DEFORMED EARLY JURASSIC PORPHYRY CU(-AU) DEPOSITS AT MINTO AND WILLIAMS CREEK, CARMACKS COPPER BELT, WESTERN YUKON TERRITORY

The Minto (~9 million tones; 1.73% Cu, 0.48 g/t Au, 7.5 g/t Ag) and Williams Creek (Carmacks Copper; ~15.5 million tones; 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. 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 hosts, which now occur as rafts and screens of variable size within slightly younger, undeformed phases of the Granite Batholith. Previously proposed genetic models for the deposits include metamorphosed VMS, metamorphosed redbed copper, or deformed porphyry-style mineralization. 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 on post-mineralization intrusive phases at Minto and Williams Creek indicate that they were emplaced at a depth of >9 km. 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. Pb and S isotopic studies of both deposits indicate a magmatic source for the contained metals and sulphur. Our results indicate that the two deposits are Cu(-Au) porphyry deposits that formed during the final stages of Early Jurassic arc activity and were tectonically buried and subsequently deformed and metamorphosed during the last stages the mineralization process. Cu/Au ratios and field observations show extensive supergene mobility of Cu especially at Williams Creek that has complicated earlier interpretations of the deposits.