Chapter 2: The Dynamics of Permeability Enhancement and Fluid Flow in Overpressured, Fracture-Controlled Hydrothermal Systems
Authors: Cox Stephen F. 
Published: March 2021

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.