Carmacks Copper Cu-Au-Ag Deposit: Mineralization and Postore Migmatization of a Stikine Arc Porphyry Copper System in Yukon, Canada

Abstract

Late Triassic to Early Jurassic porphyry Cu mineralization is common in British Columbia, yet there are few age-equivalent porphyry occurrences in Yukon. This study presents new data for the enigmatic Carmacks Copper Cu-Au-Ag deposit in south-central Yukon, Canada, which is hosted in amphibolite facies metamorphic inliers within the Early Jurassic Granite Mountain batholith. Sulfide mineralization occurs mainly as net-textured bornite and chalcopyrite in leucosome, and as chalcopyrite ± pyrite blebs and disseminations in amphibolite and quartz-plagioclase-biotite schist. Several studies suggest that the Carmacks Copper deposit and the nearby Minto deposit are related to porphyry belts in British Columbia, but constraining the timing of alteration, mineralization, and metamorphism has been difficult. This study establishes a geologic and high-precision geochronologic framework for sulfide mineralization and its host rocks at the Carmacks Copper deposit, using Re-Os dating of molybdenite, and chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) analysis of both whole zircon grains and laser-cut fragments of complexly zoned zircon grains. Our data indicate that the igneous protolith of the metamorphic inliers formed at 217.53 ± 0.16 Ma, followed by peak metamorphism at amphibolite facies at 205.82 ± 0.23 Ma, which occurred prior to Granite Mountain batholith emplacement but subsequent to Cu-Au-Ag mineralization of the protolith. An early phase of the Granite Mountain batholith was emplaced at 199.84 ± 0.14 Ma, followed by the main phase at 195 to 194 Ma. A second generation of metamorphic zircon in migmatite at 196.01 ± 0.12 Ma represents a partial melting event associated with Granite Mountain batholith emplacement. Two petrographically distinct populations of molybdenite are present in unstrained, net-textured copper sulfides. A sample dominated by strained molybdenite yielded an 187Re/187Os age of 212.5 ± 1.0 Ma, which represents the minimum mineralization age of the protolith. A sample dominated by euhedral grains yielded an 187Re/187Os age of 198.5 ± 0.9 Ma, constraining the maximum age of sulfide remobilization. These results indicate that primary mineralization is >212.5 Ma and potentially coeval with the ~217.5 Ma generation of Late Triassic magmatism. The mineralized protolith, best interpreted as the potassic alteration zone of a Late Triassic (~217–213 Ma) porphyry Cu-Au system, was metamorphosed to amphibolite facies at ~206 Ma, and subsequently migmatized during 200 to 194 Ma intrusion of the Granite Mountain batholith. The chalcopyrite-bornite-dominant assemblage in neosome precipitated from an immiscible Cu-Fe-S melt phase that partly consumed xenocrystic molybdenite and reprecipitated new molybdenite grains. The Carmacks Copper deposit and the related Minto deposit are remnants of a Late Triassic porphyry belt, where a significant fraction of the original metal endowment was likely lost through digestion of mineralized rocks by midcrustal magma in the Early Jurassic. These Yukon deposits are rare examples of metamorphosed porphyry Cu systems in the global geologic record, where rapid tectonic burial following mineralization was the principal factor in their preservation.

Late Triassic to Jurassic Magmatic and Tectonic Evolution of the Intermontane Terranes in Yukon, Northern Canadian Cordillera: Transition From Arc to Syn‐Collisional Magmatism and Post‐Collisional Lithospheric Delamination

“…The K‐feldspar megacrystic border phase of the Tatchun batholith yielded an older CA‐TIMS 206 Pb/ 238 U date of 203.32 ± 0.06 Ma (Figure 4). Although this is the only precisely dated latest Triassic phase in this study, the presence of 205–203 Ma zircon antecrysts in the Granite Mountain batholith (Kovacs et al., 2020; Sack et al., 2020) and an imprecise date of 204.9 ± 3.3 Ma from the Pelly pluton (Knight et al., 2013) suggest that this early phase may be more widespread than presently documented in the Minto suite. Inherited zircons are documented in the Granite Mountain, Minto, and Tatchun plutons (Figure 3; Kovacs et al., 2020; Sack et al., 2020).…”

“…The mineralization is dated at ca. 217–213 Ma and occurs in variably metamorphosed and migmatized rafts and xenoliths of foliated intermediate to mafic meta‐igneous rocks of probable Stikine suite affinity, enveloped within massive plutonic phases of the Minto suite (Kovacs et al., 2020; Sack et al., 2020).…”

“…Limited CA-TIMS data for the Bennett suite precludes exact estimate of the intrusive rate for these plutons, however when considering previous TIMS dates a tempo of ∼3 Myr is also indicated. (Kovacs et al, 2020;Sack et al, 2020).…”

“…A new CA‐TIMS date from a Pyroxene Mountain pluton at Mount Nansen, west of Carmacks (Figure 2; previously dated at ca. 211 Ma by LA‐ICPMS; Klöcking et al., 2016), has an age of 216.16 ± 0.07 Ma (Sack et al., 2022) and CA‐TIMS analysis of zircon fragments in a mineralized raft at the Carmacks Copper deposit indicate a protolith age of 217.53 ± 0.16 Ma (Kovacs et al., 2020). These results suggest that the Pyroxene Mountain suite is the northern extension of the Stikine suite in central Yukon and that Stikinia is locally anchored on the Yukon‐Tanana terrane.…”

“…These plutons were intruded after crustal thickening and burial of the Yukon‐Tanana terrane was initiated at ca. 205–200 Ma (Clark, 2017; Kovacs et al., 2020) and during the following phase of exhumation recorded along the flanks of the WT. Crustal thickening was accommodated along a series of southwest‐verging shear zones and thrust faults, of which the Needlerock, Tadru, Yukon River, and Reindeer faults are inferred remnants (Figure 3; Colpron et al., 2002, 2003; Parsons et al., 2018; Ryan et al., 2013, 2014).…”