Geologic description
The E Pluribus Unum Mine is in a northwest-trending zone of intensely folded and sheared black phyllite and graphitic schist that can be traced for nearly 5,000 feet (Redman and others, 1989). The shear zone is at least 130 feet wide and contains quartz-stringer veins 1-inch thick in 6-foot-thick zones, and massive quartz veins up to 2.5 feet thick in the noses of folds. The veins contain arsenopyrite, chalcopyrite, galena, pyrite, minor sphalerite, and stibnite. The stringer veins average about 0.3 ppm gold and the massive quartz veins average about 30.2 ppm gold (Roehm, 1936; Redman and others, 1989). U.S. Bureau of Mines samples contained up to 59.9 ppm gold (Redman and others, 1989). The U.S. Bureau of Mines collected a 300-pound metallurgical sample for a cyanide amenability test that resulted in 92 percent recovery of gold using 23 pounds of sodium cyanide. The E Pluribus Unum deposit was discovered in 1906 and produced gold intermittently until 1940. The total documented production was 154 ounces of gold, 44 ounces of silver, 11 pounds of copper, and 242 pounds of lead from 106 tons of ore. There are 3 adits and several trenches, and the deposit was drilled by Houston Oil and Minerals in 1985 (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). |
