Constraining the role of carbonate assimilation on spinel stability in oxide ores of the Flatreef, Bushveld Complex, South Africa
- Authors: Dyan, Siyasanga
- Date: 2021-04
- Subjects: Oxide minerals -- South Africa -- Bushveld Complex , Transvaal Supergroup (South Africa) , Magmas -- South Africa -- Bushveld Complex , Petrogenesis -- South Africa -- Bushveld Complex , Spinel group – South Africa -- South Africa -- Bushveld Complex , Dolomite -- South Africa -- Bushveld Complex
- Language: English
- Type: thesis , text , Masters , MSc
- Identifier: http://hdl.handle.net/10962/174578 , vital:42490
- Description: The northern limb of the Bushveld Complex shows significant evidence for footwall rock- magma interaction, as a result of the emplacement of magmas onto the Transvaal Supergroup sediments. The Platreef of the northern limb is known to have involved extensive contamination of the magma. The lateral extension of the Platreef, the Flatreef, is less contaminated and comprises PGE-mineralization and thick chromitite layers. This enables successful stratigraphic correlation to the Upper Critical Zone of the eastern and western limbs of the Bushveld Complex. This study aims at addressing the influence of dolomitic floor rock contamination on the formation of spinels of the Flatreef and how they may differ to their occurrences in the Bushveld Complex elsewhere. Three main drill cores (UMT-345, UMT-335, and UMT-094) from the deep drilling program by Ivanhoe Mine, north of Turfspruit, were logged and sampled. The drill cores contained rocks contaminated by dolomite in varying degrees, depending on the proportion of carbonate xenoliths present. A total of sixty-two samples were obtained from the UG-2- equivalent chromitite seam, down into the hybrid contaminated units (Footwall Assimilation Zone; FAZ). Petrographic examination of the drill cores revealed that the feldspathic pyroxenite and chromitite layers are the most pristine lithologies in the Flatreef. The chromitite layers occur as a semi-massive to massive ores. The FAZ rocks are mainly dominated by the abundance of Al-rich Cr-spinels (Mg#30-80), clinopyroxenes with a high Ca-Tschermak component (up to 35 mol.%), olivines (Fo72-84), and plagioclase (An31-78). Geochemical characteristics of the feldspathic pyroxenite and chromitite seams include low CaO/Al2O3 and Ca/SiO2 ratios, the low abundance of REE and HFSE. In contrast, the FAZ samples display high CaO/Al2O3 and CaO/SiO2 ratios, suggesting mobilization of CaO-rich fluids derived from the associated dolomite xenoliths. Observations from lithostratigraphic element profiles indicate spikes in CaO within FAZ units relative to the feldspathic pyroxene, indicating a secondary source of CaO linked to proximity to carbonate xenoliths intersected in the core. The assimilation-fractional crystallization model performed with the Upper Critical Zone parental melt and dolomite produced assemblages dominated by spinel, olivine, clinopyroxene, and plagioclase, relating to those of the most contaminated rocks of footwall assimilation zones. The model also showed that large quantities of CO2 were produced during assimilation. High amounts of CO2-fluids mobilized in the melt would have interacted with the melt and increased the overall oxidation conditions. Oxygen fugacity (ƒO2) values were constrained from the spinels in the FAZ and chromites in chromitite seams. Spinels within the most contaminated rocks of the FAZ recorded relatively high ƒO2 values ranging between NNO-0.2 and NNO+1.8 (relative to the Nickel-Nickel-Oxide buffer. High ƒO2 values in spinels from the FAZ suggest that the melt interacted with greater amounts of oxidative CO2-fluids during decarbonation reactions. Such high redox conditions could have triggered the saturation and crystallization of spinels. Comparison of mineral compositions and ƒO2 of UG-2 chromites from the Flatreef with UG-2 from the eastern limb, western limb, chromitites from the Platreef and Uitkomst Complex reveal that Flatreef chromites are of most similar to those of the Platreef Uitkosmt Complex. The northern limb and Uitkomst Complex chromites have high ƒO2 values (NNO+0.3 to NNO+1.2), indicating their apparent link to the Malmani dolomite. Interaction of magma with the Malmani dolomite would have produced high quantities of CO2, triggering an increase in oxidizing conditions. Such an effect is most prominent in the Flatreef chromitites that are in contact with the FAZ. The chromites are characterised by high ƒO2 (NNO+1.2) and distinct compositions (low Cr2O3, high Fe3+/Fetotal, and TiO2). Thus, it is proposed that carbonate assimilation in the Flatreef triggered the precipitation of these distinctive chromites proximal to FAZ, due to liberation of substantial amounts of CO2-rich fluids with a highly oxidative capacity. Carbonate assimilation in layered intrusions can be used as a monitor potential precipitation of chromites. , Thesis (MSc)--Rhodes University, Faculty of Science, Department of Geology, 2021
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- Date Issued: 2021-04
A reappraisal of the origin of the Hotazel Fe-Mn Formation in an evolving early Earth system through the application of mineral-specific geochemistry, speciation techniques and stable isotope systematics
- Authors: Mhlanga, Xolane Reginald
- Date: 2020
- Subjects: Manganese ores -- South Africa -- Hotazel , Manganese ores -- Geology , Iron ores -- South Africa -- Hotazel , Iron ores -- Geology , Geochemistry -- South Africa -- Hotazel , Isotope geology -- South Africa -- Hotazel , Geology, Stratigraphic -- Archaean , Geology, Stratigraphic -- Proterozoic , Transvaal Supergroup (South Africa) , Great Oxidation Event
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/146123 , vital:38497
- Description: Marine chemical sediments such as Banded Iron Formations deposited during the Archean-Palaeoproterozoic are studied extensively because they represent a period in the development of the Earth’s early history where the atmospheric O₂ content was below the present levels (PAL) of 21%. Prior to the Great Oxidation Event (GOE) at ca. 2.4 Ga, highly ferruginous and anoxic marine environments were dominated by extensive BIF deposition such as that of the Griqualand West Basin of the Transvaal Supergroup in South Africa. This basin is also thought to record the transition into the first rise of atmospheric O₂ in our planet, from the Koegas Subgroup to the Hotazel Formation dated at ca. 2.43 Ga (Gumsley et al., 2017). Two drill cores from the north eastern part of the Kalahari Manganese Field characterized by a well-preserved and complete intersection of the cyclic Mn-Fe Hotazel Formation were studied at a high resolution (sampled at approximately one-meter interval). Such high-resolution approach is being employed for the first time in this project, capturing in detail the three manganese rich layers intercalated with BIF and the transitions between these lithofacies. The micro-banded BIF is made up of three major phases, namely Fe-Ca-Mg carbonates (ankerite, siderite and calcite), magnetite, and silicates (chert and minor Fe-silicates); laminated transitional lutite consist of mainly hematite, chert and Mn-carbonates, whereas the manganese ore layers are made up of mostly calcic carbonates (Mn-calcite and Ca-kutnahorite) in the form of laminations and ovoids, while Mn-silicates include dominant braunite and lesser friedelite. All three lithofacies are very fine grained (sub-mm scale) and so petrographic and mineralogical observations were obtained mostly through scanning electron microscope analysis for detailed textural relationships with focus on the carbonate fraction. Bulk geochemical studies of the entire stratigraphy of the Hotazel Formation have previously provided great insights into the cyclic nature of the deposit but have not adequately considered the potential of the carbonate fraction of the rocks as a valuable proxy for understanding the chemistry of the primary depositional environment and insights into the redox processes that were at play. This is because these carbonates have always been attributed to diagenetic processes below the sediment-water interface such as microbially-mediated dissimilatory iron/manganese reduction (DIR/DMR) where the precursor/primary Fe-Mn oxyhydroxides have been reduced to result in the minerals observed today. The carbonate fraction of the BIF is made up of ankerite and siderite which co-exist in a chert matrix as anhedral to subhedral grains with no apparent replacement textures. This suggests co-precipitation of the two species which is at apparent odds with classic diagenetic models. Similarly, Mn-carbonates in the hematite lutite and manganese ore (Mn-calcite, kutnahorite, and minor rhodocrosite) co-exist in laminae and ovoids with no textures observed that would suggest an obvious sequential mode of formation during diagenesis. In this light, a carbonate-specific geochemical analysis based on the sequential Fe extraction technique of Poulton and Canfield (2005) was employed to decipher further the cyclic nature of the Hotazel Formation and its primary versus diagenetic controls. Results from the carbonate fraction analysis of the three lithofacies show a clear fractionation of iron and manganese during primary – rather than diagenetic - carbonate precipitation, suggesting a decoupling between DIR and DMR which is ultimately interpreted to have taken place in the water column. Bulk-rock concentration results for minor and trace elements such as Zr, Ti, Sc and Al have been used for the determination of either siliciclastic or volcanic detrital inputs as they are generally immobile in most natural aqueous solutions. These elements are in very low concentrations in all three lithofacies suggesting that the depositional environment had vanishingly small contributions from terrigenous or volcanic detritus. In terms of redox-sensitive transition metals, only Mo and Co appear to show an affinity for high Mn facies in the Hotazel sequence. Cobalt in particular attains a very low abundance in the Hotazel BIF layers at an average of ~ 4 ppm. This is similar to average pre-GOE BIF in South Africa and worldwide. Maxima in Co abundance are associated with transitional hematite lutite and Mn ore layers, but maxima over 100ppm are seen in within the hematite lutite and not within the Mn ore proper where maxima in Mn are recorded. This suggests a clear and direct association with the hematite fraction in the rocks, which is modally much higher in the lutites but drops substantially in the Mn layers themselves. The similarities of bulk-rock BIF and modern-day seawater REE patterns has been used as a key argument for primary controls in REE behaviour and minimal diagenetic modification. Likewise, the three lithofacies of the Hotazel Formation analysed in this study all share similar characteristics with a clear seawater signal through gentle positive slopes in the normalised abundance of LREE versus HREE. Negative Ce anomalies prevail in the entire sample set analysed, which has been interpreted before as a proxy for oxic seawater conditions. However, positive Ce anomalies that are traditionally linked to scavenging and deposition of primary tetravalent Mn oxyhydroxides (e.g., as observed in modern day ferromanganese nodules) are completely absent from the current dataset. The lack of a positive Ce anomaly in the manganese ore and peak Co association with ferric oxides and not with peak Mn, suggests that primary deposition must have occurred within an environment that was not fully oxidizing with respect to manganese. The use of stable isotopes (i.e., C and Fe) was employed to gain insights into redox processes, whether these are thought to have happened below the sediment-water interface or in contemporaneous seawater. At a small scale, all lithofacies of the Hotazel Formation record bulk-rock δ¹³C values that are low and essentially invariant about the average value of -9.5 per mil. This is independent of sharp variations in overall modal mineralogy, relative carbonate abundance and carbonate chemistry, which is clearly difficult to reconcile with in-situ diagenetic processes that predict highly variable δ¹³C signals in response to complex combinations of precursor sediment mineralogy, pore-fluid chemistry, organic carbon supply and open vs closed system diagenesis. At a stratigraphic scale, the carbonate δ¹³C (-5 to -13‰) variations between the different lithologies could instead represent temporal changes in water-column chemistry against well-developed physico-chemical gradients, depth of deposition and biological processes. The low iron isotope values recorded in the hematite lutite and manganese ore samples can be attributed to fractionation effects of initial oxidation of ferrous iron to form Fe-oxyhydroxides in the shallow parts of the basin, from an already isotopically highly depleted aqueous Fe-pool as proposed previously. The slightly higher but still negative bulk-rock δ⁵⁶Fe values of the host BIF can be attributed to water-column Fe isotopic effects at deeper levels between primary Fe oxyhydroxides and an isotopically heavier Fe(II) pool, which was subsequently preserved during diagenetic recrystallization. All above findings were combined into a conceptual model of deposition for the three different lithologies of the Hotazel Formation. The model predicts that free molecular oxygen must have been present within the shallow oceanic environment and implicates both Mn and Fe as active redox “players” compared to classic models that apply to the origin of worldwide BIF prior to the GOE. The deposition of the Hotazel strata is interpreted to have occurred through the following three stages: (1) BIF deposition occurred in a relatively deep oceanic environment above the Ongeluk lavas during marine transgression, where a redoxcline and seawater stratification separated hydrothermally sourced iron and manganese, in response to an active Mn-shuttle mechanism linked to Mn redox cycling. Abundant ferrous iron must have been oxidized by available oxygen but also by oxidised Mn species (MnOOH) and possibly even some soluble Mn(III) complexes. Through this process, Mn(III) was being effectively reduced back into solution along with cobalt(III), as Mn(II) and Co(II) respectively, thus creating maxima in their concentrations. A drawdown of Fe(OH)₃ particles was therefore the only net precipitation mechanism at this stage. Carbonate species of Fe and the abundant magnetite would possibly have formed by reaction between the ferric hydroxides and the deeper Fe(II) pool, while organic matter would also have reacted in the water-column via DIR, accounting for the low δ¹³C signature of Fe carbonate minerals. (2) Hematite lutite formation would have occurred at a relatively shallower environment during marine regression. At this stage, reductive cycling of Fe was minimal in the absence of a deeper Fe(II) reservoir reacting with the ferric primary precipitates. Therefore, DIR progressively gave way to manganese reduction and organic carbon oxidation (DMR), which reduced MnOOH to form Mn(II)-rich carbonates in the form of kutnahorite and Mn-calcite. Co-bearing Fe(OH)₃ would have precipitated and was ultimately preserved as Co-bearing hematite during diagenesis. (3) Deposition of manganese-rich sediment occurred at even shallower oceanic depths (maximum regression) where aerobic organic carbon oxidation replaced DMR, resulting in Ca-rich carbonates such as Mn-bearing calcite and Ca-kutnahorite, yet with a low carbon isotope signature recording aerobic conditions of organic carbon cycling. Mn(III) reduction at this stage was curtailed, leading to massive precipitation of MnOOH which was diagenetically transformed into braunite and friedelite. Simultaneous precipitation of Co-bearing Fe(OH)₃ would have continued but at much more subdued rates. Repeated transgressive-regressive cycles resulted in the cyclic BIF-hematite lutite- manganese ore nature of the Hotazel Formation in an oxidized oceanic environment at the onset of the Great Oxidation Event, which was nonetheless never oxic enough to drive Mn(II) oxidation fully to its tetravalent state. The mineralogy and species-specific geochemistry of the Hotazel strata, and more specifically the carbonate fraction thereof, appear to faithfully capture the chemistry of the primary depositional environment in a progressively evolving Earth System. This project opens the door for more studies focusing on better constraining primary versus diagenetic depositional 2020 Hotazel Fe and Mn deposition mechanisms of iron and manganese during the period leading up to the GOE, and possibly re-defining the significance of Fe and Mn as invaluable redox proxies in a rapidly changing planet.
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- Date Issued: 2020
A bulk and fraction-specific geochemical study of the origin of diverse high-grade hematitic iron ores from the Transvaal Supergroup, Northern Cape Province, South Africa
- Authors: Moloto, William
- Date: 2017
- Subjects: Iron ore -- South Africa -- Transvaal Supergroup , Hematite -- South Africa -- Transvaal Supergroup , Transvaal Supergroup (South Africa)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/50546 , vital:25998
- Description: The Paleoproterozoic Transvaal Supergroup in the Northern Cape Province of South Africa is host to high-grade, Banded Iron Formation-hosted hematite iron-ore deposits and is the country’s most important source of iron to date. Previous studies suggest the origin of these iron ores to be ancient supergene, and that the ore forming process would have therefore pre-dated deposition of the basal Mapedi shales of the Olifansthoek Supergroup that unconformably overlies the Transvaal strata. The nature of the protolith to the ores has been suggested to be largely BIF of the Asbestos Hills Subgroup, and mainly the Kuruman BIF. The work presented in this thesis seeks to provide insights into the diversity of processes that are likely to have been involved during the genesis of these high-grade iron ores, in the context of constraining the pre-ore lithologies and the relative role of supergene-style, largely residual enrichment processes versus any possible metasomatic hydrothermal effects. This study had as primary focus the application of combined bulk and fraction-specific geochemical applications on representative iron-ore samples from four different localities in the Northern Cape Province, namely King/Khumani, Beeshoek, Heuninkranz and Hotazel. The collected samples show a variety of textures and also capture different pre-unconformity stratigraphic sections of BIF. The key objective was to assess whether the fraction-specific analytical results could provide any firm constraints for the origin of the ferrous and non-ferrous matrix fractions of the ores, namely whether they represent any combinations of protolith residue, allochtonously-introduced detritus or hydrothermally-derived material, and whether the results are comparable and consistent across all samples studied. In particular, constraints were sought as to whether the ore protolith was exclusively BIF or may potentially have contained at least a fraction of other lithologic types, such as shale; and whether there is sufficient evidence to support solely a supergene model for the ores or the data suggest other more epigenetic models of ore formation involving the action of hydrothermal fluids Bulk-rock geochemical analyses reveal the overwhelming dominance of Fe-oxide (as hematite) in all samples, at concentrations as high as 99 wt.% Fe2O3. Major and trace-element abundances of all samples were re-calculated assuming only iron addition from the postulated protolith (average BIF and shale), and the results revealed atypical enrichments in the iron ores by comparison to average BIF, and more shale-like relative abundances when normalised against the Post-Archaean Average Shale (PAAS). Specifically, BIF-normalised diagrams show relative enrichments by as much as 53-95% for Al2O3; 11-86% for TiO2; and 4-60% for P2O5. By contrast, PAAS-normalised values display enrichments of 1-3% for Al2O3, 0.2-3% for TiO2, and 3-13% for P2O5. Similar observations can be made for the greatest majority of trace elements when normalised against average BIF as compared to normalisation against PAAS. A suite of trace element that include alkali earths (e.g. Ba, Sr) and transition metals (e.g. Ni, Zn) show enrichments that are unrelated to the apparently detrital siliciclastic fraction of the ores, and are therefore linked to a possible hydrothermal input. Fraction-specific extractions were performed via the adaptation of existing dissolution protocols using oxalic acid (iron-oxide fraction) followed by HF digestion (silicate-fraction). The analyses of the produced aliquots using ICP-MS techniques, focused mainly on the REE abundances of the separated ferrous and non-ferrous matrix fractions and their comparisons to bulk-rock REE signatures. The results lend further support to the suggestion that the ore samples contain a predominant shale-like signal which does not directly compare to published REE signatures for supergene or hydrothermal BIF-hosted iron-ore deposits alike. The data therefore collectively point to a post-unconformity epigenetic hydrothermal event/s of iron ore-formation that would have exploited not only BIF but also shale as suitable pre-ore protolith.
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- Date Issued: 2017
Mineralogical and geochemical constraints on the origin, alteration history and metallogenic significance of the Manganore iron-formation, Northern Cape Province, South Africa
- Authors: Papadopoulos, Vlassis
- Date: 2017
- Subjects: Banded iron formation , Transvaal Supergroup (South Africa) , Groups (Stratigraphy) South Africa , Lithostratigraphy , Petrology South Africa , Geochemistry South Africa
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/65189 , vital:28702
- Description: The Manganore iron-formation (MIF) of the Transvaal Supergroup is host to the most important high-grade iron ore bodies in South Africa. Prevailing models for ore genesis invoke supergene processes performing during a long period of erosion, oxidation and weathering under tropical lateritic conditions while the role of potential hydrothermal processes is not addressed. Lack of detailed petrographical and geochemical data necessitated reexamination of the MIF through new and existing drill core exploration material. Thorough petrographical investigation revealed a multi-event complex alteration history involving hydrothermal activity. Iron and silica mobility during alteration is demonstrated by a series of replacement, overprinting, crosscutting textures, extensive silicification and hematitization. Metasomatized textures such as pseudomorphs of primary magnetite, carbonate minerals and chert pods/lenses point to an alteration occurring in layer- controlled fronts and link stratigraphically the MIF to Kuruman and Griquatown iron- formations. Whole-rock geochemical data verify textural observations suggesting strong enrichment of iron or silica in meter-scale horizons, expressed by different generations of quartz and hematite. High-grade iron ore is highly enriched in TiO2 and Al2O3 compared to the protolith while both BIF and iron ore display highly increased concentrations of trace elements (transition metals and HFSE). Oxygen isotopes from different quartz textures reveal little to none isotopic exchangement during alteration whereas O isotopes from hematite are in concert to values from literature and suggest two different generations of hematite. A total of 20 minerals apart from quartz and hematite were documented. An earlier alkali/HFSE alteration event that is believed to have affected the overlying Gamagara shales is recorded in the BIF by the presence of muscovite, apatite, rutile, zircon and xenotime. A later and possibly ongoing event of succeeding hydrothermal pulses involves mainly sulphates (gypsum, baryte, celestine), pyrite, carbonates (siderite, calcite) and silicates (berthierine and tourmaline). Alkali-bearing brines persistently exploit the BIF mainly through karstification-related secondary porosity, are evidently carrying iron and are proposed to participate in or control the iron enrichment by facilitating removal of silica. The source of metals, sulfur and carbon is attributed to the underlying Campbellrand dolomites and especially to the upper Gamogaan Formation. The unconformable contact between BIF and the overlying shales is suggested as a suitable fluid conduit for the development of the observed BIF and shale-derived high-grade hematite iron ore.
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- Date Issued: 2017
Genesis of BIF-hosted hematite iron ore deposits in the central part of the Maremane anticline, Northern Cape Province, South Africa
- Authors: Land, Jarred
- Date: 2014
- Subjects: Hematite -- South Africa -- Northern Cape , Anticlines -- South Africa -- Northern Cape , Geology, Stratigraphic -- Proterozoic , Hydrothermal deposits -- Northern Cape , Rare earth metals -- Northern Cape , Iron ores -- Geology -- Northern Cape , Transvaal Supergroup (South Africa)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5095 , http://hdl.handle.net/10962/d1020905
- Description: The Paleoproterozoic Transvaal Supergroup in the Northern Cape Province of South Africa is host to high-grade BIF-hosted hematite iron-ore deposits and is the country’s most important source of iron to date. Previous work has failed to provide a robust and all-inclusive genetic model for such deposits in the Transvaal Supergroup; in particular, the role of hydrothermal processes in ore-genesis has not been adequately clarified. Recent studies by the author have produced evidence for hydrothermal alteration in shales (Olifantshoek Supergroup) stratigraphically overlying the iron-ore intervals; this has highlighted the need to reassess current ore-forming models which place residual supergene processes at the core of oregenesis. This thesis focuses on providing new insights into the processes responsible for the genesis of hematite iron ores in the Maremane anticline through the use of newly available exploration drill-core material from the centre of the anticline. The study involved standard mineralogical investigations using transmitted/reflected light microscopy as well as instrumental techniques (XRD, EPMA); and the employment of traditional whole-rock geochemical analysis on samples collected from two boreholes drilled in the centre of the Maremane anticline, Northern Cape Province. Rare earth element analysis (via ICP-MS) and oxygen isotope data from hematite separates complement the whole-rock data. Iron-ore mineralisation examined in this thesis is typified by the dominance of Fe-oxide (as hematite), which reaches whole-rock abundances of up to 98 wt. % Fe₂O₃. Textural and whole-rock geochemical variations in the ores likely reflect a variable protolith, from BIF to Fe-bearing shale. A standard supergene model invoking immobility and residual enrichment of iron is called into question on the basis of the relative degrees of enrichment recorded in the ores with respect to other, traditionally immobile elements during chemical weathering, such as Al₂O₃ and TiO₂. Furthermore, the apparently conservative behaviour of REE in the Fe ore (i.e. low-grade and high-grade iron ore) further emphasises the variable protolith theory. Hydrothermally-induced ferruginisation is suggested to post-date the deposition of the post-Transvaal Olifantshoek shales, and is likely to be linked to a sub-surface transgressive hydrothermal event which indiscriminately transforms both shale and BIF into Fe-ore. A revised, hydrothermal model for the formation of BIF-hosted high-grade hematite iron ore deposits in the central part of the Maremane anticline is proposed, and some ideas of the author for further follow-up research are presented.
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- Date Issued: 2014