U-Pb isotope systematics and apparent ages of uranium ores, Ambrosia Lake and Smith Lake districts, Grants mineral belt, New Mexico

Orebodies occur in continental sandstones of the Upper Jurassic Morrison Formation and comprise two main types of mineralization; primary ore formed early in the history of the host rock and resulted in tabular, peneconcordant orebodies rich in organic material; redistributed ore apparently was formed by oxidative destruction and reconcentration of primary ore at some time after Laramide deformation and resulted in roll-type orebodies lacking organic material. U-Pb isotope ages of four redistributed ores from the Ambrosia Lake District give concordant to nearly concordant ages of 3.3 to 12.5 m.y., supporting the concept that most redistributed ore formed in the late Tertiary or later. U-Pb isotope ages of primary ores from the Ambrosia Lake District show pronounced normal discordance, evidently due to coupled, continuous loss of Pb and radioactive daughters of ²³⁸ U. Data for ores from the Smith Lake District consistently give pre-Tertiary apparent ages.