Pebble

Porphyry copper deposit in Alaska, United States

Names and other identifiers

GMRAP ID 4
Alternate names Pebble Copper, Pebble Beach
Tract name Western Alaska Range (003pCu5004)
Site status deposit

Geographic location

Country United States
State or province Alaska
Geographic location -155.295555556, 59.898333333
Geologic map unit -155.295555556, 59.898333333
Nearby data

Geologic information

Deposit type Porphyry copper
Age (Ma) 90.4Ma
Age determination method Re-Os molybdenite
Age range 90.4 ±0.6 Ma to 89.5 ±0.3 Ma
Age reference Lang and others (2013)
Minerals anhydrite, apatite, biotite, bornite, chalcocite/digenite, chalcopyrite, chlorite, covellite, epidote, galena, gold, magnetite, molybdenite, pyrite, sericite, siderite, sphalerite
Associated rocks breccia, diorite, diorite porphyry, granodiorite porphyry, latite, pyroxenite, rhyolite, andesite, basalt, conglomerate, granodiorite, graywacke, mudstone, siltstone, tonalite
Tectonic setting continental margin
Stratigraphic age Late Cretaceous

Commodity and development information

Development status Prospect
Major commodities Ag, Au, Cu, Mo
Minor commodities As, Pb, Sb, Te, Zn
Tonnage (Mt) 7,510Mt
Cu grade (%) 0.416%
Mo grade (%) 0.024%
Au grade (g/t) 0.33g/t
Contained copper 31,200,000t
Gold-Moly ratio 14

Reference information

Comments Pebble is a giant, classic calc-alkalic porphyry copper-gold-molybdenum deposit. It is composed of the East and West zones which represent two coeval hydrothermal centers within a single system. The West zone extends from surface to ~500-m depth and is centered on four small granodiorite plutons emplaced into flysch, diorite, and granodiorite sills, and alkalic intrusions and breccias. The much higher grade East zone extends to at least 1,700-m depth and is hosted by a larger granodiorite pluton and adjacent granodiorite sills and flysch. The granodiorite plutons merge with depth. On the east side of the deposit, high-grade mineralization has been dropped 600 to 900 m into the NE-trending East graben, where the deposit remains undelineated to the east and to depth.The large size and high hypogene grades of the Pebble deposit may reflect a combination of multiple stages of metal introduction with vertically restricted, lateral fluid flow induced by hornfels aquitards in flysch (Lang and others, 2013). Pebble contains the largest gold resource of any known porphyry. Molybdenite at Pebble is enriched in rhenium (averaging 906 ppm); pyrite in pyrophyllite alteration zones contains up to 3 ppm palladium. The mineralization was accompanied by several types of alteration (prehydrothermal hornfels, deep sodic-calcic, potassic, sodic-potassic, illite, advanced argillic, qsp halo, propylitic). As of November, 2010, the resources of the Pebble prospect had been calculated at 4 levels of copper-equivalent cut-off (CuEQ) (Ghaffari and others, 2011; Pebble Partnership, 2010). The measured and indicated mineral resources at the lowest report cutoff grade (0.3% CuEQ) are reported as 5,942 Mt at 0.42% Cu, 0.35 g/t Au, and 250 ppm Mo. In addition, inferred mineral resources (same cutoff) are reported as 4,835 Mt at 0.24% Cu, 0. 26 g/t Au, and 215 ppm Mo.
Study area name United States
References Anglo American PLC (2007); Bouley and others (1995); Bundtzen and others (1993); Lang and others (2007); Lang and others (2013); Nokleberg and others (1987); Northern Dynasty Minerals Ltd. ( 2007); Schrader and others (2001); Young and others (1997); Singer and others (2008)
Site reference U.S. Geological Survey National Mineral Resource Assessment Team (2002)
Anglo American PLC, 2007, Anglo American establishes 50:50 partnership in Pebble project with Northern Dynasty: Anglo American PLC press release, about July 31, 2007, 2 p., access date unknown, at http://www.angloamerican.co.uk/article/?afwsource_key=1E55D23-79. (Web page no longer available. See http://www.minesandcommunities.org/article.php?a=6470.)
Bouley, B.A., St. George, P., and Wetherbee, P.K., 1995, Geology and discovery at Pebble Copper, a copper-gold porphyry system in southwest Alaska, 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. 422–435.
Bundtzen, T.K., Swainbank, R.C., Clough, A.H., Henning, M.W., and Hansen, E.W., 1993, Alaska’s mineral industry 1993: Alaska Division of Geological and Geophysical Surveys Special Report 48, 83 p. and tables and figures. (Also available at http://www.dggs.alaska.gov/pubs/id/2647.)
Lang, J., Payne, J., Rebagliati, M., Roberts, K., Oliver, J., McLaughlin, J., 2007, The super-giant Pebble copper-gold-molybdenum porphyry deposit, southwest Alaska [abs.], in Arizona Geological Society, Ores and Orogenesis—Circum-Pacific Tectonics, Geologic Evolution, and Ore Deposits—A Symposium Honoring the Career of William R. Dickinson, Tucson, Ariz., September 24–30, 2007: Arizona Geological Society Program with Abstracts, p. 120–121.
Lang, J.R., Gregory, M.J., Rebagliati, C.M., Payne, J.G., Oliver, J.L., and Roberts, Keith, 2013, Geology and magmatic-hydrothermal evolution of the giant Pebble porphyry copper-gold-molybdenum deposit, southwest Alaska: Economic Geology, v. 108, p. 437–462. (Also available at http://dx.doi.org/10.2113/econgeo.108.3.437.)Nokleberg, W.J., Bundtzen, T.K., Berg, H.C., Brew, D.A., Grybeck, D., 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. (Also available at http://pubs.usgs.gov/bul/1786/report.pdf.)Northern Dynasty Minerals Ltd., 2007, Welcome to Northern Dynasty: Northern Dynasty Minerals Ltd. press release, December 12, 2007, 2 p. Accessed [date unknown], at http://www.northerndynastyminerals.com/ndm/Home.asp. (Press release not available. See http://investorshub.advfn.com/Northern-Dynasty-Minerals-Ltd-NAK-9923/.)Schrader, C.M.; Crowe, Doug; Turner, Kent; and Stein, H.J., 2001, 40Ar/39Ar and Re-Os geochronology of the Pebble Copper Cu-Au-Mo porphyry deposit, southwest Alaska [abs.]: Geological Society of America Abstracts with Programs, v. 33, no. 6, p. 418. (Also available at https://gsa.confex.com/gsa/2001AM/finalprogram/abstract_27798.htm.)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/.
Young, L.E., St. George,P., and Bouley, B.A., 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.