The Bertholène uranium deposit — Mineralogical characteristics and UPb dating of the primary U mineralization and its subsequent remobilization: Consequences upon the evolution of the U deposits of the Massif Central, France

Abstract

In the southern part of the Massif Central, France, U ores of the Rouergue area are spatially related to the unconformity of the Permian-Carboniferous sedimentary terranes overlying the Variscan basement. The Bertholène uraniferous orebody is located in the Palanges Precambrian orthogneiss, close to the unconformity and in the vicinity of a highly deformed part of the orthogneiss massif marked by the occurrence of mylonitic bands for which a Variscan age is assumed. The U ore is mainly expressed as coffinite, but primary U-oxide was also identified. Hydrothermal alteration stages have been defined: a first albitization stage is expressed on a regional scale and successive stages of argillization, silicification, and lastly carbonate crystallization have been described in the mining district. The U-oxide formation followed the silicification stage. UPb systematics were used on coffinite and U-oxide to define the evolution of the Bertholène deposit. 1. (1) Primary U-oxide was found to be 173±9 Ma old. This phase of U concentration (or remobilization) has also been dated in the two other U deposits of the southern part of the Massif Central (Lodève, Pierres Plantées) as well as in deposits located in Limousin and Morvan (France). The event which mobilized the U in these deposits is related to the consequences of the Liassic pre-rifting phase that preceded the opening of Ligurian ocean during Malm times. 2. (2) Complex phenomena have affected the coffinite UPb system; UPb and PbPb data show that: 2.1. (a) Unsupported isotopically heterogeneous radiogenic Pb was in the coffinites during their genesis and may reflect the role of primary U-oxide as their source. 2.2. (b) In some coffinites, Rn loss phenomena controlled mainly the evolution of the UPb system. Two main types of coffinites were identified and the UPb data indicate that the coffinitization event occurred during Oligocene times (40-30 Ma) or from the Oligocene to actual times.