The roles of primary kimberlitic and secondary Dwyka glacial sources in the development of alluvial and marine diamond deposits in Southern Africa

The source area of Dwyka Group glacial sedimentary rocks in southern Africa contains a province of pre-Karoo diamondiferous kimberlites. Ice-flow vectors and facies variations indicate that diamonds and kimberlitic indicator minerals, acquired in this source region during the Dwyka glaciation, were transported to and deposited in areas adjacent to the modern Atlantic coast of southern Africa. Diamonds and kimberlite garnets, recovered from the Koa River gravel deposits on the Bushmanland Plateau, were probably derived by weathering of these Dwyka Group rocks. Along the west coast of South Africa and Namibia, marine and fluvial diamond concentrations of Cretaceous, Miocene and Plio-Pleistocene ages were also partly derived from Dwyka sources in both the Karoo and Kalahari Basins, as well as from other secondary sources such as glacial and fluvial sedimentary rocks of the Gariep Complex, Nama Group and Table Mountain Group. On the coastal plain, Cretaceous fluvial deposits formed during scarp retreat under podzolic weathering conditions. Miocene and younger fluvial and marine deposits formed by weathering processes that involved stripping of back-escarpment Karoo cover. Due to the presence of oversized diamictite-clast trapsites, concentrations of diamonds accumulated on exposed pre-Karoo surfaces during extended periods of crustal stability. Brief episodes of increased precipitation and uplift during the Miocene and Pliocene caused the flushing of these concentrations, in discrete events, via a few major drainages, into marine and near-coastal fluvial and aeolian settings. The majority of inland alluvial diamond deposits are located in a broad belt to the north and west of the Cretaceous diamondiferous kimberlite clusters in central South Africa. This distribution is probably the result of north-westward-flowing fluvial systems, inland of the Great Escarpment, which eroded significant thicknesses (up to 1.5 km) of Karoo cover rocks and drained into the palaeo-Molopo (Kalahari) River across the buried Cargonian Highlands. Lag concentrations of diamonds ultimately formed on the African Surface in traps such as karstic depressions and fluvial channels. Post-African weathering exposed south-westward-draining pre-Karoo glacial valleys along the margins of the Cargonian Highlands and established the Vaal-Harts-upper Orange River system, which reworked the older African-Surface regolith into secondary Miocene and younger colluvial and alluvial gravel deposits. Additional diamond contributions came from limited post-African weathering of primary kimberlite and secondary Dwyka sources. These gravel deposits extend as far as Prieska on the upper Orange River and may not have contributed large quantities of diamonds to the west-coast diamond resource. The inland alluvial deposits, therefore, have a significant primary Cretaceous kimberlite source, whereas those on the west coast were derived from multiple sources that include a substantial primary pre-Karoo kimberlite component via secondary Dwyka Group glacial redistribution. (C) 2003 Elsevier Ltd. All rights reserved.