Mt. Eielson or Copper Mountain

Prospects, Inactive

Commodities and mineralogy

Geographic location

Quadrangle map, 1:250,000-scale MM
Quadrangle map, 1:63,360-scale B-1
Latitude 63.3979
Longitude -150.3319
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Mount Eielson or Copper Mountain district (Reed, 1933; Gates and Wahrhaftig, 1944; Muir, Thomas, and Sanford, 1948; Cobb, 1980 [OFR 80-363]) contains numerous zinc-lead replacement deposits. The main group of deposits is on the north flank of Mt. Eielson and extends easterly for at least four miles from Glacier Creek on the east side of Muldrow Glacier to Bald Mountain south of Sunrise Creek. For this record, the site is at the approximate center of a group of nineteen claims which were actively explored in about 1922 to 1924. The site also is at the approximate center of the most intensely mineralized area identified in the district. The main part of the district is covered by locations 37-40 of Cobb (1972 [MF 366]) and by location 46 of MacKevett and Holloway (1977).

Geologic setting

Geologic description

The country rocks in the Mt. Eielson district are probably Paleozoic (Devonian?) and consist of limestone, calcareous argillite, slate, and schist. These rocks are cut by gabbro and locally overlain(?) by Triassic(?) greenstone. Tertiary coal-bearing rocks and Nenana Gravel form a local basin that probably underlies much of the alluvium-filled valley of Thorofare Creek on the north side of the district (Reed, 1933, plate 24). The mineral deposits are largely hosted by the limestone and calcareous argillite.
The pre-coal-bearing rocks are intruded by granodiorite and porphyritic granodiorite (Reed, 1933). The granodiorite mass near Mt. Eielson is approximately on strike with the east-northeasterly-elongated McGonagall granodiorite batholith (Reed, 1961), and is probably related to it (Decker and Gilbert, 1978; Cole, 1998). The McGonagall granodiorite is north of the Denali fault. It is equivalent to the Foraker granodiorite that is exposed south of the fault. Reed (1933) considered the granodiorite at Mt. Eielson to be Jurassic, but it is now known to be Late Eocene or Early Oligocene (Reed and Lanphere, 1973, 1974; Decker and Gilbert, 1978; Cole, 1998). The mineralization in the district is closely associated with the granodiorite and porphyritic granodiorite; it probably formed in Oligocene time, during the waning stages of McGonagall plutonism.
The granodiorite is greenish-gray and contains abundant phenocrysts of oligoclase-andesine and fewer of hornblende. K-feldspar veins and locally replaces plagioclase. The porphyritic granodiorite is more variable; some is relatively free of phenocrysts, and some contains large feldspar and hornblende phenocrysts as much as 1.5 inches long (Reed, 1933, p. 257-58). The hornblende is extensively altered to chlorite, and secondary epidote, sphene, calcite, and sericite occur widely in the rock. Locally the granodiorite contains pyrite and pyrrhotite; this rock weathers to dark rusty brown limonite. The main granitic mass on Mt. Eielson is even-grained granodiorite that grades upward into porphyritic granodiorite that forms many sill-like and dike-like apophyses in the calcic country rocks. The calcic strata generally dip northerly at a low angle.
Reed (1933, plates 22, 23, 24 and figures 35 and 36) shows that the major structure in the district is a steeply-dipping fault that strikes east and is exposed about 1/2 mile south of Mt. Eielson. Rocks north of the fault are uplifted vertically.
The mineral deposits are in stratiform, epidotized and silicified layers that formed by replacing favorable calcic argillite and limestone beds. The ore minerals replace the epidote-silica rock. The deposits occur in a belt at least four miles long which can be traced eastward from Muldrow Glacier to Bald Mountain.
The ore is banded. According to Reed (1933, p. 273), sulfide-rich bands 1/16th to 1 inch thick alternate with bands of epidote-silica rock. The most abundant gangue minerals are members of the epidote group, dominantly clinozoisite. Garnet occurs rarely; quartz and calcite are minor gangue minerals. Quartz occurs most commonly as a fine-grained replacement product, but occasionally forms veins. Locally, layers as much as 40 feet thick are mineralized. Because of glacial cover, tracing the individual mineralized beds is difficult, but some appear to be continuous for distances of hundreds of feet.
Sphalerite, mainly fine-grained, is the most abundant ore mineral, followed by coarsely crystalline galena. Chalcopyrite is less abundant but may increase in content near the contact with the underlying massive granodiorite. The chalcopyrite is paragenetically younger than the sphalerite and galena. Pyrite is common, and pyrite and pyrrhotite occur as disseminations in the granodiorite. Pyrargyrite was tentatively identified microscopically in the galena. Scattered high-silver values occur in the pyrargyrite-bearing lodes and in some galena-rich lodes, but silver content is generally less than three ounces per ton. Gold is not detected in most samples, but a few assays show 0.02-3 ounces per ton. Azurite and malachite and rare native copper are oxidation products of chalcopyrite and probably rare tetrahedrite. Metallurgical studies by the U. S. Bureau of mines identified the oxidized lead and zinc minerals cerussite and smithsonite in the near-surface ore (Muir, Thomas, and Sanford, 1947). The Bureau's studies were carried out in cooperation with detailed field mapping by the U. S. Geological Survey (Gates and Wahrhaftig, 1944).
Nickel, in garnierite, millerite, and possibly pentlandite also occurs in the district. Samples containing these minerals were submitted to the Territorial Department of Mines by F. B. Jiles in 1924 and 1926 and by W. J. Shannon in 1929 (Joesting, 1941-43, p. 18-19). The minerals probably occur in contact-metamorphosed magnesian carbonate rocks.
Geologic map unit (-150.334153745439, 63.3974066442692)
Mineral deposit model Polymetallic replacement deposits; Zn-Pb-(Cu) skarn deposits (Cox and Singer, 1986; models 19a, 18c).
Mineral deposit model number 19a, 18c
Age of mineralization The mineralization is mid-Tertiary, roughly contemporaneous with the Eocene or Oligocene emplacement age of the granodiorite (Reed and Lanphere, 1973, 1974; Decker and Gilbert, 1978; Cole, 1998).
Alteration of deposit Introduction of epidote-group minerals and silicification of calcic argillite. Granodiorite is propylitized; the hornblende has been chloritized. Sericite is locally present in the granodiorite. Locally conspicuous surface oxidation of iron-, copper-, lead-, and zinc-bearing minerals.

Production and reserves

Workings or exploration
The district was discovered in 1920 by Joe and Fannie Quigley. More deposits were found in 1921 by O. M. Grant and others. The area was actively explored from 1921 to 1924; some prospecting continued into the 1940s. The area was explored by W. E. Dunkle in 1923 for Kennecott Copper Corporation (Fairbanks Daily News-Miner, February 23, 1923.) Dunkle's miners drove at least one adit about 100 foot long; there are at least two other short adits and many prospect pits.
Because of the extent of the mineralized area, it was of substantial interest to government agencies, whose investigations began soon after discovery with the work of Davis (1923). Other early government investigations included those by Capps (1927), Moffit (1933), and Reed's definitive study (1933). World War II triggered investigations by the U.S. Geological Survey (Gates and Wahrhaftig, 1944) and U.S. Bureau of Mines (Muir, Thomas, and Sanford, 1947). The area was briefly studied by Chadwick (1975) on behalf of the U.S. Park Service. Earlier studies were summarized by Berg and Cobb (1967). Hawley and Associates (1978) also summarized earlier studies in an investigation related to Alaska National Interest Lands.
Muir, Thomas, and Sanford (1947) reported poor recovery of lead and zinc in flotation tests. At least part of the poor recovery was due to the partly oxidized sample material available for test work. Chadwick (1975) proposed that there is about 100,000 tons of material in talus that might support a small custom mill. Earlier estimates of resources had been made by Reed (1933), Gates and Wahrhaftig (1944), and Twenhofel (1953).
Indication of production None
Reserve estimates There are no measured reserves. Reed (1933) estimated a resource of many hundreds of thousands of tons of ore containing about 10 percent combined lead and zinc. Gates and Wahrhaftig (1944) estimated 200,000 tons of ore in place and in talus. Twenhofel (1953) estimated a similar tonnage grading about 5 percent zinc, 3 to 5 percent lead, and 0.2 to 0.3 percent copper. Chadwick (1975) estimated that there was about 100,000 tons of material in talus containing about 10 percent combined lead and zinc, and suggested that this material and other outcropping high-grade ore might support a small concentrating mill.

Additional comments

The peak known as Mt. Eielson was originally called Copper Mountain. The area is entirely in Denali Park and Preserve. It was originally in Mount McKinley National Park. At the time of the mineral discovery in 1920 until the 1970s, claim location and mining were legal in the National Park. There has been no substantial work in the area since the 1940s.
In general, all of the mineral occurrences on the north flank of Mt. McKinley historically have constituted the Kantishna district. Reed (1933) and others, however, segregated the intensely mineralized area near Mt. Eielson as the Mt. Eielson or Copper Mountain district.

References

MRDS Number A011239

References

Chadwick, R. H. W., 1975, Gross mineral appraisal of Mt. McKinley National Park, Katmai National Monument, proposed Lake Clark National Park: Unpublished report, National Park Service, Alaska.
Cole, R. B., 1998, Early Tertiary post-subduction volcanism and deformation along the north side of the McKinley fault, Alaska [abs]: Geological Society of America. Abstracts with program, v. 30, p. 177.
Reed, B.L., and Lanphere, M.A., 1973, Alaska-Aleutian Range batholith--Geochronology, chemistry, and relation to circum-Pacific plutonism: Geological Society of America Bulletin, v. 84, no. 8, p. 2583-2610.
Reed, B.L., and Lanphere, M.A., 1974, Offset plutons and history of movement along the McKinley segment of the Denali fault system, Alaska: Geological Society of America Bulletin, v. 85, p. 1883-1892.
Reporters C.C. Hawley
Last report date 12/15/2000