Uraninite-Water Interactions in an Oxidizing Environment

Abstract

Exceptionally low jg ¹⁸O values of primary uraninite and pitchblende (i.e. -32 per mil to -19.5 per mil) from Proterozoic unconformity-type uranium deposits in Saskatchewan, Canada, in conjunction with theoretical uraninite-water oxygen isotope fractionation factors suggest that primary uranium mineralization is not in oxygen isotopie equilibrium with clays and silicates. The low jg ¹⁸O values have been interpreted to have resulted from the recrystallization of primary uranium mineralization in the presence of modern meteoric fluids having low jg ¹⁸O values of ca. -18 per mil. The absence of apparent alteration in many of the uraninite and pitchblende samples requires that the uranium minerals exchange oxygen isotopes with fluids, with only minor disturbances to their original chemical compositions and textures. However, experiments on the interaction between water and natural uraninites, from these deposits, and detailed electron micro-probe analyses of natural uraninite and pitchblende indicate that, in the presence of water, old uraninite rapidly alters to curite (Pb₂U₅O₁₇4H₂O). The hydration of uraninite to curite releases uranium and calcium into solution and becquerelite (Ca(UO₂)₆O₄(OH)₆H₂O) is precipitated. In the presence of Si-saturated waters, uranium silicate minerals, soddyite ((UO₂)₂(SiO₄)2H₂O) and kasolite (Pb(UO₂)SiO₄H₂O are precipitated in addition to, curite and schoepite ((UO₂)₈O₂(OH)₁₂(H₂O)₁₂). The mineral paragenesis observed in these experiments is similar to sequences observed in oxidized zones in uranium deposits and UO₂-water experiments. Therefore, it is unlikely that natural uraninite and pitchblende can simply exchange oxygen with an oxidizing fluid without a concomitant change in phase chemistry or structure, nor will oxidation of uraninite lead to the formation of U₃O₇. as predicted by theoretical calculations used in natural analogue studies for the disposal of high level nuclear waste.