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International audience; Uranium (U) deposits can undergo oxidizing events, subsequent to which the U may be redistributed or precipitated under distinct mineralogical forms. Occurrences of U6+ minerals can be the result of either (i) in situ oxidation and alteration of previous U4+ minerals or (ii) direct precipitation from U6+-rich solutions. Samples from Imouraren, the largest U6+ore deposit known in the world, located in the Tim Mersoï Basin, Northern Niger, have been studied to determine the nature, distribution, and timing of occurrences of U and related minerals. The host rock is composed of sandstones and analcime-rich siliciclastic rocks of Jurassic age. The U ore is largely dominated by acicular hexavalent U minerals and by a very small amount of uraninite. A comprehensive geochemical model that describes the origin and geochemical setting of this style of economically important mineralization has been developed. The petrographic and mineralogical studies show evidence of in situ uraninite alteration into uranophane and metatyuyamunite, documented by textural relationships and chemical analyses using SEM and electron microprobe. The textural relationship between the U4+ and U6+ minerals is supported by the near-identical REE signatures of the primary uraninite and related secondary U minerals. Ion microprobe U-Pb dating (from 21 to ~ 1 Ma) confirms that primary uraninites were altered to U6+ minerals under supergene conditions during Cenozoic oxidation events in western Africa, in relation with the well-known stages of intense continental weathering that have affected western Africa from the early Cenozoic to recent times.

Faults are mostly considered as low-permeability zones due to their frequent sealing, but they can also contribute significantly to fluid movements in sedimentary basins, and subsequently to temperature anomalies. In favorable cases, paleo-temperatures and pressures can be determined from fluid-inclusion and chlorite thermometry. Using this approach, we estimate the temperature gap between incoming fluids and host rocks along the Arlit Fault, a major fault affecting the whole series of continental siliciclastic formations from Paleozoic to Late Jurassic in the Tim Mersoi Basin (Niger). Strong alteration halos and uranium deposits occur along this regional north–south-trending fault, as well as a secondary network of NE–SW faults. Sandstone porosity is partially reduced by authigenic quartz and Fe-rich trioctahedral chlorites. The chlorites precipitated directly from the solutions under reducing conditions, prior to uranium-oxide deposition. The temperatures and pressures are estimated to be around 115–150 °C and 80–120 bars respectively, and are remarkably similar in all formations from the Carboniferous to the Jurassic. These temperatures are much higher, around 60–80 °C, than those estimated at maximum burial, and document hot fluid flow along the major fault. This thermal anomaly is inferred to be related to the opening of the Atlantic Ocean during the Cretaceous, before the onset of the main exhumation stages during the mid-Tertiary. This study documents the role of faults as significant conduits for fluid flow, and their role in the mass and heat transfer related to the formation of economic deposits.