The Modern Seafloor Hydrothermal Sulfide Ore Body Internal Fluid Process Simulation Study

Some topics associated with the large seafloor hydrothermal deposit, such as the mixing processes between hydrothermal fluids and entraining seawater, the evolving patterns of hydrothermal fluids, the distribution characteristics and governing factors of the thermal and flow fields within the deposit, have been systematically simulated using numerical approach. Meanwhile, based on the previous hydrothermal experimental reactors, the newly reactor that can be used to simulate modern seafloor hydrothermal system has been designed.The associated studies are presented in the following. Firstly, the mineral and geochemical compositions of the sulfide samples dredged from the TAG hydrothermal mound, Mid-Atlantic Ridge, are analyzed. Secondly, the thermodynamic model of the hydrothermal fluid/ seawater mixing within the large hydrothermal deposit is developed, taking example for the TAG hydrothermal mound. The mixing processes between the hydrothermal fluid and the seawater heated to different extent are computed by means of EMIX, revised software (MIX) of fluids mixing processes modeling. Thirdly, the physical model of hydrothermal sulfide deposit with three-layers, is developed. Using the software HYDROTHERAL developed and issued freely by USGS (United States Geological Survey), the influences of media permeability, topography and large fractures on the distributions of thermal and flow fields within the deposit are studied. At last, a newly experimental reactor are developed, which can be used to the study the rock-water interactions and the diffusing processes of hydrothermal plume.Accordingly, some key questions related with the large hydrothermal deposit are discussed, such as, the interior structures of the TAG deposit; the geological processes governing the mineral and geochemical compositions of the TAG deposit; the genetic relationships between the black and white smokers fluids in the TAG hydrothermal field; varying trends of the hydrothermal fluids compositions and the mineral precipitations during the mixing processes of hydrothermal fluids and the entraining seawater; the geological factors taking effect on the growth process of the large seafloor hydrothermal sulfide deposit.Based on these studies, some conclusions have been drawn, as presented in the following: (1) Differences found in components concentrations of white smoker and black smoker fluids in TAG hydrothermal field can be accounted for by the interaction of the heated seawater and the end-member hydrothermal fluid at 400℃. The temperature of the heated seawater must be less than 150℃.(2) Anhydrite precipitation within the large deposit is mainly controlled by the seawater heating process and the mixing process of hydrothermal fluid and entraining seawater. Conductive cooling alone can not result in abundant sulfides precipitation from the fluid. After hydrothermal fluid vents out of the deposit, the abrupt cooling and mixing with seawater should be the main mechanism that governs sulfides precipitation.(3) 330～310℃is a special temperature zone in the mixing processes between the seawater and the hydrothermal fluid.(4) Heterogenic structure within the deposit has great influence on the distribution of the temperature fields. The distributions of black smokers in mature hydrothermal deposit are related with many factors, such as heat source and permeability.(5) The sloping oceanic top has a little influence on the thermal and flow fields within the hydrothermal sulfide deposit. The relief basement is one of the dominated factors that control the thermal flow fields within the hydrothermal sulfide deposit.(6) The function of the fractures is constrained by some factors within the large sulfide deposit with single focused thermal source, such as the thermal flux of bottom boundary, the permeability and the characteristics of the fractures.