Geoscience Map 1993-5: Geology of the Yehiniko Lake and Chutine River Area, Northwestern B.C.
(NTS 104G/11W, 12)
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Geoscience Map 1993-5 presents the geology and mineral occurrences of the Yehinko Lake and Chutine River area (104G/11W, 12) in northwestern British Columbia. The 1:50 000 scale geological map includes a legend, geological cross sections, and a reference source inset map.
Permian or older rocks comprise a package of foliated volcanic sand sedimentary rocks, including pillow basalt and recrystallized limestone. Stikine assemblage strata range in age from Lower to Upper Permian. Upper Carboniferous strata include a unit with foliated, pyroxene-feldspar phyric andesitic flows and sills, crystal tuff, and lithic lapilli tuff; and a unit with foliated argillite, siltstone, calcareous siltstone, and conglomerate. Lower to Upper Permian strata are mainly calcarenite with minor chert layers and nodules. Early Permian to perhaps Middle Triassic strata consist of chert, ribbon chert, siliceous siltstone, tuff and mudstone.
Upper Triassic Stuhini Group strata consist of a lower unit dominated by volcanic rocks and an upper unit dominated by sediments. The lower unit contains mafic volcanic rocks, including pyroxene-phyric flows and breccias; intermediate volcanic rocks including tuff, epiclastics, and conglomerate; plagioclase-rich tuffaceous wacke; and bladed plagioclase-phyric basalt or andesite flows; and undifferentiated volcanic rocks. The upper unit contains mainly sedimentary rocks including tuffaceous siltstone and wacke; and sandstone, siltstone, argillite and limestone lenses. Above the sedimentary unit are undifferentiated volcanic and sedimentary rocks. Lower to Middle Jurassic Hazelton Group is represented by a rhyolite flow unit; limy sandstone, feldspathic wacke and arenite with limestone lenses; dacite to basaltic andesite flows, tuff-breccia and tuff; rhyodacite/dacite ignimbrite flows; olivine basalt flows; and undifferentiated volcanic rocks. The Upper Cretaceous (?) to Paleocene Brothers Peak Formation of the Sustut Group consists of polymictic conglomerate, and lesser sandstone; rhyolite ash to lapilli tuff; and rhyolite to rhyodacite flows. The Eocene Sloko Group has trachyandesite and basalt flows; and locally welded dacite to rhyolite, and tuff-breccia and tuff.
Middle to Late Triassic Alaskan-type ultramafic rocks include olivine clinopyroxenite and dunite that comprise the oldest intrusive rocks in the area. The Middle to Late Triassic Stikine Suite intrusions consist of quartz monzonite, hornblende-biotite granodiorite, monzodiorite, quartz diorite, diorite and potassium feldspar megacrystic tonalite. The Late Early Jurassic Texas Creek Suite intrusions are monzodiorite and quartz monzonite. The late Early Jurassic Cone Mountain Suite has granodiorite and monzodiorite intrusions. The Middle Jurassic Three Sisters Suite contains porphyritic monzonite to monzodiorite; hornblende granite to quartz monzonite stocks with associated dikes; and hornblende biotite granite. The Eocene Hyder Suite of intrusions includes biotite granite that is locally potassium feldspar-megacrystic, hornblende-biotite granodiorite, and felsite. Tertiary and older dikes are andesite, basalt, felsite, olivine basalt, rhyolite and syenite.
Structural style in the area is dominated by brittle deformation and faulting and controlled by competency contrasts between volcanic and sedimentary rocks. Three or possibly four episodes of deformation have been recognized, but ages are poorly constrained. The earliest structures are synmetamorphic, and pre-Triassic, possibly Carboniferous, age.
These structures, and related northeast-striking penetrative foliations, are deformed by west-trending post-Triassic (?) folds. Northeast-directed northwest trending reverse faults in the southeast half of the map-sheet and northeasterly trending northeast-directed folds and contractional faults in the northwest half of the mapsheet are post-Early Jurassic.
Metallogeny in the area is related to plate boundary subduction processes and two separate mineralizing events, one in the latest Late Triassic and one in the Eocene. Each is characterized by a different base and precious metal suite. A Late Triassic to Early Jurassic alkalic volcanic centre at Galore Creek, to the south, hosts ten synvolcanic copper-gold deposits.
The largest, the Stikine Copper Limited central zone contains 125 000 000 tonnes of material with an estimated grade of 1.06 per cent copper, 0.4 gram per tonne gold and 7.7 grams per tonne silver. A similar deposit occurs at Copper Canyon. Pervasive potash metasomatism and retrograde calcsilicate alteration characterize the mineralized zones. Smaller tonnage, Eocene silver-rich base metal veins are associated with calcalkaline intrusions and hosted in northeasterly-striking faults. Alteration and gangue minerals are quartz, sericite, and iron carbonate assemblages. Volcanogenic massive sulphide deposits (e.g., Tulsequah Chief) are an untested but viable exploration target in the Paleozoic Stikine assemblage rocks.
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