The copper-bearing basic rocks of Namaqualand,South Africa

Copper mineralisation is associated with rocks of basic/intermediate composition which occur as a widespread swarm of relatively small irregularly-shaped bodies within the gneissic terrain of the Okiep District. These basic rocks comprise the so-called Koperberg noritoid suite of Namaqualand and constitute one of the three major copper producers in South Africa. While a metasomatic origin was briefly favoured for the noritoids, most researchers have emphasized the cross-cutting nature of the noritoids relative to their host rocks and prefer an origin involving magmatic injection. High noritoid ⁸⁷Sr/⁸⁶Sr initial ratio values indicated to some workers that mobilisation of suitable crustal materials was more attractive than intrusion of mantle-derived melts to account for the origin of the noritoid suite. The present study has shown that the structure, texture and mineralogy of the noritoids do not lend themselves to interpretation of the noritoids as pristine intrusive rocks, and indicate that the noritoids have undergone high-grade metamorphism after emplacement. The geochemistry of the noritoid suite reveals a pronounced alkaline affinity and mineralogical control of the chemistry characterises the rocks as cumulates. The dimensions of the noritoids appear unfavourable for development of cumulate rocks and it is considered possible that the present configuration of the noritoids results from dismemberment of larger intrusive bodies as a result of shear deformation during high-grade metamorphism. It has not been possible to identify a magma parent to the noritoid suite, but we believe the parent to have been an alkaline mantle-derived magma which suffered contamination by granitic anatects in the lower crust. Recognition of the metamorphic history of the Namaqualand noritoids renders a comparison between them and the Caraiba noritoids of interest.