Abstract

Disequilibrium studies involving the determination of total U and the activity ratios of 234U/238U and 234U/230Th or activities of 230Th, 226Ra, and 210Pb were carried out on samples from three surficial (generally within 5 m of the surface) uranium deposits in south-central British Columbia that give apparent 234U/230Th ages of 1000–20 000 years. As a result of the young ages, the deposits have not yet reached radioactive secular equilibrium and, therefore, yield very little gamma activity.The deposits formed from groundwaters that leached labile uranium from intermediate to felsic igneous rocks. Two accumulation mechanisms concentrate the uranium: evaporation, and adsorption onto organic matter.The uranium content and the activities of the various daughter nuclides are highly variable within and between the various deposits studied. Some of the variations can be explained in terms of the accumulation processes. In the evaporative process the highest value of uranium and daughter nuclides will be found at the surface, whereas in those deposits in which adsorption is the dominant mechanism these nuclides are found in association with buried organic matter. Under these circumstances, accumulations will be influenced by the flow of groundwater from different sources and also depend on whether daughter nuclides remain immobile or are leached after formation.

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