Redox dependent sulfur partitioning between liquid and vapor: implications for magmatic-hydrothermal ore genesis and volcanic degassing

Magmatic-hydrothermal ore deposits form in response to metal precipitation from fluids released from crustal magma reservoirs. Following release from the magma, these fluids often go through phase separation involving the condensation of a saline liquid phase (=brine). The brine tends to stay in the deeper part of the hydrothermal system, whereas the vapor ascends to form epithermal ore deposits. Therefore, chemical contrast between these two phases may be deterministic for the spatial distribution of mineralization in magmatic-hydrothermal systems, and also affects volcanic sulfur outputs during quiescent periods. As sulfur is a key constituent for ore metal transport and precipitation, understanding its partitioning between the vapor and brine phases is critically important.

We conducted experiments to determine the effect of oxygen fugacity on the partitioning of sulfur between vapor and brine at P-T conditions relevant for the magmatic-hydrothermal transition by trapping coexisting vapor and brine phases as synthetic fluid inclusions in quartz and subsequently analyzing them for sulfur concentrations by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The results show that at relatively low fO₂ at which sulfur is predominantly present in 2- oxidation state, sulfur shows strong preference for the vapor phase with liquid/vapor partition coefficients around 0.2 at a liquid/vapor salinity contrast above a factor of 6. However, with increasing fO₂, sulfur partitioning into the liquid phase increases corresponding to the transition of the redox state of sulfur from 2- to 4+ and 6+. In relatively alkaline fluids at the fO₂ of the MnO-Mn₃O₄ buffer, the preferential partitioning of sulfur to the liquid phase is nearly as strong as that of NaCl. Liquid condensation from SO₂-rich single-phase aqueous fluid at T = 875^oC and P = 200 MPa were observed by in situ Raman spectroscopic experiments and sulfate was identified as the dominant sulfur species in the condensed liquid. It is thus apparent that liquid condensation in oxidized magmatic hydrothermal systems is a redox process that yields sulfate-bearing brines.

A consequence of the above observations is that brines in typical porphyry Cu (-Au) ore forming systems will have the capacity to precipitate ore metal sulfides without externally derived sulfur, provided that some sulfate can be reduced to sulfide and the HCl produced during metal sulfide precipitation is consumed via fluid-rock interaction or is carried away by the vapor phase to higher levels of the system. In addition, a large fraction of the sulfur budget of the primary magmatic volatile phase in oxidized active arc volcanic systems may be stripped by brine condensation during quiescent degassing periods and may never reach the Earth’s atmosphere.