Fault-controlled metasomatic alteration of Early Proterozoic sedimentary manganese ore at Mamatwan Mine, Kalahari manganese field, South Africa

The Kalahari manganese field is situated some 60 km northwest of Kuraman in the Northern Cape Province. It is the largest known land-based manganese deposit. Low-grade sedimentary Mamatwan-type ore (containing up to 38 mass % Mn) dominates and consistitute about 97% of the ore reserves estimated approximately 13 500 Mt. The ore is composed of finely intergrown braunite and Mn-ricn carbonates such as kutnahorite and Mn-calcite. Four distinct alteration events which affected the braunite. lutite since its deposition can be observed at Mamatwan mine. Relativc age relationships between these events are established, but the absolute timing of the events remains unknown. These four alteration events are: a) low-grade metamorphism associated with metasomatism along specific stratigraphic zones within the ore body; b) reduction of ore around veins and joints with hematite-calcite-sulphide mineralization caused by epithermal fluids; c) intensive brecciation along faults associated with the introduction of groundwater-related fluids; and d) supergene weathering along the sub-outcrop of the ore body beneath the Kalahari unconformity by descending meteoric fluids.