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The mechanical and fluid pressure evolution of the Argo fault zone, St Ives goldfield, Western Australia: an example of an Archaean, shear-hosted, mesothermal gold system

Matthew A. Crawford and Stephen F. Cox

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

The development of low displacement, moderate to high-angle reverse faults during the formation of the Argo gold deposit, within a tholeiitic gabbro host-rock, involved a four stage evolution of deformation and associated hydrothermal alteration.  Fault zone evolution and Au mineralization were associated with high fluid flux, fault-valve behaviour in a severely-misoriented fault zone.  The far-field maximum principal stress was approximately east-west and horizontal, and the far-field minimum principal stress was sub-vertical.  The fault system developed at approximately 400°C in a transitional brittle-ductile regime, near the base of the continental seismogenic regime. Initial Stage 1 deformation involved ductile shear and the development of potassic (biotite-rich) alteration assemblages and associated reaction-weakening in shear zones; few quartz extension veins were formed. Stage 2 is marked by onset of predominantly brittle shear failure at elevated pore fluid factors, and was associated with widespread development of matrix-supported, dilational breccias in fault zones, and a change to sodic (albite-carbonate-quartz) alteration styles. Extension veins have limited development. Stage 3 is also dominated by brittle shear failure, and characterized by a change to quartz-carbonate assemblages in fault-fill veins and breccias. In contrast to Stage 2, large arrays of extension veins are well-developed adjacent to faults.  In Stage 4, widespread sub-horizontal quartz-carbonate-biotite extension veins were developed, but shear failure was limited.

Failure mode diagrams in pore fluid factor ~ differential stress space (Figure 1) illustrate how the structural evolution and styles of mineralisation in the Argo fault system reflect a response to progressive changes in relative rates of change of pore fluid factor and differential stress during individual fault-valve cycles. High fluid fluxes and rapid rates of recovery of fluid pressures, relative to rates of recovery of shear stress after slip events, have maintained the system at near-lithostatic fluid pressures and very low differential stresses during gold mineralization.

The structural and rock mechanics study has been complemented by detailed microstructural, microchemical and stable isotope studies of hydrothermal alteration and vein mineral assemblages to characterise variations in the intensity and style of alteration in space and time in the Argo shear system.  This work, together with analysis of gold grade distribution, is providing new insights about structural and geochemical controls on gold deposition at scales ranging from the deposit scale, down to metre-scales.