Constraints on Cr-PGE Mineralisation Models: Geochemical and petrological studies in the Middle Group 1 and 3 Chromitites, Western Limb, Bushveld Complex, South Africa
- Authors: Arunachellan, Yogendran
- Date: 2022-10-14
- Subjects: Chromite South Africa Bushveld Complex , Mineralogy South Africa Bushveld Complex , Geochemical surveys South Africa Bushveld Complex
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362730 , vital:65357
- Description: The Bushveld Complex in South Africa has been of interest in various research groups for decades, along with diverse theories regarding its origin, formation, and emplacement. These theories include magma mixing, contamination, and changes in the chamber's ephemeral parameters. Of interest for our current study is the formation of the middle group chromitite layers in the Western Limb. In this research, we aimed to determine the emplacement mechanism of the MG group chromitites by scrutinising the MG 1 and MG 3 layers. In core KD 151, the focus was placed on the MG 3 and MG 1 chromitite layers and their associated silicate rocks; on these regions of the core, time was spent for detailed observations. The differences and similarities of these layers were explored as the study advanced. The objectives were to determine the in-situ or proximal crystallisation of the chromitite by evaluating mineral textures and compositions. This required that we determine the characteristics of the immediate HW (hanging wall) and FW (footwall) to these chromitites, with insights into the relationship that anorthositic zones may offer and examine the PGE profiles of the chromitites in contrasting lithological settings. The sampled borehole was in the Western Limb of the Bushveld Complex; the immediate HW, chromitite layers and FW were divided into sections (2.5 x 5 cm) along selected horizons for a microscale study. The preliminary results of a study on the sub-economic Middle Group (MG) layers within the Critical Zone (CZ), contrasting the MG 1 (Lower CZ) and MG 3 (Upper CZ) chromitite layers of the Rustenburg Layered Suite of the Bushveld Complex, South Africa were analysed. The MG 3 and 1 suites of silicate rocks show disequilibrium textures between the pyroxenes and plagioclase, forming discontinuous olivine rims. These reaction rims are interpreted as products of magmatic aqueous fluid-facilitated reactions with minerals in a sub-solidus state. Deformation of the plagioclase was also noted in MG 1; this fracturing indicates either the transportation of these minerals or compaction by an overlying crystal mush. The MG 3 package of plagioclase, pyroxene and chromite compositions range from An67-78, En71-86 and Cr# of 68-84, respectively. The MG 1 package of plagioclase, pyroxene and chromite compositions are An64-91, En79-88 and Cr# of 70-80, respectively. The Cu/Pd ratio decreases from the base of the FW as it approaches the base of the chromitite, then remains low within the chromitite layer and finally increases upwards in the HW. These trends are observed in both the MG 3 and 1 package, therefore not influenced by the sulphide control. There is a decrease in both IPGE and PPGE upwards as the HW progresses into the chromitite. The FW levels of Ir and Ru increase upwards immediately adjacent to the FW contact, while Pd and Rh concentrations decrease. The whole-rock Mg# indicates a decrease in the MG 3 with a uniform increase in the HW and an erratic upward increase observed in the FW. The chondrite normalised PGE plots show a bell-shaped curve which is evident for the En content of the pyroxenes and the content of plagioclase with the highest values in the chromitite layer itself. The Cr# of the chromite decreases upwards with the highest values along the FW. The Cu/Pd ratio for MG 1 indicates chromitite control rather than sulphide control of the PGE. The whole-rock Mg# decreases in MG 1 with higher uniform values observed in the HW and FW. The geochemical and petrological data from the MG group study revealed that in situ fractional crystallisation seems unlikely as the sole mechanism for their formation in the CZ. A model is suggested in which the migration and transport of a magmatic slurry type suspension with accompanying hydrous fluids would likely have resulted in the emplacement of these chromitite packages along with the PGE mineralisation of the chromitites. , Thesis (MSc) -- Faculty of Science, Geology, 2022
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Trace element and sulphur isotope variations of sulphides in the Koperberg Suite, O’okiep Copper District, Namaqualand, South Africa: implications for formation of sulphides and the role of crustal sulphur assimilation
- Authors: Marima, Edmore
- Date: 2022-04-06
- Subjects: Sulfur Isotopes , Magmatism South Africa Namaqualand , Sulfides , Koperberg Suite (South Africa) , Copper sulfide , Sulfur Absorption and adsorption
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/291117 , vital:56820
- Description: The major economic copper sulphide deposits hosted in the late Mesoproterozoic intrusions of the Koperberg Suite in the O’okiep Copper District immediately overlie sulphur-bearing paragneisses of the Khurisberg Subgroup in an otherwise low-sulphur granitic basement. The dominant sulphide assemblage (chalcopyrite and bornite) hosted in the Koperberg Suite is also atypical of the intermediate solid solution (iss) assemblage (chalcopyrite and pyrrhotite) observed in most Cu-Ni magmatic sulphide deposits. This study presents sulphur isotope and in-situ trace element analysis of sulphides from the Koperberg Suite and the Khurisberg Subgroup with the view of placing constraints on the role of sulphide-bearing supracrustal metasedimentary of the Khurisberg Subgroup as a source of additional sulphur in the genesis of these deposits, and ore-forming (sulphide formation) processes which result in trace element variations registered by sulphides hosted in the Koperberg Suite. The high concentrations (up to 2100 ppm) of monosulphide solid solution (mss)-incompatible trace elements (e.g., Te, Se, Bi, Ag, Pb), and the depletion in Ni and Co (<40 ppm) of sulphides hosted in the Koperberg Suite are instead consistent with the derivation of such sulphides from a Cu-rich sulphide melt which segregated from a Ni-rich sulphide melt prior to magma emplacement in the middle crust, in agreement with one of the petrogenetic models for the Koperberg Suite proposed in the existing literature. The low S/Se ratios ( ̴650-10300) of sulphides hosted in the Koperberg Suite and the high S/Se ratios ( ̴18800-56000) registered by the main sulphide phase (pyrite) in the Khurisberg Subgroup argues against crustal contamination of the Koperberg Suite magmas by the Khurisberg Subgroup. The S/Se and Cu/S ratios of coexisting bornite and chalcopyrite hosted in the Koperberg Suite are positively correlated with the bornite modal abundance in the Koperberg Suite. Such trends are interpreted to be consistent with progressive oxidation of sulphide melt, a process which results in the crystallisation of iss-bornite assemblage and/or replacement of iss with bornite due to the enrichment of Cu and depletion in S of the sulphide melt. The oxidation of sulphide melt is likely to have been effectuated by the fractional crystallisation of mss in a prior sulphide melt segregation event and/or the fractional crystallisation of Fe2+-dominated silicate phases. Fractionation of the Cu-rich melt sulphide melt (segregated from mss) also tends to enrich the residual sulphide melts in Se. Thus, the chalcopyrite-dominated assemblage with S/Se ratios of ̴1300-10200 observed in the less basic rocks in the Koperberg Suite (leucodiorites and leuconorites) is interpreted to have formed from the least evolved sulphide melt, whereas the bornite-dominated assemblage with S/Se ratios of ̴650-5500 observed in the more mafic members of the Koperberg Suite (orthopyroxenites and norites) is interpreted to have formed from the most evolved sulphide melt. The ẟ34S isotopic signatures in sulphides of the Koperberg Suite (-1.4 to +1.91‰) and the proposed contaminant, the Khurisberg Subgroup (-1.2 to +3.5‰), overlap with the those of the Koperberg Suite below the Khurisberg Subgroup (+0.74‰) and typical mantle-derived magmatic rocks (0 ± iv 2‰). Hence, the sulphur isotope variations are inconclusive as an indicator of possible crustal sulphur assimilation into the intruding mantle magma. However, considering the trace element systematics and the sulphur isotope data, the Koperberg magmas likely attained sulphur saturation at deeper crustal levels. , Thesis (MSc) -- Faculty of Science, Geology, 2022
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Genetic relationships between migmatites and the Swartoup Pluton in the Swartoup Hills (central Namaqua Belt)
- Authors: Schmeldt, Graeme Alvin
- Date: 2021-10-29
- Subjects: Migmatite South Africa Northern Cape , Intrusions (Geology) South Africa , Metamorphic rocks South Africa Northern Cape , Metamorphism (Geology) South Africa Northern Cape , Onseepkans (South Africa) , Namaqualand (South Africa) , Anatexis , Swartoup , Koenap
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10962/192162 , vital:45201
- Description: The central Namaqua Metamorphic Complex can be characterised by long-standing high-temperature (up to granulite/amphibolite facies) conditions between _ 1300 and 1100Ma, inevitably resulting in widespread metamorphism and plutonism. Hosted within a NW–SE striking antiformal structure about 40 km east of Onseepkans, Northen Cape, South Africa, in the Swartoup Hills, lies the Swartoup Pluton. The Swartoup Pluton was sampled and described in hand specimen and thin section. The study area was photographed, with all data presented in this study. The various rock types are readily discerned in the field due to their characteristic weathering colours and overall fabrics. The Swartoup granodioritic body is hosted within metasediments of the Bysteek and Koenap Formations, of the Arribees Group. The package was later intruded by another later granitoid, the Polisiehoek Granite-gneiss. The Bysteek Formation, a wall rock to the S-type Swartoup Pluton, reacted at the contact with the igneous body resulting in localised feldspathic granites and granodiorites with prominent, often euhedral, garnet, pryoxene and titanite. The Swartoup Pluton is divided into two subgroups. The first is characterised by higher P2O5 contents, _ 0.3 – 0.4 wt.%, shown with a narrower constraint on its Rb contents, _ 80 – 160 ppm, than the second, with _ 0.14 – 0.4 wt.% P2O5 and 20 – 310 ppm Rb. Meanwhile the Polisiehoek Granite-gneiss shows _ 50 – 420 ppm Rb and _ 0.04 – 0:1 wt% P2O5. As a whole, the Swartoup Pluton is characterised by somewhat elevated CaO concentrations (_ 1.5 – 6.0 wt.%), relative to calculated averages of granites (1.8 wt.% CaO, Le Maitre, 1976) and granodiorites (3.9 wt.% CaO, Le Maitre, 1976). Whilst most of the Swartoup specimens were classified as granodiorites, some orthopyroxene-bearing monzodiorite and orthopyroxenebearing monzonite were locally found and sampled. However, much of the body appears to be granodioritic to granitic in composition. The Polisiehoek Granite-gneiss is characterised by its orange-brown weathering colour in the field, sheared texture, lower P2O5 and higher total alkali content than the Swartoup Pluton. The Polisiehoek Granite-gneiss is a highly fractionated S-type granite, as shown by plots of (a) (Na2O + K2O)/CaO and (b) FeOT/MgO versus Zr + Nb + Ce + Y (Whalen et al., 1987; Zhang et al., 2019) and also of (c) (Al2O3 + CaO)/(FeOT + Na2O + K2O) versus 100 × (MgO + FeOT + TiO2)/SiO2 (after Sylvester, 1989). Classification schemes identify the Polisiehoek Granite-gneiss as either a granite (TAS diagram, after Middlemost, 1994) or alkali granite (R1R2 diagram, after De la Roche et al., 1980). , Thesis (MSc) -- Faculty of Science, Geology, 2021
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Mineralogical, geochemical and lead isotopic analysis of the lead mineralization of the Skorpion Deposit, south western Namibia
- Authors: Uazeua, Kakunauua
- Date: 2019
- Subjects: Zinc ores -- Namibia , Formations (Geology) -- Namibia , Mineralogy -- Namibia , Lead -- Metallurgy -- Namibia , Lead -- Isotopes -- Namibia
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/68391 , vital:29250
- Description: The Skorpion none-sulphide Zinc Deposit is located in the para-autochtonous Port Nolloth Zone of the Gariep Belt, which overlays the Lower-Proterozoic Orange River Group basement rocks (Corrans et al., 1993). Situated in close proximity to the larger Rosh Pinah Zn-Pb deposit, the Skorpion Deposit contained a resource of 24.6 Mt at 10.6 % Zn and unquantified Cu and Pb prior to mining. To date, zinc has been the only metal exploited, with minor amounts of copper as a by-product. This study aims at understanding the mineralogical composition of the Skorpion lead mineralization and understanding the relationship between lead and the major metals such as zinc and copper in order to form a basis for further work that could determine the potential of processing lead as a by-product. As part of the study, work was also done on lead isotopes mainly with the aim of understanding the mineralization genesis and to determine the differences between the Skorpion and Rosh Pinah deposit which rationalize the inferior economic potential of the Skorpion lead mineralization. Results of the study have shown that majority of the lead mineralization is hosted by the felsic metavolcanics as galena and subordinately in the metasiliciclastics as pyromorphite, a lead manganese phosphate. In terms of the mineral textures, the lead minerals appear to be mainly secondary phases that have been remobilized and reprecipitated around pyrite, within pyrite cracks and intergrown with minerals such as chalcocite and greenockite. Lead has been mainly concentrated along fault zones. The elevated pyromorphite concentrations tend to occur within gossanous zones in close association with iron and manganese oxides. These textures represent supergene enrichment of a sulphide proto ore. However, contrary to copper and zinc mineralization, lead was not remobilized far from the proto ore merely as a function of its poor mobility in acidic fluids (Reddy et al., 1995). This substantiates the concentration of secondary lead in the felsic metavolcanics and to a much lesser extent, in the metasiliciclastics. Both secondary zinc and copper were reprecipitated in the metasiliciclastics, further away from the sulphide proto ore, hosted mainly by the felsic metavolcanics. The average lead isotope ratios of 206Pb/204Pb (17.26), 207Pb/204Pb (15.60) and 208Pb/204Pb (37.42) resemble results provided by Frimmel (2004) for both the Skorpion and Rosh Pinah deposits. For the Skorpion samples from Frimmel (2004) had the following average ratios: 206Pb/204Pb (17.29), 207Pb/204Pb (15.59) and 208Pb/204Pb (37.51). The Rosh Pinah samples had the following average ratios: 206Pb/204Pb (17.17), 207Pb/204Pb (15.61) and 208Pb/204Pb (37.45). These results indicate lead derivation from the lower 2.0 Ga Eburnean pre-Gariep basement in agreement with and Frimmel et al. (2004). The host felsic metavolcanics might have been derived from melting of the basement rocks during the formation of the Adamastor Ocean. In comparison to the Rosh Pinah deposit lead isotope signatures, the Skorpion lead isotopes overlap with the Rosh Pinah deposit isotopes, but have a much narrower range. This is an indication of a much shorter lived and potentially faster mineralization event contrary to the SEDEX type Rosh Pinah deposit. The smaller tonnage of the Skorpion deposit, its inferior lead concentrations and the elevated radiogenic lead isotopes point toward a VMS deposit which was formed in a small graben fed by shallow conduits during a short lived mineralization event. Sedimentary rocks covered the forming deposit at a fast rate and impaired the deposit advancement. The interaction between the upper crustal rocks and the mineralizing fluids is what may have resulted in the elevated radiogenic lead signature. In contrast to this, SEDEX deposits such as the Rosh Pinah Deposit, are generally fed by deep seated conduits that allow more longer lived leaching of metals from the underlying basement rocks and generally allow minor influence from upper crustal rocks.
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