Low-grade Mamatwan-type sedimentary manganese ore has been upgraded to high-grade Wessels-type ore in the Kalahari manganese field. The alteration appears to be of hydrothermal (metasomatie) origin closely related to zones of ferruginization (hematitization) developed along major north-south- and minor east-west-trending normal faults and fractures. The faults and fractures appear to have acted as channelways for the hydrothermal fluid eausing metasomatie alteration of the manganese ore.Manganese ores display a very consistent zonation from zones of ferruginization along faults to unaltered sedimentary manganese ore in the cores of fault blocks. This zonation begins with least altered Mamatwantype sedimentary ore, finely laminated, and composed of finely intergrown braunitc and Ca-Mn carbonates with minor hematite and abundant carbonate ovoids. Parallel to normal faults and zones of ferruginization the ore has been sequentially altered to brannite II-brannite (new), bixbyite-hansmannite, and hausmannite-rich ore. Immediately adjacent to zones of ferruginization a second stage of brannite-rich ore has been developed. The high-grade ore appears porous and massive and original sedimentary bedding is obliterated.The formation of high-grade ore appears to be related to a process of silica and carbonate leaching from sedimentary protore by hydrothermal fluids. In proximity to the feeder channels for the hydrothermal fluids the manganese ore was replaced by hematite. Some of the manganese was reprecipitated to form very high grade (> or =50 wt % Mn) manganese ore immediately adjacent to zones of ferruginization. During waning stages of hydrothermal fluid flow, silica and some calcium carbonate leached from sedimentary manganese ore apparently reprecipitated close to and within the fluid channelways. This led to the formation of a secondary stage of braunitic manganese ore developed as a transition between zones of ferruginization and high-grade hansmannite-rich ore. At the same time zones of ferruginization became calcified and some rare silicate minerals were formed.