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

Gold- and Silver-Rich Massive Sulfides from the Semenov-2 Hydrothermal Field, 13\ub031.13\u2032N, Mid-Atlantic Ridge: A Case of Magmatic Contribution?

The basalt-hosted Semenov-2 hydrothermal field on the Mid-Atlantic Ridge is host to a rather unique Cu-Zn\u2013 rich massive sulfide deposit, which is characterized by high Au (up to 188 ppm, average 61 ppm, median 45 ppm) and Ag (up to 1,878 ppm, average 490 ppm, median 250 ppm) contents. The largest proportion of visible gold is associated with abundant opal-A, which precipitated after a first generation of Cu, Fe, and Zn sulfides and before a second generation of Fe and Cu sulfides. Only rare native gold grains were found in earlier sulfides. Fluid inclusions in opal-A associated with native gold indicate precipitation at 300\ub0 \ub1 40\ub0C from fluids of salinity higher than that of seawater (3.5\u20136.8 wt % NaCl equiv). According to laser ablation-inductively coupled plasma-mass spectrometry analyses, invisible gold is concentrated in secondary covellite (23\u2013227 ppm) rather than in the primary sulfides (1,000 ppm) than all other sulfides (1 are more consistent with a mafic signature. Thermodynamic modeling of hydrothermal fluids produced by reactions between various proportions of seawater and basalt or peridotite at 350\ub0C shows that mineral assemblages broadly similar to those of the Semenov-2 deposit can precipitate from fluids produced in a mafic environment, but that Au and Ag minerals are not predicted to precipitate from such fluids over a wide temperature range. These results suggest that an additional contribution to the hydrothermal system is required in order to achieve saturation in precious metals. A magmatic input is suggested by the occurrence of plagiogranites and tonalites dredged on sea floor in the Semenov area, which could be a potential source of Au-rich magmatic fluids, and by mineralogical and geochemical similarities with magma-related, low- to intermediate-sulfidation epithermal systems, namely high Au and Ag grades, high Au/(Cu + Zn + Pb) and Au/Ag ratios, and presence of Ag, Bi, and Te minerals. The likely crucial role of silicic melts in producing high Au and Ag grades suggests that exploration for precious metal-rich, volcanic-hosted massive sulfide deposits should be primarily directed to sites in which evolved igneous rocks occur on sea floor. Both in modern and ancient mafic-hosted deposits, zones characterized by abundant deposition of silica could be good clues to the presence of significant gold

Barite-rich massive sulfides from the Semenov-1 hydrothermal field (Mid-Atlantic Ridge, 13\ub030.87\ub4 N): Evidence for phase separation and magmatic input

The ultramafic-hosted Semenov-1 hydrothermal field is a part of the Semenov group of sulfide deposits located at 13\ub030\ub4 N Mid-Atlantic Ridge. The hydrothermal deposits comprise finecrystalline, colloform, nodular, and banded barite-marcasite-pyrite assemblages, which are a result of moderately- to low-temperature venting of (Fe, Ba)-rich hydrothermal fluids. The precipitates contain low concentrations of Cu (0.02\u20130.55 wt%) and Zn (0.01\u20130.08 wt%), and variable Au content (0.35\u20134.76 ppm). Fe-disulfides are relatively enriched in Au and typical low-temperature trace elements (As, Ag, Pb, Mn, Tl). The concentrations of Au and of most trace elements are higher in the earliest, fine-grained Fe-disulfides and decrease in the late coarse-crystalline pyrite. Based on fluid inclusion data, barite precipitated from moderately- to low-temperature (244\u201383 \ub0\u421), low-salinity (0.6\u20133.8 wt % NaCl eq.), carbonate-sulfate-aqueous fluids, which incorporated a low-density component produced by fluid phase separation. The presence of CO2 and SO2 in fluid inclusions in barite, as revealed by Raman spectroscopy, may indicate a magmatic volatile contribution to the hydrothermal fluid. The main amount of marcasite and pyrite was formed at temperatures below 240 \ub0\u421, after the precipitation of barite. The occurrence of isocubanite, chalcopyrite and pyrrhotite in precipitates from station 292 indicates a later high-temperature (~300 \ub0\u421) overprint. The sulfur isotopic composition of barite (+21.0 and +21.3 \u2030) closely matches that of seawater. The isotopic composition of sulfur in sulfides from station 186 (\u20133.26 to \u20130.08 \u2030) suggests a possible contribution of light reduced sulfur derived from disproportionation of magmatic SO2 or from leaching of basalt that has degassed SO2. The higher \u3b434S values in sulfides from station 292 (\u20130.08 to +1.53 \u2030) may reflect a heavy sulfur contribution from reduced seawater sulfate. Although the Semenov-1 field is considered to be associated with serpentinized ultramafic rocks (Beltenev et al., 2007), the mineralogical composition of the hydrothermal precipitates, particularly, their high barite and pyrite contents, is more typical of EMORB-hosted seafloor hydrothermal deposits (e.g., Lucky Strike or Menez Gwen fields). Thermodynamic modeling of fluid\u2013rock hydrothermal systems using a flow reactor model demonstrates that basalt\u2013seawater interaction generates fluids that can produce only minor amounts of barite, irrespective of the Ba content of the basalt. Addition of a magmatic gas to the system produces highly acidic hydrothermal fluids capable to extract larger amounts of Ba and Fe, and form mineral assemblages similar to the studied ones. In contrast, interaction between ultramafic rock (peridotite) and seawater, with or without magmatic gas, results in precipitation of barite-free mineral assemblages. The above results do not support an ultramafic signature for the Semenov-1 hydrothermal system and suggest a mafic control on the hydrothermal fluid composition