The first step of the project was to refine the current understanding of REE hydrothermal transport and concentration. Following our previous studies on REE transport by acidic (pH < 3) Cl- and S-rich fluids (Louvel et al., 2015; Guan et al., 2020), a special focus was given to quantify the role of carbonate and fluorine-rich alkaline fluids (pH > 8) in REE ore-forming processes.
The aqueous solubility and speciation of both LREE (La, Sm) and HREE (Gd, Yb) was thus investigated up to 500°C and 80 MPa in Na2CO3±NaF solutions, conducting in-situ X-ray absorption measurements at the European Synchrotron Radiation Facility in Grenoble, France. The experimental results provide direct evidence that the role of alkaline fluids in the formation of REE deposits may have been underestimated up to now. In particular, we found that the formation of carbonate (CO32-) complexes in alkaline fluids not only promotes the simultaneous transport of REE, fluoride, and carbonate in a single fluid, but also enable an efficient fractionation between widespread LREE and highly sought-after HREE and may thus exert an unexpected control on ore grade.
In a second step, high-pressure, high-temperature experiments were performed to determine the stability conditions of pyrochlore, which is a main Nb and REE resource in carbonatite intrusions, and further evaluate the distribution of REE between these two phases. The experiments were conducted at 1100°C and 1 GPa in piston-cylinder apparatus and the concentration of Nb, Ta, Hf, REE, U and Th were determined through microanalyses at the Institute for Mineralogy. The effect of CaCO3/Na2CO3 concentrations, F, P2O5 and H2O were investigated over 7 different starting materials and are currently being interpreted.
The results on the hydrothermal mobility of REE were presented at the 2019 Goldschmidt Conference in Barcelona, Spain and the 3rd European Rare Earth RESources (ERES) Conference, held online in October 2020. Combined experimental and thermodynamic simulations of Y-Cl hydrothermal complexes have already been published in ‘Geochimica et Cosmochimica Acta’ in collaboration with a team from Monash University (Melbourne, Australia). A second manuscript presenting the potential role of REE-carbonate complexation in ore formation has been recently submitted to ‘Nature Communication’s. Two additional publications should be presented by the end of 2021 to ‘Chemical Geology’ and ‘Ore Geology Reviews’ to summarize our experimental work on REE hydrothermal speciation.
The results on pyrochlore stability and REE partitioning between carbonatite melts and pyrochlores will be presented at the 2021 Goldschmidt Conference, to be held online in June 2021, and in a manuscript to be submitted to ‘Contributions in Mineralogy and Petrology’. New experiments conducted by collaborators from Brazil and a Master student at WWU Muenster will follow on our study and further investigate the effect of Fe on Nb and REE mineralization in carbonatite melts.
