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Pebble Copper Prospect, Bristol Bay District, Lake and Peninsula Borough, Alaska, USA

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Small quartz-galena-sphalerite veins occur about 0.5 mile south of the Pebble Copper deposit (Bouley and others, 1995).
Location: The Pebble Copper deposit is at an elevation of about 1,150 feet about 2 miles north of locally named Frying Pan Lake (elevation 943 feet) in the headwaters of the Koktuli River. The site is in the SW1/4 SE1/4 sec. 21, T. 3 S., R. 35 W., Seward Meridian. The location is accurate within a few hundred feet for the center of the outcrop area of this large deposit.
Geology: Pebble Copper is a porphyry copper-gold-molybdenum deposit in the outer part of an Upper Cretaceous (89.7 Ma) tonalite-granodiorite batholith (Detterman and Reed, 1980). The batholith intrudes flysch of Jurassic or Cretaceous age. The west and south parts of the batholith comprise five textural and compositional phases ('the porphyry suite'): biotite diorite, equigranular granodiorite, porphyrytic granodiorite, biotite granodiorite, and intrusive breccia. Breccias appear to have developed with each intrusive phase; the largest breccia body is weakly mineralized (Bouley and others, 1995). The highest grade parts of the deposit are associated with porphyritic granodiorite in which plagioclase phenocrysts are replaced by potassium feldspar and sericite, and amphibole is replaced by granular greenish biotite. The 'porphyry suite' and the sedimentary country rocks at Pebble Copper are variably mineralized depending on host rock composition and proximity to the center of mineralization. Sulfides occur in stockwork veinlets near the core of the deposit, and are disseminated in peripheral phases. Most of the copper-and gold-bearing minerals occur either in hairline veinlets or in larger veins having well-developed potassic vein selvages. Copper sulfides, chiefly chalcopyrite, also occur as disseminations. Molybdenite occurs in late quartz veinlets that crosscut introduced potassic minerals. Metallic minerals identified at Pebble Copper include pyrite, chalcopyrite, and molybdenite, along with minor bornite, covellite, chalcocite, digenite, and magnetite. Gold is fine-grained, generally only a few microns in diameter, and generally occurs at or near pyrite-chalcopyrite grain boundaries (Bouley and others, 1995). Galena and sphalerite are reported in quartz veins on the periphery of the deposit. Alteration minerals include abundant secondary biotite and potassium feldspar and lesser amounts of ferroan dolomite, albite, and quartz. Characteristic accessory minerals are anhydrite, apatite, and rutile. The deposit contains more than 1 billion tonnes of material that grades about 0.3 percent copper and 0.34 gram of gold per tonne. A higher grade core zone of about 54 million tonnes averages 0.54 percent copper and 0.46 gram of gold per tonne (Bouley and others, 1995; Youngman, 2003). Cross-section 7 of Bouley and others (1995) suggests that the higher grade core overlies a stock or cupola of the porphyritic granodiorite. Mineralized material typically contains 200-300 parts per million of molybdenum: molybdenite probably would be recovered as a concentrate in a future mining operation. The deposit is oxidized to a depth of as much as 100 feet. Some secondary chalcocite is developed, but in general the richer ore seems to be related to variations in hypogene rather than supergene mineralization. Gossan of goethite and other oxidized minerals is developed locally, especially over peripheral pyrite-rich parts of the mineral deposit. Pebble Copper has been restudied since 2001 by Northern Dynasty Minerals. A regional IP program indicates that it is one of several deposits in an open-ended, 90-square-kilometer (34.8-square-mile) IP/chargeability anomalous area (Youngman, 2003). Northern Dynasty Minerals drilled 68 holes totaling 11,000 meters (about 36,100 feet) in the anomalous area in 2002, and identified new deposits described in records IL002, -003, and -004. The nearby Sill or 25 Gold (IL005) epithermal gold deposit was discovered by Cominco during reconnaissance studies of the area, and has been further explored by Northern Dynasty. The plutonic hostrocks of the Pebble Copper deposit are alkalic (potassic), and are similar to potassium-rich plutons that host copper-gold deposits throughout the world (Bouley and others, 1995; Muller and Groves, 1995).
Workings: Cominco Alaska Exploration began to explore Pebble Copper in 1986. Early exploration was based on color anomalies observed by local pilots. Drilling started in the1988 season and continued through 1992, when twelve holes were drilled to further delineate the higher grade porphyry zone found in drilling campaigns between 1988 and 1992. The general objectives were to: 1) define the extent of the porphyry system above a cutoff grade of 0.2 percent copper; 2) delineate the grade and shape of the deposit within the 0.2 percent area; and 3) delineate more precisely the size and grade of the higher grade core. Other work included a limited IP survey, environmental monitoring, and bench-scale metallurgical testing (Bouley and others, 1995). After about a decade of inactivity, work was resumed by Canada-based Northern Dynasty Minerals, Ltd., an affiliated company of the Hunter-Dickinson Group. The Northern Dynasty work began by expanding Cominco's IP survey. This work identified an open-ended, 90-square-km area (about 34.8 square miles) with anomalous IP chargeability response. This area includes the Pebble Copper deposit. In 2002, Northern Dynasty drilled about 11,000 meters (about 36,100 feet), partly at Pebble Copper, but mostly elsewhere in the geophysically anomalous area.
Age: Late Cretaceous. The Pebble Copper deposit occurs near the margin of an Upper Cretaceous (89.7 Ma) tonalite-granodiorite batholith intrusive into Jurassic or Cretaceous flysch (Detterman and Reed, 1980: Bouley and others, 1995).
Alteration: The dominant pervasive alteration at Pebble Copper is potassic. Secondary biotite has replaced primary mafic minerals and groundmass, and potassium feldspar and biotite have replaced plagioclase (Bouley and others, 1995). Ferroan dolomite, albite, anhydrite, rutile, and quartz were introduced locally. The deposit is oxidized to a depth of 100 feet.
Reserves: The inferred mineral resource in the Pebble Copper deposit was calculated by Cominco as 1,000 million tonnes of material averaging 0.3 percent copper and 0.34 gram of gold per tonne (Bouley and others, 1995); a core area contained more than 50 million tons of material grading more than 0.5 percent copper and almost 0.5 gram of gold per tonne (Youngman, 2003). Subsequent work on the property has increased the resource estimate. Based on all field work through 2002, there is a total of about 6.8 billion pounds of copper and 13 million ounces of gold in a resource of somewhat more than 1 billion tonnes of mineralized material at Pebble Copper. The higher grade core zone has been recalculated as 141 million tonnes grading 0.48 percent copper and 0.67 gram of gold per tonne (Northern Dynasty Minerals, 2003).

Commodities (Major) - Au, Cu; (Minor) - Ag, Mo
Development Status: None
Deposit Model: Porphyry Cu-Au-(Mo) (Cox and Singer, 1986; model 20c).

Mineral List

28 entries listed. 21 valid minerals.

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Detterman, R.L., and Reed, B.L., (1980) Stratigraphy, structure, and economic geology of the Iliamna quadrangle, Alaska: U.S. Geological Survey Bulletin 1368-B, 86 p.

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: Canadian Insitute of Mining, Metallurgy, and Petroleum, Special Volume 46, p. 422-435.

Muller, Daniel, and Groves, D. I., (1995) Potassic igneous rocks and associated gold-copper mineralization: in No. 56, Lecture notes in Earth Sciences, Springer-Verlag, New York, 210 p.

Youngman, Bruce, (2003) New discoveries within the giant Pebble copper-gold sulfide system [abs]: 2003 Cordilleran Exploration Roundup, Vancouver, B.C., Canada, p. 40-41.

Singer, D.A., Berger, V.I., and Moring, B.C. (2008): Porphyry copper deposits of the world: Database and grade and tonnage models, 2008. US Geological Survey Open-File Report 2008-1155.

Eppinger, R.G., Kelley, K.D., Fey, D.L., Giles, S.A., Minsley, B.J., and Smith, S.M., (2010) USGS exploration geochemistry studies at the Pebble porphyry Cu-Au-Mo deposit, Alaska—pdf of presentation: U.S. Geological Survey Open-File Report 2010–1225, 64 p.

Kelley, Karen D.; Lang, James R.; Eppinger, Robert G. (2013) The Giant Pebble Cu-Au-Mo deposit and surrounding Region, Southwest Alaska: Introduction. Economic Geology, 108: 397 - 404

External Links

Northern Dynasty Minerals, 2003, Internet Home Page, March 14, 2003:

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