Geochemical constraints on the tectonic setting of basaltic host rocks to the Windy Craggy Cu-Co-Au massive sulphide deposit, northwestern British Columbia

Windy Craggy is an approximately 300 Mt Cu-Co-Au volcanogenic massive sulphide (VMS) deposit in northwestern British Columbia, Canada. The Windy Craggy deposit is hosted by the Middle Tats Volcanics (MTV), a Late Triassic volcano-sedimentary sequence of intercalated mafic pillowed to massive volcanic flows and sills and calcareous argillite that are part of the Alexander terrane. The host footwall and hangingwall flows and sills are predominantly alkalic basalts (Nb/Y > 0.70). MTV alkali basalts at Windy Craggy are enriched in light rare earth elements (LREEs) >100X chondrite compared to chondrite, have steep REE patterns [(La/Yb)cn = 7.1–25.4], and generally lack the Ta and Nb depletions relative to primitive mantle (e.g. [Nb/Th]pm = 0.68–1.94) characteristic of arc environments, although most have [Nb/La]pm < 1. By contrast, volcanic rocks away from the deposit (and regionally; Lower Tats Volcanics, LTV) as well as late dikes that cross-cut all lithologies including metamorphic and deformational fabrics are sub-alkalic tholeiitic to calc-alkaline basalts and basaltic andesites that are less enriched in the LREEs (10–100X chondrite), have less steep REE patterns [(La/Yb)cn = 0.41–10.6], and show well-developed Ta and Nb depletions (arc signatures; [Nb/Th]pm = 0.20–0.79), consistent with formation in an oceanic arc environment. The co-occurrence of tholeiitic/calc-alkaline arc rocks with alkalic rocks indicates that the LTV (former) and MTV (latter) formed from melts that were influenced to varying degrees by subducted oceanic crust, and likely formed within a back-arc basin setting formed on a rifted oceanic arc. There is no geochemical or isotopic evidence for major involvement of continental crust. The LTV basalts likely were produced by progressive depletion in the source by partial melting of mantle overlying the subducting oceanic crust. The presence of the MTV alkalic Windy Craggy rocks overlying the LTV is consistent with the presence of a slab-window, perhaps related to subduction of a spreading centre, which allowed more enriched magmas to reach the surface with only minimal interaction with subduction-modified mantle. The presence of this slab-window might have provided the mechanism for the generation of anomalously high heat flow close to the seafloor, which initiated and sustained vigorous, long-lived hydrothermal activity necessary for the precipitation of large accumulations of massive sulphide. To our knowledge, this is the first example of a large VMS deposit associated with a slab-window.