Albertus J. B. Smith

The Iron Formations of Southern Africa

Iron formations (IFs) are Fe-rich chemical sedimentary rocks that show a unique distribution through Precambrian time, with abundant deposition from approximately 3.8 Ga, reaching a volumetric peak at 2.5 Ga, disappearing at 1.8 Ga and returning at 0.8 to 0.6 Ga. They are important paleoenvironmental proxies, recording possible ancient marine water signatures. IFs also host the largest Fe ore deposits in the world. IFs can be classified based on three criteria: Texture; Mineralogy; and Stratigraphic setting. The geological record of Southern Africa contains examples of all IF types as based on all three classification criteria that also span most of the geological time periods that mark abundant IF deposition. Meso- to Neoarchean greenstone belt-hosted (Algoma-type) IFs occur within the majority of the Kaapvaal and Zimbabwe Cratons’ greenstone belts. Some of the world’s oldest Superior-type IFs, which occur within marine successions that mark stable shelf depositional settings, occur within the Mesoarchean Witwatersrand and Pongola Supergroups on the Kaapvaal Craton. The volumetric bulk of the IFs in Southern Africa occur in the Neoarchean to Paleoproterozoic Transvaal Supergroup on the Kaapvaal Craton, which contains multiple Superior-type IFs throughout its stratigraphy. Of these, the thickest and best developed IF is the approximately 2.5 Ga Asbesheuwels-Penge IF. Neoproterozoic, Rapitan-type IFs occur in the Gariep Belt of the Northern Cape Province of South Africa and southern Namibia as well as in the Damara and Otavi Belts of northern Namibia. These are associated with glacial diamictites and were deposited during the global Sturtian glaciation. The two major geochemical components in IFs are Fe2O3 and SiO2, with the Superior-type IFs of southern Africa generally having higher Fe2O3 and lower SiO2 contents than Algoma- and Rapitan-type IFs. Some IFs in the Pongola and Transvaal Supergroups are also enriched in MnO. The rare earth element contents of IFs generally indicate that they were precipitated from marine water, with Archean and Paleoproterozoic occurrences showing significant hydrothermal inputs. The stable C isotopes of Fe-rich carbonates in Superior-type IFs are depleted in 13C which suggest that it was sourced from organic C, implying biological activity during IF deposition. The depositional models developed for the Superior-type IFs of Southern Africa take into account lateral mineralogical facies variations in IFs, with Fe-silicate facies more proximal, Fe-carbonate facies intermediate and Fe-oxide facies more distal to the paleo-coastline. The precipitation of Fe was thought to have occurred through the oxidation of dissolved, hydrothermally-derived Fe2+ to Fe3+ by Fe-oxidizing bacteria, with the preserved mineralogical facies being formed during diagenesis or metamorphism. Although free oxygen is not required for Fe oxidation by photoferrotrophic bacteria, studies on Mn contents, Mo isotopes and the sequence stratigraphy of Fe-enrichment suggest that free oxygen was present during IF deposition in some instances. The Rapitan-type IFs of Southern Africa, due to their association with the Sturtian glaciation, are thought to have been deposited as a by-product of the global-scale glacial activity. Almost complete glacial ice cover would have led to reduced water bodies building up dissolved Fe2+, with melting of the ice sheets causing the oxidation and precipitation of Fe. Enrichment of IF to Fe ore took place by either top-down supergene (ore overlying oxidized IF) or bottom-up hydrothermal (ore underlying oxidized IF) processes that leached SiO2 and oxidized all Fe-bearing minerals. The ore-forming fluids likely had high Eh and high pH. The largest and best known supergene Fe ore deposits of Southern Africa are the Asbesheuwels Subgroup-hosted deposits at Sishen, Khumani, Beeshoek and Kolomela in the Nothern Cape Province of South-Africa. The best known hydrothermal Fe ore deposit is the Penge IF-hosted deposit at Thabazimbi in the Limpopo Province of South Africa. Other smaller Fe ore deposits occur in the Transvaal Supergroup of South Africa and in the greenstone belts of the Zimbabwe Craton.