Abstract
The Paleoproterozoic Kalahari Manganese Field hosts the world’s largest land-based manganese resource within several structurally preserved deposits. Of the five structurally preserved remnants, the Main Kalahari Deposit (MKD) is by far the largest and most well studied. The chemical sedimentary manganese formations (MnFs) are hosted in a cyclical depositional sequence known as the Hotazel Formation which comprises three sequences of banded iron formation (BIF), transitional hematite lutite (HL), and MnF. The BIFs and MnFs of the Hotazel Formation transition into the Mooidraai Formation, which is a succession of deep and shallow marine carbonate units. The Voëlwater Subgroup comprises the Mooidraai, Hotazel, Ongeluk, and Makganyene formations, however the study only focuses on the upper three (Mooidraai, Hotazel, Ongeluk) formations due to their conformable nature and proposed genetic link, as is presented in this study.
Past studies have primarily focused on selected mining and prospect areas, as well as individual and clustered groups of drill cores which covered limited portions of the MKD. Past studies have also primarily focused on the economically important lower manganese bed (MnF1), and sub-economic upper manganese bed (MnF3), with a great deal of focus placed on post depositional hydrothermal and metamorphic alteration of the primary Mn ores. Little research has been done on the host BIF beds, the middle manganese bed (MnF2), and transitional HL beds, as well as the overlying Mooidraai Formation carbonates.
This study is the first of its kind to assess the entire Voëlwater Subgroup and to characterise and define the relationship between all the chemical sedimentary units on a basin wide scale. The study utilised drill cores from across the entire MKD, with focus placed on selecting drill cores which preserved the most complete and unaltered Voëlwater Subgroup stratigraphy. Detailed logs were compiled along with selecting representative samples of the lithologies of interest to characterise the internal lithostratigraphy of the Ongeluk, Hotazel, and Mooidraai formations, as well as to
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characterise the mineralogical, geochemical, and petrographic nature of the various lithologies.
Mineralogical, geochemical, and petrographic analysis of the BIFs, MnFs, and HL beds revealed the development of several mineralogically distinct facies. A total of six mineralogical facies were defined for the unaltered BIFs, comprising three end members (oxide, silicate, carbonate), and three mixed facies (oxide-silicate, oxide-carbonate, and oxide-carbonate-silicate). A total of five facies were defined for the unaltered MnFs, grading from a Mn-carbonate dominant facies 1, to a Mn-oxide dominant facies 5.
A clear mineralogical facies trend is evident within the Hotazel Formation, ranging from oxide-dominant mineral facies in the lower beds, silicate-dominant in the middle beds, and carbonate-dominant in the upper beds. This indicates that both redox and non-redox related processes were active throughout different stages of the depositional history of the Hotazel Formation, as well as having variable levels of contribution to the observed iron (Fe) and manganese (Mn) accumulations.
The HL beds exhibit mineralogical characteristics intermediate between the BIFs and MnFs. Five facies were identified based on mineralogy and modal abundances. Textural characteristics were used to define three Ongeluk Formation volcanic rock facies (amygdaloidal, hyaloclastite, massive/pillowed). Nine distinct lithofacies were defined for the Mooidraai Formation, each of which is indicative of a specific depositional environment primarily related to water depth.
Petrographic analyses of the chemical sedimentary rocks revealed that three distinct phases of mineral paragenesis could be identified, which are: 1) primary mineral precipitates formed directly from seawater; 2) early diagenetic oxides, carbonates, and silicates of varying compositions; and 3) late diagenetic to metamorphic oxides, carbonates, and silicates.
The major element geochemistry revealed the cyclicity of the Hotazel Formation in terms of Fe and Mn deposition, as well as the gradual increase in carbonate content from the base of the Hotazel Formation to the top. The REY and trace element data suggests that the Fe and Mn were derived from low-temperature hydrothermal fluids, most likely from
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an intermediate source rock, and precipitated in a low energy stable marine environment with a distinct seawater geochemical signature. Negative cerium anomalies preserved in the MnFs indicate the strong effect of oxidative processes during deposition. Carbonate carbon isotopes are generally depleted in their δ13CPDB values, indicating a mix of marine carbon and carbon derived from the oxidation of organic carbon. This suggests that the redox reaction between Fe3+ and/or Mn3+/4+ and organic carbon was a significant diagenetic process to form the observed Fe- and Mn-rich carbonates.
The processes under which Fe and Mn accumulations occurred within the basin which led to the cyclical deposition of the BIF, MnFs, and HL beds have been addressed here as well. It is suggested that gradual fluctuations in Eh levels due to sea-level rise and fall in conjunction with initial near-complete removal of Fe2+ prior to Mn2+ oxidation account for the geochemical separation of Fe and Mn. Localised inter-basinal physical and chemical conditions and precipitation processes influenced the rate of Fe and Mn accumulation resulting in the observed thickness variability of the individual beds, as well as influencing the formation and development of the various mineralogical facies.
Based on the findings and interpretation of the data presented in this study, it is proposed that deposition of the Hotazel Formation occurred in an O2 stratified, semi-restricted back-arc basin, with variable footwall topography, similar to the modern-day Black Sea basin. This basin was spatially linked to a landward carbonate shelf (Mooidraai Formation). The data indicates that the Fe and Mn were likely sourced from the footwall Ongeluk Formation volcanic rocks, and that during times of sea-level low stand (regression phase), high concentrations of Fe2+ and Mn2+ accumulated in anoxic bottom waters within a well stratified, restricted basin. A shift towards a transgressive phase resulted in increased circulation and oxygenation of the restricted basin, which would have gradually increased Eh-levels, allowing for the progressive oxidation and precipitation of Fe followed by Mn across a redoxcline. The cyclicity of the Hotazel Formation is attributed to multiple such transgression and regression phases, along with periodic Fe and Mn recharge events.
The deposition of Fe and Mn ceased and was replaced by the deposition of various carbonate lithofacies of the Mooidraai Formation which occurred due to the advancement of a prograding carbonate platform towards the north of the MKD. The lithofacies of the
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Mooidraai Formation also reflect a basin with significant footwall morphology, as well as a depositional environment that was subject to rapid changes in sea-level over its depositional history.