Late Early to Middle Jurassic Hazelton Group volcanism and its tectonic setting, McBride River area, northwest British Columbia

Continued mapping and geochronologic studies on the northeast margin of Stikinia east of Dease Lake indicate that Late Triassic plutonic rocks of the Cake Hill pluton are unconformably overlain by late Early to Middle Jurassic rocks in the upper part of the Hazelton Group. The unconformity, which has been traced laterally for 50 km, spans at least 30 m.y. and represents one of the few examples of unroofed Stuhini arc in northern Stikinia. The unconformity is overlain by sedimentary rocks of the Spatsizi Formation (up to 0.2 km thick, Toarcian) and volcanic rocks of the Horn Mountain Formation (at least 3.5 km thick, Aalenian-Bajocian). The recently-defi ned Horn Mountain Formation is unusual because it postdates typical Late Triassic to Early Jurassic arc volcanism in northern Stikinia, and was deposited during accretion of the Stikine and Cache Creek terranes. In the lower part of the Horn Mountain Formation, partly or wholly subaqueous, massive green augite-plagioclase-phyric volcanic breccia pass upward to a subaerial volcanic edifi ce composed of interlayered maroon augite-plagioclase-phyric fl ows, volcanic breccia and tuff. Felsic volcanic rocks cap the succession. The Horn Mountain Formation is cut by the Three Sisters pluton (ca. 173-169 Ma, AalenianBajocian) and McBride River pluton. The McBride River granodiorite has hitherto been interpreted as Early Jurassic (ca. 184 Ma), but based on crosscutting relationships we suggest a Middle Jurassic or younger age to be more plausible. The Bowser Lake Group (Bajocian) conformably overlies the Horn Mountain Formation; it records initiation of erosion from the Stikinia-Cache Creek tectonic welt. Chert and limestone clastbearing pebble to cobble conglomerate (>330 m thick) is interpreted to have formed close to range front faults along the building orogen. The coarse clastic facies transitions to interbedded subaerial siltstone, sandstone, chert clast-bearing conglomerate, and mafi c fl ows (>430 m thick) farther south. In the northern part of the study area, the Horn Mountain Formation and Bowser Lake Group are structurally overlain by rocks of the Whitehorse trough, juxtaposed along the Kehlechoa thrust fault (south vergent). In the hanging wall, Sinwa Formation limestones (Upper Triassic) are unconformably overlain by Takwahoni Formation sedimentary rocks (Lower Jurassic). The Takwahoni Formation succession comprises a lithologically variable unit of fi ne-grained siliciclastic rocks, polymictic conglomerate and volcanic rocks (Sinemurian?) that appears to grade upward to a thick unit of interbedded sandstone and siltstone (Pliensbachian). In the northernmost part of the map area, rocks of the Whitehorse trough are structurally overlain by rocks of the Cache Creek terrane along the King Salmon thrust fault (south vergent). At Tanzilla and McBride, northwest of the study area, the Horn Mountain Formation hosts large advanced argillic alteration systems. However in the present area, evidence of alteration and mineralization is more modest. Four grab samples from two intrusion-related hydrothermal alteration zones west and southeast of the McBride River returned no anomalous metal values. Several Cu-Ag mineral occurrences with limited alteration footprints are hosted in the middle and upper part of the Horn Mountain Formation in the southeast part of the map area. Polymetallic veins near the Kehlechoa and King Salmon thrust faults locally contain signifi cant gold and silver.