Computer Science
Scientific paper
Jun 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.2677b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 12, pp.2677-2687
Computer Science
2
Scientific paper
The middle Proterozoic Koperberg Suite occurs as a swarm of differentiated mafic dikes in a granulite facies terrane in the northwestern Cape Province of South Africa. Rock types include pyroxenite, norite, anorthosite, diorite, and glimmerite and host an important repository of copper ore. This paper reports petrographic and stable isotope data that places new constraints on the magmatic and metamorphic evolution of the Koperberg Suite and associated copper mineralization. Oxygen isotope ratios of the basic bodies are significantly enriched in 18 O ( 18 O plag = 6.2 to 9.6%; 18 O opx = 6.2 to 7.4%; and 18 O rock = 5.9 to 8.3%) relative to mantle-derived basic rocks. This enrichment appears to be the result of crustal contamination. Granite-gneiss country rocks exhibit 18 O values in the range from 7.8-8.7%. The systematics of oxygen isotope data for coexisting minerals suggest that in addition to crustal contamination further modification took place due to low-temperature, post-peak metamorphic exchange, involving circulating fluids that had undergone isotopic exchange with the surrounding gneisses. These fluids are probably related to the upper greenschist grade metamorphism associated with the regional Pan-African Orogeny (500-550 Ma). This is indicated by the lack of temperature concordancy based on ° 18 O values of several mineral pairs involving orthopyroxene, plagioclase, biotite, and magnetite. Sulfur isotope ratios, from diorite- and norite-hosted Sulfides, are light relative to mantle values and co-genetic mineral pairs show a lack of temperature concordancy. The ranges in 34 S values for the sulfides are: 34 S cpy -1.5 to -3.8% 0 , 34 SS bo -1.9 to -4.1% 0 , and 34 S po - 1.9 to -2.5% 0 . Bulk ore samples show a systematic decrease in 34 S values with increasing Cu / S ratios . This is seen as being diagnostic for an oxidation process and the degassing of a SO 2 -rich vapour phase. Petrographic observations and mineral chemistry data support this conclusion and indicate that the devolatilization event was caused by granulite facies metamorphism, followed by a later, lower temperature alteration event. In our genetic model we envisage a mantle origin for the Koperberg Suite. The original magma was modified through assimilation and fractional crystallization processes. Later modification occurred at the level of emplacement through differentiation and metamorphism. Crustal contamination can explain the origin of the enriched O-isotope values, the high levels of incompatible trace elements as well as the high initial 87 Sr / 86 Sr ratios in the basic bodies.
Boer Rudolf H.
Cawthorn Grant R.
Meyer F. M.
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