Braided Creeks; Cathedral Creek; Manhattan Prospect, Alaska Peninsula District, Lake and Peninsula Borough, Alaska, USA
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Location: This site represents a large color anomaly. The anomaly is approximately 3 miles north-south and 9 miles east-west, covering parts of T. 42 S., R. 59 and 60 W., of the Seward Meridian, encompassing the headwaters of Milk, Cathedral, and Braided Creeks (Cobb, 1972: MF-374, locality 1; U.S. Bureau of Mines, 1973, localilties 11, 12, 14, 15, and 18; MacKevett and Holloway, 1977, localities 1, 8-16, 18, and 19; Cox and others, 1981, locality 2; Nokleberg and others, 1987, locality AP 10). The map site is plotted at the location of the Manhattan prospect, near the headwaters of Braided Creek (map C-2). The location is accurate to within 1/2 mile.
Geology: Rocks in this prospect area include a large diorite stock of Oligocene age and smaller andesite and dacite plug domes of Miocene and Oligocene ages. The diorite stock cuts dacite flows of Oligocene age and sedimentary rocks of the Meshik Formation (Detterman and others, 1981). Mineralization within the main part of the stock has been described as a large, discontinuous sulfide system with low order copper and molybdenum anomalies (Fields, 1977). Cathedral and Milk Creeks drain an area distinguished by a cluster of silt samples containing anomalous values in copper and molybdenum. Fields, however, did not report any outcropping copper-molybdenum mineralization in this area, although chalcopyrite in quartz-chlorite veins was found in float. Nokleberg and others (1987) reported the occurrence of copper and molybdenite in zones of sericitic alteration within intrusives in the area. Pervasive pyrite (2 to 7 percent) is present in the sedimentary rocks and to a minor degree in the volcanic rocks peripheral to the stock. In some areas the pyrite is accompanied by epidote, chlorite, and clay alteration. Other areas exhibit pervasive sericitization. In the eastern part of the area of the color anomaly, base and precious metal veins crop out at the Manhattan prospect, near the headwaters of Braided Creek. Approximately 20 veins have been discovered, traceable for up to 700 feet on strike. These are up to several feet in thickness and contain arsenopyrite, chalcopyrite, galena, gold, pyrargyrite, pyrite, pyrrhotite, quartz, sphalerite, and tetrahedrite. The veins cut the Chignik Formation and dacite porphyry. They trend N 20 to 45 E It has been suggested that these veins represent a peripheral zone to the Cathedral Creek sulfide system (Wilson and Cox, 1983). At the Manhattan prospect Pan American Exploration (now Amoco) carried out geophysical surveys and drilled 14 holes for a total of 2,539 meters in 1965-1967. Bear Creek Mining Company briefly explored the area in 1975. In 1979 Resource Associates of Alaska sampled a 2-3 foot thick vein traceable for 700 feet. Their assay results are as follows: copper to 2400 ppm, gold to 0.596 ounce per ton, lead to 1.06 percent, silver to 16.29 ounces per ton, zinc to 9.5 percent (Anderson and others, 1979). In 1984 Anaconda drilled approximately 6 holes on this vein system. They reported metal values of 0.25 to 0.5 ounce gold per ton, 1 to 10 ounces silver per ton, and 5 to 10 percent lead-zinc. Young and others (1997) estimate a resource at the Manhattan prospect of 90,000 to 180,000 tonnes grading 0.6 ounce gold per ton, 10 ounces silver per ton, and 15 percent lead-zinc. Subsequent owners claim a resource of 575,000 tons grading 0.39 ounce gold equivalent per ton (Alaska Earth Science, 1997).
Workings: Fourteen holes were drilled at the Manhatten prospect on Braided Creek by Pan American Exploration in 1965-67 for a total of 2539 meters. Bear Creek Mining Company examined and sampled the prospect for 3 days in 1975. Resource Associates of Alaska sampled a polymetallic vein here in 1979. Anaconda drilled approximately 6 holes on the Braided Creek showings in 1984. Sampling of 1 vein by Resource Associates of Alaska assayed as follows: copper to 2400 ppm, gold to 0.596 ounce per ton, lead to 1.06 percent, silver to 16.29 ounces per ton, and zinc to 9.5 percent. Anaconda reported metal values of 0.25 to 0.5 ounce gold per ton, 1 to 10 ounces silver per ton, and 5 to 10 percent lead-zinc.
Age: Tertiary or younger.
Alteration: The sedimentary rocks peripheral to the stock exhibit pervasive sericitization in some areas and epidote, chlorite, clay alteration in others. Chloritization in the stock is common and localized areas of quartz-sericite alteration have been reported.
Reserves: Young and others (1997) list a resource of 90,000 to 180,000 tonnes grading 10 ounces silver per ton, 0.6 ounces gold per ton, and 15 percent lead-zinc. Subsequent owners claim 575,000 tons grading 0.39 ounce gold equivalent per ton (Alaska Earth Science, 1997).
Commodities (Major) - Ag, Au, Cu, Pb, Zn; (Minor) - As
Development Status: None
Deposit Model: Polymetallic vein and possible upper level of a prophyry copper system (Cox and
11 entries listed. 11 valid minerals.
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Alaska Earth Science, 1997, Mineral and oil and gas potential of the Bristol Bay region, southwestern Alaska: prepared for the Bristol Bay Native Corporation, 11 p. (Report held by Alaska Earth Sciences, Inc., Anchorage, Alaska.) Anderson, G.D., Fitch, G.M., Lappie, D.W., Lindberg, P.A., and Fankhauser, R.E., 1979, Exploration and evaluation of Bristol Bay Native Corporation Lands, Vol. II, Book 1: Prepared for Houston Oil and Minerals Company by Resource Associates of Alaska, 78 p. (Report held by Alaska Earth Sciences, Inc., Anchorage, Alaska.) Brown, D.L., Crabtree, E.H., and Smith, F.L., 1965, Mineralogic study of a complex sulfide deposit, vein number 1, project Manhatten, Alaska: Prepared for Pan American Petroleum Corporation by The Colorado School of Mines Foundation Inc., 55 p. (Report held by Alaska Earth Sciences, Inc., Anchorage, Alaska.) Cobb, E.H., 1972, Metallic mineral resources map of the Chignik quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-374, 1 sheet, scale 1:250,000. Cobb, E.H., 1980, Summaries of data and lists of references to metallic and selected nonmetallic mineral deposits in fifteen quadrangles in southwestern and west-central Alaska: U.S. Geological Survey Open-File Report 80-909, 103 p. Cox, D.P., Detra, D.E., and Detterman, R.L., 1981, Mineral resource maps of the Chignik and Sutwik Island quadrangles, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF- 1053-K, 2 sheets, scale 1:250,000. Detterman, R.L., Miller, T.P., Yount, M.E., and Wilson, F.H.,1981, Geologic map the Chignik and Sutwik Island quadrangles Alaska: U.S. Geologic Survey Miscellaneous Investigations Series Map I-1229, 1 sheet, scale 1:250,000. Fields, E.D., 1977, 1976 Annual report: Alaska search, Chignik area-Bristol Bay region: Bear Creek Mining Company, 44 p., 22 map sheets. (Report held by the Aleut Corporation, Anchorage, Alaska.) MacKevett, E.M., Jr., and Holloway, C.D., 1977, Map showing metalliferous mineral deposits in the western part of southern Alaska: U.S. Geological Survey Open-File Report 77-169-F, 38 p., 1 sheet, scale 1:1,000,000. Nokleberg, W.J., Bundtzen, T.K., Berg, H.C., Brew, D.A., Grybeck, D.J., Robinson, M.S., Smith, T.E., and Yeend, W., 1987, Significant metalliferous lode deposits and placer districts of Alaska: U.S. Geological Survey Bulletin 1786, 104 p., 2 plates, scale 1:5,000,000. U.S. Bureau of Mines, 1973, Alaska 1:250,000-scale quadrangle map overlays showing mineral deposit locations, principal minerals, and number and type of claims: U.S. Bureau of Mines Open-File Report 20-73, 95 overlays (updated in 1986, 1987). Wilson, F.H., 1980, Late Mesozoic and Cenozoic tectonics and age of porphyry copper prospects, Chignik and Sutwik Island quadrangles, Alaska Peninsula: U.S. Geological Survey Open-File Report 80-543, 94 p., 5 sheets, scales 1:250,000 and 1 inch = 1,000 feet. Wilson, F.H., and Cox, D.P., 1983, Geochronology, geochemistry, and tectonic environment of porphyry mineralization in the central Alaska Peninsula: U.S. Geological Survey Open-File Report 83-783, 24 p. Young, L.E., St. George, P., and Bouley, B., 1997, Porphyry copper deposits in relation to the magmatic history and palinspastic restoration of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral deposits of Alaska: Economic Geology Monograph 9, p. 306-333.