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Evolution of the Oman Tethyan Continental Margin |
Mineralogisch-Petrogr. Institut, Baltzerstrasse 1, CH-3012 Bern, Switzerland
The host rocks of the manganese in the Al Hammah Range (125 km S of Muscat) are red radiolarian cherts of Tithonian to Neocomian age, continuously exposed over 1000 km2. The stratigraphic sequence starts with fine-grained turbiditic limestones followed by a transition zone of coloured silt-mudstones and cherts which grade into almost CaCO3– free cherts, topped by 1–5 m of silicified limestones. The base of the Wahrah Formation is unknown. Sedimentological and geochemical features indicate that the cherts are of biogenic, non-hydrothermal origin. The stratiform manganese deposits are the result of sedimentary and tectonic enrichment processes. The single Mn-layers formed during periods of condensed sedimentation at the sediment-water interface. The different Mn-chert types (disseminated and layered ore and black Mn-rich cherts) can be explained by varying sedimentation rates of radiolaria, clay and manganese precipitation. Geochemically, the manganese deposits are characterized by high Mn/Fe ratios and by an extremely low minor element (e.g. Ni, Cu, Co) content. Using geochemical discrimination diagrams, the manganese deposits of the Wahrah Formation would be of hydrothermal origin. This contrasts with the sedimentary characteristics of the manganese-bearing cherts and with the field observations. Because of the Tithonian to Neocomian age of the Wahrah Formation cherts, the manganese enrichment can not be related to the formation of the Semail ophiolite. It is proposed that they might relate to a long distance discharge from the Owen fracture zone during the formation of the Masirah ophiolite.
