Organic composition of sulphide ores in the Okinawa Trough and its implications

A preliminary organic geochemical study shows that the sulphide ores from t he hydrothermal deposit of the Okinawa Trough are generally low in the tota l organic carbon and extremely low in the soluble organic matter. In the al iphatic hydrocarbon fraction, the n-allcanes range from C-15 to C-35, with usual maxima in the middle n-C-20 region and strong odd-carbon number predo minance when n > C-25(CPI = 1.2). The dominant analog in the aromatic fract ion is phenanthrene, a polynuclear aromatic hydrocarbon, which provides evi dence for hydrothermal activity. The organic matter derived mainly from mar ine planktonic and terrigenous vascular plants is entrapped in a high-tempe rature regime such as an active chimney and cooled quickly in the sulphide ores on the seafloor. Organic matter and sulphides are definitely products of a high-temperature alteration. The biomarker compounds indicate that the ores are formed under low Eh and pH conditions - a reducing to anoxic envi ronment which is favourable for sulphates to be reduced into sulphides by b iogenic (bacterial) or abiogenic activity.