Geologic description
The Peterson Mine was discovered in 1897. Workings include 4 shafts, 11 adits, and numerous trenches (Redman and others, 1989). The deposit consists of quartz veins in northwest-striking phyllite and greenstone. A chloritized, 80- to 100-foot-thick diorite sill forms the footwall of the veins and the phyllite along this footwall contains finely disseminated pyrite and pyrrhotite. Augite lamprophyre also occurs in the footwall and augite melaphyre flows and breccias crop out east of the mine. The quartz veins form tabular bodies and lenses that trend north-northwest and dip shallowly to the northeast. They average 3 to 5 feet wide but locally are up to 30 feet thick (Knopf, 1912). The quartz contains arsenopyrite and native gold, and the ore averages 0.3 ounce of gold per ton. Between 1916 and 1982, at least 544 tons of ore were mined that yielded 211 ounces of gold and 8 ounces of silver. Some gold was also recovered between 1905 and 1915, but production records are not available (Redman and others, 1989). U.S. Bureau of Mines samples of quartz veins contained up to 15.3 ppm gold. The Bureau collected a 350 pound metallurgical sample in 1988 for cyanide amenability, flotation, and cyanide-leach-with-assay-screen-analysis tests. The deposit was drilled by FMC Corporation in 1988 (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). |
