Abstract

The uranium deposits of the La Crouzille district which are the subject of this study (Margnac and Fanay mines) are located in the Saint-Sylvestre granitic plutonic complex, approximately 20 km north of Limoges (Haute-Vienne). The Saint-Sylvestre pluton in its entirety and the Saint-Sylvestre granite in particular were first reinterpreted on the basis of new observations made in the mines along the contact zone between the Brame and Saint-Sylvestre granites (Peny area). A similar origin for members of the Saint-Sylvestre plutonic complex, recognized earlier by Chenevoy (1958), was confirmed. Structural information (synthesized by Autran and Guillot, 1974) and isotopic data (Duthou, 1977) are reinterpreted in a new tectonic framework. The genesis of the leucogranitic Saint Sylvestre pluton is believed to be comparable to that of Himalayan leucogranites.The existence of two granitic facies (Brame and Saint-Sylvestre) is due to magmatic and deuteric processes (muscovitization, albitization) operating within the same plutonic complex. These phenomena obliterated the primary features and caused a reequilibration of the initial catazonal paragenesis (sillimanite-orthoclase) to mesozonal conditions (muscovite-biotite).The emplacement of the two granitic facies took place around 350 to 360 m.y. The age determination of 315 m.y. is now interpreted to be the result of partial reequilibration of Rb-Sr isotopes.The intrusion of lamprophyres at 285 m.y. corresponds with brittle fracture tectonic activity, in contrast to the earlier plastic deformation. A similar sequence of magmatic and tectonic styles is found elsewhere in the Hercynian chain. Two fracture systems are present (EW-NS and NW-SE). Lamprophyric magmatic activity produced localized hot spots. This heat flux caused the reheating of the cold waters which impregnated the plutonic complex and produced a convective fluid circulation as in modern geothermal systems. The fluids were channeled into faults and fractures of the east-west-north-south fracture system causing the micaceous episyenitization of the granite (dissolution of quartz, total muscovitization of plagioclase and biotite, and partial replacement of orthoclase). An earlier type of episyenite (feldspathic episyenite produced by quartz dissolution, chloritization of biotite, and feldspathization of muscovite) is considered to have been produced during the plastic P 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.

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