Combined U-Pb isotopic signatures of U mill tailings from France and Gabon: A new potential tracer to assess their fingerprint on the environment

Uranium milling activities have produced high volumes of long-lived radioactive processed wastes stored worldwide in near surface environment. The aim of this study is to highlight relevant tracers that can be used for environmental impact assessment studies involving U mill tailings. A multi-tracer study involving elemental content, U-238 decay products disequilibria and stable Pb isotopes was performed in different types of U mill tailings (alkaline, acid, neutralized acid) collected from five Tailings Management Facilities in France (Le Bosc, L'Ecarpiere, Le Bernardan, and Bellezane) and Gabon (Mounana). Our results showed that U and Pb concentrations range between 30 and 594 ppm and 66-805 ppm, respectively. These tailings have a strong disequilibrium of (U-234/U-238) and (Th-2(30)/U-238) activity ratios (1.27-1.87 and 6-65, respectively), as well as higher Pb-206/Pb-207 (1.86-7.15) and lower Pb-208/Pb-207 (0.22-2.39) compared to geochemical background ((U-234/U-238) and (Th-230/U-238) equal to unity; Pb-206/Pb-207 = 1.20; Pb-208/Pb-207 = 2.47). In situ analyzes (SEM, SIMS) showed that Pb-bearing phases with high Pb-206/Pb-207 are related to remaining U-rich phases, S-rich phases and potentially clay minerals or oxyhydroxides. We suggest that the combination of the Pb-206/Pb-207 with the (U-234/U-238) ratio is a relevant tool for the fingerprinting of the impact of U milling activities on the environment.