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Quartzite, siltite, argillite, and mica schist; Middle Proterozoic Lemhi Group; southern Belt province

Intermediate Precambrian sediments, feldspathic quartzite and siltstone of southern Idaho.
StateIdaho
NameQuartzite, siltite, argillite, and mica schist; Middle Proterozoic Lemhi Group; southern Belt province
Geologic ageMiddle Proterozoic
Original map labelYs; Y1s; Y3s; Y?s
CommentsY3sm includes greenschist to albite-epidote-amphibolite-facies metasedimentary rocks of the Lemhi Gp, as currently defined in ID090 and ID025. Much now classified as Lemhi Gp (Y3s) was included in Yellowjacket Fm (Y1s) on map ID001. Where possible, polygons of Y1s have been reclassified as Y3sm according to ID025. At the Blackbird mine, siliceous meta-exhalite and schistose clinochlore-garnet-biotite schist and hornfels host Cu-Co-Au mineralization in Y3sm (ID025, ID089).
Primary rock typequartzite
Secondary rock typesiltstone
Other rock typesmeta-argillite; exhalite; mica schist; lamprophyre
Lithologic constituents
Major
Metamorphic > Argillite (Greenschist)biotite argillite in turbidites
Sedimentary > Clastic > Siltstone (Bed)biotite siltite (meta-siltstone) in turbidites
Metamorphic > Quartzite (Bed)feldspathic quartzite in turbidites
Incidental
Metamorphic > Schist (Hornfels, biotite)schistose biotite hornfels with garnet porphyroblasts
Igneous > Hypabyssal > Lamprophyre (Dike or sill)porphyritic mafic dikes and sills
Sedimentary > Chemical > Chert (Bed)metachert after siliceous exhalite with Cu-Co mineralization
Map references
Bond, J.G., Kauffman, J.D., Miller, D.A., and Venkatakrishnan, Ramesh, 1978, Geologic map of Idaho: Moscow, Idaho, Idaho Bureau of Mines and Geology, with contributions from U.S. Geological Survey, scale 1:500,000.
Unit references
Bond, J.G., Kauffman, J.D., Miller, D.A., and Venkatakrishnan, Ramesh, 1978, Geologic map of Idaho: Moscow, Idaho, Idaho Bureau of Mines and Geology, with contributions from U.S. Geological Survey, scale 1:500,000.
Evans, K.V. and Green, G.N., 2003, Geologic map of the Salmon National Forest and vicinity, east-central Idaho: U.S. Geological Survey Geological Investigations Series I-2765, scale 1:100,000 with pamphlet, 19 p.
Rember, W.C., and Bennett, E.H., 1979, Geologic map of the Dubois quadrangle, Idaho: Moscow, Idaho, Idaho Bureau of Mines and Geology Map Series, Dubois 2o quadrangle, scale 1:250,000.
Ruppel, E.T., 1975, Precambrian Y sedimentary rocks in east-central Idaho: in Ruppel, E.T., Ross, R.J., Jr., and Schleicher, David, Precambrian and lower Ordovician rocks in east-central Idaho: U.S. Geological Survey Professional Paper, 889-A, p. A1-A23.
Hahn, G.A. and Hughes, G.J., Jr., 1984, Sedimentation, tectonism and associated magmatism of the Yellowjacket Formation in the Idaho cobalt belt, Lemhi County, Idaho: in Hobbs, S.W., ed., The Belt, abstracts and summaries; Belt Symposium II: Montana Bureau of Mines and Geology Special Publication 90, p. 65-67.
Tysdal, R.G., 2000, Revision of Middle Proterozoic Yellowjacket Formation, central Idaho: U.S. Geological Survey Professional Paper 1601-A, p. A1-A13.
Nash, J.T., and Hahn, G.A., 1989, Stratabound Co-Cu deposits and mafic vocaniclastic rocks in the Blackbird mining district, Lemhi County, Idaho: in Boyle, R.W., Brown, A.C., Jefferson, C.W., Jowett, E.C., and Kirkham, R.V., eds, Sediment-Hosted Stratiform Copper Deposits: Geological Association of Canada, Special Paper 36, p. 339-356.
Geographic coverageButte - Custer - Lemhi

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