A geological comparison of the Sishen and Sishen South (Welgevonden) iron ore deposits,Northern Cape Province,South Africa

Abstract

The Palaeo-Proterozoic Transvaal Supergroup in the Northern Cape Province of South Africa is host to the largest known resources of high-grade hematite ore on the Southern African continent. These ores are currently being exploited at Sishen and Beeshoek. Hematite occurrences on the Sishen South exploration project (some 65 km south of Sishen mine) are, at present, the focus of an intensive investigation. At both the Sishen and Sishen South deposits, mineralisation is found within lithologies belonging to the Transvaal and Oliphantshoek Supergroups. The Transvaal Supergroup comprises a basal, carbonate platform sequence (the Campbell Rand Subgroup) overlain by thick units of banded iron formation belonging to the Asbestos Hills Subgroup. The Asbestos Hills Subgroup is unconformably overlain by sedimentary rocks of the Gamagara Subgroup. The latter Subgroup has been correlated with the lowermost unit of the Oliphantshoek Supergroup. The Sishen and Sishen South deposits are located close to the western margin of the Kaapvaal craton and have been subjected to intensive, structural deformation. Both localities have undergone folding, faulting and thrusting. Early regional tectonism (2400-1700 Ma) played a critical role in the genesis and preservation of the ores from younger erosive events. At both deposits, four similar iron ore occurrences have been recognised. The ores have developed in specific depositional environments and hence display unique physical, chemical and metallurgical properties. First, the bulk of the high-grade hematite comprises laminated and massive ores, which are found in the upper parts of the Asbestos Hills Subgroup. The laminated and massive ores are best developed in basinal and pseudo-graben structures. Second, Collapse breccia ores of medium- and lower-grade are preserved within deep palaeosinkholes. The sinkholes have developed within the dolomites of the Campbellrand Subgroup. Finally, Lower-grade conglomeratic and gritty ores constitute the remainder of the deposits. These ores are found in the overlying clastic succession of conglomerates, shales and quartzites belonging to the Gamagara Subgroup. Although there are many stratigraphic and lithological similarities between the Sishen and Sishen South deposits, there are also important differences, which will impact on the profitable exploitation of the ores. The most striking contrast between the deposits is their size and geometry. Sishen mine is a single pit operation with an opencast mineable reserve of 895 Mt. The Sishen orebody is thicker, more continuous and significantly larger than the Sishen South deposit. The Sishen South deposit has a potential opencast reserve of 228 Mt. It comprises a number of small and narrow, isolated orebodies, spread over an area of ∼64 km2. There is a significant difference in grade between the deposits. On average, the orebodies of the Sishen South deposit have a higher iron content (+0·25%Fe), less potassium (-0·090%K₂O) and less phosphorus (-0·019 %P). The Sishen South deposit has less interbedded shale in the laminated and massive ores. The footwall of these high-grade ores is a light-density, intrusive sill, compared to banded iron-formation at Sishen. With a proposed heavy-medium separation plant at Sishen South, the impact of dilution on product grades is expected to be less than that currently experienced at Sishen. There is evidence at both localities to supportdifferent ore-forming processes.Most researchers are of the opinion that the laminated and massive ores are the product of supergene enrichment of primary banded iron-formations belonging to the Asbestos Hills Subgroup. Processes of laterisation and even basinal dewatering have been proposed to explain this supergene enrichment process. The exact mechanisms and events which led to the upgrading of the main ore horizon are, however, yet to be clarified. After the enrichment of the primary iron-formations (∼2200 Ma), the massive and laminated ores were folded into basinal and dome structures. Karstification took place in the vicinity of the Maramane anticline, which led to the development of sinkholes within which subsidence and collapse took place. These structures facilitated the later upgrading and preservation of the breccia ores. Some 400 Ma later, the laminated and massive ores were subjected to rapid mechanical erosion. Conglomeratic and gritty ores were deposited in extensive, alluvial fan systems at the base of the Gamagara Subgroup. These lithologies heralded the change to an oxidising palaeo-environment and the appearance of the first red-bed sequence in the region. The Sishen and Sishen South deposits developed in the same palaeo-environment and were subjected to similar tectonic deformation. There are, however, a number of differences between the deposits, which will impact on the proposed exploitation of the Sishen South orebodies.