The Characteristics of the Seafloor Massive Sulfide Deposits at the Hakurei Site in the Izena Hole, the Middle Okinawa Trough

Abstract

The seafloor massive sulfide (SMS) deposits, which were found in the Izena Hole, the middle Okinawa Trough, show two modes of occurrence, namely, large sulfide mounds on the seafloor and massive sulfide bodies beneath sediments. A 610-cm long sequence of alternating beds of sediment and ore layers was drilled to investigate the lower ore body, at high rates of core recovery (average 64 %), at the northern part of the Hakurei Site in the Izena Hole, the middle Okinawa Trough. Observation on polished sections of the core samples under reflection microscope indicates that the ores (18–166 cmbsf (cm below seafloor)) above the sediment layer (166–221 cmbsf) are porous and contain acicular barite and globular sphalerite with small barite inclusions. These textures are considered to have formed as a result of rapid cooling. On the other hand, ores (221–510 cmbsf) below the sediment layer (166–221 cmbsf) had a massive layer of sphalerite and galena on top (black ore), a middle layer of pyrite and chalcopyrite-rich layer (yellow ore) and a bottom layer of sphalerite and large barite.