UBC Theses and Dissertations
Mid-crustal Cu-Au mineralisation during episodic pluton emplacement, hydrothermal fluid flow, and ductile deformation at the Minto deposit, YT, Canada Hood, Shawn Bruce
The genesis of the Minto copper-gold deposit, YT, Canada, has been variously interpreted since its discovery although no existing model accounts for ductile deformation as a control on mineralization. Results from this study show that Minto ore is hosted within ductiley deformed granitoid host rocks emplaced as multiple intrusions into an actively deforming environment, with the variably sheared host rocks separated by incipiently deformed granodiorites essentially barren of mineralisation. Contacts between deformed/mineralized rock and incipiently deformed/barren rock range from abrupt to gradational, and are the product of pre-existing igneous contacts, variably partitioned deformation, or a combination of these. Deformation of granitoids is interpreted to have controlled fluid flow, with associated alteration promoting further deformation and fluid flow. Potassic alteration, in the form of biotite-magnetite, is the dominant alteration associated with mineralisation, and analysis of alteration using isocon diagrams indicates that K, Fe, Si, Cu, Au, and Ag have been added during alteration, although mass has been lost overall due to a relative reduction in Na and Ca. Host rock intrusion, mineralization, and deformation are interpreted via geochronology and crosscutting relationships as ongoing over at least 5 m.a., from about 202 Ma until about 197 Ma, based on U-Pb SHRIMP geochronology of zircons in granitoids and Re-Os ICP-MS geochronology of molybdenite. The trend of mineralisation is now coincident with the strike of foliation on short steep limbs and of axial planes of folded foliation. This geometry may not be representative of original processes, but of remobilization of ore during continued deformation. The above observations, coupled with data from existing studies, strongly suggest Minto is representative of deposit generation within an arc subduction environment at depths not typically considered for copper-gold deposit formation.
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