Porphyry-type ore deposits are major resources of copper and gold, precipitated from fluids expelled by crustal magma chambers. The metals are typically concentrated in confined ore shells within vertically extensive vein networks, formed through hydraulic fracturing of rock by ascending fluids. Numerical modeling shows that dynamic permeability responses to magmatic fluid expulsion can stabilize a front of metal precipitation at the boundary between lithostatically pressured up-flow of hot magmatic fluids and hydrostatically pressured convection of cooler meteoric fluids. The balance between focused heat advection and lateral cooling controls the most important economic characteristics, including size, shape, and ore grade. This self-sustaining process may extend to epithermal gold deposits, venting at active volcanoes, and regions with the potential for geothermal energy production.

[1] We present the first fully transient 2-D numerical simulations of black smoker hydrothermal systems using realistic fluid properties and allowing for all phase transitions possible in the system H 2 O-NaCl, including phase separation of convecting seawater into a low-salinity vapor and high-salinity brine. We investigate convection, multiphase flow, and phase segregation at pressures below, near, and above the critical point of seawater. Our simulations accurately predict the range in vent salinities, from 0.05 to 2.5 times seawater salinity measured at natural systems. In low-pressure systems at $1500 m water depth, phase separation occurs in boiling zones stretching from the bottom of the hydrothermal cell to the seafloor. Low-salinity vapors and high-salinity brines can vent simultaneously, and transient variations in vent fluid salinities can be rapid. In highpressure systems at roughly $3500 m water depth, phase separation is limited to the region close to the underlying magma chamber, and vent fluids consist of a low-salinity vapor mixed with a seawater-like fluid. Therefore, vent salinities from these systems are much more uniform in time and always below seawater salinity as long as phase separation occurs in the subseafloor. Only by shutting down the heat source can, in the high-pressure case, the brine be mined, resulting in larger than seawater salinities. These numerical results are in good agreement with long-term observations from several natural black smoker systems.

Magmatic-hydrothermal ore deposits document the interplay between saline fluid flow and rock permeability. Numerical simulations of multiphase flow of variably miscible, compressible H 2 O-NaCl fluids in concert with a dynamic permeability model can reproduce characteristics of porphyry copper and epithermal gold systems. This dynamic permeability model uses values between 10 À22 and 10 À13 m 2 , incorporating depth-dependent permeability profiles characteristic for tectonically active crust as well as pressure-and temperature-dependent relationships describing hydraulic fracturing and the transition from brittle to ductile rock behavior. In response to focused expulsion of magmatic fluids from a crystallizing upper crustal magma chamber, the hydrothermal system self-organizes into a hydrological divide, separating an inner part dominated by ascending magmatic fluids under near-lithostatic pressures from a surrounding outer part dominated by convection of colder meteoric fluids under near-hydrostatic pressures. This hydrological divide also provides a mechanism to transport magmatic salt through the crust. With a volcano at the surface above the hydrothermal system, topography-driven flow reverses the direction of the meteoric convection as compared to a flat surface, leading to discharge at distances of up to 7 km from the volcanic center. The same physical processes at similar permeability ranges, crustal depths, and flow rates are relevant for a number of active systems, including geothermal resources and excess degassing at volcanos. The simulations further suggest that the described mechanism can separate the base of free convection in high-enthalpy geothermal systems from the magma chamber as a driving heat source by several kilometers in the vertical direction in tectonic settings with hydrous magmatism. These root zones of high-enthalpy systems may serve as so-called super-critical geothermal resources. This hydrology would be in contrast to settings with anhydrous magmatism, where the base of the geothermal systems may be closer to the magma chamber.

A new and economically attractive type of geothermal resource was recently discovered in the Krafla volcanic system, Iceland, consisting of supercritical water at 450 °C immediately above a 2-km deep magma body. Although utilizing such supercritical resources could multiply power production from geothermal wells, the abundance, location and size of similar resources are undefined. Here we present the first numerical simulations of supercritical geothermal resource formation, showing that they are an integral part of magma-driven geothermal systems. Potentially exploitable resources form in rocks with a brittle–ductile transition temperature higher than 450 °C, such as basalt. Water temperatures and enthalpies can exceed 400 °C and 3 MJ kg−1, depending on host rock permeability. Conventional high-enthalpy resources result from mixing of ascending supercritical and cooler surrounding water. Our models reproduce the measured thermal conditions of the resource discovered at Krafla. Similar resources may be widespread below conventional high-enthalpy geothermal systems.