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Unnamed (near Helen Todd Lake)

Occurrences, Undetermined

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Commodities and mineralogy

Main commodities Pb
Ore minerals galena; magnetite; pyrite
Gangue minerals calcite

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale A-5
Latitude 55.204
Longitude -131.523
Nearby scientific data Find additional scientific data near this location
Location and accuracy This site represents two occurrences at 1000 feet elevation on the hillside west and southwest of Helen Todd Lake. The occurrences are in sections 10 and 11, T. 77 S., R. 92 E., of the Copper River Meridian. They are about 0.4 mile apart, along an unmapped logging road parallel to, and just below, the crestline of a northeast-trending ridge. The map site is approximately midway between the two occurrences. The site corresponds to locs. 28 and 29 in Berg (1972 [I 684]), and to loc. 15a, b in Karl (1992). The location is accurate within about 0.1 mile.

Geologic setting

Geologic description

The country rocks in the area of the site are undivided, metamorphosed Silurian and Ordovician volcanic, sedimentary, and intrusive rocks; and Silurian trondhjemite that intrudes the Silurian and Ordovician rocks (Berg, 1972; Berg and others, 1988). The rocks were regionally metamorphosed to greenschist-grade phyllite and semischist in Late Cretaceous time.
The occurrences are in foliated, sericitized, trondhjemite, and consist of pyrite, magnetite, and galena in sparse calcite veinlets up to 0.25 inch thick and 2-3 inches long (Berg, 1972, locs. 28, 29; Karl, 1992, loc. 15a, b) The sulfide minerals are also widely disseminated in the trondhjemite.
Geologic map unit (-131.524681212689, 55.2036552914447)
Mineral deposit model Disseminated sulfide and oxide mineralization of undetermined origin.
Age of mineralization The veins probably are Late Cretaceous or younger.
Alteration of deposit The trondhjemite hostrock is sericitized and contains disseminated sulfide minerals.

Production and reserves

Indication of production None

References

MRDS Number 10307696

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