Geochemistry and metamorphism of the Prieska Zn-Cu deposit, South Africa

Abstract

The Prieska orebody is a 50-million-ton massive sulfide Zn-Cu deposit situated near the eastern border of the middle Proterozoic Namaqua province of southern Africa.The strata-bound, stratiform deposit lies in a terrane which has been affected by high-grade metamorphism and intense deformation. Textures now seen in the ore and country rocks are not primary but reflect the complex tectono-metamorphic history. The deposit is contained in a peraluminous unit underlain by a metadacite and overlain by a composite sequence containing metapelites, metabasic rocks, and a variety of reworked volcanic rock types. Amphibolite layers that originated as intrusive basaltic dikes and sills are found in all of these rock types.It is suggested that the massive sulfide ore formed above hydrothermally altered metadacite. The rock types thus derived include a gedrite-bearing assemblage, the metamorphic equivalent of a chloritic alteration pipe, and a quartz-perthite-sillimanite gneiss representing silicified and sericitized sediments at the floor of the discharge basin. Overlying these rocks is the massive sulfide unit. Calcite-anhydrite-barite rocks immediately above the ore may have formed a sulfate-carbonate capping during sulfide deposition.The Prieska deposit and other smaller sulfide deposits in the same region have geochemical features (including metal composition and Pb and S isotope characteristics) that distinguish them from the important massive sulfide deposits of the central part of the Namaqua province. Whereas the Prieska deposits are related to hydrothermal action in a volcano-sedimentary sequence, no metamorphic equivalents of alteration pipes have been found at the sediment-hosted Aggeneys-Gamsberg deposits.