Abstract

The economically important high-grade manganese ores of the Imini district, Morocco, are exceptional because of their unusually high Mn/Fe ratios and exceptional enrichment in Ba and Pb. There is ample evidence that the three strata-bound manganese orebodies of the Imini district formed in a laterally extensive karst cave system, associated with internal sediment, and developed in a shallow marine dolostone succession of Cretaceous age. The manganese ores occur in dolostone breccias and ferruginous clays that represent the earliest phase of internal sediment in the cave system. Later phases of cave fill are ferruginous without manganese enrichment. Ore formation, karstification, and meteoric dolomitization are all related to an extended period of exposure and terrestrial weathering, prior to the deposition of terrestrial red beds and evaporites of Upper Cretaceous age that overlie the ore-bearing dolostone succession above an erosional unconformity. The manganese ores formed when warm, acidic Mn2+-bearing meteoric water migrated from the elevated regions of the Anti Atlas region into the exposed carbonate succession. Alkali feldspar-rich igneous basement rocks were the source for Mn, Pb, and Ba. Metals were deposited in a zone of mixing between metal-bearing, reducing meteoric water and oxygenated ground water resident in the cave system.

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