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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. 

Rare earth element (REE) deposits are found in association with carbonatite and alkaline systems, where metasomatism plays a significant role in the late-stage transport and enrichment of REE [1]. Fluorite is a common gangue mineral in these mineral deposits and can incorporate varying REE concentrations by substitutions of REE3+ for Ca2+. Fluorite-hosted fluid inclusions contain significant REE concentrations [2], providing a potential record of the hydrothermal ore-forming fluids. The fluorite-fluid REE partitioning mechanisms, however, are largely unknown. To date, only one study [3] measured the partitioning of REE between fluorite and aqueous fluid at 60 °C. Here, we evaluate these REE partitioning mechanisms by combining laboratory experiments with characteristics of natural fluid inclusions that provide a range of salinities and homogenization temperatures relevant to natural systems. Batch-type experiments will be conducted between 100 and 250 °C in Teflon-lined reactors, in which millimeter-sized natural fluorite crystals (Cooke’s Peak, New Mexico) will be reacted with fluids of varying initial REE concentration, pH, and salinity. Kinetic experiments were carried out at 150 °C to test for attainment of a steady state between the fluorite crystals and the aqueous solutions. The reacted fluorite crystals will be studied using SEM, CL and EMPA. Major cations and anions in the quenched fluids will be analyzed using IC and ICP-OES; REE will be determined using solution ICP-MS. These results will permit deriving REE fluorite-fluid partition coefficients. Fluid inclusions in hydrothermal fluorite veins from the fluorite-bastnäsite REE deposits in the Gallinas Mountains in New Mexico are studied to constrain temperatures, salinities, and REE concentrations of hydrothermal ore-forming fluids in alkaline systems. Fluid inclusion assemblages were identified in growth zones and will be further studied using microthermometry. Previous studies found maximum temperatures of 400 °C in sulfate-rich NaCl-KCl brines [4]. The goal will be to link partition coefficients derived from the experiments to the REE partitioning behavior found in the natural fluorite. [1] Gysi et al. (2016), Econ. Geol. 111, 1241-1276; [2] Vasyukova and Williams-Jones (2018) Chem. Geol. 483, 385-396; [3] van Hinsberg et al. (2010), Geology 38, 847-850; [4] Williams-Jones et al. (2000), Econ. Geol. 95, 327-341.