Abstract

The discovery of a major uranium deposit in the calcretes of Western Australia confirms the economic importance of this type of occurrence. The calcrete is a surficial limestone formed by near-surface groundwater; uranium derived by weathering of nearby granite occurs as carnotite in vugs and fractures in the calcrete. Vanadium has been important in fixing uranium as insoluble carnotite and appears to have been derived from the surrounding laterites. Since the calcretes are unequivocally of surficial and recent origin, the uranium must have been similarily formed.The genetic importance of such surficial processes to the origin of sedimentary deposits of the Colorado type is easily visualized. However, a surficial origin for uranium in pitchblende-vein deposits also satisfactorily explains their spatial and temporal relationships to unconformities and uranium-rich granitic complexes. Vein deposits in the Northern Australia uranium province are good illustrations of the features attributed to surficial sources of uranium in pitchblende-vein deposits.

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