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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Mesoproterozoic volcanism, metallogenesis and tectonic evolution along the western margin of the Kaapvaal Craton
- Authors: Bailie, Russell Hope
- Date: 2010-06-07T06:52:22Z
- Subjects: Geology , Volcanism , Geochemistry , Structural geology , Kaapvaal Craton (South Africa)
- Type: Thesis
- Identifier: uj:6866 , http://hdl.handle.net/10210/3298
- Description: D.Phil. , The western margin of the Archean Kaapvaal Craton, at its contact with the polydeformed and metamorphosed Proterozoic Namaqua Province, is host to four volcanosedimentary successions of Mesoproterozoic age (1.1-1.3 Ga) that occur in close spatial and temporal association to each other. These are the Areachap Group, the Leerkrans Formation of the Wilgenhoutsdrif Group and the two volcanosedimentary successions that comprise the Koras Group. There has been protracted debate as to the exact nature, origin, age and tectonic evolution of these successions, particularly as they occur immediately adjacent to an important crustal suture. A comprehensive whole rock and isotope geochemical study, complemented by zircon-based geochronology where necessary, was thus carried out to characterize and compare the volcanic rocks associated with these four successions. The results are used to assess the role of the four volcanosedimentary successions during the development of the Mesoproterozoic suture between the Kaapvaal Craton and the Namaqua Province during the ~1.2-1.0 Ga Namaquan Orogeny. The geochemical study of the Areachap Group examined a suite of lithologies from different locations along the ~280km long outcrop belt, with the aim of testing the lateral continuity and integrity of this highly metamorphosed and deformed succession. As the bulk of the samples collected were from diamond drill core intersecting volcanogenic massive sulphide (VMS) Zn-Cu deposits it was only appropriate to extend the investigation to assess the metallogenesis and relation of these deposits to their host rock sequences. This included a survey of the sulphur isotope composition of sulphides and sulphates that comprise the Zn-Cu deposits. Furthermore, the architecture and origin of the world-class Copperton deposit, the largest Zn-Cu deposit of the Areachap Group, was examined. For this purpose, available literature data were collated and complemented by new geochemical and geochronological information. Sm-Nd isotopic systematics and U-Pb zircon ages suggest a coeval origin and close genetic link between the metavolcanic rocks of the Leerkrans Formation of the Wilgenhoutsdrif Group and the Areachap Group. Both successions record the establishment of an eastward-directed subduction zone on the western margin of the Kaapvaal Craton. The Areachap Group represents the highly metamorphosed and deformed remnants of a Mesoproterozoic (ca. 1.30-1.24 Ga) volcanic arc that was accreted onto the western margin of the Kaapvaal Craton at ~1.22-1.20 Ga, during the early stages of the Namaquan Orogeny. The igneous protoliths within the Areachap Group are low- to medium-K tholeiitic to calc-alkaline in composition ranging in composition from basaltic through to rhyolitic. Tholeiitic basalts, represented by volumetrically minor amphibolites within the succession have Sm-Nd isotopic characteristics indicative of derivation from a depleted mantle source as denoted by their positive Nd(t) values. The lithogeochemical results highlight the fact that, despite differences in lithological architecture on a local scale, the Areachap Group exhibits coherent geochemical characteristics along its entire strike length.
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