Abstract

Uranium, thorium, and lead concentrations and lead isotopic results for whole-rock samples from the Owl Creek Mountains, Wyoming show that the granitic rocks are Precambrian W in age and that isotopic disequilibrium exists between ²³⁸U. and ²⁰⁶Pb. Most of this disequilibrium is best explained by recent uranium loss and in one case a recent uranium gain. From this we infer that anomalous uranium concentrations in the vicinity of the Owl Creek Mountains may have resulted from reconcentration of uranium that was mobilized within the granite during or after Laramide time.

²³²Th and ²⁰⁸Pb data do not fit a single-stage evolution pattern and therefore suggest significant mobility of thorium or lead. Isotopic compositions of lead in feldspars are characterized by a radiogenic lead component that is secondary in origin, and we conclude that much of the disequilibrium in the Thorium-Lead system is due to recent lead mobility. Three samples collected near the Cambrian weathering surface exhibit very pronounced disequilibrium in the Thorium-Lead system. For these samples most of the disequilibrium is probably due to thorium mobility during the late Precambrian.

Alpha-spectrometric analyses of three drill-core samples show that pronounced disequilibria exist in the upper portion of the ²³⁸U-decay chain. This indicates that uranium in the granite has been mobilized to a depth of at least 122 m within the last 150,000 years.

Petrologic data suggest that the granite was crustally derived by partial melting and that it crystallized under conditions of high oxygen fugacity and water activity. This origin accounts for the initially high concentrations of uranium and thorium in much of the granite, and may explain why uranium and probably thorium has been moved so easily within the granite.