Depositional Setting and Structural Evolution of the Archean Perseverance Volcanogenic Massive Sulfide Deposit, Matagami Mining District, Quebec, Canada

Abstract

The Perseverance volcanogenic massive sulfide deposit (5.1 Mt at @ 15.8% Zn, 1.24% Cu, 29.4 g/t Ag, 0.4 g/t Au), Matagami district, Abitibi greenstone belt, consists of four pipelike orebodies discordant to local bedding in a shallow-dipping portion of the South Flank volcanic succession. Most of the ore is hosted by rhyolitic lavas of the Watson unit (2725.9 ± 0.8 Ma) and is overlain by a thinly laminated tuffaceous unit known as the Key Tuffite, as well as by rhyodacitic lavas of the Dumagami unit (2725.4 ± 0.7 Ma). The presence of sulfide zones predominantly hosted in the footwall rhyolite, relicts of Key Tuffite within the ore, and intricate sulfide replacement of the laminated tuff are all features consistent with the formation of most of the deposit via subseafloor replacement. The permeability of steep synvolcanic structures likely controlled the migration of metal-bearing fluids and simultaneously allowed the downward infiltration of seawater, causing mixing, cooling, and sulfide deposition in the subseafloor environment. Stratigraphic relationships suggest that massive sulfide formation was active at Perseverance while tuffaceous sedimentation was ongoing on the seafloor. Progressive growth of the massive sulfides along subvertical structures was accompanied by the development of pipelike sericite-chlorite (± talc) alteration halos. Both the hanging wall and the footwall units are characterized by significant MgO mass gains, K 2 O-Na 2 O mass losses, and high alteration index values (e.g., chlorite-carbonate-pyrite index, alteration index). However, these geochemical changes are less intense and widespread in the hanging wall than in the footwall. The record of a decreasing alteration above the orebodies was likely caused by the emplacement of the rhyodacite while massive sulfide deposition and hydrothermal activity were still ongoing. Primary relationships between mineralization and the host lithofacies were overprinted by intense deformation during the main event of compression induced by regional north-south shortening. Deformation is controlled by contrasting rheologies within the sulfide assemblage and between the orebodies and their host rocks. Structural modifications include the transposition of sulfide ores subparallel to the main foliation, the formation of piercement veins at the interface with the host rocks, and the generation of a series of secondary textures within the orebodies, including a vertical mineralogical banding. The distribution of Cu in the deposit is interpreted to be affected by the mechanical remobilization of chalcopyrite during deformation, whereas the zonation of Zn is likely primary. Hydrothermal alteration halos adjacent to the orebodies have accommodated significant strain, as indicated by the presence of tight folds in the Key Tuffite unit and a marked schistosity. Strain localization in the vicinity of the deposit highlights the relative ductility of the ore assemblage with respect to the host volcanic succession and resulted in a spatial association between hydrothermal alteration and deformation, despite the lack of a genetic relationship.