REGIONAL SETTING AND GEOLOGICAL EVOLUTION OF THE OKIEP COPPER DISTRICT, NAMAQUALAND, SOUTH-AFRICA

The Palaeo- to Mesoproterozoic rocks of the Okiep Copper District, loc ated in the Bushmanland Subprovince of the Namaqua-Natal Metamorphic P rovince, display a polyphase metamorphic, deformational, and intrusive history related to the similar to 1.0 - 1.2 Ga Namaqua tectogenesis. Recent lithological, geochronological, isotopic, geochemical, and stru ctural studies indicate that the Namaquan evolution of these rocks com prised two main events. At similar to 1200 - 1250 Ma, a Palaeo- to Mes oproterozoic supracrustal sequence and its Kheisian basement were defo rmed (D-1) and intruded by voluminous granitoids and subsidiary mafic sills. The main tectonothermal event, which climaxed in low-P granulit e facies metamorphism of the rocks at similar to 1030 Ma, was characte rized by concomitant crustal thickening (D-2 and D-3 events) and mafic to granitoid magmatism. The synchronism between the high-grade metamo rphism, crustal thickening, and the intrusion of mantle-derived magmas is interpreted in terms of a model involving advection of heat into t he crust via intraplating of mafic magmas triggered by thinning of the mantle lithosphere during D-2 thickening. Some of these magmas attain ed mid-crustal levels and are the source of the copper mineralization in the Okiep District. Their intrusion under granulite-grade condition s slightly prior to the peak of metamorphism explains their unusual mo rphological, isotopic, and geochemical characteristics, as well as the unusual isotopic, chemical, and mineralogical character of the sulphi de parageneses.