New faults v. fault reactivation: implications for fault cohesion, fluid flow and copper mineralization, Mount Gordon Fault Zone, Mount Isa District, Australia

Abstract

Abstract Fluid flow leading to mineralization can occur both on newly formed faults and on faults that are reactivated subsequent to their initial formation. Conventional models of fault reactivation propose that, under high pore-fluid pressures, misorientated faults may reactivate due to low fault cohesion. Timing and orientation data for a mineralized Palaeo- to Mesoproterozoic terrain (Mount Gordon Fault Zone (MGFZ)) indicate that multiple successive new orientations of predominantly strike-slip faults developed (between 1590 and c . 1500 Ma), requiring that during the younger deformations some earlier formed faults were too cohesive and/or had insufficient pore-fluid pressures (or other potential fault-weakening effects) to induce reshear. Low pore-fluid pressures were probably not to blame for failed reactivation on all older faults because some young faults did form or reactivate due to high pore-fluid pressures, as evidenced by jigsaw-fit dilatant breccias, hypogene copper mineralization in veins and breccia infill, and subvertical tensile quartz veins aligned subparallel to σ 1 . The assumption that old faults consistently have little or no cohesion appears to be incorrect in this terrain. Many older faults display prominent quartz veins along their length, which may explain this conclusion. Furthermore, faults with high cohesion may have acted as barriers and compartments, so that intersections between them and newly formed faults host mineralization, not because of reactivation, but because of interaction between new faults and cohesive materials defined either by fault precipitates or rock juxtaposition. Together, these results and observations provide new, simple tools to stimulate copper exploration within the region and in fault-hosted terrains.