Barite from the Saf’yanovka VMS deposit (Central Urals) and Semenov-1 and Semenov-3 hydrothermal sulfide fields (Mid-Atlantic Ridge): a comparative analysis of formation conditions

Abstract

Different genetic types of barite from colloform and clastic pyrite-rich ores from the weakly metamorphic Saf’yanovka volcanogenic massive sulfide (VMS) deposit (Central Urals) were studied in comparison with barite from the Semenov-1 and Semenov-3 seafloor hydrothermal fields (mid-Atlantic Ridge). Hydrothermal barite generally occurs as radial aggregates of tabular crystals in contrast to compact aggregates of tabular crystals with stylolite boundaries of anadiagenetic barite from the Saf’yanovka clastic ores. The Sr content in barite shows no relationship with the genetic types. The δ34S values of hydrothermal barite from both ancient and modern colloform sulfides match those of Silurian–Devonian and contemporary seawater, respectively. The lower δ34S (avg +19.6 ‰) of hydrothermal barite from the Semenov-3 clastic sulfides indicates light sulfur contribution from oxidation of fluid H2S. The higher δ34S (avg +28.1 ‰) of anadiagenetic barite from the Saf’yanovka clastic ores reflects partial thermochemical reduction of seawater sulfate. Hydrothermal barite from the Saf’yanovka and Semenov-1 colloform ores formed from low- to moderate-T (172–194 °С and 83–233 °C, respectively) relatively low salinity (1.6–4.5 and 0.6–3.8 wt% NaCleq, respectively) fluids, which underwent phase separation. Hydrothermal barite from Semenov-3 clastic sulfides associated with chalcopyrite crystallized from higher-T (266–335 °С) higher-salinity (4.8–9.2 wt% NaCleq.) fluids. The high salinity may indicate a contribution from a magmatic fluid. Anadiagenetic barite from Saf’yanovka was formed from moderate-T (140–180 °С), low- to moderate-salinity (1.4–5.4 wt% NaCleq) pore fluids. Combining our new data with those for other seafloor hydrothermal barite occurrences, the following systematics can be defined. Barite associated with pyrite-rich sulfides forms at relatively low to moderate temperatures (<230 °C), barite associated with polymetallic-rich sulfides forms at moderately high temperatures (210–280 °C), and barite in assemblage with chalcopyrite records the highest temperatures (265–335 °C). The main source of sulfur for hydrothermal barite is seawater with a small contribution of isotopically light sulfur from partial oxidation of H2S. A contribution from isotopically heavy sulfur remaining after thermochemical or bacterial partial reduction of seawater sulfate appears to be common in diagenetic/anadiagenetic barite.