Shellabarger Pass; Shellebarger Pass Prospects, Yentna District, Matanuska-Susitna Borough, Alaska, USA
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Location: Reed and others (1978, locality 14) show these occurrences two miles north of Shellabarger Pass in the southwest quarter of Section 35, T.2 N. R. 19 W., of the Seward Meridian. Several massive sulfide bodies lie within 1/2 mile radius of this location.
Geology: Several massive sulfide bodies lie within 1/2 mile radius of this location. According to Reed and Eberlein (1972), the Shellabarger Pass volcanogenic massive sulfide (VMS) deposits are comprised of at least six individual bodies. The massive sulfide bodies are lenticular deposits with exposed dimensions on the order of 50 to 100 feet by 17 to 27 feet, composed of 60% sulfides in a very fine grained mixture of pyrite, marcasite, sphalerite, chalcopyrite, galena and pyrrhotite. Sphalerite, chalcopyrite and galena make up about 15% of total sulfides. Some areas of heavily disseminated sulfides are localized along shear zones. Sulfide minerals are intergrown with the gangue minerals siderite, calcite, quartz, and dolomite. Sulfides and gangue occur in massive, lenticular sulfide bodies, as replacements of carbonate-rich beds, and as fracture fillings, mainly in chert and siltstone. Main sulfide bodies may be proximal to basaltic flow fronts. Regionally the basalt has high background copper values of 200 to 300 ppm. The grade of the massive bodies averages 1 to 1.5% copper, 0.8 to 1.7% zinc, 0.9 to 2.4 oz/ton silver and less than 0.5% lead. Gold content varies from 0.0006 to 0.15 oz/ton. The sulfide bodies tend to be zoned, with highest chalcopyrite concentrations in basal parts and minor sphalerite in or near hanging walls (Reed and Eberlein, 1972; Nokleberg and others, 1994). Hydrothermal alteration is extensive in the footwall but is rare to absent in hanging wall (Reed and Eberlein, 1972). Indicated tonnage of known bodies is on the order of 50,000 tonnes (Reed and Eberlein, 1972). Nokleberg and others (1994) report an estimated several hundred thousand tonnes of unknown grade. The host rocks are interpreted to be part of the Mystic terrane, a displaced fragment of the late Paleozoic and early Mesozoic North American Cordillera continental margin (Reed and Nelson, 1980; Decker and others, 1994; Nokleberg and others, 1994). The deposits occur in a north-trending structural trough of Triassic to Jurassic marine sediments and mafic volcanics (hypersthene normative tholeiites that includes pillow basalt, agglomerate, and breccia) in a eugeoclinal sequence which rests unconformably over Paleozoic sedimentary rocks consisting of interbedded chert, dolomite, siltstone, shale, volcanic graywacke, and basaltic aquagene tuff (Bundtzen and Gilbert, 1983; Reed and Nelson, 1980). It is unclear whether the deposits are hosted in Paleozoic or Mesozoic rocks. There is evidence to suggest the volcanic host rocks are either upper Triassic (Norian?) or latest Devonian-earliest Mississippian. Bundtzen and Gilbert (1983) suggest that the host rocks to the Shellabarger VMS deposits could be Late Triassic. Bundtzen and others (1997) report Late Triassic fossils in from a sedimentary/basalt sequence in the Mystic terrane to the north and east in the adjacent McGrath quadrangle. Reed and Nelson (1980) consider the basalts to be post-Late Devonian age (possibly Mississippian), indicated by distinctive spherical concretions of fluoroapatite present in sedimentary breccia and conglomerate that apparently underly the pillow basalt at Shellabarger Pass. The source of the concretions is considered to be a phosphatic chert unit that overlies the Middle and Upper Devonian limestone. A nearby pillow basalt occurs in a chaotic sequence of sedimentary rocks that contains fossils of late Middle Devonian(?) (Reed and Nelson, 1980).
Workings: Reconnaissance mapping, stream silt and rock sampling are all that has been published on this property. Private concerns have held the property at various times in the past and may have completed a more detailed evaluation, the results of which are not publicly available.
Age: Triassic or Jurassic (Nokleberg and others, 1987, p. 48; Nokleberg and others,1994, p. 877) or latest Devonian-earliest Mississippian (Reed and Nelson, 1980).
Alteration: Hydrothermal alteration (mineralogy unspecified) is extensive in the footwall but is rare to absent in hanging wall (Reed and Eberlein, 1972).
Reserves: Indicated tonnage of known bodies is on the order of 50,000 tonnes (Reed and Eberlein, 1972). Nokleberg and others (1994) report an estimated several hundred thousand tonnes of unknown grade.
Commodities (Major) - Ag, Au, Ba, Cu, Pb, Zn
Development Status: No
Deposit Model: Besshi massive sulfide (Cox and Singer, 1986; model 24b).
11 entries listed. 11 valid minerals.
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Bundtzen, T.K., and Gilbert, W.G., 1983, Outline of geology and mineral resource of upper Kuskokwim region, Alaska: Alaska Geological Society 1982 Symposium on Western Alaska, v. 3, p. 101-117. 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. C.C. Hawley and Associates, Inc., 1978, Mineral appraisal of lands adjacent to Mt. McKinley National Park, Alaska: U.S. Bureau of Mines Open-File Report 24-78, 277 p., 12 sheets. Clark, A.L., and Cobb, E.H., 1972, Metallic mineral resources map of the Talkeetna quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-369, 1 sheet, scale 1:250,000. Cobb, E.H., and Reed, B.L., 1980, Summaries of data on and lists of reference to metallic and selected nonmetallic mineral deposits in the Talkeetna quadrangle, Alaska: U.S. Geological Survey Open-File Report 80-884, 106 p. Decker, J., Bergman, S.C., Blodgett, R.B., Box, S.E., Bundtzen, T.K., Clough, J.G., Coonrad, W.L., Gilbert, W.G., Miller, M.L., Murphy, J.M., Robinson, M.S., and Wallace, W.K., 1994, Geology of southwestern Alaska, in Plafker, G. and Berg, H.C., eds., The Geology of Alaska: Geological Society of America DNAG, The Geology of North America, v. G-1, p. 285-310. MacKevett, E.M., Jr., and Holloway, C.D., 1977, Map showing metalliferous and selected non-metalliferous mineral deposits in the eastern part of southern Alaska: U.S. Geological Survey Open-File Report 77-169-A, 99 p., 1 sheet, scale 1:1,000,000. Nokleberg, W.J., and (seven) others, 1994, Metallogeny and major mineral deposits of Alaska and Metallogenic map of significant metalliferous lode deposits and placer districts of Alaska, in Plafker, G. and Berg, H.C., eds., The Geology of Alaska: Geological Society of America, DNAG, The Geology of North America, Vol.. G1, p. 855-904, Plate 11, scale 1:2,500,000. 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. Reed, B.L., and Eberlein, G.D., 1972, Massive sulfide deposits near Shellabarger Pass, southern Alaska Range, Alaska: U.S. Geological Survey Bulletin 1342, 43 p. Reed, B.L., and Nelson, S.W., 1980, Geologic map of the Talkeetna quadrangle: U.S. Geological Survey Miscellaneous Investigations Map I-1174, 1 sheet, scale 1:250,000. Reed, B.L., Nelson, S.W., Curtin, G.C., and Singer, D.A., 1978, Mineral resources map of the Talkeetna quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-870-D, 1 sheet, scale 1:250,000.