Chen Huayong-200805 PhD d

8/10/2019 Chen Huayong-200805 PhD d

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THE MARCONA - MINA JUSTA DISTRICT, SOUTH-CENTRAL

PER: IMPLICATIONS FOR THE GENESIS AND DEFINITION OF

THE IRON OXIDE-COPPER (-GOLD) ORE DEPOSIT CLAN

by

Huayong Chen

A thesis submitted to the Department of Geological Sciences and Geological Engineering

In conformity with the requirements for

the degree of Doctor of Philosophy

Queens University

Kingston, Ontario, Canada

May, 2008

Copyright Huayong Chen, 2008


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I am looking for IOCG deposits

Frontispiece:

The mysterious Nazca Lines (a hummingbird) in the Caete basin, 70 kmnorth of

Marcona


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ABSTRACT

The Marcona district of littoral south-central Per represents the largest concentration of iron

oxide-copper-gold deposits in the Central Andes. Hydrothermal activity occurred episodically

from 177 to 95 Ma and was controlled by NE-striking faults.

At Marcona, emplacement of massive magnetite orebodies with subordinate, overprinted

magnetite-sulphide assemblages coincided with a 156-162 Ma episode of eruption of andesitic

magma in the Jurassic arc, but mineralization is hosted largely by underlying, Lower Paleozoic

metaclastic rocks. The magnetite orebodies exhibit smoothly curving, abrupt contacts, dike-like to

tubular apophyses and intricate, amoeboid interfingering with dacite porphyry intrusions,

interpreted as evidence for the commingling of hydrous Fe oxidic and silicic melts. An evolution

from magnetite - biotite - calcic amphibole phlogopite assemblages, which are inferred to have

crystallized from an Fe-oxide melt, to magnetite - phlogopite - calcic amphibole - sulphide

assemblages coincided with quenching from above 700C to below 450C and with the

exsolution of aqueous fluids with magmatic stable isotopic compositions. Subsequent,

subeconomic chalcopyrite - pyrite - calcite pyrrhotite sphalerite assemblages were deposited

from cooler fluids with similar 34S, 18O and 13C values, but higher D, which may record the

involvement of both seawater and meteoric water.

The much younger (95-110 Ma), entirely hydrothermal, Mina Justa Cu (-Ag) deposit is

hosted by Middle Jurassic andesites intruded, on a district scale, by small dioritic stocks at the

faulted SW margin of an Aptian-Albian shallow-marine volcano-sedimentary basin. Intense

albite-actinolite alteration (ca.157 Ma) and K-Fe metasomatism (ca.142 Ma) long preceded the

deposition of magnetite-pyrite assemblages from 500-600C fluids with a magmatic isotopic

signature. In contrast, ensuing chalcopyrite - bornite - digenite - chalcocite - hematite - calcite

mineralization was entirely the product of non - magmatic, probably evaporite-sourced, brines.


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Marcona and Mina Justa therefore represent contrasted ore deposit types and may bear

minimal genetic relationships. The former shares similarities with other Kiruna-type magnetite

(-apatite) deposits. In contrast, the latter is a hydrothermal system recording the incursion of

fluids plausibly expelled from the adjacent Caete basin. Non-magmatic fluids are inferred to be

a prerequisite for economic Cu mineralization in the Cu-rich IOCG deposits in the Central Andes

and elsewhere.


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ACKNOWLEDGEMENTS

This thesis research would not have been completed without the support of many people. First I

present my esteem and appreciation to my co-supervisor, Dr. Alan Clark, who originally gave me

the chance to come to Canada to continue my graduate study and his fatherly caring for my

studies during these years in Kingston, and more important, for his extremely strict but definitely

reasonable guidance in my Ph.D. project. Finally, although not completely, he opens my hard

granite brain using his sharp axe, to make me more like an economic geologist, who should

know everything. I also greatly thank my co-supervisor, Dr. Kurt Kyser, for his great patience in

allowing an over proud but mindless Ph.D. student to grow up slowly. I will remember his

endless understanding of my difficult situation and more important, his guidance in my research,

concise but extremely important. I also thank my supervisors for their funding for both my

research and living, dominantly from their grants funded by Natural Sciences and Engineering

Research Council of Canada (NSERC), and partly from the scholarships offered by Queens

University.

Shougang Hierro Per SA, Chariot Resources and Rio Tinto are thanked for their

cooperation in my field work, particularly for comfortable lodging in San Juan. Mr. Yuming

Chen, former chief geologist of Marcona Mine, is especially thanked for his great help at the very

beginning of this project. I thank Mr. Nicholas Hawkes and Timothy Moody for their

understanding and permission to sample Mina Justa drill cores, and also the protection afforded

by their local mine team, who ensured my safe return to Canada, without falling into ancient

mining workings.

At Queens, I benefited greatly from the advice and help of Kerry Klassen for isotope

analysis, April Vuletich for Laser-TOF-ICP-MS analysis, and Alan Grant for X-ray studies. Dr.

Gema Olivo is thanked for her help and permission to use the fluid inclusion equipment and

digital-camera microscope at Queens. Roger Innes and Jerzy Advent prepared thin and polished


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sections. I also thank Thomas Ullrich and Peter Johns for their help and advice in Ar-Ar and

microprobe analysis at UBC and Carleton University, respectively. I especially give my thanks to

Joan Charbonneau, who was the bridge between Alan and me during my thesis correction, and

other Geology staff members, Dianne Hyde, Linda Brown and Ellen Mulder for their kind help.

Thanks to many grad students and postdocs for their useful discussions and help. Special thanks

are due to Al Montgomery, Greg Lester, Jorge Benavides, Chan Quang, Amelia Rainbow, Mike

Cooley, Dave Love, Farhad Bouzari, Rui Zhang, Jingyang Zhao and Luis Cerpa.

Last, but not least, I would like thank my dear wife here in Kingston for her support,

encouragement and sacrifices, especially when she has to work hard on her own Ph.D. project.

Also I thank all my Chinese friends in Kingston, especially those from the Kingston Chinese

Alliance Church: my life will be difficult without their help.


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STATEMENT OF ORIGINALITY

I hereby certify that all of the work described within this thesis is the original work of the author.

Any published (or unpublished) ideas and/or techniques from the work of others are fully

acknowledged in accordance with the standard referencing practices.

Huayong Chen

May, 2008


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TABLE OF CONTENTS

Abstractiii

Acknowledgments...v

Statement of Originalityvii

Table of Contents...viii

List of Figures......x

List of Tables.xiv

Chapter 1. Introduction

The Iron Oxide-Copper-Gold Ore Deposit Clan: Problematic Definition and

Genesis1

The Scientific Contributions of This Study...............18

Thesis Organization...21

Chapter 2. The Longlived, Marcona-Mina Justa Iron-Copper District, Per: Implications

for the Origin of Cu-poor and Cu-rich IOCG Mineralization in the Central Andes

2.1 Abstract22

2.2 Introduction..24

2.3 Regional and District Geologic Setting31

2.4 The Marcona Magnetite Deposit..40

2.5 Paragenetic Relationships of the Marcona Orebodies..53

2.6 The Mina Justa Cu (-Ag-Au) Deposit..............69

2.7 Stable Isotope Geothermometry...84

2.8 40Ar/39Ar Geochronology.88

2.9 Discussion

2.9.1 An Oxide Melt Origin for the Main Marcona Magnetite Orebodies?............97

2.9.2 Evolution of the Marcona-Mina Justa District..103

2.10 Conclusions..115

Chapter 3. Contrasted Fluids and Reservoirs in the Contiguous Marcona and Mina Justa

Iron-Oxide Cu (-Au-Ag) Deposits, South-Central Per

3.1 Abstract..119

3.2 Introduction120


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3.3 Ore Deposit Geology..........123

3.4 Alteration and Mineralization

3.4.1 Marcona Magnetite Deposit..1263.4.2 Mina Justa Cu Deposit..129

3.5 Sampling and Analytic Techniques130

3.6 Results

3.6.1 Fluid Inclusions.133

3.6.2 Stable Isotope Geochemistry.150

3.7 Discussion

3.7.1 Fluid Evolution in the Marcona Deposit...160

3.7.2 Fluid Evolution in the Mina Justa Deposit1653.7.3 Implications for Cu-mineralizing Fluids in IOCG Deposits.169

3.8 Conclusions173

Chapter 4. Conclusions

4.1 Marcona A Unique Kiruna-type, Magnetite Deposit in the Middle Jurassic

Metallogenetic Sub-province of the Central Andes...174

4.2 Mina Justa Cu (-Ag) Deposit A Major Cu-rich IOCG Deposit in the Cretaceous

CentralAndes.177

4.3 The Protracted History of Alteration and Mineralization in the Marcona-Mina Justa

District and Other IOCG Centres...183

4.4 Implications for the Genesis of IOCG Deposits: A Redefinition and Reclassification of

the IOCG Clan185

References194

Appendix A: Analytical Techniques.....230

Appendix B: Summerized 40Ar/39Ar Analytical Data for Hydrothermal Minerals from Marcona

and Mina Justa...234

Appendix C: Fluid Inclusion Database.243

Appendix D: LA-TOFICP-MS Database..263


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LIST OF FIGURES

Chapter 1

Figure 1-1. Global Distribution of IOCG and districts and important deposits7

Figure 1-2. Grade-tonnage data for Cu-rich IOCG deposits and Cu-poor Fe oxide

deposits.9

Figure 1-3. The relationships between Cu-equivalent ore metal resource and deposit size for

Cu-rich IOCG, porphyry Cu-Mo and porphyry Cu-Au deposits10

Figure 1-4. The relationships between Cu-equivalent ore metal resource and deposit size for

Cu-rich IOCG globally, porphyry Cu (-Mo-Au) deposits in Chile-Per, SW USA-Sonora

and British Columbia, Canada11

Figure 1-5. Alteration and mineralization zonation in Cu-rich IOCG deposits..13

Figure 1-6. Alteration and mineralization zonation in Cu-poor iron oxide deposits..16

Figure 1-7. Schematic cross section illustrating the model for alteration zoning in IOCG

deposits...17

Figure 1-8. Locations of the Cu-rich IOCG deposits, principal iron deposits and manto-type

deposits in Per and Chile...19

Chapter 2

Figure 2-1. Locations of Cu-rich IOCG deposits, principal iron deposits and manto-type deposits

in Per and Chile.25

Figure 2-2. Geology of the Marcona-Mina Justa district30

Figure 2-3. Summarized stratigraphic column for the Marcona-Mina Justa district..32

Figure 2-4. Major host-rocks of the Marcona and Mina Justa deposits..34

Figure 2-5. Schematic stratigraphic columns of the Ro Grande Formation in the Can Ro

Grande, Marcona and Pampa de Pongo areas.37

Figure 2-6. Geology of the area surrounding the Marcona deposit and Mina Justa

prospect...41

Figure 2-7. Schematic cross-section of Marcona mine area...42

Figure 2-8. Cross-section of the Mina 1 and Mina 4 orebodies, Marcona.43

Figure 2-9. Three-dimensional imaging of the areal relationships of the E-Grid Marcona

magnetite orebodies44

Figure 2-10. NE-striking, NW-dipping Mina 7 orebody46


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Figure 2-11. Dike-like apophyses of magnetite extending from the hanging wall of the Mina 11

orebody and cutting strongly foliated and folded Marcona Formation..46

Figure 2-12. Megascopic features of the contacts between magnetite orebodies and MarconaFormation host rocks in the Marcona mine........................47

Figure 2-13. Large-scale relationships between magnetite orebodies and dacite porphyry in the

Marcona deposit..48

Figure 2-14. Panorama of part of Mina 3 open pit showing crudely planar or convoluted contacts

between massive magnetite bodies and their host rocks.49

Figure 2-15. Crudely spheroidal bodies of dacite porphyry enclosed by massive magnetite at the

contact between the Mina 3 magnetite orebody and dacite porphyry.50

Figure 2-16. Angular blocks of massive magnetite enclosed in a matrix consisting of ovoid bodiesof fine-grained magnetite51

Figure 2-17. Vuggy texture of magnetite and euhedral octahedral magnetite crystals...51

Figure 2-18. Dislocation of magnetite orebody by late Mina Justa and Huaca normal faults52

Figure 2-19. Alteration and mineralization paragenesis of the Marcona deposit...54

Figure 2-20. Stage I precursor alteration at Marcona.56

Figure 2-21. Marcona Stage M-II Na metasomatism.58

Figure 2-22. Mineralogical and textural relationships of Marcona main magnetite Stage (M-III)

and magnetite-sulphide stage (M-IV).60Figure 2-23. Mineralogical and textural relationships of Marcona polymetallic sulphide stage

(Stage M-V)64

Figure 2-24. Marcona Stage M-VI chlorite-talc-serpentine alteration...65

Figure 2-25. Marcona late veins (Stage M-VII).66

Figure 2-26. Hypogene alteration and mineralization paragenesis of the Mina 11 orebody..67

Figure 2-27. Paragenetic relationships of Mina 11 orebody...68

Figure 2-28. Geological map of Mina Justa Cu deposit, hosted by the upper Ro Grande

Formation70Figure 2-29. Cross-sections through major Mina Justa orebodies..72

Figure 2-30. Mineralogical and structural zonation of the Mina Justa orebodies, based on logging

of selected drill cores..73

Figure 2-31. Alteration and mineralization paragenesis of the Mina Justa deposit75

Figure 2-32. Albitization and actinolite alteration (Stage J-I) and K-Fe metasomatism (Stage J-II)

at Mina Justa...77


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Figure 2-33. Mineralogical and textural relationships of Mina Justa actinolite alteration (Stage

J-III)78

Figure 2-34. Platy Stage J-V magnetite (after Stage J-IV hematite) intergrown with calcite, quartzand chalcopyrite..............................................................................79

Figure 2-35. Mineralogical and textural relationships of Mina Justa magnetite-pyrite alteration

(Stage J-V)..81

Figure 2-36. Mineralogical and textural relationships of Mina Justa Cu mineralization (Stage

J-VI)............................83

Figure 2-37. Laser-induced 40Ar/39Ar age spectra, with Ca/K and Cl/K ratios for each heating step,

and inverse isochron plots for samples from Marcona alteration and mineralization

stages..90Figure 2-38. Laser-induced 40Ar/39Ar age spectra, with Ca/K and/or Cl/K ratios for each heating

step, and inverse isochron plots for samples from Mina Justa alteration and mineralization

stages...95

Figure 2-39. Potential compositional trajectories of volcanic rock suites from different

locations102

Figure 2-40. Laser-induced 40Ar/39Ar plateau ages for Marcona and Mina Justa alteration and

mineralization stages.105

Figure 2-41. Cartoon of the evolution of the Marcona deposit.106Figure 2-42. Cartoon of the evolution of the Mina Justa deposit.113

Chapter 3

Figure 3-1. Geology of the area surrounding the Marcona deposit and Mina Justa

prospect.123

Figure 3-2. Geological map of the Mina Justa Cu deposit...125

Figure 3-3. Alteration and mineral paragenesis of the Marcona deposits127Figure 3-4. Mineralogical and textural relationships of fluid inclusion-hosting assemblages from

the Marcona..128

Figure 3-5. Alteration and mineral paragenesis of the Marcona deposits....129

Figure 3-6. Mineralogical and textural relationships of fluid inclusion-hosting assemblages from

the Mina Justa deposits.130

Figure 3-7. Fluid inclusion types in the Marcona and Mina Justa deposits..135


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Figure 3-8. Histograms of homogenization temperatures (Th), eutectic temperatures (Te) and

ice-melting temperatures (Tm(ice)) for fluid inclusions of various paragenetic stages from

Marcona and Mina Justa...140Figure 3-9. Relationships among Te, Tm(ice)and Thfor fluid inclusions of the Marcona major

sulphide stage (M-V)143

Figure 3-10. Relationships among Te, Tm(ice)and Thfor fluid inclusions of the Mina Justa Cu

mineralization stage (J-VI)...146

Figure 3-11. LA-TOF-ICP-MS data for cation concentrations in fluid inclusions of the Mina Justa

Cu ore-forming fluid149

Figure 3-12. Interelement correlations based on TOF data..151

Figure 3-13. 34

S values of sulphides and calculated ore-forming fluids from Marcona and MinaJusta mineralization and alteration stages.156

Figure 3-14. Calculated D and 18O values of fluids at Marcona and Mina Justa..161

Figure 3-15. 18Ofluid/melt-temperature relationships for main mineralization and alteration stages at

Marcona and Mina Justa and other IOCG deposits..162

Figure 3-16. 34S values of ore fluids at Marcona and Mina Justa and other major IOCG

deposits.162

Figure 3-17. Calculated 13C and 18O values of hydrothermal fluids, Marcona and Mina Justa

mineralization and alteration stages..164Figure 3-18. Cartoon showing the evolution of the main magnetite mineralization and major

sulphide stage at Marcona.165

Figure 3-19. Genetic model for the magnetite-pyrite alteration and Cu mineralization at Mina

Justa...168

Chapter 4

Figure 4-1. Model for the Middle-to-Late Jurassic mineralization in the Central Andes.177

Figure 4-2. Model for the Early Cretaceous mineralization in the Central Andes...181Figure 4-3. Cartoon illustrating the settings of IOCG deposits.192


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LIST OF TABLES

Chapter 1

Table 1-1. Tonnages and Grades of Selected IOCG and Allied Deposits3

Chapter 2

Table 2-1. Tonnages and Grades of Selected IOCG and Allied Deposits..26

Table 2-2. Selected Tonnage/Grade Data for Marcona Orebodies.42

Table 2-3. Representative Electron Microprobe Data for Alteration Minerals, Marcona

Iron Deposit55

Table 2-4. Representative Electron Microprobe Data for Hydrothermal Silicates and Sulphides

from Mina Justa76

Table 2-5. Oxygen and Sulphur Isotope Geothermometry, Marcona and Mina Justa...86

Table 2-6. Summary of 40Ar/39Ar Ages from Marcona and Mina Justa.89

Chapter 3

Table 3-1. Locations and Types of Fluid Inclusions, Marcona and Mina Justa...138

Table 3-2. Summary of Fluid Inclusion Petrography and Microthermometric Data, Marcona and

Mina Justa....139

Table 3-3. Average Laser Ablation Time of Flight ICP-MS Analyses of Single Quartz -hosted

Fluid Inclusions, Mina Justa.148

Table 3-4. Stable Isotopic Compositions of Minerals from Marcona..152

Table 3-5. Stable Isotopic Compositions of Minerals from Mina Justa...154

Table 3-6. Summary of Sulphur Isotopic Composition of Minerals and Ore-forming

Fluids155

Table 3-7. 18O, D and 13C Values of Minerals and Fluids from Various Stages at Marcona and

Mina Justa159

Table 3-8. Copper Ore-forming Fluids Involved in IOCG Mineralization and Other Deposit

Types.168

Chapter 4

Table 4-1. Characteristics of Major Cu-poor Kiruna-type Iron Deposits.176

Table 4-2. Salient Features of the Major Central Andean Cu-rich IOCG deposits..179


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Table 4-3. Proposed Revised Classification of IOCG Deposits...188


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Chapter 1

INTRODUCTION

THE IRON OXIDE-COPPER-GOLD ORE DEPOSIT CLAN:

PROBLEMATIC DEFINITION AND GENESIS

The past half-century has seen major progress in the clarification of the genesis of the majority of

the long-established classes of hydrothermal ore deposits, including porphyry copper

(-molybdenum and/or gold), epithermal base and precious metal, skarn/carbonate replacement,

Mississippi Valley-type, and both volcanic- and sediment-hosted massive sulphide systems. At

the same time, newly recognized forms of mineralization, such as Carlin-type gold-silver and

unconformity-controlled uranium deposits, have been extensively documented, even if they

attract continued genetic argument. Against this background, the iron oxide-copper-gold

(IOCG) clan, first promulgated by Hitzman et al. (1992), stands out as a focus of fundamental

controversy, extending even to doubts as to its coherence and specificity. IOCG deposits do not

necessarily represent strictly new discoveries: several such deposits in southern and northern

Africa have been sources of copper for over 2000 years, while, among sulphide-poor deposits

widely assigned to the clan, Malmberget and Kiirunavaara, Norbotten, Sweden, have been major

sources of iron since, respectively, 1888 and 1901 (Grip, 1978), and the Grngesberg district of

central Sweden was a major producer from the 18thCentury to 1980. Moreover, vein systems rich

in hematite and/or magnetite, such as have long been mined for copper and gold in the Central

Andes, have been included in the clan by Sillitoe (2003) and others.

It could, however, be argued that an abundance of such extremely common minerals as

magnetite or hematite constitutes a vulnerable basis for the definition of a copper sulphide ore

deposit class. Thus, why should IOCG deposits not be subsumed under the magnetite-rich


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copper-gold sub-class of the porphyry clan (e.g., Ulrich et al., 2002), and what are their

relationships to magnetite- and hematite-rich gold (-copper) porphyry mineralization (Vila and

Sillitoe, 1991)? Similarly, the widespread development of calc-silicate alteration, largely

amphibolitic but locally rich in diopside and garnet, and the calcareous host-rocks of some IOCG

deposits, suggest affinities with skarn mineralization. As an extreme example, is the small

Amolanas deposit of northern Chile (3 Mt@ ~ 2.7 percent Cu) an IOCG system? There, nodular

clusters of chalcocite, bornite and chalcopyrite intergrown with and rimmed by hematite were

emplaced along flow bands and ductile-to-brittle fractures during vesiculation of a subaerial

rhyolite lava flow erupting at the margin of a Paleocene intra-arc graben hosting thick gypsiferous

evaporites (Lortie and Clark, 1976). Although differing in many respects from those in which

most IOCG deposits occur, this environment retains several key features, including the

dominance of Cu-rich sulphides, scarcity of pyrite and association with hematite, which are

characteristic of the clan.

Indeed, it was only the 1975 discovery of the Olympic Dam hematitic breccia complex in

South Australia (Woodall, 1993; Haynes, 2006), subsequently a world-class source of Cu and Au

as well as the largest single uranium producer (Hitzman and Valenta, 2005), that prompted the

establishment of the IOCG clan as a distinct entity. The direct association (Hayneset al., 1995)

of major copper sulphide mineralization with an anorogenic alkali feldspar granite stock, rather

than with the inherently copper-rich intermediate (granodioritic - quartz dioritic), orogenic

granitoid suite which globally hosts the superficially similar porphyry copper deposits, was seen

as evidence for a fundamental ore genetic distinction.

Over the past several decades, intensive exploration for deposits broadly comparable to

Olympic Dam has met with only modest success, with the greenfield discovery of only a handful

of large copper-rich examples in both Precambrian and Phanerozoic terranes of both orogenic and

anorogenic origin (Table 1-1 and Fig. 1-1). Among these, significant copper has been produced at


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Table 1-1. Tonnages and Grades of Selected IOCG and Potentially Allied Iron and Copper

Deposits and ProspectsDeposit * Age

(Ma)

Tonnage

(Mt)

Fe

(%)

Cu

(%)

Au

(g/t)

Ag

(g/t)

Other

metals etc. (%)

Data

source

Gawler Craton - Curnamona district, Australia

Olympic Dam ~1590 7738 ne (Hem) 0.87 0.3 1.6 U3O8

(0.29kg/t)

1

Acropolis-Oak Dam ~1600 560 50 (Mt) ? ? 2, 3

Prominent Hill ~1600 102 ne (Hem) 1.5 0.5 3.5 3, 4

Carrapateena ~1600 ? 905 m1) ne (Hem) 2.1 1.0 5

Kalkaroo ~1605 30 ne (Mt) 0.28 0.14 6

Cloncurry district, Australia

Ernest Henry 1504-1530 167 ne (Mt) 1.1 0.54 Co (0.05) 7

Osborne ~1540 15.2 ne (Mt) 3.0 1.05 6

Starra ~1503 6.9 ne (Mt) 1.7 4.8 6

Mount Elliott Mesoproterozoic 3.3 ne (Mt) 3.6 1.8 6

Eloise ~1530 3.1 ne (Mt) 5.5 1.4 8

Tennant Creek district, Australia

Warrego ~1830 5 ne (Mt-Hem) 2.6 7.0 Bi (0.3) 6

Gecko ~1830 4.7 ne (Mt-Hem) 3.8 0.7 14.0 Bi (0.2) 6

Peko ~1830 3.5 ne (Mt-Hem) 4.0 3.5 6

Peruvian Coastal Belt

Ral-Condestable ~115 >32 ne (Mt>Hem) 1.7 0.9 6.0 9

Monterrosas ~115 1.9 ne (Mt) 1.1 6 20 10, 11

Marcona 156-162 1940 55.4 (Mt) 0.12 trace Zn 12, 13

Mina Justa 95-104 347 ne (Mt-Hem) 0.71 0.03 3.83 13, 14

Pampa de Pongo Hem) 0.95 0.22 3.1 24

Punta del Cobre 110-117 >120 ne (Mt>Hem) 1.5 0.2-0.6 Zn 24

Productora ~130 30-70 1) ne (Mt) 0.3-0.6 trace U, Co 25

* significant Cu (>1 Mt) and/or Au (> 1 Moz) producers and prospects underlined.


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Deposit/Location Age (Ma) Tonnage

(Mt)

Fe

(%)

Cu

(%)

Au

(g/t)

Ag

(g/t)

Other

metals (%)

Data

source

Panulcillo ~115 ~15 ne (Mt) ~ 1.45 0.1 11, 20

El Espino ~108 30 ne (Mt) 1.2 0.15 11, 26

El Soldado 2) ~108 >200 ne (Hem) 1.4 27

Andacollo 3) 98-104 300 ne (Hem) 0.7 >0.25 28

Andean Cenozoic arcs

Antauta 23 ? ne (Hem) trace trace trace Mo, W, REE 29

Amolanas 62 >3 ne (Hem) 2.5-3.0 trace trace 30

El Laco ~2.3 500 >60 (Mt) trace 31

Carajs district, Brazil

Salobo ~2580 789 ne (Mt) 0.96 0.52 55 32, 33

Igarap Bahia/Alemo ~2575 219 ne (Mt) 1.4 0.86 34Sossego 2200-2300 245 ne (Mt) 1.1 0.28 35

Cristalino ~2719 500 ne (Mt) 1.0 0.3 35, 36

Mexico

San Fernando ~100 31 m 1) ne (Mt) 0.96 37

Boleo2) < 8 445 ne (Hem) 0.71 Co (0.06) 38

Zn (0.69)

Cerro de Mercado Oligocene >100 62 (Mt) trace 39, 2

Southwest U.S.A

Salton Sea Pleistocene ne (Hem) present Zn (present) 40

Copperstone Mid-Tertiary 2.1 ne (Mt) present 16.2 2Iron Springs Mid-Tertiary 450 47 (Mt) trace 2

Pumpkin Hollow ~170 312 12.3 (Mt) 0.44 41

Southeast Missouri, U.S.A

Pea Ridge ~1470 200 55 (Mt) trace trace REE 42, 2

Boss-Bixby Mesoproterozoic 70 20 (Mt) 0.7 42, 2

Pilot Knob Mesoproterozoic 22 45 (Mt) trace 42, 2

Adirondacks - U.S.A

Benson Neoproterozoic 200 25 (Mt>Hem) trace 43, 2

Lyon Mt. Neoproterozoic 25 25 (Mt) trace 43, 2

Mineville Neoproterozoic >10 42 (Mt) trace REE 43, 2

Dover Mesoproterozoic 26 49 (Mt) trace 43, 2

Sanford Lake Mesoproterozoic 30 Mt-Ilm 36 (TiO2),

V and P

44

Mid-Atlantic U.S.A

Cornwall Triassic >100 45 (Mt) 0.2 trace Co 45, 2

Grace Triassic 100 44 (Mt) 0.06 Co (0.2) 45, 2

Michigan Native Cu Mesoproterozoic 5 Mt Cu4) ne (Mt-Hem) 0.6-2.6 trace As 46


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(Mt)

Fe

(%)

Cu

(%)

Au

(g/t)

Ag

(g/t)

Other

metals (%)

Data

source

Canada

Nico (Great Bear) 1850-1880 21.8 1.08 Co- 0.13 47

Bi- 0.16

Sue Diane (Great Bear) 1850-1880 17 ne (Mt) 1.72 2.7 48

Wernecke ~1600 >20 Mt-Hem 0.35 0.17 49

Minto Early Jurassic 16.7 ne (Mt) 1.54 0.56 5.95 50,51

Iron Range ~1470 ~1 m 1) ne (Mt) 1.81 1.0 18 Pb, Zn 52

Coppercorp Mesoproterozoic 1.1 ne (Hem) 1.46 trace >200 53

Lac Tio Mesoproterozoic 120 hemo-ilmenite 32 (TiO2), V 54

Kwyjibo ~972 61 m

1)

ne (Mt-Hem) 0.26 1.6 (max) 29 (max) REE, U 55, 49Mont-de-lAigle ~400 11 m 1) ne (Mt-Hem) 1.0 2.2 (max) 56

Fennoscandinavia

Kiirunavaara-Luossavaara ~1880 2600 62 (Mt) trace (P - ~1.0) 2

Malmberget Paleoproterozoic 840 55 (Mt-Hem) trace (P - 0.7) 2

Rakkurijrvi 1853-1862 ? ne (Mt) trace trace 57

Pahtohavare ~1880 1.7 ne (Mt) 1.9 0.9 58, 2

Tjrrojkka ~1770 53 (3.2) 52 (Mt) trace (0.9) 59

Aitik 3) 1730-1890 606 ne (Mt) 0.38 0.21 60, 61

Raajrvi, Misi region 2017-2123 6.6 47 (Mt) trace trace V 62

Grngesberg Paleoproterozoic 400 55 (Mt) trace trace W 63, 2Bidjovagge, Norway Paleoproterozoic? ? ne (Mt) ? ? 64

Russia-Central Asia

Magnitogorsk, S. Ural Devonian 500 45 (Mt) 2

Peschnask, S. Ural Devonian 173 46 (Mt) 0.61 2

Teyskoe, Altai-Sayan Devonian 373 32 (Mt) t race REE, U ? 2

Kachar, Kazakhstan Carboniferous 2000 45 (Mt) 2

Sokolovsk, Kazakhstan Carboniferous 967 41 (Mt) 2

Korshunovsk, Siberia Permo-Triassic 609 34 (Mt) trace 2

Other regions in Eurasia

Bayan Obo, China 420-555 1500 35 (Mt) REE (0.6) 65, 66

Nb (0.13)

Hankou (Daye), China Jurassic >330 55 (Mt) 0.2 2

Luohe, China Jurassic >100 ne (Mt) 0.41 67

Sin Quyen, Vietnam Mesoproterozoic 52.8 ne (Mt) 0.91 0.44 REE (0.7) 68, 2

Khetri, India Neoproterozoic 140 ne (Mt-Hem) >1.1 0.5 69, 2

Madhan, India Neoproterozoic 66 ne (Mt) >1.1 >0.3 >2 69, 2

Bafq district, Iran Cambrian 1500 20-60 (Mt) trace P 70


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(Mt)

Fe

(%)

Cu

(%)

Au

(g/t)

Ag

(g/t)

Other

metals (%)

Data

source

Avnik, Turkey Ordovician >104 14-58 (Mt) trace 2

Ossa Morena, Spain 330-350 Ma >180 25-66 (Mt) 0.11-0.4 15 71

Africa

Guelb Moghrein 5),

Mauritania

550-720 23.6 ne (Mt) 1.9 1.4 Co (143g/t) 72

Phalaborwa, S. Africa ~2060 850 ne (Mt) 0.5 REE, Ni 73,74

OOkiep, S. Africa2) ~1100 >25 ne (Mt) 1.7 Co, Zn 75

Chimiwungo, Zambia2) Pan-African 766 ne (Hem) 0.66 0.1 Co (0.01) 76, 2

Malundae, Zambia2) Pan-African 161 ne (Hem) 0.89 0.03 Co (0.014) 77, 2

Kasempa, Zambia Pan-African 229 66 (Mt) 77, 2

Kalengwa, Zambia Pan-African 1.6 ne (Hem-Mt) 6.5 77, 2Mhangura, Zimbabwe2) Mesoproterozoic 76 ne (Mt) 1.2 15 78, 79

Hemhematite,Mtmagnetite,Ilmilmenite. 1) mineralized drill-core intersection. 2) commonly classified as manto-type

or sediment-hosted Cu deposits. 3) predominant mineralization classified as porphyry Cu-Au type. 4) total production

of copper to 1968. 5) Akjoujt district. ne - not economic.

References: 1- BHP Billiton, 2007; 2- Williams et al.,2005; 3- Skirrow et al.,2002; 4- Oxiana Resources, 2006; 5-

Teck Cominco Limited, 2007; 6- Williams and Pollard, 2003; 7- Mark et al.,2006; 8- Baker et al.,2001; 9- de Haller et

al., 2006; 10- Injoque, 2002; 11- Sillitoe, 2003; 12- Shougang Hierro Per SA., 2003; 13- this study; 14- Chariot

Resources, 2006; 15- Cardero Resource Corp., 2005; 16- Hawkeset al.,2002; 17- Ramrez et al.,2006; 18- Ruiz and

Peebles, 1988; 19- Oyarzun et al., 2003; 20- Hopper and Correa, 2000; 21- Benavides et al., 2007; 22- Vila et al.,1996;23- Far West Mining Ltd. 2007; 24- Marschik and Fontbot, 2001; 25- Ray and Dick, 2002; 26- Correa, 2003; 27-

Boric et al.,2002; 28- Reyes, 1991; 29- Clark and Kontak, 2004; 30- Lortie and Clark, 1976; 31- Rhodes et al.,1999;

32- Souza and Vieira, 2000; 33- Requia et al.,2003; 34- Tallarico et al.,2005; 35- Monteiro et al.,2008; 36- Huhn et

al.,2000; 37- Cruise et al.,2007; 38- Conly et al.,2001; 39- Lyons, 1988; 40- McKibben and Elders, 1985; 41- Gander

et al.,2007; 42- Day et al.,2001; 43- Friehauf et al.,2002; 44- Force, 1991; 45- Rose et al.,1985; 46- White, 1968; 47-

Fortune Minerals, 2007; 48- Corriveau, 2005; 49- Hunt et al.,2005; 50- Sherwood Copper Corp., 2007; 51- Tafti and

Mortensen, 2003; 52- Eagle Plains Resources Ltd., 2005; 53- Richards and Spooner, 1989; 54- Gross et al., 1997; 55-

Gauthier et al., 2004; 56- Simard et al.,2006; 57- Smith et al., 2007; 58- Lindblom et al.,1996; 59- Edfelt et al.,2005;

60- Wanhainen et al.,2003; 61- Wanhainen et al.,2005; 1999; 62- Niiranen et al.,2005; 63- Ripa, 1999; 64- Ettner et

al., 1994; 65- Smith and Henderson, 2000; 66- Smith and Wu, 2000; 67- Pan and Dong, 1999; 68- Mclean, 2002; 69-

Knight et al., 2002; 70- Torab and Lehmann, 2006; 71- Tornos et al., 2005; 72- Kolb et al.,2006; 73- Vielreicher et al.,

2000; 74- Groves and Vielreicher, 2001; 75- Stumpfl et al.,1976; 76- Equinox Minerals Ltd., 2007; 77- Nisbet et al.,

2000; 78- Maiden et al.,1984; 79- Maiden and Master, 1986.

Ernest Henry, Queensland (167 Mt @ 1.1% Cu, 0.54g/t Au); La Candelaria (470 Mt @ 0.95% Cu,

0.22g/t Au) and Mantoverde (400 Mt @ 0.52% Cu, 0.11g/t Au), Chile; Sossego (245 Mt @ 1.1%


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Figure 1-1.Global distribution of major IOCG districts and important deposits, including potentially allied iron a

from Corriveau, 2005). Detailed data are listed in Table 1-1.


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Cu, 0.28g/t Au), Brazil; the Khetri district, India (>200 Mt @ 1.1% Cu, 0.5g/t Au); and (perhaps

a dubious candidate: Wanhainen, 2005) Aitik, Sweden (606 Mt @ 0.38%Cu, 0.21g/t Au), while

the Salobo, Carajs, Brazil (789 Mt @ 0.96%Cu, 0.52g/t Au) and Prominent Hill, Australia (102

Mt @ 1.5%Cu, 0.5g/t Au) deposits will begin production in 2008. Elsewhere, the major

Phalaborwa (Palabora) Cu (-magnetite-apatite-vermiculite-pentlandite-baddeleyite) deposit,

South Africa, although associated with a Paleoproterozoic carbonatite centre, has been

persuasively assigned to the IOCG clan (Groves and Vielreicher, 2001). Further, the iron

oxide-rich Mantos Blancos (Chile), Chimiwungo (Zambia) and Mhangura (Zimbabwe) copper

deposits, commonly described as of manto or sediment-hosted type, share some similarities

with copper-rich IOCG deposits (Maiden and Master, 1986; Maksaev and Zentilli, 2002;

Williams et al.,2005).

It is, however, apparent that the vast majority of deposits which have been assigned to the

clan, including Kiirunavaara itself, are magnetite-rich but sulphide-poor, containing, if any, only

subeconomic copper and gold. Williams et al. (2005) have therefore recommended that the

IOCG designation be restricted to deposits with economic copper and/or gold (Fig. 1-2), although

maintaining a genetic connexion between Cu-rich and Cu-poor mineralization. The IOCG clan

thus suffers from an inherent dichotomy: most such systems failed to generate significant

sulphide mineralization, no matter how intense the precursor magnetite deposition. Further,

with the exception of Olympic Dam, these deposits are dwarfed by supergiant and behemothian

(sensuClark, 1993) porphyry copper systems, whether Mo- or Au-rich (Fig. 1-3A). Nonetheless,

in terms of the relationships between copper-equivalent ore metal resource and deposit size (Fig.

1-3), IOCGs compare closely to both Cu-Mo and Cu-Au porphyry systems. Further, even

omitting from consideration the uranium in the Olympic Dam deposit, the metal resourcevs.ore

tonnage correlation coefficient (Figs. 1-3 and 1-4) implies that the aggregate processes of base

and precious metal concentration in Cu- and Au-rich IOCG deposits may have been more


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Figure 1-2.Tonnage-grade data for Cu-rich and Cu-poor deposits which have been assigned to the iron oxide-cWilliams et al., 2005). Most Fe deposits contain traces of Cu. Those commonly described as manto-type or sbracketed. The Aitik and Andacollo deposits are underlined because their major mineralization is of porphyry Cu-


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Figure 1-3.A. The relationships between Cu-equivalent ore metal resourceand deposit sizeforCu-rich IOCG, porphyry Cu-Mo (PCMD) and porphyry Cu-Au (PCGD) deposits (cf. Clark,1993). B. an enlarged part of A to show the relationships where tonnage < 4500 Mt. IOCG,

PCMD and PCGD all show good correlations (R2 > 0.8) between Cu-equivalent metal andtonnage. The slopes are: IOCG (0.011) > PCMD (0.0083) > PCGD (0.0074). If uranium isincluded for Olympic Dam (OD) the slope for IOCG mineralization would be much higher (A).IOCG data are from references listed in Table 1-1. Data for porphyry deposits are from USGS(2005). Metal prices (Cu, Mo, Au, Ag and U) at 2007 levels.


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Figure 1-4.A. The relationships between Cu-equivalent ore metal resourceand deposit sizefor

Cu-rich IOCG deposits globally, and porphyry Cu (-Mo-Au) deposits (PCD) in Chile-Per, SWUSA-Sonora and British Columbia. B is an enlarged part of A to show the relationships wheretonnage < 4500 Mt. IOCG and PCD all show good correlations (R2 > 0.85) betweenCu-equivalent metal and tonnage. The slopes are: IOCG (0.011) >= Chile-Per PCD (0.01) > SWUSA-Sonora PCD (0.0056) > BC (0.0044). IOCG data are from references listed in Table 1-1.Porphyry deposits are referenced from Clark (1993) and USGS (2005). OD-Olympic Dam.


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efficacious than in porphyries, including those of the Central Andes. In part, this must reflect the

sulphur-, and hence pyrite-deficient, nature of IOCGs but, as suggested by Clark (1993), may be

evidence for unusually high ore metal contents in the hydrothermal fluids.

Nonetheless, although aggressive exploration for Cu- and Au-rich IOCG mineralization

continues, it is improbable that many deposits as large and rich as Olympic Dam remain to be

discovered. Similarly, Kiirunavaara is likely to remain the largest magnetite deposit in the

expanded clan. It should be emphasized that, in the broadly analogous porphyry copper field,

many of the largest deposits, e.g., Bingham, Morenci, El Teniente and Chuquicamata, were

similarly among the first to be discovered and delimited. However, a large number of major

porphyry copper deposits have been identified and developed in the past half-century, and many

of these are much larger than all known IOCGs except for Olympic Dam (Fig. 1-3A). Large

IOCGs, and especially examples rich in copper and gold, must therefore be considered, in global

terms, to be both scarce and of significantly lower inherent potential than porphyry copper

systems. These deficiencies may directly reflect an origin dependant on an improbable

conjunction of geological processes.

Ore Genetic Uncertainties

Proximity to granitoid stocks, intense, commonly high-temperature hydrothermal alteration, and

extensive hydrothermal brecciation have been interpreted by many (e.g., Sillitoe, 2003; Pollard,

2006) as supporting a direct genetic relationship between IOCG deposit development and

hydrous fluid exsolution from crystallizing silicate melts, the abundance of Fe, Cu, Au and,

locally, Co being ascribed to a mafic parental magma. Such basically magmatic-hydrothermal

models prompt analogies with, particularly, molybdenite-poor porphyry copper-gold deposits, the

vast majority of which are magnetite-rich (e.g.,Ulrich et al., 2002; Pollard and Taylor, 2002).


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However, several characteristic, if not ubiquitous, features of Cu-rich IOCGs, such as intense

albitization, Ca metasomatism (commonly amphibolization), hematite- and calcite-rich sulphide

mineralization (Fig. 1-5) and the scarcity of pyrite, as well as the overall sequence of

alteration-mineralization events (e.g., Ullrich and Clark, 1999), are difficult to reconcile with

thermally retrograde melt-aqueous fluid equilibria (Candela, 1989a and b). In addition, many

major IOCG provinces of both Proterozoic and Phanerozoic age exhibit a close correlation with

mid-latitude sedimentary/ volcanic basins which either incorporate evaporite sequences or

preserve their metamorphic relics, i.e., regional scapolite-albite (-tourmaline) assemblages

(Frietsch et al., 1997). Such relationships underlie the proposal of Barton and Johnston (1996;

2004) that the brines responsible for Cu (-Au) sulphide mineralization in IOCG deposits were

derived wholly or in part through the dewatering of intra-orogenic or anorogenic rift basins.

Stable isotope evidence for such a non-magmatic origin, at least for sulphur and oxygen, was first

presented by Ullrich and Clark (1999) and Ullrich et al. (2001) for La Candelaria, the most

important Cu-rich IOCG deposit in the Central Andes, but the incursion of basinal fluids had

earlier been documented at Olympic Dam by Haynes et al. (1995). A preliminary classification of

hydrothermal IOCG deposits by Hunt et al. (2007) ascribes all major IOCG deposits to hybrid

magmatic non-magmatic fluids. Although the ore metals, i.e.,Cu and Au, in these deposits may

be, at least in part, magma-derived (Ullrich and Clark, 1999), such inherently complex genetic

models have direct implications for the occurrence of, and hence exploration for, Cu-rich IOCGs,

as is exemplified by the studies of Benavides et al. (2006, 2007) in the Mantoverde district of

northern Chile.

A requirement for exotic sulphur in the development of Cu-rich IOCGs would highlight

not only the problematic interrelationships of precursor magnetite and sulphide mineralization,

but also the processes of magnetite formation per se. Whereas bodies of massive magnetite

antedating Cu sulphide deposition in numerous IOCG deposits clearly resulted from intense


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hydrothermal Fe metasomatism (e.g., La Candelaria; Ullrich and Clark, 1997; Rakkurijrvi:

Smith et al., 2007; Mantoverde: Benavides et al., 2007), magnetite apatite actinolite

mineralization elsewhere has been interpreted as the product of phosphatic and silicic Fe oxide

melts. Such an origin, first advocated at Kiirunavaara by Geijer (1910), was convincingly

documented by Lundberg and Smellie (1979) for the nearby Mertainen deposit. The involvement

of oxide melts has since been proposed for numerous orebodies in the Cretaceous Chilean iron

belt (Nystrm and Henrquez, 1994; Henrquez et al., 2003), but is best exemplified by the

Pliocene magnetite volcanic flows in the El Laco district of northern Chile (Park, 1972; Naslund

et al., 2002). A growing body of experimental data has confirmed the development of stable

immiscibility between silicate and Fe oxide liquids in the systemNa2O + K2O + Al2O3 + MgO

FeO + MnO + TiO2 + CaO + P2O5 SiO2 (Philpotts, 1982), and the geological occurrence of

oxidic melts with up to 60 weight percent FeO, 37 weight percent TiO2and 22 weight percent

P2O5 has been demonstrated (Clark and Kontak, 2004). Apparently magmatic magnetite -

apatite bodies are areally juxtaposed with unambiguously hydrothermal hematitic Cu-Au veins

and breccias in numerous districts, e.g., the Carmen area of northern Chile (Gelcich et al., 2005),

and commonly exhibit similarities in their alteration facies. This implies that Fe oxide melts may

be integral to IOCG-type sulphide mineralization, perhaps through their vesiculation through

either aqueous fluid (Matthews et al., 1995) or chloride (Broman et al., 1999) saturation.

However, the importance of oxidic melts and, therefore, the recognition of the

magmatic/hydrothermal interface, remain controversial (Rhodes et al., 1999; cf. Sillitoe and

Burrows, 2002). Similarly problematic is the relationship between Kiruna-type

magnetite-apatite deposits and ilmenite- or rutile-rich nelsonite bodies, which are only rarely

associated with copper sulphide mineralization (e.g., McLelland et al.,1994; Clark and Kontak,

2004).


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In the light of the controversy regarding the contribution of Fe oxide melts to IOCG

mineralization, it is paradoxical that the extensive hydrothermal alteration enveloping

sulphide-poor iron deposits such as Kiirunavaara, the Chilean iron belt and El Laco (Fig. 1-6) is

Figure 1-6.Alteration and mineralization zonation in Cu-poor iron oxide deposits.

A - Generalized cross-sectional reconstruction of the Kiruna district, Sweden (modified fromHitzman et al., 1992). B- Schematic diagram of intrusive magnetite mineralization and alterationenvelope that formed in andesitic host rock, El Laco, Chile (Rhodes et al., 1999). (Ab-albite,Act-actinolite, Apt-apatite, Bar-barite, Cal-calcite, Cpx-clinopyroxene, Fl-fluorite, Hem-hematite,Mt-magnetite, Qtz-quartz, Ser-sericite)


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remarkably similar to that associated with Cu-rich deposits incorporating metasomatic and, hence,

hydrothermal magnetite, including Ernest Henry and La-Candelaria Punta del Cobre (Fig. 1-5),

thereby permitting integrated ore deposit models such as that in Figure 1-7.

Figure 1-7. Schematic cross-section illustrating the model for alteration zoning in IOCG depositsproposed by Hitzman et al. (1992), with the addition of representative Cu-poor systems. Thethree major alteration zones, viz. Na-Ca, K (Kspar/biotite) and hydrolitic, show vertical and,locally, lateral zonation. NB. Na-Ca alteration is commonly represented by albitization,marialitization and Ca-amphibolitization (and, less widely, garnet and diopside formation inCa-poor protoliths), and hence records either Na- or Ca- metasomatism. Hematite and brecciasare dominant in the upper levels, magnetite and veins (breccias) at depth. Cu-rich IOCG andCu-poor iron oxide mineralization is focused at various levels, with different alteration patternsand mineral assemblages.

The Present Study

The problematic roles of both Fe-oxide melts and exotic hydrothermal fluids have hindered the

development of a uniform model for the genesis of IOCG mineralization, as well as the

establishment of exploration protocols for Cu-Au rich members of this clan. The research

documented in this thesis addresses these two salient problems in the context of a major


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concentration of unmetamorphosed Mesozoic IOCG deposits in the Central Andean orogen.

Hosted in part by Middle Jurassic andesitic arc volcanics, the Marcona magnetite deposit,

south-central Per (Fig. 1-8), is much the largest-known locus of iron oxide mineralization along

this well-studied convergent plate margin, and probably represents the second-largest

Kiruna-type deposit globally (Table 1-1). With only subeconomic chalcopyrite, Marcona is

directly analogous to the iron deposits of the Cretaceous Chilean iron belt. In contrast, the

contiguous Mina Justa Cu(-Ag) prospect (Fig. 1-8), the only demonstrably economic

mineralization of this type discovered over the past three decades in the Central Andes and hosted

entirely by Middle Jurassic andesites, is representative of Cu-rich IOCG deposits globally.

Together with the nearby Pampa de Pongo magnetite deposit (Hawkes et al., 2002), these

contrasted deposits occur in well-defined stratigraphic and tectonic contexts and provide a basis

for clarification of the processes responsible for magnetite and copper concentration in the IOCG

clan.

In a wider context, this research has implications for the delimitation of this controversial

family of ore deposits. Given the extreme variations in salient features of deposits assigned to the

IOCG clan, particularly with regard to geological setting and the types and sequence of alteration,

it must be asked whether such inconsistencies go beyond those exhibited by porphyry copper

deposits, for which Gustafson (1978) coined the term variations on a theme.

The Scientific Contributions of This Study

This research project in the Marcona-Mina Justa area has resulted in several distinct, but

interrelated, contributions. These are:


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Figure 1-8.Locations of Cu-rich IOCG deposits, principal iron deposits and manto-type depositsin Per and Chile (from Clark et al., 1990; Hawkes et al., 2002; Maksaev and Zentilli, 2002;Sillitoe, 2003; Oyarzun et al., 2003 and Benavides et al., 2007).NB.The twelve major orebodiesoccurring in an area of 25 km2at Marcona incorporate a magnetite tonnage similar to that in thefive largest Cretaceous Chilean iron mines.


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1- the characterization and redefinition of paragenetic relationships in the Marcona magnetite

deposit, and clarification of the relationships between magnetite and multiple-stage, but

non-economic, sulphide precipitation. Megascopic relationships strongly suggest an immiscible

melt origin for the Marcona orebodies, a model supported by fluid inclusion and stable isotope

analysis, although the incursion of modified seawater is evident in the subsequent polymetallic

sulphide stage;

2- the first documentation of the paragenetic relationships of the Mina Justa deposit, where Cu

mineralization significantly postdated magmatic-hydrothermal magnetite-pyrite alteration. The

various reservoirs of the fluids responsible for the magnetite and the later Cu mineralization

stages are defined on the basis of fluid inclusion and stable isotope analysis, as well as other

geological information. Low-temperature basinal brines are inferred to be responsible for the

formation of the Mina Justa Cu orebodies;

3- the recognition of the protracted and multistage evolution (from 177 to 95 Ma) of the district,

incorporating the Jurassic (Callovian) Marcona Fe deposit and the Cretaceous (Albian) Mina

Justa Cu deposit, on the basis of paragenetically-constrained 40Ar/39Ar geochronology. Marcona

and Mina Justa, although spatially associated, represent two entirely independent mineralization

events, with the suggestion that the intra- or retroarc environment may be favorable for IOCG

mineralization at widely separated intervals; and

4- a proposed reclassification of the broad and problematically-defined IOCG clan.


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Thesis Organization

This dissertation is constructed in a manuscript format which fulfills the requirements of the

Queens University School of Graduate Studies and Research. Two major manuscripts are

prepared for publication and form the main body of this thesis:

Chapter 2:

Chen, H., Clark, A.H., and Kyser, T.K., The longlived, Marcona-Mina Justa iron-copper district,

Per: implications for the origin of Cu-poor and Cu-rich IOCG mineralization in the Central

Andes: to be submitted toEconomic Geology

Chapter 3:

Chen, H., Kyser, T.K., and Clark, A.H., Contrasted fluids and reservoirs in the contiguous

Marcona and Mina Justa iron-oxide Cu (-Au-Ag) deposits, south-central Per: to be submitted to

Mineralium Deposita

General conclusions are drawn in Chapter 4, addressing the significance of the contrasted

Marcona and Mina Justa IOCG mineralization, summarizing the Mesozoic metallogenesis of the

Central Andes, and concluding with a new classification of IOCG deposits.


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Chapter 2

THE LONGLIVED MARCONA-MINA JUSTA IRON-COPPER DISTRICT,

PER: IMPLICATIONS FOR THE ORIGIN OF CU-POOR AND CU-RICH

IOCG MINERALIZATION IN THE CENTRAL ANDES

2.1 AbstractThe IOCG subprovince of littoral south-central Per incorporates Marcona, the preeminent

Andean magnetite deposit (1.9 Gt @ 55.4% Fe and 0.12% Cu), and, 3-4 km distant, Mina Justa,

one of the few major Andean IOCG deposits with economic copper (346.6 Mt @ 0.71% Cu, 3.8

g/t Ag and 0.03 g/t Au). Fe oxide and Cu sulphide mineralization was controlled by NE-striking

reverse (Marcona) and normal (Mina Justa) faults transecting a Middle Jurassic

(Aalenian-to-Oxfordian) andesitic shallow-marine arc and a succession of contiguous, plate

boundary-parallel, Late Jurassic to mid-Cretaceous, volcano-sedimentary basins. Detailed

documentation of alteration and mineralization relationships, supported by stable isotope

geothermometry and 40Ar/39Ar geochronology, reveals an episodic history of Mesozoic magmatic

and hydrothermal processes extending for at least 80 m.y., from ca.177 to 95 Ma.

At Marcona, initial hydrothermal activity occurred in the Aalenian (177 Ma) and Bajocian

(171 Ma), when high-temperature Mg-Fe metasomatism of Paleozoic siliciclastics underlying the

nascent Ro Grande Formation arc, generated, respectively, cummingtonite and

phlogopite-magnetite assemblages. Subsequent, widespread albite-marialite alteration (Na-Cl

metasomatism) largely predated, but overlapped with, the emplacement of an en echelon swarm

of massive magnetite orebodies with subordinate, overprinted magnetite-sulphide assemblages.

Magnetite and uneconomic Cu and Zn sulphide mineralization coincided with a 156-162 Ma


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episode of andesitic eruption which terminated the growth of the arc, but was hosted largely by

siliceous metaclastic rocks approximately 300 m below the ocean-floor. The magnetite

orebodies exhibit abrupt, smoothly curving contacts, dike-like to tubular apophyses and intricate,

amoeboid interfingering with dacite porphyry intrusions, and there is no convincing megascopic

or microscopic evidence for large-scale Fe metasomatism. The largest, 400 Mt Minas 2-3-4

orebody is interpreted as a bimodal magnetitite-dacite intrusion comprising immiscible melts

generated through the dissolution of metasedimentary quartz in parental andesitic magma.

From 162 to 159 Ma, an evolution from magnetite-biotite-calcic amphibole phlogopite

fluorapatite assemblages, which are inferred to have crystallized from a hydrous Fe-oxide melt

containing < 30 weight percent combined CaO, MgO, K2O and SiO2, to magnetite - phlogopite -

calcic amphibole - pyrrhotite - pyrite assemblages coincided with quenching from above 800C to

below 450C and the concomitant exsolution of dilute aqueous brines. Subsequently, at

156-159 Ma, chalcopyrite - pyrite - calcite pyrrhotite sphalerite galena assemblages were

deposited from lower-temperature ( 360oC) brines that may have incorporated modified

seawater.

Hydrothermal activity was thereafter focused in the Mina Justa area, where Middle Jurassic

andesites experienced intense albite-actinolite alteration atca.157 Ma and K-Fe metasomatism at

ca.142 Ma. The development of the Mina Justa deposit proper began much later, adumbrated by

ca.109 Ma actinolitization and the 101-104 Ma deposition of bodies of massive, metasomatic,

magnetite and pyrite from 500-600oC hydrothermal fluids. Mid-Cretaceous hydrothermal activity

was areally associated with the intrusion of small diorite stocks along the faulted southwestern

margin of the Aptian-Albian, arc-parallel, Caete basin. Finally, at 95-99 Ma, hypogene argentian

chalcopyrite-bornite-digenite-chalcocite mineralization, with abundant calcite and hematite, was

emplaced as two ~ 400 m long, ~ 200 m thick, gently-dipping, tabular arrays of breccia and

stockwork, cored by earlier magnetite-pyrite lenses.


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Marcona and Mina Justa, although contiguous, represent contrasted ore deposit types. The

former, like El Laco, the second-largest Andean magnetite deposit, is interpreted as the product of

Fe oxide melts coexisting with dacitic magmas within a failing andesitic arc, albeit in a

subaqueous rather than subaerial environment. The weak associated magmatogene Cu-Zn

sulphide mineralization, generated through melt vesiculation, contrasts with the considerably

higher-grade Cu- and Ag-rich orebodies at Mina Justa, which were the product of cool, oxidized,

hydrothermal fluids plausibly expelled from the Caete basin during its tectonic inversion. This

major IOCG district demonstrates that the largest, orthomagmatic, Central Andean magnetite

deposits may bear no direct genetic relationship to copper-rich centres, while the 142 Ma K-Fe

alteration at Mina Justa is evidence that IOCG-type hydrothermal activity may itself terminate

before generating significant Cu sulphide mineralization, despite occurring in a favourable

tectonic and stratigraphic environment.

2.2 Introduction

Iron oxide-copper-gold (IOCG) mineralization, first formally defined by Hitzman et al.(1992),

has been a major exploration target since the discovery of the enormous Olympic Dam Cu-U-Au

(-REE) deposit in 1975. However, few large IOCG centres with economic gold or, even, copper

grades have been identified, and the clan remains incompletely defined or delimited. Because

most early-identified IOCG systems, e.g., these of the Gawler Block of South Australia, the

Eastern Mount Isa Block of Queensland and the northern Fennoscandinavian Shield, are of

Proterozoic age, it was initially proposed that this type of mineralization was restricted to that

period (Hitzman et al., 1992). However, the Central Andean orogen, and especially the

volcano-plutonic arcs of Jurassic and Cretaceous age exposed in the Cordillera de la Costa of

northern Chile and central and southern Per, are now recognized as hosting major IOCG

mineralization (Fig. 2-1). Nonetheless, the great majority of the Andean deposits assigned to the


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IOCG clan by Sillitoe (2003) and others are exploited only for magnetite, having negligible

sulphide contents. Only the La Candelaria Punta del Cobre (Ryan et al.,1995) and Mantoverde

(Vila et al., 1996; Zamora and Castillo, 2001) districts in Chile and the much smaller Ral

Condestable deposit cluster in Per (de Haller et al., 2006) and Panulcillo deposit, Chile (Hopper

and Correa, 2000), as well as numerous vein systems (Ruiz, 1965), have produced significant

copper, and only La Candelaria Punta del Cobre and Ral Condestable have been important

sources of gold (Table 2-1). In the Andean context, however, Mantos Blancos and other so-called

Figure 2-1. A - Locations of Cu-rich IOCG deposits, principal iron deposits and manto-typedeposits in Per and Chile (from Clark et al., 1990; Hawkes et al., 2002; Maksaev and Zentilli,2002; Sillitoe, 2003; Oyarzun et al., 2003 and Benavides et al., 2007). B - Simplified geologicmap of the IOCG mineralization belt of south-central Per (modified from Vidal et al., 1990),illustrating the extent of the mid-Cretaceous Caete intra-arc extensional basin (Atherton andAguirre, 1992).


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Table 2-1. Tonnages and Grades of Selected IOCG and Allied Deposits

in the Central AndesDeposit Tonnage

(Mt)

Fe (%) Cu (%) Au (g/t) Ag (g/t) Data source

Ral-Condestable >32 ne 1.7 0.3 6.0 de Haller et al., 2006

Marcona 1 ~1940 2 55.4 0.12 trace Shougang Hierro Per SA., 2003 3

Mina Justa 347 ne 0.71 0.03 3.83 Chariot Resources, 2006 3

Pampa de Pongo 953 44.7 trace Cardero Resource Corp., 2005 3

Mantos Blancos 4 500 ne 1.0 Ramrez et al., 2006

El Laco 5 500 >60 trace Rhodes et al., 1999

Chilean iron belt 6,7 2000 60 trace Oyarzun et al., 2003

Teresa de Colmo 70 ne 0.8 trace Hopper and Correa, 2000

Cerro Negro 249 ne 0.4 0.15 Sillitoe, 2003

Mantoverde 8 400 ne 0.52 0.11 Benavideset al., 2007

Santo Domingo Sur 140 ne 0.59 Far West Mining Ltd. 2007 3

Candelaria

Punta del Cobre 9

470

>120

ne

ne

0.95

1.5

0.22

0.2-0.6

3.1 Marschik and Fontbot, 2001

Marschik and Fontbot, 2001

Panulcillo ~15 ne ~ 1.45 0.1 Hopper and Correa, 2000

El Espino 30 ne 1.2 0.15 Sillitoe, 2003

1) Annual production ~5.5 Mt concentrates (2003-2005); 2) including 389 Mt ore production (1952-2002); 3)

unpublished reports; 4) classified as a hematite-rich hydrothermal breccia deposit or manto-type; 5) sulphide-barrenPliocene iron deposit; 6) including five large deposits (200-400 Mt), viz. Boquern Chaar, Los Colorados,

Algarrobo, Cristales and El Romeral; and many small deposits with 20-100 Mt; 7) Annual production ~7.3 Mt

concentrates (2003-2004; Compania Minera del Pacifico annual report, 2004); 8) Hypogene protore; 9) comprising

several discrete deposits; ne: not economic

manto, stratabound or breccia-type Cu (-Ag) deposits (Fig. 2-1A), although incorporating only

minor hematite (Ramrez et al., 2006), may be genetically related to Fe oxide rich IOCG

systems (Vivallo and Henrquez, 1997), with which they are locally juxtaposed (Orrego et al.,

2006).

IOCG deposits are defined primarily by an abundance of magnetite and/or hematite.

Whereas the latter may be unambiguously associated with Cu mineralization, as at Olympic Dam

and Mantoverde, the genetic link between magnetite and chalcopyrite deposition is tenuous in


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many centres, and it is not clear why most deposits in the Andes and elsewhere did not evolve

from a magnetite-dominated (IO) to an economically significant chalcopyritegold - rich

(CG) stage. This uncertainty in the ore-genetic modeling directly reflects controversy regarding

the nature and, particularly, sources of the hydrothermal fluids which are clearly implicated in the

Cu sulphide facies of numerous deposits. Thus, Pollard (2000, 2001, 2006), Perring et al.(2000),

Marschik and Fontbot (2001) and Sillitoe (2003) interpret IOCGs on the basis of magmatic-

hydrothermal models, and hence as broadly analogous to the magnetite-rich porphyry Cu-Au clan,

the evolution of which is controlled primarily by silicate melt-aqueous fluid equilibria. Oliveret

al. (2004) and Marshall and Oliver (2006) concur in the importance of magma-derived fluids,

although concluding that their extensive interaction with country rocks is a prerequisite for the

development of the ore-forming brines. In contrast, Barton and Johnson (1996, 2000, 2004) argue

that the global geological and paleogeographic setting and evolution of IOCG systems imply that

the incursion of exotic, in part evaporite-sourced, brines is essential to economic Cu (-Au)

mineralization. The involvement of diverse fluids has indeed been recognized in several

sulphide-rich IOCG deposits, including Olympic Dam (Haynes et al., 1995; Johnson and

McCulloch, 1995), Ernest Henry (Mark et al., 2006; Kendrick et al., 2007), Tennant Creek

(Skirrow and Walshe, 2002), Tjrrojkka (Edfelt et al.,2005) and Sossego (Monteiro et al., 2008).

Moreover, light stable isotope and fluid inclusion data specifically supporting the involvement of

evaporite- or seawater- derived brines during Cu (-Au) mineralization have been reported by

Ullrich and Clark (1999) and Ullrich et al. (2001) for La Candelaria (but cf. Marschik and

Fontbot, 2001; Pollard, 2006), by Ripley and Ohmoto (1977) and de Haller et al. (2002) for

Ral-Condestable, by Wanhainen et al. (2003) for Aitik, by Hunt et al. (2005, 2007) for the

Wernecke breccias, and by Benavides et al. (2006, 2007) for Mantoverde and its satellites.

Predictably, therefore, a commodious classification which incorporates magmatic and


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non-magmatic hydrothermal fluid origins has been recently advocated for IOCG deposits by

Williams et al.(2005).

A radically different perspective on the genesis of IOCG mineralization is provided by the

proposal that the majority of magnetite-dominated, so-called Kiruna-type (Geijer, 1931),

deposits are the product of silica-poor, iron oxide - rich melts (e.g.,Nystrm and Henrquez, 1994;

Naslund et al., 2002; Henrquez et al., 2003), the existence of which has considerable field,

petrographic and experimental support (e.g.,Park, 1961; Philpotts, 1967; Lundberg and Smellie,

1979; Lester, 2002; Clark and Kontak, 2004; Lled, 2005). Whereas the contribution of oxide

melts to IOCG genesis remains controversial (cf.Rhodes et al.,1999; Sillitoe and Burrows, 2002,

2003), Naslund et al. (2002) argue that the extensive hydrothermal alteration associated with

bodies of inferred magmatic magnetite directly reflects a high volatile content which permits

dense oxide melts to rise to the shallow crust. Moreover, although their involvement has not

been confirmed in IOCG deposits with economic Cu-Au mineralization, it is possible that oxide

melts may have been the source of either hypersaline melts generated through Cl saturation

(Broman et al., 1999) or brines released on second boiling (Naslund et al., 2002).

Our purpose herein is to contribute to this argument through documentation of the

Marcona-Mina Justa district, south-central Per, an Andean IOCG centre with both major iron

oxide and significant Cu sulphide mineralization. Representing the largest-known concentration

of high-grade magnetite ore in the Central Andes (Table 1-1), the Marcona deposit is centred in

Ica Department at Latitude 15o

12' S, Longitude 75o

7' W (Figs. 2-1 and 2-2), 10-15 km from the

Pacific coast and at elevations of below 800 m a.s.l. Hosted by Paleozoic metasedimentary and

Jurassic andesitic and sedimentary strata, it has present reserves of 1551 Mt grading 55.4 percent

Fe and 0.12 percent Cu, and had produced 358 Mt of ore by 2002 (Shougang Hierro Per SA.,

2003). Annual magnetite production in recent years has exceeded 2 Mt of magnetite concentrates,

attaining 4.5-5.0 Mt in 2005 and 2006. Cu, Co, Ni, Zn, Pb, Ag and Au are enriched in parts of the


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deposit but, with the exception of artisanal copper mining, have not been recovered. The sulphur

and copper contents of magnetite pellets are restricted to, respectively, 0.010 and 0.015 weight

percent through flotation.

3-4 km NE of the Marcona mine (Fig. 2-2), the Mina Justa Cu-(Ag) prospect has an

indicated resource of 346.6 Mt at an average grade of 0.71 percent Cu (soluble and insoluble), ~

3.8 g/t Ag and ~ 0.03 g/t Au at a cut-off grade of 0.3 percent Cu, and an inferred resource of

127.9 Mt at 0.6 percent Cu (Mining Journal, 2006). Located at Latitude 15o 10' S, Longitude 75o

5' W, the deposit was discovered by Rio Tinto Mining and Exploration and is under development

by Chariot Resources. The wider district includes (Fig. 2-1) a second giant magnetite deposit,

Pampa de Pongo, hosted largely by Jurassic andesites and intercalated sedimentary strata.

Located 30 km southeast of Marcona-Mina Justa (Fig. 2-2; Hawkes et al., 2002), this incorporates

an inferred resource of 953 Mt grading 44 percent Fe and with erratically distributed chalcopyrite

and gold (Cardero Resource Corporation, 2005). In addition to these exocontact deposits,

numerous magnetite and/or hematite-rich veins, some rich in Cu and Au, cut dioritic - to -

monzogranitic plutons of the mid-Cretaceous Coastal Batholith in the Acar - Cobrepampa district

(Fig. 2-2; Caldas, 1976; Injoque, 1985), which includes the formerly productive La Argentina

mine.

Although Hudson (1974) interpreted the Marcona magnetite deposit as a partially remobilized,

Paleozoic sedimentary ironstone, it has been generally considered to represent an epigenetic

replacement-type or skarn system (e.g.,Atchley, 1956; Atkin et al., 1985; Injoque, 1985; Vidal et

al., 1990). Injoque (2002) and Hawkes et al. (2002) assigned it to the IOCG clan but, despite

extensive study, key aspects of the mineralization have been neglected, in particular the forms

and contact relationships of the numerous orebodies which constitute the magnetite reserve.

Moreover, the heterogeneity of the metasedimentary and volcanic host-rocks has hindered the

recognition of alteration/metasomatism directly related to mineralization. We herein document in


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Figure 2-2. Geology of the Marcona-Mina Justa district (modified from Caldas, 1978; Hawkeset al., 2002)


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detail the magnetite bodies and their host-rocks, in the context of a revised paragenetic model,

40Ar/39Ar geochronology and light stable isotopic constraints on the temperatures of ore formation.

We specifically address the possible contribution of Fe oxide melts to the formation of this largest

Andean IOCG centre, and attempt to identify the timing of the magmatic-hydrothermal transition.

Complementary studies of fluid inclusion microthermometry and chemistry, and an assessment of

the hydrothermal reservoirs, will be reported elsewhere. Detailed documentation of the

paragenetic and age relationships of the Mina Justa Cu (-Ag, Au) deposit incorporates the

observations of Moody et al.(2003) and Baxter et al.(2005).

2.3 Regional and District Geological Setting

The wider Marcona-Mina Justa district (Figs. 2-1 and 2-2) exposes erosional remnants of a

succession of volcano-plutonic arcs which regionally range in age from latest-Triassic to

Holocene, evidence for a protracted but episodic history of supra-subduction zone magmatism

along the convergent margin of the South American Plate. However, the local magmatic record

is dominated by Middle Jurassic volcano-sedimentary and hypabyssal units and by

mid-Cretaceous granitoid plutons, both associated with IOCG-type mineralization. Stratigraphic

relationships in the wider Marcona area, incorporating data from Caldas (1978), Vidal et al.

(1990), Hawkes et al.(2002) and this study, are summarized in Figure 2-3.

Pre-Mesozoic units

The IOCG belt of southern Per is underlain by high-grade metamorphic rocks of the

allochthonous Paleoproterozoic-to-Mesoproterozoic (Wasteneys et al., 1995; Loewy et al., 2004)

Arequipa Massif, the northern part of the Arequipa-Antofalla basement domain, which was

accreted onto the Amazonian Craton during the ca. 1.0-1.3 Ga Grenville-Sunsas orogeny (Loewy

et al., 2004; Chew et al., 2007). Comprising schists, gneisses, granites and migmatites cut by


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basic and pegmatitic dikes, the basement complex is unconformably overlain by Neoproterozoic

and Paleozoic sedimentary strata and, more extensively, volcanic and sedimentary rocks of

Mesozoic age (Fig. 2-2; Caldas, 1978; Hawkeset al., 2002). The Neoproterozoic strata comprise

Figure 2-3. Summarised stratigraphic column for the Marcona-Mina Justa district. (modifiedafter Caldas, 1978, Injoque, 1985; Hawkes et al., 2002, and Loewy et al., 2004).


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the ca.700Ma glacial diamictites of the Chiquero Formation and the San Juan Formation, its

dolomitic cap (Chew et al., 2007). The overlying, unfossiliferous but probably Ordovician,

Marcona Formation (Caldas, 1978) hosts the majority of the economic magnetite orebodies at

Marcona. This ~ 1,500 m-thick metasedimentary package was described by Atchley (1956) as

dominated by phyllites, hornfelsic phyllites and hornfelses, but clastic tectures are widely

preserved and spaced cleavage and decussate microscopic textures are more extensively

developed than penetrative foliation. Recognizable protoliths include siltstones, sandstones and

minor quartz arenites, all dominated by quartz with lesser interstitial plagioclase, biotite, sericite

and chlorite (Fig. 2-4A). Dolomitic marble and crystalline limestone occur in the north and

southeast parts of the deposit, but are not major ore-hosts. This unit was considered by Atchley

(1956) to be overlain by a ~ 850 m-thick succession of metasedimentary and metatuffaceous

strata assigned to a Paleozoic Cerritos Formation, a term still employed on mine geological maps

(Marcona Mining Company, 1968). However, the great majority of the rocks thus described are

exceptionally rich in magnetite, actinolite and alkali feldspar, and are herein considered to

represent either metamorphosed and metasomatized Marcona Formation lithologies or, more

widely, strongly metasomatized Jurassic andesites and intercalated volcaniclastic and clastic

sediments. The Marcona Formation is intruded by monzogranites, granodiorites and

gabbro-diorites of the post-kinematic, 425 4 Ma (Mukasa and Henry, 1990; Vidal et al., 1990),

San Nicols batholith (Fig. 2-2). In the mine area metaclastic and metacarbonate members

widely exhibit cordierite + biotite muscovite (Fig. 2-4B) and tremolite quartz (Fig. 2-4C)

assemblages, respectively, but fine-grained diopside- and forsterite felses also occur in apparently

unmetasomatized sections. Nonetheless, neither Atchley (1956) nor Injoque (1985) ascribed these

apparent hornblende hornfels and pyroxene hornfels assemblages to thermal metamorphism by

the San Nicols intrusions.


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Figure 2-4. Major host-rocks of the Marcona and Mina Justa deposits. A - Weakly-altered

Marcona Formation arkosic siltstone: fine-grained original quartz clasts (>75%) and finer-grainedinterstitial plagioclase, with minor secondary chlorite and sericite (MA3-4-A; Mina 3 open pit,south wall, 580 m; transmitted light, crossed nicols). B- Cordierite + biotite muscovite hornfels,Marcona Formation (MA3-4-B; Mina 3 open pit, south wall, 580 m; plane-polarized transmittedlight). Cordierite occurs as ovoid poikiloblasts with muscovite + quartz envelopes. Biotite isdistributed in matrix. These assemblages commonly form mafic laminae in arkosic siltstone. C-Tremolite quartz in metacarbonate horizon of Marcona Formation. Late magnetite + actinoliteassemblage replaced metamorphic tremolite and residual calcite (DDM4-6-1; drill hole DDM4-6;124 m; transmitted light, crossed nicols). D- Amygdular Rio Grande Formation andesite hostingthe Mina Justa orebodies, with vesicles filled by chlorite and carbonates. Plagioclase (An30) andhornblende occur as phenocrysts (MJ-4; surface sample; trench in the upper zone; scannedpolished-thin section). E- Ocite dike cutting Mina Justa orebodies. Plagioclase (An~50; 1.5 cm)

and hornblende occur as phenocrysts, with intense sericite-chlorite alteration. The matrix iscomposed by fine-grained plagioclase, quartz, augite, hornblende and minor magnetite (MJ-2;surface sample; trench in the upper zone; plane-polarized transmitted light). F- Dacite porphyryat Marcona. Plagioclase (An30-40) commonly occurs as phenocrysts. Hornblende, quartz andfine-grained plagioclase constitute the groundmass (MA7-10; Mina 7 open pit, NW wall, 680 m;transmitted light, crossed nicols)


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Mesozoic Stratigraphy

The Marcona iron deposit is, at least in part, of mid-Jurassic age (K-Ar data of Injoque, 1985) and

is partially hosted by Middle Jurassic shallow-marine sedimentary and volcanic strata. The

relative ages of these units and the mineralization are therefore critical to an understanding of the

environment of ore formation. Caldas (1978) comprehensively documented the Mesozoic

stratigraphic relationships in the San Juan 10,000 quadrangle, in which both Marcona and Mina

Justa lie, and in the contiguous Acar and Yauca quadrangles. The Jurassic and Cretaceous

strata of the wider Marcona area are subdivided (Figs. 2-2 and 2-5), with decreasing age, into the

Ro Grande, Jahuay, Yauca and Copara Formations (Caldas, 1978). The ages of the three older

formations are well established on faunal grounds, but those of the Copara Formation and the

dominantly hypabyssal andesitic-dacitic Bella Unin complex which intrudes it, as well as the

post-Yauca Formation hypabyssal Tunga Andesite, are poorly defined (Caldas, 1978).

The Ro Grande Formation hosts the Mina Justa deposit and several orebodies of the

Marcona mine (Injoque, 1985; Hawkes et al., 2002; Moodyet al., 2003). The type-section of

this ~ 3000 to 4000 m-thick, generally NE-striking and NW-dipping, succession is exposed in the

Monte Grande area in the Can Ro Grande, NW of Marcona (Fig.2-2; Regg, 1956, 1961). It

incorporates (Fig. 2-5) a 500 m lower member comprising a polymictic basal conglomerate

overlain successively by mudstones, sandstones, limestones, rhyolitic to andesitic breccias and

rhyolitic to andesitic flows (Romeuf et al., 1993). This association, probably corresponding to

Atchleys (1956) Cerritos Formation at Marcona (Fig. 2-5), is overlain by at least 2000 m of

gently folded red sandstones, shales, limestones and brecciated andesitic flows with high-K

calc-alkaline to shoshonitic compositions (Fig. 2-4D; Aguirre, 1988). The formation is

quasi-pervasively affected by non-deformational zeolite facies metamorphism (Aguirre and

Offler, 1985; Aguirre, 1988).


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Figure 2-5. Schematic stratigraphic columns of the Ro Grande Formation in the Can RoGrande, Marcona and Pampa de Pongo areas (Atchley, 1956; Regg, 1956; Caldas, 1978; Injoque,1985; Aguirre, 1988).

Faunal assemblages (Cox, 1956; Regg, 1956) define a Middle Jurassic (Dogger) age for the

lower, largely sedimentary, member of the formation, and the occurrence of planammatoceras

and hammatoceras (W.J. Arkell, inRegg, 1956) and theBredya manflasensis faunal assemblage

(Roperch and Carlier, 1992) indicates that sedimentation was underway in the Aalenian,i.e., prior

to 174.0+1.0-7.9 Ma and after 178.0

+1.0-1.5 Ma (Plfy et al., 2000). Ro Grande Formation volcanism

persisted into the Oxfordian (Cox, 1956; Regg, 1956; Caldas, 1978)i.e., ca.156.5+3.1-5.1 to 154.7


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+3.8-3.3 Ma (Plfyet al., 2000), but sedimentation and volcanism in the upper part of the formation

were apparently interrupted between ca. 166 and 164 Ma (Fig. 2-5). The very low-grade

metamorphism affects andesitic units above this unconformity, and therefore either took place, or

resumed, after the Bathonian.

In the Pampa de las Treinta Libras area NE of Marcona, the overlying ~ 1,000-m - thick,

NW-striking, Jahuay Formation (Regg, 1961; Figs. 2-2 and 2-3) incorporates a basal, largely

andesitic succession, overlain by a sequence of limestones and sandstones, the latter exhibiting

lateral transitions to conglomerates, intercalated with dacitic porphyry flows and cut by sills and

plugs of andesite. The upper members of the formation are dominantly shales and limestones.

Faunal assemblages extend from the lower to the upper Tithonian,i.e., 141.8+2.5-1.8to 150.5

+3.4-2.0Ma

(Plfy et al., 2000), but sedimentation may have begun earlier in the late Kimmeridgian (Caldas,

1978). Variable discordance with the underlying Ro Grande Formation is interpreted by Caldas

(1978) as the result of the development of a tectonic dome, centred on the Marcona mine area and

recorded by thick conglomerates in the Jahuay Formation and the local absence of Tithonian

strata. The Jahuay Formation accumulated in the NW-trending pre-Andean depression (Caldas,

1978).

The succeeding Neocomian Yauca Formation crops out (Fig. 2-2) to the east of the Marcona

deposit and comprises 1,500 m of strongly-faulted shales, mudstones and sandstones (Caldas,

1978; Injoque, 1985; Hawkes et al., 2002). The ~ 1000 m thick, probably Aptian to lower Albian,

Copara Formation, unconformably overlying the Yauca Formation, is composed of

conglomerates with volcanic clasts, feldspathic sandstones, graywackes and shales (Caldas, 1978;

Injoque, 1985), underlying a dominantly intermediate pyroclastic sequence, all intruded by the

andesitic Bella Unin complex. The Copara Formation accumulated in the apparent southern

extremity of the Caete Basin (Atherton and Aguirre, 1992). This mid-Cretaceous, plate


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boundary-parallel, intra-arc extensional feature is, to the northwest, infilled by calc-alkaline,

high-alumina and K-rich andesites and dacites.

The dike swarms, sills and small plugs assigned to the Tunga Andesite intrude the Yauca

Formation and older units (Caldas, 1978; Fig. 2-3). The most characteristic lithology is a coarsely

porphyritic rock with large ( 1.5 cm) glomerocrysts of labradorite and sparse augite

phenocrysts, termed ocites by Rio Tinto geologists (Hawkes et al., 2002) by analogy with the

broadly contemporaneous, strikingly porphyritic andesites of the Ocoa Formation in the Copiap

area of northern Chile (Thomas, 1958). Essentially identical textures are, however, shown by

several Ro Grande Formation andesitic flows in the Mina Justa area, a potential source of

stratigraphic confusion.

Coastal Batholith

Granitoid plutons of the Coastal Batholith (Pitcher et al., 1985) intrude Neocomian and older

strata in the Acar-Cobrepampa area (Fig. 2-2; Dunin-Borkowski, 1970; Caldas, 1978). U-Pb

zircon age data are lacking for this part of the Arequipa segment of the batholith, but K-Ar

(Cobbing, 1998) and Rb-Sr (Snchez, 1982) dates for, respectively, the Acar diorite and

Cobrepampa monzonite-monzogranite suggest that large-scale intrusion locally began at ca.109

4 Ma, shortly after emplacement of the Bella Unin complex. Small, undated, dioritic stocks,

7-8 km ESE and SE of the Mina Justa prospect (Caldas, 1978), may be correlative with the larger

intrusions to the east.

Cenozoic stratigraphy and landforms

The Mesozoic and older strata and intrusions are overlain discordantly by a Tertiary sequence

locally assigned to the Pisco, Millo and Sencca Formations (Fig. 2-2). The 450 m-thick, Miocene

to Pliocene, Pisco and Millo Formations, composed of shallow-water marine sediments and


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subordinate fine-grained volcaniclastics (Caldas, 1978; Hawkes et al., 2002), extend from the

Pisco basin in the north (Devries, 1998) to the south of Lomas (Fig. 2-2), where they contain

abundant whale fossils (Brand et al., 2004). The Lower Miocene volcaniclastics of the Pisco

Formation record an early Neogene pulse of volcanic activity recognized throughout the Central

Andes (Noble et al., 1990). A vestigially preserved, 50m, rhyodacitic ash-flow tuff, assigned

arbitrarily to the Sencca Formation (Caldas, 1978; Injoque, 1985), gives a 40Ar/39Ar biotite

plateau age of 9.13 0.25 (2 ) Ma (Quang et al., 2001). This unit overlies the Loma de Marcona

Surface, the youngest of four regionally developed erosional pediments preserved in the district.

Subsequently, the area was depressed below sea-level, blanketed by uppermost Pliocene beach

deposits (Ortlieb and Machar, 1990), and again uplifted, a process recorded by a remarkable

series of tectonically-controlled marine terraces (Broggi, 1946), the younger of which formed in

response to the local subduction of the aseismic Nazca Ridge.

2.4 The Marcona Magnetite Deposit

The Marcona mine now comprises eight open pits in a ~ 25 km2area elongated from WNW to

ESE (Fig. 2-2). Traditionally, a crudely en echelon array of 12 major magnetite orebodies

(minas) and 55 smaller cuerpos are recognized (Fig. 2-6). However, the three zones

exploited by the largest, 3 km-long pit, viz. Mina 2, Mina 3 and Mina 4, represent interconnected

segments of a single orebody (Table 2-2). Approximately 60 per cent of the reserve, comprising

the so-called E-grid orebodies, is hosted by the Marcona Formation (Fig. 2-4B), and the

remainder, the N-13 type orebodies, by the lower members of the Ro Grande Formation (i.e.,

Atchleys (1956) Cerritos Formation; Fig. 2-7). The hypogene grades (Table 2-2) of the larger

orebodies hosted by the Paleozoic metasediments average 57-58 per cent Fe, significantly

exceeding those of 41-48 per cent for the orebodies in Jurassic strata. Whereas the total sulphur

content of the mineralization is consistent at ~ 3 wt. percent, the copper content is more variable,


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Figure 2-6.Geology of the area surrounding the Marcona deposit and Mina Justa prospect. Line A-A illustrates tthrough the Marcona mine (Modified from Rio Tinto, Marcona JV exploration report, June 2003). Insert shows ar


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Figure 2-7.Schematic cross-section of Marcona mine area (A-A in Figure 2-6). Ornaments as inFigure 2-6. The magnetite orebodies are extensively dislocated by faults (modified from Hawkeset al., 2002).

Table 2-2. Selected Tonnage/Grade Data for Marcona Orebodies 1

orebody Minas 2-3-4 Mina 5 Mina 7 Minas 9-10 Mina 14 Mina 11 Mancha N-13

Host rock Marcona

Fm.

Marcona

Fm.

Marcona

Fm.

Marcona

Fm.

Chen Huayong-200805 PhD d

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