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1. What controls the mobility of rare earth elements (REE) in critical mineral deposits in acidic vs. alkaline hydrothermal fluids?

   https://doi.org/10.7185/gold2023.19801  

   Gysi, Alexander; Hurtig, Nicole; Waters, Laura; Harlov, Daniel; Miron, George Dan; Migdissov, Artaches; Zhu, Chen; Kulik, Dmitrii (July 2023, European Association of Geochemistry)

   (Per)alkaline complexes and carbonatites evolve through a complex sequence of magmatic-hydrothermal processes. Most of them are overprinted by late auto-metasomatic processes which involves the mobilization, fractionation and/or enrichment of critical elements, such as the rare earth elements (REE) [1]. However, our current ability to predict the behavior of REE in high temperature aqueous fluids and interpret these natural systems using geochemical modeling depends on the availability of thermodynamic data for the REE minerals and aqueous species. Previous experimental work on REE solubility has focused on acidic aqueous fluids up to ~300 °C and considered chloride, fluoride and sulfate as important ligands for their transport [2]. However, magmatic-hydrothermal systems that form these critical mineral deposits may cover a wider range of fluid chemistries spanning acidic to alkaline pH as well as temperatures and pressures at which the fluids are supercritical. A few recently published studies have shown that other ligands (e.g., REE carbonates and/or combined fluoride species) could become important in near-neutral to alkaline fluids [3,4], and that REE mobility can also be increased in saline alkaline fluids reacted with fluorite [5]. Here we present new hydrothermal REE hydroxyl/chloride speciation data and REE phosphate/hydroxide minerals [6,7], calcite and fluorite solubility experiments as a function of pH, salinity and temperature. We use an integrated approach to link a wide array of experimental techniques (solubility, calorimetry, and spectroscopy) with thermodynamic optimizations using GEMSFITS [8], and present the development of a new experimental database for REE and its integration into the MINES thermodynamic database (https://geoinfo.nmt.edu/mines-tdb). The latter permits simulating hydrothermal fluid-rock interaction and ore-forming processes in critical mineral deposits to better understand the behavior of REE during metasomatism. 

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2. Thermodynamic modeling of hydrothermal REE partitioning in critical mineral deposits

   Gysi, A.P.; Hurtig, N.; Water, L.; Migdisov, A.; Dub, P.; Zhu, C. (July 2022, Goldschmidt)

   Critical mineral deposits form through an interplay of magmatic-hydrothermal processes in carbonatites and (per)alkaline systems during their emplacement in the Earth’s crust. Hydrothermal aqueous fluids can lead to the mobilization, transport, and deposition of the rare earth elements (REE) coupled to development of alteration zones at the deposit scale [1]. However, unraveling the underlying processes that affect the solubility of REE in these geologic fluids is a challenge in high temperature and pressure fluids [2]. A holistic approach is key to understand the controls of fluid-rock interaction in mobilizing REE in critical mineral deposits. Through a joint effort, we formed a new U.S. geoscience critical minerals experimental–thermodynamic research hub between New Mexico Tech, Los Alamos National Laboratory and Indiana University. The goal of this project is to conduct frontiers research on the behavior of critical elements in supercritical aqueous fluids by integration of a wide array of high temperature solubility experiments complemented by spectroscopic measurements and molecular dynamic simulations. Here we present current advances to simulate a significant vein paragenesis of barite + fluorite +calcite +bastnäsite-(Ce) observed in many critical mineral deposits. A case study will be presented from the Gallinas Mountains REE-fluorite hydrothermal breccia deposit in New Mexico. Using the GEMS code package [3] and the MINES thermodynamic database (https://geoinfo.nmt.edu/mines-tdb), we highlight our current capabilities and limitations to simulate the behavior of REE in these hydrothermal fluids and minerals. A thermodynamic model is presented to simulate the partitioning of REE between calcite- and fluorite-fluid based on recent and ongoing experimental and thermodynamic work on the synthesis of REE doped minerals [4] and REE speciation in acidic and alkaline fluids. We further show how to integrate multiple experimental datasets and develop new thermodynamic models based on the new research efforts from the research hub and future directions to improve our prediction capabilities of REE complexation in supercritical fluids. [1] Gysi et al. (2016), Econ. Geol. 111, 1241-1276; [2] Migdisov et al. (2016), Chemical Geology 439, 13-42. [3] Kulik et al. (2013), Comput Geosci 17, 1–24. [4] Perry and Gysi (2020), Geochim. Cosmochim. Acta 286, 177-197. 

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3. The thermodynamic stability of monazite for vectoring the hydrothermal mobility of REE in ore deposits

   Van Hoozen, C.J.; Gysi, A.P. (January 2018, Goldschmidt conference)

   The growing applications of the rare earth elements (REE) to the high-tech and green technology industry has led to an increased interest in the thermodynamic behavior of REE minerals and their aqueous complexes within the geochemical community. The REE minerology and rock chemistry of REE deposits can display significant variations during hydrothermal overprint 1,2 . Monazite (CePO 4 ) is a common mineral in these deposits and, in several cases, can be attributed to hydrothermal mineralization processes. The giant REE deposit in the Bayan Obo carbonatite 3 and the IOA deposit in Pea Ridge 4,5 contain monazite displaying significant compositional and textural variations that may provide useful vectors of fluid-rock interaction and ore deposition processes. To quantify the meaning of these variations using thermodynamic modelling will require a robust thermodynamic dataset for REE minerals and their aqueous complexes 6 . In this study, a series of hydrothermal batch solubility experiments have been conducted using LaPO 4 , PrPO 4 , NdPO 4 and EuPO 4 to assess the compatibility of available calorimetric data of these minerals and the thermodynamic data of the aqueous REE species. Additional calorimetric and XRD measurements were carried out to determine the thermodynamic properties of a series of monazite solid solutions. Solubility experiments were carried out in aqueous HClO 4 -H 3 PO 4 -bearing solutions at temperatures between 100 and 250 °C at saturated water vapor pressure. The equilibrium constants (K s0 ) determined for each endmember was then evaluated as a function of temperature and extrapolated to standard conditions of 25 °C and 1 bar. The results indicate significant differences in retrieved solubilities in comparison to the available literature data. We will demonstrate the impact of this new thermodynamic data by analyzing the results of several batch system equilibrium simulations using the GEMS code package (http://gems.web.psi.ch) and the MINES thermodynamic database (http://tdb.mines.edu). Our current and future thermodynamic data will be implemented in this thermodynamic framework and allow for the more accurate prediction of the hydrothermal behavior of REE in mineral deposits. 

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4. Simulation of REE mobility and evolution of F-NaCl-CO2-bearing fluids in hydrothermal calcite and fluorite ore-forming veins

   https://doi.org/10.7185/gold2021.6309  

   Costa Filho, D.; Gysi, A.P. (July 2021, Goldschmidt)

   Rare earth element (REE) deposits are commonly associated with carbonatites and (per)alkaline rocks where hydrothermal magmatic fluids can play a significant role in REE mobilization and deposition [1]. Thermodynamic modeling permits predicting the evolution of ore-forming fluids and can be used to test different controls on hydrothermal REE mobility including temperature, pressure, the solubility of REE minerals, aqueous REE speciation and pH evolution associated with fluid-rock interaction. Previous modeling studies either focused on REE fluoride/chloride complexation in acidic aqueous fluids [2] or near neutral/alkaline fluids associated with calcite vein formation [3]. Such models were also applied to interpret field observations in REE deposits Bayan Obo in China and Bear Lodge in Wyoming [3,4]. Recent hydrothermal calcite-fluid REE partitioning experiments provide new data to simulate the solubility of REE in calcite, REE carbonates/fluorocarbonates at high temperatures [5, 6]. We studied the competing effects controlling the mobility of REE in hydrothermal fluids between 100 and 400 °C at 500 bar. Speciation calculations were carried out in the Ca-F-CO2-Na-Cl-H2O system using the GEMS code package [7]. The properties of minerals and aqueous species were taken from the MINES thermodynamic database [3,5]. The Gallinas Mountains hydrothermal REE deposit in New Mexico was used as a field analogue to compare our models with the formation of calcite-fluorite veins hosting bastnäsite. Previous fluid inclusion studies hypothesized that the REE were transported as fluoride complexes [8] but more recent modeling studies have shown that fluoride essentially acts as a depositing ligand [2]. Here we show more detailed simulations predicting the stability of fluorite, calcite and REE minerals relevant to ore-forming processes in carbonatites and alkaline systems. [1] Gysi et al. (2016), Econ. Geol. 111, 1241-1276; [2] Migdisov and Williams-Jones (2014), Mineral. Deposita 49, 987-997. [3] Perry and Gysi (2018), Geofluids; [4] Liu et al. (2020), Minerals 10, 495; [5] Perry and Gysi (2020), Geochim. Cosmochim. Acta 286, 177-197; [6] Gysi and Williams-Jones (2015) Chem. Geol. 392, 87-101;[7] Kulik et al. (2013), Computat. Geosci. 17, 1-24; [8] Williams-Jones et al. (2000), Econ. Geol. 95, 327-341 

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5. The hydrothermal solubility of monazite-(Ce) and xenotime-(Y)

   Gysi, A.P.; Harlov, D.; Miron, G.D. (January 2018, Goldschmidt conference)

   CePO4 and YPO4 are major components in monazite and xenotime, respectively, which are common hydrothermal phases in REE mineral deposits. Both minerals also occur as secondary minerals in iron-oxide-apatite deposits [1,2], and as accessory phases in high-grade metamorphic rocks where they display varying degrees of metasomatism. Studying the cause of their compositional variations using thermodynamic modeling may provide geochemical signals for interpreting P- T-x of crustal fluid-rock interaction. The thermodynamic properties of monazite and xenotime have been determined using several calorimetric methods [3], but only a few solubility studies have been undertaken, which test the reliability of both the calorimetric data and thermodynamic properties of associated REE aqueous species [4]. Combining available calorimetric data with the REE aqueous species from Haas et al. [5], implemented in the Supcrt92 database [6], yields several orders of magnitude differences when compared with our solubility measurements. To reconcile these discrepancies, we have used the GEMS code package [7,8] and GEMSFITS [9] for parameter optimization, and re- evaluated the standard Gibbs energies for aqueous REE species, while maintaining consistency with available calorimetric measurement of the REE phosphates. This study points to a need to revise the thermodynamic properties of the REE hydroxyl species, which will have an impact on the calculated solubilities of the REE phosphates and our understanding of the mobility of REE in hydrothermal fluids. Our new experimental data will be implemented in the MINES thermodynamic database (http:// tdb.mines.edu) [10] for modeling the chemistry of crustal fluid-rock equilibria. 

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