11:45 AM - 12:00 PM
[SCG45-11] Fluid-flow simulation for clarifying ore generation processes in the Izena Hole hydrothermal field, Okinawa Trough
Keywords:hydrothermal flow simulation, TOUGH2, temperature, boiling, seafloor massive sulfide deposit, cap layer
Seafloor hydrothermal deposits have a high potentiality for new metal resources. An understanding of the genesis of the deposits is indispensable to exploring the deposits. In the previous study, we conducted a hydrothermal flow simulation for the Iheya North hydrothermal field, Okinawa Trough and clarified that a formation of the cap layer induced the lateral flow and boiling of the hydrothermal fluids, which caused the metallic mineral deposition below the seafloor (Tomita et al., 2020). In this study, we aim to verify the applicability of the Iheya North mineral precipitation model to other seafloor hydrothermal deposits by selecting the Izena Hole hydrothermal field, Okinawa Trough as a case study field and a hydrothermal flow simulation using TOUGH2.
The field observations such as the drilling data, seismic survey, and heat flux measurement were used to construct a proper three-dimensional numerical model that contains two hydrothermal sites, Jade and Hakurei sites. In the simulation, the seafloor was set as the top boundary where the temperature and pressure were fixed at 4 ℃ and hydrostatic conditions, respectively. The physical properties of rocks were set by the data of seafloor drillings, and the permeabilities (hereinafter, k) of the five elements were adjusted with trial-and-error approaches so that the heat flux and temperature differences would be acceptably small with consideration of heat balance. To clarify the general fluid flow pattern and the temperature and pressure distributions, the model domain was simply divided into five geologic elements, conduit (high k), fault (high k), volcanic basement (medium k), cap layer (low k), and sediment (low k), by excluding geological and hydrological heterogeneities. The conduits were set vertically from the bottom to seafloor at the middle area common to Jade and Hakurei sites. The hydrothermal fluid of 360 °C was injected from the conduit bottoms, and the hydrothermal fluid was set to discharge from the conduit surfaces.
As a result of the simulation, the hydrothermal fluid ascending along the conduit was trapped by the cap layer near the surface and flowed laterally in addition to the outflow from the seafloor. The calculated heat fluxes generally corresponded with the measured values. The hydrothermal fluids that ascended along the conduit boiled under the seafloor at both the Jade and Hakurei sites due to the pressure decrease.
In the Hakurei site, a two-layer structure of exposed ore bodies on the seafloor and lower ore bodies below the seafloor were found in the northern mound around the C9027 drilling hole. We considered the origin of this two-layer structure in three stages through the simulation results and observations. In the early stage, hydrothermal fluids discharging from black smokers formed the upper seafloor massive sulfide (SMS) deposits on the seafloor. In the middle stage, cap layers (anhydrite and clay minerals) formed below the seafloor, which induced the lateral flow and boiling under the cap layers. The presence of cap layer below the seafloor was confirmed by the drilling data. After formation of the cap layers, liquid-dominated hydrothermal fluids rich in metals flow laterally below the cap layers, forming the lower SMS deposits at tens of meters depth below the seafloor. In the late stage, the center of hydrothermal activity shifted from the northern mound to the Dragon chimney, because the pore spaces were filled with mineral precipitation and then k was largely decreased at the mound.
In conclusion, this study clarified that the Iheya North mineralization model could be applied to the Izena Hole.
Reference
Tomita, S. A., Koike, K., Goto, T., & Suzuki, K. (2020). Numerical simulation-based clarification of a fluid-flow system in a seafloor hydrothermal vent area in the middle Okinawa Trough. Geophysical Research Letters, 47, e2020GL088681. https://doi.org/10.1029/2020GL088681
The field observations such as the drilling data, seismic survey, and heat flux measurement were used to construct a proper three-dimensional numerical model that contains two hydrothermal sites, Jade and Hakurei sites. In the simulation, the seafloor was set as the top boundary where the temperature and pressure were fixed at 4 ℃ and hydrostatic conditions, respectively. The physical properties of rocks were set by the data of seafloor drillings, and the permeabilities (hereinafter, k) of the five elements were adjusted with trial-and-error approaches so that the heat flux and temperature differences would be acceptably small with consideration of heat balance. To clarify the general fluid flow pattern and the temperature and pressure distributions, the model domain was simply divided into five geologic elements, conduit (high k), fault (high k), volcanic basement (medium k), cap layer (low k), and sediment (low k), by excluding geological and hydrological heterogeneities. The conduits were set vertically from the bottom to seafloor at the middle area common to Jade and Hakurei sites. The hydrothermal fluid of 360 °C was injected from the conduit bottoms, and the hydrothermal fluid was set to discharge from the conduit surfaces.
As a result of the simulation, the hydrothermal fluid ascending along the conduit was trapped by the cap layer near the surface and flowed laterally in addition to the outflow from the seafloor. The calculated heat fluxes generally corresponded with the measured values. The hydrothermal fluids that ascended along the conduit boiled under the seafloor at both the Jade and Hakurei sites due to the pressure decrease.
In the Hakurei site, a two-layer structure of exposed ore bodies on the seafloor and lower ore bodies below the seafloor were found in the northern mound around the C9027 drilling hole. We considered the origin of this two-layer structure in three stages through the simulation results and observations. In the early stage, hydrothermal fluids discharging from black smokers formed the upper seafloor massive sulfide (SMS) deposits on the seafloor. In the middle stage, cap layers (anhydrite and clay minerals) formed below the seafloor, which induced the lateral flow and boiling under the cap layers. The presence of cap layer below the seafloor was confirmed by the drilling data. After formation of the cap layers, liquid-dominated hydrothermal fluids rich in metals flow laterally below the cap layers, forming the lower SMS deposits at tens of meters depth below the seafloor. In the late stage, the center of hydrothermal activity shifted from the northern mound to the Dragon chimney, because the pore spaces were filled with mineral precipitation and then k was largely decreased at the mound.
In conclusion, this study clarified that the Iheya North mineralization model could be applied to the Izena Hole.
Reference
Tomita, S. A., Koike, K., Goto, T., & Suzuki, K. (2020). Numerical simulation-based clarification of a fluid-flow system in a seafloor hydrothermal vent area in the middle Okinawa Trough. Geophysical Research Letters, 47, e2020GL088681. https://doi.org/10.1029/2020GL088681