EMPLACEMENT FEATURES OF CUPRIFEROUS NORITOIDS IN THE OKIEP COPPER DISTRICT, NAMAQUALAND, SOUTH-AFRICA

Intrusive features of cupriferous noritoids from the deeply eroded Pro terozoic granite-gneiss terrane of the Okiep Copper District in South Africa illustrate the emplacement of basic magmas into a mid- to lower -crustal environment. The small, easterly trending intermediate to bas ic intrusions of the so-called Koperberg Suite display a wide variety of geometries, occurrence and emplacement modes, including dike-, sill - and plug-like bodies. Coeval granulite-facies metamorphism, partial melting and ductile deformation of country rock gneisses during intrus ion of the basic magmas led to emplacement and dilation processes of t he intrusions which differ significantly from diose described for dike s and sills at shallow crustal levels. Regional tectonic stresses are largely negligible due to very low differential stress (sigma1 - sigma 3) during deformation and intrusion, which permits emplacement at high angles to the principal compressive stress and which follows favorabl y inclined structural anisotropies. Dilation of the basic intrusions i s largely determined by compositional differences of host rocks and oc curs by thermal erosion, i.e., assimilation of wall rock gneisses. Low length-to-width ratios of basic to ultrabasic dikes and predominantly diapir-like outlines of mesocratic bodies are indicative of low visco sity contrasts between the mafic magmas and high-grade metamorphic hos t rock gneisses. Buoyancy-controlled ascent of the basic to intermedia te intrusions is expressed by a broad compositional zoning of basic bo dies with respect to the granite-gneiss stratigaphy. Various intrusion modes within the stratigraphic column of the granite-gneiss sequence reflect a lithological and structural stratification of the seemingly homogeneous granite-gneiss terrane.