U-Pb sytematic analysis and the formation of the Lodeve uranium deposit (France)

In the Lodeve basin, the uraniferous mineralization associated with the autunian pelitic layer 0 is usually considered as syngenetic or early diagenetic. U-Pb isotopic data performed on U bearing whole rocks demonstrate occurrence of radon losses, mainly in the 238U decay series; the 207Pb-235U geochronometer is particularly suitable to date U-Pb systems disturbed by such radon losses. For the autunian layer 0, two U-Pb mobilization phases have been respectively recognized at 173+-6 Ma and 108+-5 Ma. The oldest phase is the most clearly expressed in the studied samples, no memory of a permian age could be recognized in the U-Pb systems. The first U-Pb mobilization occurred 170 Ma ago, during a distension phase of the continental crust. This phase induced circulations of mineralized fuilds, illite recristallization in the permian pelites at temperatures ranging from 100 to 2000C and emplacement of a mantellic volcanism recently dated at 155+-6 Ma. A galena from the Autunian of the Lodeve basin, the feldspars of the surrounding variscan granitoids and galenas of mesozoic stratiform deposits in the ''Causses'', present similar Pb isotopic composition, which is in agreement with the hypothesis that some metals of the autunian pelites originated in the surrounding weathered variscan continental crust. (orig.)