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A major goal of this conference is to tackle the challenges described above and build a new network of scientists and professionals with different expertise, including but not limited to experimental geochemistry/chemistry, thermodynamic/geochemical modeling and databases, reactive mass transport modeling, molecular dynamic simulations, element extraction/separation technologies, theoretical thermodynamics and equations of state, and mineralogy, ore deposits, and processes in natural systems. Another important aspect is the participation of students and training the next generation of leaders in the field of critical minerals and thermodynamics. Participants at this meeting include scientists from academia and national laboratories, graduate and undergraduate students, as well as liaisons from industry, governmental agencies, and geological surveys. This five-day meeting includes daily talks, keynotes, small workshops, discussion sessions, and two evenings of poster sessions for students. One day includes an excursion to the nearby Lemitar Mountains carbonatite rare earth elements deposit to discover the geology of New Mexico and allow participants to link geosciences with other areas of basic energy sciences. We will also organize a geochemical modeling workshop using our “in-house” MINES thermodynamic database (Gysi et al., 2023) to show an application of thermodynamics to modeling critical mineral deposits. 

are earth elements (REE) are becoming increasingly important in modern society due to their numerous uses in manufacturing of components for green and high-tech energy industries. Studying the mechanisms of REE mineral formation in geologic systems is vital for understanding where and how these mineral deposits form. Previous studies of REE mineral deposits have shown that hydrothermal fluids can play a key role in the mobilization and enrichment of REE (Williams-Jones et al., 2000; Gysi et al., 2016; Vasyukova and Williams-Jones, 2018). Fluorite is ideal to study the behavior of REE because of their compatibility in its structure and it is a ubiquitous hydrothermal vein mineral found together with REE fluorocarbonates (i.e., bastnäsite and parisite). However, the controls on hydrothermal fluid-mineral REE partitioning in these deposits are not yet fully understood. In this study, we present petrographic observations of fluorite veins and fluid inclusions from the Gallinas Mountains REE-bearing fluorite veins/breccia deposit in New Mexico (McLemore, 2010; Williams-Jones et al. 2000). The Gallinas Mountains deposit notably contains hydrothermal fluorite and bastnäsite, and is associated with ~30 Ma alkaline igneous rocks intruded into Permian sedimentary rocks (McLemore, 2010). The goal of this study is to better understand the cause of REE variations in fluorite as a function of temperature and salinity of the fluids, and to determine how the REE concentrations change in barren and mineralized veins. Optical microscopy and cold-cathode cathodoluminescence (CL) is used to distinguish different fluorite generations and fluid inclusion types. Scanning electron microscopy (SEM) is used to identify REE minerals, zonation in fluorite, and acquire elemental compositions of different vein minerals. 

The Lemitar Mountains carbonatite (Fig. 1A) is a 515 Ma rare earth element (REE) mineral deposit in New Mexico comprising over one hundred carbonatite dikes intruded into Proterozoic igneous rocks [1, 2]. The carbonatite displays grades of up to 1.1 % total REE and showcases variable degrees of hydrothermal autometasomatism and overprinting of the surrounding host rocks through fenitization and veining [1-3]. In this study, we employ a combination of petrography, optical cold-cathode cathodoluminescence and scanning electron microscopy to delineate the mineral paragenesis of the carbonatites and the associated crosscutting hydrothermal veins (Fig. 1). The determination of trace element concentrations in apatite was achieved using LA-ICP-MS. Fluid inclusions were studied in thick sections using optical microscopy, microthermometry and a confocal Raman spectroscopy to assess their salinity, homogenization temperature, and chemical composition. 

Rare earth elements (REE) are critical elements found in monazite, xenotime, and hydrated REE phosphates which typically form in hydrothermal mineral deposits. Accurate predictions of the solubility of these REE phosphates and the speciation of REE in aqueous fluids are both key to understanding the controls on the transport, fractionation, and deposition of REE in natural systems. Previous monazite and xenotime solubility experiments indicate the presence of large discrepancies between experimentally derived solubility constants versus calculated solubilities by combining different data sources for the thermodynamic properties of minerals and aqueous species at hydrothermal conditions. In this study, these discrepancies were resolved by using the program GEMSFITS to optimize the standard partial molal Gibbs energy of formation (ΔfG°298) of REE aqueous species (REE3+ and REE hydroxyl complexes) at 298.15 K and 1 bar while keeping the thermodynamic properties fixed for the REE phosphates. A comprehensive experimental database was compiled using solubility data available between 25 and 300 °C. The latter permits conducting thermodynamic parameter optimization of ΔfG°298 for REE aqueous species. Optimal matching of the rhabdophane solubility data between 25 and 100 °C requires modifying the ΔfG°298 values of REE3+ by 1–6 kJ/mol, whereas matching of the monazite solubility data between 100 and 300 °C requires modifying the ΔfG°298 values of both REE3+ and REEOH2+ by ∼ 2–10 kJ/mol and ∼ 15–31 kJ/mol, respectively. For xenotime, adjustments of ΔfG°298 values by 1–26 kJ/mol are only necessary for the REE3+ species. The optimizations indicate that the solubility of monazite in acidic solutions is controlled by the light (L)REE3+ species at <150 °C and the LREEOH2+ species at >150 °C, whereas the solubility of xenotime is controlled by the heavy (H)REE3+ species between 25 and 300 °C. Based on the optimization results, we conclude that the revised Helgeson-Kirkham-Flowers equation of state does not reliably predict the thermodynamic properties of REE3+, REEOH2+, and likely other REE hydroxyl species at hydrothermal conditions. We therefore provide an experimental database (ThermoExp_REE) as a basic framework for future updates, extensions with other ligands, and optimizations as new experimental REE data become available. The optimized thermodynamic properties of aqueous species and minerals are available open access to accurately predict the solubility of REE phosphates in fluid-rock systems. 

The rare earth elements (REE) are essential for the high-tech and green technology industries, and used, for example, in computers, smartphones, and wind turbines. The REE are considered critical minerals and can be highly enriched in certain magmatic-hydrothermal systems including alkaline complexes and carbonatites. Almost all of the critical mineral deposits show a complex overprint by hydrothermal processes during their genesis. However, our understanding of the mobility in these ore- forming systems and our knowledge about the stability of REE minerals is still very limited. The MINES thermodynamic database is an open-access database and continuously updated with the most up to date thermodynamic data for REE aqueous species and minerals. This database also includes rock-forming minerals and permits simulating the mineralogy and alteration geochemistry that relates to the formation of these critical mineral deposits. This study gives a short overview of the MINES thermodynamic database and the GEMS code package for simulating the formation of hydrothermal calcite, fluorite and bastnäsite-(Ce) veins relevant to interpreting critical mineral deposits. 

(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. 

The MINES thermodynamic database (version 23) is a revised internally consistent thermodynamic dataset for minerals, aqueous species, and gases for simulating geochemical processes at hydrothermal conditions (≤5 kbar and ≤600 °C) with a focus on ore forming processes. The database follows a rolling release approach with new file versions becoming available once updated. The version number corresponds to the year of the most recent file creation and the number after the decimal separator indicates an upgrade during the year of release. The database is currently intended to be used with the GEMS geochemical modeling program ( http://gems.web.psi.ch/ ). Future versions will include human-readable data in .xlsx, .csv, and JSON files with all the data values, units, and references.