The St Ives mesothermal gold system, Western Australia - a case of golden aftershocks?

Abstract

The late orogenic evolution of the Archaean greenstone sequence in the Eastern Goldfields Province of the Yilgarn Craton in Western Australia is characterised by strike-slip tectonics and locally very high, fault-controlled, fluid fluxes. Fluid flow was associated with the formation of many fault-hosted and shear-zone-hosted gold deposits, which are commonly clustered adjacent to high displacement faults or shear zones. In the St Ives goldfield, near Kambalda, fluid flow in a gold-producing hydrothermal system was localised within arrays of low displacement faults and shear zones, which form part of the NNW trending, crustal-scale, Boulder-Lefroy fault system (BLFS). The numerous ore-hosting structures are kinematically related to sinistral to sinistral-oblique slip on the Playa Fault, which is a 20-km-long splay of the 200-km-long Boulder-Lefroy Fault. Most of the known gold mineralisation at St Ives occurs within an area of 20 km2 immediately south-west of the Playa Fault. The distribution of low displacement faults and shear zones that host gold mineralisation is related to the presence of a kilometre-scale contractional jog (the Victory jog) and an associated imbricate thrust array on the Playa Fault. By analogy with modern seismogenic systems, the low displacement structures that localised fluid flow and gold mineralisation in the St Ives goldfield are interpreted as aftershock structures whose development was driven by major slip events on the BLFS. For large slip events on the BLFS, and mainshock rupture arrest at the Victory jog, modelling of co-seismic static stress changes indicates that most ore-hosting structures are localised within a crustal volume whose stress state was closer to failure as a consequence of Coulomb stress transfer. The modelling supports an interpretation that aftershock networks can form a high permeability damage zone that localises fluid flow and gold mineralisation within particular parts of crustal-scale fault systems. Both co-seismic stress transfer and time-dependent changes in fluid pressures, during post-seismic fluid redistribution, are implicated in driving the growth of low displacement, gold-hosting fault networks in the St Ives goldfield. Stress transfer modelling has application for area selection in exploration programs targeting mesothermal gold systems. Clustering of deposits hosted by aftershock fracture networks is favoured by the presence of major, long-lived jogs or bends that can repeatedly arrest ruptures propagating along high displacement faults.