Seward; Thomas Prospect, Juneau Mining District, Juneau, Alaska, USAi

Regional Level Types
Seward; Thomas Prospect	Prospect
Juneau Mining District	Mining District
Juneau	City Borough
Alaska	State
USA	Country

Location: The Seward prospect is at an elevation of about 2,600 feet, about 3 miles east of Pt. Sherman on Lynn Canal and 1.5 miles south-southwest of Lions Head Mountain in the Kakuhan Range. It is in the saddle above the Comet mine (JU036), in the NW1/4 section 10, T. 35 S., R. 62 E. of the Copper River Meridian. The location is accurate.
Geology: The Seward prospect consists of a quartz vein in Jualin diorite. It is 5- to 12-feet-thick, strikes N, dips E, and has been traced on the surface for approximately 1,100 feet. Early samples (before 1906), assayed up to 24 ppm gold (Redman and others, 1989). The quartz contains free gold, but little or no sulfides (Kensington Mines, 1914). Workings include a 63-foot shaft with a 15-foot drift at the bottom, and 2 open cuts. The shaft and open cuts were completed during the 1890's. Placid Oil Co. drilled 3 core holes on the prospect in 1983; core samples assayed up to 0.7 ounce of gold per ton (Chris Croff, oral communication, 1987). The prospect is currently (2001) under control by Coeur Alaska. The Seward prospect is in the Berners Bay district at the north end of the Juneau Gold Belt. The district is characterized by a series of structurally-controlled, mesothermal, gold-bearing quartz veins. Most of the veins are in Early Cretaceous (105 Ma) Jualin Diorite, which intrudes Upper Triassic metabasalt. The Jualin Diorite is generally massive, jointed, blocky, quartz monzonite to quartz monzodiorite. Gold occurs in low-sulfide, quartz-carbonate veins that contain pyrite and tellurides; the veins are marked by distinctive ankeritic alteration zones. There are both extensional and shear veins that generally strike north to northwest and dip east. Discrete vein systems are defined by one or more through-going quartz veins, many of which are in shear zones. Levielle (1991) and Knopf (1911) describe other gangue minerals near vein margins including albite, chlorite, muscovite, and lesser tourmaline, rutile, and apatite. Hydrothermal alteration adjacent to the veins is characterized by reddish-brown ferroan dolomite (Miller and others, 1995). Other alteration includes sericitization of plagioclase, chloritization, sulfidization of mafic minerals, and albitization of feldspars (Leveille, 1991). Pyrite is the most abundant sulfide mineral, with lesser amounts of chalcopyrite, galena, sphalerite, arsenopyrite, and tetrahedrite. Gold occurs in the native state, in pyrite, and in various telluride minerals such as calaverite, hessite, and petzite (Leveille, 1991; Redman and others, 1989). The vein paragenesis consists of early quartz, carbonates, albite and pyrite, followed by deposition of base and precious metals. Gold, galena and the tellurides were the last to be deposited (Leveille, 1991). The age of hydrothermal muscovite from veins at Kensington Mine (JU029) varies from 53.4 Ma to 56.5 Ma (Miller and others, 1994). This coincides with the 55 Ma age of the other mesothermal gold vein deposits in the Juneau Gold Belt (Goldfarb and others, 1997).
Workings: Workings completed during the 1890's include a 63-foot shaft with a 15-foot drift at the bottom, and 2 open cuts. Placid Oil Co. drilled 3 core holes on the prospect in 1983; core samples assayed up to 0.7 ounce of gold per ton (Chris Croff, oral commun., 1987). The prospect is currently (2001) under control of Coeur Alaska.
Age: The age of mineralization in the Berners Bay district is about 55 Ma, the same as the other mesothermal gold-quartz-vein deposits in the Juneau Gold Belt (Goldfarb and others, 1997).
Alteration: Hydrothermal alteration adjacent to the veins is characterized by reddish-brown ferroan dolomite alteration (Miller and others, 1995). Other alteration includes sericitization of plagioclase, chloritization and sulfidization of mafic minerals, and albitization of feldspars (Leveille, 1991).

Commodities (Major) - Au
Development Status: None
Deposit Model: Low-sulfide Au-quartz vein (Cox and Singer, 1986; model 36a)

Other Databases

Link to USGS - Alaska:	JU037

Other Regions, Features and Areas containing this locality

References