3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
3 results
Search Results
Item Application of hyper-spectral remote sensing to assess contamination associated with gold mining in the Witwatersrand Gold Basin, South Africa(2015-02-05) Maya, MafuzaEfflorescent minerals are a common feature of the soil surface in seasonal environments where evapotranspiration (ET) exceeds precipitation (P), and are formed by the evaporation of salt solutions from the soil during periods of drying. On the Highveld gold fields, ET exceeds rainfall by approximately two-and-a-half times during the dry season, and soils overlying acid mine drainage and along polluted stream banks can become covered by distinctively coloured mineral efflorescent crusts. Whereas some efflorescent minerals are relatively insoluble and present a negligible environmental hazard (for example, gypsum), others may be readily soluble and contain high concentrations of potentially toxic metals (for example, copiapite, jarosite and uranyl sulphate). During periods of rainfall, such salts are washed further afield and into surface water bodies and act as sources of episodic pollution. The presence of some efflorescent minerals can be detected from their characteristic reflectance signatures using remote-sensing (RS) of the electromagnetic spectrum. The species of efflorescent minerals present is a useful indication of the spatial extent of sub-surface contamination, and also of the chemical conditions of the substrate, in particular the concentration of total dissolved solids, pH and redox conditions. The aim of this study was therefore to assess the use of remote-sensing on indicator efflorescent minerals as a cost-effective aid in the spatial mapping of acid rock-drainage polluted soils and water-bodies. This study describes the range of efflorescent crusts identified on different land-use areas and soil classes in a Highveld gold-mining region. Crusts were first measured in-situ under natural sunlight using a portable analytical spectral radiometer (ASD) as well as using X-ray diffraction (XRD). They were then dissolved in deionized water and the resulting salt solutions allowed to evaporate prior to analysis under controlled lighting conditions. Spectra were post-processed and compared with iv geological spectral reference libraries. The salt solutions were also analyzed for metal and sulphate content and the results were used to establish evaporation models from which mineral precipitation could be predicted. Minerals identified in the visible near-infra red (VNIR) region included iron oxides (hematite and goethite), and the sulphate mineral jarosite. In the short wave infra-red (SWIR) region clay minerals of the smectite group were dominant. Gypsum and Al-Mn-Mg-Na sulphate salts were identified in the SWIR region as mixtures occurring with clay minerals. Minerals identified in the VNIR-SWIR region were all confirmed by X-Ray diffraction (XRD). Upon dissolution, geochemical modeling revealed that gypsum and jarosite are the most common minerals expected to precipitate. The precipitation of gypsum and jarosite indicates persistent acidic conditions after dissolution of mineral salts. Gypsum and jarosite were also accurately identified by hyper-spectral spectroscopy and confirmed by XRD and geochemical modeling. Agreement between spectral interpreted minerals and geochemically precipitated mineral phases demonstrated the ability of hyper-spectral data in detecting efflorescence minerals on the soil surface. Using partial least squares regression (PLSR) combined with bootstrapping, reflectance spectrum was significantly correlated with geochemical variables.Item An investigation of mineralisation controls in the upper section of the Platreef in the southern sector, on Turfspruit, Northern Limb, Bushveld Complex, South Africa(2014-06-12) Kekana, Sello MelvynGeochemical, mineralogical and tenor variation studies were carried out on the cores from UMT040, UMT064 and UMT063, located in the Flatreef on Turfspruit, in the southern sector of the Platreef. The investigation comprised three objectives (1) to identify controls on mineralisation in the upper section of the Platreef, (2) to construct a 3D PGE tenor model, and (3) to test whether the new geological interpretation on Turfspruit correlates with the eastern and western limbs of the Bushveld Complex. The above-mentioned holes were used for geochemical analyses of major and trace elements; and UMT064 was also used for orthopyroxene microprobe analyses. For construction of the tenor model, a total of 276 drill holes were used. The distance between drill holes for geochemical studies is 145 m and 175 m respectively; and for the tenor model is approximately 100 m. The outcomes of the study have demonstrated that the upper section of the Platreef comprise at least four lithological units i.e. the topmost portion of T1, lowermost portion of T1, T2 Upper and T2 Lower that can be interpreted to have been formed by four separate pulses of magma. Considering the possibility of the T2 Lower being metamorphosed calc-silicates, this reduces the number of magma pulses to three. This is supported by the mineralogy, geochemical content and tenor variations. The T1 is made up of an orthopyroxenite (feldspathic in places), the T2 Upper comprised a pegmatoidal orthopyroxenite (also feldspathic in places) and the T2 Lower is made up of a harzburgite. High grade PGE, Ni and Cu mineralisation was found to occur within two zones in the upper section of the Platreef i.e. the T1m and T2 (Upper and Lower). Mineralisation occurs in the form of base metal sulphides such as pyrrhotite, pentlandite and chalcopyrite; and PGEs are dominated by amphoterics (PGE-arsenides, tellurides, antimonides and bismuthinides). These amphoterics are associated with base metal sulphides. The sulphides in both the T1m and T2 are interstitial. PGE minerals such as michenerite, sperrylite, hollingworthite and cooperite are present in the study area. In the T2, an increase in sulphide content and PGE grades are generally associated with the presence of the “main” chromitite stringer/ seam at the contact between a feldspathic pyroxenite (T1) and the coarse-grained to pegmatoidal pyroxenite (T2 Upper). Several other chromitites might be present above and below this contact, and it has been noted that not all of those chromitites are associated with an increase in sulphide content and higher PGE grades. PGE grades were found to be higher in T2 Upper than in T2 Lower, whereas, the base metals (particularly Ni) are generally higher in T2 Lower. In the T1m, mineralisation occurs close to the contact between the feldspathic pyroxenite and orthopyroxenite, but only the orthopyroxenite hosts the sulphides. Chromitite stringers rarely occur in the T1m. Where present, they generally occur at the top of the mineralised zone. The sulphides in both the T1m and T2 are thought to have resulted from separate and different processes. Those in the T1m are thought to have resulted from a fractional segregation process, whereas the sulphides in the T2 were thought to have been emplaced in bulk or as a mush. The 3D PGE tenor model has demonstrated that the tenor in both the T1 and T2 are higher than previously thought for the northern limb of the Bushveld Complex. Tenors in the T1 reach up to 700 ppm, whereas tenors of about 1000 ppm were recorded in the T2. These tenors are comparable to the tenors measured from the sulphide melt inclusions elsewhere in the northern limb. Tenors are the highest in the northwestern part of Turfspruit and they gradually decrease towards the southeast. Both the T1 and T2 are dominated by a tenor of about 100 ppm. In the T1, a tenor of 250 ppm is dominant in the northwestern and southeastern parts of the study area, whereas in the T2 it is only dominant in the northwestern part. Vertical variation in tenors has shown that the tenors are the highest in the topmost portion of the T1 and at the top of T2 Upper (below the T1-T2 contact). The T1 has been found to be comparable with the Merensky Reef due to the following: (a) similarities with respect to major and trace element compositions of whole rock analyses; (b) Cr/MgO ratios which are greater than 80; and (c) similarities in the pyroxene content excluding the MgO/((MgO)+FeO) ratio and CaO contents; and in the PGE tenor of the sulphides. In addition, the T1 has many of the characteristics of the model proposed by Naldrett et al (2009) for the formation of the Merensky Reef in the western Bushveld. However, the differences between the T1 and the Merensky Reef were also noted i.e. the MgO/((MgO)+FeO) ratios in this study are lower (averaging 0.71) than the Mg# reported for the Merensky Reef (Mg#78-83), the upper Critical Zone (Mg#78-84) and elsewhere in the Platreef north of Turfspruit. The CaO levels (representing the wollastonite component of the orthopyroxene) at Turfspruit are higher (>2 wt %) compared to the Merensky Cyclic Unit and other parts of the northern limb where the CaO levels are below 2 wt %. This is an indication that the pyroxene composition at Turfspruit is more evolved compared to the Merensky Reef.Item Platinum-group mineral assemblages in the Platreef on Tweefontein, Northern Bushveld complex, South Africa(2013-02-04) McCutcheon, ShaunaThe Platreef occurs at the base of the Northern Limb of the Bushveld Complex and is variably mineralised with PGE, Cu, and Ni. The Platreef varies in thickness from a few meters to a few hundred meters and rests on progressively older sediments of the Transvaal Supergroup and Archaean granite basement northwards. Recent studies have highlighted the importance of magmatic processes, contamination of the magma by footwall rocks and syn- and post metasomatic fluid activity on the observed mineralisation. Along the Platreef strike, the PGE grade profiles are generally top-loaded from Overysel to Tweefontein North and more variable and bottom loaded from Tweefontein Hill southwards emphasizing the importance of the change in mineralisation style at Tweefontein in relation to the whole Platreef. This study presents the first significant PGM data on the Tweefontein farm, including ten boreholes along strike, providing insight into the distinctly different PGE mineralisation styles observed. Samples were selected based on assay data, varying rock types, stratigraphic position and proximity to geological features. The selected samples were investigated using petrography, geochemistry and the automated SEM techniques of QEMSCAN and MLA. Over 9000 PGM were analysed forming one of the most comprehensive PGM studies on the Platreef to date. The lowermost footwall intersected along the Tweefontein strike is banded ironstone of the Penge Formation. This is overlain by a metasedimentary footwall package, of variable thickness, derived from the shales and dolomites of the Duitschland Formation. Iron-rich, recrystallised, noritic sills occur at the base of the Platreef and are thought to represent sills which intruded prior to the emplacement of the Platreef. A pre- and possibly syn-Bushveld structural control resulted in irregular floor topography defined by a topographic footwall high in the central Tweefontein area and topographic depressions at Tweefontein North and Tweefontein Hill. The depression areas at Tweefontein are similar to the footwall basins at Turfspruit to the south, in which the Platreef is more lithologically complex compared to the footwall high areas. The footwall basins at Tweefontein and Turfspruit contain basal massive and submassive sulphides, which may not necessarily carry significant PGE grade. The Platreef lithologies at Tweefontein are composed of pyroxenites and norites with minor harzburgitic lithologies and contain numerous cross-cutting granitic veins. Xenoliths/interlayers of metamorphosed Duitschland lithologies occur primarily near the base of the Platreef, but also in the middle and upper Platreef sequence reflecting roof pendants. Unlike the Platreef on the farms adjacent to Tweefontein, the Platreef and footwall lithologies are relatively unaltered, but localised serpentinisation and chloritisation occur within harzburgitic lithologies and metasedimentary interlayers. Based on the stratigraphy and geochemical characteristics, the Platreef at Tweefontein can be subdivided into the upper and lower Platreef. The upper Platreef subdivision occurs in the top 20-40 m of the sequence and is defined by higher Mg#, Cr, Cr (ppm)/MgO and Pt/Pd values compared to the lower Platreef. In addition, the majority of the grade and base metal sulphide (BMS) content is enriched in the upper versus the lower Platreef, particularly for the northern and central parts of Tweefontein. The upper and lower Platreef may have been derived from different magma sources based on the “R Factor” concept proposed by Campbell and Naldrett in 1979 whereby the abundance of the PGE relative to the BMS content is linked to the proportion of magma with which the sulphide ore equilibrated (Naldrett, 2005b). Previous detailed geochemical studies from Tweefontein Hill southwards highlighted compositional breaks in the Platreef sequence thought to represent distinct sill-like intrusions (Hutchinson and Kinnaird, 2005; Kinnaird, 2005; Manyeruke et al., 2005; Nyama et al., 2006). They reported a more primitive sill at the top of the Platreef, which correlates to the upper Platreef at Tweefontein. The lower Platreef is therefore likely to represent a different sill intrusion. A relatively homogenous pyroxenitic package characterises the upper Platreef, although a more heterogeneous package is observed close to and at Tweefontein Hill. At Tweefontein North, the base of the upper Platreef is often marked by a chromitiferous package comprising a pegmatoidal feldspathic pyroxenite unit, up to 6 m thick, capped by a chromitite layer. Due to similar stratigraphy and high PGE grades, this distinct horizon has been compared to the Merensky Reef found elsewhere in the Bushveld Complex. The predominant base metal sulphides (BMS) in the Platreef at Tweefontein are pyrrhotite, pentlandite, chalcopyrite with minor pyrite aligned with that found elsewhere along the Platreef strike. There is an increase in BMS content, primarily pyrrhotite, towards the base of the Platreef with massive and submassive sulphide development near the base and in the footwall, particularly at Tweefontein Hill. Sulphur isotopes and detailed mineralogical studies at Turfspruit have shown that the addition of S, As and Sb into the magma from the Duitschland footwall triggered the development of a PGE-poor sulphide liquid which was then able to mix, modify and dilute the magmatic sulphides (Hutchinson and McDonald, 2008). Due to the similarity in footwall between Turfspruit and Tweefontein, these proposed processes help to explain the increase in BMS towards the base and the development of basal massive and submassive sulphides, which are not necessarily associated with significant PGE grade. At Tweefontein North, the processes dominating the top-loaded PGE mineralisation were primarily magmatic. The PGM assemblage, hosted by base metal sulphides and magmatic silicates, is dominated by Pt-and Pd-bismuthides and -tellurides with minor PGE-sulphides and Pt-arsenides. PGE-sulphides occur in the Platreef where the chromitiferous horizon is developed, which may indicate an environment low in volatile activity and one of the most primary mineralisation styles along the Platreef strike. The footwall high, which separates the depressions at Tweefontein North and Tweefontein Hill may have kept the Platreef at Tweefontein North relatively protected from additional processes affecting Tweefontein Hill. In contrast, assimilation of the Duitschland footwall is thought to play a key role in the development of the variable but predominantly bottom-loaded PGE mineralisation at Tweefontein Hill. The PGM assemblage is Pd-dominant characterised by Sb-, As- and Bi-bearing PGM, reflecting the incorporation of Sb, As and Bi from the Duitschland footwall. The association of the PGE mineralisation with the extensive basal sulphide development implies that the mineralisation at Tweefontein Hill probably occurred due to the gravitational settling of a sulphide liquid containing a mix of sedimentary and PGE-hosting magmatic components. Due to a significant PGM-BMS association in the mineralised footwall and metasedimentary interlayers/xenoliths, a downward migrating sulphide melt is believed to be the main mechanism responsible for the redistribution of PGE, predominantly Pd, into the mineralised metasedimentary lithologies. Finally, the Platreef and footwall lithologies may be locally modified by late-stage felsic and hydrothermal fluids to form bismuthide- and arsenide-dominant PGM assemblages, primarily hosted in quartz and serpentine respectively. This study shows the PGM and sulphide mineralisation at Tweefontein to be multifaceted, involving magmatic processes, assimilation of the Duitchland footwall into the Platreef magma and late-stage hydrothermal and felsic fluid activity. Footwall composition and irregular floor topography, resulting in depression areas at Tweefontein North and Tweefontein Hill, are believed to play a key role in what processes become significant along the Tweefontein strike. This research represents a significant contribution to the understanding of the distinctly different PGE mineralisation styles at Tweefontein and allows for a complete comparison of the Platreef PGE mineralisation from Overysel to Turfspruit.