Geological and Technical Aspects of Marine Diamond Exploration in Southern Africa

Abstract

Abstract Exploration has indicated the presence of diamonds over a distance of some 1400 km along the southern African continental shelf, and extending from about 100 m above sea level to at least 150 m below sea level. This range could be extended to the 500 m isobath following the granting of new concessions between the 200 m and 500 m isobaths along the South African continental margin. Despite this very wide-ranging occurrence of diamondiferous deposits on the coastal plain and continental Shelf, little is known of the nature of the deposits. The occurrence of diamondiferous deposits is generallyassociated with high energy, shallow marine depositional environments. The concentration of diamonds and consequent mineral enrichment of these deposits are as a result of the continual reworking of older mineralized deposits, and subsequent redeposition of diamonds preferred trap sites. Reworking was mainly through the agencies wave-induced bottom currents with sea-level changes, as well as fluvial and aeolian erosional processes on emerged marine deposits. Marine diamond exploration has been under vay since the early sixties. At present there are more than thirty exploration companies holding precious mineral concession rights along the southern African west coast and continental margin, and only about 15% of the entire continental margin area has not been granted Despite this apparent interest, large areas still remain unexplored or surveyed. The current status of knowledge is mostly on the inner shelf and the middle shelf to a water depth of about 150 m. No studies of the surficial geology of the outer shelf have yet been undertaken. Exploration techniques have been developed using detailed Interpretation of high resolution geophysical data and wave refraction programmed to identified the geological features. Combined, these techniques are used to establish the most prospective depositional areas and enable marine diamond exploration is to identify the geological features most likely to be associated with these deposits. It is the aim of this paper to highlight the more salient geological features of marine diamondiferous deposits. This may assist future geophysical exploration programmes in their focus to identify these deposits. Introduction The ‘diamond coast’ of southern Africa extends for over 1500 km from St Helena Bay in the south to Conception Bay in the north (Figure l). The southern African diamond fields have attracted the attention of numerous exploration companies ever since the first diamond was discovered outside Luderitz, Namibia, in 1908. In the following ninety years, over I00 million carats of mostly gem-quality diamond have been recovered from the diamondiferous deposits. The diamonds occur in a number of different depositional environments and these are the subject of this paper. Interest in the marine diamonds grew steadily since active exploration and prospecting started in the early sixties and today the interest is at its highest ever.