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Resumen:
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Polymetallic mineralization hosted by rift marginal carbonate preserves a comprehensive archive that enables reconstruction of the initiation and evolution of rifting. The present study focuses on a Miocene shallow marine succession and related polymetallic (Fe-Mn-Zn) ores at the Um Greifat area along the Red Sea coast in the Central Eastern Desert of Egypt. The study area represents a unique Early-Middle Miocene syntectonic marginal carbonate platform of shallow marine and coastal plain facies. The carbonate-clastic succession comprises two units, lower and upper, that are separated by a magnesite bed. Dolomitization, karstification, fracturing and brecciation are the main diagenetic features, which allow reconstructing fluctuations of sea level induced by tectonic uplift and rifting.
The polymetallic nonsulfide mineralization comprises mainly Fe-Mn-Zn oxides, forming a stratiform and strata-bound ore bodies in the lower unit of the succession and/or lenticular and cavernous masses in the upper unit. The mineralogy is dominated by chalcophanite, hetaerolite, goethite, hematite and smithsonite besides zincite, pyrolusite, cryptomelane, franklinite, coronadite, quartz and alunite-natroalunite. Less common siderite, pyrite and chalcopyrite are observed. Crystal morphologies, texture and distribution of the ferromanganese minerals were modified depending on the textures, porosity, open-spaces and discontinuities of their host carbonate. The mode of occurrence, texture, mineralogy and geochemistry of the polymetallic ores suggest a two epigenetic-supergene origin. The early stage encompassed precipitation of sulfides, silica and/or circulation of highly acidic metal-rich hydrothermal solutions through faulted carbonate blocks. Dissolution and intensive oxidation of primary sulfides, Fe+2 and Mn+2 in oxic and supergenetic conditions resulted in the formation of polymetallic oxides that represent the late stage. Major uplifting and exhumation by the Red Sea extension, along with circulation of low-temperature hydrothermal fluids through faulted areas, initiated ore mineral precipitation under near-neutral to mildly acidic conditions. The interplay of diagenesis, deformation, tectonism and hydrothermalism was likely enhanced by rift-induced geothermal activity in the Red Sea basin and adjacent areas.
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