Browsing by Author "Greyling, Lynnette"

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    Geochemistry, stable isotopes and fluid inclusion studies of the Otjikoto gold deposit, Central Namibia
    (2018) Negonga, Lisias; Greyling, Lynnette; Harris, Chris
    The Otjikoto gold deposit is located within the Damara Orogenic Belt which is part of the system of Pan-African mobile belts. The Damara Belt is a northwest trending orogenic belt which formed through sequences of spreading, rifting, subduction and from continental collision between the Congo, Kalahari and Rio Delta Plata Cratons between 900 Ma and 450 Ma. Mineralisation at Otjikoto is hosted by a group of metamorphic rocks which consist of biotite schists from the Okonguarri Formation, albitites from the Oberwasser member and marble units from the Okawayo member. The gold mineralization occurs in a sheeted vein network system with veins which are positioned parallel towards the foliation of the albitites and hornfels of the Oberwasser Member. The vein types include both quartz and carbonate hosted vein types. The vein mineralogy includes magnetite, pyrite and pyrrhotite which are associated with amphibole, quartz, almandine garnets, and free gold. A fluid inclusion study carried out on the vein quartz showed the co-existence of the aqueous, carbonic and aqueouscarbonic fluid inclusions within the Okonguarri Formation. The aqueous inclusions have moderate to high salinities with salinities ranging from 30.0 to 44.5 wt. % NaCl eq. in aqueous inclusions with halite crystals and from 0.9 to 22.9 wt. % NaCl eq. in inclusions without halite crystals. The total homogenisation temperatures, Th tot (L), ranged from 151 to 488 °C in the aqueous-carbonic inclusions and from 156 to 444 °C in the aqueous inclusions. The melting temperature of CO2 (TmCO2) ranged from -60.3 to -57.5 °C in carbonic inclusions and from - 60.2 to -57.2 °C in aqueous-carbonic inclusions. It is proposed that the aqueous and carbonic inclusions (CO2-CH4) from the Okonguarri Formation are the end members of fluid immiscibility. The presence of CO2 and CH4 is indicated by the low TmCO2 values and may be justified by the decomposition of the carbonaceous metasediments which are found in the stratigraphy at Otjikoto. The δ 18O values range from of 8.35 ‰ to 8.75 ‰ for the magnetite and 17.51 ‰ to 18.47 ‰ for the vein quartz samples. Analysis of the microprobe data on the amphiboles shows that the analysed grains are made up of anthophyllites. Microprobe data of the chlorites showed that the majority of the chlorites are made up of ripidolites. Temperatures of chlorite formation were calculated to be in the range of 221°C to 358 °C by using chlorite geothermometric equations. Major element variation diagrams show a strong negative correlation of TiO2, Al2O3, MgO, Na2O and FeOt vs SiO2. Variation diagrams were also plotted for the trace elements vs Zr. The Sr, Y, La and Ce vs Zr plots show a strong positive correlation.
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    Process mineralogical characterisation of the Kansanshi copper ore, NW Zambia
    (2016) Jacobs, Tamzon Talisa; Becker, Megan; Greyling, Lynnette
    Kansanshi mine is the largest copper producer in Africa. The deposit is mineralogically and texturally complex due to supergene enrichment resulting in the presence of a variety of primary and secondary copper minerals. This necessitates the processing of ore through three separate circuits: sulphide flotation, mixed flotation and oxide leach, followed by solvent extraction and electro-winning. This study revisits the process mineralogy of the ore using modern mineralogy tools, which for such a large and complex deposit cannot but deliver significant value. Specific focus is given to copper mineralisation and the flotation of the sulphide ores in compliment to another MSc study from the Centre for Minerals Research focusing on mixed ore flotation (Kalichini, 2015). A series of hand samples and grab samples representing the variation in mineralogy and texture of the Kansanshi ore, as well as two run of mine sulphide ore flotation feed samples were used for this investigation. Process mineralogical characterisation entailed optical microscopy, XRF, QXRD, QEMSCAN and EPMA investigations, alongside a series of laboratory scale batch flotation tests of two sulphide ores at two grinds (80% passing 150 μm, 80% passing 212 μm). Copper mineralisation at Kansanshi occurs as both vein-hosted mineralisation, and to a lesser extent sediment-hosted mineralisation. Later breccia-hosted and supergene mineralisation have overprinted all previous mineralisation styles. Chalcopyrite is the main ore mineral for both vein-hosted and sediment-hosted mineralisation styles.
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    The geochemistry and petrogenesis of the Saltpeterkop carbonatite complex near Sutherland, Northern Cape, South Africa
    (2018) Marageni, Manoka; Janney, Philip; Greyling, Lynnette
    The Saltpeterkop Carbonatite Complex is a Late Cretaceous (≈76 Ma) volcanic and shallow intrusive magmatic feature located approximately 20 km southeast of Sutherland in the Northern Cape. It is unusual among southern African carbonatite complexes in that it has not been deeply eroded, and retains a significant vestige of its original volcanic features. The main geologic expression of the Complex is a ≈1.5 km diameter tuff ring, located on top of prominently updomed and fractured Beaufort Group (Karoo) sediments, that appears to have formed as the result of a major diatreme-type eruption. The volcaniclastic breccias making up the tuff ring have been heavily altered and silicified by hydrothermal activity, and thick (mm to tens of cm) Fe oxide-rich crusts, which appear to represent the alteration products of Fe-rich carbonatites, are common in this area. Outside of the central ring structure are numerous shallow intrusions (dykes, sills and irregular shapes), satellite breccia pipes and pipe-shaped intrusions that host fresh to only moderately altered igneous rocks. The main igneous rock types include (in decreasing order of abundance): carbonatite, potassic trachyte, olivine melilitite and ultramafic lamprophyre. This thesis provides the first detailed petrographic and geochemical description of these rocks (e.g., major and trace elements) and attempts to explain several aspects of their petrogenesis. The olivine melilitites and ultramafic lamprophyres are the most primitive igneous rocks in the complex and have experienced only minor to moderate extents of differentiation, respectively. They apppear to have been derived by low-degrees of partial melting of a carbonated, likely phlogopite-bearing mantle source. The lamprophyres appear to have been derived by melting at shallower depths than the melilitites based on REE constraints. The carbonatites range from relatively primitive to highly differentiated and they form a nearly continuous compositional range with the ultramafic lamprophyres and melilitites. This seems to argue against a major role for liquid immiscibility in their origin. Their REE content (up to 2 wt.% total REE oxides) correlates with their extent of differentiation. The potassic trachytes are plausibly linked to melts of mafic lower continental crust that has been metasomatised by hydrous potassium-rich carbonatitic melts and which have experienced significant fractional crystallization and assimilation of upper crustal sedimentary rocks during emplacement.
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    The stratigraphic and structural controls on copper-gold mineralization at Cassenha hill prospect, within the Archean to Paleoproterozoic Angolan shield, Congo craton, South Western Angola
    (2021) vaz Sidre, Stelvio; Harris, Chris; Greyling, Lynnette
    The Cassenha Hill copper-gold prospect is situated in the Catabola area, Huambo Province, southwestern Angola. Geologically, the prospect is part of the Angolan Central Eburnean Zone (CEZ) and consists of Paleoproterozoic metasedimentary rock sequences which have been intruded by Eburnean granitoids at ± 2.1 Ga. The prospect itself comprises an area of 180 Km2 and has been intermittently explored since 2005 by Rift Valley Resources (RVR) and its associates. The Cassenha Hill prospect is characterized by the occurrence of partially altered and highly oxidized metasedimentary rocks (meta-mudstone, meta-siltstone, carbonate-rich rock, meta-sandstone, breccia, and quartzite), and altered isolated granitoids. This study represents the first detailed study of the prospect and aims to shed light on the characteristics of the various rock types (i.e., host and barren rocks), their source/provenance, styles of alteration, and the origin and/or type of the fluid responsible for the mineralization. Techniques applied include borehole core logging, petrography, whole-rock geochemistry (XRF and ICP-MS), and stable isotope geochemistry on samples collected on the surface and from exploration boreholes drilled as part of RVR exploration program. Overall, the petrographic and geochemical studies undertaken in the rocks of the Cassenha Hill prospect indicate the following: (1) The rocks are moderately to strongly fractured; (2) The rocks have experienced weak to moderate chemical weathering; (3) The rocks are compositionally immature and originated from felsic provenance; (4) The rocks are sulfur-poor, and lacking in sulfide minerals (5) The rocks are enriched in LREE and LILE elements and depleted in HREE and HFSE elements; (6) The mineralization is associated with chloritization alteration and predominantly occurs within and/or at the edges of quartz/chlorite-rich veins/fractures and is not restricted to any rock type. Two ore stages could be identified, namely, hypogene ore (stage I) consisting of pyrite ± chalcopyrite ± other copper sulfides, and supergene ore (stage II) consisting of malachite ± azurite ± chrysocolla, which represents the prevalent mineralization at the prospect. The δ 18O value of quartz veins range from +12.81 to +13.53‰, while the δD of chlorite minerals range from -51 to -45‰. Therefore, assuming fluid-rock interaction took place at ≈ 350oC, due to the presence of quartz, the fluid had δ 18OH20 values of about +8‰, which are typical of magmatic waters. On the other hand, at a temperature of ≈ 350oC, and with the difference between chlorite and water being -33.5‰, such fluids would have had δDH20 values of about -20‰ (δ 18OH20= -4‰), which are typical of meteoric waters. Although the various mineral phases have yielded different isotopic signatures, this study suggests that water is ultimately of meteoric origin but exchanged with hydrogen-poor magmatic rock, thus maintaining the meteoric signature. The continuous influx of meteoric waters within the fracture system led to the development of copper oxides such as malachite, azurite and chrysocolla, which possibly originated from the insitu oxidation of the hypogene sulfides. This signature, together with other field, petrographic and geochemical observations allows one to, tentatively, suggest that the Cassenha Hill prospect represents an extension of a polymetallic vein-type of porphyry Cu deposit that has been subjected to supergene processes at the weathering profile.