Permeability and fluid
pathways in fracture-controlled hydrothermal systems are governed by a dynamic
competition between permeability-creation processes and permeability-destruction
processes. Permeability evolution is coupled with the evolution of fluid
pressure and stress states. Key permeability-creation processes are micro-scale
to macroscopic fracture growth, and the generation of damage zones during
co-seismic slip on faults or aseismic creep on faults and shear zones. The
competing permeability-destruction processes include crack-healing and sealing,
compaction and pore cementation. High rates of permeability destruction in
hydrothermal systems require ongoing permeability enhancement to sustain high
fluid fluxes and ore genesis.

In
aseismic regimes, competition between permeability-creation processes and
permeability-destruction processes can lead to continuous flow in deep level
hydrothermal systems. However, in seismogenic regimes, rapid co-seismic
permeability-enhancement is followed by progressive permeability-destruction in
the intervening interseismic periods, and leads to episodic flow. The results of
high pressure experiments are used to illustrate how pore fluid factors and
stress states
are
dynamically coupled with permeability evolution in hydrothermal
regimes.
Particularly in the
seismogenic mid- to upper crust, large changes in permeability during the
seismic cycle govern relative rates of change of fluid pressure around active
faults and shear zones. Rapid, post-seismic recovery of pore fluid pressures,
relative to rates of shear stress recovery, leads to growth of hydrofracture
networks prior to successive fault rupture events.

Pre-rupture
hydrofracture networks do not develop where post-rupture fluid pressure recovery
is modest relative shear stress recovery. At elevated temperatures, low rates of
pore fluid pressure recovery relative to shear stress recovery promote ductile
shear failure and grain-scale permeability enhancement within shear zones prior
to rupture events.

The
influence of pore fluid pressure cycling, shear stress cycling, and deformation
processes in controlling the evolution of permeability, flow localisation, and
flow anisotropy at the deposit scale are illustrated for mesothermal and
epithermal lode gold systems.