Geochemistry and mineralogy of supergene altered manganese ore below the Kalahari unconformity in the Kalahari manganese field, Northern Cape Province, South Africa
- Authors: Du Plooy, Andries Petrus
- Date: 2009-01-28T09:38:57Z
- Subjects: Geology , Geochemistry , Mineralogy , Petrology , Manganese ores , Northern Cape (South Africa)
- Type: Thesis
- Identifier: uj:14834 , http://hdl.handle.net/10210/1958
- Description: M.Sc. , It is the focus of the study to qualitatively describe and then quantify the mineralogical and geochemical changes associated with the supergene alteration of carbonate-rich braunite lutite (Mamatwan-type ore) immediately below the Kalahari unconformity along the southeastern suboutcrop perimeter of the Hotazel Formation in the Kalahari deposit. It was also the objective of this study to determine the timing and duration of supergene alteration. Samples for polished thin sections were carefully selected from eight representative boreholes to be representative of all the lithostratigraphic zones and ore types. The thin sections were used to study mineralogy by means of reflected light microscopy and scanning electron microscopy. X-ray powder diffractometry on representative powder samples were used to study the mineralogy and geochemistry of the samples. Microprobe analyses were also performed on the representative samples. Finally the samples were submitted for 40Ar/39Ar geochronology. In this supergene enrichment zone carbonates are leached (associated with an increase in porosity) and Mn2+/Mn3+ -bearing minerals (kutnahorite, Mn-calcite an braunite) are altered to supergene Mn4+-bearing mineral phases (todorokite and manganomelane) and minor quartz. This process upgrades ore from 38 wt% Mn to ore with more than 40 wt% Mn. Element fluxes, enrichment and depletion of major and trace elements were quantified by mass balance calculations. Na2O, K2O, Sr, Ba, Zn and H2O were enriched, while Mn3O4, Fe2O3, CaO, MgO, P, B and CO2 were leached from the ore during supergene alteration. Results of this study suggest that the development of Post African I erosional surface may have taken place 45 Ma ago. The bottom of the weathering profile gives a well-defined peak at ca. 5 Ma that may possible coincide with the development of Post African II erosional surface. The major characteristics of the alteration process of the unaltered Mamatwan-type ore to supergene altered braunite lutite can be summarized as follow: • Leaching of Mn carbonates and Mn2+/Mn3+-oxides. • Formation of Mn4+-oxyhydroxides and quartz. • Decrease in relative density of the ore. • Increase in porosity of the ore. • Leaching of Mn3O4, Fe2O3, CaO, MgO, P, B, CO2. • Enrichment of Na2O, K2O, Sr, Ba, Zn, H2O. Chemical weathering processes along the Cenozoic Kalahari unconformity appear to have affected the manganiferous lithologies of the Hotazel Formation from 45 Ma onwards to 5 Ma. The weathering front processes very slowly through the Mn-rich braunite lutite (<10m in 40 Ma; <0.25m/Ma); producing a very uniform and microcrystalline supergene mineral assemblage with distinct characteristics.
- Full Text:
- Authors: Du Plooy, Andries Petrus
- Date: 2009-01-28T09:38:57Z
- Subjects: Geology , Geochemistry , Mineralogy , Petrology , Manganese ores , Northern Cape (South Africa)
- Type: Thesis
- Identifier: uj:14834 , http://hdl.handle.net/10210/1958
- Description: M.Sc. , It is the focus of the study to qualitatively describe and then quantify the mineralogical and geochemical changes associated with the supergene alteration of carbonate-rich braunite lutite (Mamatwan-type ore) immediately below the Kalahari unconformity along the southeastern suboutcrop perimeter of the Hotazel Formation in the Kalahari deposit. It was also the objective of this study to determine the timing and duration of supergene alteration. Samples for polished thin sections were carefully selected from eight representative boreholes to be representative of all the lithostratigraphic zones and ore types. The thin sections were used to study mineralogy by means of reflected light microscopy and scanning electron microscopy. X-ray powder diffractometry on representative powder samples were used to study the mineralogy and geochemistry of the samples. Microprobe analyses were also performed on the representative samples. Finally the samples were submitted for 40Ar/39Ar geochronology. In this supergene enrichment zone carbonates are leached (associated with an increase in porosity) and Mn2+/Mn3+ -bearing minerals (kutnahorite, Mn-calcite an braunite) are altered to supergene Mn4+-bearing mineral phases (todorokite and manganomelane) and minor quartz. This process upgrades ore from 38 wt% Mn to ore with more than 40 wt% Mn. Element fluxes, enrichment and depletion of major and trace elements were quantified by mass balance calculations. Na2O, K2O, Sr, Ba, Zn and H2O were enriched, while Mn3O4, Fe2O3, CaO, MgO, P, B and CO2 were leached from the ore during supergene alteration. Results of this study suggest that the development of Post African I erosional surface may have taken place 45 Ma ago. The bottom of the weathering profile gives a well-defined peak at ca. 5 Ma that may possible coincide with the development of Post African II erosional surface. The major characteristics of the alteration process of the unaltered Mamatwan-type ore to supergene altered braunite lutite can be summarized as follow: • Leaching of Mn carbonates and Mn2+/Mn3+-oxides. • Formation of Mn4+-oxyhydroxides and quartz. • Decrease in relative density of the ore. • Increase in porosity of the ore. • Leaching of Mn3O4, Fe2O3, CaO, MgO, P, B, CO2. • Enrichment of Na2O, K2O, Sr, Ba, Zn, H2O. Chemical weathering processes along the Cenozoic Kalahari unconformity appear to have affected the manganiferous lithologies of the Hotazel Formation from 45 Ma onwards to 5 Ma. The weathering front processes very slowly through the Mn-rich braunite lutite (<10m in 40 Ma; <0.25m/Ma); producing a very uniform and microcrystalline supergene mineral assemblage with distinct characteristics.
- Full Text:
Metasedimentary manganese ores of the Serra do Navio deposit, Amapa Province, Brazil
- Authors: Chisonga, Benny Chanda
- Date: 2009-01-27T07:18:17Z
- Subjects: Geology , Manganese ores , Petrology , Mineralogy , Fluid inclusions , Geochemistry , Amapá (Territory) Brazil
- Type: Thesis
- Identifier: uj:14828 , http://hdl.handle.net/10210/1952
- Description: M.Sc. , Please refer to full text to view abstract
- Full Text:
- Authors: Chisonga, Benny Chanda
- Date: 2009-01-27T07:18:17Z
- Subjects: Geology , Manganese ores , Petrology , Mineralogy , Fluid inclusions , Geochemistry , Amapá (Territory) Brazil
- Type: Thesis
- Identifier: uj:14828 , http://hdl.handle.net/10210/1952
- Description: M.Sc. , Please refer to full text to view abstract
- Full Text:
Characterisation of the lowermost manganese ore bed of the Hotazel Formation, Gloria Mine, Northern Cape Province
- Authors: Van Staden, Anelda
- Date: 2009-01-29T12:09:24Z
- Subjects: Geology , Manganese ores , Northern Cape (South Africa)
- Type: Thesis
- Identifier: uj:14853 , http://hdl.handle.net/10210/1975
- Description: M.Sc. , This dissertation describes the N1 manganese ore bed at Gloria Mine in the Kalahari Manganese Field, Northern Cape Province. It also compares the ore bed at Gloria Mine with the correlative bed further to the south at Mamatwan Mine. The ore bed at Gloria Mine can be subdivided into ten texturally distinct zones that are laterally consistent throughout the mine lease area. The mineralogy and geochemistry of the various lithostratigraphic zones are described from two drill cores (GL28 and GL24), situated away from any known structural features or unconformities that could have affected the properties of the Ore. The ore in drill core GL28 has a mineralogical composition similar to that of typical Mamatwan-type ore described at Mamatwan Mine with braunite and kutnahorite as the main minerals. However, in drill core GL24 the ore has a very different mineralogical composition although it is texturally and geochemically rather similar to Mamatwan-type ore. The ore is composed of hausmannite, calcite and jacobsite and is apparently related to a post-depositional alteration event that did not effect Mamatwan-type ore in the Mamatwan Mine area. This altered ore is similar in composition to low-grade leastaltered manganese ores in the cores of fault blocks at Wessels and N’Chwaning Mines i.e. the area known for its hydrothermally altered high-grade manganese ores in the northern part of the Kalahari Manganese Field. In addition to the above, the N1 manganese ore bed at Gloria Mine also underwent ferruginisation close to certain joints and normal faults. No obvious alteration could be detected where the ore bed is unconformably overlain by Dwyka diamictite, nor associated with a thrust fault displacing the ore.
- Full Text:
- Authors: Van Staden, Anelda
- Date: 2009-01-29T12:09:24Z
- Subjects: Geology , Manganese ores , Northern Cape (South Africa)
- Type: Thesis
- Identifier: uj:14853 , http://hdl.handle.net/10210/1975
- Description: M.Sc. , This dissertation describes the N1 manganese ore bed at Gloria Mine in the Kalahari Manganese Field, Northern Cape Province. It also compares the ore bed at Gloria Mine with the correlative bed further to the south at Mamatwan Mine. The ore bed at Gloria Mine can be subdivided into ten texturally distinct zones that are laterally consistent throughout the mine lease area. The mineralogy and geochemistry of the various lithostratigraphic zones are described from two drill cores (GL28 and GL24), situated away from any known structural features or unconformities that could have affected the properties of the Ore. The ore in drill core GL28 has a mineralogical composition similar to that of typical Mamatwan-type ore described at Mamatwan Mine with braunite and kutnahorite as the main minerals. However, in drill core GL24 the ore has a very different mineralogical composition although it is texturally and geochemically rather similar to Mamatwan-type ore. The ore is composed of hausmannite, calcite and jacobsite and is apparently related to a post-depositional alteration event that did not effect Mamatwan-type ore in the Mamatwan Mine area. This altered ore is similar in composition to low-grade leastaltered manganese ores in the cores of fault blocks at Wessels and N’Chwaning Mines i.e. the area known for its hydrothermally altered high-grade manganese ores in the northern part of the Kalahari Manganese Field. In addition to the above, the N1 manganese ore bed at Gloria Mine also underwent ferruginisation close to certain joints and normal faults. No obvious alteration could be detected where the ore bed is unconformably overlain by Dwyka diamictite, nor associated with a thrust fault displacing the ore.
- Full Text:
A geometallurgical evaluation of the ores of the northern Kalahari manganese deposit, South Africa
- Authors: Chetty, Deshenthree
- Date: 2010-04-19T07:55:36Z
- Subjects: Geology , Manganese ores , Kuruman (South Africa)
- Type: Thesis
- Identifier: uj:6794 , http://hdl.handle.net/10210/3223
- Description: D. Phil. , The Kalahari Manganese Deposit (KMD) is the largest of five erosional relics of the Hotazel Formation that are located near Kuruman in the Northern Cape Province of South Africa. Manganese ores are exploited from the lowermost of three manganiferous beds that are interbedded with banded iron-formation (BIF) and hematite lutite, that together constitute the Hotazel Formation. Two major ore types have been delineated previously, viz. low grade braunite lutite of the Mamatwan-type, and high grade oxidic ores of the Wessels-type, with the latter spatially restricted to the northern KMD. Genesis of the ores was temporally distinct, with the Mamatwan-type ore considered as a sedimentary-diagenetic precursor to the hydrothermally altered Wessels-type ore. Drill core samples from the Nchwaning-Gloria area of the northern KMD were analysed, with the aim to better characterise ore genesis, with emphasis on ore alteration. A second part of the study aimed at the application of mineralogical and geochemical information to aspects of ore smelting for the production of Mn alloy for use in the steel industry. Methods employed were drill core logging, X-ray diffraction (XRD), petrography, electron probe microanalysis (EPMA), major and trace element (including REE) analysis (employing artificial neural networks for evaluation of elemental trends), and stable isotope (C and O) analysis. Significant effort was invested in method development for quantitative mineralogical modal analysis using Rietveld refinement of XRD data. The study shows that a number of ore types can be differentiated in the northern KMD on the basis of mineral assemblage, grade, texture and geochemical characteristics. The ores are broadly classified into least altered (LA), partially altered (PA) and advanced altered (AA) types. The LA ores are low grade (<40 wt%Mn) Mn lutites, with dolomite-group carbonate a significant component in addition to braunite. Serpentine is a ubiquitous trace mineral, and boron is a characteristic trace element hosted predominantly by braunite in these ores. Ores of the PA type comprise either braunite-hausmannite-calcite or hausmannite-calcite assemblages, are fine to coarse grained, and display intermediate Mn grades (40-45 wt%Mn). They exhibit a transitional trace element signature. Advanced altered ores may be classified into five different types, based on mineral assemblages that contain hausmannite and/or braunite as significant minerals. Carbonates occur predominantly in the form of calcite, present in minor to trace proportions. Textures vary from fine to very coarse grained, and high Mn grades (typically >45 wt%Mn), are recorded. Trace elements of significance include Zn, associated with hausmannite, B, associated with massive braunite and a number of trace minerals, and P, typically present in trace quantities of apatite. In terms of ore genesis, mineralogical, geochemical and geological considerations suggest that Mn (and Fe) originated from submarine hydrothermal vents, from which it travelled in hydrothermal plumes, prior to rapid deposition ~2.2 Ga ago. Diagenesis followed soon after deposition, through redox reactions involving organic matter and higher oxides of Mn to produce the braunite-carbonate assemblage primarily observed in LA ores. The carbonate:oxide ratio and nature of the carbonates varied slightly depending on fluctuations in organic matter flux to the sediment, as well as marine bicarbonate concentrations. Metamorphism, in relation to diagenesis and metasomatism, is poorly understood, but is perceived to have resulted in serpentine formation, as observed in LA and PA ores.
- Full Text:
- Authors: Chetty, Deshenthree
- Date: 2010-04-19T07:55:36Z
- Subjects: Geology , Manganese ores , Kuruman (South Africa)
- Type: Thesis
- Identifier: uj:6794 , http://hdl.handle.net/10210/3223
- Description: D. Phil. , The Kalahari Manganese Deposit (KMD) is the largest of five erosional relics of the Hotazel Formation that are located near Kuruman in the Northern Cape Province of South Africa. Manganese ores are exploited from the lowermost of three manganiferous beds that are interbedded with banded iron-formation (BIF) and hematite lutite, that together constitute the Hotazel Formation. Two major ore types have been delineated previously, viz. low grade braunite lutite of the Mamatwan-type, and high grade oxidic ores of the Wessels-type, with the latter spatially restricted to the northern KMD. Genesis of the ores was temporally distinct, with the Mamatwan-type ore considered as a sedimentary-diagenetic precursor to the hydrothermally altered Wessels-type ore. Drill core samples from the Nchwaning-Gloria area of the northern KMD were analysed, with the aim to better characterise ore genesis, with emphasis on ore alteration. A second part of the study aimed at the application of mineralogical and geochemical information to aspects of ore smelting for the production of Mn alloy for use in the steel industry. Methods employed were drill core logging, X-ray diffraction (XRD), petrography, electron probe microanalysis (EPMA), major and trace element (including REE) analysis (employing artificial neural networks for evaluation of elemental trends), and stable isotope (C and O) analysis. Significant effort was invested in method development for quantitative mineralogical modal analysis using Rietveld refinement of XRD data. The study shows that a number of ore types can be differentiated in the northern KMD on the basis of mineral assemblage, grade, texture and geochemical characteristics. The ores are broadly classified into least altered (LA), partially altered (PA) and advanced altered (AA) types. The LA ores are low grade (<40 wt%Mn) Mn lutites, with dolomite-group carbonate a significant component in addition to braunite. Serpentine is a ubiquitous trace mineral, and boron is a characteristic trace element hosted predominantly by braunite in these ores. Ores of the PA type comprise either braunite-hausmannite-calcite or hausmannite-calcite assemblages, are fine to coarse grained, and display intermediate Mn grades (40-45 wt%Mn). They exhibit a transitional trace element signature. Advanced altered ores may be classified into five different types, based on mineral assemblages that contain hausmannite and/or braunite as significant minerals. Carbonates occur predominantly in the form of calcite, present in minor to trace proportions. Textures vary from fine to very coarse grained, and high Mn grades (typically >45 wt%Mn), are recorded. Trace elements of significance include Zn, associated with hausmannite, B, associated with massive braunite and a number of trace minerals, and P, typically present in trace quantities of apatite. In terms of ore genesis, mineralogical, geochemical and geological considerations suggest that Mn (and Fe) originated from submarine hydrothermal vents, from which it travelled in hydrothermal plumes, prior to rapid deposition ~2.2 Ga ago. Diagenesis followed soon after deposition, through redox reactions involving organic matter and higher oxides of Mn to produce the braunite-carbonate assemblage primarily observed in LA ores. The carbonate:oxide ratio and nature of the carbonates varied slightly depending on fluctuations in organic matter flux to the sediment, as well as marine bicarbonate concentrations. Metamorphism, in relation to diagenesis and metasomatism, is poorly understood, but is perceived to have resulted in serpentine formation, as observed in LA and PA ores.
- Full Text:
Ongeluk volcanism in relation to the Kalahari manganese deposits
- Authors: Schutte, Sabine Silke
- Date: 2011-11-30
- Subjects: Geology , Manganese ores , Volcanism , Northern Cape (South Africa) , Kalahari (South Africa)
- Type: Thesis
- Identifier: uj:1746 , http://hdl.handle.net/10210/4101
- Description: D.Phil. , The Ongeluk Formation is a laterally extensive sequence of ≈2200 Ma tholeiitic basaltic andesites in the upper Griqualand West Sequence of the northern Cape Province. The stratigraphic thickness is about 500 m and the Ongeluk Formation underlies the ore-bearing strata of the Kalahari Manganese Field. The formation comprises massive lavas, pillow lavas and hyaloclastite beds in close association. These rocks were extruded under water in a marginal basin within the continental setting of the Kaapvaal Craton. The Hekpoort Basalt Formation of the Transvaal is magmatically cogenetic with the Ongeluk, having indistinguishable geochemistry and sharing a stratigraphically related hiatus in Cr values. The best age estimate for the two formations is 2193 ± 71 Ma, from Rb-Sr data of two previous workers for Hekpoort samples. The Ongeluk Formation shows a mild "regional" geochemical alteration and a profound "Kalahari" alteration beneath the Kalahari Manganese Field. Geochemical screening was used to reconstruct the magmatic composition from a selected dataset. Three stages in the development of regional alteration are ascribed to sea water-rock interaction at different temperatures, and have distinct geochemical signatures. The pervasive Kalahari alteration is characterised by a purple colouration and the decoupled alteration of alkali and high field strength elements. It is due to the development of major hydrothermal systems close to a volcanic vent which are analogous to modern mid-ocean ridge systems. A multi-system isotopic study showed that most of the isotope systems were modified by sea-floor alteration. The similarity of the 2237 ± 23 Ma Pb-Pb errorchron age with the Rb-Sr Hekpoort age reflects changes in U-Pb ratios with minor changes in Pb isotope ratio. Evidence was found in the Rb-Sr system for a minor disturbance at ≈ 1100 Ma, also reported by previous workers. This event is related to the Namaqua tectogenesis, while no isotopic evidence was found for the enigmatic ≈ 2200 to 1750 Ma Kheis orogeny, regarded as the cause of thrust faulting in the region. A genetic connection between the Ongeluk lava and the Kalahari Manganese deposits was established. The manganese ores contain evidence for both marine and hydrothermal contributions to chemical sedimentation. Negative Ce anomalies characterise an oxygenated sea in which the interaction between global oceanic and continental influences is seen. Heavy rare earth enrichment reflects volcanic hydrothermal exhalations from the Kalahari Ongeluk system. Mass balance calculations show that the entire 9 billion tons of Kalahari Manganese ore could have been derived from the Ongeluk Formation. A new model describing the origin and evolution of the Kalahari Manganese Field places a strong emphasis on the role of the syngenetic hydrothermal exhalation and upgrading.
- Full Text:
- Authors: Schutte, Sabine Silke
- Date: 2011-11-30
- Subjects: Geology , Manganese ores , Volcanism , Northern Cape (South Africa) , Kalahari (South Africa)
- Type: Thesis
- Identifier: uj:1746 , http://hdl.handle.net/10210/4101
- Description: D.Phil. , The Ongeluk Formation is a laterally extensive sequence of ≈2200 Ma tholeiitic basaltic andesites in the upper Griqualand West Sequence of the northern Cape Province. The stratigraphic thickness is about 500 m and the Ongeluk Formation underlies the ore-bearing strata of the Kalahari Manganese Field. The formation comprises massive lavas, pillow lavas and hyaloclastite beds in close association. These rocks were extruded under water in a marginal basin within the continental setting of the Kaapvaal Craton. The Hekpoort Basalt Formation of the Transvaal is magmatically cogenetic with the Ongeluk, having indistinguishable geochemistry and sharing a stratigraphically related hiatus in Cr values. The best age estimate for the two formations is 2193 ± 71 Ma, from Rb-Sr data of two previous workers for Hekpoort samples. The Ongeluk Formation shows a mild "regional" geochemical alteration and a profound "Kalahari" alteration beneath the Kalahari Manganese Field. Geochemical screening was used to reconstruct the magmatic composition from a selected dataset. Three stages in the development of regional alteration are ascribed to sea water-rock interaction at different temperatures, and have distinct geochemical signatures. The pervasive Kalahari alteration is characterised by a purple colouration and the decoupled alteration of alkali and high field strength elements. It is due to the development of major hydrothermal systems close to a volcanic vent which are analogous to modern mid-ocean ridge systems. A multi-system isotopic study showed that most of the isotope systems were modified by sea-floor alteration. The similarity of the 2237 ± 23 Ma Pb-Pb errorchron age with the Rb-Sr Hekpoort age reflects changes in U-Pb ratios with minor changes in Pb isotope ratio. Evidence was found in the Rb-Sr system for a minor disturbance at ≈ 1100 Ma, also reported by previous workers. This event is related to the Namaqua tectogenesis, while no isotopic evidence was found for the enigmatic ≈ 2200 to 1750 Ma Kheis orogeny, regarded as the cause of thrust faulting in the region. A genetic connection between the Ongeluk lava and the Kalahari Manganese deposits was established. The manganese ores contain evidence for both marine and hydrothermal contributions to chemical sedimentation. Negative Ce anomalies characterise an oxygenated sea in which the interaction between global oceanic and continental influences is seen. Heavy rare earth enrichment reflects volcanic hydrothermal exhalations from the Kalahari Ongeluk system. Mass balance calculations show that the entire 9 billion tons of Kalahari Manganese ore could have been derived from the Ongeluk Formation. A new model describing the origin and evolution of the Kalahari Manganese Field places a strong emphasis on the role of the syngenetic hydrothermal exhalation and upgrading.
- Full Text:
Genesis and characteristics of the Wolhaarkop breccia and associated manganore iron formation
- Schalkwyk, Gert Abraham Cornelius
- Authors: Schalkwyk, Gert Abraham Cornelius
- Date: 2009-01-28T09:43:23Z
- Subjects: Geology , Manganese ores , Iron ores , Breccia , Postmasburg (South Africa)
- Type: Thesis
- Identifier: uj:14848 , http://hdl.handle.net/10210/1970
- Description: M.A. , Hematized iron formation known as the Manganore iron formation is slumped into sinkhole structures in the Campbellrand Subgroup, Transvaal Supergroup, on the Maremane dome. These iron deposits are underlain by manganiferous breccias known as the Wolhaarkop Breccia. Known iron and manganese deposits of this type occur in an arc from Sishen in the north to Postmasburg in the south. The area is not being mined for manganese at the moment due to the relatively high grade of the Kalahari manganese field situated to the north of this area. The iron deposits, though, are some of the richest in the world. The aim is to establish the mode of origin for the Wolhaarkop Breccia. The Wolhaarkop Breccia is interpreted as being a residual ancient manganese wad from a karst environment in manganese rich dolostones of the Campbellrand Subgroup. This siliceous breccia contains authigenic megaquartz and angular poorly sorted clasts of chalcedony and quartz, set in a braunite-hematite matrix. Fluid inclusions in the authigenic quartz of the Wolhaarkop Breccia have been studied to establish the source of the fluid responsible for quartz precipitation in the Wolhaarkop Breccia, and indirectly, for the formation of the Wolhaarkop Breccia. Thermometric data was used to determine the maximum possible pT and depth conditions under which the quartz might have been precipitated. Fluid chemistry was determined using the bulk crush-leach method to shed some light on the fluid origin. It was established that the fluid responsible for chert recrystallization and precipitation of authigenic quartz and chalcedony had a meteoric source. Considering the results of the above-mentioned analysis, it was concluded that the iron and manganese deposits were formed during a cycle of uplift followed by subsidence. During the period of uplift, erosion in a karst environment and enrichment of iron formation in a supergene environment concentrated manganese as a manganese wad, and iron as a residual iron-oxide laterite. Meteoric water was the main fluid present during this period. Later, during a stage of subsidence, the Wolhaarkop Breccia underwent diagenesis and later lower greenschist-facies metamorphism. During a final stage of uplift the deposit was exposed to the atmosphere again, the dolostones were weathered away and the residual Manganore iron formation and Wolhaarkop Breccia were exposed to supergene alteration.
- Full Text:
- Authors: Schalkwyk, Gert Abraham Cornelius
- Date: 2009-01-28T09:43:23Z
- Subjects: Geology , Manganese ores , Iron ores , Breccia , Postmasburg (South Africa)
- Type: Thesis
- Identifier: uj:14848 , http://hdl.handle.net/10210/1970
- Description: M.A. , Hematized iron formation known as the Manganore iron formation is slumped into sinkhole structures in the Campbellrand Subgroup, Transvaal Supergroup, on the Maremane dome. These iron deposits are underlain by manganiferous breccias known as the Wolhaarkop Breccia. Known iron and manganese deposits of this type occur in an arc from Sishen in the north to Postmasburg in the south. The area is not being mined for manganese at the moment due to the relatively high grade of the Kalahari manganese field situated to the north of this area. The iron deposits, though, are some of the richest in the world. The aim is to establish the mode of origin for the Wolhaarkop Breccia. The Wolhaarkop Breccia is interpreted as being a residual ancient manganese wad from a karst environment in manganese rich dolostones of the Campbellrand Subgroup. This siliceous breccia contains authigenic megaquartz and angular poorly sorted clasts of chalcedony and quartz, set in a braunite-hematite matrix. Fluid inclusions in the authigenic quartz of the Wolhaarkop Breccia have been studied to establish the source of the fluid responsible for quartz precipitation in the Wolhaarkop Breccia, and indirectly, for the formation of the Wolhaarkop Breccia. Thermometric data was used to determine the maximum possible pT and depth conditions under which the quartz might have been precipitated. Fluid chemistry was determined using the bulk crush-leach method to shed some light on the fluid origin. It was established that the fluid responsible for chert recrystallization and precipitation of authigenic quartz and chalcedony had a meteoric source. Considering the results of the above-mentioned analysis, it was concluded that the iron and manganese deposits were formed during a cycle of uplift followed by subsidence. During the period of uplift, erosion in a karst environment and enrichment of iron formation in a supergene environment concentrated manganese as a manganese wad, and iron as a residual iron-oxide laterite. Meteoric water was the main fluid present during this period. Later, during a stage of subsidence, the Wolhaarkop Breccia underwent diagenesis and later lower greenschist-facies metamorphism. During a final stage of uplift the deposit was exposed to the atmosphere again, the dolostones were weathered away and the residual Manganore iron formation and Wolhaarkop Breccia were exposed to supergene alteration.
- Full Text:
- «
- ‹
- 1
- ›
- »