Hydrothermal transport, deposition, and fractionation of REE: Experimental data and thermodynamic calculations

English Abstract:

Evidence collected from natural systems suggests that hydrothermal transport and deposition often play a key role in concentrating the REE to economically exploitable levels. Understanding the chemical mechanisms responsible for the aqueous mobilization/deposition of the REE at elevated temperature is, thus, crucial for developing predictive models of REE ore formation. The stability of the aqueous complexes which the REE form with various ligands at elevated temperature is the key factor controlling the concentration and mobilization of REE in hydrothermal systems. An additional control is imposed by the solubility of the minerals which bind and cause precipitation of the metals of interest.

Over the past two decades, a substantial body of experimental data has been collected on the stability of REE species in aqueous fluids at high temperature. These data suggest that the speciation of the REE in hydrothermal solutions up to 300ºC is significantly different from that predicted by the theoretical models, which are still widely employed to evaluate REE mobility in nature. Moreover, in contrast to an earlier belief, these data demonstrate a pronounced dependence of the stability of some species on the atomic number of the REE, an effect that can cause REE fractionation in some hydrothermal processes.

In this contribution, we report the results of thermodynamic calculations re-evaluating the mechanisms responsible for the hydrothermal transport and deposition of the REE, based on the latest experimental data. We also evaluate the fractionation effects that can be observed during leaching/deposition in various natural environments.