Fluocerite as a precursor to rare earth element fractionation in ore-forming systems

Emerging renewable energy technologies and low-carbon transportation rely heavily on the unique optical and magnetic properties of the rare earth elements. The medium to heavy rare earth elements, neodymium to lutetium, are most sought by industry but are the least abundant in nature. Only a small proportion of known rare earth element deposits are enriched in these elements. Identifying additional sources of medium to heavy rare earth elements for resource exploration requires improved understanding of the mechanisms responsible for the formation of such highly fractionated deposits. Here we report the results of experiments demonstrating a mechanism that could lead to enrichment of medium to heavy rare earth elements in ore-forming hydrothermal systems. In our experiments, we simulated natural hydrothermal systems by heating a column containing apatite and fluorite through which we pumped a chloride-rich solution bearing rare earth elements. Analysis of our experiments shows that the fluoride mineral fluocerite can serve as a precursor phase that fractionates rare earth elements before it is subsequently converted to a thermodynamically more stable mineral. Our findings identify geological settings in which fluocerite is observed or predicted to occur as potential exploration targets for deposits enriched in medium to heavy rare earth elements. Experiments under simulated hydrothermal conditions suggest that the mineral fluocerite may serve as an intermediate phase that fractionates the rare earth elements in ore-forming systems.