Twin Hills Prospect, Kantishna District, Denali Borough, Alaska, USA
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Location: The Twin Hills prospect (Cobb, 1980, p. 109 [OFR 80-363]) is on the west side of a north-flowing tributary of Pirate Creek. The prospect is near location 35 of Cobb (1972 [MF 366]). Its location in the NE1/4NE1/4 sec. 30, T. 18 S., R. 15 W., Fairbanks Meridian, matches Moffit's (1933, p. 323) topographic description, but there are other nearby locations that could match his description. Cobb's location is also near the position of the Twin Hills lode described by Heiner and Porter (1972, Mt. McKinley quadrangle, serial number 67). The location selected for this record is probably correct within 1/2 mile. The Twin Hills prospect is number 44 of MacKevett and Holloway (1977).
Geology: The Twin Hills prospect is in the Eocene or Oligocene McGonagall granitic pluton, as mapped by Reed (1961, pl. 1). The deposit is probably in the faulted contact zone of the pluton. Moffit (1933, p. 323) noted porphyritic granite, silicified banded limestone, and an unidentified altered rock, possibly argillite or diabase, at the prospect. This altered rock contains radiating fibers of green hornblende. The prospect consists of veins and disseminated deposits. A quartz vein at least 10 feet wide contains pyrrhotite, chalcopyrite, magnetite, possibly sphalerite, and fibrous hornblende. White porphyry near the vein assayed about about 0.11 ounce of gold per ton, along with detectable silver, lead, and zinc (Moffit, 1933, p. 323). Moffit traced float containing sphalerite, magnetite, and lesser amounts of galena easterly to the ridge crest, where the float contains pyrrhotite and magnetite. The Twin Hills lode is one of a number of deposits in a mineral belt along the north flank of the McGonagall pluton. The deposits are probably roughly contemporaneous with the Eocene or Oligocene emplacement age of the McGonagall pluton.
Workings: The Twin Hills lode and nearby deposits were located by prospector W. J. Shannon in 1930. Mr. Shannon prospected this general area between about 1920 and 1939. There are no known workings. The prospect is inactive.
Age: The deposit formed subsequent to the Eocene or Oligocene intrusion of granitic rocks probably related to the McGonagall pluton (Reed and Lanphere, 1974; Decker and Gilbert, 1978; Cole, 1998).
Alteration: Propylitic alteration, including formation of fibrous hornblende on fracture surfaces; silicification of limestone; 'leaching' of white porphyry (Moffitt, 1933, p. 323).
Commodities (Major) - Ag, Au, Cu, Pb, Zn; (Minor) - Fe
Development Status: No
Deposit Model: Polymetallic veins and pluton-hosted gold (Cox and Singer, 1986; model 22c).
7 entries listed. 6 valid minerals.
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Cobb, E. H., 1972, Metallic mineral resources map of the Mount McKinley quadrangle, Alaska: U. S. Geological Survey Miscellaneous Field Studies Map MF-366, 1 sheet, scale 1:250,000. Cobb, E.H., 1980, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Mount McKinley quadrangle, Alaska: U. S. Geological Survey Open-File Report 80-363, 150 p. 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. Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p. Decker, J. E., and Gilbert, W. G., 1978, The Mt. Galen volcanics-A new middle Tertiary volcanic formation in central Alaska Range: Alaska Division of Geological and Geophysical Surveys Geological Report 59, 11 p. Heiner, L.E., and Porter, Eve, 1972, Alaska Mineral Properties, volume 2: University of Alaska, Mineral Industry Research Laboratory Report 24, 669 p. MacKevett, E.M., Jr., and Holloway, C.D., 1977, Map showing metalliferous and selected non-metalliferous mineral deposits in the eastern part of southern Alaska: U.S. Geological Survey Open-File Report 77-169-A, 99 p., 1 sheet, scale 1:1,000,000. Moffit, F. H., 1933, The Kantishna district: U. S. Geological Survey Bulletin 836, p. 301-338. 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. Reed, J. C., Jr., 1961, Geology of the Mt. McKinley quadrangle, Alaska: U. S. Geological Survey Bulletin 1108-A, p. A1-A36.