Application of Failure Mode Diagrams to the Formation of Orogenic Gold Deposits in the Classic Val-d’Or Gold District, Canada, With Emphasis on the Newly Discovered Ormaque Deposit

Abstract The Ormaque gold deposit is located within the Val-d’Or mining district of the Archean Abitibi greenstone belt, Quebec, where the influential fault-valve model for the formation of orogenic gold deposits was first applied. The deposit is hosted within the synvolcanic C porphyry, which was emplaced in volcaniclastic rocks of the ca. 2704 Ma Val-d’Or Formation. Ormaque consists of gently dipping, extensional quartz-tourmaline-carbonate veins flanking reverse faults. The veins are surrounded by alteration halos, which contain tourmaline, carbonate, and pyrite; these are locally overprinted by late albite and pyrite. Native gold and Au-Ag-Te-Bi–bearing polymetallic minerals occur as inclusions and fracture-fill in pyrite, tourmaline, and carbonate, and as disseminated grains along grain boundaries. Ormaque is dominated by extensional veins and differs from other Val-d’Or deposits, which are either dominated by fault-fill veins (e.g., Triangle, Plug No. 4, and Mine No. 3) or comprise both fault-fill and extensional veins in more equal proportions (e.g., Sigma, Lamaque, and Mine No. 2 deposits). All deposits are hosted by misoriented faults, which either cut through ca. 2685 Ma competent plugs (first group of deposits) or C porphyry (second group of deposits). Ormaque further differs from these deposits by a near lack of fault-fill veins and its more severely misoriented, steeply N- and S-dipping faults. The latter are reactivated, and extensional veins form at supralithostatic fluid pressures and low differential stress. With increasing misorientation angle, the maximum differential stress for shear reactivation of faults decreases, and the range of differential stresses over which extensional veins can form increases. The higher reactivation angles (80°) of the faults at Ormaque likely favored the formation of extensional veins rather than fault-fill veins over a wider range of differential stress and resulted in a predominance of extensional veins. Using failure mode diagrams, the study highlights how differences in reactivation angles may relate to the reactivation of preexisting high-angle faults and rotation of new faults during regional horizontal shortening, which led to differences in deposit style across a single mining district.