Two types of uranium orebodies occur in continental sandstones of the Upper Jurassic Morrison formation: 1) 'primary' tabular peneconcordant bodies rich in interstitial organic material (containing most of the U) and older than the overlying Cretaceous Dakota sandstone and 2) younger discordant ore-bodies associated with Tertiary faults, containing little organic material (coffinite is the U-carrier) and considered to be redistributed primary ore. U/Pb isotope ages of four of the younger ore-bodies from the Ambrosia Lake district are concordant at 3.3-12.5 m.y. U/Pb isotope ages of primary ores from the same district show pronounced normal discordance, explained by continuous loss of Pb and 238U daughters and, locally, by U-gain during Tertiary redistribution. The primary ores, without Tertiary U-gain, plot on the concordia diagram close to a calculated continuous diffusion curve for coupled loss of Pb and 238U daughters. The concordia intercept of this curve corresponds with an age of approx 130 m.y. Ore-bodies in the Smith Lake district have characteristics both of primary (co-extensive with organic material) and redistributed (discordant) types. They show less coherent U/Pb isotope systematics but give only pre-Tertiary apparent ages.-G.J.N.
Additional publication details
U-Pb isotope systematics and apparent ages of uranium ores, Ambrosia Lake and Smith Lake districts, Grants mineral belt, New Mexico.