Doctoral Degrees (Earth Sciences)
Permanent URI for this collection
Browse
Browsing Doctoral Degrees (Earth Sciences) by browse.metadata.advisor "Kisters, Alexander F. M."
Now showing 1 - 4 of 4
Results Per Page
Sort Options
- ItemGeological setting and evolution of the Omitiomire Cu deposit in the Southern Zone accretionary prism of the Damara Belt, Namibia(Stellenbosch : Stellenbosch University, 2017-12) Kitt, Shawn; Kisters, Alexander F. M.; Buick, Ian; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: Shear Zone hosted Cu mineralisation in the Deep Level Southern Zone accretionary prism of the Damara Belt is associated with deformation and fluid flow related to the exhumation of the Mesoproterozoic Ekuja basement gneiss dome during the Pan-African convergence of the Congo and Kalahari Cratons. In terms of tectonic setting, metamorphic grade and age structure, the mineralisation has no real equivalent in Namibia and seems to defy current classification schemes of Cu deposits. This study aims to describe the geometry and controls of mineralisation and the sources of fluids and metals in Omitiomire Cu deposit, and constrain the internal dynamics and processes that led to the imbrication of 100 km2 slivers of basement gneisses with the overlying prism metasediments. The Omitiomire Cu deposit (137 Mt at 0.54% Cu) is hosted by a low-angle, late-Damaran (ca. 520 Ma) shear zone system, referred to as the Omitiomire Shear Zone (OSZ), that developed around an older (ca. 1100–1060 Ma), late Mesoproterozoic intrusive breccia between a suite of mafic rocks (originally lava flows) and later tonalitic gneisses. The chalcocite-dominated mineralisation is associated with biotite-epidote assemblages that formed through the progressive hydration of the original mafic rocks to amphibole-biotite gneiss and biotite-epidote schist during deformation and fluid infiltration along the OSZ. Stable isotope (O, H and S) data indicate upper-greenschist-to middle amphibolite-facies conditions, a low fluid-rock ratio and a local redistribution of sulfur during fluid flow and mineralisation. This points to the remobilisation by epigenetic fluid flow of an older Cu mineralisation event. The geometry of the mineralisation is controlled by the spatial coincidence of the OSZ and the inherent heterogeneity of the original intrusive amphibolite–tonalite breccia. The gently undulating, shallowly-east dipping orebody is composed of several mineralised lenses that are contained in the regional S2 fabric. High-grade ore shoots are parallel to a prominent N-S trending L2 stretching lineation and are correlated with the cumulative number and thickness of several thin quartz-biotite-epidote-chalcocite shear zones at the contacts of interleaved schists and felsic gneisses. The kinematics and the association of the Cu mineralisation with retrograde assemblages in the OSZ link the mineralisation with the exhumation of the Ekuja Dome. The timing of exhumation is constrained to between 526.4 ± 3.5 and 521.9 ± 3.6 Ma by 40Ar/39Ar dating of biotite from the OSZ. Peak metamorphic assemblages in amphibolite gneisses from the Ekuja dome record PT conditions of 8.5-9.15 kbar and 635-655 °C, which equates burial to ca. 35 km. In contrast, PT estimates and U-Pb xenotime age determinations in the overlying prism metasediments show that peak metamorphic conditions of 7-9.25 kbar and 640-675 °C were only reached at ca. 515 Ma. This suggest that exhumation of the Ekuja dome started some ~10 Ma before maximum burial and peak metamorphism was reached in the overlying prism sediments and reflects the complexities of subduction-exhumation processes in accretionary complexes. The results of this study highlight the striking similarities with basement dome hosted deposits in the Lufilian Arc of Zambia and suggest the existence of a contiguous convergent margin along the leading edge of the Congo Craton that was active to at least ca. 515 Ma.
- ItemGeology, structural evolution and controls of hydrothermal gold mineralization in the Eastern Karagwe-Ankole fold belt, North Western Tanzania(Stellenbosch : Stellenbosch University, 2016-03) Koegelenberg, Corne; Kisters, Alexander F. M.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: In central-east Africa, the north-western margin of the Archaean Tanzania Craton (TC) is overlain by imbricated, low grade, volcano-sedimentary rocks of the Karagwe-Ankole belt’s (KAB) Eastern Domain (ED). Centred in the ED, vast stretches of a sheared and Au mineralized basement-cover contact are exposed along margins of the Mugera-Nyakahura (MN) inlier. To date, no detail research has been done in the area and the regional geology has been described from only broad reconnaissance studies. As part of an exploration project the first high resolution geological maps of key prospects and larger, more encompassing, scale maps of the basement-cover region was compiled. Mapping was supplemented by a regional scale structural traverse of the ED and selected sampling for analysis of micro-structures, geochronology and oxygen isotopes. The collective structural data has indicated that basement gneisses of the MN inlier may be considered as part of a forethrusted tectonic wedge caused by regional top-to-the-SE thick skinned thrusting. Above and in front of the wedge diagnostic back-thrusts and the reversal of kinematic fabrics in weak, often graphitic, metapelitic rocks of the Muyaga Group depict top-to-the-NW, hinterland-directed, tectonic transport along the main “roof” detachment. To the east under-thrusting of the coarse clastic Bukoba Group by the Muyaga Group has also created a distinct triangle zone at the frontal termination of the KAB. 39Ar-40Ar muscovite ages of detachment mylonites in the easternmost, and latest developing, parts of the KAB has constrained timing of D2 to at least 1326 ± 10 Ma. This age corresponds with the youngest end of the main phase of granite plutonism and mafic dyke emplacement in the KAB (1380 – 1328 Ma) and may point towards a Mesoproterozoic collisional event between the Congo- and Tanzania Cratons. U-Pb detrital zircon ages of the Muyaga- and Bukoba Groups have indicated uplift, erosion and subsequent reworking of Muyaga Group sediments and layered volcanics into the Bukoba basin after 1780 Ma, but before 1568 Ma. The Bukoba Group thus correlates with the Bwezigoro Group in SW Uganda, indicating the presence of an extensive Paleo- and/or Mesoproterozoic foreland basin overlying the western margin of the TC and Uganda Block. Lastly, controls of D2 fluid flow and mineralization along the low angle phyllonitic detachment are linked to NE trending ramp structures that were most favourable for the initiation of slip and development auriferous quartz vein networks. Upwards into the Muyaga Group progressive fold amplification and eventual fold-lock of second order anticlines, cored by competent and chemically re-active ferruginous mafic sills, are responsible for the late-kinematic development of auriferous quartz veins. Oxygen isotope values of D2 quartz veins and host rocks have indicated that fluids are derived from dehydrated clastic sediments of the Kagera Supergroup and, as such, may suggest that gold associated with greenstones of the TC have not been remobilized during D2 fold-and-thrust development in the ED. Collectively these findings greatly enhance the understanding of the geological evolution of the KAB’s easternmost parts and provides future research and exploration with a much improved geological background.
- ItemGeometry and emplacement controls of dolerite sill complexes in the Karoo basin(Stellenbosch : Stellenbosch University, 2020-04) Coetzee, Andre; Kisters, Alexander F. M.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: This dissertation is a compilation of three case studies that integrates drilling, field and geophysical data on dolerite sill complexes across the Karoo basin to constrain the emplacement processes, feeder systems and regional controls during the formation of the Karoo Large Igneous Province (KLIP) at ca. 183 Ma. Chapter 4 describes the relationships between several saucer-shaped sills in the sedimentary strata of the northern Karoo basin and underlying Archaean to Neoproterozoic basement strata of the Kalahari craton. The three-dimensional geometry of numerous individual sills across an area of 7400 km² were reconstructed using extensive drill hole and mining data up to a depth of 2.5 km. Saucer-shaped sills, largely confined to the Karoo Supergroup and representing various geometries, dip towards distinct narrow, funnel-like dolerite structures seated exclusively within the basement layers that likely represent a connection to deep-seated feeders. Importantly, many of these funnel-shape structures are localised near or rooted in older, dykes and normal faults in the basement strata. This highlights the significance of preexisting basement structures in facilitating magma transport through the upper crust and into the Karoo basin. Furthermore, the wide spatial distribution of magma feeders across the Kalahari craton is more aligned with thermal incubation of the sub-lithospheric mantle than the development of mantle plumes beneath Gondwana. Chapter 5 constrains the emplacement controls of sills in the central Karoo basin through field and drill hole data from the Victoria West sill complex. The sill complex consists of five successively emplaced saucers-shaped sills across an area of 2000 km2 that formed though magmatic underaccretion beneath earlier sills. Rigid and fully crystallised earlier sills served as stress barriers to the upwards propagation of later sill feeders which resulted in a nested sill structure marked by a sill-in-sill configuration. The close spatial overlap between the individual saucer-shaped sills and NW-SE trend of the sill complex suggest the reutilization of the same feeder system, most likely the rim of a large underlying sill. Emplacement controls of the Victoria West sill complex indicates sill formation occurred under a mild compressional stress regime at the onset of Karoo magmatism. The changeover from sill complexes at 184-180 Ma to later dyke swarms at 182-174 Ma suggest a switch in the prevailing stress field prior to Gondwana break-up. Thermal loading followed by lithospheric subsidence due to the rapid emplacement of >350 000 km3 of sills and outpouring of >106 km3 of lavas across southern Africa and Antarctica may have triggered extension and the subsequent intrusion of dyke swarms along cratonic margins. Therefore, inherited basin and lithospheric architecture was crucial in the later development of Karoo magmatism. Chapter 6 represents a study of the distinct variations of sill geometries and associated intrusive structures across the Karoo basin in relation to the basal elevation of the Drakensberg Group lavas. These spatial variations in the mode of emplacement of sills are attributed to emplacement depth in addition to changes in sedimentary facies and driving magma pressures. The northern Karoo basin is characterised by shallow (<500m) emplacement depths with sills typically showing small (<10 km) diameters and thicknesses up to 40 m. The central Karoo basin represent intermediate emplacement depths of up to 700 m with sill diameters over 30 km and thicknesses below 100 m. Advanced emplacement depths of <2 km is found in the southern Karoo basin where sills reach sizes of 50-80 km and thicknesses of 35 m. Local dyke networks show similar depth dependent relationships across the Karoo basin. Dense systematic and non-systematic dyke networks are associated with respective shallow (<500 m) and intermediate (<700 m) emplacement depths whereas greater depths (<2 km) are largely devoid of dykes. Additionally, the asymmetric distribution of different sedimentary rock types and southward thickening of layers in addition to differential magma pressures across the Karoo basin further influenced sill emplacement processes and associated structural features. Generally, the wide spatial and stratigraphic occurrence of sills throughout the Karoo basin is more consistent with many distributed feeders emplaced in the underlying basement rocks than a single plume-related source along the rift margins of southern Africa.
- ItemThe processes of melt segregation, magma ascent and pluton emplacement in the continental crust of the Damara Belt, Namibia(Stellenbosch : Stellenbosch University, 2016-03) Hall, Duncan James; Kisters, Alexander F. M.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: Theoretical models of meso- and macroscale granitic melt/magma transport commonly invoke a prominent role for dilatant fracturing of the wall rocks, but supporting field evidence is in many cases only scantly presented. This dissertation is a compilation of three case studies that integrate some of the theoretical models with field-based aspects from the obliquely exposed south Central Zone magmatic arc of the Damara Belt in Namibia. Chapter 3 describes interconnected leucogranite networks that accommodate the transport of melt at the outcrop-scale in homogeneous, high-grade near-source gneisses in the western parts of the south Central Zone. The net-structured leucogranite networks include small intrafolial stringers, larger shear-band-hosted veins and still larger fracture-hosted leucogranite sheets, which display a size-based hierarchical distribution that results from the self-organisation of the near-source melt transport pathways. The development of the networks is controlled intrinsically by variations in the rate of melt supply. During periods of elevated melt supply and under conditions of low differential stress, melt-induced extensional fracturing generates hydraulic gradients that result in the formation of melt sheets which represent potential far-field ascent conduits (dykes sensu lato). However, the orientations of the sheets are controlled by wall-rock structures and regional and local stress fields, resulting in this case in the preservation and eventual crystallisation of shallowly-dipping leucogranite sheets that were unfavourably orientated to accommodate buoyancy-driven melt/magma drainage. Chapter 4 describes the actual drainage of magma along steeply-dipping fracture conduits initiated in well-layered upper amphibolite-facies basement gneisses ~30 km SE of the outcrops presented in chapter 1. Here, the moderately-dipping gneisses contain large-scale leucogranite networks that include deca- to hecto metre-scale subvertical leucogranite lenses interpreted to document the near-source accumulation of magma necessary for efficient far-field ascent. Field relationships also document the drainage of the subvertical lenses at critical lengths and volumes in excess of 100 m and ~2.4×105 m3 respectively. These values as well as the field relationships are broadly consistent with theoretical models of magma transport along mobilised (self-propagating) hydrofractures. Self-propagating hydrofractures propagate at their tips and close simultaneously along their tails, leaving behind only very subtle and easily overprinted evidence, which may account for the elusiveness of granite conduits in the mid-crust. This work presents the first field evidence of large-scale magma drainage along self-propagating hydrofractures and has important consequences for the subsequent transport of magma through and eventual incremental emplacement within the subsolidus crust. Chapter 5 examines in more detail the implications of fracture-controlled magma ascent for granite pluton emplacement in lower amphibolite-facies wall rocks ~80 km northeast of the near-source features presented in chapters 3 and 4. Three closely-spaced tabular granitoid plutons that were emplaced roughly contemporaneously at widely-varying structural levels are investigated. Each of the plutons demonstrates the effect of mechanical contrasts in the wall-rock that favour fracture arrest rather than ascent, particularly rigidity and rheological contrasts. However, contrary to the theoretical models, it is shown that rigidity contrasts which cause fracture arrest need not be associated with lithological contacts and the role of increasing differential stress during the fracture-controlled ascent of magma towards the brittle-to-ductile transition is highlighted. The three chapters outlined above highlight the importance of fracture-controlled magma transport at all scales in the Damara Belt and substantiate some of the theoretical concepts, but also highlight prominent discrepancies between the models and natural systems.