Mechanisms of faulting and permeability enhancement during epithermal mineralisation: Cracow goldfield, Australia

The geometries, kinematics, and failure mechanisms of epithermal fault-vein networks are examined at the Cracow goldfield, Queensland, Australia, and compared with observations of active geothermal areas. Quartz-carbonate cementation and precious metal mineralisation is confined to a network of steeply dipping faults (>70°). Breccia textures indicate fault rock formed from repeated events, involving large components of dilation during fracture, wall-rock fragmentation, and mineral precipitation. New fault rock tended to form on the margins of pre-existing fault rock, generating thick zones up to 10 m wide (fault cores) and accumulating up to 300 m normal offsets. The fault-vein networks are segmented, corrugated over tens of metres along strike, and contain complex fracture networks in step-over zones. Mineralisation is not associated with any specific fault location, but occurs along planar segments of faults, in step-over zones, at fault tips and where fault dips change. The wall rocks surrounding the faults contain networks of shear and extension veins, with a large range of orientations. Furthermore, kinematic indicators on the faults are broadly normal dip-slip, but vary to oblique and strike-slip with no obvious relationship to geometry or location along the fault. Inconsistent fault kinematics and the range of wall rock vein orientations are attributed to transient changes in stress state, due to intrusion of nearby dykes. As a result, permeability would have been enhanced by dilatancy in fault rock and wall rock fractures, when corrugated normal faults temporarily failed in oblique or strike-slip events. Other permeability enhancement mechanisms likely included failure driven by high fluid pressures, and failure where faults steepen (near the Earth's surface or at jogs). Mineralisation was associated with repeated, transient pulses of fluid flow rather than a steady-state process.