Microstructural characterisation of sulphide and oxide minerals in magmatic sulphide ores

[Truncated abstract] Magmatic sulphide ore deposits are a major source of economically valuable commodities such as Ni, Cu and platinum group elements (PGE). These deposits form as a result of the segregation and concentration of sulphide liquid droplets from mafic and ultramafic magmas. The geochemistry and petrology of these systems has been extensively studied, as have regional and outcrop scale aspects of the structural geology of deformed deposits. However, microstructural work on igneous rocks in general and magmatic ore systems in particular has received relatively little attention to date. Microstructural studies can help in understanding the large scale deformation process, through investigation of the development of micro-scale textures such as crystallographic preferred orientations and development of exsolution features. Investigation of intragrain microstructures can identify the presence of dislocation arrays within the crystals that accompany grain-scale deformation and potentially facilitate element diffusion, and allow fundamental understanding of the grain-scale process that control the bulk rheology of sulphide ores during deformation. Understanding the relative strengths and deformation mechanisms of the common mineral species is relevant to an ongoing controversy about the role of mechanical differentiation of polyphase sulphide assemblages in controlling short range variability in ore tenors. This thesis describes the first study to employ electron backscatter diffraction (EBSD) analysis to investigate textures and intragrain microstructures within magmatic sulphide and oxide minerals. The study focuses on two types of deposits, komatiitic hosted Ni sulphides (Yilgarn Craton, Western Australia) and layered intrusion-hosted PGE-rich disseminated sulphides (Bushveld Complex, South Africa). Electron backscatter diffraction data is also coupled with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and Nano-scale secondary ion mass spectrometry (NanoSIMS) data to understand how deformation controls mineral chemistry at the micro-scale. In addition, EBSD microstructural results have been coupled with X-ray micro-tomography data to gain a greater understanding of the 3D morphology of the sulphides and other phases. Through the study of differently deformed komatiite hosted Ni deposits from the Yilgarn craton, and tectonically undisturbed layered intrusions, the Bushveld Complex, this thesis addresses the following objectives: a) obtaining fundamental information on physical properties and deformation mechanisms of pyrrhotite, pentlandite and pyrite under different tectonic conditions, b) trace element behaviour during deformation of sulphides and c) behaviour of sulphides and associate phases in tectonically undisturbed magmatic environments...