Abstract: The Okinawa Trough is an important part of Ryukyu subduction system and the sulfide research in this region is important for uncovering the hydrothermal mineralization characteristics in incipient back-arc basin. Many researches have demonstrated that the mineralization characteristics of CLAM hydrothermal system are different from those of others in Okinawa Trough. However, the cause of these differences are still unclear. The mineralogical characteristics and sphalerite in situ trace element contents of CLAM Fe-rich type sulfides are determined systematically. On these basis, we propose the unique mineralogical compositions and geochemical characteristics in CLAM are controlled by sulfur fugacity of hydrothermal fluid. The studied samples are mainly composed of pyrrhotite and low-temperature sphalerite. The occurrence of large amounts of pyrrhotite implies that the hydrothermal fluid possesses low sulfur fugacity. During the mineralization process, both temperature and sulfur fugacity conditions for the mineralization evolved from low to high. The high Fe contents also support the low sulfur fugacity condition during the CLAM sulfide mineralization. The calculation indicates that the sulfur fugacity value of hydrothermal fluid is –15, which is equal to those of other sediment-hosted hydrothermal systems. The decomposition of organism in sediments during hydrothermal circulation is the major cause of low sulfur fugacity in CLAM fluid.