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Unnamed (on Blashke Islands)

Occurrence in Alaska, United States with commodities Chromium, Copper, Nickel, PGE
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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 10308063
Record type Site
Current site name Unnamed (on Blashke Islands)

Geographic coordinates

Geographic coordinates: -132.88984, 56.12173 (WGS84)
Relative position Occurrence is a mafic-ultramafic intrusive complex about 2 miles in diameter that forms the southern half of the Blashke Islands. Coordinates are for the approximate center of the intrusion. Locality is easily accessible by water and the rocks of the complex are well exposed along the coast and in the internal waterways between the islands.

Site location context

Political divisions (FIPS codes)

Prince of Wales-Hyder(Census area)

Alaska(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

Petersburg A-3(quadrangle 1:63,360 scale)

Petersburg S(quadrangle 1:100,000 scale)

Petersburg(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

Prince of Wales(hydrologic unit)

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
Chromium Critical Primary
Copper Primary
Nickel Critical Primary
PGE Critical Primary

Materials information

Materials Type of material
Chalcopyrite Ore
Chromite Ore
Platinum Ore
Palladium Ore
Iridium Ore
Ruthenium Ore
Osmium Ore
Rhodium Ore

Alteration

  • (Local) None other than hornfelsing of the country rock adjacent to the complex.

Mineral occurrence model information

Model code 23
USGS model code 9
Deposit model name Alaskan Cr-Pt (PGE)
Mark3 model number 120

Nearby scientific data

(1) -132.88984, 56.12173
ListMap

Economic information

Comments on the geologic information

  • Geologic Description = The site is a classic Early Cretaceous, zoned or Alaska-type mafic-ultramafic intrusive complex about 2 miles in diameter whose circular form is reflected in the arcuate pattern of the islands themselves. The body has long been known and has best been described in detail by Kennedy and Walton (1946), Walton (1951), Himmelberg and others (1946), and Himmelberg and Loney (1995) as follows. The complex consists of a large central core of dunite surrounded by successive shells of wehrlite, olivine clinopyroxenite, and gabbro. The concentric geometry of the body suggests that it is a steeply dipping to subvertical cylinder; it is essentially undeformed. The complex intrudes Early Silurian-Devonian Descon Formation that consists mainly of of graywacke, with conglomerate, limestone, shale, and volcanic rocks. The contact of the intrusive complex is marked by a hornfelsed zone about 300-700 feet wide. . As described by Kennedy and Walton, (1946), chromite is a sparse, but ubiquitous accessory mineral in the dunite core. Disseminated sulfides, mainly pyrrhotite and chalcopyrite, occur near the boundary between pyroxenite and gabbro. There is a large aggregate tonnage of material containing 1%-2% sulfides. Analyses of the more sulfide-rich gabbro indicate as much as 0.016% Cu, 0.05% Ni, and less than 0.1 ounce per ton Pt-group elements. Other rock analyses show 0.004 ounces per ton Au, 0.04 ounces per ton Pd, and a trace of Pt. Rock samples collected by Clark and Greenwood (1972) contained an average of 0.011 ppm of both Pt and Pd with maxima of 0.020 ppm of each.
  • Age = the complex is Late Cretaceous; K-Ar ages from 93.4 to 102.0 Ma (Himmelberg and Loney, 1995).

Economic information about the deposit and operations

Development status Occurrence

Comments on exploration

  • Status = Probably inactive

Mining district

District name Petersburg

Comments on the production information

  • Production Notes = None

Comments on the reserve resource information

  • Reserves = There are no economic reserves; however, there are many occurrences of ore minerals and metals of economic interest (Kennedy and and Walton, 1946). Chromite is a sparse, but ubiquitous accessory mineral in the dunite core. Disseminated sulfides, mainly pyrrhotite and chalcopyrite, occur near the boundary between pyroxenite and gabbro. There is a large aggregate tonnage of material containing 1%-2% sulfides. Analyses of the more sulfide-rich gabbro indicate as much as 0.016% Cu, 0.05% Ni, and less than 0.1 ounce per ton Pt-group elements. Other rock analyses show 0.004 ounces per ton Au, 0.04 ounces per ton Pd, and a trace of Pt. Rock samples collected by Clark and Greenwood (1972) contained an average of 0.011 ppm of both Pt and Pd with maxima of 0.020 ppm of each.

Comments on the workings information

  • Workings / Exploration = Apparently no claims have ever been staked on this occurrence. This is not surprising considering that the highest elevation is about 120 feet and that most of the complex is barely above sea level and within a few hundred yards of the ocean. The complex has been visited by generations of geologists as a classic, easily accessible example of an Alaska-type mafic-ultramafic complex; inevitably it has been sampled for the metal suite that is often associated with such rocks, e.g. Cu, Ni, Co, Cr, and PGE. However, the metal content of samples from the body have so far proved consistently low, reflecting the geochemical background in these types of rocks rather than anything close to an economic concentration. The Bureau of Land Management (Mass, Bittenbender, and Still, 1995) recently indicated that the complex has low economic potential.

Reference information

Bibliographic references

Comments on the references

  • Primary Reference = Kennedy and Walton, 1946; Himmelberg and Loney, 1995

General comments

Subject category Comment text
Deposit Model Name = Alaskan or Zoned type mafic-ultramafic complex; portions may have PGE potential (Cox and Singer, 1986; model 9)

Reporter information

Type Date Name Affiliation Comment
Reporter 01-NOV-98 H.C. Berg and D.J. Grybeck 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.