On the Connection between Sulfide Inclusions in Olivine from Tolbachik Volcano and Fluids from Mafic Cumulates beneath the Klyuchevskoy Group Volcanoes

The high activity of the Klyuchevskaya group of volcanoes in the Holocene suggests the accumulation of large volumes of solidified magmas with a low melt content (cumulates) and ultramafic-mafic intrusions in the earth's crust beneath it. In combination with the high fluid flux characteristic of the zone of rapid subduction of an ancient oceanic plate, this creates conditions for the formation of a fluid-magmatic ore-forming system. Sulfide inclusions in olivine, found in the eruption products of the Tolbachik volcano, may provide information about the composition of the fluid of such ore-forming systems. The interaction of a low-water reduced fluid with an oxidized (NNO+1.3) basaltic melt with a dissolved sulfur content of 2000–3000 ppm was theoretically modeled. It is shown that at a local fluid content of about 1–2 wt.%, sulfur in the melt is reduced and a sulfide melt is formed. The reduction of sulfur in the melt can also be caused by the dissolution of SO, which is the main form of sulfur in the fluid with oxygen fugacity O NNO+1.5. The reducing effect is explained by the higher degree of oxidation of sulfate sulfur in the melt (S) than the degree of oxidation of SO sulfur in the fluid (S). According to the modeling results, sulfide melt appears when 2000–3000 ppm sulfur is dissolved in the melt in the form of SO. When interacting with a barren fluid with a low content of precious metals (PM), droplets of sulfides with a low PM content are formed, corresponding to the background composition of the magma. According to experimental data, in the reduced low-water fluid, the solubility of Pt and Pd in the form of carbonyls is high with low solubility of Au, whereas at high oxygen fugacity (NNO+1+1.5) the solubility of gold is very high. When magma interacts with ore-forming fluids containing the first tens of ppm of precious metals, a sulfide melt is formed, enriched in Au (oxidized fluid) or Pt (reduced fluid). The liquidus temperature of olivine increases due to local dehydration of the magmatic melt when interacting with a low-water fluid (or oxidized brine), which leads to rapid growth of olivine at high undercooling. The localization of phase transitions at the boundary of fluid bubbles facilitates the capture of sulfide droplets by growing olivine crystals. The rare occurrence of sulfide droplet inclusions in olivine from Tolbachik volcano may be due to rapid dissipation of magma-fluid interaction effects at low average content of injected fluid, resulting in the sulfide phase dissolving in the magma.