Chemostratigraphy of the lowermost iron-manganese cycle of the Hotazel Formation, and implications for its primary depositional environment

Masoabi, Ntseka Thomas

Title: Chemostratigraphy of the lowermost iron-manganese cycle of the Hotazel Formation, and implications for its primary depositional environment
Creator: Masoabi, Ntseka Thomas
Subject: Chemostratigraphy
Subject: Great Oxygenation Event
Subject: Manganese ores Geology South Africa Northern Cape
Subject: Banded iron formation
Date Issued: 2022-10-14
Date: 2022-10-14
Type: Academic theses
Type: Master's theses
Type: text
Identifier: http://hdl.handle.net/10962/362938
Identifier: vital:65376
Description: The giant Kalahari Manganese Field (KMF), located in the Northern Cape Province, South Africa, comprises approximately half of the world’s manganese resources, estimated at about eight billion tons at grades ranging from 20-48 wt%. The KMF is linked to a period in geological time when the Earth’s atmospheric and oceanic conditions underwent a major transition from oxygen-deficient to oxygen-enriched conditions – an event famously referred to as the Great Oxidation Event (GOE) that occurred around 2.4 Ga. The KMF deposits are hosted in Banded Iron Formation (BIF) of the Paleoproterozoic Hotazel Formation in the uppermost Transvaal Supergroup. The sedimentary Mn ores are interbedded with Hotazel BIF in the form of three alternating depositional cycles of BIF, transitional hematite lutite and laminated, carbonate-rich manganese ore. The lowermost and thickest of the three cycles is the most economically significant and has been mined for several decades on a large scale from the southernmost KMF. In this study, two drill cores from the southern KMF were inspected, logged and sampled at a high resolution of approximately half-meter interval per sample. The selected cores, namely G774, capturing the lower portion of the Hotazel Formation from the Mamatwan locality, and MP-56, capturing the corresponding portion from the Middleplaats locality, are geographically proximal to each other, with a horizontal distance of roughly 3 km separating the two of them. The G774 drill core is characterized by a conspicuously thick manganese layer covering a thickness of 50 m, with the overlying BIF reaching a total thickness of 11 m. The MP-56 drill core, on the other hand, has a relatively thinner corresponding manganese layer of 30 m in thickness, while the overlying BIF layer exhibits a thickness of 24 m. The extent of sampling up-section was constrained by an apparently coeval black shale layer which represents the chosen upper stratigraphic marker for the lower part of the Hotazel section in the broader area that is under investigation in this thesis. That way, a high resolution chemostratigraphic approach was employed to elucidate the potential factors contributing to the relative sedimentary lateral thickness variations seen across the southernmost KMF. High-resolution geochemical data were used to explore relationships and signals that might constrain relative precipitation rates for iron and manganese against detrital species, fluctuating redox conditions in the original environment of deposition, and chemostratigraphic correlation. All geochemical data (i.e., major oxides, minor and trace elements and carbonate carbon isotopes) were obtained respectively through employing X-ray Fluorescence (XRF), Laser Ablation Inductively Coupled Mass Spectrometry (ICP-MS), and Gas-source mass spectrometry. Comparative considerations made between the bulk geochemistry of the two sequences (i.e., Mamatwan and Middleplaats sections) reveal that periods of high-Mn deposition in the Hotazel Formation appear to be very Ca-carbonate rich (as indicated by high CaO, LOI and Sr concentrations). This, in turn, suggests that the Mn abundance is in the Hotazel ores is controlled mainly by the silicate phase braunite and is diluted by the deposition of Ca-carbonate through time. Bulk-rock concentration results for trace elements of the High Field Strength Element (HFSE) group (namely Zr, Hf, Y, Nb and Sc) were utilized to constrain the rates of either clastic and/or volcanic detrital inputs, as they traditionally represent refractory mineral particles of a common detrital/volcanic origin. The two chemosedimentary sequences preserve these elements in very low and thus quantitatively negligible concentrations – suggesting that the Hotazel depositional environment received very low and insignificant influx of a terrigeneous detrital component. A selection of these elements was therefore used to deduce, with caution, the relative as opposed to absolute precipitation rate of the major chemical constituents (i.e., Fe + Si vs Mn + carbonate), assuming a constant detrital flux through time. It was found that the relative abundances of Zr, Y and Nb is roughly 1.5 – 2 times as high in the BIF lithofacies relative to the Mn ones at both localities. This led to the inference that the Mn-enriched portion of the sediment must have been deposited at approximately twice the rate that the Fe-rich (BIF) portion was originally deposited. In terms of redox-sensitive elements, the elements Co and Mo seem to reveal the most valuable insights into the redox environment of primary chemical deposition. Cobalt displays a unique pattern in that its highest concentration is attained at the hematite lutite transitions (similarly with the REE in this regard), while very low and seemingly invariant concentration is exhibited within the core of the main orebodies. The same pattern seems to be reproduced to a degree by the corresponding bulk MgO component, whereby MgO abundance maxima are associated with the basal hematite lutite and the hematitic flanks of the Mn-ore zone, while the core of the Mn-rich layer attains relatively low and essentially invariant MgO concentrations. This implicates a close and direct association of Co with the hematite fraction of the rocks and a concurrent enrichment in Mn-rich carbonate (dolomite). On the other hand, Mo seems to have a direct and clear association with peak MnO2 content of the rocks, which in turn presents a high possibility of Mo having adsorbed onto primary Mn-oxyhydroxides in the water column, thus providing evidence that Mn-oxide must have acted as an important Mo sink, at least locally. Finally, the carbonate-carbon isotope results provide a useful tool that brings the two stratigraphic sections “together“, in conjunction with other correlatable chemostratigraphic parameters (e.g. Co, Mg). The results demonstrate that bulk carbon fluxes and isotopic signals in the sediments must reflect primary processes of deposition, and that correlation across two apparently disparate lithostratigraphic sections can be effected. The key finding is that, at times, manganese deposition in one part of a vii stratified basin was evidently accompanied by simultaneous BIF deposition at another, thus painting a very complex picture of massive primary chemical precipitation of Fe and Mn at the dawn of the GOE.
Description: Thesis (MSc) -- Faculty of Science, Geology, 2022
Format: computer
Format: online resource
Format: application/pdf
Format: 1 online resource (164 pages)
Format: pdf
Publisher: Rhodes University
Publisher: Faculty of Science, Geology
Language: English
Rights: Masoabi, Ntseka Thomas
Rights: Use of this resource is governed by the terms and conditions of the Creative Commons "Attribution-NonCommercial-ShareAlike" License (http://creativecommons.org/licenses/by-nc-sa/2.0/)

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