Lithostratigraphic correlation and mineralogical facies characterisation of the banded iron and manganese formations of the Palaeoproterozoic Hotazel Formation of the Voëlwater Subgroup Transvaal Supergroup, Northern Cape Province, South Africa

Abstract The Palaeoproterozoic Kalahari Manganese Field (KMF) hosts the world’s largest land-based manganese (Mn) resource within five structurally preserved deposits, of which the Main Kalahari Deposit (MKD) is, by far, the largest and best studied. The Mn ores occur as chemical sedimentary manganese formation (MnF) hosted in the cyclical depositional sequence of the Hotazel Formation which comprises three symmetrical sequences of banded iron formation (BIF), transitional hematite lutite (HL), and MnF. Conformably underlying and overlying the Hotazel Formation are the volcanic Ongeluk and carbonate-rich Mooidraai formations, respectively. This study presents the first detailed account of basin-wide lithostratigraphic correlation, thickness variation, and mineralogical facies characterisation and distribution for the four BIF and three MnF beds of the Hotazel Formation. Mineralogical, geochemical, and petrographic analyses of unaltered stratigraphy reveal the existence of six distinct BIF and five MnF mineralogical facies developed across the MKD. Lithostratigraphic analysis of the BIFs and MnFs indicate remarkable lateral stratigraphic continuity within individual beds showing no signs of lateral overlap between BIF and MnF. The individual BIF and MnF beds do, however, exhibit lateral as well as vertical mineralogical facies changes, as well as considerable lateral thickness variability with a somewhat inverse thickness relationship evident in most cases. Petrographic observations indicate the occurrence of primary (direct seawater precipitates) to early diagenetic mineral phases, as well as late-diagenetic minerals, comprising a variety of oxide, carbonate, and silicate minerals for the BIFs and MnFs. One way ANOVA analysis (post-hoc Tukey) of the whole rock Fe2O3, SiO2, MnO, and CaO concentrations for the BIFs and MnF revealed that apart from the carbonate dominant BIF and MnF facies, there is no statistically significant difference between the mean values of Fe2O3, SiO2, MnO, and CaO for the various facies. Based on the lithostratigraphic, mineralogical, and geochemical data presented here, it is proposed that deposition of the Fe and Mn-rich chemical sediments occurred within a semi-restricted back-arc basin, which possessed considerable physiochemical heterogeneity throughout the basin. It is interpreted that deposition of the BIF and MnF beds occurred within a redox-stratified marine environment in which considerable variation existed within sub-basins with regards to soluble metal concentrations, water depth, water circulation, oxygen levels, salinity, microbial activity, organic carbon input, as well as Eh-pH conditions. The interpreted heterogeneity and overlapping redox and non-redox precipitation processes within the basin are believed to have resulted in the observed mineralogical facies and thickness variability of the BIF and MnF beds.