Alteration-mineralization at the Bernardan U deposit (Western Marche, France); the contribution of alteration petrology and crystal chemistry of secondary phases to a new genetic model

The Bernardan uraniferous district is located in the Western Marche Complex (French Massif Central) and is represented by disseminated uranium deposits. The U orebodies occur as episyenitic pipes: vuggy granites that resulted from quartz leaching. Observations of deeper structures confirm the widespread occurrence of coffinite and the absence of visible primary U minerals. Barren episyenites also exist and may be either connected or unconnected with uraniferous zones. This study describes the alteration history of the episyenitic pipes and discusses its effects on the present U distribution. The physicochemical conditions of alteration events are supported by detailed alteration petrography, fluid inclusion studies, stable isotope data, and rare earth element contents of secondary minerals including carbonates, fluorite, and clay minerals of the illite-smectite conversion series. Isotopic ages have been collected using K-Ar radiochronology on alkali feldspars and illite-smectite mixed layers. From the petrography of episyenites, three successives crystallization stages are evident: 1. Geodic crystallization of secondary K feldspars + or - quartz + dolomite +- fluorite occurred in all types of episyenitic pipes. This stage followed the formation of episyenites (308-315 Ma). Secondary minerals began to form from saline and 18 O-rich fluids at high temperature, up to 360 degrees C for dolomite, in closed systems, as indicated by the absence of significant fracture network. 87 Sr/ 86 Sr ratios in carbonates are compatible with the values of alumino-potassic granitic bodies from the Western Marche Complex. This event is interpreted as the result of progressive cooling of residual fluids derived from fractional crystallization of the leucogranites of the Bernardan deposits. 2. The second stage occurred in the Dogger epoch (170-140 Ma). It consisted of argillization of some episyenitic pipes in response to infiltration of 18 O-rich fluids that originated in sedimentary basins. This alteration stage occurred at 100 degrees C. The close association of U silicates with clay minerals suggests that U was transported by these fluids. However, its source may not lie in the sedimentary basins. 3. The distribution of mineralization has been modified by present-day supergene mobilization which has led to oxidation and the formation of high-grade ore near the top of the deposit and in horizons adjacent to major faults or fractures subject to infiltration.