The oxide-gold, supergene and hypogene zones at the Casino gold-copper-molybdenum deposit, west central Yukon

Special Volume, Vol. SV 46, No. 1995, 1995

The Casino deposit, located in the Dawson Range mountains in the west-central Yukon Territory, is unique among Canadian porphyry deposits. It hosts significant concentrations of copper, gold and molybdenum, has a substantially preserved leached cap (oxidegold), and a well-developed copper-enriched supergene zone. Rocks of the Paleozoic or older Yukon Metamorphic Complex are intruded by the Mid-Cretaceous Dawson Range Batholith. The batholith, mainly granodiorite, is the dominant country rock to the deposit. Copper-gold-molybdenum mineralization is focussed on and genetically related to the breccia and microbreccia pipe comprising part of the 72 Ma to 74 Ma Casino Intrusive Complex. The latter is ovoid in shape, about 1 km in a north-south direction and 2 km in an east-west direction. It consists of a central, steeply plunging, conical breccial microbreccia pipe, fine-grained quartz monzonites, intrusion breccias, and plagioclase porphyritic intrusions that may be subvolcanic in origin. A late latite explosion breccia pipe and related dikes intrude near the microbreccia body. Roughly centred on the microbreccia pipe, both the alteration and mineralization are zoned. Innermost is the potassic alteration suite consisting of K-feldspar, biotite, magnetite, anhydrite, gypsum, pyrite, chalcopyrite, molybdenite, and gold. It is discontinuously surrounded by an overprinting phyllic suite consisting of quartz, pyrite, sericite, lesser tourmaline and clays. The phyllic facies appears depleted in economic mineralization. Weakly developed propylitic alteration occurs only to the north of the deposit. A 70 m thick, gold-bearing, leached cap overlies a well- developed supergene enrichment blanket. The dominant supergene copper minerals are chalcocite, digenite, and covellite. They occur as coatings, fracture fillings, and replacements of pre-existing hypogene sulphides. Preliminary geological reserves are: oxide-gold zone in the leached cap combined with the supergene oxide-copper zone - 28 million tonnes grading 0.68 gl t Au, 0.11% Cu, and 0.024% Mo; supergene sulphide zone- 86 million tonnes grading 0.41 g!t Au, 0.43% Cu and 0.031% Mo; hypogene zone- 445 million tonnes grading 0.27 g/t Au, 0.23% Cu and 0.024% Mo.
Keywords: Porphyry deposit, Yukon, Palezoic rocks