Abstract Many ore-producing hydrothermal systems form within intrinsically low permeability host rocks during fracture-controlled flow in overpressured fluid regimes. The generation and localization of fracture-controlled fluid pathways in these systems involves dynamic coupling between fluid flow, fluid pressures, stress states, and deformation processes. In high fluid flux settings, fracture-controlled permeability enhancement is driven largely by fluid pressurization rather than by tectonic loading. The orientation of the stress field plays a critical role in governing the orientations of activated fractures. Permeability destruction by fracture sealing and cementation of fragmented rock is rapid relative to the lifetimes of hydrothermal systems. Accordingly, repeated regeneration of permeability is necessary to sustain the high fluid fluxes required for ore formation. The evolution of permeability is thus controlled by a dynamic competition between permeability enhancement processes and permeability destruction processes. During fluid pressurization, the failure modes, and hence growth of fluid pathways, are particularly sensitive to differential stress and the relative cohesive strengths of faults and intact rock. The fluid pressure, stress regimes, and mechanical properties of host rocks thus influence whether deposit styles are dominated by extension veins, fault-fill lodes in optimally oriented or unfavorably oriented faults, or lode development in viscous shear zones. Many fracture-controlled hydrothermal systems in intrinsically low permeability host rocks form at very low differential stresses and near-lithostatic fluid pressure regimes. Large-scale fluid injection experiments and contemporary seismicity in fluid-active settings indicate that the characteristic response to injection of large volumes of overpressured fluids into fault zones in low-permeability host rocks is earthquake swarm seismicity. Injection-driven swarm sequences enhance permeability via thousands of microseismic slip events over periods of days to many weeks. The accumulation of net slip in ore-hosting faults involves up to thousands of separate swarm sequences. Injection-driven earthquake swarms provide a very dynamic hydrothermal environment for ore formation. Incremental growth of ore deposits occurs during short bursts of high fluid flux during swarm sequences that are separated by long intervening periods in which there is little or no flow. Rapidly recurring slip events during swarms drive repeated and rapid changes in fluid pressures, flow rates, and stresses. If injection-driven growth of a fracture network breaches a hydrologic barrier between differently pressurized regimes, ensuing rapid depressurization can be a key driver of ore deposition. Although shear failure is an inherently dilatant process that increases permeability by up to many orders of magnitude, permeability distribution in fault zones is extremely heterogeneous. Permeability enhancement in active fault zones is favored by the presence of relatively competent host rocks. Permeability is particularly enhanced within some types of fault stepovers and bends. Fracture damage around rupture termination zones, fault branch lines, and fault intersections may also generate high fluid flux pathways. Directions of flow anisotropy along predominantly linear, high-dilation damage zones in faults are strongly influenced by fault kinematics. Permeability, fluid pressures, and flow rates evolve dynamically during injection-driven rupture sequences. Changes in flow rates and fluid pressures during the lead-up to a swarm, during rupture sequences themselves, and immediately after cessation of a swarm have impacts on ore deposition processes such as gradient reactions, fluid-rock reaction, phase separation, and fluid mixing. Fluid pressurization in the lead-up to a rupture sequence enhances within-fault permeability and may promote aseismic growth of extension fracture arrays. Repeated microseismic slip events dramatically and locally enhance permeability, cause sudden fluid pressure drops in the rupture zone, and transiently disrupt flow patterns. Rupture propagation is associated with coseismic dynamic fracture damage that further enhances permeability, especially in fault sidewalls and near rupture terminations. Immediate postrupture permeability enhancement can be associated with implosion processes and stress relaxation around rupture terminations. Major loss of permeability is associated with fracture sealing during rapid depressurization in the immediate aftermath of swarms. During successive rupture sequences, changes in permeability distributions in faults are expected to lead to complex changes in flow paths. Within individual faults, the highest fluid fluxes tend to be localized within long-lived fracture damage sites that are repeatedly reactivated over a substantial part of the lifetime of a hydrothermal system.

Abstract In the Taemas area, New South Wales, Australia, a swarm of hydrothermal calcite and quartz veins is hosted in upright, open to close folded limestones and shales. Overprinting relationships and vein geometries demonstrate that the vein swarm formed progressively during fold growth and associated reverse faulting. Textures preserved in veins reveal that veins formed via hundreds to thousands of individual dilation and mineral precipitation events. Bedding-parallel flexural slip during fold growth was associated with laminated vein development, and limb-parallel stretching during fold growth was associated with the formation of bedding-orthogonal extension veins. The presence of subhorizontal extension fractures and severely misoriented reverse faults imply that fluid pressures exceeded lithostatic levels, at least transiently, during the development of the vein swarm. Vein δ 18 O compositions increase upwards through the Murrumbidgee Group in response to a progressive reaction of an externally derived, upwards-flowing low-δ 18 O fluid (of probable meteoric origin) with host limestones. Vein δ 18 O and 87 Sr/ 86 Sr compositions vary spatially and temporally within the same outcrop, and within individual veins. These variations are inferred to be caused by the ascent of packages of fluid along constantly changing flow pathways caused by multiple permeability creation–destruction cycles associated with fault slip and fault sealing. Vein trace and rare earth element (REE) concentrations are more variable, probably reflecting rapid rock buffering along fluid pathways on length scales of less than 10 m. Our results indicate that fluid-flow pathways change dynamically during crustal shortening, with pathways switching between states of low and high permeability during episodic fault slip and associated fracture development. Supplementary material: Two appendices are available at http://www.geolsoc.org.uk/SUP18492 .

Abstract 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 -13 m 2 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.