Preliminary bedrock geology of the Tanzilla and McBride area
BCGS Open File 2017-9
by B.I. van Straaten, R. Gibson, and J. Nelson
The Tanzilla – McBride area, including parts of NTS sheets 104I/03, 04, 05 and 06, is immediately east-southeast of the community of Dease Lake in northwestern British Columbia. Detailed field mapping (1:20,000-scale) in 2015 and 2016 revealed a previously poorly understood volcanic succession, the Horn Mountain Formation, a late Early to Middle Jurassic unit about 5.4 km thick in the upper part of the Hazelton Group.
The oldest units in the area are mafic volcanic rocks of the Stuhini Group (Triassic) that are cut by the Cake Hill pluton (Late Triassic). Separated by a regional unconformity, these units are overlain by the Spatsizi Formation (Hazelton Group, late Pliensbachian to Toarcian), a sedimentary succession up to 1 km thick. The Spatsizi Formation grades laterally and vertically to the Horn Mountain Formation. Units in the lower part of the Horn Mountain Formation include massive green augite-plagioclase-phyric volcanic breccia and rare grey coarse platy plagioclase-phyric lapilli tuff and pillows that were, at least in part, deposited in a subaqueous environment. Overlying units of interlayered maroon augite-plagioclase-phyric flows, volcanic breccia and tuff suggest increasingly greater volumes of volcanism and the formation of a subaerial volcanic edifice. During a hiatus in volcanism, these rocks were cut and hydrothermally altered by a 173 Ma (Aalenian) porphyry. These units are overlain by Bajocian mafic volcanic flows in the upper part of the Horn Mountain Formation. The Horn Mountain Formation is cut by the Three Sisters pluton (ca 173-169 Ma, Aalenian-Bajocian), and is unconformably overlain by sedimentary rocks of the Bowser Lake Group (Bajocian). In the northern part of the map area, folded Takwahoni Formation siliciclastic rocks deposited in the Whitehorse trough (Laberge Group, Pliensbachian to Toarcian(?)) are in the hanging wall of the south-verging Kehlechoa thrust fault, and structurally overlie the Horn Mountain volcanic succession. The Snowdrift Creek pluton (Late Jurassic) cuts the Kehlechoa fault. The Takwahoni Formation is structurally overlain by undifferentiated limestone, siliciclastic, volcanic and ultramafic rocks of the Cache Creek terrane in the hanging wall of the King Salmon thrust fault.
At least three magmatic-hydrothermal events are recognized in the map area. Late Triassic porphyry-style copper mineralization occurs at the Gnat Pass developed prospect and nearby Moss showing. The Horn Mountain Formation hosts aerially extensive gossans at Tanzilla and McBride (both early-stage porphyry projects) interpreted as Middle Jurassic in age. At Tanzilla, an advanced argillic lithocap overlies porphyry-style alteration at depth. Quartz-sericite-pyrite to potassic alteration with anomalous copper and molybdenum is hosted by a syn-mineral 173 Ma plagioclase porphyry. Our mapping extends the advanced argillic alteration at Tanzilla for at least 17 km along strike. It is interpreted as a lithocap formed by acidic hydrothermal fluid flow along an unconformity or fault in the upper Horn Mountain Formation. At the McBride showing, widespread quartz-sericite-pyrite and local potassic alteration hosts elevated copper and gold in volcanic rocks of the middle Horn Mountain Formation. Molybdenum mineralization is locally in the Snowdrift Creek pluton (Late Jurassic) and its immediate wall rocks, and in a satellite stock to the south.