The geochemistry of continental hydrothermal systems

Hydrothermal systems on the continents are of great significance because they are primary sources of economically important metals and geothermal energy, they are tourist attractions, they support bathing and health resorts, and they host extreme life forms. Research on hot springs and their deposits provide clues to early life on Earth and possibly on Mars and have led to major breakthroughs in biotechnology. Aqueous and gas-rich hydrothermal fluids also contribute to a range of volcanic hazards including the destabilization of volcanic edifices, acting as propellant in steam-driven hydrothermal explosions, reducing effective stresses in mudflows (lahars), emitting toxic and potentially lethal gases, and transporting toxic metals to watersheds. The main goals of this review are to summarize the state of knowledge on the chemistry of continental hydrothermal systems and highlight the myriad processes that operate under a wide range of temperatures, pressures, chemical compositions, and oxidation states.