Reconnaissance geology of Aishihik Lake, Snag and part of Stewart River map-areas, west-central Yukon

Aishihik Lake, Snag and part of Stewart River map-areas caver an area of more than 13,000 square miles, almost entirely within western Yukon Plateau, in west-central Yukon Territory. Metamorphosed pelitic and igneous rocks of Proterozoic and/ or Paleozoic age, included in the Yukon Group, underlie much of the region. They are intruded by large batholiths of hornblende, granodiorite, quartz monzonite and pink quartz monzonite of various Mesozoic ages. In the east part of the region Jurassic and Lower Cretaceous coarse clastic, immature sedimentary rocks of the Laberge Group and Tantalus Formation are faulted against this crystalline terrane. A suite of acid high-level plutonic, sub­volcanic and volcanic rocks, of Eocene age, the Nisling Range Alaskite and Mount Nansen Group, discordantly intrudes and overlies the older rocks. Basalt and conglomerate of the Carmacks Group (Eocene to Miocene) overlie all older rocks with distinct unconformity and are laid down on a surface of moderate relief that closely approximates the modern topography. Major and minor structures in the metamorphic rocks trend gener­ally northwest and the folded layering is a coarse schistosity that mimics a crenulation foliation. Linear elements in the rocks predate the latest folding and metamorphism which probably took place during the Early Mesozoic. Rocks of the Laberge Group are thrown into large southwestward verging folds. No recrystallization accompanied this deformation which took place in mid­Cretaceous time. Mineralization discovered to date is of two types. Copper- and tungsten-bearing magnetite skarns in marble of the Yukon Group or in prob­able Triassic volcanic rocks are found mainly in Aishihik Lake map-area. Molybdenite and chalcopyrite, disseminated in rocks of the Nisling Range Alaskite-Mount Nansen Group suite, occur in Snag and northern Aishihik Lake map-areas.