Casino porphyry copper-molybdenum deposit ia in the Dawson Range, midway between Dawson City and Whitehorse, Y.T. Mid-Cretaceous granitic rocks of the Klotassin batholith form the backbone of the Dawson Range and have intruded the Yukon Hetamorphic Complex of Paleozoic or earlier age. A 70 m.y. old volcanic unit, the Casino complex, intruded the Klotassin batholith, and is cogenetic with Casino deposit. Hear the deposit extrusive volcanic rocks are unknown but formation of a subvolcanic plug of feldspar porphyry was followed by an explosive event that formed a steeply plunging, conical breccia pipe. This permeable pipe, about 2,000 ft. (670m.) by 1,200 ft. (400m.) at the surface localized hydrothermal fluids that formed large concentrically zoned alteration patterns during upward and outward percolation. A potassic alteration facies core, about 1,500 ft. (500m.) in diameter, is centered approximately on the breccia pipe, and is characterized by secondary magnetite, biotite and potassium feldspar. This core is surrounded by phyllic (quartz, sericite, sulphide) alteration that extends about 1,000 ft. (330m.) into adjacent rocks of the Klotassin batholith. Chalcopyrite and molybdenite are concentrated in the phyllic zone along the potassic alteration side of a pyrite halo. Peripheral, weakly developed zones of argillic (clay-carbonate minerals) and propylitic (chlorite) alterations are present. This characteristic location of economically significant minerals within a zonal distribution of alteration minerals provides an important exploration guide for porphyry-type deposits in the area. Breccia formation and hydrotherrnal zoning appear interrelated and probably result from escape of metal-bearing saline solutions from "wet" magmas derived from an underlying Benioff zone associated with subduction of an oceanic plate. Supergene enrichment, preserved because the area is unglaciated, probably occurred mainly in the Paleogene and resulted in an increase in the grade of copper by an average factor of 1.7 through precipitation of chalcocite in a subhorizontal enriched zone. Copper added to this zone was extracted from up to 500 ft. (l70a.) of overlying capping rock. Controls for enrichment processes include grade of original hypogene copper, favourable breccia occurrence and alteration, and presence of pyrite. A plate-tectonic model relates the genesis of Upper Cretaceous to Tertiary porphyry-type deposits to the evolution of the western and central Canadian Cordillera. Existence of two Benioff zones is assumed from definition of two distinct younging trends of intrusive centres. The first Benioff zone, initiated west of the Queen Charlotte Islands near the Middle Triassic, continued activity until the early Tertiary when 50 m.y. old granitic rocks and associated porphyry deposits near the eastern boundary of the Coast Crystalline Belt were formed. The second Benioff zone, initiated near the earliest-Cretaceous, extended under the western margin of the North America plate and produced stocks and associated porphyry deposits that become younger from west to east across the Intennontane Belt. Intrusive activity associated with both Benioff zones ceased at about the same time, 50 m.y. ago, implying that they became imbricated. As a result, the North America plate overrode the Insular plate. Doubling of these plates is reflected in the late Mesozoic and Tertiary uplift and erosion of the Coast Crystalline Belt. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

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. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate