Emplacement Features of Cupriferous Noritoids in the Okiep Copper District, Namaqualand, South Africa

Exploration & Mining Geology, Vol. 3, No. 3, 1994

ALEXANDER P.M. KISTERS, Economic Geology Research Unit, University of the Witwatersrand, Johannesburg, Republic of South Africa. JACOBUS E. POTGIETER, Gold Fields of South Africa, NW Cape Exploration, Republic of South Africa. E. GUY CHARLESWORTH, Department of Geology, University of the Witwatersrand, Johannesburg, Republic of South Africa. CARL R. ANHAEUSSER, Economic Geology Research Unit, University of the Witwatersrand, Johannesburg, Republic of South Africa. ROGER L. GIBSON, Department of Geology, University of the Witwatersrand, Johannesburg, Republic of South Africa. MICHAEL K. WATKEYS, Department of Geology and Applied Geology, University of Natal, Republic of South Africa

Intrusive features of cupriferous noritoids from the deeply eroded Proterozoic 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 basic 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 intrusion of the basic magmas led to emplacement and dilation processes of the intrusions which differ significantly from those described for dikes and sills at shallow crustal levels. Regional tectonic stresses are largely negligible due to very low differential stress (a{ - aj during deformation and intrusion, which permits emplacement at high angles to the principal compressive stress and which follows favorably inclined structural anisotropies. Dilation of the basic intrusions is largely determined by compositional differences of host rocks and occurs 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 viscosity contrasts between the mafic magmas and high-grade metamorphic host-rock gneisses. Buoyancy-controlled ascent of the basic to intermediate intrusions is expressed by a broad compositional zoning of basic bodies with respect to the granite-gneiss stratigraphy. Various intrusion modes within the stratigraphic column of the granite-gneiss sequence reflect a lithologjcal and structural stratification of the seemingly homogeneous granite-gneiss terrane.