Mineralogical composition and its significance of hydrothermal sulfides from the Longqi hydrothermal field on the Southwest Indian Ridge

As a typical ultraslow-spreading oceanic ridge with huge amount of hydrothermal sulfide deposits, the Southwest Indian Ridge is known for its unique mineralizaiton environment.In this paper, we presented hereby the detailed mineralogy of the hydrothermal metal sulfides recovered from the Longqi hydrothermal field of the Ridge.Our results suggested that the mineral assemblage of the deposits is highly weathered and dominated by pyrrhotite and pyrite, along with a little of sphalerite (Fe) and chalcopyrite (isocubanite).In addition, minor natural gold grains are sometimes observed.There are three metallogenic stages in addition to the latest seafloor weathering (1) the high temperature stage characterized by pyrite, pyrrhotite and chalcopyrite (isocubanite); (2) the medium temperature stage marked by the mineral assemblage of pyrite and sphalerite; (3) the low-temperature stage characterized by the mineral assemblage of colloform pyrite.The seafloor weathering is characterized by Fe-oxyhydroxide.The precipitation of the main sulfide minerals is presumed to be at a temperature above 335 ℃ and experienced episodic instantaneous temperature as high as 400 ℃.The Co-rich pyrrhotite in this metal sulfide deposits has changed from monoclinic pyrrhotite+ pyrite to hexagonal pyrrhotite+ pyrite, indicating a rapid coolingprocess of the hydrothermal fluid during the mineralization.