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Coupling between deformation processes and fluid flow in the Earth’s crust

Stephen F. Cox

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

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.