THE MODE OF OCCURRENCE OF HYDROTHERMAL DEPOSITS FROM THE ANA-SITE, WEST OFFSHORE OF KUME-JIMA ISLAND, OKINAWA TROUGH

A deep-sea hydrothermal fluid vent site, named ANA site, is located at 1050 m water depth in the bottom of caldera of Daisan Kume Knoll in the west offshore of Kume-jima island mid-Okinawa Trough. The ANA site was discovered by R/V Yokosuka in 2015 (Nakamura, 2015). Harigane et al. (2015) reported that the Kume Knoll consists of several volcanic body with caldera and is mainly composed of dacite, rhyolite and sediments. We carried out KR15-16 cruise of R/V Kairei in 2015, to accurately locate the vent field and collect hydrothermal fluids and rock samples.

We collected three types of samples which are active chimney, dead chimney and sedimentary rock in the ANA-site during the KK#667 and KK#673 dives of Remotely Operated Vehicle (ROV) Kaiko Mk-IV. The white colored active chimney, exhausting high-temperature hydrothermal fluid at >200 ^°C, is mainly composed of coarse-grained fragile sulfate and bright orange colored arsenic minerals covered sulfate. Black colored dead chimney is composed of massive sulfide with minor amount of native sulfur. Because several dead chimneys kept their original shape, therefore, it is easy to observe the texture of formation process of the chimney. Sedimentary rock is composed of mainly sand stone with layered black colored sulfate and native sulfur.

In this study, we will report the chemical composition and fluid inclusion analysis of sulfide minerals in detail and discuss the formation process of the hydrothermal deposit in ANA-site.