Formation conditions of a tetragonal uranium oxide: α-U3O7 in the Brousse-Broquies Basin (Aveyron, France)

The uraniferous mineralization in the Brousse-Broquies Basin, spatially associated with the unconformity between a metamorphic basement and a Stephano-Permian nonmetamorphic cover, is very unique among the uranium deposits in Western Europe. It is essentially in the form of the tetragonal uranium oxide α-U3O7. The prism (100) and/or (110) is associated with the tetragonal dipyramid (hhl). Chemical analysis shows that Pb may have been separated out; this phenomenon is related with a recent recrystallization of these oxides as indicated by the globular nature of the crystal surfaces of α-U3O7. Deposition conditions have been deduced from mineral assemblages in the sequence of alterations and paragenesis. It implies a slightly alkaline pH. The temperature of formation, deduced from fluid-inclusion study, is 140 ± 250C and the fluid is a brine rich in NaCl and containing divalent cations. The presence of α-U3O7 for this temperature indicates a fO2 around 10-30 atm. 42 refs.; 16 figs.; 5 tabs