Mineralogy, geochemistry and microbiology insights into precipitation of stibnite and orpiment at the Daiyon-Yonaguni Knoll (Okinawa Trough) hydrothermal barite deposits

Abstract

Samples of active chimneys, chimney flanges and massive sulfides from the Daiyon-Yonaguni Knoll hydrothermal field are composed of major barite and minor stibnite and orpiment. Barite is inferred to precipitate from focused-discharge fluids composed of >40% hydrothermal end-member fluid at T = 100-240°C, whereas the stibnite and orpiment are later and lower temperature precipitates. The hydrothermal fluids from this field were subject of sub-seafloor boiling and phase separation and, consequently, are brine-rich depleted in volatile and enriched in non-volatile elements. Boiling and phase separation exerted major control on the rare earth elements (REE) partitioning in the vent fluids: high-chlorinity high-temperature fluids were enriched in light REE and low-chlorinity low-temperature fluids were enriched in heavy REE. Y/Ho molar ratio and Ce anomaly of the vent fluids suggest that the seawater has not completely reacted with the basement rocks and has not equilibrated with them. The trace element concentrations in the hydrothermal deposits suggest a complex interplay among hydrothermal, hydrogenetic and microbial processes. Sulfur isotope composition of the sulfides suggests that the sulfide S is a mixture of both basement rock and seawater S with a higher proportion of the basement rock S. The sulfate dissolved in the fluids was subjected to reduction during a slow mixing of hydrothermal fluid and seawater within the chimney walls of the Tiger and Abyss vents and this resulted in a heavy S-isotope composition of the vent fluid sulfate. Lead isotope composition of the hydrothermal deposits indicates mixing relationships suggesting that Pb and potentially other metals with similar geochemical behavior were derived from two or three sources. The Pb isotopes in the hydrothermal deposits imply that an enriched source, either sediments or extended continental lithosphere, and a depleted source, potentially back-arc mafic volcanics, are present in the area of Daiyon-Yonaguni Knoll. Filamentous orpiment found in the deposits is supposed to be either heavily mineralized fungal hyphae or pure abiogenic biomorphs. Presence of carbonaceous matter on and around the orpiment filaments suggests for microbial activity during filament formation. The filaments experienced temperature of 209.1±37.1°C which falls within the temperature range of the Daiyon-Yonaguni Knoll vent fluids. Stability phase diagrams modeling reveals that the stability of stibnite does not depend on the vent fluid chlorinity, but depends on the vent fluid temperature: the area of stibnite stability increases with decreasing vent fluid temperature and results in stibnite precipitation at low log10a of Sb2S42- and less reduced environment (Eh still <0). Orpiment is stable in a wide range of log10a of H2AsO4-, in reduced conditions and at high S activity. Barite is stable in wide range of log10a of Ba2+ and precipitates in slightly reduced to slightly oxic conditions.