anderson et al., 2013 geological analysis of aeromagnetic data from southwestern alaska: ...

8/13/2019 Anderson Et Al., 2013 Geological Analysis of Aeromagnetic Data from Southwestern Alaska: Implications for Explor

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Introduction

DISCOVERY of the Late Cretaceous Pebble porphyry Cu-Au-Mo deposit in southwestern Alaska, which has a current mea-

sured and indicated resource of 5,940 Mt at 0.42% copper,0.35 g/t gold, and 0.025% molybdenum, with a 0.30 wt % cop-per cutoff (Lang et al., 2013), has made this remote regionhighly prospective for porphyry copper deposits of similarage. The original discovery was made by Cominco geologistsin 1988 (Bouley et al., 1995). Drilling has since shown that thedeposit contains the largest gold resource and fifth largestcopper resource of any known porphyry deposit (Lang et al.,2013).

Because of extensive Tertiary and Quaternary cover (Det-terman and Reed, 1973; Beikman, 1974; Eakins et al., 1978;Case and Nelson, 1986; Wilson et al., 2006), mineral explo-ration in southwestern Alaska relies to a large extent on geo-physical techniques. Regional-scale aeromagnetic data may

be particularly useful because they cover a large area andporphyry copper deposits are commonly found in linear, oro-gen-parallel belts (Sillitoe and Perell, 2005; Sillitoe, 2010).Intrusions hosting such deposits are dominantly of magnetite-series affinity (Ishihara, 1981; Seedorff et al., 2005) and thesecan be mapped using magnetic data.

The present study shows that regional- and district-scaleaeromagnetic data are particularly useful in helping establishthe local geologic setting of the Pebble deposit. Filtering

techniques permit mapping of the igneous rocks associatedwith the Pebble deposit and allow the recognition of areaswithin the region with magnetic patterns similar to Pebble,several of which are associated with the outcrop of intrusive

rocks that are age equivalent to those hosting the Pebble de-posit. These data suggest the presence of a regional andlargely concealed Late Cretaceous magmatic arc in south-western Alaska that is highly prospective for porphyry copperdeposits similar in age to the 90 Ma Pebble deposit.

Geologic Setting

Regional geology

The geology of southwestern Alaska consists of a series ofamalgamated lithotectonic terranes that have been accretedto the North American craton since the Jurassic (Fig. 1; Wal-lace et al., 1989; Decker et al., 1994; Plafker and Berg, 1994;Hampton et al., 2010; Goldfarb et al., 2013). Overlapping

these exotic terranes are syn- to postcollisional flysch depositsand volcaniclastic strata that formed in the foreland basins ofthe accreting terranes (Wallace et al., 1989; Hampton et al.,2010; Goldfarb et al., 2013). Subduction of oceanic crust dur-ing the Jurassic and Cretaceous led to the formation of nu-merous volcano-plutonic complexes, some of which host por-phyry copper deposits.

The Pebble deposit is related to igneous rocks that postdatethe deposition of the Late Jurassic to Late CretaceousKahiltna assemblage (Wallace et al., 1989; Kalbas et al. , 2007;Hampton et al., 2010). Rocks of the Kahiltna assemblage crop

Geological Analysis of Aeromagnetic Data from Southwestern Alaska:Implications for Exploration in the Area of the Pebble Porphyry Cu-Au-Mo Deposit

ERIC D. ANDERSON,1,2, MURRAYW. HITZMAN,2 THOMAS MONECKE,2 PAUL A. BEDROSIAN,1

ANJANA K. SHAH,1 AND KAREN D. KELLEY1

1 U.S. Geological Survey, Mail Stop 973, Denver, Colorado 802252 Colorado School of Mines, Golden, Colorado 80401

Abstract

Aeromagnetic data are used to better understand the geology and mineral resources near the Late Creta-ceous Pebble porphyry Cu-Au-Mo deposit in southwestern Alaska. The reduced-to-pole (RTP) transformationof regional-scale aeromagnetic data shows that the Pebble deposit is within a cluster of magnetic anomalyhighs. Similar to Pebble, the Iliamna, Kijik, and Neacola porphyry copper occurrences are in magnetic highsthat trend northeast along the crustal-scale Lake Clark fault. A high-amplitude, short- to moderate-wavelengthanomaly is centered over the Kemuk occurrence, an Alaska-type ultramafic complex. Similar anomalies arefound west and north of Kemuk. A moderate-amplitude, moderate-wavelength magnetic low surrounded by amoderate-amplitude, short-wavelength magnetic high is associated with the gold-bearing Shotgun intrusivecomplex.

The RTP transformation of the district-scale aeromagnetic data acquired over Pebble permits differentiation

of a variety of Jurassic to Tertiary magmatic rock suites. Jurassic-Cretaceous basalt and gabbro units and LateCretaceous biotite pyroxenite and granodiorite rocks produce magnetic highs. Tertiary basalt units also pro-duce magnetic highs, but appear to be volumetrically minor. Eocene monzonite units have associated magneticlows. The RTP data do not suggest a magnetite-rich hydrothermal system at the Pebble deposit.

The 10-km upward continuation transformation of the regional-scale data shows a linear northeast trend ofmagnetic anomaly highs. These anomalies are spatially correlated with Late Cretaceous igneous rocks and inthe Pebble district are centered over the granodiorite rocks genetically related to porphyry copper systems. Thespacing of these anomalies is similar to patterns shown by the numerous porphyry copper deposits in northernChile. These anomalies are interpreted to reflect a Late Cretaceous magmatic arc that is favorable for addi-tional discoveries of Late Cretaceous porphyry copper systems in southwestern Alaska.

Corresponding author: e-mail, [email protected]

2013 Society of Economic Geologists, Inc.Economic Geology,v. 108, pp. 421436


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154W156W158W

60N

59N

50 km

Ordovician-Cretaceous sedimentary rocks of the

Farewell terrane

Triassic-Jurassic volcaniclastic and sedimentary

rocks of the Peninsular terraneJurassic plutonic rocks of the Talkeetna arc

Sedimentary rocks

Triassic metamorphic rocks

GlacierQuaternary deposits

Igneous and metamorphic rocks

Cretaceous-Paleogene volcano-plutonic rocks

Jurassic-Cretaceous flysch deposits (Kahiltna

assemblage and Kuskokwim Group); volcaniclastic

and sedimentary rocks of the Togiak terrane

Fault Mineral occurrence City

Kemuk

Shotgun

Pebble

Iliamna

Neacola

Iliamna

Cook

Inlet

Iliamna

Lake

BruinB

ayfaul

t

Mulch

atnafa

ult

LakeC

larkfau

ltChil

ikadrotna

Green

stone

s

Dillingham

Kijik

Bonanza

HillsHo

litnaf

ault

Figs. 2 & 5

Alaska U.S.A.

Canada

AnchorageStudyArea

Pacific Ocean

Arctic Ocean

154W156W158W

60N

59N

Togiak

terrane

Kahi

ltnabas

in

Cook

Inlet

154W156W158W

60N

59

N

Cook

Inlet

Taylor Mountains Lake Clark

Dillingham area

RioIliamna

A B C

D

Kusko

kwim

basin

Peninsular

terra

ne

terrane

Farewell

Pebble

FIG. 1. Location and regional geology maps of southwestern Alaska. A. Index map. B. Lithotectonic terrane map of theregion. Exotic oceanic terranes (Peninsular and Togiak terranes) were accreted to the southern margin of Alaska. These ex-otic terranes are overlapped by flysch deposits in the Kahiltna and Kuskokwim basins. The Pebble deposit is hosted in theflysch rocks in the Kahiltna basin. C. Index map showing the outlines of the regional aeromagnetic surveys. D. Generalizedgeologic map modified from Wilson et al. (2006). Triassic sedimentary rocks were intruded by Jurassic island-arc igneousrocks, and all are overlain by flysch deposits. These rocks were later intruded and overlain by Cretaceous to Paleogene vol-cano-plutonic rocks. Also shown are mineral occurrences in the study area and the locations for Figures 2 and 5.


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out along a ~400-km-long, NE-striking belt within theKahiltna basin (Fig. 1). They are mainly flysch deposits inter-preted to have been derived primarily from the igneous rocksof the Peninsular terrane to the southeast, with local contri-butions from the Triassic Chilikadrotna Greenstones to thenorthwest (Wallace et al., 1989; Wilson et al., 2006).

The Peninsular terrane is a Triassic to Jurassic island-arc

complex that was accreted to the North American craton bythe Early Cretaceous (Detterman and Reed, 1980; Jones etal., 1987; Ridgway et al., 2002; Trop et al., 2002, 2005; Clift etal., 2005). The terrane includes mafic to andesitic flows andvolcaniclastic rocks, limestone, and mudstone. These rocksstructurally overlie and are intruded by Jurassic plutonicrocks of the Talkeetna arc (Reed and Lanphere, 1973; Reedet al., 1983; Rioux et al., 2010). The plutonic rocks includegabbroic to granitic compositions, but are dominated byquartz diorite and tonalite rocks (Detterman and Reed, 1980;Reed et al., 1983).

The Togiak terrane, located west of the Peninsular terrane,is an island-arc complex that contains Late Triassic to EarlyCretaceous basaltic to dacitic volcanic and volcaniclastic rocks

and graywacke (Decker et al., 1994). These rocks were ac-creted to the North American craton during the middle tolate Early Cretaceous (Box, 1985).

The Peninsular and Togiak terranes are separated by theFarewell terrane (Fig. 1) that contains a Lower Paleozoicthrough Lower Cretaceous continental margin sequence ofsedimentary rocks (Decker et al., 1994). Flysch deposits ofthe Lower to Upper Cretaceous Kuskokwim Group uncon-formably overlie rocks of the Farewell and Togiak terranesand were structurally emplaced over the Kahiltna assemblage(Wallace et al., 1989; Decker et al., 1994).

Cretaceous to Paleogene igneous rocks crop out extensivelyin southwestern Alaska (Fig. 1; Reed and Lanphere, 1973; Wal-lace and Engebretson, 1984; Moll-Stalcup, 1994; Bundtzen

and Miller, 1997; Wilson et al. , 2006; Amato et al., 2007). Theintrusive rocks include granite, granodiorite, syenite, mon-zonite, and diorite. Limited gabbroic rocks have been mapped.The volcanic rocks are mostly of rhyolitic, dacitic, and basalticcompositions.

Several NE-striking crustal-scale faults have been recog-nized in southwestern Alaska (Fig. 1; Detterman et al., 1976;Beikman, 1980). The Lake Clark fault extends from LakeClark northeast into the Alaska-Aleutian Range batholith(Detterman et al., 1976). Based on offsets measured on

aeromagnetic anomaly maps, Haeussler and Saltus (2005) es-timated that there has been approximately 26 km of dextraloffset along the Lake Clark fault since the Eocene. Ivanhoe(1962) suggested that approximately 10 km of offset has oc-curred since the Tertiary, based on offsets of geologic units.The Pebble district is located southwest of the mapped extentof the Lake Clark fault. The fault has not been recognized

west of Lake Clark (Detterman and Reed, 1973, 1980; Plafkeret al., 1975; Koehler, 2010).The Mulchatna fault is parallel to the Lake Clark fault

(Beikman, 1980). In general, the fault coincides with theboundary between the Kuskokwim and the Kahiltna basinsand is interpreted to have significant dextral strike-slip and/ordip-slip displacement since the Late Cretaceous to Paleocene(Wallace et al., 1989). The Bruin Bay fault separates rocks ofthe Alaska-Aleutian Range batholith from sedimentary rocksto the southeast. The fault is interpreted as a high-angle re-verse fault (Detterman and Hartsock, 1966; Detterman et al.,1976). The Holitna fault is interpreted to be northwest sideup high-angle reverse with a dextral component (Wilson etal., 2006).

Known mineral occurrences in the Pebble region

The Pebble region hosts numerous mineral occurrences(Fig. 1; Table 1). In addition to the Pebble deposit, porphyry-type mineralized rocks have been recognized in the Kahiltnabasin at the Kijik, Neacola, and Iliamna prospects. The Kijikprospect contains veins and disseminated pyrite, pyrrhotite,chalcopyrite, and galena in porphyritic dacite that has under-gone extensive propylitic and silicic alteration (Nelson et al.,1985; Young et al., 1997; U.S. Geological Survey, 2008). AtNeacola, disseminated pyrite and bornite are found in a gra-nodiorite with K/Ar hornblende ages of approximately 95 Ma(Young et al., 1997; U.S. Geological Survey, 2008). The Il-iamna Cu (Au-Mo) occurrence has porphyry-style mineral-

ized rock that is of uncertain age (J. Harrop, writ. commun.,2011).The Kuskokwim basin and Togiak terrane also host a num-

ber of mineral occurrences (Fig. 1). The Shotgun occurrencecontains arsenopyrite, chalcocite, chalcopyrite, pyrrhotite,and sphalerite in quartz veins that cut hornfels adjacent to 70to 68 Ma granitic intrusions (Rombach and Newberry, 2001).Arsenopyrite- and stibnite-bearing quartz veins cutting dior-ite, quartz monzonite, and quartz diorite are common at theBonanza Hills prospect (U.S. Geological Survey, 2008). The

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TABLE 1. Mineral Deposits and Occurrences in Southwestern Alaska

Name Commodities (minor) Type Age (Ma) Reference

Bonanza Hills Au, Ag Polymetallic vein 64 Nelson et al. (1985), Eakins et al. (1978)

Iliamna Cu (Au, Mo) Porphyry unknown J. Harrop (writ. commun., 2011)

Kemuk PGE, Fe, Cu, Zn Alaska-type ultramafic complex 86 Humble Oil Refining Company (1959),Foley et al. (1997), Iriondo et al. (2003)

Kijik Cu, Au, Mo, Pb, Zn Porphyry Early Tertiary(?) Nelson et al. (1985)

Neacola Cu, Mo Porphyry 95 Young et al. (1997)

Pebble Cu, Au, Mo (Ag, Pd, Re) Porphyry 90 Lang et al. (2013)

Shotgun Au (As, Bi, Ce, Cu, T, W, Zn) Reduced intrusion-related gold; 70 Rombach and Newberry (2001)porphyry


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quartz monzonite has a K/Ar biotite age of approximately 64Ma (Eakins et al., 1978). The Kemuk prospect containsanomalous platinum, titaniferous magnetite, chalcopyrite, na-tive copper, and sphalerite within an Alaska-type ultramaficcomplex that is composed of clinopyroxene-, olivine-, andhornblende-bearing rocks (Humble Oil Refining Company,1959; Foley et al., 1997; U.S. Geological Survey, 2008). Bi-

otite from the ultramafic rocks has been dated at 86 Ma(Iriondo et al., 2003). The occurrence is buried by 30 m tomore than 140 m of glacial overburden and was identified bydrilling that followed up an aeromagnetic anomaly (HumbleOil Refining Company, 1959).

Geology of the Pebble district

The Pebble district contains igneous rocks that range fromJurassic to early Tertiary in age (Gregory and Lang, 2009;Lang et al., 2013). These rocks intrude Jurassic to Cretaceousflysch deposits of the Kahiltna assemblage. Associated withthese igneous rocks are the Pebble porphyry Cu-Au-Mo de-posit and several other porphyry-related occurrences (Fig. 2).

The Pebble deposit is spatially and temporally associatedwith igneous rocks that were emplaced during multiplepulses of magmatism. The oldest igneous rocks in the dis-trict are Jurassic-Cretaceous basalt and related gabbroic in-trusions (Bouley et al., 1995). These rocks crop out at the

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15510'W15520'W15530'W

5955'N

5948'N

5 km

Kaskanak

batholith

EastG

raben

Koktuli

Mtn

37 Zone

308 Zone

38 Zone65 Zone

Sill Zone

Pebble

Tertiary

Volcaniclastic rocksBasalt, andesite, latite,

rhyolite

Monzonite

Granodiorite

Monzonite

Monzodiorite

Diorite Gabbro

Cretaceous Jurassic-Cretaceous

Mineral Occurrence Porphyry Epithermal Skarn Pebble Fault

Flysch

Pyroxenite Basalt

FIG. 2. Generalized geologic map of the Pebble district. Jurassic-Cretaceous basalt and related gabbroid intrusions andflysch deposits were intruded by Cretaceous igneous rocks. The Pebble deposit is associated with calc-alkaline stocks that aregenetically related to granodiorite rocks of the Kaskanak batholith. Tertiary-aged monzonite intrusions and bimodal volcanicrocks are also within the district.


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northern and southern ends of the district. Biotite pyroxen-ite and diorite sills, cropping out to the southwest of the de-posit, were emplaced about 98 to 96 Ma (Lang et al., 2013).Diorite sills host much of the porphyry-style ores. Drillingindicates that the Pebble deposit is genetically related to atleast two granodiorite to quartz monzodiorite stocks that un-derlie the eastern and western sides of the deposit and have

U-Pb dates of 91 to 89 Ma (Lang et al., 2013). The mainlyunaltered Kaskanak batholith, composed almost entirely ofgranodiorite, crops out 5 km west of the Pebble deposit andhas a U-Pb date of 90 Ma (Bouley et al., 1995).

The host rocks of the Pebble deposit have been exten-sively altered (Lang et al., 2013). The earliest type of hydro-thermal alteration is sodic-calcic. The sodic-calcicalteredrocks are most strongly developed at depth where the min-eral assemblage is characterized by calcite, albite, chlorite,rutile, and epidote. Primary magnetite in the host rocks wasaltered to hematite. The sodic-calcic alteration assemblagegrades upward into a K-silicate alteration assemblage that ischaracterized by K-feldspar, biotite, and quartz, but lacksmagnetite. Zones of sodic-calcic and potassic-altered

rocks are surrounded by a weak propylitic alteration halo indiorite sills that locally contain hydrothermal magnetite.Pervasive illite-altered rocks overprint both the sodic-calcicand potassic alteration assemblages. The Pebble depositconsists of sulfides that are both disseminated and inquartz-carbonate stockwork veins. Dominant sulfide miner-als are pyrite, chalcopyrite, bornite, and molybdenite (Langet al., 2013).

The Pebble district contains several other Cretaceous por-phyry-related occurrences, including the 37, 308, 38, and 65zones (Fig. 2; Lang et al., 2013). The 37 zone is within dioriteand gabbro and consists of Cu-Au mineralized rocks associ-ated with magnetite-bearing, garnet-, pyroxene-, carbonate-,and epidote-calc-silicatealtered rocks and sulfide-bearing

veins. The 308 and 38 zones are in similar settings with horn-blende-quartz-monzodiorite to granodiorite porphyry hostrocks that are cut by porphyritic monzonite. At 308 and 38,Cu-Au-Mo mineralized rocks are associated with K-sili-catealtered rocks that are surrounded by quartz-sericite-pyrite- and propylitic-altered rocks. The 65 zone containsanomalous molybdenum, copper, and gold in an area of in-tensely altered rocks that includes quartz-sericite-pyrite,propylitic, and lesser K-silicate assemblages.

The Pebble district has undergone multiple episodes offaulting (Lang et al., 2013). A pre-, syn-, and posthydrother-mal brittle-ductile fault zone trends northeast along the east-ern side of the Pebble deposit. Eocene and older brittle ex-tensional faulting led to the formation of the NE-trending

East Graben (Fig. 2). Following its formation, the Pebble de-posit was tilted 20 E and the eastern part of the deposit wasdownthrown as much as 900 m.

Post-Pebble age rocks include porphyritic monzonite intru-sions that were emplaced at ~65 Ma into basalts of unknownage in the East Graben (Lang et al., 2013). Monzonite intru-sions and bimodal volcanic rocks were formed at ca. 46 Maand are concentrated in the area of Koktuli Mountain (Langet al., 2013). The bimodal volcanic rocks are associated withthe Sill zone, a low-sulfidation, epithermal-style mineralizedzone dated at ~46 Ma (Schrader et al., 2001).

Aeromagnetic Data

Aeromagnetic data have long been used as an aid in map-ping and understanding porphyry-style deposits (Grant, 1985;Woods and Webster, 1985; Roy and Clowes, 2000; Behn et al.,2001; Hildenbrand et al., 2001; Hoschke, 2008). Magneticanomalies are caused by two different kinds of magnetism: in-duced and remanent (permanent). The induced component

is mostly determined by the magnetic susceptibility of therock, which is a dimensionless proportionality constant re-ported in International System of Units (SI) that indicates theability of the rock to become magnetized in the presence of amagnetic field and is predominantly a function of magnetitecontent. The remanent component depends on the thermal,mechanical, and magnetic history of the rock and is indepen-dent of the field in which it is measured. In general, the in-duced component is predominant but the reverse is true inmany igneous rocks. Rocks with higher magnetic susceptibil-ities produce magnetic anomalies with larger variation in theobserved magnetic field than do rocks with lower magneticsusceptibilities (Table 2).

Granitoids can be classified as being either of two series:

magnetite or ilmenite (Ishihara, 1981). Magnetite series rocksare typically derived from oxidized mafic melts generated inthe mantle wedge during subduction processes. Ilmenite se-ries magmas may be similarly derived, but they have contri-butions from continental crustal rocks that provide carbon asa reducing agent (Ishihara, 1981). The magnetite and il-menite series rocks can be distinguished by their magnetiteand ilmenite content with magnetite series containing morethan 0.1 vol % magnetite and ilmenite series having an il-menite + magnetite content of less than 0.1 vol % (Ishihara,1981). Thus, magnetite series rocks have relatively highermagnetic susceptibilities. Magnetite and ilmenite series plu-tonic belts in the northern North America Cordillera havebeen mapped by integrating lithological, geochemical, and

geochronometric data with aeromagnetic characteristics andmagnetic susceptibility measurements (Hart et al., 2004). Theigneous rocks associated with the Pebble porphyry copper


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TABLE 2. Magnetic Susceptibility of Common Igneous Rocks(from Ford et al., 2008)

Magnetic Susceptibility (SI 103)

Rock type Min. Max. Average

Rhyolite 0.2 35 N.A.Andesite N.A. N.A. 160Granite 0 50 2.5Quartz diorite, dacite 38 191 83

Diorite 0.6 120 85Diabase 1 160 55Basalt 0.2 175 70Gabbro 1 90 70Peridotite 90 200 250Pyroxenite N.A. N.A. 125Monzonite, latite 33 135 85Acid igneous rocks 0 80 8Basic igneous rocks 0.5 97 25Trachyte 0 111 49Syenite 0 111 49

N.A. = not available


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deposit belong to the magnetite series (Lang et al., 2013) andtherefore are readily recognized in aeromagnetic surveys.

Aeromagnetic data may be collected at multiple resolu-tions. Data resolution depends on both flight line spacing andthe height above ground that the survey is flown. High-reso-lution data are collected along closely spaced flight lines at alow observation height and provide a more detailed map of

the distribution of causative sources than do lower resolutiondata collected at higher observation heights along morewidely spaced flight lines. Because the strength of the mag-netic field decreases with the cube of the distance to thesource, a shallow source body has a shorter wavelength mag-netic anomaly than a deep source body. Thus, the variationsin wavelengths of magnetic anomalies can help to determinethe depth to a causative body. The contrasting magnetic pat-terns within an aeromagnetic data set allow for differentiationof geologic units that have different magnetic properties andsusceptibilities.

Regional-scale aeromagnetic data from southwestern Alaska

Five regional-scale aeromagnetic surveys were flown in the

Pebble region along NW-trending flight lines (Fig. 1; Table3). The data from the individual surveys (Connard et al.,1999; U.S. Geological Survey, 2002, 2006a, b) were mergedinto a single, continuous data set with a grid cell size of 400 m(Anderson et al., 2011) and were processed using industry-standard techniques (Luyendyk, 1997). The resulting data setcovers approximately 82,500 km2 over four 1:250,000-scalequadrangle map sheets (Taylor Mountains, Lake Clark,Dillingham, and Iliamna).

The merged regional aeromagnetic data were furtherprocessed utilizing the reduced-to-pole (RTP) transformmethod (Baranov and Naudy, 1964; Blakely, 1995) to betteralign magnetic anomalies with causative geology. The RTPtransformation is a technique that recalculates total magnetic

intensity data as if the inducing magnetic field had a 90 in-clination, as is the case at the north magnetic pole. This oper-ation removes the dependence of magnetic data on the mag-netic inclination and thus minimizes anomaly asymmetry dueto magnetic inclination and locates anomalies above theircausative bodies. The RTP transformation was applied usingan inclination of 72.6 and declination of 18.5. Because ofthe relatively steep inclination of the magnetic field in south-western Alaska, there is only minimal difference between theobserved magnetic anomaly and the RTP transformation.

The RTP transformation of the regional aeromagnetic datashows a number of contrasting magnetic anomalies (Fig. 3).The dominant magnetic trend is NE-SW, parallel to the major

crustal-scale faults and belts of exposed igneous rocks. Thepatterns in the magnetic anomaly data generally correlatewith known lithotectonic terranes. The area between theLake Clark and Mulchatna faults is characterized by bothhigh and low magnitude magnetic anomalies within the sedi-mentary rocks of the Kahiltna basin. The Togiak terrane ismagnetically similar to the Kahiltna basin and contains two

prominent magnetic highs. The igneous rocks to the south ofthe Lake Clark fault are characterized by magnetic highs. Tothe northwest of the Mulchatna fault, sedimentary rocks ofthe Kuskokwim basin display broad, moderate magnitudemagnetic anomalies.

The Kahiltna basin shows a complex magnetic pattern. Thebasin has a relatively low magnetic background with scat-tered magnetic highs of various dimensions and amplitudes.The broad magnetic low correlates with both sedimentaryrocks and some Cretaceous to Paleogene igneous rocks(Figs. 1, 3). The broad magnetic low has been referred to asthe southern Alaska magnetic trough and interpreted to re-flect Mesozoic sedimentary rocks (Saltus et al., 1999). Insharp contrast, clusters of magnetic anomaly highs align

northeast-southwest along the Lake Clark fault. The Pebbledistrict is contained within one such high, where Cretaceousto Paleogene igneous rocks crop out (Fig. 1), suggesting thatCretaceous to Paleogene igneous rocks have both a low andhigh magnetic signature. Southwest of the Pebble district,clusters of magnetic highs are present beneath Quaternarycover. These highs are composed of moderate- to high-am-plitude and moderate- to long-wavelength anomalies. Thehighs are ovoid in shape with axial dimensions of about 4030 km and long axes trending N-NE, and thus are transverseto the Lake Clark fault. The highs are spaced between 20 and60 km apart. Between the highs are low- to moderate-ampli-tude, short-wavelength anomalies that are commonly associ-ated with mapped Cretaceous to Paleogene igneous rocks. A

northeasterly linear trend of low- to moderate-amplitude,short-wavelength magnetic high anomalies 10 to 20 km southof the Mulchatna fault corresponds to the ChilikadrotnaGreenstones.

The Peninsular terrane is dominated by magnetic anomalyhighs. The magnetic highs have been referred to as the south-ern Alaska magnetic high and interpreted to reflect arc-re-lated rocks and their basement (Saltus et al., 1999, 2007). Thehigh-amplitude, short- to moderate-wavelength anomalies ex-tend for more than 200 km along a NE-SW trend. The conti-nuity of the magnetic highs is suggestive of a broad belt ofmagnetite-rich rocks. Two N-NEtrending linear magneticlows are within the magnetic highs. The southeastern low

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TABLE 3. Survey Specifications for Regional- and District-Scale Aeromagnetic Data Sets

Survey Year Nominal height (m) Flight line spacing (m) IGRF1 Area (km2) Reference

Taylor Mountains 2004 305 1600 2000 18,870 U.S. Geological Survey (2006a)Lake Clark 1977 305 1600 1980 21,130 Connard et al. (1999)Dillingham 2005 305 1600 2000 20,716 U.S. Geological Survey (2006b)Rio 2000 305 1600 1995 9,553 U.S. Geological Survey (2002)Iliamna 2000 305 1600 1995 16,207 U.S. Geological Survey (2002)Pebble 2007 60 200 2007 426 Unpublished

1 IGRF = International Geomagnetic Reference Field


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correlates with steeply dipping sedimentary rocks alongthe Bruin Bay fault. The northern low follows the contactbetween Talkeetna arc rocks and younger volcano-plutonicrocks, but does not show a direct correlation with mappedrock types. A broad, long-wavelength magnetic anomaly highis observed east of the Bruin Bay fault, suggesting a relativelydeep magnetic source related to the Talkeetna arc beneaththe Cook Inlet (Saltus et al., 2007).

The Kuskokwim basin is characterized by a broad, moder-ate-amplitude, long-wavelength magnetic anomaly high. Themagnetic pattern suggests a relatively deep magnetic source.The Kuskokwim basin also shows moderate-amplitude, mod-erate-wavelength magnetic lows surrounded by moderate-amplitude, short-wavelength magnetic highs, as is illustrated

by the gold-bearing Shotgun intrusive complex. Moderate-amplitude, short-wavelength magnetic highs surrounded bymoderate-amplitude, short-wavelength magnetic lows arefound 40 km to the east and west of Shotgun. In both loca-tions, there are limited exposures of Cretaceous to Paleogeneigneous rocks (Fig. 1). Also, 30 km to the south of Shotgun isa high-amplitude, short- to moderate-wavelength magnetichigh that correlates with outcropping Cretaceous to Paleo-gene igneous rocks. The Bonanza Hills produce a moderate-amplitude, short-wavelength magnetic high surrounded by aring of magnetic lows.

The Togiak terrane is dominated by two prominent high-amplitude, short- to moderate-wavelength high magneticanomalies (Fig. 3). One of the anomalies is caused by near-surface magnetite-rich pyroxenite that has been drilled at theKemuk occurrence where laboratory results indicate that re-manent magnetization predominates over induced (HumbleOil Refining Company, 1959; Foley et al., 1997). The largeranomaly 35 km to the southwest is inferred to be related to~84 Ma biotite granite and syenite (Iriondo et al., 2003).

In addition to the RTP transformation, an upward continu-ation transformation of the aeromagnetic data was performedto calculate the magnetic field at an elevation higher than thatat which it was originally measured. This transformation at-tenuates near-surface effects and accentuates the anomalies

from deeper magnetic sources (Kellogg, 1953; Blakely, 1995).Thus, this transformation highlights the longer spatial wave-length anomalies at the expense of the shorter wavelengthanomalies. The aeromagnetic data from southwestern Alaskawere upward continued from the nominal survey height of305 to 10,000 m (Fig. 4).

The upward continued data from the Kahiltna basin show alinear trend of magnetic anomaly highs that correlate withknown porphyry-style occurrences (Fig. 4; from southwest tonortheast: Iliamna, Pebble, Kijik, and Neacola). Intrusions atPebble and the Neacola prospect have similar Late Cretaceous


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59N

50 km

108 168 207 241 264 283 301 318 332 347 359 371 381 402 458 531 618 729 916

nT

Kemuk

Shotgun

Pebble

Iliamnaproperty

Neacola

Iliamna

Brui

nBa

yfault

Mulch

atnafa

ult

LakeC

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ltCh

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otnaG

reenstone

s

Dillingham

Kijik

Bonanza

Hills

Peninsu

larterra

ne

Farewell terrane

Kuskokwim basin

Kahi

ltnab

asin

Togiak

terrane

Holitn

afault

Cook

Inlet

Fig. 8

FIG. 3. Map showing the RTP transformed aeromagnetic data. The Pebble deposit is located within a cluster of magneticanomaly highs. Similar clusters are found to the northeast and southwest along the crustal-scale Lake Clark fault. Dashed

white lines indicate lithotectonic terranes from Figure 1. Location for Figure 8 is shown.


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isotopic ages (Young et al., 1997; Lang et al., 2013). The up-ward continued magnetic anomalies in the Peninsular terranecorrelate with igneous rocks of Jurassic age. South of the LakeClark fault, magnetic highs in the Peninsular terrane corre-late with a cluster of intrusive rocks with ages similar to thoseof the Pebble deposit. The magnetic anomalies in theKuskokwim basin and Togiak terrane are similar. The twomagnetic intrusive centers in the Togiak terrane have radio-metric dates similar to the age of the Pebble deposit. Themagnetic anomaly 30 km to the south of Shotgun does nothave a known dated igneous rock. The 70 to 60 Ma intrusive

rocks associated with Shotgun and Bonanza Hills are not evi-dent in the upward continued data, nor are many of the ig-neous rocks in the Kuskokwim and Kahiltna basins with ra-diometric dates between 74 and 55 Ma.

Aeromagnetic data from the Pebble district

A high-resolution aeromagnetic survey was flown over thePebble district in 2007 (Table 3). Flight lines were spaced 200m apart at a nominal height of 60 m above the surface. Thedata were processed using industry standard data processingtechniques (Luyendyk, 1997).

The RTP map of these data shows many moderate- to high-amplitude, short-wavelength anomalies characteristic of near-surface magnetic sources (Fig. 5). Superimposed on thispattern are both moderate- and high-amplitude, long-wave-length anomalies, suggestive of deeper magnetic sources. Themagnetic anomalies mostly trend north to northeast.

The NE-striking, high-amplitude (~3000 nT), long-wave-length magnetic highs in the southwestern corner of the districtcorrelate with mapped basalt and gabbro units (Fig. 5). Simi-lar amplitude anomalies are found on the northern side of theKaskanak batholith, where they also correlate with known ex-

posures of basalt and gabbro. Neither the basalt nor gabbrounits have reported magnetite content or magnetic susceptibil-ities, but magnetic susceptibilities are inferred to be relativelyhigh (Table 2). The longer wavelength anomalies associatedwith the mapped basalt may indicate the rocks are relativelythick or underlain by gabbro. Both sets of anomalies are su-perimposed on a moderate-amplitude (~800 nT), long-wave-length magnetic high anomaly. This broad magnetic anomalycorrelates with mapped granodiorite within the Kaskanakbatholith. The granodiorite has up to 2% magnetite (Bouleyet al., 1995) and a magnetic susceptibility of about 30 103

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RadiometricAges: 74 - 55 Ma 135 - 183 Ma101 - 84 Ma42 - 21 Ma

Kemuk

Shotgun

Pebble

Iliamna

property

Neacola

Iliamna

Brui

nBa

yfaul

t

Mulch

atnafa

ult

LakeC

larkfau

lt

Dillingham

Kijik

Bonanza

Hills

Peninsular

terra

ne

Farewell terrane

Kuskokwim basin

Kahi

ltnab

asin

Togiak

terrane

Holitn

afault

Fig. 8

250 289 302 312 321 329 339 348 353 360 368 382 412 441 475 513 548 593 647nT

FIG. 4. Map showing aeromagnetic data upward continued to 10 km. Also shown are radiometric age dates (U.S. Geo-logical Survey, 1999; Iriondo et al., 2003; Amato et al., 2007). Igneous rocks older than 84 Ma are associated with magneticanomaly highs, whereas younger rocks can be found on both magnetic anomaly highs and lows. Dashed white lines indicatelithotectonic terranes from Figure 1. Location for Figure 8 is shown.


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SI. The large magnetic low within the Kaskanak batholith cor-relates well with known volcaniclastic rocks (Fig. 2), indicat-ing they may be nonmagnetic.

The RTP data do not show a distinct magnetic anomalyover the Pebble deposit (Fig. 5). Instead, the data show a

magnetic gradient from high to low (west to east) over the de-posit. This may be due to an increase in hydrothermal mag-netite content in the shallower western part of the deposit(Lang et al., 2013). Magnetic susceptibility measurements indrill holes indicate a slight increase in magnetite along aneast-to-west traverse across the deposit (Fig. 6). Alternatively,the magnetic gradient may simply indicate that the granodi-orite of the Kaskanak batholith deepens to the east as estab-lished by drilling.

Zones with porphyry-style mineralization elsewhere in thedistrict display magnetic highs (Fig. 5). These magnetic

anomalies may be due to the presence of either hydrothermalmagnetite or relatively shallow underlying magnetic rocks.

The prominent high-amplitude, moderate-wavelengthmagnetic high in the center of the district (Fig. 5) correlateswith a mapped biotite pyroxenite unit. The biotite pyroxenite

has >10% magnetite (Bouley et al., 1995; Lang et al., 2013)and a magnetic susceptibility around 300 103 SI. Theanomaly is >8000 nT and more than an order of magnitudelarger than other anomalies in the district (Fig. 7). This mag-netic anomaly extends ~2.5 km to the south and ~1.5 km tothe east of the pyroxenite outcrop, suggesting that the pyrox-enite unit extends to the south and east under cover.

Moderate- to high-amplitude (~400 nT), short-wavelengthanomalies trend north to northeast through the district paral-lel to the trend of the East Graben (Fig. 5). These anomaliesare similar to those generated above Tertiary volcanic rocks


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A A

B B

-925 -390 -294 -217 -151 -86 -32 17 73 136 207 286 379 481 586 700 828 1010 9100

nT

gabbros

basalts

basalts

gabbros

granod

iorites

monzonitesKaskanak

batholith

Koktuli

Mtn

pyroxenit

es

EastG

raben

Mineral Occurrence Porphyry Epithermal Skarn Pebble

FIG. 5. Map showing RTP transformed aeromagnetic data for the Pebble district. The simplified geologic units from Fig-ure 2 are outlined in black. The broad magnetic anomaly high on the west side of the district is associated with granodioriterocks in the Kaskanak batholith. Igneous rocks older than the Kaskanak batholith show up as magnetic highs, whereas

younger igneous rocks are both magnetic highs and lows. The magnetic highs associated with Tertiary igneous rocks have sig-nificantly shorter wavelengths than the anomaly associated with the Kaskanak batholith. The Tertiary monzonite intrusionsproduce magnetic lows. Also shown are the locations of the profiles depicted in Figures 6 (A-A') and 7 (B-B').


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within the East Graben. Magnetic susceptibility measure-ments of these volcanic rocks indicate that they are highlymagnetic, but they have limited spatial extent (Fig. 6). Simi-lar magnetic anomalies to the north of Kaskanak batholithand to the northeast of Koktuli Mountain suggest the pres-ence of additional near-surface volcanic rocks.

A magnetic low spatially related to known outcrops of Ter-tiary monzonite rocks overlies Koktuli Mountain (Figs. 5, 7)and indicates that these rocks are less magnetic than the gra-nodiorite of the Kaskanak batholith. Low-sulfidation epither-mal style mineralized zones are found on the east side of Kok-tuli Mountain. Hydrothermal alteration associated with the

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2156000 N

2158000 N 2158000 N

+277 ft

-277 ft

1.0

3.1

5.1

7.2

9.2

11.3

13.3

15.4

17.4

19.5

1396000 E 1398000 E 1400000 E 1402000 E 1404000 E 1406000 E 1408000 E 1410000 E 1412000 E

-900 m

-600 m

-300 m

300 m

-400

-200

0

200

400

Susceptibility

103SI

A A

A A

A A

A

B

C

Reduced-to-Pole

Reduced-to-Pole

Long wavelength Short wavelength

TertiaryCretaceous

East Graben

nT

0 m

-1200 m

Hydrothermal magnetite?

Basalt

FIG. 6. Cross section A-A' through the Pebble deposit. A. Profile of the RTP data illustrating long- and short-wavelengthanomalies. B. Map view of the RTP data along the cross section. The resource outline is shown in white. Warm colors indi-cate magnetic highs and cool colors reflect magnetic lows. C. Cross section showing magnetic susceptibility measurementsin drillcore. In general, the deposit shows low magnetic susceptibility. However, elevated values in the shallow west side ofthe deposit suggest the presence of magnetite which may be contributing to the elevated RTP data. Tertiary volcanic rocks

show high magnetic susceptibilities within the East Graben that correlate with RTP anomalies. Also shown in C is a dashedline indicating the Cretaceous-Tertiary boundary. See Figure 5 for section location.

Cretaceous

granodiorites(Kaskanak batholith)

Tertiary

monzonites(Koktuli Mountain)

Cre

taceous

pyroxen

ites

Tertiary

basalts(East Graben)

B B

360000 365000 370000 375000 380000

Easting (meters)

RTP(nT)

8000

0

2000

4000

6000

FIG. 7. Magnetic field profile B-B' across the Pebble district. The amplitude of the magnetic field is dominated by py-roxenite. The Cretaceous granodiorite produces more intense magnetic anomalies than the Tertiary monzonite. Tertiarybasalt produces short-wavelength magnetic high anomalies. See Figure 5 for profile location.


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mineralization may have been magnetite destructive, reduc-ing the magnetization of the igneous rocks.

Discussion

The regional aeromagnetic data show that the Pebble dis-trict lies within a cluster of magnetic anomaly highs. The dis-trict-scale aeromagnetic data indicate that the magnetic highs

correlate with mapped biotite pyroxenite, granodiorite, gab-bro, and basalt. The biotite pyroxenite and granodiorite rockshave been dated between 98 and 89 Ma, which coincides withthe age of porphyry-style mineralization in the Pebble district(Lang et al., 2013). The Jurassic-Cretaceous gabbro andbasalt units have not been dated, but field mapping indicatesthat these rocks are older than the granodiorite rocks of theKaskanak batholith. Tertiary basalt within the East Grabenappears to be volumetrically minor compared to the granodi-orite rocks. The Tertiary monzonite rocks near Koktuli Moun-tain produce magnetic lows. Thus, the cluster of magnetichighs in the regional RTP data indicates that geology similarto that at Pebble extends under cover for an additional 30 kmnortheast of the Pebble deposit (Fig. 3).

The aeromagnetic response to the rocks in the Shotgun andBonanza Hills areas differs from the cluster of magnetic highssurrounding the Pebble deposit (Fig. 3). The Shotgun areashows moderate-amplitude, moderate-wavelength magneticlows surrounded by moderate-amplitude, short-wavelengthmagnetic highs. Such anomalies are commonly associatedwith reduced intrusion-related gold systems where plutonshave low magnetic response, but are surrounded by magnetichighs reflecting pyrrhotite in hornfels zones (Hart, 2007).Magnetic susceptibility measurements and geochemical datafrom the Shotgun intrusive complex suggest that the reducedgranitoids in the hornfelsed Kuskokwim Group rocks are il-menite series (Rombach and Newberry, 2001).

The magnetic anomalies surrounding Bonanza Hills showan annulus similar to that around Shotgun but the highs andlows are reversed. Similar magnetic anomalies are found 40km west and east of Shotgun. The magnetic anomalies sur-rounding Bonanza Hills are further complicated by the anom-alies associated with the nearby Chilikadrotna Greenstones.More work is needed to fully understand these magnetic

anomalies, but it is clear they differ significantly from theanomalies around Pebble.Palinspastic restoration of Alaska places the igneous rocks

associated with the Pebble deposit and Neacola prospectwithin a magmatic belt subparallel to a Late Cretaceous sub-duction zone (Young et al., 1997). Hart et al. (2004) suggestedthe presence of a magnetite-series magmatic belt betweenthe Pebble deposit and the Neacola prospect. The magnetic-field data support the presence of such a magmatic belt andsuggest it is extensive, with a length of more than 400 km(Fig. 4). The aeromagnetic data are interpreted to reflect thelocation of magnetite-series intrusive rocks within this belt.

The regional aeromagnetic data include clusters of magnetichighs similar to those at Pebble along a NE trend parallel to

the Lake Clark fault (Fig. 3). These clusters are most obviousin the upwardly continued data (Fig. 4). In the Pebble district,the upward continued anomaly is centered over the Kaskanakbatholith and the genetically related hydrothermal systemsare on the flanks (Fig. 8). The upward continuation anomaliesare suggestive of a relatively deep magnetic source (Blakely etal., 1985; Ressel and Henry, 2006; Saltus et al., 2007). Alter-natively, these anomalies could indicate the presence of volu-minous, magnetic rocks at the surface, such as basalt flows, al-though there is little geologic evidence to support thispossibility (Fig. 2). The upward continued magnetic anomalyin the Pebble district is interpreted to reflect the granodioriterocks of the Kaskanak batholith. On the flanks of the anomaly


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Kaskan

ak

batholith

5 km

KoktuliMtn

East Graben

C

15510'W15520'W15530'W

5955'N

5948'N

5 km

KoktuliMtn

East Graben

B

15510'W15520'W15530'W

5955'N

5948'N

5 km

KoktuliMtn

A

15510'W15520'W15530'W

59

55'N

5948'N

Kaska

nak

batholith

Kaska

nak

batholith

Kaska

nak

batholith

East Graben

FIG. 8. Comparison of the interpreted magnetic data sets over the Pebble district. Warm colors indicate magnetic highsand cool colors reflect magnetic lows. A. District-scale RTP data showing that magnetic anomaly highs are largely over theKaskanak batholith. Also evident are basalts in the East Graben. Younger monzonite rocks on Koktuli Mountain are imagedas a magnetic low. B. Zoomed in regional-scale RTP data showing that the Kaskanak batholith correlates with clusters of mag-netic anomaly highs. In contrast are the magnetic anomaly lows on Koktuli Mountain. The graben basalts are evident but areless resolved than in the district-scale data. C. Zoomed-in regional-scale upward continued data show that the anomaly is cen-tered over the Kaskanak batholith and that the Cretaceous age hydrothermal systems are found on the flanks of the anomaly.


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and batholith are several genetically related porphyry de-posits or occurrences (Pebble, 37, 308, 38, and 65 zones).Porphyry deposits in other districts commonly are found inclusters flanking a batholithic body (Seedorff et al., 2005).

The Kijik occurrence, 75 km to the northeast of the Pebbledistrict, is located on the edge of an upward continued anomaly.The intrusive rocks at this occurrence have not been dated.

However, spatially associated with this anomaly are threemafic to intermediate dikes with 40Ar/39Ar plateau ages fromhornblende between 101 and 97 Ma (Amato et al., 2007),which are only slightly older than the igneous rocks at Pebble.The geochemistry of these calc-alkaline dikes is similar toboth typical arc- and rift-related rocks. Whether mineraliza-tion at Kijik is genetically related to these calc-alkaline rocksis unknown, but the similar aged igneous rocks together withthe interpreted aeromagnetic signature make this area highlyprospective for porphyry-style deposits.

Approximately 80 km northeast of Kijik, and within anotherupwardly continued anomaly, is the Neacola porphyryprospect, with ~95 Ma K/Ar hornblende ages on associatedigneous rocks (Young et al., 1997). Like the Pebble district,

the Neacola hydrothermal system is located on the flanks ofan upward continued anomaly. Rocks with radiometric agedates between 101 and 84 Ma, similar to the intrusive rocksat Pebble, are exposed along the flanks of this anomaly.

Upward continued magnetic anomalies also extend to thesouthwest of the Pebble deposit (Fig. 4) in an area dominatedby Quaternary surficial deposits (Fig. 1). A magnetic anomaly90 km to the southwest of the Pebble district coincides withporphyry-style mineralization at the Iliamna prospect that is ofuncertain age. This hydrothermal system is on the flanks of theupward continued anomaly. The anomaly 40 km farther to thesouthwest of the Iliamna prospect has not been investigated.

The granitic intrusive rocks at Shotgun and Bonanza Hillshave radiometric age dates between 70 and 64 Ma (Eakins et

al., 1978; Rombach and Newberry, 2001). These rocks do notproduce magnetic anomalies within the upward continueddata, suggesting they lack a relatively deep magnetic source.

Upward continued anomalies also are present off-strike ofthe Lake Clark fault in the Togiak terrane and Kuskokwimbasin. The anomaly at Kemuk is associated with 86 Ma ultra-mafic rocks. The anomaly 35 km to the southwest of Kemukhas 84 Ma igneous rocks associated with it; however, little elseis known. The RTP data show the anomaly has a similar wave-length to the magnetic anomaly over Kemuk; however, theaxial dimension of the anomaly is significantly longer. TheRTP data show several highs along the axis of the compositeanomaly, suggesting a series of magnetic sources along theaxis. The highs have similar dimensions to the Kemuk anom-

aly and may indicate additional Alaska-type ultramafic com-plexes. Further work is needed to better understand thiscomplex anomaly. The magnetic anomaly 30 km to the southof Shotgun is similar to the prominent magnetic anomaly overKemuk. Both anomalies are elongated along a north-south di-rection approximately 15 km in size, and fringed by a mag-netic low. Although speculative, the similarities suggest thepoorly understood magnetic anomaly to the south of Shotgunmay represent another Alaska-type ultramafic complex.

High-resolution aeromagnetic data from the Pebble districtindicate that the area contains a variety of igneous rocks with

different magnetic properties. The oldest rocks associatedwith the Pebble age igneous event are 98 to 96 Ma biotite py-roxenites. These rocks are strongly magnetic and form a high-amplitude, moderate-wavelength magnetic high in the centerof the district. The youngest Cretaceous igneous rocks in thePebble area are the granodiorites of the Kaskanak batholith.These rocks correlate with moderate-amplitude, long-wave-

length magnetic highs that are interpreted to be the mainsource for the upward continued anomaly over the Pebbledistrict.

Porphyry deposits are associated with large volumes of hy-drothermally altered rock (Lowell and Guilbert, 1970; Silli-toe, 2010). Magnetite may be produced or destroyed duringalteration, thus making high-resolution aeromagnetic data apotentially effective exploration tool. The high-resolution,district-scale aeromagnetic data over the Pebble deposit donot show a distinct magnetic high or low; however, otherhydrothermal systems in the district do correlate with mag-netic highs. More work is needed to understand the magneticresponse of the hydrothermal systems in the district. In gen-eral, RTP transformation of the high-resolution data appears

better suited for mapping igneous rocks, rather than zones ofhydrothermal alteration.The geometry of the upward continued magnetic anom-

alies from the regional aeromagnetic data in the Pebble re-gion is similar to clusters of porphyry copper centers alongthe Domeyko fault system within the late Eocene to earlyOligocene porphyry copper belt of northern Chile (Fig. 9).These clusters are spaced 20 to 100 km apart and are locally5 to 30 km across (Sillitoe, 2010). The upwardly continuedmagnetic anomalies in the Pebble area of southwestern Alaskadisplay similar spacing and dimensions. The Alaska anomaliesappear to be aligned along the Lake Clark fault system, whichmay be analogous to the Domeyko fault system, an arc-paral-lel, crustal-scale structure.

Large aeromagnetic anomalies along the Domeyko faultsystem have been interpreted to reflect common parental in-trusive complexes above which porphyry copper depositscluster (Behn et al., 2001). Goldfarb et al. (2013) suggest thatthe Lake Clark fault system, which forms the boundary be-tween the Peninsular terrane and Kahiltna basin, provided acrustal-scale pathway for ascent and emplacement of subduc-tion-related magmas during the Late Cretaceous. Suchcrustal-scale structures are critical for the formation of por-phyry copper systems (Richards, 2003). The upward contin-ued magnetic anomalies in southwestern Alaska thus mayrepresent potential intrusions from which porphyry coppersystems evolved. The upward continued anomaly over thePebble district has five porphyry occurrences within it (Peb-

ble, 37, 308, 38, and 65 zones), suggesting multiple hydro-thermal systems within a single upward continued anomaly.Magmatic belts have been previously recognized in south-

western Alaska (Fig. 10). Wallace and Engebretson (1984)and Moll-Stalcup (1994) defined three subparallel NE-strik-ing magmatic belts: (1) the 74 to 55 Ma Alaska Range-Tal-keetna Mountains belt, (2) the 76 to 60 Ma KuskokwimMountains belt, and (3) the 66 to 47 Ma Yukon-Kanuti belt.Petrological, geochemical, and isotopic studies indicate thatthe igneous rocks in these belts are subduction related (Wallaceand Engbretson, 1984; Moll-Stalcup, 1994). The Kuskokwim

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Mountains belt largely coincides with the Kuskokwim mineralbelt (Bundtzen and Miller, 1997). The subduction-related184 to 164 Ma Talkeetna arc is oriented parallel to these belts(Reed and Lanphere, 1973; Reed et al., 1983; Rioux et al.,2010). Isotopic dating indicates that the belts generally youngto the northwest, suggesting landward arc migration (Moll-Stalcup, 1994; Rioux et al., 2010). The age of the intrusiverocks associated with the Pebble deposit is outside the rangeof ages of known magmatic arcs in the area (Fig. 10).

The upward continued magnetic anomalies along the LakeClark fault are interpreted to reflect a Late Cretaceous (10189 Ma) magnetite-series magmatic belt within which the Peb-ble porphyry copper deposit formed (Fig. 10). The magneticsignature of this belt differs significantly from the younger

magmatic belts; in particular, differing from the Late Creta-ceous to early Tertiary precious metal-bearing KuskokwimMountains belt where the plutonic rocks are largely ilmenite-series (Bundtzen and Miller, 1997) and further illustrated atthe gold-bearing Shotgun intrusive complex. The Jurassicplutonic rocks of the Talkeetna arc produce intense, relativelycontinuous magnetic highs that are interpreted to reflect arc-related rocks and their basement (Saltus et al., 1999, 2007).The newly recognized belt of magnetic anomaly highs is in-terpreted to reflect batholiths composed of magnetite-seriesigneous rocks around which hydrothermal systems may have

developed, as illustrated by the Kaskanak batholith and themultiple Late Cretaceous hydrothermal systems in the Peb-ble district. At a broad scale, the inferred igneous rocks asso-ciated with the magnetic anomalies support a landward(northwest) subduction related arc migration. Because por-phyry copper deposits are commonly found in linear, orogen-parallel belts (Sillitoe and Perell, 2005; Sillitoe, 2010), thisnewly recognized belt of magnetic anomalies might representa highly prospective area for porphyry Cu exploration in thenorthern North American Cordillera.

Conclusions

Large parts of southwestern Alaska that are extensively cov-ered by Tertiary and Quaternary deposits are highly prospec-

tive for porphyry-style deposits similar to the giant Pebbleporphyry Cu-Au-Mo deposit. Regional- and deposit-scaleaeromagnetic data provide a means of looking through youngercover rocks to delineate the distribution of magnetic igneousrocks. Analysis of regional aeromagnetic data indicates thatthe Pebble deposit is located within a cluster of magnetichighs. Magnetically similar clusters, some that contain intru-sive rocks with ages similar to those at Pebble, form a >400-km-long belt extending to both the northeast and southwestof Pebble. These clusters are concentrated along the crustal-scale Lake Clark fault. Upward continuation analysis of these


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FIG. 9. Spatial comparison of porphyry copper deposits in northern Chile and southwestern Alaska. Porphyry copper de-posit clusters in northern Chile and upward continued anomalies in southwestern Alaska are outlined in black and overlainon digital elevation models. The porphyry clusters in northern Chile have been attributed to a common parental intrusivecomplex at depth that is imaged in magnetic anomalies (Behn et al., 2001). Both the clusters of porphyry deposits in Chileand the upward continued anomalies in southwestern Alaska are concentrated along major, crustal-scale structures. Suchstructures may provide pathways for intruding magmas associated with porphyry deposit formation.

50 km

50 km

Northern Chile

Southwest Alaska

Domeyko fault system

Lake Clark fault

A

B

Copaquire

Conchi

Pebble

Ujina

Collahuasi

Quebrada Blanca

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Esperanza

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anomalies suggests that they are associated with relatively deepmagnetic sources, such as the Kaskanak batholith in the Peb-ble district, and that hydrothermal systems are located on theflanks of the anomalies. The geometry of the upward contin-

ued anomalies shows similarities to the late Eocene to earlyOligocene porphyry copper belt of northern Chile. The up-ward continued anomalies are thus interpreted to reflect amainly buried, early Late Cretaceous magmatic arc (10189Ma) that is subparallel to younger Late Cretaceous to Paleo-gene magmatic belts to the northwest (present-day coordi-nates) and to an older Jurassic magmatic arc to the southeast.The subparallel orientation of the early Late Cretaceous mag-matic arc to the younger and older arcs, as well as a landward(northwest) younging trend, indicates similar subductionzone geometry. Thus, the upward continued anomalies may

represent subduction-related oxidized magmatism prior tolandward arc migration where the intrusions become morereduced in character.

Acknowledgments

The Pebble Limited Partnership between Northern Dy-nasty Mineral and Anglo American is thanked for access todata and logistical support for field work. We are grateful toJames Lang for his comments on the igneous petrology andgeology of the Pebble deposit. Misac Nabighian is thankedfor his assistance with the aeromagnetic data processing andreviews of the manuscript. Reviews by Dave John, TerryHoschke, Howard Golden, William Atkinson, Jr., and RichGoldfarb have substantially improved earlier versions of themanuscript.

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155W160W165W

160W

165W

60N

200 km

65N

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map

exten

t

Yukon-Kanuti (66 - 47 Ma)

Kuskokwim Mountains (76 - 60 Ma)

Alaska Range - Talkeetna

Mountains

(74 - 55 Ma)

Talkeetna arc (184-164

Ma)

Shotgun

Kemuk

Pebble NeacolaKijik

Bonanza Hills

PebbleAnchorage

CookInlet

Iliamnaproperty

-143 -105 -84 -69 -56 -46 -37 -30 -23 -16 -8 0 12 25 38 56 80 120 194

nT

FIG. 10. Map showing compilation of aeromagnetic data upward continued to 10 km (Saltus, unpub. data) and previouslyrecognized magmatic belts in southwestern Alaska (Reed and Lanphere, 1973; Wallace and Engebretson, 1984; Moll-Stal-cup 1994; Bundtzen and Miller, 1997). The upward continued anomalies described here are subparallel to the known mag-matic belts and are attributed to magnetite-series granitoids that are along the landward margin of the Jurassic Talkeetna arc.The intense magnetic anomalies in the Talkeetna arc reflect island arc-related rocks and their basement and extend acrossCook Inlet (Saltus et al., 1999, 2007). The younger magmatic belts, most notably the Kuskokwim Mountains where plutonicrocks are largely ilmenite-series, are not as evident in the upward continued data. The white polygon depicts the trend of theupward continued anomalies in southwestern Alaska that may be intrusive centers of similar igneous rocks as found at thePebble deposit.


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