Recent mobilization of U-series radionuclides in the Bernardan U deposit (French Massif Central)

Several nuclides of the 238U (234U, 230Th, 226Ra, 210Pb) and 235U (231Pa, 227Ac) decay series have been measured through gamma spectrometry in the U-bearing episyenites of the Bernardan mine (northwest Massif Central, France), in order to study recent mobilizations in the U-deposit and their time-scales. (230Th/238U) and (231Pa/235U) ratios show that U has been little affected during the last 350 ky, especially in the U-rich samples. In contrast, 226Ra was subjected to a general redistribution, with a clear tendency for Ra to be lost from the U-rich samples and added to U-poor samples. The highest Ra excesses are present in low-U samples, and are related to adsorption onto iron hydroxides. Ra appears to have been redistributed within the mineralised episyenitic bodies, which seem to remain in 226Ra–238U radioactive equilibrium, a result consistent with the earlier study of Leroy [1984. Episyénitisation dans le gisement dʹuranium du Bernardan (Marche): Comparaison avec des gisements similaires du Nord-Ouest du Massif Central français. Miner. Depos. 19, 26–35]. As shown by the 226Ra and 210Pb data, the Ra redistribution was very active between 8 and 0.1 ky, and this might be a consequence of the hydrological changes in groundwater circulation accompanying the Holocene climate warming.