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Middle Fork

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Porphyry copper deposit in Washington, United States

Names and other identifiers

GMRAP ID 21
Alternate names Three Brothers, Clipper, Condor, Hemlock/Porter
Tract name Cascade Arc (003pCu9014)
Site status deposit

Geographic location

Country United States
State or province Washington
Geographic location -121.366666667, 47.5
Geologic map unit -121.366666667, 47.5
Nearby data
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Geologic information

Deposit type Porphyry copper
Age (Ma) 25Ma
Age determination method unknown
Age range 25 Ma
Age reference Singer and others (2008)
Minerals ankerite, arsenopyrite, azurite/malachite, barite, biotite, carbonate, chalcocite, chalcopyrite, chlorite, cosalite, covellite, epidote, galena, gold, molybdenite, prehnite, pyrite, pyrrhotite, sericite, siderite, sphalerite
Associated rocks breccia, quartz diorite porphyry, granodiorite, quartz diorite, quartz monzonite, tonalite
Tectonic setting continental margin
Stratigraphic age Oligocene

Commodity and development information

Development status Occurrence
Major commodities Cu
Tonnage (Mt) 93Mt
Cu grade (%) 0.41%
Mo grade (%) 0.02%
Contained copper 380,000t
Gold-Moly ratio -9999

Reference information

Study area name United States
References Lasmanis (1995); Patton and others (1973); Singer and others (2008)
Site reference U.S. Geological Survey National Mineral Resource Assessment Team (2002)
Lasmanis, Raymond, 1995, Regional geological and tectonic setting of porphyry deposits in Washington State, in Schroeter, T.G., ed., Porphyry deposits of the northwestern Cordillera of North America: Canadian Institute of Mining, Metallurgy and Petroleum Special Volume 46, p. 77–102.
Patton, T.C., Grant, A.R., and Cheney, E.S. 1973, Hydrothermal alteration at the Middle Fork copper prospect, central Cascades, Washington: Economic Geology, v. 68, p. 816–830. (Also available at http://dx.doi.org/10.2113/gsecongeo.68.6.816.)Singer, D.A., Berger, V.I., and Moring, B.C., 2008, Porphyry copper deposits of the world—Database and grade and tonnage models, 2008: U.S. Geological Survey Open-File Report 2008-1155, [45] p. and digital data, accessed December 31, 2013, at http://pubs.usgs.gov/of/2008/1155/.