Dr Hannah Hughes
Metals in the mantle
My research aims to understand the sources and budget of metals and sulphur spatially and through time to assess if/how this influences mineralisation in the crust. I do this by studying the controls on platinum-group element (PGE), chalcophile element and semi-metal (often classed as ‘critical’ and/or precious metals) mobility in the mantle, particularly the lithospheric (upper) mantle. These elements are intimately linked with the mobility of sulphur, often present as sulphide minerals, through the processes of partial melting and metasomatism. Mantle xenoliths and sulphide inclusions in diamonds can be used as direct insights into the abundance of these elements at depth. Lamprophyric rocks (produced by very small degrees of partial melting) can also provide useful insights into the composition of the upper mantle. By linking these xenolith, diamond inclusion and lamprophyre-based observations with the regional metallic fingerprints of large igneous provinces (LIP) and other magmatic rocks at the Earth’s surface, my co-workers and I assess how the metal basket of some ore deposits, such as Ni-Cu-PGE mineralisation, may be controlled by previous tectonic events (or ‘preconditioned’). This preconditioning may have the ability to change precious metal ratios (e.g., Pt vs. Pd) in mantle-derived magmas, and thereby change the metal basket of subsequent mineral deposits. The melting conditions and even source depth of mantle plumes may similarly have a significant effect on their PGE geochemistry and ultimately their fertility for precious metals - as investigated in an ongoing study with Jordan Lindsay.
Ultramafic rocks (comprising dunite, harzburgite, wherlite or serpentinite) are an important but poorly understood component of ancient crust (Archaean cratons) in greenstone belts and tonalite-tronjhemite-granodiorite (TTG) basement terranes. While many ultramafic units have been mapped and undergone limited petrographic studies, modern geochemical methods have not been widely applied to these rocks, in such a way as to constrain their origin(s) and the implications regarding Archaean geodynamic settings. Through a study by George Guice, we aim to generate a series of geochemical tools that will constrain the origin(s) of suites of Archaean ultramafic bodies from two contrasting cratonic regions – the Kaapvaal Craton and the North Atlantic Craton. This will reveal the extent to which ultramafic units of apparently different origin(s) became dismembered and juxtaposed against one another – in essence, the processes governing ‘cratonisation’ itself. An associated aim of the project will be to examine the relationship between Archaean igneous ultramafic processes and mineralisation potential (i.e., craton-specific mineralization).
Underground mine gas outbursts and lamprophyres
Lamprophyre dykes represent a serious hazard to mining excavations and developments in some areas of the Bushveld Complex. Falls of ground, catastrophic blow-outs and wall rock failures caused by explosive gas pockets are known to occur where lamprophyres have been exposed during mining. In collaboration with Bushveld mines and funded by the Royal Academy of Engineering, we investigate the mineralogy and composition of these lamprophyre dykes and their gases, we can contribute pragmatic feedback to the mining industry with applied mineralogical and geotechnical advice on how to mitigate these dangers.