Age of uranium mineralization in the Gas Hills and Crooks Gap districts, Wyoming, as indicated by U-PB isotope apparent ages

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 observe range to 12 to 41 m.y. 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. 25 refs