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1
Everitt, Simon James. "Evolution of the UG2 unit, Bushveld Complex, South Africa : mineral composition and petrological evidence." Thesis, Rhodes University, 2013. http://hdl.handle.net/10962/d1001573.
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Several disequilibrium textures are found to occur within the hanging wall and footwall of the UG2 chromitite layer of the Bushveld Complex, South Africa. These textures include plagioclase chadacrysts found included within orthopyroxene and clinopyroxene as well as the orthopyroxenes exhibiting round crystal boundaries that appear to be resorbed. Textures found within the UG2 stratigraphy such as linear boundaries and 120° triple junctions at interfaces of adjacent plagioclase or pyroxene grains also suggest that recrystallization has taken place. The presence of both disequilibrium textures and recrystallization textures would suggest that a complex emplacement history has occurred. Ideally, this would be expected to be manifested by minerals of the same type but which are texturally distinct showing different composition. However this has been found not to be the case; minerals that suggest disequilibrium textures show similar compositions to the minerals which appear to have formed in equilibrium. This is also the same for recrystallized crystals which show the same compositions as crystals that have not been recrystallized. For example tabular clinopyroxene, which has a compositional range of En 44.6 to En 50.5, is indistinguishable from clinopyroxene occuring as discontinuous rims, En 44.3-48.2, and as intergranular necking connecting primocrysts of orthopyroxene ( En 44.3-50.4). Similarly, plagioclase occurring as inclusions with An 66.3-76.0 is indistinguishable from plagioclase occurring as zoned or recrystallized interstitial grains ( An 69.0- An 77.4). Compositional variation has however, been found to be controlled to an extent by stratigraphy in that minerals show different compositions within one layer to the same minerals within another layer, consistent with an evolving magma composition. It is concluded therefore that while composition is not texturally controlled it is to an extent stratigraphy controlled and that the evidence collected within the study supports two models for the formation of chromite within the Bushveld complex. The evidence is consistent with a combination of the magma mixing model and magma injection model to account for the textures and compositional variations found within the study. The evidence may also show support for models involving late modification of minerals by magmatic fluids but not as prominently as for the models mentioned aboveMicrosoft� Word 2010
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2
Curl, Edward Alexander 1972. "Parental magmas of the Bushveld Complex, South Africa." Monash University, Dept. of Earth Sciences, 2001. http://arrow.monash.edu.au/hdl/1959.1/9080.
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Gwatinetsa, Demand. "Distribution of iron-titanium oxides in the vanadiferous main magnetite seam of the upper zone : Northern limb, Bushveld complex." Thesis, Rhodes University, 2014. http://hdl.handle.net/10962/d1013281.
Full textAbstract:
The main magnetite seam of the Upper Zone of the Rustenburg Layered Suite (SACS, 1980) on the Bushveld Complex is known to host the world‘s largest vanadium bearing titaniferous iron ores. The vanadiferous titanomagnetites, contain vanadium in sufficient concentrations (1.2 - 2.2 per cent V₂O₅) to be considered as resources and vanadium has been mined historically by a number of companies among them Anglo-American, Highveld Steel and Vanadium and VanMag Resources as well as currently by Evraz Highveld Steel and Vanadium Limited of South Africa. The titanomagnetites contain iron ore in the form of magnetite and titanium with concentrations averaging 50-75 per cent FeO and 12-21 per cent TiO₂. The titaniferous iron ores have been historically dismissed as a source of iron and titanium, due to the known difficulties of using iron ore with high titania content in blast furnaces. The economic potential for the extractability of the titaniferous magnetites lies in the capacity of the ores to be separated into iron rich and titanium rich concentrates usually through, crushing, grinding and magnetic separation. The separatability of iron oxides and titanium oxides, is dependent on the nature in which the titanium oxide occurs, with granular ilmenite being the most favourable since it can be separated from magnetite via magnetic separation. Titanium that occurs as finely exsolved lamellae or as iron-titanium oxides with low titania content such as ulvospinel render the potential recoverability of titanium poor. The Upper Zone vanadiferous titanomagnetites contain titanium in various forms varying from discrete granular ilmenite to finely exsolved lamellae as well as occurring as part of the minerals ulvospinel (Fe₂TiO₄) and titanomagnetite (a solid solution series between ulvospinel and magnetite) . Discrete ilmenite constitutes between 3-5 per cent by volume of the massive titanomagnetite ores, and between 5-10 per cent by volume of the magnetite-plagioclase cumulates with more than 50 per cent opaque oxide minerals. The purpose of this research was to investigate the mineralogical setting and distribution of the iron and titanium oxides within the magnetitite layers from top to bottom as well as spatially along a strike length of 2 000m to determine the potential for the titanium to be extracted from the titanomagnetite ores. The titanomagnetites of the Upper Zone of the Bushveld Complex with particular reference to the Northern Limb where this research was conducted contains titanium oxides as discrete ilmenite grains but in low concentrations whose potential for separate economic extraction will be challenging. The highest concentration of titanium in the magnetite ores is not contained in the granular ilmenite, but rather in ulvospinel and titanomagnetite as illustrated by the marked higher concentration of TiO₂ in the massive ores which contain less granular ilmenite in comparison to the disseminated ores which contain 3 to 8 percentage points higher granular ilmenite than the massive ores. On the scale of the main magnetite seam, the TiO₂ content increases with increasing stratigraphic height from being completely absent in the footwall anorthosite. The V₂2O₅ content also increases with stratigraphic height except for in one of the 3 boreholes where it drops with increasing height. The decrease or increase patterns are repeated in every seam. The titanomagnetites of the main magnetite seam display a variety of textures from coarse granular magnetite and ilmenite, to trellis ilmenite lamellae, intergranular ilmenite and magnesian spinels and fine exsolution lamellae of ulvospinel and ferro-magnesian spinels parallel to the magnetite cleavage. The bottom contact of the main magnetite seam is very sharp and there is no titanium or vanadium in the footwall barely 10cm below the contact. Chromium is present in the bottom of the 4 layers that constitute the main magnetite seam and it upwards decreases rapidly. In boreholes P21 and P55, there are slight reversals in the TiO₂ and V₂O₅ content towards the top of the magnetite seams.
4
Lovegrove, Daniel Paul. "Rates and mechanisms of metamorphic processes derived from thermal aureole studies." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.249305.
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Sargeant, Fiona. "The seismic stratigraphy of the Bushveld Igneous Complex, South Africa." Thesis, University of Liverpool, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.250322.
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Twala, Mthokozisi Nkosingiphile. "Use of multispectral remote sensing data to map magnetite bodies in the Bushveld Complex, South Africa : a case study of Roossenekal, Limpopo." Diss., University of Pretoria, 2019. http://hdl.handle.net/2263/75756.
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Mineral detection and geological mapping through conventional ground survey methods based on field observation and other geological techniques are tedious, time-consuming and expensive. Hence, the use of remote sensing in mineral detection and lithological mapping has become a generally accepted augmentative tool in exploration. With the advent of multispectral sensors (e.g. ASTER, Landsat and PlanetScope) having suitable wavelength coverage and bands in the Shortwave Infrared (SWIR) and Thermal Infrared (TIR) regions, multispectral sensors, along with common and advanced algorithms, have become efficient tools for routine lithological discrimination and mineral potential mapping. It is with this paradigm in mind that this project sought to evaluate and discuss the detection and mapping of magnetite on the Eastern Limb of the Bushveld Complex, using specialized common traditional and machine learning algorithms. Given the wide distribution of magnetite, its economic importance, and its potential as an indicator of many important geological processes, the delineation of magnetite is warranted. Before this study, few studies had looked at the detection and exploration of magnetite using remote sensing, although remote sensing tools have been regularly applied to diverse aspects of geosciences. Maximum Likelihood, Minimum Distance to Means, Artificial Neural Networks, Support Vector Machine classification algorithms were assessed for their respective ability to detect and map magnetite using the PlanetScope Analytic Ortho Tiles in ENVI, QGIS, and Python. For each classification algorithm, a thematic landcover map was attained and an error matrix, depicting the user's and producer's accuracies, as well as kappa statistics, was derived, which was used as a comparative measure of the accuracy of the four classification algorithms. The Maximum Likelihood Classifier significantly outperformed the other techniques, achieving an overall classification accuracy of 84.58% and an overall kappa value of 0.79. Magnetite was accurately discriminated from the other thematic landcover classes with a user’s accuracy of 76.41% and a producer’s accuracy of 88.66%. Despite the Maximum Likelihood classification algorithm illustrating better class categorization, a large proportion of the mining activity pixels were erroneously classified as magnetite. However, this observation was not merely limited to the Maximum Likelihood classification algorithm, but all image classifications algorithms. The overall results of this study illustrated that remote sensing techniques are effective instruments for geological mapping and mineral investigation, especially in iron oxide mineralization in the Eastern Limb of Bushveld Complex.Dissertation (MSc)--University of Pretoria, 2019.
Geology
MSc
Unrestricted
7
Koegelenberg, Corne. "Experimental evidence for sulphide magma percolation and evolution : relevant to the chromite bearing reefs of the Bushveld Complex." Thesis, Stellenbosch : Stellenbosch University, 2012. http://hdl.handle.net/10019.1/20043.
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Thesis (MSc)--Stellenbosch University, 2012.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.
AFRIKAANSE OPSOMMING: Pt mineralisasie in die Bosveld Kompleks is kenmerkend gefokus op die chromatiet riwwe, alhoewel die riwwe geassosieer is met lae volumes basismetaal sulfiedes relatief tot Pt graad. Verdeling van Pt (Dsil/sulp) vanaf silikaat magma in onmengbare sulfiedvloeistof is klaarblyklik onvoldoende om Pt konsentrasies in chromatiet lae te verduidelik, a.g.v. die wanverhouding wat bestaan tussen ‘n baie groot R-faktor wat benodig word en die relatiewe silikaat rots volumes. Gevolglik, in die eksperimentele studie probeer ons beter insig kry oor moontlike Pt graad verhogingsprosesse wat plaasvind in die BK sulfied magma. Ons ondersoek die benattingseienskappe van sulfied vloeistof relevant tot chromiet- en silikaat minerale, omdat dit die sleutel maatstaf is vir die beheer van sulfied vloeistof deursypeling deur die kumulaat opeenhoping. Addisioneel het ons ook ondersoek hoe die fraksionering van sulfied vloeistof vanaf MSS kristalle, gevorm binne die hangende melanoriet muur, moontlik die sulfied samestelling en Pt graad binne ontwikkelde sulfied smelt kan beïnvloed. Twee stelle van eksperimente is gedoen: Eerstens, by 1 atm om ondersoek in te stel oor fase verwantskappe tussen 900OC en 1150OC, binne ‘n Pt-verrykte sulfied magma samestelling relevant tot die BK; Tweedens, by 4 kbar, tussen 900OC tot 1050OC, wat die afwaartse deursypeling van sulfied magma deur veelvuldige lae van silikaat minerale en chromatiet. Addisionele 1 atm eksperimente is gedoen binne ‘n chromiet gedomineerde chromiet-sulfied mengsel, om te toets of interaksie met chromiet die sulfied sisteem affekteer deur Fe2+ te verwyder of by te dra. Primêre observasies is soos volg: Ons het bevind sulfiedsmelt is uiters mobiel, die mediaan dihedrale hoek tussen sulfiedsmelt en minerale van chromiet en silikaat lae is 11O en 33O onderskydelik. Dit is ver onder die deursypelings drumpel van 60O vir natuurlike geologiese stelsels. In silikaatlae vorm die sulfiedsmelt vertikale netwerke wat deursypeling bevorder. Inteendeel, uiters effektiewe benatting van sulfiedsmelt binne chromatiete vertraag sulfied deusypeling. Tussen kristal kapilêre kragte verhoog smelt retensie, dus dien chromatiete as ‘n opgaarmedium vir sulfiedsmelt. S oorversadigte sulfied vloeistof loogsif Fe2+ vanuit chromiet en veroorsaak ‘n verhoging in Fe-konsentraie. Die gereageerde chromiet buiterante is daarvolgens verryk in Cr-spinêl eind-ledemaat. Die addisionele byvoeging van Fe2+ aan sulfied magma veroorsaak die kristalisasie van Fe-ryke Mss en verlaag dus die S-konsentrasie van die sulfied smelt. Dit verlaag Pt oplosbaarheid en lei tot die formasie van Py allooie binne-in chromatiete. Ten einde, ontsnap Cu-ryke sulfied smelt deur die onderkant van die chromatiet lae. Die observasies is direk van toepassing op die gemineraliseerde chromatiet riwwe van die Bosveld Kompleks. Ons stel voor dat sulfied magma, potensiaal ingespuit vanuit die mantel saam nuwe inspuitings van silikaat magma, deur die hangende silikaat kumulaat bo chromatiet lae deurgesypel het en ‘n betekenisvolle volume Fe-Mss gekristalliseer het. Chromatiet lae het gefunksioneer as lokvalle vir afwaartsbewegende, ontwikkelde, Cu-, Ni-, en Pt-ryke sulfied vloeistowwe. Dit word ondersteun deur die algemene verskynsel dat chromatiete hoër persentasies van Ni, Cu en Pt relatief teenoor die hangende muur silikaat lae het. Wanneer sulfied smelt in kontak is met chromiet, word dit geforseer om Mss te kristalliseer soos Fe2+ geloogsif word, waarvolgens die smelt se S konsentrasie verder verlaag word. Dit veroorsaak die presipitasie, as Pt allooie, van groot proporsies opgeloste Pt vanuit sulfied smelt. Deur die prosesse te kombineer, kan dit moontlik verduidelik word hoekom chromatiet riwwe in die Bosveld Kompleks Pt/S verhoudings veel hoër is as aanrakende melanoriet lae.
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Venter, Andrew Derick. "Air quality assessment of the industrialized western Bushveld Igneous Complex / Andrew Derick Venter." Thesis, North-West University, 2011. http://hdl.handle.net/10394/8530.
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South Africa has the largest economy in Africa, with significant mining and
metallurgical activities. A large fraction of the mineral assets is concentrated in the
Bushveld Igneous Complex (BIC), with the western limb being the most exploited.
Although the western BIC is considered to be an air pollution hotspot, inadequate air
quality data currently exists for this area.
To partially address this knowledge gap, a comprehensive air quality monitoring station
was operated for more than two years at Marikana in the western BIC. Basic
meteorological parameters, precipitation, Photosynthetic Photon Flux Density (PPFD),
trace gas concentrations (SO2, NO, NOx, O3, and CO), physical aerosol parameters
(particle number and air ion size distributions, as well as aerosol light absorption) and
total PM10 mass concentration were measured.
Compared with South African and European ambient air quality standards, SO2, NO2 and
CO concentrations were generally below the air quality standards, with average
concentrations for the sampling period of 3.8ppb (9.9μg/m³), 8.5ppb (15.9μg/m³) and
230ppb (270μg/m³), respectively. The major source of SO2 was identified as high-stack
industry emissions, while household combustion was identified as the predominant
source of NO2 and CO. In contrast, O3 exceeded the eight-hour moving average standard
(61ppb / 120μg/m³) 322 times per year. The main contributing factor was identified to be
the influx of regional air masses, with high O3 precursor concentrations. PM10 exceeded
the current South African 24-hour standard (120μg/m³) on average 6.6 times per year, the
future 2015 standard (75μg/m³) 42.3 times per year and the European standard (50μg/m³)
120.2 times per year. The PM10 average concentration for the sampling period was
44μg/m³, which exceeded the current European and future (2015) South African annual
average standard (40μg/m³), emphasising the PM pollution problem in the western BIC.
The main source of PM10 was identified as household combustion.Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2012
9
Manyeruke, Tawanda Darlington. "Compositional and lithological variation of the Platreef on the farm Nonnenwerth, northern lobe of the Bushveld Complex implications for the origin of platinum-group elements (PGE) mineralization /." Thesis, Pretoria : [s.n.], 2008. http://upetd.up.ac.za/thesis/available/etd-01192009-164657/.
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Botha, Pieter W. S. K. "The mineralogy and geochemistry of the Rooikoppies iron-rich ultramafic pegmatite body, Karee Mine, Bushveld Complex, South Africa [electronic resource] /." Pretoria : [s.n.], 2008. http://upetd.up.ac.za/thesis/available/etd-01272009-172307/.
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Kleemann, Gunther Johann. "The Geochemistry and petrology of the roofrocks of the Bushveld complex east of Groblersdal." Diss., University of Pretoria, 1985. http://hdl.handle.net/2263/29478.
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Teigler, Bernd. "Mineralogy, petrology and geochemistry of the lower and lower critical zones, Northwestern Bushveld Complex." Thesis, Rhodes University, 1991. http://hdl.handle.net/10962/d1005590.
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This study of the lower part of the Rustenburg Layered Suite in the Western Bushveld Complex is based mainly on drill core samples from three localities, which are approximately 130 km apart. The NG-sequence, situated in the northwestern sector of the complex (Union Section, R.P.M.) extends from the floor of the complex to the base of the upper Critical Zone. The sequence is ca. 1800 m thick and it comprises mainly ultramafic cumulates, namely pyroxenites, olivine pyroxenites, harzburgites and dunites. Norites and anorthos ites are present only in minor proportion. Within the upper half of the NG-sequence ten prominent chromitite layers are correlated with the LGI MG4-interval. Correlation is also established between published sequences and the two other sequences studied, located 8 km and 55 km, respectively, east of Rustenburg. Whole-rock chemical data (major and trace elements), microprobe and Sr isotope data are presented. Petrographic studies provide modal analyses and measurements of grain size. All petrographic, mineralogical and other geochemical data point to an origin of the cumulates of the NG-sequence by crystallization from liquids of the U-type lineage and derivatives thereof. No evidence is found for the involvement of parental liquids with a distinctly different composition or crystallization order (A-liquids). However, subtle compositional variations of the parental liquids are evident in slight changes of the Cr content in orthopyroxene or in variations of Sr isotope ratio. The NG-sequence is characterized by intervals with reversed fractionation trends caused by repeated influxes of pristine magma (during periods of high magmatic activity) resulting in a high degree of rejuvenation. These intervals are overlain by others with a normal fractionation trend, interpreted as cumulates formed in periods with low or no magmatic activity, in which fractional crystallization controlled bulk composition of the evolving liquid. The Lower Zone in the NG-sequence is dominated by a progressive shift towards more primitive compositions, while in the Critical Zone fractionation was the major operating process in the magma chamber. However, during deposition of the pyroxenitic lower Critical Zone several replenishment events occurred, during which fresh Cr-rich magma was emplaced. Massive chromitite layers were deposited after mixing between the newly emplaced magma and the resident residual liquid shifted bulk compositions into the primary field of chrome-spinel. Cumulus plagioclase crystallized after bulk composition of the residual liquid was driven to the orthopyroxene plagioclase cotectic by continued fractional crystallization; this occurred once in the Lower Zone, yielding a single, thin norite layer, and again in the upper Critical Zone of the NG-sequence. A facies model is proposed based on the stratigraphic and compositional variations along strike in the Western Bushveld Complex. This model explains the variations by means of the position of the sequence with regard to a feeder system. The olivine- and orthopyroxene-rich, but plagioclase-poor NG-sequence represents the proximal facies, while the SF-sequence (poor in ferromagnesian phases, but plagioclase-rich) is developed as a distal facies, close to the Brits graben.
13
Mooney, David G. "An evaluation of small scale open cast mining of UG2 in the Bushveld complex." Thesis, Rhodes University, 1997. http://hdl.handle.net/10962/d1005555.
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The current weak state of the platinum market, as well as the large inventory of platinum group metals held by Russia, necessitates that the South African platinum mining industry must carefully evaluate the benefits and disadvantage of small-scale mining of shallow, open castable ore resources. Until the late 1980's, these resources were ignored due to the metallurgical complexities of treating oxidized ore, as well as the mind set that existed within the South African mining industry which militated against open cast mining in the Bushveld. During the latter part of the 1980's and early 1990's, advances in the metallurgical treatment of oxidized ore, specifically the UG2, as well as operational problems, created the impetus to begin the exploitation of these resources. Small-scale open cast mining has become viable due to the development of suitable mining methods that facilitate mining practices acceptable to the Department of Mineral and Energy Affairs, in terms of environmental legislation. Metallurgical advances and growing experience, especially with respect to the UG2, enables reasonable platinum group metals recovery from oxidized ore. The problems experienced in doing this can, and are being overcome. With growing public awareness of environmental issues, particularly related to the mining industry, the requirement to ensure that the small-scale open cast mine site is well managed is paramount. A methodology for the evaluation is presented along with a case study of a small-scale UG2 open cast pit. Evidence is presented that shows that these small-scale open cast mining operations are extremely profitable and require minimal capital expenditure. However, caution is advised when evaluating Merensky Reef open cast operations because of the problems that they present.
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Kleemann, G. J. "The geochemistry and petrology of the roof-rocks of the Bushveld Complex east of Groblersdal." Pretoria : [s.n.], 1985. http://upetd.up.ac.za/thesis/available/etd-11152006-125021.
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Van, Wyngaardt Grizelda. "Temporal assessment of atmospheric trace metals in the industrialised western Bushveld Complex / van Wyngaardt G." Thesis, North-West University, 2011. http://hdl.handle.net/10394/6950.
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The presence of trace transition metal species in the atmosphere can be attributed to
the emission of particulate matter into the atmosphere by anthropogenic activities, as
well as from natural sources. Trace metals emitted into the atmosphere can cause
adverse health–related and environmental problems. At present, limited data exists
for trace metal concentrations in South Africa. In this investigation, the general aim
was to determine the concentrations of trace metals in atmospheric aerosols in the
industrialised western Bushveld Igneous Complex, as well as to link the presence of
these species in the atmosphere to possible sources in the region.
The measurement site was placed in Marikana, a small rural town situated 35 km
east from Rustenburg in the North West Province of South Africa. It is surrounded
by numerous industrial and metallurgical operations. MiniVolumeTM samplers and
Teflon® filters (2 ;m pores) were utilised to collect PM2.5 and PM10 particulate
samples. The MiniVolumeTM samplers were programmed to filter 5 litres of air per
minute for 12 hours per day, over a six–day period. The starting time for sampling
was altered every six days, in order to obtain both day and night samples. Sampling
was performed for a period of one year.
The collected samples were chemically analysed with inductively coupled plasma
mass spectroscopy (ICP–MS). Surface analysis of the sampled filters was performed
with a scanning electron microscope (SEM) in conjunction with energy–dispersive
spectroscopy (EDS). The dataset was also subjected to factor analysis in an attempt
to identify possible sources of trace metal species in the atmosphere.
The concentrations of 27 trace metals (Be, B, Na, Mg, Al, K, Ca, Ti, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, As, Se, Pd, Cd, Ba, Pt, Au, Hg, Tl, Pb, U) were determined. Pd, Hg,
Tl, U, Ca, Co, As, Cd, Ba and Au were above the detection limit 25% or less of the
time during the sampling period. With the exception of Ni, none of the trace metals
measured at Marikana during the sampling period exceeded local and international
standards. Higher Ni levels were possibly due to base metal refining in the region.
Pb, which is the only metal species that has a standard prescribed by the South
African Department of Environmental Affairs (DEA), did not exceed any of the standards. It is also significant to refer to Hg that was below the detection limit of the
analytical instrument for the entire sampling period.
The impact of meteorological conditions revealed that wet removal of atmospheric
PM10 trace metals was more significant than the wind generation thereof. During the
dry months, the total trace metal concentrations in the PM10 fraction peaked, while
PM10 particles were mostly washed out during the wet season. Wind speed showed
an unexpected inverse pattern compared to wet deposition. A less significant
seasonal trend was observed for the trace metal concentrations in the PM2.5 fraction,
which was attributed to a faster replenishment of smaller particles into the
atmosphere after rain events.
Separation of trace metal concentrations into PM10–2.5 and PM2.5 fractions indicated
that 79% of the total trace metal levels that were measured were in the PM2.5
fraction, which indicated a strong influence of industrial and/or combustion sources.
Fractionalisation of each of the trace metal species detected showed that for each
metal species, 40% and more of a specific metal was in the PM2.5 fraction, with Cr,
V, Ni, Zn and Mn occurring almost completely in the PM2.5 fraction.
Surface analysis with SEM supported results from the chemical analysis, which
indicated that a large fraction of the particles was likely to originate from
anthropogenic activities and from wind–blown dust. SEM–EDS also detected nonmetallic
S that is usually associated with the Pt pyrometallurgical industry that is
present in the western Bushveld Igneous Complex.
Correlations between Cr, V, Ni, Zn and Mn revealed that the main sources of these
species were pyrometallurgical industries. Explorative factor analysis of the
unprocessed and Box–Cox transformed data for all 27 metals detected, resolved four
meaningful emission sources, i.e. crustal, vanadium related, base metal related and
chromium related. Comparison of trace metal species to other parameters measured
(e.g. CO, BC) also indicated pyrometallurgical activities and wind–blown dust to be
the main sources of trace metals in this region.Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2011.
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Mavimbela, Philane Knowledge. "Metamorphism in the contact aureole of the eastern limb of the Bushveld complex, South Africa." Diss., University of Pretoria, 2013. http://hdl.handle.net/2263/41190.
Full textAbstract:
The 2.06 to 2.054 Ga Bushveld Igneous Complex intruded into the sedimentary rocks of the Transvaal Supergroup and generated an extensive contact metamorphic aureole mainly developed in the upper Pretoria group. The studied samples represent the Silverton Daspoort and Timeball Hill formations and are divisible into garnet bearing hornfels (DY918, DY954 and DY956) and garnet-free staurolite-bearing metapelites (DY916, DY982 and DY987). The garnet-bearing hornfelses marks the garnet zone within the aureole and the garnet formation is controlled by different reactions forming from 490 to 630 0C. On the other hand, the garnet free staurolite-bearing Fe-Al rich metapelites define the staurolite zone restricted to the Timeball Hill formation. The recorded P-T conditions in G0 and G1 garnets of the DY954 hornfels imply that the two garnets formed under different conditions indicating two stages of metamorphism. However, the Lu-Hf isotope systematics of these garnets records a 2061 Ma age for all garnet porphyroblasts in both the DY918 and DY954 hornfelses, which support co-genetic garnet growth regardless of their stratigraphic positions. Therefore, the 2061 Ma garnet age denote the emplacement age of the Lower Zone and Critical Zone magmas which was synchronous with the extrusion of the Rooiberg Group volcanics. The fact that all analysed garnets do not record the 2059 – 2054 intrusion of the Main Zone and Upper Zone magmas probably means that the crystallisation temperatures of the later magma pulse was not significant enough to shift the Lu-Hf isotopic signatures. Euhedral staurolites are widespread within the Fe-Al rich metapelites with grain sizes of up 4mm; texturally the majority of them have been altered or overgrown by biotite and chloritoid. The alteration or of these staurolite porphyroblasts is due to isobaric cooling during uplift, and the St-Bt assemblage represent the peak equilibrium conditions and marks the upper stability limit of the Chl-Ctd assemblage.Dissertation (MSc)--University of Pretoria, 2013.
gm2014
Geology
unrestricted
17
Otto, Tahnee. "Texture development in titaniferousmagnetites found in Layer 21 in the Bushveld Igneous Complex, South Africa." Diss., University of Pretoria, 2017. http://hdl.handle.net/2263/63292.
Full textAbstract:
Very limited studies have been conducted on titaniferous-magnetite exsolution textures and
the conditions needed for the formation of these textures. Published research surrounding
the exsolution textures consider only a particular element of the oxide, or a specific
condition. The Upper Zone of the Rustenburg Layered Suite in the Bushveld Igneous
Complex boasts 25 magnetite layers. The 21st layer counted from the Main Magnetite layer
is called Layer 21, which is the uppermost titaniferous-magnetite layer in the Upper Zone.
Uncommon exsolution textures in the titaniferous-magnetite grains were studied in order to
gain a greater understanding of the formation of oxide exsolution textures. The exsolution
texture presents itself as a three-dimensional framework of ulvöspinel-rich lamellae together
with magnetite prisms. This is called a cloth texture exsolution. The data collected for this
investigation included Scanning Electron Microscope (SEM) images, elemental weight
percentage data, and SEM line scans. Previous studies do not show exsolution textures that
are exactly similar to the exsolution textures seen in Layer 21, although the same basic type
of microtexture can be seen. This indicates that conditions such as temperature, pressure,
and oxygen fugacity, as well as the bulk mineral chemistry, plays a large role in the formation
of the exsolution texture. A rough model has been provided that considers all of the
information collected in previous studies in order to start the development of a complete
model. Another model has been provided explaining the physical appearance of the cloth
texture exsolution. The exsolution textures need to be investigated on a three-dimensional
basis in order to develop a more accurate understanding of why the titaniferous-magnetite
exsolution textures are different from location to location.Dissertation (MSc)--University of Pretoria, 2017.
Geology
MSc
Unrestricted
18
Viljoen, Willemien. "Phase relations in the system Cu-Fe-Ni-S and their application to the slow cooling of PGE matte." Pretoria : [s.n.], 2005. http://upetd.up.ac.za/thesis/available/etd-10132005-100921/.
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Manyeruke, Tawanda Darlington. "The petrography and geochemistry of the Platreef on the farm Townlands near Potgietersrus, northern Bushveld Complex." Pretoria : [s.n.], 2005. http://upetd.up.ac.za/thesis/available/etd-04282005-110052/.
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Smith, Jennifer Williamina. "The nature and origin of PGE mineralization in the Rooipoort area, northern Bushveld Complex, South Africa." Thesis, University of Leicester, 2014. http://hdl.handle.net/2381/31995.
Full textAbstract:
The Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member within the northern limb of the Bushveld Complex is a PGE-Ni-Cu mineralized, layered package of mafic cumulates. This magmatic sulfide deposit is developed at the equivalent stratigraphic position to the Platreef, being overlain by Main Zone gabbronorites and in places resting unconformably on metasediments from the Transvaal Supergroup. Parental magmas to the GNPA member were of a ‘hybrid’ composition containing both B1 and B2/B3 type magma components which were strongly crustally contaminated and S saturated at the time of emplacement. At depth, the assimilation of crustal S was crucial for ore genesis. Although parental magma(s) experienced a second localised contamination event, interaction with the local footwall at the time of emplacement, did not have any control on the genesis of sulfide mineralization. A single primary sulfide liquid, enriched in PGE, Ni, Cu and semi-metals was distributed throughout the succession during multiphase emplacement of the GNPA member. The distribution and mineralogy of platinum-group and chalcophile elements results from the complex behaviour of these elements during both sulfide fractionation and hydrothermal processes. The primary assemblage is characterised by IPGE-rich pyrrhotite, IPGE-, Rh, and Pd-rich pentlandite, chalcopyrite, and associated Pt-As and Pd-Bi-Te minerals. Secondary assemblages in addition contain Pd- and Rh-rich pyrite and millerite, and discrete minerals including Pd antimonides and arsenides. Whilst correlations between the GNPA/Platreef and Upper Critical Zone remain relatively speculative, the northern limb deposits are thought to have formed from compositionally similar or related magmas, which were poorer in Mg, richer in Ca and Fe and Pd dominant relative to the magma(s) that formed the Upper Critical Zone. It is proposed that with depth the Platreef may progressively transform into a layered succession that is exposed south of the Ysterberg-Planknek Fault and represented by the GNPA member. The Platreef can therefore possible be viewed as a marginal facies of the GNPA member, and sulfide-rich magma which escaped up the margins of the northern limb chamber.
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Letsoele, Christopher Sbusiso. "The occurrences of ultramafic pegmatites at Dwarsrivier Mine, Lower Critical Zone, Eastern Bushveld Complex, South Africa." Diss., University of Pretoria, 2017. http://hdl.handle.net/2263/63298.
Full textAbstract:
This study presents a detailed petrological and geochemical investigation of the concordant ultramafic
pegmatites of the Lower Critical Zone at Dwarsrivier Mine, in the eastern Bushveld Complex. The
pegmatites are coarse-grained and of variable size and mineralogy. They occur within the LG-6
chromitite layer as well as in the silicate rocks of the Rustenburg Layered Suite. Field observations into
their occurrence, and their association with the LG-6 chromitite layer, suggest that the pegmatites
intrude, deform and disrupt the LG-6 chromitite. The mineralogy of the pegmatites is similar as the
surrounding pyroxenite rocks, although they are texturally different from the surrounding pyroxenite
host rocks. They comprise variable amounts of clinopyroxene, orthopyroxene, olivine, plagioclase,
biotite and accessory amphiboles. Based on dominant mineralogy, the pegmatites can be grouped into
clinopyroxene-, orthopyroxene- and olivine-rich pegmatites.
The major and trace element composition of the pegmatites is different from the surrounding pyroxenite
host rocks. The whole rock geochemistry of the pegmatites indicate that the pegmatites have a slightly
higher concentration of incompatible trace elements compared to the pyroxenite host rocks, although
compositional overlaps exist. Whole rock Mg# and trace element ratios suggests that the pegmatites
are the products of less fractionated liquid(s) than their surrounding pyroxenite rocks.
When the whole rock geochemical data of the pegmatites are compared to other similar concordant
pegmatites in the eastern Bushveld Complex, as well as the discordant pegmatites from the western
Bushveld Complex, it is established that the pegmatites at Dwarsrivier mine are not genetically related
to the discordant, intrusive iron-rich pegmatites. They are also different from the concordant pegmatites
found elsewhere in the eastern Bushveld Complex.
The pegmatites are interpreted as the products of a volatile-rich melt which promoted the
recrystallization or sub-solidus enlargement of the pyroxenes. The occurrence of cumulus olivine at the
stratigraphic position of the Lower Critical Zone is exceptional. Olivine crystalized as a result of
continuous phase shifts within the pegmatite melt. Some of the pegmatites are serpentinised by late
hydrothermal fluids. The olivine-rich pegmatites display the most prominent serpentinisation, and are
composed of a mineral assemblage dominated by serpentine and magnetite.Dissertation (MSc)--University of Pretoria, 2017.
Assmang Dwarsrivier Mine
Geology
MSc
Unrestricted
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Bamisaiye, Oluseyi Adunola. "Visualisation, 3D Modelling and Spatial Analysis of the Rustenburg Layered Suite, Bushveld Igneous Complex, South Africa." Thesis, University of Pretoria, 2015. http://hdl.handle.net/2263/65903.
Full textAbstract:
Adequate knowledge of the regional subsurface geometry depth relationship between the limbs and distribution of mineral zones within the Rustenburg Layered Suite (RLS) is required for better understanding of the emplacement geometry, distribution of economic mineral zones and structural evolutionary issues. This will lead to improved exploration prospects that could assist in less degradation mining activities and environmental hazard control and management. Incompleteness of surface outcrops and limited availability of seismic data has been a hindrance to this. This research focused on the determination from available borehole data, the geometry and depth relations to modern topography of the RLS. Extensive Geostatistical analysis of hundreds of borehole log data was carried out to better constrain the complex geologic structural framework and architecture of the RLS. This has helped to identify and visualize the subsurface stratigraphic units, their geometric forms and improved the understanding of the geology and structure of the RLS.Thesis (PhD)--University of Pretoria, 2015.
Geology
PhD
Unrestricted
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Mwenze, Tshipeng. "The implications of Sr and Nd isotope data on the genesis of the Platreef and associated BMS and PGE mineralisation, Bushveld Igneous Complex, South Africa." University of the Western Cape, 2019. http://hdl.handle.net/11394/6922.
Full textAbstract:
Philosophiae Doctor - PhDThe Platreef is a platinum group elements (PGE) deposit located in the Northern limb of the Bushveld Igneous Complex (BIC). It is a series of mafic and ultramafic sills that are overlain by rocks from the Main Zone (MZ) of the BIC. In comparison to PGE deposits (i.e., Merensky Reef and the UG-2 chromitite) occurring in the Critical Zone (CZ) of the Eastern and Western Limbs of the BIC, which are less than 1 m in thickness, the Platreef is 10 to 400 m in thickness and is comprised of a variety of rocks. PGE mineralisation in the Platreef is not confined to a specific rock type, and its distribution and styles also vary with depth and along strike. Despite the numerous researches that have been conducted, the genesis of Platreef is still poorly understood. New major and trace elements in conjunction with Sr–Nd isotope data, generated from whole-rock analyses of different Platreef rocks, were collected from four drill cores along its strike. The data were examined to determine the source of the magmas and identify the processes involved in its genesis. The study also aimed at establishing whether a genetic link exists between the Platreef magmas and the magmas that formed the Lower Zone (LZ), CZ and MZ in the Rustenburg Layered Suite (RLS) of the BIC. The petrography revealed that the Platreef in the four drill cores consists of harzburgite, olivine pyroxenite, pyroxenite, feldspathic pyroxenite and norite. Based on the textural and modal mineralogy variations, feldspathic pyroxenite was subdivided into five types (I, II, III, IV and V). The variation in the average contents of MgO, LaN/YbN and ΣREE for the Platreef rocks are consistent with the modal mineralogy from the least to the most differentiated rocks. However, the Sr–Nd isotope data of the Platreef rocks have revealed two distinct groups of samples with decreasing ɛNd2060. Group 1 consists of pyroxenite and feldspathic pyroxenite II, III and V having ɛNd2060 values that range from –8.4 to –2.9, and 87Sr/86Sr2060 values from 0.707281 to 0.712106. The Platreef rocks of group 2 consist of olivine pyroxenite and feldspathic pyroxenite Type I with ɛNd2060 ranging from –12.6 to –10.8, and 87Sr/86Sr2060 ranging from 0.707545 to 0.710042. In comparison to the LZ, CZ and MZ rocks, which have ɛNd values ranging from –8.5 to –5.1, and 87Sr/86Sr ranging from 0.704400 to 0.709671, Platreef pyroxenite of group 1 have lower negative ɛNd2060 values (from –3.8 to –2.9) and higher 87Sr/86Sr2060 values from 0.709177 to 0.710492, whereas feldspathic pyroxenite of group 1 have overlapping ɛNd2060 values (from –8.4 to –4.9) but also higher 87Sr/86Sr2060 values (from 0.707281 to 0.712106). Instead, the Platreef olivine pyroxenite and feldspathic pyroxenite in group 2 highly negative ɛNd2060 values and overlapping 87Sr/86Sr2060 values. It is therefore suggested that the Platreef magmas derived from the partial melting of an heterogeneous mantle source comprising depleted mantle melts and both metasomatized slightly unradiogenic Nd enriched melts and highly unradiogenic Nd enriched melts from the subcontinental lithospheric mantle. These magmas ascended via the continental crust using different paths and interacted with rocks of different Sr–Nd isotopic compositions which resulted in the formation the hybrid magmas. The study speculates that sulphide saturation in the Platreef magmas was reached in the staging chambers at depth, and the varying styles of the PGE mineralisation in the Platreef rocks are the result of the varying degree of partial melting of the heterogeneous source for their magmas. In conlusion, this study suggests that the genesis of the Platreef is much more complex and should be considered very much independent from processes involved in the genesis of the RLS in the Eastern and Western Limbs of BIC in agreement with earlier studies.
NRF Inkaba ye Africa Iphakade
2020-08-31
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Holwell, David. "Roles of magmatism, contamination and hydrothermal processes in the development of Platreef mineralization, Bushveld Complex, South Africa." Thesis, Cardiff University, 2006. http://orca.cf.ac.uk/56160/.
Full textAbstract:
The Platreef is a highly complex, pyroxenite-hosted Ni-Cu-PGE deposit. It is located at the base of the northern limb of the Bushveld Complex, South Africa, in direct contact with a variety of country rock sediments and Archaean basement. The interaction of the Platreef magma with these diverse country rock lithologies during emplacement had a profound effect on the style and distribution of the mineralization on both regional and local scales. Geometrically, the Platreef was emplaced as a thin, sill-like intrusion, with pre-formed PGE- rich sulfide droplets entrained within the magma. Sulfur saturation is likely to have occurred in a deep staging chamber or conduit prior to emplacement, and where immiscible sulfide droplets became enriched in PGE, base metals and semi metals. After emplacement, the PGE and semi-metal rich sulfide liquid cooled to form 'primary' assemblages of IPGE-rich pyrrhotite, IPGE-, Rh- and Pd-rich pentlandite, chalcopyrite, Pt and Pd tellurides and bismuthides and electrum within the feldspathic pyroxenites of the Platreef. Assimilation and metamorphism of some sedimentary footwall rocks, particularly the dolomites of the Malmani Subgroup, released large volumes of volatiles into the Platreef magma. This hydrothermal activity redistributed PGE and base metal sulfides (BMS) into the footwall rocks, and in places overprinted the 'primary' assemblages, and occasionally decoupled PGE from BMS, with the petrology of the reef and footwall, and the mineralogy of the platinum-group minerals, significantly affected. Each locality along the strike of the Platreef with a different footwall lithology has its own unique hydrothermal history directly related to the nature of the local floor rocks. For example, where the floor is composed of anhydrous basement gneiss, volatile activity was relatively insignificant, and partial melting of the floor allowed the percolation of PGE-rich sulfide liquid to penetrate the footwall. The nature of the floor rocks also controls the type and amount of contamination in the Platreef. Sulfides in country rock shales, for example, are assimilated into the Platreef magma and locally upgrade the S content at such localities, although this did not trigger S saturation. Footwall sulfates, such as anhydrite, cannot be assimilated, but can interact with the Platreef sulfides through hydrothermal leaching when sufficient fluids had been released during assimilation. After emplacement of the Platreef, a significant period of cooling occurred, such that the Platreef was almost completely crystallized, during which time some ductile deformation occurred. The gabbronoritic hangingwall magma was then emplaced, forming a magmatic unconformity over the Platreef, occasionally exploiting shear zones to intrude finger-like bodies down into the Platreef. Where the hangingwall magma assimilated mineralized Platreef thin zones of PGE mineralization developed at its base. The magmatic intrusion of the Platreef can be considered to be distinct from that of other magmatic units in the northern limb of the Complex. Its Ni-Cu-PGE sulfide mineralization is orthomagmatic in origin however, complex interaction with a variety of country rock lithologies has locally altered the style and distribution of the mineralization, to form unique mineralogical associations and assemblages along strike. The understanding of local country rock control on features such as the mineralogy of the ores and the extent of remobilization into the footwall is critical in optimizing exploration, mining and mineral processing techniques.
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Fischer, Lennart Alexander [Verfasser]. "The Upper Zone of the Bushveld Complex, South Africa: Parental Magma and Crystallization Processes / Lennart Alexander Fischer." Hannover, 2018. http://d-nb.info/1160378800/34.
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Fraser, Nicholas Tweedie. "A Metamorphic Study of Pretoria Group Sediments Found at the Dwarsrivier Pass, Bushveld Igneous Complex, South Africa." Diss., University of Pretoria, 2019. http://hdl.handle.net/2263/76009.
Full textAbstract:
The study takes place in the Dwarsrivier area which lies on the border between Mpumalanga and Limpopo, to the North-West of Lydenberg, at an exposed road cutting. Within the road cutting, there is a unique portion of exposed rock which is light in colour and identified as a calc-silicate. The calc-silicate material is present as a package of rock and is surrounded top and bottom by pyroxenite. The surrounding rock belongs to the Bushveld Igneous Complex (BIC), which is the largest known layered intrusion on the planet and is host to numerous mines. The sample area is within the Critical Zone of the BIC and the host rock consists of pyroxenite which is crystalline and mafic. The calc-silicate package originates from the Pretoria Group sediments, which hosts the BIC, and has undergone varying degrees of metamorphism and mineralisation. The metamorphism formed and allowed for the preservation of two rare minerals, namely wüstite and chlorospinel. Numerous tests were performed on the samples, including SEM point scans to identify these rare minerals and to better understand how the calc-silicate package was preserved in the BIC. A model was created to explain the occurrence of the calc-silicate slab and surrounding features. The previous model involved the slab rising up through the BIC, but the proposed model in this thesis is that the calc-silicate was part of the roof rock which then delaminated, and subducting into the ductile magma of the BIC.Dissertation (MSc)--University of Pretoria, 2019.
Geology
MSc
Unrestricted
27
Fischer, Lennart A. [Verfasser]. "The Upper Zone of the Bushveld Complex, South Africa: Parental Magma and Crystallization Processes / Lennart Alexander Fischer." Hannover, 2018. http://d-nb.info/1160378800/34.
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Andrews, Marcelene. "The use of chemostratigraphy and geochemical vectoring as an exploration tool for platinum group metals in the Platreef, Bushveld Igneous Complex, South Africa : a case study on the Tweefontein and Sandsloot farms." University of the Western Cape, 2015. http://hdl.handle.net/11394/5229.
Full textAbstract:
>Magister Scientiae - MScThe Platreef is known for its complexity and its heterogeneous lithologies, coupled with an unpredictable PGE and BMS mineralisation. The motivation behind this study was to aid mining geologists in targeting mineralisation irrespective of the farm. It is known that the Platreef generally overlies different footwall lithologies at individual farms. Thus, the aims of this study were firstly to investigate the potential of chemostratigraphy by delineating indices indicative of distinctive lithological layers. These indices were then tied to the second aim; which were to use geochemical vectoring, which is process-based, to target the PGEs at two different farms. This study included three drillcores: from the farms Sandsloot (SS339) and Tweefontein (TN754 and TN200). The footwall units at Tweefontein are shales of the Duitschland Formation and the Penge banded iron formation; and at Sandsloot it is the Malmani Subgroup dolomites. Samples included 121 quarter cores, used for petrographical and geochemical studies. The elemental rock composition was determined by XRF and ICP-OES analyses. The approach also included statistical and mass balance methods to understand the geological and geochemical controlling processes. Initially, the Platreef package at both farms was petrographically divided into three main layers: pyroxenite, and two distinctive feldspathic pyroxenites (FP-I and FP-II). However, the pyroxenites were also further separated as P-I and P-II, because of a higher notable difference in the degree of alteration within P-I. Progressive degrees of metasomatism were further observed in the lithologies, e.g. within the Platreef package, where feldspathisation was potentially the main metasomatic process. Many geochemical plots (corroborated by the petrographical and mass balance results) illustrated that the feldspathisation were linked to an increase in the content of Al₂O₃ and CaO, and coupled with a decrease in content of Fe₂O₃ and MgO. Together with other geochemical trends, geochemically distinct units of the Platreef package could be discriminated with a metasomatism index (MI; CaO + 10Na₂O / CaO + 10Na₂O + Fe₂O₃ + MgO). The ensuing MI is lowest for the P-II pyroxenite and shows a progressive increase through FP-I, P-I to the highest values in FP-II. Geochemical layering were also observed in the calcsilicates and hornfels; e.g. a progressive decrease in the content of Fe₂O₃, Al₂O₃, Ce, Co, Cu, Ni, Zn, Zr, Au, Pd and Pt from the hornfels subunits H-I, H-II to H-III and an increase in of SiO₂, Fe₂O₃, TiO₂, SO₃, Co, Cu, Ni, Rb, V and Zn content from CS-I, CS-II to CS-III. Correlating the pyroxenites and feldspathic pyroxenites spatially from one drillcore to another were hindered, hence, chemostratigraphy were not completed. In terms of vectoring, it was essential to establish a possible link between the metasomatism index and the nature and style of the PGE and/or BMS mineralisation. The Hornfels subunit H-I and calcsilicate subunit CS-III were the main carriers of BMS and PGE. The Platreef package were more complicated: P-I (low PGE, low BMS); P-II (low PGE, high BMS); FP-II (high PGE, low BMS); and FP-I (high PGE, high BMS). Element indices (e.g. Cu+Ni and Co+Zn) were developed to define a consistent gradient indicative of these ore subunits. A validation process to assess the metasomatism index (MI), base metal indices and PGE distribution within the individual drillcores (TN754, TN200 and SS339) were then undertaken. The results were that the MI ranges were similar in all drillcores, and discriminated the subunits of the Platreef package, gabbronorites and even the calcsilicates. The base metal ratios (e.g. Ni/Co and Cu/Co) were indicative of the PGE rich zones. Trends of the base metal ratios reflected a strong positive relationship with the MI within the Platreef package and the calcsilicates. However, the opposite trend is observed with the hornfels. In conclusion, the MI could potentially be a strong vector of high PGE and BMS mineralisation. It is also possible to discriminate lithologies within the Platreef package with the MI. However, it should be noted that the limitation of this study is that the results are based on three drillcores. The Platreef is heterogeneous at individual farms and extremely diverse across the northern limb. Therefore, future research could be undertaken to validate these findings, by using a bigger drillcore database.
National Research Foundation
29
Armitage, Paul Edward Blake. "Development of the Platreef in the northern limb of the Bushveld Complex at Sandsloot, Mokopane District, South Africa." Thesis, University of Greenwich, 2011. http://gala.gre.ac.uk/9079/.
Full textAbstract:
The Platreef is a Ni-Cu-PGE mineralised tabular body at the base of the Rustenburg Layered Suite in the northern limb of the Bushveld Complex. The reef lies unconformably on a footwall (floor) sequence of Transvaal Supergroup sedimentary rocks and Archaean granite/gneiss basement, and is overlain by a hangingwall (roof) of Main Zone gabbronorites. Structural relationships suggest that the Platreef was emplaced as a broadly horizontal sill-like sheet into the Transvaal Supergroup, but local variations in its thickness and path of intrusion were caused by pre-existing structures in the country rocks. As the Platreef cooled and was nearly crystallised, ductile deformation occurred, possibly as an episode in a longer event. Main Zone magma was emplaced above the deformed, nearly consolidated Platreef and eroded the uppermost portion, locally assimilating mineralised reef. The Main Zone magma also intruded into shear zones as thin dykes down through the Platreef and metasedimentary floor. Structural patterns around a prominent dome in the floor rocks suggest that regional deformation may still have been active when the earliest Main Zone layers were developing, but ceased by Upper Main Zone time. Other studies of the Platreef beyond Sandsloot have shown that its earliest Ni-Cu-PGE mineralisation was orthomagmatic, largely preserved where the floor rocks are unreactive basement granite/gneiss. However, interaction between he Platreef magma and surrounding sedimentary rocks has produced different mineralogical associations and assemblages that were influenced by the local floor and roof rocks along the strike of the reef. At Sandsloot, the floor rocks are represented by reactive siliceous dolomites of the Malmani Subgroup. The Platreef magma caused contact metamorphism and metasomatism of the dolomites, releasing volatiles that entered the reef. These hydrothermal fluids stripped PGE from primary sulphides and redistributed the PGE within the reef and into the metasedimentary country rocks. In places, primary platinum group minerals were overprinted by lower-temperature species.
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Mitchell, Andrew Alexander. "The petrology, mineralogy and geochemistry of the main zone of the Bushveld Complex at Rustenburg Platinum Mines, Union Section." Thesis, Rhodes University, 1988. http://hdl.handle.net/10962/d1001563.
Full textAbstract:
Union Section of Rustenburg Patinum Mines is situated in the northwestern part of the Bushveld Complex, some twenty kilometres north of the Pilanesberg Alkaline Complex. The mining lease area covers a roughly triangular segment of Lower, Critical and Main Zone rocks, transgressed to the north and south by magnetite-bearing ferrogabbro of the Upper Zone. The Main Zone at Union Section is the focus of this study. The prime source of sample material for the study is the deep exploration borehole SK2, but additional, supplementary samples were collected on surface and underground, as well as from a second surface exploration borehole, SK4. In line with the recommendations of SACS (1980), the top of the Critical Zone, and therefore the base of the Main Zone, is taken to be the top of the Bastard Cyclic Unit. Sharpe (1985) suggested that the succession from the base of the Main is an isotopically separate entity Zone up to the Pyroxenite Marker from the rest of the Bushveld layered succession. This is not strictly true, as there is evidence that more than one parental magma was involved in the formation of this interval. It is, however, true that there are fundamental differences, particularly in isotopic makeup, between the Main Zone rocks below the pyroxenite Marker and those above (the latter having been assigned by Molyneux (1970) to subzone C of the Main Zone). Kruger et al. (1986, in press) suggested that the Pyroxenite Marker marks the base of the Upper Zone, and this convention is adhered to here. The implication of this is that the rocks which formerly constituted subzone C of the Main Zone are now considered part of the Upper Zone. The Main Zone rocks below the pyroxenite Marker were originally subdivided by Molyneux (1970) into two subzones, A and B. The results of the present study indicate that this subdivision is not justified. Instead, eight units have been distinguished in the Main Zone on geochemical, petrological and mineralogical bases. Each of these units is characterized by a coherent set, or progression, of chemical and petrological characteristics. The specific assignment of genetic connotations to these units has been deliberately avoided , at least until further studies of the Main Zone prove this to be justified. The demarcation of the eight units is illustrated in the composite diagram (Fig. 34) in the back pocket of this work, and the reasons for the subdivisions are listed in Table 6 (at the end of chapter 7 of this thesis). Until the late 1970's, it was thought that most layered cumulates formed by crystal settling (Wager and Brown, 1968). More recently, there has been a fundamental conceptual change, and many workers now believe that most cumulate rocks formed by in situ crystallization at the floor and walls of the magma chamber (McBirney and Noyes, 1979, Irvine, 1980a; Campbell, 1987). There is, however, some evidence for the physical separation of phases undergoing cotectic crystallization, particularly in the Upper Critical Zone and lower part of the Main Zone (Eales et al., 1986). This process, which has been alluded to in the past by various authors (Ferguson and Botha, 1963; Vermaak, 1976) involves the flotation of early-formed plagioclase crystals due to their positive bouyancy in tholeiitic liquids. The result is an apparent decoupling of the chemistry of pyroxene and plagioclase, as in unit IV of the Main Zone, where plagioclase becomes more anorthitic upwards, whilst pyroxene becomes more iron-rich. There is some substantial evidence, particularly in reversals in the strontium isotope initial ratio and the orthopyroxene Mg/(Mg+Fe) ratio , for multiple intrusion in the Main Zone. Although the largest and most important magma influx in the Main Zone was a high-R₀ aluminous tholeiite, as suggested by Sharpe (1985), the intrusive history of the Main Zone is believed to be far more complex than Sharpe (op. cit.) suggested. Significantly, there is strong evidence for small influxes of Upper Zone-type (Fe-rich tholeiite) magma in the upper reaches of the Main Zone. These are believed to be precursors to the major influx of Upper Zone-type magma at the pyroxenite Marker (Kruger et al, 1986, in press). The fate of intercumulus liquids in cumulate rocks has recently recieved substantial attention (Sparks et al., 1985; Morse, 1986; Barnes, 1986: Campbell, 1987). It is believed that the migration, or at least redistribution, of intercumulus liquids has played a vital role in modifying fractionation trends in the Main Zone. More importantly, the accumulation of late-stage intercumulus liquids is believed to be responsible for the formation of the Fe-rich ultramafic pegmatite bodies that interrupt the layered cumulates in borehole SK2Adobe Acrobat 9.53 Paper Capture Plug-in
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Van, Huyssteen Darryn Ashley. "Mineralogical variation in the basal Upper Zone, Bushveld Igneous Complex, South Africa: implications for ore genesis and mineral extraction." Thesis, Rhodes University, 2017. http://hdl.handle.net/10962/5060.
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Haikney, Susan Ann. "The nature of olivine-rich cumulate rocks of the lower critical and lower zones of the northwestern Bushveld Complex." Thesis, Rhodes University, 1993. http://hdl.handle.net/10962/d1005592.
Full textAbstract:
Boreholes NG1 and NG2 were drilled on the farm Nooitgedacht 406 KQ to intersect the lower Critical and lower Zones of the western Bushveld Complex. The aim of this study is to describe the textural features and chemical characteristics of the olivine-bearing rocks in the intersections, as determined by petrographic studies, XRF analysis and microprobe analysis. The olivine-bearing rocks are dunites, harzburgites and olivine pyroxenites. They comprise olivine and orthopyroxene, with minor chromite, clinopyroxene and plagioclase, and their textures vary between adcumulate, mesocumulate and poikilitic. The sequence intersected can be broadly correlated with that in the eastern Bushveld Complex. Of the whole-rock inter-element ratios, the MMF (MgO)/[MgO+FeO])ratio is the clearest indicator of cyclicity. The olivine-rich rocks are more primitive than the associated rocks, and seem to become more primitive with height in most intervals. The plagioclase in the olivine-bearing rocks is unusually sodic in corrposition, having a maximum Na₂0 content of 8.12%. A comparison of olivine and plagioclase compositions with those in other intrusions has revealed that the only other major intrusion with sodic plagioclase is the Kiglapait intrusion of Canada. In the Kiglapait intrusion the sodic plagioclase occurs in conjunction with fayalitic olivine as opposed to the forsteritic variety of this study. Chemical variations in the rocks sampled indicate that periodic replenishment of the magma from which the rocks crystallised must have occurred. In some of the olivine-bearing intervals where little fractionation is evident, replenishment seems to have been continuous. In other intervals fractionation appears to have continued uninterrupted for significant periods, prior to rejuvenation by fresh influxes of magma.
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Wall, Corey James. "Establishing the age and duration of magmatism in large open-system layered intrusions from the high-precision geochronology of the Neoarchean Stillwater Complex and Paleoproterozoic Bushveld Complex." Thesis, University of British Columbia, 2016. http://hdl.handle.net/2429/58716.
Full textAbstract:
The Neoarchean Stillwater Complex (Montana, USA) and the Paleoproterozoic Bushveld Complex (South Africa), two of the world’s largest layered intrusions, have been cornerstones for the study of magmatic processes in the Earth’s crust. Mafic layered intrusions are natural laboratories for assessing the emplacement, crystallization, and cooling mechanisms of mantle-derived basaltic magmas. Most layered intrusions do not yet have robust geochronological frameworks from the base to the top of their stratigraphic successions. Zircon is recognized as a relatively common accessory mineral in the Stillwater and Bushveld intrusions and crystallized from highly fractionated interstitial melt at near-solidus temperatures (980-720°C). High-precision geochronologic frameworks established for both intrusions by U-Pb zircon dating, combined with trace element and hafnium isotope compositions of zircon, reveal extended durations of magmatism (3-5 million years) and non-stratigraphic or out-of-sequence ages for both intrusions. Dating of platinum group element deposits in both intrusions (J-M Reef, Stillwater; Merensky Reef, Bushveld) indicates that they are intrusion-wide time markers that crystallized synchronously over large distances (>300 km, Bushveld). The recognition that zircon can be successfully extracted from mafic-ultramafic rocks associated with magmatic ore deposits provides new opportunities for assessing the timing and duration of mineralization processes in layered intrusions worldwide. Zircon from a thick anorthosite horizon in the Stillwater Complex has been identified as a reference material for U-Pb geochronology of Archean rocks (>2.5 Ga) and fills an important gap in the geologic timescale for the application of precise and accurate U-Pb geochronology. Collectively, the dating results indicate that both the Stillwater Complex and Bushveld Complex do not represent the products of progressively crystallized magma chambers but instead formed as stacks of amalgamated sills representing repeated injections of magma at different stratigraphic levels. These conclusions call into question current concepts regarding the origin of layered intrusions and challenge us to rethink our understanding of the timescales of magma processes throughout Earth history.Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
Graduate
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Botha, Michael James. "Petrology and geochemistry of the lower group chromitites and host rocks on the farm Zandspruit 168 JP, Western Bushveld Complex." Thesis, Rhodes University, 1988. http://hdl.handle.net/10962/d1001565.
Full textAbstract:
The eight Lower Group chromitite layers of the Ruighoek Pyroxenite in the area west of the Pilanesberg (LGl - LG7), on the farm Zandspruit 168 JP, were sampled in borehole cores drilled through the succession. The stratigraphic succession faIls within Cameron's (980) units B to E of the Critical Zone. The lowermost layer of the Lower Group, the LG 1 chromitite, is located some 440 metres below the MG 2 (Middle Group) chromitite layer, above which the first appearance of cumulus plagioclase in the Complex is seen. By convention, this horizon is designated the top of the Ruighoek Pyroxenite in the Western Bushveld Complex. Homogeneous units of chromite-bearing orthopyroxenite (bronzitite), exhibiting inconspicuous layering defined in terms of variations in orthopyroxene grain-size host all but one of the Lower Group layers; the LG 4 chromitite layer is exposed within an olivine-rich subunit 23 metres in thickness (C₃ subunit). The cumulative thickness of chromitite is 2,92 metres or 0,8 per cent of the studied section, which is 381 metres in thickness. Minimum and maximum thicknesses of the LGl - LG7 layers exposed in drill core are 17 and 81 centimetres, respectively, with minor chromitite layers ranging between 2 and 5 centimetres in thickness. Weighted mean Cr₂0₃ contents of units B to E vary between 1,17 and 3,22 per cent, with the latter estimate representative of the D₂ subunit which hosts the LG 6 chromitite layer. The LG 6 is correlated with the Steelpoort layer of the Eastern Bushveld Complex, and varies between 76 and 81 centimetres in thickness under a large portion of the farm Zandspruit. An undisturbed succession striking N15°E and dipping 12 - 15°E is depicted within the studied area, which is bounded on the eastern side by the north-striking Frank fault. Major folding of the layered succession is evident to the north of the area, where the layering adjacent to the trace of the fault dips 35° to the southwest. Particular attention is paid in the present study to (a) the nature of chromitite layers and their host rocks, (b) the contrast between the mineral chemistry of weakly disseminated chromite and grains within massive ore layers, (c) concentrations of Cr, V, Ni, Co, Sc and Ti in orthopyroxene in relation to stratigraphic height, and levels of Sr, Ba and Zr associated with hypothetically pure, intercumulus plagioclase feldspar, and (d) possible mechanisms which induce crystallization of chromitite layers containing 50 per cent Cr₂0₃ from magma with a Cr content of less than 1 000 ppm. Electron microprobe studies of chromite in relation to mineralogical and textural environment clearly reveal that (a) the proportions of Cr and Al cations are linked to paragenesis: higher Al/Cr ratios characterize olivine-bearing domains, whereas grains intergrown with plagioclase feldspar exhibit low AI/Cr ratios, and (b) Al contents rise with a decline in Mg/(Mg + Fe²⁺) from high values to a value of 0,450, then decrease with a further decline in Mg/(Mg + Fe²⁺). The paragenetically later trend is emphasized in a large population of chromite grains which escaped early encapsulation in orthopyroxene crystals and continued to grow in the environment of intercumulus plagioclase. Within-and between-sample compositional variation of grains in silicate-rich domains is modelled in terms of in situ growth increments, diffusive homogenization of zonal structures, and residence time within interstitial melt. Fractionation trends, as measured by Mg/(Mg + Fe²⁺) ratios in whole-rock and/or microprobe studies of orthopyroxene, are reversed in relation to stratigraphic height towards the top of the B unit and in the overlying C unit. These data are supported, for example, by lower vanadium contents and higher Ni/Sc ratios in hypothetically pure orthopyroxene. Small olivine crystals in chromite-rich domains are enriched in Ni relative to coarse-grained olivine in adjacent dunite: a feature attributed to early isolation of primocrysts from magma in the former case, and in situ equilibration between olivine crystals and Ni-depleted residual melt in the latter case . Similarly, rising Ni contents and Mg/(Mg + Fe²⁺) ratios of orthopyroxene with increasing stratigraphic height in the footwall of the LG 6 chromitite layer, linked to a progressive decline in orthopyroxene grain-size, are effects which may arise out of early separation of interstitial melt from orthopyroxene cumulates. A model is thus proposed which (a) links the thickness of chromitite layers to the vertical separation between successive layers or the thickness of fine-grained orthopyroxenite in the footwall, (b) ascribes copious nucleation of chromite to liquid mixing of this footwall derived, Cr - depleted contaminant with influxes of hot, primitive magma, and (c) tenders the notion that the present modal proportion of mesostasis in the footwall of a chromitite layer serves as a reciprocal measure of the volume of fractionated exudate
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Maier, Wolfgang Derek. "Geochemical and petrological trends in the UG2-Merensky unit interval of the upper critical zone in the Western Bushveld Complex." Thesis, Rhodes University, 1992. http://hdl.handle.net/10962/d1005563.
Full textAbstract:
One of the most remarkable features of the layered sequence of the Bushveld Complex is its lateral consistency in lithology. This work has established a geochemical and lithological correlation along 170 km of strike of the interval between the UG2 chromitite and the Merensky Reef within the Upper Critical zone of the western limb of the Bushveld Complex. The correlation is based on geochemical investigations of 10 borehole intersections and lithological comparisons of more than 20 borehole intersections around the western lobe of the complex. The basic data presented include 123 whole-rock analyses for major and 12 trace elements, 97 analyses for ' 12 trace elements, and ca. 5500 microprobe analyses of all major phases. Patterns of cryptic variation are established. Some layers (the UG2 chromitite and pyroxenite) show considerable consistency with regard to geochemistry and lithology. Others can be traced along most of the investigated strike length, such as the Lone Chrome Seam, the Footwall Marker anorthosite and the immediate anorthosite footwall to the Merensky Unit. Most of the distinguishable members within the study section, however, show great variation along strike (i.e., the Lower and Upper Pseudoreef Markers, the central noritic sequence in the southern arm of the western limb and parts of the immediate Merensky Reef footwall succession). Several models have been evaluated to interpret the geochemical and lithological data. The author comes to the conclusion that the degree of lithological consistency depends on the variability of magmatic parameters within different parts of the chamber. The most important of these parameters are: (i) the size of fresh primitive influxes and consequently the heat flux, (ii) the composition of the residual liquid, and (iii) the frequency of the influxes. Fresh influxes of more or less similar composition thus spread out along the floor if the residual liquid was less dense than the fresh primitive liquid, but intruded the chamber as a plume where plagioclase had crystallized for some time and the residual liquid had become relatively dense. The size of the influx may be regarded as a measure of the amount of heat flux from the feeder into the chamber. A large influx created uniform physicochemical conditions in the chamber whereas a smaller influx created a strong lateral gradient of physicochemical parameters in the chamber, with subsequent differences in viscosity, density, convection currents, yield strength and thus different mixing behaviour of different liquids. Furthermore, a persistent heat flux from the feeder may have delayed crystallization of successive phases in those parts of the chamber proximal to the feeder . Therefore, new influxes would have been deposited on a footwall of varying thickness and lithology in response to different degrees of crystallization and accumulation along strike. The development of a normal cyclic unit (chromititeharzburgite-pyroxenite-norite (+anorthosite?)) may thus have been interrupted at various stages in different parts of the chamber. The ability to correlate anorthosites over great strike distances implies that their formation did not follow entirely random processes but was dependent on specific magmatic conditions which prevailed over laterally extensive portions of the chamber at certain stages during the evolution of the crystallizing liquid.
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De, Klerk William Johan. "Petrogenesis of the upper critical zone in the Western Bushveld Complex with emphasis on the UG1 Footwall and Bastard units." Thesis, Rhodes University, 1992. http://hdl.handle.net/10962/d1005598.
Full textAbstract:
This study is an account of the stratigraphic sequence, the petrography, mineralogy (microprobe investigations of orthopyroxene, clinopyroxene, olivine and plagioclase feldspar), and whole-rock major- and traceelement geochemistry of the silicate cumulates of the Upper Critical Zone in the western Bushveld Complex. Two parts of the study - an investigation of a 350m column incorporating the MG3 and UGI Footwall Units, and a comparison of two additional Upper Critical Zone profiles with a previously compiled profile between the UGI and Bastard Units - are focused on RPM Union Section in the northwestern sector of the Complex. The third part is a detailed vertical and lateral investigation of the Bastard Unit at the top of the Critical Zone, which draws on sampling and data compilation from seventeen profiles in the western limb of the Complex. The MG3 Unit (45m) is made up of a lower chromitite layer overlain by a norite-pyroxenite-anorthosite sequence while the UGlFW Unit (295m) is composed of a related series of lower chromitite layers (MG4) overlain by a pyroxenite-norite-anorthosite sequence capped by the UGI chromitite layer. These mafic cumulates display a distinctive pattern of oscillating cryptic variation in whole-rock Mg/(Mg+Fe), FeO/Ti0₂, Cr/Co and Ni/V ratios through the sequence. Sympathetic oscillations are recorded for compositions of orthopyroxene and plagioclase feldspar and eight subcycles are recognised through the UGlFW Unit. The entire sequence is characterised by the presence of small, spheroidal, embayed and irregularly shaped plagioclase grains which are poikilitically enclosed in cumulus orthopyroxene grains of both pyroxenites and norites. This texture is indicative of partial resorption of pre-existing feldspar primocrysts within the melt prior to their being incorporated into the host orthopyroxene grains. Textural, geochemical and isotopic data suggest that this sequence was built up by periodic additions of fresh, relatively primitive liquid into fractionated resident liquid, and subsequent mixing within the magma chamber. The Bastard Unit sequence, described in Chapter 4, is the last and most complete cyclic unit (c. 60m) of the Critical Zone, and its upper contact defines the boundary between the Critical and Main Zones of the Complex. This Unit can conveniently be sub-divided into a lower part, where orthopyroxene occurs as a cumulus phase, and the upper part which is composed entirely of anorthosite (Giant Mottled Anorthosite). The basal part of the Unit (≤ 18m) comprises a thin chromitite layer < O.5cm) overlain by a pyroxenite-melanorite-norite-leuconorite sequence. The basal pyroxenite is orthocumulate in character and rapidly gives way to norites and leuconorites. A distinct threefold subdivision emerges within the Giant Mottled Anorthosite which is predominantly an adcumulate which becomes orthocumulate in character at its top. Apart from minor deviations in thicknesses these lithologies are recorded over the entire strike-length covered in this study. Profiles of cryptic variation are compiled for orthopyroxene, plagioclase and whole-rock data and show that the Bastard Unit displays a characteristic pattern which is maintained throughout the western Bushveld Complex. A minor yet distinctive reversal in cryptic variation is revealed at a level which is stratigraphically variable within the lower Giant Mottled Anorthosite, and results in a double cuspate pattern. A remarkable feature of the basal Bastard pyroxenites is that although the modal proportion of mafic to felsic constituents varies systematically away from the northwestern sector, the Mg/(Mg+Fe) ratio of orthopyroxenes remains constant at 0.804 over a lateral strike distance of 171km. Within the upper part of the Unit the orthopyroxene is markedly Fe-rich and it is here that inverted primary pigeonite appears for the first time as a cumulus phase. In addition, K-feldspar, oscillatory zoned plagioclase grains and high levels of incompatible trace elements are noted at this level. On the basis of the data presented it is concluded that the Bastard Unit represents the crystallisation of a final, relatively large influx of hotter primitive liquid, with upper Critical Zone affinities, and subsequent mixing with a column of cooler (less dense) supernatant liquid which had in part hybridized with the overlying Main Zone magma. It is hypothesised that this new liquid was emplaced as a basal flow beneath supernatant liquid and that it initiated the deposition of mafic cumulates at its base. The supernatant liquid is interpreted as representing the fractionated residuum produced by crystallisation of earlier cyclic units, with plagioclase on the liquidus, and that it contained an abundance of small plagioclase primocrysts in suspension. Development of the Unit can be viewed as a two-stage process. In the lower half of the unit, chemical and physical parameters typical of the new magma dominated the crystallisation process, and resulted in cumulates very similar to other relatively complete Upper Critical Zone units. In the upper, leucocratic sequence, above a minor reversal, crystallisation was from a liquid which was the product of mixing of a minor pulse of primitive liquid with the reservoir of hybridized supernatant liquid. Although the Bastard Unit is not continuous around the entire Western limb of the Complex, it is concluded that it developed in a single, or connected, magma chamber and that its irruptive feeder zone was located in the proximal northwestern facies of the Complex.
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Edwards, Hunter R. "A multi-isotopic geochemical investigation of the Lower Zone, Bushveld Complex, South Africa: implications for a crustal component for parental magmas." Master's thesis, Faculty of Science, 2020. http://hdl.handle.net/11427/32606.
Full textAbstract:
The current study focuses on the Lower Zone of the Bushveld Complex, South Africa using multiple geochemical and isotopic systems to determine the origin of crustal signatures, i.e. crustal assimilation or recycled crust in the mantle source, present throughout the Rustenburg Layered Suite (RLS) such as elevated 18O values. These geochemical and isotopic systems include major elements, trace elements, highly siderophile elements, oxygen isotopes, 87Sr/86Sr, 143Nd/144Nd, and Os-Os isotopes. Samples come from the Nooitgedacht Borehole 2 (NG2) at Union Section of the western limb of the Bushveld Complex, which sampled the Lower Zone. The 87Sr/86Sri (0.7043 – 0.7086) and Ndi values (-7.40 - -4.97) calculated in this study are in agreement with published data for the Bushveld. The majority of NG2 samples contain 18O greater than mantle peridotite (5.50‰) and MORB (5.70‰), in which NG2 18O ranges from 5.60 up to 8.00‰ for olivine, orthopyroxene, and clinopyroxene separates. These high 18O values suggests the Bushveld magmas sourced a crustal reservoir, either through crust assimilation or recycled crustal materials in the mantle source. This is the first study utilizing the Re-Os isotope system for the Lower Zone. The Osi values for the NG2 suite range from -4.37 to +35.9, which overlap with published data for the Critical Zone and the Platreef, the only previous Re-Os studies on the Bushveld. However, there are no previously reported negative Osi values for the Bushveld. The range in Osi values for the NG2 samples suggest mixing of at least two geochemical reservoirs. In addition to Lower Zone NG2 samples, sample NG2-773.65 is a chilled margin sample at the base of the NG2 borehole that contains high 18O ( 18O = 9.42 – 9.78‰) and radiogenic Osi (Osi = +62.5), in which the crust and/or recycled crust in the mantle source caused these high values. Sample NG2-734.64 contains the second lowest 18O ( 18O = 5.67‰) and most unradiogenic Osi (Osi = -4.37) values for the NG2 suite, evidence for a harzburgitic SCLM (H-SCLM) mantle source component. A lack of correlations for Osi values with 18O and 87Sr/86Sri values are indices for crustal assimilation processes. This lack in correlation may suggest a crustal component in the mantle is more likely than assimilation of the crust during ascent of the magmas toward the surface. The geochemical data presented in this study suggest the Lower Zone parent magma had a H-SCLM mantle source component in addition to the assimilation of the crust and/or the eclogitic SCLM (E-SCLM).
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Doig, Heather Leslie. "Strike comparison of the compositional variations of the lower group and middle group chromitite seams of the critical zone, Western Bushveld complex." Thesis, Rhodes University, 2000. http://hdl.handle.net/10962/d1005618.
Full textAbstract:
The variations in the composition, specifically the Cr20 S content and the Cr:Fe ratio, and the morphology of the Lower Group (LG) and Middle Group (MG) chromitite seams of the Critical Zone (CZ) across the western Bushveld Complex, including the Ruighoek and Brits sections, is investigated by means of whole-rock chemical data, both major and trace elements analysis, XRD and electron microprobe data. As a result ofthe paucity of exposed or developed LG1 - LG5 chromitite seams in the western Bushveld Complex, this study is confined to the investigation of the compositional variations of the LG6 to MG4 chromitite seams. In only one section, the Ruighoek section, was the entire succession of chromitite seams, from the LG1 - MG4, exposed. The silicate host rocks from the LG6 pyroxenite footwall to the collar of the CC2 drillcore (lower uCZ) in the Rustenburg section were sampled. This study reviews the compositional trends of the silicate host rocks, as the compositional variations of the chromitite seams reflect the chemical evolution of the host cumulate environment and, to a lesser degree, the composition onhe interstitial mineral phases in the chromitite seams. The compositional variations of the LG and MG chromitite seams are attributed to the compositional contrast between the replenishing magma and the resident magma. The chemical trends of the LG and MG chromitite layers and the host cumUlate rOCKS do not support the existence of two compositionalfy dissimilar magmas in the CZ, rather the cyclic layering of the CZ and the chemical variations of the chromitite seams are attributed to the mixing of primitive magma with the resident magma, both of which have essentially similar compositions. The compositional variations of the LG and MG chromitite seams along strike away from the supposed feeder site (Union section) to the distal facies (Brits section) are attributed to the advanced compositional contrast between the resident magma and the replenishing primitive magma pulses. The CZ is characterized by reversals in fractionation trends and this is attributed to the compositional evolution of the parental magma and not to the replenishment of the resident magma by influxes of grossly dissimilar magma compositions. The Cr20 S content and the Cr:Fe ratio of the MG chromitite layers increase from the Ruighoek (near proximal) section to the Brits section (distal facies). This is attributed to the advanced compositional contrasts between the resident magma and the replenishing primitive magma. In contrast, the Cr20 3 content and Cr:Fe ratios ofthe LG6 and LG8a chromitite seams decreases eastwards from the Ruighoek section. The average Cr:Fe ratio for the western Bushveld Complex is between 1.5 and\2.0, nonetheless, a progressively lower Cr:Fe ratio is noted from the LG1 chromitite up through to the MG4 chromitite seam in the Ruighoek section. tn the LG2 - LG4 chromitite interval a deviation to higher.lratios is encountered. A progressive substitution of Cr by AT and Fe in the Cr-spinel crystal lattice characterizes the chromitite succession from the LG1 seam up through the chromitite succession to MG4. The petrogeneSiS of the chromitite seams of the CZ is attributed to magma mixing and fractional crystallization of a single magma type.
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Largatzis, Savvas Anthony. "Petrogenetic implications for the Merensky Reef: a platinum-group element distribution study from wide-reef facies in the western Bushveld Complex, RSA." Thesis, Rhodes University, 2016. http://hdl.handle.net/10962/3167.
Full textAbstract:
Despite decades of research and its economic importance, the formation of the Merensky Reef still remains controversial. This study reports on the distribution of platinum-group elements within widereef facies in an attempt to identify petrogenetic controls in the formation of the Merensky Reef. Widereef Merensky was sampled from Eland Platinum Mines in the western Bushveld. Macroscopic investigation of the drillcore identified a basal chromitite stringer overlying an anorthositic footwall. The reef comprised a pyroxenite unit while the hangingwall comprised noritic, leuconoritic and anorthositic units (upwards the stratigraphy). Furthermore, an anorthositic seam was identified within the pyroxenite reef, near the top of the unit. Ophitic textures of orthopyroxene oikocrysts comprising inclusions of plagioclase chadacrysts suggest that the crystallization of plagioclase preceded the crystallization of orthopyroxene. Furthermore, plagioclase and orthopyroxene were shown to be in mineral disequilibrium with one another. Pervasive hydrous alteration features throughout the Merensky Reef suggest late stage deuteric alteration. Mineral chemistry of plagioclase cores recorded ranges for An content in the Merensky Reef as follows: An72-79 in the anorthositic footwall, An71-77 in the chromitite stringer, An45-78 in the pyroxenite reef unit, An47-73 in the anorthosite reef unit, An72-76 in the norite hangingwall, An75-77 in the leuconorite hangingwall and An72-77 in the anorthosite hangingwall. This suggest that the reef units were more evolved than the footwall and hangingwall units. Furthermore, plagioclase showed reverse zoning in the anorthosite footwall unit while normal zoning was identified in the anorthosite reef unit. This suggested that the footwall unit underwent reheating and re-equilibration with a hotter, more primitive magma (also evident in recrystallization textures) while the anorthositic reef unit cooled relatively slowly and interstitial plagioclase present within this unit equilibrated with a trapped, more evolved liquid. The pyroxenite reef unit shows enrichment in incompatible elements and corresponding negative Eu anomalies, indicating the presence of trapped liquids. Cu, Ni and S concentrations remained low throughout the reef with the exception of a peak underlying the anorthositic seam and further enrichment underlying this peak. Platinum-group element geochemistry identified two major peaks: an upper peak which coincided with the peaks for Cu, Ni and S, and showed preferential enrichment in Pd and Au relative to other PGE, and a lower peak which coincided with the presence of chromitite and showed the preferential enrichment of Os, Ir, Ru, Rh and Pt relative to Pd, Au, Cu and Ni. The formation of the lower peak was consistent with a model involving the co-precipitation of chromite and PGE clusters (as PGM) while the upper peak was attributed to a model involving the collection of PGE by an immiscible sulphide liquid. Moreover, high Cu/Pd and Pt/Pd ratios in the lower pyroxenite unit indicated a process involving sulphide fractional segregation and scavenging while the inverse, present within the upper pyroxenite unit, suggested a more dynamic system involving the introduction of PGE-undepleted magma and S during simultaneous sulphide precipitation. Furthermore, a separation of PPGE peaks from IPGE peaks was observed within the pyroxenite unit, indicating a different partitioning behavior between PPGE and IPGE. The separation of these peaks is attributed to a sulphide liquid fractionation model while depletion haloes occurring in the proximity of the main PGE peaks was suggested to form through an Ostwald-ripening type mechanism. The results of this study are consistent with a model for the formation of the Merensky Reef involving a combination of geochemical processes, including sulphide segregation and fractionation, as well as multiple replenishments of magma.
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Ramushu, Adam Puleng. "Geochemical and Petrographic Characterization of the Transition Boundary between the MG2 package to MG3 package at Dwarsrivier Chrome Mine, Bushveld Complex, South Africa." University of the Western Cape, 2018. http://hdl.handle.net/11394/6407.
Full textAbstract:
Magister Scientiae - MSc (Earth Science)This study area is situated within the Eastern Bushveld complex at Dwarsrivier chrome mine, which is approximately 30 km from Steelpoort and 60km from Lydenburg in the Mpumalanga province. The primary aim of the project is to identify the petrological and geochemical characteristics that can be used to distinguish the various rock types of feldspathic pyroxenites, chromitites, anorthosites and chromitite pyroxenites and determine whether the various rock types are from the MG2 package and MG3 package were formed from a single or multiple magma pulses. The geochemical and mineralogical variation studies were carried out using cores from borehole DWR74 and DWR172 located on the farm Dwarsrivier 372 KT. Using the combination of various multivariate statistical techniques (factor, cluster and discriminant analysis) multi element diagrams and trace element ratios, the outcome of the study demonstrated that each of the four rock types can be sub-divided into two groups.
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Mandende, Hakundwi. "Geochemical and petrographic characterization of platreef pyroxenite Package p1, p2, p3 and p4 units at the akanani prospect area, bushveld Complex, South Africa." University of the Western Cape, 2014. http://hdl.handle.net/11394/4350.
Full textAbstract:
>Magister Scientiae - MScThis study is focused on the Akanani prospect area, approximately 25 km north-west of the town of Mokopane, Limpopo Province where exploration geologists at the study area have classified the ‘pyroxenitic’ units into P1, P2, P3 and P4 units upward in order of succession with height based on their textures, mineralogy and colour. The primary aim of this study is to distinguish the distinctive geochemical and mineralogical characteristics that can be used to identify each unit (P1 to P4) and in so doing create major geochemical, petrographic and mineralogical variables that will help or facilitate the exploration for and recovery of PGE and BMS mineralisation. Geochemical and mineralogical variation studies were carried out on the cores from ZF044, ZF045, ZF048, ZF057, ZF078, ZF082 and M0023, located in the Platreef at the Akanani Prospect area on the farms Moordkopje 813LR and Zwartfontein 814LR. Using a combination of various multivariate statistical techniques (factor, cluster and discriminant analysis) and mineralogical studies (CIPW norm, microprobe analysis, petrography), the outcomes of the study have demonstrated that the Platreef at Akanani comprise at least four lithological units i.e. the basal pyroxenite portion referred to as the P1 unit comprises chromitite, pyroxenites and feldspathic pyroxenites with associated Cr, TiO2, chromite, pyroxenes, hematite and Fe2O3, the mineralized section of the P2 unit is characterized by harzburgite, serpentinized harzburgite and in places orthopyroxenites are present consistent with high MgO and LOI contents, the feldspathic portion referred here as the P3 unit is characterized by a feldspathic pyroxenite containing higher Al2O3, Na2O, K2O, albite, hypersthene and SiO2 and the top most portion of the P4 unit comprising CaO, Diopside, ilmenite, anorthite, apatite and P2O5 that can be interpreted to have formed by three separate magma pulses. Considering the possibility that the P4 unit is a hybrid melt of assimilated Platreef that interacted with intruding Main Zone magma, this reduces the number of magma pulses to two. The classification of P1, P2, P3 and P4 units of the Platreef at Akanani shows that the criteria used by mining personnel to classify the four lithological units is not definitive and therefore are not highly reliable. Although various multivariate statistical techniques were employed relatively similar elemental associations were obtained highlighiting the importance of this approach. The strongly positive correlation between sulphides, PGEs and chromite at Akanani is consistent with an orthomagmatic deposit that had been disturbed by significant hydrothermal activity, while in places a good BMS-PGE relationship is commonly associated with the main chromitite stringers in P1. Mineral and whole rock compositions of silicate rocks highlight the strongly magnesian nature of the ultramafic P2 unit. Mineral chemistry studies of chromite, orthopyroxene, olivine, clinopyroxene and plagioclase are consistent with the multi- emplacement model. Convective exchange resulted in the enrichment of iron at the bottom of the stagnant chamber, while incompatible elements migrated upwards consistent with iron depletion with stratigraphic height. Injection of P1 magma and subsequent mixing with country rocks gave rise to the formation of chromitites and addition of plagioclase component to the intruding magma. A normal fractionation trend is suggested between P2 and P3 consistent with enrichment of MgO in P2 and enrichment of Al2O3, Na2O, SiO2 and K2O in P3. The An% of 84.4 of plagioclase coupled with CaO enrichment in P4 is suggestive of some Main Zone influence and can be interpreted as resulting from partial melting and recrystallization of P3 in response to the intrusion of the Main Zone magma is suggested for the formation of the P4 unit. There exists a good correlation between the modal mineralogy and mineral chemistry as determined optically, the norm as determined by the CIPW norm and the whole-rock geochemical results as determined by multivariate statistics and conventional methods.
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"A mineralogical and petrographic study of prematoids and layered rocks of the upper critical zone of the western Bushveld Complex, South Africa." Thesis, 2015. http://hdl.handle.net/10210/14325.
Full textAbstract:
D.Phil.This study which describes rocks of the Upper Critical Zone of the Bushveld Complex is subdivided into three parts. The main rock type of the Upper Critical Zone, the noriteanorthosite, is the subject of the first part. Inclusions in chromite and plagioclase were studied. The inclusions' in chromite were observed at different stages of their formation. The chromite crystals usually overgrow plagioclase, pyroxene and hydrous minerals (biotite, amphibole and clinozoisite) trapping them at grain boundaries or triple junctions of chromite host grains. With 'continuous growth of the host minerals the inclusion starts changing its shape from elongated to circular and the hydrous mineral proportion of the inclusion increases. Simultaneously amphibole changes its composition from pargasitic to tremolitic...
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Coetzee, Hendrik. "The tectono-chronological evolution of the Bushveld complex." Thesis, 1995. http://hdl.handle.net/10539/22842.
Full textAbstract:
A dissertation submitted to the Faculty of Science of
the University of the Witwatersrand for the degree of
Master of ScienceDetailed high precision geochronological studies have been performed on the 2054 Ma old Bushveld Complex, in an attempt to unravel its tectonic and thermal evolution in the period immediately following intrusion and crystallisation. The geochronological techniques used have been specifically chosen to sample specific temperature episodes in the cooling of the Complex, rather than to necessarily provide an accurate emplacement age, The Bushveld Complex is seen in this study as part of the Bushveld Magmatic Province, rather than as an isolated intrusion, The geochronological data are therefore interpreted in the context of the current understanding of the Proterozoic tectonic and thermal history of the Kaapvaal Craton. The development of clean chemical methods and accurate geochronological methods are essential to this type of study. The reduction of laboratory blanks, especially for lead and the development of laboratory techniques for the analysis of small samples therefore played an important part in this study. It has been possible to lower analytical blanks, especially lead blanks to levels where the analysis of small samples is possible. In addition, the zircon evaporation technique was attempted. Phlogopite micas from the Critical Zone of the Bushveld Complex give a wlde range of Rb-Sr model ages, some almost 100Ma older than the preferred age. This indicates a period of hydrothermal alteration of considerable duration at the same time as the intrusion. The slightly young Rb-Sr age recorded for all the mica and whole rock data collected for this study indicates the alteration of the micas which is evident from petrographic and electron microprobe studies. U-Pb and Pb-Pb zircon ages are also Significantly younger than the preferred age, indicating a degree of alteration. This is also seen in the discordance of the zircons seen in the U-Pb data.
AC2017
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Shelembe, Refilwe Pamela. "The Merensky Unit, Lonplats' Mines, western Bushveld Complex." Thesis, 2008. http://hdl.handle.net/10539/5910.
Full text
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Wabo, Hervé. "Paleomagnetism of post-Transvaal sill complexes, selected dykes and the Uitkomst Complex - relation to the Bushveld Complex." Thesis, 2014. http://hdl.handle.net/10210/8790.
Full textAbstract:
Ph.D. (Geology)The Paleoproterozoic (i.e. 2500 Ma to 1600 Ma) apparent polar wander path (APWP) for the Kaapvaal craton (KC) is not well constrained, due to the lack of reliable paleopoles and absence of numerical ages for existing poles. In addition, the duration of emplacement, and timing of remanence acquisition of the Rustenburg Layered Suite (RLS) and other units of the Bushveld Large Igneous Province (LIP) are still unclear. During the present paleomagnetic study, samples were collected from the small intrusions that occur around the RLS and that are believed to be related to the Bushveld LIP for the establishment of new paleomagnetic and virtual geomagnetic poles. In addition, samples from post-Transvaal sills and dykes were targeted for U-Pb dating and geochemical analyses. Geochronological and geochemical data helped to constrain the timing of the newly defined paleopoles. These paleopoles were used in conjunction with previously published ones from KC to evaluate the APWP for this craton during the Paleoproterozoic. Two of the studied post-Transvaal sills in the eastern KC revealed U-Pb ages that are identical to the age recently reported from the Marginal Zone of the RLS. Geochemical signatures of sill samples were in very good agreement with the newly obtained ages. New ages and geochemical data provided constraints on the magnetic components recorded by the sills. The results confirm the existence of B1 Bushveld magma-related sills on KC as well as pre and post-Bushveld sills as previously suggested. Particularly, dataset from the B1 Bushveld magma-related sills allowed for understanding the magnetic history of the RLS at the early stages (Marginal Zone) of its formation. Paleomagnetic sampling of the Uitkomst Complex provided constraints on the remanence acquisition of this complex and also helped to understand the timing of the Bushveld magmatism outside of the main complex. Paleomagnetic data from a post-Transvaal dolerite dyke swarm near Lydenburg revealed a complex magnetic history. Characteristic magnetic components constrained by geochemical analyses were not similar to the RLS, but indicate probable relationship to other units of the Bushveld LIP. The new ages generated in this study coupled to those previously obtained from the upper layers of the RLS suggested that this suite emplaced within a time period of at least 4 million years. Paleomagnetic results from the B1 Bushveld magma-related sills and available data from the upper layers of the RLS reveal that during the RLS emplacement, the Earth’s magnetic field reversed at least eight times. These results, together with data from the Lydenburg dykes, further indicate a minimum of nine changes in polarity of the Earth’s magnetic field during the formation of the Bushveld LIP. During the present study, new pole positions of different reliability were added to the existing paleomagnetic database for the KC. Paleopoles from the Paleoproterozoic database of the KC (including those generated in the present study) were used to propose a new APWP for this craton from ~2200 Ma to ~1800 Ma. Particularly, poles from the B1 Bushveld magma-related sills and Uitkomst Complex provide the information to identify striking features in the APWP of the Paleoproterozoic KC.
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Letts, Shawn Andrew. "The palaeomagnetic significance of the Bushveld Complex and related 2 Ga magnetic rocks in ancient continental entities." Thesis, 2008. http://hdl.handle.net/10539/4889.
Full textAbstract:
The Kaapvaal Craton was the scene of two major magmatic events around 2.0 Ga,
namely the Bushveld Complex and the Phalaborwa Complex. Both complexes
have been the subject of numerous palaeomagnetic studies during and prior to the
1980s. Despite these studies, systematic inconstancies for emplacement ages, in
particular for the Bushveld Complex, have been found between the
palaeomagnetic findings and well constrained ages. The greatest concern with the
Bushveld Complex results are the large spread in pole positions previously
determined for the different zones. This has been interpreted in prior studies to
indicate that the Bushveld Complex was emplaced and cooled below the Curie
temperature of magnetite over a time span of 50 my. The results obtained
previously for the Phalaborwa Complex appear to be out of position (~16°) with
respect to those for the Bushveld Complex. This is of concern because new
geochronological data show that the Rustenburg Layered Suite of the Bushveld
Complex was emplaced approximately 1 my after the Phalaborwa Complex.
These inconsistencies have prompted the current re-investigation of the
palaeomagnetic results for both the Bushveld Complex and the Phalaborwa
Complex.
New palaeomagnetic data collected from all zones of the Rustenburg Layered
Suite from the Eastern, Northern and Western Lobes of the Bushveld Complex,
yielded palaeomagnetic poles that eliminated the spread in the apparent polar
wander path. This observation is in agreement with precise age data, constraining
the time period of emplacement of the complex to ~ 6 my. Resulting beddingcorrected
high blocking components from all zones produced better groupings,
thereby supporting a primary magnetic signature and indicating that the complex
was intruded in a near-horizontal position. Dual polarities identified within each
zone of the complex and positive reversal tests have identified one of the oldest
known reversals of the Earth’s magnetic field.
iii
Palaeomagnetic data from the Phalaborwa Complex have produced a pole position
that is in close proximity to those obtained from the coeval Bushveld Complex.
In an attempt to achieve a better understanding of tectonic events occurring in the
Kaapvaal Craton a number of dual polarity dykes within the Bushvled and
Phalaborwa Complexes were palaeomagnetic analysed. Results revealed that the
acquired pole positions are in agreement with ~1.9 Ga dykes, indicating the
possibility that the dykes occurring in both complexes are part of the same
magmatic event.
Palaeopoles generated during this study were used in refining the Kaapvaal Craton
apparent polar wander path around 2.0 Ga, and in conjunction with other welldefined
2.0 Ga poles for the Kaapvaal Craton, a robust cratonic pole was produced
that was used in Precambrian palaeographic reconstructions with emphasis on the
postulated Vaalbara continent and the Columbia supercontinent. Palaeomagnetic
reconstruction derived in this study has cast doubt on the existence of the
Vaalbara continent at 2.0 Ga. Although, some support is given to the existence of
the Columbia supercontinent at the same period.
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Zintwana, Masibulele P. "The petrology and geochemistry of the marginal and lower zones in the Clapham Trough, Eastern Bushveld Complex." Thesis, 2016. http://hdl.handle.net/10539/19360.
Full textAbstract:
Submitted in fulfilment of the requirements for a Master of Science degree in Geology, in the Department of Geosciences, University of the Witwatersrand, Johannesburg, South Africa.
2015This study undertook to re-evaluate the conventional historic interpretation that accepted the Marginal Zone as representative of the chill phase to the earliest emplacement of Lower Zone magmas. The Clapham Trough preserves a thick sequence of the Marginal Zone rocks, at least 220 m thick. Poor exposures and incomplete stratigraphy of the rock succession that occurs between the floor and the Marginal Zone rocks presented great limitations to earlier studies, and led earlier workers to accepting that the base of the Bushveld Complex is the Marginal Zone norite. This study presents results from the 692 m CH6 drilled core, which intersects the Marginal-Lower Zone boundary in the Clapham Trough. The base of the CH6 drill core consists of melanorite (with less than 40 % cumulus plagioclase), which is conformable with the underlying, thick Basal Ultramafic Sequence (BUS, described in Wilson and Chunnett, 2010; and Wilson, 2012) separating the Marginal Zone rocks with the floor rocks of the Magaliesberg Formation. The amount of cumulus plagioclase in the Marginal Zone increases with increasing stratigraphic height such that the top units of the Marginal Zone are norite-leuconorites (typically 45-65 % cumulus plagioclase), bordering on anorthosite. The progressive changes in the modal variations led to the subdivision of the Marginal Zone norite to a basal Mafic Norite and a xenolith-bearing Shelter Norite. The latter is deemed a correlative of the Xenolithic Norite described at Olifants River Trough. Coupled with the increasing amount of cumulus plagioclase, the An# increases with stratigraphic height. The An# fractionation trend is reversed from that of the co-existing orthopyroxene observed in the same interval (An63-74 vs. En81-70). The reversed An# compositions are an abnormal differentiation trend. The compositional disequilibrium between co-existing orthopyroxene and plagioclase formed from in-situ crystallization with floatation of plagioclase, through convection, separating the cotectic phases. All the data in the Marginal Zone show that these rocks have continuous fractionation trends with no interruptions. The Marginal Zone rocks are cumulus rocks that formed through fractional crystallization in a temporarily closed magma chamber. The present work showed unequivocally that the Marginal Zone is a product of differentiation of earlier emplacement of B1-magma, and cannot be representative of either a chill zone or composite sills. The appropriate (parental) liquid composition of the Marginal Zone formed after 30 % crystallization of the B1-magma. The postulated liquid composition is 6 wt. % MgO and 56.7 wt. % SiO2. The entire Marginal Zone succession would have formed from about 30-54 % crystallization of the B1-magma. The crystallization of the Marginal Zone was ended abruptly by the emplacement of a new batch of B1-magma, which must have mixed with the residual magma that must have ponded atop Marginal Zone cumulates after 54 % crystallization. The mixing of the evolved residual magma and the primitive B1-magma formed the liquid postulated to be parental to the Lower Zone A (10.59 wt. % MgO and 57.10 wt. % SiO2). The Transitional Pyroxenite bears all the evidence of mixing between magmas of contrasting compositions, forming the 10-30 m gradational boundary unit between the Marginal Zone and Lower Zone A (correlative of the Lower Orthopyroxenite Subzone described at Olifants River Trough). The Lower Orthopyroxenite Subzone at the Clapham Trough is almost a mono-mineralic rock succession with generally constant orthopyroxene composition (En87-86), with the exceptions at marker norite horizons (En84-82; An83-81). The constant compositions observed in Lower Zone A are attributed to contemporaneous emplacement of new magma and differentiation, which maintained the composition of the parental magma.
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Coomber, Stephen John. "Gravity modelling in the western Bushveld Complex, South Africa, using integrated geophysical data." Thesis, 2009. http://hdl.handle.net/10539/6962.
Full textAbstract:
A 10 km x 10 km study area in the western Bushveld Complex, south of the Pilanesberg Complex, was selected for testing the inversion of vertical component gravity (Gz) data to determine the geometry of the Bushveld Complex/Transvaal Supergroup contact. This contact has a density contrast of ~0.350 g.cm-3 making it a suitable target for gravity inversion. The resulting 3D gravity model agrees well with the 3D seismic interpretation, indicating that the depths determined from the seismic data are appropriate. The gravity inversion could be extended laterally to investigate regions without seismic data coverage. This methodology may prove useful where upwellings in the floor of the Bushveld Complex distort seismic data, but can be imaged by gravity inversions.
The Gz dataset was created from converted Airborne Gradient Gravity (AGG) data, combined with upward continued ground Gz gravity data, providing extensive coverage. This combined dataset was used in an interactive, iterative 3D gravity inversion methodology used to model the geometry of the Bushveld Complex/Transvaal Supergroup contact and densities of the Bushveld Complex, Transvaal Supergroup and Iron-Rich Ultramafic Pegmatoids (IRUPs). The resulting 3D gravity model provides an acceptable first-pass model of the Bushveld Complex/Transvaal Supergroup contact. In the shallow south-west region of the study area, the steeply dipping contact was determined from borehole intersections. 3D seismic data was the only constraint towards the north-east, where the contact flattens out to a sub-parallel contact, at ~2 000 m depth. In the north-western section, the Bushveld Complex/Transvaal Supergroup contact is fault-bounded by a conjugate set of the Rustenburg Fault, causing the Bushveld to onlap the Transvaal sediments. In the southern region, the contact changes as the conjugate fault dies out, and the Bushveld Complex becomes layered/sub-parallel to Transvaal sediments. This, and other geological features (e.g. faulting, folding, dykes), can be explained in relation to the regional tectonic history, relating to motion along the Thabazimbi-Murchison Lineament (TML). Pre-Bushveld emplacement NW-SE far-field stress caused NW trending extensional features in the region (e.g. Rustenburg Fault). Re-orientation of the compressive force to NE-SW, in syn- to post-emplacement, caused compressive features in the region (e.g. open folds with axes trending NW).
Ground gravity data (100 m x 100 m station- and line-spacing) were also inverted to obtain a 3D model of the overburden, constrained by borehole data. However, the inversion failed to satisfy the gravity data and borehole data simultaneously, relating to difficulties in modelling the regional gravity field and the gradational nature of the weathered contact. Several rapid variations in overburden thickness were mapped, with particular success in the high frequency ground gravity survey (30 m x 30 m station- and line-spacing) with the identification of a deeply weathered (~10 m deep) channel relating to an mapped fault.
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VanTongeren, Jill A. "The 'Ins' and 'Outs' of the Bushveld Complex Upper Zone." Thesis, 2011. https://doi.org/10.7916/D86M3DTZ.
Full textAbstract:
This dissertation is an investigation into the geochemical and geodynamic evolution of the Upper Zone of the Bushveld Complex. The Bushveld Complex is one of the few large layered intrusions in which the entire cumulate stratigraphy is preserved and well-exposed from its base to its roof. Despite this unique feature, relatively little is known about the nature of magmatic differentiation in the uppermost portions of the Bushveld. As a first order, I quantify the chemical composition of the preserved stratigraphy (i.e. the bulk composition) from the geochemical base of the Upper Zone (the Pyroxenite Marker) to the contact with the roof. On the basis of major element modeling and trace element equilibria I show that the bulk composition is not representative of the original magma composition, and therefore some magma must be missing. I propose that the Rooiberg Group lavas and/or Rashoop granophyres, which make up the immediate roof of the intrusion, represent the missing magma. A further test of the magma-loss hypothesis comes from the trace element contents of apatites in the uppermost 625 m of the Upper Zone stratigraphy. Comparison of the equilibrium liquid compositions calculated from these apatites with the Rooiberg and/or Rashoop roof rocks shows that they are a geochemical match. My results also indicate a role for large-scale (>625 m) liquid immiscibility at the top of the Bushveld. This is the first documented evidence for liquid immiscibility based on the compositions of mineral phases, not melt inclusions; and it is the first quantitative evidence for large-scale immiscibility in the Bushveld Complex. Quantification of the parent magma composition at the Pyroxenite Marker allows me to not only estimate the `outputs' from the magma chamber, but also to constrain the `inputs'. A geochemical record of magma input and mixing is recorded in the cumulate stratigraphy for approximately 350 m below the Pyroxenite Marker. Using the evolution in mineral compositions I calculate the composition and proportion of incoming magma to the Upper Zone, as well as the style of input. The composition of the incoming magma is then compared to other known pulses of magma into the Bushveld Complex in order to put constraints on the source contributions and formation dynamics of the intrusion as a whole.
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Lum, Jullieta Enone. "Plagioclase compositions in the upper zone of the bushveld complex." Thesis, 2011. http://hdl.handle.net/10539/10270.
Full textAbstract:
The approximately 2 km thick Upper Zone of the Rustenburg Layered Suite represents a series
of differentiated rocks with the appearance of magnetite, olivine and apatite in that order, as
cumulus phases in the stratigraphy. Plagioclase is the dominant cumulus mineral throughout the
sequence; hence its composition provides a continuous record of differentiation. 126 plagioclase
mineral separates have been obtained and analyzed from bore core drilled through the Upper
Zone. Anorthite content decreases from An60 close to the base of the Upper Zone to An30 at the
top. Six reversals were identified in a previous study occurring over a vertical interval of 16 to 69
m and in which the An increases upward by 3 to 6%. A detailed examination of one prominent
reversal in the present study shows that the reversal is gradual. Sr in plagioclase concentrations
increase erratically upward from about 400 ppm to 800 ppm. Ba contents in plagioclase vary
irregularly between 200 and 500 ppm throughout the lower 1.5 km of stratigraphy and only
increase to about 700 ppm towards the top of the sequence. The reversals which are observed in
the An content are not observed in the Ba and Sr contents of plagioclase. Data for Sr and Ba in
plagioclase exist for the Skaergaard intrusion and have not been quantitatively modeled before.
Hence, Rayleigh fractionation models are developed to quantitatively model both intrusions. For
the Skaergaard intrusion, Sr in plagioclase increases slowly from 400 at the base to about 600
ppm at 80% crystallized. It then increases rapidly to 1300 ppm at the end of crystallization. Ba
varies erratically between 79 ppm and 200 ppm until 90% of crystallization from where its
concentration increases to over 600 ppm at the end of crystallization.
The most simplistic model for the evolution of such trace elements in a layered intrusion is to
assume a homogenous magma chamber crystallizing a mineral assemblage in constant
proportions, with constant partition coefficients and with a constant proportion of trapped liquid
so that bulk D is constant throughout crystallization. However, these assumptions are totally
inappropriate for natural situations because mineral assemblages, their proportions, trapped
liquid fractions and partition coefficients change as magmas fractionate. It therefore becomes
necessary to subdivide the crystallization into a very large number of small Rayleigh
fractionation stages, and change any or all of these four variables at each stage.For the Skaergaard intrusion, the following variables are used. Values for F are estimated from
the percentage of crystallization of the entire intrusion which is then subdivided into 100 small
stages of fractionation. Observed plagioclase mode and trapped liquid fractions are set to change
from 55 to 33% and 45 to 4% upwards respectively so that cumulus plagioclase mode can be
calculated. Utilizing the conventional Rayleigh modeling by calculating a starting liquid content
from D=CM/CL and applying the Rayleigh Law to the section does not fit typical parental magma
compositions and observed data for plagioclase. Consequently, a value for the Skaergaard
parental magma within the range of typical Ba contents of starting liquid is then used to model
the section and allowance is made for the trapped liquid shift (TLS) effect. The Ba content of
calculated re-equilibrated plagioclase differs by a factor of 3 from the calculated original
cumulus plagioclase content. Incorporation of the TLS effect produces the observed range of
values for Ba in plagioclase. In all the models, a constant DBa of 0.4 is used throughout
fractionation because the model is relatively insensitive to changing DBa in plagioclase. Sr which
is compatible into plagioclase has a smaller TLS effect, thus the model is more sensitive to
variations in DSr in plagioclase. The best fit with the observed data is obtained if DSr in
plagioclase is varied smoothly from 1.7 at the start of fractionation to 1.9 at the end.
In the case of the Upper Zone of the Bushveld Complex, values for F, DBa in plagioclase,
plagioclase mode and TL fraction are used to set up a calculation similar to the Skaergaard
intrusion. A constant DBa in plagioclase of 0.4, DSr in plagioclase from 1.5 to 2.1, observed
plagioclase mode from 65 to 45% and a constant proportion of trapped liquid of 10% are used.
Different starting liquid compositions are then tried until values are obtained for which Ba and Sr
contents of calculated re-equilibrated plagioclase are in agreement with analytical data. Based on
modeling of the Upper Zone of the Bushveld Complex, Ba contents of calculated re-equilibrated
plagioclase also differ significantly from respective original cumulus contents due to the TLS
effect. Therefore, applying the model to the Upper Zone of the Bushveld Complex validates the
importance of allowing for the TLS effect. The computed liquid contents at the base of the UZ
are 273 and 325 ppm for Sr and Ba respectively. In view of the reversals in mineral composition
and small scale variations in the Sr and Ba contents of plagioclase, various magmatic processes
such as magma addition and internal overturn which have been proposed to occur in the Upper Zone are inferred to be possible. There are a number of uncertainties with the modeling, hence
none of these processes can be conclusively ruled out.