Terry M. Seward

Hydrosulphide complexing of Au (I) in hydrothermal solutions from 150–400°C and 500–1500 bar

Abstract The solubility of gold has been measured in aqueous sulphide solutions at temperatures between 150°C and 500°C and pressures of 500–1500 bar over a wide range of pH and total dissolved sulphur concentrations. The solubilities ranged from 0.002–1 mg/kg (1 × 10−8 to 5 × 10−6 m) in experiments with low total sulphur and acid pH, and from 2–108 mg/kg (1 × 10−5 to 5 × 10−4 m) in solutions wit)1 high total reduced sulphur concentrations and near neutral pH. The solubilities generally increased with increasing temperature, pH, and total dissolved sulphur. At near neutral pH, an inverse correlation between solubility and pressure was observed, whereas in acid pH solutions, above 150°C, increasing pressure also increased the solubility. In near neutral pH solutions a solubility maximum was observed. This maximum is due to the species Au(HS)2−. However, with increasing temperature, in accordance with the shift of pK1 of H2S towards more alkaline pH, the maximum solubility also shifts to higher pH-values and consequently, at high temperatures the species stable at lower pH will dominate. It has been unambiguously proven that over a wide range of temperatures and pressures in reduced sulphur-containing hydrothermal solutions of low pH, the stoichiometry of the dominant Au (I)-hydrosulphide complex, is AuHS0. High temperature and high pressure: equilibrium constants for the formation of the Au(I)-hydrosulphide complexes, AuHS0, and Au(HS)2−, pertaining to the equilibria Au(s) + H2S = AuHS0 + 1 2 H2(g) (1) and Au(s) + H2S + HS− = Au(HS)2− + 1 2 H2(g), (2) have been calculated. The nonlinear least squares fitted equilibrium constant for reaction (1) varies from log K(1) = −6.81 at 150°C/500 bar to a maximum of −5.90 at 200°C/1500 bar and decreases again at higher temperatures (-7.83 at 400°C/500 bars). For reaction (2), a similar variation occurs: log K(2) = −1.45 at 150°C/500 bar to −1.03 at 250°C/500 bar and −1.75 at 400°C/1500 bar. The thermodynamic functions for the Au(I)-hydrosulphide formation reactions and the cumulative and stepwise formation constants were derived after transforming the above reactions into isocoulombic form. The equilibrium constants were derived after transforming the above reactions into isocoulombic form. role in the transport and deposition of gold in ore depositing environments which are characterised by low pH fluids.

Magmatic vapor contraction and the transport of gold from the porphyry environment to epithermal ore deposits

Fluid-phase stability relations combined with thermodynamic modeling using fluid-inclusion analyses and new gold-solubility experiments lead to an integrated geological interpretation linking epithermal gold mineralization and porphyry-style ore formation to the cooling of hydrous magma chambers. The essential chemical requirement for gold transport to low temperatures is an initial excess of sulfide over Fe in the magmatic fluid, which is best achieved by condensing out Fe-rich brine from a buoyant, low- to medium-salinity vapor enriched in volatile S. This vapor can contract directly to an aqueous liquid, by cooling at elevated pressure above the critical curve of the salt-water fluid system. Physical and chemical conditions are matched when magmatic fluid is released through a gradually downward-retracting interface of crystallizing magma beneath a porphyry stock, predicting the consistent zoning and overprinting relations of alteration and mineralization observed in magmatic hydrothermal systems.

The adsorption of gold(I) hydrosulphide complexes by iron sulphide surfaces

The adsorption of gold by pyrite, pyrrhotite, and mackinawite from solutions containing up to 40 mg/kg (8 μm) gold as hydrosulphidogold(I) complexes has been measured over the pH range from 2 to 10 at 25°C and at 0.10 m ionic strength (NaCl, NaClO4). The pH of point of zero charge, pHpzc, has been determined potentiometrically for all three iron sulphides and shown to be 2.4, 2.7, and 2.9 for pyrite, pyrrhotite, and mackinawite, respectively. In solutions containing hydrogen sulphide, the pHpzc is reduced to values below 2. The surface charge for each sulphide is therefore negative over the pH range studied in the adsorption experiments. Adsorption was from 100% in acid solutions having pH < 5.5 (pyrite) and pH < 4 (mackinawite and pyrrhotite). At alkaline pH’s (e.g., pH = 9), the pyrite surface adsorbed 30% of the gold from solution, whereas the pyrrhotite and mackinawite surfaces did not adsorb. The main gold complex adsorbed is AuHS°, as may be deduced from the gold speciation in solution in combination with the surface charge. The adsorption of the negatively charged Au(HS)2− onto the negatively charged sulphide surfaces is not favoured. The X-ray photoelectron spectroscopic data revealed different surface reactions for pyrite and mackinawite surfaces. While no change in redox state of adsorbent and adsorbate was observed on pyrite, a chemisorption reaction has been determined on mackinawite leading to the reduction of the gold(I) solution complex to gold(0) and to the formation of surface polysulphides. The data indicate that the adsorption of gold complexes onto iron sulphide surfaces such as that of pyrite is an important process in the “deposition” of gold from aqueous solutions over a wide range of temperatures and pressures.

A spectrophotometric study of hydrogen sulphide ionisation in aqueous solutions to 350°C

Abstract The ultraviolet spectra of dilute aqueous H 2S/HS − solutions have been measured up to 350°C. From these data, the molal equilibrium constant, K 1, for the first ionisation of H 2S has been calculated. Over the temperature range from 25 to 350°C at saturated vapour pressures, K 1 varies from 6.99 (25°C) to 8.89 (350°C). Standard thermodynamic data for H 2S (aq) and HS − are presented for the temperature range 25 to 300°C at 100 bar. The shift of the charge-transfer-to-solvent spectrum of HS − with increasing temperature is attributable to an expansion of hydrosulphide ion solvation environment.

An X-ray absorption (EXAFS) spectroscopic study of aquated Ag+ in hydrothermal solutions to 350°C

Abstract The first direct measurements over a range of temperatures from 25–350°C are reported for cation-oxygen (water) distances in aqueous solutions at equilibrium saturated vapour pressures. The silver-oxygen (first shell water) bond lengths in AgNO 3 and AgClO 4 solutions were obtained using extended X-ray absorption fine structure (EXAFS) measurements on the silver K-edge. At 25°C and 1 bar, the Ag + ion is coordinated by four inner sphere water ligands with an average silver-oxygen distance of 2.32 (±0.01) A. With increasing temperature to 350°C, the silver-oxygen (H 2 O) bond length decreases by about 0.10 (±0.01) A and the coordination number decreases from 4 to ∼3. The Debye-Waller factors remain approximately constant with increasing temperature. Silver-nitrate ion pairing (as AgNO 0 3 ) was observed only at 200 and 300°C in an 0.10 AgNO 3 /3.00 m HNO 3 solution for which the silver-nitrogen (nitrate) distances of 3.24 and 3.27 A, respectively, were obtained.

An EXAFS study of solvation and ion pairing in aqueous strontium solutions to 300°C

Abstract X-ray absorption fine structure (EXAFS) measurements on 0.10-m strontium-containing solutions from 25 to 300°C at saturated vapour pressure indicate that the first shell Sr 2+ –oxygen (water) bond length decreases from 2.57 to 2.52 (±0.01) A. Over the same temperature range, the number of coordinated first shell water molecules decreases from 8 to 6. Simulations of a 1.35-m SrCl 2 using both rigid (SPC) and flexible (BJH) water models over the range from 25 to 330°C also show a decrease in the Sr 2+ –oxygen distance of 0.03 A, which is independent of density. EXAFS measurements and molecular dynamics simulations of concentrated SrCl 2 solutions up to 270°C and 300°C, respectively, show the presence of polynuclear cluster molecules, further underlining the importance of such species in defining the solute chemistry of hydrothermal solutions in the earth’s crust. These data represent the first direct spectroscopic confirmation of solute clustering in high temperature aqueous solutions of alkali metal or alkali earth metal salts.

The hydrosulphide/sulphide complexes of copper(I): experimental determination of stoichiometry and stability at 22°c and reassessment of high temperature data

Abstract The solubility of chalcocite has been measured over the pH range 4 - 11.5 in aqueous sulphide solutions in order to determine the stoichiometry and stability of the Cu(I) hydrosulphide/sulphide complexes at room temperature. A flow-through column was used as an alternative method for the measurement of the solubilities. Non-linear least squares fitting of the results gave the following stoichiometries and stability constants at 22°C for I = 0.0: Cu + +2 HS − =Cu( HS ) 2 − log β 122 =+17.18±0.13 2Cu + +3 HS − =Cu 2 S ( HS ) 2 2− +H + log β 232 =+29.87±0.14. The stability of a third complex expected in the low pH region has been estimated: Cu + + HS − =Cu HS 0 log β 111 ≈+13. The Cu(HS) 2 − complex will predominate in the near-neutral region at intermediate to high sulphide concentrations (>0.001 mol kg −1 ) while Cu 2 S(HS) 2 2− will only be important at basic pH values and high sulphur concentrations. At lower sulphur concentrations ( −1 ), CuHS° is the dominant hydrosulphide complex. In natural anoxic bottom waters and porewaters, sulphide concentrations fall in the region where both Cu(HS) 2 − and CuHS° may contribute significantly to total copper solubility. In order to test the applicability of the low temperature speciation model at elevated temperature, the solubility data of Crerar and Barnes (1976) were refit using CuHS° + Cu(HS) 2 − . The data show an excellent fit with this model and the following equations for the temperature dependence (25 ≤ T ≤ 350°C) of the cumulative stability constants were derived: log β 111 =3.798+ 2752 T log β 122 =−614.3+ 6.702×10 4 T − 5.920×10 6 T 2 +83.06 ln T where T is temperature in Kelvin. Speciation calculations show that for a hydrothermal fluid at 300°C with sulphur concentration buffered at pyrite-pyrrhotite-magnetite, pH = 4-6, the dominant hydrosulphide complex will be either CuHS° or Cu(HS) 2 − depending on the pH. In lower pH solutions, CuHS° is expected to be the dominant hydrosulphide complex at most geological sulphur concentrations. Comparison with Cu(I) chloride complexes shows that, at 300°C, CuCl 2 − will predominate when chloride concentrations exceed 0.1 to 1.0 mol kg −1 at pH values buffered at potassium feldspar-muscovite-quartz. As temperature decreases, the stability of the chloride complexes declines and therefore hydrosulphide complexes predominate over an increasingly wider range of chloride concentration. In hydrothermal solutions, copper transport as hydrosulphide complexes reaches mineralizing levels only at high total sulphur concentrations and basic pH values. Under more acidic conditions and lower total sulphur, chloride complexing is required for the transport of sufficient copper to form economic mineralization.

Convective hydrothermal C02 emission from high heat flow regions

Abstract In addition to volatiles released from volcanoes, the flux of CO2 to the atmosphere from other sources (e.g., metamorphism and subsurface magmatism) represents an important aspect of the global carbon cycle. We have obtained a direct estimate of the present-day atmospheric CO2 flux from convective hydrothermal systems within subaerial, seismically-active, high heat flow regions. Geothermal systems of the Salton Trough (California, U.S.A.) and the Taupo Volcanic Zone (New Zealand) provide benchmarks for quantifying convective hydrothermal CO2 fluxes from such regions. CO2 fluxes from the Salton Trough ( ∼ 109 mol yr−1) and the Taupo Volcanic Zone (∼ 8·109 mol yr−1) were computed using data on convective heat flow and the temperatures and CO2 concentrations of reservoir fluids. The similarity in specific CO2 flux ( ∼ 106 mol km−2 yr−1) from these two disparate geologic/tectonic settings implies that this flux may be used as a baseline to compute convective hydrothermal CO2 emission from other areas of high heat flow. If this specific flux is integrated over high heat flow areas of the circum-Pacific and Tethyan belts, the total global CO2 flux could equal or exceed 1012 mol yr−1 Adding this flux to a present-day volcanic CO2 flux of ∼ 4·1012 mol yr−1 the total present-day Earth degassing flux could balance the amount of CO2 consumed by chemical weathering ( ∼ 7·1012 mol yr−1).

Hydrosulfide/sulfide complexes of copper(I): Experimental confirmation of the stoichiometry and stability of Cu(HS) 2 to elevated temperatures

Abstract The solubility of chalcocite has been measured over the temperature range 35–95°C at pH 6.5–7.5 in aqueous hydrosulfide solutions in order to determine the stability constants of the Cu(HS) 2 − complex. A heated flow-through system was used in which solutions are collected at temperature to avoid the problem of copper precipitation due to quenching. The quality of the data was sufficient to resolve a 0.1 log unit increase of the dissolution/complexation equilibrium constant with each 10°C increase in temperature. The equilibrium constants were fit using previously published methods to obtain the values of thermodynamic parameters for the Cu(HS) 2 − complexation reaction. To compare results with predictive techniques, one-term and two-term isocoulombic extrapolation methods were applied to the stability constants measured below 100°C. The two-term extrapolation to 350°C showed excellent agreement with the derived constants proving its applicability to soft metal–soft ligand interactions. The one-term method gave a reasonable agreement but deviated about one logarithmic unit at 350°C. This is attributed to differences in energetic, volumetric, and structural properties of the reactants and products. Speciation calculations show that at low temperatures ( −1 ) while at higher temperatures, chloride complexes will be dominant under most geological conditions. Only in solutions with high reduced sulfur content and alkaline pH values will hydrosulfide complexes predominate and may play a role in the generation of economic copper mineralization.

Stability of chloridogold(I) complexes in aqueous solutions from 300 to 600°C and from 500 to 1800 bar

Abstract The solubility of gold has been measured in the system H2O+H2+HCl+NaCl+NaOH at temperatures from 300 to 600°C and pressures from 500 to 1800 bar in order to determine the stability and stoichiometry of chloride complexes of gold(I) in hydrothermal solutions. The experiments were carried out in a flow-through autoclave system. This approach permitted the independent determination of the concentrations of all critical aqueous components in solution for the determination of the stability and stoichiometry of gold(I) complexes. The solubilities (i.e. total dissolved gold) were in the range 9.9 × 10−9 to 3.26 × 10−5 mol kg−1 (0.002–6.42 mg kg−1) in solutions of total dissolved chloride between 0.150 and 1.720 mol kg−1, total dissolved sodium between 0.000 and 0.975 mol kg−1 and total dissolved hydrogen between 4.34 × 10−6 and 7.87 × 10−4 mol kg−1. A nonlinear least squares treatment of the data demonstrates that the solubility of gold in chloride solutions is accurately described by the reactions, Au(s) + 2Cl − + H + = AuCl 2 − + 0.5 H 2 ( g ) K s ,020 Au(s) + H 2 O = AuOH(aq) + 0.5 H 2 ( g ) K s ,001 where AuCl2− predominates in acidic chloride solutions and AuOH(aq) in neutral to alkaline chloride and chloride-free solutions. The solubility constant, logKs,020, increases with increasing temperature and decreases with increasing pressure from a minimum of −5.43 (±0.29) at 300°C and 500 bar to a maximum of −0.15 (±0.16) at 600°C and 1000 bar, with the pressure effects becoming more important with increasing temperature. The equilibrium solubility constant for AuOH(aq) has been previously determined by Stefansson and Seward (2003) . The solubility of gold at pH >5 was found to be independent of chloride concentration up to 1 mol kg−1 and identical to the solubility of gold with respect to AuOH(aq). The stability of AuClOH− was estimated to be 3 to 6 orders of magnitude less stable than AuOH(aq) and AuCl2− in hydrothermal solutions. Hence, gold(I) chloride complexes play an important role in transporting gold in aqueous acidic chloride solutions above 400°C.