TECTONO-MAGMATIC AND METALLOGENIC EVOLUTION OF THE LATE TRIASSIC TO MIDDLE JURASSIC HAZELTON GROUP IN NORTHWEST BRITISH COLUMBIA

Abstract

The Hazelton Group (Stikine terrane) in NW British Columbia represents a regionally extensive, Late Triassic to Middle Jurassic successor arc. It records two economically important metallogenic episodes associated with the final stages of subduction- and rift-related magmatism in the Stikine terrane, and collision of Stikinia with inboard and outboard terranes and their accretion to Laurentia. This comprehensive study attempts to indicate the environments most likely to contain exploitable mineral resources. Volcanic and sedimentary rocks of the lower Hazelton Group were deposited along two arc axes and in an intervening basin. Near Terrace, British Columbia, lower Hazelton Group (Telkwa Formation) volcanism initiated by 204.8±0.3 Ma and continued for >10 My, creating 16 km of stratigraphy. The volcanic rocks have bimodal silica concentrations and geochemical characteristics of subduction-related island-arc magmatism. Mafic to intermediate rocks comprise a mantle-derived differentiated suite; rhyolites formed from anatexis of arc crust caused by magmatic underplating. Circa 174 Ma bimodal volcanic rocks of the upper Hazelton Group Iskut River Formation define a 300 by 50 km discontinuous belt in northwestern B.C. (the Eskay rift). They represent the final episode of Jurassic magmatism in the Stikine terrane, and are interpreted to have been deposited in sub-basins that opened during arc-scale transcurrent shearing during collision of Stikinia and surrounding terranes. Two geochemically distinct types of tholeiitic basalts are present; both resemble back-arc basin basalts formed by melting of asthenospheric and sub-arc mantle sources. Group 1 is isotopically more juvenile, and less enriched in non-conservative incompatible elements then group 2 basalts, which are affected by crustal contamination. The formation of latest Triassic to Early Jurassic porphyry-Cu deposits in Stikinia is temporally linked to initiation of Hazelton Group magmatic-arc activity ca 205 Ma. The porphyry intrusions are interpreted to be derived from subduction-related, oxidized, metalliferous magmas that underplated Stikinia. Circa 174 Ma VMS deposits (e.g. Eskay Creek and Anyox) are linked to rifting during post-subduction, orogenic-scale, terrane collisions. The VMS deposits are found in the southern portion of the Eskay rift, in association with group 1 basalts, which are interpreted to have formed from rapidly ascending magma in an advanced rift setting.