Thesis (MSc) -- Stellenbosch University , 1995. / ENGLISH ABSTRACT: A number of small greenstone bodies of the Bridgetown Formation are exposed as
elongated lenses and dykes within metasediments of the Malmesbury Group in the
Western Cape Province, South Africa. The Malmesbury Group is part of the
Neoproterozoic to Cambrian (Namibian) Saldania Subprovince which is the southern
continuation of a Pan-African mobile belt system.
A detailed geological and geochemical study was conducted on the largest outcrop
of the Bridgetown Formation, situated 20km east of the town Moorreesburg.
The Bridgetown Formation consists of a meta-volcano-sedimentary sequence that
experienced polyphase deformation and metamorphism up to the lower greenschist
facies. Tectonically, the Bridgetown Formation is included in the Boland tectonic
domain, east of the Piketberg-Wel lington fault zone that is suggested to run Skm
west 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 differentiated
alkaline metabasalt with a within-plate tectonomagmatic fingerprint. ii) An
intermediate unit of poorly differentiated tholeiitic metabasalt, intruded by a younger
tholeiitic metabasite with a low degree of differentiation. Both members of the
intermediate unit have ocean-floor basalt (P-type MORB) and island arc basalt
fingerprints. iii) An upper unit of poorly differentiated as well as more evolved
alkaline metabasalts, interlayered with metatuff with an alkaline basaltic composition,
metasedimentary rocks with a marine origin, and graphitic schists and muscovitequartz
schists, both with a continental crust provenance. iv) An overlying
metasedimentary sequence including dolomite, massive and oolitic chert, jasper and
jaspilite.
The Bridgetown Formation probably also comprises a lower metamorphosed
ultramafic unit, indicated by the association of Ni- and Cr-rich talc bodies, Ni-and errich
banded 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 metavolcanics
resemble Hawaiian volcanism , indicated by an initial deep water stage of alkaline
magmatism, followed by main tholeiitic edifice and post-caldera alkaline magmatism.
Post-caldera alkaline magmatism occurred contemporaneously with deposition . of
sediments and chemical precipitates (carbonates and cherts).
The Bridgetown metavolcanics have no magmatic association with either the
Bloubergstrand volcanics or mafic and intermediate plutonic rocks in the
Malmesbury Group. However, some physical and geochemical similarities exist
between the Bridgetown Formation and the age related Grootderm Formation of the
Marmora Terrane (Gariep Supergroup) which is considered to represent ophiolitic
material. The Bridgetown Formation probably represents segments of oceanic crust,
including seamounts of oceanic islands, which were tectonically emplaced in an
accretionary prism zone during subduction of oceanic crust underneath the Kalahari
Craton, 600 to 700 Ma ago. This resulted in the present spatial configuration of
various small greenstone bodies in the Malmesbury Group.
To date no exploitable mineral deposits have been found 1n the Bridgetown
Formation. However, Au and As anomalies in stream sediment and soil samples,
taken in the Spitskop area, require further attention. lt is suggested that the gold
and arsenic is hosted in brittle deformed clear to milky quartz veins which developed
at zones of competency contrasts in all the li tholog ies in the Spitskop area. / AFRIKAANSE OPSOMMING: 'n Aantal klein groenskisliggame van die Bridgetown Formasie is blootgestel as
verlengde lense en gange binne metasedimente van die Malmesbury Groep in die
Wes-Kaap Provinsie, Suid-Afrika. Die Malmesbury Groep is deel van die
Neoproterozo·iese tot Kambriese (Namibiese) Saldania Subprovinsie wat die
suidelike voortsetting is van 'n Pan-Afrikaanse mobiele gordel sisteem.
'n Gedetaileerde geologiese en geochemiese studie is gedoen op die grootste
dagsoom van die Bridgetown Formasie, gelee 20km oos van die dorp Moorreesburg.
Die Bridgetown Formasie bestaan uit 'n metavulkanies-sedimentere opeenvolging
wat pol ifase vervorming en metamorfisme tot en met die laer groenskis fasies
ondergaan het. Die Bridgetown Formasie word hier in die Boland tektoniese domein
ingedeel deur die Piketberg-Wellington verskuiwingsone 5km wes van Heuningberg,
subparallel a an die Bergrivier, te plaas. Dit stem ooreen met Rabie ( 197 4) se
oorspronkl ike verdeling van die tektoniese domeine.
Die Bridgetown Formasie bestaan uit: i) 'n Basale eenheid wat hoofsaaklik bestaan
uit min gedifferens ieerde alkali-metabasalte met binneplaat tektonomagmatiese
eienskappe. ii) 'n lntermediere eenheid wat bestaan uit min gedifferensieerde
tholeiitiese metabasalt en 'n jonger intrusiewe tholeiitiese metabasiet wat 'n lae
graad van differensias ie ondergaan het. Beide intermediere eenhede het
oseaanvloer-basalt (P-t ipe MORB) en eilandboog basaltiese eienskappe. iii) 'n
Boonste eenheid wat bestaan uit min gedifferensieerde asook meer
gedifferensieerde alkal i-metabasalte, tussengelaagd met metatuf met 'n alka libasaltiese
samestelling; metasedimentere gesteentes met 'n mariene oorsprong, en
grafitiese ski ste en kwarts-muskoviet skiste, beide met 'n kontinentale kors
oorsprong . iv) 'n Oorliggende metasedimentere opeenvolging wat dolomiet,
massiewe en ooli tiese chert, jaspis en jaspiliet insluit.
Die Bridgetown Formasie slu it moontlik ook 'n onderliggende gemetamorfiseerde
ultramafiese eenheid in; aangedui deur die assosiasie van Ni- en Cr-ryke
ta lkl iggame, Ni- en Cr-ryke gebande chert, chlorietskis en klein dolomiet-talk-chloriet
liggame by Spitskop, gelee direk noordwes van die hoof groensteenliggaam.
Die opeenvolg ing van magmatisme en die geochemie van die metavulkaniese
gesteentes stem ooreen met Hawaiiese vulkanisme, naamlik 'n diepwater stadium, gekarakteriseer deur alkaliese magmatisme, gevolg deur 'n hoof tholeiitiese
opbouing en post-kaldera alkaliese magmatisme. Die post-kaldera alkaliese
magmatisme het gelyktydig plaasgevind met afsetting van sedimente en chemiese
presipitate (karbonate en cherte ).
Die Bridgetown metavulkaniese gesteentes het geen magmatiese assosiasie met
6f die Bloubergstrand vulkaniese gesteentes 6f mafiese en intermediere plutoniese
gesteentes in die Malmesbury Groep nie. Fisiese en geochemiese ooreenkomste
bestaan egter tussen die Bridgetown Formasie en die Grootderm Formasie van die
Marmora Terrein (Gariep Supergroep) wat beskou word as ofiolitiese materiaal. Die
Bridgetown Formasie verteenwoordig moontlik segmente van oseaankors,
insluitende oseaan-eilande, wat tektonies in 'n melange sone ingeplaas is tydens
subduksie van oseaankors onder die Kalahari Kraton in (600 tot 700 Mj gelede). Dit
verklaar die huidige ruimte like verspreiding van verske ie klein groensteenliggame in
die Malmesbury Groep.
Tot en met hede is geen ontginbare mineraalafsettings in die Bridgetown Formasie
ontdek nie. Au en As anomalie in stroomsediment- en grondmonsters, geneem in
die Spitskop area, behoort egter verdere aandag te geniet. Daar is voorgestel dat
die goud en arseen voorkom in brosvervormde helder tot melkerige kwartsare wat
ontwikkel het in swak sones in al die litologie in die Spitskop area.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/54889 |
| Date | 06 1900 |
| Creators | Slabber, Nina |
| Contributors | Scheepers, R., 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 | English |
| Type | Thesis |
| Format | 98 leaves : ill. |
| Rights | Stellenbosch University |
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