Twin Hills

Prospect, Inactive

Commodities and mineralogy

Main commodities Ag; Au; Cu; Pb; Zn
Other commodities Fe
Ore minerals chalcopyrite; galena; magnetite; pyrrhotite; sphalerite
Gangue minerals hornblende; quartz

Geographic location

Quadrangle map, 1:250,000-scale MM
Quadrangle map, 1:63,360-scale B-1
Latitude 63.3306
Longitude -150.4847
Nearby scientific data Find additional scientific data near this location
Location and accuracy 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).

Geologic setting

Geologic description

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.
Geologic map unit (-150.486947910819, 63.3301006086986)
Mineral deposit model Polymetallic veins and pluton-hosted gold (Cox and Singer, 1986; model 22c).
Mineral deposit model number 22c
Age of mineralization 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 of deposit Propylitic alteration, including formation of fibrous hornblende on fracture surfaces; silicification of limestone; 'leaching' of white porphyry (Moffitt, 1933, p. 323).

Production and reserves

Workings or exploration 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.
Indication of production None

Additional comments

The prospect is in Denali National Park and Preserve.

References

MRDS Number A011258

References

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., 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 12/13/2000