D.Phil. / The Ongeluk Formation is a laterally extensive sequence of ≈2200 Ma tholeiitic basaltic andesites in the upper Griqualand West Sequence of the northern Cape Province. The stratigraphic thickness is about 500 m and the Ongeluk Formation underlies the ore-bearing strata of the Kalahari Manganese Field. The formation comprises massive lavas, pillow lavas and hyaloclastite beds in close association. These rocks were extruded under water in a marginal basin within the continental setting of the Kaapvaal Craton. The Hekpoort Basalt Formation of the Transvaal is magmatically cogenetic with the Ongeluk, having indistinguishable geochemistry and sharing a stratigraphically related hiatus in Cr values. The best age estimate for the two formations is 2193 ± 71 Ma, from Rb-Sr data of two previous workers for Hekpoort samples. The Ongeluk Formation shows a mild "regional" geochemical alteration and a profound "Kalahari" alteration beneath the Kalahari Manganese Field. Geochemical screening was used to reconstruct the magmatic composition from a selected dataset. Three stages in the development of regional alteration are ascribed to sea water-rock interaction at different temperatures, and have distinct geochemical signatures. The pervasive Kalahari alteration is characterised by a purple colouration and the decoupled alteration of alkali and high field strength elements. It is due to the development of major hydrothermal systems close to a volcanic vent which are analogous to modern mid-ocean ridge systems. A multi-system isotopic study showed that most of the isotope systems were modified by sea-floor alteration. The similarity of the 2237 ± 23 Ma Pb-Pb errorchron age with the Rb-Sr Hekpoort age reflects changes in U-Pb ratios with minor changes in Pb isotope ratio. Evidence was found in the Rb-Sr system for a minor disturbance at ≈ 1100 Ma, also reported by previous workers. This event is related to the Namaqua tectogenesis, while no isotopic evidence was found for the enigmatic ≈ 2200 to 1750 Ma Kheis orogeny, regarded as the cause of thrust faulting in the region. A genetic connection between the Ongeluk lava and the Kalahari Manganese deposits was established. The manganese ores contain evidence for both marine and hydrothermal contributions to chemical sedimentation. Negative Ce anomalies characterise an oxygenated sea in which the interaction between global oceanic and continental influences is seen. Heavy rare earth enrichment reflects volcanic hydrothermal exhalations from the Kalahari Ongeluk system. Mass balance calculations show that the entire 9 billion tons of Kalahari Manganese ore could have been derived from the Ongeluk Formation. A new model describing the origin and evolution of the Kalahari Manganese Field places a strong emphasis on the role of the syngenetic hydrothermal exhalation and upgrading.