Nickel mineralisation in the Jamestown ophiolite, Barberton greenstone belt, South Africa
MetadataShow full item record
The Bon Accord oxide deposit was found in the Bon Accord farm in Barberton, Mpumalanga Province, South Africa. It represented a remarkable deposit that was hosted within highly serpentinised meta-peridotites of the Onverwacht Group of the Barberton Supergroup in the Barberton Greenstone Belt. The aims of this research are to investigate methods that can be applied in modelling the characteristics of the nickel oxide mineralisation in the deposit, as well as establish parameters that can be used to find similar deposits. A variety of analytical techniques (Appendix A) were used to investigate the samples. The ultramafic rocks of the Bon Accord nickel oxide deposit are highly deficient in sulphur but have unusually high nickel mineralisation (NiO ~36%) and comprise unusually high nickel-rich mineral assemblages. The nearly chondritic average ratios (Gd/Yb)N ≈1, and Al2O3/TiO2 ≈17 of the in Bon Accord nickel oxide body are similarities shared with komatiites, and may be thought to represent a possible link between the two rock suites and their processes of formation. Bon Accord oxide and its altered host rocks are characterised by slightly enriched light rare earth elements and flat heavy rare earth elements in both chondrite and primitive mantle normalised plots. The formation model tends to correlate with mantle thermal plume activities possibly induced by either subduction of cold lithosphere or by lithosphere subduction in a back arc environment. Such environments are suitable for the production of the observed crustal characteristics seen in the Bon Accord host rocks and also present a possible mechanism for the exposure at surface of what would have been a dense nickel-rich mass. The density of Bon Accord oxide deposit requires that the mantle thermal plume(s) responsible for transportation, must have been large with sufficient energy, and high temperatures >1800°C to accommodate and maintain such a process without losing all the material and energy during ascent. The occurrence of material which is highly enriched chalcogenic elements (As and Sb) but sulphur deficient led to the formation of unknown mineral phases serve which could serve as an indication of favourable environmental properties to form the nickel enrichment. While the relatively flat PGE patterns with a Pd negative anomaly and low Pd/Ir ratios (<1), low Ni/Cu and Pd/Ir and relatively enriched Pd/Pt ratios in the nickel sulphide host rock samples imply formation in an environment that contained some sulphur phases that were replaced by late stage sulphur deficient processes, similar to those observed in komatiites. Enrichment could have been from activities associated with post-magmatic, low-temperature hydrothermal oxidation of primary magmatic sulphide phases and the associated sulphur loss which possibly led to significant upgrading of the originally already concentrated nickel mineralisation. The variable magnetic susceptibility measurements observed in the Bon Accord oxide and host rock samples were influenced by temperature, grain size and chemical composition. Bon Accord oxide samples are mainly characterised by ferromagnetic materials with relatively large positive values and susceptibility readings in excess of 2000. Bon Accord host rocks are characteristic of both diamagnetic and paramagnetic properties with readings recorded almost <100. The high magnetic properties of Bon Accord oxide samples probablymare due to the abundant occurrence of Ni- magnetite trevorite.