Protores and Country Rocks of the Nsuta Manganese Deposit (Ghana)

Nsuta (Ghana) is one of the most important Precambrian manganese deposits of the world. It is hosted in a package of metamorphosed volcanic and sedimentary rocks belonging to the 2.1 Ga old Birimian Supergroup. Metavolcanic greenstones, derived from a basaltic to andesitic protolith contain intercalations of various carbonate phyllites and silicate marbles. These metasediments which represent initial mixtures of pelitic to psammitic siliciclastic material and carbonates are, especially the marbles, enriched in manganese. They represent the protores of the Nsuta deposit, which were later upgraded to minable ores by supergene weathering. The primary ore-forming process occurred in a shallow-water shelf environment, where manganese carbonates were precipitated below the redox interface, i.e. under mildly reducing conditions. Mineral assemblages in the greenstones and metasediments testify to peak-metamorphic conditions of roughly 500-degrees-C and 5 kbar fluid pressures. During this stage, the fluid phase was dominated by H2O. The retrograde metamorphic evolution is documented in chloritization of biotite and garnet, replacement of actinolite by chlorite and calcite, and breakdown of kutnahorite to form submicroscopic domains of calcite, rhodochrosite and ankerite. Judging from reequilibration of aqueous fluid inclusions and the subsequent trapping of a CO2-dominated fluid phase, the retrograde uplift path was anticlockwise.