Porphyry-type copper mineralisation is developed in a quartz-feldspar porphyry pluton of the 1900-1730 Ma Vioolsdrif Intrusive Suite, which crops out near the lower Orange River in southern Namibia. Petrographic evidence for hydrothermal alteration is widespread, but most intense in a zone near the central part of the elongate porphyry body. Both the porphyry and metavolcanic country rocks are affected. Potassic alteration that often characterises Phanerozoic porphyry-type copper mineralisation is difficult to distinguish in the Haib deposit because of the high-K nature of the calc-alkaline host rocks. Stable isotope patterns indicate depletion in 18O (represented by low δ18O whole rock and mineral values) and deuterium (low δD values) during hydrothermal alteration, which suggests the introduction and circulation of heated meteoric fluids. Preliminary studies have identified two types of fluid, an early high-salinity high-temperature fluid trapped in primary inclusions, and a late-stage low-temperature low-salinity fluid trapped as secondary inclusions in healed fractures.