The sedimentology and diagenesis of copper-bearing rocks of the southern margin of the Damaran Orogenic Belt, Namibia and Botswana

Abstract

Late Proterozoic sediments of the Doornpoort, Klein Aub and Ghanzi Formations outcrop on the southern foreland of the Damaran Orogenic Belt, and contain stratiform copper ore deposits. This thesis presents sedimentological, diagenetic and isotopic studies of these sediments, defines the controls on copper mineralization and presents a model for ore genesis. The above-mentioned Formations are reclassified into the Doornpoort Formation, which is divided into the Klein Aub, Dordabis, Eskadron and Lake Ngami Members. The Doornpoort sediments were deposited between c. 1,000 MIR and c. 850 MYR age, unconformably on a mineralized basement which yields Eburnian (2,000 ± 200 NYR) and Kibaran (1,100 ± 200 MYR) ages. They consist of alluvial fan, lacustrine, aeolian, playa and mud flat sediments, deposited in a semi-arid to arid climate. Four types of alluvial fan complex are recognized and divided into two morphological groups: 1) thick conglomeratic fans with rapid lateral facies changes into aeolian, lacustrine and playa deposits, and 2) thin laterally persistent complexes with intercalated mud flat and lacustrine sediments. Pan complexes of differing morphological type can be correlated using megasequences (10's to 100's m. thickness of sediment). Thin, broad fans were probably formed by regional uplift and unconfined sediment discharge, whereas thick, narrow fan complexes were produced by localized uplift with sediment discharge through stable, restricted feeder zones. Despite the destructive mineralogical and textural effects of Damaran deformation and associated lower greenschist facies metamorphism, a generalized diagenetic sequence is identified for the formation of red and green Doornpoort sediments. Red sediments formed in alkaline, oxidizing diagenetic conditions and contain evidence of dissolution and replacement of detrital grains. In situ alterations probably led to the release of various elements into interstitial waters, which later recombined to form authigenic minerals and overgrowths. A paragenetic sequence of authigenic minerals is recognized in the aeolian Red Bed sandstones of the Witvlei fan complex, parts of which can be identified in surrounding fluviatile and playa sediments. Green sediments may contain copper and iron sulphides, which are intergrown with diagenetic minerals in the Witvlei playa sediments. Copper mineralization is restricted to reduced playa and marginal lacustrine sediments, which are closely associated with Red Bed deposits. Sedimentological evidence suggests that copper distribution was determined by ground and surface water drainage and then controlled by lacustrine palaeocurrents. Textural and sulphur isotopic data indicates that copper was fixed in the sediments during diagenesis, as sulphides, by the bacterial reduction of groundwater sulphate. Both sedimentological and isotopic data point to a basement origin for the copper and silver. A model for ore genesis is proposed, involving the release of copper and silver from mineralized basement rocks during Late Proterozoic semi-arid to and weathering and the transport of metals in solution as copper sulphate, which on contact with neutral or alkaline aqueous solutions, was converted to insoluble basic copper carbonate and carried in suspension. Metalliferous waters collected in topographically low areas on the alluvial fan surface, in playas and lakes. Particulate basic carbonates were deposited with the suspended sediment load and reworked by lacustrine palaeocurrents, forming placer deposits. The metals were subsequently fixed as sulphides during diagenesis