At depths greater than several kilometers in the crust, elevated temperature, elevated confining pressure, and the presence of reactive pore fluids typically drive rapid destruction of permeability in fractured and porous rock. Ongoing deformation is required to regenerate permeability and facilitate the high fluid flux necessary to produce hydrothermal ore systems. A dominant influence on the development of fluid pathways in hydrothermal systems is provided by stress states, fluid pressures, and reactions that drive permeability enhancement and compete with permeability destruction processes.

Fluid redistribution within hydrothermal systems at depth in the crust is governed largely by hydraulic gradients between upstream fluid reservoirs and the downstream regions of permeable networks of active faults, shear zones, and related structures that drain reservoirs. Pressure-driven flow leads to generally upward migration of fluids, although permeability anisotropy and tortuous flow paths may cause a significant along-strike component to fluid migration. Devolatilization reactions in prograding metamorphic regimes play a key role, not only in fluid production but also in generating transitory elevated permeability in deep crustal reservoirs. Active deformation and the development of high pore-fluid factors (the ratio of fluid pressure to vertical stress) in fluid reservoirs also drive permeability enhancement via grain-scale microfracturing and pervasive development of meso- to macroscale hydraulic fracture arrays. In the upstream, high-temperature parts of hydrothermal systems, pervasive fluid flow through the crust may occur via episodic migration of fluid-pressure pulses. Recent observations suggest that propagating fluid-pressure pulses may create transient permeabilities as high as 10^-13m² in the deep crust.

Flow focusing occurs wherever networks of active, high-permeability shear zones, faults, or other permeable structures, penetrate pressurized fluid reservoirs. These structures drain reservoirs and provide pathways for fluid redistribution to higher crustal levels. Contrasting styles of flow are expected between flow pathways in the aseismically deforming lower half of the crust and pathways within the seismogenic regime in the upper half of the crust. Below the seismic-aseismic transition, steady-state creep processes favor near -constant permeabilities and continuous fluid flow. In the seismogenic regime, large changes in fault permeability during the seismic cycle produce episodic flow regimes. In particular, large earthquake ruptures that propagate down from the upper crust into deeper level fluid reservoirs generate major, transitory perturbations to fluid pressure gradients. Episodic fluid redistribution from breached, overpressured (i.e., suprahydrostatic) reservoirs has the potential to generate large fluid discharge and high fluid/rock ratios around the downstream parts of fault systems after large rupture events. Hydrothermal self-sealing of faults, together with drainage of the hydraulically accessible parts of reservoirs between earthquakes, progressively shuts off flow along fault ruptures. Permeability enhancement due to rupture events may also drive transitory flow of fluids, derived from shallow crustal reservoirs, deep into fault zones after earthquakes.

As earthquakes migrate around fault systems in the upper , seismogenic part of the crust, permeability distribution and fluid pathways can evolve in complex ways. To achieve the necessary time-integrated fluid fluxes, the formation of large ore systems in this regime requires redistribution of fluid batches predominantly through small segments of fault systems during numerous rupture cycles. Sustained localized flow at the ore field scale is favored by development of long-lived, actively deforming, high-permeability structures such as fault jogs or fault intersections on high displacement faults. These structures can produce pipelike pathways linking deep reservoirs and shallower crustal levels. The generation of aftershock networks also influences fluid redistribution and discharge around the downstream ends of main-shock rupture zones. The distribution of these networks, and their repeated reactivation, is influenced by stress changes caused by main-shock rupture and by postseismic migration of fluid-pressure pulses away from the downstream ends of main-shock rupture zones.

At the deposit scale, in fracture-controlled hydrothermal systems, the highest fluid flux occurs where the apertures, densities, and connectivities of fractures are greatest. The locations and geometries of these sites are governed by fluid-driven permeability enhancement in structurally controlled sites such as jogs, bends, and terminal splays, typically in low displacement faults and shear zones, as well as by fault intersections, competence contrasts, and fold-related dilation. Permeability anisotropy in structural pathways can influence deposit-scale flow directions and shapes of ore shoots.