U-Pb isotope analyses of 15 ore samples from mines in the Gas Hills and Crooks Gap districts of Wyoming suggest that mineralization in both districts occurred more than 26 m.y. ago, with at least some uraninite-coffinite formation before 35 m.y. ago. Pb/U apparent ages from highest quality samples scatter in a range of 26 to 35 m.y., and inclusion of all samples increases the observed range to 12 to 41 m.y. The U-Pb isotope systematics of uraninite-coffinite and pyrite in the ores record the migration and loss of both stable and radioactive uranium daughters, so that pyrite in the ores invariably contains unsupported radiogenic Pb which is anomalously enriched in 206 Pb. Correction for Pb loss from three of the highest quality ore samples, using the pyrite radiogenic Pb, suggests a maximum age of uraninite-coffinite formation distinctly less than the approximately 55 m.y. age of the host rock. The results of this study indicate that the last ore-forming events are much older than many previous estimates, and rule out the possibility of major late Tertiary mineralization in these districts.

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