The geology and geochemistry of the Bridgetown Formation of the Malmesbury Group, Western Cape Province

[摘要] ENGLISH ABSTRACT:A number of small greenstone bodies of the Bridgetown Formation are exposed aselongated lenses and dykes within metasediments of the Malmesbury Group in theWestern Cape Province, South Africa. The Malmesbury Group is part of theNeoproterozoic to Cambrian (Namibian) Saldania Subprovince which is the southerncontinuation of a Pan-African mobile belt system.A detailed geological and geochemical study was conducted on the largest outcropof the Bridgetown Formation, situated 20km east of the town Moorreesburg.The Bridgetown Formation consists of a meta-volcano-sedimentary sequence thatexperienced polyphase deformation and metamorphism up to the lower greenschistfacies. Tectonically, the Bridgetown Formation is included in the Boland tectonicdomain, east of the Piketberg-Wel lington fault zone that is suggested to run Skmwest of Heuningberg and subparallel to the Berg River. This agrees with Rabie's(1974) original subdivision of the tectonic domains.The Bridgetown Formation consists of: i) A basal unit of poorly differentiatedalkaline metabasalt with a within-plate tectonomagmatic fingerprint. ii) Anintermediate unit of poorly differentiated tholeiitic metabasalt, intruded by a youngertholeiitic metabasite with a low degree of differentiation. Both members of theintermediate unit have ocean-floor basalt (P-type MORB) and island arc basaltfingerprints. iii) An upper unit of poorly differentiated as well as more evolvedalkaline metabasalts, interlayered with metatuff with an alkaline basaltic composition,metasedimentary rocks with a marine origin, and graphitic schists and muscovitequartzschists, both with a continental crust provenance. iv) An overlyingmetasedimentary sequence including dolomite, massive and oolitic chert, jasper andjaspilite.The Bridgetown Formation probably also comprises a lower metamorphosedultramafic unit, indicated by the association of Ni- and Cr-rich talc bodies, Ni-and errichbanded chert, chlorite schist and small dolomite-talc-chlorite bodies at Spitskop,situated directly northwest of the main greenstone body.The sequence of eruptive stages and the geochemistry of the metavolcanicsresemble Hawaiian volcanism , indicated by an initial deep water stage of alkalinemagmatism, followed by main tholeiitic edifice and post-caldera alkaline magmatism.Post-caldera alkaline magmatism occurred contemporaneously with deposition . ofsediments and chemical precipitates (carbonates and cherts).The Bridgetown metavolcanics have no magmatic association with either theBloubergstrand volcanics or mafic and intermediate plutonic rocks in theMalmesbury Group. However, some physical and geochemical similarities existbetween the Bridgetown Formation and the age related Grootderm Formation of theMarmora Terrane (Gariep Supergroup) which is considered to represent ophioliticmaterial. The Bridgetown Formation probably represents segments of oceanic crust,including seamounts of oceanic islands, which were tectonically emplaced in anaccretionary prism zone during subduction of oceanic crust underneath the KalahariCraton, 600 to 700 Ma ago. This resulted in the present spatial configuration ofvarious small greenstone bodies in the Malmesbury Group.To date no exploitable mineral deposits have been found 1n the BridgetownFormation. However, Au and As anomalies in stream sediment and soil samples,taken in the Spitskop area, require further attention. lt is suggested that the goldand arsenic is hosted in brittle deformed clear to milky quartz veins which developedat zones of competency contrasts in all the li tholog ies in the Spitskop area.

[发布机构] Stellenbosch University