Tectonostratigraphic evolution of the Swartland region and aspects of orogenic lode-gold mineralisation in the Pan-African Saldania Belt, Western Cape, South Africa

[摘要] ENGLISH ABSTRACT:The Swartland region in the western Cape, South Africa, covers approximately 5000km2 and forms part of the Pan-African Saldania Belt that represents thesouthernmost extremity of the Pan-African orogenic belts in southern Africa. Regionalmapping of the Swartland area shows that lithologies can be classified usingpredominantly structural and to a lesser extent lithological criteria. This led to theproposal of a new classification, were rocks of the previous classification of theMalmesbury Group are divided into two new groups, namely the Swartland andMalmesbury groups.The Swartland group can be divided into the Berg River and Moorreesburgformations, a series of quartz-chlorite-muscovite-feldspar schists, quartz schists,graphitic schists and limestones; and the Bridgetown formation, a series ofmetavolcanic rocks with WPB-MORB affinities that possibly represent seafloor.Deposition of the sediments is suggested to have occurred concurrently withdeformation in an accretionary prism/fore-arc and was initiated with the opening ofthe lapetus Ocean at ca. 600 Ma. This early deformation event, Dt (ca. 575 Ma), onlyaffected the Swartland group and exhibits pervasive bedding transposition, thrustingand imbrication of units creating a tectonostratigraphic sequence. Where identified,kinematic indicators and fold vergence indicate a top-to-the-west transport directionduring the early, low-angle Di deformation.The Malmesbury group overlies the Swartland group, being locally separated by anunconformity. The Malmesbury group is a succession of conglomerates, grits andshales (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 after560 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 MaCape Granite Suite. The Franschhoek Formation, formally part of the MalmesburyGroup is now classified, along with the inferred ca. 535-510 Ma Magrug andPopulierbos Formations of the previous Klipheuwel Group. The redefined Klipheuwelgroup documents a change in depositional environment from the continentalslope/ocean trench, marine and flyschoid deposits of the Malmesbury group tocontinental, fluvial half-graben and graben deposits. Exhumation, extensive erosionand the formation of a peneplain, was followed by the deposition of the TableMountain Sandstone Group around 550-510 Ma.The Spitskop gold prospect, located 10 km south of Piketberg, represents the firstidentified occurrence of mesothermal gold mineralisation in the Saldania Belt.Metamorphic devolatilisation of the Swartland group during Di led to the scavengingand transportation of gold along shallow-dipping shear zones that are containedwithin the early, sub-horizontal So/Si tectonic fabric. Pervasive fluid movement in theSpitskop area led to elevated gold values compared to background valuesthroughout the lithologies at Spitskop. The lack of any economic-grade goldmineralisation is probably related to the absence of suitably orientated structures,such as high-angle faults, that are commonly believed to represent the prerequisitefor large fluid throughputs that could result in economic-grade gold deposits. Themineralisation at Spitskop, however, provides a genetic model for further explorationof gold in the Swartland group.

[发布机构] Stellenbosch University