Geologic evolution of the Yukon River: implications for placer gold

Abstract The greatest placer potential in northwest Canada lies in preglacial drainage systems. The largest placer gold deposits are associated with Pliocene pre-glacial fluvial gravels of the Dawson Range (in particular Klondike Plateau) of west-central Yukon. The abundance and/or the lack of gold concentration in this area were controlled by the ability of the streams to aggrade in response to changes caused by extensional faulting in the Tintina Trench, differential uplift, and regional denudation. Gold bearing gravels in the northern and eastern slopes of the Dawson Range are found in low order tributaries of the pre-glacial south-flowing Yukon River. Glaciation played an important role in masking and changing the direction of drainage, and creating new channels in this region. The earliest known drainage changes relate to Late Miocene formation of the Tintina Trench, and Early Pliocene uplift of the St. Elias Mountains. Prior to these events the Yukon River drained south into the Gulf of Alaska. Initial regional glaciation (2.6–2.9 Ma) diverted the southward drainage toward the northwest, becoming part of the Kwikhpak River in Alaska. The present Yukon River drainage basin is about 27.5% larger than the ancestral Kwikhpak River basin. The evolution of the river is described here in three stages (1) drainage in pre-Late Miocene, (2) drainage in Early Pliocene and (3) drainage after earliest Pliocene glaciation.