Mineralogical and geochemical characteristics of hydrothermal minerals collected from hydrothermal vent ffields in the Southern Mariana spreading center

Archaean and Pika sites) in the Southern Mariana Trough was investigated to clarify the mineralogical and geochemical characteristics of the hydrothermal minerals. The Snail site and the Yamanaka site are located on the crest of the backarc spreading ridge (on-axis). The Pika site sits atop an off-axial volcano, 4.9 km distant from the spreading axis, while the Archaean site sits on the flank of the spreading ridge, about 1.5 km distant from the axis. In the four hydrothermal sites, chimney and mound sulfides are composed mainly of pyrite, marcasite, sphalerite and chalcopyrite. Conduits and interstices of these sulfide minerals are filled with late barite and anhydrite. Mineralizations of off-axial hydrothermal fields have mineralogical and geochemical signatures (Fe-rich and low fS2 condition) similar to those of mineralizations at sediment-free mid-ocean ridge settings. By contrast, mineralizations of on-axial sites show distinctly Zn and Pb-rich (high fS2 condition) signatures similar to those found in arc or rifting settings. Sulfur isotopic compositions of collected sulfide minerals show a similar diversity with a mid-ocean ridge range for the off-axial sites and an island-arc range for the on-axial sites. These isotopic signatures could be explained by varying proportions of magmatic sulfur leached from the underlying volcanic rocks and reduced seawater sulfates.