VOLCANIC-HOSTED SULFIDE-BARITE DEPOSIT FROM THE KITA-BAYONNAISE SUBMARINE CALDERA, IZU-OGASAWARA ARC, NORTHWESTERN PACIFIC

Volcanic-hosted sulfide and barite mineralisation with high Au content (19.3 ppm) was discovered by submersible study (Shinkai 2000) on the eastern floor of the Kita-Bayonnaise submarine caldera on the Shichito -Iwojima Ridge, Izu-Ogasawara Are, northwestern Pacific. The hydrother mally mineralized rhyolitic tuff is partly coated by Mn oxyhydroxide, and associated with hydrothermal Mn crusts and debris of mussel shells in the vicinity of the hydrothermally altered tuff-breccias. The mine arlized rhyolitic tuff occurs as a fragment of a large block (about 2 x I x I m) on talus, while the crusts occur on the caldera floor as a few thin patches several meters across or smaller Ore-mineral composit ion of the mineralized rhyolitic tuff consists mainly of sulfides and barite. Randomly interstratified sericite and montmorillonite is the o nly hydrothermal clay mineral from glass shards in the mineralized rhy olitic tuff The mineralized tuff is significantly enriched in Zn, Fe, Cu, Bq Au, Ag, and As, and has high Au/Ag ratio (0.036 to 0.071) compa red with other marine volcanogenic sulfides Au is probably present as submicrometer-sized particles within sulfide minerals or is contained within the crystal lattice of the sulfide minerals. The delta(34)S val ues of sulfides of subsamples from the mineralized rhyolitic tuff indi cate a fairly narrow range between +3.4 and +4.8 parts per thousand bu t are characterized by lighter isotopes in a drusy cavity compared wit h vein and disseminated sulfides. The sulfide sulfur is mainly from vo lcanic rocks of the caldera basement. A single barite sulfur isotopic value (+20.7 parts per thousand) and salinity of fluid inclusions in b arite are close to modern seawater values, suggesting a major contribu tion of seawater sulfate to the barite precipitation.