Ore and gangue minerals of seafloor hydrothermal deposits in the Mariana Trough

Abstract

Chimney and mound samples collected by Shinkai 6500 from two hydrothermal vent fields in the Mariana Trough were studied mineralogically and geochemically. One of hydrothermal vent fields is so called Forecast Vent Field at the depth of 1,450 m in the South Mariana Trough, and another deeper field is so called Alice Spring Field at the depth of 3,600 m in the Middle (or Central) Mariana Trough. Ore and gangue minerals from the both hydrothermal fields are similar, but the quantity of them is different in each field. Main ore minerals in the South Mariana Trough are sphalerite, marcasite, chalcopyrite and tetrahedrite-tennantite, and those in the Middle Mariana Trough are wurtzite, chalcopyrite and tetrahedrite-tennantite. As gangue minerals, gypsum is abundant and barite is poor in the South Mariana Trough, but, in contrast, anhydrite and barite are rich in the Middle Mariana Trough. Chemical compositions of ore minerals are analyzed by EPMA. Sphalerie contains FeS up to 2.5 mol %. The trapping temperature of fluid, i.e. formation temperature is obtained after pressure correction for the measured homoge- nization temperature of fluid inclusion. Pressure-corrected trapping temperature of fluid inclusion in wurtzite and anhydrite in the South Mariana Trough averages 240°C, and that in wurtzite in the Middle Mariana Trough averages 278°C. The measured maximum temperatures during this survey are 210°C and 267°C in the South Mariana Trough and Middle Mariana Trough, respectively. The salinity of fluid is determined from melting temperature depression of fluid inclusion. Sulfur isotopes of sulfide and sulfate minerals are examined, and bulk chemical compositions of chimneys are analyzed. It is noteworthy that some chimneys contain gold of the order of 10 to 30 ppm.