ABSTRACT This three-day field trip focuses on the stratigraphy and the structural characteristics of the late- and post-Taconian sedimentary basins of the southern Québec Appalachians, with a particular emphasis on N-to-S and W-to-E structural and lithological variations. In order to discuss various aspects of the regional structural evolution of these basins, we will visit a series of key outcrops following three sections, the Beauce/Thetford-Mines sections, the Sherbrooke section, and the Coaticook section. RÉSUMÉ Cette excursion de trois jours se concentre sur la stratigraphie et les caractéristiques structurales des bassins sédimentaires tardi- et post-Taconien des Appalaches du sud du Québec, en mettant l’accent sur les variations structurales et lithologiques du nord au sud et d’ouest en est. Afin de discuter des divers aspects de l’évolution structurale régionale de ces bassins sédimentaires, nous visiterons une série d’affleure ments clés en suivant trois sections, soient les sections de Beauce/Thetford-Mines, de Sherbrooke, et de Coaticook.

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.

The Rivière-des-Plante ultramafic Complex lies along the Baie Verte–Brompton line in southern Québec and has previously been interpreted as an ophiolitic mélange. It is bounded on the northwest by a northwest-dipping thrust fault and unconformably overlain by conglomerates belonging to the Saint-Daniel Mélange to the southeast. It consists of harzburgite, serpentinite, ophicalcite, gabbro, granite, and granofelsic to mylonitic fragmental rocks. The latter have been interpreted as “exotic” metasedimentary rocks correlative with those of the Chain Lakes massif of western Maine. Our mapping suggests that the Rivière-des-Plante ultramafic Complex is not a mélange, but rather a deeply eroded ophiolitic remnant mostly represented by mantle peridotites that correlate with those of the Thetford-Mines ophiolite. The granofelsic to mylonitic rocks represent xenolith-bearing granitoids crosscutting the peridotites rather than “exotic” blocks derived from the Chain Lakes massif. These granites are similar to ca. 470 Ma peridotite-hosted granitoids of the Thetford-Mines ophiolite, which were generated by anatexis of the Laurentian margin during ophiolite obduction. A comparison of metasedimentary rocks of the Chain Lakes massif with those of the southern Québec Laurentian margin, as well as stratigraphic and geochronological data for both the southern Québec and western Maine Appalachians, suggests that the Chain Lakes likely represents more or less in situ Laurentian margin, and that metamorphism and anatexis dated at 469 Ma may have been caused by the obduction of the southern Québec ophiolites.