Sulfur Isotope Composition of Olivine Gabbronorites from a Mineralized Apophysis of the Yoko-Dovyren Intrusion, Northern Transbaikalia, Russia

High-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 δ 34 S 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 δ 34 S 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 δ 34 S (+8.65 ± 0.34‰, n = 5). Lower values of δ 34 S 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 δ 34 S = +2.20‰ and δ 34 S = +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 (δ 34 S = 0 and +2‰) and a provisionally chosen contaminant (δ 34 S = +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 δ 34 S values in rocks from the basal part of the apophysis may be explained, under the assumption that contact-metamorphic H 2 S-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.