Geologic environment, mineralogy, and fluid inclusions of the Bois Noirs-Limouzat uranium vein, Forez, France

The genesis of the Bois Noirs-Limouzat uranium deposit involves a very complex history. The Bois Noirs granite resulted from the anatexis of uranium-rich sediments near granulite facies conditions. The temperature is estimated to have been at least 800 degrees C accompanied by a low H ₂ O partial pressure (possibly resulting from the presence of carbon dioxide). This magma was syntectonically emplaced along the east-west structures in a nonmetamorphic environment. Progressive crystallization and differentiation proceeded inward from the margins toward the core of the intrusion. A fluid phase formed after a large part of the magma had crystallized. This fluid migrated toward the core and altered the primary magmatic minerals, quartz, orthoclase, oligoclase, and biotite, to quartz, microcline, albite, and chlorite. The alteration of the primary accessory minerals, sphene, zircon, monazite, and xenotime, resulted in the partial liberation of their uranium content. When all the magma had crystallized, the fluid phase migrated outward to precipitate uraninite, with an associated quartz-muscovite alteration. This critical step produced an easily leachable uranium source in the granite in the form of uraninite.Thermal activity was maintained in the Bois Noirs area from the emplacement of the granite, at 335 m.y., until the intrusion of the quartz porphyry, at 270 m.y. The enrichment of the Bois Noirs granite in K, U, and Th could have provided an essential or additional heat source for the hydrothermal convective circulation. Water of a supposed meteoric origin, mixed with carbon dioxide of a deep origin, produced the remobilization of the uranium contained in uraninite. The east-west foliation of the granite was reopened and tectonic activity produced open zones filled with porous, crushed, granite breccias. The fluid which dissolved the uraninite crystals in the granite was rich in carbon dioxide, and the uranium was transported mainly in the form of uranyl monocarbonate (UO ₂ CO ₃ ) ⁰ with a significant amount of H ₂ S in solution. The fluid pressure, which was nearly lithostatic in the deep zones of the granite (265 to 775 bars), became progressively hydrostatic (100 to 300 bars) near the breccia zones where pitchblende was deposited. The CO ₂ concentration decreased from a maximum of 3 mole percent to less than 1 mole percent. Part of the remaining dissolved CO ₂ could have reacted with ions such as Al (super +3) and Ca (super +2) liberated by the alteration of the plagioclase to form a carbonate phase. The uranyl carbonate complex was destabilized and the hexavalent uranium was immediately reduced by the oxidation of the reduced sulfur species in solution.Marcasite and pyrite deposition alternated with pitchblende. Oxidation of the reduced sulfur species in solution during pitchblende deposition stopped the growth of iron sulfides. Pitchblende deposition took place between 77 degrees and 100 degrees C.After this initial stage five other stages of mineral deposition occurred under different conditions of temperature and fluid composition. Pitchblende was transformed to coffinite during the second stage without new uranium being introduced. Important "per descensum" remobilization of the primary uranium ore occurred during the Oligocene uplifting of the Montagne Bourbonnaise horst, with precipitation, mainly with bearings in a range of N 165 degrees to N 135 degrees , of hexavalent uranium minerals and sooty pitchblende.

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