Petrogenetic and metallogenic significance of mafic-ultramafic dykes and volcanic sequences in the thompson nickel belt: preliminary results (parts of nts 63j, 63o, and 63p)

New field observations obtained from mafic and mafic to ultramafic intrusions and volcanic sequences in the Thompson Nickel Belt (TNB) indicate that the larger "dyke-like" intrusions occur along major tectonic boundaries and potentially record the development of precollisional extensional faults. Ongoing and related geochemical and geochronological studies should help to establish the temporal and genetic relationships between the dykes, volcanic sequences and mineralized ultramafic sills in the TNB. Currently, this is not possible in light of recent geochronological data for the 2.09-1.86 Ga "Molson dyke swarm" and the lack of age information for the TNB. The best preserved and perhaps the largest intrusion in the TNB occupies the Grass River lineament. Here, a layered mafic-ultramafic intrusion (or multiple intrusions) was emplaced along a fault that, in places, separates the Archean Superior Province from the Ospwagan Group on the east side of the TNB. The Grass River intrusion appears to be younger than adjacent mafic volcanic rocks and, accordingly, could postdate all of the sedimentary rocks in the Ospwagan group. The occurrence of apparently exhalative sulphide mineralization in pillow basalt-argillite-iron formation associations is recognized at two different locations in the TNB. This type of sulphide mineralization may have predated some of the mafic-ultramafic magmatism in the TNB and could have provided an alternative to the Pipe Formation as a source for external sulphur an essential element in the generation of large, magmatic Ni sulphide deposits.