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Abstract. We investigate groundwater flow near cooling plutons with a computer program that can model multiphase flow, temperatures up to 1200øC, thermal pressurization, and temperature-dependent rock properties. A series of experiments examines the effects of host-rock permeability, size and depth of pluton emplacement, single versus multiple intrusions, the influence of a caprock, and the impact of topographically driven groundwater flow. We also reproduce and evaluate some of the pioneering numerical experiments on flow around plutons. Host-rock permeability is the principal factor influencing fluid circulation and heat transfer in hydrothermal systems. The hottest and most steam-rich systems develop where permeability is of the order of 1045 m 2. Temperatures and life spans of systems decrease with increasing permeability. Conduction-dominated systems, in which permeabilities are <1046 m 2, persist longer but exhibit relatively modest increases in near-surface temperatures relative to ambient conditions. Pluton size, emplacement depth, and initial thermal conditions have less influence on hydrothermal circulation patterns but affect the extent of boiling and duration of hydrothermal systems. Topographically driven groundwater flow can significantly alter hydrothermal circulation; however, a low-permeability caprock effectively decouples the topographically and density-driven systems and stabilizes the mixing interface between them thereby defining a likely ore-forming environment.

The permeability of continental crust is so highly variable that it is often considered to defy systematic characterization. However, despite this variability, some order has been gleaned from globally compiled data. What accounts for the apparent coherence of mean permeability in the continental crust (and permeability-depth relations) on a very large scale? Here we argue that large-scale crustal permeability adjusts to accommodate rates of internal and external forcing. In the deeper crust, internal forcing -fluxes induced by metamorphism, magmatism, and mantle degassing -is dominant, whereas in the shallow crust, external forcing -the vigor of the hydrologic cycle -is a primary control. Crustal petrologists have long recognized the likelihood of a causal relation between fluid flux and permeability in the deep, ductile crust, where fluid pressures are typically near-lithostatic. It is less obvious that such a relation should pertain in the relatively cool, brittle upper crust, where nearhydrostatic fluid pressures are the norm. We use first-order calculations and numerical modeling to explore the hypothesis that upper-crustal permeability is influenced by the magnitude of external fluid sources, much as lower-crustal permeability is influenced by the magnitude of internal fluid sources. We compare model-generated permeability structures with various observations of crustal permeability.

Near‐critical extrema in the properties of water may influence flow patterns in hydrothermal systems, but singularities in equations of state for H2O at its critical point have inhibited quantitative modeling. Posing governing equations in terms of pressure (P) and enthalpy (H) avoids these singularities and facilitates computation. Numerical simulations with a P‐H based model show little near‐critical enhancement in heat transfer for systems in which flow is driven by fixed pressure drops. However, in density‐driven systems, near‐critical variations in fluid properties can enhance convective heat transfer by a factor of 10² or more (“superconvection”) if permeability is sufficiently high. Near‐critical two‐phase processes (“heat pipes”) are at least equally effective at dissipating thermal energy. The restriction to high‐permeability environments within a fairly narrow P‐H window suggests that superconvection may be quite rare in natural systems

Source Rock Maturity Immature (%R o values < 0.6) Oil window (%R o values 0.6 to 1.3) Gas window (%R o values 1.3 to 2.0) Overmature (%R o values > 2.0) Outline of basin Outline of maximum extent of Devonian shale-Dashed where concealed Sample with %R o value Sample with conodont CAI value Sample with SCI value 0 50 100 150 200 250 miles 0 50 100 150 200 250 300 350 kilometers Michigan basin Illinois basin Appalachian basin Conodont Color Alteration Index (CAI) Liptinite Fluorescence Thermal Alteration Index (TAI) Hydrocarbon Generation / Preservation Events Vitrinite Reflectance in %R o (mean)