Chemical and Isotopic Composition of Sulfide Minerals from the Noho Hydrothermal Field in the Okinawa Trough

Abstract

Studies of the element contents and isotopic characteristics of sulfide minerals from seafloor hydrothermal sulfide deposits are a significant method of investigating seawater-fluid mixing and fluid-rock and/or sediment interactions in hydrothermal systems. The seafloor hydrothermal sulfide ores from the Noho hydrothermal field (NHF) in the Okinawa Trough (OT) consist of pyrrhotite, isocubanite, sphalerite, galena, and amorphous silica. The Rh, Ag, Sb, and Tl contents mostly increase in galena as the fluid temperature decreases in the late ore-forming stage. In the sulfide minerals, the rare earth elements are mainly derived from the hydrothermal fluids, while the volcanic rocks and/or sediments are the sources of the sulfur and lead in the sulfide minerals. After the precipitation of galena, the redox state becomes oxidizing, and the pH value of the fluid increases, which is accompanied by the formation of amorphous silica. Finally, neither pyrite nor marcasite has been observed in association with pyrrhotite in the NHF sulfides, likely indicating that the amount of sulfur was limited in this hydrothermal system, and most of the residual Fe was incorporated into the sphalerite. This suggests that the later pyrite and/or marcasite precipitation in the seafloor hydrothermal sulfide deposit is controlled by the sulfur content of the fluid. Furthermore, it is possible to use hydrothermal sulfides and their inclusions to trace subseafloor fluid circulation processes.