M.Sc. / Please refer to full text to view abstract

D.Phil. / The western margin of the Archean Kaapvaal Craton, at its contact with the polydeformed and metamorphosed Proterozoic Namaqua Province, is host to four volcanosedimentary successions of Mesoproterozoic age (1.1-1.3 Ga) that occur in close spatial and temporal association to each other. These are the Areachap Group, the Leerkrans Formation of the Wilgenhoutsdrif Group and the two volcanosedimentary successions that comprise the Koras Group. There has been protracted debate as to the exact nature, origin, age and tectonic evolution of these successions, particularly as they occur immediately adjacent to an important crustal suture. A comprehensive whole rock and isotope geochemical study, complemented by zircon-based geochronology where necessary, was thus carried out to characterize and compare the volcanic rocks associated with these four successions. The results are used to assess the role of the four volcanosedimentary successions during the development of the Mesoproterozoic suture between the Kaapvaal Craton and the Namaqua Province during the ~1.2-1.0 Ga Namaquan Orogeny. The geochemical study of the Areachap Group examined a suite of lithologies from different locations along the ~280km long outcrop belt, with the aim of testing the lateral continuity and integrity of this highly metamorphosed and deformed succession. As the bulk of the samples collected were from diamond drill core intersecting volcanogenic massive sulphide (VMS) Zn-Cu deposits it was only appropriate to extend the investigation to assess the metallogenesis and relation of these deposits to their host rock sequences. This included a survey of the sulphur isotope composition of sulphides and sulphates that comprise the Zn-Cu deposits. Furthermore, the architecture and origin of the world-class Copperton deposit, the largest Zn-Cu deposit of the Areachap Group, was examined. For this purpose, available literature data were collated and complemented by new geochemical and geochronological information. Sm-Nd isotopic systematics and U-Pb zircon ages suggest a coeval origin and close genetic link between the metavolcanic rocks of the Leerkrans Formation of the Wilgenhoutsdrif Group and the Areachap Group. Both successions record the establishment of an eastward-directed subduction zone on the western margin of the Kaapvaal Craton. The Areachap Group represents the highly metamorphosed and deformed remnants of a Mesoproterozoic (ca. 1.30-1.24 Ga) volcanic arc that was accreted onto the western margin of the Kaapvaal Craton at ~1.22-1.20 Ga, during the early stages of the Namaquan Orogeny. The igneous protoliths within the Areachap Group are low- to medium-K tholeiitic to calc-alkaline in composition ranging in composition from basaltic through to rhyolitic. Tholeiitic basalts, represented by volumetrically minor amphibolites within the succession have Sm-Nd isotopic characteristics indicative of derivation from a depleted mantle source as denoted by their positive Nd(t) values. The lithogeochemical results highlight the fact that, despite differences in lithological architecture on a local scale, the Areachap Group exhibits coherent geochemical characteristics along its entire strike length.

>Magister Scientiae - MSc / The group of granites on the eastern margin of the Mesoproterozoic Namaqua sector of the polydeformed and highly metamorphosed Namaqua-Natal Province of southern Africa is known as the Keimoes Suite. The suite includes mixtures of diverse rock types not belonging to a single intrusive series and so it should be subdivided into more than one intrusive suite. The exact definition, extent, distribution and petrogenesis of these granites have been poorly defined in the past, with various authors defining the suite differently due to the lack of proper geochronology and geochemical data. The exact contact between the Namaqua sector and Kaapvaal Craton together with the role of the suite to the Namaqua tectonic evolution is still unclear. The granites of the Keimoes Suite are thought to mark the contact between the Namaqua sector and the Kaapvaal Craton. This study seeks to address the above mentioned problems by making use of new geochronology, isotope, major and trace element geochemistry together with petrography. The granites of the Keimoes Suite were previously grouped based on their degree of deformation. The geochronology, undertaken as part of this study, has proven that this classification is unfounded. The degree of foliation in these granites appears to be largely controlled by the abundance of platy minerals, such as biotite and muscovite, together with the intrusion mechanism, with deformational processes, such as shearing, playing a secondary role. The geochronology, together with geochemistry has helped to redefine the previously defined Keimoes Suite so that two well defined separate suites are recognized and the third is poorly defined due to lack of more samples of that age group. The new classification or grouping of the granites of the eastern Namaqua sector allows a more detailed examination of the tectonic evolution of this region. A member of the 1225 to 1200 Ma early syn-tectonic granites, the Josling Granite, shows a strongly developed foliation and was derived from a depleted source with a relatively low continental crustal component. This granite intruded during the time of arc accretion, and is associated with, and partly responsible for the D₁ deformation and M₁ metamorphism recognized in most of the rocks of the eastern terranes of the Namaqua sector. In terms of age, the syn-tectonic granites of the Augrabies Suite extend from 1200 to 1120 Ma and were largely derived from depleted sources with variable but more substantial amounts of continental crustal components as compared to the early syn-tectonic granite. The granites of this suite intruded during the period of peak D₂ deformation with peak magmatism between 1180 - 1135 Ma, and particularly around 1150 Ma, during the peak of metamorphism (M₂) caused by, and associated with these voluminous intrusions. The Keimoes Suite can now be defined as comprising granites of late- to post-tectonic age relative to the 1.2 - 1.08 Ga Namaquan Orogeny with magmatism occurring on the western side of the Kaapvaal Craton. The 1116 to 1066 Ma Keimoes Suite intruded during the stage of the Namaquan Orogeny in which there was continued indentation of the Kaapvaal Craton into the Namaqua sector with wrenching and shearing causing the development of rifting into which the granites intruded. The Keimoes Suite granites were derived from continental crustal sources and incorporated varying degrees of depleted source components. The intrusives and extrusives of this age occured after the main collisional event between the Namaqua Sector and the Kaapvaal Craton and are associated with the D₃ deformational event, imparting the thermal conditions leading to the M₃ metamorphic event of the rocks within both the Kakamas and Areachap Terranes. The suites mark the suture between the Archean Kaapvaal Craton and the Proterozoic Namaqua sector. The compositions of the granites of the individual suites were mainly controlled by the source with the degree of partial melting exerting a major control. The proportion of entrained peritectic assemblages and accessory minerals played a major role in controlling the compositions of the granites, particularly those of the trace elements. Variations within the compositions of the same suite are due to source heterogeneities. Generally, fractionation processes played a secondary role in influencing the composition of the granites. / Council for Geoscience and National Research Foundation

M.Sc. / The Kaapvaal craton is one of few regions on earth with an almost continuous record of wellpreserved supracrustal rocks ranging in age from ~3.5 Ga to the late Paleoproterozoic at ~1.75 Ga. In this study diagenetic carbonates from the Paleoarchean Buck Reef Chert and Joe’s Luck Formation of the Swaziland Supergroup, the Mesoarchean Thalu and Promise Formations of the Mozaan/Witwatersrand Supergroups and the Paleoproterozoic Timeball Hill and Silverton Formations of the Transvaal Supergroup were sampled and analyzed. The aim of the study was to determine possible variations in the composition of the carbonates through time and their significance especially with regards to microbial activity in diagenetic systems in early Earth history. Results indicate similar petrographic observations and geochemical signatures in diagenetic carbonates of iron formations in the Buck Reef Chert, Joe’s Luck and Griquatown Iron Formation. The carbonates all tend to be siderites with iron derived from hydrothermal input and all are depleted in 13C relative to Peedee Belemnite standard. It suggested that siderite formed as a result of microbial respiration. Microbes degrade organic matter and reduce iron in this process. This resulted in the depletion in 13C and in the precipitation of siderite. However in order for iron reduction to have occurred the reduced iron first had to be oxidized. This most probably occurred through iron oxidizing chemolithoautotrophs under microaerophilic conditions. Diagenetic carbonate concretions of the Thalu and Promise Formations are manganiferous and are highly depleted in 13C relative to PDB. There is also strong evidence for hydrothermal input of manganese and iron into the system because of positive europium anomalies. The carbonates from both of the formations strongly suggest the presence of some free oxygen. The reasoning behind this conclusion is as follows: The depletion of 13C in the carbonates points to microbial decomposition of organic matter and manganese respiration (the decomposition of organic matter by microbial MnO2 reduction) is shown to be the most reasonable process that led to the formation of the carbonate concretions. The implication is that MnO2 must first have been precipitated and that can only be achieved in the presence of free oxygen with the oxidation reaction often catalyzed by manganese oxidizing chemolithoautotrophs. The carbonates of the Timeball Hill and Silverton Formationsare calcites ad contain little no iron. There is also little or no evidence for hydrothermal input and the basin appears to be a clastic dominated. It is generally accepted that a major rise in oxygen in the oceans and the atmosphere occurred at about 2.32 Ga. This rise in oxygen levels is reflected in the diagenetic calcite concretions of the Silverton Formation. Both iron and manganese reduction where not very effective because of the depletion in the basin water of these two elements, organic carbon taken up in the calcite concretions, indicated by negative δ13CPDB carbonate values, was most probably derived from aerobic and/or nitrate respiration. The most important conclusion from this study is that sufficient free oxygen and hence oxygenic photosynthesis were present to oxidize both Fe and Mn at least as far back as the Paleo-Mesoarchean.