M.Sc. (Geology) / Nel, B.P. (2013). Petrography and geochemistry of iron formations of the Paleoproterozoic Koegas Subgroup, Transvaal Supergroup, Griqualand West, South Africa. MSc thesis (unpublished), University of Johannesburg, Aucklandpark, pp. 133. The Early Paleoproterozoic Koegas Subgroup comprises a succession of siltstone, mudstone, iron-­‐formation, chert and carbonate rocks that overlies the iron-­‐formations of the Asbestos Hills Subgroup with sharp contact. It is overlain with erosional unconformable contact by glaciogenic diamictites of the Makaganyene Formation. This study focused on the lithostratigraphy, mineralogy and geochemistry of the iron-­‐ formations of the Koegas Subgroup based on fresh diamond drill core samples obtained during the Agouron scientific drilling project in South Africa in 2004. The iron formations the Koegas Subgroup are represented by a few important lithotypes, occurring in distinct sedimentary facies, which formed in unique depositional and diagenetic environments. The iron formations consist essentially of four facies, namely silicate lutite, mixed silicate-­‐siderite lutite, siderite lutite and siderite peloidstone A repetitive sedimentary cycle consisting of fine-­‐grained chemical lithotypes grading upward into reworked chemical lithotypes is evident throughout the Koegas Subgroup iron formations. Silicate lutite formed in deep water settings well below the wave base along a chemocline. Siderite lutite formed in shallower parts of the basin through transformation of primary ferric iron precipitate by iron respiration in presence of organic carbon. Peloidstone formed above normal wave base in shallow water by reworking of earlier siderite lutite deposits. The REE geochemistry provides important clues as to the depositional environment of the iron formation as follows. Depletion in LREE and enrichment in HREE combined with positive Y are typical of ocean water indicate that the iron formations were deposited in a marine environment. Positive Eu anomaly suggest the presence of a hydrothermal component in the ocean water from which the iron formations were deposited. Negative Ce anomalies indicate that somewhere in the marine system Ce3+ was oxidized to Ce4+ oxide, probably in the presence of free oxygen in the ocean water column (Bau and Dulski, 1996). The negative Ce anomalies seen in the Koegas iron formations are the oldest currently known from iron formations. As such the Ce anomalies most probably signify an increase in the oxygenation state of the ocean immediately prior to the rise of atmospheric oxygen as defined by Guo et al. (2009).

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.