Abstract
Auriferous quartz veins have traditionally been invoked as the source of the extensive placer deposits of the Klondike District. Exploration for plausible bedrock sources has proved challenging because of a lack of outcrop and the discontinuous nature of the veins, which are hosted in complex units of polydeformed schist. The discovery of a unit of disseminated gold in schist along the Lone Star ridge has raised the possibility that quartz veins may not be the sole source of the detrital gold.
The Klondike Gold district is located in north-west Yukon and is part of the Tintina Gold belt which extends through the Yukon and into Alaska. The discovery of the White Gold District in 2004 precipitated an exploration boom accompanied by studies of regional metallogeny. Subsequently both the Klondike and White goldfields were ascribed to a major period of orogenic mineralisation in the Mid-Late Jurassic.
Mineralised discordant quartz veins comprise subhedral milky quartz and contain visible gold associated with minor pyrite. The veining at Lone Star is interpreted to be part of a series of orogenic style veining across the Klondike which formed as a single stage (Rushton et al. 1993). Disseminated gold is present in a unit of schist thought to be a rhyo-dacite which also contains syngenetic sulphide lenses (Mortensen 1990). A model proposed by Mackenzie et al. (2007) suggests that the syngenetic layer may be a source for gold in the discordant veins (MacKenzie et al. 2007), but the genetic relationship between the auriferous schist and the auriferous veins has hitherto been unclear.
Detailed petrographic studies revealed that the parageneis of gold is late and associated with a discrete phase of hyaolophanee (Ba Kspar), Fe–Mg carbonate and telluride minerals. Imaging of the quartz veins using cathodoluminescence revealed four stages of quartz emplacement (Q1–Q4) with gold occurring in Q3. This phase of quartz displays bright CL, forms less than 5% of the vein and fractures pyrite. Gold occurs along the fractures of pyrite with hyaolphane at a number of localities. The mineralogical association and late paragenetic timing of gold in the schist and veins suggests that they may be part of the same event. A new model is proposed whereby a single aurifierous fluid permeated pre-existing fractures in veins and the schist.
Trace element analysis of the quartz and gold are currently being investigated to better characterise the auriferous fluid. The discovery of gold within the schist and a single auriferous quartz stage has permitted an informed critique of previous exploration strategies which have focussed on the (mostly barren) quartz veins whilst being unaware of a potential low grade high tonnage target. Additionally, the new model may provide a solution to discrepancies between gold inventories in auriferous quartz veins and their local placer expressions both at Lone Star and elsewhere.