UBC Undergraduate Research

A Petrographic and Fluid Inclusion Comparison of mid-Cretaceous gold bearing veins near the 5 Moz Coffee Deposit, west-central Yukon Lee, Well-Shen


The Dawson Range district in west-central Yukon is host to various important mineral deposits, including several fault and fracture-hosted gold systems, such as the significant 5 Moz Coffee gold deposit. Three gold-bearing hydrothermal vein systems of inferred mid-Cretaceous age (~92-96 ma), sharing the same Au-As-Sb relationship as Coffee are located nearby. A field, petrographic and microthermometric investigation concludes that the Boulevard and Sugar prospects share a common auriferous vein generation with similar ore and gangue minerals. Auriferous veins are strongly controlled by steeply dipping, brittle shear-structures with cataclasite zones as well as disseminated gold-hosting arsenian pyrite replacing mafic minerals and magnetite within the host rock- features also observed at Coffee. Boulevard, Sugar and Longline (a past-producer of gold in the Dawson Range) share similar low salinity (0-8 wt% NaCl), CO₂ rich (~0.22 mol%), methane bearing fluids, suggesting a possible co-genetic relationship. All three systems share similar trapping conditions (~300 °C, 1 kbar), however, modeled isochores do not intersect arsenian pyrite stability fields. The Toni Tiger molybdenum occurrence, shares similar fluid characteristics and mineralization age to Boulevard auriferous veins in the absence of a causative pluton. An orogenic molybdenum model is proposed whereby pacification of magnetite buffers allowed for fluid redox fluctuations to precipitate locally sourced molybdenum during the same gold deposition event at Sugar and Boulevard. An epizonal orogenic gold deposit model is proposed for the Dawson Range gold systems, whereby trans-crustal shear zones tap deep-crust gold-bearing metamorphic fluids, advected by residual heat from the cooling Dawson Range batholith (emplaced ~100 Ma).

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