The iron and manganese formations of the Paleoproterozoic Hotazel Formation in the southern Kalahari Manganese Field of South Africa: Paleoenvironmental and depositional implications at the onset of the Great Oxidation Event

The ∼2.4 Ga Hotazel Formation of the Transvaal Supergroup of South Africa marks the oldest major chemical sedimentary Mn accumulation and hosts the largest known land-based Mn deposit on Earth. Its age places it in temporal proximity to the Great Oxidation Event (GOE), potentially making its deposition a major paleoenvironmental event. However, the extent t to which it records a local versus a global event remains unclear. This paper assesses the stratigraphy, mineralogy and geochemistry of an unaltered drill core intersection of the Hotazel Formation close to the southern limit of the region. The stratigraphy in the study area is typical of the broader region. It is marked by three depositional cycles of banded Fe formation (BIF), hematite lutite (HL) and Mn formation (MnF). The observed sedimentology indicates that the Fe to Mn to Fe depositional cycles correspond to sea-level rise and fall, with the BIFs generally deposited in shallower water than the MnFs. Quartz and most Fe-Mn-rich oxides and silicates appear to be early diagenetic whereas the 13 C-depleted carbonates and coarser magnetite are paragenetically later. The implication is that all minerals, except for quartz, greenalite and stilpnomelane, formed from Fe 3+ - and Mn 3+/4+ -oxyhydroxide precursors. The Fe and Mn were most likely sourced as Fe 2+ and Mn 2+ from low-temperature hydrothermal plumes in a deeper, reduced water column. The main mode of deposition was the biologically mediated oxidation of Fe 2+ and Mn 2+ in shallower, oxygenated waters, implying that it is a biogenic deposit. The formed Fe 3+ - and Mn 3+/4+ -oxyhydroxides selectively scavenged different trace elements depending on the surface charge of their stable ligands, forming the trace element patterns observed in this study. Fe oxidation would have needed to reach near completion before Mn oxidation could proceed. The results imply that the Hotazel Formation was deposited in a semi-restricted basin with inverted bathymetry. Both a continental shelf with an offshore barrier and a continental back-arc basin would be suitable depositional settings to account for most of the features. A major implication is that the Hotazel Formation might have recorded an isolated, unique depositional setting rather than a global paleoenvironmental signature. It also implies that for land-based sedimentary Mn deposits the tectonic setting, basin architecture and chemical sedimentology are the critical exploration vectors rather than the age of the sedimentary sequence.