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Structural features of the Bokkeveld Group (Cape Supergroup) in the Eastern Cape, South Africa Gideon Brunsdon 1 , P.W.K. Booth 2 , J.A. Mondon 3 1. NMMU, South Africa, gideon.brunsdon@nmmu.ac.za 2. NMMU, South Africa, peter.booth@nmmu.ac.za 3. NMMU, South Africa, jean-luc.mondon@nmmu.ac.za ABSTRACT Previous studies of the Bokkeveld Group focussed mostly on the sedimentation and sedimentary features of this group with a view to interpreting the depositional environment. This study aims to contribute new information on the structural features of the Bokkeveld Group north-west of Uitenhage in the Eastern Cape, where there is evidence of folding and thrust faulting, similar to that occurring in the under- and overlying groups of the Cape Supergroup. Field work to date shows that predominantly argillaceous beds of siltstones and mudstones, with lesser arenites and thin conglomerates make up the Bokkeveld Group. This group conformably overlies the Table Mountain Group of predominantly arenaceous beds. Both groups have been folded into asymmetric anticlines and synclines, with a prominent, south-dipping pervasive cleavage developed in the Bokkeveld Group. The fissile nature of argillaceous beds on outcrop have required that very careful observations of the all important bedding-cleavage relationship in the field are made, so that correct interpretations of facing direction of strata can be applied. Thrust faults showing fore-and backthrusts in the Bokkeveld Group are interpreted to have formed during a main deformation phase of the Cape Orogeny, during the Permian. Samples of mainly argillaceous rocks have been collected and mineral compositions identified with the aid of a petrographic microscope and a scanning electron microscope (SEM). Bedding in argillaceous shales of the Gydo Formation shows a graded texture with layers of coarse grained silicate minerals alternating with fine grained chlorite. Chlorite, muscovite and quartz make up the major minerals in the shales, and accessories include zircon, Rare Earths minerals and opaques (iron ore). In places bedding planes dip predominantly to the northeast, but this is probably a function of poor outcrop because bedding does dip in the opposite direction elsewhere in the study area, indicating the presence of folding in the Bokkeveld Group. A pervasive cleavage dips consistently to the southeast, and appears to be thrust fault related. It is however possible that some of the cleavage development might also be closely related to folding in the Bokkeveld Group. This relationship needs to be carefully verified through further microscopic studies. The results of this study will inevitably point to the fact that strata have been disrupted by structures such as folds, thrust faults and cleavages to the extent that the stratigraphic order of the Bokkeveld Group in this area will have to be reassessed. KEYWORDS: Bokkeveld Group, cleavage, folded, faulted strata 12
An Investigation of Nickel Mineralization in the Northwestern Part of the Barberton Greenstone Belt, South Africa N. Chabangu, M. Tredoux, F. Madala University of the Free State, P. O. Box 339, Bloemfontein, 9300, South Africa chabangun@ufs.ac.za, mtredoux@ufs.ac.za, fulufhelomadala@yahoo.com ABSTRACT The scope of this research is to evaluate the most probable origin of the Bon Accord (BA) Nickel-Oxide body and the possible mechanisms responsible for its formation, since it appears to be the only type of this deposit found in the world. Its occurrence seems to be confined to the Bon Accord farm near Barberton in Mpumalanga, South Africa. It may represent a very rare type of rock. The Barberton Greenstone Belt (BGB) formed during the Archean Eon. It represents > 3.0 Ga rocks in a greenschist facies belt which might contain remnants of ancient oceanic crust. The BGB consists of a strongly folded, ENE-trending, mid-Archean (3.6-3.1 Ga), volcano-sedimentary remnant, entirely surrounded by a variety of granitoids of the Kaapvaal Craton. Geophysical data has shown that the greenstone belt reaches a maximum thickness of 7.5 km and on average 3-5 km. The objectives of the study include researching the following: 1) The relationship between the Nickel-Oxide BA body and the associated low grade sulphide mineralizations. 2) The evaluation of the full set of major elements found in BA using modern methods and the implication of these data for BA and its probable origin. 3) How the current data can be reconciled with previously proposed models for the formation of BA. The work done to date: Eleven samples were analysed, ten of these samples were collected from the BA body during this current research program. The extra sample was derived from a previous research program and was donated to this study by Prof Grant Cawthorn. The analysis done on the ten samples from BA, include mineralogical investigations, XRD, XRF work while susceptibility readings were taken on all the eleven samples because of the high magnetic remanence of BA. The work done until now also includes field trips to the BA deposit. A new geological map of BA and the surrounding areas is currently under production. Future work includes sample analysis and interpretation using the SEM, EMP and ICP-MS. KEYWORDS: Barberton Greenstone Belt, Bon Accord, Nickel Oxide, Sulphides 13
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