The sedimentology and diagenesis of copper-bearing rocks of the southern margin of the Damara orogenic belt, Namibia and Botswana

Abstract

Abstract provided by author:

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 arid 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 wafers 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

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