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


Steven Micklethwaite1, Stephen F. Cox1,2

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


This project applies the principles of the triggering mechanisms and triggering effects of active fault systems to understand gold mineralisation in ancient fault systems. Earthquakes generate small elastic stress changes, which in turn trigger other earthquakes and many thousands of aftershocks. Each aftershock is a fault slip event that enhances the permeability of the crust and high-frequencies of aftershocks tend to occur on faults with the same dimensions as those faults that host gold mineralisation in Australian gold camps. Previously, we have shown that at crustal depths of 10-20 km, orogenic-type gold deposits occur where co-seismic stress changes around a fault are likely to have triggered clusters of aftershocks (Micklethwaite and Cox, 2004, 2006; Micklethwaite, 2007). Therefore Stress Transfer Modelling helps us understand the dynamics of ancient fault systems and acts as a valuable predictive tool for the exploration industry.

Stress Transfer Modelling is now being extended to gold mineralised fault systems that developed in near-surface crustal environments (1-6 km) during episodes of normal faulting and high geothermal gradients, such as epithermal gold deposits and Carlin-type gold deposits. Active faulting in volcano-tectonic environments provides a modern day analogue for epithermal fault systems (e.g. Taupo Zone, New Zealand; Coso geothermal field, USA). Using a combination of detailed field work, contemporary observations of geothermal fields, plus stress transfer modelling, we have found there is a complex interaction between faulting, intrusive activity, and potential mineralisation. Fault slip events and permeability enhancement can be triggered by dyke intrusions, or other fault slip events, and these triggering mechanisms control fault growth and fault spacing. Importantly, dyke intrusion transiently changes local stress states, with a profound influence on fault kinematics, and the location permeability enhancement and mineralisation (figure 1).

In addition, collaboration with Dr Heather Sheldon (CSIRO, Perth, Western Australia) is enabling us to use Damage Mechanics Theory to understand why small stress changes generated by fault-slip events exert such a strong control on resulting aftershocks, fluid flow and ultimately fault-related mineralisation (Sheldon and Micklethwaite, 2007).

Figure 1. (A) 10000's of earthquakes (represented by black dots) were triggered by the intrusion of a dyke (white line) off the coast of Japan (Toda et al., 2002). The interconnected earthquake swarms represent massive permeability enhancement and Stress Transfer Modelling is able to predict their distribution by calculating the positive stresses (red) generated by intrusion of the dyke. Normal earthquakes were triggered on the flanks of the dyke and strike-slip earthquakes were triggered in the tails. If the dyke intrudes in a slightly different position, some of the normal faults can be reactivated as strike-slip faults. (B) Normal fault systems have complex linkages and fault shapes. Fluids can flow along-strike through fault jogs, but they can also flow upwards through fracture-related permeability in relay zones, and to a limited extent through damage zones associated with their fault cores. (C) Normal faults develop corrugated strikes. In mineralised regions the intrusion of a dyke can change the stress state on a fault from one decade to the next, leading to dramatic changes in fault-plane permeability when there is a slip event. This directly affects where mineralization will be focused.

 

Micklethwaite, S., (2007) The significance of linear-trends and clusters of fault-related mesothermal lode-gold mineralization, Economic Geology, 102, 000-000.
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.
Sheldon, H., Micklethwaite, S., (2007) Damage and permeability around faults: Implications for mineralization, Geology, 35, Issue 10, 903-906.