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Introduction

A GROUP of iron oxide-rich Cu-Au(-Zn-Ag) deposits defines abelt along the eastern margin of the composite coastal

batholith, southeast of Copiap, Chile (Fig. 1, Table 1). Thisbelt, referred to as the Punta del Cobre belt, includes theCandelaria deposit with mineable reserves of 470 Mt at 0.95percent Cu, 0.22 g/t Au, and 3.1 g/t Ag (Marschik et al., 2000),the Punta del Cobre district, and several middle- and small-sized mines with estimated combined reserves plus produc-tion of >120 Mt at 1.5 percent Cu, 0.2 to 0.6 g/t Au, and 2 to

The Candelaria-Punta del Cobre Iron Oxide Cu-Au(-Zn-Ag) Deposits, Chile

ROBERT MARSCHIK,*

Lehrstuhl fr Lagerstttenlehre, Institut fr Mineralogie, TU Bergakademie Freiberg,Brennhausgasse 14, 09596 Freiberg/Sachsen, Germany

AND LLUS FONTBOT

Section des Sciences de la Terre, Universit de Genve, Rue des Marachers 13, 1211 Genve 4, Switzerland

Abstract

Several iron oxide-rich Cu-Au(-Zn-Ag) deposits define an approximately 5-km-wide and at least 20-km-longbelt along the eastern margin of the coastal batholith near Copiap, Chile. This belt includes the large Cande-laria mine and a group of middle- and small-sized mines in the Punta del Cobre district, which is located about3 km northeast of the Candelaria deposit. Estimated geologic resources of the belt are on the order of 700 to800 million metric tons (Mt) at 1.0 percent Cu. The ore occurs in veins, breccia, and stringer bodies, and in re-placement bodies that are roughly concordant to bedding. The orebodies are hosted mainly by volcanic andvolcaniclastic rocks of the Punta del Cobre Formation and, in places, also occur in volcaniclastic intercalationsin the lower part of the overlying Early Cretaceous Chaarcillo Group. Most of the larger orebodies in the belt

are located where northwest-trending brittle faults intersect the contact between massive volcanic and vol-caniclastic rocks. These northwest faults and a major northeast-trending ductile shear zone control portions ofthe ore of the Candelaria deposit.

Chalcopyrite is the only hypogene Cu mineral. The Cu-Au ore is characterized by abundant magnetiteand/or hematite and by locally elevated concentrations of Ag, Zn, Mo, and light rare earth elements. The oreis hosted mainly in zones with biotite-potassium feldspar calcic amphibole epidote alteration at Candelaria.In the Punta del Cobre district, ore in the deeper parts of the deposits is similarly associated, whereas at shal-low levels it occurs in zones of biotite-potassium feldspar, or albite-chlorite calcite alteration.

Mineralization at Candelaria-Punta del Cobre took place under relatively oxidized conditions manifested bythe formation of specular hematite. In parts of the district, the pseudomorphic replacement of early specularhematite by magnetite during the main iron oxide formation marks a shift toward more reduced conditions orhigher temperatures. The bulk of the magnetite probably formed at temperatures of about 500 to 600C. Themain sulfide stage followed with formation of pyrite and chalcopyrite at temperatures of >470 to 328C. Sub-sequent martitization of the magnetite points to a temperature decrease. Cooling of the hydrothermal systemis also indicated by the homogenization temperatures of 236C of saline fluid inclusions in late-stage calcite.

Oxygen isotope combined with microthermometric data suggest that magmatic fluids or nonmagmatic fluidsequilibrated with magmatic silicates were dominant during the main copper mineralization. Relatively light oxy-gen isotope signatures of fluids in equilibrium with late calcite suggest mixing with a nonmagmatic fluid (e.g.,basinal brines or meteoric waters) during the late stages of hydrothermal activity. Sulfur isotope ratios of chal-copyrite, pyrite, pyrrhotite, and sphalerite from the Bronce, Candelaria, Las Pintadas, Santos, and SocavnRampa deposits range from 34SCDTvalues of 0.7 to +3.1 per mil. This narrow range of sulfur isotope ratiosnear 0 per mil is consistent with sulfur of magmatic origin. Anhydrite from the Candelaria mine parageneticallyoverlaps with chalcopyrite. Fluid inclusions in this anhydrite homogenize between 340 and 470C and it has34SCDTvalues between 14.5 and 17.5 per mil. A sulfate-sulfidevalue of 13.4 per mil for a sample with coexistinganhydrite and chalcopyrite is consistent with sulfide-sulfate fractionation at temperatures on the order of 400C.Ore lead isotope signatures are homogeneous and similar to those of least altered volcanic host rocks and nearbyintrusive rocks. Radiometric ages, including new 40Ar/39Ar ages for hydrothermal alteration at Candelaria, pointto a main Cu-Au mineralization event at Candelaria-Punta del Cobre at around 115 Ma. The ages indicate thatore formation was broadly coeval with batholithic granitoid intrusions and with regional uplift. They furtherimply that the Cu-Au(-Zn-Ag) deposits formed at shallow crustal levels (


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3 km northeast of Candelaria (Figs. 1 and 2). These mines to-gether with Candelaria display mineralization and associatedalteration features that are similar to those found in depositsof the iron oxide (Cu-U-Au-REE) class as defined by Hitz-man et al. (1992) based on Proterozoic examples. Genetic hy-potheses proposed to explain the formation of the deposits ofthis class include hydrothermal models invoking magmatic

fluid-dominated (e.g., Gow et al., 1994; Rotherham et al.,1998; Williams, 1998; Williams et al., 1999; Pollard, 2000) orsaline nonmagmatic fluids (e.g., Battles and Barton, 1995;Haynes et al. 1995; Barton and Johnson, 1996, 2000; Bartonet al., 1998). Metallogenic aspects and models for the ore for-mation at Candelaria and/or Punta del Cobre have been dis-cussed, e.g., by Camus (1980), Hopf (1990), Ryan et al. (1995),Marschik and Fontbot (1996), Ullrich and Clark (1999),Marschik et al. (2000), and Mathur et al. (2002). We present asummary of field and analytical data on the Candelaria depositand the Punta del Cobre district. We discuss the mineralogy,

paragenetic sequence, and the distribution of the principal al-teration assemblages at the district scale and present a com-pilation of available data regarding the timing of ore forma-tion and mineralization processes.

Geologic Context

The Candelaria-Punta del Cobre iron oxide Cu-Au(-Zn-Ag)

deposits occur in the Chilean coastal cordillera (Fig. 1). Thedeposits are located to the east of the nearby main branchesof the Atacama fault zone, which stretches over 1,000 kmalong the Chilean coast. The Atacama fault zone is a subduc-tion-linked arc-parallel strike-slip fault system that has beenactive at least since Jurassic times (e.g., Scheuber and An-driessen, 1990; Brown et al., 1993; Scheuber et al., 1995;Dallmeyer et al., 1996). This fault system controlled mineral-ization of many of the iron deposits of the Chilean iron beltthat also occur in the Chilean coastal cordillera (e.g., Book-strom, 1977; Thiele and Pincheira, 1987; Espinoza, 1990;

CANDELARIA-PUNTA DEL COBRE IRON OXIDE Cu-Au(-Zn-Ag) DEPOSITS, CHILE 1801

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Paipote smelter

La Candelaria

Punta Negra

Alcaparrosa

Trinidad

2730' S

7015' W

0 2

km

Tailing impoundments

Mine in operation

Mine closed

Las Pintadas

CSZ

FSZ

OSZ

Viita Azul

Mine area

Open pit

Q. Los Algarrobos

Q.

Los

Toros

Q. Nantoco

Q. Las Pintadas

Atacama Gravels(Miocene)

Alluvium(Recent)

CopiapBatholith(Early Cretaceous)

Intrusive rocks

(undifferentiated)

Tonalite (Kt)

Monzodiorite (Kmd)

Quartz monzonite (Kqm)

Diorite (Khd)

111.5 0.4 Ma

109.9 0.4 Ma

Q. Melndez

Tierra Amarilla

Nantoco

Carola

Sacovn Rampa

Chaarcillo Group

Bandurrias Group

Punta del Cobre Formation

Shear zone

CSZ

FSZ

OSZ

High-angle fault

Trinidad

Santos

109.9 1.7 Ma

111 3 Ma

Candelaria shear zone

Florida shear zone

Ojancos shear zone

Resguardo

Mantos de Cobre

Bronce

Legend

Q. Florida

Marta-Venus

Manto Monstruo

San Gregorio (Cu-Au)

San Francisco (Cu-Au)

Transito (Au)

Pun

tad

elCo

bre

distr

ict

Copiapriver

Las Pintadas district

Ojancos Nuevo district

Ladrillos district

Low or medium angle fault

Axis of the Tierra Amarilla Anticline

Carola

Atacama-Kozan

Project

La Tigresa (Cu-Au)

K-Ar

40 39Ar/ Ar

40 39Ar/ Ar

Kmd

Kqm

Kt

KhdA

A'

Khd

(Tithonian(?)-pre-late Valanginian)

(late Valanginian to Aptian-Albian)

(late Valanginian to late Aptian)

FIG. 2. Geologic map of the Candelaria-Punta del Cobre area. The Lar mine mentioned in the text was located where theCandelaria pit is shown. The position of the schematic cross section of Figure 6 is indicated.


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Mnard 1995). In the Copiap area, the Chilean iron belt isrepresented, e.g., by the Cerro Imn (Espinoza et al., 1994)and Cerro Negro Norte deposits (Vivallo et al., 1995; Fig. 1).The Candelaria-Punta del Cobre iron oxide Cu-Au(-Zn-Ag)deposits and most of the deposits of the Chilean iron belt arehosted in Early Cretaceous arc-derived volcanic and volcani-clastic rocks adjacent to intermediate plutons of the Chilean

coastal batholith.Stratified rocks exposed in the Candelaria-Punta del Cobrearea represent a facies transition of a continental volcanic arcto the west and northwest (Bandurrias Group) and a shallowmarine back-arc basin to the east and southeast (ChaarcilloGroup). Sedimentation in the back-arc basin commenced inBerriasian time (Early Cretaceous) with the deposition of theupper part of the volcanic-volcaniclastic Punta del CobreFormation, which underlies the late Valanginian to Aptian(Early Cretaceous) carbonate rocks of the Chaarcillo Group(Abundancia-Nantoco, Totoralillo, and Pabelln formations;accumulated thickness 1,7002,000 m). Basin inversion,

which started in late Aptian times (possibly at around 115Ma), eventually resulted in the partial erosion of the back-arcsequence (Segerstrom and Parker, 1959; Zentilli 1974; Jur-gan, 1977). Granitoid plutons of the Copiap batholith in-truded the back-arc deposits in the western portion of thearea, causing an extensive contact metamorphic aureole (Till-ing, 1962, 1963, 1976). The Copiap batholith consists of sev-eral calc-alkaline intrusions ranging from diorite to quartzmonzonite (SiO2 5068 wt %). These plutons are intruded, inplaces, by altered aplitic dikes (SiO2 7276 wt %). Dikes areabundant at the eastern margin of the batholith, near Cande-laria. Hydrothermally altered dacite porphyry dikes and sillsthat locally contain minor sulfide mineralization and postorelamprophyric dikes occur at Candelaria and in the Punta delCobre district. Portions of the Copiap batholith west of Can-

delaria are marginally affected mainly by intense sodic(-cal-cic) alteration that presumably is related to the ore formation(see below). Potassium-argon ages of batholith intrusionsrange between 119 to 97 Ma (Arvalo, 1994, 1995). The40Ar/39Ar ages of 111.5 Ma for a monzodiorite and 109.9 Mafor a granodiorite-tonalite near Candelaria have been deter-mined (Fig. 2; Arvalo, 1999).

District Stratigraphy

The oldest rocks in the Candelaria-Punta del Cobre areabelong to the Tithonian(?)-pre-late Valanginian Punta delCobre Formation (Marschik and Fontbot, 2001), whichhosts most of the iron oxide Cu-Au(-Zn-Ag) orebodies. The

Punta del Cobre Formation (Fig. 3) is divided into the vol-canic Geraldo-Negro Member (>500 m) and the overlyingpredominantly volcaniclastic Algarrobos Member (>800 m indrill cores; Marschik and Fontbot, 2001). The Geraldo-Negro Member is further subdivided into the Lower An-desites (>300 m) that consist of altered massive andesitic vol-canic rocks, and the Melndez Dacites (up to 200 m)comprising intensely alkali-metasomatized lava domes andflows of originally dacitic composition that overlie the LowerAndesites east of the Copiap River, in the Punta del Cobredistrict, sensu stricto (Fig. 4). The Algarrobos Member isformed by a succession of coarse, poorly bedded volcaniclas-tic conglomerates and breccias with centimetric to decimetric

clasts that contain several intercalations of fine-grained sedi-ments such as siltstones, arenites, coarse sandstones, andmicroconglomerates, commonly on the order of 10 to about40 m thick, and also lenses of massive volcanic andesitic tobasaltic rocks. The Algarrobos Member is characterized bymarked lateral changes in thickness and facies. It passes ver-tically and laterally into the overlying calcareous Chaarcillo

Group (Abundancia or Nantoco Formations) with a contactdefined by the first continuous bed of massive limestone or itsmetamorphosed equivalent. The Algarrobos Member con-tains several horizons of economic or petrologic importancethat were defined in the Punta del Cobre district and, to acertain extent, can be correlated within the Candelaria-Puntadel Cobre area. These horizons include: the Basal Breccia,the Trinidad Siltstone, and the Upper Lavas. The Basal Brec-cia (up to 25 m) is a red sedimentary breccia, in places con-glomeratic, that contains interdigitations of sandstone andgrades into red sandstone toward the south. It is exposed inthe Punta del Cobre district, where it overlies the MelndezDacites. A similar horizon was identified in a drill core fromQuebrada Los Algarrobos, where a hematite-bearing redsandstone rests on the Lower Andesites. The Basal Brecciahosts stratiform orebodies and the top of the Basal Brecciacommonly marks the upper limit of the mineralization in thePunta del Cobre district. The Trinidad Siltstone (up to 60 m)that overlies the Basal Breccia is mainly composed of silt- andsandstone, chert, and tuffaceous sedimentary rocks, which lo-cally contain elongate, decimetric to metric clasts of brec-ciated limestone. The Trinidad Siltstone is characterized bystrong lateral changes in facies and thickness. In the SocavnRampa mine, it covers a local erosional surface above theBasal Breccia. The Trinidad Siltstone can be correlated withbiotitized originally fine-grained tuffaceous rocks at Cande-laria. Whereas this unit is commonly barren in the Punta del

Cobre district, it hosts high-grade ore at Candelaria. TheUpper Lavas (up to 45 m) form a discontinuous horizon oflenses of basaltic to basalt-andesitic lavas and volcanic brec-cias with a primitive geochemical signature of less differenti-ated magmas. This magmatic event is interpreted to reflectthe incipient opening of the Early Cretaceous marine back-arc basin south of Copiap (Marschik and Fontbot, 2001).

Structural Framework

The dominating structural elements in the Candelaria-Punta del Cobre area are a large northeast-trending antiform(Tierra Amarilla anticlinorium), a southeast-verging fold-thrust system (El Bronce fold-thrust system; Arvalo and

Grocott, 1997), and a dense set of north-northwest to north-west-trending high-angle sinistral transcurrent faults. The lat-ter control parts of the mineralization (e.g., Camus, 1980;Marschik and Fontbot, 1996). Additionally, northeast- andeast-northeasttrending high-angle and moderately (3050)

west dipping faults and sinistral east-northeasttrending high-angle faults are present. Mylonitic shear zones and cataclasticrocks locally form the contact between intrusive and EarlyCretaceous country rocks (e.g., Ojancos shear zone). Ductiledeformation is also recorded in the Candelaria-Florida shearzones, affecting volcanic and volcaniclastic rocks close to thebatholith contact (Fig. 2). Such deformation is manifested innorth-northeasttrending, 30 to 70 west-dipping zones of

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intensely foliated K-metasomatized (biotite) rocks (Fig. 5a).The Candelaria and Florida shear zones are possibly seg-ments of a major shear-fault zone that may find its continua-tion in the Inca de Oro area (Sylvester and Palacios, 1992;Sylvester and Linke, 1993). The Candelaria shear zone pre-dated the copper mineralization and is the oldest deformation

recognized in the Candelaria mine to date. The Candelaria-Florida shear zones are cut and displaced by sinistral north-northwest to northwest-trending high-angle faults, the east-northeasttrending high-angle faults, and by the broadlynortheast trending moderately west dipping faults. Shearingmust have occurred between Berriasian (the age of the de-formed rocks) and Aptian (age of mineralization) times, i.e.,at a depth equivalent to or less than the maximum thicknessof the late Valanginian to late Aptian Chaarcillo Group(2,000 m plus eroded material) that overlies the shearedrocks. Therefore, ductile deformation took place far abovenormal ductile-brittle transition and the Candelaria andFlorida shear zones are interpreted to represent thermally

moderated ductile deformation related to batholith emplace-ment, as suggested for other ductile shear zones associated

with high-level intrusions now exposed in the Early Creta-ceous magmatic arc of northern Chile (e.g., Grocott et al.,1993; Wilson et al., 2000).

Mineralization and AlterationOre occurrences

In the Punta del Cobre belt, copper ore occurs as massiveveins (Fig. 5b), in the matrix of hydrothermal breccias, as dis-continuous veinlets or stringers in the altered host rocks (Fig.5c) or superposed on massive magnetite replacement bodies(Fig. 5d), and as replacements and pore infilling of bodiesroughly concordant with stratification (mantos, Fig. 5e). Largerorebodies are commonly located where northwest- to north-northwesttrending faults intersect the contact between themassive volcanic rocks of the Geraldo-Negro Member and theoverlying volcaniclastic Algarrobos Member (Figs. 3, 4, 6, and 7).


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"Magnetite Manto"

"K-feldspar Breccia"

"Tuffs (and volcaniclasticsediments)"

"Lower Andesites" (A)

"Lower Andesites" (B)

AlgarrobosMember

CHAARCILLO GROUP

Upper Lavas

Trinidad Siltstone

Lower Andesites

Scapolite pyroxene amphibole garnet skarnwith garnetite horizons or quartz hornfels

Biotite hornfels or altered (K-feldspar or albite,biotite, amphibole) massive volcanic rocks;scapolite, and amphibole veinlets

Quartz and pyroxene hornfels orpyroxene-scapolite-garnet skarn overbiotite hornfels; amphibole veinlets

Volcaniclastic breccia with clasts intensely alteredto K-feldspar in matrix of mainly magnetite commonlyplus amphibole

Stratiform magnetite body plus amphibole alteration

Intense biotitization (brown and/or greenbiotite) plus quartz and magnetiteamphibole pink garnet, locally minorcordierite; overprinted by pervasive andfracture-controlled amphibole alteration

Biotite-quartz-magnetite-albite/K-feldsparalteration, in places, overprinted by fracture-controlled or pervasive amphibole

Biotite-quartz-magnetite-Na plagioclasealteration, locally minor K-feldsparand/or minor amphibole

Dacite dike/sill

Candelaria terminology(e.g., Ryan et al. 1995;Ullrich and Clark, 1999)

"Metasediments"

Geraldo-Negro

Member

up to 40 m

"Upper Andesites"up to 200 m

(include biotite hornfelses)

40-100 m

>350 m

200-300 m

40-100 m

>350 m

Main alteration assemblagesLithologyRegional stratigraphy

PUNTADELC

OBREFORMATION

Dacite dike

Albite and/or K-feldspar

FIG. 3. Schematic lithostratigraphic column of the Punta del Cobre Formation and the vertical distribution of the mainalteration mineral assemblages at the Candelaria mine. The fracture-controlled calcic amphibole alteration is indicated.However, there are several other veining events (see paragenetic sequence, Fig. 9). The metasediment unit at Candelaria isroughly equivalent to the Abundancia Formation (Chaarcillo Group).


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In the Punta del Cobre district, sensu stricto, concordantstratiform bodies are hosted mainly by the Basal Breccia thatoverlies the Melndez Dacites. Subordinately, they occur alsoreplacing small lenses of clastic sediments within or brec-ciated tops of volcanic flows. The mantos are commonly

underlain by veins or elongated breccia bodies. The lattermay split up into veins at depth. The subvertical orebodies,

which constitute the main portion of the mineralization, areemplaced along the northwest- to north-northwesttrendingfaults and are mainly confined to the volcanic Geraldo-Negro


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Basal Breccia

Altered massive volcanicrocks of originally daciticcomposition

AlgarrobosMember

Geraldo-NegroMember

PUNTADELC

OBREFORMATION

Upper Lavas

Trinidad Siltstone

Lower Andesites

Chlorite-carbonatesericitehematite alteration

Locally horizons of red chertmagnetite

Chlorite and carbonate alteration,plus hematite and/or magnetite

K-feldspar-chlorite/biotitecalcite quartz

Predominantly red andgreen chert, siltstone andtuffaceous rocks

Altered massive volcanicrocks and volcanic brecciain volcaniclastic sediments

Biotite-quartz-K-feldspar/albitechlorite

Volcaniclastic breccia andconglomerate, siltstone,and chert

Altered massive volcanicrocks of originally andesiticcomposition

Red sedimentary breccia,in places conglomeratic,with sandy interdigitations

Fracture-controlled amphibole

Albite-quartz-biotite/chlorite

Albite-chlorite calcite quartz

Melndez Dacites

CHAARCILLO GROUP

up to 60 m

up to 25 m

up to >200 m

>300 m

up to 45 m

Lithology Main alteration assemblagesRegional stratigraphy

Limestone and volcani-clastic rocks

FIG. 4. Schematic lithostratigraphic column of the Punta del Cobre Formation and vertical distribution of the main al-teration mineral assemblages in the Punta del Cobre district.

FIG. 5. (a). Outcrop of the Candelaria shear zone, which is a north-northeasttrending, on average 50 west-dipping zoneof intense foliated and K-metasomatized rocks. (b). North-northwesttrending vein of massive chalcopyrite plus minor pyritein the Candelaria south pit. (c). Stringers of chalcopyrite-pyrite in intensely iron-metasomatized volcanic or volcaniclasticrocks, Candelaria north pit. (d). Massive magnetite with superposed amphibole and chalcopyrite-pyrite, Candelaria south pit.(e). Manto ore hosted in the lower part of the Basal Breccia at the Carola mine. (f). Chalcopyrite-pyrite veinlet cuts mag-netite replacements, illustrating that the iron oxide formation preceded sulfide mineralization, Candelaria orebody. (g). Chal-copyrite-pyrite associated with amphibole cut and use postmagnetite albite veinlets, Candelaria orebody. (h) Amphibole con-taining interstitial chalcopyrite and small crosscutting chalcopyrite veinlets in previously K-metasomatized (K feldspar) rock.(i) Pyrite-chalcopyrite and K feldspar cut magnetite and the superposed pervasive amphibole alteration, Candelaria orebody.(j). Magnetite with superposed amphibole alteration both cut by a quartz plus K feldspar veinlet that was later used by chal-copyrite-pyrite; K feldspar commonly also uses the earlier quartz veinlets. (k) Epidote-pyrite cutting potassium-metasoma-tized (K feldspar) rock; the epidote-pyrite veinlet was later used by amphibole plus minor chalcopyrite; note the small am-phibole veinlet that cuts the epidote-pyrite alteration. (l) Chalcopyrite-pyrite cutting an anhydrite veinlet. (m) Greenbiotite-quartz-magnetite alteration at Candelaria; chalcopyrite occurs similarly associated with green biotite. (n). Early spec-ular hematite (hmI) from the Carola mine. (o) Pseudomorphic replacement of early specularite (hmI) by magnetite(mushketovite), relics of the original hematite are locally preserved. (p). Magnetite replacing and overgrowing hematite(hmI) during the main iron oxide mineralization. (q). Pyrrhotite as infilling between pyrite, chalcopyrite occurs in fracturesof pyrite. (r). Late hematite (hmII) and martitized magnetite.


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b)

e)

h)

k)

n)

q)

c)

f)

i)

l)

o)

r)

a)

d)

g)

j)

m)

p)

mt

mushketovite

hm I

mt

py

mt

hm II

hm I

py

cpy

hm I

po

cpy

bio

mtqtz

amph

kspar

cpy

amph

epi-py

kspar

cpy

ab

amph-cpy-py

amph

kspar mtqtz +cpy-py

anh

cpy-py

mt cut by anh

amph

kspar

py-cpy

mt

cpy-py

mt

mt

dike

shear zone

mt plus cpy-py

amph cuts epi-py

mt

cpy

anhpy

1 cm

1 cm1 cm

1 cm

1 cm1 cm

1 cm

1cm

1 mm

1 mm 1 m

1 m1 m

1 m

FIG. 6. Schematic west-southwesteast-northeast section through the Candelaria-Punta del Cobre area. Orebodies atPunta del Cobre are projected and are not to scale. The location of the cross section is shown in Figure 2.


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%) and biotite-quartz-magnetite plus sodic plagioclase and/orK feldspar in the Lower Andesites show a strong spatial rela-tionship with ore. Potassic assemblages pass at shallower lev-els into sodic assemblages of albite-chlorite calcite quartz(with Na2O up to 10 wt %). Sodium metasomatized rocks areconsidered to represent a peripheral part of the hydrothermalsystem. However, there is also ore in albitized rocks without

evidence of related potassic alteration, e.g., in parts of the So-cavn Rampa, Resguardo, and Mantos de Cobre mines (Fig.7). Sodic alteration is best developed in the Melndez Dacitesand particularly in the upper part of the lava dome south ofQuebrada Melndez and north of the Manto Verde mine. In-tensity of carbonatization and chloritization tends to increasehigher in the stratigraphic section, i.e., toward the volcanic-sedimentary rock contact and beyond, whereas the generally

weak to moderate developed silicification diminishes. Chlo-rite-calcite hematite assemblages are typically found in thesedimentary rocks of the Basal Breccia and the Trinidad Silt-stone and the overlying barren lavas and volcanic breccia ofthe Upper Lavas (Figs. 4 and 7).

Dacite porphyry dikes are affected by various types of hy-drothermal alteration. At Candelaria, these dikes commonlyshow sodic (albite) or potassic (K feldspar) alteration deeperin the section, and a sodium or potassium metasomatized corein an envelope of scapolite-garnet-pyroxene magnetite en-doskarn with or without minor chalcopyrite-pyrite in theupper part. Dacite dikes in the Chaarcillo Group above theCandelaria orebody and in a similar level at Quebrada ElBuitre are affected by sodic-calcic alteration. In the Punta delCobre district, the dacite porphyries are intensely sodium orpotassium metasomatized (e.g., the Trinidad and Carolamines) and, locally, host a sulfide-bearing stockwork (Marschikand Fontbot, 1996). Postore lamprophyric dikes that usedsimilar tectonic structures as the Cu mineralization in the

Punta del Cobre district are commonly affected by carbonati-zation and chloritization.Extensive areas of sodic or sodic-calcic alteration occur in

plutonic, subvolcanic or volcanic, and sedimentary rocks alongthe eastern margin of the Copiap batholith (Fig. 8). Sodic al-teration of igneous rocks commonly results in strong albitiza-tion, which leaves a bleached white- or light gray-coloredrock. It is particularly well developed south and southwest ofthe Candelaria tailings pond. Locally observed albite-amphi-bole minor epidote assemblages may be caused by super-position of calcic amphibole on previous albitization or alter-natively by a different sodic-calcic alteration event similar tothat described by Dilles and Einaudi (1992) in the Ann Mason

porphyry system. An early pervasive albitization that pre-ceded potassic alteration is recognized in the Punta del Cobredistrict (Marschik and Fontbot, 1996). Andesitic host rocksat Candelaria were also affected by pervasive sodic prior topervasive potassic alteration and mineralization (spilitizationof Ullrich and Clark, 1999). We correlate this early albitiza-tion with the albitization found in igneous and sedimentaryrocks west and southwest of the Candelaria deposit.

Voluminous sodic scapolite-rich assemblages developed inrocks of the Abundancia Formation around Candelaria. Sodicscapolite commonly is associated with calcic amphiboleand/or pyroxene, minor epidote andraditic garnet butmay occur without significant amounts of these calc-silicates.

These scapolite-rich beds may represent metamorphosedevaporitic horizons that are described to occur in the LowerCretaceous rocks in the area (Segerstrom, 1962; Hopper andCorrea, 2000).

Albitization west and southwest of Candelaria is locally as-sociated with minor disseminations of pyrite trace chal-copyrite and/or veinlets and disseminations of hematite,

whereas rocks affected by scapolite amphibole and/or py-roxene alteration may locally host small magnetite chal-copyrite-pyrite mantos (e.g., the El Bronce mine; Daz, 1990)or contain traces of pyrite chalcopyrite, commonly in vein-lets. Albitization occurs as discordant alteration within thedistrict (e.g., Quebrada, Melndez; southwest of Candelaria;at the junction of Quebrada Nantoco and Quebrada LosToros). In contrast, scapolite is largely confined to the layeredrocks of the Chaarcillo Group and the uppermost part of thePunta del Cobre Formation near the batholith contact. It isfound also in veinlets, commonly together with amphibole, inthe altered margins of the batholith.

Thermal contact metamorphism caused mineralogicalchanges without significant modification of the original geo-chemical composition in the affected andesitic volcanic rocksadjacent to the batholith contact. Calcic amphibole-rich as-semblages developed in volcanic rocks close to the batholithcontact grading into epidote-chlorite-rich assemblages far-ther outboard to the east (Marschik and Fontbot, 1996).These contact metamorphic assemblages locally grade intohydrothermal assemblages of the alkali metasomatized rocksthat in places host the ore deposits.

Ore mineralogy

The ore consists essentially of magnetite and/or hematite,chalcopyrite, and pyrite, with local pyrrhotite, sphalerite,trace quantities of molybdenite, and arsenopyrite (see below

and Fig. 9). Native gold occurs mainly as micron-sized inclu-sions in chalcopyrite. Ryan et al. (1995) described gold-fillingmicrofractures in pyrite and Hopf (1990) an Hg-Au-Ag alloy.Minerals in the poorly developed supergene oxidation andenrichment zones include malachite, chrysocolla, massive andsooty chalcocite, and covellite (Sillitoe and Clark, 1969).Gangue mineralogy consists predominantly of quartz and an-hydrite at Candelaria and calcite and/or quartz at Punta delCobre. Tourmaline and traces of fluorite occur locally (Hopf,1990). The Candelaria, Carola, and Trinidad deposits locallycontain several hundreds parts per million of light rare earthelements (Fig. 10, Table 3; Marschik et al., 2000). Allanite isthe only rare earth element-bearing mineral identified under

the microscope so far. A detailed description of the hypogeneore minerals and their distribution is given in Hopf (1990)and Ryan et al. (1995).

Paragenetic sequence

The paragenetic sequences in the Candelaria deposit and inthe Punta del Cobre district are similar (Fig. 11). Differencesbetween the deposits arise from their relative position withinthe hydrothermal system, as detailed below.

Intense iron metasomatism accompanied by early potassicalteration and silicification postdates large-scale early perva-sive albitization. Both Ti-rich brown (TiO2 >2.0 wt %) andgreen hydrothermal biotite (TiO2


9/27


10/27


0361-0128/98/000/000-00 $6.00 1809

ab-amphepi

Pervasive

Igneousrocks

Atthebatholith

Peripheral

Nonetotrace

Na2O(6-9wt%)>

Discordant,

(sodicscap

albitization;

marginWof

todistal

CaO(1.5-2.5wt%);

transitionaltothe

chlsericite)

fracture

Candelaria

K2O>1wt%

almostpure

controlledand

albitizationSWof

dissemin

ated

Candelaria;possiblya

amphibole

resultofsuperposition

ofearlyalbitization

andlateamph

alterationorcaused

byasodic-calcic

eventcoevalwiththe

scap-pxamph

formationabove

Candelaria

Upper

Amphbio

Pervasive

Volcaniclastic

Nearthebatholith

Distal

None

Na2O(5-8wt%)

Variableproportions

greenschistchlepi

andvolcanic

contact;PuntaN

egra;

CaO(3-5wt%)MgO

ofamphandbio;

facies

sericite

rocks

inplacesatQ.La

high(upto7wt%);

commonlywithout

Pepita;nearAtacama-

K2Ovariable(upto

significant

Kozanprospect;

3wt%)

metasomaticchanges

UpperLavasat

inhost-rock

Candelaria

composition

Lower

Epi-c

hl-c

te

Pervasive

Volcanicand

Q.Nantoco,Q.L

os

Distal

None

CaO8-10wt%;

Withoutmetasomatic

greenschistqtzamph

volcaniclastic

Toros,QRivera

Na2O3-4wt%;

changesinrock

facies

rocks

andWofit

K2O

1-3wt%

composition

(propylitic

alteration)

1Abbreviations:ab=albite,amph=amphibole,bio=biotite,chl=chlorite,c

te=calcite,epi=epidote,kspar=Kfeldspar,plag=plagioclase,px=pyroxene,q

tz=quartz,scap=scapolite

KaFm.=

AbundanciaFormation,L.O

.I.=

lossofignitionQ.=

Quebrada(valleyo

rgorge)

2Positionrelativetoapostulatedcenteroftheoresystem

3Typicalvaluesofwhole-rockanalysis

stronglydependonFecontents

CombinedCa-KorNa-Caalterationm

ineralassemblages,inplaces,maybetheresultofsuperpositionofCaonpreexistingKorNaalteration

DatabasedonMarschikandFontbot(1996)

TABLE2.(Cont.)

Spatially

Typical

Alteration

Mineral

Relative

associated

Relativetime

whole-rock

type

assemblages1

Style

Hostrocks

Location

position2

mineralization

relationship

analysis3

Comments


11/27


0361-0128/98/000/000-00 $6.00 1810

W E

750 m

700 m

650 m

El Bronce deposit

0 50

m

1000

m

Orebody, 0.4 % Cu contour

600 m

500 m

400 m

300 m

200 m

100 m

sea level

Candelaria deposit

bio-qtz-mtkspar plus abundant Ca amph

kspar or Na plag-bio-Ca amph

bio-qtz-alm gt cordcommonly plus Ca amph

to scale

to scale

scap gt px Ca amph

Na plag-qtz-bio-mt

kspar minor Ca amph

scap gt

sea level

200 m

400 m

600 m

800 m

Socavn Rampa mine

Resguardo mine Carola mine

Punta del Cobre district

Hematite associated with ore

Magnetite-rich ore

Massive magnetite

ab-chl-cte qtz

Ca-amph overprint

Santos mine

bio-qtz + Na plag or kspar

kspar-chl/bio cte qtz

not to scale

chl-cte-hm

Magnetite-hematite predominance

hmmt

FIG. 7. Distribution of iron oxide minerals and main alteration types in the Candelaria-Punta del Cobre iron oxide Cu-Au(-Zn-Ag) deposits. Sections through the El Bronce deposit (after Daz, 1990), the Candelaria deposit (after Martin et al.,1997), and the Punta del Cobre district (modified from Flores, 1997) are shown. The El Bronce deposit was located abovethe Candelaria deposit to the west of the Bronce fault (Fig. 2). Abbreviations are given in Table 2.


12/27

with widespread pervasive magnetite alteration. Green-col-ored biotite is commonly the biotite variety that occurs in fo-liated (sheared) domains of the Candelaria deposit (Fig 5 aand m). In places, sheared massive green biotite cuts dis-placed fragments of massive magnetite replacements.

Iron metasomatism resulted in the formation of specularhematite (hm I, Fig. 9), mainly in dilatational fractures and

open spaces (Fig. 5n), and coevally, of massive magnetite re-placement bodies during an early mineralization stage. Sub-sequent pseudomorphic replacement of fracture-controlledspecularite (hm I) by magnetite (mushketovite; Ramdohr,1980) and new formation of magnetite (mt I) records a shifttoward more reduced conditions and/or higher temperatures(Figs. 5o and p, and 9). The main copper mineralization oc-curred after the early main iron oxide mineralization. Chal-copyrite pyrite crosscutting magnetite or early specularite

with interstitial chalcopyrite pyrite are typically observed inthe deposits of the Punta del Cobre belt (Fig. 5f). The exis-tence of two temporally distinct major hydrothermal stages isdemonstrated by the occurrence of physically separated veins

of massive magnetite hematite without sulfides and massivechalcopyrite-pyrite without iron oxides, which in places occurin contact with each other (e.g., in the Carola mine).

Chalcopyrite-pyrite texturally postdate a widespread albite,a calcic-amphibole, and a quartz K feldspar veining event atCandelaria. In the northern part of the Candelaria orebody,albite veinlets, occasionally plus minor scapolite, cut the early

biotite-quartz-magnetite alteration. These albite veinlets inturn are cut and used by main copper mineralization. Albiteveining in the Candelaria orebody is interpreted to occur co-eval with sodic scapolite alteration in the overlying Abundan-cia Formation. Calcic-amphibole may largely overlap with themain-stage chalcopyrite-pyrite. Calcic amphibole associated

with chalcopyrite-pyrite cutting or using previously formedalbite veinlets is common (Fig. 5g). Similarly, pervasive Kfeldspar albite alteration is cut by calcic amphibole epi-dote. The association of calcic amphibole with interstitialchalcopyrite-pyrite and pre- or post-amphibole K feldspar re-placements or veinlets is common (Fig. 5h and i). Crosscut-ting relationships in a few places indicate that there are at


0361-0128/98/000/000-00 $6.00 1811

Paipote smelter

La Candelaria

Punta Negra

2730' S

7015' W

0 2

kmLas Pintadas

CSZ

FSZ

OSZ

Q. Los Algarrobos

Q.

Los

Toros

Q. Nantoco

Q. Las Pintadas

Tierra Amarilla

Nantoco

CSZ

FSZ

OSZ

Candelaria shear zone

Florida shear zone

Ojancos shear zone

Q. Florida

Punta

delC

obre

dist r

ict

Copiapriver

Alteration type (at the surface)

Biotite and/or K-feldspar(chlorite calcite quartz)

Ca-amphibole biotite

Calcic-potassic(mainly Ca-amphibole epidote, biotite, K-feldspar)

Epidote-chlorite calcite

Garnet pyroxene scapolite skarn

Pervasive albitization(chlorite calcite quartz)

Albite or Na-plagioclase amphiboleepidote minor scapolite

Na-scapolite pyroxene amphibole skarn(garnetite horizons)

Biotite Ca-amphibole(foliated rocks)

Na or Na-Ca alteration

K or Ca-K alteration

Thermal contact metamorphic alteration

Viita Azul

Q. Melndez

FIG. 8. Surface distribution of main alteration mineral assemblages in the Candelaria-Punta del Cobre area. See geologicmap in Figure 2 for stratigraphic units.


13/27

least two generations of calcic amphibole. Chalcopyrite of oneand the same event may have various types of associated alter-ation minerals, depending on the position within the hydro-thermal system. Veinlets of quartz-mushketovite with inter-stitial pyrite-chalcopyrite, veinlets of quartz K feldspar chalcopyrite-pyrite locally plus trace molybdenite cutting cal-cic amphibole (Fig. 5i), veinlets of chalcopyrite-pyrite withquartz-chlorite plus biotite cutting biotitized rocks, veinlets ofanhydrite-chalcopyrite, veinlets of calcic amphibole chal-copyrite cutting and using epidote pyrite veinlets (Fig. 5j),

veinlets of chalcopyrite pyrite sphalerite cutting anhydrite(Fig. 5k; see below), among others, are observed at Candelaria.

Pyrite accompanies chalcopyrite but chalcopyrite as infill-ing in skeletal and cataclastic pyrite and other textural obser-

vations suggest that pyrite (py I) began to form earlier thanchalcopyrite. A later minor phase of pyrite (py II) veinlets cutmassive chalcopyrite veins. The fact that pyrrhotite com-monly but not exclusively predates chalcopyrite, crosscuttingrelationships and other textural evidence suggest that pyrrhotiteis roughly contemporaneous with the first stage of main chal-copyrite formation (Fig. 5q; see also Hopf, 1990; Ryan et al.,1995; Ullrich and Clark, 1999). Anhydrite cuts and is cut bychalcopyrite and/or pyrite and, in some instances, occurs inter-grown with chalcopyrite (Fig. 5k). Post-anhydrite chalcopyrite


0361-0128/98/000/000-00 $6.00 1812

Ductile/brittle

at CandelariaUplift Brittle

Molybdenite

Iron Oxide Stage Sulfide Stage

Time

CANDELARIA

Punta del Cobre

V

P+V

P

Chalcopyrite

Pyrite

Hematite

Magnetite

Sphalerite

Pyrrhotite

Gold

Chalcopyrite

Pyrite

Hematite

Magnetite

Sphalerite

Gold

Calcite

Tourmaline

Albite

Quartz

Biotite

Ca-amphibole

K-feldspar

Chlorite

P

P

Albite

Quartz

Biotite

Ca-amphibole

K-feldspar

I

I

I

I II

Upper part

Lower part

V

V+P

V

V+P

Almandine-rich garnet

P

P

Pervasive biotitization

Pervasive silicification

Upper part

V

V+P

V

P

TEN

Loc

Loc

V

TEN

I II

I II

Epidote

V

V

Tourmaline?

AnhydriteV

Calcite

Late

Stage

V

V

P+V

Loc

Loc

II

II

V

V

Chlorite

V

V

Cordierite

Grunerite-cummingtonite

Widespreadscapolitefollowed

byandraditicgarnetformation

abovetheCandelariaorebody

TEN

Loc

Loc

?

Lower part

TEN

FIG. 9. Paragenetic sequences of main ore, alteration, and gangue minerals in the Candelaria orebody and the Punta delCobre district. Peak contact metamorphic calc-silicate minerals, occurring outside the orebody, are not represented in thisFigure (see text). The thickness of the lines roughly represents the relative abundance of the particular mineral but has noquantitative implications regarding other mineral species. The most relevant distinguishable veining events at Candelaria areshown. Peak contact metamorphism at Candelaria, indicated by the gray shaded field, is represented by the widespread sodicscapolite-pyroxene-andraditic garnet alteration above the orebody (see also Fig. 8). Peak contact metamorphism occurred atthe end of the early potassic alteration, i.e., after the biotite-quartz-magnetite alteration. It was about the time at which de-formation style changed from ductile-brittle to brittle at Candelaria and the available ages and the geologic context suggestthat this was broadly coeval with regional uplift. The various hematite, magnetite, and pyrite generations mentioned in thetext are indicated (see also Fig. 3). Abbreviations: P = pervasive alteration, TEN = tentatively, V = veinlets, ? = uncertain.


14/27

locally contains sphalerite inclusions. There is a spatial associ-ation of sphalerite with epidote-allanite-chalcopyrite-pyrite-anhydrite at Candelaria.

Gold is present native as inclusions mainly in chalcopyriteand pyrite (Hopf, 1990; Ryan et al., 1995). The exact parage-netic position of molybdenite is poorly constrained due to itsscarcity. Ullrich and Clark (1999) place the formation ofmolybdenite at the beginning of the main copper mineraliza-tion. In the few molybdenite-bearing samples available forthis study, molybdenite appears to postdate calcic-amphiboleand possibly occurs at the end of the main chalcopyriteformation. A textural relationship between anhydrite andmolybdenite has not been observed. Apatite occurs as

rosettes of prismatic needles of about 2 mm intergrown withquartz at the Carola mine (Hopf, 1990) and is also common atCandelaria (Ryan et al., 1995). According to Hopf (1990), ap-atite formed after tourmaline.

Hypogene metallic mineralization at a district scale endedwith the deposition of hematite (hm II) and the martitizationof the previously formed magnetite (mt I; Fig. 5r). The late

hematite (hm II) event may correlate with specularite calciteveins recognized at a regional scale, which locally contain tracesof magnetite, pyrite, and rarely chalcopyrite. Late calcite

veins and veinlets cut the alteration and veinlet types previ-ously mentioned. However, there are a few examples in whichchalcopyrite and pyrite postdate calcite veining (e.g., in theupper part of the Carola mine). Minor chabazite and phillip-site have been microscopically identified in late veinlets. Peakof the contact metamorphic calc-silicate alteration occursafter the iron oxide formation and before the main Cu min-eralization, because contact metamorphic andraditic garnetpostdates sodic scapolite, which cuts biotite hornfelses and bi-otitized volcanic rocks of the Upper Lavas on top of the Can-delaria orebody. Andraditic garnet in turn is cut by calcic-am-phibole and by pyrite chalcopyrite veinlets that arecorrelated with the main mineralization stage (see above inthe alteration section). The formation of biotite hornfels inthe upper part of the Punta del Cobre Formation and lowerpart of the Chaarcillo Group is correlated with the barrenbrown biotite-almandine cordierite alteration in theTrinidad Siltstone and brown biotite-quartz-magnetite in the

volcanic and volcaniclastic rocks at Candelaria.

Metal and gangue mineral zonation

A marked zonation is observed in the magnetite-hematitedistribution at a district scale and within the ore deposits. De-posits lying closer to the batholith contact tend to have mag-

netite rather than hematite, even in relatively high strati-graphic positions (e.g., El Bronce or Las Pintadas, bothhosted in rocks assigned to the Abundancia Formation). Vir-tually all of the early specularite (hm I) at Candelaria is nowtransformed into mushketovite (mt I). Early specular hematite(hm I) is preserved in the upper parts of the Carola and So-cavn Rampa mines, i.e., in the marginal parts of the hydro-thermal system. Second-generation hematite (hm II) and themartitization of the previously formed magnetite (mt I) canbe best observed in the Punta del Cobre district (Fig. 5r). AtCandelaria, veinlets of late specular hematite (hm II), with as-sociated minor pyrite-chalcopyrite, are rare and martitiza-tion of magnetite is uncommon. Pyrrhotite is restricted to the

upper central part of the northern Candelaria orebody (Ryanet al., 1995) and occurs locally in the Carola mine (Hopf,1990).

A redox boundary is observed at the upper limit of the oresystem in the Punta del Cobre district, represented by oremantos hosted in the Basal Breccia. The lower and internalparts of the mantos show the same greenish color as the un-derlying alkali metasomatized and chloritized volcanic rocks,but toward the top and margins, i.e., approaching the oxidizedcontinental rocks of the Basal Breccia, the color changes tored due to an increase of hypogene hematite.

In general, magnetite tends to be associated with potas-sic assemblages, whereas hematite is common in sodium


0361-0128/98/000/000-00 $6.00 1813

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

10

100

1000

Candelaria mine

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

10

100

1000

Socavn Rampa-Trinidad mines

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu1

10

100

1000

Carola mine

PC 1316

PC 1317

PC 1336

PC 1345

PC 1347

PC 1421

PC 1464

PC 1479

PC 1516

PC 148

PC 165

PC 188

PC 198

PC 207

PC 208

PC 217

PC 219

PC 221

PC 620

PC 704

PC 834

PC 1356

FIG. 10. Chondrite-normalized rare earth element pattern of rocks fromthe Candelaria, Carola, and Socavn Rampa-Trinidad mines, illustrating localenrichments in light rare earth elements that occur with these deposits. Ref-erence chondrite of Nakamura (1974).


15/27


0361-0128/98/000/000-00 $6.00 1814

TABLE 3. Whole-Rock Analysis of Ore Samples from Selected Ore Deposits of the Punta del Cobre Belt

PC 1336 PC 1345 PC 188 PC 198 PC 219 PC 704 PC 834 PC 1356Sample Candelaria Candelaria Carola Carola Carola Socavn Rampa Socavn Rampa TrinidadLocation Trinidad Algarrobos Melndez Melndez Melndez Melndez Melndez MelndezUnit Siltstone Member Dacites Dacites Dacites Dacites Dacites DacitesAlteration Ca-K K K K-Na K Carbonate K K

SiO2 41.69 45.02 54.05 56.41 46.05 51.21 46.71 53.33TiO2 0.62 0.52 0.53 0.51 0.46 0.51 0.72 0.46Al2O3 12.00 11.62 12.68 13.73 11.01 8.59 15.02 12.48Fe2O3* 21.98 30.10 15.19 11.36 19.40 21.14 15.81 17.61MnO 0.16 0.07 0.05 0.07 0.10 0.12 0.10 0.09MgO 6.36 2.14 2.29 2.58 6.21 4.09 2.76 4.41CaO 4.39 1.01 0.29 1.13 1.34 2.91 1.28 1.05Na2O 0.17 0.09 0.06 3.10 0.00 0.00 0.04 0.00K2O 4.51 7.64 8.54 4.42 4.98 0.08 7.68 6.09P2O5 0.47 0.10 0.12 0.16 0.15 0.23 0.15 0.14L.O.I. 0.91 0.25 2.52 3.29 6.06 7.84 4.74 3.29Total 93.25 98.56 96.31 96.77 95.76 96.72 95.01 98.95

Au (ppb) 474 78 199 152 686 480 499 15

ppmAg 10.9


16/27

metasomatized rocks. These relationsips can be interpretedin terms of more internal and high-temperature vs. externaland low-temperature portions of the ore-forming hydrother-mal system. For example, the Socavn Rampa mine, which ischaracterized by sodic rather than potassic assemblages, is es-sentially devoid of magnetite, whereas hematite is widespread.In contrast, the Trinidad mine, which constitutes the northerncontinuation of the Socavn Rampa mine, has magnetite-richore, including massive magnetite bodies (J. Ponce, pers. com-mun., 1996), associated with potassic assemblages suggestingthat the deposit represents another mineralization center be-

tween the Santos mine to the north and the Carola mine to thesouth. Within the Carola mine, massive magnetite bodiesoccur only below mining level 10 (362 m a.s.l) in the westernpart of the mine. In its eastern part, magnetite bodies are ab-sent and the Cu/Fe is higher (Lino, 1984; Hopf, 1987).

The distributions of quartz and calcite are similar to thoseof magnetite and hematite. In Candelaria and in deep parts ofthe Punta del Cobre district, quartz veinlets are widespreadand silicification is common, whereas calcite occurs essen-tially in thin veinlets. In contrast, calcite veins and veinlets areabundant in the upper portions of the Punta del Cobre dis-trict, where pervasive carbonate alteration is common. Anhy-drite is confined to the lower part of Candelaria orebody and

locally occurs in the Trinidad and Carola mines.At Candelaria, highest Cu-Au grades occur in the TrinidadSiltstone in the uppermost part of the deposit, whereas mostof the ore is hosted in the volcaniclastic and volcanic rocksbelow this horizon (Ryan et al., 1995). Gold concentrationsshow a good positive correlation with copper contents(Marschik et al., 2000). Gold grades in the deposits of thePunta del Cobre district are variable and appear to be slightlyhigher than at Candelaria (Table 3) and there may be a Cu/Aufractionation at a district scale.

Local enrichments of Zn are recognized at the Carola andCandelaria mines. At deep levels of the Carola mine (below335 m a.s.l.), Zn grades may exceed 2 wt percent in some

stopes (N. Pop, pers. commun., 1997). In the upper levels ofthe Carola mine, sphalerite is virtually absent and massive oreusually contains below 100 ppm Zn (Hopf 1987). At Cande-laria, two horizons have been identified in which Zn contentslocally are about 1 wt percent (W. Martin, pers. commun.,2000). These horizons occur roughly at the upper contact ofthe Lower Andesites and at the lower contact of the Trinidad

Siltstone. Highest concentrations of light rare earth elementsoccur locally near these same contacts (Marschik et al., 2000).

Sulfur Isotope Geochemistry

Sulfur isotope analyses on 47 samples from the Bronce (n =2), Candelaria (n = 34), Las Pintadas (n = 1), Santos (n = 4),and Socavn Rampa mines (n = 6) were carried out at the Sta-ble Isotopes Laboratory of the University of Lausanne, usingan online elemental analyzer-continuous flow-isotope ratiomass spectrometer and in the G.G. Hatch Isotope Laborato-ries, University of Ottawa. Pyrrhotite, sphalerite, pyrite, an-hydrite, and chalcopyrite that predates, is intergrown with,and postdates anhydrite, were analyzed. These samples repre-sent the most relevant paragenetic positions (see Fig. 9). The34S values of chalcopyrite, pyrite, pyrrhotite, and sphaleritefrom the deposits lie between 0.7 and +3.1 per mil relativeto Caon Diablo Troilite (CDT; Table 4, Fig. 11). Sulfidesfrom the Socavn Rampa mine have 34SCDTvalues between0.7 and +0.5 per mil, and those from the Santos mine lie be-tween 0.3 and +1.1 per mil. Sulfides from Candelaria have34SCDT values of 0.3 and 3.1 per mil, whereas anhydrite

yielded values between 14.5 and 17.5 per mil. We could notconfirm relatively heavy values for sulfides reported by Ull-rich and Clark (1999; up to 5.7 for the main-stage and7.2 for the late-stage mineralization). A general shift to-

ward lower 34SCDT values upsection in the Santos and So-cavn Rampa mines is interpreted to reflect oxidation of the

ore fluid approaching the Basal Breccia (Marschik et al.,1997a). This is in agreement with the abundance of magnetitein the lower parts and the predominance of hematite in theupper parts of the deposits of the Punta del Cobre district. Asimilar trend is also observed on a district scale where there isa general decrease in 34SCDTvalues from Candelaria, via San-tos toward the Socavn Rampa mine (Fig. 11).

Geochronology

The 40Ar/39Ar analyses on biotite and amphibole separatesfrom the Candelaria deposit were carried out by theGeochronological Research Laboratory of the New MexicoBureau of Mines and Mineral Resources, Socorro, New Mex-

ico, United States, using the furnace incremental-heatingtechnique. These analyses were carried out to complementpreviously reported data, which are incorporated in the dis-cussion below. The analytical results are given in Table 5. An-alytical methods and parameters are detailed in the Appendix.

Brown biotite (sample PC 99139) from the barren perva-sive biotite-almandine-rich garnet cordierite alteration inthe tuffaceous rocks that correlate with the Trinidad Siltstonehas been analyzed. The age spectrum has two plateaus, oneformed by heating steps D to F with 74.7 percent of the gas re-leased and another plateau from steps F to H with 61 percentof the gas released (Fig. 12; Table 5). Each of the plateaus

would conform the definition of a plateau age of at least three


0361-0128/98/000/000-00 $6.00 1815

chalcopyrite

pyritechalcopyrite-pyrite

anhydrite

pyrrhotitesphalerite

34 S (permil)

-2 0 2 4 6 8 10 12 14 16 18

El Bronce

Las Pintadas

Socavn Rampa

Santos

Candelaria

FIG. 11. Sulfur isotope variations within selected deposits of the Punta delCobre belt. Sulfides from Candelaria show a 34S range from 0.3 to 3.1 permil. Anhydrites have values between 14.5 and 17.5 per mil. Sulfides from allother mines studied are similar to those from Candelaria. However, a ten-dency toward lower 34S values from the internal (Candelaria mine) via in-termediate (Santos mine) to the external portions of the system (Socavn

Rampa mine) can be observed.


17/27


18/27


0361-0128/98/000/000-00 $6.00 1817

lensewithinandesiticrock

PC98166

Candelaria

Chalcopyrite

Massivechalcopyritevein

Algarrobos

1.1

PC677

Socavn

Chalcopyrite

Patchyreplacementsof

BasalBreccia/

-0.7

cutbypyriteveinlets

Member

Rampa

chalcopyriteandpyrite

Melndez

Dacitescontact

PC98193

Candelaria

Chalcopyrite

Volcaniclasticmicro-

Algarrobos

2.9

PC790

Socavn

Chalcopyrite

Chalcopyrite-pyritevein

Lower

0.5

conglomerate,patchy

Member

Rampa

Andesites

epidote,amphibole

veinlets,K-feldspar

envelopsandreplacements,

minorchalcopyrite-pyrite

PC98195

Candelaria

Chalcopyrite

Chalcopyritefrom

Algarrobos

0.3

PC795

Socavn

Chalcopyrite

Chalcopyrite-pyriteas

Melndez

0.4

andesiticwallrock

Member

Rampa

brecciainfill

Dacites

PC98202

Candelaria

Chalcopyrite

Massivepyrite-

Algarrobos

1.0

PC834

Socavn

Chalcopyrite

Pyrite-chalcopyrite-

BasalBreccia

0.0

chalcopyriteinthe

Member

Rampa

specularite(hmI)

lowestpartofthe

AlgarrobosMember

PC99044

Candelaria

Chalcopyrite

Chalcopyriteintergrown

Lower

1.9

PC678

Socavn

Chalcopyrite

Calcitecenteredpyrite-

BasalBreccia/

-0.5

(cogenetic)withanhydrite

Andesites?

Rampa

chalcopyriteveinlet

Melndez

Dacitescontact

PC99152

Candelaria

Chalcopyrite

Chalcopyrite-sphalerite

Algarrobos

0.3

PC795

Socavn

Pyrite

Chalcopyrite-pyriteas

Melndez

0.4

veinletcuttinganhydrite

Member

Rampa

brecciainfill

Dacites

PC99125

Candelaria

Pyrrhotite

Pyrrhotite-chalcopyrite

Algarrobos

2.0

veinlet

Member

TABLE4.(Cont.)

Sample

Mine

Mineral

Description

Unit

34S

Sample

Mine

Mineral

Description

Unit

34S

(CDT,)

(CDT,)

110

120

130

140

90

100

10

40

60

80

100

B

750C

850

D920

E1000

F1075

G1110

H1180

I1210

J1250

%R

adiogenic

Apparentage(Ma)

K/Ca

(b) Age spectrum of sample PC 99131 biotite

20 30 40 50 60 70 80 90 100

Cumulative percentage Ar released39

115.14 0.18MSDW = 1.1

1000

100

10

1

Total fusion age = 114.90 0.54

0

App

arentage(Ma)

0.01

0.1

D

E

F G

H

I J

116.6 1.2MSDW = 4.9**

%R

adiogenic

K/Ca

(c) Age spectrum of sample PC 98137 amphibole



10 20 30 40 50 60 70 80 90 1000

110

120

130

140

90

100

0

40

80

1020

1080

1120

1160

1200

1300 1400

10 20 30 400

110

120

130

140

90

100

40

60

80

100

20

Apparentage(Ma)

1000920850

750

0.1

10

1000

B

C D E F G H

116.51 0.26

MSWD = 0.9

%R

adiogenic

K/Ca

(a) Age spectrum of sample PC 99139 biotite

50 60 70 80 90 100



10751110 1180

FIG. 12. The 40Ar/39Ar age spectra of alteration minerals from the Cande-laria deposit. (a). Age spectrum of brown biotite from the barren biotite-al-mandine-rich garnet cordierite alteration. (b). Age spectrum of green hy-drothermal biotite from the biotite-quartz-magnetite alteration that hostschalcopyrite pyrite veinlets and disseminations (see also Fig. 5m). (c). Agespectrum of amphibole intergrown with chalcopyrite-pyrrhotite.


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consecutive steps that overlap within their errors and whichcontain 50 percent of the gas released (Fleck et al., 1977).However, there are marked decreases in the K/Ca from F toG, and from step G to H, which indicate mineral inclusions.

We consider a 40Ar/39Ar weighted mean plateau age of 116.51 0.26 Ma (all errors at 2) calculated from heating steps Dto F as representative. The total fusion age of this biotite sam-ple is 115.85 0.54 Ma.

Green hydrothermal biotite (sample PC 99131) from thefoliated pervasive biotite-quartz-magnetite alteration with

veinlets of quartz-chlorite green biotite, chalcopyrite, andminor pyrite, and disseminated chalcopyrite intergrown withthe biotite gave a 40Ar/39Ar weighted mean plateau age of115.14 0.18 Ma, based on heating steps C to I, which con-tain 96 percent of the gas released (Fig. 12; Table 5). The totalfusion age of this biotite sample is 114.9 0.54 Ma.


0361-0128/98/000/000-00 $6.00 1818

TABLE 5. Results of 40Ar/39Ar Analysis

ID Temp 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 39ArK K/Ca 40Ar* 39Ar Age 1 (C) (103) (1018 mol) (%) (%) (Ma) (Ma)

PC 99139, 3.69 mg biotite, J = 0.0039298, NM-131, Lab no. = 51807-01A 650 12.53 0.0343 20.230 29.2 14.9 52.1 0.6 45.70 1.10B 750 18.12 0.0089 7.357 71.1 57.2 87.9 2.1 109.48 0.55C 850 17.66 0.0046 1.501 258.9 110.5 97.4 7.5 117.95 0.30D 920 17.23 0.0024 0.666 610.6 217.0 98.7 20.2 116.74 0.23E 1000 17.16 0.0017 0.535 876.8 307.8 98.9 38.4 116.51 0.23F 1075 17.13 0.0024 0.542 2105.9 216.8 98.9 82.2 116.31 0.22G 1110 17.04 0.0181 0.639 642.1 28.1 98.8 95.5 115.55 0.22H 1180 17.38 0.1392 1.669 185.2 3.7 97.1 99.4 115.83 0.31I 1210 17.78 0.2021 3.263 10.0 2.5 94.5 99.6 115.40 3.00J 1250 18.08 0.4306 1.962 9.3 1.2 96.9 99.8 120.10 3.40K 1300 18.48 0.2846 2.852 10.5 1.8 95.4 100.0 120.90 2.90Total gas age n = 11 4809.5 189.4 115.85 0.541Plateau MSWD = 0.9 n = 3 steps D-F 1746.3 239.0 74.7 116.51 0.261

PC 99131, 4.60 mg biotite, J = 0.0039313, NM-131, Lab no. = 51809-01A 650 16.63 0.0643 28.850 39.9 7.9 48.6 0.5 56.50 1.30B 750 19.01 0.0069 6.840 165.2 74.2 89.2 2.5 116.45 0.39C 850 17.69 0.0045 3.081 536.4 113.6 94.7 9.0 115.07 0.25D 920 17.04 0.0037 0.661 837.2 137.7 98.7 19.2 115.50 0.26

E 1000 16.94 0.0041 0.393 1055.5 123.3 99.2 32.0 115.41 0.25F 1075 16.89 0.0019 0.282 1924.0 272.2 99.4 55.4 115.30 0.25G 1110 16.87 0.0016 0.278 1555.7 311.5 99.4 74.3 115.12 0.23H 1180 16.85 0.0013 0.378 1550.2 393.6 99.2 93.1 114.82 0.22I 1210 16.86 0.0014 0.377 442.8 368.6 99.2 98.5 114.90 0.23J 1250 16.79 0.0091 1.447 66.8 56.2 97.3 99.3 112.31 0.53K 1300 16.98 0.0071 2.943 36.1 72.0 94.7 99.8 110.62 0.87L 1650 27.63 0.14 28.690 19.2 3.6 69.3 100.0 130.90 2.10Total gas age n = 12 8229.1 256.0 114.90 0.541Plateau MSWD = 1.1 n = 7 steps C-I 7901.9 264.2 96.0 115.14 0.181

PC 98137, 14.43 mg hornblende, J = 0.0039261, NM-131, Lab no. = 51805-01A 800 209 9.573 665.3 6.28 0.053 6.3 0.7 92 21.2B 850 61.71 12.45 178.700 1.1 0.041 16.1 0.8 69.70 30.50C 950 49.73 20.07 122.400 2.2 0.025 30.7 1.1 106.40 14.90D 1020 27.79 16.43 40.530 36.3 0.031 61.9 5.1 119.20 1.60E 1080 19.31 19.01 11.880 376.2 0.027 89.8 47.0 120.30 0.35F 1120 17.40 13.68 6.244 265.6 0.037 96.0 76.5 115.70 0.35G 1160 19.95 13.96 12.550 15.9 0.037 87.3 78.3 120.50 2.30H 1200 18.56 14.59 9.974 14.3 0.035 90.7 79.9 116.70 2.00I 1300 17.84 14.15 6.943 98.0 0.036 95.1 90.8 117.61 0.50J 1400 18.77 13.69 9.532 73.5 0.037 91.1 99.0 118.39 0.73K 1650 39.74 13.47 66.540 9.1 0.038 53.4 100.0 145.60 4.70Total gas age n = 11 898.3 0.033 118.4 1.51Plateau MSWD = 4.92 n = 5 steps F-J 467.3 0.037 32.9 116.6 1.21Isochron MSWD = 6.52 n = 5 steps F-J 114.4 2.31

Notes:Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interferring reactionsIndividual analyses show analytical error only; plateau and total gas age errors include error in J and irradiation parametersn = number of heating stepsK/Ca = molar ratio calculated from reactor produced 39ArK and 37ArCa1 2error2 MSWD outside 95% confidence interval


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Euhedral amphibole (sample PC 98137) in a matrix of mas-sive chalcopyrite-pyrrhotite has a somewhat disturbed agespectrum (Fig. 12; Table 5). Steps A to C are heterogeneous,giving ages younger than 110 Ma with large errors. Steps D toE yield an age of about 120 Ma, whereas steps F to J yieldages that increase from 115.70 0.70 to 118.39 1.4 Ma. Theabrupt change in ages between steps E and F correlates with

an increase in both K/Ca and radiogenic yield, which indi-cates the presence of inclusions within the amphibole grains.Steps F to J do not conform the definition of a plateau age.However, steps F to H contain 32.9 percent of the gas re-leased and overlap within their errors. Steps I and J have asimilar K/Ca ratio and comparable radiogenic yield to steps Fto I, which leads us to believe that a pseudoplateau age of116.6 1.2 Ma calculated from steps F to J is representative(Fig. 12; Table 5) and relevant. An isochron age of 114.4 2.3Ma was calculated from these same steps. The total fusion ageof this amphibole sample is 118.4 1.5 Ma.

Discussion

Alteration and mineralization patterns allow the establish-ment of a descriptive model for an idealized Candelaria-Punta del Cobre-type iron oxide Cu-Au(-Zn-Ag) system. Mostof the larger orebodies in the belt are located at the intersec-tion of northwest- to north-northwesttrending brittle faults

with the contact of massive volcanic and volcaniclastic rocks.These northwest- to north-northwesttrending brittle faultsand a major north-northeasttrending ductile structure ap-pear to be the main fluid conduits.

In the internal parts of this system, represented by Cande-laria and the deeper parts of the mines in the Punta del Cobredistrict, ore is associated with mainly calcic-potassic (calcicamphibole epidote-biotite, K feldspar) alteration (see alsoTable 2). Magnetite is abundant and occurs predominantly as

massive replacement bodies that grade laterally into intensepervasive iron metasomatized (magnetite) volcanic and vol-caniclastic rocks. Both early and late hematite are locally pre-sent in minor quantities. Quartz veins and veinlets are com-mon and silicification is widespread. Calcite veinlets andcarbonatization are minor or absent. Anhydrite locally occursin veins and veinlets.

The intermediate parts of the system are represented bythe central upper portion of the Santos and Carola mines andare characterized by intense potassic (biotite and/or Kfeldspar) alteration, in places with subordinate calcic amphi-bole alteration. Pervasive biotitization of the host rock gradesinto intense chloritization toward more shallower levels, i.e.,

external parts. Magnetite occurs in veins and disseminated inthe host rocks. Hematite is common and more abundant thanin the internal parts of the system.

In the external parts of the system, represented by the So-cavn Rampa mine, mineralization is associated with intensesodic (albite) alteration and chloritization. Pervasive carbonati-zation and calcite veins are common and may be intense,

whereas silicification and quartz veins are essentially absent.Hematite is the predominant iron oxide mineral, whereas mag-netite is uncommon. However, iron oxides may be minor or vir-tually absent in some places with significant amounts of sulfides.

The structural control and geometry of the ore, particularlythat of the Punta del Cobre district, suggest that ascending

fluids were focused by north-northwest to northwest-trend-ing faults to form subvertical bodies in volcanic rocks. Wherethese fluids reached permeable horizons they spread out lat-erally to form concordant lens-shaped orebodies, e.g., inbrecciated flow tops and intercalated volcaniclastic horizons,but mainly in the Basal Breccia, the lowermost part of thesediments of the Algarrobos Member that overlies the vol-

canic host rocks (Marschik and Fontbot, 1996).Fluid inclusions in postmagnetite quartz, which containsinterstitial chalcopyrite from the Candelaria deposit homoge-nize at 370 to >440C (Marschik et al., 2000). Similar ho-mogenization temperatures of 328C for hypersaline CO2-rich fluid inclusions in quartz from Candelaria are reportedby Ullrich and Clark (1999). Liquid-vapor inclusions in anhy-drite that formed coevally with the main copper mineraliza-tion at Candelaria homogenize between 340 and 470C andliquid-vapor inclusions in calcite at temperatures of 180C(Marschik et al., 2000). Microthermometric measurementson fluid inclusions in postore calcite from the Punta delCobre deposits indicate that this late-stage fluid was alsosaline, containing 12 to 24 wt percent NaCl equiv and 13 to23 wt percent CaCl2 (Marschik et al., 1997b). Homogeniza-tion temperatures obtained are between 125 and 175C.Rabbia et al. (1996) reported homogenization temperaturesof fluid inclusions in calcite from the Carola mine between175 and 236C.

Quartz associated with chalcopyrite from the Candelariadeposit has 18OSMOWvalues between 11.2 to 12.6 per mil(Marschik et al., 2000). A fluid in isotopic equilibrium withthis quartz at temperatures between 370 and 440C has18OSMOWvalues between 5.9 and 8.9 per mil (using isotopicfractionation factors of Friedman and ONeil, 1977). This iso-topic composition is compatible with a fluid of magmatic originor a nonmagmatic fluid equilibrated with silicates at high

temperature. Oxygen isotope ratios of calcite from the Carolamine are between 15.4 and 15.9 per mil (Rabbia et al., 1996).The 18OSMOWvalues of calcite from the Santos and SocavnRampa mines range from 14.3 to 15.3 per mil, and those ofcalcite from the Candelaria deposit are between 11.7 and 11.9per mil (Marschik et al., 2000). A fluid in equilibrium with thecalcite from the Carola mine would have 18OSMOWvalues ap-proximately between 4.6 and 7.7 per mil for a temperaturerange of 175 to 235C. Accordingly, a fluid in equilibrium

with the calcite from the Santos and Socavn Rampa mineswould have an oxygen isotope composition between 2.8 and+4.7 per mil and with the calcite from the Candelaria depositbetween 5.4 and +1.3 per mil at temperatures of 100 to

180C. The relatively low oxygen isotope ratios of a fluid inequilibrium with the late calcite at Santos and SocavnRampa suggest that mixing between a magmatic fluid (orequilibrated with magmatic rocks) with a nonmagmatic fluid(e.g., basinal brine or meteoric waters) took place during thelate stages of hydrothermal activity.

Sulfur isotope ratios of sulfides from several mines of thePunta del Cobre belt determined for this study are fairly uni-form. The narrow range of 34SCDT (0.7 and +3.1) near 0per mil is consistent with a magmatic sulfur source and in-compatible with significant input of sulfur derived from evap-orites. The latter would imply more oxidized conditions thanindicated by the paragenetic study and a larger spread in the


0361-0128/98/000/000-00 $6.00 1819


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34SCDTvalues. The 34SCDTvalues of anhydrite from Cande-laria between 14.5 and 17.5 per mil overlap with those sug-gested for Cretaceous seawater (Claypool et al., 1980). How-ever, similar 34S values for sulfates are reported also fromporphyry copper and high-sulfidation gold deposits (e.g.,Ohmoto and Rye, 1979; Arribas, 1995). The sulfate-sulfide for asample with coexisting anhydrite and chalcopyrite is 13.4 per

mil, which is consistent sulfide-sulfate fractionation at tem-peratures on the order of 400C (Ohmoto and Rye, 1979;Ohmoto and Lasaga, 1982). Taking the paragenetic positionof anhydrite within the main sulfide stage and the homoge-nization temperatures of fluid inclusions in anhydrite(340470C) into account (Fig. 9), the data are consistent

with a predominantly cooling magmatic sulfur-bearing fluid.As discussed above, mineralization occurred under fairly

oxidized conditions manifested in the formation of early spec-ular hematite (hm I; Figs. 5n and 9). Pseudomorphic replace-ment of this early specular hematite by magnetite (mushke-tovite; Fig. 5o) and deposition of magnetite (mt I; Fig. 5p)could be explained by an increase in temperature as the hy-

drothermal system reached its climax at the site of ore forma-tion (waxing stage) and/or by interaction of the oxidized fluidwith Fe2+ abundantly contained in the andesitic host rock. Al-teration mineral associations indicate that the depositsformed by near-pH neutral solutions. The oxidizing characterof the fluid explains its metal transport capacity.

The bulk of the iron oxide mineralization probably oc-curred at temperatures of about 500 to 600C. Thesetemperature estimates are based on biotite-garnet Fe-Mgexchange geothermometry on early (pre-chalcopyrite) alter-ation in the tuffaceous rocks of the Algarrobos Member atCandelaria, determined by Ullrich and Clark (1997). Themain sulfide stage follows with the formation of pyrite andchalcopyrite. Ore formation temperatures of about 400 to

500C at the Carola mine were estimated based on miner-alogic evidence (Hopf, 1990). Homogenization temperaturesof fluid inclusions in quartz and anhydrite from the Cande-laria deposit suggest temperatures in the range of 328 to>470C for the main copper mineralization at Candelaria(Ullrich and Clark, 1999; Marschik et al., 2000; see above).Further cooling of the hydrothermal system is indicated bythe homogenization temperatures of 236C of fluid inclu-sions in late-stage calcite (Rabbia et al., 1996; Marschik et al.,1997b, 2000). Martitization of magnetite and new formationof hematite (hm II) during the latest stage of mineralizationare consistent with progressive cooling of the system. An in-crease in oxygen fugacity may also contribute to cause this

mineralogical change. That the ore fluid had higher oxygen fu-gacities at the upper limit and distal portions of the ore systemis also suggested by the above-mentioned variations towardlighter sulfur isotope composition of chalcopyrite and pyrite.

Preliminary Pb isotope studies (Marschik et al., 1997a;Marschik and Chiaradia, 2000) show that the sulfide mineralsand the least altered volcanic and intrusive rocks in the Can-delaria-Punta del Cobre area have similar isotopic composi-tions. The relatively narrow isotopic compositional range ofthe ore minerals, with 208Pb/204Pb values between 38.2 and38.4, 207Pb/204Pb values between 15.57 and 15.59, and 206Pb/204Pb values between 18.45 and 18.62, indicates that the hy-drothermal fluid had a homogeneous Pb isotope composition.

Initial 187Os/188Os values of 0.36 0.1 for an isochron calcu-lated from data of hydrothermal magnetite and sulfide fromCandelaria (Mathur, 2000; Mathur et al. (2002), and an initial187Os/188Os value of 0.33 for sulfide from the Bronce mine aresimilar to calculated initial 187Os/188Os values that range from0.20 that 0.41 for magmatic magnetite in nearby batholithicrocks. The similarities in initial 187Os/188Os values of the ore

and magmatic oxides and in the Pb isotope compositions ofsulfides and granitoid plutons is consistent with the hypothe-sis that the batholithic magmas could be the metal source forthe Candelaria-Punta del Cobre iron oxide Cu-Au deposits.The hydrothermally altered dacite porphyry dikes, althoughbarren or only weakly mineralized, could represent an ex-pression of an underlying igneous body that was responsiblefor mineralization.

We underline the significance of the presence of specularhematite (later pseudomorphously replaced by magnetite,mushketovite) as an indicator for the oxidized character of thefirst mineralization stage. The presence of early hematite orof mushketovite is a typical feature of the deposits of the Can-delaria-Punta del Cobre Cu-Au(-Zn-Ag) system and is proba-bly a widespread phenomenon in iron oxide Cu-Au systems(e.g., Sossego, Brazil; (e.g., Sossego, Brazil, R. Marschik,unpub. data; Salobo, Brazil, Requia, 2002; Starra, Cloncurrydistrict, Australia; Rotherham, 1997; and Ral-Condestable,Per, Haller, 2000). In other deposits it may have been over-looked. For instance, bladed magnetite from the EmmieBluff deposit, Stuart Shelf, Australia (photo plate of fig. 2b,Gow et al., 1994), is likely to represent mushketovite.

A variety of data are consistent with, but not unequivocallyindicative of, magmatic fluid contribution to this hydrother-mal system. This evidence includes: (1) oxygen isotope com-position of quartz associated with chalcopyrite representingthe main mineralization at Candelaria; (2) presence of hyper-

saline CO2-rich fluid inclusions (Ullrich and Clark, 1999) inthe main ore stage, as well as of saline fluid inclusions in late-stage calcite (Marschik et al., 1997b); (3) the 18OSMOW com-position of 4.4 to 7.7 per mil of a fluid in equilibrium withcalcite from the Carola mine (Rabbia et al., 1996); (4) the ox-idized character of the ore fluid; (5) the narrow range of34SCDTvalues of sulfides near 0 per mil; and (6) the age ofthe mineralization coeval with nearby intrusive activity. Thesimilarities and transitions in terms of fluid composition andhost-rock alteration of the Candelaria-Punta del Cobre de-posits with porphyry skarn systems (e.g., Einaudi et al., 1981,Einaudi, 1982; Lang et al., 1995) provide additional empiricalarguments that magmatic-sourced fluid is an essential part of

the ore system.Cooling and fluid mixing are probably the main precipita-tion mechanisms at Candelaria-Punta del Cobre. Mineraliza-tion typically, but not exclusively, occurred near the contact ofmassive, relatively impermeable volcanic rocks and overlyingporous and permeable volcaniclastic sediments. These sedi-ments and the upper part of the faulted volcanic rocks weremost likely saturated with either basinal brines, formation, ormeteoric waters, which could contribute to the light oxygenisotope signatures mentioned above. An ascending salinemagmatic(?) metal and sulfur-bearing fluid would have beendiluted and cooled, causing saturation with respect to sulfidesas it interacted with this external fluid, thereby precipitating


0361-0128/98/000/000-00 $6.00 1820


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the ore minerals. During the early stages of mineralization,reduction of the oxidized fluid through interaction with the

volcanic host rocks could have been an additional precipita-tion mechanism. On the basis of the present data, however, itis not possible to dismiss a participation of evaporite-sourcedbrines as proposed by Barton and Johnson (2000) and Ullrichand Clark (1999).

Alteration and mineralization ages

Determination of alteration and mineralization ages atCandelaria-Punta del Cobre has been the objective of several

contributions. Figure 13 is a compilation of the relevant agedata, relating the absolute ages to the stratigraphic record.The mineralization age at Candelaria is best represented bytwo Re-Os molybdenite ages of 114.2 0.6 and 115.2 0.6Ma (Mathur, 2000; Mathur et al., 2002). Previously publishedalteration ages cluster around 116 to 114 and 112 to 110 Ma(e.g., Marschik et al., 1997b; Arvalo, 1999; Ullrich and Clark,

1999). The new alteration ages presented in Table 5 and Fig-ure 12 fall into the 116 to 114 Ma age range. The 40Ar/39Arweighted mean plateau age of 115.14 0.18 Ma for green hy-drothermal biotite associated with chalcopyrite-pyrite from


0361-0128/98/000/000-00 $6.00 1821

80

90

100

110

120

130

140

Age (Ma)

Albian

Aptian

Cenomanian

Turonian

Berriasian

Valanginian

Hauterivian

Barremian

Mineralization age is 114.7 Ma

Coniacian

Santonian

Campanian

Regional cooling to 200-150C

(closure temperature of K-feldspar)

P. del Cobre

Totoralillo

Stratigraphic position of the main orebodies

Cerrillos

Hiatus

(basin inversion,

transpression)

Unconformity

Pabelln

Stratigraphy Stage

Nantoco-Abundancia

Overburden at the time of

mineralization is less than 3 km

170

0- 2

00

0m

Minimum age of mineralization(post-ore thermal events?)

40Ar/39Ar amphibole pseudo plateau age (sample PC 98137, this study)

Rb-Sr isochron (Marschik et al., 1997)

40Ar/39Ar biotite inverse isochron age (Marschik et al., 1997)

40

Ar/39

Ar biotite plateau age (Ullrich and Clark, 1999)40Ar/39Ar biotite total fusion age (Marschik et al., 1997)

Re-Os molybdenite ages (Mathur, 2000)

40Ar/39Ar biotite plateau age (sample PC 99139, this study)

40Ar/39Ar biotite plateau age (sample PC 99131, this study)

Batholithemplacement

(main mineralization)

40Ar/39Ar whole rock plateau age (Marschik et al., 1997)

40Ar/39Ar whole rock inverse isochron age (Marschik et al., 1997)

40Ar/39Ar biotite isochron age (Arvalo, 1999)

40Ar/39Ar amphibole plateau age (Ullrich and Clark, 1999)

K-Ar whole rock age (Marschik et al., 1997)

40Ar/39Ar whole rock isochron age (Arvalo, 1999)

FIG. 13. Summary of isotopic ages of rocks and minerals of the Candelaria-Punta del Cobre iron oxide Cu-Au deposits.Correlation of absolute and relative stratigraphic ages, according to Gradstein et al. (1995). Metallic mineralization at Can-delaria-Punta del Cobre commenced as marine conditions ceased in the Atacama back-arc basin.


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Candelaria (sample PC 99131, Fig. 12, Table 5) is identicalwithin errors with a 40Ar/39Ar inverse isochron age of 114.9 1.0 Ma for ore-related green hydrothermal biotite at theSantos mine and a less precise total fusion age of 114.6 1.4Ma, similar to biotite from the Resguardo mine in the Puntadel Cobre district (Marschik et al., 1997b). These biotite agesare similar to the Re-Os molybdenite ages and are interpreted

to have recorded the same alteration event that predated andaccompanied parts of the Cu-Au mineralization. The pseudo-plateau age of 116.6 1.2 Ma of amphibole intergrown withchalcopyrite and pyrrhotite (sample PC 98137; Fig. 12, Table5), although imprecise and only taken as an approximation, iscompatible with the Re-Os molybdenite and the 40Ar/39Ar bi-otite ages mentioned above. Brown biotite from the barrenbiotite-almandine cordierite alteration in the Tuff unit(Trinidad Siltstone) at Candelaria with a weighted meanplateau age of 116.51 0.26 Ma (sample PC 99139, Fig. 12,Table 5) is somewhat older than the green biotite. Ullrich andClark (1998, 1999) report 40Ar/39Ar plateau ages averaging114.2 0.8 Ma for similar biotite from the barren biotite-quartz-magnetite alteration and a 40Ar/39Ar plateau age of111.7 0.8 Ma for amphibole associated with chalcopyritefrom Candelaria. According to these authors, this later am-phibole age represents the age of copper mineralization. The40Ar/39Ar isochron ages of 110.7 1.6 Ma for biotite fromdeformed mineralized biotite-rich rocks and 111.0 1.4 Mafor a whole-rock chip of the same sample from Candelaria(Arvalo, 1999), and a 40Ar/39Ar total fusion weighted meanage (2 analyses) of green hydrothermal biotite from the Res-guardo mine (Punta del Cobre district) yielding 111.6 1.4Ma (Marschik et al., 1997b) are identical within errors withthe amphibole plateau age of Ullrich and Clark (1998, 1999).These younger alteration ages of 110 to 112 Ma could indi-cate that there is another, possibly paragenetically similar

alteration and mineralization event. This event could be re-lated to nearby 110 to 112 Ma batholithic intrusions (Fig. 2).

Implications for tectonic setting and depth of mineralization

There is evidence for episodes of Middle to Late Creta-ceous transpression (e.g., Sylvester and Palacios, 1992; Laraet al., 1996; Arvalo and Grocott, 1997; Taylor et al., 1998) as

well as extension (Mpodozis and Allmendinger, 1993) at a re-gional scale. However, their exact timing is only poorly con-strained. The age data summarized here have implications forthe structural evolution of the area. Parts of the iron oxideand the bulk of the Cu ore at Punta del Cobre are hosted bynorth-northwest to northwest-trending high-angle sinistral

transcurrent brittle faults, which were active by Aptian times(115 Ma). At Candelaria, mineralization occurred in thesesame north-northwest to northwest faults and in the north-northeasttrending Candelaria shear zone. The latter showsindication of minor movements during the early iron metaso-matism but was essentially inactive as a ductile shear zone atthe time of main Cu mineralization during which brittle con-ditions prevailed.

The northwest-trending high-angle faults play an importantrole in structural models for Middle-Late Cretaceous trans-pressional conditions for the Atacama region. Age and fielddata presented here are compatible with the onset of gener-alized transpressional conditions in the region already at the

time of mineralization. This is consistent with the geologicrecord indicating cessation of marine sedimentation in theback-arc and regional uplift in the late Aptian (e.g., Segerstromand Parker, 1959; Zentilli, 1974; Jurgan, 1977, Prez et al.,1990).

Taking into account that by around 115 Ma (Aptian) theChaarcillo Group (late Valanginian to Aptian) should have

reached its maximum thickness of 1,700 to 2,000 m, and thatthe ore deposits essentially occur in the upper part of thePunta del Cobre Formation, it is possible to estimate thatmineralization took place at about a 2- to 3-km depth (Fig.13). The change from ductile-brittle conditions during min-eralization could be explained by a temperature decrease that

went along with the regional uplift in the late Aptian.

Related iron oxide-rich deposits

Other Cretaceous ore deposits in the Andean Cordillerapresent similarities to the Candelaria-Punta del Cobre ironoxide Cu-Au(-Zn-Ag) system, although some have been inter-preted using other genetic models. These deposits includethe Manto Verde (K-Ar sericite ages are 117 3 and 121 3Ma; Vila et al., 1996), Manto Ruso (Orrego and Zamora,1991), and Teresa de Colmo iron oxide Cu-Au deposits (Hop-per and Correa, 2000) in Chile, and the Ral-Con

marschik and fontbote-2001

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