Fluid density control on vapor-liquid partitioning of metals in hydrothermal systems

Abstract

Hot aqueous fluids, both vapor and saline liquid, are primary transporting media for metals in hydrothermal-magmatic systems. Despite the growing geological evidence that the vapor phase, formed through boiling of magmatic ore-bearing fluids, can selectively concentrate and transport metals, the physical-chemical mechanisms that control the metal vapor-liquid fractionation remain poorly understood. We performed systematic experiments to investigate the metal vapor-liquid partitioning in model water-salt-gas systems H2O-NaCl-KCl-HCl at hydrothermal conditions. Measurements show that equilibrium vapor-liquid fractionation patterns of many metals are directly related to the densities of the coexisting vapor and liquid phases. Despite differences in the vapor-phase chemistry of various metals that form hydroxide, chloride, or sulfide gaseous molecules of contrasting volatile properties, water-solute interaction is a key factor that controls the metal transfer by vapor-like fluids in Earth's crust. These findings allow quantitative prediction of the vapor-liquid distribution patterns and vapor-phase metal transport in a wide range of conditions. Our density model accounts well for the vapor-brine distribution patterns of Na, Si, Fe, Zn, As, Sb, and Ag observed in fluid inclusions from magmatic-hydrothermal deposits. For Au and Cu, the partitioning in favor of the liquid phase, predicted in a sulfur-free system, contrasts with the copper and gold enrichment observed in natural vapor-like inclusions. The formation of stable complexes of Cu and Au with reduced sulfur may allow for their enhanced transport by sulfur-enriched magmatic-hydrothermal vapors.

Fe isotope fractionation during phase separation in the NaCl–H2O system: An experimental study with implications for seafloor hydrothermal vents

“…The Fe/Cl ratio of the liquid remained essentially constant. This is expected from mass balance constraints following the metered addition of a NaCl-Fe-H 2 O source fluid of fixed composition, together with the previously well-established partitioning behavior of Fe that strongly favors the liquid (Bischoff and Rosenbauer, 1987;Pester et al, 2014;Pokrovski et al, 2005). In P-T region 2 (464-466 • C, 29.8-24.7 MPa), halite-vapor coexistence was achieved, which was associated with a sharp increase in the Fe/Cl ratio of the vapor (Fig.…”

“…Laboratory experiments have demonstrated that charged species, especially divalent transition metals such as Fe, show a strong preference for the liquid phase during vapor-liquid equilibria at elevated temperatures and pressures (Bischoff and Rosenbauer, 1987;Pester et al, 2014;Pokrovski et al, 2005). At the same time, a number of field studies (Beard et al, 2003;Rouxel et al, 2008;Sharma et al, 2001) have considered phase separation as a possible mechanism to induce Fe isotope fractionation in seafloor hydrothermal vent fluids.…”

Trace element geochemistry of the 1991 Mt. Pinatubo silicic melts, Philippines: Implications for ore-forming potential of adakitic magmatism

“…Such a fluid derived from the dehydrated oceanic slab is likely to be important in the ore metal enrichment of the adakitic magmas. Because of the highly oxidizing conditions during adakite magma differentiation which destabilize sulfide phases, these metals are easily mobilized likely in the form of chloride complexes by Cl-bearing aqueous fluids (Keppler and Wyllie, 1991;Stolper and Newman, 1994;Keppler, 1996;Frank et al, 2002;Pokrovski et al, 2005) derived from descending, dehydrated oceanic crust. Moreover, the SO 2 ligand which becomes the dominant sulfur species at the expense of H 2 S in the oxidized fluid might also form soluble complexes with chalcophile metals and, thus, further facilitate their mobility.…”

“…The soft chalcophile metals (Au, Cu) form much more stable complexes with soft ligands like HS À than with moderately hard Cl À and OH À /H 2 O in aqueous fluids (e.g., Pokrovski et al, 2005). However, the abundant anhydrite crystallization in melt and fluid (e.g., Pasteris et al, 1996) should result in a decrease of the fluid-phase sulfur contents, causing Cu, Au, Pb and Zn complexation with the far more abundant Cl À in the pre-eruptive fluid, whereas As, Sb and Mo are expected to form soluble hydroxide complexes (e.g., Wood and Samson, 1998;Pokrovski et al, 2005Pokrovski et al, , 2002. High-Al adakitic magmas related to Au/Cu and Cu/Mo mineralization are peraluminous (González-Partida et al, 2003;Reich et al, 2003;Qu et al, 2004;Whalen et al, 2004).…”

Antimony speciation in saline hydrothermal fluids: A combined X-ray absorption fine structure spectroscopy and solubility study

“…Vapor phase is known to be more acid and enriched in volatile HCl than the dense brine as show experiments on brine-silicate-vapor systems (e.g., Shinohara and Fujimoto, 1994;Frank et al, 1998). Because antimony chloride and hydroxy-chloride species are more volatile than SbðOHÞ 3 (e.g., Pokrovski et al, 2005b), they are expected to preferentially partition into the vapor phase. This is confirmed by the recent vapor-liquid fractionation measurements which show low values for Sb vapor-liquid partition coefficients, largely in favor of the NaClrich liquid at neutral pH (K Sb = [m Sb in vapor]/[m Sb in liquid]; K Sb $ 0.01-0.001 at 350-450°C).…”