Geology, hydrothermal alteration, and genesis of the world-class Canadian Malartic stockwork-disseminated Archean gold deposit, Abitibi, Quebec

Abstract

The Canadian Malartic Mine consists of an Archean low-grade bulk tonnage gold deposit (reserves of 10.7 Moz Au @ 0.97 g/t) hosted by clastic metasedimentary rocks of the Pontiac Group and subalkaline porphyritic quartz monzodiorite and granodiorite located immediately south of the Larder Lake-Cadillac Fault Zone. The quartz monzodiorite and granodiorite yield syn-Timiskaming U-Pb zircon ages of 2677 and 2678 Ma, respectively. Gold mineralization is characterized by zones of quartz-carbonate vein stockwork and disseminated pyrite with a Au-Te-W-S-Bi-Ag}Pb}Mo metallic signature. These ore zones are dominantly oriented subparallel to the northwest-striking S2 foliation and to the east-striking and south-dipping Sladen Fault, thus forming northwest-southeast and east-west mineralized trends. Molybdenite from highgrade ore yielded a Re-Os age of ca. 2664 Ma. In both the sedimentary rocks and the quartz monzodiorite, the proximal and distal alteration zones are characterized by the presence of calcite and ferroan dolomite, respectively. In the sedimentary rocks, the ore zones show a wide distal biotite alteration halo with proximal assemblages comprising albite and/or microcline. The quartz monzodiorite comprises a distal hematitic alteration zone that is overprinted by proximal microcline + albite + quartz replacement zones. This study suggests that at Canadian Malartic deposit, the gold mineralization and its distribution are largely controlled by D2 faults, shear and high-strain zones developed in the hinge zone of F2 folds, and by the Sladen Fault. A ?2678 Ma syn-Timiskaming magmatic-hydrothermal early phase of gold mineralization can be inferred by the metallic signature or the ore, the presence of mineralized stockworks, the potassic alteration (biotite/microcline), and association with porphyritic intrusions. The main characteristics of the Canadian Malartic deposit is thus best explained by syn-D2 deformation gold (ca. 2670-2660 Ma) superimposed onto, or remobilized from, a gold-bearing magmatic/hydrothermal system related to Timiskaming-age porphyritic intrusions emplaced along a major fertile fault zone.