Unnamed (on Blashke Islands)

Occurrence, Probably inactive

Commodities and mineralogy

Main commodities Cr; Cu; Ni; PGE
Ore minerals chalcopyrite; chromite; pt-group minerals

Geographic location

Quadrangle map, 1:250,000-scale PE
Quadrangle map, 1:63,360-scale A-3
Latitude 56.122
Longitude -132.888
Nearby scientific data Find additional scientific data near this location
Location and accuracy 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.

Geologic setting

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.
Geologic map unit (-132.889705568343, 56.1216385319531)
Mineral deposit model Alaskan or Zoned type mafic-ultramafic complex; portions may have PGE potential (Cox and Singer, 1986; model 9).
Mineral deposit model number 9
Age of mineralization The complex is Late Cretaceous; K-Ar ages from 93.4 to 102.0 Ma (Himmelberg and Loney, 1995).
Alteration of deposit None other than hornfelsing of the country rock adjacent to the complex.

Production and reserves

Workings or 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.
Indication of production None
Reserve estimates 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.
Production notes None

References

MRDS Number 10308063

References

Taylor, H.P., 1967, The zoned ultramafic complexes of southeastern Alaska, in Wyllie, P.J., ed., UIltramafic and Related Rocks: New York, J. Wiley and Sons, p. 97-121.
Reporters H.C. Berg (Fullerton, California) and D.J. Grybeck (USGS)
Last report date 11/1/1998