Big Lake

Prospect, Active

Commodities and mineralogy

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

Geographic location

Quadrangle map, 1:250,000-scale JU
Quadrangle map, 1:63,360-scale D-4
Latitude 58.8364
Longitude -135.0166
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Big Lake prospect is at an elevation of 1,200 feet, about 0.7 mile from the northeast side of Johnson Creek and 1 mile northwest of Berners Bay. It is in the NE1/4 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 Big Lake prospect was discovered in 1988 by International Curator Resources, Ltd. It consists of a series of parallel quartz veins that strike northwest and dip 70NE in Jualin Diorite. The veins crop out along the ridge crest for nearly 1,300 feet; they extend toward the Yankee Boy prospect (JU051), about 2,300 feet away. The Big Lake and Yankee Boy veins are probably part of the same vein system that consists of a zone about 300 feet wide. There are two distinct types of Big Lake quartz veins; one is massive and the other consists of thin, white veinlets. The massive veins are distinguished by a pervasive yellow-brown iron stain, sheared margins, and 1 to 10 percent pyrite; they are in sheared, chloritic diorite that contains about 1 percent disseminated pyrite. The small, white, veinlets range in thickness from one-eighth inch to three feet, and lay parallel to each other in zones up to fifty feet wide. The veins contain isolated blebs of pyrite, chalcopyrite, malachite, and specular hematite. Alteration adjacent to the veins is characterized by sericite, pervasive interstitial carbonate, and moderate to strong silicification (Barnett and others, 1989). In 1988, International Curator Resources Ltd. collected numerous samples of the Big Lake veins and Placer Dome U.S. drilled one core hole in 1989. The average for twenty grab samples collected along strike for 1200 feet was 0.454 ounce of gold per ton (Barnett, 1989). The drill hole had several anomalous intercepts; one 2.5-foot core sample assayed 0.109 ounce of gold per ton (Barnett and others, 1989).
Geologic map unit (-135.018408970574, 58.8360738892417)
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 Alteration adjacent to the veins is characterized by sericite, pervasive interstitial carbonate, and moderate to strong silicification. Local oxidation of copper minerals.

Production and reserves

Workings or exploration The Big Lake prospect was discovered in 1988. International Curator Resources, Ltd. collected numerous samples of the veins and Placer Dome U.S. drilled one core hole in 1989.
Indication of production None

References

References

Barnett, J.C., 1989, Jualin gold project, 1988 progress report, Berners Bay District: Unpublished report for International Curator Resources, Ltd., 75 p.
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