Browsing by Author "Le Roex, Anton"

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    A study of mantle inclusions in the Koffiefontein Kimberlite pipe, South Africa
    (1980) CARDOSO_P; Gurney, John; Le Roex, Anton
    Mantle derived rock and mineral fragments from the Koffiefontein Kimberlite Pipe have been studied in the form of xenoliths, megacrysts, concentrate minerals and inclusions in diamonds. Mantle derived xenoliths are unusuaLly scarce. The predominant xenoliths are pyroxenites. Peridotites are rare and eclogites are very rare. The pyroxenites have been assigned to five categories. In general, they show a clear metamorphic textural history with widespread evidence of exsolution, polygonisation and deformation. Some of these pyroxenites are interpreted to represent the crystallisation products of basaltic magma at depth. Some have been metasomatised. The pyroxene chemistries suggest that more than one pyroxenite body has been sampled. The peridotites are very similar to the common peridotites of N. Lesotho and are thought to represent depleted mantle. The Koffiefontein diamonds contain both eclogitic and peridotitic inclusions. The inclusions show evidence for lack of equilibration, and one appears to link tog~ther the two parageneses mentioned above. The presence of (MgFe)O as an inclusion is reported. The megacrysts studied consisted only of garnets, and a few pyroxenes. They are interpreted to have crystallised from a magma which cooled slowly and was relatively deficient in Tio2 to account for the scarcity of ilmenite. The heavy mineral concentrate contained garnet, orthopyroxene, opaque minerals and clinopyroxene in order of abundance. The opaque minerals were chromite and ilmenite. The chemical compositions of most of the concentrate grains do not match the mineral compositions found in xenolith s, or megacrysts. A small number of concentrate grains have compositions ma tched by the diamond inclusions. Attempts to a pply geothe r mobarometry to the pyroxenites and to the diamond inclusions gave conflicting results which areascribe d to lack of equilibration between co-existing mineral phases and/or within individual mineral grains. The Koffiefontein xenolith suite is unusual for the relative abundance of pyroxenite and the scarcity of peridotite. The concentrate is unusual for the relative abundance of orthopyroxene particularly and of chromi te.
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    The development of the national accelerator centre proton microprobe as an analytical tool in geochemistry
    (1995) Van Achterbergh, Esmé; Le Roex, Anton; Gurney, John J
    This thesis describes work performed to establish and demonstrate a quantitative trace element microanalysis technique for geological material using protons accelerated by the Van de Graaff Accelerator at the National Accelerator Centre (NAC) in Faure near Cape Town. The method relies on the analysis of Proton Induced X-ray Emission (PIXE) spectra, interpreted with the help of the GeoPIXE software package. The use of the Si(Li) energy dispersive detector provides simultaneous multi-element detection at the parts-per-million (ppm) level, and a scanning beam facility permits trace element distributions to be studied at these levels. The calibration of the detector efficiency and the thicknesses of selectable X-ray attenuating filters was performed using pure elemental samples. This involved the accurate determination of the target to detector distance, the thickness of the active volume of the Si(Li) detector crystal, the thicknesses of all the absorbing layers between the sample and the detector crystal, and the assessment of the effects of incomplete charge collection in the detector.
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    Geochemical fingerprinting of carbonate wall rock alteration at major base metal sulphide deposits in the Otavi Mountain Land, Namibia
    (1998) Chetty, Deshenthree; Frimmel, Hartwig E; Le Roex, Anton
    The Otavi Mountain Land is a base metal ore province in which base metal sulphide deposits are hosted by platform carbonates in a foreland fold-and-thrust belt on the northern edge of the PanAfrican Damara Belt. Deposits have been classified as the Berg Aukas- and Tsumeb- types, based on differences in ore association, stratigraphic position and geochemistry of ores and gangue carbonates. Mineralisation at each of these deposits is accompanied by carbonate alteration in the form of dolomite and calcite veins, carbonate recrystallisation, calcitisation and carbonate silicification. Optical cathodoluminescence imaging, electron probe micro analysis, X-ray fluorescence spectrometry, X-ray diffraction, high performance ion chromatography, proton probe micro analysis, stable isotope techniques, and fluid inclusion microthermometry were employed (i) to differentiate between carbonate generations associated with the alteration and mineralisation, particularly for the more economic Tsumeb-type deposits, represented by Tsumeb and Kombat, as well as in comparing between Berg Aukas- and Tsumeb-type deposits; (ii) to set constraints on the fluids effecting such alteration and associated mineralisation; (iii) to determine the relationship of the Khasib Springs deposit, for which little geochemical data exists, to deposits of the Tsumeb-type, and (iv) to identify those parameters which are diagnostic of Tsumeb-type mineralisation.
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    The geochemistry of a suite of eclogite xenoliths from the Rietfontein Kimberlite, South Africa
    (2000) Appleyard, Clare M; Le Roex, Anton
    The Rietfontein kimberlite is an off-craton kimberlite pipe, located west of the Kaapvaal Craton at 26.75°, 20.04°E and hosts a range of xenocryst lithologies, including peridotite, eclogite and a suite of megacryst minerals. This study focuses on a suite of eclogite xenoliths, which were subject to a detailed petrographical and geochemical study, aimed at their characterisation and comparison to eclogites from on-craton and other off-craton localities. Garnet, clinopyroxene, accessory and secondary minerals were analysed for major element compositions using electron microprobe techniques and garnet and clinopyroxene trace element compositions were determined by Laser Ablation Inductively-Coupled-Plasma Mass Spectrometry (LA-ICP-MS) techniques. Oxygen isotopic compositions of five garnet samples were obtained using laser flourination techniques, followed by analysis by gas source mass spectrometry.
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    The geochemistry of selected mid-ocean ridge basalts from the Southern mid-Atlantic ridge (40°- 55°S)
    (2000) Le Roux, Petrus Jacobus; Le Roex, Anton
    In 1993, dredge-sampling from 40.6°S to 525.5°S, during cruise 9309 of the R/V Maurice Ewing, greatly increased the number of basalts recovered from this section of the southern Mid-Atlantic Ridge (SMAR), and significantly expanded the global suite of samples from slow-spreading ridges.
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    Geochemistry of selected South African group I, group II and transitional kimberlites located on and off the Kaapvaal craton
    (2004) Coetzee, Megan; Le Roex, Anton
    Eighteen Jurassic to Cretaceous South African kimberlites representative of group 1, group 11 and transitional varieties that have been emplaced through both the Archean Kaapvall craton (on-craton) and Proterozoic Namaqua-Natal belt (off-craton), have been selected for a comparative study aimed at characterising their geochemistry and source region compositions, as well as understanding the petrogenetic processes that have affected them. The petrography of the analysed kimberlites is similar to typical group 1 and group 11 kimberlites, characterised by deformed and anhedrarl olivine and phlogopite macrocrysts, with more subhedral to euhedral olivine and phlogopite phenocrysts and microphenocrysts, set in a groundmass of mostly serpentine, calcite and phlogopite (group 1 kimberlites), or calcite, serpentine, phlogopite and diopside (group 11 kimberlites). The transitional kimberlites tend to show intermediate characteristics, with the on-and off-craton transitional kimberlites showing more similarity to group 1 and group 11 kimberlites, respectively.
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    The geochemistry of the Gough Island basalts and their mantle source region
    (2008) Long, David J; Le Roex, Anton; Class, Cornelia
    Ten Gough Island primary magma compositions were calculated for use in constrained forward modelling. The primary magma compositions were calculated by the addition of 14-28% clinopyroxene and equilibrium olivine, in equal proportions, back into the primitive lava compositions in 1 % increments until an Mg# of 69 was reached. Constrained forward modelling methods were used in determining the chemical and mineralogical nature of the mantle source region as well as the required melting parameters required in producing the Gough Island suite of lavas. The models indicate that the full compositional range, as well as the crossing REE patterns at Dy, can be produced through ~5-8% equilibrium melting of a mantle source region composed of garnet lherzolite or ~30-50% melting of a garnet pyroxenite source.
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    Orientation of the maximum horizontal stress (SHmax) in the Orange basin and Outeniqua sub-basins of South Africa : neotectonic implications
    (2015) Logue, Andrew Selkirk; Le Roex, Anton; Ben-Avraham, Ben; Heidbach, Oliver; Andreoli, Marco
    South Africa lies within an intraplate setting, characterised by sparse, scattered, low-magnitude seismicity not easily correlated with known neotectonic features. Recent seismicity has repeatedly been shown to result from the reactivation of pre-existing geological structures, typically large faults or shear zones, which are preferentially-aligned to the contemporary stress field. Expanding the catalogue of stress data for the region is therefore vital to better understand the regional stress field pattern and aid in identifying potentially seismogenic structures. In these intraplate regions, reliable high-quality indicators of maximum horizontal compressive stress (SHmax) are difficult to obtain in the absence of earthquake-derived focal mechanism solutions (so called "beach balls"). In South Africa however, extensive hydrocarbon exploration drilling in the Mesozoic offshore provide an opportunity to utilise the borehole breakout technique to derive quality SHmax orientation data. In collaboration with the World Stress Map Project (WSM), and utilising borehole logs provided by the Petroleum Agency South Africa (PASA), training and software was provided to review, analyse and visualise borehole breakouts observed in 4-arm calliper logs from exploration areas in the Outeniqua and Orange basins. Digital music software can limit the forms of music we create by using interfaces that directly copy those of the analogue instruments that came before. In this study we report on a new multi-touch interface that affords a completely new form of drum sequencing. Based on ideas from Avant-guard music and embodied interaction, a technology probe was created and then evaluated by a wide range of users. We found that for users with no musical training, and for users with a large amount of musical training, the software did allow them to be more creative. However, users with limited training on existing sequencing software found the new interface challenging.
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    A pertographic and geochemical study of selected peridotitic and pyroxenitic xenoliths from the three kimberlite localities in the Lac de Gras region, Northwest Territories, Canada
    (2002) Doyle, Patricia Mary; Le Roex, Anton; Gurney, John
    Eighty-five peridotitic and pyroxenitic xenoliths from the Arnie, Pigeon and Misery kimberlites in the Lac de Gras region, Northwest Territories, Canada were selected for inclusion in this study. The three kimberlites are situated within a 40 km radius of one another on the BHP property, and all are diamond-bearing. The Misery kimberlite is presently being mined, and the Pigeon kimberlite is part of · the future BHP-DiaMdt Ekati mining plan. A petrographic study of the xenoliths using both transmitted light microscopy and binocular microscopy was followed by major anti trace element analysis. Major element compositions of individual minerals were determined using a wavelength dispersive electron microprobe, and trace element abundances were determined using laser ablation ICP-MS. Pressures and temperatures of equilibration were then determined using 'garnet-olivine, garnet-orthopyroxene and trace element geothermobarometers (TNi, Per).
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    Petrogenesis of the Northwest corner intrusive phases, Dutoitspan kimberlite, South Africa
    (2008) Hanekom, Adri; Le Roex, Anton
    The Outoitspan Group 1 kimberlite pipe forms part of the well-known cluster of pipes located in and around the city of Kimberley, South Africa. Eight macroscopically distinct intrusive phases, i.e. D2 Type 2, D2 Type 3, D2/D5, D5, D18, Type 5, D16 and the D16 dyke are present in the Northwest Corner area of the mine. Microscopically they range from macrocrystic to aphanitic hypabyssal (magmatic) kimberlites with varying amounts of opaque minerals, monticellite and phlogopite. Olivine is the dominant macrocryst phase and alteration varies from unaltered to highly serpentinised. These intrusive phases also contain variable amounts of crustal xenoliths.
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    Petrogenesis of the Swartruggens and Star Group II kimberlite dyke swarms, South Africa
    (2004) Coe, Nancy; Le Roex, Anton; Gurney, John J
    The Swartruggens (156 Ma) and Star (128 Ma) kimberlites are two Group II, diamondiferous, hypabyssal kimerlite dyke swarms, situated in the Northern Province and the Free State respectively, South Africa. Representative samples from all dykes exposed in the mining operations, the Main and Changhouse Dykes, South Fissure and the barren Muil Dyke at Swartruggens, and the Wynandsfontein, East Star, Clewer, Byrnes and Barren dykes at Star, have been analysed for their major and trace element contents and Sr, Nd and Hf isotope compositions. Primary kimberlite magma chemistry is subjected to considerable modification due to the incorporation of both mantle and crustal material during ascent to the surface, crystal fractionation, and post-emplacement alteration by deuteric fluids. This study aims to constrain the effects of these processes, and thus to identify least-modified, close-to-primary, parental magma compositions, with the view to understanding the source region characteristics of, and the petrogenetic processes giving rise to, these kimberlites.
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    Structural setting of the Jordan Northern Highlands : an integrated study using surface and sub-surface geological data by utilizing GIS Technology
    (2011) Al Tarawneh, Maha Atallah; Le Roex, Anton; Ben-Avraham, Zvi
    This study was designed, for the identification of the General structural style of this part of Jordan by: (1) Definition of dominant structural styles in the JNH: such as the main structural elements, basins, and highs; (2) Inference of the structural setup and its tectonic significance, and (3) Identification of the influence of the Dead Sea Transform on the geological setting of the JNH area.
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    Sulphide Re-Os characterisation and nitrogen aggregation state of the Ellendale diamonds, Kimberley Province, Australia
    (2008) Smit, Karen Vena; Gurney, John J; Richardson, Steve; Le Roex, Anton; Shirey, Steve; Smith, Craig
    A suite of sulphide-bearing diamonds recovered from the Ellendale 4 and 9 pipes in the Ellendale diamond province of lamproite intrusions in north-western Australia have been investigated for their nitrogen aggregation state and the Re-Os isotope geochemistry of the sulphide inclusions. The Ellendale lamproites, dated at ca. 20 Ma (Allsopp et at., 1985) intrude the King Leopold Mobile Belt just south of the Kimberley craton and are thus an atypical, off-craton diamondiferous locality. The diamonds contain roughly equal proportions of peridotitic and eclogitic inclusions (Hall & Smith, 1984; Griffin et at., 1988; Jaques et at., 1989). The diamonds in this study range in size from 0.26 to 0.92 carats and are dominated, with the exception of one octahedron, by highly resorbed tetrahexahedroida.