AbstractHigh-precision analysis of sulfur isotope composition was carried out for sulfide fractions from ten samples of olivine gabbronorite that composes a thick (approximately 300 m) swell of a ore-bearing apophysis that is parallel to the basal part of the Yoko-Dovyren massif in northern Baikal area, Russia. The δ34S values were found out to widely vary from +11‰ to –1.9‰. The maximum enrichment in isotopically heavy sulfur was identified within the basal horizon, which is 10 m thick, whereas the minimum values of δ34S were observed near the upper contact of the intrusive body. Sulfide droplets in chilled picrodolerite from the lower contact zone (Pshenitsyn et al., 2020) show a narrow range of δ34S (+8.65 ± 0.34‰, n = 5). Lower values of δ34S ranging from +2.09 to +2.53‰ are characteristic of the sulfide-rich net-textured ores, the mineralized olivine gabbronorite, and a cutting leucogabbro dike. The sulfur isotope compositions of two samples of pyrite-bearing rocks from the host carbonate–terrigenous rocks display discrete values of δ34S = +2.20‰ and δ34S = +9.40 ± 0.14‰ at a whole-rock sulfur concentration up to 3.5 wt %. Simple scenarios of the additive mixing of isotope-contrasting reservoirs corresponding to a juvenile magmatic source (δ34S = 0 and +2‰) and a provisionally chosen contaminant (δ34S = +9.4‰) are demonstrated to require a high degree of assimilation of host rocks (as much as 60–80%) and complete isotope equilibration of the hybrid system. In the contact picrodolerite with rare globular sulfides, the mixing mechanism is inconsistent with the estimated sulfur solubility in its parental magma: approximately 0.08 wt % (Ariskin et al., 2016). The high δ34S values in rocks from the basal part of the apophysis may be explained, under the assumption that contact-metamorphic H2S-bearing fluid was introduced into the magmatic system, by the thermal decomposition of pyrite coupled with dehydration of the host rocks. The proposed mechanism does not require a volume assimilation of crustal materials and is consistent with petrological and geochemical characteristics of the Dovyren magmas and derivative cumulates.