Berners Tunnel

Prospect, Active

Commodities and mineralogy

Main commodities Au
Other commodities Ag; Cu; Pb; Zn
Ore minerals chalcopyrite; gold; pyrite
Gangue minerals ankerite; calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale JU
Quadrangle map, 1:63,360-scale D-4
Latitude 58.8294
Longitude -135.0172
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Berners Tunnel is at an elevation of about 150 feet in a short, unnamed north tributary to Johnson Creek. It is 3.5 miles south of Lions Head Mountain and 2 miles north-northwest of Berners Bay, in the SE1/4 section 14, T. 35 S., R. 62 E. of the Copper River Meridian. The location is accurate.

Geologic setting

Geologic description

Note: Beginning in the 1990s, this and other old mines and prospects in the vicinity were consolidated by Coeur-Alaska into a single property (Birak, 2006). They have carried out major new drilling and underground exploration that better defines the mineralization, largely blurs the geologic distinction among the old deposits, and extends the ore bodies greatly. They have developed a coherent modern interpretation of the mineralization as a single deposit which Coeur-Alaska intends to mine as a unit. This deposit is described separately in ARDF as the Jualin (Coeur-Alaska) deposit (JU262). For geologic reference purposes and for their historic value, the pre-Coeur-Alaska data for this and the other old mines and prospects that Coeur-Alaska consolidated are retained in ARDF.
The Berners Tunnel was begun in 1914 to drain water from the Jualin Mine (Redman and others, 1989). The tunnel, which trends northwesterly was never completed. During construction, workers collected ten-foot rock-chip samples along its 5,000-foot length. Two intervals contained elevated gold values. Between stations 1912 and 2042 from the portal, a 130-foot zone averaged 0.12 ounce of gold per ton; between stations 2112 and 2212, a 100-foot zone averaged 0.087 ounce of gold per ton. The first 400 feet of the tunnel was in glacial till and colluvium and the portal has long since collapsed. In 1989, Placer Dome U.S. drilled 3 core holes along the trace of the Berners Tunnel. The drill holes cut numerous anomalous zones along a major NW-trending shear zone. The richest intercept averaged 0.028 ounce of gold per ton. The drill core consists of moderately to strongly sheared diorite with widespread potassic and sericitic alteration. The potassic alteration suite includes epidote, chlorite, and pyrite. Locally, the sheared diorite is highly silicified giving it a bleached appearance. This silicified diorite contains relatively abundant, disseminated pyrite and chalcopyrite, and quartz-pyrite veinlets (Barnett and others, 1989).
Geologic map unit (-135.019008681971, 58.8290740356768)
Mineral deposit model Low-sulfide Au-quartz vein (Cox and Singer, 1986; model 36a).
Mineral deposit model number 36a
Age of mineralization Hydrothermal muscovite from veins in the Jualin mine are 53.2 Ma to 55.5 Ma (Miller and others, 1994). This coincides with the 55 Ma age of the other mesothermal gold veins in the Juneau Gold Belt (Goldfarb and others, 1997).
Alteration of deposit The drill core consists of moderately to strongly sheared diorite showing widespread potassic and sericitic alteration. The potassic alteration suite includes epidote, chlorite, and pyrite. Locally, the sheared diorite is highly silicified giving it a bleached appearance.

Production and reserves

Workings or exploration The site consists of a 5,000-foot tunnel which is caved at the portal. Placer Dome U.S. drilled 3 core holes along the trace of the tunnel in 1989.
Indication of production None

References

References

Barnett, J.C., Vandel, J.C., Monks, J.I., and Johnson, G.S., 1989, Jualin gold project, 1989 progress report: Unpublished report for Placer Dome U.S., Inc., 75 p.
Birak, D.J., 2006, Kensington gold project: Unpublished Technical Report for Cour d'Alene Mines, 116 p. (posted on www/.sedar.com, April 20, 2006).
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.
Miller, L.D., Goldfarb, R.J., Gehrels, G,E., and Snee, L.W., 1994, Genetic links among fluid cycling, vein formation, regional deformation, and plutonism in the Juneau gold belt, southeastern Alaska: Geology, v. 22, p. 203-206
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.
Reporters J.C. Barnett and L.D. Miller (Juneau, Alaska ); D.J. Grybeck (Port Ludlow, WA)
Last report date 6/5/2008