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

To define the discriminatory features of different genetic types of barite, hydrothermal and diagenetic barites from colloform and clastic pyrite–rich ores from the weakly metamorphic Saf’yanovka volcanogenic massive sulfide deposit (Devonian, Central Urals) were studied in comparison with that from similar modern seafloor deposits from the mid-Atlantic Ridge (Semenov-1 and Semenov-3 hydrothermal fields). Hydrothermal barites from all the studied deposits exhibit similar morphology: they occur as tabular crystals or their aggregates. In contrast, diagenetic barite from clastic ores of the Saf’yanovka deposit occur as compact aggregates of deformed, broken, or slightly curved tabular crystals with stylolite boundaries. The variable Sr contents in the studied barites show no relationship with the genetic types. The average δ34S values of hydrothermal barite from both ancient and modern colloform sulfides (+22.9 ‰, Saf’yanovka deposit; +21.2‰, Semenov-1 field) match those of Silurian–Devonian and contemporary seawater, respectively. The lower δ34S of hydrothermal barite from clastic sulfides of the Semenov-3 field (+19.6 ‰), which is associated with high-Se, high-temperature chalcopyrite, indicates light sulfur contribution from oxidation of fluid H2S. The higher average δ34S of diagenetic barite from clastic ores of the Saf’yanovka deposit (+28.1‰) is interpreted to reflect partial thermochemical reduction of seawater sulfate due to interaction with ferrous minerals and/or organic matter. In spite of different geodynamic setting, hydrothermal barite from colloform ores from the Saf’yanovka deposit (back-arc basin) and Semenov-1 field (slow-spreading mid-oceanic ridge) were formed under similar low- to moderate-temperature conditions (172–194 °С and 83–233 °C, respectively) from relatively low-salinity fluids (1.6–4.5 and 0.6–3.8 wt.% NaCleq, respectively). Variations in salinity values from higher- to lower-than seawater reflect phase separation in the parent fluids. High contents of CO2, CH4, and N2 (up to 1.58, 0.05, and 0.006 mol%, respectively) in fluid inclusions from the Saf’yanovka deposit are attributed to reactions with abundant hydrothermal fauna and C-bearing sediments. The presence of SO2 and CO2 in fluid inclusions from the Semenov-1 field is ascribed to contributions from a magmatic fluid. Hydrothermal barite from Semenov-3 clastic sulfides crystallized at higher-temperature (266–335 °С) from higher-salinity fluids (4.8–9.2 wt.% NaCleq.). The high salinity may again indicate a contribution from a magmatic fluid, consistent with high measured CO2 content in fluid inclusions (1.6 mol%). Diagenetic barite from the Saf’yanovka clastic ores was formed at moderate temperatures (140–180 °С) from low- to moderate-salinity pore fluids (1.4–5.4 wt.% NaCl eq.). The variable salinity may reflect contributions from various water sources, e.g., connate seawater, silicate dehydration, and transformation of primary hydrothermal barite with low-salinity fluid inclusions. Combining our new data with those for other seafloor hydrothermal barites the following systematics can be defined. Barite precipitated on chimney rims or associated with pyrite-rich, colloform, massive 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 is seawater for both hydrothermal and diagenetic barite; additional contribution of isotopically light sulfur from partial oxidation of H2S or of isotopically heavy sulfur from bacterial sulfate reduction may occur in hydrothermal barite, whereas a contribution from isotopically heavy sulfur remaining after thermochemical or bacterial partial reduction of seawater sulfate appears to be common in diagenetic barite.