Bimodal Tertiary volcanic rocks are exposed along the Tintina Trench in central Yukon in the Glenlyon, Grew Creek, and Ketza areas. Basalt and rhyolite are interbedded with coarse sedimentary rocks and preserved in grabens. The basalt forms both subaqueous and subaerial flows, hydrovolcaniclastic, pyroclastic, and autoclastic deposits. Rhyolite forms intrusions, lava flows, and volcaniclastic deposits. The volcanic and sedimentary rocks probably formed in a series of extensional basins whose original size and shape is yet to be determined. At Grew Creek, gold-bearing chalcedony veinlets and associated argillic alteration are near rhyolite dykes. In the Glenlyon Area, quartz, chalcedony, and fluorite veins and associated silicification are in and near rhyolite dykes.
