Abstract
For more than 80 years of mining the economically important Imini district has been the main producer of manganese in Morocco. Three stratabound manganiferous orebodies in the district are hosted in a 10-m-thick Cenomanian-Turonian dolostone that records the effects of laterally extensive dissolution and epigenetic processes. The predominance of pyrolusite, accompanied by minor amounts of hollandite group minerals (cryptomelane, hollandite s.s., coronadite), lithiophorite, and romanechite, provide a high grade ore having more than 70 wt% MnO and low contents of SiO2, Fe2O3, and P2O5. One metallogenic model suggests the main mineralization stage was related to a period of terrestrial exposure and weathering, leading to formation of an extensive karst system, following host rock deposition and dolomitization. This model envisages Mn mineralization prior to the deposition of Coniacian-Maastrichitan (Senonian) red beds, carbonates, and evaporites. A variety of data shows that the ore beds formed by epigenetic processes, in which Mn precipitation occurred with increasing pH and Eh. These physicochemical conditions are linked to dolomite dissolution and the influx of oxygenated meteoric waters, probably in contact with deeper O2-depleted meteoric groundwaters, and to surficial saline solutions and/or microbial activity. The Cenozoic construction of the Atlas Mountains probably remobilized or at least reworked previously formed manganese accumulations.
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Acknowledgments
We are grateful to the staff of the Imini mine and to Société Anonyme Chérifienne d’Etude Minière (SACEM) for hospitality, field authorizations, and guidance on sampling sites. We warmly thank Prof. A. Bernard of the Université Libre de Bruxelles for SEM images and analytical advice, as well as Y. Missenard and B. Saint-Bézar for helpful discussions on structural features of the Imini area.
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Dekoninck, A., Leprêtre, R., Saddiqi, O., Barbarand, J., Johan, Y. (2016). The High-Grade Imini Manganese District—Karst-Hosted Deposits of Mn Oxides and Oxyhydroxides. In: Bouabdellah, M., Slack, J. (eds) Mineral Deposits of North Africa. Mineral Resource Reviews. Springer, Cham. https://doi.org/10.1007/978-3-319-31733-5_22
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