Chapter 8 Magmatic-Hydrothermal-Structural Evolution of the … · 2020. 8. 8. · 167 Introduction...

167

IntroductionTHE PEBBLE deposit is located 320 km southwest of Anchor-age in southwest Alaska (Fig. 1). Resources in all categories atPebble total 10.78 billion metric tons (Bt) of mineralized rockwhich contain 80.6 billion pounds (Blb) copper, 5.6 Blb ofmolybdenum, and 107.4 Moz of gold (Table 1; Gaunt et al.,

2010). The Pebble deposit is the only significant porphyry de-posit thus far discovered in southwest Alaska and an under-standing of its principal characteristics is essential to develop-ing exploration strategies in this highly prospective but largelycovered and underexplored region.

The Pebble deposit formed at about 90 Ma in associationwith subduction-related hornblende granodiorite porphyryintrusions. It comprises the East and West zones, which areapproximately equal in size, with slightly lower grade miner-alization in the center of the deposit where the peripheries ofthe two zones merge. The West zone, which was discovered

Chapter 8

Magmatic-Hydrothermal-Structural Evolution of the Giant Pebble Porphyry Cu-Au-Mo Deposit with Implications for Exploration in Southwest Alaska

JAMES R. LANG1,† AND MELISSA J. GREGORY2,*1 Hunter Dickinson Inc., 15th Floor, 1040 West Georgia Street, Vancouver, British Columbia, Canada V6C 2V6

2 Pebble Limited Partnership, P.O. Box 267, Iliamna, Alaska 99606

AbstractThe Pebble deposit is located ~320 km southwest of Anchorage, Alaska. It is one of the largest porphyry

deposits known, with a total resource of 10.78 billion metric tons (Bt) of mineralized rock divided between thecontiguous West and East zones.

The oldest rocks in the Pebble district are Jurassic-Cretaceous Kahiltna flysch, which contains interbeddedbasalt and associated gabbro intrusions. These were cut between 99 and 96 Ma by coeval granodiorite and dior-ite sills, followed by alkalic intrusions and related breccias. Subalkalic hornblende granodiorite porphyry plu-tons were emplaced at ~90 Ma and include the Kaskanak batholith and smaller stocks related to Cu-Au-Momineralization. Porphyry mineralization has been dated between 89.5 and 90.4 Ma by Re-Os on molybdenite.Late Cretaceous volcanic and sedimentary rocks completely conceal the East zone. Eocene volcanic rocks andsubvolcanic intrusions occur east and southeast of the Pebble deposit and glacial sediments are widespread.

The East and West zones represent two coeval hydrothermal centers within a single system. The West zoneextends from surface to ~500-m depth and is centered on four small granodiorite plugs emplaced into flysch,diorite and granodiorite sills, and alkalic intrusions and breccias. The much higher grade East zone is hostedby the larger East zone granodiorite pluton and adjacent granodiorite sills and flysch and extends to at least1,700-m depth below surface. The granodiorite intrusions merge with depth. Lower grade, less extensive min-eralization occurs in the center of the deposit where the peripheries of the East and West zones converge. Onthe eastern side of the deposit, faulting dropped high-grade mineralization 600 to 900 m into the NE-trendingEast graben where the deposit remains open.

Variations in hypogene grade and metal ratios reflect multiple stages of metal introduction and redistribu-tion. Premineralization hornfels formed around the Kaskanak batholith. Early disseminated and vein-hostedCu-Au-Mo mineralization formed with potassic alteration in the East zone and sodic-potassic alteration in theWest zone. In the East zone, potassic alteration is underlain by weakly mineralized sodic-potassic ± calcic alteration. Slightly younger quartz veins introduced additional molybdenum. Illite ± kaolinite alteration over-printed the early alteration assemblages and variably redistributed copper and gold. Late-stage advancedargillic alteration is associated with high-grade Cu-Au mineralization in the East zone; it was controlled by asynhydrothermal brittle-ductile fault zone and comprises a core of pyrophyllite alteration associated with chal-copyrite bounded to the west by sericite alteration with hypogene bornite, digenite, covellite, and trace enar-gite and tennantite. Copper was removed by quartz-sericite-pyrite alteration that forms a halo to the depositand yields outward to propylitic alteration. A weakly mineralized quartz-illite-pyrite cap is preserved in theupper central part of the deposit. Weak supergene mineralization is present only in the West zone where theLate Cretaceous cover sequence was eroded. The large size and high hypogene grades of the Pebble depositmay reflect a combination of multiple stages of metal introduction with vertically restricted, lateral fluid flowinduced by hornfels aquitards in flysch.

The Pebble deposit occurs in one of several large, deep-seated magnetic anomalies which occur at the intersection of crustal-scale structures both parallel and at high angles to an arc, which formed in southwestAlaska during the Cretaceous. This setting is similar to fertile porphyry environments in northern Chile andsuggests that southwestern Alaska is highly prospective for porphyry exploration.

† Corresponding author: e-mail, [email protected]*Present address: Mineral Deposit Research Unit, Department of Earth

and Ocean Sciences, University of British Columbia, 6339 Stores Rd, Van-couver, British Columbia, Canada V6T 1Z4.

© 2012 Society of Economic Geologists, Inc.Special Publication 16, pp. 167–185

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in 1989 by Cominco America (Bouley et al., 1995), containsmoderate hypogene grades of Cu-Au-Mo mineralization anda thin zone of weakly developed supergene mineralization.The West zone is only exposed in one outcrop and is other-wise concealed by glacial sediments. The East zone was dis-covered by Northern Dynasty Minerals in 2005 (Rebagliatiand Lang, 2009) and contains high-grade hypogene mineral-ization without a supergene component. It is completely con-cealed by Late Cretaceous volcanic and sedimentary rocks.

This paper presents a general overview of the Pebble de-posit, and it is based largely on recent detailed studies byLang et al. (2013) and Gregory et al. (2013). In this manu-script, the regional setting of the deposit and the geology andstructure of the Pebble district are described, as are the char-acteristics of hydrothermal alteration, vein types, and metaldistribution and their geologic controls. The discussion fo-cuses on the characteristics which led to the large size and

high hypogene grade of the Pebble deposit. A generalizedmagmatic-hydrothermal-structural model is presented andexploration strategies are considered.

Discovery HistoryExploration and discovery have been ongoing in the Pebble

area since 1984 and have been described by Bouley et al.(1995) and Rebagliati and Lang (2009). The Iliamna area wastargeted by Cominco America for its potential to host epi-thermal- and intrusion-related gold deposits (Bouley et al.,1995). Small, auriferous epithermal veins were discoveredabout 15 km south of Pebble on Sharp Mountain (Fig. 2) dur-ing regional reconnaissance in 1984. Surface prospecting oncolor anomalies in 1987 led to discovery of epithermal veinsin the Sill deposit, located 5 km southeast of Pebble (Fig. 2).Geochemically anomalous soil and talus samples were alsocollected from what was later recognized to be the western

168 LANG AND GREGORY

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KB

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BonanzaHills

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kilometers

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SEDIMENTARY AND METAMORPHIC ROCKSAlaska-Aleutian Range-Tertiary/Triassicsedimentary rocks

Cretaceous/Jurassic flysch deposits

Lime Hills province-Jurassic/Ordoviciansedimentary rocks

Metamorphic rocks

Quaternary deposits Snow/ice

IGNEOUS ROCKS

Cretaceous-Tertiary igneous rocks

Alaska-Aleutian Range-Tertiary igneous rocks

Alaska-Aleutian Range-Jurassic igneous rocks

FIG. 1. Location of the Pebble deposit and regional geologic setting of southwest Alaska. Dashed lines separate terranes:CzC = Cenozoic cover, FT = Farewell terrane, KB = Kuskokwim basin, PT = Peninsular terrane, TT = Togiak terrane. Blackcircles are the location of known mineral deposits. Major dextral strike-slip faults indicated by solid lines. Modified slightlyfrom Anderson et al. (2013).


side of the Pebble West zone. Subsequent mapping, soil sam-pling, and small induced polarization surveys led to drill dis-covery of the Pebble West zone in 1989. The initial drill holeswere collared in strongly auriferous, peripheral quartz-sericite-pyrite alteration located west of the Pebble deposit,whereas the deposit was intersected in the sixth drill hole,which was collared near the only surface exposure of the de-posit, a very tiny outcrop of altered intrusion cut by quartzveins. An inferred resource of 430 Mt, grading 0.35% Cu and0.41 g/t Au, accompanied by low-grade molybdenum, was es-timated in 1993 (Bouley et al., 1995).

Northern Dynasty Minerals acquired the property in 2001.They recognized a significant potential to expand and upgradethe deposit because 90 and 50% of the widely spaced drillingto that time ended in mineralization that graded >0.30 and>0.50% copper equiv, respectively (copper-equiv calculationdefined in Table 1). Extensive drilling in 2003 and 2004 in-creased the inferred resource in Pebble West to 2,737 Mt,grading 0.27% Cu, 0.30 g/t Au, and 0.015% Mo. Grade, alter-ation, and vein patterns recognized during delineation drillingon the eastern side of the West zone in late 2004 led to drill dis-covery of the high-grade Pebble East zone in 2005 (Rebagliatiand Lang, 2009). The deposit was drilled to its current re-source in 2008 (Table 1). Between 2002 and 2009, explorationdrilling based on soil geochemistry and induced polarizationsurveys discovered additional centers of intrusion-related min-eralization in the 38, 37, 52, 25, 308, and 65 zones (Fig. 2).

As of 2012 the Pebble Limited Partnership, an equal-rightsventure formed in 2007 between subsidiaries of Anglo Amer-ican plc and Northern Dynasty Minerals Ltd. to advance theproject, is completing an economic prefeasibility study andcontinuing with district exploration.

Regional SettingThis summary of the regional tectonic, magmatic, and metal-

logenic setting of southwest Alaska is based largely upon Gold-farb et al. (2013, and contained references). The allochthonousWrangellia superterrane comprises amalgamated oceanic arcs

that approached North America sinistrally in the early Meso-zoic. Foreland flysch basins (Kalbas et al., 2007), includingthe Kahiltna basin which hosts the Pebble deposit, formedbetween Wrangellia and pericratonic terranes and previouslyamalgamated allochthonous terranes of the Intermontanebelt (Wallace et al., 1989; McClelland et al., 1992). Beginningbetween 115 and 110 Ma, the foreland basins were folded,complexly faulted, and subjected to low-grade regional meta-morphism (Bouley et al., 1995) as Wrangellia accreted toNorth America by the late Early Cretaceous (Detterman andReed, 1980; Hampton et al., 2010). Convergence changed todextral transpression between 97 and 90 Ma (Goldfarb et al.,2013) and most deformation was thereafter accommodatedby dextral strike-slip faults parallel to the continental margin(Fig. 1). These include the major Denali fault, located in cen-tral Alaska, and subparallel faults to the southeast; the Pebbledeposit is located between strands of the dextral Lake Clarkfault zone (Fig. 1; Shah et al., 2009). Intrusions at Pebble areundeformed (Goldfarb et al., 2013) and were probably em-placed during a period when at least local extension occurredacross southwest Alaska in the mid-Cretaceous (e.g., Pavlis etal., 1993). The relative importance of extensional versus com-pressional structures to formation of the Pebble deposit is notwell constrained, although a synhydrothermal compressionalfault has been recognized within the deposit (see below).

Multiple episodes of magmatism and associated mineral-ization are recognized across southwest Alaska (Fig. 1; Gold-farb, 1997; Young et al., 1997) and began with the onset ofdextral transpression following basin collapse in the MiddleCretaceous (Goldfarb et al., 2013). Alaska-type alkalic ultra-mafic complexes were emplaced at Kemuk (Foley et al., 1997;Iriondo et al., 2003) and at Pebble (Bouley et al., 1995) be-tween about 100 and 88 Ma. Subalkalic intrusions emplacedbetween 97 and 90 Ma are associated with porphyry Cu-Mo± Au mineralization at Pebble (Bouley et al., 1995; Lang etal., 2013), Iliamna (P. St. George, pers. commun., 2010), andNeacola (Reed and Lanphere, 1973; Young et al., 1997; Fig.1). Porphyry Cu-Au ± Mo deposits which formed between 75

PEBBLE Cu-Au-Mo DEPOSIT, SW ALASKA: MAGMATIC-HYDROTHERMAL-STRUCTURAL EVOLUTION 169

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TABLE 1. Estimated Mineral Resources and Contained Metal in the Pebble Deposit, Alaska

Resource estimations Contained metal

Cut-off1 Million (% Cu-equiv) Cu-equiv (%) metric tons Cu (%) Au (g/t) Mo (ppm) Cu (Blb) Au (Moz) Mo (Blb)

Measured and indicated resources0.30 0.78 5942 0.42 0.35 250 55.0 66.9 3.280.40 0.83 5399 0.45 0.36 260 53.6 62.5 3.090.60 0.98 3668 0.55 0.41 293 44.5 48.3 2.371.00 1.29 1449 0.76 0.52 341 24.3 24.2 1.09

Inferred resources0.30 0.35 4835 0.24 0.26 215 25.6 40.4 2.290.40 0.66 2845 0.32 0.30 259 20.1 27.4 1.620.60 0.89 1322 0.48 0.37 289 14.0 15.7 0.841.00 1.20 353 0.69 0.45 379 5.4 5.1 0.29

Notes: Copper-equiv (%) calculations: West zone: Cu% + (Au g/t × 69.6%/85% × 29.00/40.79) + (Mo% × 77.8%/85% × 75.58/40.79); East zone: Cu%+ (Au g/t × 76.8%/89.3% × 29.00/40.79) + (Mo% × 83.7%/89.3% × 75.58/40.79)

1 Copper-equiv grades were calculated using metal prices of $1.85/lb copper, $902/oz gold, and $12.50/lb molybdenum, normalized to estimated metalrecoveries; details are provided by Gaunt et al. (2010)


and 60 Ma are related to intermediate to felsic, subalkalic in-trusions (Hart et al., 2010; Goldfarb et al., 2013) at Whistler(Couture and Siddorn, 2007; located about 250 km northeastof Pebble), Kijik, the Bonanza Hills (Anderson et al., 2013),and Shotgun (Rombach and Newberry, 2001; Fig. 1). Volu-minous Eocene volcanic rocks and subvolcanic intrusionscover much of the region and locally host epithermal preciousmetal mineralization (Bundtzen and Miller, 1997).

District GeologyPostmineralization cover is extensive in the Pebble district

and therefore geologic interpretation (Fig. 2) largely derivesfrom exploration drill holes, mapping of talus, and airbornegeophysical surveys. The summary which follows is based on

Lang et al. (2013) who tabulate all isotopic dates and describerock types in greater detail.

Rock types

Kahiltna flysch is the oldest rock type in the district and hasa minimum age of 99 Ma based on crosscutting intrusions. Itcomprises a normal-facing succession of gently folded basinalturbidites, which include immature, laminar to thick-beddedsiltstone, mudstone, subordinate wacke, and rare lenses ofpebble conglomerate with an intermediate igneous prove-nance. Basalt flows and minor mafic volcaniclastic rocks areinterbedded with flysch in the southwest and north parts ofthe district and are cut by irregular gabbroic bodies of uncer-tain age (Fig. 2).


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D

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{EastGraben

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kana

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Kahiltna FlyschJurassic-Cretaceous Rocks

Cretaceous Rocks

Eocene Rocks

Alkalic Intrusions & Breccias (~96 Ma)

Sedimentary & Volcanic Cover Sequence

Monzonite Porphyry Intrusions (~65 Ma)

Late Cretaceous to Paleocene Rocks

Boundaries of ‘Cover Sequence’

Pebble Deposit – Resource Outline

Known Centersof Mineralization

Eocene

Cretaceous

Brittle FaultsBrittle-DuctileFault Zone (BDF)

Diorite Sills (~96 Ma)

Granodiorite Sills (~96 Ma)

Granodiorite Stocks (~90 Ma)

Basalt

Gabbro

Volcanic Rocks & Subvolcanic Intrusions

}

Area of Figure 4BDF

1 Gold Zone

25 Gold Zone

65 Zone

Sill Zone

Sharp MtnZone

308 Zone

37 Zone

52 Zone

38 Zone

FIG. 2. Interpreted geology of the Pebble district. In the eastern part of the map area, the dark yellow dashed lines indi-cate the limits of the Late Cretaceous cover sequence, which has been removed so that the geology of the Pebble deposit isvisible. See text for detailed discussion.


Several types of intrusions were emplaced into the Kahiltnaflysch between 99 and 96 Ma (Bouley et al., 1995; Schrader,2001; Hart et al., 2010; Lang et al., 2013). Porphyritic dioritesills are widespread, whereas hornblende granodiorite por-phyry sills occur only in the immediate vicinity of the Pebbledeposit. A suite of alkalic intrusions includes quartz-free, tex-turally variable biotite pyroxenite, syenomonzonite, mon-zonite and monzodiorite porphyry intrusions, and associatedintrusion breccias. The alkalic rocks extend from the Pebbledeposit into the 25 zone about 5 km to the south but drill in-tersections in the southeast part of the district indicate thatthey are widespread (Fig. 2).

Hornblende granodiorite porphyry stocks were emplacedat approximately 90 Ma (Bouley et al., 1995; Schrader, 2001;Lang et al., 2013). They include the Kaskanak batholith andnumerous, smaller, petrographically similar bodies foundmostly within, or proximal to, zones of porphyry mineraliza-tion around the margins of the batholith (Fig. 2). These rocksare described below.

The Kahiltna flysch and mid-Cretaceous intrusive and vol-canic rocks are unconformably overlain by well-bedded sedi-mentary and volcanic rocks informally called the Cover se-quence. The Cover sequence occurs mostly on, and thickenstoward, the east side of the district but also occurs on top, andto the west and south, of the Kaskanak batholith (Fig. 2). Thethickness of the Cover sequence reaches at least 1,950 mwithin the East graben, but elsewhere it is mostly <600 mthick. The succession is normal-facing and includes pebble toboulder conglomerate, wacke, siltstone and mudstone, basaltflows, dikes and sills, and basalt to rhyolite volcaniclastic frag-mental rocks. An unaltered andesite dike, interpreted to berelated to the Cover sequence, was dated at 84.1 ± 0.3 Ma byAr-Ar methods on biotite (Schrader, 2001).

Two postmineralization hornblende monzonite porphyryintrusions returned isotopic dates of ~65 Ma (Lang et al.,2013). One intrusion cuts basalts of the Cover sequence (seebelow) within the East graben and a second was intersectedby drilling 3 km north-northeast of the deposit (Fig. 2).

Volcanic and subvolcanic intrusive rocks in the east andsoutheast parts of the district have radiometric ages of 46 to48 Ma (Bouley et al., 1995; Schrader, 2001; Lang et al., 2013).Rock types include rhyolite dikes, brecciated rhyolite flows,fine-grained, equigranular to porphyritic biotite-bearinghornblende latite subvolcanic intrusions and coarse-grainedhornblende monzonite porphyry.

Pleistocene to Recent glacial sediments cover much of thedistrict. The sediments are typically <30 m thick but drill in-tersections range up to 160 m in the southeast part of the dis-trict. Ice-flow directions over the Pebble deposit were to thesouth-southwest and the glaciers responsible for the deposi-tion of the youngest of these sediments had retreated byabout 11 Ka (Detterman and Reed, 1973; Hamilton andKlieforth, 2010).

Structure

District-scale structural history is poorly understood due toa paucity of outcrop and marker horizons. The Kahiltna flyschexhibits shallow to moderate dips to the east, south, andsoutheast which may, in part, reflect doming around theKaskanak batholith. Folds in the flysch are mostly open and

formed before hydrothermal activity at Pebble (Goldfarb etal., 2013).

Faults occur throughout the Pebble district (Fig. 2). A NE-trending, synhydrothermal brittle-ductile fault zone on theeast side of the Pebble deposit is described separately below.Most faults are brittle normal or normal-oblique structureswhich cut all rock types in the district and have been largelyinferred from discontinuities in airborne magnetic and elec-tromagnetic data. The most prominent faults strike north-northeast and northwest, with fewer striking east. Major NE-trending normal faults bound the East graben and displacehigh-grade mineralization on the east side of the Pebble de-posit (Fig. 2). Many brittle faults cut Eocene rock types butprecursor structures may have been periodically reactivatedsince the mid-Cretaceous (L. Rankin, pers. commun., 2011).

Geology of the Pebble DepositThe Pebble deposit is hosted by Kahiltna flysch, diorite,

and granodiorite sills, intrusions and breccias of the alkalicsuite and granodiorite stocks and is cut by numerous faults(Figs. 3, 4).

Rock types

Kahiltna flysch within the deposit is mostly well-beddedsiltstone with <10% wacke interbeds and has an overall dip of<25° to the east. Bouma sequences (Bouley et al., 1995),graded beds, and load casts demonstrate that the stratigraphyis normal-facing. Basalt interbeds are not present but a gab-broic body occurs immediately to the northwest of the de-posit (Fig. 3).

The oldest intrusions within the Pebble deposit are dioriteand granodiorite sills emplaced into flysch. Fine-grained dior-ite sills have a subporphyritic texture defined by plagioclase,hornblende, and minor pyroxene. They are found only in thewestern half of the deposit (Fig. 4) where they form two large,laterally continuous bodies which can exceed 100 m in thick-ness and numerous, narrow, discontinuous bodies. Granodi-orite sills range from fine to medium grained but are all min-eralogically similar to each other. They contain phenocrysts ofacicular hornblende and equant plagioclase and relativelysmaller phenocrysts of apatite in a very fine grained ground-mass of K-feldspar, quartz, and minor magnetite. Granodior-ite sills are laterally continuous, range from a few to nearly300 m in thickness and thicken to the northeast, possibly to-ward an unidentified source intrusion. They comprise theUpper and Lower sills, which extend across the entire deposit(Fig. 4), and sill 3 which has only been incompletely delin-eated at depth in the East zone.

The southwest quadrant of the Pebble deposit is dominatedby alkalic suite rocks (Fig. 3), which comprise a chaotic mix-ture of at least four stages of intrusion and several undividedbreccias. Early intrusions comprise various medium-grained,biotite monzonite porphyries that commonly contain scatteredK-feldspar megacrysts up to several centimeters in size. Laterintrusions are fine-grained porphyritic biotite monzodiorite.All of these intrusive phases contain abundant angular to sub-rounded xenoliths of flysch, diorite sill, possible granodioritesill and, in the younger monzodiorite phase, older phases ofalkalic intrusion. The concentration of xenoliths increases to-ward the margins of the younger biotite monzodiorite phase


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and transition into breccias which contain subangular to sub-rounded fragments of flysch, diorite sill, and early alkalic in-trusions in a matrix of rock flour. Fragments range from a fewmillimeters to at least tens of meters in size. Some longer drillintersections of diorite and granodiorite sills can be correlatedwith undisturbed bodies outside the breccias but, more com-monly, fragment types are mixed and transported. The alkalicsuite is currently modeled as a single megabreccia body dueto its internal, small-scale complexity.

The Pebble deposit is centered on a group of five very sim-ilar hornblende granodiorite porphyry intrusions (Fig. 3).These comprise the larger East zone pluton and four smallerbodies in the West zone. Uranium-lead zircon ages clusteraround 90 Ma and overlap with Re-Os dates of about 89.6 Maon molybdenite (Schrader, 2001; Lang et al., 2013). The northcontact of the East zone pluton is close to vertical and its roofcontacts dip shallowly to the west; it continues to the south,where the shape is not well defined and has been droppedinto the East graben by the ZG1 fault (Fig. 4). The smallerstocks in the West zone have outward-dipping contacts. Re-cent deep drilling suggests that these intrusions likely coa-lesce below 1,000-m depth.

The East zone is entirely concealed by the east-thickeningCover sequence which is up to 670 m thick above the easternpart of the deposit (Fig. 4). The contact between the Pebbledeposit and the Cover sequence ranges from razor sharp to

structurally disrupted. The lower part of the Cover sequencecomprises a thick basal conglomerate overlain by interbeddedpebble conglomerate, wacke, siltstone, and mudstone,whereas the upper part comprises mainly volcaniclastic rocksand lesser flows of basalt to andesite composition (Fig. 4).Within the East graben, the Cover sequence is 1,300 m thicknorth of the ZE fault and at least 1,950 m thick to its southand comprises basalt flows with lesser clastic sedimentary in-terbeds. Basalts within the East graben were intruded atabout 65 Ma by two sheet-like bodies of unaltered horn-blende monzonite porphyry.

Eocene rocks are rare within and proximal to the Pebbledeposit. They comprise narrow rhyolite dikes, a pink horn-blende monzonite intrusion intersected at depth in the cen-tral part of the East graben, and a rhyolite flow breccia at thetop of the East graben south of the ZE fault.

Structure

The Kahiltna flysh forms an M-shaped anticline with axesthat plunge shallowly to the east-southeast (Rebagliati andPayne, 2006). Folding did not affect the overlying Cover se-quence but bedding in both units is grossly subparallel andlater tilting of about 20° to the east affected both rock typesas a single package (Fig. 4). Tilting can only be constrained toyounger than the aforementioned minimum age of 84 Ma onthe Cover sequence.


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limit ofestimatedresources

western limit ofCover Sequence

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EAST

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FIG. 3. Interpreted geology of the Pebble deposit. The Late Cretaceous cover sequence occurs to the east of the dark yel-low line and has been removed for clarity. Cross-section A-A' is shown in Figure 4. The brittle-ductile fault zone (BDF) isindicated by the cross-hatched pattern. The dashed outline of the estimated resources at a 0.3% copper-equiv cut-off is usedas a reference point for alteration and grade distribution in Figure 5. White areas are either undrilled or Cretaceous rocktypes below the Cover sequence are unknown. See Figure 2 for geology legend.


An important brittle-ductile fault zone has been identifiedon the eastern side of the Pebble deposit (Figs. 3, 4). Thebrittle-ductile fault zone is a zone of distributed deformationdefined by cataclastic seams and healed breccias. It strikesnorth-northeast, extends at least 3 km to the limit of drillingalong strike, and is vertical or dips steeply to the west. Thebrittle-ductile fault zone is at least 200 m in width but is trun-cated on the east by the ZG1 fault (Fig. 4). The brittle-ductilefault zone is a dextral-oblique/reverse structure with a maxi-mum of 400 m lateral displacement and an unconstrainedvertical component (S. Goodman, pers. commun., 2008).Grade and vein density patterns and a higher intensity of de-formation in flysch compared to the East zone pluton indicatethat the brittle-ductile fault zone was active before, during,and after both magmatism and hydrothermal activity (Lang etal., 2013). The characteristics of deformation along the brit-tle-ductile fault zone, and its timing relative to hydrothermalactivity at Pebble, support at least a local compressional envi-ronment during the formation of the deposit. The brittle-duc-tile fault zone does not penetrate the Cover sequence.

Postmineralization brittle faults within the Pebble depositconform to district-scale patterns (Fig. 3). Most have between15 and 90 m of normal offset except for the ZA fault whichhas 30 m of reverse movement. Normal displacement on theE-trending ZE fault increases from <50 m in the west toabout 300 m in the east. The ZF fault has a minimum of 250m of normal displacement and juxtaposes mineralized sodic-potassic alteration against poorly mineralized propylitic andquartz-sericite-pyrite alteration on the west (Fig. 3). The NE-trending ZG1 normal fault forms the western boundary of theEast graben and has displacement of 650 m in the north and900 m in the south; the subparallel ZG2 fault has between270 and 550 m of additional normal displacement. W-dippingnormal faults have been identified east of Pebble and mayform the east side of the East graben. Brittle faults host basaltdikes related to the Cover sequence and were therefore ac-tive by 84 Ma.

Hydrothermal Alteration and Vein TypesAlteration types at Pebble are described as hornfels, potas-

sic, sodic-potassic, sodic-potassic ± calcic, illite ± kaolinite,advanced argillic, quartz-illite-pyrite (quartz-illite-pyrite),quartz-sericite-pyrite, and propylitic assemblages (Figs. 4, 5).These assemblages are associated with a wide variety of veintypes. This naming system is Pebble specific and reflects themineralogy and chemistry of each alteration assemblage.Sericite at Pebble is defined as fine-grained, crystalline whitemica, whereas illite refers to very fine grained, noncrystallinewhite mica (Harraden et al., 2013). Advanced argillic alter-ation uses the naming convention of Meyer and Hemley(1967), although this alteration at Pebble differs somewhatfrom the original definition as discussed below.

Prehydrothermal hornfels

Massive, biotite-rich, pyrite-bearing hornfels formed adja-cent to the Kaskanak batholith immediately prior to hydro-thermal activity at Pebble. The hornfels aureole is narrow tothe south of Pebble but is much wider in the vicinity of thedeposit, which magnetic data suggest is underlain by thebatholith (Shah et al., 2009; Anderson et al, 2013). Hornfels


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A A’

300 m

300

m

300 m

300

m

ZG1Fault

Cover Sequence

East

Graben

Kahiltna flyschAlkalic intrusions & breccias

Granodiorite sills

Mostly volcanic rocks

Upper Sill

Lower Sill

Granodiorite plutons

Cover Sequence

Cretaceous Rock Types

Diorite sills

Mostly clastic sedimentary rocks

Area with brittle-ductiledeformation textures

Geology

A A’ZG1Fault

Sodic-potassic

Pyrophyllite-bearing

Quartz-illite-pyrite (QIP)

Sericite (with bornite, digenite)

Potassic

Quartz-sericite-pyrite (QSP)

Alteration

Copper

Gold

Molybdenum

Copper-Equivalent

Partial QSPoverprint

??

??

FIG. 4. Geology, alteration, and metal distribution in section A-A'. Loca-tion of section is shown in Figure 3 and grade legends in Figure 5. Gradesare shown as they appear in the resource block model (Gaunt et al., 2010)and copper-equiv grades are calculated with the equation in Table 1.



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Copper

<0.15%

0.15-0.35%

0.70-1.00%

>1.00%

0.35-0.70%

Copper

Alteration

1000 m

1000 m

N

N

Gold

<0.10 g/t

0.10-0.30 g/t

0.50-1.00 g/t

>1.00 g/t

0.30-0.50 g/t

Gold

Molybdenum

<100 ppm

100-300 ppm

300-500 ppm

>500 ppmMolybdenum

magnetite-bearing

resourceboundary

mostlypotassic

mostlysodic-

potassicNQV

>20% quartzvein density

SQV

Copper-Equivalent

Alteration

0.25-0.50%

1.00-1.50%

0.50-1.00%

<0.25%

>1.50%Copper-Eq.

Sodic-potassic

Pyrophyllite-bearing

Quartz-illite-pyrite (QIP)

Sericite (bornite, digenite)

Potassic

Quartz-sericite-pyrite(QSP)

QIPPropylitic with QSP overprint

Grade Contour Values

FIG. 5. Plan view of alteration and metal distribution in the Pebble deposit. Grades are shown as they appear in the re-source block model (Gaunt et al., 2010) at the contact between the deposit and the overlying Cover sequence, which hasbeen removed. Copper-equiv grades are calculated with the equation in Table 1. For geologic reference, the resource out-line matches that shown in Figure 3. A simplified distribution of alteration types is shown in the map at upper left, NQV andSQV are the northern and southern domains that contain >50% quartz veins, as discussed in the text.


affects all pre-90 Ma rock types. In the deposit, it is most in-tense within flysch, which was highly susceptible to brittlefracture, although poor development of alteration envelopesaround veins suggests it had a low permeability compared toother rock types.

Early alteration types

Potassic alteration occurs throughout the deposit (Fig. 5). Itis strongest in flysch and granodiorite sills above, adjacent,and within the East zone pluton and in small areas in theWest zone (Gregory and Lang, 2009; Gregory et al., 2013).The assemblage is K-feldspar, quartz, biotite, trace to minorankerite or ferroan dolomite, apatite, and rutile (e.g., Fig.6A). Sulfides comprise subequal, disseminated chalcopyriteand pyrite with minor disseminated molybdenite and, locally,minor bornite (Gregory and Lang, 2009; Gregory et al.,2013). The proportion of biotite relative to K-feldspar corre-lates with the original Fe-Mg concentration of affected rocksand is highest in flysch and diorite sills (e.g., Fig. 6B).

Sodic-potassic alteration is the principal assemblage in theWest zone and occurs below potassic alteration in the Eastzone (Figs. 4, 5). It comprises albite, biotite, carbonate min-erals, quartz, and minor magnetite, along with pyrite, chal-copyrite, and minor molybdenite (Gregory and Lang, 2011,2012; Gregory et al., 2013). Anhydrite is only locally present,appearing at 730 m below both the surface in the West zoneand the unconformity in the East zone. Except for the pres-ence of albite and minor magnetite, sodic-potassic alterationis very similar to potassic alteration. A magnetite-rich subzoneof sodic-potassic alteration is associated with magnetite-bear-ing M veins (discussed below), which occur in the central partof the West zone (Fig. 5), within or proximal to Fe-rich dior-ite sills (Fig. 6C). In the East zone, potassic alteration yieldswith depth to a weakly mineralized sodic-potassic ± calcic alteration (e.g., Fig. 6D). This alteration contains locallyabundant epidote and calcite and typically <2% pyrite-chal-copyrite-molybdenite but is otherwise very similar to sodic-potassic alteration. Potassic alteration consistently overprintsboth sodic-potassic and sodic-potassic ± calcic alteration butthe three alteration types are interpreted to be coeval, withthe apparent relative timing a consequence of telescoping orchanging fluid chemistry during cooling.

Rare truncations of quartz-sulfide veins associated withpotassic alteration at the contacts between granodiorite silland granodiorite pluton, and rarely as xenoliths within gran-odiorite plutons, suggests that more than one substage ofearly potassic and/or sodic-potassic alteration may be pre-sent at Pebble (D. Braxton and A. Wilson, pers. commun.,2008; see discussion in Lang et al., 2013). The current data,however, do not yet permit delineation of these potentialsubstages.

Veins associated with early alteration types

At least 80% of quartz-sulfide veins at Pebble comprisefour types which formed during early alteration. These veintypes span a continuum of textural and mineralogical charac-teristics and each shows variations that may correlate with lat-eral and/or vertical position within the deposit. The namingconventions used below are Pebble specific and, althoughsimilar to standard porphyry vein nomenclature (e.g., Gustafson

and Hunt, 1975; Clark, 1993; Gustafson and Quiroga, 1995),they are not exactly equivalent to the vein types describedfrom their type deposits.

Type A veins: The earliest veins are type A. Type A1 is themost common and is similar to the A veins of Gustafson andHunt (1975). They occur mostly within the East zone plutonwhere they form anastomosing arrays of sinuous, discontinu-ous veins. They typically have diffuse contacts with host rockand locally have narrow K-feldspar alteration envelopes.They contain clear quartz, minor white K-feldspar and <1 to2% pyrite, lesser chalcopyrite, and rare molybdenite. Theyoccur within zones of pervasive, weakly mineralized potassicalteration.

Vein type A2 has characteristics transitional between quartzveins and pegmatite and occurs at depths below about 1,000m within the East zone pluton. These veins have selvages ofK-feldspar and cores of coarse-grained quartz, contain clotsof brown biotite, and trace chalcopyrite, molybdenite, and/orpyrite. They lack alteration envelopes.

Vein type A3 occurs at depths below 750 m in the East zonepluton and is distinguished from type A1 by intense envelopesof white K-feldspar.

Type B veins: Type B veins crosscut type A veins and arethe most widespread at Pebble. They are coincident withpotassic, sodic-potassic, and sodic-potassic ± calcic alterationand are most abundant within and proximal to the East zonepluton. The B veins at Pebble occur as several variants whichhave general similarity to B veins as described by Gustafsonand Hunt (1975).

The most common variant is type B1. They are planar, con-tinuous, and have sharp contacts with host rocks (e.g., Fig.6B). They contain clear quartz accompanied by trace to minorbiotite, K-feldspar, apatite and/or rutile and, rarely, anhydrite.Total sulfide concentration is typically 2 to 5% and comprisessubequal pyrite and chalcopyrite, minor molybdenite and, lo-cally, minor bornite. Well-mineralized alteration envelopesare ubiquitous and contain mostly K-feldspar in felsic hostrocks with a higher proportion of brown biotite in more mafichost rocks.

Type B2 veins are temporally equivalent to type B1 butoccur mostly at depths >800 m in the East zone where theycoincide with sodic-potassic ± calcic alteration. They are dis-tinguished from type B1 veins by weaker alteration en-velopes, lower sulfide concentration, green chlorite pseudo-morphs after coarse aggregates of brown biotite (e.g., Fig.6D), and minor to rare calcite and epidote.

Type B3 quartz veins are most common in the north-cen-tral and south-central part of the East zone and extend to1,800-m depth below surface, the vertical limits of drilling.These veins are distinguished by abundant molybdenite,which ranges from low concentration selvages to semimassiveand massive seams, and the near absence of other sulfides. K-feldspar alteration envelopes are weak and sporadic. Type B3veins cut type A, B1 and B2 veins and locally cut type C veinsbut their timing relative to late advanced argillic alterationhas not been clearly established.

Type M veins: Magnetite-rich M veins (e.g., Fig. 6C), whichare generally similar to veins defined by Clark (1993), are as-sociated with magnetite-bearing sodic-potassic alterationwithin and proximal to diorite sills in the West zone and may


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7360 3357’

A

7387 3035’

E

7363 3428’

D

7374 2075’

F

8422 2919’

G

7398M 237’

I

6338 1992’

B

7399M 286’

C

8406 1570’

H

FIG. 6. Examples of alteration types in hand sample. Number in lower left of each photograph refers to drill hole num-ber and depth in feet; scale bars are 5 mm. A. Potassic-altered granodiorite pluton with disseminated chalcopyrite and cutby type B1 veins. B. Potassic-altered flysch with early A1 veins cut by B1 veins. C. Sodic-potassic−altered diorite cut by typeM quartz-magnetite-carbonate veins with K-feldspar envelopes. D. Sodic-potassic ± calcic−altered granodiorite pluton cutby a type B2 vein at lower left. E. Strongly illite-altered granodiorite pluton cut by type A1 veins. F. Sericite-altered gran-odiorite pluton with disseminated pyrite-bornite. G. Pyrophyllite-altered granodiorite pluton with disseminated chalcopyrite.H. Quartz-sericite-pyrite−altered granodiorite sill cut by a type D pyrite vein. I. Quartz-illite-pyrite−altered flysch with dis-seminated pyrite.


be a variant of type C veins. They are planar to irregularand contain magnetite and quartz with lesser ankerite andK-feldspar. Sulfide concentration is commonly >10% andcomprises chalcopyrite, pyrite, and minor molybdenite. Min-eralogically similar alteration envelopes are narrow but ubiq-uitous.

Type C vein: These veins are least common within and im-mediately adjacent to the East zone pluton but are the mostabundant vein type in the central and west parts of the de-posit. They cut A and B veins, except possibly the B3 variant,and overlap in time with M veins. Type C veins at Pebble aredefined according to their timing relative to other vein typesand do not resemble the C veins defined by Gustafson andQuiroga (1995). At Pebble, type C veins contain quartz, lo-cally abundant ankerite or ferroan dolomite, and minor totrace K-feldspar, magnetite, and biotite. Sulfide concentra-tion is typically 10% or greater and comprises subequal pyriteand chalcopyrite, highly variable molybdenite, and rare bor-nite and arsenopyrite. These veins have wide, very prominentalteration envelopes with mineralogy that mirrors vein fill.Copper grades of up to several percent occur where alter-ation envelopes to type C veins coalesce. Some narrow veinsin flysch have zoned alteration envelopes similar to the type Cveins but are locally cut by type B1 veins; these may form anearly vein type paragenetically similar to the type EB veins ofGustafson and Quiroga (1995).

Vein density: The combined density of vein types A, B, andC across most of the Pebble deposit is between 5 and 15 vol%. Vein density is used in the sense of Haynes and Titley(1980) and does not include the effect of the alteration en-velopes to the veins. Lower concentrations occur near themargins of the deposit and at depth, whereas higher densitiesare within or proximal to the East zone pluton and, locally, inhost rocks proximal to granodiorite plutons in the West zone.Vein density does not correlate with rock type and extendssmoothly across geological contacts.

Vein densities of between 50 and 90% occur in two areas ofthe East zone and are named the NQV and SQV zones (Fig.5). The increase from background to high densities is grada-tional over distances of 100 to 500 m. The very high vein den-sities in these areas probably reflect repeated refracturingand dilation of the affected zones. The zone of high quartzvein density north of the ZE fault (NQV, Fig. 5) is broadlycylindrical and veins are undeformed; it extends up to 600 mbelow the Cover sequence, below which vein density revertsto deposit average. The second zone (SQV, Fig. 5) is a steep,tabular zone, which coincides with the brittle-ductile faultzone, and remains open at >1,500-m depth. It is truncated onthe east by the ZG1 fault but limited drilling indicates that itcontinues into the East graben. Veins in the second zone arecommonly deformed and locally brecciated. The quartz inthese zones is most similar to vein types A1 and/or B1 and isinterpreted to have formed during or prior to early potassicalteration.

Intermediate stage of illite ± kaolinite alteration

Illite and illite-kaolinite alteration, both accompanied byminor pyrite, overprint potassic and sodic-potassic alteration,respectively, throughout the deposit. Alteration ranges fromincipient replacement of feldspar phenocrysts to strong and

pervasive alteration of all minerals in the host rock (e.g., Fig.6E). Fracture or fault control is rarely apparent. It is weakestin flysch in the West zone and strongest at intermediatedepths within the East zone pluton.

Late advanced argillic alteration

Advanced argillic alteration is spatially related to the brit-tle-ductile fault zone and is associated with the highest Cu-Augrades in the deposit. This alteration has a mineral assem-blage dominated by quartz-sericite-pyrophyllite-pyrite-chal-copyrite-bornite-digenite. This is consistent with advancedargillic alteration as described by Meyer and Hemley (1967).The assemblage at Pebble has two end members—quartz-sericite-pyrite-bornite-digenite-chalcopyrite (Fig. 6F) andquartz-pyrophyllite-sericite-chalcopyrite-pyrite (Fig. 6G) thatoccur as spatially discrete, adjacent zones. The pyrophyllite-rich zone occurs along the brittle-ductile fault zone and isbounded to the west by an upward-flaring envelope ofsericite-rich alteration (Fig. 4). Limited drilling confirms thatadvanced argillic alteration continues into the East graben.Pyrophyllite alteration is accompanied by quartz, pyrite, andchalcopyrite. Trace diaspore, zunyite, kaolinite, and dickitehave been identified by shortwave infrared spectroscopy (A.Thompson, pers. commun., 2007). Pyrite concentrationmostly exceeds 5% and is much higher than in nearby earlypotassic alteration. Pyrophyllite alteration is broadly coinci-dent with, and overprints, the southern zone of high quartzvein density and affects host rock between early, deformedquartz-sulfide veins. Quartz veins associated with pyrophyllitealteration are minor and contain massive to semimassivepyrite ± chalcopyrite. Pyrophyllite alteration has not beenidentified in the northern zone of high quartz vein density.

Sericite alteration occurs mostly in the upper 300 m of thedeposit on the southern, downthrown side of the ZE fault butrelicts are also preserved to the north. Alteration is pervasiveand dominated by sericite, which selectively replacesfeldspars and illite-altered feldspars. Pyrite concentration isintermediate between pyrophyllite and early potassic alter-ation. Sericite alteration is characterized by a high sulfidationassemblage of bornite, covellite, digenite, tennantite-tetra-hedrite and, locally, trace enargite. These minerals rim andreplace chalcopyrite and pyrite that precipitated duringpotassic and illite alteration. Associated veins are minor,quartz-rich, with variable pyrite and locally have envelopes ofquartz, sericite, pyrite, and high sulfidation copper minerals.

The advanced argillic alteration at Pebble is of a slightlylower sulfidation state and less acidic than other examples ofthis alteration worldwide (e.g., Oyu Tolgoi; Khashgerel et al.,2009). When modeled in fO2-pH space at 350°C, these two as-semblages occur at two slightly different sets of physiochem-ical conditions (Fig. 7). Pyrophyllite-rich alteration with chal-copyrite occurs at a slightly lower pH and fO2, whereassericite-rich alteration with pyrite-bornite occurs at higherpH and fO2 (Fig. 7). These small changes in pH and fO2 can bethe result of changes in temperature, fluid-wall rock ratios,and buffering effects as a result of fluid-wall rock interactions.Sericite-rich assemblages are typically higher temperature(~350°C) than pyrophyllite-rich assemblages (300°−350°C;Johnson et al., 1992). At Pebble, pyrophyllite-rich assem-blages occur in rocks with a high density of quartz veins,


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whereas sericite-rich alteration occurs in areas of low veindensity—this suggests that different degrees of fluid-wallrock interaction may have influenced the resulting alteration.

Hedenquist et al. (1998) found that pyrophyllite can occurin two separate environments due to (1) condensation of avapor phase resulting in the very high sulfidation and highlyacidic conditions typical of the epithermal environment, and(2) fluids cooling from the sericite stability field resulting insimpler pyrophyllite-sericite assemblages that occur atdeeper levels of the porphyry-epithermal hydrothermal sys-tem. The sericite- and pyrophyllite-rich assemblages at Peb-ble are more consistent with the second mechanism for pyro-phyllite formation.

Quartz-sericite-pyrite and quartz-illite-pyrite alteration

A halo of texture-destructive quartz-sericite-pyrite alter-ation extends at least 4 km to the south of the deposit and 1.5km to the north. It is present in mid-Cretaceous rocks in thenorthern part of the East graben but is only weakly developedwest of the ZF fault. The alteration comprises quartz, sericite,and 8 to 20% pyrite, with minor to trace ankerite, rutile, ap-atite and, rarely, pyrrhotite (e.g., Fig. 6H). On the north andsouth sides of the East zone, quartz-sericite-pyrite alterationfronts dip steeply outward and transition from potassic alter-ation over distances of tens to about 200 m. Weak quartz-sericite-pyrite alteration occurs sporadically throughout theWest zone. The quartz-sericite-pyrite alteration is associatedwith pyrite-rich type D veins (Gustafson and Hunt, 1975) thatcontain quartz, trace rutile, chalcopyrite, and ankerite, sur-rounded by intense, mineralogically similar alteration en-velopes. The quartz-illite-pyrite alteration (e.g., Fig. 6I) is interpreted to represent a zone that was once weak quartz-sericite-pyrite alteration that occurred at the transition

between potassic/sodic-potassic alteration and true quartz-sericite-pyrite alteration and that has been overprinted bylow-temperature illite alteration during collapse of the hydro-thermal system (the same intermediate alteration overprintthat occurs in the potassic and sodic-potassic alterationzones). Therefore, this alteration zone represents an erosionalremnant of a quartz-sericite-pyrite cap, which partially re-placed earlier potassic and/or sodic-potassic alteration in theupper, central part of the deposit (Figs. 4, 5).

Propylitic alteration

Propylitic alteration lies beyond, and is replaced by, quartz-sericite-pyrite alteration. It extends at least 5 km to the southand to the limit of drilling 3 km to the north of the deposit andweak effects occur throughout the eastern half of theKaskanak batholith. Propylitic alteration comprises chlorite,epidote, calcite, quartz, and magnetite, minor albite andhematite, and trace chalcopyrite and contains <3% commonlycubiform pyrite. The assemblage is associated with type Hveins (locally defined) which are mineralogically similar topervasive propylitic alteration. Polymetallic type E veins (lo-cally defined) occur mostly in areas of propylitic and quartz-sericite-pyrite alteration south of the deposit and locally cutsodic-potassic alteration in the West zone. They containquartz, calcite, pyrite, sericite, sphalerite, galena, minor chal-copyrite, and typically trace arsenopyrite, tennantite-tetra-hedrite, freibergite, argentite, native gold, and gold tellurides;weak sericite alteration envelopes are common.

Metal DistributionMineralization in the West zone is mostly hypogene with

thin supergene and leached capping, whereas mineralizationin the East zone is entirely hypogene.

Distribution of mineralization

The Pebble deposit, as preserved, has a flat-tabular geo-metry. Grades of copper and gold diminish below 400 m inthe West zone but extend much deeper in the East zone, par-ticularly within and proximal to the brittle-ductile fault zone(Fig. 4). Both copper and gold grades diminish gradually to-ward the north and south margins of the deposit until poorlymineralized quartz-sericite-pyrite alteration is encountered(Fig. 5). The highest grades of mineralization occur in theEast zone and in smaller domains in the West zone, whereasgrades in the area between these centers are more modestand extend to shallower depths. Molybdenum exhibits a morediffuse pattern with significant grades to 1,800 m, the verticallimit of drilling (Fig. 4); two areas in the north-central andsouth-central parts of the East zone contain more abundanttype B3 veins and have elevated molybdenum grades com-monly higher than 0.05% (Fig. 5). In the west, weakly miner-alized propylitic and quartz-sericite-pyrite alteration andwell-mineralized potassic alteration are juxtaposed by the ZFfault. In the east, mineralization is downdropped into theEast graben by the ZG1 fault.

Supergene mineralization

A thin, incompletely developed leach cap occurs locallyacross the top of the West zone but is rarely more than 10 mthick. Underlying supergene mineralization has an average


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mus

covi

te

mus

covi

te

K-f

eld

spar

pyr

ophy

llite

CHALCOPYRITE

BORNITE

CHALCOPYRITE

BORNITE

HSO4-

SO42-

HS-

H2S

PYRITE

PYRRHOTITE

HEMATITE

MAGNETITE

-15

-20

-25

-30

-35

-40

-45

Log

f O2

pH0 1 2 3 4 5 6 7 8 9 10 11 12

350ºC200 bars

Sericite-pyrite-bornite assemblage

Pyrophyllite-sericite-chalcopyrite-pyrite assemblage

FIG. 7. Log fO2-pH diagram showing stability fields for the sericite-pyrite-bornite assemblage and the pyrophyllite-sericite-chalcopyrite-pyrite assem-blage at 350°C and 200 bars. Phase boundaries and predominant fields foraqueous species calculated using SUPCRT92 (Johnson et al., 1992). As-sumptions: aS = 0.1 (Hezarkhami et al., 1999), K/Na = 0.2 (Roedder ,1984,using data from Tracy, 2001, indicating the presence of halite-bearing fluidinclusions without sylvite).


thickness of only 22 m and exhibits a rapid transition to un-derlying hypogene mineralization. Supergene sulfides com-prise chalcocite, covellite, and minor bornite which variablyreplace hypogene chalcopyrite and typically form only thinrims on pyrite (Gregory and Lang, 2009; Gregory et al.,2013). Supergene copper grades are locally up to 50% higherthan underlying hypogene grades. Kelley et al. (2011) re-ported dates of 6 and 8 Ma on two grains of jarosite, whichthey interpreted to have been glacially transported from thesupergene zone in the Pebble West zone.

Hypogene mineralization

Metal signatures at Pebble correlate closely with alteration.Much of the Cu-Au-Mo mineralization in the deposit was in-troduced during early potassic, sodic-potassic and sodic-potassic ± calcic alteration. Copper is present primarily aschalcopyrite accompanied locally by minor bornite and tracetennantite-tetrahedrite. The pyrite to chalcopyrite ratio is closeto one in the East zone but is higher in the West zone wherequartz-illite-pyrite and quartz-sericite-pyrite alteration are lo-cally present. Gold occurs primarily as electrum inclusions inchalcopyrite with minor amounts hosted by silicate alterationminerals; pyrite contains high-fineness gold (<10 wt % Ag),electrum, and rare gold telluride inclusions (Gregory et al.,2013). Diorite sills which contain magnetite-rich alteration andtype M veins have higher gold concentrations relative to cop-per than other early alteration types. Molybdenite occurs bothin quartz veins and intergrown with disseminated chalcopyrite.

Incipient to weak illite ± kaolinite alteration had little effecton grade, whereas strong illite alteration reduced the grade ofcopper and gold but left molybdenum largely undisturbed.Gold liberated during illite ± kaolinite alteration was recon-stituted as high-fineness inclusions in new or recrystallizedpyrite (Gregory and Lang 2009; Gregory et al., 2013).

Advanced argillic alteration zones have much higher gradesof copper and gold, but very similar molybdenum, compared toearly potassic alteration. Pyrophyllite alteration contains highconcentrations of pyrite and chalcopyrite and both mineralshost inclusions of high-fineness gold. This is the only zone inthe deposit where high-fineness gold inclusions occur in chal-copyrite and this chalcopyrite therefore can be distinguishedfrom chalcopyrite that precipitated during older potassic al-teration that has electrum inclusions (Gregory et al., 2013).The increase in copper grade in sericite alteration is due to theprecipitation of high sulfidation copper minerals and pyritethat replaced early chalcopyrite (Gregory and Lang, 2009).Gold occurs as high-fineness inclusions in pyrite and high sul-fidation copper minerals and as electrum in relict chalcopy-rite related to early potassic alteration (Gregory et al., 2013).

The two zones of high quartz vein density initially may havebeen low-grade domains which formed during early potassicalteration. The zone along the brittle-ductile fault zone is nowstrongly mineralized where overprinted by pyrophyllite alter-ation. The zone north of the ZE fault has average to lowgrades of copper and gold except in small volumes over-printed by sericite alteration.

Late quartz-sericite-pyrite alteration removed most copperand molybdenum but typically gold is present in concentra-tions between 0.15 and 0.5 ppm (Lang et al., 2008). The effectof quartz-illite-pyrite alteration on copper and molybdenum

grade is similar to that of quartz-sericite-pyrite alteration butthe depletion is less extreme. In both cases, gold occurs ashigh-fineness inclusions in younger generations of pyrite(Gregory et al., 2013).

Relationships between copper, gold, and molybdenum dur-ing early alteration are shown in Figure 8. Copper and molyb-denum are not significantly correlated. A subpopulation withhigh grades of molybdenum and with low copper plausibly re-flects the effect of late type B3 veins. Gold and copper arebroadly correlated where sodic-potassic alteration has af-fected granodiorite plutons in the West zone; a similar trendis evident in a subset of data from the deposit as a whole, al-though remaining data are uncorrelated. In other alterationtypes, there is no significant correlation between the threemain metals at Pebble.

Rhenium and palladium are present in concentrations ofpotential economic interest. The average rhenium concentra-tion in molybdenite metallurgical concentrates is 780 to 966ppm (Lang et al., 2013). Palladium occurs in concentrationsof up to 3 ppm in pyrite but only within pyrophyllite alter-ation zones (Gregory et al., 2013) where numerous drill in-tersections up to tens of meters in length have returned val-ues between 0.1 and 1.17 ppm.

The absolute grades of copper, gold, and molybdenumwithin the intensely altered cores of the East and West zonesare not significantly influenced by rock type, plausibly as a re-flection of the fluid-buffered environment. One exception isthe association of high Cu-Au grades with Fe-rich diorite sills,a relationship common in porphyry deposits (e.g., Ray, Ari-zona; Phillips et al., 1974). Where alteration is less intense,particularly near the margins of the deposit, comparativelyless permeable, hornfelsed flysch consistently has lowergrades than adjacent intrusive rock types.

Other zones of mineralization in the district

Numerous zones of intrusion-related and epithermal-stylemineralization have been discovered in the Pebble districtbut none have been fully explored (Fig. 2). These include: (1)Eocene epithermal Au-Ag veins in the Sill zone and on SharpMountain; (2) porphyry-style Cu-Mo-Au mineralization, sim-ilar in age and characteristics to the Pebble West zone, in the38 and 308 zones; (3) propylitic alteration associated withpolymetallic mineralization and hosted by 90 Ma granodioriteintrusions in the 52 zone; (4) Cu-Au mineralization of in-ferred mid-Cretaceous age associated with calc-silicate alter-ation in the 37 zone; (5) strongly anomalous Mo, Cu, Au, andother metals related to siliceous breccias and strong quartz-sericite-pyrite−style alteration of uncertain age over an areaof at least 5 km2 in the 65 zone; and (6) gold, Cu-Au, and basemetal mineralization associated with 96 Ma alkalic intrusionswithin and north of the 25 zone. The only intrusive event notassociated with hydrothermal activity is the 65 Ma mon-zonites. Detailed descriptions of these mineralized zones areavailable in Lang et al. (2013), Rebagliati and Haslinger(2003), Haslinger et al. (2004), Rebagliati et al. (2005), andRebagliati and Payne (2006, 2007).

DiscussionStructural controls have influenced the location of the Peb-

ble deposit at all scales. Anderson et al. (2009, 2010, 2011,


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2013) inverted regional aeromagnetic data and proposed thatPebble is hosted by one of at least five large centers of deep-seated magmatic activity in southwest Alaska, which they in-terpreted to be the roots of a Cretaceous arc. These centersare distributed parallel to the Lake Clark and similar, re-gional-scale, strike-slip faults, and potential deep-seated crossfaults are suggested by regional gravity data (L. Rankin, pers.commun., 2011). This setting is similar to that of major por-phyry deposits in northern Chile (e.g., Tosdal and Richards,2001; Anderson et al., 2010).

At district and deposit scale, fault orientations are consis-tent with connecting structures between strands of the LakeClark fault zone (Shah et al., 2009, 2011). Movement on manyof these faults occurred after formation of Pebble (Goldfarb

et al., 2013) but near coincidence of the younger ZG1 brittlefault and the synhydrothermal brittle-ductile fault zone sug-gests that older precursor structures were important. The re-gional to local setting at Pebble therefore manifests a columnof crust that experienced long-lived, episodic magmatism anddeep-seated structural activity conducive to emplacement oflarge magmatic sources of fluids and metals, intrusion of mul-tiphase cupolas at shallow paleodepth, and maintenance ofhydrothermal plumbing systems.

Multiple stages of metal introduction contributed to thehuge tonnage and, in the East zone, to the high hypogenegrades of mineralization at Pebble (Fig. 9) and can plausiblybe related to multiple phases of intrusion and fluid generation(e.g., Proffett, 2003; Sillitoe, 2010). High-level intrusions


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0

1

2

3

4

0.0 0.5 1.0 1.5 2.0 2.5 3.0Copper (%)

0

1

2

3

4

0.0 0.5 1.0 1.5 2.0 2.5 3.0Copper (wt%)

0

500

1000

1500

2000

2500

3000

0.0 0.5 1.0 1.5 2.0 2.5 3.00

500

1000

1500

2000

2500

3000

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Gol

d (g

/t)

Mol

ybd

enum

(pp

m)

Sodic-Potassic & Potassic Alteration inAll Rock Types (except West Zone stocks)

n = 3358 n = 3964

Sodic-Potassic Alteration in GranodioriteStocks, Pebble West Zone

Au:Cu = 1

Au:Cu = 1

Subgroup:High Mo at low Cu

Copper ( wt%)Copper ( wt%)

FIG. 8. Relationship between gold, copper, and molybdenum in the Pebble deposit. Data are restricted to samples dom-inated by early potassic and/or sodic-potassic alteration. Data in the diagrams at left include results from diorite sills, alkalicintrusions, granodiorite sills, the East zone granodiorite pluton, and flysch; samples from granodiorite plutons in the Westzone are shown separately in the diagrams at right.


related to Pebble are probably fractionates of the underlyingKaskanak batholith, which represents a volume of magma suf-ficient to generate the fluids necessary to form Pebble (Fig.9A; cf., Richards, 2003, 2005). Ascent of fluids from this un-derlying source was focused along the several granodioritecupolas and also, in the East zone, along the brittle-ductilefault zone. Lang et al. (2013) have suggested that mineraliza-tion related to early potassic, sodic-potassic and sodic-potas-sic ± calcic alteration may comprise an earliest Cu-Au-(Mo)substage, related either to thus far unidentified or blind in-trusions (Fig. 9B), and a later substage of Cu-Mo-(Au) relatedto the confirmed granodiorite porphyry plutons (Fig. 9C).

The latter relationship may explain the relatively higher gradesof Mo within the East zone pluton (Fig. 4). This was followedclosely by widespread molybdenum-rich type B3 veins, whichplausibly reflect late fluids generated during advanced frac-tionation of the magmatic source of the early Cu-Au-Mo min-eralization (Fig. 9C; Candela and Holland, 1986). The depositcooled sufficiently to form widespread illite ± kaolinite alter-ation, which redistributed copper and gold, possibly during aperiod of magmatic-hydrothermal quiescence. A late, discretepulse of fluid, plausibly related to a blind intrusion emplacedat depth along the brittle-ductile fault zone, formed advancedargillic alteration and precipitated additional, high-grade Cu-Au


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west eastBDF

PEBBLEDEPOSIT

Stage 1A: Possible Early Cu-Au-(Mo) (potassic)

Stage 0: Pre-Hydrothermal Setting

Stage 1B: Intermediate Cu-Mo-(Au) (potassic) Stage 2: Late Au-Cu (advanced argillic)

??

??

?? ???? ?? ??

A

B C DArea of Figs. 9B-D

current erosion levelKaskanak Batholith

Kahiltna Flysch

crystallized carapace

partially crystallized magma

Diorite andGranodiorite Sills

hornfels

horn

fels

Lateral Flow InducedBy Hornfels in Flysch

West Zone East Zone Unexplored(East Graben)

HypothesizedIntrusionsBelowLevel ofDrilling

Fluids Derived From Kaskanak Magma at Depth

Mineralization SpatiallyRelated to Intrusions

Late Mo-RichFluids

IntrusionsIntersectedAt Levelof Drilling

Hypothesized IntrusionAlong BDF Below Levelof Drilling

MineralizationFocused Along BDF

FIG. 9. Magmatic-hydrothermal-structural model for the Pebble deposit. Diagrams are cartoons which highlight possiblescenarios for the major stages of metal introduction to the deposit and are not constructed to scale. See text for detailed dis-cussion. A. Setting of the Pebble deposit with respect to an interpretation of the preerosional geometry of the Kaskanakbatholith. B. Possible early Cu-Au-(Mo) mineralization related to early potassic alteration. Intrusive apophyses emplacedbelow level of drill testing. C. Possible later Cu-Mo-(Au) mineralization related to potassic alteration associated with em-placement of granodiorite plutons exposed in the deposit. D. Late Cu-Au mineralization formed during advanced argillic al-teration. Fluids from an intrusion at depth were focused along the brittle-ductile fault zone. Modified slightly from Lang etal. (2013).


mineralization in the East zone (Fig. 9D). The quartz-sericite-pyrite alteration formed outboard of and variablyoverprinted the other alteration types as the hydrothermalsystem waned; the quartz-illite-pyrite alteration is most likelya relict cap of quartz-sericite-pyrite alteration that was par-tially converted to illite. Some degree of metal variationwithin each of the major mineralization substages may alsohave been influenced by evolution of fluids from a singlemagmatic source and reutilization of structural flow path-ways. The multiple stages of intrusion, alteration, and metalintroduction at Pebble are compatible with a protractedhydrothermal history (cf. Sillitoe and Mortensen, 2010).

There is no evidence to support more than about 20° ofeastward rotation of the Pebble deposit. Its current flat-tabu-lar geometry is considered to reflect its original orientationabove the eastern flank of the Kaskanak batholith (Fig. 9A).Although the amount of eroded mineralization is unknown,limited fluid inclusion data (Tracy, 2001) and analogy with themetal ratio studies of Proffett (2009) and Murakami et al.(2010) suggest formation at a paleodepth of less than 2 km;much of the deposit may therefore be preserved . The currentdeposit geometry may, in part, reflect vertical restriction offluid flow by hornfels aquitards in flysch and promotion of asignificant component of lateral fluid flow along the relativelymore permeable diorite and granodiorite sills (Fig. 9A; cf. Sil-litoe, 1997). This scenario explains the upward flaring of latesericite alteration (Fig. 9D), the spatial coincidence of late ad-vanced argillic alteration with early alteration types, the ab-sence of hydrothermal breccias, the distinct base to the NQVzone of high density quartz veins, and the lower average metalgrades encountered in flysch at the margins of the deposit.Longer fluid residence may also have contributed to higherconcentrations of veins and disseminated mineralization.

Magmatic-Hydrothermal-Structural SummaryA provisional geologic history of the Pebble deposit conceiv-

ably manifests the following major components: (1) depositionof Kahiltna flysch prior to 99 Ma; (2) intrusion of diorite andgranodiorite sills at about 96 Ma; (3) emplacement of alkalicsuite plutons and associated breccias at about 96 Ma; (4) in-trusion of the Kaskanak batholith and formation of hornfels atabout 90 Ma; (5) emplacement at depth of intrusions coevalwith the Kaskanak batholith and generation of fluids responsi-ble for an early substage of Cu-Au-(Mo) mineralization; (6)formation of the later potential substage of Cu-Mo-(Au) min-eralization and zones of high vein density in association withgranodiorite plutons within the Pebble deposit; (7) additionalmolybdenum mineralization related to type B3 veins; (8) cool-ing, formation of illite ± kaolinite alteration, and redistributionof copper and gold; (9) additional intrusions at depth along thebrittle-ductile fault zone and formation of advanced argillicalteration and associated Cu-Au mineralization; (10) cessationof magmatic-hydrothermal activity; (11) uplift and depositionof the Cover sequence in a dynamic erosional environmentduring the Late Cretaceous; (12) rotation of the entire de-posit 20° to the east; (13) erosion of the Cover sequence fromthe West zone and formation of supergene mineralization by8 Ma; (14) Pleistocene to Recent glaciation, removal of pale-osupergene mineralization, and downice dispersion; and (15)glacial retreat by about 11 Ka and formation of additional

minor supergene mineralization. The accuracy of the modelwill be tested and revised as more detailed geologic, fluid in-clusion, isotopic, and geochronological studies are completed.

Exploration ConsiderationsPebble is a world-class porphyry Cu-Au-Mo deposit and

whether it is an isolated giant, like the deposits at Butte andBingham Canyon, or is part of a major, emerging metallo-genic belt remains to be learned. Regardless of this uncer-tainty, first-order exploration targeting for Cretaceous por-phyry deposits in southwest Alaska should focus on the largemagmatic-structural centers (Anderson et al., 2013), whichare known to host mid- and Late Cretaceous deposits (Younget al., 1997; Goldfarb et al., 2013). Viable drill targets withinthese areas are, however, likely to be concealed by extensiveEocene rock and Quaternary glacial cover. Detailed mappingof surficial glacial deposits and determination of ice-flow di-rections (e.g., Hamilton and Klieforth, 2010) are critical to in-terpretation of soil and till geochemical surveys and heavy in-dicator mineral studies (Kelley et al., 2009, 2011; Rebagliatiand Lang, 2009; Eppinger et al.. 2013). Each of these meth-ods has produced high-contrast anomalies which indicate thepresence of Pebble but the anomalies have been glacially dis-persed up to at least 11 km south of the deposit (Rebagliatiand Lang, 2009). These methods will be most successfulwhere deposits are more like the Pebble West zone and over-lain only by glacial cover.

Exploration in areas with young, unmineralized cover rockswill be more challenging. Eppinger et al. (2009, 2013), Smithet al. (2009), and Kelley et al. (2010) report that high-resolu-tion ICP-MS analysis of surface waters and certain types ofselective extraction geochemical analysis of soils indicate thepresence of concealed mineralization in Pebble East belowthe Cover sequence. The copper isotope composition of sur-face waters also vectors to concealed mineralization in bothPebble East and West (Mathur et al., 2011, 2013). In bothcases, patterns may reflect upflow of ground waters alongmajor faults. Surface waters are widespread in southwestAlaska and hydrogeochemical exploration methods can bebroadly applied. Biogeochemical surveys have been unsuc-cessful at Pebble (Kelley et al., 2010).

Geophysical surveys will be key exploration tools. Drillingat the margins of strong induced polarization chargeabilityhighs led to discovery of the 38, 308, 37, 52, and 25 zones inareas covered only by glacial sediments (Rebagliati and Lang,2009). Airborne magnetic and electromagnetic surveys havebeen less effective in directly identifying zones of mineraliza-tion in the Pebble district but provide important constraintson geology and structure. Three-dimensional inversion ofgeophysical datasets significantly improves resolution todepth and should incorporate in situ measurements of thephysical properties of mineralized and barren rock types bydownhole geophysical logging (Pare et al., 2012).

The expression of the top part of the Pebble hydrothermalsystem is poorly known. The location of quartz-illite-pyrite al-teration and control on advanced argillic alteration by thebrittle-ductile fault zone suggests that the deposit was cappedby strongly altered, geochemically anomalous quartz-sericite-pyrite or quartz-illite-pyrite alteration, potentially complicatedby a superimposed advanced argillic lithocap and by vertical


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restriction of fluid flow by hornfels aquitards. The intense,laterally extensive, geochemically anomalous alteration andmolybdenum-bearing siliceous breccias in the 65 zone (Fig.2) suggest that this area may be a high-level analogue to Peb-ble, which requires drill testing to depth. The distribution ofzones of alteration and mineralization in the Pebble district isat a scale similar to or larger than that in many major por-phyry districts which contain clusters of major deposits (Fig.10). This supports a potential for additional major discoveriesin the Pebble district, and elsewhere in southwest Alaska,which may require deep drilling to adequately test.

AcknowledgmentsThe Pebble Limited Partnership, Anglo American, and

Northern Dynasty Minerals are thanked for permission topublish this manuscript. The content was greatly improved byreviews by Steve Kesler, Alan Wilson, and Jeff Hedenquistand by discussions with the many project geologists who haveworked at Pebble since 2003. The authors are grateful toRobin Smith and Brian McNulty for preparing many of thegraphics.

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chemistry, geology, aeromagnetics, Landsat, and digital elevation models

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Bri

ttle

ritt

le-D-D

uct

ilect

ileF

aultlt

Zo

ne

Radomiro Tomic5,731 Mt @ 0.56% Cu

ChuquicamataDistrict, Chile

Andina-Los Bronces-LosSulfatos District, Chile

Oyu Tolgoi MineralTrend, Mongolia

Pebble DistrictAlaska

Chuquicamata6,298 Mt @ 0.62% Cu

Sill Zone(Eocene)

38 PorphyryZone

308 PorphyryZone

Mina Sur266 Mt @ 1.24% Cu

MM823 Mt @ 1.04% Cu

Wes

t Fis

sure

Fau

lt

Major Faults

Possible MajorStructural Corridor

Chuquicamata Norte1,395 Mt @ 0.35% Cu

Ulaan KhudCu-(Au-Mo) Discovery

Pro

bab

leR

egio

nalS

truc

tura

lCon

trol

Untested Anomaly

Heruga Deposit970Mt @ 0.48% Cu

10 km

Open

Open

Only ReconnaissanceExploration South

of Pebble

Pebble

Mount SharpZone (Eocene)

65 PorphyryZone

25 GoldZone

N N N

N

South Deposits Group656Mt @ 0.60% Cu

Hugo North Deposit1,639Mt @ 1.41% Cu

Hugo South Deposit490Mt @ 1.05% Cu

52 PorphyryZone

37 SkarnZone

1 GoldZone

Andina16,908 Mt @ 0.63% Cu

>0.5% Cu

San Enrique - Monolito3,700 Mt @ 0.70% Cu

Los Sulfatos4,500 Mt @ 0.90% Cu

Los Bronces3,186 Mt @ 0.39% Cu

FIG. 10. Highly simplified representation of the distribution of mineralized zones in the Pebble district in comparison toother porphyry districts which contain clusters of major deposits. Each area is illustrated at identical scale. Distribution ofdeposits and resources for Oyu Tolgoi and Chuquicamata were obtained from various corporate websites. Information ondeposits in the Andina diagram was obtained from Irarrazaval et al. (2010).


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Chapter 8 Magmatic-Hydrothermal-Structural Evolution of the … · 2020. 8. 8. · 167 Introduction...

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