Experimental study of arsenic speciation in vapor phase to 500°C: implications for As transport and fractionation in low-density crustal fluids and volcanic gases

Abstract

The stoichiometry and stability of arsenic gaseous complexes were determined in the system AsH 2 O Ϯ NaCl Ϯ HCl Ϯ H 2 S at temperatures up to 500°C and pressures up to 600 bar, from both measurements of As (III) and As (V) vapor-liquid and vapor-solid partitioning, and X-ray absorption fine structure (XAFS) spectroscopic study of As (III)-bearing aqueous fluids. Vapor-aqueous solution partitioning for As (III) was measured from 250 to 450°C at the saturated vapor pressure of the system (P sat) with a special titanium reactor that allows in situ sampling of the vapor phase. The values of partition coefficients for arsenious acid (H 3 AsO 3) between an aqueous solution (pure H 2 O) and its saturated vapor (K ϭ mAs vapor /mAs liquid) were found to be independent of As (III) solution concentrations (up to ϳ1 to 2 mol As/kg) and equal to 0.012 Ϯ 0.003, 0.063 Ϯ 0.023, and 0.145 Ϯ 0.020 at 250, 300, and 350°C, respectively. These results are interpreted by the formation, in the vapor phase, of As(OH) 3 (gas), similar to the aqueous As hydroxide complex dominant in the liquid phase. Arsenic chloride or sulfide gaseous complexes were found to be negligible in the presence of HCl or H 2 S (up to ϳ0.5 mol/kg of vapor). XAFS spectroscopic measurements carried out on As (III)-H 2 O(ϮNaCl) solutions up to 500°C demonstrate that the As(OH) 3 complex dominates As speciation both in dense H 2 O-NaCl fluids and low-density supercritical vapor. Vapor-liquid partition coefficients for As (III) measured in the H 2 O-NaCl system up to 450°C are consistent with the As speciation derived from these spectroscopic measurements and can be described by a simple relationship as a function of the vapor-to-liquid density ratio and temperature. Arsenic (III) partitioning between vapor and As-concentrated solutions (Ͼ2 mol As/kg) or As 2 O 3 solid is consistent with the formation, in the vapor phase, of both As 4 O 6 and As(OH) 3. Arsenic (V) (arsenic acid, H 3 AsO 4) vapor-liquid partitioning at 350°C for dilute aqueous solution was interpreted by the formation of AsO(OH) 3 in the vapor phase. The results obtained were combined with the corresponding properties for the aqueous As(III) hydroxide species to generate As(OH) 3 (gas) thermodynamic parameters. Equilibrium calculations carried out by using these data indicate that As(OH) 3 (gas) is by far the most dominant As complex in both volcanic gases and boiling hydrothermal systems. This species is likely to be responsible for the preferential partition of arsenic into the vapor phase as observed in fluid inclusions from high-temperature (400 to 700°C) Au-Cu (-Sn,-W) magmatic-hydrothermal ore deposits. The results of this study imply that hydrolysis and hydration could be also important for other metals and metalloids in the H 2 O-vapor phase. These processes should be taken into account to accurately model element fractionation and chemical equilibria during magma degassing and fluid boiling.

The solubility and speciation of molybdenum in water vapour at elevated temperatures and pressures: Implications for ore genesis

“…The formation of these complexes augments metal solvation to such an extent that an appreciable amount of metal (up to 6.6 ppb Au, 101 ppb Ag, and 280 ppm Cu) can be transported in the vapour phase at physiochemical conditions encountered in nature. Similar experiments by Pokrovski et al (2002) have shown that, at temperatures up to 500°C and pressures up to 600 bars, arsenious acid (H~S03) partitions preferentially into HCI and H 2 S-bearing water vapour. The authors of this study attributed the high solubility of As to the formation of a gaseous arsenic hydroxide complex.…”

The coupled geochemistry of Au and As in pyrite from hydrothermal ore deposits

“…Consequently, it is unlikely that such a significantly unbalanced charge (close to 10 %) could be accommodated by euhedral grains of pyrite, supporting the notion that surface kinetics are more effective in incorporation of Au in arsenian pyrite. The formation of Fe-deficient arsenian pyrite would then require a source of Fe 2+ , which may be satisfied by partial dissolution of pre-existing (diagenetic) pyrite, and formation of local reducing conditions that would promote destabilization of Au-S complexes and subsequent Au-deposition (Pokrovski et al, 2002b), or an external source of Fe from dissolution of carbonates (Hofstra and Cline, 2000). Thus, the proposed mechanism is consistent not only with geological evidence but also with the previously reported mineralogical features of Carlin pyrite, e.g., cationic deficiencies, abundant trace metal contents (and variety), and nonsystematic distribution of Au (Johan et al, 1987;Fleet and Mumin, 1997;Hofstra and Cline, 2000).…”

“…10). The preferential partitioning of As and Au into the magmatic vapour above 450-550 o C (Pokrovski et al 2002b), and the higher amounts of these elements in fluid inclusions than in pyrite, suggest that only a fraction of the Au and As carried by the ore fluid is consumed by pyrite in porphyry systems, while the rest of As and Au may be transported into the shallower epithermal level. Figure 7c clearly demonstrates an increase in the concentrations of Au and As in pyrite during the transition from porphyry to epithermal environments.…”

Phase Equilibria in Binary and Ternary Hydrothermal Systems

“…It is designed to study salt solubility in homogeneous vapor phase or supercritical fl uid at temperatures up to 500 °C and pressures up to 1000 bars (Zakirov and Sretenskaya, 1994;Pokrovski et al, 2002). The inner volume of the highpressure vessel -a reactor (made of titanium alloy) is about 120 cm 3 , whereas the inner volume of the sampler (10) can be varied from 1 to 20 cm 3 depending on the task imposed.…”