Union Bay (near peak 2535)

Occurrence, Active

Commodities and mineralogy

Main commodities Cr; Fe; Ir; Os; Pd; Pt; Rh
Ore minerals chromite; magnetite

Geographic location

Quadrangle map, 1:250,000-scale CR
Quadrangle map, 1:63,360-scale D-1
Latitude 55.76226
Longitude -132.05195
Nearby scientific data Find additional scientific data near this location
Location and accuracy This is a generic site for the entire Union Bay mafic-ultramafic complex, which crops out over an area of about 6 by 7 miles. Many early reports describe the complex and the mineral deposits in it as a single entity. This site is near the center of the dunite core of the complex, where several chromite occurrences have been reported. It is at peak 2535, which is about 0.5 mile north-northeast of the center of section 34, T. 70 S., R. 87 E. Since 2001, there has been considerable exploration of several prospects within the complex for platinum-group elements; these are described separately (CR005-012). In addition, a large block of claims was staked for iron in the the 1960's; the deposit at those claims is described in ARDF record CR004.

Geologic setting

Geologic description

This site includes the entire Union Bay mafic-ultramafic intrusive complex, which outcrops over an area of about 6 by 7 miles. The complex is the largest of numerous small, Cretaceous mafic-ultramafic intrusive bodies scattered in a belt along the length of southeastern Alaska (Ruckmick and Noble, 1959; 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. Clark and Greenwood (1972 [PP 800-C, p. C21-27]) report a major increase in the volume of the dunite core due to its serpentinization.
Early workers called attention to magnetite scattered through the clinopyroxenite and to small pods and lenses of chromite in the dunite, but no deposits of significant size were identified prior to the 1960s (Budddington and Chapin, 1929; Kennedy and Walton, 1946; Twenhofel, 1953; Kaufman, 1958; Condon, 1961). Columbia Iron Mining Company searched for iron ore in the area from 1954 to 1970 (Noel 1966; Fischer, 1975; Maas and others, 1995). They patented 18 claims at the west end of the complex (CR004), and identified a resource of about 1 billion tons of material with 18 to 20 percent total iron and about 2 percent titanium. Clark and Greenwood (1972 [PP 800-C, p. C157-160]) carried out the first systematic survey to ascertain the platinum-group-element (PGE) content of the complex from 50 samples they collected of the various rocks. Their samples averaged 0.093 part per million (ppm) platinum and 0.023 ppm palladium; the maximum value was 1,600 ppm platinum, 0.200 ppm palladium, 0.062 ppm rhodium, and 0.215 ppm iridium. Anaconda Minerals did a reconnaissance survey of the complex but concluded there was no likelihood of an economic platinum-group-element deposit in it (Anaconda Collection, American Heritage Center, University of Wyoming, files 6804.01, 7503.09, and 7503.12). In the early 1990s, Maas and others (1995) collected placer samples in several streams on the north side of the complex and reported considerable anomalous platinum and palladium in the concentrates. They identified ferroan platinum, native osmium, osmium-iridium, and hollingworthite (a rhenium-platinium-palladium arsenide) in the concentrates and suggested that the source was the clinopyroxenite and wehrlite on the north side of the complex. In 2000, Freegold Ventures Limited began exploring for PGE in the complex and they have located a number of prospects that they are currently working on in a joint venture with Lonmin PLC (www.freegoldventures.com/s/Home.asp; March 1, 2004). These prospects are described in detail as separate sites (CR005-012).
Van Treeck and Newberry (2003) studied these PGE deposits in detail and concluded that the PGE minerals are hydrothermal in origin and are associated with veins and lenses of magnetite that cut the mafic and ultramafic rocks of the complex. An early generation of hydrothermal magnetite associated with diopside dikes was formed from 575 to 700 degrees C; the PGE minerals are associated with an intermediate stage of hydrothermal activity marked by the deposition of magnetite and hornblende alteration that occurred between 475 to 575 degrees C; and a later generation of magnetite rimmed by interlayered chlorite and serpentine formed at less than 475 degrees C. They identified the following PGE minerals: ferroplatinum, erlichmanite, iridosmine, platiniridium, and several unnamed rhenium-arsenic-sulfur, rhenium-iron, platinum-antimony, and platinum-iridium-sulfur minerals. The source of the hydrothermal fluids is unknown but the absence of quartz suggests that the fluids are related to the mafic and ultramafic rocks.
Geologic map unit (-132.05363628193, 55.7619009865071)
Mineral deposit model Chromite, magnetite, and PGE associated with an Alaska-type mafic-ultramafic complex (Cox and Singer, 1986; model 9).
Mineral deposit model number 9
Age of mineralization Chromite, magnetite, and PGE associated with the emplacement of a Cretaceous mafic-ultramafic complex.
Alteration of deposit Most of the early workers did not specifically note alteration apart from that associated with the intrusion of the complex. See CR005-012 for a modern interpretation of the alteration associated with the PGE deposits currently (2004) being explored in the complex.

Production and reserves

Workings or exploration The Union Bay mafic-ultramafic complex has been sampled sporadically by government and industry geologists and engineers since at least the 1930s for chromite, magnetite, and platinum-group-elements. However, the sampling was more reconnaissance than systematic in nature until exploration of the complex for PGE began in 2001. The work on the several prospects that are currently (2004) being explored (CR004-CR007 and CR010-CR012) includes systematic outcrop sampling and diamond drilling.
Indication of production None
Reserve estimates In the 1960s, a resource of 1 billion tons of material that contains 18 to 20 percent iron and about 2 percent titanium was identified near the west end of the complex.



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.
Cannon, Bart, 1993, Electron microprobe analysis of Alaskan PGE samples: Cannon Microprobe, Seattle Washington, 4 p. (Unpublished report held by the Bureau of Land Management, Mineral Information Center, Juneau, Alaska.)
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.
Murray, C.G., 1972, Zoned ultramafic complex of the Alaskan type: feeder pipes of andesitic volcanoes: Geological Society of America Memoir 132, p. 313-335.
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., 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.
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.
Taylor, H.P., Jr., 1969, Origin of magnetite in zoned ultramafic complexes of southeastern Alaska, in Wilson, H.D.B., ed., Magmatic Ore Deposits: Economic Geology Monograph 4, p. 209-230.
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