Geoscience Map 1993-6: Geology of the Tahltan Lake Area, Northwestern B.C.
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Geoscience Map 1993-6 presents the geology and mineral occurrences of the Tahltan Lake area (104G/13) in northwestern British Columbia. The 1:50 000 scale geological map includes a legend, geological cross sections, and a reference source inset map.
Stikine assemblage strata are of Paleozoic age. Permian or older rocks comprise a package of foliated volcanic sand sedimentary rocks, including recrystallized limestone. Upper Carboniferous strata include a unit with foliated, pyroxene-feldspar phyric andesitic flows and sills, crystal tuff, and lithic lapilli tuff, and massive andesite; and a unit with foliated argillite, siltstone, calcareous siltstone, and conglomerate. Upper Carboniferous to Lower Permian strata are mainly sericitic ash tuff and tuffaceous siltstone, chert, and calcareous siltstone. Lower 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.
Amphibolite bodies in the area are of uncertain age. Middle to Late Triassic intrusions consist of diorite to granodiorite. The Late Triassic to Early Jurassic Copper Mountain Suite of intrusions includes syenite with local potassic feldspar megacrysts, quartz monzonite, and magnetite-biotite clinopyroxenite bodies. The Early Jurassic Texas Creek Suite has monzodiorite and quartz monzonite intrusions. 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. Several structural culminations are cored by rocks of the Stikine assemblage and bounded by northeast-striking high angle reverse faults.
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. To the north, carbonate-hosted disseminated Au-Ag characterizes the former Golden Bear mine, and there is potential for epithermal Au-Ag: low sulphidation mineralization. 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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