UBC Theses and Dissertations
Mineralogy, geochemistry, and geochronology of the Northern Dancer tungsten-molybdenum deposit, Yukon and British Columbia Brand, Allison Aurora
The Northern Dancer (formerly Logtung) deposit is a low-grade, large-tonnage W-Mo intrusion-hosted porphyry system located on the Yukon-BC border within the central belt of the Yukon-Tanana Terrane. Discovered in 1976, it has an inferred resource estimate of 242 Mt at 0.10% W0₃ and 0.047% MoS₂. Mineralization is developed within late Cretaceous monzonitic granite (109.4 ± 0.9 Ma to 110.5 ± 0.8 Ma), comagmatic crosscutting felsic dikes (111.7 ± 0.7 Ma), and adjacent hornfelsed metasedimentary rocks, which were metamorphosed by early Jurassic diorite plutons (187.7 ± 2.6 Ma) located southwest and northeast of the deposit. The source of the metals is inferred to be the monzonite and felsic dikes. Four spatially overlapping vein types host the majority of mineralization, although minor dissemination occurs in some metasedimentary host rocks. The earliest mineralized veins (Type 1) are quartz-garnet-diopside dominant with accessory molybdoscheelite and fluorite. Crosscutting the Type 1 veins are quartz-feldspar-fluorite veins (Type 2) with accessory scheelite, pyrite, and prominent alteration halos (inner epidote-chlorite and outer homblende zones). Quartz-epidote veins (Type 3) are generally restricted to the felsic dike system and contain the majority of molybdenite mineralization. Polymetallic sheeted quartz-beryl-scheelite-molybdenite veins (Type 4) which crosscut all earlier veins and most felsite phases can be up to 1 m wide, and continue outside the defined deposit zone. Ore mineral assemblage and scheelite composition varies by vein!host environment; Type 1 veins contain molybdoscheelite only (avg. 4.85 wt.% MoO₃ or 0.09 Mo apfu), Type 2 veins contain purer scheelite (1.13 wt.% MoO₃ or 0.02 Mo apfu), Type 3 veins contain primarily molybdenite ± scheelite (0.73 wt.% MoO₃ or 0.02 Mo apfu), and Type 4 veins contain scheelite + molybdenite ± beryl (0.92 wt.% MoO₃ or 0.02 Mo apfu). Garnet compositions are intermediate between grossular and andradite end-members in metasedimentary rocks, and are spessartine-rich in felsic intrusive rocks; variations in fluorine content of the fluid may be reflected by F-content in garnet (up to 1.62 wt.% F or 0.39 F apfu). Pyroxene compositions are intermediate between hedenbergite and diopside end-members, and possess temporally evolving Fe²⁺/Fe³⁺ ratios, suggesting changing redox conditions. Whole-rock geochemical and Rietveld analyses suggest that high F content/fluorite abundance correlate with high W content/scheelite abundance.
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