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Structural controls on the superposition of highsulfidation epithermal mineralisation into porphyry

copper-molybdenum deposits: lessons fromRosario, northern Chile

D.R. Cooke1, G.J. Masterman*1, R.F. Berry, J.L. Walshe2 and P.A. Gow**2

1 CODES, University of Tasmania, Private Bag 79, Hobart, TAS 7001, Australia,d.cooke@utas.edu.au

* current address: Bolnisi Gold NL, Chihuahua, Mexico

2 CSIRO Exploration and Mining, 26 Dick Perry Av, Kensington, Western Australia 6151

** current address: XStrata Exploration, Mt Isa, Australia

Abstract

The Eocene-Oligocene porphyry belt of northern Chile contains the world’s largest accumulationof porphyry-related copper metal. Conflicting models exist regarding the relative importanceof strike-slip and reverse faulting during the emplacement of the porphyry and epithermal systems,based on regional studies and also from work at the Chuquicamata deposit. In contrast, at theCollahuasi district, high sulfidation state copper-silver mineralisation was superimposed intothe core of the Rosario copper-molybdenum porphyry deposits along normal faults duringgravitational collapse of the Domeyko Cordillera. Exhumation of the porphyry environmentoccurred rapidly during this event, allowing near-surface epithermal mineralisation (< 200mpaleodepth) to be juxtaposed into the potassic altered core of the porphyry deposit (estimateddepth of formation: 1300 m) in the space of approximately one million years. Mass wastingafter major episodes of tectonic uplift provides an effective method of hypogene upgrading ofporphyry ores by high sulfidation mineralisation.

Introduction

Genetic relationships between porphyry and high sulfidation state (HS) epithermal mineralisationare well-established (e.g., Arribas et al., 1995; Hedenquist et al., 1998). However, reasons as towhy some porphyry and HS deposit couplets are separated spatially, whereas others aresuperimposed into the same space remain obscure.

Many giant Eocene-Oligocene porphyry copper-molybdenum deposits occur in the DomeykoCordillera of northern Chile, including behemoths such as Chuquicamata, La Escondida andRosario, together with other major deposits such as El Abra, El Salvador, Radomiro Tomic, ElAbra, Mansa Mina, Toki, Gaby and La Fortuna (Fig. 1). Several of the largest deposits arehybrid porphyry-epithermal systems, with high sulfidation state mineralisation superimposedinto the potassically altered core of the porphyry deposit (e.g., Chuquicamata, Rosario, LaEscondida, El Salvador; Fig. 1). Northern Chile is therefore an ideal location to investigatelikely mechanisms of superposition (or telescoping) of the epithermal environment into thecore of the porphyry system.

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Geological setting

Since the Paleozoic, the geological evolution of northern Chile has included periods of terrainaccretion, passive margin and back-arc basin sedimentation, protracted subduction and arcmagmatism. Intermittent episodes of magmatic activity have occurred since the Jurassic, withthe principal magmatic arc migrating eastwards from the Coastal Cordillera (Jurassic) to theLongitudinal Valley (Cretaceous), Precordillera (Eocene-Oligocene) and Western Cordillera(Miocene-Recent).

Late Cretaceous-Early Tertiary volcanism was terminated by the Incaic orogeny. The Incaicorogeny marks a change in the tectonic regime from extensional to strong orogeny-normalshortening (Scheuber and Reutter, 1992). The change in deformation regime is interpreted to bethe result of a change in plate configuration in the south east Pacific between 110 and 70 Ma.Southward migration of the Aluk-Farallon spreading centre resulted in a reduction in the angleof convergence along the South American margin.

Figure 1. Map showing the location of the Collahuasi district relative to other major copper and golddeposits in Chile and western Argentina. Metallogenic belts for the five major copper provinces arealso shown. Dashed contour lines are the depths to the Wadati-Benioff zone. Modified from Munteanand Einaudi (2000) and Masterman et al. (2005).

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Porphyry and high sulfidation mineralisation formed between 42 and 31 Ma in the Precordillera,with the giant porphyry deposits forming towards the end of this metallogenic epoch. Theolder, smaller deposits in the southern end of the belt (El Salvador, Potrerillos; Fig. 1) are gold-enriched relative to their northern counterparts.

Migration of the northern Chilean magmatic arc 100-280 km to the east over the last 200 millionyears resulted in the overprinting of the back-arc environment by a magmatic arc, followed bysubsequent forearc environment. Interaction of the back-arc architecture, together with earlierarchitectures (Palaeozoic terrane boundaries, sutures, etc) with the magmatic arc during the Eocene-Oligocene was fundamental to the formation of the Eocene-Oligocene porphyry province.

Domeyko Fault System

A variety of terms have been used to describe the dominant N-S trending fault system evidentwithin the Precordillera of northern Chile. These include the West Fissure (Falla Oeste in Spanish),Domeyko Fault System, and West Fissure Fault System. The terminology ‘west’ is derived fromthe type locality at the Chuquicamata Mine where a single branch of this fault system definesthe western margin of the ore deposit (e.g., Lindsay et al., 1995; Ossandon et al., 2001).

The Domeyko Fault System has in part controlled emplacement of the Eocene-Oligoceneporphyries of northern Chile. The northern portion of the Domeyko Fault System representsthe reactivated eastern margin of the main Jurassic back-arc basin, and hosts a complex set ofbroadly N-S structures. Eocene-Oligocene magmatism was structurally focussed and attaininggreatest volumes where the Domeyko Fault System intersects other structures, either transversetransfer structures, or N-S thrusts (inverted syn-sedimentary normal faults?)

Lindsay et al. (1995), among others, argued that strike-slip fault movements on the West Fissurecontrolled the emplacement of the Chuquicamata porphyry deposit. The plate convergencevector provided by Pardo-Casas and Molnar (1987), based on plate reconstructions fromprominent ocean floor magnetic anomalies, suggests a constant ENE-directed convergence from49 Ma, with a minor change to ESE between 26-20 Ma. This convergence direction shouldhave translated into a dominantly dextral sense of strike-slip movement on the Domeyko FaultSystem. However, movement sense indicators on the fault system show a highly variable senseof movement. Notably the sense of movement commonly interpreted as associated withmineralisation is sinistral (Reutter et al., 1996).

In contrast to the strike-slip models, McClay et al. (2002) and Skarmeta et al. (2003) haveargued that the convergence angle was too high for major strike-slip fault movements along theDomeyko Fault System during the Eocene-Oligocene. Instead, they have shown that theDomeyko Fault System is dominated by thrust faults, some of which have reactivated basin-bounding normal faults associated with Jurassic back arc sedimentation.

Collahuasi district

A cluster of Eocene-Oligocene porphyries occur in the Collahuasi district (Fig. 2), including thesupergiant Rosario (3.11 Gt @ 0.82 % Cu, 0.024 % Mo and 0.01 g/t Au), and the giant Ujina(636 Mt @ 1.06 % Cu), and Quebrada Blanca porphyry deposits (400 Mt @ 0.83 % Cu, 0.015% Mo and 0.1 g/t Au; Camus, 2002). The district also contains high sulfidation state epithermalcopper-silver veins (including La Grande and Rosario), the intermediate sulfidation stateMontcezuma epithermal silver vein system and the Huinquintipa exotic copper deposit.

Epithermal veins in the Collahuasi district have been mined since at least 1400 AD, with thepeak of epithermal mining activity occurring between 1907 and 1920 (Moore and Masterman,2002). The porphyry potential of the district was realised with the advent of modern explorationin the latter half of the twentieth century, although significant difficulties were encountered inbringing the mines into production. Claims staked in the late 1950s resulted in the discovery of

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Figure 2. Generalized geology of the Rosario, Cerro La Grande and Quebrada Blanca areas. Theoutline of copper mineralization at the Rosario and Quebrada Blanca porphyry centres is shown, aswell as vein-hosted Cu-Ag-(Au) massive sulfide occurrences at Poderosa and Cerro La Grande. High-grade silver occurs in a laminated intermediate-sulfidation quartz vein at Monctezuma. Modified fromMasterman et al. (2005) after Munchmeyer et al. (1984).

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the supergene-enriched Quebrada Blanca deposit in 1977 (production commenced in 1994).Subsequent exploration resulted in the discovery of Rosario in 1979 (first mined in 2002) andUjina in 1991 (initial production in 1998).

Hypogene upgrading at the Rosario Porphyry Cu-Mo Deposit

At the Rosario porphyry deposit, high sulfidation epithermal veins have been superimposedinto the core of the porphyry system, resulting in significant hypogene upgrading of the porphyryore (Masterman et al., 2005). This process produced ‘reverse alteration zonation’ with centraldomains of advanced argillic and phyllic alteration that have overprinted more laterally extensivepotassic and propylitic alteration zones. Similar alteration patterns have been reported atChuquicamata (Ossandon et al., 2001).

The Rosario porphyry was emplaced at 34.4 ± 0.4 Ma (Masterman et al., 2004). Potassic alterationand the mineralised quartz vein stockwork formed at depths of around 1,300 m below thepaleosurface, based on fluid inclusion results (Masterman et al., 2005). Exhumation of theporphyry system allowed for superposition of massive sulfide epithermal veins and advancedargillic alteration into the core of the porphyry system at 32.6 ± 0.3 Ma. The epithermal veinsat Rosario formed at depths of approximately 200 m below the paleosurface, implying that atleast 1 km of rock was eroded at Rosario over a period of approximately 1 m.y.

The Rosario Porphyry intruded immediately after the Incaic tectonic phase (Fig. 3a), implying thatit was emplaced as the Domeyko Cordillera underwent gravitational collapse. Gravitational slidingalong normal faults, such as the Rosario Fault, potentially accelerated exhumation and helped topromote telescoping of the high-sulfidation environment onto the Rosario Porphyry (Fig. 3b).

Figure 3. Northeast-southwest schematicsection showing a model of divergentgravitational collapse inferred to have affectedthe Collahuasi district. a) Most of the lateEocene shortening was accommodated byisoclinal folds in the Mesozoic sedimentary andvolcanic rocks. Note that the Permian basementwas uplifted relative to the Mesozoic sequencesalong deep which may have included theDomeyko and Loa fault systems. Thin-skinneddeformation (e.g., reverse faults) wasaccommodated along low-angle thrusts andinverted basin-margin faults. Magmas ascendedfrom a mixing, assimilation, storage andhomogenization (MASH) zone at the base of thecrust to levels of neutral buoyancy in the middle-to-upper crust. They did not erupt, butcrystallized and produced high-level, intrusion-centred brittle-ductile veins (e.g., the early-stageveins at Rosario). b) Partial collapse of theorogenic belt is inferred to have occurred at theend of the Incaic Orogeny. Crustal units weredetached along gravity slides that werepotentially connected to thrusts in the foreland.Detritus from erosion was either collected inbasins above the detachments or transportedout of the system. Exhumation changed theenvironment from lithostatic to hydrostatic at the site of ore formation and coincided with formation ofintermediate and late-stage veins at Rosario. That porphyry and superimposed high-sulfidation stylemineralization occur at the same crustal level implies protracted intrusive activity at Rosario and theexistence of a well-developed and replenished MASH zone at the base of the crust. Adapted for theCollahuasi district from a diagram in Rey et al. (2001). Modified after Masterman et al., (2005).

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This mechanism has proven highly effective at generating giant, high-grade hypogene resourcesthroughout northern Chile, and should be a focus for exploration in other porphyry provinces.

Acknowledgments

This study was part of AMIRA International project P511. We are grateful to the Centre for OreDeposit Research (CODES), AMIRA International, CSIRO Exploration and Mining and CompañiaMinera Doña Inés de Collahuasi (CMDIC) for providing financial, logistical and technical support.Manuel Durán is thanked for approving and funding the work at Collahuasi. We appreciatepermission to publish from AMIRA International and CMDIC. Thanks also to Jorge Skarmetafrom CODELCO for numerous insights into the structural evolution of northern Chile.

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Author

David Cooke is an Associate Professor and leader of the Ore Formation research program atCODES, the Australian Research Council’s Centre of Excellence in ore deposits at the Universityof Tasmania. He gained a BSc(hons) from La Trobe University and a PhD from MonashUniversity. David and his students have been researching porphyry and epithermal mineraldeposits from around the Pacific rim for the past twenty years. David is the 2005 Thayer Lindsleylecturer for the Society of Economic Geologists.