The mobility of uranium and associated trace elements in the Bates Mountain Tuff, central Nevada

The Bates Mountain tuff is a rhyolitic ash flow which crops out over a large area in central Nevada. Samples of fresh glassy, devitrified and weathered (to clay) rocks from the four cooling units that make up the formation were analysed for U, Th, Li, Fe, Mn, Mo and F to test if this tuff formation might be a ready source for uranium on leaching by ground waters. The Th content is apparently unchanged by devitrification and its enrichment in clay samples is considered to be residual. Therefore Th is assumed to be immobile and is used as the independent variable for evaluating changes in the other elements. Uranium is evenly distributed in the fresh volcanic glasses and apparently also in the devitrified rocks, where it may however be associated with particular devitrification minerals. Assuming homogeneous distributions for all the trace elements in each cooling unit, the data are explained by a loss of U, Mo, Li and F on devitrification and a further loss of U and Mo but a gain of Li and F on alteration to clay.