Department of Earth Sciences

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Browsing Department of Earth Sciences by Author "Belcher, Richard William"

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    Tectonostratigraphic evolution of the Swartland region and aspects of orogenic lode-gold mineralisation in the Pan-African Saldania Belt, Western Cape, South Africa
    (Stellenbosch : Stellenbosch University, 2003-12) Belcher, Richard William; Rozendaal, Abraham ; Kisters, A. F. M.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The Swartland region in the western Cape, South Africa, covers approximately 5000 km2 and forms part of the Pan-African Saldania Belt that represents the southernmost extremity of the Pan-African orogenic belts in southern Africa. Regional mapping of the Swartland area shows that lithologies can be classified using predominantly structural and to a lesser extent lithological criteria. This led to the proposal of a new classification, were rocks of the previous classification of the Malmesbury Group are divided into two new groups, namely the Swartland and Malmesbury groups. The Swartland group can be divided into the Berg River and Moorreesburg formations, a series of quartz-chlorite-muscovite-feldspar schists, quartz schists, graphitic schists and limestones; and the Bridgetown formation, a series of metavolcanic rocks with WPB-MORB affinities that possibly represent seafloor. Deposition of the sediments is suggested to have occurred concurrently with deformation in an accretionary prism/fore-arc and was initiated with the opening of the lapetus Ocean at ca. 600 Ma. This early deformation event, Dt (ca. 575 Ma), only affected the Swartland group and exhibits pervasive bedding transposition, thrusting and imbrication of units creating a tectonostratigraphic sequence. Where identified, kinematic indicators and fold vergence indicate a top-to-the-west transport direction during the early, low-angle Di deformation. The Malmesbury group overlies the Swartland group, being locally separated by an unconformity. The Malmesbury group is a succession of conglomerates, grits and shales (Piketberg Formation), grading into greywackes, shales, siltstones, sandstones and minor limestones of the Tygerberg and Porterville formations. Sedimentation probably commenced after ca. 575 Ma and lasted until shortly after 560 Ma. Both the Swartland and Malmesbury groups were then deformed by the deformation event, D2 (ca. 552-545 Ma), and were intruded by the 552 to 510 Ma Cape Granite Suite. The Franschhoek Formation, formally part of the Malmesbury Group is now classified, along with the inferred ca. 535-510 Ma Magrug and Populierbos Formations of the previous Klipheuwel Group. The redefined Klipheuwel group documents a change in depositional environment from the continental slope/ocean trench, marine and flyschoid deposits of the Malmesbury group to continental, fluvial half-graben and graben deposits. Exhumation, extensive erosion and the formation of a peneplain, was followed by the deposition of the Table Mountain Sandstone Group around 550-510 Ma. The Spitskop gold prospect, located 10 km south of Piketberg, represents the first identified occurrence of mesothermal gold mineralisation in the Saldania Belt. Metamorphic devolatilisation of the Swartland group during Di led to the scavenging and transportation of gold along shallow-dipping shear zones that are contained within the early, sub-horizontal So/Si tectonic fabric. Pervasive fluid movement in the Spitskop area led to elevated gold values compared to background values throughout the lithologies at Spitskop. The lack of any economic-grade gold mineralisation is probably related to the absence of suitably orientated structures, such as high-angle faults, that are commonly believed to represent the prerequisite for large fluid throughputs that could result in economic-grade gold deposits. The mineralisation at Spitskop, however, provides a genetic model for further exploration of gold in the Swartland group.