Greenback

Prospects, Inactive

Alternative names

Terminus

Commodities and mineralogy

Main commodities Ag; Au; Cu
Other commodities Mn; Pb; Zn
Ore minerals arsenopyrite; azurite; chalcanthite; chalcopyrite; galena; gold; malachite; pyrite; pyrolusite; pyrrhotite; sphalerite
Gangue minerals calcite; garnet; quartz

Geographic location

Quadrangle map, 1:250,000-scale MM
Quadrangle map, 1:63,360-scale A-3
Latitude 63.1996
Longitude -151.1134
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Greenback and Terminus prospects (Cobb, 1980, p. 46, 106 [OFR 80-363]) are two of four claims staked end to end which extend from upper Slippery Creek to upper Iron Creek on the northwest flank of Mount McKinley. The westernmost claim is the Terminus. The easternmost claims in the four claim group are the Old Sourdough and Magnet (MM172). The coordinates are for the approximate center of the end line between the Terminus and Greenback claims (see map by Earl R. Pilgrim, fig. 4.1-B, Hawley and Associates, 1978). It is in the NW1/4NW1/4 sec. 12, T. 20 S., R. 19 W., Fairbanks Meridian. The claims trend northeast and lie beween about 4500 and 5000 feet in elevation. The location is probably accurate to 500 feet. The Greenback and Terminus claims are locations 30 and 31 of Cobb (1972 [MF 366]) and 40 of MacKevett and Holloway (1977).

Geologic setting

Geologic description

The Greenback and Terminus claims trend northeasterly, subparallel to a belt of Paleozoic shale, limestone, and mafic intrusives that are intruded by sill-like granitic dikes of probable mid-Tertiary age. These rocks are on the north flank of the McGonagall batholith, which also trends northeast (Reed, 1961; Reed and Lanphere, 1973, 1974). Near the west end of the south endline of the Greenback claim a contact between limestone and a granitic sill strikes slightly east of north and dips 60 east. The discovery location on the Greenback claim is in the limestone, as mapped by Pilgrim (Hawley and Associates, 1978, fig. 4.1-B).
Moffit (1933, p. 321) reported that mineral deposits on the Greenback and Terminus claims are associated with apophyses of granite dikes. Minerals include pyrrhotite, chalcopyrite, sphalerite, galena, and, in altered limestone, garnet. The sulfide minerals are locally partly oxidized to azurite, chalcanthite, malachite, and possibly pyrolusite. Sedimentary beds on the Terminus claim locally are replaced by chalcopyrite, galena and sphalerite.
In 1936-37, W. E. Dunkle prospected gold-bearing lodes associated with granitic dikes in the Slippery Creek area. The dikes strike east-northeast to northeast, about the same as the strike of the sedimentary country rocks. The location of the gold deposits relative to the Greenback-Terminus-Magnet-Old Sourdough claims is uncertain. The deposits are continuous for at least one mile and possibly as much as three miles and probably coincide with the claims. The deposits are in the wallrocks of a granitic dike 3 to 20 feet thick. Both walls of the dikes contain quartz-calcite veins 2.5 to 12 feet thick. The veins contain about 10 percent pyrite and arsenopyrite. At one point, the hanging (south?) wall is cut by a 10-foot vein, and the footwall by a 12-foot vein, making that part of the deposit a total of about 42 feet thick, including 20 feet of intervening dike material. The dike material is oxidized and contains narrow ribbons of quartz carrying about 8 percent pyrite. Although this material does not pan gold, it evidently contains free gold, inasmuch as about 50 percent of the assay gold in surface samples was soluble in cyanide (W. E. Dunkle to J. G. Baragwanath, written communication, 1936). Samples collected by Dunkle assayed $5.60 to $15.00 (0.16 to 0.43 ounce) in gold per ton over thicknesses of 2.5 to 15 feet. .
The age of mineralization is probably middle Tertiary (Oligocene), about contemporaneous with emplacement of the youngest phases of the McGonagall batholith.
Geologic map unit (-151.115653041096, 63.1990802554526)
Mineral deposit model Low- to moderate-sulfide gold-quartz deposits; disseminated and replacement lodes of copper, lead, and zinc (Cox and Singer, 1986; models 22b and 22c).
Mineral deposit model number 22b and 22c
Age of mineralization The mineralization is probably middle Tertiary (Oligocene), somewhat younger than the emplacement age of the McGonagall batholith, which has been dated as Late Eocene or Early Oligocene (Reed and Lanphere, 1974; Cole, 1998).
Alteration of deposit Replacement and silicification of dikes along their walls; replacement (sulfidation) of sedimentary and volcanic host rocks. Secondary (supergene) alteration caused by acidic, oxygenated, near-surface water produced chalcanthite, azurite, malachite, and possibly pyrolusite.

Production and reserves

Workings or exploration The Greenback and Terminus claims were located on prospects discovered by W. J. Shannon before 1929. Although the region was extremely remote, Shannon made a large number of significant discoveries. About 1935, he interested W. E. Dunkle, a mining operator then affiliated with Pardners Mines of New York. Shannon optioned the claims to Dunkle in 1936. The option included the antimony-bearing lodes at the Straightaway Glacier (MM176) and Merinser (MM174) prospects. Shannon then staked more claims to cover a solid block of land 3 miles long in the Slippery Creek-Iron Creek area (Heiner and Porter, 1972, Mt. McKinley quadrangle, serial number 39; W. E. Dunkle letter dated Sept. 10, 1936 to J. G. Baragwanath). Dunkle brought in men and equipment to prospect the claims (Smith, 1938, p. 35; Capps, 1940, p. 188). Dunkle also flew a small tractor from the Golden Zone mine to the Shannon properties in a Ford Trimotor, the first time a tractor had been flown across the Alaska Range (Fairbanks Daily News-Miner, April 22, 1937). Dunkle optioned the property to Anaconda Copper Co. in 1937. Dunkle and Anaconda drove a short adit, possibly on the Merinser claim, and collected a bulk sample of the vein material. The sample assayed 0.16 ounce of gold per ton, no antimony, 2.04 percent arsenic and 5.2 percent sulfur. The samples were tested in Anaconda's laboratory at Butte, Montana; the tests showed that the unoxidized ore was highly refractory (written communs., Francis Cameron to M. H. Gidel, July 1937, and F. F. Frick to Reno Sales, November 1937). Results of the sampling and testing showed that a resource on the order of tens of millions of tons was available at the prospect, but that it was too low-grade and refractory to be developed at the remote location in Slippery Creek. The claims were returned to Shannon. At some later time Shannon abandoned the claims, but they were restaked in about 1967 by Arley Taylor of Kantishna (Heiner and Porter, 1972; Chadwick, 1976). Chadwick believed that the Greenback was a good prospect, but did not have enough information to evaluate it.
Indication of production Undetermined
Reserve estimates Low-grade resource of tens of millions of tons.
Production notes Production for testing only.

Additional comments

The prospects are in Denali National Park and Preserve. They constiture a possibly significant gold resource. Written communications between W. E. Dunkle and J. G. Baragwanath are in C. C. Hawley's Dunkle files. The Anaconda written communications are in the collection at the Western Heritage Center, University of Wyoming.

References

MRDS Number A011221; A011256

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.
Heiner, L.E., and Porter, Eve, 1972, Alaska Mineral Properties, volume 2: University of Alaska, Mineral Industry Research Laboratory Report 24, 669 p.
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 12/12/2000