Clarification of Regional Seafloor Hydrothermal System in a Japan Back-Arc Basin by Numerical Simulation with TOUGH2

Abstract

Hydrothermal activity occurs by a fluid circulation system heated by magma, forming a variety of economically important mineral deposits. To identify the dynamic behavior of seafloor hydrothermal systems, a number of numerical modeling have been constructed in the past three decades. However, the large-scale temperature distribution and hydrothermal flow pattern have not yet been clarified. These thermal characteristics are quite important to estimate the high potential zones of important metal minerals, because metal-bearing minerals can be formed in the system largely depending on temperature. Based on that background, we selected the Iheya North Knoll as a case study, and applied hydrothermal flow simulation using TOUGH2 to reveal distribution of physical properties and hydrothermal flow pattern in the seafloor hydrothermal system of Japan back-arc basin. In the numerical model, we set seafloor as the top boundary where temperature and pressure were fixed at 4 ℃ and hydrostatic condition, respectively, and optimized the physical properties of rocks from the field survey data. This simulation yielded a plausible large-scale temperature distribution and flow pattern of the hydrothermal fluids, which correspond to the observed values even around the discharge area where the flow tends to be complicated. The effectiveness of the model was verified through the sensitivity analysis, and the location and physical properties of the conduit and the permeability of volcanic basement were identified as significant factors controlling the accuracy of hydrothermal flow simulation. Our next step is to build a more advanced numerical model considering the thermal alteration and chemical reaction of rocks with fluids.