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M.Sc. / Please refer to full text to view abstract

Student Number : 9210081T - MSc dissertation - School of Geosciences - Faculty of Science / The occurrence of large, massive iron oxide deposits throughout the Bushveld Complex, South Africa, and its associated roof-rocks is well known. The style of mineralisation and the associated alteration exhibits many characteristics of iron oxide-copper-gold (IOCG) type deposits. The contained mineralisation is dominated by iron oxide and fluorite and is accompanied by a diverse polymetallic association, with anomalous fluorite, copper, gold, barite, uranium and LREE. The Ruigtepoort orebody, located in the western Bushveld Complex, is such an example and is surrounded by some 20 smaller occurrences in the upper stratigraphic portions of the Bushveld Complex, all displaying strong structural control. These IOCG bodies occur as narrow veins, hydrothermal breccias, subhorizontal sheets, or as pipe-like intrusions usually utilising pre-existing structures. Set in red Nebo granite, the mineralised core consists of severely chloritised rock that is haloed by progressively less-altered granite. The alteration passes from the chlorite core to more hematite-phyllosilicate-dominated alteration, to sericite-illite-dominated alteration; followed by the relatively fresh country granite. These alteration haloes dissipate rapidly away from the body over only a few metres. Sodic-calcic alteration described in other IOCG is not locally observed. Extensive zones of barren feldspar-destructive alteration exist, including K-metasomatism, sericitisation and silicification. Multiple alteration episodes appear to have occurred, resulting in extensive overprinting and a very complex paragenesis. The primary mineral assemblage consists of Fe-chlorite, fluorite, quartz, hematite, and specularite, with accessory pyrite and chalcopyrite. Multiple generations of hematite, quartz, fluorite and chlorite are also observed. At other localities, the assemblage is dominated by magnetite-actinolite-britholite. Significantly enriched concentrations of Au (2 g/t), Cu (0,45 wt%), Ba, Y and LREE are encountered in the small, mineralised core. A fluid mixing model is proposed characterised by an initial highly-saline, sulphur-poor magmatic fluid which mixed with a lower temperature oxidised, surficial fluid. Structure was probably a significant factor in determining the initial distribution of hydrothermal centres and the overall morphology of the entire system. Subsequently, continuous brecciation, alteration, mineral precipitation and fault activity helped develop the hydrothermal centres into a complex array of variably mineralised, lenticular, pipe-like and irregularly shaped breccia bodies.

Geochemical, 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.

M.Sc. / This study focuses on the Merensky Reef (MR) occurring within the Two Rivers Platinum mine property in the farm Dwarsriver 372 KT, on the Southern sector of the Eastern Limb of the Bushveld Complex. Five MR exploratory drill core intersections were obtained. Petrographic and mineral chemical characteristics of these drill core samples focused on the characterization of minerals like clinopyroxene, orthopyroxene, plagioclase, chromite and olivine. Data of the cryptic variation of orthopyroxene, plagioclase and chromite, from a 10 m interval (approximate thickness of the section studied); from footwall through the MR to the hangingwall lithologies at Dwarsriver are described in this study. Locally the vertical cryptic variation of these minerals is broadly consistent with regional trends of the RLS. The lateral variation (i.e. along strike) is less pronounced; however, locally these minerals appear to be chemically evolving moving to the south of the property. Footwall orthopyroxene compositions vary from a minimum of En66 and reach a maximum of En84. Those of the MR range from En71 to En85. Hangingwall orthopyroxene compositions range from En60 and reach a maximum of En80. Plagioclase compositions in the footwall units range from a minimum of An69 and reach a maximum of An85. Those of the MR range from a minimum of An35 to a maximum of An84. This wide range in plagioclase compositions is believed to be as a result of the increased presence of fluids within the MR interval. The hangingwall plagioclase compositions range from An64 to An84. By analogy of the Western Limb, where the lithologies of the Northwestern sector are believed to be proximal to the feeder of this limb; the local lateral variation in the present study suggests that the lithologies of either the Central or Western sectors are most probably proximal to the feeder for the Eastern Limb. PGM assemblages associated with and adjacent to the MR chromitite stringers were evaluated using an MLA. Data obtained from this technique is in broad agreement with regional studies of the MR. With the aid of wholerock PGE assays the MLA technique has proven to be a powerful tool in evaluating PGM assemblages relatively quickly, from a few carefully selected samples. The mineralogical associations of the PGM with the gangue and host minerals have shown three main associations. These are the associations of chromite, BMS and silicates with the PGM, of which the base metal sulfide (BMS) association is remarkable given that these have a relatively low modal abundance. The relatively high mineralogical association of the BMS with PGM has been explained by a model involving a base metal sulfide liquid which possibly scavenged chalcophile and siderophile elements. Chromite chemistry and modal analyses of MR secondary silicate phases, which peak adjacent to the chromitite stringers, suggests elevated fluid overprinting within and adjacent to the chromitite stringers. The upper chromitite stringers, however, have higher abundances of PGM phases that are believed to be secondary in origin relative to the basal chromitite stringers. Generally the PGM associated with the upper chromitite stringers are also bigger in size averaging 70 μm as opposed to 27 μm for those associated with the basal chromitite stringers. The increase in grain size of the PGM along with the higher modal abundance of secondary PGM phases associated with the upper stringers is believed to be as a result of fluids. These fluids although affecting both the upper and basal chromitite stringers, appear to have had a relatively higher influence on the upper chromitite stringers. The most common PGMs encountered in this study are isoferroplatinum, sperrylite, michenerite, maslovite, cooperite, laurite and braggite.

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.

M.Sc. (Geology) / Please refer to full text to view abstract

The N.N.W.-S.S.E. striking Rustenburg Fault zone, in the western Transvaal Basin, South Africa, has been mapped, in order to unravel its tectonic history. Thickness differences in the Daspoort Formation of the Pretoria Group on opposite sides of the Fault suggest that the Fault was active during Pretoria Group sedimentation, with normal faulting producing localised second-order basins on the down-thrown side of the Fault. In post-Pretoria Group times, but before the intrusion of the Bushveld Complex at -2050 Ma, the area surrounding the Fault zone underwent two compressive events. The first was directed N.E.S. W., producing S.E.-N.W. trending folds, and the second was directed N.W.-S.E., producing N.E.-S.W. trending folds. The second set of folds refolded the first set to form typical transitional Type 1-Type 2 interference folding, and this compression ultimately caused reactivation of the Rustenburg Fault, so that dextral strike-slip movement displaced the Pretoria Group sediments by up to 10.6 km. The subsequent intrusion of the Bushveld Complex into the adjacent strata intensely recrystallised, and often assimilated, the strata along the Fault zone. The fault rocks within the Fault zone were also recrystallised, destroying any pre-existing tectonic fabric. Locally, the Fault zone has been assimilated by the Bushveld Complex. After the intrusion of the Bushveld Complex, little movement has occurred along the Fault, especially where the Fault passes under areas occupied by the Bushveld Complex. It is thought that the crystallisation of the Bushveld Complex has rheologically strengthened the neighbouring strata, preventing them from being refaulted. This model presented above is at variance with previous assumptions that continuous regional extension during Pretoria Group sedimentation culminated in the intrusion of the Bushveld Complex. / Dissertation (MSc)--University of Pretoria, 1997. / gm2015 / Mining Engineering / MSc / Unrestricted

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

Thesis (MSc)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: It is widely accepted that sulphide is the carrier and concentrator of PGEs during magmatic mineralization episodes in the Merensky Reef (MR). PGE concentration peaks and sulphide volume percent peaks are very closely correlated. Koegelenberg, (2011), showed in an experimental investigation that sulphide movement through a cumulate silicate and cumulate oxide pile behave in such a way that sulphide melt gets trapped in chromitite layers. When looking at the compositional distribution of sulphide within the MR it is noted that not only does the sulphide volume percent varies with MR stratigraphy but also the sulphide composition. Sulphide composition is more Cu-rich in the chromitite layers and more Fe and Ni dominated in the hanging wall to the chromitite layers. Until now the more Cu-rich assemblage of the chromitite layers are accepted to be of a sulphide melt composition compared to the Fe and Ni dominated Monosulphide Solid Solution or MSS composition in the hanging wall. In this study we used an experimental approach with a sulphide starting composition thought to exist as the parental sulphide composition of the MR to investigate the phase relations with changing temperature. It is found that the sulphide composition in the chromitite layers represent a sulphide melt composition at 1000 ± 50ºC. At 1000ºC, 50% of the sulphide system would exist as a melt. This Cu-rich melt would have segregated from the MSS and be trapped in the chromitite layer. Also at 1000ºC the partitioning of the Pt would have induced a secondary enrichment step of the Pt concentration in melt through the partitioning of Pt between a sulphide melt and a sulphide solid phase. The experimental evidence in this study points towards a possible source for the parental sulphide magma to the MR, which could have been a slightly Cu enriched mantle sulphide composition. Also, the secondary enrichment of Pt through sulphide melt fractionation at 1000ºC plays an important role in the shaping of the ore body. / AFRIKAANSE OPSOMMING: Dit word wydliks aanvaar dat die sulfied fraksie van die Merensky Rif (MR) die draer en die konsentrasie agent is vir Platinum Groep Elemente (PGE`s) gedurende mineralisasie episodes. PGE konsentrasie en sulfied volume persentasie is op `n hoogtepunt by gelyke stratigrafiese posisies in the MR. Koegelenberg, (2011), het deur middel van eksperimente bewys dat `n sulfied smelt deur `n voorafbestaande kumulaat laag kan beweeg en dat veranderende fisiese eienskappe tussen sulfied smelt en silikaat kristal en sulfied smelt en chromiet kristal, die sulfied smelt sal opsuig en verhoud om verder deur te suipel. Dit is egter oplettend dat nie net die sulfied volume persentasie varieer as `n funksie van die MR stratigrafie nie, maar ook die sulfied samestelling. Die meer Cu-ryke sulfied samestelling in die chromiet lae word aanvaar as `n sulfied smelt fraksie en die meer Fe en Ni dominerende sulfied samestelling in die oorhangende wandgesteentes verteenwoordig die Monosulfied Vaste Oplossing (MVO) soliede fase. In hierdie studie maak ons gebruik van eksperimentele petrologie tesame met `n begin samestelling verteenwoordigend van die oorsprong sulfied samestelling van die MR, om die fase verwantskappe van hierdie spesifieke samestelling te ondersoek. Dit word gevind dat die fraksionering tydens die vorming van die MR plaasgevind het by ongeveer 1000 ±50 C. By hierdie temperatuur is 50% van die sisteem teenwoordig as `n smelt fase. Hierdie Cu-verykte smelt was daartoe instaat om deur die silikaat laag te suipel, geskei te raak van die Fe en Ni dominerende MVO en vasgevang te word in die chromiet lae. Hierdie fraksionering van die sulfied smelt het ook `n sekondêre effek gehad op die verspreiding van Pt tussen sulfied smelt en sulfied soliede fases. Hierdie eksperimentele bewyse dui eerstens op die moontlikheid van `n sulfied smelt in die MR wat sy oorsprong vanuit `n effense Cu-verykte mantel bron kan hê, en tweedens op die belangrikheid van `n sekondêre proses vir Pt re-distribusie tydens die vorming van die MR.