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.
The dynamic behavior of magmatic hydrothermal systems entails coupled and nonlinear multiphase flow, heat and solute transport, and deformation in highly heterogeneous media. Thus, quantitative ...analysis of these systems depends mainly on numerical solution of coupled partial differential equations and complementary equations of state (EOS). The past 2 decades have seen steady growth of computational power and the development of numerical models that have eliminated or minimized the need for various simplifying assumptions. Considerable heuristic insight has been gained from process‐oriented numerical modeling. Recent modeling efforts employing relatively complete EOS and accurate transport calculations have revealed dynamic behavior that was damped by linearized, less accurate models, including fluid property control of hydrothermal plume temperatures and three‐dimensional geometries. Other recent modeling results have further elucidated the controlling role of permeability structure and revealed the potential for significant hydrothermally driven deformation. Key areas for future research include incorporation of accurate EOS for the complete H2O‐NaCl‐CO2 system, more realistic treatment of material heterogeneity in space and time, realistic description of large‐scale relative permeability behavior, and intercode benchmarking comparisons.
Thermohaline convection of subsurface fluids strongly influences heat and mass fluxes within the Earth's crust. The most effective hydrothermal systems develop in the vicinity of magmatic activity ...and can be important for geothermal energy production and ore formation. As most parts of these systems are inaccessible to direct observations, numerical simulations are necessary to understand and characterize fluid flow. Here, we present a new numerical scheme for thermohaline convection based on the control volume finite element method (CVFEM), allowing for unstructured meshes, the representation of sharp thermal and solute fronts in advection‐dominated systems and phase separation of variably miscible, compressible fluids. The model is an implementation of the Complex Systems Modelling Platform CSMP++ and includes an accurate thermodynamic representation of strongly nonlinear fluid properties of salt water for magmatic‐hydrothermal conditions (up to 1000°C, 500 MPa and 100 wt% NaCl). The method ensures that all fluid properties are taken as calculated on the respective node using a fully upstream‐weighted approach, which greatly increases the stability of the numerical scheme. We compare results from our model with two well‐established codes, HYDROTHERM and TOUGH2, by conducting benchmarks of different complexity and find good to excellent agreement in the temporal and spatial evolution of the hydrothermal systems. In a simulation with high‐temperature, high‐salinity conditions currently outside of the range of both HYDROTHERM and TOUGH2, we show the significance of the formation of a solid halite phase, which introduces heterogeneity. Results suggest that salt added by magmatic degassing is not easily vented or accommodated within the crust and can result in dynamic, complex hydrologies.
We present a new numerical scheme for multiphase convection of salt water at magmatic‐hydrothermal conditions based on the control volume finite element method. In a series of benchmarks with HYDROTHERM and TOUGH2, we find very good agreement of the simulated hydrothermal systems. Simulations at high‐temperature, high‐salinity conditions outside of the range of these models show the influence of solid halite on dynamic flow behavior and suggest that salt from magmatic degassing is not easily vented or accommodated within the crust.
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 ...H2O‐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 high‐pressure 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.
Sub-seafloor hydrothermal convection at mid-ocean ridges transfers 25% of the Earth's heat flux and can form massive sulfide ore deposits. Their three-dimensional (3D) structure and transient ...dynamics are uncertain. Using 3D numerical simulations, we demonstrated that convection cells self-organize into pipelike upflow zones surrounded by narrow zones of focused and relatively warm downflow. This configuration ensures optimal heat transfer and efficient metal leaching for ore-deposit formation. Simulated fluid-residence times are as short as 3 years. The concentric flow geometry results from nonlinearities in fluid properties, and this may influence the behavior of other fluid-flow systems in Earth's crust.
The origin of crustal‐scale silicic magmatism remains a matter of debate, and notable uncertainty exists concerning the physical mechanisms that drive ascent and emplacement of felsic magmas in upper ...crustal regions. A 2‐D numerical model demonstrates that injection of mantle‐derived mafic magma into a partially molten hot zone in the lower crust can drive felsic magma ascent and intrusion into upper crustal levels. The injection of mafic magma induces overpressure in the reservoir, which increases crustal stresses and triggers development of brittle/plastic shear zones, and can drive significant surface uplift. The emerging topography causes a nonuniform overpressure distribution in the reservoir and can trigger felsic magma ascent along crustal shear zones. Based on systematic numerical experiments, we investigate the influence of crustal strength and injection rate. The initial upper crustal strength controls the degree of crustal faulting and surface uplift and therefore whether felsic magma ascent can be initiated or not. The final upper crustal strength influences the depth and final style of felsic intrusion. The injection rate of mafic magma determines the time scale of overpressure growth and surface uplift stage. In contrast, the duration of the subsequent felsic ascent and intrusion emplacement stages remains nearly constant. Our results imply that mafic underplating and intrusion into the lower crust may not only be a prime control for the generation of felsic magmas in the lower crust but may also be an important physical driving mechanism for felsic magma ascent and intrusion into upper crustal levels.
Key PointsNumerical simulation of felsic magma ascent triggered by mafic injectionAscent and intrusive behavior of felsic magma governed by upper crustal strengthInjection rate of mafic governs the timescale of the system evolution
High‐resolution numerical simulations give clear insights into the three‐dimensional structure of thermal convection associated with black‐smoker hydrothermal systems. We present a series of ...simulations that show that, at heat fluxes expected at mid‐ocean ridge spreading axes, upflow is focused in circular, pipe‐like regions, with the bulk of the recharge taking place in the near‐axial region. Recharging fluids have relatively warm temperatures. In this configuration, the system maximizes its heat output, which can be shown to be linked to nonlinearity in the fluid properties. Furthermore, we present a series of simulations with different permeability scenarios. These show that when permeability contrasts are moderate, convection maintains this pipe‐like fluid flow structure. The permeability contrast has a dominant effect on flow patters only at early, immature, stages of convection, focussing upflow in high‐permeability regions and downflow in low‐permeability regions. In such early stages of convection, diffusive vent styles can emerge, which look remarkably similar to diffuse vent fields in natural systems. Finally, simulations in which permeability is defined as a function of temperature indicate that the brittle‐ductile condition is likely to occur at temperatures not lower than 650°C. At lower brittle‐ductile transition temperatures, the system cannot remove the heat delivered from the magma chamber and vent temperatures are substantially lower than 400°C. This result is in agreement with estimates of the brittle‐ductile transition temperature from rock‐mechanical studies and the occurrence of earthquakes in the oceanic lithosphere.
The pressure dependence of deuterium-hydrogen (D-H) fractionation in water to 500°C and 200 megapascals has been calculated from high-temperature, high-pressure spectroscopic data. Pressure effects ...have a maximum at the critical temperature of water (20 per mil between 22 and 200 megapascals). Even larger effects are predicted for vaporlike densities from molecular dynamics simulations and molecular orbital calculations. Pressure effects explain many of the large discrepancies in published mineral-water D-H fractionation curves. Possible applications to natural examples include mineral-water isotope geobarometry.
Various thermodynamic properties of H2O that are defined as pressure or temperature derivatives of some other variable, such as isothermal compressibility (β, pressure derivative of density), ...isobaric thermal expansion (α, temperature derivative of density), and specific isobaric heat capacity (cf, temperature derivative of enthalpy), all show large magnitudes near the critical point, reflecting large variations in fluid density and specific enthalpy with small changes in temperature and pressure. As a result, mass (related to fluid density) and energy (related to fluid enthalpy) transport in this PT region are sensitive to changing PT conditions. Addition of NaCl to H2O causes the region of anomalous behavior, here defined as the critical region, to migrate to higher temperatures and pressures. The critical region is defined as that region of PT space in which the dimensionless reduced susceptibility χ~ ≥ 0.5. When NaCl is added to H2O, the critical region migrates to higher temperature and pressure. However, the absolute magnitudes of thermodynamic properties that are defined as temperature and/or pressure derivatives (α, β, and cf) all decrease with increasing salinity. Thus, the mass and energy transporting capacities of hydrothermal fluids in the critical region become less sensitive to changing PT conditions as the salinity increases. For example, quartz solubility can be described as a function of fluid density, and because density becomes less sensitive to changing PT conditions as salinity increases, quartz solubility also becomes less sensitive to changing PT conditions as fluid salinity increases. Similarly, fluxibility describes the ability of a fluid to transport heat by buoyancy‐driven convection, and fluxibility decreases with increasing salinity. Results of this study show that the mass and energy transport capacity of fluids in the Earth's crust are maximized in the critical region and that the sensitivity to changing PT conditions decreases with increasing salinity.
Pure H2O exhibits anomalous behavior in the vicinity of the critical point, reflected by large variations in density and specific enthalpy with small changes in temperature and pressure. Mass and energy transport properties that are temperature or pressure derivatives of density and specific enthalpy thus show large variability near the critical point. Addition of NaCl to H2O causes the critical region in which fluid properties exhibit anomalous behavior to migrate to higher temperatures and pressures.
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