Encio Gulch (formerly Riddle Gulch)

The Encio Gulch (formerly Riddle Gulch) is a gold mine located in Alaska.

About the MRDS Data:

All mine locations were obtained from the USGS Mineral Resources Data System. The locations and other information in this database have not been verified for accuracy. It should be assumed that all mines are on private property.

Mine Info

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Site Name

Primary: Encio Gulch (formerly Riddle Gulch)

Commodity

Primary: Gold
Secondary: Tungsten
Secondary: Silver
Secondary: Bismuth

Location

State: Alaska
District: McGrath

Land Status

Not available

Holdings

Not available

Workings

Not available

Ownership

Not available

Production

Not available

Deposit

Record Type: Site
Operation Category: Past Producer
Operation Type: Unknown
Years of Production:
Organization:
Significant:

Physiography

Not available

Mineral Deposit Model

Model Name: Shoreline placer Ti

Orebody

Not available

Structure

Not available

Alterations

Alteration Type: L
Alteration Text: Auriferous gravels partially cemented by ferricrete, probably due to ground water oxidation.

Rocks

Not available

Analytical Data

Not available

Materials

Ore: Scheelite
Ore: Bismuth
Gangue: Hematite
Gangue: Ilmenite
Gangue: Magnetite
Gangue: Sphene
Gangue: Thorianite
Gangue: Zircon

Comments

Comment (Geology): Age = Quaternary

Comment (Exploration): Status = Inactive

Comment (Commodity): Ore Material = Bismuth (native)

Comment (Deposit): Model Name = Placer Au-PGE (Cox and Singer, 1986; model no. 39c)

Comment (Deposit): Other Comments = Note: Not the same creek as the one currently named Riddle Gulch on USGS topographic maps. See also Hidden Creek (MD066), Holmes Gulch (MD072) and Birch Gulch (MD073).

Comment (Reference): Primary Reference = Brown, 1926

Comment (Commodity): Ore Material = placer gold

Comment (Commodity): Ore Material = Native bismuth

Comment (Workings): Workings / Exploration = Exploration and production began in Encio Gulch in 1922 by E.M Whalen. Exploration and production resumed in 1926 and continued on a small scale until 1932. Small scale production was finished during development of a small cut in 1937. A hydraulic mining method that deployed a boom dam arrangement was the principle mining method prior to 1932.

Comment (Geology): Geologic Description = Encio Gulch is the upper extension of the placer deposit in Hidden Creek (see Hidden Creek placer; MD066). The gulch occupies a first order incision that exhibits a stream gradient of nearly 500 feet/mile (95 m/km). Much of the previous production activities occurred near the confluence of Encio Gulch and Hidden Creek, where the hydraulic gradient became about 250 feet/mile. The paystreak varied from 25-to-55 feet (8-17 m) wide, and averaged about 35 feet (11 m) in width. About 10 feet (3 m) of overburden laid on top of a 4-foot (1.2 m) thick paystreak. Bedrock under the paystreak consists of altered monzonite of the Nixon Fork pluton, which has a K-Ar age of 68 Ma (Bundtzen and Miller, 1997). Monzonite boulders up to 6 feet (2 m) in diameter were commonly encountered during past mining activities. Two records of gold fineness from Encio Gulch yielded values of 857 and 886, which differs from the higher gold fineness average of 928 found in Hidden Creek below Encio Gulch (Mertie, 1936).

Comment (Production): Production Notes = In 1922, 57 ounces of gold and 6 ounces of byproduct silver were recovered from 570 cubic yards of pay gravels in Riddle (now Encio) Gulch. In 1937, about 6 ounces of placer gold was recovered from an undisclosed amount of pay gravels in Encio Gulch. Production from 1926 to 1932 was probably lumped with the production total on Hidden Creek (MD066). Reed and Miller (1971) imply that small scale production activities took place in Encio Gulch during 1968.

References

Reference (Deposit): Cobb, E.H., 1974, Synopsis of mineral resources and geology of Alaska: U.S. Geological Survey Bulletin 1307, 53 pages.

Reference (Deposit): Brown, J.S., 1926, The Nixon Fork country: U.S. Geological Survey Bulletin 783. p. 97-144.

Reference (Deposit): Fisher, D.G., and Juilliand, J.D., 1986, Mineral deposit types and their characteristics: U.S. Bureau of Land Management Technical Bulletin 3031-1, 134 pages.

Reference (Deposit): Patton, W.W., Jr., Moll, E.J., Dutro, J.T., Jr., Silberman, M.L., and Chapman, R.M., 1980, Preliminary geologic map of Medfra quadrangle, Alaska: U.S. Geological Survey Open-File Report 80-811-A, 1 sheet, scale l:250,000.

Reference (Deposit): Herreid, G.H., 1966, Geology and geochemistry of the Nixon Fork area, Medfra quadrangle, Alaska: Alaska Division of Mines and Minerals Geologic Report 22, 34 p., 1 sheet, scale 1:40,000.

Reference (Deposit): Reed, B.L., and Miller, T.P., 1971, Orientation geochemical soil survey at the Nixon Fork Mines, Medfra quadrangle, Alaska: U.S. Geological Survey Bulletin 1312K, p. K1-K21.

Reference (Deposit): Mertie, J.B., Jr., 1936, Mineral deposits of the Ruby-Kuskokwim region, Alaska: U.S. Geological Survey Bulletin 864-C, 115-245.

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