Diamonds delivered to the West Coast of southern Africa from erosion of the Kaapvaal-based kimberlites and lamproites

Abstract In the early 1900s diamonds were discovered along the Atlantic coast of southern Africa in Cainozoic alluvial deposits. Most of these were derived from erosion of Mesozoic primary sources on the Kaapvaal Craton. Previous attempts to quantify the amount of diamonds that were eroded from inland lamproites variety Kaapvaal (lamproites vK), and kimberlites varied between 10,000 and 3,000 Mct (million carats). Based on new geomorphological techniques, the post-Gondwana erosion rates and landscape denudation of southern Africa are reviewed. The Early and Middle Cretaceous were peak periods of landscape denudation, which almost coincide with the emplacement of lamproites vK and kimberlites, respectively. This study demonstrates that the eroded lamproites vK and kimberlites, since emplacement, have released 852.7 and 1,336.4 Mct respectively. Assuming that 50% of the carats was lost through breakage during transport and that some 1% was retained in fluvial terraces, it leaves 1,074.6 Mct that were delivered to the West Coast. The palaeo-Orange/Vaal river system has evolved spatially, supplying diamonds via different exit points along the coast. Initially, the transport of diamonds was by the Karoo River delivering some 209.5 Mct eroded from the 120 Ma old lamproites vK to the coast near the present Olifants River. Before the 85 Ma kimberlite event the river outfall to the Atlantic Ocean shifted to the general area of today’s Lower Orange River. This outlet received 865.1 Mct from these Cretaceous lamproites vK and kimberlites, but there are also indications that a small number of diamonds from older primary sources are present.