Seafloor hydrothermal clay alteration at Jade in the back-arc Okinawa Trough: Mineralogy, geochemistry and isotope characteristics

Seafloor hydrothermal activity at Jade has resulted in extensive alteration of the host epiclastic sediments and pumiceous tuffs, forming mica, kaolin s (kaolinite and halloysite), Mg-rich chlorite, talc, montmorillonite, and a mixed-layer mineral of dioctahedral chlorite and montmorillonite (Chl/Mon t). Clay mineral assemblages show a vertical variation, which reflects vari able amounts of cold seawater incorporated into hot hydrothermal fluids in subsurface sediments and tuff. However, mixing alone cannot explain the occ urrence of abundant kaolin minerals at Jade. The formation of kaolin minera ls requires much more acidic fluid than expected from simple mixing of hydr othermal fluids and cold seawater. Low pH values are likely attained by oxi dation of H2S either dissolved in the hydrothermal fluid or released from t he fluid during decompression. The fluid reaching the seafloor is discharge d into cold seawater, which caused precipitation of sulfides close to vents and native sulfur and barite at the margins of the vent areas. Halloysite, barite and anhydrite show Sr isotope compositions similar to ma rine Sr, indicating the derivation of marine Sr directly from seawater or b y the dissolution of calcareous nannoplanktons. The isotopic compositions o f kaolinite (delta(18)O = +7.4 parts per thousand, delta D = -23 parts per thousand), Chl/Mont (delta(18)O = +7.0 parts per thousand, delta D = -32 pa rts per thousand), and mica (delta(18)O = +5.4 to +9.9 parts per thousand, delta D = -30 to -26 parts per thousand) suggest fluids of a heated seawate r origin. The O isotopic data yielded formation temperatures of 170 degrees C for kaolinite, 61 to 110 degrees C for halloysite, and 145 to 238 degree s C for mica. Barite delta(34)S values (+21.0 to +22.5 parts per thousand) are very simil ar to the marine sulfate value, confirming that the barite formation took p lace due to mixing of Ba bearing hydrothermal fluids and sulfate-rich seawa ter. Native sulfur shows a large variation in delta(34)S in one hand specim en probably because of rapid disequilibrium precipitation of S during fluid exhalation on the seafloor. Sulfur in hydrothermal fluids is usually consu med to form metal sulfides. Therefore, abundant native sulfur at Jade sugge sts high H2S/metals ratios of the hydrothermal fluids. The alteration assemblages and isotopic data of hydrothermal minerals from Jade are very similar to those of Kuroko-type barite deposits of middle Mio cene age, which formed from fluids of high S/metals ratios at less than 200 degrees C. At Jade, there is only one black smoker actively discharging high temperatu re (similar to 320 degrees C) fluid, but there are many fossil sulfide chim neys and mounds in the area. The mineralogy and high Au and Cu in these pre cipitates suggest highly metalliferous hydrothermal activity in the past. T hese activities likely resulted in discharge of hydrothermal plumes and fal l-outs of sulfides and sulfates on the seafloor. These fall-outs were incor porated in sediments far from the vent areas. They are now recorded as high metal contents in sediments with no petrographic and mineralogical evidenc e of in-situ hydrothermal activity. Some are high as 8,100 ppm for Cu, 12,5 00 ppm for Zn, 1,000 ppm for As, 100 ppm for Ag and 21,000 ppm for Pb. Detr ital grains of montmorillonite in such sediments are coated with Fe-oxyhydr oxides during the suspension in seawater before settling on the seafloor. T he depths of such metal anomalies in sediments suggest high levels of metal liferous hydrothermal activities from 1,800 to 300 ybp. (C) 1999 Elsevier S cience Ltd.