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Monazite-(Ce) and xenotime-(Y) occur as secondary minerals in iron-oxide-apatite (IOA) deposits, and their stability and composition are important indicators of timing and conditions of metasomatism. Both of these minerals occur as replacement of apatite and display slight but important variations in light (e.g. La, Ce, Pr, Nd, etc.) and heavy (e.g. Y, Er, Dy, Yb, etc.) REE concentrations [1,2]. The causes for these chemical variations can be quantified by combining thermodynamic modeling with field observations. Major challenges for determining the stability of these minerals in hydrothermal solutions are the underlying models for calculating the thermodynamic properties of REE-bearing mineral solid solutions and aqueous species as a function of temperature and pressure. The thermodynamic properties of monazite and xenotime have been determined using several calorimetric methods [3], but only a few hydrothermal solubility studies have been undertaken, which test the reliability and compatibility of both the calorimetric data and thermodynamic properties of associated REE aqueous species [4,5]. Here, we evaluate the conditions of REE metasomatism in the Pea Ridge IOA-REE deposit in Missouri, and combine newly available experimental solubility data to simulate the speciation of LREE vs. HREE, and the partitioning of REE as a function of varying fluid compositions and temperatures. Our new experimental data will be implemented in the MINES thermodynamic database (http:// tdb.mines.edu) for modeling the chemistry of crustal fluid-rock equilibria [6]. [1] Harlov et al. (2016), Econ. Geol. 111, 1963-1984;[2] Hofstra et al. (2016), Econ. Geol. 111, 1985-2016; [3] Navrotsky et al. (2015), J. Chem. Thermodyn. 88, 126-141; [4] Gysi et al. (2015), Chem. Geol. 83-95; [5] Gysi et al. (2018), Geochim. Cosmochim. Acta 242, 143-164; [6] Gysi (2017), Pure and Appl. Chem. 89, 581-596. 

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. 

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. 

Societal demand for critical metals used in the high-tech and green industries has led to an increased interest in REEs associated with ore deposits. Hydrothermal mineralization of monazite (CePO4) in various REE deposits can display significant variations in REE mineralogy and rock chemistry. Monazite displays textural and REE compositional variations, such as those observed in the giant Bayan Obo REE deposit in China and in the Pea Ridge iron-oxide-apatite (IOA) deposit in Missouri. The coupling of compositional variations of monazite with thermodynamic modeling of fluid-rock interaction processes may provide a useful vectoring tool in these ore deposits. However, interpreting these geochemical fingerprints requires building an internally consistent thermodynamic dataset for REE minerals and their relevant aqueous complexes. In this study, a series of hydrothermal solubility experiments were carried out using synthetic monazite crystals (i.e., LaPO4, PrPO4, NdPO4, and EuPO4) to assess the consistency of reported mineral calorimetric data and the thermodynamic data of the aqueous REE complexes. The solubility experiments were conducted in aqueous HClO4-H3PO4–bearing solutions at temperatures between 100° and 250°C and at saturated water vapor pressure. Equilibrium constants (Ks0) for the dissolution reaction of monazite end members were retrieved as a function of temperature and extrapolated to standard conditions of 25°C and 1 bar. Results indicate significant differences between the new solubility data and those reported in the literature. We demonstrate the impact of these new thermodynamic data in a series of fluid-rock interaction models using the GEMS code package (http://gems.web.psi.ch) and the MINES thermodynamic database (http://tdb.mines.edu). The simulated monazite stability can be correlated to field observations and allows for the prediction of the behavior of REE in hydrothermal fluids and their association to alteration observed in ore deposits. 

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. 

Hydrothermal ore deposits involve complex fluid-rock interactions, where alteration zones yield information on physicochemical conditions of ore-forming processes and fluid pathways. Lithogeochemical vectors can be used to delineate alteration zones and potentially yield information for targeting ore zones for the exploration of mineral deposits. To be able to recognize these geochemical and mineralogical signatures, it is imperative to understand the underlying ore-forming processes and link them to geochemical vectors. Numerical modeling permits tracking changes in rock chemistry and metal precipitation mechanisms by simulating various alteration and fluid evolution scenarios such as boiling, fluid mixing, fluid-rock interaction, and changes in redox, pH, and temperature. The open-access MINES thermodynamic database (http://tdb.mines.edu) has been recently launched for modeling fluid-rock interaction and the chemical changes related to ore-forming processes, using the program GEM-Selektor (http://gems.web.psi.ch). The philosophy behind the MINES database is to focus on testing a series of numerical modeling projects used to simulate various ore deposits. We present a number of case studies where GEM-Selektor has been successfully applied to simulate ore-forming processes. These include Cu transport and mineralization at the Kansanshi Cu-Au deposit hosted in metasedimentary rocks, and the metasomatism of pegmatites and mobilization of rare earth elements (REEs) in the Strange Lake REE-Zr-Nb mineral deposit. We also use this method to demonstrate the coupling of complex chemical equilibria with reactive mass transport along a fluid flow path for simulating the deposition of Pb and Zn in a Mississippi Valley-type (MVT) deposit and the alteration associated with volcanogenic massive sulfide (VMS) deposits. The long-term goal of this project is the generation of a geochemical vectors database for interpreting field data, using knowledge gained from numerical simulations combined with field observations. This link between fundamental and applied research is expected to significantly advance ore vectoring for the exploration industry.