Geology and origin of the manganese ore of Um Bogma, Sinai

The manganese deposits of Um Bogma form lenses in dolomitic beds at the base of a Carboniferous sequence. The ore lenses are 1 to 200 meters in diameter, and up to 6 meters thick. The larger lenses are differentiated into manganese-rich cores and iron-rich margins. The manganese minerals include divalent and tetravalent oxides and hydroxides : psilomelane, pyrolusite, polianite, manganite, hausmannite and pyrochroite. Hematite and goethite are the main iron-bearing minerals. From stratigraphie and pétrographie evidence, it is concluded that the ore-deposit is of sedimentary origin, formed syngenetically in tidal pools. The chemical and mineralogie differentiation of the lenses was a post-depositional process, and resulted, presumably, from Eh-pH differences between the cores and the margins.