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.

Abstract Metamorphic rocks produce fluids as devolatilization occurs during prograde metamorphism or as melts (which act as temporary repositories for fluids) crystallize during the early stages (>650°C) of cooling in high-grade metamorphic terranes. Metamorphosed shales and graywackes, which make up much of the sedimentary component of the upper crust, initially contain approximately 4 wt percent H 2 O, which may be liberated during the metamorphic cycle. These fluids may combine with others derived from external sources (e.g., synmetamorphic igneous intrusions or surface-derived fluids), and have the potential to transport heat, cause metasomatism, alter the rheology of the rocks, or form ore deposits. Metamorphic fluid flow in the crust is probably initially widespread, as fluids are derived from much of the rock mass, and then becomes increasingly channeled as fluids are focused along higher-permeability layers or along structures such as faults or shear zones. This type of flow path promotes ore genesis as metals can be scavenged from a large volume of rocks, with deposition occurring where fluids are focused and flowing down temperature. Most metamorphic fluids are dominated by H 2 O, with variable CO 2 and minor amounts of other species (e.g., F, Cl, B, and S). At high to moderate metamorphic grades, H 2 O and CO 2 are miscible at all XCO 2 values unless significant salt is present. Such fluids transport some metals (e.g., Cu, Au, Ag) relatively efficiently but not base metals. Thus, the variety of metamorphogenic ore deposits will be limited unless input of saline fluid from other sources (e.g., igneous bodies) occurs, or the terrane is composed of significant volumes of meta-evaporites. However, remobilization of preexisting orebodies may occur during metamorphism and deformation.