Unnamed (western Gravina Island)

Prospect in Alaska, United States with commodities Silver, Copper, Molybdenum

Geologic information

Identification information

Deposit ID 10094029
MRDS ID A012320
Record type Site
Current site name Unnamed (western Gravina Island)

Geographic coordinates

Geographic coordinates: -131.83566, 55.2628 (WGS84)
Relative position This prospect is at an elevation of about 150 feet on western Gravina Island, about 0.3 mile from tidewater, and about 1.0 mile south-southeast of triangulation point 'Pug.' The site is in section 23, T. 76 S., R. 89 E., of the Copper River Meridian. It corresponds to loc. 52.1 in Elliott and others (1978). The location is accurate within 0.1 mile.
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Geographic areas

Country State
United States Alaska

Commodities

Commodity Importance
Silver Primary
Copper Primary
Molybdenum Secondary

Materials information

Materials Type of material
Chalcopyrite Ore
Hematite Ore
Molybdenite Ore
Pyrite Ore
Dolomite Gangue
Quartz Gangue

Alteration

  • (Local) Limestone locally is dolomitized, and rhyolite locally contains conspicuous hydrothermal hematite. Local iron- and copper-staining.

Mineral occurrence model information

Model code 85
USGS model code 22c
Deposit model name Polymetallic veins
Mark3 model number 46

Host and associated rocks

  • Host or associated Associated
    Rock type Volcanic Rock (Aphanitic) > Felsic Volcanic Rock > Rhyolite
  • Host or associated Host
    Rock type Volcanic Rock (Aphanitic) > Felsic Volcanic Rock > Rhyolite

Nearby scientific data

(1) -131.83566, 55.2628

Comments on the geologic information

  • Geologic Description = The country rocks at this prospect consist of Upper Triassic rhyolite and rhyolite tuff (Berg, 1973, Berg and others, 1988). This formation is depositionally overlain by, and locally in fault contact with, Upper Triassic carbonate, clastic, and basaltic volcanic rocks. The strata were regionally metamorphosed to prehenite-pumpellyite or to low-greenschist grade in Late Cretaceous time. Locally, the limestone is dolomitized, and the rhyolite contains conspicuous hydrothermal hematite. ? the deposit is in iron-stained metarhyolite(?) and consists of a ten-foot-wide breccia zone containing quartz, hematite, dolomite, chalcopyrite, and pyrite (Berg, 1973, p. 37). An analysis of a sample of the mineralized breccia showed 1.5 ppm Ag and 2% Cu (Koch and Elliott, 1978; OFR 78-156A). In 1970, workings consisted of a water-filled adit that probably was driven in the early 1900s. Prospecting in this area in the 1960s and 70s reportedly revealed disseminated chalcopyrite and pyrite, and traces of molybdenite and secondary copper minerals in complexly sheared, hydrothermally altered metamorphic and intrusive country rocks (Berg, 1973, p. 37). Assuming that the mineralized breccia postdates the regional metamorphism, the deposit is Late Cretaceous or younger.
  • Age = Late Cretaceous or younger.

Economic information

Economic information about the deposit and operations

Development status Prospect
Commodity type Metallic

Mining district

District name Ketchikan

Comments on the workings information

  • Workings / Exploration = In 1970, workings consisted of a water-filled adit that probably was driven in the early 1900s. An analysis of a sample of the mineralized breccia showed 1.5 ppm Ag and 2% Cu (Koch and Elliott, 1978 [OFR 78-156A]). Prospecting in this area in the 1960s and 70s reportedly revealed disseminated chalcopyrite and pyrite, and traces of molybdenite and secondary copper minerals in complexly sheared, hydrothermally altered metamorphic and intrusive country rocks (Berg, 1973, p. 37).

Reference information

Links to other databases

Agency Database name Acronym Record ID Notes
USGS Mineral Resources Data System MRDS A012320
USGS Alaska Resource Data File ARDF KC054

Bibliographic references

  • Deposit

    Berg, H.C., 1973, Geology of Gravina Island Alaska: U.S. Geological Survey Bulletin 1373, 41 p.

  • Deposit

    Elliott, R.L., Berg, H.C., and Karl, Susan, 1978, map and table describing metalliferous and selected nonmetalliferous mineral deposits, Ketchikan and Prince Rupert quadrangles, Alaska: U.S. Geological Survey Open-file Report, 78-73-B,17 p., scale 1:250,000.

  • Deposit

    Koch, R.D., and Elliott, R.L., 1978, Analyses of rock samples from Ketchikan quadrangle, southeastern Alaska: U.S. Geological Survey Open-File Report 78-156-A, 162 p., 1 sheet, scale l:250,000.

  • Deposit

    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 MF-1807,27 p., scale 1:250,000.

Comments on the references

  • Primary Reference = Berg, 1973

General comments

Subject category Comment text
Deposit Model Name = Polymetallic veins? (Cox and Singer, 1986; model 22c)

Reporter information

Type Date Name Affiliation Comment
Reporter 01-JUL-1999 H.C. Berg U.S. Geological Survey