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Continental; Chevelle; Cannery Creek Prospects, Ketchikan District, Wrangell-Petersburg Borough, Alaska, USA

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Location: This site consists of three separate prospects that were first located in 2003. The coordinates are at the Continental prospect, which is the middle and best sampled of the three; it is about 1.1 mile west-southwest of Mount Burnett and about 0.2 mile northeast of the center of section 25, T. 70 S., R. 86 E. The Chevelle prospect is about 0.7 mile northeast of the Continental prospect, and the Cannery Creek prospect is about 1.2 mile southwest of the Continental prospect. The locations of these prospects are known only from a small-scale map on a web page but they are probably accurate to within 0.2 mile.
Geology: The Continental, Chevelle, and Cannery Creek 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 the 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; as of March 1, 2004). The Continental, Chevelle, and Cannery Creek prospects were discovered in 2003 (see CR 006-012 for the others). Detailed geologic data are not available, but, like the other PGE prospects located in the Union Bay mafic-ultramafic complex since 2001, these prospects are in werhlite and magnetite-bearing clinopyroxenite, cut by diopside dikes. Thirteen of the grab samples of magnetite veins, pyroxenite, and wehrlite collected at the Continental prospect contained 3.160 to 14.950 grams of platinum per ton and 0.168 to 0.940 grams of palladium per ton. Preliminary channel sampling at the Continental prospect obtained values of up to 6.2 grams of platinum per ton over 3.3 feet. The results of sampling at the Chevelle prospect have been encouraging, and a zone of copper-, platinum-, and palladium-bearing sulfides was discovered northeast of the mouth of Cannery Creek. Van Treeck and Newberry (2003) studied these PGE deposits in detail and concluded that the PGE minerals are hydrothermal in origin and 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 formed from 575 to 700 degrees C; the PGE minerals are associated with magnetite and hornblende alteration formed from 475 to 575 degrees C; and there is 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: Only surface sampling and limited channel 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. 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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