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,
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,
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
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.