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Introduction

THE LANCONES basin of northwestern Peru (also known asthe Celica-Lancones basin; Jaillard et al., 1999) represents aCretaceous marine volcanic and sedimentary succession withlarge tonnage deposits of base and precious metal-bearingmassive sulfides around Tambogrande (Tegart et al., 2000;

Winter et al., 2004). The basin overlaps both the WesternCordillera and the para-Andean depression of northwesternPeru and southwestern Ecuador (Fig. 1) and represents thenorthernmost of a series of Late Jurassic to Early Cretaceous

continental margin, arc-related rift basins extending along theSouth American western margin through Peru (Huarmey and

Caete basins or western Peruvian trough; Myers, 1974; Cob-bing et al., 1981; Atherton et al., 1983), Chile (Coast Range;Vergara et al., 1995), and Argentina (Rocas Verdes; Dalziel,1981; Hanson and Wilson, 1991). In comparison to the othersegments of the marginal rift system, the Lancones rift ormarginal basin is well preserved and includes the arc andforearc components as well as accreted allochthonous crustalblocks known as the Amotape Range (Fig. 2).

In this paper the geology of the Lancones basin, with em-phasis on the volcanic successions and the eastern portion ofthe basin, is reviewed based on recent mapping and detailedexamination of diamond drill core. Field evidence in particular

Volcanic Stratigraphy and Geochronology of the Cretaceous Lancones Basin,Northwestern Peru: Position and Timing of Giant VMS Deposits

LAWRENCE S. WINTER,1,,* RICHARD M. TOSDAL,1 JAMES K. MORTENSEN,2 AND JAMES M. FRANKLIN3

1Mineral Deposit Research Unit, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4

2Department of Earth and Ocean Sciences,University of British Columbia,6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4

3Franklin Geoscience Ltd., 24 Commanche Drive, Nepean, Ontario, Canada K2E 6E9

Abstract

A ~10-km-thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the CretaceousLancones basin in northwestern Peru and underlies part of the Huancabamba deflection. The marine volcanicsuccessions show markedly different compositional features and depositional facies consistent with a maturingarc within a shallowing marine basin. The earliest volcanism accompanying rifting was dominated by basalticpillow lava and breccia with lesser aphyric to feldspar-quartz porphyritic felsic volcanic rocks. These volcanicsuccessions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sul-fide (VMS) deposits, which are inferred to have formed in a localized but relatively deep marine setting. U-Pbzircon dating of felsic volcanic rocks associated with VMS deposits at Tambogrande indicates ages from 104.8 1.3 to 100.2 0.5 Ma for the ore-bearing volcanic sequence. The timing of onset of rift-related volcanism isnot well constrained but is therefore of middle Albian age or older. Subsequent latest Albian to Turonian vol-canism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields U-Pb zircon agesof 99.3 0.3 to 91.1 1.0 Ma. These later volcanic successions are intercalated and overlain by siliciclastic andcarbonate sedimentary sequences prevalent in the western forearc section of the Lancones basin. Finally, thebasin was intruded by Late Cretaceous to Tertiary granitoids of the Coastal batholith.

The genesis of the Cretaceous Lancones basin and other equivalent volcanic rift-related, marginal basins inwestern South America, including the western Peruvian trough, is related tectonically to the break-up of Gond-wana. Early volcanism and associated VMS deposits formed in the Lancones basin during the Albian coincidedwith the initial rifting stage, prior to active oceanic spreading, between South America and Africa. During thistime the relatively stationary western margin of continental South America was undergoing extension and rift-ing due to a westward and oceanward retreating arc, resembling a Mariana arc-type setting. The Mochicaorogeny marks the termination of rifting, subsidence, and related volcanism along the western margin of SouthAmerica. This orogenic event also broadly coincides with the onset of spreading of the South Atlantic and west-

ward drift of the South American continent. Subsequent volcanism in the Lancones basin was more continen-tal arclike under an Andean-type scenario.

Corresponding author: e-mail, [email protected]*Present address: Altius Minerals Corporation, Suite 202, 66 Kenmount

Road, St. Johns, Newfoundland and Labrador, Canada A1B 3V7.

2010 by Economic Geology,Vol. 105, pp. 713742 Submitted: June 9, 2009Accepted: February 23, 2010

Economic GeologyBULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

VOL. 105 JuneJuly NO. 4


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has been used to redefine the formations. This documentationrepresents a detailed account of the volcanic successions of theLancones basin and places the Tambogrande volcanogenicmassive sulfide (VMS) deposits in a stratigraphic and geotec-

tonic context. These VMS deposits represent the largest of themassive sulfide deposits in Peru (Vidal, 1987) and representthe most significant group of VMS deposits in South America.Resources in all categories include TG1: 109 million metrictons (Mt) grading 1.6 percent Cu, 1.0 percent Zn, 0.5 g/t Au,and 22 g/t Ag, plus 16.7 Mt grading 3.5 g/t Au and 64 g/t Ag inoxide ore; TG3: 82 Mt grading 1.0 percent Cu, 1.4 percent Zn,0.8 g/t Au, and 25 g/t Ag; and B5: resource not defined (Man-hattan Minerals, 2002). Furthermore, they are within theupper 3 percent of all bimodal-mafictype VMS deposits(Barrie and Hannington, 1999) globally with respect to con-tained tons of base metals (Franklin et al., 2005).

The first U-Pb zircon ages for volcanic rocks from the Lan-cones marginal basin of northwestern coastal Peru are pre-sented herein and used to establish a chronostratigraphicframework for the volcanic successions of the Lancones basin,

thus helping constrain the timing of VMS formation. The vol-canic depositional setting including the VMS environs of theLancones basin and the evolution of the setting through timeare presented and compared to the volcanic stratigraphy ofthe other penecontemporaneous Cretaceous sequences inPeru. Finally, we discuss the Mesozoic volcanic arc in the con-text of the tectonomagmatic evolution of South America. Be-cause northwestern Peru lies in a major oroclinal bend, theHuancabamba deflection, additional constraints on the strati-graphic, volcanological, and tectonic development of this re-gion serves to improve our understanding of the tectonomag-matic history the Andes as a whole.

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Ecuador

Chile

Peru

Colombia

Boli

via

Brazil

69o 18

o

10o

2o

6o

14o

81o

77o

73o18

o

OLM

OS

MASSIFAMOTAPE

RANGE

PARA-ANDEAN

DEPRESSIONS

BRAZILCRATON

INTE

RMONTANE

DEP

RESSIONS

INTERMONTANE

DEPRESSIONS

EASTERN CORDILLERA

OCEANTR

ENCH

SUBANDEANBELT

WESTERN CORDILLE

RA

ALTIPLANO

CORDILLERADE LA COSTA

HuancabambaDeflection

0 100 200 300 400

CaeteB

asin

teB

i

10o

81o

77o

73o 69

o2o

6o

14o

Maria Teresa

Aurora Augusta

Santa Cruz de Cocachara district

Palma

Cerro Lindo

Balducho

LB

HB

CB

Studyarea

Tambogrande

Cerro

Blanco

Moro

FIG. 1. Morphostructural units of the Peruvian Andes (modified after Benavides-Cceres, 1999). Cretaceous marginalbasinsLancones (LB), Huarmey (HB), and Caete (CB)are superimposed. Also shown are the locations of other VMSdeposits and prospects (circles) from Vidal (1987) and Steinmller et al. (2000).


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Tectonic Setting

The Lancones basin is situated at the major oroclinal bendin the Andes, the Huancabamba deflection, which separatesthe north-northwesttrending Peruvian Andes from the

northeast-trending Ecuadorian Andes (Mgard, 1987; Mi-touard et al., 1990; Fig. 2). The tectonic evolution of theJurassic to Tertiary western margin of South American,specifically to the north of the Huancabamba deflection, waslargely a function of terrain accretion as well as variable plateconvergence directions and rates (Soler and Bonhomme,1990), the latter influenced by the late stages of Gondwanabreakup in the Early Cretaceous. These events triggered sub-duction along the western margin of the continent and markthe oldest phase of the Andean Cycle (Benavides-Cceres,1999). Throughout the Jurassic, a southeast-directed subduc-tion system was responsible for continental arc volcanism

along the Ecuadorian segment (Litherland et al., 1994),whereas a sinistral transform system dominated the Peruviansegment (Fig. 3A; Jaillard et al., 2000). A shift toward north-east-directed convergence occurred in the Early Cretaceous.

This is indicated by the termination of the arc along theEcuadorian segment.The Amotape terrane is a microcontinental block of Paleo-

zoic or older metasedimentary rocks and Triassic metaplu-tonic rocks based on U-Pb zircon ages (Noble et al., 1997;

Winter, 2008). Within the Amotape terrane in southernEcuador, high pressure metamorphosed oceanic rocks yieldcooling ages of ~132 to 110 Ma (Arculus et al., 1999; Bosch etal., 2002) that record accretion to continental South Americaduring the Neocomian. The allochthonous Amotape terrane

was transported northward and accreted in the Early Creta-ceous with northeast-trending dextral faults developed during

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LIMA PERU

BRAZIL

BOLIVIA

CHILE

TAMBOGRANDE

ECUADOR COLOMBIA

Pacific Ocean

0 500

Km

AmotapeRa

nge

Olm

os

Mass

if

Pa

cific

Oce

an

SechuraBasin

0 50Km

A

B

Peru

5 S

o

4 So

81 Wo

80 Wo

6 So

Las Lomas

Ecuador

Intrusive Rocks

Granite-Diorite

Undifferentiated

Forearc clastic sequence

Volcanic Arc Sequence

Tertiary/Quaternary

Late Cretaceous

Early Cretaceous

Precambrian/Paleozoic

Amotape Range / Olmos Massif

Subsurface Fault, dip directionSurface Fault (known, inferred)

VMS Deposit

Piura

TG1TG3

B5

study area

FIG. 2. A. Location map for the Tambogrande project. B. Regional map showing major tectonostratigraphic units ofcoastal northwestern Peru. The locations of VMS deposits (TG1, TG3, and B5) in the Tambogrande area are also shown andthe field area of this study outlined. Modified after Jaillard et al. (1999) and Tegart et al. (2000).


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clockwise rotation (Mourier et al., 1988; Fig. 3B). The accre-tion is temporally linked to, and likely triggered, the westwardrelocation of the plate boundary which manifested as a newsubduction zone along the north-northwesttrending Peru-

vian segment. Under this Mariana-type arc system, steep sub-duction and slab roll-back caused extension and attenuationin the overriding continental plate and resulted in rifting andthe formation of the Lancones basin and the western Peru-

vian trough along a north-northwest trend. This was followedby the deposition of marine sequences and the eruption oflarge volumes of mafic-dominated arc volcanic rocks (Fig. 3C;Benavides-Cceres, 1999). A component of dextral shear andassociated clockwise rotation may have been related to theopening of the Lancones basin in Albian times (Winter et al.,2002). Gravity modeling of crustal structure along the Peru-

vian continental margin indicates that a dense arch-like struc-ture of 3.0 g/cm3 underlies portions of the volcanic-domi-nated western Peruvian trough and possibly represents theintrusion of rift-related basic material into the continentalcrust (Jones, 1981).

In late Albian times the geodynamical cycle shifted towardAndean-type subduction and marked the first Andean com-pressive tectonism, i.e., the Mochica phase of Mgard (1984),and gave rise to subsequent continental arc volcanism andplutonism such as the Upper Cretaceous Coastal batholith(Fig. 3D). An increasing convergence rate through the Albian(Soler and Bonhomme, 1990), temporally linked to the open-ing of the South Atlantic, may have been responsible for thistransition in subduction zone setting. A series of postrift col-lisional events along the Northern Andes in Ecuador short-ened the Lancones basin and contributed an additional com-ponent of clockwise rotation (Mitouard et al., 1990). Theaccretion of the Pallatanga terrane in the Late Cretaceousand the Silante and Macuchi island arcs in the late Eocene to

early Oligocene (Fig. 3E; Hughes and Pilatasig, 2002; Spik-ings et al., 2005; Vallejo et al., 2009) led to the current terraneconfiguration (Fig. 3F). This compressive tectonic regime,driven by active subduction of the Nazca plate under theSouth American plate, continues to the present day. Severaldeformation events are recognized in northwestern Peru inthe Late Cretaceous and Tertiary (Jaillard et al., 1999), in-cluding a mid- to late-Albian tectonic phase correlated withthe Mochica orogeny defined in the central and northernAndes by Mgard (1984). According to Reyes and Caldas(1987) and to mapping conducted in this study, this deforma-tion is manifested mostly as broad open folds in the easternLancones basin, in general with northeast- to southwest-strik-ing, Andean-normal, fold axes (Figs. 4, 5). A series of west-

northwesttrending topographic lineaments are identifiablefrom the satellite data but do not appear to have resulted inany mapable displacement (Fig. 4). These linears may be rep-resentative of the same fracture system that controlled theemplacement of the plutonic rocks.

Data derived from petroleum exploration within the basinprovides additional constraints on the Mesozoic structural his-tory of the area. Alencastre (1980) reported a number offaults, interpreted from borehole and geophysical data to be asnormal or block and transcurrent faults (Fig. 1). Seismicallyimaged faults in the Tertiary as well as Cretaceous strata strikeeast-northeast to northeast. Tegart et al. (2000) suggested

northeast-trending subsurface Paleozoic faults imaged to thesouthwest under cover of the Sechura basin (Alencastre,1980) can be projected to the Lancones basin and inferredthese to be primary graben-bounding faults controlling the lo-cation of Cretaceous volcanic rocks and VMS deposits.

As with the segment of the Coastal batholith within thewestern Peruvian trough, the batholith in northwestern Peruhas been emplaced within the marginal volcanic arc succes-sion of the Lancones basin (Fig. 1). Limited plutonism has oc-curred in the Copa Sombrero Group to the west. Althoughmuch of the Tambogrande district has been intruded by vari-ous phases of the Coastal batholith, more voluminous pluton-ism is known to the east of the map area (Fig. 4). The LasLomas complex, a 15-km-wide zoned gabbroic to granitic in-trusion in the center of the map area yields U-Pb and Ar-Arages that suggest the time of emplacement for these rocks

was 88 to 47 Ma (Winter, 2008). No intrusive phases havebeen identified as synvolcanic in origin.

Regional Geology

The Lancones basin is limited to the east-southeast andsouthwest to north by continental crustal blocks that repre-sent the Jurassic to Early Cretaceous prerift Andean marginand were topographic highs during deposition in the Meso-zoic (Cobbing et al., 1981). To the southeast, the Paleozoic(?)Olmos massif is probably a reactivated margin of the Ama-zonian craton (Macfarlane, 1999). This poorly defined terraneconsists of pre-Ordovician greenschist facies pelitic to psam-mitic rocks overlain by platform carbonate rocks of Triassic toEarly Jurassic age, considered equivalent to the Marangeanticline farther southeast in central Peru (Cobbing et al.,1981; Reyes and Caldas, 1987; Mourier et al., 1988; Lither-land et al., 1994). Bordering the Lancones basin to the south-

west, west, and north are Paleozoic or older metasedimentary

rocks and Triassic granitic rocks of the Amotape Range(Mourier et al., 1988; Aspden et al., 1995; Noble et al., 1997;Winter, 2008).

Volcanic and sedimentary rocks of the Cretaceous Lan-cones basin can be subdivided into a volcanic arc in the eastand forearc in the west. The rocks are exposed over 135 kmin strike length and ~150 km in width through northwesternPeru and southwestern Ecuador. The basin extends beneathTertiary cover in the southwest for an additional 50 km (Fig.2). The volcanic arc rocks form a succession that is up to 80km wide and consists of mafic to felsic volcanic and volcani-clastic rocks. These uppermost sequences of the volcanic arcsuccession grade into sedimentary rocks which dominate the

western forearc portion of the Lancones basin (Jaillard et al.,

1999). The forearc turbiditic subbasin was filled with the 3-km-thick Copa Sombrero Group that interfingers at the baseof the group but, for the most part, overlaps and buries the

volcanic arc sequence (Morris and Aleman, 1975; Chvez andNuez del Prado, 1991; Jaillard et al., 1996, 1999). Late Cre-taceous to mid-Tertiary marine sequences as well as Pleis-tocene and recent sediments unconformably cover all olderrocks.

Volcanic Stratigraphy

The stratigraphic units that define the volcanic arc sequenceof the Lancones basin include a wide spectrum of volcanic


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Time

Necomianto Albian(?)

Albian(lowercontact

unknown)

Latest Albianto Turonian

late

Albian?

LateCretaceous

Stratigraphy

Late Triassic

to Proterozoic

Unconformity

Turonian

and younger

EarlyCretaceous

Tertiary -Quaternary

Phas e

1

Phas e

2

massive sulphide deposits

Unconformity

100m

TG1-TG3

SR

Cerro El Ereo FormationCoarse feldspar porphyritic volcanic &

subvolcanic rocks and breccias, minorbedded cobble-size breccias to feldspar-pyroxene-crystal-rich tuffs.

Cerro San Lorenzo FormationPillow basalt-dominated sequence, lesserbreccias. Aphyric dacite flows/breccia(SR),rare quartz phyric rhyodacite stocks(PR).Massive sulphidedeposits(MS).

San Pedro GroupDeformed, thin bedded chert to sandstones.

Amotope or Olmos massifs

Continental crustal rocks.

La Bocana FormationBasalt to basaltic-andesite and rhyoliticvolcanic and volcaniclastic rocks,including quartz-xtal and lithic tuffs, massflow deposits. Minor siliciclastic rocks.

Lancones Formation

Siliciclastic to carbonaterocks and lesserreworked volcaniclasticunits.

Sedimentary/Volcanic Sequences

PR

MS

MS

Basaltic-andesite flows

crystal and lithic felsic tuff

basaltic lavas, dominantlypillows and hyaloclastite

chert

meta-sedimentary, meta-intrusive rocks

mafic crystal and lithictuffs

Crowded plagioclasemafic flows and breccias

MS

shale- siltstone-sandstone

limestone, calcareoussiliciclastics

turbidite sequence

basalt or basaltic-andesitevolcaniclastic rocks

rhyolitic lava flowsand associated breccias /felsic dykes and plugs

Dacite

MudstonesRhyoliticvolcaniclastic

RhyoliteBreccia

Mafic dykes

LEGEND

PRSR

post-mineralization rhyolite

syn-mineralization rhyolite

FIG. 6. Schematic stratigraphic column of the eastern portion of the Lancones basin. Inset section shows the Tambo-grande area in more detail.


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Though outcrop is poor to nonexistent in the vicinity of thedeposits, an extensive drill core library was available for study.Construction of a rhyolite-dacite volcanic complex intimatelyassociated with the VMS deposits is documented by Winter etal. (2004). The extent of the felsic complex is not completelydelimited, but at TG1 and TG3 it is a minimum of 2 km in di-ameter and with a composite thickness of up to at least 300 m.

Basalt is variably feldspar (020%) and augite (05%) por-phyritic or microporphyritic and is typically vesicular with 2 to10 percent, 1- to 5-mm amygdules (Fig. 7A). Near the B5 de-posit (Fig. 1), basalt is locally scorialike and includes breccia

with millimeter-scale clasts dominated by bubble wall shards(Fig. 7B, C). Massive to pillowed basaltic flows are the mostcommon lithofacies and form monotonous nondescript inter-

vals up to several hundred meters thick. Pillows are typically0.5 to 1.0 m in diameter (Fig. 7D), commonly with radial frac-tures and onion-skintype concentric flow foliations (Fig.7E). Individual pillow lava flow units are decameters thickand associated with a variety of breccia types (Fig. 7F). Pillowfragment breccia is derived from the collapse of pillow

mounds that vary from proximal talus breccia to distal faciesshowing evidence of mass transport, e.g., reverse sorting (Fig.7G). Basaltic autoclastic breccia within the Cerro SanLorenzo Formation includes hyaloclastite and autobreccia.Hyaloclastite has angular and cuspate, mostly pebble-sizebreccia clasts with distinct clast outlines and common jigsaw-fit textures due to quench fragmentation in seawater (Fig.7H). Autobreccia, formed due to viscosity variations within acooling and flowing lava, display amoeboid-shaped clasts withabundant, fine (


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associated in situ breccias and proximal resedimented volcani-clastic units. Quartz porphyritic lavas are not recognized nearthe VMS deposits and are generally uncommon throughout

the Cerro San Lorenzo Formation. However, quartz-plagio-clase porphyritic rhyolite forms late dikes or stocks within the

volcanic complex at TG1 and elsewhere in the formation.Dacitic lava flows and breccias are conspicuous in the imme-diate hanging wall of the TG1 and TG3 deposits. The daciteis characterized by distinctive pale gray-green, aphyric tofeldspar porphyritic textures, and is commonly amygdule rich(Fig. 8D).

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E

G

F

H

I

Dk

V

MF

1 m

1 m

FIG. 7. (Cont.)

A

B

FIG. 8. Drill core photographs of intermediate and felsic rocks from theCerro San Lorenzo Formation. A. Massive feldspar porphyritic rhyolite.Scale units are in millimeters. TG1 area. B. Flow-banded rhyolite autobrec-cia; these breccias typically grade into lavas. Textures partly masked by quartzand sericite alteration. TG3 area. C. Rhyolitic, unsorted, clast-supported, vol-caniclastic rock with pebble-size clasts and massive sulfide fragments (near

the TG3 deposit). D. Green-gray, feldspar porphyritic dacite with large flow-foliated chlorite-quartz pipe amygdules. Hanging wall to TG3 deposit.

C

D


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Sedimentary rocks in the Cerro San Lorenzo Formation arelimited to sparse, thin-bedded to laminated, black, carbona-ceous mudstones in decimeter-thick units. Although volumet-rically minor, these pelagic sedimentary rocks are ubiquitousthroughout this formation. No other sedimentary rock typesare known to occur in the Cerro San Lorenzo Formation.

Cerro El Ereo FormationThough the lower contact with the Cerro San Lorenzo For-

mation is not exposed, the upper contact with the La BocanaFormation is conformable and relatively sharp. This sequencehas a rather limited geographic distribution in the western-central part of the area and does not extend north of the LasLomas pluton (Fig. 4). This study restricts the rocks includedtherein to those of similar character that crop out on the west-ern side of the study area, specifically in the vicinity of CerroEl Ereo (Figs. 4, 5) and not as widespread as suggested by

Reyes and Caldas (1987). The Cerro El Ereo Formation hasan approximate thickness of up to 2,000 m.

The Cerro El Ereo Formation is an entirely mafic volcanicsequence defined by distinctive and monotonous coarsecrowded feldspar porphyritic breccia and minor coherent lavaflows and dikes. Subhedral feldspar phenocrysts to glomero-crysts range from 1 to >10 mm, averaging 4 to 5 mm (Fig.9A). Amygdaloidal lava flows are uncommon, though amyg-daloidal clasts are locally present in breccia. Volcaniclasticrocks in this formation are typically nonstratified, matrix-sup-ported, subangular to subrounded boulder breccias (Fig. 9B),or cobble- to pebble-sized lithic and feldspar crystal tuff (Fig.9C). Minor, thin-bedded feldspar and rare pyroxene crystaland ash tuff, possibly reworked, occur near the upper contactof the formation (Fig. 9D). The sequence is also distinctive inthe complete absence of felsic volcanic and sedimentaryrocks. No pillow lava or autoclastic breccia units similar to


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A

C

B

D

W

CA

CM

10 cm

1 cm

FIG. 9. Field photographs of mafic rocks from the Cerro El Ereo Formation. A. Typical porphyritic textures of the CerroEl Ereo Formation porphyritic basalt. Sample contains ~20 percent feldspar phenocrysts to >1 cm in a nonamygdaloidal,aphanitic matrix. Subvolcanic or thick-flow facies. B. Bleached-looking, boulder-size, subround clasts (C) of basalt feldsparporphyry in a fine matrix of dark gray feldspar porphyritic material (M). Clasts show in situ breccia textures (jigsaw-fit) at-tributed to progressive fragmentation of blocks during transport. C. Unsorted, nonstratified basaltic cobble- to pebble-sizedlithic and feldspar crystal-bearing volcaniclastic rock. The sample contains an equal proportion of aphyric to weakly feldsparporphyritic (W) clasts and coarse feldspar (C) porphyry clasts. Amygdaloidal clasts (A) are present but are generally not com-mon. Clast margins often are not easily discernable. D. Thin- to thick-bedded feldspar crystal to ash-sized tuff; reworked fa-cies at top of formation.


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those that are common in the Cerro San Lorenzo Formationare known in the Cerro El Ereo Formation. The dominanceand textural uniformity of poorly vesicular, matrix-supported,nonstratified breccia, with distinctive porphyritic juvenile and

variably abraded clasts, and the limited geographic extent ofthe unit suggest that the formation may be the result of a lo-calized and unique volcanic edifice.

La Bocana Formation

The La Bocana Formation marks a return to bimodal vol-canism of basaltic-andesite and rhyolitic rocks that is compo-sitionally different from the Cerro San Lorenzo Formation

(Winter, 2008) and has a greater abundance of volcaniclasticrocks. The presence of pyroclastic deposits, including crystal-rich tuff, may indicate a shift to a relatively more shallow

water depositional setting when compared to the Cerro SanLorenzo Formation. Reyes and Caldas (1987) reported anupper Albian age for this sequence based on fossil evidence.The La Bocana Formation has an estimated thickness of3,500 m with conformable upper and lower contacts.

Mafic rocks in this formation include highly vesicular thickflows and dikes (Fig. 10A) with well-developed flow foliations(Fig. 10B). Flows are observed to grade into autoclastic depositsof unsorted coarse breccia (Fig. 10C). Breadcrust texture is

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M T

M

A

C

E

B

D

F

So

granodioritedyke

2 cm

Flo

wfo

liatio

n

dir

ection

FIG. 10. Field photographs of mafic rocks from the La Bocana Formation. A. Moderately west dipping, thick massivebasaltic-andesite flows. Felsic stocks and dikes cut perpendicular to bedding. B. Basaltic andesite dikes with strongly flow fo-liation defined by flattened and large vesicules (silica amygdules up to 30 cm). C. Polylithic, basaltic-andesitedominated,mass-flow deposit. Note the fractures and in situ fragmentation of the clasts due to mass transport (see arrows). D. Crackedand brecciated outer crust of basaltic andesite lava flow and interstitial hyaloclastite resulting from quenching of exposedlava. E. Mafic lobes (M) injected into felsic quartz-crystal tuffs (T). Tuffs show soft-sediment deformation textures andmafic flows show columnar jointing indicating tuffs were nonwelded and/or nonlithified during deposition of mafic flows. F.Lithic and quartz-feldspar crystal rhyolitic tuff. Coloration of the domains are a result of secondary recrystallization toquartzo-feldspathic (light) and chloritic (+clay) assemblages due to devitrification of glass component.


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illustrated by chilled and fractured margins of lava flows withinterstitial hyaloclastic breccia (Fig. 10D). Pillow lava is pre-

sent in only a few localities. However, polygonal jointed maficlava flow lobes, apophyses, and dikes in volcaniclastic depositssuch as felsic crystal tuffs (Fig. 10E) are common.

Felsic rocks range from coherent facies of quartz- and/orfeldspar- porphyritic lava (lava flows, domes, dikes) to crys-tal-, lithic-, and pumice-bearing tuffs (Figs. 2, 10F, G). TheLa Bocana Formation is the only sequence in the Lanconesbasin to include felsic tuffaceous rocks indicative of pyro-clastic eruptions. Another feature of the felsic volcanic rocksis that sparse granitic xenoliths are present in several flowsand were derived either from older crustal basement or fromrelated igneous plutonic roots carried in penecontemporane-ous eruptions.

Volcaniclastic rocks make up a significant proportion of this

unit and range from chaotic, matrix-supported, angular, boul-der-size breccia (Fig. 10H) to well sorted, pebble-size brecciaand decimetric layers of cross-bedded volcanic sandstone(Fig. 10I). The boulder breccia occurs as either talus depositsrelated to fault scarps or mass flows. Other deposits of well-sorted coarse clastic rocks suggest a high-energy shallow-ma-rine to fluvial environment. Basaltic-andesite generally domi-nates the clast composition, though felsic volcanic clasts alsooccur. Locally, calcareous sedimentary clasts are abundant andoccur as blocks up to boulder size. Mafic and felsic volcanicrocks are intercalated with the volcaniclastic units and suggestthat volcanism was also active in a shallow-marine setting.

Lancones Formation

The Lancones Formation consists of a basal sequence of

polylithic, basaltic to andesitic, volcaniclastic units. Thesethick-bedded and variably stratified breccia are intercalatedwith siliciclastic and calcareous sedimentary units, includinglimestone, calcareous sandstone, siltstone, and graywacke(Fig. 11A, B). Sedimentary rocks become more abundant to-

ward the top of the unit. Reyes and Caldas (1987) reportedfossils in the age range of late Albian and Early Cenomanian.These rocks are interpreted to have been deposited in a rela-tively shallow marine environment gradational into the fore-arc sedimentary sequences of the Copa Sombrero Groupsenso lato in the western Lancones basin (Figs. 4, 11).

U-Pb Geochronologic Data

Sample preparation and analytical procedures for U-Pbgeochronology are described in Appendix 1. Two U-Pb tech-niques were utilized because of the subtle inherited zirconcores to many of the zircon populations. Thermal ionizationmass spectrometry (TIMS) and sensitive high resolution ionmicroprobe-reverse geometry (SHRIMP-RG) analytical dataare presented in Tables A1 and A2. Rock sample and zircondescriptions are provided in Table 1 and shown in the map(Fig. 5). A total of 13 U-Pb zircon analyses are discussed indetail below. Summarized results are also plotted onschematic regional- and mine-scale stratigraphic sections(Fig. 12). All errors for these ages are presented as 2unless


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G H

H

I

FIG. 10 (cont.). G. Rhyolitic quartz crystal-rich and lithic tuff with flow-banded rhyolite clasts. H. Coarse, boulder brec-cia with chaotic, unsorted, subround (pillow?) clasts. Talus breccia. I. Medium-bedded, well-sorted, and locally cross-bedded(arrow), pebble- to sand-sized, mafic-dominated volcaniclastic rocks.


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otherwise specified. TIMS analyses were completed during2002 to 2004 and SHRIMP-RG work was performed in 2003and 2004.

Felsic and intermediate volcanic rocks sampled from theproject area are generally aphyric or weakly porphyritic andwere typically found to yield low quantities of zircon concen-trates. In addition, some samples analyzed by TIMS produced

an older age due to an inherited Proterozoic component.Therefore, a combination of TIMS and SHRIMP-RG analy-ses was selected due to the variability of zircon contents as

well as to characterize igneous crystallization and inheritedcomponents. Among the samples processed, with two ex-ceptions, only quartz porphyritic varieties of felsic volcanicrocks contain zircon. Nine rock samples of dacitic to rhyolitic

726 WINTER ET AL.

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A B

FIG. 11. A. Thin-bedded arenaceous sequence from the Lancones Formation; massive unit at top of outcrop is a dioritesill. B. Thin-bedded limestones and limey arenites.

Time

Unconformity


Albian(lowercontact

unknown)

LatestAlbianto Turonian

lateAlbian?

LateCretac

eous

Stratigraphy

Late Triassicto Proterozoic

Unconformity

Turonianand younger

?

EarlyCretaceous

Tertiary -Quaternary

Pha s e 1

Pha s e

2

100

m

99.3 +/-0.3 LW-086

97.0 +/-0.4 LW-010

98.9 +/-0.7 LW-078

91.1 +/-1.0 LW-043

104.8 +/-1.3 TG1-136

103.2 +/-1.0 LW-016

99.8 +/-1.6 LW-013

100.2 +/-0.5 TG1-111

90.3 to 95.3 LW-077

72.4 - 79 Ma

and 88 Ma

47-55 Ma

FIG. 12. Schematic stratigraphic column of the eastern portion of the Lancones basin. Legend as in Figure 6. Inset sec-tion shows the Tambogrande VMS section in more detail. Age data from this study are shown along with sample numbers intheir relative stratigraphic positions. Ages of plutonic rocks from Winter (2008).


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volcanic rocks from the eastern Lancones basin were dated byU-Pb zircon methods, including three samples from theCerro San Lorenzo Formation and six samples from the LaBocana Formation. Four additional samples from the La Bo-cana Formation displayed strong inheritance and did not

yield igneous ages. U-Pb zircon ages are presented from fourTIMS and six SHRIMP-RG analyses. A further 25 samples

were processed but did not yield zircons.

Volcanic rocks of the Cerro San Lorenzo Formation

Sample TG1-136 (rhyolitic volcaniclastic) represents theimmediate hanging-wall strata to the TG1 massive sulfide de-posit. This and all other samples yielded zircons that weremostly clear, colorless, stubby to elongate, euhedral prisms.The sample produced a small yield of


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0361-0128/98/000/000-00 $6.00 729

200

600

0.02

0.04

0.06

0.08

0.10

0.12

0.14

02 04 06 08 0 100 120

238U/

206Pb

207Pb/206Pb

200

600

0.02

0.04

0.06

0.08

0.10

0.12

0.14

02 04 06 08 01 00 12

238U/

206Pb

207Pb/206Pb

600

200

0.02

0.04

0.06

0.08

0.10

0.12

0.14

02 04 06 08 0 100 120

238U/

206Pb

207Pb/206Pb

200

600

0.02

0.04

0.06

0.08

0.10

0.12

0.14

02 04 06 08 0 100 12

238U/

206Pb

207Pb/206Pb

600

200

0.02

0.04

0.06

0.08

0.10

0.12

0.14

02 04 06 08 0 100 120

238U/

206Pb

207Pb/206Pb

Age = 98.9 0.7 Ma(MSWD = 1.1)

206

238

Pb/U

age(Ma)

Age = 99.8 1.6 Ma(MSWD = 3.2)

206

238

Pb/U

age(Ma)

Age = 103.2 1.0 Ma

(MSWD = 1.3)

206

238

Pb/U

age(Ma)

80

90

100

110

120

Age = 104.8 1.3 Ma

(MSWD = 1.8)

206

238

Pb/

Uage(Ma)

80

90

100

110

120

80

90

100

110

120

80

90

100

110

120

Age = 91.1 1.0 Ma(MSWD = 3.7)

206

238

Pb/U

age(Ma)

80

90

100

110

120

LW-078

,Rhyolite BoulderVolcaniclasticLa BocanaFormation

TG1-136

,Cerro San Lorenz oFormation

Rhyolite PebbleBreccia

LW-013

,La BocanaFormation

Rhyolite QuartzPorphyry Dyke

LW-016

,Rhyolite QuartzPorphyry DykeCerro San Lorenz oFormation

D

LW-043

,

Rhyolite Quartz-Feldspar PorphyryDykeLa BocanaFormation

A B

C

E

FIG. 13. 238U/206Pb vs. 207Pb/206Pb Tera-Wasserburg plots (Teraand Wasserburg, 1972) for various volcanic rock samples from theCerro San Lorenzo and La Bocana Formations. Error ellipses are2. Dashed lines indicate data points omitted vs. solid lines/grayellipses for data included in the age calculation. Inset figures showbox plots for all sample points for 207Pb-corrected 206Pb*/238U data

with error bars at 2.Open boxes are omitted, whereas solid boxeswere included in the age calculation. Ages given are 206Pb/238Uwith 2uncertainties.


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B and each of the two more discordant analyses yield calculatedupper intercept ages of 1.16 and 1.51 Ga, suggesting a mainlyMesoproterozoic average for the inherited zircon component.The 206Pb/238U age of fraction B is surprisingly young (73.5 Ma),however, indicating either that this sample has a considerably

younger crystallization age than all of the other dated samplesor that this fraction experienced much stronger postcrystal-lization Pb loss than was evident from the systematics of anyof the other samples. A crystallization age for this sample can-not be assigned on the basis of the available analytical data.

730 WINTER ET AL.

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90

92

94

96

98

B

C

D

E

F

A

0.090 0.094 0.098 0.102

0.0135

0145

0.0155

206 238Pb/ U ages of6 fractions

range from 90.3 & 95.3 Ma

LW-077

,Dacite Flow/

Autobreccia La BocanaFormation

94

96

98

100

102

104

A

B

C

D

E

0.095 0.105 0.1150.0145

0.0155

0.0165

TG1-111

,Cerro San Lorenzo

Quartz-FeldsparPorphyry Rhyolite Dyke

Formation

94

96

100

A

CD

E

0.095 0.099 0.107

0.0144

0.0150

0.0156

Mean Pb/ U age of206 238 A-C-D = 97.0 0.4 Ma

LW-010

,

Dacite Feldspar

Porphyry La BocanaFormation

40

80

120

160

200

240

280

320

360

400

A

B

C

0.00 .2 0.40 .6 0.8

0.0

0.02

0.04

0.06

to 1.51 Ga

to 1.16 Ga

2-pt. regressions through BL.I. = 67.8-70.7 Ma

LW-051

,La BocanaFormation

Quartz-LithicRhyolite Tuff

92

96

100

104

A

BC D

E

F

0.092 0.098 0.104

0.013

0.017

Mean Pb/ U age of206 238

B & C = 99.3 0.3 Ma

A&D have inheritanceE&F have Pb-loss

LW-086,

La Bocana FormationRhyolite Boulder Breccia

D

A B

C

E

Mean Pb/ U age ofA & E = 100.2 +/- 0.5 Ma206 238

206Pb

/238U

206Pb/238U

206Pb

/238U

206Pb/238U

206Pb/238U

207Pb/235U

207

Pb/

235

U

207Pb/235U

207

Pb/

235

U

207Pb/235U

FIG. 14. 207Pb/235U vs. 206Pb/238U U-Pb concordia plots for var-ious volcanic rock samples from the Cerro San Lorenzo and LaBocana Formations.


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Sample LW-066 (quartz porphyritic rhyolite breccia) yieldeda small quantity of zircon analyzed by SHRIMP-RG. Mostsamples show strong Pb loss and unrealistically young206Pb/238U ages, whereas older ages were measured ranging to1898 Ma. Two data points yield 206Pb/238U ages of 99.4 and99.8 Ma and are in agreement with the range of ages deter-mined for other volcanic rocks in this area (97.099.3 Ma).

Zircons from samples LW-026 (quartz-feldspar porphyriticrhyolite dike) and LW-033 (quartz-feldspar porphyritic rhyo-lite breccia) also yield dominantly older and inherited206Pb/238U ages ranging up to ~2540 Ma. Single zircons fromeach of samples LW-026 and LW-033 yield 206Pb/238U ages of90.1 and 90.9 Ma, respectively. These ages are in agreement

with ca. 91 Ma rhyolite (LW-043) sampled in this part of theLancones Formation.

Discussion

Depositional evolution of the Lancones basin

The 10-km-thick stratigraphic package that defines the vol-

canic arc sequence of the Lancones basin represents a largebimodal eruptive event in the mid-Cretaceous within a mar-ginal basin that presently is 150 km wide. The stratigraphysuggests a progressively shallowing basin and evolving depo-sitional environment through the mid-Cretaceous. Althoughthe continental crust at the Pacific margin would have beensignificantly thinner than the present-day western Andes, theamount of crustal attenuation and subsidence was substantial.An ensialic rift event related to a Mariana-type suprasubduc-tion zone is envisaged to account for the volcanism. The totalinferred thickness of the Lancones basin successions impliesa synsubsidence depositional environment. Rifting, crustalsubsidence, volcanism, and sedimentation were synchronous.

The Cerro San Lorenzo Formation is interpreted as a rela-

tively deep water facies based on the absence of pyroclasticrocks and the presence of pelagic sedimentary rocks. Al-though neither direct nor unequivocal evidence of paleowaterdepth, a volcanic succession comprised entirely of massive topillowed lava flows and associated autoclastic deposits may beinterpreted as a result of deep-water suppression of eruptinglavas resulting in effusive-only eruptions (Cas, 1992; Batizaand White, 2000; White et al., 2003). A deep-water environ-ment is also consistent with the inferred depositional settingand deposit type, i.e., metal budget and hydrothermal alter-ation, of VMS deposits at Tambogrande (Tegart et al., 2000;

Winter et al., 2004). Specifically, the deposits have high cop-per and zinc to lead ratios, lack significant gold enrichment,and display no evidence of boiling of hydrothermal fluids,

suggesting a relatively high hydrostatic pressure controlled byrelatively deep water depth (Franklin et al., 2005; Hanning-ton et al., 2005).

The Cerro San Lorenzo Formation lacks abundant siliclas-tic or distal volcaniclastic deposits despite the inferred deep-

water setting in a narrow rifted continental basin. With conti-nental topographic highs inferred at both margins, a greaterproportion of sedimentary rocks might be expected in a deepbasin or at least close to the margins of the basin. Indeed, theSan Pedro Group, interpreted as Albian in its upper parts, hasbeen mapped along the eastern portions of the Lanconesbasin (Reyes and Caldas, 1987) and may be the slope facies of

the eastern margin of the basin in the early synrift phase. TheCopa Sombrero Group forearc sedimentary sequence tempo-rally overlaps with the entire volcanic arc succession along the

western margin. Clearly sedimentation was active during thevolcanic episodes. Therefore, it is possible that the Cerro SanLorenzo Formation represents an ocean ridge-type structure,analogous to back-arc volcanic ridge edifices in modern set-tings. For comparison, the Manus basin continental back-arcrift hosts an active 700-m-high bimodal volcanic ridge flankedby siliciclastic rocks (Taylor et al., 1995). If the Cerro SanLorenzo Formation had been built as a relatively topograph-ically high volcanic edifice, the interstratification of volcanicand/or volcaniclastic and sedimentary rocks may be limited tothe flanks of the volcanic ridge.

Timing and duration of the volcanic arc

U-Pb TIMS and SHRIMP-RG zircon ages presented inthis paper permit an estimation of the timing and duration ofthe arc volcanism in the Lancones basin (Fig. 12). Volcanismcommenced at a minimum of ca. 105 Ma (Albian) and con-

tinued to at least ca. 91 Ma (Turonian), suggesting a minimumvolcanic arc lifespan of about 14 Ma. The upper age limit ofthe volcanic succession is further constrained with U-Pb zir-con ages of 78 to 88 Ma for granitic rocks of the Coastalbatholith that intruded into the Lancones basin. In theEcuadorian segment of the Lancones basin, granodiorite atthe Los Linderos porphyry Cu-Mo-Au prospect (Chiaradiaand Paladines, 2004) cuts basaltic rocks and is dated at 88.4 1.0 Ma (Winter, 2008). Compared to the youngest Lanconesbasin volcanic rocks dated in this study (91.1 1.0 Ma), thereis a minimal temporal transition from arc volcanism to arcplutonism. Likewise, a temporal overlap from volcanism toplutonism is recognized within the Coastal batholith in the

western Peruvian trough (Pitcher et al., 1985; Soler and Bon-

homme, 1990; Polliand et al., 2005).The duration of volcanism is also well constrained based onthe data reported herein (Fig. 12). The Cerro San LorenzoFormation ranges from an oldest age of 104.8 1.3 Ma (sam-ple TG1-136) to a minimum of 100.2 0.5 Ma (sample TG1-111), indicating a minimum lifespan of about 3 to 6 Ma. A fel-sic dike in the uppermost Cerro San Lorenzo Formationdated at 99.8 1.6 Ma (sample LW-013) is considered to bea feeder dike to the La Bocana Formation. The La BocanaFormation volcanic rocks are limited to a maximum age of99.3 0.3 Ma (sample LW-086) to a lowermost age of 91.1 1.0 Ma (LW-043) and suggest a minimum duration for vol-canism of 7 to 10 Ma.

No volcanic rocks from the Cerro El Ereo Formation

were suitable for dating methods employed in this study;however, the approximate age range can be deduced fromthe given ages of the overlying La Bocana Formation andunderlying Cerro San Lorenzo Formation (Fig. 15). Vol-canic rocks of the Cerro El Ereo Formation must be

younger than 100.2 Ma. However, the minimum age re-quires some consideration of the stratigraphy and spatialdistribution of the various formations. First, the oldest vol-canic rocks in the La Bocana Formation occur in the north-ern part of the map area and are dated at 99.3 Ma (or 99.8Ma for felsic dikes in the uppermost Cerro San LorenzoFormation near the inferred contact with the La Bocana


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Formation). These values suggest a potential 3 Ma intervalfor eruptions of the Cerro El Ereo Formation lavas. How-ever, the contact between the Cerro San Lorenzo Formationand La Bocana Formation is uncertain and the Cerro ElEreo Formation is not present in the north part of the maparea (Fig. 5). Conversely, south of the Las Lomas complex,

the La Bocana Formation is known to be conformable abovethe Cerro El Ereo Formation. Age constraints in this part ofthe La Bocana Formation are much younger, such as 91.1 1.0 Ma, indicating a time interval that spans at least 8 to 11Ma for duration of the Cerro San Lorenzo and La BocanaFormations. Under this scenario in the southern Lanconesbasin, the Cerro El Ereo Formation would have potentiallybeen deposited contemporaneously with the deposition ofthe lower part of the La Bocana Formation to the north.Due to the monotonous basaltic volcanism and paucity ofsedimentary rocks in the Cerro El Ereo Formation, a singlelarge volcanic event is postulated that may not have requireda significant time interval.

Age of massive sulfide deposits

All known VMS deposits and prospects are associated withthe Cerro San Lorenzo Formation, the lowermost of the vol-canic sequences of the Lancones basin. The Cerro SanLorenzo Formation represents the initial magmatic pulse ofthe marginal basin and/or rift sequence and appears to havebeen deposited in a relatively deep water environment duringa period of maximum subsidence. During subsequent stagesof volcanism (i.e., the Cerro El Ereo, La Bocana, and Lan-cones Formations), successions become progressively more

volcaniclastic- and sediment-rich and appear to have been de-posited in relatively shallow water as late-rift, basin-fill se-quences. Therefore, the association of VMS deposits with theCerro San Lorenzo Formation may be linked to the estab-

lishment of large-scale hydrothermal systems in the early-rifttectonic stages, during the most likely period of high magmainput, maximum heat flow, and high permeability due to rift-related extensional and transcurrent faulting.

Winter et al. (2004) provided a detailed reconstruction forthe TG1 and TG3 VMS deposits whereby equivalent time-stratigraphic horizons can be correlated between each of theTG1 and TG3 deposits. Therefore, the formation of thesemassive sulfide deposits is considered of practically identicalage. The B5 VMS deposit, located 10 km to the south, couldnot be represented on a continuous section with TG1 andTG3 but is hosted in similar volcanic strata.

The minimum age of massive sulfide formation at TG1 iswell constrained by several U-Pb zircon dates. The immedi-ate hanging-wall felsic volcaniclastic rocks have been dated at104.4 1.9 Ma (sample TG1-136). The sample is a sand- topebble-size, moderately well sorted, dacitic volcaniclastic unitthat, based on the volcanic reconstructions of the depositional

environment (Winter et al., 2004), represents a distal facies ofa sea-floor rhyolite flow-dome complex. The rhyolite eruptionwas most likely synchronous with or slightly postdates massivesulfide formation. Additional age constraints on the depositsare thus provided by a rhyolite dike that intrudes the TG1 de-posit. This felsic unit represents a postmineralization phaseand provides a minimum age limit of 100.2 0.5 Ma for for-mation of the TG1 and TG3 deposits. Footwall rocks to the

VMS deposits did not yield zircons or other minerals suitablefor U-Pb dating, and therefore the maximum age for the mas-sive sulfide system could not be determined.

Comparison of the Lancones basin to thewestern Peruvian trough

Geochronologic and stratigraphic data presented hereinverify the broadly contemporaneous evolution of the volcanicsuccessions of the Lancones basin and the western Peruviantrough (Cobbing et al., 1981). Stratigraphically, the CasmaGroup that filled the western Peruvian trough has a totalthickness of up to 9 km and is dominated by mafic with lesserfelsic volcanic rocks (Myers, 1974; Offler et al., 1980; Cob-bing et al., 1981; Soler and Bonhomme, 1990), comparable tothe 8 to 10 km of mafic-dominated bimodal strata of the Lan-cones basin (Fig. 16). Volcanism recorded by the CasmaGroup was largely terminated during the late Albian-EarlyCenomanian, indicating a short-lived but vigorous volcanicevent.

The Mochica phase, which represents the first contrac-

tional orogeny in the central Peruvian Andes (Mgard, 1984),affected the Casma Group at the Albian-Cenomanian bound-ary. This effectively represents the termination of the rift-stagemarine volcanism. Volcanic rocks postdating the Mochicaphase were interpreted to be products of shallow-marine tosubaerial volcanism (Cobbing et al., 1981) and these erup-tions overlapped the initial phases of the Coastal batholith(Pitcher et al., 1985; Soler and Bonhomme, 1990). A marinetransgression and the deposition of shelf carbonates in the

western Peruvian trough are recorded during the Cenoman-ian to Turonian (Jaillard and Soler, 1996). Although deforma-tion related to the Mochica orogeny is not well documented

732 WINTER ET AL.

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Rhyolite

Las Lomascomplex47-88 Ma

104.8 - 100.2 Ma

99.3 Ma

91.1 Ma

Cerro San LorenzoFormation

La BocanaFormation

LanconesFormation

NorthSouth

Cerro El Ereo

Formation

contacts

unknown

contactnotexposed

not exposed

conformablecontact

conformable

General lithology

siliclastic

Mafic/felsicvolcanic

Mafic

volcanic

Mafic/felsicvolcanic

99.8 Ma

Dyk

e

?

FIG. 15. Schematic stratigraphy and U-Pb zircon ages that constrain the volcanic formations in the Lancones basin.


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in the volcanic rocks of the Lancones basin, the timing of the

contractional episode coincides with the transition from theCerro San Lorenzo Formation to the La Bocana Formation.Moreover, the facies change from deep-(?) water, pillow lava-dominated volcanic rocks of the Cerro San Lorenzo Forma-tion to mixed pyroclastic, siliciclastic, and carbonate rocks,representing dominantly shallow water environments of theLa Bocana Formation and younger sequences suggests amajor geodynamic change, the timing of which is similar tothat described for the upper Casma Group (Cobbing et al.,1981). The stratigraphic thickness, relative age, volcanic fa-cies, and inferred depositional environment of the westernPeruvian trough is remarkably similar to the Lancones basinand suggests that both basins evolved along similar pathways.

We note, however, that there are some metallogenic differ-

ences between VMS deposits of the Lancones basin and thoseof the western Peruvian trough (i.e., Huarmey and RioCaete basins; Vidal, 1987). Most notable is the fact thatmany western Peruvian trough VMS deposits are relativelysmall, tend to be more zinc and lead dominant, and also pre-cious metal poor when compared to the giant, pyritic, copper-,zinc-, and gold-bearing deposits of the Lancones basin.

Tectonic implications

The fundamental processes required to generate a marginalrift basin related to subduction along a continental margin areconsidered to be fairly well understood with the prerequisites

being a subducting and sinking ocean slab and a trench line

that migrates oceanward at a greater rate than the overridingplate, termed slab roll-back (Hamilton, 1995). Benavides-Cceres (1999) suggested that the Lancones basin marginalrift resembled a Mariana-type arc (Karig et al., 1978) whichforms when the overriding plate, either oceanic or continen-tal, is attenuated and rifted due to tensional stresses associ-ated with a steeply dipping slab, roll-back, and arc migration.The stratigraphic and geochronologic data in this study helpto reconcile the tectonic regime related to the formation ofthe Lancones basin and, by inference, the western Peruviantrough.

The formation of the western Peruvian trough is temporallylinked with a major global tectonic event, the final break-upof Gondwana, which culminated in the opening of the South

Atlantic Ocean in the Early Cretaceous (Scotese, 1991). Morespecifically, the Albian represents the time of final separationof South America from Africa and opening of the equatorialSouth Atlantic ca. 105 Ma (Sibuet et al., 1984; Scotese, 1991).The lack of significant westward motion of the South Amer-ica plate during the Albian was of major importance to theformation of the western Peruvian trough and Lancones basinas a steeply subducting but retreating oceanic slab along thePacific margin of South America triggered oceanward (west-

ward) migration of the overlying volcanic arc (Soler and Bon-homme, 1990). The result was extension and rifting of theoverlying western continental margin of South America and


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Unconformity


Albian(lowercontact

unknown)

LatestAlbianto Turonian

lateAlbian?

LateCretaceous

Turonianand younger

EarlyCretaceous

?

CerroEl Ereo Fm.

Cerro SanLorenzo Fm

San PedroGroup

Lancones Fm

LANCONES BASIN

Casma (Lower)

Casma (Upper)

Atacongo, Pamplona,Morro Solar Fms.

Unconformity

La Bocana Fm

Punte Piedra Fm?

WESTERN PERUVIAN TROUGH

?

MOCHICA OROGENY

FIG. 16. Comparison of schematic volcanic stratigraphy of the Lancones basin and western Peruvian trough (modifiedfrom Myers, 1974; Offler et al., 1980; Cobbing et al., 1981) with emphasis on age correlation. Legend as in Figure 6.


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subsequent arc volcanism, leading to a paleotectonic regimethat may have resembled the modern-day Bransfield Strait(Barker and Austin, 1998) or the Miocene Japan Sea (Jolivetand Tamaki, 1992).With the onset of spreading in the SouthAtlantic and the westward drift of the South American conti-nent, the development of the Lancones basin and western Pe-ruvian trough terminated (Soler and Bonhomme, 1990). Thistransition from extensional to contractional tectonics ismarked by the Mochica orogeny (Mgard, 1984). In the Lan-cones basin, shallow-marine to terrestrial volcanism followedin a less extended margin and evolved to granitoid plutonismunder a continental arc regime.

Inheritance in zircons and implications for basement rocks

Only minor inheritance is suggested from the zircon datafrom rocks of the Cerro San Lorenzo Formation but signifi-cantly more inheritance is obvious from volcanic rocks fromthe La Bocana Formation. Although there is somewhat of asampling bias with more samples collected from the La Bo-cana Formation than from the Cerro San Lorenzo Formation,

the greater inheritance of zircon by felsic rocks of the La Bo-cana Formation may indicate a more important continentalcrust component in the source and/or pathways of these lavas.Three of ten samples from the La Bocana Formation displaya variety of inherited ages up to 2,500 Ma. Strong zircon in-heritance is indicative of continental crust in the genesis ofthese rocks, especially in the younger sequences. This con-cept has implications for basement architecture and magmagenesis and may suggest that rapid advection of mantle-de-rived magmas occurred in the early stages of rifting, thus pro-ducing arc magmatism which underwent limited crustal in-teraction, either with crustal basement rocks or sedimentaryrocks derived thereof. In contrast, in later stages of the arcduring the waning of rifting and reduced extension, pooling of

magmas resulted in relatively more crustal contamination.This concept is consistent with a model for VMS formation atTambogrande whereby early-rift, ore-associated magmas

were able to transfer more heat to upper crustal levels anddrive VMS-forming hydrothermal cells (Winter, 2008).

Conclusions

The depositional history of the Lancones basin is recordedby a ~10-km sequence of bimodal volcanic and volcaniclasticrocks representing at least 14 Ma of volcanic activity from ca.105 to 91 Ma based on reliable U-Pb zircon ages. Two mainphases of volcanism comprise the volcanic arc sequence:

1. The oldest, early-rift phase, recorded by the Cerro San

Lorenzo Formation, represents deep-water, bimodal, mafic-dominated volcanic rocks comprised of lava flows, autoclasticbreccias, and minor pelagic sedimentary rocks. Four U-Pbzircon ages ranging from 104.8 1.3 to 100.2 0.5 Ma suggestmiddle to late Albian ages for this formation. VMS deposits atTambogrande are hosted within the Cerro San Lorenzo For-mation and have a minimum age of 104.8 1.3 Ma, as con-strained by the presynmineralization felsic volcanic rocks. Im-mediate footwall premineralization volcanic rocks did not

yield any minerals suitable to U-Pb dating methods.2. Late-rift volcanism is recorded by the Cerro El Ereo, La

Bocana, and Lancones Formations. These formations are

dominated by deposits of shallower marine facies and repre-sent bimodal volcanism. Five U-Pb zircon ages range from99.3 0.3 to 91.1 1.0 Ma.

The volcanic phases are broadly chronologically correlatedwith major tectonomagmatic events, specifically the openingof the South Atlantic Ocean in the Early Cretaceous during

the final break-up of Gondwana. Early-rift volcanism was aresult of strong crustal attenuation and rifting of the westernmargin of a relatively static South American continent duringthe Albian times and correlates with preopening or rift stageof the South Atlantic at equatorial latitudes. The Lanconesbasin during early-rift volcanism would have resembled amodern-day back-arc basin or rifted arc-type setting. Activespreading in the South Atlantic Ocean and westward drift ofthe South American continent correlates broadly with theMochica orogeny in the western Andes during the Albian.This contractional tectonic event marks the termination ofthe Lancones basin early-rift volcanic event. Late-rift volcan-ism is more typical of Andean arc magmatism and was the re-sult of subduction beneath a less extensional margin duringthe late Albian to Turonian.

Acknowledgments

The authors gratefully acknowledge financial and in-kindsupport from Mediterranean Resources Ltd. (formerly Man-hattan Minerals) as part of LSWs Ph.D. dissertation andspecifically thank Peter Tegart for initiating this project. AnNSERC Collaborative Research and Develop grant to RMTis also recognized. Support from the Hugh E. Mckinstry grant(SEG Foundation) and the Egil H. Lorntzsen and Thomasand Marguerite MacKay Memorial scholarships (Universityof British Columbia) to LSW are also greatly appreciated.Andy Carstensen, Cristian Soux, Gord Allen, Brian Thurston,Kosta and Sefika Lesnikov, Ed Lyons, Arturo Cordova, AllanSan Martin, and Miguel Jimenez are thanked for many valu-able discussions and input. Geochronologic work was sup-ported by Rich Friedman (U-Pb TIMS) and Tom Ullrich (Ar)at UBCs Pacific Centre for Isotopic and Geochemical Re-search. Claire Chamberlain (MDRU) is thanked for conduct-ing some of the SHRIMP work at Stanford. Earlier reviews ofthis manuscript by Steve Piercey and Derek Wilton and for-mal reviews by Jan Peter and Kelly Russell were invaluable.Economic Geology reviewers Lluis Fontbote and Cesar Vidaland associate editor Ross Sherlock provided excellent cri-tiques that greatly improved this paper. This is MDRU con-tribution P-263.

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Zircon was separated from approximately 10 to 15 kg offresh rock material used of each sample. Samples for U-Pb

dating were processed using a Rhino jaw crusher, a Bicodisk grinder equipped with ceramic grinding plates, and aWilfley wet-shaking table equipped with a machined Plexi-glass top, followed by conventional heavy liquids and mag-netic separation using a Frantz magnetic separator. Sam-ples were handpicked using a binocular microscope.

Thermal Ionization Mass Spectrometry (TIMS) U-Pbanalyses were done at the Pacific Centre for Isotopic andGeochemical Research at the University of British Columbia.Mineral fractions for analysis were selected based on grainquality, size, magnetic susceptibility, and morphology. All zir-con fractions were air abraded prior to dissolution to mini-mize the effects of postcrystallization Pb loss, using the tech-nique of Krogh (1982). Samples were dissolved in

concentrated HF and HNO3 in the presence of a mixed233-

235U-205Pb tracer for 40 h at 240C in PTFE or PFA micro-capsules contained in high-pressure vessels (Parr bombs).Sample solutions were then dried to salts at ~125C, re-bombed and redissolved in 3.1N HCl for 12 h at 210C. Sep-aration and purification of Pb and U employed ion exchangecolumn techniques modified slightly from those described byParrish et al. (1987). Pb and U were sequentially eluted intoa single beaker and loaded together on a single zone refinedRe filament using a phosphoric acid-silica gel (SiCl4) emitter.Isotopic ratios were measured using a modified single collec-tor VG-54R thermal ionization mass spectrometer equipped

with an analogue Daly photomultiplier. Measurements weredone in peak-switching mode on the Daly detector. U and Pb

analytical blanks were in the range of


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FIG. A1. SEM cathodoluminesence images of zircons showing location of spot analyses for SHRIMP-RG data.


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LW-033-quartz-feldsparporphyriticrhyolitedike

1.1

0.000267

47

.047

3.8

.075

4.6

364

2097

0.23

.003

0.9

4.9

21.8

0.2

.0433

6.1

0.02

6.2

.0034

0.9

.148

1.1

0.000302

17

.183

0.6

.142

1.9

94

475

0.57

.485

0.7

70.7

2490.6

24.2

.1794

0.8

11.94

1.1

.4827

0.7

.676

2

0.000082

52

.082

1.1

.139

1.3

86

1671

0.45

.199

0.7

33.9

1167.5

7.6

.0804

1.4

2.21

1.6

.1989

0.7

.431

4

0.000883

43

.052

5.1

.126

5.1

727

98

0.36

.001

1.3

2.5

9.0

0.1

.0393

16.7

0.01

16.8

.0014

1.4

.086

5

0.005973

32

.086

9.2

.183

5.9

30

3219

0.32

.016

1.9

1.4

98.2

2.2

.0144

4.3

6

0.033547

8

.480

3.1

1.264

3.9

365

1689

0.79

.008

1.2

3.1

22.8

3.3

.0030

15.6

7

0.000056

49

.083

0.7

.091

1.2

154

1533

0.30

.218

0.4

99.6

1271.5

5.3

.0820

0.8

2.46

1.0

.2177

0.4

.447

10

0.000037

78

.051

1.1

.020

2.7

94

156

0.06

.037

0.3

48.7

234.0

0.8

.0505

1.4

0.26

1.5

.0369

0.3

.235

11

0.003089

5

.093

2.9

.122

6.1

314

98

0.10

.030

0.3

82.7

179.9

1.5

.0479

15.7

0.19

15.8

.0282

0.5

.033

12

0.000089

16

.052

1.2

.018

3.1

82

198

0.05

.036

0.3

51.8

225.8

0.8

.0507

1.3

0.25

1.4

.0357

0.3

.250

13

0.000091

35

.129

0.7

.192

0.9

132

532

0.57

.252

0.5

52.3

1378.0

12.1

.1280

0.8

4.44

1.0

.2513

0.5

.551

14

1

.060

2.5

.053

4.0

15

242

0.16

.097

1.0

8.1

596.5

5.9

.0618

2.5

0.83

2.7

.0972

1.0

.372

15

0.000128

86

.057

2.5

.206

2.0

99

170

0.66

.066

0.9

8.8

410.3

3.6

.0554

3.9

0.50

4.0

.0658

0.9

.223

TABLEA2.

(Cont.)

207corr

204

204

2

04

Meas

Meas

Meas

Corr

Rad

206Pb/

corr

corr

corr

Spot

204Pb/

%

207Pb/

%

208Pb/

%

Th

U

232Th/

206Pb/

%

206Pb

238U

1

207Pb/

%

207Pb/

%

206Pb/

%

Err

name

206Pb

err

206Pb

err

206Pb

err

(ppm)

(ppm)

238U

238U

err

(ppm)

Age

err

206Pb

err

235U

err

238U

err

corr

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