Geology and genesis of copper deposits and associated host rocks in and near the Quill Creek area, southwestern Yukon
Creators: name: Campbell, Susan Wendy
Publisher: name: University of British Columbia
Year: 2010

description: The Kluane Ranges are underlain by a sequence of stratified rocks ranging in age from Permian to Triassic, cut by .Cretaceous and Tertiary intrusions. Flow and pyroclastic rocks of Lower Permian Station Creek Formation were probably part of a volcanic arc environment. Argillaceous and tuffaceous rocks of the Transition Zone of Station Creek Formation and overlying sedimentary rocks of Lower Permian Hasen Creek Formation were deposited in a subaqueous environment, possibly a back-arc basin. Upper Triassic basalts of the Nikolai Greenstone are largely subaerial and were probably a product of rift volcanism. Lower Triassic Kluane gabbro-ultramafic complexes are sill- or sheet-like bodies, divisible into a Lower Group (within the Transition Zone) that consist of peridotite and dunite with associated nickel-copper sulphide deposits, and an Upper Group (within Hasen Creek rocks) that consist of gabbro and peridotite but with no known associated nickel-copper deposits. The ultramafic rocks are piagioclase-bearing and chemically are pyroxenitic and peridotitic komatiites. Bulk chemical composition of the Quill Creek complex is similar to that of pyrolite and calculations show the complex could have formed from an upper mantle diapir, affected by 40 percent partial melting and composed of residual olivine crystals and ultramafic liquid. Prominent copper lode deposits in the Kluane Ranges include: (1) vein and disseminated types in Station Creek volcanic rocks; (2) nickel-copper associated with Kluane complexes; and (3) vein-type in the Nikolai Greenstone. A narrow, positive range of sulphur isotopic compositions for vein sulphides in Station Creek Formation contrast sharply with a large range for those in the Nikolai Greenstone. The former deposits resulted from only local mobilization of sulphur (and metals) during metamorphism in a closed system, whereas mineralization in the Nikolai Greenstone involved considerable variation in chemical parameters of ore fluids in an open system with more than one source of sulphur. Anomalously negative sulphur isotopic values for nickel-copper sulphide bodies resulted from contamination of magmatic sulphur by 30 to 60 percent sulphur from wall rocks. Much of this contamination occurred prior to or during segregation of an immiscible sulphide liquid as at Quill Creek, with further contamination during injection of still liquid sulphide bodies into footwall rocks in the case of Canalask deposit. Substantial contaminant.sulphur may have been crucial in achieving sulphur saturation of magmas of Lower Group Kluane complexes and subsequent formation of associated nickel-copper sulphide deposits. Isotopic data for both magmatic sulphides and sulphur-bearing country rock support a strong case for a general sulphur contamination model for nickel-copper deposits related to mafic and ultramafic intrusions on a worldwide scale. Pyrite in wallrock is considered to be the most likely general source of sulphur contributed to ultramafic magmas and several different stages of sulphur contamination are possible throughout consolidation of the magmas and its associated sulphides. Sulphur transfer to the magma can occur by bulk country rock assimilation or by fluid ingress where the fluid derives sulphur from the contact zone by incongruent melting of pyrite or by complete dissociation of pyrite. Important features of this model, having economic implications, are: (1) emplacement of the ultramafic complex as a magma; (2) presence of sulphur-bearing country rocks; and (3) presence of a sulphur-depletion halo adjacent to the ultramafite.