A process-based understanding of magmatic critical metal deposits

‘Critical’ metals are those which modern society relies upon for economical and technological progress but have inherent supply risks. Many critical metals are facing rising demand due to their use in green energy transition technologies (e.g. wind turbines, electric vehicles), in particular rare earth elements (REEs) and lithium (Li). These metals have become increasingly topical in the past few years as countries seek to diversify and secure their supply chains. In nature, many REE and Li deposits are associated with igneous rock types. For example, alkaline silicate intrusions and associated carbonatites are the world’s major hosts of REEs, whereas granitoids and pegmatites are a major Li source. However, we lack a detailed theoretical understanding of how REE or Li behave in these igneous systems, particularly the magmatic processes that control their enrichment and may subsequently lead to mineralisation. This knowledge is essential to diversify the critical metal supply, support technological progress and, for green energy materials, meet net-zero targets.  My project tackles this timely societal and geological challenge through an ambitious extension of a traditionally metamorphic geology tool (phase equilibria modelling) to an igneous and economic geology problem, creating a process-based understanding of magmatic critical metal deposits, with a particular focus on REEs and Li.
My project takes an observation-driven phase equilibria modelling approach to understand the magmatic processing of critical metals. Natural case studies (e.g. Greenland, southwest England and West Africa) provide a crucial reference frame for all modelling. Fieldwork is the starting point to collect rock samples and make observations, followed by detailed geochemical analysis at the whole-rock, thin section and individual mineral scale (using e.g. electron probe microanalysis and laser-ablation trace element mapping). We then integrate phase equilibria modelling, using a new suite of thermodynamic models developed during this project, to understand the observed major and accessory mineral behaviour as a function of a range of physio-chemical variables (e.g. pressure, temperature, oxidation state). This approach allows us to decode the formation conditions of the igneous system of interest. By simultaneously tracking the trace element evolution of all phases via partition coefficients, we have developed a powerful approach to quantify the controls on the ‘sweet-spots’ for magmatic critical metal enrichment in nature. During the renewal of this fellowship, our work will focus on extending our thermodynamic modelling capabilities to include carbonatite and fluorine-bearing melts, developing methods to account for ‘accessory’ minerals (e.g. eudialyte) that can host substantial REEs, and applying our workflow to Li-bearing granites following a focus on REEs in the first years of the project.
By understanding the magmatic processes that have led to world-class critical metal deposits such as the REE-rich Ilimaussaq intrusion in Greenland, or the Li-rich Cornwall granites in the UK, we can predict where to locate other similar deposits. To maximise the potential of our research, this project brings together a team of globally-renowned academic and industrial project partners in addition to the research team, providing world leading expertise spanning from the pluton scale down to mineral grains, and from critical metal policy to active industry. The innovative framework pioneered by this fellowship can be generalised to other magmatically-processed critical raw materials, providing a bold long-term program of research that could revolutionise industrial approaches to critical metal exploration in igneous settings.