Effects of mass transfer, compaction and secondary porosity on hydrothermal upgrading of Paleoproterozoic sedimentary manganese ore in the Kalahari manganese field, South Africa

In the northwestern part of the Kalahari manganese field low-grade carbonate-rich Mamatwan-type ore is altered to high-grade oxide-rich Wessels-type ore in association with normal faults. Mass balance calculations, based on the assumption that manganese was geochemically immobile, suggest that upgrading of the manganese ore can be attributed to leaching of Mg, Ca, CO2 and SiO2 from the sedimentary ore with residual enrichment of Mn. Hydrothermal alteration resulted in development of about 10 to 20% of secondary porosity in the ores and the orebed was compacted to two thirds of its original stratigraphic thickness.