Encio Gulch (formerly Riddle Gulch)

Mine, Inactive

Commodities and mineralogy

Main commodities Au
Other commodities Ag; Bi; W
Ore minerals native bismuth; placer gold; scheelite
Gangue minerals hematite; ilmenite; magnetite; sphene; thorianite; zircon

Geographic location

Quadrangle map, 1:250,000-scale MD
Quadrangle map, 1:63,360-scale A-4
Latitude 63.224
Longitude -154.747
Nearby scientific data Find additional scientific data near this location
Location and accuracy 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.

Geologic setting

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 pay streak 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 pay streak. Bedrock under the pay streak 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).
Geologic map unit (-154.749321762003, 63.2233924755016)
Mineral deposit model Placer Au-PGE (Cox and Singer, 1986; model 39c).
Mineral deposit model number 39c
Age of mineralization Quaternary
Alteration of deposit Auriferous gravels partially cemented by ferricrete, probably due to ground water oxidation.

Production and reserves

Workings or 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.
Indication of production Yes; small
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.

Additional 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).