Browsing by Author "Reid, D L"
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- ItemRestrictedGeochemistry of the early Cretaceous Koegel Fontein anorogenic igneous complex, South Africa(2011) Curtis, C G; Trumbull, R B; de Beer, C H; Harris, C; Reid, D L; Romer R LThe Koegel Fontein Igneous Complex in the Western Cape Province of South Africa is a high-level granite-syenite intrusive suite with minor plugs and dykes of felsic and mafic composition. Age dating and field relations show that the felsic rocks intruded as two sequential series, beginning with syenites and the Kerskloof suite of felsic dykes (andesite-trachyandesite, trachyte-rhyolite) at about 144 Ma, followed by the Rietpoort granite pluton and quartz-porphyry dykes at about 134 Ma. The intmsion of the mafic dykes and plugs took place between the two felsic series. The early Cretaceous age of the Koegel Fontein associates its formation with Gondwana breakup and establishment of the volcanic rifted margin in southwest Africa. This paper reports results from the first geochemical saidy of the Koegel Fontein Complex. Two groups of mafic units are di.stinguished based on mineralogy, field characteristics and chemical composition. The tholeiitic group comprises basalt and basaltic andésites, the alkaline group ranges from basanite-tephrite to phono-tephrite and basaltic trachy-andesite. Distinctions of incompatible element concentrations (K, P, Ti) and ratios (Y/Nb, Zr/Nb) indicate the presence of two .separate magma series. Regional dolerite dykes (Gariep dykes and others) associated with contemporary magmatism along the continental margin occur in and near the complex. These regional dolerites are much more homogeneous compositionally than the mafic dykes of Koegel Fontein. Compositionally, they closely resemble the less-differentiated tholeiite dykes. The felsic units at Koegel Fontein show geochemical and isotopic evidence for two magma series, which correspond to the two intrusive episodes suggested by geochronology and field relations. The first series comprises the volumetrically minor intrusions of syenites, the Kerskloof dykes and the Rooivleitjie granite, with high values (eNd(t) from -0.4 to -1.8) indicating a strong mantle input. Similarities of trace element signatures suggest an affinity with the alkaline mafic units. The second series of felsic magmas makes up the bulk of the Koegel Fontein complex, with the large Rietpoort granite pluton and quartz-porphyry dykes (eNd(t) = -4.8 to -6.9). These rocks may be related to the tholeiitic mafic series but their isotopic composition indicates a large crustal component. The granitic rocks at Koegel Fontein have geochemical characteristics regarded as typical for anorogenic (A-type) granites, including high contents of Y, Nb, U and Th. In this and in many other features, the Koegel Fontein complex resembles the contemporary intrusive complexes in the Damaraland of northwest Namibia. Like the Damaraland granites, the Koegel Fontein magmas are of mixed origin, and their isotopic composition reflect the proportion of mantle vs. crustal source contributions.
- ItemRestrictedInvestigation of the potential for mineral carbonation of PGM tailings in South Africa(Elsevier, 2011) Vogeli, J; Reid, D L; Becker, M; Broadhurst, J; Franzidis, J.-PIncreasing atmospheric CO2 concentration is currently of considerable concern in terms of global warming. A possible technology that can contribute to the reduction of CO2 emissions is its sequestration by mineral carbonation. In this study, tailings from several different platinum mines in South Africa will be mineralogically characterised and their potential for mineral carbonation reviewed. Mg and Ca-rich minerals (plagioclase, olivine, orthopyroxene, clinopyroxene) present in the tailings are good candidates for mineral carbonation, which mimics natural weathering processes in which these minerals react with gaseous CO2 to form Ca or Mg carbonates. Since the reaction is influenced by particle surface area, the ultra fine grained nature of the PGM tailings provides another reason for the promise of PGM tailings for mineral carbonation. A preliminary ranking of the tailings samples and their efficacy for mineral carbonation has been developed according to whether the samples showed harzburgtic (e.g. Northam Platinum mine), pyroxenetic (e.g. BRPM) or noritic mineral assemblages. This information and understanding will assist in identifying opportunities and guiding the development of engineered facilities for the sequestration of CO2 by means of mineral carbonation.
- ItemRestrictedPetrogenesis of the False Bay Dyke Swarm Cape Peninsula South Africa: Evidence for basement assimilation(Geological Society of South Africa, 2011) Backeberg, N R; Reid, D L; Trumbull, R B; Romer, R LThe False Bay Dyke Swarm represents igneous activity associated with the opening of the South Atlantic during the early Cretaceous, and can be considered a distal component of the Etendeka-Paraná Large Igneous Province. In contrast to the intense magmatism in Namibia and Brazil, with the Paraná-Etendeka continental flood basalts and huge dyke swarms, the False Bay swarm is considered to be the product of a relatively low magma-flux in a passive rift setting. Previous work suggested that the False Bay dykes consist of a single magma type, with a marked differentiation from olivine-tholeiite basalt to ferro-tholeiitic andesite, accompanied by crustal contamination. This study uses new trace element and radiogenic Sr and Nd isotope analyses to better constrain the processes of magma evolution in the dykes. Combined assimilation − crystal fractionation (AFC) modelling suggests a first stage of nearly closed system fractionation of a gabbroic assemblage (olivine + clinopyroxene + plagioclase + magnetite), accompanied by progressively larger amounts of crustal assimilation at intermediate and late stage. The AFC models show that the exposed country rocks, including Cape Granites and Malmesbury Group metasediments, are compositionally unfavorable for producing the observed assimilation trends in the dykes. Instead, a more suitable crustal assimilant would be Mesoproterozoic granitic gneisses similar to those exposed in the neighbouring Namaqua Province, which may underly the Cape Peninsula.
- ItemRestrictedThe Bushmanland Group supracrustal succession, Aggeneys, Bushmanland, South Africa: Provenance, age of deposition and metamorphism(2007) Bailie, R; Reid, D LThe central portion of the structurally complex and poly-metamorphosed Namaqua Province of western South Africa is underlain by a series of pelitic and psammitic metasediments, termed the Bushmanland Group, which is host to world class Broken Hill-type Pb-Zn-Cu-Ag deposits at Aggeneys. Geochemical, U-Pb SHRIMP zircon geochronology and Sm-Nd isotopic determinations of the metasediments at Aggeneys were undertaken to determine the age and nature of the provenance, as well as to confine the age of deposition of the Bushmanland Group in this area. The metasediments were derived from moderately-weathered granodiorites to granites, or their volcanic equivalents, with ~2.65 to ~2.25 Ga Sm-Nd TDM model ages. Mixed sources are likely from the geochemical signatures of the metasediments as well as the varied detrital zircon ages, with two main age groupings present of ~2.12 to ~1.97 Ga and ~1.88 Ga to ~1.64 Ga. The upper part of the succession has a narrower compositional range and was derived from a more mafic provenance than the basal portion. The provenance is likely to have been a combination of ~1.8 Ga Gladkop Suite granitic gneisses found to the west and the ~2.1 to ~1.9 Ga intrusive and volcanosedimentary rocks of the Richtersveld Subprovince to the northwest. The metasediments were deposited in a passive margin setting and underwent extensive reworking in addition to recycling. The time of deposition of the Bushmanland Group is constrained between the age of the youngest detrital zircon (~1.64 Ga) and the onset of the regionally extensive Kibaran Orogeny at ~1.2 Ga. Metamorphic overgrowths and rims on detrital zircons record two distinct events, the ~1.21 to ~1.18 Ga Kibaran Orogeny, characterised by extensive granitic magmatism, peak D2 deformation and peak upper amphibolite facies grade M2 metamorphism, and the more ubiquitous ~1.04 to ~1.01 Ga Namaquan Orogeny characterised by weaker M3 metamorphism.