Crooked Creek Mines, Circle Mining District, Yukon-Koyukuk Census Area, Alaska, USAi

Regional Level Types
Crooked Creek Mines	Group of Mines
Circle Mining District	Mining District
Yukon-Koyukuk Census Area	Census Area
Alaska	State
USA	Country

First placer diamonds documented in Alaska were found on Crooked Creek.
Location: The location is at the mouth of Sawpit Creek, where mining took place in 1981. Mining in 1952 and 1973 to 1981 was near the mouth of Mammoth Creek and east for several km. Crooked Creek is the drainage below the junction of Porcupine and Mammoth Creeks. It flows 30 km in an easterly direction through the town of Central before joining Birch Creek. The Steese Highway parallels Crooked Creek between the Hot Springs Fault and the town of Central. Placer mining has been confined to the 8 km of the creek immediately downstream from where the Hot Springs fault crosses Crooked Creek (Yeend, 1991, p. 17).
Geology: Quartzitic schist is present upstream from the Hot Springs fault and along the upstream tributaries, and it makes up most of the detritus in the creek gravel. A small granite outcrop is present upstream along Mammoth Creek (Lampright, 1996). All but this uppermost part of the creek lies within the Tintina fault zone. At the Hot Springs Fault junction the Crooked Creek flood plain dissects late Pleistocene fan gravel that forms a prominent 20 meter high bench to the north. To the south, several less prominent stair-stepped bench levels of late Pleistocene fan gravel grade down to the Crooked Creek flood plain. The alluvial gravel in Crooked Creek is composed predominantly of well-rounded to subrounded clasts as much as 15 cm in diameter. A pebble count in Crooked Creek approximately 4 kilometers downstream from the Hot Springs fault reveals a composition of 43 percent quartz-mica schist, 32 percent quartzite, 21 percent quartz, and 4 percent weathered granite (Yeend, 1991). Gold-bearing gravel that is 2 to 5 meters thick overlies false bedrock with clay-rich, altered cobble gravel. Mining excavations in the area where the Hot Springs fault crosses the Crooked Creek valley exposed fault gouge, altered schist bedrock, and orange gravel. Locally, where river scour has eroded the intervening gray gravel, the overlying muck rests directly on the orange gravel (Yeend, 1991, p. 19). Downstream from the Hot Springs fault are many locations where the gray gravel extends vertically into the orange gravel. Gray gravel locations within the orange gravel are interpreted as former sites of ice wedges (Kline, 1985). The gray gravel was able to fill in as the ice wedges melted and the creek washed over the uneven surface (Lampright, 1996). The gold is thought to be contained within the gray gravel that overlies the clay-rich orange gravel. Some gold extends into the upper orange gravel, but for the most part the orange gravel acts as a trap for the fine gold. Placer gold occurs primarily in the lower 1 to 2 meters of gray alluvial gravel, which is generally more consolidated that the overlying gravel and sometimes includes blocks of the underlying orange gravel (Yeend, 1991). The paystreak in Crooked Creek is as much as 400 meters wide and 1 to 2 meters thick (Yeend, 1991). Wood fragments are scattered through the gold bearing gravel. The upper gravel unit yielded an age from wood of approximately 1,480 B.P. This young age implies a continuous reworking of these gravels (Yeend, 1991). Gold flakes are very flattened, commonly 1 to 3 mm in largest dimension. (Menzie, 1983) Values range from 0.01 to 0.03 ounces per cubic yard. Upstream from the fault, the values are lower, approximately 0.00625 ounces per cubic yard, as reported by miners on the basis of exploratory sampling (Lampright, 1996). Placer mining has been confined to the 8 km of the creek immediately downstream from where the Hot Springs fault crosses Crooked Creek (Yeend, 1991, p. 17). Placer mining occurred in 1952 and from 1973 through the 1980's. Bob Cacy of Points North conducted geochemical and magnetic surveys on Crooked Creek in 1993 and 1994 (Bundtzen and others, 1993; Swainbank and others, 1994). The first diamonds in the Circle Quadrangle were discovered on Crooked Creek. In 1982, Jim Regan discovered a 0.3 ct diamond in a sluice box. Frank Warren discovered an even larger, 1.4 ct diamond, during placer mining in 1984 (Eakins and others, 1985). In 1986, Paul Manuel recovered a third diamond from the Crooked Creek gravels about 1500 ft downstream from the Warren discovery (Forbes and others, 1987, p. 7). Subsequent investigations by experienced diamond producers indicated no trace minerals indicative of kimberlite or lamproite source rocks.
Workings: Placer mining has been confined to the 8 km of the creek immediately downstream from where the Hot Springs fault crosses Crooked Creek (Yeend, 1991, p. 17). Placer mining occurred in 1952 and from 1973 through the 1980's. Bob Cacy of Points North conducted geochemical and magnetic surveys on Crooked Creek in 1993 and 1994 (Bundtzen and others, 1993; Swainbank and others, 1994). The first diamonds in the Circle Quadrangle were discovered on Crooked Creek. In 1982, Jim Regan discovered a 0.3 ct diamond in a sluice box. Frank Warren discovered an even larger, 1.4 ct diamond, during placer mining in 1984 (Eakins and others, 1985). Subsequent exploration specifically designed to recover diamonds has been unsuccessful.
Production: Values range from 0.01 to 0.03 ounces of gold per cubic yard. Upstream from the fault, the values are lower, approximately 0.00625 ounces per cubic yard, as reported by miners on the basis of exploratory sampling (Lampright, 1996). Total amount of production is unknown.

Commodities (Major) - Au; (Minor) - Diamond
Development Status: Yes; small
Deposit Model: Placer gold deposit (Cox and Singer, 1986; model 39a)

Other Databases

Link to USGS - Alaska:	CI013

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References