Occurrence of Hydrothermal Alteration Minerals at the Jade Hydrothermal Field, in the Izena Hole, Mid-Okinawa Trough

Mineralogical and geochemical features of hydrothermal alteration minerals in the sediment cores from the Jade hydrothermal field in the Izena Hole, mid-Okinawa Trough, were studied by XRD, EPMA and TEM-EDS analyses. A core sample 1186MBL collected from the surface sediment near the sulfide chimney venting high temperature fluid up to 320 °C was characterized by occurrence of kaolinite, with sulfide minerals such as sphalerite and galena. The kaolinite would be related to be formed under acidic condition caused by oxidation and dissolution of the sulfide minerals by penetrating seawater. Core samples (1188MB, 1193MB) were collected from the surface sediment in the vicinity of clear hydrothermal fluid venting of ~100 °C, which is located in 400 m distant from the sulfide chimney. In these cores, occurrence of chlorite and smectite was identified. The chlorite in the core 1188MB had chemical composition close to Al-rich chlorite which is classified as sudoite, although chlorite found in other hydrothermal fields in the Okinawa Trough is characterized as significantly Mg-rich chlorite. Core samples of up to 4–6 m length were also collected near the low temperature fluid venting to study alteration in deep layers. One of two long core samples (BMS-J-2) was characterized by chlorite and illite assemblage below 380 cmbsf, while the other (LC-J-2) was characterized by abundant occurrence of K-feldspar below 300 cmbsf. Occurrence of euhedral crystals of K-feldspar in size up to several tens μm suggests the formation by precipitation from high temperature fluid.