Unnamed (near Magnetic Point)

Prospects, Undetermined

Commodities and mineralogy

Main commodities Fe; Ti
Other commodities Cr
Ore minerals chromite; ilmenite; magnetite

Geographic location

Quadrangle map, 1:250,000-scale CR
Quadrangle map, 1:63,360-scale D-1
Latitude 55.78344
Longitude -132.17374
Nearby scientific data Find additional scientific data near this location
Location and accuracy The center of this block of claims is about 1.9 miles west of Mount Burnett and about 0.5 mile east-southeast of the center of section 23, T. 70 S., R. 86 E. The outline of the claim block is shown on Figure 44 of Maas and others (1995).

Geologic setting

Geologic description

This block of claims is at the west end of the Union Bay mafic-ultramafic intrusive complex, which outcrops over an area of about 6 by 7 miles (Ruckmick and Noble, 1959). The complex is the largest of numerous small, Cretaceous mafic-ultramafic plutons scattered in a belt along the length of southeastern Alaska (Lanphere and Eberlein, 1966; Brew and Morell, 1983; Gehrels and Berg, 1992). Many of these plutons are concentrically zoned, an unusual characteristic that has led to their classification as 'Alaska-type,' or 'Alaskan,' complexes (Noble and Taylor, 1960; Taylor and Noble, 1960; Wyllie, 1967; Jackson and Thayer, 1972). As mapped by Ruckmick and Noble (1959) and reinterpreted by Himmelberg and Loney (1995), the Union Bay complex consists of an outer layer of gabbro that is succeeded inward by magnetite clinopyroxenite, wehrlite, and a core of dunite. The dunite forms a vertical pipe about a mile in diameter. It is bordered on the east by narrow, nearly-vertical shells of wehrlite and clinopyroxenite, and on the west by a thick, layered sequence of wehrlite, clinopyroxenite, and gabbro that forms either a large recumbent fold or a lopolith. The complex intrudes probably Upper Jurassic and Lower Cretaceous argillite, tuff, and graywacke of the Gravina sequence (Gehrels and Berg, 1992). The bedded rocks are thermally metamorphosed to schist and gneiss for about 1,000 feet from the intrusive contact. Himmelberg and Loney (1995) suggest that the complex was emplaced during the last stages of Cretaceous regional folding, when the dunite underwent plastic deformation that resulted in a preferred orientation of the olivine.
Early workers called attention to magnetite occurrences scattered through the clinopyroxenite and to small pods and lenses of chromite in the dunite, but no deposits of significant size were identified for many years (Budddington and Chapin, 1929; Kennedy and Walton, 1946; Twenhofel, 1953; Kaufman, 1958; Condon, 1961). Columbia Iron Mining Company explored the complex for iron ore from 1954 to 1970 (Noel 1966; Fischer, 1975; Maas and others, 1995). They patented 18 claims in clinopyroxenite at the west end of the complex and identified a resource of about 1 billion tons of material with 18 to 20 percent total iron and about 2 percent titanium. There was at least some diamond drilling on the claims. The magnetite is a primary component of the clinopyroxenite, although at least some was deposited from hydrothermal solutions, probably during emplacement of the complex (Van Treeck and Newberry, 2003).
Geologic map unit (-132.175425041318, 55.7830815987669)
Mineral deposit model Magnetite in clinopyroxenite of an Alaska-type mafic-ultramafic complex (Cox and Singer, 1986; model 9).
Mineral deposit model number 9
Age of mineralization The magnetite is a primary and hydrothermal component of clinopyroxenite in a Cretaceous mafic-ultramafic complex.

Production and reserves

Workings or exploration Much surface sampling and at least some diamond drilling on the claims.
Indication of production None
Reserve estimates The deposit has a resource of 1 billion tons of material with a grade of 18 to 20 percent iron and about 2 percent titanium.

References

References

Brew, D.A., and Morell, R.P., 1983, Intrusive rocks and plutonic belts of southeastern Alaska: Geological Society of America Memoir 159, p. 171-193.
Jackson, E.D., and Thayer, T.P., 1972, Some criteria for distinguishing between stratiform, concentric, and alpine peridotite-gabbro complexes: International Geological Congress, 24th, Montreal, 1972, Proceedings, Section 2, p. 289-296.
Lanphere, M. A., and Eberlein, G. D., 1966, Potassium-argon ages of magnetite-bearing ultramafic complexes in southeastern Alaska (abs.): Geological Society of America Special Paper 87, p. 94.
Noble, J.A., and Taylor, H.P. Jr., 1960, Correlation of the ultramafic complexes of southeastern Alaska with those of other parts of North America and the world: Internnational Geological Congress, 21st, Copenhagen, 1960, Report, Part 13, p. 188-197.
Ruckmick, J.C., and Noble, J.A., 1959, Origin of the ultramafic complex at Union Bay, southeastern Alaska: Geological Society of America Bulletin, v. 70, 981-1018.
Taylor, H.P,. and Noble, J.A., 1960, Origin of the ultramafic complexes in southeastern Alaska: International Geological Congress, 21st, Copenhagen, Report, p. 175-187.
Van Treeck, C.J., and Newberry, Rainer, 2003, The Union Bay platinum prospect, SE Alaska, a hydrothermal PGE deposit (abs.): Canadian Insitute of Mining, Metallurgy, and Petroleum, Conference Montreal, May 4-7, 2003, 1 p.
Wyllie, P.J., 1967, Zoned ultramafic complexes, in Ultramafic and related rocks: New York, John Wiley and Sons, p. 83-84.
Reporters D.J. Grybeck (Applied Geology)
Last report date 5/1/2004