Isotopic ages of uranium deposits in the Todilto Limestone, Grants district, and their relationship to the ages of other Colorado Plateau deposits
William R. Berglof
Uranium-lead isotopic data suggest an age of 150-155 Ma for uraninite ores from the Todilto Limestone Member of the Wanakah Formation in the Grants uranium district. The deposits apparently began to form relatively soon after deposition of the host rock. The precise age of the Todilto is uncertain, and the interval between sedimentation and mineralization, possibly 10 Ma or more, cannot be determined from isotopic data alone. Some Todilto isotopic ages are concordant or nearly concordant; others are discordant, usually showing the pattern of normal discordance that is common in low-temperature uranium minerals. Discordance most likely resulted from loss of radiogenic lead and migration of intermediate daughters in the 238U decay chain. Geologic and isotopic studies indicate that uranium deposits of several different ages are present throughout the Colorado Plateau, and that primary mineralization developed early in the history of the Chinle and Morrison Formations and the breccia pipes of the Grand Canyon. Some younger ores formed by redistribution of uranium from older deposits. Interpretations of Todilto isotopic data are consistent with these concepts. In the Grants region, ore in the Todilto may have existed before the much larger deposits in the Morrison Formation began to form, but age data supporting this conclusion are not definitive.
- Berglof, William R., 1992, Isotopic ages of uranium deposits in the Todilto Limestone, Grants district, and their relationship to the ages of other Colorado Plateau deposits, in: San Juan Basin IV, Lucas, Spencer, G.; Kues, Barry S.; Williamson, Thomas E.; Hunt, Adrian P., New Mexico Geological Society, Guidebook, 43rd Field Conference, pp. 351-358.