Geology of the Latham and Pallen Creek area, northwestern British Columbia: Distinguishing the Tsaybahe group, Stuhini Group, and Hazelton Group, and the onset of Triassic arc volcanism in northern Stikinia.

The oldest units in the Latham Creek and Pallen Creek area are penetratively deformed meta-sedimentary and volcanic rocks and limestones of the Stikine assemblage (upper Paleozoic). These rocks are overlain (likely unconformably) by a volcano-sedimentary sequence informally referred to as the Tsaybahe group (Lower-Middle Triassic), which is succeeded by the Stuhini Group (Upper Triassic). We subdivide the Tsaybahe group into a sedimentary unit of fi ne-grained siliciclastic rocks and minor chert, and a volcanic unit of monomictic tuff breccia with plagioclaseaugite-phyric volcanic clasts. Tsaybahe volcanic rocks appear texturally similar to the overlying Stuhini Group, but are separated based on their stratigraphic position atop of the Stikine assemblage, rare Middle Triassic biostratigraphic ages, low magnetic susceptibility, and low response on regional aeromagnetic surveys. Stuhini Group volcanic rocks include massive monomictic tuff breccia and lapilli-tuff with augite-plagioclasephyric volcanic clasts, have a high magnetic susceptibility, and display a high and variable response on regional aeromagnetic surveys. Triassic and older stratifi ed rocks are cut by Late Triassic stocks and plutons ranging from ultramafi c to gabbro, hornblende-rich quartz diorite and hornblende quartz monzonite in composition. Triassic units generally lack penetrative tectonic fabrics and are deformed into map-scale open folds. An outlier of volcano-sedimentary rocks assigned to the upper part of the Hazelton Group is inferred to unconformably overlie the Triassic rocks. The succession (~500 m thick) includes two sedimentary units that are overlain by a maroon volcanic unit, which is capped by a felsic volcanic unit. Based on lithological and stratigraphic criteria, the sedimentary units are assigned to the Spatsizi Formation and the volcanic units to the Horn Mountain Formation. Three Middle Jurassic plutons are exposed in the area, ranging in composition from biotite-hornblende quartz diorite to biotite monzogranite. Developed within or adjacent to these plutons are zones of alteration and mineralization containing locally elevated copper, gold, silver and/or molybdenum in fractures, veins, skarns, and gossans. Augite-phyric mafi c volcanic units in each of the Tsaybahe group, Stuhini Group, and Horn Mountain Formation, although temporally distinct, are texturally similar. Compared to widespread exposures of mafi c volcanic rocks of the Stuhini Group in northern Stikinia, occurrences of the Tsaybahe group and its correlatives are rare. However, owing to a lack of age constraints, we consider that Tsaybahe group exposures may have been included in the Stuhini Group and suggest that the unit is more extensive than currently recognized. The Tsaybahe group may represent nascent Middle Triassic arc volcanism before widespread Upper Triassic Stuhini arc activity.