REE, Y and U concentration at the fluid–iron oxide interface in late Cenozoic cryptodolines from Southern Belgium

Cryptodolines dug in Mesozoic U-rich calcarenites from the Mons Basin (S. Belgium) have been studied, to determine to which extent U and associated trace elements (Th, REE) have been mobilized during the weathering event(s). Special attention is paid to the trace elements immobilization/trapping processes. Analytical methods used include major/trace elements geochemistry (emission ICP; ICP-MS), U/Th isotope geochemistry (α-spectrometry) and mineralogy (XRD, SEM, microprobe). Our results indicate that, although weak in intensity, the weathering process resulted in selective 234U (vs. 230Th) mobilization, in the last 750 Kyr. A recent (Quaternary) age is thus proposed for the weathering process. Mobilized U(VI) is concentrated in U-rich (Y, REE)-phosphate (xenotime/rhabdophane/churchite) coatings, neoformed on Fe–Co–Mn oxyhydroxides resulting from the weathering of the calcarenites. The neoformation process of U-rich accessory minerals on high-surface-area host minerals is of special interest in the containment of mobilized radionuclides within the immediate vicinity of surface radioactive waste deposits undergoing weathering processes.