Diagenetic carbonates and biogeochemical cycling of organic matter in selected Archean-Paleoproterozoic sedimentary successions of the Kaapvaal Craton, South Africa
- Authors: Cochrane, Justin Michael
- Date: 2010-06-03T05:38:44Z
- Subjects: Stratigraphic geology , Geochemistry , Petrology , Mineralogy , Sedimentation and deposition , Kaapvaal Craton (South Africa)
- Type: Thesis
- Identifier: uj:6855 , http://hdl.handle.net/10210/3288
- Description: M.Sc. , The Kaapvaal craton is one of few regions on earth with an almost continuous record of wellpreserved supracrustal rocks ranging in age from ~3.5 Ga to the late Paleoproterozoic at ~1.75 Ga. In this study diagenetic carbonates from the Paleoarchean Buck Reef Chert and Joe’s Luck Formation of the Swaziland Supergroup, the Mesoarchean Thalu and Promise Formations of the Mozaan/Witwatersrand Supergroups and the Paleoproterozoic Timeball Hill and Silverton Formations of the Transvaal Supergroup were sampled and analyzed. The aim of the study was to determine possible variations in the composition of the carbonates through time and their significance especially with regards to microbial activity in diagenetic systems in early Earth history. Results indicate similar petrographic observations and geochemical signatures in diagenetic carbonates of iron formations in the Buck Reef Chert, Joe’s Luck and Griquatown Iron Formation. The carbonates all tend to be siderites with iron derived from hydrothermal input and all are depleted in 13C relative to Peedee Belemnite standard. It suggested that siderite formed as a result of microbial respiration. Microbes degrade organic matter and reduce iron in this process. This resulted in the depletion in 13C and in the precipitation of siderite. However in order for iron reduction to have occurred the reduced iron first had to be oxidized. This most probably occurred through iron oxidizing chemolithoautotrophs under microaerophilic conditions. Diagenetic carbonate concretions of the Thalu and Promise Formations are manganiferous and are highly depleted in 13C relative to PDB. There is also strong evidence for hydrothermal input of manganese and iron into the system because of positive europium anomalies. The carbonates from both of the formations strongly suggest the presence of some free oxygen. The reasoning behind this conclusion is as follows: The depletion of 13C in the carbonates points to microbial decomposition of organic matter and manganese respiration (the decomposition of organic matter by microbial MnO2 reduction) is shown to be the most reasonable process that led to the formation of the carbonate concretions. The implication is that MnO2 must first have been precipitated and that can only be achieved in the presence of free oxygen with the oxidation reaction often catalyzed by manganese oxidizing chemolithoautotrophs. The carbonates of the Timeball Hill and Silverton Formationsare calcites ad contain little no iron. There is also little or no evidence for hydrothermal input and the basin appears to be a clastic dominated. It is generally accepted that a major rise in oxygen in the oceans and the atmosphere occurred at about 2.32 Ga. This rise in oxygen levels is reflected in the diagenetic calcite concretions of the Silverton Formation. Both iron and manganese reduction where not very effective because of the depletion in the basin water of these two elements, organic carbon taken up in the calcite concretions, indicated by negative δ13CPDB carbonate values, was most probably derived from aerobic and/or nitrate respiration. The most important conclusion from this study is that sufficient free oxygen and hence oxygenic photosynthesis were present to oxidize both Fe and Mn at least as far back as the Paleo-Mesoarchean.
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Origin of high-grade hematite ores at Thabazimbi Mine, Limpopo Province, South Africa
- Authors: Netshiozwi, Simon Thizwilondi
- Date: 2009-01-28T09:42:32Z
- Subjects: Hematite , Sedimentation and deposition , Geology , Thabazimbi ( South Africa)
- Type: Thesis
- Identifier: uj:14842 , http://hdl.handle.net/10210/1965
- Description: M.Sc. , High-grade hematite ores at the Thabazimbi Mine, Limpopo Province, occur as stratabound bodies in the Early Paleoproterozoic Penge Iron Formation of the Transvaal Supergroup. Iron ores occur at three distinct positions in the Penge Iron Formation (i) basal ore bodies located immediately above a thin oxidised shale unit that marks the base of the Penge Iron Formation in the Thabazimbi area and that may be interpreted as a structural contact towards the underlying dolostones of the Malmani Subgroup; (ii) ore bodies developed immediately above a prominent mafic sill in the Penge Iron Formation; (iii) small, lenticular ore bodies developed in the iron-formation without apparent structural control. Ore bodies in all three stratigraphic positions formed on the expense of the Penge Iron Formation protore, they share very similar mineralogical and textural attributes and can be subdivided into three major ore types with respect to their mineralogy and physical characteristics, namely, (a) carbonate-hematite ore; (b) hard hematite ore; (c) supergene modified ore. Further subdivision into subtypes is possible based on textural attributes. The first stage of iron ore formation at the Thabazimbi deposit is marked by oxidation of ferrous minerals (carbonates and grunerite) and their replacement by hematite. Efficient leaching and replacement of chert in the iron-formation to produce high-grade hematite ores characterizes the second stage of alteration. Stable isotope and fluid inclusion evidence point to a hydrothermal origin of the iron ores. Two hydrothermal fluids were identified, namely a highly saline Ca-Mg-rich brine (S = 27 wt% NaClequiv, TH = 160ºC) and a Nadominated fluid of intermediate salinity (S = 10 wt% NaClequiv, TH = 130ºC) that is possibly of meteoric origin. The results obtained in this study are used to propose the following sequence of mineralising events for the Thabazimbi iron ore deposit: (i) Deposition of iron-formation and diagenesis; (ii) contact metamorphic alteration related to the intrusion of the Bushveld igneous complex; (iii) metasomatic oxidation, leaching and residual upgrading that is tentatively linked to structurallycontrolled hydrothermal fluid flow; (iv) supergene modification of existing high-grade ore bodies in post-Gondwana times along the old African land surface.
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The nature and origin of the polymetallic Salt River massive sulfide deposit, Northern Cape Province, South Africa
- Authors: Osburn, Keith Craig
- Date: 2012-06-07
- Subjects: Namaqua Metamorphic Province , Sedimentation and deposition , Geology (Kakamas, South Africa) , Salt River sulfide deposit , Petrology , Metamorphic rocks , Geology, Stratigraphic
- Type: Thesis
- Identifier: uj:8667 , http://hdl.handle.net/10210/5022
- Description: M.Sc. , The Salt River deposit is a poly-metallic base metal deposit with a Zn-Cu-Pb metal content that occurs southwest of the town of Kakamas within the Northern Cape Province, South Africa. The Salt River deposit occurs within the Geelvloer Formation of the Bushmanland Subprovince of the Proterozoic Namaqua Metamorphic Province (NMP). This study constitutes the first detailed study of the host rock succession to the Salt River deposit, by investigating the lithostratigraphy, petrography geochemistry and geochronology. During the course of the study, various styles of wall-rock alteration were identified and investigated to determine their effect on the host rock succession. A further aim of this study was to classify the Salt River deposit and compare it to neighboring deposits occurring in the NMP. Geochronological studies were undertaken to define the age of mineralization. Detailed logging of exploration diamond drill core combined with petrographic investigation was used to define thirteen distinct lithotypes. The stratigraphy is dominated by felsic grey gneisses and mafic amphibolites, minor calc-silicate rocks, granitic augen gneisses, pegmatites and two lithologies that represent the metamorphosed equivalents of hydrothermally-altered host rock. Lithostratigraphic investigations yielded a rather uniform succession containing four distinct marker beds defined by their common occurrence and ease of correlation across various boreholes.
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