Geologic description
The deposit at the Eagle River Mine consists of heavily-faulted quartz veins along shear zones in slate and graywacke that are cut by a a few thin, intrusive sheets of green, augite melaphyre. The quartz veins are up to 15 feet thick, vary from 25 to 100 feet long, and are repeatedly terminated by faults. Ground conditions are poor and many of the veins are in slump blocks and sloughed areas. The veins generally strike northwest, dip 70 NE, and contain pyrite, pyrrhotite, arsenopyrite, native gold, galena, sphalerite, chalcopyrite, and minor native copper. High gold values are generally associated with high sulfide content, mainly arsenopyrite and galena. Honeycombed quartz with no sulfides also carries considerable free gold, as does quartz with much pyrrhotite (Spencer, 1906; Knopf, 1912; Cobb, 1978 [OFR 78-374]). The Eagle River Mine was discovered in 1902. The mine workings include over 30,000 feet of underground workings, including the 3,000-foot Flume Tunnel. The mine had a 20-stamp mill and produced 19,451 ounces of gold and 8,855 ounces of silver between 1904 and 1915. Ore mined from the upper workings was 90 percent free-milling and ore mined in the lower workings was 56 percent free-milling. An additional 100 ounces of gold, 5 ounces of silver, and 1 pound of lead were recovered from the tailings in 1940. U.S. Bureau of Mines samples contained up to 50.3 ppm gold, 42.5 ppm silver, 1.27 percent lead, and 0.7 percent zinc. In 1981 and 1982, Placid Oil Co. drilled several holes in the mine area. Houston Oil and Minerals Co. drilled additional holes in 1985. Reserves at the Eagle River Mine have been estimated as high as 750,000 tons of ore with an average of 0.2 ounce of gold per ton (Kurtak and Maas, 1988). The U.S. Bureau of Mines however, estimates an inferred resource of 53,100 tons of ore with an average of 0.28 ounce of gold per ton (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). |
