SOCIETY OF ECONOMIC GEOLOGISTS DISTINGUISHED LECTURE: THE HYDROTHERMAL MOBILITY OF THE RARE EARTH ELEMENTS: IMPLICATIONS FOR ORE GENESIS

Although the Rare Earth Elements (REE) were long considered immobile during hydrothermal alteration, and were used extensively to interpret igneous petrogenesis, it is now clear that they can acquire considerable mobility in hydrothermal fluids. Indeed, there are a number of REE ore deposits, including the World’s largest deposit, Bayan Obo, China, which formed largely as a result of the transport of the REE by hydrothermal fluids. Moreover, for many other deposits, there is strong evidence that REE mobilisation may be essential to the economic viability of the mineralisation. Until recently, aqueous fluoride complexes were widely thought to be the main agents of REE transport in hydrothermal fluids, on the grounds that fluorine-bearing minerals are common in REE deposits and the REE form very strong complexes with fluoride ions. However, thermodynamic modelling of natural systems suggests that this is not the case, because of the relatively strong association of HF and the very low solubility of simple REE-fluoride salts¹. Instead, chloride complexes appear to be mainly responsible for REE transport in many ore-forming systems. Nonetheless, we do not exclude the possibility that other ligands, including sulphate and carbonate ions, may play a role in REE transport in some environments. A feature of some REE deposits is preferential mobility of the light REE. This can be explained by experimental data, which show that the stability of the lanthanides decreases with increasing atomic number at temperatures above 150°C. The reverse is true at lower temperature. A model that may be applicable to many deposits in which hydrothermal mobilisation is important to REE ore formation is one in which concentration of the REE occurs due to interaction of acid REE-Cl-bearing fluids with pH-buffering rocks such as limestone or marble, or mixing with pH-neutral fluids, and/or decreasing temperature.

¹Williams-Jones, A.E., Migdisov, A.A., and Samson, I.M. (2012) Elements, 8, 355-360.