Intrusion-Related Geothermal Systems

Many geothermal systems are intimately associated with intrusion and volcanism along convergent plate margins, within rifts, and at hot spots. Areas of active structural deformation, especially extension and transtension, localize shallow intrusion and favor formation and maintenance of fracture networks that promote convection of deeply circulated meteoric water. Such systems efficiently deliver heat and mass flow from the intrusive realm at depths >3 km to the Hydrosphere. Extensive fracture networks are formed and maintained by the interplay of tectonic stresses with magma emplacement, and the episodic expulsion of magmatic fluids during recharge, eruption, and crystallization. Hydrothermally driven fluid flow along upflow and outflow paths alters reservoir rocks to characteristic secondary mineral assemblages that reflect the chemistry and temperature of the system. A capping hydrothermal clay-rich layer commonly limits communication of deep (>0.6 km), high-temperature (220–350 °C) neutral-chloride brine from near-surface groundwater circulating above this altered zone. The chemical characteristics of fluids reaching the surface locally along structures, and the distribution of clay alteration as revealed by electrical resistivity surveys, are used to prospect for exploitable systems. Of all geothermal systems, those related to magmatic intrusion are the most prolific source of electricity worldwide. If properly developed and managed, they provide a very long-term source of indigenous energy without combustion of fossil fuels.