Geochemical evolution of the Richtersveld area, South Africa, as deduced from regional geochemical maps of stream sediments

The Richtersveld area, bounded by 16° and 18° E and 28° and 29° S, has been mapped geochemically using stream sediment samples collected from first-order streams on a one-kilometre grid. The samples have been analysed for 20 elements and geochemical maps on a scale of 1:250,000 have been prepared.The basement consists of the Orange River Group which has been intruded by the Vioolsdrift Suite between 1700 and 1900 Ma. Intrusive into the Vioolsdrift Suite is the Richtersveld Suite, consisting of alkaline granite, syenite and diorite, which intruded at 970 Ma.The basement and the Richtersveld Complex are overlain by the younger Gariep Complex and in the south the Vioolsdrift Suite is overlain by the still younger Nama basin. The last intrusive event was the emplacement of three granitic plutons at 550 Ma.Geochemical maps for Rb and Y are used to characterize the geochemistry of the basement rocks. These maps show that the northern portion of the Vioolsdrift Suite is granodioritic whereas the southern portion is granitic. The boundary between the two granitic terrains is delineated.The distribution of Rb in the sandstone of the Gariep Complex reflects the Rb variation in the Vioolsdrift Suite and the Richtersveld Complex from which it was derived. The Rb in the sandstone occurs in feldspar thus demonstrating that mechanical weathering dominated during the its formation. The distribution of Y confirms both the proposed source areas and weathering conditions. Yttrium occurs in placer deposits in the sandstones of the Gariep Complex.The deep water facies of the Gariep Complex reflects the provenance areas vaguely, but due to homogenization in the basin the distribution of Y is more homogeneous in the shale. The effect of chemical traps such as dolomitic limestone is, however, apparent.The sporadic high concentration levels of Rb and Y in the sandstone of the Nama basin indicate that the granitic portion of the Vioolsdrift batholith was the source area, that mechanical weathering dominated and that placer deposits formed during the deposition of the fluvial sandstones. The dolomitic limestones, however, show evidence of the chemical fractionation of Y, indicating that chemical leaching and concentration took place during this phase.Elements such as Sn, W, Be, B, Th, Nb and Ta, all of which occur in the southern Vioolsdrift batholith, could have concentrated as placers in the Gariep and Nama sandstones. Elements such as U, Y, P, and Cu, of which substantial deposits are known, did not concentrate in the sandstones, but formed accumulations in the limestone and carbonaceous limestone of the younger sediments. Geochemical maps for Y, P and U show examples of hitherto unknown metallogenic provinces in the deep water sediments.Geochemical maps for Ti, Zr and P can be used to define the boundaries and relative concentration of diamonds within individual deposits and to identify new exploration targets.The pegmatite metallogenic province is divided into a northern P-enriched province containing accumulations of Y, Zr, Th, U and rare-earths, and a southern Nb-enriched province. The latter contains several Ta- and Be-producing pegmatites. The extension of these two pegmatitic provinces, which are also associated with the widespread W-mineralization, to the west of the Nama basin is also suggested.