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

Steven Micklethwaite1,2, Stephen F. Cox1

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Now at: CODES, Private Bag 126, University of Tasmania, Hobart, TAS 7001, 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 the Carlin-type gold deposits.

Both earthquakes and intrusive events generate small elastic stress changes in the crust, which have been linked to the triggering of thousands of aftershocks and the enhancement of permeability. We developed stress transfer modelling (STM) to understand the dynamics of ancient fault systems and act as a valuable predictive tool for the exploration industry (Micklethwaite and Cox, 2004, 2006; Micklethwaite, 2007). We have also been able to link co-seismic stress changes to wall rock damage generation and permeability, using Damage Mechanics Theory (Sheldon and Micklethwaite, 2007).

In 2008, we applied STM to near-surface hydrothermal environments (0.5-2 km; Carlin goldfield, Nevada), and deep hydrothermal environments (15-25 km; Agnew goldfield, West Australia). In the Carlin goldfield, mineralisation is broadly stratiform but also related to the upper tips of ~5 km-long normal dip-slip faults. Debate exists as to whether fluid migration was controlled by active fault processes, reaction-enhanced porosity generated in specific lithologies, or convection through fracture networks that were indefinitely open due to the low confining stresses of near-surface environments. In the Agnew goldfield, mineralisation forms a linear trend of pod-like bodies on the western limb of a regional fold. Fault rocks containing ore-grade mineralisation are dominated by ductile shear textures, with only small percentages of brittle deformation textures such as breccias and veins. Metamorphic grades suggest the goldfield formed in the mid-crust possibly at the base of a structure, but it was not clear whether visco-elastic creep processes, or co-seismic damage controlled fluid migration and mineralization.

In both case studies, STM predictions were made in three dimensions, and it was demonstrated that mineralization could be matched by the unique stress patterns generated at the tips of fault ruptures; indicating co-seismic elastic stress changes are a first-order control on permeability enhancement and mineralisation. This potentially represents a breakthrough for Carlin-related research and promises to resolve a long-standing debate.

Micklethwaite, S. and Cox, S.F., 2004. Fault segment rupture, aftershock-zone fluid flow, and mineralization.  Geology, 32, 813-816.
Micklethwaite, S. and Cox, S.F., 2006.  Progressive fault triggering and fluid flow in aftershock domains.  Earth and Planetary Science Letters, 250, 318-330.
Micklethwaite, S., 2007. The significance of strings and clusters of fault-related mesothermal lode gold mineralization. Economic Geology. 102, issue 6, 1157-1164.
Sheldon, H.A. and Micklethwaite, S., 2007. Damage and permeability around faults: Implications for mineralization. Geology, 35, 903-906.