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Encio Gulch (formerly Riddle Gulch)

Past Producer in Alaska, United States with commodities Gold, Silver, Bismuth, Tungsten
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Sections on this page
  1. Identification information
  2. Geographic coordinates
  3. Site location context
  4. Geographic areas
  5. Commodities
  6. Materials information
  7. Alteration
  8. Mineral occurrence model information
  9. Nearby scientific data
  10. Economic information about the deposit and operations
  11. Mining district
  12. Links to other databases
  13. Bibliographic references
  14. General comments
  15. Reporter information

Geologic information

Identification information

Deposit ID 10001653
MRDS ID A012427
Record type Site
Current site name Encio Gulch (formerly Riddle Gulch)
Related records 10112491

Geographic coordinates

Geographic coordinates: -154.74989, 63.22337 (WGS84)
Relative position 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.

Site location context

Political divisions (FIPS codes)

Yukon-Koyukuk(Census area)

Alaska(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

Medfra A-4(quadrangle 1:63,360 scale)

Medfra S(quadrangle 1:100,000 scale)

Medfra C(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

Stony River(hydrologic unit)

Upper Kuskokwim River(hydrologic accounting unit)

Southwest(hydrologic subregion)

Alaska(hydrologic region)

Federal lands

Doyon, Limited(ANCSA Region)

ANCSA Region NTVPIC(Type of land area)

NTVPIC(Federal land areas administered by NTVPIC)

Geographic areas

Country State
United States Alaska

Commodities

Commodity Importance
Gold Primary
Silver Secondary
Bismuth Critical Secondary
Tungsten Critical Secondary

Comments on the commodity information

  • Ore Material = placer gold
  • Ore Material = Native bismuth
  • Ore Material = Bismuth (native)

Materials information

Materials Type of material
Bismuth Ore
Scheelite Ore
Hematite Gangue
Ilmenite Gangue
Magnetite Gangue
Sphene Gangue
Thorianite Gangue
Zircon Gangue

Alteration

  • (Local) Auriferous gravels partially cemented by ferricrete, probably due to ground water oxidation.

Mineral occurrence model information

Model code 121
USGS model code 39c
Deposit model name Shoreline placer Ti

Nearby scientific data

(1) -154.74989, 63.22337
ListMap

Economic information

Comments on the geologic information

  • 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).
  • Age = Quaternary

Economic information about the deposit and operations

Development status Past Producer
Commodity type Metallic

Comments on exploration

  • Status = Inactive

Mining district

District name McGrath

Comments on the production information

  • 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.

Comments on the workings information

  • 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.

Reference information

Bibliographic references

Comments on the references

  • Primary Reference = Brown, 1926

General comments

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

Reporter information

Type Date Name Affiliation Comment
Reporter 04-MAY-98 Bundtzen, T.K. Pacific Rim Geological Consulting

Beyond USGS

Supplemental information added by qvyshift.com. Not part of the original USGS MRDS record.

Maps centered on this site

Authoritative Alaska resources

These are landing pages for further research — the state agencies don't currently expose per-mine deep links.