Epithermal mineralisation related to carbonatites: a key potential source of critical heavy rare earth elements for clean energy

Rare earth elements (rare earths) are a group of 16 elements with similar chemical properties, which naturally occur together. The rare earths neodymium, praseodymium, samarium, dysprosium, and terbium are highly economically important as they are used in high-strength permanent magnets. These magnets are essential for producing electric motors in electric vehicles and for generating power from wind turbines. Demand for these technologies is forecast to grow substantially to 2026 in order to meet clean energy targets and reduce CO2 emissions. Demand for rare earth magnets will also follow this upward trend, but there are issues relating to the stable provision of rare earth ore. One issue is that existing production is limited to China, who control over 90% of the World's rare earth supply. This limited supply is unstable, as demonstrated by China's 2010 restriction of rare earths to Japan over a territorial dispute. A second issue is that not enough 'heavy' rare earths (e.g., terbium and dysprosium), are currently mined compared to the 'light' rare earths (e.g. neodymium). This is because heavy rare earths are naturally less abundant, but also because the current major source of rare earths, a rock type termed a 'carbonatite', is predominantly only light rare earth-rich.

During the proposed project, I will address the problem of limited heavy rare earth supplies by investigating a new type of heavy rare earth mineralisation occurring near existing light rare earth-rich carbonatite deposits. If mined together, these combined deposit types could provide the correct mix of rare earths required for the magnet industry. However, there are several problems facing mineral exploration companies who wish to find and exploit these deposits:
- It is unclear where they are located, why they occur, or how they vary,
- There is no clear way to easily find these new deposits,
- It is currently uncertain if it is possible to economically mine these deposits.

My proposed research, undertaken in collaboration with UK-based mineral exploration companies, will resolve these issues through the development of 'geomodels'. Geomodels re-imagine ore deposits in terms of their formation process; in this case, likely to be a volcanic system interacting with water to form hot springs. When hot springs form mineral deposits deep underground they are termed 'epithermal' deposits. I will test if some of the aspects of an epithermal deposit geomodel are applicable to heavy rare earth element mineralisation around carbonatites.

Geologists have a wide toolkit in order to develop and test geomodels. A key starting point, however, is to study the relationships between different rocks and minerals in order to understand the timing of mineralisation. This is obtained from field and microscopic observations of rock samples. Small variations in what a mineral is made of can reveal the timing and conditions in which it formed. In this project, I will investigate the composition of minerals in order to determine the timing of mineralisation, as well as fluid temperature and composition. Furthermore, analyses of microscopic amounts of trapped water within minerals will be undertaken to back-calculate the original formation temperature. These techniques will be used on samples from five different deposits from different depths in the Earth. Observations from the five test localities will be compared with an epithermal model, in order to produce a final geomodel suitable for the mineral exploration industry.

The resulting geomodel will be of benefit to industry by enabling the rapid discovery and assessment of new heavy rare earth resources and reducing the cost and environmental impact of mineral exploration through better-informed prospecting. In the long run, exploitation of epithermal heavy rare earth deposits will lead to a secure and balanced supply of the heavy rare earths, essential for magnet production, into the UK supply chain.