Uranium isotope variation within vein-type uranium ore deposits

Isotopic composition of uranium has previously been used to infer the depositional redox environment of ura-nium 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 U-238/U-235 and U-234/U-238 ratios. These characteristics are explored via high precision mass spectrometric mea-surement 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 delta U-238 values from -0.16 to +0.03 (+/- 0.04) parts per thousand and -1.6 to -64.7 (+/- 0.4) parts per thousand for delta U-234 (+/- 2SD). These variations can be attributed to redox-driven isotope fractionation processes and/or U redistribution during localised leaching and re-precipitation. Ana-lyses 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 parts per thousand for delta U-238 and 62.0 +/- 0.6 parts per thousand for delta U-234). These findings highlight the possibility of significantly different delta U-238 and delta U-234 of uranium ore concentrate from the same mine even if manufacturing processes remain unchanged.