The geological evolution of the eastern Richtersveld

Includes bibliographical references.The lithology of the early Proterozoic (-1950-1750 my) Richtersveld is characterised by predominantly calc-alkaline volcanics, (De Hoop Subgroup) overlain by and intercalated with quartzitic metasediments into which the mainly granodioritic rocks of the Vioolsdrif Intrusive Suite (VIS) were emplaced. Metavolcanics and sediments of the De Hoop Subgroup have been subdivided into formations, and their paleogeographic environments have been deduced from lithological criteria. In the north eastern Richtersveld systematic reconstruction permitted the recognition of four distinct depositional domains: (a) a high altitude volcanic terrain in the northeast, characterised by a great thickness of an entirely volcanic succession. (b) a synvolcanic graben structure in the southeast; infilled by melanocratic volcanics and immature sediments. (c) a low-lying volcanic terrain in the southwest, possibly representinga coastal plain, lithologically characterised by volcanic debris and redeposited volcanics of domain (a).d) marine quartzites in the south, indicating a marine ingression from the southwest towards the end of the volcanic cycle ,In the southeastern Richtersveld more strongly metamorphosed acid to intermediate volcanics overlie metasediments of the Namaqua Metamorphic Complex(NMC).Within the Vioolsdrif Intrusive Suite four intrusive phases can be distinguished. The oldest phase, a fine-grained granodiorite, is only tentatively assigned to the Vioolsdrif Intrusive Suite and probably constitutes a subvolcanic member of intermediate metavolcanics of the Kookrivier Formation. It was followed by syntectonic even-grained granodiorite and by posttectonic granite and leucogranite. Textural analysis of the undeformed granite phase provided information as to its intrusion and crystallisation history and the sequence of crystallisation. Since the latter allows the estimate of the water content of the initial magma, and as water content of the magma determines - according to some models -geological position and extent of porphyry ore deposits, predictions could be made as to the probable site and extent of possible porphyry ore deposits, which are in agreement with preliminary results obtained during field work. The metamorphic and tectonic development probably started soon after or even during deposition of the supracrustal rocks. Low grade metamorphism of low pressure type (andalusite-sillimanite facies series sensu Miyashiro, 1973) in the central northeastern Richtersveld (associated with tight to isoclinal Fi-folds of different magnitudes) and migmatisation in the south and northeast (mainly postdating F1) suggest an essentially zoned metamorphic complex with increasing grade of metamorphism towards the margins of the Richtersveld