Sulfur species and gold transport in arc magmatic fluids

The sulfur species present in magmatic fluids affect the global redox cycle, the Earth's climate and the formation of some of the largest and most economic ore deposits of critical metals. However, the speciation of sulfur under conditions that are relevant for upper crustal magma reservoirs is unclear. Here we combine a prototype pressure vessel apparatus and Raman spectroscopy to determine sulfur speciation in arc magmatic fluid analogues in situ over a range of geologically relevant pressure-temperature-redox conditions. We find that HS^-, H₂S and SO₂ are the main sulfur species in the experimental fluids, while the concentrations of S⁶⁺ species and S₂ ^- and S₃ ^- sulfur radical ions are negligible, in contrast to previous experimental work. The measured gold solubilities in the experimental fluids are highest when sulfur is dominantly present as HS^- and H₂S species and greatly exceed thermodynamic predictions. Our results imply that HS^-, rather than sulfur radicals, accounts for the high solubilities of gold in magmatic-hydrothermal fluids. We also find that magmatic sulfur degassing is a redox process under oxidizing conditions and may lead to additional magma oxidation beyond that imparted by slab-derived fluxes and crystallization.