Doctoral Degrees (Earth Sciences)

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The anatectic history of Archaean metasedimentary granulites from the Ancient Gneiss Complex, Swaziland
(Stellenbosch : Stellenbosch University, 2012-03) Taylor, Jeanne; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: This study is an investigation of the anatectic history of high-grade paragneisses from the Ancient Gneiss Complex (AGC) in Swaziland. The work involved an integrated field, metamorphic, geochemical, geochronological and structural study of metasedimentary granulites from three separate, but spatially related areas of outcrop in south-central Swaziland, which were subjected to multiple high-grade partial melting events throughout the Meso- to Neoarchaean. The project has aimed to constrain the age(s) and conditions of metamorphism, so as to contribute to the understanding of geodynamic processes in the Barberton and AGC granite-greenstone terranes, as well as to investigate certain physical and chemical aspects of anatexis in the migmatites. The metamorphic record retained in these rocks, constrained by phase equilibria modelling as well as zircon and monazite SHRIMP and LA-ICP-MS geochronology, informs on the state of the mid- to lower-crust of the southeastern Kaapvaal Craton during key events associated with early lithosphere assembly and crustal differentiation. It also suggests that the region is comprised of more than one high-grade terrane. Two of the areas investigated experienced high-temperature metamorphism at ca. 3.23-3.21 Ga, in addition to a major 830-875º C, 6.5-7.6 kbar anatectic event at ca. 3.11-3.07 Ga. Intermediate and younger high-temperature events are recorded at ca. 3.18 Ga, ca. 3.16 Ga and 2.99 Ga. The timing of these metamorphic events coincided with the amalgamation of the eastern domain of the proto-Craton via subduction and accretion of micro-continental fragments at ca. 3.23 Ga, including the Barberton Greenstone Belt (BGB) and AGC terranes, as well as discrete episodes of crustal differentiation and potassic granitic magmatism between ca. 3.23 and 3.10 Ga. The third area investigated holds no record of Mesoarchaean metamorphism, but instead experienced a 830- 855 ºC, 4.4-6.4 kbar partial melting episode at ca. 2.73 Ga. This broadly coincided with the formation of a large continental flood basalt province, the ca. 2.71 Ga Ventersdorp LIP, and widespread intracratonic granitic magmatism on the Craton towards the end of the Neoarchaean. An explanation for the contrast in metamorphic record in the two terranes may be that the 2.71 Ga granulites represent a much younger sedimentary succession, and that granulites from the older terrane were left too restitic, after substantial partial melting during the Mesoarchaean, to record subsequent high-grade events. Finally, this study documents the details of S-type granitic magma production and extraction from a typical metapelitic source. Using the 2.73 Ga granulites from the AGC as a natural field laboratory, a case is made for the selective entrainment of peritectic garnet to the magma as a mechanism for generating relatively mafic, peraluminous S-type granite compositions. The work demonstrates the evolution of entrained peritectic garnet in such magmas, and is in strong support of a ‘peritectic phase entrainment’ process by which relatively mafic granite magmas are produced from melts which, in theory, should be highly leucocratic.
Austral summer and winter trace metal distributions in the Southern Ocean and Antarctic seasonal sea ice
(Stellenbosch : Stellenbosch University, 2021-03) Loock, Jean Christian; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The metabolic efficiency of carbon (CO2) sequestering microorganisms, such as marine phytoplankton, is subject to the availability of trace metal nutrients such as Manganese (Mn), Cobalt (Co) and Cadmium (Cd) among others. However, much of the Southern Ocean is characterised by a paucity of trace metals, resulting in low productivity despite excess availability of the macronutrients Nitrate (NO3), Phosphate (PO4) and Silicate (SiO4). Areas exhibiting this paradoxical condition are classified as High Nutrient Low Chlorophyll (HNLC). The paucity is related to the regional remoteness, which increases vulnerability to weak inputs via traditional sources (e.g. atmospheric dust, riverine discharge, and continental margins). Thus, regional productivity may also rely on alternative supply pathways such as internal recycling, hydrothermal vent inputs, winter resupply, and indirect inputs from the seasonal sea ice inventories. However, our understanding of these processes, and by association their impact on global carbon cycling, remains relatively limited with large gaps in the database, particularly concerning the winter state and trace metals in sea ice. This study endeavoured to constrain these poorly understood facets by reporting observations from a novel trace metal dataset spanning three research cruises (1 summer, 2 winter). This includes the first winter data for Cd (Chapter 2), and Co and Mn (Chapter 3) from within the HNLC Southern Ocean (GEOTRACES GIPY05_e transect), and the first multi-element trace metal (Mn, Fe, Co, Cu, Ni, Cd and Pb) and sympagic (sea ice) phytoplankton community data for pancake sea ice in winter (WOCE IO6 transect). Concentrations are reported for the total dissolvable (T, unfiltered) and dissolved (D, <0.2 μm) fractions. Analogous distributions of TCd and DCd were found in the austral summer across the sector, albeit in the surface (and mixed layer) where TCd > DCd owing to depletion of DCd. This results in partitioning of the Cd fractions (ΔTM = total – dissolved) as DCd is incorporated into the TCd fraction. Larger ΔCd values correlated to productivity, but further dependent on the phytoplankton community structure (diatom links) and metabolically related trace metal concentrations. Winter reoccupation of the Antarctic Circumpolar Current (ACC) revealed that TCd and DCd concentrations had increased within the mixed layer, while ΔCd values decreased (consistent with lower winter productivity). Moreover, despite elevated mixed layer concentrations in winter, and weak uptake, DCd maintained a highly predictable linear correlation to PO4 (summer = 592x – 539, R2 = 0.93; winter = 584x – 515, R2 = 0.87). I propose that the relationship is dominated, or set, by mixing with the upwelling UCDW within the Antarctic Zone (AAZ) but may ultimately be susceptible to uptake in the northernly advecting surface waters. In summer, LCo concentrations ranged from ca. 5.15 pmol/kg in surface waters in the Sub-Tropical Zone (STZ) up to ca. 37.7 pmol/kg proximal to the Sub-Antarctic Front (SAF). High concentrations in the surface near the SAF were linked to the Antarctic Intermediate Water (AAIW). In summer, surface DMn concentrations ranged from ca. 0.034 nmol/kg at 60°S in the Weddell Gyre to ca. 0.96 nmol/kg in the STZ. Extremely low concentrations (<0.125 nmol/kg) also characterised surface waters near the Polar Front (PF) at 50°S and at 65°S in the Weddell Gyre, and may co-limit productivity in the region. Thus, supplementation of Mn even via sporadic hydrothermal inputs, such as at 54°S (DMn = 0.67 nmol/kg at 300 m) may be hugely beneficial. LCo and DMn concentration increased significantly in winter in the surface and mixed layer along with decreased ΔCo and ΔMn. LCo and DMn remained strongly coupled in summer and winter, albeit with linear correlation plots unique to the season. Ice cores drilled from pancake ice samples collected within the Antarctic marginal ice zone (MIZ) at 61°S, 30°E revealed that DFe concentrations, and to an extent DMn, were heavily elevated (up to 40 times) in sea ice relative to their open seawater concentrations. Cores were analysed in segments (3 x ± 10 cm). Two trace metal cores were melted using different methods (direct melting and matrix matched melting), but the results were consistent in all segments (no melting method bias) and compared well to ranges reported in external studies. Chlorophyll-a (Chl-a) concentrations were also significantly elevated (>10 times) in sea ice relative to under ice seawater. Trace metal and Chl-a concentrations followed a high (top), low (middle), high (bottom) distribution within the core segments (c-shaped distribution). The phytoplankton community structure was dominated by diatoms and Phaeocystis.
Bulk geochemical, biomarker and leaf wax isotope records of Mfabeni peatland, KwaZulu Natal, South Africa since the late Pleistocene
(Stellenbosch : Stellenbosch University, 2016-03) Baker, Andrea; Routh, Joyanto; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Southern Africa is a topographically diverse region that is influenced by temperate, sub-tropical and tropical climates encompassing varying rainfall zones. The core regional contemporary climatic drivers are the large sea surface temperature (SST) gradients between the Atlantic and Indian Oceans, and seasonal fluctuations in the Inter Tropical Convergence Zone (ITCZ). Our understanding of how these two mechanisms interacted in the past and how ecosystems responded to these climate drivers is ambiguous, mainly due to a lack of continuous archives as a consequence of the region‘s semi-arid climate. The Mfabeni peatland is a 11 m thick continuous peat sequence that has been dated to ca. 47 kcal yr BP. It is the only known coastal peatland record in the summer rainfall zone of Southern Africa to transcend the Last Glacial Maximum (LGM), and gives us the opportunity to reconstruct high-resolution palaeoenvironment records under both glacial and interglacial conditions on the south eastern coastline of the African continent. A diverse set of geochemical techniques and analysis (bulk C and N elemental and stable isotopes; different biomarkers and leaf wax δ13C isotope) was undertaken on a 810 cm long core to reconstruct primary productivity, organic matter (OM) sources, rates of OM remineralisation, peatland hydrology and relative contributions of C3 and C4 plant matter into the peat deposit. These geochemical climatic indicators were used to infer precipitation intensities and relative temperatures at time of sedimentation and, in conjunction with other regional archives, the dominant mechanisms (Indian Ocean SST changes versus changes in the position of ITCZ) driving climatic fluctuations since the late Pleistocene were explored. We established the Mfabeni peatland to be a well-preserved and unique palaeoecological archive that recorded both environmental and climatic signals throughout the depositional history of the peatland. Even though the dominant OM source of the peat was terrestrial and emergent plants, there were definitive periods of predominant submerged macrophyte input, suggesting elevated water levels. A general positive trend was observed between the temperature and moisture proxies, however the local plant physiology (n-alkane chain lengths; ACLalk) and plant types (terrestrial vs aquatic and their influence on OM lability; CPIalk) was dominated by moisture availability as opposed to temperature variations, arguably due to the relatively moderate cooling experience in the sub-tropics during the LGM. The leaf wax C isotope data set established variability in the proportional balance of C3 and C4 plants, with interchanges between plant clades and inter family C3 and C4 switches in response to changes in environmental conditions. However, plant assemblage shifts were absent during some of the more ephemeral climatic events which we concluded was due to local hydrological overprinting. The Mfabeni archive correlates strongly with Mozambique Channel SST records, suggesting the dominant climate forcing factor in south eastern Africa to be the evaporation potential and advection of moisture from the adjacent Indian Ocean since the late Pleistocene. It was also noted that the Mfabeni record exhibited overall opposite environmental responses to Northern Hemisphere climatic events, suggesting an anti-phase coupling between the two hemispheres.
The Darling granite batholith
(Stellenbosch : Stellenbosch University, 1972-09) Schoch, A. E.(Aylva Ernest); Verwoerd, W. J.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
The Darling batholith is characterised by large scale hybridisation, but mainly consists of the coarsely porphyritic Darling granite. This granite changes gradationally into a biotite-rich variety which occupies a roughly elliptical area with a major, northwesterly trending axis of 30 km. The biotite granite envelops a large irregular body of hybrid granodiorite. Small intrusions of younger granite occur within the batholith namely the Klipberg and Contreberg granites and possibly the biotite-rich Dassenberg granite. Dassen Island is underlain by fine-grained granite which could be related to either the younger or coarsely porphyritic granites. A prominent northwesterly trending mylonite zone can be traced through Darling to Swartberg, and ultimately to Trekoskraal in the Saldanha batholith, but is not continuous since it occasionally changes into gneissic granite and is also interrupted by the younger intrusives. Quantitative mapping included measurement of matrix grain size, average maximum phenocryst length, xenolith distribution density, quartz nodule distribution density and average size, lineation, dark mineral index and gneissosity. On Dassen Island the distribution of tourmaline nodules was , determined. The results are displayed as small scale contour maps which show strong correlation between the various parameters. The average values of matrix grain size, average phenocryst length and xenolith distribution density are respectively 2-5mm, 20-60mm and 0-1,5 per m² for the Darling granite, and 1-2 mm, 5-20 mm and 1-9 per m² for the hybrid granodiorite. It was found that the matrix grain size decreases with increase in hybridisation. The spotty distribution pattern of tourmaline nodules on Dassen Island indicates addition of boron by assimilation of metamorphites and a late stage liquid immiscibility process. The granites :.have normal mineralogy and the K-feldspar of the phenocrysts is maximum microcline (Δ = 0,9 - 1,0). The hybrid granodiorite contains much pinitised cordierite and locally garnet. The deeply pleochroic bioti te is probably of the 2M1 polytype and has a higher Fe:Mg ratio in the hybrid granodiorite than in the granite (2,8 - 3,0 vs. 2,2 - 2,3). The intimately associated chlorite seems to be of the Ia polytype. The cordierite is of the normal and low temperature type with average intensity index of 2,7 , distortion index of 0,3 and 2 a of 63°. The xenoliths are predominantly quartzitic metagraywackes, but lime-rich types holding sphene and diopside were occasionally encountered. Thirteen new chemical analyses and thirty-one previously published analyses are used to calculate average composite analyses of the various rock types. The results of calculations employing Barth standard cell values indicate that the hybrid granodiorite could have originated by reaction between granite magma and Malmesbury quartzitic metagraywacke and pe-lite with a little limestone. A "granite differentiation index" based on weight percentages of (Ti02 + MgO + FeO + Fe2O3) and (Si02 + Na2O + K2O) shows a linear relationship between the granites in probable order of age. The magmatic differentiation trend is separated from the hybridisation trend on a 6alk - 2(al - alk) - (100 - 2al) diagram. Mesonorms and their cordierite variants are used to effect comparison with the experimental granitic system of von Platen (1965). The Darling and Contreberg granites plot near the relevant cotectic surfaces. A pilot experimental study of melting behaviour indicates that the Contreberg granite is closer to a minimum melt composition than the Darling granite. Comparison of alkali values with a M Na2O - M K2O Schreinemakers diagram of Korzhinskii (1959), shows that the alkali ratio of the older analyses may be incorrect, and indicates that the dark minerals have a greater effect on plagioclase composition than the amount of K-feldspar. The classification of granites by means of Harpum diagrams is shown to have little relevance to the reconstruction of the ancient thermodynamical variants. The Darling granite is correlated with the Hoedjies Point granite of the Saldanha batholith and on geochronological evidence probably corresponds in age (500- 600 m.y.) with the Cape Peninsula granite. The younger granites of Darling are tentatively correlated with the Cape Columbine granite of the Saldanha batholith. The northeastern boundary of the Darling batholith is a major fault, the Colenso fault, which is considered to extend as far as Northwest Bay, Saldanha. It is proposed that the Darling batholith occupies a down-faulted block within a graben and that the hybrid granodiorite represents a remnant synform of the roof rocks intruded by the granite. The younger granites constitute only four percent by volume of the batholith and may represent anatectic melts from a nearby subjacent source.
Developing protocols for XCT scanning of dense mineral ore samples with applications to geology and minerals processing
(Stellenbosch : Stellenbosch University, 2020-04) Bam, Lunga C.; Miller, Jodie A.; Becker, Megan; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: X-ray computed tomography (XCT) is a non-destructive technique capable of producing 3Dmineralogical and textural information from drill cores. The discrimination between mineralogical information of the drill cores was optimised by using the developed linear attenuation coefficient data bank that can automatically provide linear attenuation coefficient information. The discrimination between the minerals was further optimised by using the determined optimal scanning parameters. XCT technique is most effective when scanning low density samples or minerals with low linear attenuation coefficients. However, when scanning high density samples, the technique suffers from the lack of X-ray penetration which results in beam hardening. Beam hardening affects the true representation of mineralogical and textural information and this leads to the misrepresentation of the mineralogical and textural information. Beam hardening is not easily quantifiable because its impact on the sample information is not uniform and can result in a loss of sample information. To address this, it was proposed to use an aluminium standard when scanning high density samples which acted as a standard in order to quantify the degree of beam hardening in each slice of the sample volume. The aluminium standard sample not only quantified the degree of beam hardening but also determined the optimal sample size for scanning where no sample information is lost. The optimal sample size for scanning was determined to be 4mm when scanning samples with SG > 3. Even though the impact of beam hardening was minimised when using the optimal sample size the degree of beam hardening still affected the discrimination between minerals. This lead to the development of a simplified dual energy method in order to optimise the discrimination between minerals that are affected by beam hardening and result in high levels of noise within the images. The developed simplified dual energy method uses a combination of scanned volume data volume together with the simulated image. This combination has an advantage over the traditional dual energy method that uses two scanned volume data which is more time consuming. The simplified dual energy method effectively discriminated mineralogical information with no artefacts as compared to the traditional dual energy method which result in edge artefacts. The utilisation of the aluminium standard and the simplified dual energy method resulted in the reliable quantification of porosity information and 3D chalcopyrite grain size distribution (GSD). The quantified porosity information was largely in agreement with QEMSCAN results which show the importance of using the aluminium standard when scanning high density ore samples. The quantified 3D chalcopyrite GSD had a similar trend to the 2D QEMSCAN data but with coarser GSD as expected. This shows the effectiveness of the developed simplified dual energy method to optimise the discrimination of chalcopyrite in dense ore mineral samples. The reliable quantification of porosity and chalcopyrite information is important in minerals processing. Porosity is a component of texture and it is of relevance to physical processing where chalcopyrite is important in terms of inherent rock strength, its breakage, liberation properties and establishing geometallurgical units. The reliable quantification of the textural information using XCT shows that the technique can be adopted and adapted to any ore type with even more complex textures or mineralogies.
Differentiation and stabilisation of the Archean continental crust, the example of the northern edge of the Kaapvaal craton, South Africa
(Stellenbosch : Stellenbosch University, 2017-03) Vezinet, Adrien; Moyen, Jean-Francois; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The Earth is one of the only solid celestial bodies displaying, at present day, an internal activity significant enough to modify its surface. This activity is mostly pictured by plate tectonic motions, which triggers the magmatic activity of mid-ocean ridges as well as subduction zones and mountain ranges development. These geodynamics are the locus of the creation of the oceanic crust, the continental crust and the reworking of older crustal material respectively that deeply modify both physical and chemical properties of the Earth’s lithosphere. One of the main consequences of these processes is the chemical layering of the continental crust, composed of granodioritic to granitic upper levels while lower crustal levels are more mafic. Two crustal processes are assumed to be responsible for such a structure: (i) the crustal growth taking place at subduction zones and (ii) the crustal differentiation taking place within collision and accretionary orogens. The PhD work presented in this manuscript focuses on the recognition and the manifestation of Archean crustal differentiation processes. The Archean eon which represents 1/3 of the geological record is featured by both lithologies unrecognized in younger eons and cryptic geodynamics. Most of investigations concentrate on the characterisation of aluminium-rich lithologies that allow an accurate determination of the pressure-temperature evolution underwent by crustal materials during crustal thickening geodynamics. However, aluminium-rich lithologies - mainly represented by metasediments - account for only 10 % on average of Archean terranes whereas orthoderived gneisses (TTG + granite) - which also testify for crustal differentiation processes - form around 75 % of these terranes. The following contribution depicts an Archean composite grey gneiss complex located at the northern edge of the Kaapvaal craton is South Africa. Results carried out during this PhD study have major consequences on Archean geodynamics. Firstly, the Uranium-Lead isotope study of zircon grains from igneous lithologies of the northern Kaapvaal craton indicates an uninterrupted recording from ca. 2.97 Ga to ca. 2.68 Ga, namely during more than 300 Ma (which is the time that separates the Carboniferous/Permian transition from present day). This protracted recording sheds light on Archean tectonics and questions the ways and means of such geodynamics. Our isotope investigation, associated with field investigation and whole rock major and trace elements chemistry, are assumed to picture accretionary orogen geodynamics that contribute to a moderated crustal thickening. Secondly, the Lutetium-Hafnium isotope signature of the same zircon grains displays unprecedented results. Our analyses indicate that igneous lithologies from the northern Kaapvaal craton evolve on a single crustal trend through time. This is interpreted as the absence of exotic material involved in the accretionary orogen, i.e. the crustal block evolved in a thermodynamically closed system. It is therefore crucial to monitor the evolution of the grey gneiss complex forming phases through time. Geochemical analyses of dated phases show a noteworthy sequence of events with: . Emplacement of a basic complex (produced by partial melting of a depleted mantle) at ca. 2.97 Ga, which is subsequently partially melted, implying the, . Production and emplacement of TTGs during 100 Ma, from ca. 2.95 Ga to ca. 2.85 Ga, themselves subsequently differentiated into, . Granites which intrude the continental crust from ca. 2.85 Ga to ca. 2.75 Ga. These geodynamics are also marked by the emplacement of greenstone belts and associated sedimentary basins. The burying of supracrustal lithologies is recognized while ongoing accretionary orogen, however, no partial melting feature is described. After the intrusion of granites at ca. 2.85-2.75 Ga, partial melting reactions are recognized within supracrustal formations. Therefore, this study aims to demonstrate the key role of a rheologically strong granitic crust for the onset of an efficient burying process. Finally, the burying of supracrustal formations towards lower crustal levels triggers mantle metasomatism, ensued partial melting, production of mantle derived granite and therefore stabilisation of the entire crustal block. The PhD work presented here shed light on the Archean tectonics. This eon is perceived as a time where geodynamics were faster than present day due to a hotter and thus a more active Earth. Nevertheless, our results show that composite grey gneiss complexes may be built over a protracted time span, achieved through self-refinement of crustal materials. Grey gneiss complexes are only moderately investigated even though information enclosed in these lithologies is complementary with those from aluminium-rich rocks. Therefore, deeper investigations of these geological objects must be a central scope in order to improve the knowledge of the Archean eon and appears necessary for the building of even more realistic geotectonic models.
Distribution and characterization of marine iron-rich particles
(Stellenbosch : Stellenbosch University, 2013-12) Von Der Heyden, Bjorn Phillip; Roychoudhury, Alakendra N.; Myneni, Satish C. B.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: This thesis investigates questions surrounding the role that iron-rich colloids (nominally sized between 0.02 μm and 0.2 μm) and particulates (>0.2-0.45 μm) play in the context of the greater iron biogeochemical cycle. To this end, this study complements a review of reported size-fractionated iron (Fe) measurements with chemical and mineralogical data derived from synchrotron-based xray measurements. From an extensive literature review, the global surface ocean colloidal iron (cFe) pool is found to be highly dynamic, frequently exhibiting seasonal trends and nutrient-like behaviour. Spatial variability in surface ocean colloidal iron concentration is primarily a function of total iron supply, although the concentration and strength of iron-binding ligands, and inorganic thermodynamic constraints are additional influential factors. The size-fractionated study of colloidal Fe has rendered considerable evidence pointing towards direct or indirect biological utilization of this cFe pool; however, a more complete understanding of cFe-biological interaction necessarily requires better knowledge of cFe chemistry and mineralogy. To address these issues, this thesis documents the development of a novel x-ray microscopy and spectroscopy technique for determining the Fe speciation of individual Fe-rich particles under environmental conditions. Variations in the peak splitting in iron L3-edge XANES (X-ray Absorption Near-Edge Structure) spectra reflect changes in the local coordination environment surrounding the metal centre. Specifically, the energy splitting ( ΔeV) and intensity ratio of the split peaks at the L3-edge vary as a function of the Fe valence state, the number and chemistry of coordinating ligands and polyhedral distortion effects; and combinations of the two parameters are found to be characteristic of individual Fe minerals. To understand Fe speciation, the Δ eV versus intensity ratio plot was successfully applied to a variety of environmental Fe particles (greater than 20 nm diameter) collected from two ocean basins; the Southern Ocean and the south western Pacific Ocean. Speciation differences in Fe particles collected from the Southern Ocean show distinct compositional trends between the coasts of South Africa and Antarctica, with different Fe pools associated with the different oceanographic frontal zones. Despite the oxygenated nature of the seawater sampled, the presence of significant particle-hosted Fe(II) was observed in both the Southern Ocean at high latitudes, and at sampling sites proximal to the Kermadec Ridge in the Pacific Ocean. Ferrous iron particles at the latter study area were shown to be strongly associated with carbon functional groups, notably alcohol and carboxamine moieties. These findings, relating to particle chemical differences and associations with organic matter, have significant implications for our understanding of particle behaviour, their surface interactions and the role that they play in primary productivity and global elemental cycles.
The effect of winery wastewater irrigation on the properties of selected soils from the South African wine region
(Stellenbosch : Stellenbosch University, 2016-12) Mulidzi, Azwimbavhi Reckson; Clarke, Catherine E.; Myburgh, P. A.; Roychoudhury, A. V.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Due to an increase in wine production as well as an intensification of environmental legislation in South Africa, the need for guidelines for sustainable management of winery wastewater has increased. To address this, the first part of the study focused on the seasonal dynamics of the volumes and quality of undiluted winery wastewater. The soil chemical dynamics were monitored in two different soils that were irrigated with undiluted winery wastewater for three years. Over-irrigation with undiluted winery wastewater in combination with winter rainfall caused large amounts of cations, particularly K+ and Na+, to leach beyond 90 cm soil depth. Consequently, the leached elements are bound to end up in natural water resources over time. Irrigation with undiluted winery wastewater did not have a pronounced effect on soil pH(KCl). This was probably due to the decomposition of organic matter and the fact that the applied salts as well as dissolved organic or mineral acids leached beyond 90 cm depth. The practical application of irrigation with diluted winery wastewater was assessed in a pot experiment. Irrigations were applied under a rain shelter over four simulated irrigation seasons. Four soils varying in texture were irrigated with winery wastewater that was diluted to 3000 mg/L chemical oxygen demand (COD). The four soils were irrigated with municipal water as a control. The rate of K+ increase in the soil containing 20% clay was higher than in soils containing 13% clay, or less. This suggested that heavy soils will aggravate the risk of high K+ levels. The risk of Na+ accumulation increased linearly with the clay content in the soil. Low Ca2+ and Mg2+ concentrations in the diluted wastewater had no effect on the soil, irrespective of clay content. Irrigation with diluted winery wastewater increased soil pH(KCl) substantially in all soils over four simulated seasons. The soil pH increase was attributed to the addition of organic and mineral salts via the diluted winery wastewater to the soil. The effect of simulated rainfall on soils irrigated with winery wastewater was also assessed in a pot experiment. Six soils with different clay content were irrigated with winery wastewater diluted to 3000 mg/L over one simulated irrigation season. Thereafter, good quality river water simulating winter rainfall was added to the pots. The rainfall was simulated according to the long term averages of the regions were the soils originated. Leaching of cations, particularly K+ and Na+ occurred only from four of the six soils when winter rainfall was simulated. In one of the sandy soils, the simulated rainfall was too low to allow leaching. In the case of other soil where there was no leaching, high clay content of 35% in combination with low rainfall prevented leaching. Where three soils received the same amount of rainfall, more cations leached from the sandy soils compared to the two heavier soils. These trends indicated that leaching of cations was a function of soil texture and rainfall.
Evolution of the continental crust and significance of the zircon record, a case study from the French Massif Central.
(Stellenbosch : Stellenbosch University, 2018-03) Couzinie, Simon Gwenael; Moyen, Jean-Francois; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The formation of the continental crust is a major consequence of Earth di erentiation. Recent advances on this topic bene ted from improvements of analytical techniques enabling in situ measurements of U Pb Hf O isotope compositions in zircon, a widespread accessory mineral of continental igneous rocks. This study reconstructs the evolutionary path followed by the crust segment today exposed in the eastern part of the French Massif Central (FMC), a portion of the Variscan belt of Western Europe, with the aim to investigate the potential aws of the zircon record of crust evolution. In this scope, the origin and geodynamic signi cance of the constituent FMC lithological units are tackled by combining conventional petrological observations with zircon U Pb Hf O isotope data. Two major inconsistencies exist between the results obtained following this integrated approach and the conclusions drawn solely from zircon isotopic signatures, taken out of their petrological context, as is commonly performed in studies investigating crust evolution. First, zircon Hf model ages point to substantial Mesoproterozoic crust formation in the FMC whereas more than 60% of the crust is actually Neoproterozoic in age. Second, new additions to the continental crust volume during the Variscan orogeny are not recorded even though 5 to 10% of the exposed crust formed at that time. The origin of both discrepancies inherently lies in the mixed isotopic signature carried by many zircon grains. Such equivocal information can only be detected when additional petrological constrains on the zircon host rocks are available and provide guidance in interpreting the zircon record of crust evolution.
Experimental evidence for sulphide magma percolation and evolution : relevant to the chromite bearing reefs of the Bushveld Complex
(Stellenbosch : Stellenbosch University, 2012-03) Koegelenberg, Corne; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Pt mineralization within the Bushveld Complex is strikingly focused on the chromitite reefs, despite these horizons being associated with low volumes of base metal sulphide relative to Pt grade. Partitioning of Pt (Dsil/sulp) from silicate magma into immiscible sulphide liquid appears unable to explain Pt concentrations in chromitite horizons, due to the mismatch that exists between very large R factor required and the relevant silicate rock volume. Consequently, in this experimental study we attempt to gain better insight into possible Pt grade enhancement processes that may occur with the Bushveld Complex (BC) sulphide magma. We investigate the wetting properties of sulphide melt relevant to chromite and silicate minerals, as this is a key parameter controlling sulphide liquid percolation through the cumulate pile. Additionally, we have investigated how fractionation of the sulphide liquid from mono-sulphide-solid-solution (Mss) crystals formed within the overlying melanorite might affect sulphide composition and Pt grades within the evolved sulphide melt. Two sets of experiments were conducted: Firstly, at 1 atm to investigate the phase relations between 900OC and 1150OC, within Pt-bearing sulphide magma relevant to the BC; Secondly, at 4 kbar, between 900OC to 1050OC, which investigated the downwards percolation of sulphide magma through several layers of silicate (melanorite) and chromitite. In addition, 1atm experiments were conducted within a chromite dominated chromite-sulphide mixture to test if interaction with chromite affects the sulphide system by ether adding or removing Fe2+. Primary observations are as follows: We found sulphide liquid to be extremely mobile, the median dihedral angles between sulphide melt and the minerals of chromitite and silicate layers are 11O and 33O respectively. This is far below the percolation threshold of 60O for natural geological systems. In silicate layers sulphide liquid forms vertical melt networks promoting percolation. In contrast, the extremely effective wetting of sulphide liquid in chromitites restricts sulphide percolation. Inter-granular capillary forces increase melt retention, thus chromitites serve as a reservoir for sulphide melt. Sulphide liquid preferentially leaches Fe2+ from chromite, increasing the Fe concentration of the sulphide liquid. The reacted chromite rims are enriched in spinel end-member. This addition of Fe2+ to the sulphide magma prompts crystallization Fe-rich Mss, decreasing the S-content of sulphide melt. This lowers Pt solubility and leads to the formation of Pt alloys within the chromitite layer. Eventually, Cu-rich sulphide melt escapes through the bottom of the chromitite layer. These observations appear directly applicable to the mineralized chromitite reefs of the Bushveld complex. We propose that sulphide magma, potentially injected from the mantle with new silicate magma injections, percolated through the silicate cumulate overlying the chromitite and crystallized a significant volume of Fe-Mss. Chromitite layers functioned as traps for percolating, evolved, Cu-, Ni- and Pt-rich sulphide liquids. This is supported by the common phenomenon that chromitites contain higher percentages of Ni, Cu and Pt relative to hanging wall silicate layers. When in contact with chromite, sulphide melt is forced to crystallize Mss as it leaches Fe2+ from the chromite, thereby further lowering the S-content of the melt. This results in precipitation, as Pt alloys, of a large proportion of the Pt dissolved in the sulphide melt. In combination, these processes explain why chromitite reefs in the Bushveld Complex have Pt/S ratios are up to an order of magnitude higher that adjacent melanorite layers.
The formation of Earth’s early felsic continental crust by water-present eclogite melting
(Stellenbosch : Stellenbosch University, 2013-03) Laurie, Angelique; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The sodic and leucocratic Tonalite, Trondhjemite and Granodiorite (TTG) granitoid series of rocks characterise Paleo- to Meso- Archaean felsic continental crust, yet are uncommon in the post-Archaean rock record. Consequently, petrogenetic studies on these rocks provide valuable insight into the creation and evolution of Earth’s early continental crust. The highpressure (HP)-type of Archaean TTG magmas are particularly important in this regard as their geochemistry requires that they are formed by high-pressure melting of a garnet-rich eclogitic source. This has been interpreted as evidence for the formation of these magmas by anatexis of the upper portions of slabs within Archaean subduction zones. In general, TTG magmas have been assumed to arise through fluid-absent partial melting of metamafic source rocks. Therefore, very little experimental data on fluid-present eclogite melting to produce Archaean TTG exist, despite the fact that water drives magmatism in modern arcs. Consequently, this study experimentally investigates the role of fluid-present partial melting of eclogite-facies metabasaltic rock in the production of Paleo- to Meso-Archaean HP-type TTG melts. Experiments are conducted between 1.6 GPa and 3.0 GPa and 700 ºC and 900 ºC using natural and synthetic eclogite, and gel starting materials of low-K2O basaltic composition. Partial melting of the natural and synthetic eclogite occurred between 850 ºC and 870 ºC at pressures above 1.8 GPa, and the melting reaction is characterised by the breakdown of sodic clinopyroxene, quartz and water: Qtz + Cpx1 + H2O ± Grt1 = Melt + Cpx2 ± Grt2. The experimental melts have the compositions of sodic peraluminous trondhjemites and have compositions that are similar to the major, trace and rare earth element composition of HPtype Archaean TTG. This study suggests that fluid-present eclogite melting is a viable petrogenetic model for this component of Paleo- to Meso-Archaean TTG crust. The nature of the wet low-K2O eclogite-facies metamafic rock solidus has been experimentally defined and inflects towards higher temperatures at the position of the plagioclase-out reaction. Therefore, the results indicate that a crystalline starting material is necessary to define this solidus to avoid metastable melting beyond temperatures of the Pl + H2O + Qtz solidus at pressures above plagioclase stability. Furthermore, this study uses numerical and metamorphic models to demonstrate that for reasonable Archaean mantle wedge temperatures within a potential Archaean subduction zone, the bulk of the water produced by metamorphic reactions within the slabs is captured by an anatectic zone near the slab surface. Therefore, this geodynamic model may account for HP-type Archaean TTG production and additionally provides constraints for likely Archaean subduction. The shape of the relevant fluid-present solidus is similar to the shape of the pressure-temperature paths followed by upper levels of the proposed Archaean subducting slab, which makes water-fluxed slab anatexis is very dependant on the temperature in the mantle wedge. I propose that cooling of the upper mantle by only a small amount during the late Archaean ended fluid-present melting of the slab. This allowed slab water to migrate into the wedge and produce intermediate composition magmatism which has since been associated with subduction zones.
Genesis and evolution of a Neoproterozoic magmatic arc: the cordilleran-type granitoids of the Araçuaí Belt, Brazil.
(Stellenbosch : Stellenbosch University, 2018-12) Narduzzi, Francesco; Stevens, Gary; Farina, Federico; Lana, Cristiano de Carvalho; Nalini Jr., Herminio Arias; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The Araçuaí orogen (SE Brazil) is one of the largest (350,000 km2) and long-lived (ca. 630 – 480 Ma) granitic province in the world. Its wide variety of granitoids recording mid- to lower crustal P - T conditions allow direct investigation of petrological processes occurring in the deepest part of the continental crust. This study investigates the field, textural, geochemical, geochronological and isotopic evolution of the pre-collisional Galiléia Batholith (ca. 15,000 km2) outcropping in the central part of the Araçuaí orogen. This weakly foliated Neoproterozoic (ca. 632–550 Ma), metaluminous to slightly metaluminous (ASI = 0.97–1.07), calc-alkaline granitoid body is characterized by the widespread occurrence of grossular-rich garnet and epidote. This is a rare mineral association in Cordilleran-I-type granitoids and of special petrogenetic significance. Field, petrographic, and mineral chemistry evidence indicate that garnet, epidote, biotite as well as white mica crystals (low-Si phengite), are magmatic. There is no difference in bulk rock major and trace element composition between the Galiléia and other garnet-free cordilleran-type granitoids worldwide. The uncommon garnet+epidote parageneses are related to the pressure, temperature and water content conditions of magma crystallization. Comparison with the mineral assemblages and mineral compositions obtained from crystallization experiments from garnet-bearing metaluminous calc-alkaline magmas, indicates that the supersolidus coexistence of grossular-rich garnet, epidote and white mica is consistent with crystallization at pressures greater than 0.8 GPa (above 25 km depth). This shows that the Galiléia batholith was assembled in the lower crust during the accretionary/collisional stages of the Neoproterozoic Brasiliano Orogeny. However, the lifetime of deep magma chambers and the duration of magmatic activity in them remains a puzzle, contrary to young upper crustal magmatic systems. Despite being homogeneous with respect to mineralogy/texture and major/trace elements, all samples from the central part of the batholith record extreme variability in U-Pb magmatic ages from ca. 630 to 555 Ma. Trace element geochemistry and Hf isotopes from the igneous zircons – here interpreted as autocrysts (ca. 555 – 560 Ma) and antecrysts (> 560 Ma) – are all consistent with an open-system crystallization, rather than a simple cooling following fractional crystallization at the level of magma emplacement. We interpret the age variability as the result of a long-lived, uninterrupted injection of compositionally similar magmas in the lower crust during the batholith assembling. These conditions kept the system above its solidus through the 80 Ma of zircon crystallization. Unradiogenic 176Hf/177Hf and 143Nd/144Nd isotopic values of the Galileía samples indicate no direct mixing with mantle-derived magmas. This explains the scarcity of mafic products in the region. Mineral textural, geochronological and isotopic similarities with other younger and older granitic plutons constructed within accretionary / fore-arc settings, better explain the characteristics showed by the Galiléia granitoids. Thus it is suggested that this giant batholith was assembled in an accretionary prism during the Brasiliano Orogenic stages. Eventually, it is likely that during the Brasiliano/Pan-African orogeny, accretionary prism, fore- and back–arc setting were sites of voluminous silicic magmatism and commonplaces for the stabilization of continental crust and its differentiation.
The geochemical evolution of the alkaline and carbonatite complexes of the Damaraland igneous province, South West Africa
(Stellenbosch : Stellenbosch University, 1978-12) Prins, Petre; Stellenbosch University. Faculty of Science. Department of Earth Sciences.
ENGLISH ABSTRACT: The DamaraLand igneous province &n northern South West Africa contains basalt/granite, alkaline, peralkaline and carbonatite complexes. The geochemical features of the Okorusu, Kalkfeld, Ondurakorwne and Osongombo carbonatite complexes and the peralkaline Etaneno complex form the main thesis of this study X-ray fluorescence electron microprobe and wet chemical methods were used to establish the behaviour of major and trace elements (Nb , Zr, Y, Sr, Rb, Ba, Ce , Nd, La, CZ and F) during the formation of these complexes. The two largest carbonatite complexes, namely Okorusu and Kalkfeld, consist of confocal rings of syenite and nepheline syenite into which smaller plugs of carbonatite were emplaced. An early phase of Na- Fe fenitization occurred at both complexes and resulted in the formation of aegirine granites and syenites at Kalkfeld and a massive pyroxene fenite at Okorusu. At Okorusu this initial fenitization was followed by extensive brecciation and feldspathization. Syenitic rocks of Kalkfeld also underwent K-metasomatism. Concentrations of hematite ore (Kalkfeld, Osongombo, Okorusu),rare earth minerals (Ondurakorume), apatite (Ondurakorume ) and fluorite (Okorusu) are of potential or actual economic importance. Plots of oxides and trace elements versus differentiation index, K/Rb plots and plots on diagrams such as Qz-Ne- Kp and Sio2- Az2o3- Na 2o+x2o suggest that f ractionation of feldspar and nepheline (also plagioclase at Etaneno) occurred during the evolution of the alkaline magmas at these complexes. The alkaline rocks of Okorusu are much more Na- and volatile-rich than similar rocks of Kalkfeld and Ondurakorume. These high concentrations might be connected with the relatively high REE contents of the Okorusu alkaline rocks and the relatively low. REE contents of their associated carbonatitic rocks. Most of the carbonatitic rocks from the Kalkfeld group of complexes are highly enriched in the rare earth elements. Major and trace element con= tents show that the partly fenitized granite ring at Kalkfeld does not, as previously thought, belong to the alkaline suite but that it represents wallrocks of Salem granite. It is suggested that at both Kalkfeld and Okorusu sodium loss during fenitization drove the composition of the magmas from the ijolitic cotectic in the system Na20-AZ2o3- Fe 2o3- sio2, into the oxide field. This resulted in an increase in the precipitation of magnetite as indicated by the increase in modal magnetite in the rocks with lower Na2o contents. The fenitized rocks at Kalkfeld and Okorusu display trace element (Zr, Sr, Rb, Ba, F and Cl ) as well as REE contents which are similar to those of their associated alkaline rocks while the concentration of these elements in feldspathiz ed rocks corresponds to that of their associated carbonatitic rocks. The early phase of Na- metasomatism is thus considered to have been caused by fluids escaping from the crystallizing silicate magmas while the later phase of feldspcthization apparently resulted from a K- co2- rich fluid in equilibrium with the carbonate magma. A similar distinction between Na and K- metasomatism is suggested by an investigation of the chemical trends displayed by analysed fenites from other alkaline/carbonatite complexes. The carbonatite plugs of Kalkfeld and Ondurakorwne consist of multiple intrusions that are of magmatic origin. Their major element contents vary regularly with the relative age of the intrusions within each complex, as can be expected if fractionation processes were op.
Geological setting and a geometallurgical evaluation of the Namakwa Sands heavy minerals deposit, West Coast of South Africa
(Stellenbosch : Stellenbosch University, 2015-12) Philander, Carlo; Rozendaal, Abraham; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The Namakwa Sands deposit, which is situated along the west coast of South Africa features world-class titanium and zircon resources with a challenging processing character. This study employs geometallurgical principles to define and quantify key mineralogical properties of the deposit that could affect throughput, recovery and quality. In addition to geotechnical investigations, a representative suite of ore and process samples were systematically studied with light microscopy, XRF, XRD, QEMSCAN, LA-ICP-MS and EMPA. The siliciclastic, arenaceous Namakwa Sands deposit developed during the Early Pliocene (~5 Ma ago) to the Late Pleistocene and the deposit stratigraphy assimilates well into the regional West Coast Group. The mineralisation is hosted by two adjacent ore bodies, which are strikingly different in various aspects. Graauwduinen West (GD West) consists of three strandline-dune couplets set in a transitional shallow marine-aeolian environment, whereas Graauwduinen East (GD East) comprises a dune deposit sans marine influence. Multiple, superimposed duricrust horizons effectively cemented the medium-grained ore bearing sands of both ore bodies. GD West is characterised by greater oversize (+1 mm fraction) and slimes (-45 μm fraction) percentages as well as poorer levels of mineral liberation and mineral surface exposure, unfavourable processing attributes that are all related to the duricrust. Mineralised trends are conspicuously distinct for the two ore bodies, i.e. southwest-northeast for GD West and southeast-northwest for GD East. The degree of mineralisation is significantly better for GD West than GD East, but heavy mineral assemblages from GD East are marked by higher proportions of the valuable minerals zircon, rutile, ilmenite and its alteration product leucoxene. Ilmenite (FeTiO₃) is the chief valuable mineral present and about 20% of the ilmenite population is affected by various stages of alteration. Optically, two coloured varieties of rutile (TiO₂), namely yellow and the more common red type are recognised. Their major element chemistry is similar, but red rutile contains greater levels of V and Cr, but lesser quantities of Fe and Nb than the yellow variety. About 72% of the zircon (ZrSiO₄) population are optically clear, and hosts lesser quantities of the penalty elements U, Th, REE and Fe than the coloured varieties. Statistical differences in the bulk geochemistry and mineralogy of the two ore bodies indicate contrasting sediment routing dynamics. Proximal source terranes utilising fluvial-marine courses supplied the required heavy minerals budget for GD West. By contrast, the source of GD East is considered to originate mainly via an interior, fluvial-aeolian corridor. Overall, however, quantitatively the medium to high-grade facies Namaqualand Metamorphic Province is considered the key contributor to the heavy mineral population of the Namakwa Sands deposit. The complexity of the ore characteristics observed, translates into a challenging and variable processing response. Only mineral grade, liberation, magnetic deportment, particle size and particle chemistry were established as meaningful mineral recovery drivers. The recovery of valuable minerals during primary concentration, which entails wet spiral separation, is mainly an inverse function of gangue grade. This study confirms that duricrust cementing agents are the key contributor to poor mineral liberation, which result in significant tailings losses during spiral separation. Variations in the magnetic susceptibility of the heavy mineral fraction, subtract significantly from mineral recoveries during wet magnetic secondary concentration. Particle chemistry becomes an important recovery driver during final mineral separation due to the sensitive trade-off between stringent product quality specifications and mineral recovery. Ilmenite recovery for instance is mainly controlled by the intricate deportment of SiO₂, a key product quality penalty that is intimately locked with the ilmenite host as surface coatings and silicate inclusions. Similarly, the deportment of the penalty elements Fe, Ti, U and Th, which reach high concentrations in coloured zircon varieties, are complex and present major constraints to the recovery of the current zircon population. The total recovery potential is better for zircon than for the titanium minerals, which is consistent with actual mineral recovery trends. Overall, the mineral liberation and the abundance of gangue minerals, particularly garnet and pyroxene, represent the most detrimental constraints to the recovery of the valuable mineral fraction. The mineral recovery potential is markedly different for the two ore bodies. GD East demonstrates a better mineral recovery potential compared to GD West. Particle chemistry and magnetic susceptibility are the key penalties that constrain ilmenite recovery for GD East ore, whereas zircon recovery is mostly impacted by particle chemistry. Mineral liberation, particle chemistry and gangue grades are the key penalties that limit mineral recovery for GD West ore. The gains in mineral resource intelligence materialised into a tangible improvement in mineral resource utilisation over the duration of the study. Enticing opportunities to further improve mineral resource utilisation revolves around creating a better fit between ore characteristics and related metallurgical behaviour, processing technology and market requirements.
Geological 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.
Die geologie van 'n gebied in Noord-Oos Transvaal met spesiale verwysing na die verspreiding en petrografie van die rotssoorte van die Palabora-stollings-kompleks
(Stellenbosch : Stellenbosch University, 1948-12) Brandt, J. W.(Jacobus W.); Stellenbosch University. Faculty of Science. Dept. of Earth Sciences; Jacobs, Ivan
The geology and petrology of the Marble Delta
(Stellenbosch : Stellenbosch University, 1973-02) Otto, J. D. T.; Verwoerd, W. J.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.; Davids, Ricardo
The Marble Delta is an area of approximately 40 sq. km in Natal (30°22' Long., 30°40' Lat.), occupied by Precambrian marble and associated granites. The deeply dissected country was geologically mapped on a scale of I : 6 000. A new lithostratigraphic classification of the metasediments is proposed. The base of the lowermost Le Joncguet Formation (composed of dolomitic marble and siliceous beds) is not exposed. This is followed by the predominantly ca.lcitic Oribi Formation with interbedded dolomite and graphitic layers. The marble is unconformably overlain by the Cherrywillingham Formation which comprises mainly amphibolite and granulite. The .three formations together constitute the Marble Delta Group. The main petrological units are calc-silicate marble, metaquartzite, dolomite marble, cluster serpen~ine marble and ce,lcite marble. Graphite layers are considered to have originated in situ from organic remains; there is occasional evidence of ionic transfer of carbon from this graphite by magmatic fluids. The common mineral assemblages are diopside + calcite + dolomite, tremolite + calcite + diopside + quartz, calcite + quartz + dolomite, forsterite + calcite + dolomite, plagioclase + cordierite + garnet + quartz, amphibole + clinopyroxene + plagioclase, hornblende + clinopyroxene + calcite, wollastor~te + calcite + diopside. Other minerals are graphite, antigorite, chrysotile, sphene, spinel, clinohumite, chondrodite, zoisite, clinozoisite, hedenbergite, phlogopite, ilmenite, hercynite, dravite, cummingtonite, talc, apatite, microcline, saponite. The mineral assemblages resulted from regional metamorphism and polyphase contact metamorphism.
Geology, 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.
Geometry 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.
Gold mineralisation in the Kirk Range area, southern Malawi
(Stellenbosch : Stellenbosch University, 2020-12) Chisambi, Joshua, Joseph; Von der Heyden, Bjorn; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: This study aims at understanding the characteristics and controls of gold mineralisation in the Kirk Range area, southern Malawi. Malawi’s mineral inventory is not known, particularly this is attributed to a general low resolution in the knowledge of Malawi’s geology, coupled with historical under-exploration for this important economic commodity. Regional scale interpretations are afforded by in-depth investigation of airborne geophysical data, which are then supported by ground geological mapping and by microscopic observations using optical and electron microscopy, and X-ray computed tomography (XCT). We further used petrography and mineral chemistry, zircon geochronology, PT estimates, fluid inclusion microthermometry and stable isotope studies. Gold is mainly hosted in biotite schists and is structurally controlled by the NE-SW trending ductile shear zones where it occurs as flakes ranging in size between 0.24 and 4 mm. The gold mineralisation is associated with smoky quartz carbonate veins. It is associated with a pyrite-chalcopyrite-sphalerite generation of sulphide, and to a lesser extent, with sericitised biotite schist wall rock. U–Pb ages for gold bearing-schists and proximal gneisses document two distinct events: an Irumide-age period of metamorphism and a Pan African period of metamorphism. The gneisses experienced both the Irumide and Pan African events while the biotite schist experienced only the Pan African deformation. Irumide metamorphism occurred at around 1066-991 Ma and Pan African metamorphism overprinted the Irumide and occurred at around 700-485 Ma. Based on a loose PT constraint, the rocks experienced peak metamorphic conditions of around 500-600ºC and 10-15 kbars. Mineralization occurred in the early stages of Pan African event before peak metamorphism likely under greenschist facies conditions. Later deformation to amphibolite facies conditions of metamorphism during the prolonged Pan African event led to the transposition of the original mineralization. Gold mineralisation occurred at temperature of around 222-300ºC and pressure of around 2.0–2.8 kbar. Minerals of economic interest were likely deposited by fluid immiscibility and fluid rock interaction.

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