Gabbro-norite-hosted Ni-Cu-(Co)-sulphide mineralization in southern Namaqualand and its relationship to the cupriferous Koperberg Suite of the Okiep Copper District, South Africa
Small Ni-Cu-(Co) massive sulphide deposits are located 140 km south of the well-known Okiep Copper District within a polyphase deformed, granulite facies metamorphic section of the mid-Proterozoic Namaqua-Natal belt in western Namaqualand. Massive sulphides, consisting of magnetic pyrrhotite with minor exsolved cobaltian pentlandite and chalcopyrite as well as pyrite, are concentrated towards the structural (possibly original) base of an orthomagmatic gabbronorite host rock. The bodies are small, irregularly shaped and structurally attenuated parallel to the regional foliation. The gabbro-norite host displays in situ segregations of anorthosite, diorite, and glimmerite. It is juxtaposed with small bodies of non-mineralized norite and quartz-norite, collectively known as the Nuwefontein Suite, and extensively developed younger charno-enderbite. The largest two of the massive sulphide lenses have a combined in situ ore resource of 2 million metric tons at a grade of 0.88 % Ni, 0.20 % Cu, 410 ppm Co, and negligible platinum group elements. A comparative petrochemical study with the cupriferous mafic rocks of the time-equivalent Koperberg Suite of the Okiep Copper District shows the pre- to syntectonic two-pyroxene granulites to be the closest analogy to the mineralized gabbro-norites. The similar geochemical characteristics of both the Nuwefontein and Koperberg suites indicate a common upper-mantle-lower-crust derived, calc-alkaline, high-alumina, basaltic parent magma generated in a subduction-related tectonic environment. A two-stage evolutionary genetic model is proposed. The early intrusive phases consisted of gabbro-norite sills and dykes with minor associated Ni-Cu-Co massive sulphide mineralization (Hondekloof type). This was followed by later, diapir-shaped, composite mafic intrusions dominated by anorthosite (Koperberg type). Mineralization consists of disseminated chalcopyrite and bornite, hosted mainly by the last hypermelanic low-sulphur magnetite-rich phases. This interpretation is consistent with the Fram and Longhi model proposed for the generation of anorthosite complexes from basaltic magmas. It is apparent that orthomagmatic base metal mineralization associated with calc-alkaline magmatism extends beyond the perimeters of the Okiep Copper District in western and southern Namaqualand.