Geologic description
The Dull and Stephens Mine was discovered in 1906. It was developed by a 150-foot adit, a 144-foot adit, a 199-foot adit, a 297-foot adit, and numerous trenches. The property produced 32 ounces of gold by sluicing overburden in 1909 and 1914. The deposit consists of several 1- to 2-foot- thick, northeast-trending, discontinuous quartz veins and irregular masses of quartz that dip steeply north. The veins are along the contact between phyllite and greenstone (in part volcanic breccia), and contain pyrite, arsenopyrite, pyrrhotite, galena, and traces of chalcopyrite and native gold (Knopf, 1912). U.S. Bureau of Mines samples contained a weighted average grade of 2.0 ppm gold over 1.8 feet (Redman and others, 1989). This mine is in the Juneau Gold Belt, which consists of more than 200 auriferous quartz vein deposits that have produced nearly 7 million ounces of gold. These gold-bearing mesothermal quartz vein systems form a 160-km-long by 5- to 8-km-wide zone 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). |
