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Unnamed (north of Wood Cove, Forrester Island)

Occurrence, Inactive

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Commodities and mineralogy

Main commodities Mo
Other commodities Cu
Ore minerals chalcopyrite; magnetite; molybdenite; pyrite; pyrrhotite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale DE
Quadrangle map, 1:63,360-scale D-5
Latitude 54.8168
Longitude -133.5204
Nearby scientific data Find additional scientific data near this location
Location and accuracy This occurrence consists of two sample sites on the eastern shoreline of Forrester Island about 0.5 mile north of Wood Cove. They are near the center of section 30, T. 81 S., R. 80 E; the location is accurate.

Geologic setting

Geologic description

During reconnaissance surveys in 1969, Clark and others (1971) identified a porphyry molybdenum deposit near Wood Cove on the north end of Forrester Island. A similar occurrence (DE001) is on the northern tip of Forrester Island. Forrester Island consists mainly of Cretaceous(?) diorite and quartz monzonite (Clark and others, 1971; Gehrels and Berg, 1992). At the northeast corner of the island, the pluton intrudes Silurian or Ordovician(?) conglomerate that has been metamorphosed near the pluton.
At one location at this site, stockwork-type molybdenum-porphyry mineralization occurs in an area approximately 300 feet by 100 feet. The host rock varies from quartz monzonite to granodiorite; it is propylitically altered, strongly pyritized, and cut by numerous quartz veinlets containing molybdenite, epidote, and pyrite. The host rock also contains disseminated molybdenite, as well as small amounts of magnetite, chalcopyrite, and pyrrhotite. The molybdenum content of 11 samples varied from 5 to 500 parts per million (ppm) molybdenum with an average of about 200 ppm. At another locality, molybdenite occurs in quartz veins 1 to 6 inches thick that cut hornfelsed metaconglomerate near the contact with the pluton. Samples contained 500-700 ppm molybdenum.
Geologic map unit (-133.522045956501, 54.8164413583888)
Mineral deposit model Porphyry molybdenum (Cox and Singer, 1986; model 21b).
Mineral deposit model number 21b
Age of mineralization Probably related to the Cretaceous(?) diorite and quartz monzonite pluton that makes up most of Forrester Island.
Alteration of deposit The quartz monzonite and diorite host rock has been strongly propylitized; the metaconglomerate adjacent to the pluton has been hornfelsed and is cut by numerous quartz veins.

Production and reserves

Workings or exploration There are no workings and probably no exploration has taken place.
Indication of production None
Reserve estimates Clark and others (1971) suggest that the resource potential of these occurrences is extremely limited.

Additional comments

The occurrence is in the Forrester Island National Wildlife Refuge and is closed to exploration.

References

MRDS Number AO10268

References

Clark, A. L., Berg, H., C., Grybeck, Donald, and Ovenshine, A. T., 1971, Reconnaissance geology and geochemistry of Forrester Island National Wildlife Refuge, Alaska: U.S. Geological Survey Open-File Report 71-67, 1 sheet, scale 1:63,360, 9 p.
Cobb, E. H., 1972, Metallic mineral resources of the Dixon Entrance quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-434, 1 sheet, scale 1:250,000.
Cobb, E. H., 1978, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Dixon Entrance quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-863, 34 p.
Cobb, E. H., Wanek, A. A., Grantz, Arthur, and Carter, Claire, 1968, Summary report on the geology and mineral resources of the Bering Sea, Bogoslof, Simeonof, Semidi, Tuxedni, St. Lazaria, Hazy Islands, and Forrester Island National Wildlife Refuges: U.S. Geological Survey Bulletin 1260-K, p. K1-K28.
Gehrels, G.E., and Berg, H.C., 1984, Geologic map of southeastern Alaska: U.S. Geological Survey Open-File Report 84-866, 28 p., 1 sheet, scale 1:600,000.
Reporters D.J. Grybeck (Applied Geology)
Last report date 9/1/2003