Unnamed Occurrence (ARDF - MF059; east side of Queen Inlet), Juneau District, Skagway-Hoonah-Angoon Borough, Alaska, USA
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Location: The location is an area on the east side of Queen Inlet near its mouth, and about 2 miles east-northeast of the north end of Composite Island. Mineralization extends about 400 feet along the shore line and extends uphill to at least 1000 feet above sealevel. The coordinates are for the approximate center of the area. The deposit is accurately located. The location is number 39 of Cobb (1972) and number 40 of MacKevett and others (1971).
Geology: The area is underlain by a mixed beded unit of Devonian and Silurian age that is mostly volcanic. Minor amounts of slate, argillite, and marble occur with andesitic volcanics. A major fault that strikes northwest separates the Devonian-Silurian unit from hornfels and marble of undivided Paleozoic age--this fault may mark the uphill termination of the mineral occurrence (Brew and others, 1978, pl. 1A). The bedded rocks are cut by alaskite and porphyritic dikes, probably of Tertiary age. Detailed mapping shows very complex relations (MacKevett and others, 1971, pl. 10). Magnetite occurs in tactite and marble interleaved with alaskite; the marble units generally strike east-northeast and are cut by dark porphyritic andesite dikes that strike northwest. The magnetite-rich bodies are as much as 20 feet thick. The alaskite contains numerous irregular quartz veins and irregular clots, masses and disseminations of sulfides, mainly pyrite and minor chalcopyrite. The andesite dikes also contain disseminated and blebby pyrite. Except for one sample, which contained 7 percent iron, all samples (eight total) contained greater than 10 percent iron. An 18-foot chip sample of the richest magnetite zone contained 23.4 percent iron reported as ferric oxide. The samples were anomalous in copper, cobalt, and tin. The magnetite bodies are marked by anomalies of about 1000 gammas. Exposure is poor; magnetic anomalies on an uphill traverse were as much as 1300 gammas, probably indicating buried magnetite lenses.
Workings: Poorly exposed bodies of magnetite and disseminated and blebby pyrite occur in tactite, marble and alaskite. An 18-foot chip sample of the richest-appearing magnetite body contained 23.4 percent iron as ferric oxide. Seven other samples showed major (greater than 10 percent) iron. Copper, cobalt, molybdenum, tin, and zinc were detected in anomalous amounts, respectively 300 ppm, 300 ppm, 20 ppm, 30 ppm, and 300 ppm. There are no mine workings. The most detailed investigation was conducted by the Geological Survey in 1966 (MacKevett and others, 1971).
Alteration: Calc-rocks have been altered to tactite composed mainly of calcite, garnet, hornblende, pyroxene and quartz. Propylitic alteration of alaskite has introduced actinolite, calcite, chlorite, and epidote. Actinolite and chlorite, with plagioclase, have also formed in the alteration of the andesite dikes.
Commodities (Major) - Fe; (Minor) - Co, Cu, Mo, Sn
Development Status: No
Deposit Model: Contact and porphyry mineralization, magnetite (Fe) skarn (Cox and Singer, 1986
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. 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.