Hydrothermalism

Abstract

The circulation of seawater through oceanic lithosphere and the associated fluid-rock thermal and chemical exchange that occurs in the subsurface has a major effect on the composition of both seawater and oceanic lithosphere. The wide range of different mineralogical and geochemical characteristics of hydrothermal systems is a result of a combination of source rock composition, pressure-and temperature-dependent solubility controls, and possible direct contributions of metals and gases from subvolcanic magmas. These characteristics provide clues to the geology and chemical processes that are occurring in the underlying crust. The heating and subsequent venting of circulated seawater is the major mechanism for cooling hot, newly formed oceanic crust. The resulting chemical exchange includes the leaching and transport of base and precious metals from the underlying rock to the seafloor. The mixing of hot, metal-rich hydrothermal fluids with cold seawater at sites of focused black or white smoker discharge on the seafloor result in the formation of volcanogenic massive sulfide deposits that often host chemosynthetic-based ecosystems. The massive sulfide deposits that formed in ancient oceans and are now exposed on land are an important source for Cu, Zn, Pb, Au, and Ag and are mined for their metal contents. In the future, hydrothermal sulfide deposits on the modern seafloor may similarly be exploited for their rich metal content.