Abstract: Mid-Cretaceous plutons in the McQuesten River region intrude Upper Proterozoic to Mississippian miogeoclinal metasedimentary rocks of Selwyn Basin. They form a belt trending east from the Tintina Trench which can be roughly subdivided into two parallel belts. Plutonic rocks fall into three main groups: 1) biotite-muscovite (two-mica) granite in the southernmost belt which follows the trend of the McQuesten Anticline; 2) biotite-hornblende quartz monzonite, granite and granodiorite in the northern belt which follows the thrust faulted contact of the Hyland Group (Grit Unit) with the Road River Formation; and 3) hornblende-biotite syenite and associated quartz syenite, quartz monzonite, granite and tourmaline-orbicular granite along the north edge of the northern belt.

Tin-silver breccia veins and skarns are spatially associated with the two-mica granites, while tungsten-gold skarns and sheeted veins are associated with biotite-hornblende granite, quartz monzonite and granodiorite. The concentrically zoned syenite intrusion in the northern belt (ZETA) includes all the plutonic phases (two-mica granite, biotite-hornblende granitoids, and hornblende-biotite syenitoids), and links them cogenetically through the fractional crystallization process. The ZETA tin-silver veins are associated with the tourmaline orbicular granite, which is the most evolved phase of the concentrically zoned ZETA syenite intrusion.

Plutons in the McQuesten region resemble those of the Selwyn and Tombstone Plutonic Suites. They are post to syntectonic, roughly circular in shape, and intrude miogeoclinal metasedimentary rocks of ancient North America. They show a concentric zonation and are surrounded by contact aureoles. The intrusive suite is bimodal, with a southern belt consisting of evolved two-mica granites and a northern belt consisting of less evolved biotite-hornblende granites. Lavas associated with the plutons are believed to be coeval.

Authors: Emond, D.S.

NTS Mapsheet(s): 115P

Citation: Emond, D.S., 1992. Petrology and geochemistry of tin and tungsten mineralized plutons, McQuesten River Region, Central Yukon. In: Yukon Geology Volume 3, T.J. Bremner (ed.)Exploration & Geological Services Division, Indian & Northern Affairs Canada, p. 167-195.

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