Van Scheltema, Joseph2019-09-092019-09-092019https://hdl.handle.net/10539/28055A Dissertation submitted to the Faculty of Science, University of the Witwatersrand in the Fulfillment of the requirement for the degree of Master of ScienceThe Bushveld Complex is the largest known layered igneous intrusion on Earth and is vital for its Ni-Cu-PGEs resources. Recent geophysical studies indicate that the total areal extent of the Bushveld is more than 90 000 km2 (Finn et al., 2015). The northern limb of the Bushveld Complex in the Limpopo province has a sinuous N-S oriented outcrop with an exposed width of around 15 km and an exposed strike length of approximately 110 km. The northern truncation of the northern limb has been believed to be at the Hout River shear zone, however, exploration to the north of the northern limb, led by Platinum Group Metals (PTM) since 2011, revealed the presence of mineralised rocks of a Bushveld affinity, beneath the Waterberg Group sediments (Huthmann et al., 2016; Kinnaird et al., 2017). The Waterberg Project succession hosts world class mineralisation and McDonald et al., (2017) suggested that this succession continues southwards to the Aurora Project area, for up to 40 km. Sylvania Resources Limited, (2012) initiated drilling on the Harriet’s Wish farm located between the Waterberg Project to the north, and the Aurora Project to the south, with the Hout River shear zone cross-cutting the farm. The location of this study site is therefore, important in establishing the suggested continuation of the Waterberg Project mineralisation. Three drillcores looked at in this study are located to the north-, and one drillcore is located within the Hout River shear zone. To understand the Harriet’s Wish sequence, the following approaches were used: core logging to establish the lithologies; magnetic susceptibility profiles were obtained to define a boundary between the lower and upper lithologic units; petrographic study was undertaken to confirm lithologies and to identify the mineral assemblages; an ore mineralogical study was carried out to understand mineralisation distribution and controls; major and trace element geochemical study, CIPW norms and PGE tenor calculations were done to confirm stratigraphic subdivision and more fully understand the processes involved in the formation of the sequence. The results were compared to the known characteristics of the Waterberg Project and the Aurora Project successions as well as certain Platreef localities. Two lithologic units and two mineralised zones occur on Harriet’s Wish. The lower lithologic unit is composed of gabbronorite, gabbro, troctolite, olivine gabbronorite, pyroxenite and anorthosite, similar to the TGA sequence on the Waterberg Project. The upper lithologic unit is analogous to the Upper Zone on the Waterberg Project and comprises gabbronorite, gabbro, anorthosite, minor olivine-rich varieties as well as magnetite-bearing varieties and magnetitites. There is no equivalent to the Ultramafic Sequence found on Harriet’s Wish as harzburgite is absent on Harriet’s Wish. However, this unit may be present deeper in the sequence, or it may have been reworked during the emplacement of later gabbroic melts. The scarce chromite grains within the basal part of the lower lithologic unit, the relic olivine chadacrysts and the semi-dissolved olivine-rich fragments within troctolite and olivine gabbronorite could be remnants of earlier ultramafic cumulates. The lower mineralised zone is found within the lower lithologic unit of the Harriet’s Wish succession and shows some similarities to the F Zone of the Waterberg Project in terms of ore mineralisation style and base metal concentrations, although the petrographic and chemical compositions of host rock are different. The upper mineralised zone on Harriet’s Wish is geochemically analogous to the T Zone on the Waterberg Project, and the mineralisation on the Aurora Project. However, the Harriet’s Wish upper mineralised zone is distinguished by notably higher Au proportions than the Waterberg Project T Zone and the Aurora Project succession and carries the indicative quartz-chloriteamphibole-pyrite assemblage, which is common in the T Zone on the Waterberg Project, but absent on the Aurora Project. It is, therefore, suggested that the T Zone does indeed continue southwards from the proximal Waterberg facies to Harriet Wish and finally to the distal Aurora facies as suggested by Kinnaird et al., (2017) and McDonald et al., (2017). Furthermore, the lithology, geochemistry, ore mineralisation and petrography suggest that there are definitive differences in the magmatic stratigraphy across the Hout River shear zone. The idea that a separate magmatic basin or sub-chamber exists north of the Hout River shear zone is, therefore, supported by this study. It is envisaged that different processes related to the proximity to the footwall may have triggered the deposition of the lower mineralised zone. These processes include magma mixing and mingling, assimilation of xenoliths and hydrothermal fluid circulation that involves sediment-derived volatiles. The upper mineralised zone is suggested to be formed due to mixing of the resident S- and PGE-poor Main Zone-type magmas with the later influxes of the PGE- and S-rich fertile Upper Zone magma entering the chamber.enThesis