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Bear Mountain Occurrence, Sheenjek District, Yukon-Koyukuk Borough, Alaska, USA

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Part of regional E-W trend of small domes, intrusions and doubly-plunging anticlines.
Location: Location is approximate center of area 1.25 mi (2 km) (east-west) by 0.35 mi (0.55 km )(north-south) of anomalous samples 517-521 and 608-13 of Barker (1981, pl. A-3); about 4 mi (6.4 km) southwest of Bear Mountain summit; known to within 2000 ft (610 m).
Geology: Tungsten-rich porphyry molybdenum occurrence. Area of about 1 sq km underlain by altered complex of rhyolite porphyry, quartz porphyry, and a pipe-shaped core of intrusive breccia about 300 m in diameter; complex is cut by dikes of rhyolite porphyry and aplite. Mineralized area defined by Mo-rich gossan zone trending NW for 1.5 km over 40 ha area characterized by soil samples containing 600 ppm Mo or greater; nearly coincident with 30 ha area of soil samples containing 500 ppm W or greater. Total area of Mo + W mineralization about 50 ha; vertical and horizontal zonation evident. Central Mo-rich gossan zone, outer zone of wolframite-topaz mineralization in a Fe-enriched upper-level phase of the complex below a silicified cap, and peripheral zone of Pb-enrichment. Sulfides have largely been oxidized to limonite, hematite, goethite, jarosite, and secondary ferromolybdates; wolframite occurs as disseminated grains in porphyritic rock, quartz veins, and silicified gossan. Molybdenite and galena have each been noted in only one locality. Tertiary rhyolite porphyry stock intrudes Mississippian and older metasedimentary rocks (Barker, 1981, p. 33-40, 106-137).
Workings: Rock samples taken in 1983 contain 0.1-0.8% M, 0.06-0.6% W, 2.2% F, 60-120 PPM Nb as reported in Barker and Swainbank (1985); soil sample 2521 on p. 141 of Barker (1981) from area of intrusive breccia contains 4900 ppm Mo, 860 ppm W, 400 ppm Pb and 9.7 ppm U (Barker, 1981, p. 140-150).
Alteration: Core of sericitic and argillic alteration with outer zone of silicification; local peripheral areas of pyritic and propylitic alteration.

Commodities (Major) - Mo, W, Pb
Development Status: No
Deposit Model: Lode; Mo-W porphyry breccia

Mineral List



21 entries listed. 14 valid minerals.

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References

Barker, J.C., 1978, Mineral Investigations of certain lands in the eastern Brooks Range: a summary report: U.S. Bureau of Mines Open-File No. 63-78, 25 p., 5 sheets.
Barker, J.C., 1981, Mineral investigations of certain lands in the eastern Brooks Range, 1978: U.S. Bureau of Mines Open-File 37-81, 288 p., 8 sheets.
Barker, J.C., and Swainbank, R.C., 1985, A tungsten-rich porphyry molybdenum occurrence at Bear Mountain, northeast Alaska: U.S. Bureau of Mines Open-File Report 8-85, 64 p. Brosg
Econ Geol (1986) 81:1753-1759

Mineral and/or Locality  
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