The timing and nature of post-depositional iron mineralization in ore from the Wolhaarkop Dome, Northern Cape Province, South Africa

Abstract: The high-grade hematite iron ore deposits of the Griqualand West Basin in the Northern Cape, hosted in the Kuruman and Griquatown Iron Formations (IFs), are the largest known iron ore deposits in southern Africa. These deposits occur on or close to a regional anticline called the Maremane Dome; this includes the Wolhaarkop Dome, a smaller anticlinal structure west of the Maremane Dome. The Wolhaarkop Dome is the main structure in the Kolomela area and bordered by the Kheis Terrane to the west and Namaqua-Natal metamorphic province to the south. Iron mineralisation in the Wolhaarkop Dome consists of laminated, massive and breccia ore making up iron ores from the Manganore Formation. These are overlain by detritally derived iron ores of the Gamagara Formation. The most prevalent ore type is the breccia ore and the rest of the ore types are not as developed. The rest of the stratigraphy is made up of conglomerates and iron-rich shales of the Doornfontein Conglomerate Member and quartzites and iron-rich shales of the Sishen Shale Member. Several hematite textures, classified based on morphology and size, can be identified in the different iron ore types. This includes cryptocrystalline hematite, microplaty hematite, patchy hematite and martite as well as specularite. Mineralogically the iron ores are dominated by hematite. The gangue minerals present include quartz (chert), aluminium silicates, pyrite, barite, gypsum, apatite, calcite, monazite and xenotime. The oxidised banded iron formation (BIF), iron ore and shales are characterised by the enrichment of Fe2O3, Al2O3, TiO2 and K2O and depletion of SiO2, MnO, MgO, CaO, Na2O and P2O5. The rare earth element (REE) PAAS (Post-Archean Australian shale) normalised diagrams reveal an influence of multiple events. This includes a prominent supergene event supported by light REE-enriched shale-normalised diagrams combined with positive Ce anomalies. A heavy REE-enriched signature is reflected in the iron ores and these typically have higher Al and Ti concentrations, Eu anomalies different to that in the protolith. Oxygen isotopes in samples from the present study are comparable to those of the Sishen, Beeshoek and Rooinekke iron ore deposits. The formation of iron ore deposit in the Wolhaarkop Dome formed from a multi-stage model combing an initial supergene event and subsequent reworking of the earlier formed ores. This was then followed by hydrothermal event reflected in specularite veins cross cutting the iron iv ore. Using (U-Th)/4He and (U-Th)/21Ne hematite dating, laminated and massive ores from the Wolhaarkop Dome revealed ages of 1540 ±103.31 Ma and 1400 ± 94 Ma, respectively. These ages correspond to a period of cratonic stability and therefore represent resetting ages during burial metamorphism, implying that the main ore-forming event precede these ages. Younger ages in specularite veins and BIF occurring at 1100 ± 230.71 Ma, 1100 ± 49.48 Ma and 966 ± 47.32 Ma, overlap with the Kheis and Namaqua-Natal orogeny. These ages are interpreted to represent non-pervasive hydrothermal fluid-flow on the Wolhaarkop Dome that potentially could have caused further upgrading of the ore.M.Sc. (Geology