Fluid evolution of extinct seafloor massive sulfide deposits in the 26°S hydrothermal field, Mid-Atlantic Ridge

The 26°S hydrothermal field (SMAR 26°S), located at 26°12' on the slow-spreading Southern Mid-Atlantic Ridge, was first found by the Chinese Cruise in 2011. This site is primarily basaltic in composition (N-MORB) and hosts inactive hydrothermal systems. Inactive black smokers and sulfide debris are one of the most important products of submarine hydrothermal systems. This thesis focuses on research in terms of the magma and magma degassing process which influence fluid evolution. A comprehensive study of the geological setting and petrographic characteristics of sulfides in this study allowed a deeper understanding of the paragenetic sequences of sulfide mineralization; The geochemical and isotopic compositions of sulfides were obtained by using various techniques (i) to clarify the features of fluid evolution and variations on physicochemical conditions, (ii) to study the factors controlling trace elements distribution in the petrographically different zones in chimneys, and (iii) to trace the source of metallogenic elements and that of ore-forming fluid. It was possible to establish a three-endmember mixing model including seawater, magmatic fluid, and volatile-rich magmatic fluid to reveal the effects of water-rock interaction, the mixture of hydrothermal fluid with seawater, as well as magma degassing in each stage during fluid evolution, so as to exhibit the evolution characteristics of the ore-forming fluid in SMAR 26°S hydrothermal field. As one of the main findings, the metallogenic model for the SMAR 26°S differs significantly from the conventional view of seawater-rock interaction, i.e., considering that ore-forming fluids could be controlled by seawater-rock interaction and seawater-fluid mixing, it is believed that magmatic fluids, as well as volatile-rich fluids resulting from magma degassing are thought to be the principal source of ore-forming fluids and play an important role in the evolution of fluids.