LATE AND POST-ACCRETIONARY MAGMATISM AND METALLOGENY IN THE NORTHERN CORDILLERA, YUKON AND EASTERN ALASKA

Late to mainly post-accretionary intrusions and comagmatic volcanic rocks of Late Triassic to Late Cretaceous age, all thought to be related to convergent tectonics along the northwestern margin of Laurentia, are widespread in southern and western Yukon and eastern Alaska. Late Triassic and Early Jurassic (218-185 Ma) intrusions within the Yukon-Tanana Terrane (YTT) and in the northern extensions of Stikinia and Quesnellia are metaluminous, strongly oxidized, yield geochemical signatures consistent with formation in a continental arc setting, and host significant Cu-Au porphyry(?) mineralization at the Minto and Carmacks Copper deposits. Early Cretaceous (~112-99 Ma) magmatism occurs within a band >400 km wide, from the Denali Fault on the southwest far onto the North American continental margin. This phase of magmatism comprises a belt of relatively oxidized, calc-alkaline intrusions of continental arc affinity on the southwest, with a broad, NE-younging zone of relatively reduced intrusions and caldera-filling volcanics in a retro-arc setting, inboard of the coeval arc. The origin of this retro-arc magmatism is much debated; models invoking crustal thickening and partial melting in a flat-slab subduction setting, slab break-off, and/or asthenospheric upwelling in the back-arc region have been suggested. Arc magmatism during this interval was associated with Cu-(Mo) porphyries, Cu skarns and a variety of epithermal and other gold-silver vein systems. Au-bearing intrusion-related vein systems, as well as W and Cu skarns, are associated with the retro-arc magmatism. Late Cretaceous magmatism in the region includes an early (~72-78 Ma) suite of calc-alkaline intrusions in a narrow belt in the Dawson Range that host major Cu-Mo porphyry deposits (e.g., Casino, Taurus), as well as a slightly younger (72-68 Ma) and more widespread suite of intrusive and volcanic rocks (Carmacks Gp.) that were emplaced simultaneously from the Denali Fault on the southwest to the Tintina Fault on the northeast. This younger phase of magmatism includes both calc-alkaline arc signatures as well as localized alkaline phases of possible plume origin, and has a very diverse metallogenic signature, including Cu, Mo and W-Mo porphyries, Cu-Au skarns, low-sulphidation epithermal veins, and Ag-Pb-Zn carbonate replacement deposits.