Upper Slate Creek

Mines, Inactive

Commodities and mineralogy

Main commodities Au
Other commodities Ag; Cu; Hg; PGE
Ore minerals cinnabar; gold; native copper; pge; pyrite
Gangue minerals amphibole; chlorite; epidote; garnet; ilmenite; magnetite; zircon

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale A-2
Latitude 63.1657
Longitude -144.8009
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Upper Slate Creek mines are in the headwaters of the creek mainly in the W1/2 section 13 and the W1/2 section 24, T. 20 S., R. 15 E., Fairbanks Meridian, and in the Chisna Pass area southeast of Slate Creek. The location is for this record on Slate Creek on the section line that divides sections 23 and 24. The approximate location of mining on upper Slate Creek during 1979 was shown by Yeend (1981 [C 823-B and OFR 81-355]).

Geologic setting

Geologic description

Upper Slate Creek is mainly underlain by argillite of the Eagle Creek Formation of Pennsylvanian age. This unit was previously mapped as Mankomen Formation by Rose (1967). Rose also mapped breccia of unknown type and a small body of locally brecciated quartz monzonite in upper Slate Creek. A pyritic argillite unit at the head of upper Slate Creek contained a trace of gold and 0.12 ounce of silver per ton (Rose, 1967).
The 'round wash' conglomerate of Tertiary age probably unconformably overlies the Eagle Creek Formation but has been removed by erosion and glaciation. The conglomerate is locally preserved in downfaulted blocks (Yeend, 1981 [OFR 81-355]). A pebble count of placer gravel in Slate Creek shows that it consists mainly of greenstone, felsic plutonic rocks, and slate. Quartz constitutes about six percent of the material (Yeend, 1981 [OFR 81-355]).
Complex glacio-fluvial placers are present in upper Slate Creek. Extensive drilling in 1974 and 1975 delimited buried auriferous channels incised in Eagle Creek Formation that are buried by low-grade auriferous glacial outwash. A valley glacier formerly occupied the north-trending valley of upper Slate Creek. This glacier probably split, and one branch extended southeast toward Limestone Creek and a second branch extended toward Powell Gulch and Slate Creek. Drainage was modified during the last main glaciation by damming of Slate Creek by a glacier in the valley of the West Fork Chistochina River; there was a possible reversal of flow (Yeend, 1981 [OFR 81-355]). The rich gold-bearing channels formed before the last valley glaciation, which, however, brought in additional gold in the outwash gravel. In addition, in incised auriferous channels, gold was also found on bedrock surfaces sloping as much as 30 degrees (Douglas Colp, oral communication, November 2001).
Yeend (1981 [OFR 81-355]) proposed that the placer mine operating in 1979 mined an ancient colluvial-alluvial fan near the point where the flow of upper Slate Creek changed from southwest to northwest. The depth of auriferous bedrock increases to the south. The lowest pay was about 120 feet deep when mining ceased in 1982. The channel, however, continued as outlined by a few drill holes that found gold deeper than 120 feet.
Most of the gold in upper Slate Creek is flat and iron-stained; it is similar to gold found elsewhere in the district that was derived from the 'round wash' conglomerate of Tertiary age. A small amount of gold occurs in small, bright, angular nuggets; this gold is inferred to have come from veinlets in the Pennsylvanian bedrock (Walter Ashwill, oral communication, 2001). Concentrates in upper Slate Creek contain cinnabar, native copper, and sparse silvery grains of platinum group metals. The concentrates also contain magnetite and garnet. Yeend (1981 [OFR 81-355]) described the gold and concentrates that were being recovered in 1979. Gold was brassy yellow or copper colored; some had adhering quartz. Concentrate obtained from a gravel section above the mine consisted of about 30 percent magnetite, 10 percent each of ilmenite, pyrite, and chlorite, and less than 10 percent of epidote, zircon, garnet, and amphibole.
More than 20,000 ounces of placer gold were mined in upper Slate Creek between 1979 and 1982.
Geologic map unit (-144.803128959297, 63.1653044869806)
Mineral deposit model Placer Au-PGE, glacio-fluvial (Cox and Singer, 1986; model 39a).
Mineral deposit model number 39a
Age of mineralization Pleistocene.

Production and reserves

Workings or exploration
Gold was known in upper Slate Creek and was mined on a small scale before 1974. In 1974 and 1975 upper Slate Creek was drilled with churn and Becker-type drills and a clamshell rig by Ranchers Exploration and Development Co. of Albuquerque, New Mexico. Drilling found gold on steep hard bedrock surfaces and in channels incised in local bedrock. All the gravel, except for 4 or 5 feet of surface gravel, was processed. Most of the gold produced came from the deeper gravel.
The mining plant was mobilized in 1978 and in 1979 when production began. Total production from 1979 to 1982 was about 21,000 ounces of placer gold. The best year was 1980, when 8,000 ounces of gold were recovered. Ranchers discontinued mining in 1982. There has been some mining since then, but nothing to approach the scale of the Ranchers project, which repaid all exploration and development costs and returned a modest profit (Walter Ashwill and Douglas Colp, oral communications, November 2001).
Indication of production Yes; medium
Reserve estimates A gold resource remains in deep ground and probably in benches along upper Slate Creek.

References