Magnet

Prospects, Inactive

Alternative names

Old Sourdough

Commodities and mineralogy

Main commodities Au
Other commodities Cu; Pb; Zn
Ore minerals azurite; chalcopyrite; galena; gold; malachite; pyrite; pyrolusite; pyrrhotite; sphalerite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale MM
Quadrangle map, 1:63,360-scale A-3
Latitude 63.2056
Longitude -151.0899
Nearby scientific data Find additional scientific data near this location
Location and accuracy
The Magnet and Old Sourdough prospects (Cobb, 1980, p. 64, 80 [OFR 80-363]) are at the head of Iron Creek. The claims are the northeast continuation of the Terminus and Greenback claims (MM173), and are part of an approximately six-mile-long belt of mineral deposits on the north flank of the Alaska Range.
The Magnet and Old Sourdough claims are on the Iron Creek side of the Slippery Creek-Iron Creek divide, in the SE1/4 SE1/4 sec. 1, T. 20 S., R. 19 W., Fairbanks Meridian. For this record, the site is at an elevation of about 5200 feet. The location is based on a planetable topographic-geologic map of the claim block prepared by Earl R. Pilgrim in 1929 (copied as fig. 4.1-B, Hawley and Associates, 1978). The location is accurate within about 500 feet. The Magnet and Old Sourdough claims are combined as location 32 in Cobb (1972 [MF 366]) and as number 41 in MacKevett and Holloway (1977).

Geologic setting

Geologic description

The country rocks at the Magnet and Sourdough claims are limestone, shale or argillite, quartzite, and sill-like granitic dikes. The strata strike northeast and probably generally dip southeast. The sedimentary rocks are of Paleozoic age. The dikes are probably apophyses of the McGonagall pluton of mid-Tertiary age. The main outcrop area of the McGonagall pluton is about 1/4 mile southeast of the claims (Reed, 1961; Reed and Lanphere, 1973, 1974).
The mineral deposits are closely associated with the granitic dikes. Moffit (1933, p. 321-322) reported that a cut on the Magnet claim exposed dark, fine-grained silicified rock containing pyrrhotite, galena, and sphalerite. A felsic dike exposed in the canyon of Iron Creek on the Old Sourdough claim contains disseminated to massive pyrrhotite that probably is auriferous. Rock exposed upstream of the dike on the north side of Iron Creek contains pyrrhotite, sphalerite, chalcopyrite, and copper carbonates. Brecciated sedimentary country rock is cemented with malachite and azurite, and the dikes contain specks of pyrolusite. Sampling by W. E. Dunkle in about 1936 or 1937 (see MM173) showed that vein-like zones in the walls of the dikes assay about 0.16 to 0.43 ounce of gold per ton. The deposits were explored by surface and underground workings in 1937.
The mineralization probably is Oligocene, forming shortly after the emplacement of the McGonagall pluton of Late Eocene or Early Oligocene age (Reed and Lanphere, 1974).
Geologic map unit (-151.092152615565, 63.2050810509647)
Mineral deposit model Polymetallic gold lodes affiliated with granitic intrusive rocks (Cox and Singer, 1986; models 22b and 22c).
Mineral deposit model number 22b and 22c
Age of mineralization The mineralization probably is Oligocene, forming shortly after the emplacement of the McGonagall pluton of Late Eocene or Early Oligocene age (Reed and Lanphere, 1974).
Alteration of deposit Silicification and sulfidation of sedimentary rocks and granitic dikes. Local oxidation of copper and manganese minerals.

Production and reserves

Workings or exploration The Magnet and Old Sourdough claims were located before 1929 by W. J. Shannon, who explored them until about 1935. The claims were optioned by W. E. Dunkle in 1936 and explored by Dunkle and the Anaconda Copper Company in 1937. Although gold-bearing dikes were found over a strike length of at least one mile, the gold-bearing rock was refractory and the grade found during underground testing was less than on the surface. The property was returned to Shannon. The claims were relocated by Arley Taylor in 1967 (Heiner and Porter, 1972).
Indication of production None
Reserve estimates Possible low-grade gold resource.

Additional comments

The area is in Denali National Park and Preserve.

References

MRDS Number A011235; A011243

References

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 10/9/2000