A distinctive and widespread tephra bed is a very useful stratigraphic tool, especially if its age is accurately and precisely known. However, distal tephra beds, like those in the Klondike region of the Yukon, are difficult to date because of their fine grain size and contaminated character. Grain-specific methods, such as the fission-track and 40 Ar/39 Ar techniques, are essential for reliable results. Three rhyolitic tephra beds, each with a homogeneous glass composition, have been dated by the glass-fission-track method. Mosquito Gulch tephra is 1.42 ± 0.16 Ma and dates an intermediate-level terrace on Bonanza Creek. Quartz Creek tephra is 3.00 ± 0.33 Ma and confirms a late Pliocene age for the upper part of the White Channel gravel. Furthermore, the presence of the Quartz Creek tephra in an ice-wedge cast indicates that permafrost conditions must have existed in west-central Yukon at this time. The Jackson Hill tephra is 0.13 ± 0.03 Ma.

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Although the White Channel gravel (WCG) of the Klondike district, Yukon, contains gold placers which have been exploited for over a century, few sedimentological studies have been undertaken. This study reports a four stage evolution of the WCG, comprising:i. An initial downcutting period which preferentially retained gold particles on the base of the strath.ii. An aggradational stage in which gold concentration occurred within sedimentary features.iii. A lacustrine layer representing a depositional hiatus.iv. A final, more rapidly aggrading fluvial stage.Identification of the lacustrine layer has clarified the evolution of the WCG depositional fluvial systems. Architectural element analysis and detailed sedimentological observations have been synthesized to gain a clearer understanding of the spatial variations within the WCG. Additionally, the identification of plant species from pollen within the lacustrine layer provides irrefutable evidence that the Klondike district was at least 7°C warmer during the Pliocene compared to the present.

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.

Six dikes that are closely associated with the Klaza epithermal vein system in the Mt. Nansen district yield early Late Cretaceous U-Pb zircon ages (78.2-76.3 Ma); this age is similar to that obtained from the porphyry stock that hosts the Cyprus Cu-Mo-Au porphyry occurrence immediately to the southeast. These results support the interpretation that epithermal veins in the Mt. Nansen district are likely genetically related to subvolcanic magmatism. Granodiorite of the Dawson Range batholith that underlies most of the Klaza property gives a U-Pb zircon age of 107.9±0.3 Ma. These dates overlap with previously reported mid-Cretaceous U-Pb zircon ages for felsic dikes associated with the Brown-McDade and related vein and breccia deposits in the Mt. Nansen mine. The new results, together with regional dating and Pb isotopic data from western Yukon, emphasize the metallogenic importance of the “early Late Cretaceous” magmatic-hydrothermal event in this region.

Map sheets 95C/5 and 95D/8 in southeast Yukon contain eight successions of sedimentary rocks ranging from Proterozoic to Paleocene in age. Early to middle Paleozoic starta are best preserved. Proterozoic strata have been tentativel correlated with Pinguicula Group, Mackenzie Mountains Supergroup and Windermere Supergroup. Newly defined Cambrian-Ordovician Crow Formation documents shallow water to suaerial siliciclastic sedimentary rocks related to a paleogeographic high.

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We report here the results of U-Pb zircon analyses for samples collected during regional mapping of the northern Whitehorse trough. Three samples of the Nordenskiold formation, a crystal-rich volcaniclastic unit that occurs at several stratigraphic horizons in the Laberge Group, yielded concordant U-Pb zircon ages of 188.1 ± 0.4 Ma, 187.2 ± 0.4 Ma and 186.5 ± 0.3 Ma, respectively. These results clearly indicate multiple eruptive events rather than recycling of the volcaniclastic material. Zircons from a thin layer of ash tuff in the Tanglefoot formation (Laberge Group) yielded a concordant U-Pb age of 187.1 ± 0.7 Ma, confirming correlation with the Nordenskiold formation. A sample of porphyritic granite from the only pluton intruding the northern Whitehorse trough, near Carmacks, yielded a concordant U-Pb zircon age of 112.8 ± 0.2 Ma, whereas a precise age could not be resolved from zircons extracted from a porphyry dyke intruding the Tantalus Formation in the southern Whitehorse trough.