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Vol 65, No 1 (2023)
Articles
Эксперимент в решении проблем эндогенного рудообразования
3
Copper in Hydrothermal Systems: a Thermodynamic Description of Hydroxocomplexes
Abstract
The experimental data available in the literature on the solubility of Cu(met.) and Cu2O (cuprite) in water under hydrothermal conditions have been processed. Key experiments were carried out on the solubility of cuprite at 300 C, saturated vapor pressure of H2O, vs. pH of the solution. As a result, a set of values of thermodynamic properties for 25°C, 1 bar and parameters of the HKF (Helgeson-Kirkham-Flowers) and AD (Akinfiev-Diamond) equation of states for Cu(I) hydroxocomplexes was obtained, which make it possible to describe their behavior in a wide range of temperatures (0 – 600 C), pressures (1 – 3000 bar) and densities of aqueous fluid (0.01 – 1 gcm–3). As it has been shown by thermodynamic modeling the Cu+ ion is prevalent in the acidic and weakly alkaline regions of the aqueous solvent over the entire temperature and pressure range studied. The effect of the neutral CuOH hydroxocomplex begins to show up in the alkaline region at T > 300 C and grows with increasing temperature. The second copper hydroxocomplex Cu(OH)2– shows up only in the strongly alkaline region, and the temperature has almost no effect on its behavior
4-14
COUPLED SOLUBILITY OF Cu and Ag IN CHLORIDE-BEARING HYDROTHERMAL FLUIDS (350-650 °С, 1000 - 1500 BAR)
Abstract
The coupled solubility of Cu(cr) and Ag(cr) was measured in acidic chloride solutions at 350 °С/1000 bar, 450 °С/1000 bar and 653 °С/1450 bar in a wide range of chloride concentrations (0.02m HCl + (0-15m) NaCl). The experiments were performed using autoclave (350, 450 °С) and ampoule (653 °С) techniques, the dissolved metal concentrations were measured after quenching the experimental system. The equilibrium constants of the reaction combining the dominant forms of transport of the metals are determined
Cu(cr) + AgCl2- = Ag(cr) + CuCl2- K°(Cu-Ag) .
The calculated reaction constant is independent of chloride concentration. It is known that AgCl2- predominates in the studied chloride concentration range. Therefore, the main cupper form of occurrence is CuCl2- in all experimental fluids with chloride content up to 47 wt.%, which is close to the NaCl saturation limit. The constant of the coupled Cu and Ag solubility was determined for the experimental PT-parameters as lg K°(Cu-Ag) = 2.65±0.20 (350 °С/1000 bar), 2.28±0.10 (450 °С/1000 bar), 1.49±0.34 (653 °С/1450 bar). These data, together with values from the literature for temperatures from 200 to 900 °С and pressure up to 2000 bar were fitted to the density model equation: lg K°(Сu-Ag) = 1.066 + 1.108∙103∙T(K)-1 + 3.585lgd(w) – 1.443∙lg d(w)103T(K)-1, where d(w) is the pure water density. According to these data, copper is much more soluble in chloride solutions compared to silver, but the difference in the solubility decreases with the temperature increase. Reliable literature data on the silver solubility constant allow to calculate the copper solubility one
Cu(к) + HCl° + Cl- = CuCl2-+ 0.5H2(р-р) K°(Cu) ,
lg K°(Cu) = 1.39±0.20 (350 °С, 1000 bar), 1.91±0.10 (450 °С, 1000 bar), 2.06±0.34 (653 °С, 1450 bar). The new values of K°(Cu) are combined with reliable literature data to calculate the density model parameters. The obtained density model equation can be used to calculate the copper solubility constant up to 800 °C and pressures to 2000 bar: lg K°(Сu) = 6.889 – 3.298∙103∙T(K)-1 + 8.694∙lg d(w) – 4.807∙lg d(w)∙103∙T(K)-1. The solubility of chalcopyrite in the system with mineral buffer assemblages pyrite-hematite-magnetite and K-feldspar-muscovite-quartz is discussed.
15-31
Cadmium behavior in sulfur-bearing aqueous environments: insight from CdS solubility measurements at 25-80°C
Abstract
The present experimental study explores the effects of temperature and sulfur in Cd aqueous geochemistry under reduced conditions. Greenockite CdS solubility is measured in H2O-H2S-HClO4-NaHS solutions at 25-80°C as a function of pH and sulfur concentration. Based on solubility product measurements in highly acid solutions, the standard thermodynamic properties of greenockite (CdS) are revised, and the recommended value of ∆fG0298.15 for greenockite CdS(s) is -151.5±0.3 kJ mol-1. The stability of greenockite (CdS) is higher than predicted by calculations using previous literature data. At 80°C, the stability constants for Cd-HS complexes are measured for the first time, the values are 10–5.65±1.00 for CdS(s) + H+ = CdHS+, 10–6.00±0.40 for CdS(s) + H2S0(aq) = Cd(HS)20(aq), 10–3.87±0.10 for CdS(s) + H2S0(aq) + HS- = Cd(HS)3-, and 10–3.53±0.20 for CdS(s) + H2S0(aq) + 2HS- = Cd(HS)42-. Modeling of Cd behavior at 3-200°C shows that Cd-HS species are more important than previously believed. The fraction of Cd(HS)n2-n (n = 1-4) complexes increases with mH2S and decreases with T. Thus, in euxinic marine environments with mH2S ≥ 10-5, Cd speciation changes from Cd-Cl to Cd-HS. This speciation change is expected to affect Cd isotope fractionation and should be accounted for when applying Cd isotopic signature as a paleo tracer in marine sediments. The new thermodynamic data are indispensable for modeling Cd behavior in response to pH, T, and mH2S. As a function of these parameters, sulfur has the main control on Cd geochemistry being the main factor of Cd precipitation at low mH2S and favoring Cd mobilization at high mH2S.
32-45
FEATURES OF THE INTERACTION OF SHEELITE WITH HCl SOLUTIONS AT 400 AND 500 ° C, 100 MPa AND VARIOUS f(O2) (ACCORDING TO EXPERIMENTAL AND CALCULATED DATA)
Abstract
Experimental studies were carried out on the solubility of scheelite in HCl solutions in the concentration range from 0.01 to 0.316 mol•kg-1 H2O at 400 and 500°C, a pressure of 100 MPa and the fugacity of oxygen (hydrogen) specified by the buffers Cu2O-CuO, Fe3O4-Fe2O3, Ni-NiO and Co-CoO. It was found that scheelite in HCl solutions at the specified parameters dissolves incongruently. In solutions containing from 0.01 to 0.0316 m HCl, minor amounts of tungsten oxides WO3 and (or) WO3-x are found in the run products, along with scheelite. In solutions containing from 0.1 to 0.316 m HCl, the formation of calcium tungsten bronzes (CTB) CaxWO3 is observed, the average composition of which corresponds to the formula Ca0.07WO3. Based on the analysis of the experimental data obtained, the free energies of the formation of tungsten oxides WO3, WO2.9, scheelite and calcium of tungsten bronze were calculated. Using mutually agreed thermodynamic data, the solubility of Sch in solutions of HCl, (Na,K)Cl with the participation of alumino-silicates is calculated. It is shown that the scheelite has a wide area of congruent solubility in saline systems.
46-57
Metal-Sulfide Liquation Of Ore-Forming Melt In The Fe–Fe(NiCu)S–C System And Its Role In The Genesis Of Magmatic Sulfide Deposits In The Norilsky Region (By Experimental And Geological Data)
Abstract
The results of an experimental study of the melting of a graphite-saturated Fe–Fe(CuNi)S–C system at 0.5 GPa and 1150–1250°С with the addition of impurity elements Ag, Au, Re, Pt, Pd, and Rh are presented. The quenched sulfide melt forms the matrix of the sample, at 1150–1200°С it is represented by the FeNiS pyrrhotite phase (Mss) with inclusions of the FeCuS phase (Iss) anomalously enriched in Cu. At 1250°С, the sulfide melt is quenched in the form of a homogeneous pyrrhotite Ms phase of the Fe(CuNi)S composition. The change in the Ms composition of the quenched sulfide melt by the two-phase Mss + Iss association is considered as evidence of the existence of supra-liquidus Mss and Iss stratification of the sulfide Fe–Ni–Cu melt in the range of 1150–1250°С. As a result of fractionation of elements between immiscible Fe-sulfide (Ms) and Fe-metal (Mc) melts, Fe, Ni, Pt, Re, Au are predominantly concentrated in the metal melt, while Cu, Ag are concentrated in the sulfide melt. The role of the supra-liquidus Mss–Iss liquation in the genesis of sulfide mineralization of the deposits of the Talnakh ore cluster, including pyrrhotite-chalcopyrite "drops" in picritic gabbro-dolerites, as well as pyrrhotite and chalcopyrite types of ores of zonal sulfide deposits, is discussed. The role of Ms and Mc separation of the Fe-sulfide melt during carbon contamination in the Cu–Pd specialization of magmatic sulfide deposits of the Norilsk region is considered.
58-73
SILVER SULFIDES AND SELENIDES IN ORES FROM Au-Ag EPITHERMAL DEPOSITS OF THE OKHOTSK-CHUKOTKA VOLCANIC BELT
Abstract
Mineral associations and compositions of sulfoselenides and other silver chalcogenides from epithermal deposits of the Okhotsk-Chukotka volcanic belt (North-East Russia) - Rogovik, Lunnoye, Julietta, Valunistoe, Corrida and Pepenveem have been studied. Different types of relationships between S-naumannite and Se-acanthite in the ores of the studied deposits have been established. It is shown that a general pattern in the sequence of formation of silver chalcogenides is the replacement of S-naumannite and Se-bearing minerals (Se-galena, Se-stephanite) of early parageneses by Se-acanthite or acanthite at later stages. Based on the results of thermodynamic calculations and data on the composition of the main productive mineral associations, the physicochemical parameters (T, ƒS2, ƒSe2, ƒO2, pH) of ore formation at these deposits were reconstructed.
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