Drift prospecting in high relief areas of the Cordillera requires consideration of paleo-ice-flow reversals. This means rethinking the manner and degree to which glacial ice eroded, transported and deposited surficial sediments. The regional context, geomorphic landforms and sediment stratigraphy identified in the Seagull Creek valley suggest that late-glacial up-valley ice flow, although relatively short in duration, may have been the controlling process for glacial transport and deposition in this area. This interpretation has important implications for mineral exploration programs that utilize glacially transported materials for various forms of geochemical analysis. Geomorphic landforms and glacial dynamics responsible for reverse (up-valley) ice flow in Seagull Creek valley have important implications for mineral exploration on the Ross River Minerals Tay LP gold-copper property.

New geochronological and geochemical data from the ‘Windy-McKinley’* terrane provide insight into the age, correlation and paleotectonic settings of the various subdivisions of the terrane. U-Pb zircon age determinations for felsic meta-volcanic rocks of the White River formation and gabbro intrusions are Late Devonian and late Middle Triassic respectively. These new age determinations substantiate the proposed correlation of these components of ‘Windy-McKinley’ terrane with the succession on strike to the northwest which hosts the volcanogenic massive sulphide deposits in the Delta District, Alaska. Trace-element geochemical data from Triassic gabbro intrusions into the Mirror Creek and White River formations, and diabase and gabbro of the Harzburgite Peak-Eikland Mountain ophiolite suggest that magmatism in both subdivisions occurred in supra-subduction zone settings. However, the age of the ophiolite is not known, therefore mafic magmatism may not be coeval across the terrane and may have formed above different subduction zones at different times. *Quotes are used to indicate that the assignment to Windy and McKinley terranes is obsolete, but a new name has not yet been assigned.

Paleozoic platformal and basinal strata of Cassiar Terrane are separated from rocks of Yukon-Tanana Terrane to the west by an unexposed fault in southeast Glenlyon map area. Quartzite, marble, phyllite, and amphibolite are grouped in Cassiar Terrane, and no rocks of Slide Mountain Terrane are recognized. The mid-Cretaceous Glenlyon batholith contains pendants of Cassiar Terrane and is intruded by at least five andesite dykes. West of the fault, the Yukon-Tanana Terrane includes: (1) mafic volcaniclastic rocks with preserved primary textures; (2) coarse-grained quartz-feldspar grit; and (3) metasiltstone and semi-pelitic schist. The grit is intruded by foliated hornblende granodiorite, likely of early Mississippian age. Small outcrops of tectonized serpentinite were tectonically emplaced into Yukon-Tanana Terrane, and a positive magnetic anomaly parallel to the fault suggests an unexposed extension to the southwest. Two mylonite localities and evidence of brittle cataclasis up to 1 km on either side of the presumed buried fault suggest a complex structural history along this terrane boundary.

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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.

This dataset contains U-Pb isotopic data and associated ages of detrital zircon from sedimentary and metasedimentary rocks collected from southwestern Alaska and from the Yukon-Tanana upland and environs of eastern interior Alaska and western Yukon, Canada. The samples were collected as part of geological mapping and research conducted from 1996 to 2021 and funded by the Mineral Resources Program of the U.S. Geological Survey. Detrital zircon grains were separated and analyzed by four different laboratories: U.S. Geological Survey laboratories in Denver, Colorado; Apatite to Zircon, Inc. (A2Z); GeoSep Services (GSS); and the University of British Columbia. All analyses were conducted between 2007 and 2021 using laser-ablation-inductively-coupled-plasma-mass spectrometry (LA-ICP-MS) techniques.

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.

Nine pollen profiles were obtained along a broad latitudinal transect extending from the Upper Nisling Valley to the Ittlemit Lake and Bear Lakes areas in the southwest Yukon Territory. Radiocarbon dated fossil pollen records from these profiles provided evidence for the reconstruction of postglacial environmental history of the area. Surficial samples combined to aid the interpretation of the fossil data. Records of these sequences suggest that the post-glacial vegetation in the area exhibited a successional development. Immediately after the deglaciation, the area was covered by a sparse herb dominated tundra assemblage. Betula glandulosa invaded the area at about 10,000 yr BP, which initiated the replacement of herbaceous tundra by a dwarf birch shrub-tundra. Picea glauca invaded the Upper Nisling Valley in the northern part of the study area and the Ittlemit Lake Basin in the south at least around 9,000 yr BP, while 8,600 yr BP in the central Aishihik Basin, marked the establishment of forest-tundra environment in the area. Significant rise of Picea pollen at 7,500-8,000 yr BP in the north-central part of the study area indicate the establishment of a modern boreal forest, which is approximately 1,500-2,000 years earlier than that indicated by the Antifreeze Pond pollen diagram from the Snag area of the southwest Yukon. It is notable that the pollen spectra of two lacustrine sediments suggest a phase change of lakes into marsh occurring around 5,000-6,000 yr BP in the area, and change of bog conditions has also been recorded at almost the same time in the area. These events occurred simultaneously in the area and probably reflect the change of climatic conditions, which might have led to the development of permafrost in the region. The development of early post-glacial vegetation in the area exhibited an unstable pattern. This instability was primarily related to the sequential invasion and migration of taxa from late-glacial refugia. Stable vegetation associations were established in the area as a result of migrators reaching their limit of climatic tolerance during the middle Holocene. Post-glacial vegetation history in the Ittlemit Lake Basin area seems to have exhibited a different pattern.

The Tummel fault zone, a northwest-trending belt of rocks of uncertain age and/or tectonic affinity, separates Paleozoic miogeoclinal strata of Cassiar Terrane from Yukon-Tanana Terrane metavolcanic and metasedimentary rocks. Northeast of the fault, Cassiar Terrane comprises pelitic and semipelitic rocks with rare amphibolite, which are correlated with the Kechika Group. These are overlain by carbonate correlated with the Askin Group. Southwest of the fault, in Yukon-Tanana Terrane, Devono-Mississippian siliciclastic rocks are overlain by Mississippian arc volcanic rocks. Granodiorite and diorite of the Telegraph Plutonic Suite (348-350 Ma) intrude the siliciclastic rocks. Foliated greenstone, leucogabbro intrusions, serpentinite and chert occur in the Tummel fault zone. The Early Cretaceous Glenlyon Batholith intrudes strata of Cassiar Terrane. Contact metamorphism recognized across the Tummel fault zone is interpreted to have been imposed by the Glenlyon Batholith. If correct, this interpretation requires that post-mid-Cretaceous displacement across the Tummel fault zone has been minimal (~5 km).