Uranium isotope variation within vein-type uranium ore deposits

Highlights: • Localised U isotope heterogeneity is observed in vein type uranium ore deposits. • Changes in ²³⁴U/²³⁸U and ²³⁸U/²³⁵U are observed during acid leaching experiments. • These data suggest U isotope ratios change during conversion from ore to UOC. • Implications for the use of the uranium isotopic signature in nuclear forensics. Isotopic composition of uranium has previously been used to infer the depositional redox environment of uranium ore concentrates and also provide a potential signature to inform nuclear forensic investigations. This study evaluates the diagnostic power of the U isotope signature by investigating (1) the heterogeneity of U isotope compositions in samples collected from the same mine and/or vein, and (2) the influence of U ore processing on ²³⁸U/²³⁵U and ²³⁴U/²³⁸U ratios. These characteristics are explored via high precision mass spectrometric measurement of vein type uranium ore samples collected predominantly from mines located in central Portugal and Southwest England. Samples collected from the same vein and mine exhibit δ²³⁸U values from −0.16 to +0.03 (±0.04) ‰ and −1.6 to −64.7 (±0.4) ‰ for δ²³⁴U (±2SD). These variations can be attributed to redox-driven isotope fractionation processes and/or U redistribution during localised leaching and re-precipitation. Analyses of residues and leachates from small-scale batch experiments designed to simulate industrial U ore leaching procedures reveal significant positive and negative changes in isotope composition in the leachate relative to the bulk material (up to 0.21 ± 0.06‰ for δ²³⁸U and 62.0 ± 0.6‰ for δ²³⁴U). These findings highlight the possibility of significantly different δ²³⁸U and δ²³⁴U of uranium ore concentrate from the same mine even if manufacturing processes remain unchanged.