JURASSIC STRUCTURAL AND TECTONIC ARCHITECTURE OF YUKON TANANA TERRANE IN WEST CENTRAL YUKON

Yukon-Tanana terrane (YTT) in west central Yukon comprises a tectonic collage of internally complex but distinct structural slices. The highest structural sheet mainly comprises the Early Mississippian Finlayson volcanic-dominated assemblage and the polyphase Simpson Range plutonic suite locally intruding the pre-Late Devonian Snowcap assemblage (SCA). The middle structural sheet mainly includes the SCA that is overlain and intruded by the volcanic and plutonic rocks of the mid-Permian Klondike assemblage. The upper and middle sheets are separated by the Yukon River thrust, which postdates Permian metamorphism. The structurally lowest sheet comprises the volcanic, plutonic and sedimentary rocks of the White River assemblage (WRA), interpreted as parautochthonous Laurentian rocks structurally below the relatively thin nappe of YTT. The middle and lower sheets are separated by the Moose Creek fault, which is marked by lozenges of metamorphosed mantle peridotite. The WRA is intruded by gabbro of the Late Triassic Snag Creek suite, indicating that the Moose Creek fault is post-Triassic. The two sheets of YTT rocks are inferred to restore to a pre-Early Jurassic position southwest of the WRA, but in close proximity to North America at that time. The Mississippian rocks of the upper structural sheet are overlain by the Triassic volcanic rocks of the Semenof formation and intruded by the Triassic Pyroxene Mountain suite, indicating the Yukon River thrust is post-Late Triassic. We interpret the Yukon River thrust as the cause of structural thickening prior to intrusion of the latest Triassic-Early Jurassic plutonic suite which hosts the Minto mine and Williams Creek deposit. Regional Ar/Ar data suggests that intrusion of these plutons (characterized by magmatic epidote) was followed by a rapid, regional exhumation event that cooled a vast portion of YTT in the Early Jurassic. One of these extensional faults has been documented as the Willow Lake fault. The deeper-seated nature of the northern part of the Late Triassic-Early Jurassic suite may explain the apparent lack of Early Jurassic porphyries in Yukon that are more typical in British Columbia. The Yukon River thrust may have continued to be a controlling structure on later metallogenic events, such as mid-Jurassic mineralization in the Whitegold district.