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1. Hydrothermal solubility and partitioning of REE between fluids and minerals: REE phosphates, apatite, fluorite, and calcite

   Gysi, A P (August 2024, American Chemical Society)
2. Hydrothermal calcite-fluid REE partitioning experiments

   Filho, D.C.; Gysi, A.P.; Kalugin N. (July 2022, Goldschmidt)
3. Hydrothermal REE reaction paths in critical mineral deposits

   Gysi, A. P. (July 2021, Goldschmidt)

   Critical mineral deposits commonly form in magmatic-hydrothermal systems including carbonatites and/or alkaline syenites, and more evolved peralkaline granites where the rare earth element (REE) undergo a complex array of partitioning, transport and mineralization. Significant alteration and veining zones develop in these deposits and can be used to vector ore zones in the field [1]. The REE ore minerals typically reflect the characteristics of these systems, which are enriched in carbonate, fluoride, and phosphate or a combination thereof. The REE can also be incorporated into vein minerals such as calcite, fluorite and apatite where the REE3+ exchange for Ca2+ on the crystal lattice [2]. These minerals give us clues about the hydrothermal reaction paths of REE in critical mineral deposits. This study aims to: 1) present our recent findings from hydrothermal fluid-mineral REE partitioning experiments, 2) discuss thermodynamic models to simulate REE in critical mineral deposits, and 3) link the thermodynamic simulations to field observations. Hydrothermal fluid-calcite partitioning experiments were conducted between 100 and 200 °C by hydrothermal fluid mixing and precipitation [2] at near neutral to mildly alkaline pH (6 – 9). The REE concentrations in synthetic calcite crystals and aqueous fluids sampled in situ were used to fit the data to the lattice strain model [3] and using the Dual Thermodynamic approach [4]. A second type of experiment consisted of reacting natural fluorite and apatite crystals with acidic to mildly acidic (pH of 2 – 4) aqueous fluids in batch-type reactors to study the behavior of REE and mineral dissolution-precipitation reactions near crystal surfaces. The GEMS code package [5] was used to implement these new data into a thermodynamic model and simulate possible REE reaction paths in hydrothermal fluids. Two REE mineral deposits in New Mexico (Lemitar and Gallinas Mountains) present ideal case studies to illustrate how these models can be linked to field observations from natural systems. [1] Gysi et al. (2016), Econ. Geol. 111, 1241-1276; [2] Perry and Gysi (2020), Geochim. Cosmochim. Acta 286, 177-197; [3] Blundy and Wood (1994) Nature 372, 452-454; [4] Kulik (2006), Chem. Geol. 225, 189-212; [5] Kulik et al. (2013), Computat. Geosci. 17, 1-24. 

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4. Hydrothermal Rare Earth Elements (REE) Partitioning into Fluorite: A Fluid Inclusion Study from the Gallinas Mountains REE-bearing Fluorite Deposit, NM

   https://doi.org/10.56577/SM-2024.2978  

   Velmani, Aadish; Gysi, Alexander; Hurtig, Nicole (April 2024, New Mexico Geological Society)

   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. 

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5. A REE-in-plagioclase–clinopyroxene thermometer for crustal rocks

   https://doi.org/10.1007/s00410-016-1326-9  

   Sun, Chenguang; Liang, Yan (April 2017, Contributions to Mineralogy and Petrology)