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North Bradfield River

Prospect in Alaska, United States with commodities Copper, Iron, Silver, Gold, Molybdenum, Zinc
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Sections on this page
  1. Identification information
  2. Geographic coordinates
  3. Site location context
  4. Geographic areas
  5. Commodities
  6. Materials information
  7. Alteration
  8. Mineral occurrence model information
  9. Nearby scientific data
  10. Economic information about the deposit and operations
  11. Mining district
  12. Links to other databases
  13. Bibliographic references
  14. General comments
  15. Reporter information

Geologic information

Identification information

Deposit ID 10308512
Record type Site
Current site name North Bradfield River
Alternate or previous names Ptarmigan

Geographic coordinates

Geographic coordinates: -131.39181, 56.38576 (WGS84)
Relative position The above coordinates are for the approximate center of a group of mineral occurrences at an elevation of about 3300 feet on a ridge in Section 24, between two forks of upper North Bradfield River. Cobb (1978, loc. 2-4) gives the following coordinates to encompass the area of the known deposits: Lat. 56.383-56.40N; Long. 131.383-131.417W. These coordinates probably define the geographic limits of the known deposits with an accuracy of a few hundred feet.

Site location context

Political divisions (FIPS codes)

Wade Hampton(Census area)

Alaska(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

Bradfield Canal B-5(quadrangle 1:63,360 scale)

Bradfield Canal SW(quadrangle 1:100,000 scale)

Bradfield Canal(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

Southern Southeast Alaska(hydrologic accounting unit)

Southeast Alaska(hydrologic subregion)

Alaska(hydrologic region)

Federal lands

Tongass National Forest(National Forest)

National Forest FS(Type of land area)

FS(Federal land areas administered by FS)

Geographic areas

Country State
United States Alaska

Commodities

Commodity Importance
Copper Primary
Iron Primary
Silver Secondary
Gold Secondary
Molybdenum Secondary
Zinc Critical Secondary

Comments on the commodity information

  • Gangue = calc-silicate minerals

Materials information

Materials Type of material
Chalcopyrite Ore
Hematite Ore
Magnetite Ore
Malachite Ore
Pyrrhotite Ore
Calcite Gangue

Alteration

  • Calc-silicate skarn is developed in marble units; local iron staining

Mineral occurrence model information

Model code 62
USGS model code 18d
Deposit model name Skarn Fe

Nearby scientific data

(1) -131.39181, 56.38576
ListMap

Economic information

Comments on the geologic information

  • Geologic Description = The North Bradfield River prospect (MacKevett and Blake, 1963, p. D1-D21) consists chiefly of metasomatic, magnetite-skarn deposits at the northwest end of a large roof pendant of gneiss, granulite, schist, marble, and skarn in quartz monzonite of the Coast Range Batholith, which in turn is cut by dikes of quartz diorite, aplite, and alaskite. The metamorphic bedded rocks are complexly folded. MacKevett and Blake (1963, p. D15-D16) interpret the general structure of the pendant as an overturned syncline that probably extends for many miles to the southeast. Sonnevil (1981, p. B117), on the other, hand, interprets the dominant structure in the area as a homocline with northwest to northeast dips. Koch (1997, p. 24) reports that the pendant is marked by an aeromagnetic trough that roughly parallels its outcrop (U. S. Geological Survey, 1979). The deposit is in marble units of the roof pendant and consists of calc-silicate skarn that is partly replaced by massive magnetite with interstitial pyrrhotite; the magnetite is cut by veinlets of chalcopyrite. This ore contains subordinate amounts of hematite, limonite, and malachite. The orebodies, of which at least 15 are exposed, are crudely stratiform and apparently discontinuous; they range in strike length from 50-350 feet and in thickness from 2-40 feet. Koch (1997, p. 24-25) suggests that at least some of the metal concentration in these deposits is related to the emplacement of an Eocene quartz monzonite and granodiorite stock, near the contact with the Mesozoic or Paleozoic metamorphic bedded rocks that host the deposits (Elliott and Koch, 1981; Koch, 1996). Koch (1997, p. 24, 25) and Elliott and Koch (1981, p. 8, loc. 9) also report anomalous amounts of Ag, Au, Mo, and Zn in rock samples collected at and near the North Bradfield River deposits.
  • Age = Eocene

Economic information about the deposit and operations

Development status Prospect
Commodity type Metallic

Comments on exploration

  • Status = Probably inactive

Mining district

District name Petersburg

Comments on the reserve resource information

  • Reserves = No estimates of reserves have been made public. Most of the magnetite bodies that are exposed probably contain 50-65% iron and 0.1-0.5% Cu (MacKevett and Blake, 1963, p. D17). Elliott and Koch (1981, p. 8, loc. 9) state that the two structural interpretations that have been proposed (MacKevett and Blake, 1963; Sonnevil, 1981) have different implications for potential reserves.

Comments on the workings information

  • Workings / Exploration = The deposit was discovered in 1955 and was explored by private interests in the 1960's by surface stripping, an airborne magnetometer survey, and six diamond drillholes totalling 186 feet. Koch (1997, p. 24) reports that the pendant is marked by an aeromagnetic trough that roughly parallels its outcrop (U.S. Geological Survey, 1979). Koch (1997, p. 24, 25) and Elliott and Koch (1981, loc. 9) also report anomalous amounts of Ag, Au, Mo, and Zn in rock samples collected at and near the North Bradfield River deposits.

Reference information

Bibliographic references

Comments on the references

  • Primary Reference = MacKevett and Blake, 1963; Sonnevil, 1981

General comments

Subject category Comment text
Deposit Model Name = Fe skarn
Deposit Other Comments = Elliott and Koch (1981) also apply the name 'Ptarmigan' to the North Bradfield River prospect.

Reporter information

Type Date Name Affiliation Comment
Reporter 17-MAY-1998 H. C. Berg U.S. Geological Survey

Beyond USGS

Supplemental information added by qvyshift.com. Not part of the original USGS MRDS record.

Maps centered on this site

Authoritative Alaska resources

These are landing pages for further research — the state agencies don't currently expose per-mine deep links.