This Atlas presents an extensive geoscience data set for the Late Triassic to Jurassic plutons that intrude the Intermontane terranes in Yukon. Plutons of this age are associated with significant porphyry Cu ± Au ± Mo deposits in British Columbia. In Yukon, Cu–Au–Ag deposits at Minto and Carmacks Copper are hosted by an Early Jurassic batholith but are not typical porphyry deposits. This project aims to provide the regional framework for Late Triassic to Jurassic plutons and associated mineral occurrences in southern Yukon. We systematically describe over 34 plutons and present new data for most of these, including 47 U-Pb zircon dates, 44 Ar-Ar dates, 187 whole-rock geochemical analyses, 30 whole-rock Sr and Nd isotope analyses, 27 feldspar Pb isotope analyses, zircon Hf isotope analyses for 22 samples and 26 pressure estimates from aluminum-in-hornblende analyses. Complementing these analytical data are extensive photomicrographs, photographs and maps.

The geology in the Mount Nansen area, of the Dawson Range, comprises a metamorphic basement overlain and intruded by mid and Late Cretaceous magmatic rocks. Mid-Cretaceous epithermal deposits are recognized in the Brown-McDade cluster in the south, and Late Cretaceous porphyry and epithermal occurrences in the centrally located Klaza cluster. Here we report eight new U-Pb zircon crystallization ages, two CA-TIMS and six LA-ICPMS, and Pb isotopic data for igneous feldspar from six samples. A 199.06 ± 0.96 Ma crystallization age demonstrates the host rock to the Brown-McDade deposit is the Late Triassic to Early Jurassic Minto suite. We report crystallization ages of 111 ± 1.8 Ma for the Dickson Hill porphyry, 107.96 ± 0.03 and 107.86 ± 0.03 Ma for the central and border phases of the Bow Creek granite, respectively, and 107.0 ± 0.72, 107.0 ± 0.78 and 107.5 ± 0.67 Ma for equigranular and porphyritic phases of the Dawson Range granodiorite. These ages confirm a mid-Cretaceous Whitehorse suite affinity for these rocks with porphyritic textures suggesting high-level emplacement. Feldspar Pb isotopic data of igneous rocks in the Mount Nansen area become more radiogenic with time and are distinct for Late Triassic to Early Jurassic plutonic rocks versus Cretaceous rocks. Cretaceous feldspar isotopic data broadly overlaps Pb isotopic values of galena from deposits throughout the Dawson Range suggesting coeval magmatic rocks are a significant source of metals.

The Reid Lakes batholith (RLB) in southwestern McQuesten map area (115P) has previously been tentatively assigned a mid-Cretaceous age, although two K-Ar ages from the northwestern part of the batholith indicated that at least part of the body must be late Paleozoic or older. U-Pb dating of two lithologically distinct samples from the southeastern part of the RLB yields Early Mississippian crystallization ages (341.5 ± 0.7 Ma and 355.7 ± 0.9 Ma). A sample of the Moose Creek pluton, which is on the northeast side of the Tintina fault zone, but was previously interpreted to be part of the RLB, yielded a U-Pb age of 92.0 ± 0.3 Ma. The Moose Creek pluton is therefore correlated with the mainly 95-93 Ma South Lansing plutonic suite which intrudes rock units of Ancestral North America, whereas the RLB is a multi-phase, Early Mississippian intrusion into metamorphic rocks of the Yukon-Tanana terrane.

Late Triassic to Early Jurassic age (~220-185 Ma) intrusions comprise one of the most widespread and volumetrically significant plutonic suites in central and western Yukon, and eastern Alaska, but have received very limited study thus far. A new research project has been initiated that will examine the temporal, geochemical and petrotectonic evolution of this magmatic event, and the nature and origin of associated Cu, Au and PGE mineralization. The intrusions are mainly hornblende- and biotite-bearing granodiorites and quartz monzonites, although granitic phases and rare ultramafic phases (as at Pyroxene Mountain) are also present. Several bodies of coarse-grained muscovite granite that are included within the suite have been recognized in southwestern Dawson, and central and western Stewart River map areas. Most intrusions give preliminary U-Pb zircon and titanite ages of ~195 Ma to ~185 Ma, although scattered bodies give ages up to 218 Ma. Geochemical studies completed thus far indicate that most intrusions are metaluminous and formed in a volcanic arc environment, although some of the muscovite-granite phases in western Yukon are peraluminous and trend into the anorogenic (within-plate) granite field on various tectonic discriminant plots. Dating studies at Minto and Williams Creek indicate that copper-gold mineralization in both areas is hosted in part by deformed intrusions dated at ~194 Ma and is crosscut by massive, post-mineralization Granite Mountain batholith dated at ~190 Ma. The mineralization is therefore intimately associated with the Triassic-Jurassic magmatism, and we tentatively interpret the deposits as deformed copper-gold porphyries.

Granodiorite of the Coast Plutonic Complex intruded metasedimentary rocks in the Dezadeash Range of the northern Coast Belt in the late Mesozoic. Graphitic staurolite-biotite schist, associated with the Kluane Metamorphic Assemblage, underlies the western Dezadeash Range, whereas cordierite-biotite gneiss, previously correlated with the Late Proterozoic - Paleozoic Nisling Assemblage, is exposed in the eastern and southern regions. A terrane boundary was placed in the central Dezadeash Range. Recent petrographic studies reveal a southeastward increase in metamorphic grade. Prograde appearance of cordierite partly obliterated an older schistosity and caused a fabric change near the postulated terrane boundary. Furthermore, typical continental margin rocks, such as marble and quartzite, are not observed. This suggests that all metamorphic rocks in the Dezadeash Range can be correlated with the Kluane Metamorphic Assemblage, whereas Nisling Assemblage rocks occur in the Coast Mountains to the east. Therefore, the terrane boundary is located in the Dezadeash River valley, further southeast than previously thought.

Paleomagnetic conglomerate tests were run at three localities in the Conglomerate formation of the Jurassic Laberge Group in the southern Yukon. The localities are located within the Whitehorse Trough about 5, 20 and 40 km east of its margin with the intruded Coast Plutonic Complex. At each locality, 2.5-cm-diameter cores were drilled from 32 clasts of mainly igneous provenance and 8 more from the conglomerate matrix, totaling 120 cores, and yielding 190 specimens in all. The specimens were analysed using standard paleomagnetic demagnetization and testing techniques. The matrix, mostly greywacke in composition, yields characteristic remanent magnetization (ChRM) directions carried by pyrrhotite that give coherent steep downward directions. The clasts, at all three localities, yield paleomagnetic conglomerate tests that show statistically random ChRM directions. This leads to the conclusion that the clasts were not remagnetized by the event that remagnetized the matrix, which was likely caused by hydrothermal fluid flow during the Late Cretaceous to Eocene Laramide Orogeny. Furthermore, these conclusions support earlier studies by demonstrating that felsic plutons in the Whitehorse Trough likely carry primary ChRM directions, e.g., the 75 Ma Mount Lorne stock, the 109 Ma Mount McIntyre pluton, the 112 Ma Whitehorse batholith, and the 186 Ma Teslin Crossing stock.

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This report presents the geochronology results for one sample from a Late Triassic Pyroxene Mountain pluton collected in the Mount Nansen area and analyzed for U-Pb zircon dating at Boise State University. These data complement the large dataset released in the Atlas of Sack et al. (2020).

Early Jurassic plutons in west-central Yukon were emplaced during accretion of the Intermontane terranes onto the western North American margin, and their exhumation is recorded in the sedimentological and geochronological record. Here the aluminum-in-hornblende geothermobarometer is applied to the 200-197 Ma Tatchun batholith and the 188-180 Ma Aishihik batholith, to estimate crystallization depths and exhumation rates. The Tatchun batholith crystallized at 6.4-7.2 kbar (23-26 km), whereas the Aishihik batholith was emplaced at 3.4-4.2 kbar (12-15 km). The Tatchun batholith exhumed at a rate of ~1.2-2.0 mm/a after crystallization, and was likely exposed at surface by the Middle Jurassic. Although subvolcanic equivalents of the Tatchun batholith have been lost to erosion, it could be prospective for deformed copper deposits such as Minto and Carmacks Copper. Hypabyssal and water-saturated phases of the Aishihik batholith suggest that portions of the batholith could be at, or just below, an appropriate crustal level for magmatic-hydrothermal mineralization.

The early Tertiary magmatic episode in the northern Canadian Cordillera is linked to the restructuring of the Kula-North American plate system from orthogonal to oblique convergence. Resultant volcanism was widespread, and remnant successions outcrop along the eastern margin of the Coast Plutonic Complex (CPC). The Sifton Range volcanic complex of southwestern Yukon is a member of the Paleogene Sloko-Skukum Group, and comprises a 900-m thick, shallow-dipping, volcanic succession dominated by intermediate to evolved lava and pyroclastic rocks deposited in a northwesterly trending half-graben. Locally, the volcanic sequence is intruded by alkali-feldspar granites of the CPCs Nisling Plutonic Suite dated at 57.5 Ma. Felsite sills radiate from the main intrusive body, and together with numerous basaltic to dacitic dykes traverse the volcanic package. Both the felsic volcanic rocks and epizonal granitoids exhibit anomalous enrichments in large-ion lithophile elements indicating crustal contributions during the late-stage petrogenesis of the complex. In addition, the Sifton Range intrusive rocks exhibit modal mineralogy reflective of lower ambient pressures relative to the compositionally similar Annie Ned granites along the Alaska Highway between Stony Creek and Mendenhall, 20 km south of the complex. The amount of post-Eocene uplift (ca. 30 m/Ma) that exposed the contact between the intrusive and corresponding volcanic rocks is constrained by the presence of a calc-silicate bed at an elevation of 1830 m within the upper volcanic stratigraphy.