PETROCHEMISTRY OF THE OKIEP COPPER DISTRICT BASIC INTRUSIVE BODIES, NORTHWESTERN CAPE PROVINCE, SOUTH-AFRICA

Copper mineralization is associated with noritic and pyroxenitic rocks in the granulite facies metamorphic terrane in the Okiep district of Namaqualand, in the northwestern Cape province of South Africa. These bodies have been interpreted to be postmetamorphic and the mineralizat ion has been inferred to be magmatic. However, the sulfide mineralogy of many of the orebodies is dominated by bornite, whereas immiscible m agmatic sulfide ores contain pyrrhotite and chalcopyrite. Hence, the O kiep ores cannot be primary magmatic sulfides. Geochemical evidence is presented to demonstrate that the ores have undergone an extensive ox idative event in which pyrrhotite and chalcopyrite were converted to m agnetite and bornite. Up to 90 percent of the sulfur has been lost in the case of some samples from the Carolusberg mine. The high Fe2O3/TiO 2 and Cu/S ratios, the low S/Se ratio of whole-rock samples, and their variation in different orebodies provide quantitative evidence for th is process. It is possible to recalculate the original sulfide from th ese data and a Cu content of approximately 10 percent is indicated. Ho wever, these ores only contain on the order of hundreds of parts per m illion of Ni, and so the Cu/Ni ratio is extremely high compared to tha t of most sulfide ores derived from a differentiated basic magma. The textural and mineralogical evidence for this oxidation is best seen in samples from the Carolusberg mine. In many of the other mines, a late r low-temperature hydrothermal alteration has obliterated the evidence for these reactions.