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Unnamed (southwest ridge of Bush Mountain)

Occurrences, Undetermined

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Commodities and mineralogy

Main commodities Ag; Au; Cr; Cu; Mo; Ni; Pb
Ore minerals bornite; chalcopyrite; pyrite; pyrrhotite
Gangue minerals albite; calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale A-5
Latitude 55.217
Longitude -131.519
Nearby scientific data Find additional scientific data near this location
Location and accuracy This site represents four occurrences in a mile-long, northwest-trending line extending from about 500 feet elevation near the northwest shore of Helen Todd Lake, to about 1100 feet elevation on the southwest ridge of Bush Mountain. The occurrences are in sections 2, 3, and 11, T. 77 S., R. 92 E., of the Copper River Meridian. The map site is at the approximate center of the line of occurrences. The site corresponds to loc. 12a-d in Karl (1992). The location is accurate within about 0.1 mile.

Geologic setting

Geologic description

The country rocks in the area of this site include: undivided, metamorphosed Silurian and Ordovician volcanic, sedimentary, and intrusive rocks; Silurian trondhjemite; recrystallized, Upper Triassic sedimentary and volcanic rocks; and Upper Jurassic or Cretaceous flyschlike metasedimentary rocks and metavolcanic rocks (Berg, 1972 [I 684]; Berg and others, 1988; Karl, 1992). The rocks were regionally metamorphosed to greenschist-grade phyllite and semischist in Late Cretaceous time.
The mineral occurrences consist of sheared, pyritic greenstone and microgabbro containing quartz and calcite veins (Karl, 1992, loc. 12a-d). Some of the quartz veins contain pyrite, pyrrhotite, chalcopyrite, and bornite. Some of the veins are cataclastic, and the microgabbro is propylitically altered to epidote, chlorite, quartz, calcite, and sericite.
A sample of greenstone containing calcite-pyrrhotite veins assayed 500 ppm Cr and 1000 ppm Ni. Samples of greenstone, a quartz-sulfide vein, and quartz-albite-pyrite breccia contained up to 3.25 ppm Au, 70 ppm Ag, 15000 ppm Cu, 500 ppm Pb, and 10 ppm Mo. A sample of microgabbro contained 500 ppm Cr and 200 ppm Ni; samples of greenstone and quartz breccia contained up to 3 ppm Ag and 700 ppm Cu (Karl, 1992, loc. 12a-d).
Geologic map unit (-131.520681692429, 55.2166549557106)
Mineral deposit model Low-sulfide Au-quartz veins (Cox and Singer, 1986; model 36a)
Mineral deposit model number 36a
Age of mineralization Probably Late Cretaceous or younger.
Alteration of deposit Country rocks are propylitized and pyritic.

Production and reserves

Workings or exploration A sample of greenstone containing calcite-pyrrhotite veins assayed 500 ppm Cr and 1000 ppm Ni. Samples of greenstone, a quartz-sulfide vein, and quartz-albite-pyrite breccia contained up to 3.25 ppm Au, 70 ppm Ag, 15000 ppm Cu, 500 ppm Pb, and 10 ppm Mo. A sample of microgabbro contained 500 ppm Cr and 200 ppm Ni; samples of greenstone and quartz breccia contained up to 3 ppm Ag and 700 ppm Cu (Karl, 1992, loc. 12a-d).
Indication of production None

References

MRDS Number 10307682

References

Berg, H.C., 1972, Geologic map of Annette Island, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-684, 8 p., 1 sheet, scale 1:63,360.
Berg, H.C., Elliott, R.L., and Koch, R.D., 1988, Geologic map of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Mineral Investigations Series Map I-1807, 27 p., scale 1:250,000.
Reporters H.C. Berg, USGS
Last report date 7/7/1999