New Mexico Geological Society Annual Spring Meeting — Abstracts

Rare earth elements (REE) are critical minerals essential to high-tech and green energy industries. Economic concentrations of REE occur within carbonatites and alkaline deposits, where hydrothermal alteration is a key process in the evolution of these deposits (Gysi & Williams-Jones, 2013; Moore et al., 2015). Calcite is a common gangue vein mineral in these hydrothermally overprinted systems and incorporates trace elements via several possible coupled substitutions. The REE signature of calcite records fluid evolution and provides a powerful tracer of hydrothermal processes in ore-forming systems (Perry & Gysi, 2018). Hydrothermal experiments offer a controlled approach to replicate this partitioning behavior and study the underlying mechanisms of calcite-fluid REE partitioning. However, to date very few experiments have been conducted at hydrothermal conditions.
Here, we present batch-type hydrothermal experiments conducted at 200 °C and saturated water vapor pressure. Calcite was synthesized with variable initial REE concentrations (~200-750 ppb) using a method similar to Perry and Gysi (2020). Experiments were performed in a 600 ml stirred titanium Parr reactor with in situ fluid sampling over 1-2 weeks, followed by quenching and recovery of precipitated calcite. Quenched fluids were analyzed using IC, ICP-OES, and ICP-MS. Acid-digested calcite was analyzed using ICP-OES and solids were characterized by SEM-EDS. Thermodynamic calculations were conducted using the GEMS code package and the MINES thermodynamic database at New Mexico Tech (Gysi et al., 2023).
Precipitated calcite displays 25-100 μm rhombohedral crystals. The recovered calcite is enriched in light REE, as reflected by a positive slope in a mole fraction-ionic radius diagram. This results from the similar ionic radius of Ca²⁺ and the trivalent light REE³⁺. Measured light REE (e.g., La, Pr, Nd) concentrations range between ~100-950 ppm, and heavy REE concentrations (e.g., Dy, Ho, and Yb) between ~50-950 ppb. Yttrium is commonly considered a heavy REE and has an ionic radius similar to Ho. However, the concentrations of Y in the synthesized calcite are much higher (i.e., 0.0011-0.0103 mol/kg) than Ho (i.e., 0.0006-0.0056 mol/kg) or other REE. Similarly, Y exhibits the greatest relative mole fractions in calcite across experiments (1.10×10⁻⁴ to 1.02×10⁻²), followed by light REE such as La, Pr, Nd, and Ce (7.84x10⁻⁵-6.62x10⁻⁴), while heavy REE, including Dy, Ho, Yb, and Lu, have lower mole fractions (3.63x10⁻⁵-5.67x10⁻⁴).
The compositions of the quenched experimental fluids indicate that the REE concentrations decrease systematically with time, consistent with progressive partitioning into calcite. In experiments with ~200 ppb initial REE, total light REE concentrations decline from 1.38x10⁻⁶ to 8.80x10⁻⁸ mol/kg, whereas heavy REE concentrations decline from 1.89x10⁻⁶ to 2.13x10⁻⁸ mol/kg. In ~750 ppb initial REE experiments, total light REE concentrations decrease from 5.85x10⁻⁶ to 1.15x10⁻⁷ mol/kg, and heavy REE from 8.04x10⁻⁶ to 6.32x10⁻⁸ mol/kg. The most pronounced decreases occur in heavy REE after 7-8 days in the ~750 ppb initial REE experiments.
Partition coefficients (KD) for REE partitioning between the hydrothermal fluid and calcite were calculated for each REE. Y consistently exhibits the highest KD values across all experiments. In ~200 ppb initial REE experiments, KD values remain <1, indicating a preference for the hydrothermal fluid. In contrast, experiments with ~750 ppb initial REE concentrations indicate a preference for calcite with KD values > 1 within four days of reaction for Y and light REE, and within 10 days for other REE. A plot of partition coefficients vs. ionic radius of the REE3+ defines parabolic trends which can be fitted to the lattice strain model, similarly observed in fluorite-fluid partitioning experiments by van Hinsberg et al. (2010).
Ultimately, these experiments show that REE partitioning into calcite at hydrothermal conditions evolves as the system approaches equilibrium and depends strongly on the starting REE concentration. The calculated KD values shift from fluid-dominated to calcite-dominated behavior with increasing initial REE concentrations and time. Overall, we see a preferential incorporation of light over heavy REE into calcite. However, Y consistently exhibits higher KD values than Ho and the other REE, despite Y and Ho typically behaving similarly in geological systems. Nevertheless, these results are consistent with natural systems such as the Lemitar carbonatite, where hydrothermal calcite is light REE fractionated with high concentrations of Y, Ce, Nd, and La over other REE (Obringer et al., in preparation). Future work will focus on developing a thermodynamic model for REE partitioning into calcite that can better account for factors impacting partitioning other than the lattice strain model, such as REE complexation in the hydrothermal fluid, that can be applied across a wider range of fluid compositions and temperatures.

References:

1. Gysi, A. P., & Williams-Jones, A. E. (2013). Hydrothermal mobilization of pegmatite-hosted REE and Zr at Strange Lake, Canada: A reaction path model. Geochimica et Cosmochimica Acta, 122, 324–352. https://doi.org/10.1016/j.gca.2013.08.031
2. Gysi, A.P., Hurtig, N.C., Pan, R., Miron, G.D., and Kulik, D.A., (2023). MINES thermodynamic database, New Mexico Bureau of Geology and Mineral Resources, version 23, https://doi.org/10.58799/mines-tdb
3. Moore, M., Chakhmouradian, A. R., Mariano, A. N., & Sidhu, R. (2015). Evolution of rare-earth mineralization in the Bear Lodge carbonatite, Wyoming: Mineralogical and isotopic evidence. Ore Geology Reviews, 64, 499–521. https://doi.org/10.1016/j.oregeorev.2014.03.015
4. Perry, E. P., & Gysi, A. P. (2018). Rare earth elements in mineral deposits: Speciation in hydrothermal fluids and partitioning in calcite. Geofluids, 2018, Article 5382480. https://doi.org/10.1155/2018/5382480
5. Perry, E., & Gysi, A. P. (2020). Hydrothermal calcite–fluid REE partitioning experiments at 200 °C and saturated water vapor pressure. Geochimica et Cosmochimica Acta, 286, 177–197. https://doi.org/10.1016/j.gca.2020.07.018
6. van Hinsberg, V. J., Migdisov, A. A., & Williams-Jones, A. E. (2010). Reading the mineral record of fluid composition from element partitioning. Geology, 38(9), 847–850. https://doi.org/10.1130/G31112.1