The uranium mineralization of Pen Ar Ran (Armorican Massif), France: An atypical "vein type" deposit
Résumé
Twenty percent of the uranium produced in France was extracted in the Armorican Massif, in three different locations (Pontivy, Guérande et Mortagne) which belong to the "High Heat Production Belt", a 100 km wide NW-SE zone characterized by elevated contents in radioactive elements present in most of the geological formations [1], including 320 to 315 Ma old peraluminous granites [2]. In the Guérande uranium district, the Pen Ar Ran deposit is located in a deformed zone, at the contact between porphyroids and quartzitic black schists. This deformed zone corresponds to an E-W shear zone that affected both the metamorphic formations (porphyroids and black schists) and the intrusive Guérande leucogranite, during its emplacement. Mineralization is under the form of massive veins carrying pechblende and sulphide that are spatially associated with the leucogranite. However, this uranium deposit is different from the other "vein type" known in France because of the unusual nature of its uranium oxides (between pechblende and uraninite). Furthermore, the fluids responsible for this mineralization are hotter (350-400°C) than elsewhere. The question of the source of the uranium is also complex. The uranium content of the metamorphic formations is rather low (around 3 ppm) but it is also low in the leucogranite (max. 9 ppm). It is however possible that the uranium was leached out of the granite by surface derived oxidizing fluids while the granite was still at depth [3]. Therefore, the leucogranite could represent the source of the uranium. In order to test this hypothetis, we performed a comprehensive petrological, geochemical, and geochronological study of the Guérande granite and compared these data to the available data from the porphyroid and the uranium mineralization. [1] Vigneresse, J.L., Cuney, M., Jolivet, J., Bienfait, G. (1989. Tectonophysics, 159, 47-60. [2] Tartèse, R., Ruffet, G., Poujol, M., Boulvais, P., Ireland, T.R. (2011). Terra Nova, 23, 390-398. [3] Tartèse, R., Boulvais, M., Poujol, M., Gloaguen, E., Cuney, M. (2013). Economic Geology, 108, 379-386.