Effects of cationic substitutions and alteration in uraninite, and implications for the dating of uranium deposits

Uraninite grains from two sites of unconformity-type uranium mineralization, the high-grade basement-hosted McArthur River deposit and the sediment-hosted Virgin River prospect in the Paleoproterozoic Athabasca Basin in northern Saskatchewan, Canada, were investigated with a scanning electron microscope and analyzed with an electron microprobe. Electron back-scatter images, chemical compositions and calculated chemical U/Pb ages from the uraninite indicate substitution of radiogenic Pb by Ca, Si, and Fe from later fluid-circulation events after initial formation of the uraninite at ca. 1600 Ma. Conversion of uraninite to coffinite,with SiO2 UP to 10 wt%, seems to have occurred at ca. 700 Ma. The intensity and the character of the substitutions depend on both the amount and the composition of the alteration fluids, which may be influenced by the characteristics of the host rocks.