Sulfur Isotopes in Magmatic-Hydrothermal Systems, Melts, and Magmas

The first studies on sulfur isotope geochemistry were performed approximately sixty years ago (e.g., Thode et al. 1949; Trofimov 1949). Since these early measurements on terrestrial and extra-terrestrial materials, S isotopes have been used to highlight and investigate several processes, such as the evolution of the solar system; the oxidation of the Earth’s atmosphere and hydrosphere; the genesis of ore deposits and fossil fuels (coal, oil, and gases); the origin and provenance of S species in different natural fluids, including groundwater, rainwater, as well as present-day and ancient marine waters (as recorded by evaporite deposits); the S isotope fractionation in bacterially-mediated processes; the impact of anthropogenic activities, for instance mining and related acid drainage; and others. The sulfur isotopic compositions of volcanic rocks, magmatic gases, and closely related hydrothermal fluids were the subject of numerous investigations. Among these, a considerable contribution was provided by Sakai and coworkers, who elucidated the importance of degassing and sulfide separation and the different effects of these processes, depending on the redox state of the melt and, in the case of degassing, of the separated magmatic gases as well. All of these applications were made possible due to the experimental and theoretical works devoted to the determination of the fractionation factors between different S-bearing species and compounds and their temperature dependence. This review is devoted to the magmatic-hydrothermal environment as a whole, focusing on active systems and their past analogues, which are represented by different types of ore deposits. Special emphasis is given to the use of S isotopes to investigate the effects of degassing and separation of sulfide and sulfate minerals from silicate melts. For the sake of clarity, we recall the meaning of the following terms: Fluid is a general term including both liquids and gases. Vapor is a substance in the