Pebble Deposit

Site Name

Primary: Pebble Deposit
Secondary: Pebble Beach Copper
Secondary: Pebble Copper

---

Commodity

Primary: Gold
Primary: Copper
Secondary: Silver
Secondary: Molybdenum

---

Location

State: Alaska
District: Bristol Bay

---

Land Status

Land ownership: State
Note: the land ownership field only identifies whether the area the mine is in is generally on public lands like Forest Service or BLM land, or if it is in an area that is generally private property. It does not definitively identify property status, nor does it indicate claim status or whether an area is open to prospecting. Always respect private property.

---

Holdings

Type: Located Claim

---

Workings

Not available

---

Ownership

Owner Name: Northern Dynasty Minerals Ltd and Anglo American
Percent: 100.0
Info Year: 2010
Years: 2001 -

Owner Name: Cominco Ltd.
Percent: 100.0
Home Office: Canada
Info Year: 1994

Owner Name: Cominco Alaska Explorations Ltd>
Home Office: Alaska
Info Year: 1994

---

Production

Not available

---

Deposit

Record Type: Site
Operation Category: Prospect
Operation Type: Surface
Mining Method: Unknown
Discovery Year: 1989
Discovery Method: Ore-Mineral In Place
Years of Production:
Organization:
Significant: Y

---

Physiography

Not available

---

Mineral Deposit Model

Model Name: Porphyry Cu-Au

---

Orebody

Not available

---

Structure

Not available

---

Alterations

Alteration Type: L
Alteration: potassic

---

Rocks

Name: Granodiorite
Role: Host
Age Type: Host Rock
Age Young: Late Cretaceous

Name: Tonalite
Role: Host
Age Type: Host Rock
Age Young: Late Cretaceous

---

Analytical Data

Not available

---

Materials

Ore: Galena
Ore: Sphalerite
Ore: Pyrite
Ore: Molybdenite
Ore: Bornite
Ore: Magnetite
Ore: Gold
Ore: Chalcopyrite
Ore: Chalcocite
Ore: Covellite
Ore: Goethite
Gangue: Biotite
Gangue: Diorite
Gangue: Quartz

---

Comments

Comment (Geology): From ARDF 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. 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: molydenite 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).

Comment (Geology): From ARDF 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.

Comment (Exploration): From ARDF Cominco Alaska Exploration began to explore Pebble Copper in 1986. Early exploration was based on color anomalies observed by local pilots. Drilling started in the 1988 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.

Comment (Exploration): The Northern Dynasty website as of 7/2010 reports drilling at the Pebble deposit and surrounding exploration lands totaled 886,177 feet in 1,085 holes. http://www.northerndynastyminerals.com/ndm/PD_DPR.asp

---

References

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

Reference (Geology): 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.

Reference (Geology): 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.

Reference (Deposit): The Metals Economics Group (MEG) webpage for Pebble contains information about this deposit. Accessed on 6/29/2010. http://www.metalseconomics.com/
URL: http://www.metalseconomics.com/

Reference (Deposit): The Northern Dynasty Minerals Ltd website description of Pebble. http://www.northerndynastyminerals.com/ndm/Pebble.asp
URL: http://www.northerndynastyminerals.com/ndm/Pebble.asp

Reference (Deposit): Porter GeoConsultancy web page for Pebble has a description of the geology. Accessed 9/8/2010. http://www.portergeo.com.au/
URL: http://www.portergeo.com.au/

Reference (Reserve-Resource): Bundtzen, T.K., Swainbank, R.C., Wood, J.E., and Clough, A.H., 1992, Alaska's mineral industry 1991: A summary: Alaska Division of Geological & Geophysical Surveys Information Circular 35, 11 p.
Pages: 9

Reference (Reserve-Resource): ALASKA JOURNAL OF COMMERCE 1/28/91 P. 2

Reference (Deposit): METALS WEEK 1/21/91

Reference (Reserve-Resource): NORTHERN MINER, V 76/N 47, 3/9/92, P.1,19.

Reference (Reserve-Resource): COMINCO LTD., 1993 ANNUAL REPORT, P.35.

Reference (Reserve-Resource): COMINCO LTD., 1995 ANNUAL REPORT, P.37.

Reference (Deposit): CANADIAN MINES HANDBOOK 1991-92, P.108.

Reference (Geology): 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 Institute of Mining, Metallurgy, and Petroleum, Special Volume 46
Pages: 422-435

Reference (Deposit): MINING ENGINEERING, 3/91, P. 285.

Reference (Deposit): MINING JOURNAL, SUPPLEMENT, 5/20/94, P.8.

---

The Top Ten Gold Producing States

These ten states contributed the most to the gold production that built the West from 1848 through the 1930s. The Top Ten Gold Producing States.