Cooper Creek

Mine, Active

Commodities and mineralogy

Main commodities Au
Other commodities As
Ore minerals arsenopyrite; gold; magnetite

Geographic location

Quadrangle map, 1:250,000-scale SR
Quadrangle map, 1:63,360-scale B-8
Latitude 60.474
Longitude -149.8745
Nearby scientific data Find additional scientific data near this location
Location and accuracy Cooper Creek flows north into the Kenai River at 3 miles below Kenai Lake. The mine is located in the SW1/4 section 31, T. 5 N., R. 3 W., Seward Meridian at about 700 feet elevation. The creek has been worked from Stetson Creek to its mouth. The map location is in the center of the placer deposit. This is location 160 of Cobb and Richter (1972), location 173 of MacKevett and Holloway (1977), location 25 of Cobb and Tysdal (1980), and location P-86 of Jansons and others (1984). This location is accurate to within 300 feet.

Geologic setting

Geologic description

Cooper Creek is 10 miles long and occupies a narrow bedrock canyon nearly to its junction with the Kenai River. Bedrock in the area is slate and graywacke of the Valdez Group of Late Cretaceous age (Nelson and others, 1985).
Near its mouth, Cooper Creek has cut through stratified gravels of a glacial delta deposit of the Kenai River valley. A bench gravel deposited by Cooper Creek on the east side of the stream is between 100 and 200 feet above the current creek level (Moffit, 1906). The gold content of the glacial delta gravels was low, but the Cooper Creek bench gravels averaged about $0.40 per cubic yard (with gold at $20.67 per ounce) (Cobb and Tysdal, 1980). Cooper Creek bench gravels are 8 to 10 feet thick and are composed of lenses of pebbly gravel resting on a sand and clay false bedrock.
Even richer bench gravels in the canyon lay on bedrock and contained both coarse and fine gold. The largest reported nugget produced from the canyon bench gravels was worth $3.80 (gold at $20.67) (Johnson, 1912).
Gravels of the active stream channel are loose and easily handled and composed of pebbles of black slate and graywacke interspersed with some boulders of fine-grained felsic dike rocks (Hoekzema and Sherman, 1983). Boulders larger than 3 feet are rare; most boulders are between 1.5 to 3 feet. The stream gravels of the both the canyon and the delta have been worked and produced gold. The stream gravels vary considerably in thickness and, in general, are richer in the canyon than near the mouth (Johnson, 1912).
The gold is derived from three sources: from the delta deposits that flank the stream flat, from the auriferous glacial and fluvioglacial deposits in the glaciated valley of Cooper Creek, and to a slight extent, probably by post-glacial erosion of gold-bearing lodes in the bedrock of the valley (Johnson, 1912). The gold is small, flat, heavy, and not flaky (Johnson, 1912). Gold fineness is 85 percent. Gravel concentrates also contain pyrite, arsenopyrite, and magnetite (Johnson, 1912).
Geologic map unit (-149.876654697496, 60.4734232577533)
Mineral deposit model Placer Au (alluvial) (Cox and Singer, 1986; model 39a).
Mineral deposit model number 39a
Age of mineralization Quaternary.

Production and reserves

Workings or exploration
Between 1899 and 1917, creek gravels were worked by hand placer methods and hydraulic mining, both in the canyon and on the flat at the lower end of the creek (Johnson, 1912). Only minor production has occurred since.
Hydraulic operations were in progress in 1911 on the stream flat at the mouth of the creek. On account of the low stream gradient, a Ruble elevator with a 48-foot body, 10 feet wide, and a 12-foot expansion at the lower end was installed. The gold-saving attachments consisted of four sluice boxes, 12 feet long by 4 feet wide, set on a grade of 8 inches to the box length. The three lower boxes were set with steel-capped wooden cross riffles, 4 inches by 2 inches by 4 feet in size, 2 inches apart with 1-inch spaces between the steel straps (Johnson, 1912). Water for hydraulic mining was obtained from Stetson, and two other nearby creeks by an upper ditch 4 miles long, a lower ditch 1.75 miles long, and 1,300 feet of flume. Two No. 2 Hendy giants with 4-inch nozzles were available, but only one giant with a 5-inch nozzle operating under a 200-foot head was in use in June 1911 (Johnson, 1912). The usual operation was to strip the soil down to the gravel layer and then to wash all the gravel down to the false bedrock over the elevator. Most of the gold was caught in the upper boxes.
The U.S. Bureau of Mines collected four 0.1 cubic yard samples from the bench and bar deposits near the mouth of Cooper Creek. These contained 0.0018 to 0.019 ounce of gold per cubic yard (Jansons and others, 1984).
Current mining, consisting of small-scale suction dredging in the active stream channel, occurs intermittently at and just below Cooper Creeks juncture with Stetson Creek.
Indication of production Yes; small
Reserve estimates The U.S. Bureau of Mines indicated that only limited quantities of unmined gravel remain (Jansons and others, 1984).
Production notes
Moffit (1906) reported that most of the pay from Cooper Creek was taken from a single claim in a single year and a profit of 14 pounds of gold was made that year.
The U.S. Bureau of Mines estimated total production to have exceeded 1,000 ounces, of which less that 50 ounces has been produced since 1975 (Jansons and others, 1984).

References

References

Reporters Jeff A. Huber and Carol S. Huber (Anchorage)
Last report date 6/29/2000