Three-Dimensional Numerical Simulation of Gas–Liquid Two-Phase Flow in the Izena Hole, Middle Okinawa Trough, to Clarify the Generation Process of Seafloor Hydrothermal Deposits

Abstract

Although seafloor hydrothermal deposits are essential metal resources because of their considerable reserves and high metal grades, the concrete physical setting that causes their generation remains unclear. These can be clarified only through numerical simulation studies because it is impossible to observe the physical setting and actual phenomena over a long period and a wide area below the seafloor. In this study, we attempted to clarify those by simulating a three-dimensional flow system of a gas–liquid two-phase fluid and regional temperature distribution in a hydrothermal field. The Izena Hole, middle Okinawa Trough, was selected as our target field. Results show that the ore bodies on and beneath the seafloor were generated by different mechanisms. In the early stage of hydrothermal activity, most of the hydrothermal fluids ascended freely from a great depth and spouted from the seafloor and consequently generated ore bodies on the seafloor by mixing with seawater. Over time, a hardly permeable cap layer was formed by hydrothermal alteration, which caused lateral fluid flows and inhibited the inflow of cold seawater under the cap layer, resulting in a temperature increase and consequent boiling of the fluids. Mineral replacement occurring in large amounts of lateral fluid flows over tens of thousands of years is the most likely cause of the generation of subseafloor ore bodies. Therefore, cap layer formation is an essential factor in generating subseafloor ore bodies and changing the location of the ore body occurrence from on to beneath the seafloor.

Keywords

• boiling
• cap layer
• temperature distribution
• hydrothermal circulation
• tough2