Mineralogy, Sulfur Isotope and Fluid Inclusion Studies of Hydrothermal Ore at the Hakurei Deposit, Bayonnaise Knoll, Izu-Bonin Arc

摘要

Sulfide and sulfate ore samples collected from the Hakurei deposit of the Bayonnaise knoll were examined for the occurrence and chemical composition of minerals, including the sulfur isotopes and the microthermometry of fluid inclusions. Massive sulfide ore, mineralized volcanic rock, and anhydrite ore occur in descending order, from the seafloor to the bottom of the cored sample. The massive sulfide ore is dominated by sphalerite and accompanied by tennantite, chalcopyrite, and pyrite with lesser amounts of galena, enargite, and covellite. Amorphous silica is commonly precipitated on the surface of the sulfide minerals. As-bearing minerals such as tennantite, enargite, and luzonite are common, while galena and Sb-rich tetrahedrite are scarce. The mineral abundance and chemical composition of the minerals differs from that found in chimneys of the deposit. The sulfur isotope compositions in the minerals are +3.1-5.2‰ for sulfides and +19.6-21.8‰ for sulfate minerals. The homogeneous nature of the sulfur isotopes suggests that sulfur incorporated in the Hakurei deposit came from the reduction of aqueous sulfate in seawater.