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The Hatoma Koll is a submarine volcano located in the southern Okinawa trough, 24°51' N and 123°50' E. Present-day hydrothermal activity associated with high temperature (Tmax = 300??) fluid venting was observed within the crater floor at the water depth around 1500 m. While active chimney structures are characterized by sulfate mineralization, sulfide deposits were found in mound structure. We studied mineralogical and chemical composition of these hydrothermal precipitates in detail.
During the dive expedition YK07-04 of SHINKAI6500 (JAMSTEC) conducted in March 2007, a white chimney sample was collected in the vicinity of an active venting site. The white chimney is predominantly composed of quite fine anhydrite (CaSO4). Minor amount of gypsum (CaSO4?E2H2O) was found from the whole part and barite (BaSO4) was found only in the bottom part of the chimney. Minor sulfides such as sphalerite (ZnS) and galena (PbS) were also identified. While sizes of the sulfides collected from the top of the chimney were smaller than 10 ?Êm, those from the bottom part were a few hundred ?Êm. Chimney samples collected during previous expeditions conducted in 2000 and 2001 consist predominantly of barite. These samples were probably matured chimneys because amorphous silica precipitated between barite crystals. Only minor amount of sulfides such as pyrite (FeS2) and tetrahedrite ((Cu,Fe)12Sb4S13) were found along the fluid conduit. The mound deposits were mainly composed of sulfides such as wurtzite (ZnS), tetrahedrite, galena, chalcopyrite (CuFeS2) and pyrite. It is noteworthy that the mound sulfides included wurtzite as a zinc sulfide, while the white chimney contains sphalerite.
Chemical composition of the sphalerite and wurztite was determined by EPMA analysis. FeS content in ZnS is 1.0 mol% on average for the white chimney, and 3.4 mol% for the mound sulfide. This range suggests hydrothermal precipitation occurs under rather oxidative environment. MnS content in ZnS is 4.0 mol% on average for the white chimney, and 0.3 mol% for the mound sulfide. This ranges are notably high compared with other submarine hydrothermal ore deposits (e.g. <0.1 mol% for the TAG mound in the MAR, 0.2 mol% for the Middle Valley in Juan de Fuca Ridge, <0.1 mol% for the Mariner field in the Lau basin). Similar high Mn content (4.7 mol%) was reported for wurtzite associated with carbonate mineralization in the Yonaguni Koll IV hydrothermal field in the southern Okinawa trough. This unique signature would reflect chemical environment specific to the hydrothermal activities in the southern Okinawa trough.
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