Encio Gulch Mine (formerly Riddle Gulch), Yukon-Koyukuk Census Area, Alaska, USAi

Regional Level Types
Encio Gulch Mine (formerly Riddle Gulch)	Mine
Yukon-Koyukuk Census Area	Census Area
Alaska	State
USA	Country

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).
Location: The Encio Gulch placer mine is at extreme left limit head of Hidden Creek drainage basin immediately opposite the head of Ruby Creek, which dissects the Nixon Fork lode mining area (MD062) to the northwest.
Geology: 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).
Workings: 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 principal mining method prior to 1932.
Age: Quaternary
Alteration: Auriferous gravels partially cemented by ferricrete, probably due to ground water oxidation.
Production: 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.

Commodities (Major) - Au; (Minor) - Ag, Bi, W
Development Status: Yes; small
Deposit Model: Placer Au-PGE (Cox and Singer, 1986; model no. 39c)

Other Databases

Link to USGS - Alaska:	MD063

Other Regions, Features and Areas containing this locality

References