Alaska, USA

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The Pebble porphyry copper, gold, molybdenum deposit is located approximately 380 km SW of Anchorage and 27 km NW of Iliamna in the Bristol Bay region of southwest Alaska (#Location: 51° 53' 30"N, 155° 16' 39"W).

The deposit falls within the Kahiltna terrane of Alaska, near splays off the northeast-trending, crustal scale, Lake Clark transform fault. The northern section of the Kahiltna terrane is dominated by Late Triassic basalt, andesite and sedimentary rocks which are overlain by Jurassic to Cretaceous andesitic turbidites, which were intruded by intermediate to felsic Cretaceous plutons, and capped by Tertiary volcanic and sedimentary units and by Quaternary glacial deposits.

The deposit is composed of the two contiguous West and East Zones, which are believed to represent two coeval thermal and fluid centres within a single magmatic-hydrothermal system. These zones extend from surface to ~500 m depth and deeper to the east. Both zones are hosted within siltstones of the Jurassic-Cretaceous Kahiltna Flysch sequence of andesitic argillite, siltstone and wacke. These sediments were intruded by 98 to 96 Ma diorite and granodiorite sills, with the East Zone being centred on several small granodiorite cupolas. In addition, in the West Zone, there is a 96 Ma complex of alkalic biotite pyroxenite, monzosyenite and monzonite intrusions and related breccias. In addition, four small ~90 Ma hornblende granodiorite porphyry intrusives, which are also genetically related to mineralisation, occur in the West Zone, while a similar, larger granodiorite pluton is found in the East Zone. Subalkalic 91 to 89 Ma granodiorite intrusions also include the mostly fresh Kaskanak Batholith to the west. The Central Zone is a peripheral, up-dip expression of the East Zone along a series of granodiorite sills, and is in fault contact with the West Zone.

The West Zone outcrops, while the East Zone, which extends to a depth of at least 1700 m, is unconformably overlain by a sequence of Late Cretaceous to Eocene sedimentary and volcanic rocks. Both zones are unconformably overlain by thin glacial sediments.

The earliest alteration preserved in the Pebble East Zone is a sodic-calcic phase where cores and rims of igneous plagioclase phenocrysts are replaced by calcite and albite, respectively. Sodic-calcic altered granodiorite occurs at depth in the East Zone Stock and grades upwards into coeval K silicate alteration with the groundmass being replaced by fine-grained, intergrown quartz and K feldspar. Biotite has replaced hornblende phenocrysts and igneous magnetite was destroyed. The potassic alteration assemblages was accompanied by chalcopyrite ± bornite, mostly formed in quartz veins or associated with biotite-rutile clots while gold formed native inclusions in chalcopyrite. Remnants of igneous magnetite were altered to hematite. Potassic alteration was originally the most extensive assemblage in the Pebble East Pluton.

Pervasive illite overprinted both potassic and sodic-calcic alteration throughout the Pebble East Zone, with hydrothermal biotite being completely replaced by illite. The degree to which K feldspar or calcite-albite altered plagioclase phenocrysts were replaced by illite was dependent upon the strength of alteration, with complete replacement only occurring where alteration is strong, and quartz and K feldspar being preserved in the groundmass. Illite alteration converted the original chalcopyrite ± bornite assemblage to chalcopyrite-pyrite. Iron liberated from altered biotite was sequestered into pyrite.

The sodic-calcic, potassic and illite altered granodiorite is displaced by a major syn-hydrothermal fault zone which controlled younger pyrophyllite-quartz alteration. Intense quartz stockwork veining is found within the fault while granodiorite between quartz veins were altered to pyrophyllite-quartz-sericite. Pyrophyllite occurs as a replacement of K feldspar in the altered groundmass of the granodiorite, while illite altered phenocrysts were sericitised. Chalcopyrite-bornite and chalcopyrite-pyrite intergrowths formed during the potassic and illite alteration phases are typically preserved, although during intense alteration chalcopyrite was commonly replaced by pyrophyllite. Significant gold was introduced during this alteration stage with grades frequently of >1 g/t compared to to the general background <0.5 g/t Au, although Cu was not upgraded, with introduced arsenic producing tennantite which replaced chalcopyrite.

Peripheral, laterally extensive sericitic alteration overprints the margins of the deposit, and propylitic and SCC (sericite-chlorite-clay) assemblages are locally developed. Intense sericite alteration is texturally destructive with sericite and quartz completely replacing potassic and illite-altered granodiorite. Intense sericite alteration is accompanied by chalcopyrite-pyrite ± tennantite which was reconstituted as a high-sulphidation assemblage of pyrite-bornite-digenite ± enargite. Copper grades are elevated throughout weak to intense sericite alteration. Gold is stripped during the most intense sericite alteration but is unaffected by weaker alteration. The youngest pulse of alteration produced an intense quartz sericite-pyrite assemblage which extends far outside, but overprints the margins, of the deposit. Pre-existing Cu sulphides were destroyed by this late phase where overprinted, resulting in low grades.

Total resources in February 2007 were quoted by Lang et al., (2013) as:
    West and Central Zones, 4.13 Gt @ 0.29% Cu, 0.31 g/t Au, 0.015% Mo (0.30% Cu equiv. cutoff); and
    East Zone, 3.38 Gt @ 0.57% Cu, 0.36 g/t Au, 0.036% Mo (0.6% Cu
equiv. cutoff) - remaining open in four directions.

or, (Pebble Partnership website, 2009):
    Measured + Indicated Resource of 5.1 Gt + Inferred Resource of 4.0 Gt - for a total of 9.1 Gt @ 0.37% Cu, 0.32 g/t Au, 0.025% Mo.

Mineral Resources (Kelley, et al., 2010):
    Measured + Indicated Resource of 5.942 Gt @ 0.42% Cu, 0.35 g/t Au, 250 ppm Mo (at a 0.1% Cu
equiv. cutoff)
        plus Inferred Resource of 4.835 Gt @ 0.24% Cu, 0.26 g/t Au, 215 ppm Mo and is open in several directions.

NI 43-101 compliant Mineral Resources at 31 December 2017 (Northern Dynasty Minerals Limited website, 2020):
    Measured + Indicated Resource - 6.456 Gt @ 0.40% Cu, 0.34 g/t Au, 0.0246% Mo, 1.7 g/t Ag (0.76% Cu
equiv. cutoff);
    Inferred Resource - 4.454 Gt @ 0.25% Cu, 0.25 g/t Au, 0.0226% Mo, 1.2 g/t Ag (0.55% Cu
equiv. cutoff).

The most recent source geological information used to prepare this summary was dated: 2009.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Anderson E D, Hitzman M W, Monecke T, Bedrosian P A, Shah A K and Karen D. Kelley  2013 - Geological Analysis of Aeromagnetic Data from Southwestern Alaska: Implications for Exploration in the Area of the Pebble Porphyry Cu-Au-Mo Deposit: in    Econ. Geol.   v.108 pp. 421-436
Ayuso R A, Kelley K D, Eppinger R G and Forni F,  2013 - Pb-Sr-Nd Isotopes in Surficial Materials at the Pebble Porphyry Cu-Au-Mo Deposit, Southwestern Alaska: Can the Mineralizing Fingerprint be Detected Through Cover? : in    Econ. Geol.   v.108 pp. 543-562
Goldfarb R J, Anderson E D and Hart C J R,  2013 - Tectonic Setting of the Pebble and Other Copper-Gold-Molybdenum Porphyry Deposits within the Evolving Middle Cretaceous Continental Margin of Northwestern North America: in    Econ. Geol.   v.108 pp. 405-419
Gregory M J, Lang J R, Gilbert S and Hoal K O  2013 - Geometallurgy of the Pebble Porphyry Copper-Gold-Molybdenum Deposit, Alaska: Implications for Gold Distribution and Paragenesis : in    Econ. Geol.   v.108 pp. 463-482
Harraden C L, Mcnulty B A, Gregory M J and Lang J R,  2013 - Shortwave Infrared Spectral Analysis of Hydrothermal Alteration Associated with the Pebble Porphyry Copper-Gold-Molybdenum Deposit, Iliamna, Alaska : in    Econ. Geol.   v.108 pp. 483-494
Kelley K D, Lang J R and Eppinger R G,  2013 - The Giant Pebble Cu-Au-Mo Deposit and Surrounding Region, Southwest Alaska: Introduction : in    Econ. Geol.   v.108 pp. 397-404
Kelley KD, Lang J and Eppinger RG,  2010 - Exploration geochemistry of the giant Pebble porphyry Cu-Au-Mo deposit, Alaska: in    SEG Newsletter   No. 80 pp 1, 18-23
Lang J R, Gregory M J, Rebagliati C M, Payne J G, Oliver J L and Roberts K,  2013 - Geology and Magmatic-Hydrothermal Evolution of the Giant Pebble Porphyry Copper-Gold-Molybdenum Deposit, Southwest Alaska : in    Econ. Geol.   v.108 pp. 437-462
Mathur R, Munk L, Nguyen M, Gregory M, Annell H and Lang J,  2013 - Modern and Paleofluid Pathways Revealed by Cu Isotope Compositions in Surface Waters and Ores of the Pebble Porphyry Cu-Au-Mo Deposit, Alaska: in    Econ. Geol.   v.108 pp. 529-541
Robert G. Eppinger, David L. Fey, Stuart A. Giles, Eric C. Grunsky, Karen D. Kelley, Burke J. Minsley, Leeann Munk and Steven M. Smith  2013 - Summary of Exploration Geochemical and Mineralogical Studies at the Giant Pebble Porphyry Cu-Au-Mo Deposit, Alaska: Implications for Exploration Under Cover : in    Econ. Geol.   v.108 pp. 495-527

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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