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. B elgium) have been studied, to determine to which extent U and associated tr ace elements (Th, REE) have been mobilized during the weathering event(s). Special attention is paid to the trace elements immobilization/trapping pro cesses. Analytical methods used include major/trace elements geochemistry ( emission ICP; ICP-MS), U/Th isotope geochemistry (alpha-spectrometry) and m ineralogy (XRD, SEM, microprobe). Our results indicate that, although weak in intensity, the weathering process resulted in selective U-234 (Vs. Th-23 0) mobilization, in the last 750 Kyr. A recent (Quaternary) age is thus pro posed 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 radion uclides within the immediate vicinity of surface radioactive waste deposits undergoing weathering processes. (C) 1999 Elsevier Science B.V. All rights reserved.