Abstract

The Okinawa trough is an active back-arc-spreading center in which submarine hydrothermal exhalation (black chimneys) and sulfide mineralization are forming. Fluid inclusion composition in minerals of the hydrothermal feeder zones in the JADE field shows that the hydrothermal systems are extremely gas-rich. There are two independent and co-existing fluids, CO 2 -hydrocarbon and saline fluids. Microthermometry and Laser Raman Microprobe (LRM) analyses on individual fluid inclusions of the CO 2 -hydrocarbon type, dominated by CO 2 and containing minor CH 4 , C 2 H 4 , C 2 H 6 , N 2 and H 2 S, are almost equivalent to the fluid inclusion composition of a natural gas field. The saline fluid is mainly composed of H 2 O and contains minor Na (super +) and K (super +) . It is close to a H 2 O-NaCl, or H 2 O-NaCl-KCl system, with salinities from 3.7% to 7.5% NaCl eq. Before venting on the seafloor, the fluids experienced immiscible separation. One of end-member fluids is downward circulating seawater that interacted with sub-seafloor rocks. The other was gas (volatile)-rich fluid derived from a shallow magma chamber. The immiscibility separation of fluids resulted from the decrease of temperature and pressure of the hydrothermal system and continuous input of magmatic volatiles. The saline fluids in this system are discharging to form sulfide chimneys and black smoke. Meanwhile, CO 2 -rich fluid discharges as CO 2 bubbles through hydrate pipes. A large amount of CO 2 -CH 4 -H 2 S gas or fluid in the upper part of the hydrothermal system, and reaction of this gas or fluid with saline fluids, finally enables the accumulation of metal sulfides.

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