Exploration potential of stress transfer modelling in fault-related mineral deposits

Steven Micklethwaite 1 , Stephen F. Cox 1,2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Department of Earth and Marine Science, Australian National University , Canberra , ACT 0200, Australia

Transient permeability enhancement processes such as faulting and fracturing, are expected to be very important controls for fluid flow and the formation of mineral deposits in the crust. This is because rates of reaction and sealing in hydrothermal environments are extremely rapid and wall-rocks typically have intrinsically low permeabilities. By coupling structural field observations with simple Stress Transfer Models of fault-slip we aim to correlate domains of known mineralisation with areas where faulting and fracturing is predicted to have been triggered repeatedly over the lifetime of a fault system. Stress Transfer Modelling (STM) was developed by the USGS and uses boundary-element code to calculate the changes in static stress generated by fault-slip events. STM has proven valuable in predicting the spatial distribution of aftershocks and the triggering of earthquakes for hazard assessment of active fault systems. This is achieved by calculating the positive stress changes that bring pre-existing fault networks closer to failure, thus triggering aftershocks.

The inference that step-over regions in ancient fault systems were significant geometrical barriers to rupture propagation has enabled us to apply STM to fossil fault systems. The model shown displays the distribution of co-seismic stress changes generated by fault-slip events on regional shear systems associated with the New Celebration goldfield, Western Australia. Faults present in domains of positive stress change are brought closer to failure by regional fault slip events. A good correlation is observed between the location of fault-hosted gold deposits and those domains where positive stress changes are predicted, suggesting an important link exists between permeability generated by earthquake-aftershock behaviour and the migration of mineralising fluids. By using this approach in a number of studies fault-triggering has been identified as a first-order control on mid-crustal fluid flow (Micklethwaite and Cox, 2004, 2006). We are now extending our work to volcano-structural regimes, where the interaction of fault ruptures and dyke intrusions has led to the development of epithermal gold deposits.

In addition, a collaboration with Dr Heather Sheldon (CSIRO, Perth , Western Australia ) is enabling us to understand why small stress changes generated by fault-slip events exert such a strong control on resulting aftershocks and ultimately fault-related mineralisation. Experimental deformation results show that as rock is stressed large numbers of microcracks are generated that eventually coalesce into through-going failure planes. This behaviour is described and modelled by Damage Mechanics. Coupling Damage Mechanics and STM explains the temporal and spatial decay of aftershocks over time with only small elastic stress changes, and may allow us to estimate rates of fluid flow through fault systems. The success of Stress Transfer Modelling and Damage Mechanics in explaining the temporal and spatial distribution of aftershocks suggests that, near plate boundaries, the earth's crust is in a near-constant state of criticality.

References: Micklethwaite S. and Cox, S.F. (2004) Fault-segment rupture, aftershock zone fluid flow and mineralization. Geology, 32, 866-870.

Micklethwaite , S., and Cox, S.F., (2006) Progressive fault triggering and fluid flow in aftershock domains: Examples from mineralized Archaean fault systems. Earth and Planetary Science Letters , 250, 318-330.