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Thorium mineralization is frequently hosted in carbonate-bearing rocks, and thorium commonly substitutes into the structures of carbonate-bearing minerals that have precipitated from or been modified by hydrothermal fluids. Given this common association, it is reasonable to consider the hypothesis that the presence of carbonate ligands in hydrothermal solutions promotes the transport of Th through the formation of stable aqueous complexes. Our ability to evaluate this hypothesis, however, is hindered by the lack of experimental data for Th-carbonate species at conditions beyond ambient. The low-temperature data indicate that carbonate is a strong complexing agent for Th. In this contribution, we investigate the solubility of Th in carbonate-bearing fluids relevant to natural systems (0.05–0.5 m NaHCO3/Na2CO3; pHT ~ 7.8–9.8) at elevated temperature (175–250 °C). We demonstrate that, in contrast to the behavior of Th at low temperature, the stability of Th-carbonate complexes is not sufficient for them to predominate at these conditions. Instead, the solubility of Th is governed by hydrolysis reactions. Under the experimental conditions investigated, the predominant hydroxyl complexes are Th(OH)40 and Th(OH)5−. Thermodynamic formation constants were derived for these species at the temperatures considered in our experiments (log β4 = 43.34 and 44.31 at 175 and 200 °C, respectively, and log β5 = 46.15 and 47.9 at 225 and 250 °C, respectively) to permit forward modeling of Th mobility in natural systems. Our study indicates that carbonate ions are unlikely to play a role in transporting Th in hydrothermal fluids. Summarizing the results of this study and our previous studies of the solubility of Th in hydrothermal fluids, we conclude that SO42− is the primary ligand responsible for the hydrothermal transport of Th.

The solubilities of the rare earth chlorides REECl3, where REE = (La, Nd, Er), were measured in HCl bearing water vapor from 350 to 425 °C with water partial pressures ranging from 8 to 170 bar. Solubility data were fit to the Pitzer-Pabalan quasi-chemical model in order to extract thermodynamic parameters for the formation of the gaseous REE-chloride-water clusters REECl3(H2O)n. The data show that the solubility of the REE chlorides are orders of magnitude higher than salts such as NaCl or CuCl at low water fugacities, despite their sublimation energies being substantially higher. This enhanced solubility is likely due to the high enthalpy associated with binding a single water molecule to form the species REECl3(H2O), with derived enthalpies ranging from −378 to −465 kJ/mol. Addition of further water molecules to form higher order clusters (n > 1) involves enthalpy changes of ~−20 kJ/mol, and are in effect thermodynamically suppressed over the temperature range 350–425 °C. Despite the enhanced solubility of small REECl3 water clusters, simulations of boiling processes demonstrate that the REE show highly conservative behavior, partitioning strongly into the dense aqueous phase. Not surprisingly, the presence of phosphates in the system makes this effect even more pronounced, completely immobilizing the REE. This would reduce transport in both the vapor and aqueous phase to negligible levels. However, we suggest that vapor phase transport of the REE may play a significant role in systems having a relatively low partial pressure of water (below the saturation point), where the relatively high stability of the first hydrated REE chloride clusters (REECl3(H2O) and REECl3(H2O)2) will give a preference for gas transport of the REE relative to other elements. This can likely happen in systems involving a gas/melt exchange in fumarolic exhalations, where water vapor discharges at relatively low (close to atmospheric) pressures.

Ore formation in porphyry Cu-Au-(Mo) systems involves the exsolution of metal-bearing fluids from magmas and the transport of the metals in magmatic-hydrothermal plumes that are subject to pressure fluctuations. Deposition of ore minerals occurs as a result of cooling and decompression of the hydrothermal fluids in partly overlapping ore shells. In this study, we address the role of vapor-like fluids in porphyry ore formation through numerical simulations of metal transport using the Gibbs energy minimization software, GEM-Selektor. The thermodynamic properties of the hydrated gaseous metallic species necessary for modeling metal solubility in fluids of moderate density (100–300 kg/m3) were derived from the results of experiments that investigated the solubility of metals in aqueous HCl- and H2S-bearing vapors. Metal transport and precipitation were simulated numerically as a function of temperature, pressure, and fluid composition (S, Cl, and redox). The simulated metal concentrations and ratios are compared to those observed in vapor-like and intermediate-density fluid inclusions from porphyry ore deposits, as well as gas condensates from active volcanoes. The thermodynamically predicted solubility of Cu, Au, Ag, and Mo decreases during isothermal decompression. At elevated pressure, the simulated metal solubility is similar to the metal content measured in vapor-like and intermediate-density fluid inclusions from porphyry deposits (at ~200–1,800 bar). At ambient pressure, the metal solubility approaches the metal content measured in gas condensates from active volcanoes (at ~1 bar), which is several orders of magnitude lower than that in the high-pressure environment. During isochoric cooling, the simulated solubility of Cu, Ag, and Mo decreases, whereas that of Au reaches a maximum between 35 ppb and 2.6 ppm depending on fluid density and composition. Similar observations are made from a compilation of vapor-like and intermediate-density fluid inclusion data showing that Cu, Ag, and Mo contents decrease with decreasing pressure and temperature. Increasing the Cl concentration of the simulated fluid promotes the solubility of Cu, Ag, and Au chloride species. Molybdenum solubility is highest under oxidizing conditions and low S content, and gold solubility is elevated at intermediate redox conditions and elevated S content. The S content of the vapor-like fluid strongly affects metal ratios. Thus, there is a decrease in the Cu/Au ratio as the S content increases from 0.1 to 1 wt %, whereas the opposite is the case for the Mo/Ag ratio; at S contents of >1 wt %, the Mo/Ag ratio also decreases. In summary, thermodynamic calculations based on experiments involving gaseous metallic species predict that vapor-like fluids may transport and efficiently precipitate metals in concentrations sufficient to form porphyry ore deposits. Finally, the fluid composition and pressure-temperature evolution paths of vapor-like and intermediate-density fluids have a strong effect on metal solubility in porphyry systems and potentially exert an important control on their metal ratios and zoning.

Closed-system experiments were conducted to investigate the decomposition of sodium dithionite in aqueous solutions under varying pH and starting concentrations to simulate the deployment of dithionite as an in-situ redox barrier. Co-determination of dithionite and its degradation products was conducted using UV–Vis spectrometry, iodometric titration, and ion chromatography. In unbuffered solutions, dithionite reacted rapidly, whereas in near-neutral solutions (pH ∼7), it persisted for ∼ 50 days and in alkaline solution (pH ∼9.5) for >100 days. These are the longest lifetimes reported to date, which we attribute to not only excluding oxygen but also preventing outgassing of H2S. Thoroughly constraining the reaction products has led to the following hypothesized reaction: 4 S2O42− + H2O → HS− + SO32−+2 SO42− + S4O62− + H+ which represents relatively rapid degradation at near-neutral pH values. At the more alkaline pH, and over longer time scales, the reaction is best represented by: 3 S2O42− + 3 H2O → 2HS- + SO32−+3 SO42−+ 4 H+ the following kinetic rate law was developed for the pH range studied: dC i dt = S i 10 − 4.81 { H + } 0.24 { S 2 O 4 2 - } , where dC i dt is the rate of change of the i t h chemical component in the simplified equation (mole L−1 s−1) and Si is the stoichiometric coefficient of the ith chemical. The kinetic rate law was used to calculate a pseudo first order half-life of 10.7 days for near-neutral pH and 33.6 days for alkaline pH. This work implies that if hydrogen sulfide is contained within the system, such as in the case of a confined aquifer below the water table, dithionite decomposes more slowly in alkaline aqueous solution than previously thought, and thus it may be more cost-effectively distributed in aquifers than has been previously assumed.

Studies on incorporation of radionuclides into the crystal structures of phosphate minerals strongly indicate that uranium and its mobile fission products can be efficiently immobilized through uptake from aqueous solution by formation of phosphate and carbonate minerals. Aiming at development of a new engineered backfill material, we have investigated uptake of uranium at a range of temperatures similar to those expected at waste repository sites, where thermal peak of the waste package (heated by radioactive decay) is ~300°C (Greenburg and Wen 2013). The available literature data on uranium uptake by phosphates are limited to ambient temperatures (e.g. Arey et al. 1999), and to our best knowledge no experimental studies on uranium uptake by phosphates and carbonate at hydrothermal conditions have been performed.Experiments were conducted in the autoclaves at saturated water pressure and 200-350°C, where metastable phosphate (brushite) and carbonate (aragonite) were transformed to apatite/monetite and calcite in NaCl solutions. Uranium was introduced into autoclave in separate tubes in the forms of U3O8 and UO3 in experiments at reduced and oxidized conditions. Oxidation state of dissolved uranium (U4+ or U6+) was controlled by addition of solid redox buffers into autoclaves.X-ray diffraction (XRD) and backscattered electron diffraction (EBSD) of crystalline products allowed estimation of mineral transformation rate. Inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS allowed obtaining uranium content in crystals and its concentration in co-existed solution. Partition coefficient (D) of uranium was calculated as the ratio of uranium content in the solid to uranium concentration in the solution. Selected solids were examined with synchrotron-based X-ray absorption spectroscopy (XAS).Overall, our results showed: 1) brushite transforms to monetite and apatite mixture during 6 days, but up to 1 month is required for complete transformation to apatite; 2) mineralogy of the final phase (monetite or hydroxyapatite) depends on ionic strength of the solution (confirmed by thermodynamic calculations); 3) uranium is compatible with phosphate and carbonate minerals, where D could be as high as 1000; 4) uptake of U4+ by calcite is higher than that of U6+ by up to a factor of 100; 5) uranium incorporates into calcite structure as U6+ at oxidized conditions. Additional analyses are pending and results will be presented at EGU meeting. ReferencesArey J.S., Seaman J.C., and Bertsch P.M. (1999) Environ. Sci. Technol. 33, 337-342.Greenburg H.R. and Wen J. (2013) LLNL-TR639869-DRAFT, 38.

Heterogeneous distribution of trace elements (impurities) within individual calcite crystals is a phenomenon commonly observed in natural and laboratory systems. Changes in thermodynamic intensive parameters (mostly chemical potential and temperature) cannot always explain the inhomogeneous impurity patterns in calcite crystals and it has been suggested that growth rate and crystallographic orientation may exert strong effects on elemental incorporation into calcite. In addition, there are a number of experimental studies on micro-scale element (E) distribution between non-equivalent pairs of calcite vicinal faces (known as sectoral zoning); however, the variability of partition coefficients (e.g., KE = (E/Ca)calcite/(E/Ca)fluid) within individual crystals remains undetermined. In this study, we have extended the work on elemental distribution between crystal sectors to evaluation of partition coefficients of trace and minor elements (Li, B, Mg, and Sr) in calcite crystal faces (10–14) and (01−12), whose growth rates were assessed. Growth entrapment model (GEM) and lattice strain theory were applied to explain KE heterogeneity by varying near-surface diffusivity of Mg and Sr and by varying surface enrichment factor for Li, B, Mg, and Sr. Decoupling of sectoral and growth rate effects reveals that sectoral zoning plays a key role in elemental distribution. More specifically, KLi and KB vary by more than one order of magnitude and KMg varies by a factor of two within individual crystal faces. Strontium sectoral distribution is different from those of Li, B, and Mg and KSr varies by up to a factor of two. These behaviors likely reflect different mechanisms of incorporation of Li, B, Mg, and Sr into calcite.

This work evaluated the immobilization of uranium (U) through incorporation into calcite under reduced and oxidized conditions. We investigated how much U could be entrapped by calcite crystallizing in chloride solutions in autoclaves at temperatures from 162 to 300 °C. The oxidation state of U was set by controlling oxygen fugacity via redox buffers. Uranium was introduced into calcite growth media as a solid oxide compound or U aliquot. We found the uptake of tetravalent U by calcite is higher than that of hexavalent U by up to four orders of magnitude. We estimate that crystallization of a few mg of calcite immobilizes all dissolved U when 1 kg of solution is saturated with UO2 under reduced hydrothermal conditions.

Legacy industrial waste has left groundwater plumes of hexavalent Cr (Cr(VI)) that requires treatment, predominantly by reduction to Cr(III) and subsequent precipitation. One promising technology is the injection of a strong reducing agent, sodium dithionite, to create an in-situ redox barrier, but implementation requires a comprehensive understanding of the reactions occurring with dithionite injection. Batch and column experiments were conducted with aquifer sediments to determine both the significant reactions and efficacy of sodium dithionite treatment for a groundwater plume of hexavalent chromium. The batch experiments demonstrate consumption of dithionite over the experiment (disappearance by 43 d) concurrent with leaching of about 1 mM Fe from the sediments. Reactions deduced from batch experiments were incorporated into a 1-D numerical model to simulate reactions occurring during the column injection. The treatment was able to successfully reduce approximately 30 pore volumes of groundwater containing 800 µg kg−1 Cr(VI). The experiments also demonstrate that although mineral forms of Fe are important phases in the reduction of Cr(VI), Fe alone cannot account for the entire reduction capacity imparted to the sediments. Rather, the correlation between Cr and reduced S retained in the columns, combined with Scanning Electron Microscopy (SEM) of treated sediments, suggest that formation of reduced S phases contributes to the prolonged reduction capacity.

Gaseous mixtures of water vapor and neutral molecules of salt (e.g., NaCl, CuCl etc.) can be highly non-ideal due to a strong attractive interaction between salt and water molecules. In particular, this can result in high solubility of salts in water vapor and a strong dependence of solubility on vapor pressure. The analysis of salt solubility in water vapor can be done using the Pitzer-Pabalan model, which is based on the thermodynamic theory of imperfect gases. The original Pitzer-Pabalan work demonstrated that one can reproduce experimental data for NaCl solubility in vapor. No analysis was performed on the reliability of their original fits, which we believe has contributed to the lack of applications of the Pitzer-Pabalan model despite the apparent success of the original paper. In this work, we report our recent progress in developing a rigorous fitting procedure to parameterize the Pitzer-Pabalan model using experimental data. Specifically, we performed fitting of the experimental results obtained elsewhere for NaCl and CuCl. We investigate the degree of underfitting/overfitting and the sensitivity of the fitting quality to variations in the resulting fitting parameters. The results, as represented by the thermodynamic parameters describing the energetics of formation of salt-bearing water clusters, were successfully benchmarked against Gaussian 16 ab initio quantum chemistry calculations. The resulting rigorous fitting procedure presented here can now be applied to other systems.

The solubility of Yttrium (III) fluoride as a function of fluoride activity was investigated experimentally at 100, 150, 200 and 250 °C and vapor-saturated water pressure. Data obtained from the experiments were used to determine the solubility product of YF3(s), the fluoride speciation of Y and the stability constants of the corresponding complexes. The dominant Y-fluoride species at the temperatures investigated is the di-fluoride complex ( YF 2 + ). Consequently, fluoride speciation of Y differs substantially from that of the lanthanides (Ln), which dominantly form mono-fluoride complexes (LnF2+) at these temperatures. The logarithm of the solubility product (log Ksp) of YF3(s) is −20.8 ± 0.50, −21.5 ± 0.46, −22.4 ± 0.56 and −24.3 ± 0.12 at 100, 150, 200 and 250 °C, respectively. The logarithm of the formation constant (log β) of YF 2 + is 8.3 ± 0.77, 10.7 ± 0.43, 12.1 ± 0.31 and 13.3 ± 0.16 at the same temperatures, respectively. Differences in the speciation of Y from that of the lanthanide, holmium (Ho), quantitatively explain the fractionation between these geochemical twin elements, which has been reported for many fluoride-rich hydrothermal systems. Our results emphasize the usefulness of the Y/Ho ratio as a geochemical indicator in hydrothermal systems.

Knowledge of the thermodynamic properties of aqueous nickel chloride complexes is important for understanding and quantitatively evaluating nickel transport in hydrothermal systems. In this paper, UV–Visible spectroscopic measurements are reported for dissolved nickel in perchlorate, triflic acid and sodium chloride solutions at temperatures up to 250 °C and 100 bar. The observed molar absorbance of Ni 2+ in both perchlorate and triflic acid solutions is similar, and the absorbance peak migrates toward lower energy (red-shift) with increasing temperature. The spectra of nickel chloride solutions show a systematic red-shift with increasing temperature and/or chloride concentration. This allowed identification of the nickel chloride species as NiCl + , NiCl 2(aq) and NiCl 3 - , and determination of their formation constants. Based on the experimental data reported in this paper and those of previous experimental studies, formation constants for these nickel chloride complexes have been calculated for temperatures up to 700 °C and pressures up to 2000 bar. The solubility of millerite (NiS) and pentlandite (Ni 4.5 Fe 4.5 S 8 ) calculated using these constants shows that nickel dissolves in significantly higher concentrations in hydrothermal solutions than previously estimated. However, the solubility is considerably lower than for corresponding cobalt sulphide minerals. This may explain why hydrothermal nickel deposits are encountered so much less frequently than hydrothermal deposits of cobalt.

The solubility of REE(III) fluoride solids was determined in fluoride- and chloride-bearing solutions at 150, 200 and 250 C and saturated water vapor pressure. These experimental data, together with experimental data from previously published studies, were used to evaluate formation constants for chloride- and fluoride-bearing aqueous species of the entire REE(III) group at temperatures up to 300 C. The data show that the stability of these species differs significantly from that predicted theoretically. For example, contrary to the theoretical predictions, LREEF 2+ species are more stable than HREEF 2+ species at elevated temperature. The behavior of the chloride-bearing species is similar. Parameters for the Helgeson–Kirkham–Flowers (HKF) equation of state were determined for REEF 2+ , REECl 2+ and REECl2 + complexes using these experimental data and permit calculation of formation constants of these species at conditions not investigated experimentally. These data now permit the mobility of all REE in fluoride- and chloride-bearing hydrothermal systems to be reliably evaluated at intermediate temperatures and pressures. 2009 Elsevier Ltd. All rights reserved.

We have conducted experiments to evaluate the vapour–liquid fractionation of Mo(VI) in the system MoO3–NH3–H2O at 300–370 °C and saturated vapour pressure, using a two-chamber autoclave that allows separate trapping of the vapour and liquid. The measured total Mo concentrations in each phase were used to calculate a distribution coefficient, K D V / L , which increases as the density of the vapour approaches that of the liquid, and is greater than one for pH ⩽ 4. Molybdenum speciation in the vapour is described by a single complex, MoO3 H2O. By contrast, thermodynamic modeling of the distribution of Mo species in the liquid indicates that bimolybdate (HMoO4−) is the dominant aqueous species at the conditions of our experiments, and that molybdate (MoO42−) and molybdic acid (H2MoO40) are present in smaller quantities. As vapour–liquid fractionation occurs between neutral species, it is governed by the reaction H2MoO40(aq) = MoO3 · H2O(g). Fractionation is therefore controlled by the concentration of H2MoO40 in the liquid, which increases with increasing temperature and decreasing pH. Owing to the pH dependence of K D V / L , it cannot be used to describe Mo fractionation in aqueous vapour–liquid systems with compositions different than those of this study. We have therefore calculated a composition-independent (Henry’s Law) constant, K H V / L , for each experimental point, using the measured total Mo concentration in the vapour and the modeled concentration of H2MoO40 in the liquid. This constant may be applied to aqueous vapour–liquid systems of known liquid composition to estimate the concentration of Mo in a vapour for which little chemical information is available, and thereby supplement the available fractionation data for natural porphyry-forming systems. The results of this study demonstrate that at conditions typical of natural porphyry ore-forming systems, a significant amount of molybdenum fractionates into the vapour over the liquid, and the vapour may transport quantities of Mo in excess of that in the liquid at pH conditions below those of the muscovite–microcline reaction boundary.

Experiments on the incorporation of trace elements into calcite were performed, and rare earth elements (REE) were used to mark the growth zones of individual crystals. Experiments were conducted at different pH (7.7 to 8.8) and temperatures (2 °C to 24.6 °C) in NH4Cl + CaCl2 solutions, where REE were rapidly consumed by growing calcite. LA-ICP-MS line-scans yielded the distribution of (REE/Ca)calcite within individual crystals in a manner consistent with the addition of REE into fluid. A sharp decrease of (REE/Ca)calcite toward the crystal edge suggests the fast depletion of (REE/Ca)fluid due to strong REE consumption by growing calcite. An attempt was made to estimate the lower limit of the partition coefficients between calcite and fluid using selected REE/Ca data within individual calcite crystals and the amount of REE added into fluid.