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Geological Society, London, Special Publications; 1983; v. 11; p. 185-195;
DOI: 10.1144/GSL.SP.1983.011.01.19
© 1983 Geological Society of London

Duricrusts: Calcretes, Silcretes and Gypcretes

Concentration of uranium and vanadium in calcretes and gypcretes

Donald Carlisle

Department of Earth and Space Sciences, University of California, Los Angeles, Los Angeles, California 90024, USA

Uranium ore-bearing calcretes (in Western Australia and Namibia) are non-pedogenic. They result from lateral transport rather than vertical redistribution of components and they develop mainly in the capillary fringe along the axes of large stable drainages with low gradients under uniquely arid climates. Valley, deltaic and lake margin calcretes and dolocretes in Western Australia are part of an orderly succession from silica hardpan to playa salts. Distributions of uraniferous non-pedogenic and non-uraniferous pedogenic calcretes are mutually exclusive. Carnotite, the only ore mineral, results from: (1) evaporative concentration of U, V and/or K; (2) destabilization of uranyl carbonate complexes consequent to evaporative and common-ion precipitation of Ca/Mg carbonate; and (3) oxidation of V(IV) to V(V) in upwelling groundwater. Richest concentrations occur where groundwaters rise toward the evaporative zone. Pedogenic calcretes may inhibit ore-grade concentration. Carnotite in gypcrete is economically significant but less common.