SULFIDE EMPLACEMENT IN THE SOUTH RAGLAN TREND, CAPE SMITH FOLD BELT, QUEBEC

Several multi-million ton magmatic sulfide deposits are hosted by dikes of komatiitic dunite, peridotite, and pyroxenite that served as feeders to the overlying volcanic and subvolcanic deposits of the Raglan trend. Sulfide liquation was provoked by largescale assimilation of metasedimentary and metavolcanic host rocks. Assimilation of relatively sulfur-poor rocks in the dike margins led to the crystallization of large masses of olivine in the South Raglan Trend prior to sulfide saturation, leading to lower Ni/Cu ratios than at the Raglan deposits. The Mequillon, Ivakkak, Tootoo, TK, and Mesamax deposits all represent the basal terminations of blade-shaped dikes that terminated downwards against massive metabasaltic lava flows of the lowermost Beauparlant Formation. The bases of the dikes locally retain chilled margins but generally have contacts showing evidence for thermal erosion of their hosts. Basaltic wallrock has been raised to pyroxene hornfels or has been partially melted; metasediments contain pockets of pegmatite and in both cases the intrusive rocks are coarse grained ultramafic cumulates right up to the contact. The bottoms of the dikes are trough-shaped and generally contain net-textured sulfide hosted by ultramafic cumulates. Above the net-texured sulfide there are extensive domains of disseminated sulfide hosted by ultramafic rock forming the matrix to ultramafic breccias interpreted to represent foundered blocks of chilled margins from the thermally eroded upper reaches of the dike margins. Most examples show evidence of later breaching of the lowermost margin of the dike, permitting sulfide melt that had already accumulated in the dike bases to form keel-like sheets of massive sulfide beneath the main mineralized bodies. Textures and compositions of sulfide mineralization show that whereas disseminated sulfide blebs remained isolated and retained their primary compositions, the net-textured sulfide bodies experienced extensive fractionation of residual sulfide melt from early-formed monosulfide solid solution cumulates. Late sulfide melt migrated distances of hundreds of meters.