Uranium, thorium, and lead systematics in Granite Mountains, Wyoming

Abstract

Uranium, thorium and lead concentrations and isotopic compositions were determined on total rocks and a feldspar sample from widely separated parts of the Granite Mountains in central Wyoming. Linear relations defined by 206Pb/ 204Pb − 207Pb/ 204Pb and 208Pb/ 204Pb − 232Th/ 204Pb for the total rock samples define 2.8 billion-year isochrons. In contrast, 238U/ 206Pb ages are anomalously old by a factor of at least four. The low 238U/ 204Pb values, coupled with the radiogenic 206Pb/ 204Pb and radiogenic 207Pb/ 204Pb ratios, indicate that contents of uranium in near-surface rocks would have had to have been considerably greater than those presently observed to have generated the radiogenic lead. It is possible that more than 10 11 kg of uranium has been removed from the Granite Mountains, and the most feasible interpretation is that most of this uranium was leached from near-surface rocks at some time during the Cenozoic, thus providing a major source for the uranium deposits in the central Wyoming basins.