Evolving patterns of the fluids within the TAG hydrothermal field

Abstract

The mixing of seawater/hydrothermal fluid within the large seafloor hydrothermal sulfide deposits plays a key role in the formation processes of the sulfide deposits. Some issues attract considerable attentions in the study of seafloor hydrothermal system in recent years, such as the relationships among different types of vent fluids, the characteristics of chemical compositions and mineral assemblages of the hydrothermal deposits and their governing factors. Combined with the measured data of hydrothermal fluid in the TAG field, the thermodynamic model of mixing processes of the heated seawater at different temperatures and the hydrothermal fluid is calculated to understand the precipitation mechanism of anhydrite and the genetic relationships between the black and white smoker fluids within the TAG mound. The results indicate that the heating of seawater and the mixing of hydrothermal fluid/seawater are largely responsible for anhydrite precipitation and the temperature of the heated seawater is not higher than 150°C and the temperature of the end-member hydrothermal fluid is not lower than 400°C. Based on the simulated results, the evolving patterns of fluids within the TAG deposit are discussed. The mixed fluid of the end-member hydrothermal fluid and the seawater heated by wall rock undergoes conductive cooling during upflowing within the deposit and forms “White Smoker” eventually. In addition, the end-member hydrothermal fluid without mixed with seawater, but undergoing conductive cooling, vents out of the deposit and forms “Black Smoker”.