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The geology and genesis of the Coffee gold deposit in west-central Yukon, Canada : implications for the structural, magmatic, and metallogenic evolution of the Dawson Rage, and gold deposit models MacWilliam, Kathryn R.G.


The Coffee gold deposit, located in the Dawson Range, west-central Yukon, Canada, is an example of structurally-controlled, gold-only mineralization with a global resource of ~4 Moz gold. Establishing the controls on ore distribution enhances exploration potential, and provides implications for the tectonic and metallogenic evolution of the northern Cordillera, and global gold deposit models. The controls on ore distribution were identified by drill core logging, field mapping, petrography, and analytical techniques that included geochemistry, shortwave infrared spectroscopy, and geochronology. The results from these analyses allowed the geological and structural framework, and a hydrothermal model for ore formation to be established. Furthermore, a genetic model for hydrothermal events and exhumation within the Dawson Range was developed. Coffee is hosted in metamorphic rocks of the Yukon-Tanana terrane, and mid-Cretaceous plutonic rocks. Metamorphic rocks include psammitic to semi-pelitic schist and K-feldspar augen-bearing orthogneiss that were metamorphosed to lower amphibolite in the Permian. Plutonic rocks include biotite granite of the ca. 100 Ma Coffee Creek pluton, and coeval intermediate dykes. The east-trending dextral strike-slip Coffee Creek fault exerts an important control on mineralization, which is disseminated in the wall rock of high order faults and fractures, and ~1m wide breccia corridors. Mineralization comprises auriferous sulphides that are associated with an alteration assemblage of quartz-muscovite-illite-kaolinite-carbonate, which sulphidize host rock muscovite and biotite. Age determinations from hydrothermal white mica constrains mineralization to ca. 97 Ma, and the coexistence of alteration and ore minerals constrains the character of the hydrothermal fluid to ~250°C and pH ~5. Isotope data provides evidence for decoupled metal sources, and low temperature thermochronology constrains the depth of mineralization to ~5km. Coffee occurs in close spatial and temporal proximity to orogenic gold mineralization. This spatial association, the ore fluid characteristics, and the mid-Cretaceous tectonic regime argues for ore fluids sourced from metamorphic devolatilization of likely siliciclastic rocks at depth. Coffee is interpreted as an epizonal orogenic gold deposit, the result of a regionally significant gold mineralizing event. This interpretation has implications for the metallogeny and exploration of orogenic gold mineralization in the Dawson Range and northern Cordillera, as well as globally.

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