Postmineralization faults in the Beaufor gold deposit, Abitibi greenstone belt, Canada: Geometry, origin, and tectonic implications for the Val-d'Or mining district

The Beaufor mine is a mesothermal vein-type gold deposit hosted by the Bourlamaque pluton, a synvolcanic intrusion crosscutting Archean volcanic rocks of the Abitibi greenstone belt ill the Val-d'Or mining district. The auriferous system is related to south-dipping reverse shear zones and consists of up to 2-m-wide sheared and extensional sulfide-rich quartz veins. The auriferous veins system is crosscut by two generations of faults, rast-west-trending, south-dipping (similar to 60 degrees) brittle-ductile reverse faults (e.g., the Dike Ol-Central fault system) and subvertical to steeply north-dipping brittle faults (e.g., the Beaufor fault system). The Dike 01- Central fault system presents structural fabrics, kinematic. and hydrothermal characteristics similar to those of auriferous shear zones. Both are interpreted as master faults and secondary structures, respectively; with the Dike 01-Central fault system acting as a channelway for the updip draining of ore fluids that were mainly trapped in low-angle synthetic shear zones. Incremental deformation and progressive decrease of bulk fluid pressure led to strain localization and faulting of auriferous quartz veins by the Dike Ol-Central fault system. The Beaufor fault system crosscuts all structures of the Beaufor deposit. It consists of a series of oblique-reverse, dextral Faults that lack evidence for pervasive hydrothermal alteration. The main structure of that system is the east-west-trending, north-dipping Beaufor fault, which is associated with northeast southwest-trending synthetic (i.e., West shear faults) and north-south-trending antithetic faults (i.e., Postore faults). Fault zones of the Beaufor mine are reminiscent of large-scale tectonics in the Val-d'Or mining camp, Auriferous quartz-tourmaline vein deposits are commonly associated with east- west-trending high-angle reverse shear zones and hydrothermal events coeval with, or slightly postdating, peak metamorphism and regional deformation. Several fault zones of the Val-d'Or area preserve evidence for both reverse shearing and dextral strike-slip faulting. Structures recorded in the Beaufor mine suggest that reverse and dextral trancurrent Faulting is genetically unrelated and probably resulted from sequential faulting rather than transpression.