SHRIMP U–Pb dating of the Antucoya porphyrycopper deposit: new evidence for an Early Cretaceousporphyry-related metallogenic epoch in the CoastalCordillera of northern Chile

Victor Maksaev & Francisco Munizaga & Mark Fanning &

Carlos Palacios & José Tapia

Abstract The Antucoya porphyry copper deposit (300 Mtat 0.45% total Cu) is one of the largest deposits of a poorlyknown Early Cretaceous porphyry belt in the CoastalCordillera of northern Chile. It is related to a successionof granodioritic and tonalitic porphyritic stocks and dikesthat were emplaced within Jurassic andesitic rocks of the LaNegra Formation immediately west of the N–S trendingsinistral strike-slip Atacama Fault Zone. New zirconSHRIMP U–Pb data indicate that the porphyries ofAntucoya crystallized within the time span from 142.7±1.6 to 140.6±1.5 Ma (±2 σ), and late, unmineralized, NW–SE trending dacite dikes with potassic alteration andinternal deformation crystallized at 141.9±1.4 Ma. TheAntucoya porphyry copper system appears to be formedafter a change of stress conditions along the magmatic arcfrom extensional in the Late Jurassic to transpressiveduring the Early Cretaceous and provides support for anEarly Cretaceous metallogenic episode of porphyry-type

mineralization along the Coastal Cordillera of northernChile.

Keywords Porphyry copper . Andes . Chile .

Geochronology . SHRIMPU–Pb dating .

Coastal Cordillera

Introduction

The Antucoya porphyry copper deposit is one of the largestsystems of the poorly known Early Cretaceous metal-logenic belt in northern Chile (Camus 2003; Sillitoe andPerelló 2005), containing resources of 300 million tonnesof oxidized ore at 0.45% total Cu. The deposit is located130 km NNE of Antofagasta, at 1,710 m of altitude on asubdued part of the Buey Muerto hills, on the eastern slopeof the Coastal Cordillera (Fig. 1). Most geologicalinformation is from exploration work, including 43,000 mof diamond drilling and 510 m of underground galleries,done by SQM Metales S.A., the former owner of theproperty (Tapia et al. 1998). Recently, in December 2005,Antofagasta Minerals S.A. have acquired the Antucoyamining property and this company also holds the adjacentBuey Muerto claims, where a porphyry copper deposit witha geological resource of 219 Mt of oxide ore at 0.36% Cuwas discovered (Perelló et al. 2003).

We have used zircon U–Pb dating to resolve a preciseage of crystallization for three samples from the mainintrusive rock units of the Antucoya porphyry copperdeposit, and compared these results to other geochronolog-ical data, providing support for an Early Cretaceousmetallogenic epoch in the Coastal Cordillera of northern-most Chile.

V. Maksaev (*) : F. Munizaga : C. PalaciosDepartamento de Geología, Universidad de Chile,Casilla 135128, Correo 21,Santiago, Chilee-mail: vmaksaev@ing.uchile.cl

M. FanningResearch School of Earth Sciences,Australian National University,Canberra ACT 0200, Australia

J. TapiaSociedad Química y Minera de Chile,Aníbal Pinto 3228,Antofagasta, Chile

Geologic background

A variety of copper deposits, including iron oxide–copper–gold, VMS, strata-bound copper–(silver), and some por-phyry copper deposits, represent an Early Cretaceousmetallogenic episode along the Coastal Cordillera fromPeru to central Chile (e.g., Maksaev and Zentilli 2002;Camus 2003; Sillitoe 2003; Sillitoe and Perelló 2005).Although exact ages are somewhat equivocal (Munizaga etal. 1985; Boric et al. 1990), the Early Cretaceous porphyry

copper deposits represent the earliest mineralization of thistype in the evolution of the Chilean Andes.

The Antucoya porphyry copper deposit occurs within anintrusive complex composed of multiphase granodioriticand tonalitic porphyries and dacite dikes, which wereemplaced within Jurassic andesitic volcanic rocks of theLa Negra Formation. Postmineralization gravel partlycemented by nitrates, regolith, and gypcrete cover most ofthe Antucoya deposit; thus, only limited surface exposureexists. Copper-bearing ore occurs as stockwork, dissemina-tion, impregnation in altered rocks, and as breccia matrix; itis hosted by the granodioritic and tonalitic porphyries, andby magmatic to hydrothermal breccias, within an area of1.6×1.0 km (Fig. 2). Unmineralized, NW-striking dacitedikes (5 to 20 m thick) crosscut the mineralized porphyriesand hydrothermal breccias. The volcanic succession ex-posed at the Buey Muerto hills strikes 25–30°W and dipsfrom 20 to 30°NE. It is formed out of massive aphanitic toporphyritic andesites, locally interbedded with fine-grained,tuffaceous calcareous sandstone and siltstone horizons. TheAntucoya copper deposit occurs immediately west of aconspicuous N–S lineament of the main trace of theAtacama Fault Zone. This is a regional intraarc, sinistralstrike-slip fault zone, which was active during the EarlyCretaceous (e.g., Dallmeyer et al. 1996; Scheuber andAndriessen 1990). Dike emplacement and brecciation atAntucoya are controlled by NW-striking faults. These NWfaults splay from the neighboring N–S Atacama Fault Zoneand are consistent with a NW–SE compression at the timeof faulting and dike emplacement.

Two altered and mineralized porphyry intrusions occurat Antucoya. The dominant lithology is granodioriticporphyry (“Antucoya porphyry”) and lesser tonalite por-phyry (known as “fine-grained porphyry”). The granodio-ritic porphyry is formed by plagioclase phenocrysts (25%)up to 3 mm long, and perthitic K-feldspar phenocrysts up to1 mm long (5%). Plagioclase phenocrysts are altered tokaolinite, illite, calcite, and traces of anhydrite. Thegroundmass (70%) is a microcrystalline aggregate of quartzand argillized feldspar, with chlorite and opaque mineraldissemination and minor relicts of chloritized fine-grainedbiotite. The granodiorite porphyry contains a dense stock-work of quartz veinlets, some with oxidized copperminerals (atacamite, brochantite, chrysocolla, and copperwad), hematite, and limonites (after sulfides), and somewith late veins of calcite, anhydrite, and opaque minerals.

The tonalite porphyry (fine-grained porphyry) has minorrelict (phantom) plagioclase phenocrysts (ca. 10%) up to2 mm long, with strong argillic alteration and partly calcitereplacement. The groundmass (90%) is a microcrystallineaggregate of quartz and argillized plagioclase with abun-dant chlorite and disseminated opaque minerals that appearto completely replace mafic minerals and relicts of partly

Fig. 1 Location of the Antucoya porphyry copper deposit and thegeological setting of the Early Cretaceous porphyry-related copperdeposits of the Coastal Cordillera of northern Chile

chloritized microcrystalline aggregates of biotite and minorsericite. This porphyry also contains a stockwork of quartzveinlets, and late calcite veins, but also fractures coated bylimonite and copper oxide minerals. The granodiorite andtonalite porphyries have mutual intrusion relationships,suggesting either that these are composite intrusions or thatthey intruded almost simultaneously. In addition, magmatic-hydrothermal breccias occur along the contacts betweenthese two porphyries in the center of the deposit, but are alsospatially related to NW-striking faults. These are polymicticand matrix-supported, pipe-like, irregular bodies, whichrange from near vertical to SW-dipping; their recognizedvertical extent is 250 to 350 m. The uppermost part of thebreccia bodies (above the 1,400-m level) has a matrix ofhydrothermal minerals, whereas downward an altered grano-dioritic igneous matrix dominates. The breccia fragments areangular to subrounded and are composed of altered andmineralized granodioritic and tonalitic porphyries.

NW-striking, unmineralized, dark-gray dacite porphyrydikes crosscut the mineralized porphyries; these dacitedikes are formed by plagioclase phenocrysts (25%) up to4 mm long, fresh or partly sericitized, and some affected bybrittle microfracturing, partly chloritized microcrystallinebiotite and opaque minerals forming pseudomorphs afteramphibole phenocrysts (5%), and scarce quartz eyes up to2 mm in diameter. The groundmass (70%) consists ofmicrocrystalline aggregates of quartz and argillized plagio-clase microliths. Fine-grained biotite dissemination occursin the groundmass, but biotite is largely replaced bychlorite. Although the dacite dikes were intruded aftermineralization, they are affected by potassic alteration(biotite), with partial chloritization and argillic overprint.These dacite dikes occur along NW-trending faults, and areinternally affected by microfractures; this brittle deforma-tion is probably related to fault reactivations after dikeemplacement.

b)

200 m0200 m0

SW NE

1,750

1,650

1,550

1,450

1,350

Elevation (m)

a)

SW NE

1,750

1,650

1,550

1,450

1,350

Elevation (m)

c) d)

Argillic

Potassic

Chlorite-Sericite

Phyllic

7497000N

7497500N

7496500N

409000E 409500E

7497000N

7497500N

7496500N

409000E 409500EUTM metric grid UTM metric grid

LITHOLOGY

Dacite dikes

Igneous andhydrothermal breccias

Jurassic volcanic rocks

Granodiorite porphyry

Tonalite fine porphyry

Faults

HYDROTHERMAL ALTERATION

Fig. 2 Geology and hydrothermal alteration of the Antucoya porphyrycopper deposit: a geological cross-section of the deposit, b cross-section showing the distribution of the hydrothermal alteration types,

c geology of the 1,350-m level, and (d) hydrothermal alteration typesof the 1,350-m level

Tab

le1

ZirconSHRIM

U–P

bdata

forintrusiverocksfrom

Antucoy

apo

rphy

rycopp

erdepo

sit

Grain

spot

U(ppm

)Th(ppm

)Th/U

206Pb*

(ppm

)204Pb/

206Pb

±f 206%

Total

Radiogenic

Age

(Ma)

238U/206Pb

±207Pb/

206Pb

±206Pb/

238U

±206Pb/

238U

±

Sam

pleKP64

granod

iorite

porphy

rya

1.1

2915

0.51

0.6

0.00

2636

0.00

1160

2.52

44.56

1.07

0.06

890.00

450.02

190.00

0513

9.5

3.4

2.1

3015

0.51

0.6

0.00

0789

0.00

0781

4.79

44.84

0.98

0.08

690.00

710.02

120.00

0513

5.4

3.2

3.1

2914

0.47

0.6

––

3.87

42.20

0.92

0.07

970.00

570.02

280.00

0514

5.2

3.3

4.1

2414

0.60

0.5

0.00

1298

0.00

1042

5.13

42.88

1.03

0.08

970.00

860.02

210.00

0614

1.1

3.7

5.1

5037

0.73

1.0

0.00

3997

0.00

1266

2.57

43.95

0.80

0.06

930.00

330.02

220.00

0414

1.4

2.6

6.1

4118

0.43

0.8

0.00

1047

0.00

0806

2.97

45.87

0.89

0.07

240.00

510.02

120.00

0413

4.9

2.7

7.1

2913

0.46

0.6

0.00

5822

0.00

1844

4.58

42.36

0.94

0.08

540.00

800.02

250.00

0614

3.6

3.5

8.1

6036

0.60

1.2

0.00

2058

0.00

0832

2.29

43.02

0.72

0.06

710.00

290.02

270.00

0414

4.8

2.5

9.1

2515

0.62

0.5

0.00

4194

0.00

1588

4.68

41.49

0.97

0.08

620.00

600.02

300.00

0614

6.4

3.6

10.1

3613

0.36

0.7

0.00

5456

0.00

1516

3.75

42.67

0.88

0.07

880.00

780.02

260.00

0514

3.8

3.3

11.1

3725

0.67

0.7

0.00

3341

0.00

1115

4.37

46.03

0.94

0.08

340.00

430.02

080.00

0413

2.6

2.8

12.1

2913

0.46

0.6

0.00

2492

0.00

1069

5.30

41.22

0.92

0.09

120.00

500.02

300.00

0514

6.4

3.4

13.1

6432

0.51

1.2

0.00

1528

0.00

0966

2.58

44.70

0.77

0.06

940.00

410.02

180.00

0413

9.0

2.5

14.1

3920

0.50

0.8

0.00

2156

0.00

0896

2.67

43.47

0.88

0.07

010.00

550.02

240.00

0514

2.7

3.0

15.1

2311

0.49

0.5

0.00

8487

0.00

2690

6.55

41.24

1.04

0.10

100.00

620.02

270.00

0614

4.4

3.8

16.1

3315

0.46

0.7

0.00

2180

0.00

1309

3.53

42.47

0.97

0.07

710.00

990.02

270.00

0614

4.8

3.8

17.1

4218

0.43

0.8

0.00

1552

0.00

0811

3.45

43.58

0.86

0.07

630.00

720.02

220.00

0514

1.3

3.1

18.1

3818

0.47

0.7

0.00

2868

0.00

1168

3.47

43.75

0.88

0.07

640.00

680.02

210.00

0514

0.7

3.1

19.1

156

0.42

0.3

0.00

8540

0.00

4370

9.17

42.85

1.74

0.12

170.01

700.02

120.00

1013

5.2

6.3

20.1

5024

0.47

1.0

0.00

0702

0.00

0695

1.86

43.90

0.78

0.06

370.00

740.02

240.00

0514

2.5

2.9

Sam

pleKP65

tonalitepo

rphy

ryb

1.1

6847

0.70

1.3

0.00

0797

0.00

0330

2.28

45.24

0.75

0.06

690.00

260.02

160.00

0413

7.8

2.3

2.1

2612

0.46

0.5

0.00

3689

0.00

1307

5.96

42.49

1.02

0.09

630.00

690.02

210.00

0614

1.1

3.6

3.1

4527

0.59

0.9

0.00

3545

0.00

1255

1.76

44.98

0.85

0.06

280.00

440.02

180.00

0413

9.3

2.7

4.1

4133

0.81

0.8

0.00

1369

0.00

1031

3.69

45.06

1.75

0.07

810.00

410.02

140.00

0813

6.3

5.3

5.1

4021

0.52

0.8

0.00

6451

0.00

1793

3.75

43.33

0.88

0.07

870.00

410.02

220.00

0514

1.6

2.9

6.1

3719

0.51

0.7

0.00

2680

0.00

1014

3.63

43.90

0.89

0.07

770.00

570.02

200.00

0514

0.0

3.0

7.1

3313

0.40

0.6

0.00

4584

0.00

1385

4.15

44.58

1.59

0.08

180.00

580.02

150.00

0813

7.1

5.0

8.1

3421

0.60

0.7

0.00

1990

0.00

1347

3.77

43.38

1.35

0.07

890.00

430.02

220.00

0714

1.4

4.4

9.1

5223

0.44

1.0

0.00

2369

0.00

0896

1.92

44.34

1.22

0.06

410.00

310.02

210.00

0614

1.0

3.9

10.1

188

0.44

0.4

0.00

3563

0.00

2756

8.24

44.16

1.21

0.1142

0.01

050.02

080.00

0613

2.6

4.1

11.1

2411

0.45

0.5

0.00

4281

0.00

1750

5.85

41.47

1.02

0.09

550.00

710.02

270.00

0614

4.7

3.8

12.1

115

750.66

2.1

0.00

1706

0.00

0515

1.17

45.82

0.65

0.05

810.00

240.02

160.00

0313

7.6

2.0

13.1

198

0.39

0.4

0.00

7665

0.00

2870

6.10

42.05

1.09

0.09

740.0114

0.02

230.00

0714

2.4

4.3

14.1

3316

0.47

0.7

0.00

2978

0.00

0995

3.88

43.39

0.95

0.07

980.00

810.02

220.00

0514

1.2

3.4

15.1

2612

0.44

0.5

0.00

2869

0.00

1531

5.10

44.48

1.05

0.08

940.00

550.02

130.00

0513

6.1

3.4

16.1

7247

0.66

1.4

0.00

2282

0.00

0942

1.82

42.62

0.69

0.06

350.00

370.02

300.00

0414

6.8

2.4

17.1

167

0.47

0.3

0.00

7377

0.00

3087

8.47

41.55

1.14

0.1163

0.0117

0.02

200.00

0714

0.5

4.4

Tab

le1

(con

tinued)

Grain

spot

U(ppm

)Th(ppm

)Th/U

206Pb*

(ppm

)204Pb/

206Pb

±f 206%

Total

Radiogenic

Age

(Ma)

238U/206Pb

±207Pb/

206Pb

±206Pb/

238U

±206Pb/

238U

±

18.1

7135

0.49

1.4

0.00

1268

0.00

0651

1.94

43.66

0.69

0.06

440.00

370.02

250.00

0414

3.2

2.4

19.1

4923

0.46

1.0

0.00

0481

0.00

0476

3.25

42.75

0.82

0.07

480.00

370.02

260.00

0414

4.3

2.8

20.1

5935

0.59

1.2

0.00

1709

0.00

0690

1.95

43.34

0.72

0.06

450.00

280.02

260.00

0414

4.2

2.4

Sam

pleKP67

dacite

dike

c

1.1

4221

0.51

0.8

––

2.46

44.76

0.83

0.06

840.00

550.02

180.00

0413

9.0

2.7

2.1

7640

0.53

1.4

0.00

4192

0.00

1018

3.39

45.69

1.01

0.07

570.00

280.02

110.00

0513

4.9

3.0

3.1

6451

0.79

1.3

0.00

1315

0.00

1180

1.96

43.47

0.71

0.06

450.00

270.02

260.00

0414

3.8

2.4

4.1

9585

0.89

1.8

0.00

0980

0.00

0713

2.03

45.71

0.66

0.06

490.00

370.02

140.00

0313

6.7

2.1

5.1

4936

0.74

1.0

0.00

1691

0.00

0669

2.70

42.57

0.75

0.07

040.00

790.02

290.00

0514

5.7

3.0

6.1

178

0.48

0.4

0.00

2972

0.00

2549

5.76

42.63

1.83

0.09

470.00

640.02

210.00

1014

1.0

6.1

7.1

5428

0.52

1.1

0.00

2655

0.00

1028

2.37

42.71

0.74

0.06

780.00

310.02

290.00

0414

5.7

2.6

8.1

8759

0.67

1.7

0.00

1742

0.00

0551

1.67

44.96

0.68

0.06

210.00

240.02

190.00

0313

9.5

2.1

9.1

3829

0.75

0.7

0.00

2700

0.00

1096

3.85

45.41

0.89

0.07

930.00

400.02

120.00

0413

5.1

2.7

10.1

7854

0.69

1.5

0.00

1284

0.00

0768

1.72

43.70

0.68

0.06

260.00

360.02

250.00

0414

3.4

2.3

11.1

134

134

1.00

2.6

0.00

1037

0.00

0328

1.29

44.91

0.61

0.05

910.00

180.02

200.00

0314

0.2

1.9

12.1

4127

0.64

0.8

0.00

1173

0.00

0857

3.26

43.71

0.83

0.07

480.00

490.02

210.00

0414

1.1

2.8

13.1

5432

0.60

1.0

0.00

2813

0.00

0890

2.86

44.34

0.79

0.07

160.00

330.02

190.00

0413

9.7

2.5

14.1

4323

0.53

0.8

0.00

1576

0.00

0678

3.59

43.80

0.82

0.07

740.00

380.02

200.00

0414

0.4

2.7

15.1

4218

0.44

0.8

0.00

1395

0.00

0839

3.21

43.48

1.48

0.07

440.00

460.02

230.00

0814

1.9

4.9

16.1

7638

0.51

1.5

0.00

2553

0.00

0683

1.67

43.83

0.69

0.06

220.00

250.02

240.00

0414

3.0

2.3

17.1

2313

0.56

0.5

0.00

8293

0.00

2848

6.51

42.72

1.02

0.10

070.00

980.02

190.00

0613

9.5

3.8

18.1

4824

0.49

1.0

0.00

1997

0.00

0755

1.94

41.92

0.76

0.06

450.00

310.02

340.00

0414

9.1

2.7

19.1

179

0.54

0.3

0.00

4144

0.00

3327

6.79

42.62

1.15

0.10

290.00

950.02

190.00

0713

9.5

4.1

20.1

7842

0.54

1.5

0.00

1894

0.00

0878

1.28

43.61

0.67

0.05

910.00

240.02

260.00

0414

4.3

2.3

Uncertaintiesgivenat

the1σ

levelon

spot

analyses.

Error

inTem

orareferencezircon

calib

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2.7±1.6

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ued)

Four hypogene hydrothermal alteration assemblageswere recognized at Antucoya: potassic, chlorite–sericite,quartz–sericite, and propylitic. The first three alterationtypes affect the porphyries and breccias, whereas propyliticalteration is restricted to the volcanic country rocks(Arellano 2003). However, most of the recognized orebodyis affected by pervasive supergene argillic alteration (illite,dickite, and kaolinite) and oxidation (atacamite, brochan-tite, chrysocolla, copper wad, jarosite, and limonite), whichextend down to depths of 300 to 350 m from the surface,and are overprinted on previous hypogene alteration types.Supergene processes, albeit unconstrained, are thought tohave developed during the formation of the Oligocene–Miocene coastal Tarapacá pediplain (Mortimer et al. 1974).Potassic alteration mainly affects breccia bodies and thetonalitic porphyry characterized by the biotite, K-feldspar,and quartz assemblage. Fluid inclusion data indicate thatpotassic alteration took place at temperatures around 450–500°C, with fluid salinities from 7 to 68wt% NaCl equivalentrelated to liquid–vapor phase separation (Arellano 2003).A stockwork of quartz veins with chalcopyrite, pyrite, andmolybdenite with quartz–sericitic haloes occurs in theuppermost part of the deposit (Fig. 2), and an assemblageof chlorite, sericite, smectite, quartz, pyrite, and chalcopyriteoccurs within the Antucoya granodiorite porphyry andmagmatic-hydrothermal breccias. The propylitic assemblageof chlorite, quartz, albite, epidote, calcite, and pyrite occurswithin the surrounding volcanic country rocks (Arellano2003).

Materials and analytical methods

Zircon grains were separated from three samples of themain intrusive rock units at the Antucoya porphyry copperdeposit following normal mineral separation procedures:crushing, desliming, and heavy liquid and paramagneticseparation. Handpicked, prismatic dipyramidal zircon crys-tals, from 50 to 250 μm long, were mounted in epoxytogether with chips of the Temora and SL13 referencezircons, sectioned approximately in half and polished.Reflected and transmitted light photomicrographs wereprepared for all zircons, as were cathodoluminescence(CL) scanning electron microscope images. The CL imageswere used to decipher the internal structures of thesectioned grains and to ensure that the ∼20 μm of SHRIMPspot was wholly within a single age component within thesectioned grains. U–Th–Pb analyses were made usingthe SHRIMP II at the Research School of Earth Sciences,The Australian National University, Canberra, Australia,following procedures given in Williams (1998, and refer-ences therein). Each analysis consisted of six scans throughthe mass range with a Temora U–Pb reference grain

analyzed for every three unknown analyses. The data werereduced using the SQUID Excel Macro of Ludwig (2001);U/Pb ratios were normalized relative to a value of 0.0668for the Temora reference zircon, equivalent to an age of417 Ma (see Black et al. 2003). Uncertainties in the U–Pbcalibration was 0.36% for an analytical session. Uncertain-ties given for individual analyses (ratios and ages) are at the1σ level (Table 1). Tera and Wasserburg (1972) concordiaplots, probability density plots with stacked histograms, andweighted mean 206Pb/238U age calculations were carried outusing ISOPLOT/EX (Ludwig 2003). Weighted mean206Pb/238U ages were calculated and the uncertainties arereported as 95% confidence limits.

Geochronology

Samples of the granodioritic porphyry, tonalitic porphyry,and a dacite dike from the Antucoya porphyry copper weredated. The new zircon geochronologic results are summa-rized on Table 1, whereas 206Pb/238U ages (±2 σ) andrepresentative U–Pb Tera–Wasserburg concordia plots areshown in Fig. 3.

Zircons from the mineralized granodiorite porphyry ofthe Antucoya orebody (KP-64; Fig. 3a) yielded a206Pb/238U age of 142.7±1.6 Ma (16 analyzed spots).Zircons from the mineralized tonalite porphyry (KP-65;Fig. 3b) yielded a 206Pb/238U age of 140.6±1.5 Ma (18analyzed spots). Zircons from a postmineralization dacitedike from Antucoya (KP-67; Fig. 3c) yielded a 206Pb/238Uage of 141.9±1.4 Ma (16 analyzed spots). The relativelyinvariable Th/U ratios and their respective spot U–Pb ages(Table 1) are consistent with magmatic zircons, and noinherited components were detected. These characteristicstogether with the high closure temperature of zircon (e.g.,Cherniak and Watson 2000) allow us to interpret the newU–Pb data as representative of the crystallization ages ofthe respective igneous rocks. The three new zircon U–Pbages are statistically identical as they overlap within theiranalytical uncertainty (at ±2 σ level), indicating that asuccession of magma injections took place at Antucoyawithin a short period of time that falls within the time spanof analytical uncertainty of the SHRIMP U–Pb datingmethod.

Discussion

The zircon U–Pb data indicate that the succession ofintrusions of Antucoya crystallized within the time spanfrom 142.7±1.6 to 140.6±1.5 Ma. Late, NW–SE trending,dacite dikes with internal brittle and ductile deformationthat crystallized at 141.9±1.4 Ma suggest that the porphyry

system developed during a stage of active sinistral shearingalong the neighboring N–S Atacama Fault Zone. The U–Pbage for a potassic altered, but unmineralized dike representsthe minimum age for the hypogene copper mineralization.The formation of the Antucoya porphyry copper deposit isafter a change of stress conditions along the Mesozoicmagmatic arc from extensional in the Late Jurassic to

transpressive during the Early Cretaceous, according to thetectonic interpretation of northern Chile by Scheuber andGonzalez (1999).

Perelló et al. (2003) reported K–Ar ages of 137±4 Maand 132±4 Ma for hydrothermal biotite and sericite,respectively, from the neighboring Buey Muerto property,which is part of the same porphyry copper system as

03104105106107140.0

60.0

80.0

01.0

21.0

41.0

61.0

81.0

94745434149373

702 bP602 bP

031

431

831

241

641

051

451

Age

(M

a)

error bars are ±2

Mean = 142.7 ± 1.6 Ma [1.1%] 95% conf.MSWD = 0.57, probability = 0.90832 /U 602 bP

KP-64 AntucoyaGranodiorite Porphyry

data-point error ellipsesare at 68.3% confidence

221

621

031

431

831

241

641

051

451error bars are ±2σ

03104105106107140.0

60.0

80.0

01.0

21.0

41.0

61.0

1594745434149373

832 /U 602 bP

702 bP602 bP

KP-65 AntucoyaFine-grained Tonalite Porphyry

data-point error ellipsesare at 68% confidence

Mean = 140.6 ± 1.5 Ma [1.0%] 95% conf.MSWD = 0.81, probability = 0.69

Age

(M

a)

621

031

431

831

241

641

051

451error bars are ±2σ

03104105106107140.0

60.0

80.0

01.0

21.0

41.0

94745434149373

832 /U 602 bP

702 bP602 bP

KP-67 AntucoyaDacite Dike

data-point error ellipsesare at 68% confidence

Mean = 141.9 ± 1.4 Ma [0.92%] 95% conf.MSWD = 0.76, probability = 0.72

Age

(M

a)

(a)

(b)

(c)

Fig. 3 Tera–Wasserburg con-cordia plot and weighted aver-age plot of SHRIMP U–Pbisotope data for a granodioriteporphyry (sample KP-64),b tonalite porphyry (sample KP-65), and c dacite dike (sampleKP-67). Data are shown as 1 σerror ellipses; error bars andU–Pb ages are ±2 σ

Antucoya. The biotite K–Ar age overlaps with the U–Pbages within its relatively large analytical uncertainty,whereas the sericite age is slightly younger. However, theseK–Ar ages are regarded as minimum age estimates for theporphyry system and are consistent with the lower closuretemperature of micas for the K–Ar method compared to thatof zircon in U–Pb dating.

Antucoya appears to be approximately coeval with themain mineralizing stage of the Mantos Blancos copperdeposit, where hydrothermal breccias, disseminations, andstockwork sulfide mineralization were formed related todioritic and granodioritic stocks and sills dated by the40Ar/39Ar method between 141 and 142 Ma according toRamírez et al. (2006). In addition, whole rock K–Ar ages of132±8 Ma and 118±15 Ma were published for the Puntillasporphyry copper deposit (Munizaga et al. 1985; Boric et al.1990). These ages also suggest an Early Cretaceousporphyry mineralization, but the large errors of the K–Arages make it uncertain if the Puntillas deposit was formed atthe same time as Antucoya.

Conclusions

The Antucoya intrusive complex crystallized at 142.7±1.6to 140.6±1.5 Ma, which is within a relatively short timespan of less than ca. 2 Ma during the earliest Cretaceous.The emplacement of the porphyry intrusions took placewithin NW-trending faults that splay from the adjacent N–Ssinistral strike-slip Atacama Fault Zone at the onset oftranspression along the suprasubduction magmatic arc.

The U–Pb data for the Antucoya porphyry copperdeposit provide the first precise ages for an early Creta-ceous metallogenic episode of porphyry-type mineralizationalong the Coastal Cordillera of northern Chile. The samemineralizing episode appears to be represented by otherporphyry copper deposits of northern Chile. However, moreprecise ages are required to check the temporal extent of themineralizing event.

Acknowledgements This study was supported by CONICYT, Chile,through Fondecyt 1040492 grant to V. Maksaev and F. Munizaga(Universidad de Chile). We thank Massimo Chiaradia, BerndLehmann, and Carmen Holmgren for valuable reviews of themanuscript.

References

Arellano M (2003) Distribución y control de la mineralización delpórfido cuprífero Antucoya, II Región, Chile. B.Sc. thesis,Departamento de Geociencias Geológicas, Universidad Católicadel Norte, Antofagasta, Chile, p 81

Black LP, Kamo SL, Allen CM, Aleinikoff JN, Davis DW, Korsch RJ,Foudolis C (2003) TEMORA 1: a new zircon standard forPhanerozoic U–Pb geochronology. Chem Geol 200:155–170

Boric R, Díaz F, Maksaev V (1990) Geología y yacimientos metal-íferos de la Región de Antofagasta, Bol. 40. Servicio Nacional deGeología y Minería, Santiago, p 246

Camus F (2003) Geología de los sistemas porfíricos en los Andes deChile. Servicio Nacional de Geología y Minería, Chile, p 267

Cherniak DJ, Watson EB (2000) Pb diffusion in zircon. Chem Geol172:5–24

Dallmeyer RD, Brown M, Grocott J, Taylor GK, Treolar PJ (1996)Mesozoic magmatic and tectonic events within the Andean plateboundary zone, 26°–27°30′ S, North Chile: constraints from40Ar/39Ar mineral ages. J Geol 104:19–40

Ludwig KR (2001) SQUID 1.02, a user’s manual. BerkeleyGeochronology Center, CA, special publication no. 2

Ludwig KR (2003) User’s manual for Isoplot/Ex, version 3.0, a geo-chronological toolkit for Microsoft Excel. Berkeley Geochronol-ogy Center, CA, special publication no. 4

Maksaev V, Zentilli M (2002) Chilean strata-bound Cu–(Ag) deposits:an overview. In: Porter TM (ed) Hydrothermal iron oxidecopper–gold and related deposits: a global perspective, vol. 2.PCG Publishing, pp 185–205

Mortimer C, Farrar E, Saric N (1974) K–Ar ages from tertiary lavas ofthe northernmost Chilean Andes. Geol Rundsch 63:484–490

Munizaga F, Huete C, Hervé F (1985) Geocronología K–Ar y razonesiniciales 87Sr/86Sr de la Franja Pacífica de Desarrollos Hidroter-males, vol. 4. Actas IV Congreso Geológico Chileno, Antofa-gasta, pp 357–379

Perelló J, Martini R, Arcos R, Muhr R (2003) Buey Muerto: porphyrycopper mineralization in the Early Cretaceous arc of northernChile. Actas X Congreso Geológico Chileno, Concepción(electronic version)

Ramírez LE, Palacios C, Townley B, Parada MA, Sial AN, Fernandez-Turiel JL, Gimeno D, Garcia-Valles M, Lehmann B (2006) TheMantos Blancos copper deposit: an upper Jurassic breccia-stylehydrothermal system in the Coastal Range of Northern Chile.Miner Deposita 41:246–258

Scheuber E, Andriessen PAM (1990) The kinematic and geodynamicsignificance of the Atacama fault zone, northern Chile. J StructGeol 12:24–257

Scheuber E, Gonzalez G (1999) Tectonics of the Jurassic–EarlyCretaceous magmatic arc of the north Chilean Coastal Cordillera(22–26°S): a story of crustal deformation along a convergentplate boundary. Tectonics 18:895–910

Sillitoe RH (2003) Iron oxide copper–gold deposits: an Andean view.Miner Deposita 38:787–812

Sillitoe RH, Perelló J (2005) Andean copper province: tectonomag-matic settings, deposit types, metallogeny, exploration, anddiscovery. In: Hedenquist JW, Thompson JFH, Goldfarb R,Richards J (eds) Economic geology one hundredth anniversaryvolume (1905–2005). Society of Economic Geologists, Littleton,Colorado, USA, p 845–890

Tapia J, Crespo H, Collao S (1998) Antucoya, pórfido cuprífero en laCordillera de la Costa, II Región, Chile, vol. 3. In: Actas XCongreso Latinoamericano de Geología y VI Congreso Nacionalde Geología Económica, Buenos Aires, Argentina, pp 250–255

Tera F, Wasserburg GJ (1972) U–Th–Pb systematics in three Apollo14 basalts and the problem of initial Pb in lunar rocks. EarthPlanet Sci Lett 14:281–304

Williams IS (1998) U–Th–Pb geochronology by ion microprobe. In:McKibben MA, Shanks III WC, Ridley WI (eds) Applications ofmicroanalytical techniques to understanding mineralizing pro-cesses. Rev Econ Geol 7:1–35