throughout Western Australian channel and playa uranium deposits and has been observed in similar deposits in Namibia and Botswana. In addition to this economic implication, Mg‐clay mediated nucleation of dolomite potentially has global relevance as a precipitation mechanism for low temperature dolomite in sedimentary deposits where detrital and/or authigenic Mg‐clays are present. Maturation of sedimentary dolomite from disordered high‐calcian dolomite to ordered low‐calcian dolomite occurs very early in the meteoric realm making it resistant to alteration during burial diagenesis. Diagenetic resistance may be further increased by early meteoric silicification related to the degradation of associated Mg‐clays. These findings indicate that Mg‐clay associated sedimentary dolomite has potential to retain a primary isotopic signature indicative of its origin, making it a useful recorder of palaeoenvironmental conditions.