A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2016 / The Mesoproterozoic Pilanesberg Complex, South Africa, is the world’s largest alkaline intrusive complex. Mapped geological field relationships suggest the Complex has circular inward dipping layers. However, it is unclear how the dipping layers extend at depth. As a result, the 3D geometry of the Pilanesberg Complex is unknown. Modelling of the Pilanesberg Complex uses 2D forward models as well as 3D forward and inversion, gravity and magnetic data models, to set limits on the 3D geometry of the Pilanesberg Complex. The 2D Bouguer gravity models and geology maps indicate that some of the Bushveld Complex Main Zone shifted to the west of the Pilanesberg Complex during emplacement. This, and a highly faulted country rock, accounts for a portion of how the host rock was able to accommodate the Pilanesberg Complex intrusion. The geometry of the Complex is explored with test gravity models where the model of outward dipping and vertically dipping cylinders are unable to match the Bouguer gravity signal over the Complex, but the inward dipping model matched the data to provide a possible solution for the geometry of the Complex. The Pilanesberg Complex geometry is modelled with 3D magnetic inversion, 3D forward gravity models and 2.5D gravity test profiles that were all constrained by the surface geology. The different models correlate so that best data fit for the Complex is represented by an overall inward dipping structure. Surface geological measurements indicate that the northern edge of the Complex dip out to the north. The 3D forward modelling was able to produce a positive solution that matched the gravity data with a northward dipping northern edge. The dipping northern edge is also observed on the University of British Columbia, UBC, 3D gravity inversion and the Euler deconvolution gravity profile solutions. The depth of the Pilanesberg Complex from 3D forward gravity modelling is estimated to be between 5 and 6 km. The Complex is suggested to have undergone block movement where the northern block and southern block are separated by the 30 km long Vlakfontein fault, which bisects the Complex from the north-east to the south-west. The image processing contact depth, Euler deconvolution solutions and the 3D Voxi inversion model suggest that the fresh bedrock is closer to surface in the north, while the southern block appears to be approximately 1km deeper than the northern block. The northern dip and block movement are explained by complicated structural events that include trap door graben settling which hinged on the northern edge as well as faulting and external block movement during a regional lateral extensional event. The Pilanesberg Complex intruded during a larger system of alkaline intrusions, known as the Pilanesberg Alkaline Province. The intrusions are associated with the Province due to their ages and chemical affinity. This Province includes two dyke swarms that radiate to the north-west and south of the Pilanesberg Complex, as well as smaller circular clinopyroxenite intrusions throughout the Bushveld Complex. The Pilanesberg dyke system and the circular clinopyroxenite intrusions are reversely magnetised with IGRF corrected values ranging between -150 to -320 nT compared to the normally magnetised 166 to 330 nT values of the Pilanesberg Complex. This suggests that a magnetic reversal occurred between the emplacement of the Pilanesberg Complex and the dyke System. The age data of the Complex and dyke Swarm suggest a magnetic reversal could have occurred between the emplacement of the Pilanesberg Complex and the Pilanesberg dyke System. The Complex is dated at 1602 ± 38 Ma and 1583 ± 10 Ma, from two white foyaite samples from the southern edge (using 40Ar/39Ar amphibole spectrum analysis). These ages are vastly different from previously reported ages, which ranged between 1200 Ma and 1450 Ma (Harmer R., 1992; Hansen et al., 2006). The error analysis has improved considerably from the published dates making the proposed dates plausible for the intrusion of the Pilanesberg Complex as the first and main intrusion of the Pilanesberg Alkaline Province. The Pilanesberg dyke System intruded much later between 1219 ± 6 Ma to 1268 ± 10 Ma for the red syenite dyke samples (using 40Ar/39Ar on feldspars spectrum analysis) and 1139 ± 18 Ma obtained for the grey syenite dyke (using 40Ar/39Ar on amphiboles inverse isochronal analysis). The dyke Swarm dates are significantly younger than the previously published ages for the dykes, which were between 1290 Ma and 1330 Ma (Van Niekerk, 1962; Emerman, 1991). / LG2017

MSc. (Geology) / The Mesoproterozoic Pilanesberg Alkaline Complex is located in the north central region of the Kaapvaal Craton of South Africa and is one of the largest alkaline intrusive bodies in the world. It is also one of the least studied due to the fact that the dominant exposure of the complex forms part of the Pilanesberg National Park. The Pilanesberg Complex intruded into the western limb of the Paleoproterozoic Bushveld Complex, the complex is a circular structure of 530km2 appearing on the map as a set of concentric rings of different varieties of syenites, capped by extrusive pyroclastic units and lavas. The present study is focused on the petrography and mineral chemistry of the rocks and minerals in an outcrop of mainly lujavrite, which is located in the Ledig nepheline syenite previously referred to as Ledig foyaite, named after an informal settlement by the name of Ledig which is situated approximately a kilometer south of the outcrop. This unit is emplaced in the southern region of the Pilanesberg complex and is believed to be a hybrid of the white nepheline syenite and the green nepheline syenite. A number of different rock types were examined from this outcrop, however, detailed work was only carried out on the lujavrites, in particular the eudialyte bearing lujavrites. The lujavrites consist of porphyroblasts of heterogeneously-sized feldspars, feldspathoids and mafic minerals set within a finer grained material. The bulk rock geochemical data on the Ledig lujavrites indicate considerable variation, but no obvious trends. A selected number of samples collected from the ledig lujavrite outcrop were geochemically analysed, these samples sit within the nepheline syenite field and have a shoshonitic affinity. The samples analysed have a relatively wide range of SiO2 content (20.39% to 52.67%), however the majority of the samples fall between 47.07 and 52.67%, the fluorite rich sample analysed has the lowest SiO2 as well as the lowest alkali content. The lujavrites and tinguaites are silica undersaturated and rich in alkalis. All the samples analysed are alkaline in nature and fall in the ferroan field. There is some variation in the SiO2 content and a large variation in Mg# in the sample population, this appears to be due to different proportions of minerals occurring as phenocrysts. No clear trends emerge, which is in part due to the very large differences in element concentrations within rock types with similar SiO2 content and Mg#. A lack of geochemical variation stemming from Abstract differentiation was expected as the samples were collected from a single outcrop essentially within a single lithology. There are a number of different phases of eudialyte that have been identified and studied from the Pilanesberg, at both the outcrop and in the northern area of the green lujavrites. At the Ledig lujavrite outcrop, there are euhedral magmatic eudialytes, which contain nepheline inclusions as well as post-magmatic eudialytes present in the samples studied. The two textural types have distinctly different chemical compositions. The majority of the feldspars present within the Ledig lujavrites appear, from their texture, to be primary magmatic minerals, however there are also feldspars present within the lujavrites which appear to be a product of secondary unmixing of feldspars as documented by the perthitic textures. Sodalite is present as a magmatic mineral; however, it is most commonly observed in the interstitial spaces and is thought to be an alteration product of nepheline. Analcime occurs in the groundmass, forming in the intergranular reaction rim between mineral phases. The analcime is typically controlled by the shape of the interstices...