North Bradfield River

Site Name

Primary: North Bradfield River
Secondary: Ptarmigan

---

Commodity

Primary: Copper
Primary: Iron
Secondary: Molybdenum
Secondary: Silver
Secondary: Gold
Secondary: Zinc

---

Location

State: Alaska
District: Petersburg

---

Land Status

Not available

---

Holdings

Not available

---

Workings

Not available

---

Ownership

Not available

---

Production

Not available

---

Deposit

Record Type: Site
Operation Category: Prospect
Operation Type: Unknown
Years of Production:
Organization:
Significant:

---

Physiography

Not available

---

Mineral Deposit Model

Model Name: Skarn Fe

---

Orebody

Not available

---

Structure

Not available

---

Alterations

Alteration Text: Calc-silicate skarn is developed in marble units; local iron staining

---

Rocks

Not available

---

Analytical Data

Not available

---

Materials

Ore: Chalcopyrite
Ore: Hematite
Ore: Magnetite
Ore: Malachite
Ore: Pyrrhotite
Gangue: Calcite

---

Comments

Comment (Workings): 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.

Comment (Deposit): Model Name = Fe skarn

Comment (Reserve-Resource): 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.

Comment (Deposit): Other Comments = Elliott and Koch (1981) also apply the name 'Ptarmigan' to the North Bradfield River prospect.

Comment (Geology): 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.

Comment (Geology): Age = Eocene

Comment (Reference): Primary Reference = MacKevett and Blake, 1963; Sonnevil, 1981

Comment (Commodity): Gangue = calc-silicate minerals

Comment (Exploration): Status = Probably inactive

---

References

Reference (Deposit): Koch, R.D., 1997, Metalliferous mineral resource potential in the Bradfield Canal quadrangle, southeastern Alaska: U.S. Geological Survey Open-File Report 97-271, 57 p., 1 sheet, scale 1:250,000.

Reference (Deposit): MacKevett, E.M., Jr., and Blake, M.C., Jr., 1963, Geology of the North Bradfield River iron prospect, southeastern Alaska: U.S. Geological Survey Bulletin 1108-D, p. D1-D21.

Reference (Deposit): Koch, R.D., 1996, Reconnaissance geologic map of the Bradfield Canal quadrangle, southeastern Alaska: U.S. Geological Survey Open-File Report 81-728-A, 35 p., 1 sheet, scale 1:250,000.

Reference (Deposit): Elliott, R.L., and Koch, R.D., 1981, Mines, prospects, and selected metalliferous mineral occurrences in the Bradfield Canal quadrangle, Alaska: U.S. Geological Survey Open-File Report 81-728-B, 23 p., 1 sheet, scales 1:250,000 and 1:63,360.

Reference (Deposit): Sonnevil, R.A., 1981, New data concerning the geology of the North Bradfield River iron prospect, southeastern Alaska, in Albert, N.R.D., and Hudson, Travis, eds., The United States Geological Survey in Alaska, Accomplishments during 1979: U.S. Geological Survey Circular 823B, p. B117-B118.

Reference (Deposit): Cobb, E.H., 1978, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Bradfield Canal quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-922, 98 p.

Reference (Deposit): U.S. Geological Survey, 1979, Aeromagnetic map of the Petersburg area, Alaska: U.S. Geological Survey Open-File Report 79-832, 1 sheet, scale 1:250,000.

---

The Top Ten Gold Producing States

These ten states contributed the most to the gold production that built the West from 1848 through the 1930s. The Top Ten Gold Producing States.