STABLE ISOTOPIC EVIDENCE FOR CRUSTAL CONTAMINATION AND DESULFIDATION OF THE CUPRIFEROUS KOPERBERG SUITE, NAMAQUALAND, SOUTH-AFRICA

The middle Proterozoic Koperberg Suite occurs as a swarm of differenti ated mafic dikes in a granulite facies terrane in the northwestern Cap e Province of South Africa. Rock types include pyroxenite, norite, ano rthosite, diorite, and glimmerite and host an important repository of copper ore. This paper reports petrographic and stable isotope data th at places new constraints on the magmatic and metamorphic evolution of the Koperberg Suite and associated copper mineralization. Oxygen isot ope ratios of the basic bodies are significantly enriched in O-18 (del ta O-18plag = 6.2 to 9.6 parts per thousand; delta O-18opx = 6.2 to 7. 4 parts per thousand; and delta O-18rock = 5.9 to 8.3 parts per thousa nd) relative to mantle-derived basic rocks. This enrichment appears to be the result of crustal contamination. Granite-gneiss country rocks exhibit delta O-18 values in the range from 7.8-8.7 parts per thousand . The systematics of oxygen isotope data for coexisting minerals sugge st that in addition to crustal contamination further modification took place due to low-temperature, post-peak metamorphic exchange, involvi ng circulating fluids that had undergone isotopic exchange with the su rrounding gneisses. These fluids are probably related to the upper gre enschist grade metamorphism associated with the regional Pan-African O rogeny (500-550 Ma). This is indicated by the lack of temperature conc ordancy based on deltaO-18 values of several mineral pairs involving o rthopyroxene, plagioclase, biotite, and magnetite. Sulfur isotope rati os, from diorite- and norite-hosted sulfides, are light relative to ma ntle values and co-genetic mineral pairs show a lack of temperature co ncordancy. The ranges in deltaS-34 values for the sulfides are: deltaS -34cpy -1.5 to -3.8 parts per thousand, deltaS-34bo -1.9 to -4.1 parts per thousand, and deltaS-34po -1.9 to -2.5 parts per thousand. Bulk o re samples show a systematic decrease in deltaS-34 values with increas ing Cu/S ratios. This is seen as being diagnostic for an oxidation pro cess and the degassing of a SO2-rich vapour phase. Petrographic observ ations and mineral chemistry data support this conclusion and indicate that the devolatilization event was caused by granulite facies metamo rphism, followed by a later, lower temperature alteration event. In ou r genetic model we envisage a mantle origin for the Koperberg Suite. T he original magma was modified through assimilation and fractional cry stallization processes. Later modification occurred at the level of em placement through differentiation and metamorphism. Crustal contaminat ion can explain the origin of the enriched O-isotope values, the high levels of incompatible trace elements as well as the high initial Sr-8 7/Sr-86 ratios in the basic bodies.