Stephen F. Cox
Experimental, field-based, microstructural and numerical modelling
approaches are being used to explore several aspects of coupling
between deformation processes, fluid transport and reaction, especially
in fracture-controlled flow regimes.
In 2008, field-based, modelling, microstructural and microchemical
studies are being used to further explore the growth of faults and
fracture networks, and what factors lead to fluid flow becoming localised
in certain parts of fracture controlled hydrothermal systems. The
research has implications for understanding the localisation of hydrothermal
ore deposits within fracture systems, and with minerals industry
support, is providing new tools to help enhance exploration strategies.
The work is also providing fundamental insights about the roles of
reactive pore fluids in controlling fault mechanics and the growth
of fracture networks.
Field and modelling studies, in the North Carlin gold systems
in Nevada, have evaluated the role of co-seismic static stress changes
in governing the distribution of aftershock activity in controlling
localisation of fluid flow and related gold mineralisation within
particular parts of fault networks.
A study of an Archaean, mesothermal gold system near Kambalda (WA),
has shown how, during fault-valve behaviour near the base of the
continental seismogenic regime, the relative rates of recovery of
shear stress and pore fluid factors after slip events, impact on
the internal structure of fault zones, as well as the nature and
distribution of gold mineralisation.
In intrusion-related hydrothermal systems, the evolution of fluid
pathways and the geometry, distribution, and other characteristics
of vein systems, are governed by interactions between stress and
fluid pressure states, and by the orientation of stress fields during
and after magma emplacement. Stress states and the orientations of
stress fields within active intrusive systems respond very dynamically
to repeated cycles of inflation and deflation of fluid pressures
due to migration of magma and hydrothermal fluids. Repeated
variations in stress magnitudes and orientations can also be driven
by sudden co-seismic stress relief and more gradual interseismic
stress recovery associated with episodic, large earthquakes in convergent
margin settings. Additionally, geodetic and seismological studies
have demonstrated that episodic fluid migration, as well as cyclic
changes in the orientations and magnitudes of stresses, occur on
time-scales of years to decades in contemporary magmatic systems. This
occurs especially in response to eruption cycles, emplacement of
dyke swarms, and effects of nearby earthquakes. Indeed, stress
change due to magma migration is likley to be a major driver of seismicity,
and associated development of fracture-controlled fluid pathways,
up to 15 km from the sites of magma movement. Small dynamic stress
changes associated with remote, but large, earthquakes can also trigger
microseismicity and attendant fracture propagation and fluid movement
in magmatic systems.
A new project, as part of the ANU node of the Centre of Excellence
in Ore Deposits, is using this modern understanding of the dynamics
of magmatic systems as a basis for undertaking structural, microstructural
and alteration studies to analyse the evolution of stress and fluid
pressure states during the development of vein systems and faults in
intrusion-related hydrothermal systems. The broad goal is to understand
how the dynamic evolution of fracture-controlled fluid pathways impacts
on the styles of flow and ore deposition, as well as the distribution
of mineralisation. In 2008, attention has focussed on the giant Porgera
gold deposit (in the far western PNG highlands). Projects on other
intrusion-related systems are being developed for subsequent years. A
key early result is that fluid flow, in such fracture-controlled hydrothermal
systems, is probably controlled by episodic breaching of substantially
overpressured, magmatic fluid reservoirs at depth. Fluid pressure
fluctuations are associated with repeated cycles of reservoir breaching
and episodic, fluid-driven growth and sealing of fracture networks. These
processes have important implications for ore deposition. In
particular, large, transient hydraulic gradients promote rapid flow
and potentially severe chemical disequilibrium in the ore fluid.