Aspects of Palaeo-terrace Formation and Stream Sediment Petrography of the Orange and Vaal River Confluence in the Douglas Area

Abstract

The mineralogy and geochemistry of a representative number of stream sediments in combination with geomorphologic mapping of the Orange/Vaal River terraces is used to assess the provenance of the sediments and the diamond-bearing potential of the terrace material in the confluence area around Douglas, Northern Cape. Utilizing sediment profile descriptions, grain size analyses, heavy mineral separation and identification techniques we were able to distinguish sediment provenance groups at different sites in the Orange/Vaal confluence area near Douglas. First assessment of the rock fragment/mineral spectrum reveals Ventersdorp volcanics material as well as Karoo material to be predominant provenances of these sediments, and to a lesser extent glacial material derived from Dwyka series to be the source of sediments; based on the frequency of diggings, the possible diamond-bearing potential appears to be by far higher in the terraces of the lower Vaal River and along the Orange River downstream Douglas than along the Orange River section between Hopetown and Douglas. Looking specifically at the heavy minerals further studies will attempt to correlate the terraces. Also, relative ages will be determined by looking at the post-depositional alteration or weathering. First quantification of the (heavy) mineral content displays a large contribution of quartz, epidote, magnetite, and amphibole grains from Karoo sediment and volcanics, whereas altered granite and pegmatite fragments of unknown origin form the lesser part of the sediment spectrum. At Bucklands, on both sides of the confluence point, relatively large estimated gravel thicknesses (>5m) are overlain by several cycles of up to 3 m thick mud to silty sediments with occasional larger pebble to gravel occurrence, both part of the Rietputs and Riverton Formations. Orange and Vaal River sediments are sorted selectively by hydraulic processes causing the sorting of according to size, shape and density, concentrating heavy minerals in trap sites such as potholes, along bars, and in areas of turbulent water flow. Therefore river bars and slip-off slopes in an area of moderate relief appear to be prime loci of heavy mineral deposition. Principally it can be concluded that the Orange River section between Hopetown and Douglas shows less pronounced younger gravel terrace formation due to a relatively low erosion basis and a stronger incision into the Ventersdorp bedrocks; the lower Vaal River and the Orange River downstream Douglas however have a more pronounced terrace formation due to a higher erosion basis caused by bedrock morphology.