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. Plain Language Summary In seafloor hydrothermal systems, the seawater heated by magma circulates under the seafloor and forms seafloor massive sulfide (SMS) deposits. Recently, SMS deposits have attracted interest as a new metal resource, the generation mechanism and model of which must be established for accurate resource exploration. However, the mechanism is not yet fully understood. To address this problem, we applied a hydrothermal flow simulation and clarified the temperature distribution and fluid flow in the Iheya North Knoll, southwestern Japan. The result revealed that lateral flow and boiling of hydrothermal fluids occur below the seafloor. A low permeability caprock formation generated by anhydrite and clay mineral development is the most plausible cause of these occurrences, because a caprock can increase the temperature and induce boiling of fluids by suppressing the seawater inflow. This formation also makes fluid outlets less permeable, thus induces lateral flow. Consequently, vapor‐rich hydrothermal fluids poor in metals are discharged from vents, while liquid‐dominated hydrothermal fluids rich in metals flow laterally below the caprocks, forming subseafloor SMS deposits. Key Points Numerical simulation of multiphase fluid flow revealed regional temperature, fluid‐flow patterns, and physical property distributions Integration of results with geologic interpretations provided a plausible generation m