Unnamed Occurrence (ARDF - MF072; east of Brady Glacier), Juneau District, Skagway-Hoonah-Angoon Borough, Alaska, USA
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Location: This iron occurrence is at an elevation of about 1350 feet about 0.25 mile south of the southeast edge of an east-pointing arm of Brady Glacier and approximately 3.5 miles north-northwest of the head of the west arm of Dundas Bay. It is in the SW 1/4 SE 1/4 section 35, T. 39 S., R. 53 E., of the Copper River Meridian. (Kimball and others, 1978, report iron-rich skarn lenses in this vicinity from 1300 to 1700 feet elevation.) Location is accurate within 0.1 mile. The occurrence is the same as number 54 of MacKevett and others (1971), number 53 of Cobb (1971), and number 63 of Kimball and others (1978).
Geology: The country rocks in the area of this occurrence consist of a northwest-trending belt of Paleozoic marble bounded on the west by foliated granodiorite of Cretaceous age and on the east by leucocratic biotite granite and affiliated rocks of Tertiary age (Brew and others, 1978). Magnetite lenses as much as 15 feet thick and 35 feet long occur in northwest-striking, steeply dipping bodies in limy hostrocks intermixed with leucocratic granitic rock. The limy rocks are partly converted to skarn. MacKevett and others (1971, p. 72, table 9) reported more than 10 percent iron, 1000 ppm copper, and 7 to 15 ppm molybdenum in grab and selected samples collected at the site. They also reported magnetic anomalies of as much as 5000 gammas, but believed that the iron-rich lenses were too small to be commercially significant. A selected sample collected by the U.S. Bureau of Mines (Kimball and others, 1978, p. C267) contained 1400 ppm copper, 1.5 ppm silver, and greater than 20 percent iron. Samples of lenses that ranged from 4.5 to 35 feet across contained more than 30 percent iron and as much as 750 ppm copper.
Workings: There are no workings. The Geological Survey and Bureau of Mines sampled only surface exposed occurrences. Samples collected by MacKevett and others (1971) contained 7-15 ppm molybdenum, 1000 ppm copper and greater than 10 percent iron. Kimball and others (1978) reported iron to greater than 30 percent and, in one selected sample, 1400 ppm copper. The magnetite-rich skarn lenses are as much as 35 feet long and are traceable through a vertical range of at least 400 feet.
Alteration: Skarn, contact metasomatic deposit.
Commodities (Major) - Fe; (Minor) - Ag, Cu, Mo
Development Status: No
Deposit Model: Fe skarn (Cox and Singer, 1986; model 18d).
6 entries listed. 5 valid minerals.
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Brew, D.A., Johnson, B.R., Grybeck, D., Griscom, A., Barnes, D.F., Kimball, A.L., Still, J.C., and Rataj, J.L., 1978, Mineral resources of the Glacier Bay National Monument Wilderness Study Area, Alaska: U.S. Geological Survey Open-File Report 78-494, 670 p., 7 sheets. Cobb, E.H., 1972, Metallic mineral resources map of the Mount Fairweather quadrangle, AK: U.S. Geological Survey Miscellaneous Field Study Map MF-436, 1 sheet, scale 1:250,000. Kimball, A.L., Still, J.C., and Rataj, J.L., 1978, Mineral resources, in Brew, D. A., and others, Mineral resources of the Glacier Bay National Monument wilderness study area, Alaska: U.S. Geological Survey Open-File Report 78-494, p. C1-C375. MacKevett, E.M., Jr., Brew, D.A., Hawley, C.C., Huff, L.C., and Smith, J.G., 1971, Mineral resources of Glacier Bay National Monument, Alaska: U.S. Geological Survey Professional Paper 632, 90 p., 12 plates, scale 1:250,000.