The ~2.4 Ga Hotazel Formation of the Transvaal Supergroup of South Africa is best known for hosting the largest land-based manganese deposit in the world, namely the Kalahari Manganese Field. The Hotazel Formation comprises three sedimentary manganese ore beds interbedded with banded iron formation. Where the ~2.0 Ga pre-Gamagara unconformity intersects the Hotazel BIFs, medium to high grade hematite iron ore is developed. More porous, medium grade iron ore is also developed where the ~45 Ma Kalahari unconformity or faults connected to this unconformity intersects the BIFs. The thicker iron ore beds occur towards the top of the formation immediately below the pre-Gamagara unconformity and can be up to 5 m thick. Both laminated and massive ores are developed.
Above the unconformity, poorly sorted hematitic conglomeratic to arenitic ores are developed as localized facies of the laterally extensive basal Doornfontein conglomerate member of the Gamagara Formation. The laminated ore comprises fine- to medium-grained, compact, randomly orientated hematite aggregates, whereas the massive ore comprises non-laminated, medium- to coarse-grained, inequigranular hematite. The Fe2O3 content of the ores vary between ~70 and 97 wt%, with elevated MnO contents of up to ~11 wt% locally observed. The rare earth element concentrations in the ores show similar distributions to that of the original banded iron formations, but with higher contents and the light rare earth elements more enriched. The majority of ore samples show moderate to strong true positive and negative Ce anomalies, whereas true Ce anomalies are lacking in the banded iron formation. The hematite d18OSMOW values in the ore show a range of -1.4 to -0.1 . The occurrence of the Hotazel iron ores at the top of the formation above unaltered BIF and in contact with the pre-Gamagara unconformity, the strong Ce anomalies and the higher hematite d18OSMOW values suggest these ores were formed through supergene processes. It is thus most likely that descending meteoric waters marked by positive Eh and high pH leached silica and oxidized all iron-bearing phases to hematite, thus forming significant volumes of iron ore. CITATION:Smith, A J B, Gutzmer, J, Beukes, N J and Chisonga, B C, 2019. Iron ores of the Hotazel Formation, Northern Cape Province, South Africa, in Proceedings Iron Ore 2019, pp 420439 (The Australasian Institute of Mining and Metallurgy: Melbourne).