Sulfur isotope characteristics of metamorphosed Zn-Cu volcanogenic massive sulfides in the Areachap Group,Northern Cape Province, South Africa

摘要

Zn-and Cu-rich massive sulfide ores of volcanogenic origin [volcanogenic massive sulfide (VMS) deposits] occur as stratiform/stratabound lenses of variable size hosted by gneisses, amphibolites, and schists of the Areachap Group, in the Northern Cape Province of South Africa. The Areachap Group represents the highly deformed and metamorphosed remnants of a Mesoproterozoic volcanicarc that was accreted onto the western margin of the Kaapvaal Craton during the ～1.0-1.2 Ga Namaquan Orogeny. Sulfur isotope data (δ~(34)S) are presented for 57 sulfide separates and one barite sample from five massive sulfide occurrences in the Areachap Group. Although sulfides from each site have distinct sulfur isotope values, all δ~(34)S values fall within a very limited range (3.0‰ to 8.5‰). Barite has a δ~(34)S value of 18.5‰, very different from that of associated ulfides. At one of the studied sites ( Kantienpan), a distinct increase in δ~(34)S of sulfides is observed from the massive sulfide lens into the disseminated sul a distinct footwall alteration zone. Sulfide sulfide and sulfide—barite mineral pairs which recrystallized together during amphibolite- and lower granulite facies metamorphism are not in isotopic equilibrium. Sulfur isotope characteristics of sulfides and sulfates of the Zn—Cu ores in the Areachap Group are, however, very similar to base metal sulfide accumulations associated with modern volcanic arcs and unsedimented mid-ocean ridges. It is thus concluded that profound recrystallization and textural reconstitution associated with high-grade regional metamorphism of the massive sulfideores of the Areachap Group did not result in extensive sulfur isotopic homogenization. This is similar to observationsin other metamorphosed VMS deposit districts and confirms that massive sulfide ores remain effectively a closed system for sulfur isotopes for both sulfides and sulfates during metamorphism.