BCGS Geoscience Map 2017-1
Geology and geochronology of the Turnagain ultramafic-mafic intrusion, British Columbia
The Turnagain Alaskan-type intrusion in northern British Columbia belongs to a global class of ultramafic-mafic intrusions emplaced at convergent margin tectonic settings that are gaining prominence as exploration targets for magmatic Ni-Cu-PGE sulphide mineralization. The Turnagain body is unusually enriched in Ni(-Co) sulphides compared to typical Alaskan-type intrusions. In terms of contained Ni metal, the low-grade mineralization at Turnagain ranks ninth among the world’s largest deposits, constituting a total resource of 1842 Mt @ 0.21 wt % Ni and 0.013 wt % Co.
Geoscience Map 2017-1 provides a compilation of the geology (1:10,000 scale), incorporating historical and modern exploration work, and geochronological results (U-Pb, 40Ar/39Ar) for the intrusion and its host rocks. A series of transverse and longitudinal sections controlled in part by drillhole information are used to construct a conceptual 3-D model of the intrusion. Field observations and geochronology establish a multi-stage history of emplacement in the Early Jurassic, with four distinct intrusive phases spanning at least 3 million years (~188-185 Ma). The geochronological results indicate two separate episodes of sulphide mineralization hosted by different intrusive phases: an early event that produced the main Ni(-Co) resource; and a younger event that gave rise to significant Cu(-PGE) mineralization, a potential future resource. From regional geological studies and geophysical surveys, we infer that deformation of the Turnagain intrusion was during accretion of an outboard arc assemblage (Quesnellia/Yukon-Tanana terranes) onto miogeoclinal strata of the ancient North American margin. The geochronological data bracket this accretionary event to after ca. 185 Ma (mid-Early Jurassic) and before ca. 171 Ma (early Middle Jurassic).
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