Tacoma; Bonanza Prospect, Juneau Mining District, Juneau, Alaska, USAi

Regional Level Types
Tacoma; Bonanza Prospect	Prospect
Juneau Mining District	Mining District
Juneau	City Borough
Alaska	State
USA	Country

Location: The Tacoma prospect is at an an elevation of about 1,000 feet, on the south side of Sawmill Creek. It is 1 mile east of Berners Bay and 2 miles north of Echo Cove in the NE1/4 section 33, T. 36 S., R. 63 E. of the Copper River Meridian. The location is accurate within 1/4 mile.
Geology: The Tacoma prospect was discovered in 1901 and developed by 3 adits and several open cuts (Redman and others, 1989). The deposit consists of two east-west-striking, north-dipping quartz veins up to 8 feet thick in black phyllite and metavolcanic rocks. The veins are brecciated, and contain pyrite, galena, arsenopyrite, and sphalerite. The phyllite contains stringers of quartz carrying disseminated pyrite. Quartz vein samples contain up to 1.1 ppm gold, 169.8 ppm silver, and 2.78 percent lead (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).
Workings: The Tacoma prospect was discovered in 1901 and developed by 3 adits and several open cuts.
Age: 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).

Commodities (Major) - Ag, Au, Pb; (Minor) - Zn
Development Status: No
Deposit Model: Low-sulfide Au-quartz vein (Cox and Singer, 1986; model 36a)

Other Databases

Link to USGS - Alaska:	JU066

Other Regions, Features and Areas containing this locality

References