Hidden Creek: Grable and Blackburn Mine

Mine, Inactive

Commodities and mineralogy

Main commodities Ag; Au
Other commodities Bi; Sn; Th; W
Ore minerals bismuth; cassiterite; gold; scheelite; thorianite
Gangue minerals hematite; ilmenite; magnetite; zircon

Geographic location

Quadrangle map, 1:250,000-scale MD
Quadrangle map, 1:63,360-scale A-4
Latitude 63.215
Longitude -154.747
Nearby scientific data Find additional scientific data near this location
Location and accuracy The well known Hidden Creek placer gold deposit is located on Hidden Creek, a stream that flows south-southwest from Encio Gulch area near Nixon Fork lode mine to the Nixon Fork of the Kuskokwim River. The Hidden Creek placer gold deposit extends from the mouth of Encio (Riddle) Gulch downstream to the confluence of Birch Gulch and Hidden Creek, a distance of approximately 1.5 mile (2.4 km).

Geologic setting

Geologic description

The productive portion of Hidden Creek lies along a third order stream with an average gradient of 120 feet/mile. Bedrock in most of the area that was placer mined is part of the Nixon Fork monzonite pluton which has yielded a K-Ar age of 68.0 Ma (Bundtzen and Miller, 1997). Most of the stream gravels consisted of monzonite float. Limestone underlies Hidden Creek Valley about 2 miles downstream from the head of the drainage. A copper-bearing pyroxenite dike that intrudes the limestone was found in one placer mine cut, which was thought to be a local source for placer gold (Cobb, 1978; Herreid, 1966).
Unconsolidated overburden above monzonite bedrock was 10-12 feet (3.0-3.7 m) thick, and auriferous gravels averaged about 4 feet (1.2 m) thick. The pay streak width varied from 75 to 125 feet (23 to 38 m) wide and averaged about 60 feet (18 m) in width. Below Birch Gulch, the Quaternary fill quickly thickens to as much as 200 feet (60 m); this abrupt thickening of Quaternary valley fill is probably related to the change of underlying bedrock from monzonite to limestone; chemical weathering, karsting, and cavern formation in the limestone was noted by Brown (1926).
Fourteen gold fineness determinations from placer gold mined in Hidden Creek ranged from 912 to 962 and averaged 928 (Mertie, 1936; Smith, 1941; Glover, 1950). Silver was the major impurity in the placer gold, which exhibits some of the highest fineness values of any Alaskan placer gold. In addition, substantial quantities of native bismuth occurs, frequently intergrown with native gold, magnetite, ilmenite, and brown scheelite. Minor to trace amounts of barite, cassiterite, zircon, and thorianite were found in heavy mineral concentrates during mine production, however, no other minerals other than placer gold were commercially recovered. Other placer streams such as Holmes, Riddle, and Birch Gulches, which presumably have similar if not identical lode sources for the placer gold, yield significantly different fineness values, which has mystified previous workers (i.e., Smith, 1941). Such data suggests significantly different lode sources for the placer gold in these areas.
Geologic map unit (-154.749321217808, 63.2143921046013)
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 with ferricrete, due to ground water oxidation.

Production and reserves

Workings or exploration From 1922 to 1932, placer deposits were developed with hydraulic mine methods. A scraper was used to move large monzonite boulders in the mine cuts; these boulders frequently ranged from 2 to 6 feet (0.6-2 m) in diameter. In 1922, 274 ounces of placer gold were recovered from 4,000 cubic yards of pay; i.e., this mine cut contained a recovered grade of 0.06 oz/cubic yard (2.78 g/cubic meter) gold. A dragline was first used in 1935 and was used until mine shutdown in 1938. After World War II, Birch Creek and its tributaries continued to produce on a small scale until about 1960.
Indication of production Yes; small
Production notes Hidden Creek contributed about 50 percent of the placer gold production from the Nixon Fork area. Placer gold was first discovered on Hidden Creek in 1917 (Herreid, 1966). In 1922, F.A. Matthews and Louis Blackburn initiated placer gold production, which continued every season until 1932. Production resumed in 1935 and continued until 1938. Total production from 1922 to 1938 is estimated to be 4,435 ounces (138 kg) of gold and 230 ounces (7 kg) of silver. Production on Hidden Creek from 1938 to 1960 has not been accurately determined, but is not thought to be significant (Ted Almasy and Margaret Mespelt, personal communication, 1998).

Additional comments

See also Encio (Riddle) Gulch (MD063) Birch Gulch (MD073), and Holmes Gulch (MD072). The Hidden Creek: Grable and Blackburn Mine is on land selected or conveyed to Doyon Ltd. For further information, contact Doyon Ltd. at 210 1st Ave., Fairbanks, Alaska, 99701.