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Geology of Casino porphyry copper-molybdenum deposit, Dawson Range, Y.T. Godwin, Colin Inglis

Abstract

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.

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