Numerical Simulation-Based Clarification of a Fluid-Flow System in a Seafloor Hydrothermal Vent Area in the Middle Okinawa Trough

Despite many studies on seafloor hydrothermal systems conducted to date, the generation mechanism of seafloor massive sulfide (SMS) deposits is not yet fully understood. To elucidate this mechanism, this study clarifies the three-dimensional regional temperature distribution and fluid flow of a seafloor hydrothermal system of the Iheya North Knoll, middle Okinawa Trough. Lateral flow and boiling of hydrothermal fluids below a caprock were the main features found by the simulation. A caprock formation generated by anhydrite precipitation and hydrothermal alteration is the most plausible cause of these features, because caprocks can increase the temperature and induce boiling of fluids by preventing seawater inflow. Such a formation also gradually makes the top of the conduit less permeable; thus, lateral flow occurs. Consequently, vapor-rich hydrothermal fluids poor in metals are discharged from vents as white smokers, whereas liquid-dominated hydrothermal fluids rich in metals flow laterally below the caprocks, forming subseafloor SMS deposits.