O and Pb isotopic analyses of uranium minerals by ion microprobe and U–Pb ages from the Cigar Lake deposit

Abstract

We apply a rapid and accurate in situ technique to make U–Pb isotopic measurements of complexly intergrown uranium minerals and oxygen isotopic analyes of uraninite from the unconformity-type Cigar Lake uranium deposit. Secondary uranium minerals intergrown with uraninite, such as coffinite, USiO 4· nH 2O and calciouranoite, CaU 2O 7·5H 2O, were identified by high-resolution transmission electron microscopy (HRTEM). In situ U–Pb results from three stages of uraninite and coffinite define well-correlated arrays on concordia with upper intercepts of 1461±47, 1176±9, and 876±14 Ma (±1 σ). These ages are interpreted as the minimum ages of mineralization correlate with the timing of clay mineral alteration (∼1477 Ma) associated with these unconformity-type uranium deposits, the ages of magnetization events at 1600–1450 and 900 Ma from the Athabasca Basin, and the Grenvillian Orogeny at ∼1100 Ma. In situ U–Pb isotopic analyses of uraninite and coffinite can document the Pb*/U heterogeneities that occur on the scale of 15–30 μm, thus providing relatively accurate information regarding the timing of fluid interactions associated with the evolution of these deposits. The high spatial resolution and precision of the ion microprobe allow us to measure δ 18O values of 20–100 μm unaltered portions of uraninites from Cigar Lake. The range of δ 18O values (−33.9 to −20.4‰) are among the lowest reported for unconformity-type deposits, confirming that conventional fluorination analyses of material sampled at the mm-scale are insufficient to avoid contamination from isotopically heavier coffinite and calciouranoite.