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West Cobra; East Cobra Prospects, Ketchikan District, Wrangell-Petersburg Borough, Alaska, USA

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Location: The West Cobra prospect, which is at the site selected for this record, is about 0.4 mile south of Mount Burnett and about 0.4 mile east-northeast of the center of section 30, T. 70 S., R. 87 E. The East Cobra prospect is about 1.5 miles east of the West Cobra prospect. The only published location of the Cobra prospects is a small-scale map on a web page, but the site is probably accurate to within 0.2 mile.
Geology: The Cobra East and West prospects are in the Union Bay complex, 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 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. There has been considerable sporadic reconnaissance sampling of the Union Bay complex for magnetite and chromite since at least 1929 and several attempts to locate concentrations of platinum-group-elements (PGE) (See CR013 for details of exploration in the complex prior to 1995.) In the early 1990's, Maas and others (1995) collected placer samples in several streams on the north side of the complex and reported anomalous platinum and palladium in the concentrates. They also identified ferroan platinum, native osmium, osmium-iridium, and hollingworthite (a rhenium-platinium-palladium arsenide) in the concentrates and suggested that the source was in 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). The Cobra prospects were discovered in 2001. While detailed geologic data or sample analyses are not available, the Cobra prospects are similar to the other PGE prospects in the Union Bay mafic-ultramafic complex that have been discovered since 2001 (see, for example, CR004 and CR005). 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.
Workings: Apparently only surface sampling.
Age: The PGE deposits are in a Cretaceous mafic-ultramafic complex and they are probably related to its emplacement.
Alteration: An early generation of hydrothermal magnetite associated with diopside dikes 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 secondary hornblende formed between 475 to 575 degrees C; and the last hydrothermal stage, marked by the deposition of of magnetite rimmed by interlayered chlorite and serpentine, formed at less than 475 degrees C.

Commodities (Major) - Ir, Os, Pd, Pt, Rh; (Minor) - As, Cr, Fe, Sb
Development Status: None
Deposit Model: PGE minerals associated with magnetite in pyroxenites of an Alaska-type mafic-u

Mineral List



7 entries listed. 4 valid minerals.

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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. 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. 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. 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.

 
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