Abstract
The Transvaal Supergroup in South Africa hosts some of the largest iron ore deposits in the world, with deposits hosted in two regions, namely the Griqualand West and Transvaal Basins. The high-grade iron ores hosted in the Kuruman and Griquatown Iron Formations (Asbesheuwels Subgroup) within the Maremane Dome in the Griqualand West region, Northern Cape Province, South Africa, are such examples. This study focuses on the iron ore bodies at Kumba Iron Ore’s Kolomela Mine, which is situated on the southern margin of the Maremane Dome. These high-grade ores are typically preserved in karst structures, with the main ore body, comprising laminated and massive ores, lying above oxidized banded iron formations (which, together with the ores, are locally termed the Manganore Formation) and below the Pre-Gamagara Unconformity. Conglomeratic ore is hosted directly above the unconformity in the Gamagara Formation. The banded iron formation (BIF), which is brecciated in some intervals, is more iron-rich at the top close to the contact with the ore, decreasing down the stratigraphy. The laminated ore, which occur at the bottom of the ore package, are made up of alternating high lustre and low lustre laminae and contain calcite, chlorite and biotite in the veins and the pores. The massive ores, which occur above the laminated ores, are made up of fine- to coarse grained rocks and textures varying between anhedral, microcrystalline, patchy and specular. The conglomeratic ore above the massive ores is made up of massive and laminated ore clasts; and the Doornfontein conglomerates contain BIF clasts with subordinate laminated and massive ore clasts, ferruginised quartz, shales and sandstone clasts. The Doornfontein conglomerates are not of ore-grade. Significant amounts of detrital pyrite were also found in the shales and conglomerates. All the samples contain subordinate amounts of apatite, chlorite, biotite, ilmenite, allanite, ankerite, quartz and calcite as fillings in pores, veins and breccias. The general stratigraphy shows an enrichment in Fe2O3 and a depletion in SiO2 relative to unaltered Kuruman BIF, except in the Manganore BIF, where the Fe2O3 and SiO2 are similar to unaltered BIF. There is also an enrichment in Al2O3, TiO2 and P2O5 and a depletion in MnO, iv MgO, and Na2O compared to unaltered Kuruman BIF. Ba and Sr are enriched in most of the analysed samples. The enrichment of shale-normalized light rare earth elements displayed by some the massive and laminated ores reveals a significant supergene enrichment event in the area, although some samples show an enrichment in HREY, which is thought to be the result of a later hydrothermal overprint. These samples show both positive and negative Ce anomalies, which are absent in pristine Asbesheuwels BIF, suggesting a post-depositional overprint by a high Eh fluid. The oxygen isotopes trends are comparable to those shown by iron ore from Sishen, Beeshoek and Rooineke Mines, which also occur below the Pre-Gamagara Unconformity and are considered to be typical supergene iron ores. The 4He/³He and (U/Th)/21Ne ages acquired for hematite do not correspond to the approximate age of the Gamagara Unconformity (~2.2-1.9 Ga). A sample of massive ore had a hematite age of 1640 Ma ±123.1, which does not correspond to any tectono-thermal activity in the region and is thought to be attributable to resetting from burial. Samples of brecciated ore, laminated ore and oxidized Manganore BIF gave hematite ages of 1340±72.2 Ma, 1050±70.40 Ma and 1070±57.6 Ma, respectively. The brecciated ore age potentially overlaps with the onset of the ~1.29-1.17 Ga Kheis orogeny or could be due to burial reset. However, laminated ore and oxidized Manganore BIF samples overlap with the Kheis and/or Namaqua-Natal (~1.2-1.0 Ga) orogenies. Ages clearly predating these orogenies, in conjunction with stratigraphic and geochemical evidence, suggest that the main ore-forming event was a Pre-Gamagara Unconformity-related supergene event, with erosion of BIF-hosted ore from the flanks into the karst-related basins depositing the conglomeratic ore. The younger hematite ages that overlap with the Kheis and/or Namaqua-Natal orogenies suggest that a hydrothermal overprint also occurred, although it appears to be limited to more permeable lithologies.
M.Sc. (Geology)