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Eureka Prospect, Juneau District, Juneau Borough, Alaska, USA

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Location: The Eureka prospect is at an elevation of about 2,300 feet, about 2 miles northeast of Pt. Sherman on Lynn Canal and 1 mile southwest of Lions Head Mountain in the Kakuhan Range. It is in the N1/2 of the SE1/4 section 4, T. 35 S., R. 62 E. of the Copper River Meridian. The location is accurate.
Geology: The Eureka prospect is a quartz vein stockworks similar to the Kensington Mine (JU029). It strikes north-south and dips 63 E. It is about 60 feet wide and has been traced along strike for 350 feet (Redman and others, 1989). The deposit was discovered in 1897. Workings include 2 adits, several open cuts, a short raise and 360 feet of workings. The stockworks is cut by the Kensington tunnel. The Eureka prospect has an inferred reserve of 300,000 tons of ore that contain 0.17 ounce of gold per ton (Redman and others, 1989). Placid Oil Co. drilled 2 surface core holes and 7 underground drill holes in 1981 and 1982 (Chris Croff, Placid Oil Co., oral commun. 1987). The prospect is within the Kensington (JU029) project area, that is currently (2001) under control of Coeur Alaska. The Eureka 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: The deposit was discovered in 1897. Workings include 2 adits, several open cuts, a short raise and 360 feet of workings. The stockworks is cut by the Kensington tunnel. Placid Oil Co. drilled 2 surface core holes and 7 underground drill holes in 1981 and 1982 (Chris Croff, Placid Oil Co., oral commun. 1987). The prospect is within the Kensington (JU029) project area that 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).
Reserves: The Eureka prospect has an inferred reserve of 300,000 tons of ore with 0.17 ounce of gold per ton (Redman and others, 1989).

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

Mineral List



20 entries listed. 16 valid minerals.

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References

Goldfarb, R.J., Miller, L.D., Leach, D.L., and Snee, L.W, 1997, Gold deposits in metamorphic rocks in Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 151-190. Knopf, Adolph, 1911, Geology of the Berners Bay region, Alaska: U.S. Geological Survey Bulletin 446, 58 p. Leveille, R.A., 1991, Geology and gold deposits of the Jualin mine area, Berners Bay district, southeastern Alaska: Fairbanks, University of Alaska, M.S. thesis, 200 p. Miller, L.D., Goldfarb, R.J., Snee, L.W., Gent, C.A., and Kirkham, R.A., 1995, Structural geology, age, and mechanisms of gold vein formation at the Kensington and Jualin deposits, Berners Bay district, southeast Alaska: Economic Geology, v. 90, p. 343-368. Redman, E.C., Maas, K.M., Kurtak, J.M., and Miller, L.D., 1989, Bureau of Mines Mineral Investigations in the Juneau Mining District, Alaska, 1984-1988, Volume 2--Detailed mine, prospect, and mineral occurrence descriptions, Section D, Juneau Gold Belt Subarea: U.S. Bureau of Mines Special Publication, 424 p.

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