Geological setting of the volcanic-hosted Silbak Premier Mine, northwestern British Columbia, (104 A/4, B/1)
Creators: name: Brown, Derek Anthony
Publisher: name: University of British Columbia
Year: 2010

description: Detailed mapping of a 7.5 km² area at 1: 2,500 and a 1:10,000 compilation map over 60 km² have established Hazelton Group stratigraphy and structure. Hazelton Group stratigraphy begins with at least 1,000 metres of Late Triassic-Early Jurassic (210 ⁺²⁴₋₁₄ Ma; U-Pb zircon) green andesite flows, breccias and tuffs. Less than 1750 metres of green and maroon andesitic to dacitic volcaniclastic rocks overlie the andesite unit. North of Silbak Premier, at Slate Mountain, the volcaniclastic unit is overlain by up to 200 metres of a black tuff unit containing characteristic fresh biotite and white plagioclase fragments. The top of the Hazelton is a regional marker horizon, the Monitor rhyolite breccia and tuff (197 ± 14 Ma; zircon U-Pb). Hazelton volcanics are overlain by three different units. At Slate Mountain the Bowser Lake Group Bathonian/Callovian argillite and siltstone (at least 1500 m thick) lie above Hazelton rocks. Farther north on Mount Dilworth, Monitor rhyolite is succeded by black tuff or a Toarcian buff carbonate. East of Monitor Lake, less than 75 metres of Bajocian Spatsizi Group silicic shale and tuff overlies Hazelton volcanic rocks. Three intrusive episodes are discerned through isotopic dating: Early Jurassic (190 ± 2 Ma; U-Pb zircon) Texas Creek plutonic suite dacitic porphyries; Eocene Hyder suite leucocratic dykes; and oligocene-Miocene (25.2 ± 2.3 Ma; K-Ar biotite and 18 ± 6 Ma; Rb-Sr) biotite lamprophyre dykes. The Jurassic suite includes K-feldspar megacrystic "Premier porphyry" sills and dykes that are in part spatially and possibly genetically associated with mineralization. Structural features include disharmonic tight folds, ductile shear zones, and brittle faults. At least 4 phases of pre-Eocene deformation are defined by: (1) moderate west-plunging recumbent folds, (2) north-plunging inclined folds, (3) north-plunging upright folds, and (4) moderate west-plunging pencil lineations. The map area is divisible into three structural domains: the North, East and Silbak domains. The North domain is characterized by a marked structural discordance between warped Hazelton volcanic rocks and disharmonically folded Bowser Lake Group argillite and siltstone. Three phases of folding are: first phase tight to isoclinal disharmonic, recumbent folds; second phase open folds with shallow northwest-dipping axial planar cleavage; and a third phase upright, shallow north-plunging synclinorium. Structural continuity is difficult to establish due to lack of marker horizons and inferred detachments. The East domain is characterized by phase 3 gently north-northwest-plunging folds and locally east-verging asymmetric chevron folds in the Spatsizi Group. In contrast to North domain, Monitor rhyolite and/or Spatsizi Group are structurally conformable with Bowser Lake Group rxks. The Silbak domain is characterized by phase 4 pencil lineations and quartz veins. Stope geometry illustrates that mineralization occurs along two trends (1) northeast zone and (2) northwest zone of unknown phase. Steeply dipping, east-striking ductile fabrics occur in the Texas Creek batholith at the Riverside mine, Alaska and in maroon volcaniclastics along Bear River Ridge. Mylonitic fabrics at Riverside mine suggest a dextral sense of shear. A biotite lineation in the mylonitic foliation yields a totally reset Eocene K-Ar date. The width of Eocene Hyder dyke swarms indicates that there has been at least one kilometre of northeast brittle crustal extension. About 1400 metres of dextral transcurrent movement along the Long Lake-Fish Creek fault is post-Eocene dyke emplacement. oligocene-Miocene lamprophyre dykes fill fractures produced during east-west extension. Regional syntectonic greenschist grade metamorphism produced a carbonate-chlorite-sericite-pyrite mineral assemblage, probably in Middle Cretaceous time, bracketed by isotopic dating results. Hazelton Group volcanic rocks and coeval Texas Creek porphyritic rocks are subalkaline high-K to very high-K andesites and dacites. Tectonic discrimination diagrams indicate a calcalkaline, volcanic arc setting, with similar geochemical patterns to those for Andean volcanic rocks. Mineralization is hosted in Hazelton Group andesites and coeval Texas Creek porphyritic dacite sills and dykes. Mineralization and porphyry emplacement appear to have been controlled by northeast- and northwest-striking structures. Ore is predominantly discordant but locally concordant with moderately northwest-dipping andesite flows and breccias. No mineralization occurs in or above overlying maroon volcaniclastic rocks. Sericite alteration gives a Paleocene K-Ar date (63 ± 5 Ma); this is interpreted to be partially reset. The spatial link with Texas Creek K-feldspar porphyry and discordant nature of the ore suggests mineralization is Early Jurassic age and supports an epigenetic model.