Depiction of the sub-seafloor hydrothermal system in the Izena Hole, middle Okinawa Trough based on the whole rock chemical analyses and mineralogical observations of core samples

During the research cruise CK16-05 (Exp. 909), took place from November to December 2016, drilling core samples were collected at the Izena Hole, middle Okinawa Trough, Japan. In this location, the thick subseafloor sulfide orebody exists from 30 to 60 mbsf around a hydrothermal mound. In this cruise, five drilling core samples were continuously collected in the region from the mound center to east direction for about 250 m as well as three unmineralized/unaltered reference holes. In this study, we describe the alteration and mineralization process associated with subseafloor hydrothermal activity on the basis of visual core description, mineral assemblage determined by XRD analyses, and the chemical composition by ICP-MS of the collected samples.

In the mound center, the abundances of sphalerite and galena, and the concentrations of associated elements (Zn, Pb), are decreased from the core top to 20-30 mbsf. On the other hand, chalcopyrite appeared from the depth of 20m. This switch of the ore minerals indicates the continuous temperature increase of hydrothermal fluid toward the bottom. On the other hand, focused on the subseafloor orebody around the mound center, the depth of concentration peaks of Zn, Pb, Fe, and Cu (associated with ore minerals) clearly deepens from Hole C9026A (70 m eastward from the mound center) to Hole C9025A (200 m), suggesting the continuous temperature decrease of hydrothermal fluid accompanied by the subseafloor lateral movement. The alteration degree of sediments present on the orebody is generally slight to moderate, glauconite/celadonite formed by low temperature alteration of the pumice are observed, suggesting no (or very low degree of) interaction with hydrothermal fluid. These mineral and chemical composition change demonstrates the subseafloor lateral movement (diffusion) of hydrothermal fluid and indicates that this type of subseafloor orebody deposit matures below the seafloor.