The Margnac and Fanay uranium deposits of the La Crouzille District (western Massif Central, France); geologic and fluid inclusion studies
J. Leroy
4 deformation which folded the Saint-Sylvestre plutonic complex into an anticlinorium (NNE to NE fold axes).Pitchblende and pyrite mineralization immediately followed the emplacement of the lamprophyres and the micaceous episyenitization of the granite and a minimum age of 275 m.y., rather than 240 to 250 m.y., is accepted. The mineralization was precipitated from a CO 2 -rich fluid by the unmixing of complex CO 2 -H 2 O mixtures following a drop in pressure. The temperature of the solutions in the 132 episyenite column of the Margnac mine was approximately 345 degrees C.After pitchblende deposition, the fluid became progressively more water-rich during the precipitation of microcrystalline quartz and hematization of pyrite. The deposition of quartz plus marcasite and the transformation of pitchblende to coffinite took place during falling temperatures (330 degrees to 140 degrees C) related to the progressive decay of the geothermal system. The formation of the deposit ended with the deposition of fluorite, barite, and calcite, beginning at 135 degrees C and continuing to a lower temperature.Supergene modification caused the formation of secondary minerals (black products, gummite, etc.) and the adjustment of the deposits and prospects to the present erosion surface. These supergene modifications are recent. Based on a reinterpretation of U-Pb isotopic measurements, they date from about 30 m.y. and are still continuing to some extent.
This record provided courtesy of AGI/GeoRef.
