The Memphremagog polymetallic massive sulphide deposit occurs at the contact between a pillowed volcanic sequence and a detrital sedimentary sequence consisting principally of shale and greywacke. The deposit, situated about 100 km east of Montréal in the Appalachians of southeastern Quebec, is composed of massive sulphide breccia. Major minerals include pyrrhotite, pyrite, sphalerite, galena, chalcopyrite, and calcite, with minor amounts of arsenopyrite, freibergite ((Cu,Ag,Fe)12Sb4S13), meneghinite (CuPb13Sb7S24), gudmundite (FeSbS), and kersteritic stannite (Cu2(Fe,Zn)SnS4). The mineral assemblage, metal content, and primary textures are comparable to modern sea-floor sulphide mounds enclosed by thick sedimentary sequences deposited in basins such as the Guaymas Basin in the Gulf of California.Although the footwall basalt unit is interstratified with the St-Daniel Formation of the Ophiolite Belt, it possesses a transitional geochemical signature of alkaline affinity that is uncharacteristic of ophiolitic basalts. We propose that this volcanic unit was emplaced within a transtensional basin of a rifted fore-arc system that formed during the Taconic orogeny as a result of diachronous oblique collision of an island- arc system (Ascot–Weedon?) with the North American continent. During magmatic activity, a hydrothermal system was imprinted on the volcanics and underlying sediments. Subsequent hydrothermal fluid emanations led to the formation of the Memphremagog sulphide deposit, which is quite distinct from sulphide deposits generally found within ophiolite belts.