THE MECHANICAL AND FLUID PRESSURE EVOLUTION OF THE ARGO FAULT ZONE, KAMBALDA, WESTERN AUSTRALIA: AN EXAMPLE OF AN ARCHEAN, SHEAR-HOSTED, MESOTHERMAL GOLD SYSTEM

The development of low displacement, moderate to high-angle reverse faults during the formation of the Argo gold deposit involved a four stage evolution of deformation and associated hydrothermal alteration within a tholeiitic gabbro host-rock. Fault zone evolution and Au mineralization was associated with high fluid flux, fault-valve behavior in a stress regime in which the maximum principal stress was approximately east-west and horizontal, and the minimum principal stress was sub-vertical. The fault system developed at approximately 400°C in a transitional brittle-ductile regime. Initial Stage 1 deformation involved ductile shear and the development of potassic (biotite-rich) alteration assemblages and associated reaction-weakening; minor quartz extension veins were formed. Stage 2 is marked by onset of predominantly brittle shear failure, the widespread development of matrix-supported breccias in fault zones, and sodic (albite-carbonate-quartz) alteration styles. Extension veins have limited development. Stage 3 is characterized by a change to quartz-carbonate assemblages in fault-fill veins and breccias, and the widespread development of large extension vein arrays. 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 are used to illustrate how the structural evolution of the Argo fault system was 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.