Numerical simulation of submarine hydrothermal ore-deposits

Submarine hydrothermal ore-deposits are known to be rich in metal resources and are to be found abundant around the Japanese islands. In recent years, several field studies had been carried out and a lot of data on chemical and physical properties of geological samples (ore-deposits, rocks, sediments) have been gathered especially for hydrothermal fields in the Okinawa Trough. However, the mechanism of formation and accumulation of these deposits is not yet clearly understood. In this study, we investigated hydrothermal fluid flow in combination with chemical reactions of a simplified model (2-dimensional radial model) representing hydrothermal activity around a vent fluid conduit beneath the seafloor. The numerical code TOUGHREACT V3.0 – OMP, which is for coupled non-isothermal multiphase fluid flow and geochemical reactive transport was employed. Almost all model parameters are chosen based on available results of geophysical and geochemical surveys of the Iheya hydrothermal fields in the Okinawa Trough. Therefore, the geological structures were set as having diverse permeability, which reflects alternating pumiceous volcaniclastics layer and hemipelagic sediment layer.Chemical reactions including precipitation of minerals (anhydrite, quartz and sphalerite) caused by mixing of hydrothermal fluid and seawater-derived fluid and/or cooling was considered in this setting. Following two features were clarified: First, high velocity venting hydrothermal fluid ascends to the seafloor through the fluid conduit at the center of the model, whereas nearby cold water percolates downward from the seafloor and convection occurs. Second, among the alternating layer structure, preferential horizontal flow is obvious for layers with high permeability, which resulted in primary sulfide precipitation with a lateral extend. These results are likely to be consistent to recent field studies during scientific drilling into the submarine hydrothermal fields.