Periodic Reporting for period 1 - REESources (REESources: experimental investigation of the role of fluids in the formation of rare metals ore deposits.)
Reporting period: 2019-01-01 to 2020-12-31
Rare Earth Elements (REE) have become an essential resource for the transition to a carbon-free economy. Meeting the increasing demand for these critical metals requires a fundamental understanding of the geological processes that enable REE concentration in the Earth’s crust.
In a very schematic way, the enrichment process leading to the concentration of REE up to economic grade can be divided into 3 phases: (1) a magmatic enrichment phase that involves fractional crystallization of carbonatite, syenite or granite melts, (2) an hydrothermal enrichment phase that involves high-temperature fluids that mobilized and transport the REE out of their magmatic host minerals and reprecipitates them as newly enriched minerals in the magmatic intrusion or the host rocks and (3) a weathering phase where low-temperature meteoritic fluids can further concentrate the REE, especially in ion-adsorption clays.
The aim of the REESources project was to provide new insights on the magmatic and hydrothermal enrichment phases, using novel experimental designs that enable us to study the behaviour of REE upon magmatic crystallisation and hydrothermal alteration, directly at the high P-T conditions relevant to the formation of REE ore-deposits.
Especially, we identified three main research questions to be answered by the REESources project:
1. What is the effect of different ligands (Cl, F, SO42-, CO32-) on the hydrothermal mobilization of the REE? Is there an ideal composition responsible for the enrichment of more valuable HREE over LREE?
2. How are REE extracted from their magmatic source? Is hydrothermal remobilization solely related to the late evolution of alkaline systems (ie., ore-forming fluids are only produced at the very end of the magmatic intrusion’s life) or can early magmatic fluids mobilize REE at T > 500 °C?
3. How are REE elements and their valuable cousins Nb and Ta concentrated in magmatic phases? What is the role of F, but also P in the formation of ore minerals as zircon, pyrochlore or apatite?
To answer these questions, I performed a combination of in-situ spectroscopy measurements and piston-cylinder experiments at the European Synchrotron Radiation Facility (Grenoble, France) and the Institute of Mineralogy at the WWU University (Münster, Germany).
In a very schematic way, the enrichment process leading to the concentration of REE up to economic grade can be divided into 3 phases: (1) a magmatic enrichment phase that involves fractional crystallization of carbonatite, syenite or granite melts, (2) an hydrothermal enrichment phase that involves high-temperature fluids that mobilized and transport the REE out of their magmatic host minerals and reprecipitates them as newly enriched minerals in the magmatic intrusion or the host rocks and (3) a weathering phase where low-temperature meteoritic fluids can further concentrate the REE, especially in ion-adsorption clays.
The aim of the REESources project was to provide new insights on the magmatic and hydrothermal enrichment phases, using novel experimental designs that enable us to study the behaviour of REE upon magmatic crystallisation and hydrothermal alteration, directly at the high P-T conditions relevant to the formation of REE ore-deposits.
Especially, we identified three main research questions to be answered by the REESources project:
1. What is the effect of different ligands (Cl, F, SO42-, CO32-) on the hydrothermal mobilization of the REE? Is there an ideal composition responsible for the enrichment of more valuable HREE over LREE?
2. How are REE extracted from their magmatic source? Is hydrothermal remobilization solely related to the late evolution of alkaline systems (ie., ore-forming fluids are only produced at the very end of the magmatic intrusion’s life) or can early magmatic fluids mobilize REE at T > 500 °C?
3. How are REE elements and their valuable cousins Nb and Ta concentrated in magmatic phases? What is the role of F, but also P in the formation of ore minerals as zircon, pyrochlore or apatite?
To answer these questions, I performed a combination of in-situ spectroscopy measurements and piston-cylinder experiments at the European Synchrotron Radiation Facility (Grenoble, France) and the Institute of Mineralogy at the WWU University (Münster, Germany).
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.
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.
Regarding the conditions of REE hydrothermal transport and concentration, our in-situ experiments reveal a previously unexpected role of carbonate-rich alkaline fluids in the ore-forming process. These results not only reveal that the modes of REE transport in alkaline fluids differ fundamentally from those in acidic fluids, but further underline that early alkaline fluids may be key to the mineralization of hydrothermal REE-fluorocarbonates.
Overall, our findings advocate for the reevaluation of the role of alkaline fluids in the genesis of REE deposits, for instance via a more systematic characterization of fluid inclusions volatile and trace elements composition and new high P-T solubility studies. Such a step may be critical for defining the key chemical and geochemical factors controlling enrichment in the most valuable rare earth (Nd, Pr and Dy) and developing more effective exploration strategies to sustain REE production into the future.
Regarding pyrochlore stability in carbonatite melts, our measurements reveal that the amount of Nb necessary to crystallize pyrochlores from F-bearing carbonatite melts are much lower than previously thought: while Mitchell and Kjarsgaard (2004) suggested that concentrations as high as 15wt% Nb2O5 could be dissolved in melts prior to their saturation and pyrochlore crystallization at 1000 C and 0.1 GPa, our experiments show that pyrochlores are the liquidus phase in melts containing only 2wt%. As Nb is overall a trace element in the Earth, this Nb concentration appears much more realistic.
Further conclusions are yet to be drawn from the careful analysis of REE partitioning between pyrochlore and carbonate melts. Such quantitative data are yet missing and should help in the future development of geochemical models of REE magmatic concentration processes.
Overall, our findings advocate for the reevaluation of the role of alkaline fluids in the genesis of REE deposits, for instance via a more systematic characterization of fluid inclusions volatile and trace elements composition and new high P-T solubility studies. Such a step may be critical for defining the key chemical and geochemical factors controlling enrichment in the most valuable rare earth (Nd, Pr and Dy) and developing more effective exploration strategies to sustain REE production into the future.
Regarding pyrochlore stability in carbonatite melts, our measurements reveal that the amount of Nb necessary to crystallize pyrochlores from F-bearing carbonatite melts are much lower than previously thought: while Mitchell and Kjarsgaard (2004) suggested that concentrations as high as 15wt% Nb2O5 could be dissolved in melts prior to their saturation and pyrochlore crystallization at 1000 C and 0.1 GPa, our experiments show that pyrochlores are the liquidus phase in melts containing only 2wt%. As Nb is overall a trace element in the Earth, this Nb concentration appears much more realistic.
Further conclusions are yet to be drawn from the careful analysis of REE partitioning between pyrochlore and carbonate melts. Such quantitative data are yet missing and should help in the future development of geochemical models of REE magmatic concentration processes.