Geochemical Variability of Pyrite, Sphalerite, and Chalcopyrite from Submarine Hydrothermal Vents

Abstract

Pyrite, sphalerite, and chalcopyrite are major constituents of seafloor massive sulfide (SMS) deposits and have the ability to incorporate various trace elements. Here, we report trace elements geochemistry of pyrite, sphalerite, and chalcopyrite from submarine hydrothermal vents at diverse geological settings including the Central Indian Ridge, North Fiji Basin, Eastern Manus Basin, and Tonga intra-oceanic arc. The study areas are distinguished by a large range of variations in host rock compositions, physicochemical conditions of hydrothermal fluids, sulfide mineralogy, and isotopic compositions: (1) water/rock-dominated systems (Edmond, MCS-1, and KF2 1657), and (2) magmatic-hydrothermal systems (SuSu Knolls and TA25 EVF), respectively. We mainly focused on sulfide minerals around the innermost chalcopyrite linings, reflecting the peak hydrothermal activity to constrain the nature of pristine fluids with little seawater mixing. Our results show that the trace element geochemistry of pyrite, sphalerite, and chalcopyrite is mainly controlled by fluid temperature and redox state, host rock composition, magmatic input, and sub-surface and precipitation processes such as seawater mixing and selective partitioning. Although the contribution from these factors significantly varies between each vent field, the influence of different metal sources on seafloor hydrothermal mineralization is characterized by the distinct behavior of Co, As, and Hg. The water-rock interaction, and to some extent relatively reducing and high-temperature fluids, induces the preferential incorporation of Co into sulfide minerals in water/rock-dominated systems, whereas the magmatic contribution plays an important role in the production of As-Hg-rich sulfide minerals in magmatic-hydrothermal systems. This is more evident in the Co/As and Co/Hg ratios of sulfide minerals, consistent with the compiled data from the previous study. As a consequence, we suggest that the Co/As and Co/Hg ratios of pyrite, sphalerite, and chalcopyrite could be useful empirical indicators to discriminate the nature of seafloor hydrothermal mineralization in diverse geological settings.