Sulfur, along with hydrogen, oxygen, and carbon, is one of the most common volatiles in magmatic mantle processes. As a redox-sensitive element, sulfur can have a direct influence on the redox evolution of mantle rocks, melts, and fluids, and participate in processes of mantle metasomatism. Modern concepts suggest that subduction processes play a key role in the global sulfur cycle. We report the results of the first high-pressure–high-temperature experiments in olivine-sulfur and olivine-pyrite systems aimed at modeling sulfidation processes in a silicate mantle with involvement of S-bearing fluids or melts and determining a potential mechanism of sulfide formation under deep subduction conditions. It was found that at the lower temperature stage of sulfidation, the partial recrystallization of olivine was accompanied by extraction of Fe and Ni into an S-bearing fluid and, finally, an olivine, orthopyroxene, and pyrite assemblage was formed; i.e., sulfide mineralization of an ultramafic rock occurred. At higher temperatures, the complete sulfidation and recrystallization of olivine resulted in the formation of forsterite and enstatite, containing inclusions of Ni-rich sulfide melt. Strong enrichment of S-bearing fluids in Fe and Ni led to a further sulfide melt generation. It is thus experimentally demonstrated that the influence of ephemeral S-bearing fluids on ultramafic mantle rocks results in an extraction of base metals from the solid-phase silicates, modifying their mineral and chemical compositions, and providing conditions for mobilization of an ore material in the form of sulfides at pressure-temperature conditions of the lithospheric mantle.