Abstract The Malartic gold camp is located in the southern part of the Archean Superior Province and straddles the Larder Lake-Cadillac fault zone that is between the Abitibi and Pontiac subprovinces. It comprises the world-class Canadian Malartic deposit (25.91 Moz, including past production, reserves, and resources), and smaller gold deposits located along faults and shear zones in volcanic and metasedimentary rocks of the Abitibi subprovince. North of the Larder Lake-Cadillac fault zone, the Malartic camp includes 2714 to 2697 Ma volcanic rocks and ≤2687 Ma turbiditic sedimentary rocks overlain by ≤2679 to 2669 Ma polymictic conglomerate and sandstone of the Timiskaming Group. South of the fault, the Pontiac subprovince comprises ≤2685 Ma turbiditic graywacke and mudstone, and minor ultramafic to mafic volcanic rocks and iron formations of the Pontiac Group. These supracrustal rocks were metamorphosed at peak greenschist to lower amphibolite facies conditions at ~2660 to 2658 Ma, during D 2 compressive deformation, and are cut by a variety of postvolcanic intrusions ranging from ~2695 to 2640 Ma. The Canadian Malartic deposit encompasses several past underground operations and is currently mined as a low-grade, open-pit operation that accounts for about 80% of the past production and reserves in the camp. It dominantly consists of disseminated-stockwork replacement-style mineralization in greenschist facies sedimentary rocks of the Pontiac Group. The mineralized zones are spatially associated with the Sladen fault and ~2678 Ma subalkaline to alkaline porphyritic quartz monzodiorite and granodiorite. Field relationships and isotopic age data for ore-related vein minerals indicate that gold mineralization in the Canadian Malartic deposit occurred at ~2665 to 2660 Ma and was contemporaneous with syn- to late-D 2 peak metamorphism. The smaller deposits in the camp include auriferous disseminated-stockwork zones of the Camflo deposit (1.9 Moz) and quartz ± carbonate-pyrite veins and breccias (0.6 Moz) along faults in chemically and mechanically favorable rocks. The age of these deposits is poorly constrained, but ~2692 Ma postmineral dikes, and ~2625 Ma hydrothermal titanite and rutile from the Camflo deposit highlight a long and complex hydrothermal history. Crosscutting relationships and regional geochronological constraints suggest that an early episode of pre-Timiskaming mineralization occurred at >2692 Ma, shortly after the end of volcanism in the Malartic camp, and postmetamorphic fluid circulation may have contributed to concentration or remobilization of gold until ~2625 Ma. However, the bulk of the gold was concentrated in the Canadian Malartic deposit during the main phase of compressive deformation and peak regional metamorphism.

Abstract The Canadian Malartic low-grade bulk tonnage gold mine (total production and reserves of 303.3 t or 10.7 Moz at 0.97 g/t) is located in the Archean Abitibi greenstone belt, immediately south of the crustal-scale Larder Lake-Cadillac fault zone. The deposit is predominantly hosted in clastic metasedimentary rocks of the Pontiac Group and, to a lesser extent, in subalkaline porphyritic quartz monzodiorite and granodiorite. The quartz monzodiorite and granodiorite yielded syn-Timiskaming U-Pb ID-TIMS zircon ages of 2677.8 ± 1.5 and 2678.4 ± 1.7 Ma, respectively. The ore, which is characterized by a Au-Te-W-S-Bi-Ag ± Pb ± Mo metallic signature, mainly consists of quartz-carbonate vein stockworks and replacement zones with disseminated pyrite. The ore zones are dominantly oriented subparallel to a NW-striking S 2 foliation and to the E-striking and S-dipping Sladen fault, thus forming NW-SE and E-W mineralized trends. 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 made up of albite and/or microcline with pyrite. The quartz monzodiorite comprises a distal hematite-bearing alteration zone that is overprinted by proximal microcline + albite + quartz + pyrite replacement zones. The metallic signature of the ore, the presence of mineralized stockworks, the potassic alteration (biotite/microcline), and an association with ca. 2678 Ma porphyritic intrusions suggest the possibility of an early, syn-Timiskaming magmatic-hydrothermal auriferous event in the area. However, this study indicates that gold mineralization and its distribution at Canadian Malartic are largely controlled by D 2 deformation and related features such as faults, shears, and high-strain zones. Of particular importance are the S 2 cleavage developed in the hinge zone of F 2 folds, and the Sladen fault. Molybdenite from high-grade ore yielded a Re-Os age of 2664 ± 11 Ma that is compatible with a syn-D 2 timing for the bulk of the mineralization. The main characteristics of the Canadian Malartic deposit are thus best explained by a syndeformational event (D 2 ; ca. 2670–2660 Ma) potentially superimposed onto a gold-bearing magmatic/hydrothermal intrusion-related system associated with Timiskaming-age porphyritic intrusions emplaced along the crustal-scale Larder Lake-Cadillac fault zone.

Abstract The Abitibi greenstone belt, which straddles the border between Ontario and Quebec in eastern Canada, represents one of the largest and best-preserved Neoarchean greenstone belts in the world. The belt consists of E-trending successions of folded volcanic and sedimentary rocks and intervening domes of intrusive rocks. Submarine volcanism occurred between 2795 and 2695 Ma. Six volcanic assemblages have been defined, recording submarine volcanism during specific periods of time. Komatiite successions within some of these volcanic assemblages are host to magmatic sulfide deposits. However, economically more important are volcanogenic massive sulfide (VMS) deposits, which contain a total of ~775 million tonnes (t) of polymetallic massive sulfides. Approximately half of the endowment is hosted by volcanic rocks of the 2704 to 2695 Ma Blake River assemblage. VMS deposits of this assemblage also account for most of the synvolcanic gold in the Abitibi greenstone belt, totaling over 1,100 t (~35 Moz). Submarine volcanism was followed by the deposition of large amounts of sedimentary material derived from a shallow marine or subaerial hinterland, created as a result of crustal thickening during an early phase of mountain building at ≤2690 to ≤2685 Ma. Submarine volcanic rocks and the overlying flysch-like sedimentary rocks of the Porcupine assemblage were affected by large-scale folding and thrusting during at least one deformational event prior to 2679 Ma. At this time, a terrestrial unconformity surface developed between the older and already deformed rocks of the Abitibi greenstone belt and molasse-like sedimentary rocks of the Timiskaming assemblage, which were deposited between ≤2679 and ≤2669 Ma. Deposition of the Timiskaming sedimentary rocks occurred in extensional basins and was locally accompanied by predominantly alkaline volcanism and related intrusive activity. Crustal shortening and thick-skinned deformation resulted in the structural burial of the molasse-like sedimentary rocks of the Timiskaming assemblage after 2669 Ma. Panels of Timiskaming deposits were preserved in the footwall of these thrusts, which are today represented by major fault zones cutting across the supracrustal rocks of the Abitibi greenstone belt. The structural history of these fault zones is complicated by late-stage strike-slip deformation. The Porcupine-Destor and Larder Lake-Cadillac fault zones of the southern Abitibi greenstone belt as well as second- and third-order splays off these fault zones are host to a number of major orogenic gold deposits. The gold endowment of these deposits exceeds 6,200 t (~200 Moz), making the Abitibi greenstone belt one of the economically most important metamorphic terranes in the world.

Abstract Komatiitic rocks occur mainly in Archean greenstone belts, less commonly in Paleoproterozoic volcano-sedimentary belts, and only rarely in younger volcanic settings. As in most other greenstone belts worldwide, komatiitic rocks are locally abundant in the Abitibi greenstone belt but generally represent only a small proportion of the volcanic rocks in the volcanic succession. Although only locally exposed, glacially sculpted exposures of only weakly metamorphosed and mildly deformed komatiites of mineralized and unmineralized komatiites in the Abitibi greenstone belt are among the best in the world, characterized by excellent textural preservation and, in some cases, excellent mineralogical preservation. Komatiitic rocks in the Abitibi greenstone belt occur predominantly within the Pacaud (2750–2735 Ma), Stoughton-Roquemaure (2723–2720 Ma), Kidd-Munro (2720–2710 Ma), and Tisdale (2710–2704 Ma) assemblages, but have recently also been recognized in lesser abundances within the Deloro (2734–2724 Ma) and Porcupine (≤2690–≤2685 Ma) assemblages. Overall, the komatiitic rocks present in these assemblages are characterized by a wide variety of lithofacies (textural, compositional) and flow facies; however, a regional analysis of komatiite physical volcanology reveals some fundamental differences between each of the komatiite-bearing assemblages. The Kidd-Munro and Tisdale komatiite-bearing assemblages contain the largest volumes of komatiitic rocks, in particular thick, highly magnesian cumulate lava channels and channelized sheet flows. This suggests that the magma discharge rates were higher for these assemblages and/or that they formed more proximal to the eruptive site. However, the recently discovered Grasset Ni-Cu-(PGE) deposit hosted within relatively high MgO cumulate rocks that are interpreted to occur within the Deloro assemblage highlights the possibility of the other komatiite-bearing assemblages to contain similarly prospective volcanic and/or subvolcanic facies. Geochemical data indicate that regardless of age or petrogenetic affinity (Al-undepleted vs. Al-depleted vs. Ti-enriched vs. Fe-rich), almost all of the parental magmas were undersaturated in sulfide prior to emplacement and therefore represent favorable magma sources for Ni-Cu-(PGE) mineralization. Volcanological data indicate that almost all komatiite-associated Ni-Cu-(PGE) deposits in the Abitibi greenstone belt appear to be localized in lava channels or channelized sheet flows, which have the capacity to thermomechanically erode S-bearing country rocks and to efficiently transfer metals from the magma to sulfide xenomelts. Three type localities (Spinifex Ridge in La Motte Township, Pyke Hill in Munro Township, and Alexo in Dundonald Township) illustrate how physical volcanology (lava channelization) and stratigraphic environment (S source) need to operate quasi-simultaneously to allow for the genesis of significant amounts of Ni-Cu-(PGE) sulfides within a komatiitic succession. As not all komatiite magma pathways are mineralized, one of the most important challenges is to be able to distinguish potentially mineralized successions from barren successions.