Carnotite mineralization in inland drainage areas of Australia

Calcrete deposits developed adjacent to gypsite facies in arid inland drainage areas of Australia contain several economic carnotite deposits, major prospects and numerous minor occurrences. The calcareous lithofacies located in the ponded sections of internal drainage systems, are not only the main host for localization and concentration of uranium, but they also offer the greatest number and variety of depositional environments for carnotite ore development. Factors identified as fundamental in carnotite mineralization include the physiographic and hydrologic setting of the individual uraniferous drainage systems and the solution chemistry (p_CO2 ; CO₃ HCO₃, U, V, K concentrations; Eh, pH). However, variation in several basic ore characteristics (mineralogic, textural and chemical variations, radiometric disequilibrium and/or distribution) in some of the mineralized areas suggest that a major factor controlling the presence and ultimate extent of carnotite mineralization relates to the nature and extent of post-depositional modification of earlier deposits due to changing chemical/physical processes. Variation in the modes of carnotite precipitation relates directly to the textural and compositional characteristics of the host in different groundwater hydrologic zones; hence, the chemical modifications may be directly attributed to the contemporary hydrologic circulation pattern in individual drainage systems. The physical processes mainly relate to the downward gradation of vadose modification into mineralized calcretes, which may ultimately result in redistribution of ore zones within the drainages.