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1. The thermodynamic stability of monazite for vectoring the hydrothermal mobility of REE in ore deposits

   Van Hoozen, C.J. ; Gysi, A.P. ( January 2018 , Goldschmidt conference)

   The growing applications of the rare earth elements (REE) to the high-tech and green technology industry has led to an increased interest in the thermodynamic behavior of REE minerals and their aqueous complexes within the geochemical community. The REE minerology and rock chemistry of REE deposits can display significant variations during hydrothermal overprint 1,2 . Monazite (CePO 4 ) is a common mineral in these deposits and, in several cases, can be attributed to hydrothermal mineralization processes. The giant REE deposit in the Bayan Obo carbonatite 3 and the IOA deposit in Pea Ridge 4,5 contain monazite displaying significant compositional and textural variations that may provide useful vectors of fluid-rock interaction and ore deposition processes. To quantify the meaning of these variations using thermodynamic modelling will require a robust thermodynamic dataset for REE minerals and their aqueous complexes 6 . In this study, a series of hydrothermal batch solubility experiments have been conducted using LaPO 4 , PrPO 4 , NdPO 4 and EuPO 4 to assess the compatibility of available calorimetric data of these minerals and the thermodynamic data of the aqueous REE species. Additional calorimetric and XRD measurements were carried out to determine the thermodynamic properties of a series of monazite solid solutions. Solubility experiments were carried out in aqueous HClO 4 -H 3 PO 4 -bearing solutions at temperatures between 100 and 250 °C at saturated water vapor pressure. The equilibrium constants (K s0 ) determined for each endmember was then evaluated as a function of temperature and extrapolated to standard conditions of 25 °C and 1 bar. The results indicate significant differences in retrieved solubilities in comparison to the available literature data. We will demonstrate the impact of this new thermodynamic data by analyzing the results of several batch system equilibrium simulations using the GEMS code package (http://gems.web.psi.ch) and the MINES thermodynamic database (http://tdb.mines.edu). Our current and future thermodynamic data will be implemented in this thermodynamic framework and allow for the more accurate prediction of the hydrothermal behavior of REE in mineral deposits. 

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2. Simulation of REE mobility and evolution of F-NaCl-CO2-bearing fluids in hydrothermal calcite and fluorite ore-forming veins

   https://doi.org/10.7185/gold2021.6309  

   Costa Filho, D. ; Gysi, A.P. ( July 2021 , Goldschmidt)

   Rare earth element (REE) deposits are commonly associated with carbonatites and (per)alkaline rocks where hydrothermal magmatic fluids can play a significant role in REE mobilization and deposition [1]. Thermodynamic modeling permits predicting the evolution of ore-forming fluids and can be used to test different controls on hydrothermal REE mobility including temperature, pressure, the solubility of REE minerals, aqueous REE speciation and pH evolution associated with fluid-rock interaction. Previous modeling studies either focused on REE fluoride/chloride complexation in acidic aqueous fluids [2] or near neutral/alkaline fluids associated with calcite vein formation [3]. Such models were also applied to interpret field observations in REE deposits Bayan Obo in China and Bear Lodge in Wyoming [3,4]. Recent hydrothermal calcite-fluid REE partitioning experiments provide new data to simulate the solubility of REE in calcite, REE carbonates/fluorocarbonates at high temperatures [5, 6]. We studied the competing effects controlling the mobility of REE in hydrothermal fluids between 100 and 400 °C at 500 bar. Speciation calculations were carried out in the Ca-F-CO2-Na-Cl-H2O system using the GEMS code package [7]. The properties of minerals and aqueous species were taken from the MINES thermodynamic database [3,5]. The Gallinas Mountains hydrothermal REE deposit in New Mexico was used as a field analogue to compare our models with the formation of calcite-fluorite veins hosting bastnäsite. Previous fluid inclusion studies hypothesized that the REE were transported as fluoride complexes [8] but more recent modeling studies have shown that fluoride essentially acts as a depositing ligand [2]. Here we show more detailed simulations predicting the stability of fluorite, calcite and REE minerals relevant to ore-forming processes in carbonatites and alkaline systems. [1] Gysi et al. (2016), Econ. Geol. 111, 1241-1276; [2] Migdisov and Williams-Jones (2014), Mineral. Deposita 49, 987-997. [3] Perry and Gysi (2018), Geofluids; [4] Liu et al. (2020), Minerals 10, 495; [5] Perry and Gysi (2020), Geochim. Cosmochim. Acta 286, 177-197; [6] Gysi and Williams-Jones (2015) Chem. Geol. 392, 87-101;[7] Kulik et al. (2013), Computat. Geosci. 17, 1-24; [8] Williams-Jones et al. (2000), Econ. Geol. 95, 327-341 

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3. The hydrothermal solubility of monazite-(Ce) and xenotime-(Y)

   Gysi, A.P. ; Harlov, D. ; Miron, G.D. ( January 2018 , Goldschmidt conference)

   CePO4 and YPO4 are major components in monazite and xenotime, respectively, which are common hydrothermal phases in REE mineral deposits. Both minerals also occur as secondary minerals in iron-oxide-apatite deposits [1,2], and as accessory phases in high-grade metamorphic rocks where they display varying degrees of metasomatism. Studying the cause of their compositional variations using thermodynamic modeling may provide geochemical signals for interpreting P- T-x of crustal fluid-rock interaction. The thermodynamic properties of monazite and xenotime have been determined using several calorimetric methods [3], but only a few solubility studies have been undertaken, which test the reliability of both the calorimetric data and thermodynamic properties of associated REE aqueous species [4]. Combining available calorimetric data with the REE aqueous species from Haas et al. [5], implemented in the Supcrt92 database [6], yields several orders of magnitude differences when compared with our solubility measurements. To reconcile these discrepancies, we have used the GEMS code package [7,8] and GEMSFITS [9] for parameter optimization, and re- evaluated the standard Gibbs energies for aqueous REE species, while maintaining consistency with available calorimetric measurement of the REE phosphates. This study points to a need to revise the thermodynamic properties of the REE hydroxyl species, which will have an impact on the calculated solubilities of the REE phosphates and our understanding of the mobility of REE in hydrothermal fluids. Our new experimental data will be implemented in the MINES thermodynamic database (http:// tdb.mines.edu) [10] for modeling the chemistry of crustal fluid-rock equilibria. 

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4. The solubility of monazite (LaPO4, PrPO4, NdPO4, and EuPO4) endmembers in aqueous solutions from 100 to 250 °C

   https://doi.org/https://doi.org/10.1016/j.gca.2020.04.019  

   Van Hoozen, C. ; Gysi, A. P. ; Harlov, D. E. ( July 2020 , Geochimica et cosmochimica acta)

   Monazite is a light rare earth element (REE) phosphate found in REE mineral deposits, such as those formed in (per)alkaline and carbonatite magmatic-hydrothermal systems, where it occurs in association to the development of alteration zones and hydrothermal veins. Although it has been recognized that monazite may undergo replacement by coupled dissolution-precipitation processes, currently there is no model describing the compositional REE variations in monazite resulting from direct interaction with or precipitation from hydrothermal fluids. To develop such a model requires quantification of the thermodynamic properties of the aqueous REE species and the properties of the monazite endmembers and their solid solutions. The thermodynamic properties of monazite endmembers have been determined previously using calorimetric methods and low temperature solubility studies, but only a few solubility studies have been conducted at >100 °C. In this study, the solubility products (logKs0) of LaPO4, PrPO4, NdPO4, and EuPO4 monazite endmembers have been measured at temperatures between 100 and 250 °C and saturated water vapor pressure. The solubility products are reported with an uncertainty of ±0.2 (95% confidence) according to the reaction, REEPO4(s) = REE3+ + PO43−. (see table in manuscript) The REE phosphates display a retrograde solubility, with the measured Ks0 values varying several orders of magnitude over the experimental temperature range. Discrepancies were observed between the experimental solubility products and the calculated values resulting from combining calorimetric data of monazite with the properties of the aqueous REE3+ and PO43− species available in the literature. The differences between the calculated and measured standard Gibbs energy of reaction (ΔrG0) for PrPO4, NdPO4, and EuPO4 increased with higher temperatures (up to 15 kJ mol−1 at 250 °C), whereas for LaPO4 these differences increased at lower temperatures (up to 8 kJ mol−1 at 100 °C). To reconcile these discrepancies, the standard enthalpy of formation (ΔfH0) of monazite was optimized by fitting the experimental solubility data and extrapolating these fits to reference conditions of 25 °C and 1 bar. The optimized thermodynamic data provide the first internally consistent dataset for the solubility of all the monazite endmembers, and can be used to model REE partitioning between monazite and hydrothermal fluids at >100 °C. 

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5. The solubility of monazite (CePO4), SmPO4, and GdPO4 in aqueous solutions from 100 to 250 °C

   https://doi.org/https://doi.org/10.1016/j.gca.2018.08.038  

   Gysi, A.P. ; Harlov, D. ; Miron, G.D. ( January 2018 , Geochimica et cosmochimica acta)

   Monazite (CePO4) is a light rare earth element (REE) phosphate occurring as accessory mineral in metamorphic, igneous and sedimentary rocks, and is also a common mineral in REE mineral deposits. Metasomatism of monazite yields important clues about fluid-rock interaction in the crust, in particular, because its compositional variations may enable us to determine conditions of mineralization. The thermodynamic properties of monazite have been determined using several calorimetric methods, but up to the present time only a few solubility studies have been undertaken, which test the reliability of both, the thermodynamic properties of the REE phosphates and associated REE aqueous species. In this study, we have measured the solubility of the monoclinic REE phosphate end-members CePO4, SmPO4, and GdPO4 in aqueous perchloric acid solutions at temperatures from 100 to 250 °C at saturated water vapor pressure (swvp). The solubility products (Ks0) were determined according to the reaction: REEPO4 = REE3+ + PO43−. Combining available calorimetric data for the REE phosphates with the REE aqueous species from the Supcrt92 (slop98.dat) dataset, yields several orders of magnitude differences when compared with our solubility measurements. We have investigated ways to reconcile these discrepancies and propose a consistent set of provisional thermodynamic properties for REE aqueous species and REE phosphates that reproduce our measured solubility values. To reconcile these discrepancies, we have used the GEMS code package and GEMSFITS for parameter optimization by adjusting the standard Gibbs energy of REE3+ and REEOH2+ at 25 °C and 1 bar. An alternative optimization could involve adjustment of the standard Gibbs energy of REEPO4(s) and REEOH2+. Independently of the optimization method used, this study points to a need to revise the thermodynamic properties of REEOH2+ and possibly other REE hydroxyl species in future potentiometric studies. These revisions will have an impact on calculated solubilities of REE phosphates and our understanding of the mobility of REE in natural hydrothermal fluids. 

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