Geological and geochemical constrains on the genesis of the sedimentary-hosted Bou Arfa Mn(-Fe) deposit (Eastern High Atlas, Morocco)

Carbonate-hosted Mn deposits are widespread in North Africa with most of the Mn ores being hosted by Mesozoic dolomitic formations of the Moroccan Atlasic system. The Bou Arfa Mn(-Fe) deposit described herein is located in the eastern High Atlas, close to the North Atlasic Front, and has accounted for the production of ~2 Mt of ore grading from 33 to 82% Mn. The mineralization occurs within Sinemurian flat-lying dolostones as stratabound and karst-filling (veins, lenses, pockets). This mineralization is characterized by the extensive presence of pyrolusite, and minor concentrations of manganite, hausmannite, goethite and hematite. Previous studies suggested that the Bou Arfa Mn-bearing mineralization has a strictly synsedimentary origin, but the intense dolomitization and the occurrence of Mn oxides instead of Mn carbonates question this model. New petrographical, mineralogical and geochemical data show that three dolomitization phases accompanied the ore formation. The early synsedimentary dolomitization was not responsible for strong Mn enrichments, whereas a second dolomitization took place with the formation of hausmannite and manganite during early diagenesis. The precipitation mechanism involves circulation of dissolved Mn ²⁺ in pore waters, and then precipitation under diagenetic suboxic and alkaline conditions. The scarcity of Mn carbonates is likely due to competitiveness of dissolved carbonates species that have favored dolomite precipitation in the early stage. Presence of saddle dolomite in a subsequent alteration stage, crystallography of the Fe oxides, and evolution of δ ¹³C and δ ¹⁸O reflect burial diagenesis and fluid mixing in the carbonate host rock. These conditions have enabled the transformation of the primary ore into pyrolusite, when diagenetically-derived hydrothermal fluids have generated vein-filling ore. The position of the Bou Arfa site above a paleohigh formed during Atlas rifting, the presence of a faulted zone and the dolomitization of the Sinemurian series have delimited the extension of this diagenetic Mn deposit, which is in some points similar to MVT deposits of North Africa. Weathering is poorly recorded in the Bou Arfa Mn ores due to the stability of the ore-forming minerals under subaerial conditions in relation to the most recent periods of exposure of the High Atlas building.