Origin and metallogenic significance of alkali metasomatism in the Paleoproterozoic Mapedi Formation, Kalahari Manganese Field, South Africa
- Authors: Ikwen, Emmanuella Biye
- Date: 2023-10-13
- Subjects: Metasomatism (Mineralogy) South Africa , Banded iron formations , Kalahari manganese field , Sugilite , Hydrothermal alteration , Quartzite
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424632 , vital:72170
- Description: The occurrence of alkali-rich metasomatic assemblages has been widely reported in various regions of the Kalahari Manganese Field (KMF). This alkali metasomatism has been characterized by the secondary introduction of elements such as K, Na, Li, Ba, P, V, Zn, As, amongst others. This study further explores the possibility of widespread alkali metasomatism in the KMF by reporting on and examining the occurrence of sugilite and other alkali-rich minerals at the contact between the Transvaal and Olifantshoek Supergroups in the Hotazel Mine area of the north-eastern KMF. The lithologies observed at the contact show macroscopic (such as cross cutting veins) and microscopic evidence of hydrothermal alteration. Using analytical methods such as X-ray diffraction, X-ray fluorescence, and scanning electron microscopy, results showed that in the north-eastern region of the KMF, the metasomatism observed at the Transvaal-Olifantshoek contact is mainly characterized by enrichment in sodium, and the occurrence of sodium minerals, predominantly in the form of aegirine. The aegirine forms exclusively in the quartzites of the Mapedi Formation along with minerals such as sugilite, baryte, banalsite, amongst others. Albite also occurs within the quartzites, but also within the Mapedi red shales. The secondary nature of these minerals is established by geochemical comparisons with pristine, as well as alkali-metasomatized samples of the same formation which were obtained from other parts of the KMF and Postmasburg. These comparisons showed that the Mapedi quartzites in the north-eastern KMF have undergone extensive oxidation compared to samples of the same formation which were obtained from Postmasburg. The north-eastern quartzites have an average hematite abundance of 17 wt.% compared to Postmasburg quartzite which have an average of 7 wt.% hematite. Furthermore, some quartzite samples contained up to 40 wt.% in hematite content. The comparisons also showed that Mapedi quartzites from the north-eastern KMF are substantially more sodium enriched compared to Mapedi quartzites from the Postmasburg region, which on average have sodium oxide content below detection limits. Geochemical comparisons were made between pristine Hotazel Formation samples from north-western KMF (Gloria Mine) and samples obtained from the north-eastern KMF (Hotazel Mine). Results showed that the samples obtained from the top of the Hotazel Formation (in the Hotazel mine area) are likely altered hematite lutite and not Banded Iron Formation, evident by their substantially high manganese oxide content (over 30 wt.% in some cases). When compared to pristine samples, the lutite also showed evidence of hydrothermal alteration, predominantly in the form of phosphate and barium enrichment, evident by the occurrence of baryte and apatite. The alkali metasomatism occurring at the contact between the Transvaal and Olifantshoek Supergroups was shown to be predominantly characterized by enrichment in Na, K, Li, Al, Ba, Sr, and P. The metasomatism characterized in this study was also proposed to possibly post-date an earlier metasomatic event which was characterized by leaching of silica and extensive oxidation of the rocks observed at the Transvaal-Olifantshoek contact in the north-eastern KMF. The occurrence of the alkali-rich minerals outlined above geochemically parallels other alkali-rich metasomatic assemblages reported in other parts of the KMF, as well as in the Postmasburg Manganese Field. Thus, based on the consistent occurrence of secondary, alkali-rich mineral assemblages across the KMF, characterized by the common occurrence of aegirine along with minerals such as sugilite and albite, there is evidence of a large-scale alkali metasomatism in the KMF. This study also explores the possible role that the Transvaal-Olifantshoek unconformity might have played in acting as a major conduit for fluid propagation because the observed mineral assemblages occur right at the contact between the Hotazel and Mapedi Formations. The occurrence of the alkali-rich minerals predominantly around the unconformity, as well as the relative depletion of phosphates in stratigraphically deeper parts of the Hotazel suggest that the fluid metasomatism was aided by the Olifantshoek-Transvaal unconformity surface. This study concludes that there is evidence for a strong link between the metasomatism occurring at the contact between the Hotazel and Mapedi formations (in the north-eastern KMF) and what is observed in the broader KMF region. , Thesis (MSc) -- Faculty of Science, Geology, 2023
- Full Text:
- Authors: Ikwen, Emmanuella Biye
- Date: 2023-10-13
- Subjects: Metasomatism (Mineralogy) South Africa , Banded iron formations , Kalahari manganese field , Sugilite , Hydrothermal alteration , Quartzite
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424632 , vital:72170
- Description: The occurrence of alkali-rich metasomatic assemblages has been widely reported in various regions of the Kalahari Manganese Field (KMF). This alkali metasomatism has been characterized by the secondary introduction of elements such as K, Na, Li, Ba, P, V, Zn, As, amongst others. This study further explores the possibility of widespread alkali metasomatism in the KMF by reporting on and examining the occurrence of sugilite and other alkali-rich minerals at the contact between the Transvaal and Olifantshoek Supergroups in the Hotazel Mine area of the north-eastern KMF. The lithologies observed at the contact show macroscopic (such as cross cutting veins) and microscopic evidence of hydrothermal alteration. Using analytical methods such as X-ray diffraction, X-ray fluorescence, and scanning electron microscopy, results showed that in the north-eastern region of the KMF, the metasomatism observed at the Transvaal-Olifantshoek contact is mainly characterized by enrichment in sodium, and the occurrence of sodium minerals, predominantly in the form of aegirine. The aegirine forms exclusively in the quartzites of the Mapedi Formation along with minerals such as sugilite, baryte, banalsite, amongst others. Albite also occurs within the quartzites, but also within the Mapedi red shales. The secondary nature of these minerals is established by geochemical comparisons with pristine, as well as alkali-metasomatized samples of the same formation which were obtained from other parts of the KMF and Postmasburg. These comparisons showed that the Mapedi quartzites in the north-eastern KMF have undergone extensive oxidation compared to samples of the same formation which were obtained from Postmasburg. The north-eastern quartzites have an average hematite abundance of 17 wt.% compared to Postmasburg quartzite which have an average of 7 wt.% hematite. Furthermore, some quartzite samples contained up to 40 wt.% in hematite content. The comparisons also showed that Mapedi quartzites from the north-eastern KMF are substantially more sodium enriched compared to Mapedi quartzites from the Postmasburg region, which on average have sodium oxide content below detection limits. Geochemical comparisons were made between pristine Hotazel Formation samples from north-western KMF (Gloria Mine) and samples obtained from the north-eastern KMF (Hotazel Mine). Results showed that the samples obtained from the top of the Hotazel Formation (in the Hotazel mine area) are likely altered hematite lutite and not Banded Iron Formation, evident by their substantially high manganese oxide content (over 30 wt.% in some cases). When compared to pristine samples, the lutite also showed evidence of hydrothermal alteration, predominantly in the form of phosphate and barium enrichment, evident by the occurrence of baryte and apatite. The alkali metasomatism occurring at the contact between the Transvaal and Olifantshoek Supergroups was shown to be predominantly characterized by enrichment in Na, K, Li, Al, Ba, Sr, and P. The metasomatism characterized in this study was also proposed to possibly post-date an earlier metasomatic event which was characterized by leaching of silica and extensive oxidation of the rocks observed at the Transvaal-Olifantshoek contact in the north-eastern KMF. The occurrence of the alkali-rich minerals outlined above geochemically parallels other alkali-rich metasomatic assemblages reported in other parts of the KMF, as well as in the Postmasburg Manganese Field. Thus, based on the consistent occurrence of secondary, alkali-rich mineral assemblages across the KMF, characterized by the common occurrence of aegirine along with minerals such as sugilite and albite, there is evidence of a large-scale alkali metasomatism in the KMF. This study also explores the possible role that the Transvaal-Olifantshoek unconformity might have played in acting as a major conduit for fluid propagation because the observed mineral assemblages occur right at the contact between the Hotazel and Mapedi Formations. The occurrence of the alkali-rich minerals predominantly around the unconformity, as well as the relative depletion of phosphates in stratigraphically deeper parts of the Hotazel suggest that the fluid metasomatism was aided by the Olifantshoek-Transvaal unconformity surface. This study concludes that there is evidence for a strong link between the metasomatism occurring at the contact between the Hotazel and Mapedi formations (in the north-eastern KMF) and what is observed in the broader KMF region. , Thesis (MSc) -- Faculty of Science, Geology, 2023
- Full Text:
Chemostratigraphy of the lowermost iron-manganese cycle of the Hotazel Formation, and implications for its primary depositional environment
- Authors: Masoabi, Ntseka Thomas
- Date: 2022-10-14
- Subjects: Chemostratigraphy , Great Oxygenation Event , Manganese ores Geology South Africa Northern Cape , Banded iron formation
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362938 , vital:65376
- Description: The giant Kalahari Manganese Field (KMF), located in the Northern Cape Province, South Africa, comprises approximately half of the world’s manganese resources, estimated at about eight billion tons at grades ranging from 20-48 wt%. The KMF is linked to a period in geological time when the Earth’s atmospheric and oceanic conditions underwent a major transition from oxygen-deficient to oxygen-enriched conditions – an event famously referred to as the Great Oxidation Event (GOE) that occurred around 2.4 Ga. The KMF deposits are hosted in Banded Iron Formation (BIF) of the Paleoproterozoic Hotazel Formation in the uppermost Transvaal Supergroup. The sedimentary Mn ores are interbedded with Hotazel BIF in the form of three alternating depositional cycles of BIF, transitional hematite lutite and laminated, carbonate-rich manganese ore. The lowermost and thickest of the three cycles is the most economically significant and has been mined for several decades on a large scale from the southernmost KMF. In this study, two drill cores from the southern KMF were inspected, logged and sampled at a high resolution of approximately half-meter interval per sample. The selected cores, namely G774, capturing the lower portion of the Hotazel Formation from the Mamatwan locality, and MP-56, capturing the corresponding portion from the Middleplaats locality, are geographically proximal to each other, with a horizontal distance of roughly 3 km separating the two of them. The G774 drill core is characterized by a conspicuously thick manganese layer covering a thickness of 50 m, with the overlying BIF reaching a total thickness of 11 m. The MP-56 drill core, on the other hand, has a relatively thinner corresponding manganese layer of 30 m in thickness, while the overlying BIF layer exhibits a thickness of 24 m. The extent of sampling up-section was constrained by an apparently coeval black shale layer which represents the chosen upper stratigraphic marker for the lower part of the Hotazel section in the broader area that is under investigation in this thesis. That way, a high resolution chemostratigraphic approach was employed to elucidate the potential factors contributing to the relative sedimentary lateral thickness variations seen across the southernmost KMF. High-resolution geochemical data were used to explore relationships and signals that might constrain relative precipitation rates for iron and manganese against detrital species, fluctuating redox conditions in the original environment of deposition, and chemostratigraphic correlation. All geochemical data (i.e., major oxides, minor and trace elements and carbonate carbon isotopes) were obtained respectively through employing X-ray Fluorescence (XRF), Laser Ablation Inductively Coupled Mass Spectrometry (ICP-MS), and Gas-source mass spectrometry. Comparative considerations made between the bulk geochemistry of the two sequences (i.e., Mamatwan and Middleplaats sections) reveal that periods of high-Mn deposition in the Hotazel Formation appear to be very Ca-carbonate rich (as indicated by high CaO, LOI and Sr concentrations). This, in turn, suggests that the Mn abundance is in the Hotazel ores is controlled mainly by the silicate phase braunite and is diluted by the deposition of Ca-carbonate through time. Bulk-rock concentration results for trace elements of the High Field Strength Element (HFSE) group (namely Zr, Hf, Y, Nb and Sc) were utilized to constrain the rates of either clastic and/or volcanic detrital inputs, as they traditionally represent refractory mineral particles of a common detrital/volcanic origin. The two chemosedimentary sequences preserve these elements in very low and thus quantitatively negligible concentrations – suggesting that the Hotazel depositional environment received very low and insignificant influx of a terrigeneous detrital component. A selection of these elements was therefore used to deduce, with caution, the relative as opposed to absolute precipitation rate of the major chemical constituents (i.e., Fe + Si vs Mn + carbonate), assuming a constant detrital flux through time. It was found that the relative abundances of Zr, Y and Nb is roughly 1.5 – 2 times as high in the BIF lithofacies relative to the Mn ones at both localities. This led to the inference that the Mn-enriched portion of the sediment must have been deposited at approximately twice the rate that the Fe-rich (BIF) portion was originally deposited. In terms of redox-sensitive elements, the elements Co and Mo seem to reveal the most valuable insights into the redox environment of primary chemical deposition. Cobalt displays a unique pattern in that its highest concentration is attained at the hematite lutite transitions (similarly with the REE in this regard), while very low and seemingly invariant concentration is exhibited within the core of the main orebodies. The same pattern seems to be reproduced to a degree by the corresponding bulk MgO component, whereby MgO abundance maxima are associated with the basal hematite lutite and the hematitic flanks of the Mn-ore zone, while the core of the Mn-rich layer attains relatively low and essentially invariant MgO concentrations. This implicates a close and direct association of Co with the hematite fraction of the rocks and a concurrent enrichment in Mn-rich carbonate (dolomite). On the other hand, Mo seems to have a direct and clear association with peak MnO2 content of the rocks, which in turn presents a high possibility of Mo having adsorbed onto primary Mn-oxyhydroxides in the water column, thus providing evidence that Mn-oxide must have acted as an important Mo sink, at least locally. Finally, the carbonate-carbon isotope results provide a useful tool that brings the two stratigraphic sections “together“, in conjunction with other correlatable chemostratigraphic parameters (e.g. Co, Mg). The results demonstrate that bulk carbon fluxes and isotopic signals in the sediments must reflect primary processes of deposition, and that correlation across two apparently disparate lithostratigraphic sections can be effected. The key finding is that, at times, manganese deposition in one part of a vii stratified basin was evidently accompanied by simultaneous BIF deposition at another, thus painting a very complex picture of massive primary chemical precipitation of Fe and Mn at the dawn of the GOE. , Thesis (MSc) -- Faculty of Science, Geology, 2022
- Full Text:
- Authors: Masoabi, Ntseka Thomas
- Date: 2022-10-14
- Subjects: Chemostratigraphy , Great Oxygenation Event , Manganese ores Geology South Africa Northern Cape , Banded iron formation
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362938 , vital:65376
- Description: The giant Kalahari Manganese Field (KMF), located in the Northern Cape Province, South Africa, comprises approximately half of the world’s manganese resources, estimated at about eight billion tons at grades ranging from 20-48 wt%. The KMF is linked to a period in geological time when the Earth’s atmospheric and oceanic conditions underwent a major transition from oxygen-deficient to oxygen-enriched conditions – an event famously referred to as the Great Oxidation Event (GOE) that occurred around 2.4 Ga. The KMF deposits are hosted in Banded Iron Formation (BIF) of the Paleoproterozoic Hotazel Formation in the uppermost Transvaal Supergroup. The sedimentary Mn ores are interbedded with Hotazel BIF in the form of three alternating depositional cycles of BIF, transitional hematite lutite and laminated, carbonate-rich manganese ore. The lowermost and thickest of the three cycles is the most economically significant and has been mined for several decades on a large scale from the southernmost KMF. In this study, two drill cores from the southern KMF were inspected, logged and sampled at a high resolution of approximately half-meter interval per sample. The selected cores, namely G774, capturing the lower portion of the Hotazel Formation from the Mamatwan locality, and MP-56, capturing the corresponding portion from the Middleplaats locality, are geographically proximal to each other, with a horizontal distance of roughly 3 km separating the two of them. The G774 drill core is characterized by a conspicuously thick manganese layer covering a thickness of 50 m, with the overlying BIF reaching a total thickness of 11 m. The MP-56 drill core, on the other hand, has a relatively thinner corresponding manganese layer of 30 m in thickness, while the overlying BIF layer exhibits a thickness of 24 m. The extent of sampling up-section was constrained by an apparently coeval black shale layer which represents the chosen upper stratigraphic marker for the lower part of the Hotazel section in the broader area that is under investigation in this thesis. That way, a high resolution chemostratigraphic approach was employed to elucidate the potential factors contributing to the relative sedimentary lateral thickness variations seen across the southernmost KMF. High-resolution geochemical data were used to explore relationships and signals that might constrain relative precipitation rates for iron and manganese against detrital species, fluctuating redox conditions in the original environment of deposition, and chemostratigraphic correlation. All geochemical data (i.e., major oxides, minor and trace elements and carbonate carbon isotopes) were obtained respectively through employing X-ray Fluorescence (XRF), Laser Ablation Inductively Coupled Mass Spectrometry (ICP-MS), and Gas-source mass spectrometry. Comparative considerations made between the bulk geochemistry of the two sequences (i.e., Mamatwan and Middleplaats sections) reveal that periods of high-Mn deposition in the Hotazel Formation appear to be very Ca-carbonate rich (as indicated by high CaO, LOI and Sr concentrations). This, in turn, suggests that the Mn abundance is in the Hotazel ores is controlled mainly by the silicate phase braunite and is diluted by the deposition of Ca-carbonate through time. Bulk-rock concentration results for trace elements of the High Field Strength Element (HFSE) group (namely Zr, Hf, Y, Nb and Sc) were utilized to constrain the rates of either clastic and/or volcanic detrital inputs, as they traditionally represent refractory mineral particles of a common detrital/volcanic origin. The two chemosedimentary sequences preserve these elements in very low and thus quantitatively negligible concentrations – suggesting that the Hotazel depositional environment received very low and insignificant influx of a terrigeneous detrital component. A selection of these elements was therefore used to deduce, with caution, the relative as opposed to absolute precipitation rate of the major chemical constituents (i.e., Fe + Si vs Mn + carbonate), assuming a constant detrital flux through time. It was found that the relative abundances of Zr, Y and Nb is roughly 1.5 – 2 times as high in the BIF lithofacies relative to the Mn ones at both localities. This led to the inference that the Mn-enriched portion of the sediment must have been deposited at approximately twice the rate that the Fe-rich (BIF) portion was originally deposited. In terms of redox-sensitive elements, the elements Co and Mo seem to reveal the most valuable insights into the redox environment of primary chemical deposition. Cobalt displays a unique pattern in that its highest concentration is attained at the hematite lutite transitions (similarly with the REE in this regard), while very low and seemingly invariant concentration is exhibited within the core of the main orebodies. The same pattern seems to be reproduced to a degree by the corresponding bulk MgO component, whereby MgO abundance maxima are associated with the basal hematite lutite and the hematitic flanks of the Mn-ore zone, while the core of the Mn-rich layer attains relatively low and essentially invariant MgO concentrations. This implicates a close and direct association of Co with the hematite fraction of the rocks and a concurrent enrichment in Mn-rich carbonate (dolomite). On the other hand, Mo seems to have a direct and clear association with peak MnO2 content of the rocks, which in turn presents a high possibility of Mo having adsorbed onto primary Mn-oxyhydroxides in the water column, thus providing evidence that Mn-oxide must have acted as an important Mo sink, at least locally. Finally, the carbonate-carbon isotope results provide a useful tool that brings the two stratigraphic sections “together“, in conjunction with other correlatable chemostratigraphic parameters (e.g. Co, Mg). The results demonstrate that bulk carbon fluxes and isotopic signals in the sediments must reflect primary processes of deposition, and that correlation across two apparently disparate lithostratigraphic sections can be effected. The key finding is that, at times, manganese deposition in one part of a vii stratified basin was evidently accompanied by simultaneous BIF deposition at another, thus painting a very complex picture of massive primary chemical precipitation of Fe and Mn at the dawn of the GOE. , Thesis (MSc) -- Faculty of Science, Geology, 2022
- Full Text:
Lateral and vertical mineral-chemical variation in high-grade ores of the Kalahari Manganese Field, and implications for ore genesis and geometallurgy
- Authors: Motilaodi, Donald
- Date: 2022-10-14
- Subjects: Manganese ores , Geometallurgy , Hydrothermal alteration , Petrology , Mineralogy , Geochemistry
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362972 , vital:65379
- Description: The Kalahari Manganese Field (KMF) is a world-class resource of manganese ore hosted by the Paleoproterozoic Hotazel banded iron formation. KMF ores are categorised into two main types, i.e., low-grade, carbonate rich, braunitic ore (Mn≤40wt%) and carbonate-free, high-grade, Ca-braunite+hausmannite ore (Mn≥44wt%). High-grade ores, also known as Wessels type from the homonymous mine in the northernmost KMF, are thought to have formed from variable degrees of hydrothermal carbonate and silica leaching from a low-grade ore precursor, termed Mamatwan-type after the homonymous mine in the southernmost KMF. This project aims to conduct a mineralogical and mineral-chemical study of representative manganese ore samples from a suite of drillcores intersecting both the upper and the lower layers in the northern KMF, covering the areas of Wessels, N’chwaning and Gloria mines. Petrographically, the high-grade Mn ore displays great variability in three-dimensional space. Texturally, the ores exhibit a great variety of textures which may or may not show preservation of the laminated and ovoidal textures that typify the postulated low-grade protore. There is also significant variation in the mineralogical and geochemical characteristics of the high-grade Mn ores both vertically and laterally. Vertical variation includes, probably for the first time, variability between the upper and lower ore layers within individual drillcores of the Hotazel sequence. Mineralogically, the ores contain variable modal abundances of the ore-forming minerals braunite (I, II, “new”) and hausmannite, and much less so of bixbyite, marokite and manganite. Common accessories include andradite, barite and low-Mn carbonate minerals. Chemically, the dominant ore minerals braunite and hausmannite, contain Fe up to 22 and 15wt% respectively, which accounts for the bulk of the iron contained in the ores. Braunite compositions also exhibit a large range with respect to their ratio of Ca/Si. Mineral-specific trace element concentrations for the same minerals measured by LA-ICP-MS, reveal generally large variations from one element to the other. When normalized against the trace element composition of bulk low-grade precursor ore, strong enrichments are recorded for both hausmannite and braunite in selected alkali/alkali earth elements, transition metals and lanthanides, such as Sc, Co, Zn, Cu, Pb, La, and Ce. These are akin to enrichments recorded in average high-grade ore. Although there is also no obvious relationship between Fe content in both hausmannite and braunite and their trace element abundances, the drillcore that captures high-grade ore with the highest trace element concentrations appears to be located most proximal to a major fault. Results collectively suggest that high-grade Mn ores of the KMF have undergone a complex hydrothermal history with a clear and significant metasomatic addition of trace elements into ore-forming minerals. First order trends in the mineralogical and mineral-chemical distribution of the ores in space, suggest hausmannite-dominated ores near the Hotazel suboutcrop, and an apparent decline in ore quality with braunite II-andradite-barite-calcite ores as the major graben fault is approached in a southwesterly direction. The latter trend appears to be at odds with prevailing fault-controlled alteration models. Elucidating that hydrothermal history of the Wessels-type high grade Mn ores of the KMF, will be crucial to understanding the compositional controls of these ores in space, and the potential impact thereof in terms of geometallurgy. , Thesis (MSc) -- Faculty of Science, Geology, 2022
- Full Text:
- Authors: Motilaodi, Donald
- Date: 2022-10-14
- Subjects: Manganese ores , Geometallurgy , Hydrothermal alteration , Petrology , Mineralogy , Geochemistry
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362972 , vital:65379
- Description: The Kalahari Manganese Field (KMF) is a world-class resource of manganese ore hosted by the Paleoproterozoic Hotazel banded iron formation. KMF ores are categorised into two main types, i.e., low-grade, carbonate rich, braunitic ore (Mn≤40wt%) and carbonate-free, high-grade, Ca-braunite+hausmannite ore (Mn≥44wt%). High-grade ores, also known as Wessels type from the homonymous mine in the northernmost KMF, are thought to have formed from variable degrees of hydrothermal carbonate and silica leaching from a low-grade ore precursor, termed Mamatwan-type after the homonymous mine in the southernmost KMF. This project aims to conduct a mineralogical and mineral-chemical study of representative manganese ore samples from a suite of drillcores intersecting both the upper and the lower layers in the northern KMF, covering the areas of Wessels, N’chwaning and Gloria mines. Petrographically, the high-grade Mn ore displays great variability in three-dimensional space. Texturally, the ores exhibit a great variety of textures which may or may not show preservation of the laminated and ovoidal textures that typify the postulated low-grade protore. There is also significant variation in the mineralogical and geochemical characteristics of the high-grade Mn ores both vertically and laterally. Vertical variation includes, probably for the first time, variability between the upper and lower ore layers within individual drillcores of the Hotazel sequence. Mineralogically, the ores contain variable modal abundances of the ore-forming minerals braunite (I, II, “new”) and hausmannite, and much less so of bixbyite, marokite and manganite. Common accessories include andradite, barite and low-Mn carbonate minerals. Chemically, the dominant ore minerals braunite and hausmannite, contain Fe up to 22 and 15wt% respectively, which accounts for the bulk of the iron contained in the ores. Braunite compositions also exhibit a large range with respect to their ratio of Ca/Si. Mineral-specific trace element concentrations for the same minerals measured by LA-ICP-MS, reveal generally large variations from one element to the other. When normalized against the trace element composition of bulk low-grade precursor ore, strong enrichments are recorded for both hausmannite and braunite in selected alkali/alkali earth elements, transition metals and lanthanides, such as Sc, Co, Zn, Cu, Pb, La, and Ce. These are akin to enrichments recorded in average high-grade ore. Although there is also no obvious relationship between Fe content in both hausmannite and braunite and their trace element abundances, the drillcore that captures high-grade ore with the highest trace element concentrations appears to be located most proximal to a major fault. Results collectively suggest that high-grade Mn ores of the KMF have undergone a complex hydrothermal history with a clear and significant metasomatic addition of trace elements into ore-forming minerals. First order trends in the mineralogical and mineral-chemical distribution of the ores in space, suggest hausmannite-dominated ores near the Hotazel suboutcrop, and an apparent decline in ore quality with braunite II-andradite-barite-calcite ores as the major graben fault is approached in a southwesterly direction. The latter trend appears to be at odds with prevailing fault-controlled alteration models. Elucidating that hydrothermal history of the Wessels-type high grade Mn ores of the KMF, will be crucial to understanding the compositional controls of these ores in space, and the potential impact thereof in terms of geometallurgy. , Thesis (MSc) -- Faculty of Science, Geology, 2022
- Full Text:
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