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11th SAGA Biennial Technical Meeting and Exhibition
- Conference date: 16 Sep 2009 - 18 Sep 2009
- Location: Swaziland, South Africa
- Published: 16 September 2009
81 - 100 of 157 results
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Biomarker and Stable Carbon Isotope Analyses of Sedimentary Organic Matter from Lake Tswaing: Evidence for Deglacial Wetness and Early Holocene Drought from South Africa
Authors I. Kristen, H. Wilkes, A. Vieth, K.-G. Zink, B. Plessen, H. Oberhänsli, F. Schmidt and T. C. PartridgeThe 1.13-km-wide crater Tswaing (25°24’30’’ S, 28°04’59’’ E) is located approximately 40 km northwest of Pretoria at an altitude of 1045 m above sea level. The crater was formed in granitic bedrock by a meteorite impact. Sediments from Lake Tswaing document hydrological changes in southern Africa over the last 200 Ka. In this study we investigate modern plant and lake water as well as modern and ancient sediment samples from the Tswaing Crater in South Africa using biomarker and stable carbon isotope analyses on bulk OM and specific biomarker compounds. The characteristic molecular markers for higher land plants (predominantly C3-type deciduous angiosperms) in Lake Tswaing are long-chain n-alkanes (n-C27-33), n-alkanols (n-C28+30), stigmasterol, ß-sitosterol, ß-amyrin, α-amyrin and lupeol. The C17 n-alkane, tetrahymanol, gammaceran-3-one and C29 sterols dominate autochthonously produced OM. By comparing carbon isotope signatures of bulk OM and the characteristic biomarkers, we trace the modern carbon cycle in the crater environment and find indications for methanogenic activity in the lake from isotopically depleted moretene. A comparative study of core sediments reveals changes in the terrestrial (C3 vs. C4) and aquatic bioproductivity and allows insights into the variability of the carbon cycle under the influence of changing climatic conditions for the time from the end of the last glacial (Termination I) to the late Holocene, c. 14,000–2,000 calibrated years before present (yr BP). The most pronounced changes occur in the aquatic realm at c. 10,000 yr BP and 7,500 yr BP when our results imply climate swings from more humid to more arid and to gradually more humid conditions again, which can be related to a shift in the position of the Inter-Tropical Convergence Zone or to changes in the tropical atmosphere-ocean interaction.
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Signals of Mass Redistribution Observed at the South African Geodynamic Observatory Sutherland
Authors C. Kroner, H. Pflug, S. Werth, A. Güntner, B. Creutzfeldt, P. Fourie and Ph. CharlesSince the year 2000 a superconducting gravimeter (SG) has been operating at the South African Geodynamic Ob-servatory Sutherland (SAGOS). It is one of the few instruments installed in the southern hemisphere and presently the only one of its kind on the African continent. Time-dependent gravity observations carried out with supercon-ducting gravimeters are long-term stable and of high resolution (~ 1 nm/s² in the time domain) have thus the poten-tial to provide constraints for hydrological models since they completely contain the changes in all water storage components in contrast to hydrological point observations. Esp. in combination with satellite-derived temporal gravity field variations as from the GRACE mission they provide a powerful tool for the investigation of changes in continental water storage. SAGOS is located on a hill in the Karoo, a semi-arid area with an average annual precipitation in the range of 200 to 400 mm. A seasonal effect on gravity related to local hydrological variations is clearly seen in the SG record. It is in the or-der of magnitude of several nm/s². Depending on the hydrological model the large-scale hydrological impact on gravity amounts to 10 – 20 nm/s² with or without a pronounced seasonal variation. Substantial differences exist between both, the amplitudes and the features of the gravity changes derived from global hydrological models. The same applies to the observed hydrology-related gravity changes and the model-based gravity variations. The results of the recently initiated investigations will contribute to an improved local and regional quantification of hydro-logical dynamics and thus establish additional means for the management of the valuable resource water.
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Soil Water Saturation in the Cathedral Peak VI Catchment, KwaZulu-Natal
Authors B.T. Kuenene, C.W. van Huyssteen and M. HensleyDegree of water saturation and water storage capacity has been studied in the Cathedral Peak VI catchment in South Africa. Soil water content data for the Cathedral Peak VI collected since 1990 in various Water Research Commission projects were used to calculate the annual degree of water saturation (s) above 0.78 of porosity (ADs>0.78) for the soils of the catchment. The ADs>0.78 value was used to determine the soil water regime of soils of the catchment. The degree of water saturation (s) was found to be heterogeneous downslope in the catchment, controlled primarily by topography. Soils in the mid and upper slopes had a dry water regime while soils in the toe slope had a wetter water regime. The soil water storage capacity of the catchment was estimated to be approximately 248 583 m3, using rainfall and streamflow data. Results reported here increases understanding of the hydrological characteristics of soils in the Cathedral Peak VI catchment and provide key information on the role of soils in catchment hydrology.
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Seismic Structure of Southern Africa: New Constraints from Surface Waves
Authors Sergei Lebedev and Joanne AdamContinental-scale seismic tomography shows the lithospheric keels beneath the cratons of Africa as pronounced high-velocity anomalies, bottoming at depths of up to about 200 km. A pronounced sub-cratonic low-velocity zone is observed only in Tanzania, where it is neighboured by volcanic areas and may be showing hot material—possibly of mantle-plume origin—trapped or spreading beneath the thick cratonic lithosphere. Regional-scale, surface-wave array analysis in South Africa shows variations in lithospheric structure across the Kaapvaal Craton and the Limpopo Belt. A reduction in S-velocity at the bottom of the lithosphere (150-200 km depths) is required by the data both beneath the Kaapvaal Craton and the Limpopo Belt, being less pronounced beneath the latter.
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Amplitude Effects of Magnetic Signals on Depth Estimation Routines
More LessIncreased acquisition of geophysical data in conjunction with limited geological information places greater importance on accurate geophysical processing and interpretation methodologies. There are numerous semi-automated data processing routines presently being implemented which specialize in location, geometry, and depth of potential source bodies. Most routines rely on the innate relationship that magnetic signal frequency is a function of a magnetic source’s depth and geometry. An additional relationship that is often not addressed is amplitude, which is a function of a source’s magnetization. Most analytic routines incorporate some sort of mathematical entity in their fundamental equations to address magnetization effects. However, there are associated limitations of these analytic routines that must be clearly defined to accurately interpret the calculated results. The mathematical expression of tilt-angle has recently been developed into a depth-estimation routine, known as “tilt-depth”. It has been shown to work reliably on sources that generate non-complex anomalies, which include a non-dipping body under the influence of a vertical magnetic field vector. It is important to consider how depth estimations will be effected when the amplitude of the source magnetization varies. Tilt-depth solutions become more inaccurate once the source approaches the surface and increases in magnetization. This is due to the creation of more complex signal geometry and reiterates the premise that the limitations of any of depth estimation routines must be considered and mitigated.
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Investigation of the Behaviour of Platinum-group Minerals and Base Metals Sulphides During Flotation of UG2 Ore
Authors Thelma B. Lekgetho, Deshenthree Chetty and Marian TredouxPlatinum-group element (PGE) mining in South Africa originally concentrated on the Merensky reef (MR) in the western limb of the Bushveld igneous complex. However, most of the mineable MR ore in the western limb has been extracted and therefore the mines are now focussing on the Upper Group 2 chromitite (UG2) ore. The increased PGE production from the UG2 ore poses challenges to the beneficiation process because of the ore’s high chromite content and smaller (2 μm to 30 μm) platinum-group mineral (PGM) size as compared to the fairly large (10 μm to 200 μm) grains in the MR. Traditionally, PGMs have been recovered from the MR ore by sulphide flotation and this sulphide flotation is still used for the UG2 ore with continuing improvements to the circuit. Due to the uncertainty of the mode of occurrence of the PGM in the UG2 ore, this research project is using mineralogy to optimise the PGE beneficiation. We have investigated the behaviour of the PGMs and base metal sulphides (BMS) during flotation, as well as looked at whether flotation of the BMS can be used as a proxy to predict the flotation behaviour of the PGM. The work is ongoing. This investigation is being done on a primary circuit of a UG2 concentrator plant. The mineralogy of the primary feed, various concentrator cells and tailings was analysed using a mineral liberation analyser (utilizing energy dispersive X-ray fluorescence (XRF)) and X-ray diffraction for mineralogical information, and wavelength dispersive XRF for chemical assays of the bulk samples. To date, it appears that 75% of the total PGMs fed into the flotation cells is in a liberated form, while 16% is associated with the BMS and 9% with the gangue (silicates). The results show that both BMS and PGMs respond in a similar way to sulphide flotation, but behave differently in some ways, because of differences in their grain size distributions.
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The Elusive Flying Squids: Interim Results from Recent Tests with a Heli-borne EM Low Temperature SQUID System.
Authors C. L Le Roux, Ronny Stolz, Braam Du Plooy and Jaco SmitFollowing the positive outcomes and successful application of Low Temperature SQUIDs (LTS) as ground TEM B-field sensors, a research and development program was launched to try and realize similar benefits in airborne EM applications. The AeroTEM III helicopter-slung bird had been selected as the prototype platform for the integration of the IPHT / Supracon LTS EM SQUID sensor technology with a new generation experimental airborne EM system. A number of ground-based and two airborne flight tests were conducted, first in December 2007 with a 100% duty cycle Spectrem (“on-time”) waveform and then in September 2008 with an AeroTEM triangular “off-time” waveform.
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Revealing the Beattie Magnetic Anomaly and the Anatomy of the Crust of Southernmost Africa: Geophysics and Deep Sub-surface Geology Where the Cape Fold Belt and Karroo Basin Meet
Authors A. S. Lindeque and M.J. de WitThe deep crust of the southernmost margin of Africa contains unresolved tectonic features such as the Paleozoic Cape Fold Belt (CFB), the Paleozoic-Mesozoic Karroo Basin and the largest terrestrial magnetic anomaly, the Beattie Magnetic Anomaly (BMA). Without resolving these structures, our understanding of the evolution of the southern margin will be incomplete and limited. Under the auspices of the Inkaba yeAfrica framework, several geophysical datasets were acquired from 2004 to 2007, along two transects across the margin and its unique tectonic features. This research presents a tectonic model and crustal geometry, at the centre 100 km of the western transect. The model is derived from the joint interpretation of: surface geology, aeromagnetic data, nearby deep boreholes, teleseismic receiver functions, impedance spectroscopy measurements on borehole samples, near vertical reflection seismic data (NVR), shallow P- and S-wave velocity data, wide angle refraction data and magnetotelluric data. The model differentiates a four component ~ 42 to 45 km thick crust and constrains the two part BMA to a ~10 to 12 km wide northern zone, and a ~5 to 7 km wide southern zone, both at ~7 to 8 km below surface, continuing for a depth of ~5 km, and, contained in the Mesoproterozoic Namaqua-Natal mid-crust. The BMA source is interpreted to be a Namaqua-like massive to disseminated, deformed/metamorphosed strataform sulphide ore body. The model presents evidence in support of a thin-skinned tectonic thrust model for the evolution of the flat-based CFB and shows no significant fore-deep stratigraphic thickening in the Karroo Basin towards the CFB front. The tectonic model suggests a Meso-Proterozoic collision orogen setting, overprinted by a mid-Phanerozoic thin-skinned fold and thrust belt coupled to the far-field accretion/subduction margin to the south.
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Kalahari Borehole Stratigraphy from the Tsodilo Hills Area, NW Botswana: Clues to Understanding Uplift and Climate History of the Kalahari Plateau
Authors B. Linol, M. J. de Wit, F. Guillocheau and J. BruchsWe report analysed stratigraphic records across the central Kalahari highlands, including facies analysis and logging of a number of boreholes near the Tsodilo Hills of NW Botswana, which track the geological changes associated with the formation of the Kalahari Plateau. The study of these condensed sedimentary sequences from the interior of this plateau reveal that the evolution of its palaeo-environments are linked to both regional and local tectonic and climate changes, e.g. those related to the plateau’s episodic vertical movements, the opening of new oceans and continental margins flanking the plateau, and associated magmatism. Dating these geological events derived from the preserved stratigraphy is one of the main objectives of our ongoing drilling program. Reconstructions of the sedimentary architectures on profiles between boreholes, and correlations at more regional scales, are based on bio- and litho-stratigraphy. The total succession (100 to 200m thick) comprises relics of Karoo sequences that include diamictites, turbidites and deltaic deposits overlying high grade gneisses. This lower succession records significant tectonic, climate and topography changes from the carboniferous Dwyka period (350-300 Ma) to the Jurassic Stormberg magmatism (180 Ma). These successions are variably altered and intruded by Late Cretaceous (100-70 Ma) kimberlite deposits. Overlying sediments comprise a thick Kalahari sequence of carbonates and sands, usually interpreted as calcretes and aeolian deposits, respectively. In sub-surface, the carbonates are extensive and vary in thickness from 30 to 60m. These appear to be related to lacustrine processes as recorded in various biomarkers (no fossils are visible), and pedogenesis. Carbon and oxygen isotopes of the sediments provide distinct trends that will be completed by the study of a stalagmite from caves across southern Africa to test for high resolution signatures of climate changes. The uppermost unconsolidated sand-silt successions are difficult to sample by conventional drilling. New techniques will be tested later this year. Our first tests reveal sequences include muddy and carbonated intervals with sufficient plant remains to date wet-dry episodes to reconstruct relatively recent climatic variations.
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Advances in airborne EM: Introducing ZTEM
Authors Bob Lo, Jean Legault, Petr Kuzmin and Keith FiskA series of demonstration tests were conducted using the Z-TEM, airborne AFMAG system over deep targets in the Athabasca Basin of Saskatchewan, Canada. These tests were conducted in mid-2008 and were flown to test Z-TEM’s ability to detect large conductive targets at depth; deeper than conventional airborne EM methods. Data are presented over areas where the conductors are located 450-600 metres beneath the surface. As well, a case of Z-TEM following the plunge of a conductor to over 800 metres depth is shown
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Reservoir Characterization of Wells KD1, KE1, KF1, KH1, in Block 3, Orange Basin, Offshore South Africa
Authors Donovan Lombard and A. AkinluaPetroleum reservoir characterization can be described as a process that includes integrating, analyzing and understanding all available data from the well. Understanding the reservoir characteristics plays a pivotal role for the optimization towards the well performance. The two fundamental properties that govern the quality of reservoir rocks are porosity and permeability. Clay minerals in contrast, have a major influence on reservoir quality. Clay minerals occlude pore spaces, coats grain surfaces, and swells in the presence of water. Thus petroleum drilling operations and reservoir management also becomes affected. The main aim of this study is to provide a clear understanding and description on reservoir heterogeneities of four wells (i.e. KD1, KE1, KF1, and KH1) allocated within block 3 of the Orange Basin. The objective of this study are 1) to quantify the effect of clay minerals on reservoir quality (RQ), 2) to identify and characterize potential reservoir zones, 3) evaluate the reservoir properties 4) use of graphical techniques 5) implications on production of hydrocarbons. The objectives of this study will be achieved and accomplished by using wireline logs, core data, X-ray diffraction (XRD), thin-sections and scanning electron microscope (SEM) analyses.
Expected outcomes of this study includes, tight gas reservoir sands, fair to relatively high reservoir quality rocks, high net to gross ratios, variable poroperm values, increasing clay content with burial depth that influences RQ, high quartz cementation and diagenetic processes controlling RQ.
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Neutron Tomography of Geological Samples – Preliminary Results.
Authors L. Longridge, F. C. de Beer, L. Coney, P. Ogilvie and S. WebbNeutron tomographic imaging has been conducted on a variety of geological samples, including migmatitic metapelite, suevitic breccia, as well as mafic granulite, in order to investigate the success of this technique in non-destructively distinguishing their component minerals in three-dimensions. Preliminary results indicate that coarse textures found in these samples are clearly resolved, but μm-scale resolution of mineral distribution is not yet possible. The variable composition of minerals in geological samples due to solid solution results in overlapping attenuation coefficients for some minerals, and these minerals may be indistinguishable using neutron tomography. Hydrous minerals and voids present in samples are well resolved, and volumes of component phases in samples can be easily calculated, provided these phases are resolvable.
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The Integration of Borehole Geophysical logs for Geotechnical Risk Assessment at the Paardekraal 13-Level Ventilation Shaft Project
Authors Bathandwa Mabedla and Kazek TrofimczykDownhole wireline logging and borehole radar surveys were undertaken at Paardekraal in two shaft geotechnical boreholes for the planning and design of ventilation shafts for underground platinum mining. Paardekraal is one of production areas of Anglo Platinum’s Rustenburg Section, Rustenburg Platinum Mines Limited, on the Western lobe of the Bushveld Complex. The horizons mined for platinum group metals are the Merensky Reef (MR) and the UG2. Old MR support pillars are targeted to provide stable ground conditions for a vertical shaft to access to the UG2 horizon, which lies stratigraphically below the mined out MR. Studies show that most structures (jointing) in the Bushveld Complex are steep-dipping and could pose a threat to mining operations even over short distances. Mapping of structures intersecting the borehole, their location and their orientation is used to assist in highlighting zones of potential hazard and to characterize rock formations. Key geophysical tools were deployed in the Paardekraal geotechnical boreholes to complement standard drill core geotechnical analyses. The acoustic televiewer records high resolution (2 mm pixel), oriented images of the full circumference of the borehole sidewall and maps in detail the location and orientation of structural features intersecting the borehole. Density and sonic velocity logs provide information on rock strength and competence. Flowmeter, differential temperature, neutron and fluid conductivity logs identify zones of possible ground water ingress (fluid pathways). A method for integrating the structural and geotechnical information, interpreted from the geophysical logs, into a single, visual log that highlights hazardous geotechnical zones was applied. This so-called Hazard Index (HI) is a weighted combination of the following parameters, namely: intact rock strength (IRS), shear wave slowness (SWAV), fracture frequency (FRAC), joint intersection rating (INTS) (based on fracture tilt combination), water ingress (FLOW) and fracture density projected within the shaft barrel (FDEN). Borehole radar in reflection mode penetrates the formations around the borehole to distances of up to 50 m (and sometimes further). This gives information about the condition of the rock mass surrounding the borehole and the lateral extension and continuity of major structural features and dykes. Critically, the radar data was analysed for information on the location of old MR workings with respect to the proposed position of the shaft excavations and the integrity of the MR pillar surrounding the shafts.
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Aspects of Palaeo-terrace Formation and Stream Sediment Petrography of the Orange and Vaal River Confluence in the Douglas Area
Authors N. MacDonald, J.H.A. Clark, P.G. Holmes and C. GauertThe mineralogy and geochemistry of a representative number of stream sediments in combination with geomorphologic mapping of the Orange/Vaal River terraces is used to assess the provenance of the sediments and the diamond-bearing potential of the terrace material in the confluence area around Douglas, Northern Cape. Utilizing sediment profile descriptions, grain size analyses, heavy mineral separation and identification techniques we were able to distinguish sediment provenance groups at different sites in the Orange/Vaal confluence area near Douglas. First assessment of the rock fragment/mineral spectrum reveals Ventersdorp volcanics material as well as Karoo material to be predominant provenances of these sediments, and to a lesser extent glacial material derived from Dwyka series to be the source of sediments; based on the frequency of diggings, the possible diamond-bearing potential appears to be by far higher in the terraces of the lower Vaal River and along the Orange River downstream Douglas than along the Orange River section between Hopetown and Douglas. Looking specifically at the heavy minerals further studies will attempt to correlate the terraces. Also, relative ages will be determined by looking at the post-depositional alteration or weathering. First quantification of the (heavy) mineral content displays a large contribution of quartz, epidote, magnetite, and amphibole grains from Karoo sediment and volcanics, whereas altered granite and pegmatite fragments of unknown origin form the lesser part of the sediment spectrum. At Bucklands, on both sides of the confluence point, relatively large estimated gravel thicknesses (>5m) are overlain by several cycles of up to 3 m thick mud to silty sediments with occasional larger pebble to gravel occurrence, both part of the Rietputs and Riverton Formations. Orange and Vaal River sediments are sorted selectively by hydraulic processes causing the sorting of according to size, shape and density, concentrating heavy minerals in trap sites such as potholes, along bars, and in areas of turbulent water flow. Therefore river bars and slip-off slopes in an area of moderate relief appear to be prime loci of heavy mineral deposition. Principally it can be concluded that the Orange River section between Hopetown and Douglas shows less pronounced younger gravel terrace formation due to a relatively low erosion basis and a stronger incision into the Ventersdorp bedrocks; the lower Vaal River and the Orange River downstream Douglas however have a more pronounced terrace formation due to a higher erosion basis caused by bedrock morphology.
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Magnetotelluric Experiment Across Southern Barberton Greenstone Belt: a Students’ Perspective of the Field Work
Authors S. A. MacLennan, L. Moodley, U. Weckmann, O. Ritter, A. Nube, X. Chen, J. Deacon, G. Willkommen, R. Barth, R. Streich, K. Tietze and M. J. de WitThe 3.2-3.5 Ga Baberton Greenstone Belt contains some of the planets oldest structural lineaments, shear zones and tectonic melanges that may represent surface features of sutures zones. A magnetotelluric survey was conducted in April 2009 across the NE structural grain of the greenstone belt to image at depth one of the most prominent sutures in this area, the Saddleback- Inyoka Fault Zone(SIFZ). This will provide a key set of observations with which to unravel Archean tectonics. The MT field campaign took place over 6 weeks with a stringent timetable in effect throughout the campaign in order to complete the planned survey. Each team would typically check one MT station, pack one up and set a new station up in the next location every day. In the evenings the data was processed and its quality was scrutinised. The field campaign required a great deal of planning, negotiation and physical labour in order to make it a success! To complement this geophysical data, ongoing field mapping by one of us, Scott MacLennan, is focussed on a transect across the SIFZ in the area around the Stolzberg syncline. In addition, and in order to increase the accuracy of the modelling of the magnetotelluric data, a representative rock sample set of the Baberton Greenstone Belt and beyond will be collected. Conductivity measurements will be carried out on these samples in Germany in 2010. This will complement the MT work to provide a more robust conductivity model for this area of the Kaapvaal craton. The integration of these different geological and geophysical techniques will result in a data set that could help elucidate what geodynamic processes were important in shaping this remnant of Archean crust.
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The Nickel Deposit at Bon Accord, Barberton, South Africa
Authors Fulufhelo Madala and Marian TredouxThe Barberton greenstone belt in the Mpumalanga, RSA, covers an area of some 6 000 km to the southeast of the town of Nelspruit. The Bon Accord (BA) Ni-oxide deposit is located in the southwest of the area. The deposit was initially discovered in the 1920s, but technology of that time could not smelt the extremely Ni-rich material and the ore was abandoned. It was rediscovered during regional Ni exploration by Eland resources Limited (in the 1970s). It has been estimated that about 22.5 t of material was removed in the 1920s. The host rock consists of ultramafic rocks of the 3.2 Ga Onverwacht Group of the Barberton greenstone belt. The Ni-oxide deposit forms a elliptical body about 0.5 m thick and 20 m wide along its longest axis. It is situated close to a contact between the ultramafic rocks of the Jamestown igneous suite and a siliceous mylonite zone. The BA deposit has a unique mineralogy of rare Ni-rich minerals, such as liebenbergite (olivine), trevorite (spinel), willemsite (talc), nimite (chlorite), and bonaccordite (borate). The levels of Cu and S are both low (<100 ppm), and that of Ni unusually high (>30%), which means that BA differs from other all known Ni deposits chemically. The aim of this research has been to do a thorough optical microscopic and XRD investigation of unclassified samples of the BA body and surrounding rocks, identifying all minerals in the samples and comparing them with the results obtained from the previous reports. XRF analysis was also done to determine the full suite of major elements (something that was not available before) and some selected trace elements. Owing to the unusual chemistry of the BA material, special adjustments had to be made to the ‘normal’ sample preparation techniques routinely used for making fusion beads for major element XRF analysis, to prevent the sample from reacting with the Pt crucible.
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Three-dimensional Modeling of Petroleum Generation in the Southeast Flank of the Bredasdorp Basin, off the South Coast of South Africa
Authors Lihle Madyibi, Rolando di Primio and George SmithThe aim of this study was to investigate the evolution of petroleum system in the southeast flank of the Bredasdorp Basin. In this study, we have applied a two-dimensional, later up-scaled to a three-dimensional, modeling approach in order to advance our understanding of the petroleum systems evolution in the southern flank of the Bredasdorp Basin. Prior to the modelling, mapped seismic horizons were depth converted using the VoK method. The model is based on the interpretation of 64490m two-dimensional seismic-reflection profiles, as well as the analyses of well logs and cores that constrain the sedimentary facies distribution and the units’ age. Three main source rock intervals (Barremian, Aptian and Turonian) were modeled for maturity and hydrocarbon generation. The simulation was performed using Petromod® 3D package (version 10, IES Germany). Source rock intervals were assigned properties such as Total Organic Content (TOC) data from the respective wells that intersected these intervals. Calibration data to the model included vitrinite reflectance and bottom-hole temperature (BHT). Results obtained from petroleum generation indicate that the Barremian and Aptian source rocks are mature whereas the Turonian source is immature. Modelling output suggests that the said source rocks started petroleum generation as from 80 Ma but halted about 69 to 35 Ma and generating very slow from period between 12 to 0 Ma both periods of generation owing to minimal burial depth of sediments.
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Techno-advantages of Acoustic Televiewer Logging on Shaft Boreholes
Authors Mary N. Mahlatji and Warren KrynieGeophysical borehole logging of two shaft boreholes separated by 300m was used to complement geological logging of un-oriented borehole core in a geotechnical investigation aimed at mapping rock strengths and stability in advance of shaft-sinking operators. High-angle (>60º) dip magnitude fracture systems interpreted from acoustic televiewer imagery were ubiquitous throughout both holes and showed large variations around a mean strike orientation of NW-SE, in line with borehole “breakout” events also mapped from the televiewer. Fracture depths correlates well with those mapped from geological core logging. A unique, 150m wide zone of anomalously high fracture frequency density in borehole A has been confirmed from ongoing shaft-sinking as reflecting “disturbed” ground, and points to the presence of a sub-vertical structural feature intersecting the borehole. Fracture orientations over the “disturbed” ground differ considerably from those above and below this zone. Ancillary geophysical data (density) does not uniquely define the extent of the highly fractured zone. Elsewhere, shallow temperature anomalies in both boreholes possibly reflect intersections of the same aquifer
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Palaeomagnetic Results from a Neoarchaean Dyke Swarm in the Badplaas-Barberton Area, South Africa
By L.P. MaréA palaeomagnetic study is in progress on dyke swarms in the Badplaas-Barberton area and the preliminary results are presented here. The main aims of the study are to contribute towards the debate on Neoarchaean crustal evolution, and attempt to constrain the age, tectonic setting and genesis of these mainly tholeiitic dyke swarms.
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Distinguishing Between Impact Craters and Volcanic Pipes using Ground Magnetics: Field Examples from Zimbabwe
Authors Sharad Master and David J. RobertsonImpact structures produced by meteorite or cometary impacts are generally circular features, which are accompanied by a range of diagnostic shock phenomena. Many other processes can also produce circular structures that resemble impact structures. We report on two case studies from Zimbabwe, where we investigated two circular possible impact structures, in similar host rocks, using N-S traverses with a hand-held proton precession magnetometer. The West Save structure, in eastern Zimbabwe is a ~600m diameter structure, developed in Triassic sandstones of the Karoo Supergroup. The ~220m diameter Sinamwenda structure, in western Zimbabwe, near Lake Kariba, is also developed in similar Triassic Karoo sandstones. The West Save structure contains basalt and gabbroic intrusions, and is formed by differential erosion of a more indurated sandstone rim around a volcanic pipe. At West Save a strong asymmetric jagged dipole magnetic anomaly, ranging from 30140 to 31080 nT, is interpreted as a signal produced by a vertical cylinder of intrusive rock in a volcanic vent. The Sinamwenda structure, by contrast, has no trace of volcanic or intrusive rocks, and it shows not the slightest magnetic anomaly, with a constant total magnetic field value of 31000 nT, with a variation of the magnetic intensity of only 2 nT. An overturned rim, multiply striated joint surfaces, microbrecciation, and planar deformation lamellae in quartz indicate an impact origin for the Sinamwenda structure. The lack of a magnetic anomaly rules out alternative interpretations such as kimberlitic intrusions. We conclude that ground magnetic study of circular structures can easily distinguish between structures produced by impact processes (absence of magnetic anomaly), and structures produced by volcanic intrusions (dipole magnetic anomaly), if the magnetic susceptibility of the surrounding rocks is minimal.
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