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

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 by²⁰⁶Pb/²⁰⁴Pb −²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/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 low²³⁸U/²⁰⁴Pb values, coupled with the radiogenic²⁰⁶Pb/²⁰⁴Pb and radiogenic²⁰⁷Pb/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 10¹¹ 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.