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Nature and origin of the early Jurassic copper (-gold) deposits at Minto and Williams Creek, Carmacks Copper Belt, Western Yukon : subtitle examples of deformed porphyry deposits

University of British Columbia

The Minto (~9 million tonnes; 1.73% Cu, 0.48 g/t Au, 7.5 g/t Ag) and Williams Creek (Carmacks Copper; ~15.5 million tonnes; 1.01% Cu, 0.42 g/t Au) deposits represent the most significant Cu(-Au) deposits discovered thus far within the Carmacks Copper Belt of western Yukon. The origin of the mineralization is controversial and several very different genetic models have previously been proposed for these deposits, including metamorphosed volcanic massive sulphide, metamorphosed redbed copper, or deformed porphyry-style mineralization. Mineralization consists of disseminations and stringers of chalcopyrite and bornite hosted by variably deformed Early Jurassic plutonic rocks and to a lesser extent strongly metamorphosed supracrustal rocks ("siliceous ore" and "biotite schist/amphibolite") that are interpreted to represent mineralized and deformed metasedimentary wall rocks to the mineralized intrusive phases. Mineralization occurred prior to the ductile deformation that has affected the ore host rocks, which now occur as rafts and screens of variable size within slightly younger, undeformed phases of the Granite Batholith. Lithogeochemical studies of both pre- and post-mineralization intrusive phases at Minto and Williams Creek indicate that they are weakly peraluminous, sub-alkaline, and formed in a continental magmatic arc setting. Mineralized granodioritic gneiss from both deposits and post-mineralization granodiorite at Williams Creek yield U-Pb zircon ages of about 197-198 Ma and massive, post-mineralization intrusive phases yield essentially the same age. Ar-Ar dates for muscovite from selvages developed along late quartz-feldspar-epidote veins at Minto are 182-183 Ma. Al-in-hornblende geobarometry for post-mineralization intrusive phases at Minto and Williams Creek indicate that they were emplaced at a depth of >9 km. Pb and S isotopic studies of both deposits indicate a mainly magmatic source for the contained metals and sulphur. Results of the study indicate that the two deposits are most reasonably interpreted to be Cu(-Au) porphyry deposits that formed during the late stages of Early Jurassic arc activity in the region and were tectonically buried and subsequently deformed and metamorphosed immediately after formation. Cu/Au ratios and field observations indicate extensive supergene mobility of Cu, especially at Williams Creek, that has complicated previous interpretations of the deposits.

Text

2009-12-11

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http://hdl.handle.net/2429/16627

Geological Sciences

University of British Columbia

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