Boron isotopes of Manganese ores from the northern part of the Kalahari Manganese Deposit, South Africa

The world-class deposits of the Kalahari Manganese Field (KMF) in South Africa constitute a key resource of manganese (Mn) ore for the global steel industry. Many aspects of the origin of high metal-grade ores in the northernmost KMF remain unresolved, especially with respect to the complex hydrothermal history of these ores and the source/s of fluids responsible for epigenetic metal enrichment. In this study, the geochemistry of boron (B) is employed as a potential proxy for such processes, and the results are contextualised against an assumed low metal-grade precursor. Samples collected from the northern part of the Kalahari Manganese Deposit allow for the distinction of three Mn ore types, of which the precursor ore is the closest candidate, petrographically and geochemically, to primary/diagenetic Mn deposition. This precursor ore is dominated by Mn-carbonate and braunite, and records B concentrations in the order of a few 100s to 1 000s ppm and positive d¹¹B values at an average of 11 ± 6‰. Ferruginous Mn ores appear to have formed from decarbonation of precursor ore and metasomatic ferric iron addition, particularly in the stratigraphically upper Mn ore bed, which is closest to the overlying Olifantshoek and Kalahari unconformities. Ferruginous Mn ores appear depleted in B (ca. 500 ppm on average) and have lower boron isotope values (d¹¹B: 0 ± 3‰). Hydrothermally Mn-enriched ores, in contrast, are dominated by hausmannite, braunite-II and bixbyite and record highest B concentrations (up to 10 000s ppm) and d¹¹B values as high as 41‰, pointing to metasomatic introduction of B by circulating evaporitic brines. While further research is needed to comprehensively unravel the geochemical intricacies of B within the complex interplay of fluids and rocks in the Hotazel region, the findings of this study still strongly suggest that B holds promise as a tracer for alteration processes in the KMF.