New Mexico Geological Society Annual Spring Meeting — Abstracts

Rare earth elements (REE) are a rapidly expanding resource due to their growing use in advanced technologies, principally within the green energy sector [1]. In REE-bearing magmatic-hydrothermal systems, the mobility of these metals in hydrothermal fluids is controlled by the stability of REE phosphates, fluorocarbonates and zirconosilicates [2,3] and the aqueous species with which they complex. Current thermodynamic models often underpredict the solubility of these elements in high temperature hydrothermal fluids due to a lack of experimental and thermodynamic data at supercritical conditions [4]. This can result in predictions which differ from experimentally measured REE solubilities by several orders of magnitude, greatly limiting our capabilities to accurately predict REE transport and fractionation in supercritical fluids. Here we use batch-type Inconel 625 Parr reactors to investigate synthetic ErPO₄ (xenotime) solubility between 350°C at P_SAT and 450°C and 500 bar, varying starting pH (2, 3, 4, 7, and 10), and salinities (0.01, 0.1, and 0.5 molal NaCl). Chloride and hydroxyl Er complexes are important in controlling REE mobility, specifically for iron oxide apatite (IOA) deposits. IOA deposits are a relatively untapped REE-bearing resource which have recently garnered attention due to their elevated REE potential. One of the largest domestic IOA deposits is the Pea Ridge deposit in Missouri. Pea Ridge provides an excellent natural system application of REE mobility as a function of chloride and hydroxyl speciation as REE within this system are believed to have been transported almost exclusively by chloride complexes [5].

At 350 and 400°C and 0.01m NaCl, Er demonstrates high solubility at acidic pH and decreased solubility at mildly acidic conditions. For alkaline experimental solutions (starting pH ~10) solubility is less than or equivalent to those at mildly acidic pH values. However, at 450°C, Er assumes a different trend where solubility is elevated at alkaline pH. The increased solubility of Er in acidic conditions is indicative of chloride speciation whereas the high solubility observed in alkaline conditions at 450°C likely results from hydroxyl complexation. Reduced solubility at mildly acidic conditions suggests a loss of chloride complex dominance, and, at 450°C, a transition to Er hydroxyl complex dominance. As a function of increasing salinity, Er becomes more soluble at low pH and the solubility minimum shifts towards alkaline pH as a result of expanded Er chloride complex stability. Theoretical predictions of Er solubility and speciation were performed using GEM-Selektor code package [6] and the MINES database [7]. Modeled speciation trends indicate ErCl²⁺, ErCl₂⁺, and ErCl_3(aq) dominance at acidic conditions for 350, 400, and 450°C respectively and Er(OH)₄^- at alkaline conditions. However, 350 and 400°C experimental solubility trends do not follow predicted speciation in alkaline conditions, rather they more closely mirror the behavior of ErOH_3(aq). Furthermore, these models incorrectly overpredict the total solubility of Er by up to two orders of magnitude. These results highlight a need for further high temperature experimental data. Such findings will serve in providing more accurate models of natural systems, specifically IOA deposits which are strongly dependent on chloride complexation.

References:

1. [1] Migdisov et al. (2016), Chem. Geol. 439, 13-42; https://doi.org/10.1016/j.chemgeo.2016.06.005
2. [2] Gysi et al. (2015), Chem. Geol. 401, 83-95; https://doi.org/10.1016/j.chemgeo.2015.02.023
3. [3] Gysi et al. (2018), Geochim. Cosmochim. Acta. 242, 143-164; https://doi.org/10.1016/j.gca.2018.08.038
4. [4] Haas et al. (1995), Cosmochim. Acta 59, 4329-4350; https://doi.org/10.1016/0016-7037(95)00314-P
5. [5] Harlov et al. (2016), Econ. Geol. 111, 1963-1984; https://doi.org/10.2113/econgeo.111.8.1963
6. [6] Kulik et al. (2013), Comput. Geosci. 17, 1-24; https://doi.org/10.1007/s10596-012-9310-6
7. [7] Gysi et al. (2023); https://doi.org/10.58799/mines-tdb