Microchemical Studies of Placer and Lode Gold in the Klondike District, Yukon, Canada: 2. Constraints on the Nature and Location of Regional Lode Sources

Abstract

Alloy compositions and associated suites of opaque mineral inclusions have been used to define the micro-chemical signature of 3,050 placer gold grains from 34 localities and 561 gold grains from six lode localities from the northeastern, central, and southern Klondike gold district, Yukon, Canada. These data augment those from a detailed parallel study of placer-lode gold relationships in the western part of the Klondike goldfield (see Chapman et al., 2010). Gold compositions have been considered in association with chemical controls on alloy compositions to establish whether variation between signatures represents zonation within a single mineralizing event or discrete episodes of mineralization. Three main gold types have been identified each of which exhibits subtypes. Type 1 gold is generally a simple Au-Ag alloy with low Ag (12–25%) and an inclusion suite dominated by pyrite and simple base metal sulfides. Type 2 gold is generally Ag rich (18–55%) and contains Hg (≤9%). Type 3 gold (10–40% Ag ± Hg ≤ 4%) is prevalent in the south of the region. Analysis of large sample sets has allowed us to delineate the geographic extent of each gold type and their relative importance within the placer populations. Type 1 gold is the major component of placer deposits in Bonanza, Hunker, Dominion, and upper Sulphur Creeks. Type 2 gold is relatively trivial but contributes to placer gold in Bear Creek and some of the paleoplacers in White Channel Gravel in the north of the study area. Type 3 gold is actively augmenting the placer inventory in the south of the region. This study builds upon a detailed parallel study in the western Klondike which concluded that types 1 and 2 gold were emplaced by a single hydrothermal system of limited geographic extent, centered on the Lone Star Ridge. Here we propose that small, rich hydrothermal systems were also present in the upper reaches of Bear and Last Chance Creeks and Hunker Dome. If correct, this model shows that economically important orogenic gold mineralization can be formed in smaller hydrothermal systems than previously recognized. In addition, type 3 gold is widespread in the south of the study area where it appears to be present at the surface. Consideration of the abundance of the various gold types contributing to the placers has established type 1 gold to be the most important and originally present in geographically constrained vertically persistent mineralization. We propose that this type offers the best target for exploration in the northern and central Klondike. In the southern Klondike the large placers containing only type 3b gold suggest a potentially important undiscovered source in the catchment of Gold Run Creek.