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Cenozoic oxidation episodes in West Africa at the origin of the in situ supergene mineral redistribution of the primary uranium orebodies (Imouraren deposit, Tim Mersoï Basin, Northern Niger)

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Abstract

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.

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Acknowledgments

ORANO is thanked for providing scientific support for this study, which formed part of the PhD Thesis of Marah Mamane Mamadou. The authors would like to acknowledge the stimulating discussions with Dr. Denis Virlogeux from DGS ORANO on the geology of Tim Mersoï for manuscript improvement. The authors would also like to thank the GeoRessources laboratory for providing analytical data on SEM & EMP and the Petrographic and Geochemical Research Center (CRPG) for providing SIMS. This work also has benefited from access to the analytical platforms provided by the National Research Agency (ANR) via Labex Ressources21. Reviews and edits by M. Guiraud and anonymous reviewer as reviewers, and G. Beaudoin and M. Fayek as editor and AE editor, respectively, have significantly improved this manuscript, and are very much appreciated.

Funding

This study received financial support from the ORANO via CREGU.

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Authors and Affiliations

  1. GeoRessources, CNRS, CREGU, Faculté des Sciences et Technologies Campus des Aiguillettes – “Entrée 3B”, Université de Lorraine, Rue Jacques Callot, B.P. 70239, F-54506, Vandœuvre-lès-Nancy cedex, France

    Marah Mamane Mamadou, Michel Cathelineau & Rémi Schmitt

  2. Centre de Recherches Pétrographiques et Géochimiques, UMR 7358, CNRS, Université de Lorraine, 15 rue Notre Dame des Pauvres, BP 20, 54501, Vandœuvre-lès-Nancy, France

    Etienne Deloule

  3. ORANO, BG Mines, Direction Géosciences, 125 avenue de Paris, F-92320, Chatillon, France

    Marc Brouand

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  1. Marah Mamane Mamadou
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Correspondence to Marah Mamane Mamadou.

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Mamane Mamadou, M., Cathelineau, M., Deloule, E. et al. Cenozoic oxidation episodes in West Africa at the origin of the in situ supergene mineral redistribution of the primary uranium orebodies (Imouraren deposit, Tim Mersoï Basin, Northern Niger). Miner Deposita 55, 1333–1352 (2020). https://doi.org/10.1007/s00126-019-00945-w

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