The origin and evolution of the Klondike goldfields, Yukon, Canada

The world famous Klondike goldfields are located in the unglaciated part of west-central Yukon, Canada. Since their discovery over 100 years ago, they have produced an estimated 311 tonnes of gold, primarily from bench and creek placers that are fluvial in origin and range from Pliocene to Holocene in age. Historically, the placers are classified into three levels of gravel with four main units. These include the high-level White Channel Gravel (Pliocene), presently the most important gold-bearing unit, which sits nonconformably on an erosional bedrock surface (i.e., the ‘White Channel strath’) and is overlain and interbedded with the glaciofluvial Klondike Gravel (Pliocene); the intermediate-level gravel (Pleistocene), the least important economically; and the low-level gravel (Pleistocene–Holocene), historically the most important gold-bearing unit, but it has been mined three or four times now. The goldfields originated from the weathering and erosion of early Cretaceous, discordant mesothermal quartz veins, and the light grey color of the matrix of the White Channel Gravel is due mainly to weathering and diagenetic alteration by groundwater flow. The concentration of placer gold is related to a hierarchy of physical scales: at the lithofacies scale (metres), bed roughness determined sites of gold deposition; at the element scale (tens of metres), gravel bars were preferentially enriched in gold; at the reach scale (hundreds of metres), stream gradient was an important factor; at the system scale (hundreds of km), braided river environments transported large amounts of gold; and at the sequence scale (thousands of km), economic placers formed initially in the high-level White Channel Gravel and later in the intermediate-level and low-level gravel. The White Channel strath is interpreted as an erosional ‘tectonic’ terrace that formed during isostatic uplift and under conditions of dynamic equilibrium. The high-level White Channel Gravel and Klondike Gravel are interpreted as a depositional ‘climatic’ terrace that formed during a reversal in the tectonically induced downcutting, which is attributed to the initial and most extensive of the pre-Reid glaciations (3 Ma) in the Yukon. The intermediate-level gravel is interpreted as minor erosional ‘complex response’ terraces that formed during static equilibrium when there were pauses in valley-floor degradation, which are attributed to the subsequent and less extensive pre-Reid glaciations. The low-level gravel formed also during valley-floor degradation and may represent a return to dynamic equilibrium conditions. Hence, the dominant forcing mechanisms controlling the evolution of the goldfields were isostatically compensated exhumation and climatic change related to the repeated glaciation of the Yukon. In addition, the lowering of baselevel from high-level, to intermediate-level and finally to low-level gravel was accompanied by a decrease in accommodation space (as indicated by a decrease in gravel thickness), which resulted in an increase in the concentration of the placer gold.