Polyphase hydrothermal sulfide mineralization in the minami–ensei hydrothermal field, middle okinawa trough: Implications from sulfide mineralogy and in situ geochemical composition of pyrite

Abstract

The mineralogical and geochemical zonation of hydrothermal chimneys preserve important information on mineralization process and hydrothermal evolution. In this study, the sulfide mineralogy and in situ geochemical compositions of pyrite from a hydrothermal chimney in the Minami–Ensei (ME) hydrothermal field were analyzed systematically to determine the mineralization characteristics. Seven layers with different mineral compositions were observed in an individual chimney from ME, including four pyrite-dominant layers and three barite-rich layers. The barite-rich layers occur mainly as intercalations in the hydrothermal chimney, i.e., barite-rich layers and pyrite-dominant layers precipitate alternately, suggesting the occurrence of a polyphase mineralization process, which is highlighted by the in situ trace elements and sulfur isotope compositions of pyrite. The extremely low Co, Ni, Cu, and Se contents (<1 ppm) and high Pb, As, Tl, Mn, and Zn contents (up to several thousands of ppm) in pyrite indicate that the mineralization fluid temperature is unlikely to be higher than 200 °C, which allows the bacterial reduction of seawater sulfate, causing negative shifts in the δ34S values of ME pyrite (−5.00 ‰ to 4.01 ‰). The stable fluid pathway and aperiodic variations in magmatic intensity cause polyphase sulfide mineralization in an individual hydrothermal chimney in the ME hydrothermal field.