@article{fdi:010067692,
  title = {{C}ombined effect of carbon dioxide and sulfur on vapor-liquid partitioning of metals in hydrothermal systems},
  author = {{K}okh, {M}. {A}. and {L}opez, {M}. and {G}isquet, {P}. and {L}anzanova, {A}ur{\'e}lie and {C}andaudap, {F}. and {B}esson, {P}. and {P}okrovski, {G}. {S}.},
  editor = {},
  language = {{ENG}},
  abstract = {{A}lthough {CO}2 is a ubiquitous volatile in geological fluids typically ranging from a few to more than 50 wt%, its effect on metal vapor-liquid fractionation during fluid boiling and immiscibility phenomena in the {E}arth's crust remains virtually unknown. {H}ere we conducted first experiments to quantify the influence of {CO}2 on the partition of different metals in model water + salt + sulfur + {CO}2 systems at 350 degrees {C} and {CO}2 pressures up to 100 bar, which are typical conditions of formation of many hydrothermal ore deposits. {I}n addition, we performed in situ {R}aman spectroscopy measurements on these two-phase systems, to determine sulfur and carbon speciation in the liquid and vapor phases. {R}esults show that, in {S}-free systems and across a {CO}2 concentration range of 0-50 wt% in the vapor phase, the absolute vapor-liquid partitioning coefficients of metals ({K}-vap/liq = {C}-vap/{C}-liq, where {C} is the mass concentration of the metal in the corresponding vapor and liquid phase) are in the range 10(-6)-10(-5) for {M}o; 10(-4)-10(-3) for {N}a, {K}, {C}u, {F}e, {Z}n, {A}u; 10(-3)-10(-2) for {S}i; and 10(-4)-10(-1) for {P}t. {W}ith increasing {CO}2 from 0 to 50 wt%, {K}-vap/liq values decrease for {F}e, {C}u and {S}i by less than one order of magnitude, remain constant within errors (+/- 0.2 log unit) for {N}a, {K} and {Z}n, and increase by 0.5 and 2 orders of magnitude, respectively for {A}u and {P}t. {T}he negative effect of {CO}2 on the partitioning of some metals is due to weakening of hydration of chloride complexes of some metals ({C}u, {F}e) in the vapor phase and/or salting-in effects in the liquid phase ({S}i), whereas both phenomena are negligible for complexes of other metals ({N}a, {K}, {Z}n, {M}o). {T}he only exception is {P}t (and in a lesser extent {A}u), which partitions significantly more to the vapor of {S}-free systems in the presence of {CO}2, likely due to formation of volatile carbonyl ({CO}) complexes. {I}n the {S}-bearing system, with {H}2{S} content of 0.1-1.0 wt% in the vapor, {K}-vap/liq values of {C}u, {F}e, {M}o, and {A}u are in the range 0.01-0.1, those of {P}t 0.5-2.0, those of alkali metals are similar to the {S}-free system, and the partitioning of none of the studied metals is influenced by the presence of {CO}2 (up to 50 wt% in the vapor). {O}ur data thus confirm the large enhancement of volatility in the presence of reduced sulfur ({H}2{S}) due to formation of sulfide complexes for chalcophile metals such as {A}u, {P}t, {M}o and, to a lesser extent, {C}u and {F}e, as reported in previous studies of {CO}2-free water-salt systems. {T}he negligible effect of {CO}2 on vapor-liquid partitioning of the studied metals in {S}-bearing systems is due to the lack of hydration of metal sulfide species making them little sensitive to changes in water activity and solvation power of {CO}2-{H}2{O} vapor. {O}ur findings, combined with existing data over a wide range of temperature on vapor-liquid partitioning of metals in {H}2{O}-dominated systems, suggest that {CO}2 exerts mostly an indirect impact on metal fractionation, by extending vapor-liquid immiscibility to higher temperatures and pressures or depth compared to a {CO}2-free {H}2{O}-{S}-salt system. {T}he deeper vapor-liquid separation, in particular in {S}-bearing systems, is expected to cause more significant partitioning of precious metals and molybdenum ({A}u, {P}t, {M}o) into the vapor phase while base metals ({F}e, {Z}n, {C}u) remain concentrated in the salt-rich ({N}a{C}l, {KC}l) liquid phase. {I}n addition, irrespective of the presence of sulfur, an expansion of the immiscibility domain to higher temperature and pressure conditions in the presence of {CO}2 will also increase the depth of ore deposition and affect the vertical metal zonation in hydrothermal systems.},
  keywords = {{C}arbon dioxide ; {S}ulfur ; {M}etals ; {F}luid ; {V}apor ; {V}apor-liquid partitioning ; {I}mmiscibility},
  booktitle = {},
  journal = {{G}eochimica et {C}osmochimica {A}cta},
  volume = {187},
  numero = {},
  pages = {311--333},
  ISSN = {0016-7037},
  year = {2016},
  DOI = {10.1016/j.gca.2016.05.010},
  URL = {https://www.documentation.ird.fr/hor/fdi:010067692},
}