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UBC Theses and Dissertations

Genesis and post-ore modification of the migmatized Carmacks Copper Cu-Au-Ag porphyry deposit, Yukon, Canada Kovacs, Nikolett

Abstract

The Carmacks Copper and Minto Cu-Au deposits are hosted within northwest trending, variably metamorphosed, and deformed inliers engulfed by the Late Triassic to Early Jurassic Granite Mountain batholith (GMB). The origin of the deposits and their relationship to host rocks have been obscured by post-ore modification processes and their deposit genesis is controversial. Detailed study of the Carmacks Copper deposit indicates that it is hosted by Late Triassic (210.1 ± 5.3 Ma, U-Pb zircon) quartz-plagioclase-biotite schist and amphibolite, which form part of the Late Triassic Stikinia arc (Povoas Formation, Lewes River Group). The metamorphic rocks preserve an early northwest trending, steeply dipping, penetrative foliation (S₁). Hypogene copper mineralization occurs as S₁ parallel chalcopyrite stringers. The minimum age of mineralization is 212.5 ± 1.0 Ma, which is the ¹⁸⁷Re/¹⁸⁷Os age obtained for molybdenite inherited from a mineralized protolith. The ca. 198 Ma Granite Mountain batholith intrudes the metamorphic rocks and cross-cuts S₁ foliation, and thus post-dates mineralization. Emplacement of the batholith (~800°C at 5.5-6.5 kbar) caused partial melting of the metamorphic rocks that resulted in the formation of migmatite and remobilization of sulphides as an immiscible copper sulphide melt. Sulphide melt cooled and crystallized in the migmatite as net-textured bornite and chalcopyrite at 198.6 ± 0.9 Ma (¹⁸⁷Re/¹⁸⁷Os euhedral molybdenite), which is coeval with crystallization of GMB. The correlative Minto deposit is similarly characterized by net-textured copper sulphides hosted within migmatite; however, it is more digested by the GMB and thus the metamorphic rocks (Povoas Formation, Lewes River Group) are less preserved. The Carmacks Copper and Minto deposits are herein interpreted as Cu-Au porphyry deposits that were tectonically buried, deformed, and metamorphosed during the amalgamation of Stikinia and Yukon-Tanana terrane, and subsequently migmatized and further deformed during the emplacement of the GMB. These deposits form part of the Late Triassic Cu-Au porphyry mineralization that correlates with age-equivalent porphyry systems in the Stikine terrane of British Columbia.

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