Abstract
The Kalahari Manganese Field situated in the Northern Cape Province of South Africa is the largest known land-based Mn deposit on Earth. It is hosted by the Fe- and Mn-rich chemical sedimentary succession of the approximately 2.4 Ga Hotazel Formation. The Hotazel Formation forms part of the Postmasburg Group in the Griqualand West region of the Transvaal Supergroup. It is conformably underlain by the Ongeluk Formation volcanics and capped by the carbonates of the Mooidraai Formation. The Hotazel Formation is composed of four banded iron formation (BIF) units separated by three Mn formation (MnF) units with hematite-lutite transitions between the units. This study is based on a single drill core MP72 drilled on the Middelplaats farm (Northern Cape Province) located south of Hotazel. Drill core MP72 intersects the entire Hotazel Formation and is pristine with no evidence of alteration. This study investigated all the BIF and MnF units in the Hotazel Formation in drill core MP72. The results are used to produce depositional models and gain further understanding of the paleoenvironmental conditions during deposition of the Hotazel Formation.
The Hotazel Formation, as studied from drill core MP72, displays three distinct BIF mineralogical facies. The bottom of the formation is dominated by oxide facies BIF, the middle of the formation by silicate facies BIF and the top by carbonate facies BIF. The MnF units are very fine grained with evidence of ovoids and lenticles and are fairly uniform. The mineralogy of the MnF units is dominated by braunite and kutnohorite. The BIF units are coarser grained with inclusions of sandy lenses and sedimentary structures such as cross lamination which indicates deposition might have occurred above wave base in the depositional basin. The transitional units (T) and MnF units show no dynamic sedimentary structures and were likely deposited deeper in the basin. Therefore, the BIF units were deposited source proximal in shallow water whereas the MnF units were deposited source distal in a deeper setting.
The geochemical analysis depicts the nature of the Hotazel Formation with a high Fe content in the BIF units (up to 61.297 wt%) and a high Mn content in the MnF units (up to 46.090 wt%) which clearly outlines the alternating layering of BIF and MnF units. The trace elemental analysis shows that deposition occurred in a marine environment as the REY patterns are similar to those of modern sea water, which is common for many
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Paleoproterozoic BIFs. The lack of Al and Ti shows that the formation was starved of clastic input which indicates that the REY patterns observed are a result chemical deposition and not clastic input. The REY shows two distinct features, the first being the lack of a positive Eu anomaly which indicates that the fluids that sourced the Fe and Mn were low temperature (>250°C). The negative Ce anomaly sheds new light on the Hotazel Formation as the full stratigraphic sample set shows that it occurs throughout the formation and is restricted mainly to the MnF and transitional units. The direct relationship between Fe and Ce and the inverse relationship between Mn and Ce suggests that the formation of the negative Ce anomaly was mostly like due to an in-situ oxidation process. During this process Fe oxidation buffers cerium oxidation, with cerium oxidation occurring after Fe depletion and prior to Mn oxidation. The progressive oxidation of Fe to cerium to Mn would have required the presence of free oxygen in the water body. The deposition of Fe and Mn was mediated by Fe- and Mn-oxidizing bacteria based on previously published δ13CPDB values (-9.5 to -3.40/00) and the volume of Fe and Mn found in the Hotazel Formation. The Mn-oxidizing bacteria would have required free oxygen
Based on the data represented in this study and in previous literature three possible depositional models are considered: 1) dynamic hydrothermal plume and sea level model, which depicts a back-arc basin which hosts a waxing and waning hydrothermal plume; 2) static depleting plume model, which depicts a static basin which is invaded hydrothermal plume pulses; and 3) continentally derived model, which depicts a continental source of Fe and Mn which enters a marine environment via dissolved continental run-off. All the models have aspects supported by, but also contradicted by the data presented in this study, with the dynamic hydrothermal plume and sea level model seeming most likely. There are limitations to creating a depositional model based on one drill core as the features found in this drill core cannot always be extrapolated to the whole basin. However, insights from the drill core allows important paleoenvironmental conditions aspects to be better understood.
All the considered depositional models require the presence of free oxygen in the marine environment during the deposition of the Hotazel Formation. Recent age determinations place the age of the Hotazel Formation just prior to or at the onset of the Great Oxidation Event (GOE), with the latter marking the first emergence of Earth’s ozone layer. This indicates that the GOE was preceded or marked at its onset by increases of free oxygen in marine environments.