Bowser Creek-Main Mine, McGrath District, Yukon-Koyukuk Borough, Alaska, USA
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Location: The Bowser Creek-Main deposit is located along the steep, eastern margin of an active rock glacier in Bowser Creek valley; it is at an approximate elevation of 4,200 feet (1,280 m) in sec. 9, T. 24 N., R. 24 W., of the Seward Meridian. Location accurately determined; reporter visited the site in 1982 and 1988.
Geology: The Bowser Creek-Main deposit consists of replacement pods, lenses and veins of sphalerite, galena, pyrrhotite, and minor chalcopyrite, marcasite, and pyrite in skarn adjacent to felsic dikes. However, important fissure-controlled, silver-rich galena, tetrahedrite, pyrrhotite, and calcite mineralization occurs in marble away from the skarn itself. Sphalerite is iron-rich (marmatite). Galena is paragenetically late and crosscuts the earlier sphalerite-chalcopyrite veins and replacement bodies. No sulfosalts were recognized. The skarns and replacement deposits at the Bowser Creek-Main deposit are related to a dumbell-shaped, 7 square kilometer, composite pluton that intrudes a limestone rich section of the Barren Ridge Limestone, a unit of the Dillinger subterrane (Bundtzen, Harris, and Gilbert, 1997). A quartz porphyry phase of the Bowser Creek pluton has been radiometrically dated at 60.4 Ma (Bundtzen and others, 1988). Prospect sketches were provided by Reed and Elliott (1968, C 596). The richest silver concentrations occur in shear-zone controlled massive galena-sulfide veins. Reed and Elliott (1968, C 596) reported values of up to 9,635.0 grams/tonne silver, 60.00 percent lead, 14.70 percent zinc, 0.49 percent copper, and 0.4 grams/tonne gold. Bundtzen and others (1988) reported that the average of six high-grade samples of massive galena ore from shear zones in the marble front contained 2,510.0 grams/tonne silver, 23.70 percent lead, 3.44 percent zinc, 0.15 percent copper, and 110 ppb gold. Other samples collected by Bundtzen and others (1988) contained 157 ppm cobalt and 0.11 percent cadmium. Based on polished-section and assay data, silver has a highest correlation coefficient with lead (galena); cobalt is concentrated in the pyrrhotite. High cadmium values occur in sphalerite. The mineralogical source of the sporadic gold values is unknown.
Workings: The Bowser Creek-Main deposit was discovered during regional geological mapping and mineral investigations conducted in the southern Alaska Range by the U.S. Geological Survey and published in Reed and Elliott (1968, C 559 and 596) and Reed and Lanphere (1972). Portland-based, Alaskamin Mining Company and St. Eugene Mining Company, the operator for Falconbridge Mining Company, acquired the property in late 1968 (Williams, 1969) and by 1973, cut about 250 feet of surface trenches, built a 2,600-foot-long airport on the Post River, and sacked about 15 tons of high grade galena ores for air shipment to Sunshine Mining Company's Kellogg smelter in Idaho (B.L. Reed and C.N. Conwell, written communication 1982). Records from an alleged early drilling program have not been documented. Reed and Elliott (1968, C 559) reported values from surface chip samples of up to 9,635 grams/tonne silver, 60.00 percent lead, 14.70 percent zinc, 0.49 percent copper, and 0.4 grams/tonne gold. Bundtzen and others (1988) reported that the average of six high grade samples of massive galena-sulfide ores from shear zones in the marble front contained 2,510 grams/tonne silver, 23.70 percent lead, 3.44 percent zinc, 0.15 percent copper, and 110 ppb gold. Other samples collected by Bundtzen and others (1988) contained 0.11 percent cadmium and 157 ppm cobalt.
Age: Tertiary, based on 60.4 Ma age of nearby quartz porphyry intrusion; ore mineralization not dated.
Alteration: Pyrrhotite and iron-rich sphalerite (marmatite) ubiquitously weather to a deep reddish brown gossan.
Production: Approximately 15 tons of galena-rich massive sulfide ores were sacked and eventually processed at the Sunshine Mining Company smelter in Kellogg, Idaho in 1972-73 (B.L. Reed and C.N. Conwell, written communications, 1982). These ores contained about 2,332 grams/tonne silver, about 50 percent lead, and elevated zinc, cadmium, copper, and gold. Only the lead and silver were credited at the smelter (C.N. Conwell, written communication, 1982).
Commodities (Major) - Ag, Pb, Zn; (Minor) - As, Au, Cd, Co, Cu
Development Status: Yes; small
Deposit Model: Zinc-lead skarn deposits (Cox and Singer, 1986; model 18c); low temperature lea
Mineral List
12 entries listed. 11 valid minerals.
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References
Bundtzen, T.K., Harris, E.E., and Gilbert, W.G., 1997, Geologic Map of the eastern McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 97-14, 34 p., 1 sheet, scale 1:125,000. Bundtzen, T.K., Kline, J.T., Smith, T.E., and Albanese, M.D., 1988, Geology of the McGrath A-2 quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Professional Report 91, 18 p., 1sheet, scale 1:63,360. Cobb, E.H., 1972, Metallic mineral resources map of the McGrath quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-379, 1 sheet, 1:250,000 scale. Cobb, E.H., 1976, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Iliamna, Lake Clark, Lime Hills, and McGrath quadrangles, Alaska: U.S. Geological Survey Open-File Report 76-485, 101 p. Cobb, E.H., and Reed, B.L., 1981, Summaries of data on and lists of references to metallic and selected nonmetallic mineral occurrences in the Iliamna, Lake Clark, Lime Hills, and McGrath quadrangles, Alaska; Supplement to Open-File Report 76-485; Part A, Summaries to January 1, 1981: U.S. Geological Survey Open-File Report 81-1343-A, 25 p. Reed, B.L., and Elliott, R.L., 1968, Lead, zinc, and silver deposits at Bowser Creek area, McGrath A-2 quadrangle, Alaska: U.S. Geological Survey Circular 559, 17 p Reed, B.L. and Elliott, R.L., 1968, Geochemical anomalies and metalliferous deposits between Windy Fork and Post River, southern Alaska: U.S. Geological Survey Circular 596, 22 pages. Reed, B.L., and Lanphere, M.A., 1972, Generalized geologic map of the Alaska-Aleutian range batholith showing potassium-argon ages of the plutonic rocks: U.S. Geological Survey Miscellaneous Field Studies Map MF-372, 2 sheets, scale 1:1,000,000. Williams, J.A., 1969, Report of the Division of Mines and Minerals for the year 1968: Alaska Division of Mines and Minerals Annual Report 1968, 67 p.
