Microchemical Studies of Placer and Lode Gold in the Klondike District, Yukon, Canada: 1. Evidence for a Small, Gold-Rich, Orogenic Hydrothermal System in the Bonanza and Eldorado Creek Area

Abstract

Orogenic gold camps are commonly associated with placer deposits although in many cases the relationship between placer and lode source may be unclear. Placer gold deposits in Bonanza and Eldorado Creeks in the northern Klondike district goldfield, Yukon, Canada, probably yielded more than 2 million ounces (Moz) from a small geographic area, but known occurrences of gold-bearing orogenic-style veins from which the placers were derived accounted for only a few thousand ounces. The alloy chemistry and mineralogy of opaque mineral microinclusions (microchemical signatures) within these gold grains were compared to that of samples collected during a detailed placer gold sampling program. Collection of placer samples from the headwaters of watercourses in this unglaciated area allowed identification of microchemical signatures within gold populations from proximal sources which augmented information from lode gold sources to facilitate characterization of local gold mineralization. A total of 1,163 gold grains from 18 lode sources and 2,777 grains from 27 placer localities within the Bonanza and Eldorado Creek drainages were characterized in terms of the Ag and Hg content of the alloy together with the associated inclusion suite, and two main compositional ranges, along with several subtypes, were identified. Type 1 gold is a simple Au-Ag alloy but various subtypes have been identified on the basis of different ranges of Ag and/or specific inclusion assemblages. Type 2 gold is an Au-Ag-Hg alloy composition. The spatial arrangement of the gold types shows a systematic change in microchemical signature. The lowest Ag gold occurs in the Lone Star Ridge area, and the Ag content increases both in the northeast and southwest. The appearance of Te-bearing mineral inclusions in populations coincides with increasing Ag, and argentite inclusions are present only in high Ag gold toward the periphery of study area. Type 2 gold is absent at Lone Star Ridge and all localities to the east but is progressively more abundant in sample localities to the west. The geographic distribution of gold types correlates well with placer abundance (inferred from the production from previous mining activity) and the study area is bounded by creeks and gulches in which there was insufficient placer gold for economic extraction. We conclude that the Lone Star Ridge area represents the epicenter of a single hydrothermal system in which spatial and temporal fluid evolution has generated compositional zonation. Variation in Ag is ascribed to decrease in Au/Ag ( aq ) following preferential Au deposition in the center of the system. Controls on Hg contents within Au alloy are unclear but may be a consequence of mineralization temperature. The complex but consistent signatures of placer gold samples from the Eldorado Creek drainage represents liberation of types 1 and 2 gold grains which coexist in some lode occurrences. The different proportions of types 1 and 2 gold observed in placer and lode samples from west of Lone Star Ridge reflect both temporal and spatial evolution of the hydrothermal system. The main Eldorado Creek placers exhibit a signature most similar to lode gold from the Buckland and Nugget zones on Lone Star Ridge. Placer gold in upper Bonanza Creek is distinguished by a relatively narrow Ag range and the small but consistent presence of hessite in the inclusion suite. No corresponding lode occurrence for this gold type has been identified to date. This is the first time that the compositional signatures of major placer gold deposits derived from orogenic gold vein systems has been studied in this detail, and the application of basic chemical principles to fluid evolution and electrum precipitation has allowed demarcation of an area in which extremely rich gold mineralization was formerly present, while simultaneously identifying the most promising areas for further exploration for undiscovered lode gold.