Unnamed Prospects (ARDF - CR004; near Magnetic Point), Ketchikan District, Wrangell-Petersburg Borough, Alaska, USA
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Geology: 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 x 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 clinoproxenite, 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 emplacment of the complex (Van Treeck and Newberry, 2003).
Workings: Much surface sampling and at least some diamond drilling on the claims.
Age: The magnetite is a primary and hydrothermal component of clinopyroxenite in a Cretaceous mafic-ultramafic complex.
Reserves: 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.
Commodities (Major) - Fe, Ti; (Minor) - Cr
Development Status: None
Deposit Model: Magnetite in clinopyroxenite of an Alaska-type mafic-ultramafic complex (Cox an
3 entries listed. 3 valid minerals.
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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. Buddington, A.F., and Chapin, Theodore, 1929, Geology and mineral deposits of southeastern Alaska: U.S. Geological Survey Bulletin 800, 398 p. Condon, W.H., 1961, Geology of the Craig quadrangle, Alaska: U.S. Geological Survey Bulletin 1108-B, p. B1-B43. Fischer, R.P., 1975, Vanadium resources in titaniferous magnetite deposits: U.S. Geological Survey Professional Paper 926-B, p. B1-B10. Gehrels, G.E., and Berg, H.C., 1992, Geologic map of southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1867, 1 sheet, scale 1:600,000, 24 p. Himmelberg, G.R., and Loney, R.A., 1995; Characteristics and petrogenesis of Alaskan-type ultramafic-mafic intrusions, southeastern Alaska: U. S. Geological Survey Professional Paper 1564, 47 p. 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. Kaufman, A., 1958, Southeastern Alaska's Mineral industry: U.S. Bureau of Mines Information Circular 7844, 37 p. Kennedy, G.C., and Walton, M.S., Jr., 1946, Geology and associated mineral deposits of some ultrabasic rock bodies in southeastern Alaska: U.S. Geological Survey Bulletin 947-D, p. 65-84. 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. Maas, K.M., Bittenbender, P E., and Still, J.C., 1995, Mineral investigations in the Ketchikan mining district, southeastern Alaska: U.S. Bureau of Mines Open-File Report 11-95, 606 p. 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. Noel, G.A., 1966, The productive mineral deposits of southeastern Alaska: Alaska Division of Mines and Minerals, Report for the year 1966, p. 51-57, 60-68. 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. Twenhofel, W.S., 1953, Potential Alaskan mineral resources for proposed electrochemical and electrometallurgical industries in the upper Lynn Canal area, Alaska: U.S. Geological Survey Circular 252, 14 p. 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.