Pebble

Prospect, Active

Commodities and mineralogy

Geographic location

Quadrangle map, 1:250,000-scale IL
Quadrangle map, 1:63,360-scale D-7
Latitude 59.8991
Longitude -155.2807
Nearby scientific data Find additional scientific data near this location
Location and accuracy In plan view as of late 2011, the Pebble prospect covers an area about 1.9 by 2.8 miles and consists of two contiguous segments, West Pebble and East Pebble. The coordinates are at about the center of the prospect, about 9.2 miles north-northeast of Sharp Mountain and 18.7 miles northwest of Iliamna. It covers most of sections 26 to 28, T. 3 S., R. 35 W.

Geologic setting

Geologic description

Pebble is a classic calc-alkalic copper-gold-molybdenum porphyry deposit. As of early 2011, the Pebble prospect extends over an area about 1.9 by 2.8 miles and has been explored to a depth of at least 5,000 feet (Gaunt and others, 2010). The Pebble prospect consists of two contiguous segments, Pebble West and Pebble East. Pebble West was discovered by Cominco American Inc. in 1988; it is amenable to open pit mining. Pebble East was discovered in 2005 by Northern Dynasty Minerals Inc. by drilling. Pebble East does not come to the surface and is covered by a wedge of postmineral volcanic and sedimentary rocks. It has been considered for underground mining by block caving.
In 2001, Northern Dynasty acquired the right to earn a 100 percent interest in Pebble from Teck-Cominco. As of 2006, Northern Dynasty had earned the 100 percent interest, subject only to royalties on production. In 2007, Northern Dynasty agreed to a partnership with Anglo American PLC to advance the Pebble project and as of early 2011, that partnership is in force as 'The Pebble Partnership' (2011).
From 1985 to 1997, Pebble was explored by Cominco American and Teck-Cominco, from 2001 to 2007 by Northern Dynasty, and from 2007 to early 2011 by The Pebble Partnership (Gaunt and others, 2010). The property has been covered by extensive geologic mapping and sampling, by several geochemical surveys, and by several ground and aerial geophysical surveys using a variety of techniques. As of December 2009, 1,085 holes, totaling 270,107 meters, have been drilled on Pebble. The drilling consisted of 568 exploration holes, 227 geotechnical holes, 60 holes for metallurgical samples, and 230 environmental holes. As the Northern Dynasty work has progressed, there has been a succession of profession technical reports describing the deposit and the work in detail; the latest is Gaunt and others (2010). (The others are in the reference list.)
The host rocks for the Pebble deposit are Jurassic and Cretaceous flysch that has been invaded by a variety of 96 to 99 Ma intrusive rocks including diorite sills, gabbro, alkali monzonite, a breccia complex, and granodiorite sills. Those rocks have been invaded by five 89-91 Ma granodiorite stocks, two of which are the principal sources of the mineralization in West Pebble and East Pebble. The East Pebble ore body is covered by a wedge of Upper Cretaceous to Tertiary sedimentary and volcanic rocks.
The mineralization consists mainly of pyrite, chalcopyrite, and molybdenite in and adjacent to two 90 Ma granodiorite stocks (Gaunt and others, 2010). These ore minerals are disseminated and in zones of high-density quartz-vein stockworks, in the upper parts of the stocks, along granodiorite dikes that extend laterally from the stocks, and in the adjacent hornfelsed flysch and intrusive host rocks. Pebble West, which is exposed at the surface has an oxidized zone, usually no more than several tens of meters thick, and a supergene zone with deposition of chalcocite and covellite. The hornfelsed country rock is cut by a variety of veins including quartz veins, magnetite-bearing veins, pyrite-dominated veins, and polymetallic quartz-calcite veins with sphalerite galena, chalcopyrite, pyrrhotite, telluride minerals, and tetrahedrite-tennantite.
The mineralization was accompanied by several types of alteration. From younger to older, these include: 1) pre-hydrothermal hornfelsing; 2) deep sodic-calcic alteration; 3) early potassium feldspar-biotite-magnetite alteration that is associated with most of the copper-gold-molybdenum mineralization; 4) peripheral propylitic alteration; 5) an illite overprint of the the potassium silicate alteration; 6) younger advanced argillic alteration; and 7) young peripheral quartz-sericite-pyrite alteration.
As of November, 2010, the resources of the Pebble prospect have been calculated at 4 levels of copper-equivalent cut-off (CuEQ) (Ghaffari and others, 2011; Pebble Partnership, 2010). The measured and indicated mineral resources with a CuEQ of from 0.30 to 1.00 percent are from 5,942 million tonnes with a grade of 0.0.42 percent copper, 0.35 gram of gold per tonne, and 250 parts per million (ppm) molybdenum to 1,449 million tonnes with a grade of 0.76 percent copper, 0.52 gram of gold per tonne, and 341 ppm molybdenum. The inferred mineral resources with a CuEQ of from 0.30 to 1.00 percent are from 4,835 million tonnes with a grade of 0.24 percent copper, 0.26 gram of gold per tonne, and 215 ppm molybdenum to 353 million tons with a grade of 0.69 percent copper, 0.45 gram of gold per tonne, and 379 ppm molybdenum.
Geologic map unit (-155.282894137115, 59.8984106792867)
Mineral deposit model Porphyry Cu-Au-(Mo) (Cox and Singer, 1986; model 20c).
Mineral deposit model number 20c
Age of mineralization Unambiguously related to two 89-91 Ma granodiorite stocks.
Alteration of deposit The mineralization was accompanied by several types of alteration. From younger to older, these include: 1) pre-hydrothermal hornfelsing; 2) deep sodic-calcic alteration; 3) early potassium feldspar-biotite-magnetite alteration that is associated with most of the copper-gold-molybdenum mineralization; 4) peripheral propylitic alteration; 5) an illite overprint of the the potassium-silicate alteration, 6) younger advanced argillic alteration; and 7) young peripheral quartz-sericite-pyrite alteration.

Production and reserves

Workings or exploration From 1985 to 1997, Pebble was explored by Cominco American and Teck-Cominco, from 2001 to 2007 by Northern Dynasty, and from 2007 to early 2011 by The Pebble Partnership (Gaunt and others, 2010). The property has been covered by extensive geologic mapping and sampling, by several geochemical surveys, and by several ground and aerial geophysical surveys using a variety of techniques. As of December 2009, 1,085 holes, totaling 270,107 meters, have been drilled on Pebble. The drilling consisted of 568 exploration holes, 227 geotechnical holes, 60 holes for metallurgical samples, and 230 environmental holes. As the Northern Dynasty work has progressed, there has been a succession of profession technical reports describing the deposit and the work in detail; the latest is Gaunt and others (2010). (The others are in the reference list.)
Indication of production None
Reserve estimates As of November, 2010, the resources of the Pebble prospect have been calculated at 4 levels of copper-equivalent cut-off (CuEQ) (Ghaffari and others, 2011; Pebble Partnership, 2010). The measured and indicated mineral resources with a CuEQ of from 0.30 to 1.00 percent are from 5,942 million tonnes with a grade of 0.0.42 percent copper, 0.35 gram of gold per tonne, and 250 parts per million (ppm) molybdenum to 1,449 million tonnes with a grade of 0.76 percent copper, 0.52 gram of gold per tonne, and 341 ppm molybdenum. The inferred mineral resources with a CuEQ of from 0.30 to 1.00 percent are from 4,835 million tonnes with a grade of 0.24 percent copper, 0.26 gram of gold per tonne, and 215 ppm molybdenum to 353 million tons with a grade of 0.69 percent copper, 0.45 gram of gold per tonne, and 379 ppm molybdenum.
Production notes None.

References

References

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, p. 422-435.
Gaunt, J.D., Rebagliati, C.M., Lang, James, Titley, Eric, Melis, Lawrence, Barratt. Derek, and Hodgson, Stephen, 2010, Technical report on the 2009 program and update on metallurgy, Pebble copper-gold-molybdenum project, Iliamna Lake area, southwestern Alaska, U.S.A: Unpublished Technical Report for Northern Dynasty Minerals Ltd., 195 p. (posted on www.sedar.com, Mar. 18, 2010).
Ghaffari, Hassan, Morrison, R.S., de Ruiter, M.A., Zivkovia, Aleksander, Hantelmann, Tysen, Ramsey, Douglas, and Cowie, Scott, 2011, Preliminary assessment of the Pebble project, southwest Alaska: Unpublished report to Northern Dynasty Minerals Ltd., 529 p. (posted on www.sedar.com on March 22, 2011).
Haslinger, R.J., Payne, J.G., Price, S. and Rebagliati, C.M., 2004, 2003 Summary report on the Pebble Porphyry gold-copper project: Unpublished report for Northern Dynasty Minerals Ltd., 119 p. (posted on www.sedar.com, Jun. 30, 2004).
Pebble Partnership, 2010, Updated mineral resource estimate for Pebble prospect: http://www.pebblepartnership.com/sites/default/files/pub/press%20release%20feb%202010.pdf (News Release, Feb. 1, 2010).
Pebble Partnership, 2011, http://www.pebblepartnership.com/home (as of Feb 22, 2011).
Rebagliati, C.M., 2003, Summary report on the Pebble copper-gold porphyry project; Unpublished report for Northern Dynasty Minerals Ltd., 94 p. (posted on www.sedar.com, Feb. 13, 2003).
Rebagliati, C.M., and Payne, J.G., 2005, 2004 summary report on the Pebble gold-copper project: Unpublished report for Northern Dynasty Minerals Ltd., 118 p. (posted on www.sedar.com, Apr. 4, 2005).
Rebagliati, C.M., and Payne, J.G., 2007, 2006 summary report on the Pebble gold-copper-molybdenum project: Unpublished report for Northern Dynasty Minerals Ltd., 119 p. (posted on www.sedar.com, Apr. 2, 2007).
Rebagliati, C.M., Lang, James, Gaunt, David, Rennie, David, Melis, Lawrence, Barratt. Derek, and Hodgson, Stephen, 2008, Technical report on the 2007 program and update on metallurgy and resources on the Pebble copper-gold-molybdenum project, Iliamna Lake area, southwestern Alaska, U.S.A: Unpublished Technical Report for Northern Dynasty Minerals Ltd., 81 p. (posted on www.sedar.com, Apr. 1, 2008).
Rebagliati, C.M., Lang, James, Titley, Eric, Rennie, David, Melis, Lawrence, Barratt. Derek, and Hodgson, Stephen, 2009, Technical report on the 2008 program and update on metallurgy, Pebble copper-gold-molybdenum project, Iliamna Lake area, southwestern Alaska, U.S.A: Unpublished Technical Report for Northern Dynasty Minerals Ltd., 170 p. (posted on www.sedar.com, Feb. 27, 2009).
Rennie, D.W., and Srivastava, R.M., 2005, Technical report on the Pebble deposit, Alaska, USA: Unpublished technical report for Northern Dynasty Minerals Ltd., 73 p. (posted on www.sedar.com, June 14, 2005).
Reporters C.C. Hawley (Hawley Resource Group); D.J. Grybeck (Contractor, USGS)
Last report date 4/1/2012