Thesis (PhD)--Stellenbosch University, 2003. / 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. / AFRIKAANSE OPSOMMING: Die Swartland streek in die Wes-Kaap, Suid-Afrika, beslaan ongeveer 5000 km2 en
vorm deel van die Pan-Afrikaanse Saldania-gordel wat die mees suidelike deel van
die Pan-Afrikaanse orogene gordels in suidelike Afrika verteenwoordig. Regionale
kartering van die Swartland streek dui aan dat die gesteentes geklassifiseer kan word
deur oorwegend strukturele, en tot 'n mindere mate litologiese kriteria te gebruik.
Gevolglik word ‘n nuwe klassifikasie voorgestel, waar gesteentes volgens die vorige
klassifikasie van die Malmesbury groep verdeel word in twee groepe, naamlik die
Swartland en Malmesbury groepe.
Die Swartland groep kan verdeel word in die Bergrivier en Moorreesburg formasies,
‘n reeks kwarts-chloriet-muskoviet-veldspaat skis, kwarts skis, grafitiese skis en
kalksteen; en die Bridgetown formasie, ‘n reeks metavulkaniese gesteentes met
WPB-MORB affiniteite wat moontlik oseaanvloer verteenwoordig. Daar word
voorgestel dat afsetting van die sedimente gelyktydig plaasgevind het saam met
vervorming in ‘n akkresionere prisma/voorboog, geinisieer deur die opening van die
lapetus Oseaan (ca. 600 Ma). Hierdie vroee vervorming, Di (ca. 575 Ma), het slegs
die Swartland groep geaffekteer en vertoon deurdringende verplasing van
gelaagdheid, oorskuiwing en imbrikasie van eenhede en het ‘n tektonostratigrafiese
opeenvolging gevorm. Waar identifiseer, dui kinematiese aanwysers en plooi
kanteling op ‘n bokant-na-wes beweging gedurende die vroee, lae hoek Di
vervorming.
Die Malmesbury groep oordek die Swartland groep, plaaslik geskei deur ‘n
diskordansie. The Malmesbury groep bestaan uit ‘n opeenvolging konglomeraat,
grintsteen en skalie (Piketberg formasie), wat gradeer in grouwak, skalie, sliksteen,
sandsteen en ondergeskikte kalksteen van die Tygerberg en Porterville formasies.
Sedimentasie het waarskynlik begin na ca. 575 Ma en het voortgeduur tot kort na 560 Ma. Beide die Swartland en Malmesbury groepe is hierna vervorm deur D2, (ca.
552-545 Ma) en daaropvolgend ingedring deur die 552 tot 510 Ma Kaap Graniet
Suite. Die Franschhoek Formasie, voorheen deel van die Malmesbury Groep, word
nou geklassifiseer tesame met die afgeleide ca. 535-510 Ma Magrug en Populierbos
formasies as deel van die voorheen geklassifiseerde Klipheuwel groep. Die
hergedefinieerde Klipheuwel groep dui op 'n verandering in afsettingsomgewing
vanaf die kontinentale glooiing/oseaantrog, mariene en flyschoiede afsettings van die
Malmesbury groep na kontinentale, fluviale half-graben en graben afsettings.
Herblootstelling, omvattende erosie en die vorming van ‘n skiervlakte is gevolg deur
die afsetting van die Tafelberg Sandsteen Groep random 520-510 Ma.
Die Spitskop goudvoorkoms, 10 km suid van Piketberg, verteenwoordig die eerste
identifiseerde voorkoms van mesotermale goudmineralisasie in die Saldania Gordel.
Metamorfe ontvlugtiging van die Swartland groep gedurende Dt het aanleiding gegee
tot die roofuitruiling en vervoer van goud langs laaghellende skuifskeursones in die
vroee, subhorisontale S0/Si tektoniese maaksel. Deurdringende vloeistofbeweging in
die Spitskop omgewing het aanleiding gegee tot verhoogde goudwaardes in
vergelyking met agtergrond waardes dwarsdeur die litologiee by Spitskop. Die gebrek
aan ekonomiese graad goud mineralisasie is waarskynlik verwant aan die
afwesigheid van geskikte georienteerde strukture, soos hoe hoek verskuiwings, wat
oor die algemeen beskou word as ‘n voorvereiste vir die toevoer van groot
hoeveelhede vloeistof wat kon aanleiding gegee het tot ekonomiese graad
goudafsettings. Die mineralisasie by Spitskop verskaf egter 'n model vir verdere goud
eksplorasie in die Swartland groep.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/49789 |
| Date | 12 1900 |
| Creators | Belcher, Richard William |
| Contributors | Rozendaal, A., Kisters, A. F. M., Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 318 p. : ill. |
| Rights | Stellenbosch University |
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