Chert-Hosted Manganese Deposits in the Wahrah Formation: A Depositional Model

Abstract

The non-metamorphic, stratiform manganese deposits in the Wahrah Formation (Al Hammah Range, 125 km south of Muscat) are of upper Berriasian to Hauterivian age. The manganese enriched horizons are continuously exposed over several 100 km2. Sedimentological and geochemical features prove that the host rock of the manganese deposits exclusively consisting of red cherts are of biogenic, non-hydrothermal origin, deposited in a distal part of the Hawasina basin. A depositional (geochemical) environment comparable to that of a siliceous ooze facies is highly likely. Geochemically the manganese layered cherts and nodules, with their extremely high Mn/Fe ratio and the low minor element content, are similar to hydrothermal deposits associated with volcanic activity or to diagenetic manganese enrichments in hemipelagic regions in sediments with relatively high organic-C contents. Diagenetic shallow water deposits are geochemical similar to the Wahrah Formation enrichment also. Regional geological considerations, mineralogical, sedimentological and REE-patterns evidences in the Wahrah Formation are, however in conflict with these models. Therefore, we propose a alternative manganese transport and accumulation model. “Continentally” derived manganese is introduced to the sea in suspended and/or dissolved form and deposited on the continental margin and -slope. Upward migrating pore fluids, diagenetically enriched in manganese, are released to the water column and horizontally transported through the Oxygen-minimum zone towards the open basin. During selective element scavenging onto different sinking particles and the formation of manganese-rich microparticles, element separation takes place. Sedimentary features indicate that manganese microparticles, precipitated in the water column and the sea water sediment interface, are mechanically enriched to fine laminae and thicker layers. Hydrodynamically controlled separation processes during transport of clay minerals, manganese-rich microparticles and biogenic particles (main element carrier phases) explain the geochemical composition. Geochemical differences and similarities of nodular and layered ore types are attributed to later minor element redistributions.