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Structural controls on the superposition of high

sulfidation epithermal mineralisation into porphyry

copper-molybdenum deposits: lessons from

Rosario, 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,

[email protected]

* 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 worlds largest accumulation

of porphyry-related copper metal. Conflicting models exist regarding the relative importance

of 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 the

Collahuasi district, high sulfidation state copper-silver mineralisation was superimposed into

the core of the Rosario copper-molybdenum porphyry deposits along normal faults during

gravitational collapse of the Domeyko Cordillera. Exhumation of the porphyry environment

occurred rapidly during this event, allowing near-surface epithermal mineralisation (< 200m

paleodepth) 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 wasting

after major episodes of tectonic uplift provides an effective method of hypogene upgrading of

porphyry ores by high sulfidation mineralisation.

Introduction

Genetic relationships between porphyry and high sulfidation state (HS) epithermal mineralisation

are well-established (e.g., Arribas et al., 1995; Hedenquist et al., 1998). However, reasons as to

why some porphyry and HS deposit couplets are separated spatially, whereas others are

superimposed 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 and

Rosario, together with other major deposits such as El Abra, El Salvador, Radomiro Tomic, El

Abra, Mansa Mina, Toki, Gaby and La Fortuna (Fig. 1). Several of the largest deposits are

hybrid porphyry-epithermal systems, with high sulfidation state mineralisation superimposed

into the potassically altered core of the porphyry deposit (e.g., Chuquicamata, Rosario, La

Escondida, El Salvador; Fig. 1). Northern Chile is therefore an ideal location to investigate

likely mechanisms of superposition (or telescoping) of the epithermal environment into the

core of the porphyry system.


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Geological setting

Since the Paleozoic, the geological evolution of northern Chile has included periods of terrain

accretion, passive margin and back-arc basin sedimentation, protracted subduction and arc

magmatism. Intermittent episodes of magmatic activity have occurred since the Jurassic, withthe principal magmatic arc migrating eastwards from the Coastal Cordillera (Jurassic) to the

Longitudinal Valley (Cretaceous), Precordillera (Eocene-Oligocene) and Western Cordillera

(Miocene-Recent).

Late Cretaceous-Early Tertiary volcanism was terminated by the Incaic orogeny. The Incaic

orogeny marks a change in the tectonic regime from extensional to strong orogeny-normal

shortening (Scheuber and Reutter, 1992). The change in deformation regime is interpreted to be

the 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 angle

of convergence along the South American margin.

Figure 1. Map showing the location of the Collahuasi district relative to other major copper and gold

deposits in Chile and western Argentina. Metallogenic belts for the five major copper provinces are

also shown. Dashed contour lines are the depths to the Wadati-Benioff zone. Modified from Muntean

and 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. The

older, 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 million

years resulted in the overprinting of the back-arc environment by a magmatic arc, followed by

subsequent forearc environment. Interaction of the back-arc architecture, together with earlier

architectures (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 evident

within 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 from

the type locality at the Chuquicamata Mine where a single branch of this fault system defines

the 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-Oligocene

porphyries of northern Chile. The northern portion of the Domeyko Fault System represents

the reactivated eastern margin of the main Jurassic back-arc basin, and hosts a complex set of

broadly N-S structures. Eocene-Oligocene magmatism was structurally focussed and attaining

greatest volumes where the Domeyko Fault System intersects other structures, either transverse

transfer 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 Fissure

controlled the emplacement of the Chuquicamata porphyry deposit. The plate convergence

vector provided by Pardo-Casas and Molnar (1987), based on plate reconstructions from

prominent ocean floor magnetic anomalies, suggests a constant ENE-directed convergence from

49 Ma, with a minor change to ESE between 26-20 Ma. This convergence direction should

have translated into a dominantly dextral sense of strike-slip movement on the Domeyko Fault

System. However, movement sense indicators on the fault system show a highly variable sense

of 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) have

argued that the convergence angle was too high for major strike-slip fault movements along the

Domeyko Fault System during the Eocene-Oligocene. Instead, they have shown that the

Domeyko 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 the

supergiant 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 epithermal

copper-silver veins (including La Grande and Rosario), the intermediate sulfidation state

Montcezuma 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 the

peak 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 exploration

in the latter half of the twentieth century, although significant difficulties were encountered in

bringing 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. The

outline of copper mineralization at the Rosario and Quebrada Blanca porphyry centres is shown, as

well 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 from

Masterman 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) and

Ujina 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 superimposed

into the core of the porphyry system, resulting in significant hypogene upgrading of the porphyry

ore (Masterman et al., 2005). This process produced reverse alteration zonation with central

domains of advanced argillic and phyllic alteration that have overprinted more laterally extensive

potassic and propylitic alteration zones. Similar alteration patterns have been reported at

Chuquicamata (Ossandon et al., 2001).

The Rosario porphyry was emplaced at 34.4 0.4 Ma (Masterman et al., 2004). Potassic alteration

and the mineralised quartz vein stockwork formed at depths of around 1,300 m below the

paleosurface, based on fluid inclusion results (Masterman et al., 2005). Exhumation of the

porphyry system allowed for superposition of massive sulfide epithermal veins and advanced

argillic alteration into the core of the porphyry system at 32.6 0.3 Ma. The epithermal veins

at Rosario formed at depths of approximately 200 m below the paleosurface, implying that at

least 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 that

it was emplaced as the Domeyko Cordillera underwent gravitational collapse. Gravitational sliding

along normal faults, such as the Rosario Fault, potentially accelerated exhumation and helped to

promote telescoping of the high-sulfidation environment onto the Rosario Porphyry (Fig. 3b).

Figure 3. Northeast-southwest schematic

section showing a model of divergent

gravitational collapse inferred to have affected

the Collahuasi district. a) Most of the late

Eocene shortening was accommodated by

isoclinal folds in the Mesozoic sedimentary and

volcanic rocks. Note that the Permian basement

was uplifted relative to the Mesozoic sequences

along deep which may have included the

Domeyko and Loa fault systems. Thin-skinned

deformation (e.g., reverse faults) was

accommodated along low-angle thrusts and

inverted basin-margin faults. Magmas ascended

from a mixing, assimilation, storage and

homogenization (MASH) zone at the base of the

crust to levels of neutral buoyancy in the middle-

to-upper crust. They did not erupt, but

crystallized and produced high-level, intrusion-

centred brittle-ductile veins (e.g., the early-stage

veins at Rosario). b) Partial collapse of the

orogenic belt is inferred to have occurred at theend of the Incaic Orogeny. Crustal units were

detached along gravity slides that were

potentially connected to thrusts in the foreland.

Detritus from erosion was either collected in

basins above the detachments or transported

out of the system. Exhumation changed the

environment from lithostatic to hydrostatic at the site of ore formation and coincided with formation of

intermediate and late-stage veins at Rosario. That porphyry and superimposed high-sulfidation style

mineralization occur at the same crustal level implies protracted intrusive activity at Rosario and the

existence of a well-developed and replenished MASH zone at the base of the crust. Adapted for the

Collahuasi 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 resources

throughout 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 Ore

Deposit Research (CODES), AMIRA International, CSIRO Exploration and Mining and Compaia

Minera Doa Ins de Collahuasi (CMDIC) for providing financial, logistical and technical support.

Manuel Durn is thanked for approving and funding the work at Collahuasi. We appreciate

permission to publish from AMIRA International and CMDIC. Thanks also to Jorge Skarmeta

from CODELCO for numerous insights into the structural evolution of northern Chile.

References

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Skarmeta, J., McClay, K. and Bertens, A., 2003, Structural controls on porphyry copper deposits in northern

Chile: New models and implications for Cu-Mo mineralization in subduction orogens [abs.], in Dcimo

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Author

David Cooke is an Associate Professor and leader of the Ore Formation research program at

CODES, the Australian Research Councils Centre of Excellence in ore deposits at the University

of Tasmania. He gained a BSc(hons) from La Trobe University and a PhD from Monash

University. 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 Lindsley

lecturer for the Society of Economic Geologists.

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