Geologic description
The Reagan prospect was discovered in 1894 and has a 250-foot adit, 3 additional adits, a 42-foot winze, and a 40-foot shaft. Echo Bay Mines Ltd. explored the property in 1988. The deposit consists of 2 quartz veins in a shear zone along the contact between black phyllite and green phyllite. The veins dip southwest and are 1 to 3 feet thick. They contain arsenopyrite, chalcopyrite, electrum, galena, native silver, pyrite, sphalerite, and tetrahedrite. U.S. Bureau of Mines samples contained up to 52.2 ppm gold, 4,612.0 ppm silver, 1.71 percent zinc, and 0.97 percent lead. The U.S. Bureau of Mines has estimated a resource of 82,000 tons of ore of uncertain grade (Redman and others, 1989). This prospect is in the Juneau Gold Belt, which consists of more than 200 gold-quartz-vein deposits that have produced nearly 7 million ounces of gold. These gold-bearing mesothermal quartz vein systems form a zone 160 km long by 5 to 8 km wide along the western margin of the Coast Mountains. The vein systems are in or near shear zones adjacent to west-verging, mid-Cretaceous thrust faults. The veins are hosted by diverse, variably metamorphosed, sedimentary, volcanic, and intrusive rocks. From the Coast Mountains batholith westward, the host rocks include mixed metasedimentary and metavolcanic sequences of Carboniferous and older, Permian and Triassic, and Jurassic-Cretaceous age. The sequences are juxtaposed along mid-Cretaceous thrust faults (Miller and others, 1994). The sequences are intruded by mid-Cretaceous to middle Eocene plutons, mainly diorite, tonalite, granodiorite, quartz monzonite, and granite. Sheetlike tonalite plutons emplaced just east of the Juneau Gold Belt and undeformed granite and granodiorite bodies that are emplaced farther to the east are between 55 and 48 Ma (Gehrels and others, 1991). The structural grain of the belt is defined by northwest-striking, moderately to steeply northeast-dipping, penetrative foliation that developed between Cretaceous and Eocene time (Miller and others, 1994). The majority of the veins in the Juneau Gold Belt strike northwest. Isotopic dates indicate that the auriferous veins in the Juneau Gold Belt formed between 56 and 55 Ma (Miller and others, 1994; Goldfarb and others, 1997). |
