M.Sc. / Certain carbonate bearing formations in the Paleoproterozoic Pretoria Group and its Griqualand West equivalent exhibit remarkable geochemical and stable isotopic signatures. The 8'3Ccarb isotopic signatures from the Duitschland and Silverton Formations exhibit large positive excursions, which seemingly coincide with a significant increase in atmospheric oxygen between 2.4 and 2.0 Ga. The Duitschland Formation with its distinctive basal unconformity is composed primarily of limestone and dolomite units, interbedded with two compositionally different shale units and quartzite. Toward the base of the formation there is a distinct conglomeratic quartzite which forms a sequence boundary above which isotopic and geochemical signatures change dramatically. Normal marine isotopic signatures characterize the lower portion of the succession while above the sequence boundari, the carbonates are enriched in "C. This enrichment, however, appears to be the result of local processes occurring within a closed basin. Furthermore it is apparent that the Duitschland Formation (with its three distinct marker beds) is the equivalent of the Rooihoogte Formation and therefore constitutes the base of the Pretoria Group. The Mooidraai Dolomite Formation which outcrops only locally in the Northern Cape Province, is characterized by fenestral and microbially laminated dolomite. The geochemical properties are relatively homogeneous with increases in the FeO and MnO concentrations, resulting from post depositional diagenesis. The stable isotope signatures of these dolomites represent normal marine signatures. There is, however, a depletion in the 813C and 8180 signatures in the ankeritic and sideritic lithofacies, which suggests that this succession was deposited from a stratified water column with respect to the total dissolved CO2. The positive 6 13C excursion present in the carbonates of the Lucknow Formation in Griqualand West, traditionally grouped with the Olifantshoek Group can be correlated with carbonates near the top of the Silverton Formation in the Transvaal area. The latter also displays distinctly positive 6 43C values. One possibility is that if these successions were deposited in closed anoxic basins and that the isotopic anomalies are the result of local processes such as fermentive diagenesis and methanogenesis. However, the close association of the carbonates with shallow marine orthoquartzites suggests that these were deposited in an open marine system and that the positive 8 !3C values reflect a shift in the composition of the ocean water at the time of deposition of the carbonates at 2.2 Ga. Other carbonates present in the Pretoria Group, namely from the Vermont and Houtenbek Formations, display normal open marine 8' 3C values of close to zero. A systematic stratigraphic compilation of all 6 43C values available from the Transvaal Supergroup indicates that two clear-cut positive 5' 3C excursions are present. These excursions were apparently short-lived and well defined and did not occur over an extended period of time as suggested by earlier studies based on global compilations with large uncertainties in radiometric ages of deposits.

The subject of BIF genesis, particularly their environmental conditions and ocean chemistry at the time of deposition and their evolution through time, has been a subject of much contentiousness, generating a wealth of proposed genetic models and constant refinements thereof over the years. The prevailing paradigm within the various schools of thought, is the widespread and generally agreed upon depositional and diagenetic model(s) which advocate for BIF deposition under anoxic marine conditions. According to the prevailing models, the primary depositional environment would have involved a seawater column whereby soluble Fe²⁺ expelled by hydrothermal activity mixed with free O₂ from the shallow photic zone produced by eukaryotes, forming a high valence iron oxy-hydroxide precursor such as FeOOH or Fe(OH)₃. An alternative biological mechanism producing similar ferric precursors would have been in the form of photo-ferrotrophy, whereby oxidation of ferrous iron to the ferric form took place in the absence of biological O₂ production. Irrespective of the exact mode of primary iron precipitation (which remains contentious to date), the precipitated ferric oxy-hydroxide precursor would have reacted with co-precipitated organic matter, thus acting as a suitable electron acceptor for organic carbon remineralisation through Dissimilatory Iron Reduction (DIR), as also observed in many modern anoxic diagenetic environments. DIR-dominated diagenetic models imply a predominantly diagenetic influence in BIF mineralogy and genesis, and use as key evidence the low δ¹³C values relative to the seawater bicarbonate value of ~0 ‰, which is also thought to have been the dissolved bicarbonate isotope composition in the early Precambrian oceans. The carbon for diagenetic carbonate formation would thus have been sourced through a combination of two end-member sources: pore-fluid bicarbonate at ~0 ‰ and particulate organic carbon at circa -28 ‰, resulting in the intermediate δ¹³C values observed in BIFs today. This study targets 65 drillcore samples of the upper Kuruman and Griquatown BIF from the lower Transvaal Supergroup in the Hotazel area, Northern Cape, South Africa, and sets out to explore key aspects in BIF carbonate petrography and geochemistry that are pertinent to current debates surrounding their interpretation with regard to primary versus diagenetic processes. The focus here rests on applications of carbonate (mainly siderite and ankerite) petrography, mineral chemistry, bulk and mineral-specific carbon isotopes and speciation analyses, with a view to obtaining valuable new insights into BIF carbonates as potential records of ocean chemistry for their bulk carbonate-carbon isotope signature. Evaluation of the present results is done in light of pre-existing, widely accepted diagenetic models against a proposed water-column model for the origin of the carbonate species in BIF. The latter utilises a combination of geochemical attributes of the studied carbonates, including the conspicuous Mn enrichment and stratigraphic variability in Mn/Fe ratio of the Griquatown BIF recorded solely in the carbonate fraction of the rocks. Additionally, the carbon isotope signatures of the Griquatown BIF samples are brought into the discussion and provide insights into the potential causes and mechanisms that may have controlled these signatures in a diagenetic versus primary sedimentary environment. Ultimately, implications of the combined observations, findings and arguments presented in this thesis are presented and discussed with particular respect to the redox evolution and carbon cycle of the ocean system prior to the Great Oxidation Event (GOE). A crucial conclusion reached is that, by contrast to previously-proposed models, diagenesis cannot singularly be the major contributing factor in BIF genesis at least with respect to the carbonate fraction in BIF, as it does not readily explain the carbon isotope and mineral-chemical signatures of carbonates in the Griquatown and uppermost Kuruman BIFs. It is proposed instead that these signatures may well record water-column processes of carbon, manganese and iron cycling, and that carbonate formation in the water column and its subsequent transfer to the precursor BIF sediment constitutes a faithful record of such processes. Corollary to that interpretation is the suggestion that the evidently increasing Mn abundance in the carbonate fraction of the Griquatown BIF up-section would point to a chemically evolving depositional basin with time, from being mainly ferruginous as expressed by Mn-poor BIFs in the lower stratigraphic sections (i.e. Kuruman BF) to more manganiferous as recorded in the upper Griquatown BIF, culminating in the deposition of the abnormally enriched in Mn Hotazel BIF at the stratigraphic top of the Transvaal Supergroup. The Paleoproterozoic ocean must therefore have been characterised by long-term active cycling of organic carbon in the water column in the form of an ancient biological pump, albeit with Fe(III) and subsequently Mn(III,IV) oxy-hydroxides being the key electron acceptors within the water column. The highly reproducible stratigraphic isotope profiles for bulk δ¹³C from similar sections further afield over distances up to 20 km, further corroborate unabatedly that bulk carbonate carbon isotope signatures record water column carbon cycling processes rather than widely-proposed anaerobic diagenetic processes.

M.Sc. / This thesis involves a study ofthe geology surrounding the —530 Ma to —519 Ma Venetia kimberlite pipes situated between AIldays and Messina (now renamed Musina) in the Beit Bridge Terrane of the Limpopo Belt, South Africa. The Limpopo Belt is an eastnortheast trending high grade metamorphic terrane thought until recently to be the result ofa collisional event between the Kaapv_aal and Zimbabwe Cratons between 2.7 and 2.65 Ga. However, recent studies have challenged this concept and suggest that the assembly was more complex and took place over an extended period of time ending at —2.04 Ga. This study involved surface mapping of the Farms Rugen (south) and Ostrolenca, providing additional information to help with mine planning, grade control and exploration. It forms a portion of a project initiated between Venetia Mine, the Venetia- Limpopo Nature Reserve and Professor Jay Barton of RAU to geologically map in detail the area around the pipes (scale < 1:10 000) and to study various aspects of the regional geology. The rock types into which the Venetia kimberlite pipes intruded belong to the Venetia klippe, an east-west trending synclinal structure with the axial plane dipping steeply northwards. Lithologically, the Venetia klippe comprises four layered units in which interlayered granitic or arkosic quartzofeldspathic gneisses, with and without biotite and garnet, and para and ortho-amphibolite, quartzite and meta-carbonate rocks (marble and limestone), banded iron formation and calc-silicate rock occur. Geochemical analysis (SEM and electron microprobe) of the meta-carbonates (re-crystallised magnesian limestone, coarse-grained marble and fine-grained foliated marble), indicate the precursors to be magnesian limestone, dolomite and limestone. Several events have been identified during the structural evolution of the area. They include: formation of gneissic metamorphic layering, tectonic suturing between different lithologies, formation of a syncline and east-west strike-slip faulting, north-south trending folds and northeast-southwest dextral strike-slip faulting, tourmaline bearing pegmatite emplacement, dolerite intrusion, tourmaline absent pegmatite emplacement, kimberlite emplacement and reactivation of pre-existing structures. Depositional structures only in the fine-grained foliated marble are preserved, e.g. graded bedding, cross-bedding, rip-up clasts and channels. These structures suggest deposition of the carbonates in two main depositional environments; peritidal (channels and rip-up clasts) and subtidal shelf (graded bedding and cross-bedding). A study of pegmatites in the area shows two main pegmatite types: tourmaline bearing and tormaline absent, each locally displaying a zonation. Mineralogically, the pegmaties are rich in quartz and albite and lack K-feldspar so they are classified as sodic-rich or plagio-pegmatites. Step heating 40Ar/39Ar analyses of muscovite from undeformed pegmatite yields an age of —2.0 Ga, which is interpreted to represent the time of pegmatite emplacement into the Venetia klippe rocks. Structurally, the pegmatites are sheet-like bodies cross-cutting compositional layering, joints, faults, folds and the dolerite, except for the older tourmaline bearing pegmatite that has intruded along east-west faults, but not northeast-southwest trending faults. By applying the principles of a dike propagation model, the source of the Venetia pegmatites should be greater than 5X5X5 km in volume and a maximum of 10km away. An appropriate granitic source has been recognized on the farm Gotha to the south of the mine by Martina Barnett. Leucocratic granodiorite, tonalite and granite with minor xenoliths of amphibolite, quartzite and magnetite quartzite define the Gotha Granitic Complex and pegmatite decreases in abundance away from it to the north and east. Deposition of Unit 3 lithologies into a rifted basin and an ancient epeiric sea is possible. However, there is more evidence (peritidal and shelf environments of the metacarbonates) and clean mature quartzites to suggest deposition into a passive continental margin or epeiric sea similar to the Malmani dolomites of the Transvaal Supergroup.