Zircon U–Pb geochronology and geochemistry of the Cerro Colorado porphyry copper deposit, northern Chile

Accepted Manuscript Zircon U–Pb Geochronology and Geochemistry of the Cerro Colorado Porphyry Copper Deposit, Northern Chile Debbie P.W. Tsang, Simon R. Wallis, Koshi Yamamoto, Makoto Takeuchi, Hiroshi Hidaka, Kenji Horie, Brian C. Tattitch PII: DOI: Reference:

S0169-1368(17)30144-0 https://doi.org/10.1016/j.oregeorev.2017.12.019 OREGEO 2439

To appear in:

Ore Geology Reviews

Received Date: Revised Date: Accepted Date:

21 February 2017 6 September 2017 18 December 2017

Please cite this article as: D.P.W. Tsang, S.R. Wallis, K. Yamamoto, M. Takeuchi, H. Hidaka, K. Horie, B.C. Tattitch, Zircon U–Pb Geochronology and Geochemistry of the Cerro Colorado Porphyry Copper Deposit, Northern Chile, Ore Geology Reviews (2017), doi: https://doi.org/10.1016/j.oregeorev.2017.12.019

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1

Zircon U–Pb Geochronology and Geochemistry of the Cerro Colorado

2

Porphyry Copper Deposit, Northern Chile

3

Debbie P.W. Tsang1, Simon R. Wallis2,1, Koshi Yamamoto1, Makoto

4

Takeuchi1, Hiroshi Hidaka1, Kenji Horie 3,4, Brian C. Tattitch5

5

1

6

Environmental Studies, Nagoya University, Japan.

7

2

8

University of Tokyo, Japan

9

3

Department of Earth & Planetary Science, Graduate School of

Department of Earth & Planetary Science, School of Science, The

Division for Research and Education, Geoscience Group, National

10

Institute of Polar Research, Japan

11

4

12

Studies, Japan

13

5

Department of Polar Science, The Graduate University for Advanced

School of Earth Sciences, University of Bristol, United Kingdom

14 15 16

Corresponding author: Debbie P.W. Tsang

17

email address: [email protected]

18 19

Keywords: Porphyry copper deposits; U– U–Pb geochronology; Northern

20

Chile; Timing of Magmati Magmatism agmatism; sm; Volcanism. Volcanism.

21

1

22

1

Introduction

23

Up to 70% of the world’s copper and almost all of its molybdenum is

24

hosted by porphyry deposits (e.g., Seedorff et al., 2005; Sillitoe, 2010,

25

2012). Porphyry copper deposits (PCDs) dominantly form in convergent

26

plate margins spatially closely associated with volcanic arcs (e.g.,

27

Richards, 2011, 2013; Cooke et al., 2005; Sillitoe, 2010, 2012). The metals

28

are brought to shallow levels in the crust in intermediate magma bodies.

29

As these magmas crystallise, NaCl-rich brines are released, and these

30

fluids scavenge the metals from the magma before transporting them into

31

the surrounding country rock (e.g., Dilles, 1987; Candela, 1989; Cline &

32

Bodnar, 1991). Transport and precipitation of the ore-bearing NaCl-rich

33

fluids involve flow through high-porosity domains and reaction with

34

reduced sulphur to produce the metal sulphides (e.g., Holland, 1965;

35

Landtwing et al., 2005; Kouzmanov & Pokrovski, 2012). Widespread acidic

36

alteration is a prominent by-product of the ore-formation (e.g., Sillitoe,

37

2010; Richards, 2011).

38

The overall framework for PCD formation is well understood, but

39

there remain numerous key aspects of the process that are contentious or

40

unclear. In particular, the genetic relationships between PCD formation

41

and active volcanism are contested. A major eruption or major release of

42

Cu-bearing gasses would act to disperse the metal and prevent ore

43

formation, and many workers have concluded that PCD formation takes

44

place when volcanism is suppressed or absent (e.g., Cloos, 2001; Cooke et

2

45

al., 2005; Sillitoe, 2010). In contrast, several field-based studies have

46

documented evidence suggesting syn-eruption PCD formation (e.g.,

47

Hedenquist et al., 1998; Hattori & Keith, 2001; Nadeau et al., 2010, 2016).

48

Resolving this issue is important to help identify modern analogues for

49

PCDs, and will aid further exploration.

50

The South American convergent margin is associated with

51

numerous PCDs arranged in belts parallel to the modern volcanic arc (Fig.

52

1). The most prominent and economically valuable formed in the Eocene to

53

Oligocene times (e.g., Maksaev et al., 2007). Numerous studies have

54

documented

55

characteristics of these PCDs (e.g., Clark et al., 1990, 1998; Cornejo et al.,

56

1997; Maksaev et al., 2004, 2010; Sillitoe & Perelló, 2005; summary of

57

Camus & Dilles, 2001). However, the ages of these belts are mainly based

58

on K–Ar and Ar–Ar ages of K-bearing alteration minerals associated with

59

ore formation. Ar-dating of high temperature parts of PCDs is likely to

60

yield cooling ages after the last thermal maximum and do not give direct

61

information on the age of magmatic intrusion. Interpretations of Ar-dating

62

are also complicated by the possibility that extraneous Ar may be

63

introduced into these systems due to initial trapping from magma and the

64

action of hydrothermal fluids (e.g., Chiaradia et al., 2013). A better

65

indication of the crystallisation age of magma intrusion that leads to the

66

formation of PCDs is given by U–Pb dating of zircon. However, U–Pb

67

dating from the younger and better-preserved PCD belts of South America

68

has only been reported in a limited number of academic publications, and

the

geological,

geochemical

and

geochronological

3

69

most attention has been focused on supergiant PCDs (e.g., Richards et al.,

70

1999; Ballard et al., 2001; Maksaev et al., 2004; Munizaga et al., 2008)

71

(also see supplementary sheet 1).

72

In this study, we examined cores extracted from Cerro Colorado, an

73

active PCD mine site that lies at the northern end of the Paleocene–early

74

Eocene PCD belt of Chile (Fig. 1) (Camus, 2005; Charrier et al., 2007). The

75

cores supplied by the mine site sample all the main rock types of this

76

region. We carried out zircon U–Pb dating of representative samples from

77

the intrusions and host rocks, and combined the results with geochemical

78

analyses to constrain the ages of the main pulses of intrusion. We also

79

examine possible links between PCD formation and active volcanism.

80 81

2

Geological Overview of the Cerro Colorado Area

82

The BHP Billiton Cerro Colorado Mine is located about 120 km to

83

the northeast of Iquique (Fig. 2) and hosts an estimated total of 361 Mt of

84

ore with an average grade of 0.61 % Cu (BHPB, 2016). The framework for

85

understanding the geological setting of the Cerro Colorado mine was

86

established by Bouzari & Clark (2002, 2006), based on field observations of

87

the surface geology and a series of cores through the mine site, and the

88

following summary is mainly based on their work.

89

The Cerro Empexa Formation (Fig. 2) is the principal host for

90

copper mineralisation and consists of a thick regionally-developed volcanic

91

succession composed mainly of andesitic volcanic and minor continental

4

92

siliciclastic sedimentary rock units (Tomlinson et al., 2001; Bouzari &

93

Clark, 2002, 2006; Charrier et al., 2007). The Cerro Empexa Formation

94

can be divided into an earlier subgroup composed mainly of andesitic lavas

95

and breccias, lahars, minor ignimbrites, and sedimentary rocks, and a

96

later subgroup composed of andesitic to dacitic lava, breccia and tuff

97

(Tomlinson et al. 2001). The lower part of the Cerro Empexa Formation is

98

Upper Cretaceous (e.g., Galli, 1968; Tomlinson et al. 2001; Charrier et al.,

99

2007). The age of the upper part is less well known, but it may be

100

Paleocene to Eocene (Segerstrom, 1959; Muñoz, 1975; Lortie & Clark,

101

1987; Charrier et al. 2007).

102

In general, the Cerro Empexa Formation unconformably overlies

103

either Jurassic to Early Cretaceous volcanogenic and marine sedimentary

104

rocks of the Chacarilla Formation (Galli, 1968; Bouzari and Clark, 2006)

105

or Late Paleozoic to Triassic rocks of the Choiyoi Granite-Rhyolite

106

Province (e.g., Kay et al., 1989; Maksaev et al., 2014).

107

The Cerro Colorado copper mineralisation is associated with

108

intrusion of subvolcanic stocks (Fig. 3), which Bouzari & Clark (2006)

109

separate into a quartz porphyry and a biotite-quartz-plagioclase porphyry.

110

Mine geologists refer to the former as PQZ but divide the latter into two

111

separate tonalite-like units PTO 1 and PTO 2. In this study, we use

112

similar divisions as those used in the mine, but refer to PTO 1 as a

113

porphyritic quartz monzonite based on the phenocryst types (Table 1). In

114

addition, field mapping has shown there are several intrusive bodies found

115

in the border area around the mine, which are not clearly associated with

5

116

metallogenesis (Upper Cretaceous to Eocene intrusions marked on Fig. 2).

117

K–Ar and Ar–Ar dating of these bodies yielded latest Cretaceous to

118

Paleogene ages of 68–62 Ma (Huete et al., 1977; Maksaev, 1990; Agemar

119

et al., 1999; Wörner et al., 2000).

120

Zircon U–Pb age dating only available in mine reports gives ages for

121

the porphyritic tonalite, porphyritic quartz monzonite, and leucocratic

122

quartz porphyry of 52.8 ± 0.1 Ma, 54.4 ± 0.1 Ma, and 51.7 ± 0.2 Ma

123

respectively (CMCC, 2014). Unless otherwise specified, all errors for ages

124

are reported at the 2σ level or with 95 % confidence levels. An age of 63.5 ±

125

0.1 Ma is also reported from the leucocratic quartz porphyry (CMCC,

126

2014).

127

A geological history for magmatic activity at the mine site

128

culminating in a phase of intrusion around 52 Ma is compatible with the

129

52–50 Ma age range reported for Ar–Ar dating of minerals developed

130

during hydrothermal alteration (Bouzari & Clark 2002, 2006; Cotton,

131

2003, CMCC, 2014), which can be interpreted as cooling ages. However,

132

Re–Os dating of molybdenite from brecciated leucocratic quartz porphyry

133

suggests metal mineralisation at 55.5 ± 0.3 Ma (Cotton, 2003) and 53.8 ±

134

0.3 Ma (CMCC, 2014). These ages are significantly older than the Ar–Ar

135

ages of alteration and difficult to reconcile with the known U–Pb ages

136

given for the porphyritic intrusions in the mine site, which should predate

137

mineralisation but are mostly younger. However, these mineralisation

138

ages are compatible with older magmatic activity recognised in plutonic

139

rocks in the vicinity of Cerro Colorado mine.

6

140

In summary, the limited available age data in and around the Cerro

141

Colorado mine area suggest igneous intrusion, mineralisation and

142

hydrothermal alteration occurred in the time interval ca. 65–50 Ma, but

143

the data show considerable scatter; the variations may indicate multiple

144

intrusive and mineralisation events.

145 146

3

Cerro Colorado Mine Geology

147

3.1

Lithology Lithology and petrology

148

A total of 17 cores with lengths of ca. 300–1000 m, were studied at

149

the mine site. All these cores were drilled since 2008. Most of the

150

recovered core consists of either Cerro Empexa Formation or the intrusive

151

bodies. Locally the underlying basement was also intersected. The cores

152

were grouped into a NW set of 13 cores that provide a good coverage along

153

an SW–NE transect through the mine, including the main centers of Cu

154

and Mo mineralisation (Fig. 4a), and a second set of four cores that

155

provide information on a subsidiary section parallel to the first but located

156

approximately 500 m to the SE (Fig. 4b).

157 158

1) Basement units

159

Four of the deepest cores expose coarse-grained basement granite

160

(Fig. 5b) at around 800 m below surface, one of which exposed 2 m of mafic

161

gneiss (Fig. 5a) beneath the granite. The granite is composed of

162

equigranular K-feldspar, plagioclase and quartz with rare biotite with a

7

163

grain size of 1–2 cm. Graphic texture is locally developed. The granite

164

locally

165

chalcopyrite (ca. 1 volume %) and molybdenite. The feldspar is partly

166

altered into epidote and illite, and sulphide-bearing quartz veins cut the

167

granite.

contains

tourmaline

and

small

amounts

of

disseminated

168 169

2) Dominant host rock–The Cerro Empexa Formation

170

The Cerro Empexa Formation has a maximum observed thickness

171

of 800 m and consists of andesite to dacite volcanic rocks. The dominant

172

rock type is porphyritic andesite (Fig. 5c), comprising 1–3 mm phenocrysts

173

of plagioclase and quartz set in a dark fine-grained matrix. The

174

phenocrysts generally comprise 5–25 volume %, with intervals where the

175

andesite is largely aphanitic. Lithic fragments up to 5 cm are locally

176

present. Basement granite clasts are common in the basal 20 m of the

177

formation. The Cerro Empexa Formation is pervasively affected by

178

hydrothermal alteration and is the main host rock of the Cerro Colorado

179

mine. Point counting of 3 samples from Cerro Empexa unit shows the

180

presence of ca. 1.5–3.5 volume % chalcopyrite (see Table 2). Chalcopyrite

181

and molybdenite mineralisation are most closely associated with

182

stockwork veins that cut the andesite.

183 184

3) Breccia unit

8

185

A distinct breccia unit (Fig. 5d) is identified in the upper parts of

186

many of the cores and mainly exposed in the center and eastern region of

187

the mine site (Fig. 4). This unit has a thickness of up to 300 m, it contains

188

on average 15 volume %, locally up to 40 volume % of 5–30 mm sub-

189

angular to sub-rounded clasts with irregular shapes, including altered

190

volcanic lithic fragments together with quartz grains and minor

191

identifiable Cerro Empexa Formation clasts (Fig. 6 a & b) enclosed in a

192

fine-grained light-coloured, locally porous matrix consisting of quartz,

193

muscovite, sericite, clay minerals with minor calcite. The breccia shows

194

pervasive phyllic (sericitic) alteration. Disseminated chalcopyrite, bornite,

195

chalcocite (together these make up ca. 1–1.5 volume %) and pyrite are

196

present in the matrix and fine-grained molybdenite is locally abundant.

197

This unit corresponds to the bodies described by Bouzari and Clark (2006)

198

as intrusive breccias and proposed to be associated with the porphyritic

199

intrusive units.

200

The breccia unit contains clasts of Cerro Empexa and cross-cuts

201

these volcanic units (Bouzari & Clark, 2006) showing that it is relatively

202

young. The breccia unit has a matrix composed of mainly 100–400 μm

203

quartz, muscovite and even finer clay-altered material, a porous texture

204

and a high proportion of sub-rounded clasts, indicating fluidisation and an

205

intense abrasive action. These features point to the presence of a large

206

amount of gas occurring as a fluid phase during brecciation. However,

207

there is also limited evidence for crystallisation of new magmatic material

208

(see section on dating). These features suggest that the breccia unit is best

9

209

described as magmatic-hydrothermal breccia (e.g., Sillitoe 1985), which is

210

normally considered an intrusive type. However, the large amount of gas

211

involved suggests volcanic activity at relatively shallow levels. In the

212

following we refer to this breccia as volcanic, but leaves open the question

213

of whether or not the brecciation associated with volatile release reached

214

the surface.

215 216

4) Intrusive units

217

Porphyritic tonalite

218

The porphyritic tonalite (Fig. 5e) is exposed in the eastern and

219

western region of Cerro Colorado mine and shows thin sill features (Fig.

220

4a) intruding the Cerro Empexa Formation. The tonalite unit contains

221

plagioclase and biotite phenocrysts (1–4 mm) making up to 40–50 volume

222

% of the rock set in a quartz, plagioclase and biotite groundmass (300–500

223

μm). Porphyritic tonalite generally displays a greyish white colour due to

224

common silicification and sericitisation. Pseudomorphs of biotite and

225

chlorite after hornblende are commonly observed. Biotite is itself

226

commonly replaced by chlorite with rutile needles (Fig. 7a). Plagioclase

227

phenocrysts are commonly replaced by sericite, phengite and illite. Zircon

228

is a common accessory mineral. Disseminated chalcopyrite (ca. 2–2.5

229

volume %) is observed in the matrix but molybdenite is scarce.

230

Chalcopyrite-bearing stockwork veins, associated with chlorite and sericite

231

hydrothermal alteration, cut the porphyritic tonalite. The intensity of

10

232

hydrothermal alteration and copper mineralisation is greater in the

233

porphyritic tonalite than in the other porphyritic units. A localized 14–30

234

m wide brecciation zone is observed between the tonalite and Cerro

235

Empexa Formation in the eastern zone.

236

Porphyritic quartz monzonite

237

The porphyritic quartz monzonite (Fig. 5f) is exposed in the central

238

region of Cerro Colorado mine. It consists of 1–5 mm phenocrysts of

239

euhedral to subhedral K-feldspar, plagioclase, and biotite making up to ca.

240

30 volume % of the rock. Similar-sized but rarer quartz phenocrysts are

241

also present. These phenocrysts are uniformly distributed in an aphanitic

242

(< 20 μm) matrix. The matrix of the porphyritic quartz monzonite shows a

243

range of colours from creamy grey to light brown due to hydrothermal

244

alteration. Sericitisation is pervasive and replacement of plagioclase and

245

K-feldspar

246

pseudomorphs after hornblende are locally present (Fig. 7c). Rutile

247

needles are commonly observed where chlorite replaces biotite (Fig. 7d)

248

and zircon is a common accessory mineral. The porphyritic quartz

249

monzonite contains disseminated chalcopyrite (ca. 0.5–1.5 volume %) and

250

is also commonly cut by chalcopyrite- (and locally molybdenite-) bearing

251

quartz and sericite rich stockwork veins.

by

sericite,

phengite

and

illite

is

common.

Biotite

252

Brecciation features can be observed throughout most of the

253

porphyritic quartz monzonite body. Angular clasts, mainly composed of

254

the monzonitic and quartz porphyry units, and also locally xenoliths of the

255

porphyritic tonalite, Cerro Empexa Formation and basement units, are

11

256

cemented by weathered clay or quartz fill (Fig. 6 c–f). Chalcopyrite and

257

molybdenite mineralisation is commonly found along the clast boundaries

258

and in the breccia matrix. Lesser amounts are disseminated in the clasts,

259

and as part of quartz veins that cut the breccia unit.

260

Leucocratic Quartz Porphyry

261

The leucocratic quartz porphyry (Fig. 5g) is exposed in close

262

association with the porphyritic quartz monzonite. It contains more than 5

263

volume % of rounded to irregular, or rectangular-shaped (Fig. 7i) partially

264

embayed quartz “eyes” (1–5 mm diameter), and 10 — 30 volume ％ of

265

similar-sized plagioclase phenocrysts and aggregates comprised of K-rich

266

phyllosilicates and clay, which are interpreted as altered products of K-

267

feldspar. These phenocrysts are distributed within a leucocratic aphanitic

268

groundmass composed mainly of quartz, sericite, phengite, illite and other

269

clay minerals. This unit locally contains biotite and chlorite.

270

Some of the quartz eyes in strongly altered domains have unusual

271

lath shapes. The presence of clay mineral aggregates in the cores of some

272

these grains and presence of grains with outlines similar to K-feldspar

273

with Carlsbad twining suggests that at least some of the quartz eyes are

274

secondary and pseudomorphing pre-existing K-feldspar grains as a result

275

of hydrothermal alteration (Fig. 7 f–i). Similar processes have been

276

reported in other studies of hydrothermal alteration systems (e.g., Kerr et

277

al. 1950; Brauhart et al. 2001).

12

278

The leucocratic quartz porphyry commonly contains disseminated

279

pyrite in the matrix. Copper is mainly present in the form of chalcocite

280

and covellite; these minerals probably represent secondary sulphides after

281

chalcopyrite (e.g., Sillitoe & Clark, 1969; Buckley & Woods, 1984), and

282

make up < 1 volume %. Both molybdenite and chalcopyrite are rare as

283

matrix minerals, but commonly occur in quartz veins that cut the

284

leucocratic quartz porphyry. Brecciation features similar to the porphyritic

285

quartz monzonite are also observed in the leucocratic quartz porphyry

286

(Fig. 6 c–d).

287

Porphyritic granodiorite

288

In a confined region in the NE part of the mine site, some

289

porphyritic granodioritic rocks intrude the porphyritic tonalite and the

290

breccia unit. These younger rocks show considerable variations in texture

291

and abundance of quartz in groundmass, but since the occurrence is small

292

and localized, here we treat them as one unit. The porphyritic granodiorite

293

(Fig. 5h) is characterized by ca. 20–30 volume % of 1–3 mm phenocrysts of

294

mainly plagioclase, with minor K-feldspar and biotite in a dark fine-

295

grained to aphanitic, locally chloritized or sericitised groundmass. Sericite

296

partly replaces plagioclase but to a much lesser extent than the other

297

intrusive units. This unit is largely lacking copper mineralisation

298

suggesting it was intruded towards the end of ore formation.

299 300

3.2

Relative age relationships

13

301

In the field studies, particular attention was paid to contact areas

302

between the different lithologies. The leucocratic quartz porphyry and

303

porphyritic tonalite are generally distributed in distinct parts of the mine,

304

and no cross-cutting intrusive contacts were observed between them.

305

Nevertheless, enclaves that closely resemble the porphyritic tonalite are

306

commonly observed included in the leucocratic quartz porphyry unit (Fig.

307

6e), suggesting the leucocratic quartz porphyry is younger. This is in

308

agreement with previous suggestions (Bouzari & Clark, 2006; CMCC,

309

2014). In contrast, although the leucocratic quartz porphyry and the

310

porphyritic quartz monzonite occur in similar parts of the mine site,

311

similar enclave relationships were not observed. In addition, all observed

312

contacts between the leucocratic quartz porphyry and the porphyritic

313

quartz monzonite show gradual transitions (Fig. 6 h & i), and no clear

314

boundary could be drawn between these two rock types in the field.

315

The porphyritic granodiorite shows sharp intrusive contacts with

316

the porphyritic tonalite, locally with a chilled margin. These features

317

clearly indicate the porphyritic granodiorite is younger than the

318

porphyritic tonalite. Further information concerning the relative ages of

319

the intrusions comes from geochemistry and geochronology.

320 321

4

Geochemistry

322

The strong alteration of feldspars and groundmass of the leucocratic

323

quartz porphyry and its gradational contacts with the porphyritic quartz

14

324

monzonite suggest that the quartz porphyry may be a more strongly

325

altered part of the monzonite. To examine this possibility, we identified

326

alteration chemical vectors by analysing bulk rock geochemistry of

327

samples from the Cerro Empexa Formation that had undergone different

328

degrees and grades of alteration. We then compared these vectors with the

329

compositions shown the monzonite and quartz porphyry. Analytical

330

procedures and the whole rock XRF geochemical data are compiled in

331

supplementary sheet 2, together with whole rock ICP-MS geochemical

332

data provided by Cerro Colorado mine. The mine data are based on

333

dissolution of silicate minerals.

334

To examine the degree of alteration, and in particular to focus on

335

the K, Ca and Na transport, we use the (2Ca + Na + K)/Al versus K/Al

336

molar ratio plot (e.g., Madeisky, 1996, Warren et al., 2007). A unity value

337

for (2Ca + Na + K)/Al represents fresh plagioclase and having both (2Ca +

338

Na + K)/Al and K/Al values at 1 indicates the presence of biotite, K-

339

feldspar, or both of these minerals. Thus, fresh intermediate to felsic rocks

340

should plot close to 1 on the horizontal axis. K metasomatism should

341

increase the K/Al value whereas clay formation will cause a decrease in

342

the (2Ca + Na + K)/Al value. Sericite is formed as a major alteration

343

product in phyllic alteration zone as a result of K metasomatism.

344

Therefore, a good proxy for sericitisation is the molar ratio of muscovite.

345

The mass transfer vectors for the Cerro Empexa Formation (Fig. 8)

346

show a general decrease in (2Ca + Na + K)/Al and increase in K/Al,

347

plotting closer to muscovite, with greater alteration of the andesitic host

15

348

rocks. The same compositional plots for the monzonitic and quartz

349

porphyry units show identical relationships. This suggests that the

350

leucocratic quartz porphyry may have been formed by feldspar destruction

351

and sericitisation of the porphyritic quartz monzonite following the

352

alteration vectors seen in the Cerro Empexa Formation.

353 354

5

Zircon U–Pb Geochronology

355

A total of 24 samples of the main units of the Cerro Colorado mine

356

site were selected for zircon separation and U–Pb dating. Sample locations

357

are shown in Fig. 4. Samples weighing 1–2 kg were first crushed by hand

358

to < 840 µm, and then further reduced to a grain size of 105–177 µm by

359

milling and wet sieving. This range corresponds to the average size of

360

zircon grains observed in thin sections. Diiodomethane (CH2I2), was used

361

to separate the heavy mineral portions from the felsic fraction. Zircon

362

crystals were then hand-picked, mounted and polished using a series of

363

diamond pastes down to 1/4 µm size. Cathodoluminescence (CL) images of

364

zircon grains were taken using an SEM-CL unit at the Department of

365

Earth and Environmental Sciences, Nagoya University to characterize

366

zircon zonation patterns and help reveal the presence of distinct growth

367

domains (Fig. 9).

368

5.1

CL Imaging

369

The majority of the CL images of zircon grains from the intrusions

370

show well-formed crystal growth shapes with clear sector and oscillatory

16

371

zoning characteristic of magmatic zircon (Fig. 9 a–l, q). Distinct cores are

372

also commonly observed, which tend to be darker in CL and show

373

irregular shapes (Fig. 9 m–p). However, many zircon grains showing

374

concentric oscillatory growth zones with no clear break in the CL images

375

also show domains with distinct ages (e.g., Fig. 9 a, b, h, j).

376

In order to obtain ages that are representative of the final

377

emplacement age of the rocks, we focused on obtaining results from the

378

rims of zircon measured using laser ablation spot dating. CL and back

379

scattered electron images were used to select domains for analysis that did

380

not overlap boundaries between distinct cores and rims and did not

381

contain cracks or inclusions. The locations of analysed spots were verified

382

by SEM and CL observation after ablation.

383

5.2

LA-ICP-MS Isotope Analyses

384

Zircon U–Pb LA-ICP-MS analysis was carried out at the Graduate

385

School of Environmental Studies, Nagoya University using a NWR213

386

(Electro Scientific Industries, Inc., USA) laser ablation system coupled

387

with an Agilent 7700x (Agilent Technologies, USA) inductively coupled-

388

plasma mass spectrometer (ICP-MS) with Nd-YAG laser source of λ = 213

389

nm. The analyses were carried out with a laser energy of 11.7 J/cm2, a

390

repetition rate of 10 Hz, a pre-ablation time of 8 seconds, an integration

391

time of 10 seconds and with a laser spot size of 25 µm for all analyses.

392

For all analyses NIST SRM610 glass (Horn and von Blanckenburg,

393

2007), 91500 zircon (238U–206Pb age = 1062.4 ± 0.4 Ma; Wiedenbeck et al.,

17

394

1995) and OD-3 zircon (238U–206Pb age = 33.0 ± 0.1 Ma; Iwano et al., 2013)

395

were used as standard samples. The analytical procedures follow Orihashi

396

et al. (2008) and Kouchi et al. (2015).

397

Altogether 15–50 spots were analysed for each of the 24 samples

398

(Table 3, Supplementary sheet 3). U–Pb ages were calculated using Isoplot

399

version 4.15 (Ludwig, 2012). Using the probability of concordance as an

400

approach to filter U–Pb data (Ludwig, 1998), only data with probabilities

401

of concordance ≥ 0.1 (e.g., Rossignol et al., 2016) were used for

402

interpretation for the three main intrusive units (porphyritic tonalite,

403

porphyritic quartz monzonite and leucocratic quartz porphyry) and the

404

small porphyritic granodiorite unit. For the basement granite, Cerro

405

Empexa Formation and the breccia unit, data with probabilities of

406

concordance > 0.01 (e.g., Matthews & Guest, 2016) were used. An

407

additional filter was applied to exclude data with high uncertainties.

408

Individual analyses with 2σ error < 10.0% for the subvolcanic intrusive

409

units, or < 10% for the basement and host rocks units, were used for the

410

calculation (e.g., Gehrels et al., 2011; Zimmerer & McIntosh, 2013).

411 412

5.3

Results

413

1) Approach to age interpretations

414

The age results show multiple peaks indicating that inherited

415

zircon is common in all rock units. In order to estimate representative

416

ages of formation, age distribution histograms are used to identify distinct

18

417

age groups present in each rock unit (Fig. 10). A single result that gives

418

the youngest age places an older limit on the age of formation. When

419

multiple results concentrate around the same young age showing a single

420

peak, a weighted mean age of the young peak can be calculated to

421

represent the age of crystallisation. An unmixing multicomponents

422

algorithm method (Sambridge & Compston, 1994) is used if multiple peaks

423

are recognised in the age distribution. We use the mean squared weighted

424

deviation (MSWD) values to help determine if the calculated ages are

425

statistically representative of single crystallisation events (Wendt & Carl,

426

1991). For the units in which multicomponents are recognised, weighted

427

mean ages are recalculated using the constituent fractions of the youngest

428

peaks given by unmixing multicomponent model to achieve suitable

429

MSWD values (e.g., Bolhar et al., 2010; Murray et al., 2013).

430 431

2) Data description and interpretation

432

i) Basement granite

433

Three granite samples, IA12, 15IA03 and IA68 yielded plutonic

434

activity ages of 347–232 Ma (Fig. 10a). The spread in ages obtained

435

suggests inherited zircon is present. However, the range is in good

436

agreement with the reported episodic magmatism during the evolution of

437

paleo-Gondwanan margin from late Paleozoic to early Mesozoic (e.g., Kay

438

et al, 1989; Maksaev et al., 2014). Our granite ages are consistent with the

439

late Paleozoic–Triassic magmatism reported in the Collahuasi district—

19

440

the northernmost exposure of Choiyoi Magmatic Province (Munizaga et

441

al., 2008; Maksaev et al., 2014).

442 443

ii) Cerro Empexa Formation

444

Four samples were selected from different stratigraphic levels to

445

obtain dates over the full range of the Cerro Empexa Formation sequence

446

in this area.

447

Two andesite samples, 15IA01 and IA66, collected at about 800 m

448

below the surface, close to the contact with the granite basement, yielded

449

a complex age spectrum with a young peak showing a range of 83–76 Ma

450

and a weighted mean of 79.1 ± 2.6 Ma (Fig. 10b). Inherited zircon is also

451

common.

452

An andesite sample collected from 300 m below the surface, 13IA07,

453

is representative of the mid volcanic succession. Two dates of 75.1 ± 5.5

454

Ma and 68.8 ± 4.6 Ma suggest an upper Cretaceous age for this unit.

455

However, most analyses yield old ages up to 1 Ga from inherited zircon.

456

A sample of a diorite dyke, IA84, that intruded into the andesitic

457

lava sequences of the Cerro Empexa Formation yielded a spread of ages

458

mainly falling in the range 78–67 Ma with a weighted mean of 72.5 ± 1.4

459

Ma (Fig. 10c). Some older ages from inherited grains were also recorded.

460

These results show the presence of magmatic intrusions occurring at the

461

same time as the Cerro Empexa Formation volcanism. Differences in ages

462

of cores and rims of zircon grains suggest zircon growth over a period of

20

463

several millions of years possibly related to recycling and autocrystic

464

zircon crystallisation during diorite emplacement (e.g., Fig. 9p).

465 466

iii) Breccia Unit

467

Breccia samples 13IA06 and IA49 yielded a unimodal spread in

468

ages 68–54 Ma (Fig. 10d) with a weighted mean age of 57.2 ± 1.4 Ma.

469

These data show the presence of volcanic activity after the development of

470

the Cerro Empexa units, which ended about 70 Ma. Ages older than 70 Ma

471

show the presence of inherited grains. The breccia unit represents a

472

distinct phase of magmatic activity at Cerro Colorado Mine with a

473

restricted distribution.

474 475

iv) Porphyritic tonalite

476

Almost all the porphyritic tonalite zircon ages collected from 5

477

samples, 15IA09, IA29, IA80, IA81 and 1006, fall in the range 59–50 Ma.

478

The age distribution histogram (Fig. 10e) shows a single peak with a

479

weighted mean age of 53.51 ± 0.80 Ma.

480

Zircon grains for which both rims and cores were analysed

481

commonly show concordant early Eocene core ages and discordant rim

482

analyses. One explanation for the rim results is addition of extraneous Pb

483

during hydrothermal interaction of zircon with metal-rich brines (e.g.,

484

Sinha et al., 1992; Mattinson et al., 1996). The discordant results were not

485

included in the final age interpretation.

21

486 487

v) Porphyritic quartz monzonite

488

Four porphyritic quartz monzonite samples, 13IA02, 13IA04,

489

13IA11 and IA97, yield a continuous spread of zircon grain ages 64–46

490

Ma. Inherited zircon is also present. Within the 64–46 Ma range, the age

491

distribution histogram (Fig. 10f) shows a clear bimodal distribution, with

492

an obvious peak occurring at around 50 Ma and a subordinate peak at ca.

493

60 Ma. CL images of some well-formed relatively small single grains of

494

zircon suggest a single continuous phase of growth and have rim ages of

495

ca. 60 Ma (Fig. 9 j, n). This suggests the 60 Ma peak of the age distribution

496

represents a discrete phase of zircon crystallisation.

497

Due to the bimodal age distribution of ages for this sample, we use

498

the unmixing multicomponents by Gaussian deconvolution method

499

(Sambridge & Compston, 1994) to separate the two peaks. Application of

500

this algorithm results in two ages of 59.51 ± 0.8 Ma and 50.29 ± 0.65 Ma,

501

which we interpret as both representing significant phases of zircon

502

growth associated with magma crystallisation. Recalculation of the young

503

group of ages (N = 32) gives a weighted mean of 50.37 ± 0.79 Ma with an

504

MSWD of 1.6.

505 506

vi) Leucocratic quartz porphyry

507

Leucocratic quartz porphyry samples, 13IA05, 13IA10, Mo6 and

508

IA76, yielded a very similar age population set to the porphyritic quartz

22

509

monzonite with a continuous spread of zircon ages 62–48 Ma, a peak at

510

around 50 Ma and a less prominent shoulder at around 60 Ma (Fig. 10g).

511

Applying unmixing multicomponents analysis to the data yields ages of

512

60.3 ± 2.4 Ma and 51.74 ± 0.6 Ma, very similar to the ages for the

513

porphyritic quartz monzonite. Recalculation of the young group of ages (N

514

= 30) gives a weighted mean of 51.73 ± 0.88 Ma with an MSWD of 2.2.

515

This MSWD value is somewhat greater than that expected for a single age

516

population (Wendt & Carl, 1991), suggesting these age data include some

517

unresolved component possibly related to the presence of zircon

518

antecrysts.

519 520

vii) Porphyritic granodiorite

521

Two samples of the porphyritic granodiorite, IA37 and IA47, yielded

522

a range of ages of 61–47 Ma. The age distribution histogram (Fig. 10h)

523

shows two peaks: a prominent peak at ca. 50 Ma and a smaller one at ca.

524

60 Ma, similar to the porphyritic quartz monzonite and the leucocratic

525

quartz porphyry. The unmixing multicomponents analysis yields ages of

526

58.1 ± 3.2 Ma and 50.98 ± 1.7 Ma. Recalculation of the young group of ages

527

(N = 8) gives a weighted mean of 51.0 ± 1.5 Ma with an MSWD of 1.2.

528 529

5.4

Zircon U–Pb SHRIMP analysis

530

To constrain the ages of the main intrusive porphyritic bodies in the

531

Cerro Colorado mine site we selected well-formed grains that were

23

532

concordant, had simple continuous CL zoning and yielded the youngest

533

peak LA-ICP-MS ages were selected to carry out Sensitive High

534

Resolution Ion Microprobe (SHRIMP IIe) analysis at the National

535

Institute of Polar Research, Japan.

536

SHRIMP has a higher spatial resolution than LA-ICP-MS and this

537

allows several measurements to be made within a single CL band or

538

within well-defined CL domains of single grains. For SHRIMP analyses,

539

both the depth of the ablated pit and its mean diameter are considerably

540

smaller than for LA-ICP-MS. Similar to the procedure for the LA-ICP-MS

541

analyses, both backscattered electron images and CL images were used to

542

select the sites for SHRIMP analyses. One of the advantages of using the

543

SHRIMP technique is that weighted mean ages can be derived from

544

repeated measurements of a single zone within a single grain. Concentric

545

CL zones in minerals reflect contemporaneous growth surfaces and

546

measurements and their distribution allows those parts of the crystal that

547

grew at the same time to be identified. The results are summarized in

548

Table 4.

549 550

(i)

Experimental procedures

551

The analyses were carried out with an O2– primary ion beam

552

intensity of 1.1 nA and a spot size with mean diameter of approximately

553

14 µm. Analytical conditions attained a mass resolution of 5100 (M/∆M)

554

and a sensitivity of 24 cps/206Pb ppm/nA. To minimise possible

24

555

contamination, the sample surface was first cleaned by rastering using a

556

90 µm beam for 2 minutes before actual measurement

557

The standards TEMORA2 (age = 416.8 ± 0.3 Ma; Black et al., 2004)

558

and 91500 (Wiedenbeck et al., 1995) were used for U–Pb calibration and

559

for determining U concentrations, respectively. The standard OD3 with

560

reference value 32.853 ± 0.016 Ma (Lukács et al., 2015) was used as an

561

external reference material. The sample preparation, experimental

562

procedures and data reduction approach of SHRIMP measurements

563

essentially follows Horie et al. (2013).

564 565

(ii)

SHRIMP results of zircon grains from the three main porphyritic

566

lithologies

567

A zircon grain was selected from the porphyritic tonalite sample

568

15IA09. Two spots from the same CL growth zone, and one from a similar

569

domain were analysed. The probability of concordance of individual

570

analysis was first calculated using Isoplot 4.15 (Ludwig, 2012) to check for

571

data validity. All 3 spots attained values ≥ 0.94 and together they yield a

572

weighted mean

573

errors for SHRIMP analyses are represented at the 1σ level) (Fig. 11a).

574

The 53.5 Ma SHRIMP age for the tonalite is, within error, identical to the

575

age derived from LA-ICP-MS analyses.

206Pb/ 238U

age of 53.5 ± 1.2 Ma (following normal practice

576

A zircon grain was selected from the porphyritic quartz monzonite

577

sample 13IA02. Six spots from the same CL growth zone were analysed.

25

578

The probability of concordance for all 6 spots is ≥ 0.94 and together they

579

yield a weighted mean

580

age is, within error, identical to the younger age derived from LA-ICP-MS

581

analyses for the porphyritic quartz monzonite.

206Pb/ 238U

age of 49.9 ± 0.64 Ma (Fig. 11b). This

582

A zircon grain from the leucocratic quartz porphyry sample 13IA10.

583

Two spots from the same CL growth zone, and two others from a similar

584

oscillatory zircon domain were analysed. The probability of concordance

585

for all 4 spots is ≥ 0.94 and together they yield a weighted mean 206Pb/ 238U

586

age of 49.4 ± 0.78 Ma (Fig. 11c). This result is slightly younger than the

587

estimate from LA-ICP-MS analyses, but almost identical to the result for

588

the porphyritic quartz monzonite sample.

589

The SHRIMP dating confirms the inference from LA-ICP-MS age

590

dating that the leucocratic silicified quartz-porphyry-like domain and the

591

monzonite-porphyry domain were crystallizing at the same time around 50

592

Ma and the tonalitic porphyry formed 3–4 million years earlier.

593 594

6

Discussion

595

6.1

Geochronology of the Cerro Colorado area

596

The presence of 350–230 Ma granitic basement in the Cerro

597

Colorado area extends the known northern extent of the Late Paleozoic to

598

Triassic Choiyoi magmatic province by about 100 km. In addition, the lack

599

of the Jurassic–Early Cretaceous Chacarilla Formation in Cerro Colorado

26

600

area suggests that this unit was eroded away prior to the deposition of

601

Cerro Empexa Formation.

602

Our new U–Pb dating shows the presence of large amounts of

603

inherited zircon in the Cerro Empexa Formation, but the cluster of ages

604

from 80 to 70 Ma agrees well with the proposed Upper Cretaceous age for

605

this unit (Galli, 1968; Tomlinson et al. 2001; Charrier et al., 2007). Zircon

606

grains from the bodies that intrude the Cerro Empexa Formation in the

607

Cerro Colorado mine site show widespread evidence for crystallisation at

608

ca. 60 Ma. This agrees with K–Ar and Ar–Ar ages reported in earlier

609

studies for intrusive bodies of varying compositions that occur in the wider

610

region and intrude the Cerro Empexa Formation (Huete et al., 1977;

611

Maksaev, 1990; Agemar et al., 1999; Wörner et al., 2000). The breccia unit

612

that cuts Cerro Empexa Formation shows evidence for crystallisation at

613

ca. 57 Ma and is additional evidence for significant igneous activity

614

around 60 Ma. Metal mineralisation events commonly occur up to several

615

million years after the main phase of intrusion in an active magmatic

616

system (e.g., Valencia et al., 2006; Barra et al., 2013). Magmatic events at

617

60–57 Ma are, therefore, in good agreement with the Re–Os molybdenite

618

ages of 55.5 ± 0.3 Ma (Cotton, 2003) and 53.8 ± 0.3 Ma (CMCC, 2014)

619

reported from the Cerro Colorado mine site.

620

Our new dating of the intrusive bodies of the Cerro Colorado mine

621

site shows the tonalitic body is the oldest and was intruded ca. 53.5 Ma.

622

This was followed by the monzonitic–quartz porphyry body and small

623

granodioritic porphyry body all of which have emplacement ages of ca. 51–

27

624

50 Ma. All these intrusive bodies contain significant amounts of inherited

625

and autocrystic zircon and previously reported older zircon ages, including

626

the 54.4 Ma and 63.5 Ma ages given for the monzonite and quartz

627

porphyry, respectively (CMCC, 2014) are likely to reflect similar recycled

628

xenocrysts and antecrysts zircon grains (e.g., Miller et al., 2007; von

629

Quadt et al., 2011; Simmons et al., 2013). An early intrusion age of the

630

porphyritic tonalite is consistent with its strong alteration compared to

631

other intrusions and is in agreement with previous workers (Bouzari &

632

Clark, 2006).

633

The very similar ages derived for the porphyritic quartz monzonite

634

and leucocratic quartz porphyry lend support to the inference from field

635

and geochemical evidence that the leucocratic quartz porphyry is simply a

636

more altered equivalent of the porphyritic quartz monzonite. The 50 Ma

637

monzonitic–quartz porphyry body emplaced in the centre of the Cerro

638

Colorado mine is likely to have been the main intrusion that enabled ore

639

deposition in almost all rock units—including the higher Cu content

640

porphyritic tonalite and itself.

641

Ar–Ar dating of biotite from early-stage alteration and muscovite

642

from main-stage alteration in the Cerro Colorado mine site yields ages of

643

52–50 Ma (Cotton, 2003; Bouzari & Clark, 2006; CMCC, 2014). These are

644

in good agreement with our estimated intrusive ages for the porphyritic

645

units, suggesting that this is a suitable time frame for intrusion, metal

646

mineralisation and cooling, and that extraneous Ar is not a significant

647

problem in this area. The lack of Ar–Ar ages of around 60 Ma is best

28

648

explained as due to the reheating of the hydrothermal system during the

649

53–50 Ma stage of porphyry intrusions (e.g., Chiaradia et al., 2013).

650

Combining the results for the U–Pb dating with the Ar–Ar dating

651

suggests that cooling and, by inference, metal mineralisation was largely

652

complete by 50 Ma. Further evidence in support of this comes from the 51–

653

50 Ma age of the porphyritic granodiorite. This unit is largely

654

unmineralised, suggesting it formed towards the end of mineralisation-

655

related hydrothermal activity.

656 657

6.2

Syn-volcanic intrusion of the quartz monzonite porphyry

658

The porphyritic quartz monzonite shows an aphanitic groundmass

659

(Fig. 5e) implying rapid crystallisation, which could be due to rapid

660

cooling. However, this is difficult to reconcile with the observed coarser

661

grain size of the matrix of the porphyritic tonalite that is exposed at

662

similar structural levels. The clearly crystalline matrix of the tonalite

663

suggests the country rock was warm enough to prevent sudden cooling

664

after intrusion. Our new dating shows there is a limited ca. 3-million-year

665

time difference between the emplacement of these two subvolcanic units

666

implying they were emplaced at similar shallow paleo-depths and, by

667

implication, similar ambient temperatures. The lack of a chilled margin to

668

the monzonitic–quartz porphyry stock is further evidence that the

669

quenching implied by its fine grain size is not simply due to rapid cooling.

29

670

An alternative explanation that can account for the main features of

671

the monzonitic–quartz porphyry is rapid crystallisation due to sudden

672

decompression. An undercooling effect could be created during rapid

673

decompression of the melt at isothermal conditions, causing crystallisation

674

of phenocrysts and quenching of the groundmass (e.g., Cashman, 1992;

675

Cashman & Blundy, 2000; Hammer & Rutherford, 2002). Decompression-

676

induced crystallisation during magma ascent has been highlighted as a

677

potentially important process associated with formation of porphyritic

678

stocks in hydrothermal ore environments (Cashman, 2004). Rapid

679

exsolution of a volatile phase during decompression is likely to cause

680

fracturing and may be related to surface volcanism (e.g., Sillitoe, 1973,

681

1985). The strong brecciation seen in both the leucocratic quartz porphyry

682

and the host rock surrounding the monzonitic–quartz porphyry stock (Fig.

683

6 c & d) is in agreement with the proposal that this unit crystallized

684

rapidly due to rapid exsolution of a gas phase.

685

An alternative explanation for intense brecciation in PCD deposits

686

is that it results from a large volume of gas released during Cu-sulphide

687

mineralisation reactions (e.g., Blundy et al., 2015). However, in Cerro

688

Colorado mine, the main brecciated zone occurs around the monzonitic–

689

quartz porphyry stock and is not present above the main host of hypogene

690

copper mineralisation—the sericite-chlorite-clay-alteration zone—around

691

the porphyritic tonalite intrusion. These observations suggest that the

692

brecciation was caused by monzonitic–quartz porphyry emplacement

693

rather than sulphide mineralisation. The physical features associated with

30

694

the porphyritic tonalite and the monzonitic–quartz porphyry are

695

summarized in Fig. 13.

696 697

7

Conclusions

698

Zircon U–Pb dating shows the Cerro Empexa Formation in the

699

Cerro Colorado PCD mine area of N Chile is Upper Cretaceous and

700

unconformably overlies Late Paleozoic–Triassic basement granite that can

701

be correlated with the Choiyoi Magmatic Province. This extends the

702

previously known distribution of this granite unit by about 100 km

703

northward.

704

Intrusive magmatic activity associated with mineralisation in the

705

Cerro Colorado area occurred in two main phases at ca. 60 Ma and 53–50

706

Ma. The 60 Ma intrusions are mainly recognised in the broader Cerro

707

Colorado area outside of the mine site. However, there is a major volcanic-

708

related breccia unit that developed at around 57 Ma and can be linked to

709

this magmatic phase, and associated with an early phase of mineralisation

710

at ca. 56 and 54 Ma. A tonalitic porphyry intruded at ca. 53.5 Ma followed

711

by monzonite-quartz and granodioritic porphyry bodies at ca. 50 Ma. The

712

monzonite and quartz porphyry units have traditionally been treated as

713

separate intrusions, but gradational textural contacts and geochemical

714

changes coupled with indistinguishable zircon U–Pb ages suggest the

715

quartz porphyry unit is a strongly altered and silicified equivalent of the

716

monzonite.

31

717

A close relationship between volcanic activity and porphyry

718

intrusion is provided by the fine-grained matrix of the monzonite unit and

719

the associated locally intense brecciation, both of which are best explained

720

as the result of pressure release due to rapid decompression associated

721

with volcanic eruption.

722 723

8

Acknowledgements

724

This work was mainly funded by BHP Billiton and a Japanese

725

MEXT scholarship. Additional support was provided by a grant under the

726

scheme of ProjecTerrae offered by J. Chan. We thank the editor J. Mauk

727

for his constructive advice, and the two reviewers F. Bouzari and S. Kay

728

for providing useful comments to improve this piece of work. We thank S.

729

Sparks for his support of this study and insightful comments. Thanks also

730

to J. Blundy, F. Cooper, A. Rusk, D. Condon, S. Tapster, D. Tang and J.

731

Wong, for their constructive comments and advice. We also thank D.

732

Hutton for his help in the field. We would like to acknowledge K. Mimura

733

for his generous assistance in geochemistry analysis at Nagoya

734

University. We also thank M. Nozaki and T. Nagaya for guidance on CL

735

imaging in SEM labs at Nagoya University, A. Ruggiero, E. Gonzales and

736

V. Quintana for logistic help at Cerro Colorado Mine, N. Chile, I. Jetsonen

737

for assistance in zircon preparation work, and S. F. Cheuk for graphical

738

assistance.

32

739

9

Figure captions

740

Fig. 1. Summary of published

741

metallogenic provinces plotted against latitude in northern Chile (Map

742

modified after Sillitoe, 2010 & 2012). Error bars are generally smaller

743

than the size of the symbols and are not shown on the plot. All data points

744

and corresponding references are provided in supplementary sheet 1.

745

Fig. 2. Geologic map of the Cerro Colorado area (area shown in the

746

rectangle in small map) showing major structures and lithostratigraphic

747

units (after Bouzari & Clark, 2006).

748

Fig. 3. Local geological map of Cerro Colorado Mine pit area (modified

749

after Bouzari & Clark, 2006) with locations of the inspected drill cores.

750

Lithology abbreviations in legend: PTN = porphyritic tonalite; PMN =

751

porphyritic quartz monzonite; LQP = leucocratic quartz porphyry; PGD =

752

porphyritic granodiorite.

753

Fig. 4. Cross-sections derived from projecting information from core logs

754

onto two subparallel roughly E–W sections across the mine pit. a) North

755

section (A–B in Fig. 3), b) South section (C–D in Fig. 3). Lithology

756

abbreviations: CEF = Cerro Empexa Formation; VBX = breccia unit; PTN

757

= porphyritic tonalite; PMN = porphyritic quartz monzonite; LQP =

758

leucocratic quartz porphyry; PGD = porphyritic granodiorite.

759

Fig. 5. Rock types exposed in drill cores (length of scale bar is 1 cm, all

760

photos are in the same scale). a) Mafic gneiss underlying granite. b)

761

Basement granite, locally epidote-altered (left = fresh, right = epidote-

U–Pb, Ar–Ar and

Re–Os ages in

33

762

altered). c) Cerro Empexa Formation (CEF) andesite with common

763

plagioclase phenocrysts and lithic fragments. Aphanitic layers are also

764

present, in contrast to the porphyritic CEF (left: with phenocrysts and

765

clasts; right: aphanitic). d) Breccia unit (VBX). Locally abundant

766

disseminated molybdenite gives the rock a bluish grey tint, in contrast to

767

the light-coloured matrix VBX (left shows disseminated molybdenite in

768

matrix; right shows mainly a clay-mineral-rich matrix). e) Porphyritic

769

tonalite containing biotite and plagioclase phenocrysts in a matrix with a

770

grain size 300–500 µm. (f) Porphyritic quartz monzonite containing biotite,

771

two feldspars and quartz phenocrysts in an aphanitic matrix.. g)

772

Leucocratic quartz porphyry containing phenocrysts of both plagioclase

773

and K-feldspar and significant amount of subrounded quartz “eyes” in a

774

matrix of quartz and feldspars.. h) Porphyritic granodiorite, with K-

775

feldspar, plagioclase, and biotite phenocrysts in an intermediate matrix.

776

Fig. 6. Features observed in hand specimens of the breccia unit and the

777

monzonitic–quartz porphyry unit. Length of scale bar is 1 cm. (a, b)

778

Breccia unit consists of rounded to sub-rounded clasts set in a fine-grained

779

matrix. The clasts are commonly highly altered and silicified and the

780

matrix is locally rich in disseminated molybdenite (a) Cerro Empexa

781

Formation fragments occur locally. (c, d) Brecciated leucocratic quartz

782

porphyry within the subvolcanic stock. All fragments are derived from the

783

monzonitic–quartz porphyry unit. The fine-grained, clay-mineral-rich

784

matrix in between the fragments is commonly mineralized. (e, f) Xenoliths

785

(porphyritic tonalite and Cerro Empexa Formation) incorporated in the

34

786

monzonitic–quartz porphyry unit. Enclaves of porphyritic tonalite (e) seen

787

in breccia formed related to leucocratic quartz porphyry emplacement

788

suggest the monzonitic–quartz porphyry postdates the porphyritic

789

tonalite. (g– (g–i) A series of observations suggesting porphyritic quartz

790

monzonite and leucocratic quartz porphyry represent different alteration

791

grades of the same rock type. g) Sample of leucocratic quartz porphyry.

792

Rare biotite occurs in this sample. Chlorite formed as an altered product of

793

this biotite. These observations suggest the leucocratic quartz porphyry

794

has a more mafic precursor. h) Examples of transitions between

795

porphyritic quartz monzonite and leucocratic quartz porphyry, with 1–2

796

cm boundary zones, showing the contacts are gradual. i) Different degrees

797

of alteration in the monzonitic and quartz porphyry units. The altered

798

parts of the rock have turned into a quartz eye-bearing leucocratic rock,

799

but do not show clear contacts with porphyritic quartz monzonite.

800

Abbreviations: Bn = bornite; Ccp = chalcopyrite; CEF = Cerro Empexa

801

Formation; LQP = leucocratic quartz porphyry; Mo = molybdenite; PMN =

802

porphyritic quartz monzonite; PTN = porphyritic tonalite; Py = pyrite.

803

Fig. 7. Photomicrographs of examples of hydrothermal alteration in the

804

porphyritic quartz monzonite, the porphyritic tonalite, and the leucocratic

805

quartz porphyry (c & d are taken under plane polarized light; others with

806

crossed nicols). a) Rutile needles coexisting with chlorite replacing biotite

807

in the porphyritic tonalite. b) Plagioclase altered into a mixture of sericite

808

and clay minerals in the porphyritic tonalite. c) Pseudomorph of biotite

809

after hornblende in the porphyritic quartz monzonite. d) Chlorite partially

35

810

replacing biotite and coexisting with rutile needles in the porphyritic

811

quartz monzonite. e) Quartz and altered feldspar phenocrysts sitting in an

812

ultrafine-grained matrix in the porphyritic quartz monzonite. Carlsbad

813

twinning is clearly observed in one orthoclase crystal. f)– i) Examples of

814

the leucocratic quartz porphyry showing different levels of feldspar

815

destruction and subsequent replacement by quartz, forming secondary

816

quartz “eyes” in the leucocratic quartz porphyry. f) Feldspar destruction

817

forming sericite and clay minerals such as illite and smectite. g) Feldspar

818

completely replaced by sericite, phengite and clay minerals. h) Incomplete

819

replacement of illite and clay mineral pseudomorphs by quartz. i) Good

820

candidate for a quartz grain completely pseudomorphing a lath-shaped

821

feldspar with an outline resembling Carlsbad twinning. The straight sides

822

of the quartz grain make an apparent angle of 145˚, which corresponds

823

closely to the expected angle of ~147.5˚ between the (100) and the inverse

824

twin face (001) of orthoclase (e.g., Lewis, 1899). Abbreviations: Alu =

825

alunite; Bt = biotite; Chl = chlorite; Ksp = K-feldspar; Ms = muscovite; Op

826

= opaque minerals; Or = orthoclase; Pl = plagioclase; Qz = quartz; Rt =

827

rutile; Ser = sericite.

828

Fig. 8. Molar ratio plot of K/Al vs. (2Ca + Na + K)/Al showing Cerro

829

Empexa Formation with three different main alteration grades: potassic,

830

sericitic-chlorite-clay and phyllic (sericitic). Porphyritic quartz monzonite

831

and leucocratic quartz porphyry are plotted for comparison. The

832

metasomatism data arrays show that leucocratic quartz porphyry has

833

undergone significant Ca and Na loss, during feldspar destruction, and a

36

834

considerable K gain during sericitic alteration. Muscovite is plotted as a

835

proxy

836

supplementary sheet 2. Field of unaltered volcanic rocks is plotted after

837

Warren et al. (2007).

838

Fig. 9. a–o) Examples of CL images of zircon from the three main intrusive

839

units (porphyritic quartz monzonite, porphyritic tonalite and leucocratic

840

quartz porphyry). Length of scale bar is 100 µm. The circles show locations

841

of laser ablation spots. Symbols with solid lines show ages of intrusive

842

units, whereas symbols with dashed lines indicate ages of host rock units.

843

Zircon with different microtextures can be seen. The zircon type on top

844

does not show distinctive core-rim relationships, and commonly shows

845

clear sector and concentric oscillatory zoning from interior to rim. Both

846

core and rim domains of zircon grains show a spread of ages with

847

Cretaceous, Paleocene and Eocene ages. The zircon grains at the bottom

848

show distinct old cores overgrown by zoned rims. Late Paleozoic to

849

Mesozoic and also Cretaceous cores are observed, suggesting the presence

850

of zircon inherited from both basement units and the host rock Cerro

851

Empexa Formation or the breccia unit sequence. p) CL image of a zircon

852

grain from the subsurface diorite. The distinct core and rim ages are all

853

Cretaceous, suggesting new zircon that formed during the diorite intrusive

854

event grew on an older core likely related to the Cerro Empexa andesite.

855

q) CL image of a zircon grain from breccia unit shows continuous

856

oscillatory zoning. The age of rim growth is younger than zircon from the

857

Cerro Empexa Formation.

for

sericitisation.

All

geochemical

data

are

provided

in

37

858

Fig. 10. Age population histograms for all measured rock units. Weighted

859

mean ages (b–e) and ages obtained from unmixing multicomponent

860

algorithm method (f–h) calculated using Isoplot 4.15 (Ludwig, 2012) are

861

shown. Data points and the weighted mean calculations for each sample

862

are marked on Table 3 and shown on plots in supplementary sheet 3. For

863

the porphyritic quartz monzonite, the leucocratic quartz porphyry and the

864

porphyritic granodiorite, the dotted red lines represent the projection of

865

the population belonging to the spread of the first Gaussian peaks. Peak

866

ages obtained by deconvolution of the data spectra are shown with their

867

corresponding constituting fractions. Bars shown in dark grey shades

868

represent the youngest Gaussian populations. The light grey bars showing

869

subordinate peaks at ca. 60 Ma are included in the unmixing

870

multicomponents calculation (see text). Outlying data excluded in age

871

calculations are shown as white bars. For the intrusive units, data points

872

collected from distinct cores, center and rims of zircon are represented by

873

different symbols. The widespread of ages from Cretaceous to Eocene

874

suggests xenocrysts and antecrysts are common in the system. Lithology

875

abbreviations: CEF = Cerro Empexa Formation; LQP = leucocratic quartz

876

porphyry; PGD = porphyritic granodiorite; PMN = porphyritic quartz

877

monzonite;. PTN = porphyritic tonalite; VBX = breccia unit.

878

Fig. 11. Concordia ages calculated and plots generated using Isoplot 4.15

879

(Ludwig, 2012) for SHRIMP analyses of three individual zircon grains. a)

880

Three spots analysed for the porphyritic tonalite (PTN) b) Six spots

881

analysed for the porphyritic quartz monzonite (PMN); c) Four spots

38

882

analysed for leucocratic quartz porphyry (LQP). The analysed spots are

883

shown on zircon images respectively. For the porphyritic tonalite and

884

leucocratic quartz porphyry, concordia ages calculated excluding the spot

885

ages obtained from the centre of the two zircon grains yield concordia ages

886

of 53.5 ± 1.5 Ma and 49.3 ± 0.86 Ma, respectively. These are almost

887

identical to the results calculated when the two spot ages are included (a

888

and c).

889

Fig. 12. Ages determined for samples of Cerro Empexa Formation, the

890

breccia unit and the intrusive units showing continuous igneous activity

891

from Cretaceous to Eocene. Data points in black represent ages of the

892

main phase of mineralisation at ca. 53.5–50 Ma. Data points in dark grey

893

represent ages of an early phase of igneous activity at 60–57 Ma. Data

894

points in light grey and white represent ages of the ca. 80–70 Ma activity

895

related to the Cerro Empexa Formation, and other older xenocrystic

896

components, respectively.

897

Fig. 13.

898

Observed physical features related to the shallow porphyritic tonalite

899

intrusion and the monzonitic–quartz porphyry subvolcanic stock are

900

illustrated. Xenoliths are broken off from the wall rocks, introducing

901

zircon xenocrysts into the system. Antecrysts that begin to crystallize in

902

the upper crustal magma chamber are incorporated into the ascending

903

magma. Thus the zircon collected from the shallow subvolcanic

904

environment shows a wide range of ages, and polyphase zircon growth can

905

be observed. Lithology abbreviations: CEF = Cerro Empexa Formation;

Schematic geological evolution of the Cerro Colorado area.

39

906

LQP = leucocratic quartz porphyry; PGD = porphyritic granodiorite; PMN

907

= porphyritic quartz monzonite; PTN = porphyritic tonalite; VBX = breccia

908

unit.

909 910

Table 1

Nomenclature of different rock types and their alteration

911

styles at Cerro Colorado mine

912

Table 2

913

rock units. The samples were chosen from different depths from the drill

914

cores in Cerro Colorado mine. The percentages of Cu bearing minerals are

915

relatively small and vary among samples; for other major rock forming

916

minerals, the normalized point counts (calculated at an increase of every

917

100 counts) achieved consistency toward higher number of total counts.

918

Lithology abbreviations: CEF = Cerro Empexa Formation; LQP =

919

leucocratic quartz porphyry; PGD = porphyritic granodiorite; PMN =

920

porphyritic quartz monzonite;. PTN = porphyritic tonalite; VBX = breccia

921

unit.

922

Table 3

923

rocks and intrusive units in Cerro Colorado mine site. Lithology

924

abbreviations: CEF = Cerro Empexa Formation; LQP = leucocratic quartz

925

porphyry; PGD = porphyritic granodiorite; PMN = porphyritic quartz

926

monzonite;. PTN = porphyritic tonalite; VBX = breccia unit.

Summary of point counting of different minerals from each

LA-ICP-MS U–Pb geochronology data for zircon from host

40

927

Table 4

SHRIMP U–Pb geochronology data for three single zircon

928

grains selected from a porphyritic tonalite, a porphyritic quartz monzonite

929

and a quartz porphyry sample

41

930

10

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932

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1099

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1101

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1102

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1104

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1224

55

Bouzari & Clark, 2006

Hos t roc k

Cerro Empexa Formation —andesite

Mine Nome nclatu re

PDI

Rock type

Cerro Empexa Formation — andesite

Abb revi atio n use d in figu res and tabl es in this stud y

CEF

Alteration types

Meta llic mine rals

Almost all types are present: mainly sericitechlorite-clay (SCC) alteration, biotite-quartzmagnetite alteration (potassic), and sericitequartz-pyrite (± clay) (phyllic). Locally cut by quartz-albite veins (with big halos, with or without chlorite), which represents a transition from potassic to SCC (Bouzari & Clark, 2006)

Main host rock, closel y assoc iated with chalc opyri te and moly bdeni te mine ralisa tion; depe nding on altera tion types , locall y assoc iated with magn etite,

56

Volcanic breccia

Intr Intrusive usiv hydrotherm e/ al breccia

BXIPQ Z

borni te, pyrit e, and copp er oxide mine rals. Subsi diary host rock, conta ins disse minat ed chalc opyri te and borni te. Pervasive phyllic with Rich VBX sericite, quartz, pyrite and in clay minerals finegrain ed moly bdeni te. Disse minat ed pyrit e occur s in matri x. Transitional stage, phyllic alteration (quartz-sericitepyrite)

57

sub vol can ic uni ts

Quartz porphyry

Biotitequartzplagiocalse porphyry

PQZ

PTO1

Leucocratic quartz porphyry

Mainly phyllic alteration with quartz-sericite-pyrite and clay minerals. Cut by LQP quartz-sericite-clay veins and less commonly quartz-sericite (± chlorite) veins

Porphyritic quartz monzonite

Less altered compared to LQP and PTN. Biotite and chlorite replace preexisting hornblende. Cut by quartz-sericite-clay veins, and less commonly by quartz-sericite (± chlorite) veins

PM N

Disse minat ed pyrit e is com mon in matri x; Cuoxide mine rals occur locall y in matri x and veins. Moly bdeni te and chalc opyri te mainl y assoc iate with stock work phylli c veins. Disse minat ed chalc opyri te spars ely foun d in matri

58

PTO2

Porphyritic tonalite

More intensely altered compared to PMN. SCCand phyllic alteration PTN common, local potassic alteration contains biotite–quartz-magnetite veins

x; chalc opyri te and moly bdeni te mine ralisa tion mainl y assoc iated with stock work phylli c veins. Cuoxide mine rals com mon in veins in uppe r parts of the unit. Disse minat ed chalc opyri te com mon in the matri x, but

59

moly bdeni te is scarc e. Chalc opyri te closel y assoc iated with SCCand phylli c stock work veins. Locall y conta ins magn etite. Hornblende -plagioclase porphyry dyke (outcrop)

Intense phyllic alteration and pyrite veins

Dyke

Porphyritic granodiorit e (exposed in drilled core)

Essentially unaltered, locally displays low PGD degree of phyllic–chloritic alteration

Large ly unmi nerali sed but local occur rence of disse minat ed chalc opyri te

60

and rare SCCand phylli c veins. 1225 1226

61

Table 2 Summary of point cointing of different minerals from each of the rock units

Rock unit

Granit e

Num ber of sam ples

Cou nts per sam ple

Cu minerals (sulphide and oxide) in matrix (%)

KFelds par (%)

31.6– 33.6

26.8– 28.5

23.3– 24.2

1.4–2

40.7– 43.6

2

500

2

100 0

1

100 0

3.7

13.3

25.2

N.O.

6.9

VBX

2

100 0

0.6–1.3

22.6– 24.1

6.3– 7.4

N.O.

N.O.

PTN

3

100 0

2–2.6

43.1– 48.1

17.8– 25.9

N.O.

1.2– 4.4

3

100 0

0.6–1.3

9.3–17.1

15.3– 24.5

11.9– 21.9

10– 29

0.5–0.7

21.3– 24.3

3.3– 15.9

2.3– 11.8

N.O.

0.2

2.2

35.5

5.1

4.4

CEF (andes ite) CEF (diorit e)

PMN

LQP

3

PGD

1

100 0 100 0

1–1.3

Quartz* (%)

Plagio clase (%)

Chl orit e, seri Bioti cite te , (%) clay min eral s (%) 11. 4– 15. N.O. 3 21. 14.7 5– – 27. 15.2 4

1.6–2.1

N.O.

40. 3 44. 6– 48 20. 5– 31. 8 7.8 – 16. 1 21. 1– 61. 4 28. 5

Groun dmass (%)

N.O.

10.3– 18

6.1 14.7– 19.2

N.O.

27.6– 35.5

4.7– 17.2 20.7

* Percentages of quartz include identifyable microcrystalline quartz, likely formed during quartzphyllic alteration

62

N.O. = not observed 1227 1228

63

a) Zircon U–Pb results of individual basement granite samples

Sample Name 15IA03 15IA03-1 15IA03-2 15IA03-3 15IA03-4 15IA03-5 15IA03-9 15IA0310 15IA0311 15IA0312 15IA0315 15IA0316 IA12 IA12-2 IA12-8 IA12-10 IA12-13 IA12-16 IA12-17 IA12-21 IA12-22 IA12-23 IA68 IA68-1 IA68-3 IA68-5 IA68-9 IA68-10 IA68-11 IA68-12 IA68-13 IA68-14 IA68-16

Th/U 207Pb/206Pb

0.56 0.57 0.45 0.37 0.39 0.59

Error 206Pb/238U 2σ Relatively fresh with localized illitization two-feldspar granite, 890 m below 0.0607 ± 0.0051 0.0538 0.0563 ± 0.0063 0.0529 0.0534 ± 0.0032 0.0529 0.0560 ± 0.0027 0.0481 0.0540 ± 0.0045 0.0547 0.0532 ± 0.0049 0.0539

0.48

0.0581

± 0.0060

0.0553

0.62

0.0594

± 0.0051

0.0497

0.54

0.0563

± 0.0047

0.0509

0.68

0.0544

± 0.0035

0.0516

0.30

0.0599

± 0.0034

0.0881

0.35 0.43 0.32 0.33 0.54 0.49 0.26 0.49 0.60

Relatively fresh with localized illitization two-feldspar granite, 774 m below 0.0521 ± 0.0049 0.0366 0.0539 ± 0.0056 0.0448 0.0555 ± 0.0027 0.0423 0.0546 ± 0.0028 0.0366 0.0505 ± 0.0042 0.0507 0.0550 ± 0.0030 0.0406 0.0529 ± 0.0036 0.0388 0.0520 ± 0.0034 0.0542 0.0550 ± 0.0053 0.0480

0.70 0.59 0.48 0.51 0.68 0.69 0.52 0.64 0.54 0.24

Moderately epidote-altered two-feldspar granite, 889 m below surface 0.0539 ± 0.0024 0.0539 0.0566 ± 0.0031 0.0518 0.0545 ± 0.0038 0.0542 0.0571 ± 0.0033 0.0476 0.0507 ± 0.0034 0.0509 0.0556 ± 0.0026 0.0494 0.0565 ± 0.0050 0.0517 0.0528 ± 0.0028 0.0506 0.0535 ± 0.0062 0.0526 0.0553 ± 0.0040 0.0516

64

* For basement granite, data points with probability of concordance < 0.1 and > 0.01 are included

b) Zircon U–Pb results of individual Cerro Empexa Formation (CEF) samples (andesite and su

Sample Name 15IA01 15IA01-2 15IA0113 15IA0114 15IA0118

IA66 IA66-1 IA66-2 IA66-4 IA66-6 IA66-8 IA66-9 IA66-11 IA66-14

Th/U 207Pb/206Pb

0.53

Error 206Pb/238U 2σ Potassic altered, phenocrysts and clast-rich CEF andesite, 788 m below sur 0.0686 ± 0.0176 0.0124

0.76

0.0640

± 0.0125

0.0118

0.75

0.0551

± 0.0086

0.0146

0.70

0.0486

± 0.0104

0.70 0.38 0.69 0.63 0.90 0.64 0.63 0.65

0.0120 Weighted mean age of late Cretaceo

Potassic altered aphyric CEF andesite, 833 m below surface 0.0577 ± 0.0025 0.0719 0.0562 ± 0.0038 0.0969 0.0472 ± 0.0075 0.0124 0.0536 ± 0.0028 0.0496 0.0549 ± 0.0040 0.0438 0.0585 ± 0.0109 0.0126 0.0652 ± 0.0127 0.0129 0.0521 ± 0.0054 0.0746 Weighted mean age of late Cretaceo

Weighted mean age of late Cretaceous peak of lower andesite sequence (15IA01 and IA66) 13IA07 13IA07-2 13IA07-3 13IA07-9 13IA0711 13IA0714 13IA0717

0.31 0.68 0.87

Sericite-chlorite-clay altered CEF andesite, close to a section of brecciated 0.0741 ± 0.0048 0.1843 0.0496 ± 0.0162 0.0117 0.0815 ± 0.0038 0.2035

0.11

0.0663

± 0.0034

0.1407

2.28

0.0608

± 0.0036

0.0948

0.72

0.0719

± 0.0183

0.0107 Weighted mean age^ of late Cretace

65

IA84 IA84-1 IA84-5 IA84-7 IA84-9 IA84-10 IA84-12 IA84-13 IA84-14 IA84-16 IA84-18 IA84-19 IA84-20 IA84-21 IA84-22 IA84-25 IA84-26 IA84-29 IA84-31 IA84-34 IA84-35 IA84-36 IA84-37 IA84-38 IA84-39 IA84-40

0.51 0.54 0.67 0.60 0.72 0.72 0.84 0.87 0.84 0.57 0.80 0.51 0.51 0.85 0.56 0.52 0.42 0.46 0.48 0.58 0.63 0.71 0.78 0.92 0.58

Potassic altered porphyritic subsurface diorite, 745 m below surface 0.0532 ± 0.0153 0.0116 0.0504 ± 0.0122 0.0120 0.0489 ± 0.0053 0.0114 0.0722 ± 0.0204 0.0104 0.0522 ± 0.0158 0.0107 0.0506 ± 0.0169 0.0107 0.0627 ± 0.0139 0.0122 0.0495 ± 0.0051 0.0429 0.0511 ± 0.0160 0.0144 0.0762 ± 0.0222 0.0112 0.0704 ± 0.0166 0.0123 0.0598 ± 0.0153 0.0109 0.0441 ± 0.0126 0.0105 0.0530 ± 0.0139 0.0111 0.0534 ± 0.0197 0.0109 0.0437 ± 0.0122 0.0113 0.0534 ± 0.0157 0.0117 0.0435 ± 0.0151 0.0115 0.0542 ± 0.0185 0.0111 0.0685 ± 0.0162 0.0113 0.0490 ± 0.0159 0.0118 0.0649 ± 0.0125 0.0118 0.0557 ± 0.0108 0.0138 0.0606 ± 0.0129 0.0111 0.0582 ± 0.0052 0.0431 Weighted mean age of late Cretaceo

^ Error is internal 2σ. Satistically impractical to calculate MSWD value for two data points. * For host rocks, data points with probability of concordance < 0.1 and > 0.01 are included into ca DC – Distinct old core age

c) Zircon U–Pb results of individual volcanic breccia (VBX) samples

Sample Name 13IA06 13IA06-7 13IA06-9 13IA0610 13IA06-

Th/U 207Pb/206Pb

0.70 0.79 1.17 0.71

Error 206Pb/238U 2σ Phyllic altered pervasively silicified VBX, 60 m below surface 0.0467 ± 0.0188 0.0087 0.0600 ± 0.0219 0.0092 0.0448 0.0661

± 0.0111 ± 0.0181

0.0084 0.0142

66

11 13IA0612 13IA0614 13IA0618 13IA0620 13IA0621 13IA0622

IA49 IA49-1 IA49-2 IA49-6 IA49-7 IA49-8 IA49-10 IA49-11 IA49-13 IA49-14 IA49-15 IA49-16 IA49-17 IA49-20 IA49-22 IA49-26 IA49-33 IA49-34 IA49-36

0.82

0.0482

± 0.0165

0.0085

0.80

0.0567

± 0.0179

0.0106

0.88

0.0654

± 0.0235

0.0101

0.82

0.0481

± 0.0071

0.0254

0.85

0.0801

± 0.0215

0.0098

1.01

0.0591

± 0.0202

0.74 0.35 0.44 1.05 0.43 0.65 0.72 0.43 0.68 0.91 0.66 0.89 0.46 0.71 0.90 0.75 0.78 0.63

0.0087 Weighted mean age of late Cretaceous to ea

Phyllic to argillic altered VBX, 147 m below surface 0.0540 ± 0.0245 0.0096 0.0811 ± 0.0248 0.0090 0.0562 ± 0.0215 0.0088 0.0463 ± 0.0203 0.0085 0.0839 ± 0.0261 0.0097 0.0407 ± 0.0186 0.0090 0.0655 ± 0.0242 0.0098 0.0701 ± 0.0241 0.0090 0.0918 ± 0.0328 0.0086 0.0356 ± 0.0160 0.0084 0.0454 ± 0.0243 0.0085 0.0373 ± 0.0164 0.0093 0.0575 ± 0.0254 0.0097 0.0645 ± 0.0258 0.0091 0.0638 ± 0.0231 0.0086 0.0216 ± 0.0157 0.0085 0.0495 ± 0.0219 0.0086 0.0560 ± 0.0137 0.0106 Weighted mean age of late Cretaceous to ea

Weighted mean age of late Cretaceous to early

* For host rocks, data points with probability of concordance < 0.1 and > 0.01 or error > 10.0 % an Rj – Age is statistically rejected during calculation of weighted mean for volcanic breccia unit

d) Zircon U–Pb results of individual porphyritic tonalite (PTN) samples Sample

Th/U 207Pb/206Pb

Error

206Pb/238U

67

Name 15IA09 15IA096(2) 15IA09-7 15IA0921 15IA0923 15IA0926

IA29 IA29-1 IA29-3 IA29-4 IA29-6 IA29-8 IA29-11 IA29-13 IA29-15 IA29-16 IA29-24

IA80 IA80-1 IA80-2 IA80-5 IA80-6 IA80-8 IA80-10 IA80-12 IA80-16 IA80-20

IA81 IA81-1(2) IA81-9 IA81-20 IA81-32

2σ Sericite-chlorite-clay altered PTN collected from 642 m below surface 1.11 0.92

0.0449 0.0405

± 0.0145 ± 0.0135

0.0083 0.0090

0.89

0.0474

± 0.0129

0.0091

0.43

0.0539

± 0.0101

0.0084

1.27

0.0468

± 0.0130

0.0080 Weighted mean age of the Palaeocene to ea

0.79 0.91 0.80 0.87 0.99 0.94 0.91 0.89 0.72 0.72

Sericite-chlorite-clay altered PTN collected from 248 m below surface 0.0633 ± 0.0195 0.0084 0.0425 ± 0.0125 0.0119 0.0402 ± 0.0162 0.0081 0.0579 ± 0.0143 0.0080 0.0585 ± 0.0152 0.0088 0.0420 ± 0.0157 0.0081 0.0484 ± 0.0162 0.0086 0.0383 ± 0.0153 0.0079 0.0438 ± 0.0183 0.0085 0.0620 ± 0.0181 0.0082 Weighted mean age of the Palaeocene to ea

0.79 0.82 0.94 0.91 0.60 0.21 0.97 0.67 1.18

Sericite-chlorite-clay altered PTN collected from 338 m below surface 0.0558 ± 0.0185 0.0083 0.0529 ± 0.0096 0.0081 0.0583 ± 0.0199 0.0088 0.0373 ± 0.0132 0.0086 0.0570 ± 0.0206 0.0083 0.0522 ± 0.0035 0.0302 0.0517 ± 0.0138 0.0088 0.0478 ± 0.0181 0.0085 0.0426 ± 0.0309 0.0078 Weighted mean age of the Palaeocene to ea

0.85 0.82 0.60 0.65

Potassic altered PTN collected from 726 m below surface 0.0496 ± 0.0094 0.0503 ± 0.0065 0.0589 ± 0.0167 0.0580 ± 0.0142

68

0.0089 0.0088 0.0082 0.0084

Weighted mean age of the Palaeocene to ea 1006 1006-1 1006-7 1006-13 1006-16 1006-29 1006-36 1006-39

0.62 0.97 0.82 0.51 0.66 0.57 0.46

Sericite-chlorite-clay altered PTN collected from 520 m below surface from 0.0518 ± 0.0122 0.0085 0.0515 ± 0.0112 0.0080 0.0455 ± 0.0113 0.0091 0.0452 ± 0.0155 0.0081 0.0395 ± 0.0136 0.0088 0.0440 ± 0.0122 0.0082 0.0492 ± 0.0156 0.0084 Weighted mean age of the Palaeocene to ea

Weighted mean age of the Palaeocene to early DC – Distinct old core age Rj – Age is statistically rejected during calculation of weighted mean for porphyritic tonalite unit

e) Zircon U–Pb results of individual porphyritic quartz monzonite (PMN) samples

Sample Name 13IA02 13IA02-1 13IA02-2 13IA02-3 13IA02-5 13IA02-6 13IA02-7 13IA02-8 13IA0211 13IA0213 13IA0214 13IA0215 13IA0219 13IA0221

Th/U 207Pb/206Pb

0.71 0.58 0.57 0.82 0.37 0.91 0.68

Error 206Pb/238U 2σ Low intensity phyllic altered and silicified PMN collected from 150 m below 0.0624 ± 0.0214 0.0076 0.0488 ± 0.0186 0.0079 0.0440 ± 0.0090 0.0077 0.0414 ± 0.0263 0.0100 0.0440 ± 0.0150 0.0074 0.0549 ± 0.0171 0.0082 0.0513 ± 0.0202 0.0080

0.77

0.0510

± 0.0192

0.0076

1.22

0.0473

± 0.0139

0.0072

0.41

0.0534

± 0.0177

0.0077

0.79

0.0494

± 0.0096

0.0080

0.75

0.0646

± 0.0242

0.0097

0.46

0.0466

± 0.0138

0.0080

Youngest gaussian peak age obtained by dec

69

Subordinate peak age obtained by deconv 13IA04 13IA04-1 13IA04-2 13IA04-3 13IA04-4 13IA04-6 13IA04-7 13IA04-8 13IA04-9 13IA0411 13IA0413 13IA0414 13IA0418

0.42 0.65 0.46 0.63 0.32 0.83 0.61 0.51

Sericite-chlorite-clay to potassic altered PMN collected from 365 m below s 0.0629 ± 0.0220 0.0091 0.0528 ± 0.0030 0.369 0.0571 ± 0.0184 0.0090 0.0613 ± 0.0227 0.0087 0.0514 ± 0.0097 0.0097 0.0433 ± 0.0108 0.0120 0.0522 ± 0.0238 0.0081 0.0484 ± 0.0273 0.0077

0.52

0.0602

± 0.0248

0.0078

0.61

0.0469

± 0.0273

0.0078

0.69

0.0531

± 0.0231

0.0082

1.06

0.0595

± 0.0180

0.0077

Youngest gaussian peak age obtained by dec

Subordinate peak age obtained by deconv 13IA11 13IA11-1 13IA11-2 13IA11-4 13IA11-5 13IA11-6 13IA11-8 13IA1117 13IA1119 13IA1120 13IA1121 13IA1122 13IA1123 13IA1125 13IA11-

0.70 0.41 0.60 0.43 0.64 0.47

Relatively fresh PMN collected from 500 m below surface close to brecciate 0.0520 ± 0.0114 0.0123 0.0491 ± 0.0230 0.0091 0.0477 ± 0.0088 0.0113 0.0556 ± 0.0142 0.0088 0.0478 ± 0.0071 0.0112 0.0481 ± 0.0055 0.0114

0.48

0.0462

± 0.0079

0.0095

0.93

0.0592

± 0.0171

0.0090

0.97

0.0407

± 0.0156

0.0093

1.04

0.0406

± 0.0141

0.0095

0.61

0.0563

± 0.0141

0.0094

0.92

0.0406

± 0.0130

0.0086

0.16 0.53

0.0544 0.0443

± 0.0044 ± 0.0098

0.321 0.0127

70

26 13IA1127 13IA1128 13IA1130 13IA1132 13IA1133 13IA1136 13IA1137 13IA1138 13IA1139 13IA1141

0.86

0.0402

± 0.0335

0.0081

0.70

0.0377

± 0.0218

0.0091

0.75

0.0419

± 0.0200

0.0091

0.77

0.0577

± 0.0196

0.0079

0.56

0.0458

± 0.0144

0.0083

0.81

0.0396

± 0.0170

0.0078

1.05

0.0498

± 0.0184

0.0080

0.73

0.0433

± 0.0157

0.0076

1.30

0.0485

± 0.0130

0.0076

1.01

0.0474

± 0.0165

0.0071

Youngest gaussian peak age obtained by dec

Subordinate peak age obtained by deconv IA97 IA97-1 IA97-4 IA97-5 IA97-7 IA97-9 IA97-10 IA97-12 IA97-13 IA97-16 IA97-18 IA97-21 IA97-22 IA97-23

0.70 0.49 0.45 1.33 0.54 0.55 0.86 0.94 1.07 0.87 0.50 0.82 0.30

Phyllic altered PMN collected from 374 m below surface close to a section o 0.0469 ± 0.0032 0.0098 0.0492 ± 0.0083 0.0108 0.0490 ± 0.0079 0.0077 0.0540 ± 0.0135 0.0083 0.0398 ± 0.0105 0.0120 0.0518 ± 0.0108 0.0123 0.0513 ± 0.0146 0.0078 0.0487 ± 0.0204 0.0091 0.0524 ± 0.0215 0.0092 0.0510 ± 0.0133 0.0084 0.0630 ± 0.0237 0.0078 0.0590 ± 0.0142 0.0083 0.0494 ± 0.0068 0.0087

Youngest gaussian peak age obtained by dec

Subordinate peak age obtained by deconv

Youngest gaussian peak age of PMN obtained method

71

Subordinate peak age of PMN obtained by dec Youngest gaussian peak weighted mean DC – Distinct old core age

f) Zircon U–Pb results of individual leucocratic quartz porphyry (LQP) samples

Sample Name 13IA05 13IA0501 13IA0502 13IA05-4 13IA05-6 13IA05-8 13IA0510 13IA0511 13IA0512 13IA0514 13IA0516 13IA0517 13IA0519 13IA0521 13IA0523 13IA0524 13IA0525 13IA0526 13IA0529 13IA0530

Th/U 207Pb/206Pb

Error 206Pb/238U 2σ Phyllic altered LQP (~10% quartz "eyes"), 266 m below surface

0.95

0.0470

± 0.0191

0.0075

0.77 0.63 0.65 1.16

0.0522 0.0518 0.0478 0.0556

± ± ± ±

0.0147 0.0105 0.0070 0.0146

0.0077 0.0115 0.0118 0.0082

0.68

0.0559

± 0.0173

0.0078

0.59

0.0500

± 0.0087

0.0118

0.45

0.0430

± 0.0109

0.0120

1.26

0.0440

± 0.0116

0.0084

0.47

0.0501

± 0.0057

0.0144

0.88

0.0512

± 0.0172

0.0095

1.47

0.0414

± 0.0128

0.0080

0.47

0.0481

± 0.0111

0.0134

0.93

0.0599

± 0.0218

0.0085

0.44

0.0507

± 0.0041

0.0117

0.85

0.0474

± 0.0208

0.0086

0.82

0.0492

± 0.0195

0.0082

0.82

0.0498

± 0.0122

0.0086

1.16

0.0399

± 0.0125

0.0080

72

13IA0531 13IA0533 13IA0534 13IA0543 13IA0548

0.66

0.0541

± 0.0106

0.0080

0.51

0.0488

± 0.0122

0.0104

0.52

0.0509

± 0.0094

0.0115

1.12

0.0611

± 0.0185

0.0082

0.51

0.0412

± 0.0084

0.0117

Youngest gaussian peak age obtained by dec

Subordinate peak age obtained by deconv 13IA10 13IA1002 13IA1003 13IA1005 13IA1007 13IA1009 13IA1010 13IA1011 13IA1012 13IA1013 13IA1016 13IA1017 13IA1020

Phyllic (to argillic) altered LQP (~5% quartz "eyes"), 70 m below surface 0.47

0.0441

± 0.0180

0.0078

0.55

0.0535

± 0.0219

0.0076

0.69

0.0434

± 0.0162

0.0082

0.62

0.0401

± 0.0152

0.0087

0.89

0.0385

± 0.0160

0.0084

0.76

0.0531

± 0.0157

0.0094

0.63

0.0455

± 0.0164

0.0075

0.23

0.0490

± 0.0057

0.0343

0.71

0.0414

± 0.0188

0.0075

1.78

0.0527

± 0.0084

0.0075

0.77

0.0396

± 0.0109

0.0086

0.55

0.0610

± 0.0187

0.0080

Youngest gaussian peak age obtained by dec

Subordinate peak age obtained by deconv Mo6 Mo6-4 Mo6-10

0.46 0.51

Phyllic altered LQP (~10% quartz "eyes"), 347 m below surface in a xenolith 0.0611 ± 0.0179 0.0084 0.0555 ± 0.0122 0.0116

73

Mo6-12 Mo6-16 Mo6-19 Mo6-21 Mo6-22 Mo6-27

IA76 IA76-1 IA76-4 IA76-7 IA76-9 IA76-10 IA76-12 IA76-14 IA76-18 IA76-19 IA76-21 IA76-23 IA76-24

0.64 0.70 0.52 0.84 0.77 0.99

0.66 0.45 0.81 1.16 2.80 0.50 0.68 0.50 0.77 0.56 0.52 0.49

0.0455 0.0476 0.0532 0.0615 0.0512 0.0486

± ± ± ± ± ±

0.0101 0.0081 0.0145 0.0200 0.0102 0.0133

0.0114 0.0106 0.0082 0.0105 0.0082 0.0079 Weighted mean age # of the youngest ga

Phyllic altered LQP (~5-8% quartz "eyes"), 214 m below surface in a xenolit 0.0473 ± 0.0074 0.0113 0.0524 ± 0.0065 0.0488 0.0504 ± 0.0214 0.0108 0.0442 ± 0.0151 0.0083 0.0453 ± 0.0142 0.0082 0.0553 ± 0.0040 0.0462 0.0455 ± 0.0177 0.0091 0.0596 ± 0.0173 0.0081 0.0533 ± 0.0202 0.0096 0.0433 ± 0.0079 0.0115 0.0437 ± 0.0097 0.0123 0.0546 ± 0.0129 0.0120

Youngest gaussian peak age obtained by dec

Subordinate peak age obtained by deconv

Youngest gaussian peak age of LQP obtained method

Subordinate peak age of LQP obtained by dec Youngest gaussian peak weighted mean # The sample shows unimodal age distribution, weighted mean is calculated DC – Distinct old core age

g) Zircon U–Pb results of individual porphyritic granodiorite (PGD) samples Sample Name IA37 IA37-22 IA37-25

Th/U 207Pb/206Pb

0.86 0.92

Error

206Pb/238U

2σ Low intensity sericite-chlorite-clay altered PGD, 386 m below surface 0.0576 ± 0.0250 0.0094 0.0453 ± 0.0208 0.0083

74

IA37-30 IA37-32

IA47 IA47-1 IA47-2 IA47-9 IA47-13 IA47-14 IA47-20 IA47-25

1.24 1.13

0.90 0.85 1.15 1.23 0.61 0.85 1.04

0.0480 0.0599

± 0.0159 ± 0.0223

0.0091 0.0078 Weighted mean age^ of the youngest ga Weighted mean age^ ## of the subord

Relatively unaltered PGD, 70 m below surface 0.0493 ± 0.0165 0.0449 ± 0.0181 0.0591 ± 0.0204 0.0576 ± 0.0168 0.0432 ± 0.0159 0.0634 ± 0.0248 0.0411 ± 0.0203

0.0081 0.0085 0.0073 0.0081 0.0080 0.0090 0.0079

Weighted mean age # of the youngest ga

Youngest gaussian peak age of PGD obtained method

Subordinate peak age of PGD obtained by dec Youngest gaussian peak weighted mean ^ Error is internal 2σ. Satistically impractical to calculate MSWD value for two data points. # The sample shows unimodal age distribution, weighted mean is calculated ## The sample shows bimodal age distribution, however the number of data is insuffucuent for the 1229 1230

75

Table 4 SHRIMP U–Th–Pb dating results for zircon grains from a porp porphyry a) Data for zircon grain #1 from 15IA09 (Porphyritic tonalite) Spot # [samplegrain. spot]

% 206Pbc

ppm U

ppm Th

(207Pbcorrected) ppm 206Pb*

232Th /238U

IA09-1.1 IA09-1.2 IA09-1.3

0.03 0.19 0.07

46 66 62

38 42 48

0.33 0.48 0.44

0.84 0.66 0.81

(1) 206Pb/238U Age

206P A

53.4 54.3 53.0

53.4 54.2 53.0

±2.4 ±2.3 ±1.9

b) Data for zircon grain #11 from 13IA02 (Porphyritic quartz monzonite) Spot # [samplegrain. spot]

% 206Pbc

ppm U

ppm Th

(207Pbcorrected) ppm 206Pb*

232Th /238U

IA2-11.1 IA2-11.2 IA2-11.3 IA2-11.4 IA2-11.5 IA2-11.6

0.03 0.04 0.14 0.14 0.05 0.03

127 70 61 127 119 142

111 32 31 122 85 135

0.85 0.46 0.40 0.84 0.83 0.95

0.90 0.47 0.53 1.00 0.74 0.98

(1) 206Pb/238U Age

206P A

50.0 49.0 49.5 49.9 52.4 49.8

50.0 49.0 49.5 49.9 52.4 49.8

±1 ±3 ±2 ±1 ±3 ±1

c) Data for zircon grain #3 from 13IA10 (Leucocratic quartz porphyry) Spot # [samplegrain. spot]

% 206Pbc

ppm U

ppm Th

(207Pbcorrected) ppm 206Pb*

232Th /238U

IA10-3.1 IA10-3.2 IA10-3.3 IA10-3.4

-0.31 0.42 0.07

54 279 91 114

20 197 60 143

0.36 1.85 0.58 0.76

0.39 0.73 0.68 1.29

(1) 206Pb/238U Age

206P A

49.6 49.5 47.4 49.9

49.6 49.6 47.4 49.9

±2 ±1 ±2 ±2

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard c alibration was 0.39% (not included in above errors but required when comparing mounts). (1) Common Pb corrected using measured 204Pb. (2) Common Pb corrected by assuming 206Pb/238U-207Pb/235U age-concordance 1231

76

1232

77

1233 1234 1235 1236 1237 1238 1239

The highlights below summarised the core findings of our study: ・Porphyry intrusions of the Cerro Colorado Cu mine, N. Chile formed 60–50 Ma. ・Presence of a 57 Ma breccia implies syn-volcanic porphyry Cu formation. ・Fine-matrix porphyry units linked to rapid crystallisation due to decompression.

78

1240 1241

79

1242 1243

80

1244 1245

81

1246 1247

82

1248 1249

83

1250 1251

84

1252 1253

85

1254 1255

86

1256 1257

87

1258

88

1259

89

1260 1261

90

1262 1263

91

1264 1265

92

1266

93