Résumé
Fluid circulation at the unconformity of major basins with their basement is the source of avariety of mineralisations. This is the case, for example, with the uranium mineralisation at thebase of the Proterozoic Athabasaca (Canada) and McArthur (Australia) basins. In theStephano-Permian Lodève Basin in south-eastern France, uranium mineralisation is locatedclose to the unconformity. The basin has a half-graben geometry controlled by steeply north-dipping faults in the south of the basin. The mineralisation is mainly prevalent in the northernpart of the basin, which has undergone minimal burial. The mineralisation is mainlyconcentrated in a faulted zone with conjugated normal faults dipping to the south as seen atthe former Mas d’Alary operation, and also rich in levels of organic matter from the base of thePermian series. A simple idea is that oxidising fluids carrying uranium (in the form of uranyl)were collected per ascensum in these faults, possibly from the unconformity. When these fluidscame into contact with reducing lithologies, they precipitated their uranium by crystallisation ofuranium oxide. Independently, in the northern part of the Lodève Basin, the unconformity ofPermian series on Palaeozoic sediments (Cambrian dolomites or Carboniferous sandstonesand mudstones) is exposed in numerous outcrops. Very often, the discordant contact showstraces of silicification, sometimes crosscutting the bedding, sometimes pervasive along thestrata, especially the coarsest; the Autunian conglomerates, for example, show spectacularevidence of dissolution of carbonate pebbles associated with multimetric silicification. Thequestion we are addressing here is to clarify the possible link between the circulation of fluidsat the origin of the silicifications at the base of the Lodève Basin and that necessarily involvedin the formation of the uranium deposit. Our strategy is to combine field analysis withmineralogical analysis to determine the nature of the uranium-bearing phases in thesilicifications. In fact, radioactivity anomalies measured in the field indicate that uraniumenrichment does indeed exist, at least locally, in the silicifications. A numerical hydro-thermalmodelling approach will complement the analytical work and the conceptual model of fluidcirculation. From this work, carried out as part of the Uralod project (NEEDS program), wehope to obtain general information on the significance of silicification in uranium mineralisationassociated with fluid circulation along the unconformity.