Abstract

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 mineralization has been inferred to be magmatic. However, the sulfide mineralogy of many of the orebodies is dominated by bornitc, whereas immiscible magmatic sulfide ores contain pyrrhotite and chalcopyrite. Hence, the Okiep ores cannot be primary magmatic sulfides. Geochemical evidence is presented to demonstrate that the ores have undergone an extensive oxidative event in which pyrrhotite and chalcopyrite were converted to magnetite and bornite. Up to 90 percent of the sulfur has been lost in the case of some samples from the Carolusberg mine. The high Fe 2 O 3 /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 this process. It is possible to recalculate the original sulfide from these data and a Cu content of approximately 10 percent is indicated. However, these ores only contain on the order of hundreds of parts per million of Ni, and so the Cu/Ni ratio is extremely high compared to that of most sulfide ores derived from a differentiated basic magma.The textural and mineralogieal evidence for this oxidation is best seen in samples from the Carolusberg mine. In many of the other mines, a later low-temperature hydrothermal alteration has obliterated the evidence for these reactions.

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