EVOLUTION AND OCCURRENCE OF URANIUM DEPOSITS THROUGH GEOLOGIC TIME

The formation of economic concentrations of uranium is controlled by the contrasting behavior of its two principle oxidation states, the normally immobile reduced uranous ion and the generally mobile oxidized uranyl ion. During primary igneous processes the incompatible behavior of the uranous species results in concentration in late alkalic differentiates. The mobility and concentration of uranium will be strongly dependent on the redox state of the Earth's crust. A major timeline during the evolution of the Earth was about 2.4Ga when the atmosphere changed from reduced to oxidized. Previous to that time most economical concentrations of uranium occurred in paleoplacer deposits. Following the oxygenation of the hydrosphere most economic deposits, i.e. unconformity-related, sandstone-hosted, and veins, resulted from fluid transport as uranyl complexes and deposition by reduction; similar processes resulted in the subeconomic shale-hosted deposits. Differentiation within these types lies in the mode of transport, nature of reductant, steepness of the redox gradient, and host structure. Even though some deposit types appear to be concentrated at unconformity horizons, there is not a consistent relationship between deposit type and age. Concentration is more related to the establishment of geologic circumstances that result in contrasting redox states across lithologic boundaries, whether it was oxidized siliciclastic units unconformably overlying carbonaceous metamorphic rocks, or facies changes within fluvial environments in sandstone hosted deposits. Age-specific occurrences are a result of the coincidence of geologic processes favorable for the proper combination of uranium sources, redox gradients, and structure to serve as conduits for fluid flow, from crosscutting faults and breccias in unconformity-related deposits and veins, to permeable fluvial channels in sandstone-type deposits. Circumstances that are particularly favorable for mineralization include the superposition of elevated carbon dioxide levels in the hydrosphere with times of increased alkalic volcanism and deposition of volcaniclastic debris.