Timing and Formation of Porphyry Cu–Mo Mineralization in the Chuquicamata District

ARTICLE

Timing and formation of porphyry CuMo mineralizationin the Chuquicamata district, northern Chile:new constraints from the Toki cluster

Fernando Barra & Hugo Alcota & Sergio Rivera &Victor Valencia & Francisco Munizaga & Victor Maksaev

Received: 16 October 2012 /Accepted: 17 December 2012 /Published online: 11 January 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract The recently discovered Toki cluster, whichincludes the Toki, Quetena, Genoveva, Miranda, and Opacheporphyry CuMo prospects, is located 15 km southsouth-west of the ChuquicamataRadomiro Tomic mines in north-ern Chile. These prospects occur in an area of 56 km andare completely covered with Neogene alluvial deposits. In-ferred resources for the cluster are estimated at about 20 Mt offine copper, with Toki and Quetena contributing 88 % ofthese resources. Mineralization in these deposits is associatedwith tonalite porphyries that intruded andesites and dacites ofthe Collahuasi Group and intrusions of the FortunaLos PicosGranodioritic Complex. Hypogene mineralization in the Tokicluster consists mainly of chalcopyritebornite with minormolybdenite with mineralization grading outward to a chal-copyritepyrite zone and ultimately to a pyrite halo. Alterationis dominantly of the potassic type with K-feldspar and hydro-thermal biotite. Sericitic alteration is relatively restricted tolate quartzpyrite veins (D-type veins). Previous KAr geo-chronology for the cluster yielded ages within a range of 34 to40 Ma. Four new ReOs ages for Toki indicate that

molybdenite mineralization occurred in a single pulse at38 Ma. ReOs ages for three different molybdenite samplesfrom Quetena are within error of the Toki mineralization ages.These ages are concordant with a new zircon UPb age of38.60.7 Ma from the tonalite porphyry in Quetena. Two ReOs ages for Genoveva (38.10.2 and 38.00.2 Ma) are alsowithin error of the Toki and Quetena molybdenite ages. FourReOs molybdenite ages for Opache range between 36.4 and37.6 Ma. TheMiranda prospect is the youngest with an age of36 Ma. Four new ReOs ages for the Chuquicamata depositrange between 33 and 32 Ma, whereas nine new 40Ar/39Arages of biotite, muscovite, and K-feldspar range between 32and 31 Ma. Analyzed molybdenites have Re and Os concen-trations that vary between 213,099 ppm and 81,231 ppb,respectively. The highest Re and Os concentrations are foundin the Toki prospect. Three new 40Ar/39Ar ages for the Tokicluster are younger than the ReOs mineralization ages. Theage spectra for these three samples show evidence of excessargon and have similar inverse isochron ages of 35 Ma thatprobably reflect a late hydrothermal phyllic event. The newgeochronological data presented here for the Toki clusterindicate that molybdenite mineralization occurred within avery short period, probably within 2 Ma, and synchronously(at 38 Ma) in three mineralization centers (Toki, Quetena,and Genoveva). Furthermore, mineralization at the Toki clus-ter preceded the emplacement of the Chuquicamata deposit(3531 Ma) and indicates that porphyry CuMo mineraliza-tion occurred episodically over a period of several millionyears in the Chuquicamata district.

Introduction

Chile is the major copper producer in the world with morethan 5.26 Mt of copper in 2011 (www.cochilco.com). Mostof the copper resources are centered in porphyry copper

Editorial handling: J. Perello

F. Barra (*) : F. Munizaga :V. MaksaevDepartamento de Geologa, Universidad de Chile, Santiago, Chilee-mail: [email protected]

F. BarraAndean Geothermal Center of Excellence (CEGA),Universidad de Chile, Santiago, Chile

H. Alcota : S. RiveraExploraciones Mineras Andinas S.A., Santiago, Chile

V. ValenciaSchool of Environment, Washington State University, Pullman,WA, USA

S. RiveraCoro Mining Corp, Santiago, Chile

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deposits, and some of the largest deposits of this type, suchas Chuquicamata, Escondida, and Collahuasi among others,are located in northern Chile. These world-class depositshave been extensively studied and are continually underscrutiny in order to better understand how porphyry copperdeposits form and what factors control their size or theamount of mineral resources contained in a deposit.

Considering the high concentration of large porphyrycopper deposits in northern Chile that have been foundand are under exploitation, it is not surprising that greatefforts are made for exploration of the region. This concen-trated exploration effort by many international companieshas been rewarded with the discovery of new copper andgold resources (e.g., Camus and Dilles 2001; Sillitoe 2004;Sillitoe and Perell 2005). A noteworthy example is theextensive exploration program initiated in 1991 byCodelco-Chile (Behn et al. 2001; Camus 2003) that yieldedthe discovery of Opache in 1996, Genoveva in 1999, Toki in2000, Quetena in 2002, and Miranda in 2006, all within theChuquicamata district (Fig. 1). These deposits are spatiallyvery close, and as a group are known as the Toki cluster.

The tendency of porphyry copper deposits to occur in clus-ters and/or alignments has long been recognized in the Amer-ican Southwest province (Titley 1982; Damon et al. 1983), andnumerous examples have also been documented in Chile (e.g.,Potrerillos, Olson 1989; Marsh et al. 1997; La Fortuna-ElNegro, Perell et al. 1996; Quebrada Blanca-Collahuasi(Rosario)-Ujina, Masterman et al. 2004; Zaldivar-Escondida-Escondida Norte-Chimborazo, Richards et al. 2001; Padilla-Garza et al. 2001, 2004). However, it is still unclear whether thedeposits that compose a cluster are emplaced at the same timeor during a short restricted period, or are the result of a longprotracted period of magmatichydrothermal activity.

The ReOs isotopic system applied to molybdenite pro-vides valuable information on the timing of molybdenitemineralization once the suitability of samples has beendetermined (see Barra et al. 2003 and references therein).Moreover, recent studies have shown that ReOs molybde-nite ages can provide an estimation of the duration ofhydrothermal systems and the number of molybdenite min-eralization events within a single ore deposit (Jensen 1998;Selby and Creaser 2001; Barra et al. 2003, 2005; Maksaev etal. 2004; Cannell et al. 2005).

In the Toki cluster, the age of mineralization has beenindirectly determined by KAr geochronology on associatedintrusives (whole rock ages) and hydrothermal biotite(Camus 2003), but the high uncertainty associated withthese determinations does not allow for a proper evaluationof the age and the relative timing of mineralization eventsthat formed the cluster. In this contribution, we present abrief description of these recently discovered deposits (Toki,Quetena, Genoveva, Miranda, and Opache), and we providea new UPb zircon age for the tonalite porphyry in Quetena,

precise ReOs molybdenite ages, and 40Ar/39Ar ages forthese deposits and for the large Chuquicamata deposit tobetter constrain the age of formation of deposits within theChuquicamata district.

Geologic setting

The Chuquicamata district includes the Chuquicamata andRadomiro Tomic megadeposits, the exotic copper depositMina Sur, the newly developed Ministro Hales mine (for-merly known as Mansa Mina) located 5.5 km south ofChuquicamata, and the deposits of the Toki cluster (Toki,Genoveva, Quetena, Miranda, and Opache) located about15 km SSW of Chuquicamata (Fig. 1). Total identifiedresources for the Chuquicamata district are estimated at107.4 Mt of fine copper considering a cutoff Cu grade of0.2 wt% (Codelco Memoria Anual 2011). Inferred resourcesfor the cluster are estimated at about 20 Mt of fine copper,with Toki contributing with ca. 11.2 Mt and Quetena with3.8 Mt (Camus 2003; Rivera et al. 2006).

The deposits of the district are located in the Late Eo-ceneEarly Oligocene (4331 Ma) porphyry copper belt thatextends for about 1,400 km, from 18S to 31S (Sillitoe1988; Sillitoe and Perell 2005). The rocks in the area rangein age from Paleozoic to Tertiary. The main structural fea-ture observed is the West Fault, which is part of theDomeyko Fault System (Boric et al. 1990; Dilles et al.1997) and divides the district in two domains: an easterndomain and a western domain.

The oldest rocks in the area are represented by Paleozoicschists of the Limn Verde metamorphic complex that cropout in the western domain (Fig. 1). A PaleozoicEarlyMesozoic volcanosedimentary sequence composed mainlyof sedimentary rocks at the base with pyroclastic flows ofdacitic composition, andesites and andesitic breccias at thetop has been identified in the Toki area. This volcanosedimentary sequence is probably correlated with the Colla-huasi Group (Tomlinson et al. 2001; Munizaga et al. 2008),and are covered with alluvial deposits. The sequence hasalso been recognized in nearby outcrops and in the subsur-face close to the Ministro Hales deposit, where it was datedat 232303 Ma by UPb in zircon (Camus 2003). A seriesof granitoids of Late Paleozoic age occur in the district andare grouped in the Cerro Chuquicamata plutonic complex(Fig. 1) including the Chuquicamata Hills Diorite (267273 Ma hornblende and biotite KAr ages, Tomlinson etal. 2001), Mina Sur Granodiorite, and the Mesa Granite (KAr biotite age of 3054 Ma, Marinovich and Lahsen 1984;

Fig. 1 Geologic map of the Chuquicamata district showing the loca-tion of mines and prospects (Modified from Tomlinson et al. 2001;Tomlinson and Blanco 2008 and Codelco unpublished maps)

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ID-TIMS U-Pb zircon age of 296.92.1 Ma, Tomlinson andBlanco 2008). Other relevant intrusions are the East Granodi-orite of Triassic age (2291 Ma UPb in zircon, Tomlinson etal. 2001) and the Elena Granodiorite. Previous geochronologyon the Elena Granodiorite yielded ages from Jurassic to EarlyCretaceous (1223.8 Ma, KAr in biotite, Ambrus 1977). Amore recent UPb age determination in zircon indicates aTriassic age of 2272 Ma (Tomlinson et al. 2001), althoughin the Carmen area, east of the Chuquicamata mine, this unityielded UPb zircon ages of 233.12.2 and 231.42.0 Ma(Proffett and Dilles 2007). Other Triassic units in the Chuqui-camata district are represented by volcanic rocks of the Colla-huasi Group, which have been dated at 231.60.5 Ma(Tomlinson et al. 2001).

In the western domain, particularly in the southwesternquadrant of the district extensive outcrops of Jurassic rockscomposed mainly of marine sedimentary rocks (limestonesand sandstones; Caracoles Group) overlie the Paleozoicmetamorphic basement and PaleozoicTriassic CollahuasiGroup. Overlying the Jurassic sequence are Lower Creta-ceous continental sandstones of the Cerritos Bayos Forma-tion and volcanosedimentary deposits from the QuebradaMala and Icanche Formations of Late Cretaceous and Eo-cene age, respectively. The Late Cretaceous age of theQuebrada Mala Formation has been confirmed by UPbdating on zircons from a sample from the Cerro Negro area(72.53.1 Ma, Codelco internal report) immediately to thenorth of the Toki cluster (Fig. 1). West of the Chuquicamatadistrict, the Quebrada Mala Formation is intruded by quartzdiorite to quartz monzonite plutonic rocks of the Monte-cristo Intrusive Complex. UPb zircon dating using laserablation ICP-MS yielded an age of 62.70.5 Ma (Campbellet al. 2006).

Extensive EoceneOligocene magmatism is representedby the Los Picos Monzodioritic Complex, the FortunaGranodioritic Complex, and the Chuquicamata IntrusiveComplex, also known as the Chuqui Porphyry Complex(Dilles et al. 1997; Ossandn et al. 2001). The FortunaGranodioritic Complex and Los Picos Monzodioritic Com-plex crop out in the western domain and are temporally andspatially associated with the deposits of the Toki cluster,whereas the Chuquicamata Intrusive Complex is associatedwith the Chuquicamata and Radomiro Tomic deposits in theeastern domain and the Ministro Hales deposit in the west-ern domain (Fig. 1). The Fortuna Granodioritic Complex isa polyphase pluton composed of the Antena granodiorite,Fiesta hornblende granodiorite, San Lorenzo granodioriteand tonalite porphyries, and Tetera aplite porphyries (Dilleset al. 1997; Tomlinson et al. 2001; Ossandn et al. 2001).Los Picos quartz monzodiorite is the oldest unit with UPbzircon geochronology by laser ablation ICP-MS and SHRIMPages between 43.10.6 and 41.80.3 Ma (Campbell et al.2006; Table 1). The Los Picos bodies intrude the Caracoles

Group and the Quebrada Mala and Icanche Formations and isin turn intruded by the Antena granodiorite, which has beendated at 39.30.4 Ma (ID-TIMS U-Pb zircon age, Dilles et al.1997) and 38.80.6 Ma (laser ablation ICP-MS UPb zirconage, Campbell et al. 2006). The Fiesta hornblende granodio-rite, with UPb zircon ages of 37.60.7 (Dilles et al. 1997),39.40.5, and 38.20.3 Ma (Campbell et al. 2006), alsointrudes the Los Picos and the Antena granodiorite, and it isintruded by the San Lorenzo tonalite porphyries (38.20.8 Ma, Campbell et al. 2006) and Tetera aplite porphyries(Dilles et al. 1997; Tomlinson et al. 2001). The ChuquicamataIntrusive Complex is composed of four porphyry units: theWest, Fine Texture, East, and Banco porphyries. Descriptionsof all these units have been reported by Ossandn et al. (2001)and Camus (2003). Ballard et al. (2001) and Campbell et al.(2006) reported laser ablation ICP-MS and SHRIMP UPbzircon ages that range from 34.5 to 33.1 Ma (Table 1)

Post-mineralization continental clastic sedimentarysequences with ash and ignimbrite intercalations are widelydistributed in the district. These sequences have beengrouped into the El Loa Group, which comprises the Jal-quinche Formation and the Opache Formation (May et al.2005). The Opache Formation covers the Opache prospect,whereas Miocene and younger gravels cover the RadomiroTomic, Mina Sur, Ministro Hales, Genoveva, Quetena, andToki deposits (Fig. 1).

Geology of the Toki cluster

Lithological units

The deposits of the Toki cluster (Opache, Toki, Genoveva,Quetena, and Miranda) are located in the western domain ofthe Chuquicamata district at approximately 15 km SSW ofthe Chuquicamata pit and within an area of 30 km2. Theoldest rocks recognized in the area are the Limn Verdeschists of Paleozoic age (Fig. 1). This unit is followed by asequence of sedimentary rocks, pyroclastic flows, andesitesand andesitic breccias, in part correlated with the CollahuasiGroup (UPb age of 232303 Ma; Tomlinson et al. 2001)exposed in the Radomiro Tomic deposit area (Fig. 1). Thepyroclastic flows of dacitic composition have been datedusing laser ablation ICP-MS UPb zircon technique yield-ing Permian ages of 2799 Ma in the Toki area and 273.511.0 Ma at Quetena (Codelco internal report). Overlying thisPermian volcanosedimentary sequence are Jurassic sedi-ments and Cretaceous volcanic rocks.

The Fortuna Granodioritic Complex and Los Picos Mon-zodioritic Complex intrude the Paleozoic schists, and theJurassic and Cretaceous volcanosedimentary sequences. Inthe Toki area, these complexes include a series of porphyrieswith composition ranging from diorite and monzodiorite to

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granodiorite, tonalite, and dacite (Dilles et al. 1997; Riveraand Pardo 2004; Tomlinson and Blanco 2008). Mineralizationin the district is associated with breccias and tonalitic togranodioritic porphyries of the Fortuna complex, which in-trude andesites and dacites of the Collahuasi Group, the LosPicos Monzodiorite Complex, and the Antena granodiorite(Figs. 2 and 3). The latter is the main host rock for the San

Lorenzo tonalite porphyries which form elongate bodies witha NNE orientation in the Toki and Quetena area, and a WNWorientation at Opache (Fig. 2). At Toki, the San Lorenzotonalite porphyry has a zircon UPb age of 38.80.5 Ma(Rivera and Pardo 2004). At Genoveva, the host rocks to theSan Lorenzo porphyries comprise a succession of fine-grainedsandstones, laminated mudstones, local conglomerate, and

Table 1 Compiled 40Ar/39Ar and UPb ages for the Chuquicamata district

Unit Age (Ma) Method/mineral dated Reference

Los Picos quartz monzodiorite 43.10.6 UPb SHRIMP/zircon Campbell et al. (2006)

41.80.3 UPb LA-ICPMS/zircon Campbell et al. (2006)


Antena granodiorite 38.80.6 UPb LA-ICPMS/zircon Campbell et al. (2006)

39.30.4 UPb ID-TIMS/zircon Dilles et al. (1997)

Fiesta hornblende granodiorite 39.40.5 UPb LA-ICPMS/zircon Campbell et al. (2006)


37.60.7 UPb ID-TIMS/zircon Dilles et al. (1997)

San Lorenzo rhyodacitic porphyry 38.20.8 UPb LA-ICPMS/zircon Campbell et al. (2006)

East porphyry 35.20.4 UPb SHRIMP/zircon Ballard et al. (2001)

34.60.3 UPb LA-ICPMS/zircon Ballard et al. (2001)

33.70.2 ArAr/K-feldspar (potassic alteration) Reynolds et al. (1998)




31.90.2 ArAr/biotite (potassic alteration) Reynolds et al. (1998)





31.60.3 ArAr/K-feldspar (sericitic alteration) Reynolds et al. (1998)

30.90.2 ArAr/K-feldspar (sericitic alteration) Reynolds et al. (1998)

31.10.2 ArAr/sericite (sericitic alteration) Reynolds et al. (1998)

31.20.2 ArAr/biotite (sericitic alteration) Reynolds et al. (1998)

West porphyry 34.00.3 UPb SHRIMP/zircon Ballard et al. (2001)


Bench porphyry 34.10.3 UPb SHRIMP/zircon Ballard et al. (2001)



RT major 35.00.3 UPb SHRIMP/zircon Campbell et al. (2006)


31.80.3 ArAr/sericite (sericitic alteration) Cuadra and Rojas (2001)

RT minor 33.90.4 UPb SHRIMP/zircon Campbell et al. (2006)

MM porphyry 38.90.4 UPb LA-ICPMS/zircon Boric et al. (2009)

Quartz porphyry in Ministro Hales 35.50.6 UPb LA-ICPMS/zircon Boric et al. (2009)

Opache porphyry 37.31.0 UPb SHRIMP/zircon Campbell et al. (2006)


Toki tonalite 38.80.5 UPb SHRIMP?/zircon Rivera and Pardo (2004)

34.50.2 ArAr/sericite from D-vein halo Rivera and Pardo (2004)

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Fig. 2 Geologic map of the Toki cluster interpreted at the 1,900-m level. Also shown are the location of drill hole samples. Precise location andcharacterization of samples are indicated in Table 2 (Source: unpublished map, Exploraciones Mineras Andinas exploration team)

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coarse-grained sandstones, with a SHRIMP UPb zircon ageof 300.31.8 Ma for an interbedded rhyolitic tuff (Tomlinsonand Blanco 2008). These sedimentary and volcanic rocks ofPermo-Triassic age have NS strikes and dip to the west andhave been metamorphosed by the intrusion of the FortunaGranodioritic Complex and the Los Picos Monzodioritic

Complex. Porphyry copper mineralization at Toki, Quetena,and Genoveva is genetically and spatially associated with theintrusion of the San Lorenzo tonalite porphyries.

The Opache prospect is associated with a series ofporphyry units (Early and Late San Lorenzo tonaliteporphyries) that intrude diorites, tonalites, granodiorites,

Fig. 3 Cross sections of the Toki, Quetena, Genoveva, and Miranda prospects showing rock types, distribution of alteration zones, andmineralization profiles (section 7.521.400 N) (Source: unpublished sections, Exploraciones Mineras Andinas exploration team)

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monzonites, and monzodiorites of La Fortuna and LosPicos complexes (Camus 2003) and Permo-Triassicmetasedimentary rocks and andesites from the Colla-huasi Group. The San Lorenzo late porphyry is locallyknown as the Opache porphyry and has a laser ablationICP-MS UPb zircon age of 37.31.0 Ma (Campbell etal. 2006). Associated to these porphyry units are brecciabodies of igneous (Negra breccia) and hydrothermal(Main breccia) origin.

The Toki cluster (Fig. 3) is almost entirely covered withunconsolidated alluvial gravels of the El Loa Group andother undifferentiated deposits that reach a thickness of150 to 220 m in Toki and Miranda, about 70 to 100 m inQuetena, and 10 to 50 m at Genoveva. A widespread thinash layer interbedded within the gravel cover has been datedat 9.6 Ma (Rivera et al. 2006).

Structures

Two main structural systems control the emplacement ofthe mineralization at Toki, Miranda, and Quetena(Fig. 2). A NNE to NE system controls the emplace-ment of tonalite porphyry bodies and its associatedprimary mineralization. This system appears to be relat-ed to structures in the Ministro Hales area which have asimilar orientation. The second structural system (NNWto NW) controls the emplacement of late veins and themorphology of the secondary enrichment blanket andcopper oxide mineralization. A third post-mineral faultsystem with an orientation NW to WNW offsets thesupergene blanket (Rivera and Pardo 2004).

A reverse fault system (Genoveva Fault) with a NSorientation and 6065E dip is recognized between Gen-oveva and Quetena (Fig. 2) and extends further southinto the Opache area (Tomlinson et al. 2001). Also atOpache, a minor structural system with an NW to SEorientation controls the emplacement of the tonalite por-phyries and dikes, breccia bodies, hydrothermal alter-ation, and mineralization.

Alteration

The most dominant type of alteration in the cluster ispotassic alteration, characterized by K-feldspar and biotite(Fig. 3) and affecting all porphyries, equigranular intru-sions, and volcanic rocks. The margins of the minerali-zation centers are characterized by a propylitic alterationhalo with chlorite and epidote. Phyllic (sericitic) alter-ation is mainly associated with D-type quartzpyrite veinhalos. An alteration assemblage of quartz, K-feldspar,andalusite, and sericite has also been recognized in Toki,possibly associated with dacitic intrusive phases (Riveraand Pardo 2004).

In contrast to the rest of the mineralized centers of thecluster, Opache is characterized by sericitic alteration(Fig. 4), predominantly in the form of thick haloes of D-type veins. This alteration is superimposed on earlier potas-sic alteration (K-feldspar, secondary biotite, and quartz) andpropylitic (chloriteepidote) alteration assemblages.

Mineralization

Hypogene copper mineralization in the cluster is centered ontonalite porphyries of the San Lorenzo suite and constituteselongated bodies with a general NNE orientation. At Toki,Miranda, and Quetena, the mineralization is characterizedby fine-grained disseminations of bornite and chalcopyritetogether with quartzsulfide stockworks. The ore bodiescontain central parts dominated by chalcopyrite and bornitewhich is best developed at Toki over an area of approxi-mately 2,500 m by 1,000 m along a NNE-trending body.Opache is dominated by an assemblage of chalcopyrite andpyrite (Fig. 4), an assemblage that is also observed at depthin Genoveva. Typically, the hypogene mineralization is con-centrated in the central zones of each deposit, although atQuetena it tends to be localized at the contacts between thecausative porphyries and their tonalitic country rocks. Ageneral lateral zoning from a central core of chalcopyriteand bornite grading outward into a chalcopyrite-dominatedzone and more externally into a pyrite zone is recognized(Fig. 3). Minor hypogene chalcocite, digenite, and covellitealso occur in Toki (Rivera and Pardo 2004) and Miranda. Inthe propylitic halo, pyrite veins are predominantly observedwith minor sphaleritegalenacarbonate veins.

Each central ore zone contains low-grade areas (pyrite,pyrite with minor chalcopyrite) that interrupt the continuityof the ore zone, and which are associated with late pyriteveins. Figure 3 portrays the distribution of copper mineraliza-tionwithin the cluster and shows that the supergene profile hasa thickness that varies from 80 m to as much as 300 m.Copper oxides that form a blanket with a thickness that rangebetween 80 and 160 m dominate the upper section. The maincopper oxide zone is composed of chrysocolla, malachite,atacamite, azurite, tenorite, and copper wad, all of which occurin fractures and veinlets. The uppermost section of the super-gene profile consists of leached rock with clays, hematite, andgoethite, which in certain parts of the cluster interrupt thecontinuity of the oxide blanket.

Copper oxides have partly replaced secondary copperminerals (chalcocite, covellite) defining a mixed zone underthe oxide zone. This mixed zone is characterized by thepresence of atacamite, malachite, hematite, native copper,cuprite, chalcocite, covellite, and minor pyrite, chalcopyrite,and bornite. This mixed zone has a thickness that rangesbetween 20 and 120 m and grades downward into theenriched sulfide blanket, below which hypogene sulfide

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mineralization dominates. The mixed zone is best developedat Toki, associated with the central core of the prospect andlocated between faults.

The secondary enrichment blanket has a strong structuralcontrol with a NNW orientation and a thickness that variesbetween 20 and 150 m. The mineralogy of this blanket ischaracterized by chalcocite with minor covellite replacing

pyrite, chalcopyrite, and bornite. Toki also shows thebest developed enrichment blanket with a thickness ofover 300 m. A 10- to 30-m-thick layer containing exoticoxide copper mineralization (copper wad and chryso-colla) has also been recognized at Quetena, Toki, andMiranda, the full potential of which has yet to bedetermined.

Fig. 4 Cross sections of the Opache prospect showing rock types, distribution of alteration zones, and mineralization profiles (section 7.515.500 N)(Source: unpublished sections, Exploraciones Mineras Andinas exploration team)

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Table 2 Description of samples dated from the Chuquicamata, Toki, Quetena, Genoveva Miranda, and Opache porphyry copper deposits

Deposit Sample name Drill core coordinates(UTM)

Method/mineral

Depth(m)

Description

Chuquicamata CHDD-7215 7,534,636 N-510,119E ReOs/mo 540.5 qtz-mo transitional B-vein in East porphyry

CHDD-7225 7,534,821 N-510,231E ReOs/mo 728.3 qtz-mo transitional B-vein in East porphyry

CHDD-7202-1 7,535,175 N-510,197E ReOs/mo 443.3 qtz-mo transitional B-vein in East porphyry

CHDD-7202-2 7,535,175 N-510,197E ReOs/mo 386.5 qtz-mo transitional B-vein in East porphyry

CHDD-7239 7,535,553 N-510,680E ArAr/bio 117.1 Deep early potassic (biotite-K-feldspar) alteration with ccp inEast porphyry

CHDD-7229 7,535,003 N-510,343E ArAr/musc 732.6 Early potassic alteration overprinted by late sericite alteration withpy-dg-bn in East porphyry

CHDD-7229 7,535,003 N-510,343E ArAr/musc 691.0 Early potassic alteration overprinted by late sericite alterationwith py-dg-bn in East porphyry

CHDD-7237 7,535,462 N-510,624E ArAr/kfs 186.4 Early K-feldspar alteration associated to bn in East porphyry


CHDD-7215 7,534,635 N-510,119E ArAr/musc 566.0 Late sericite alteration with py-en in East porphyry


CHDD-6602 7,535,497 N-510,744E ArAr/bio 474.2 Early gray-green sericite with bn-dg in East porphyry

CHDD-6602 7535497 N-510744E ArAr/musc 857.7 Late sericite alteration with py-cv in East porphyry

Toki AD-1157A 7,519,807 N-506,088E ReOs/mo 863.5 Moderate sericite alteration, qtz-ccp-mo transitional B-vein intonalite porphyry

AD-1005A 7,520,599 N-505,996E ReOs/mo 718.5 Moderate sericite and minor chlorite alteration, ccp-motransitional B-vein in tonalite porphyry

AD-1123 7,520,199 N-506,299E ReOs/mo 503.8 Moderate sericite-chlorite-silica alteration, qtz-mo transitionalB-vein in tonalite porphyry

AD-1162 7,521,799 N-505,900E ReOs/mo 690.5 Moderate sericite and minor K-feldspar and chlorite alteration,qtz-mo transitional B-vein in hydrothermal breccia in tonaliteporphyry

AD-1123 7,520,199 N-506,299E ArAr/plg 413.6 Moderate K-feldspar and minor sericite alteration, plagioclasefrom tonalite porphyry

Quetena AD-1159 7,522,000 N-504,646E ReOs/mo 575.5 Strong K-feldspar alteration, qtz-mo transitional B-vein intonalite porphyry

AD-1185 7,521,799 N-504,099E ReOs/mo 277.3 Minor biotite-chlorite alteration, mo transitional B-vein ingranodiorite stock

AD-1705 7,522,200 N-505,000E ReOs/mo 327.6 Minor biotite-sericite alteration, ccp-mo-kfs transitionalB-vein in tonalite porphyry

AD-1159 7,522,000 N-504,646E ArAr/bio 513.0 Moderate K-feldspar alteration, biotite (primary?) in tonalite porphyry

AD-1163 7,522,000 N-504,400E UPb/zrn 499.5 Moderate K-feldspar and minor sericite alteration, zirconsfrom tonalite porphyry

Genoveva AD-966 7,521,700 N-503,200E ReOs/mo 246.0 Moderate chlorite and minor biotite alteration, qtz-moccptransitional B-vein in metandesite

AD-1012 7,521,299 N-503,401E ReOs/mo 332.7 Moderate biotite-chlorite alteration, qtz-mo transitional B-veinin tonalite porphyry

AD-1012 7,521,299 N-503,401E ArAr/bio 214.6 Moderate sericite alteration, biotite (primary?) in tonalite porphyry

Miranda DDH-2550 7,520,478 N-507,079E ReOs/mo 636.5 Moderate chlorite-sericite-K-feldspar alteration, moccptransitional B-vein in tonalite porphyry

DDH-2565 7,520,699 N-506,670E ReOs/mo 393.5 qtz-mo transitional B-vein

DDH-2569 7,520,900 N-506,800E ReOs/mo 782.7 qtz-mo-ccp transitional B-vein

Opache AD-432 7,517,507 N-503,652E ReOs/mo 363.0 Moderate biotite-chlorite alteration, mo-ccp B-vein in igneous breccia

AD-396 7,517,100 N-503,524E ReOs/mo 441.0 Moderate biotite-chlorite alteration, mo-ccp B-vein in igneous breccia

AD-430 7,517,500 N-503,408E ReOs/mo 333.0 Moderate chlorite-sericite alteration, mo transitional B-vein intonalite porphyry

All UTM coordinates are based on Datum WGS84

bio biotite, bn bornite, ccp chalcopyrite, cv covellite, dg digenite, en enargite,momolybdenite, kfsK-feldspar, plg plagioclase, py pyrite, qtz quartz, zrn zircon

638 Miner Deposita (2013) 48:629651

Sample preparation and analytical methods

Several samples from the Toki cluster were selected forisotopic dating using ReOs, 40Ar/39Ar, and UPbmethods. Sample locations are shown in Fig. 2, and inTable 2.

Molybdenite ReOs dating

Molybdenite contains high concentration of Re, usually tensto hundreds of parts per million, and essentially no commonOs; hence, all the Os contained in molybdenite is radiogenic(187Os). This property makes molybdenite a single mineralchronometer for ReOs dating that has been widely used indifferent mineral deposits.

Mineral separates for four molybdenite samples fromToki, three samples from each of Quetena, Opache, andMiranda, and two samples from Genoveva were analyzedat the ReOs lab, Department of Geosciences, University ofArizona, USA, according to the procedure described inBarra et al. (2003, 2005).

Samples were digested using the Carius tube method(Shirey and Walker 1995). About 0.5 g of sample wasloaded in the Carius tube with Re and Os spikes anddissolved in inverse aqua regia by heating in an oven at220 C for 12 h. After sample dissolution and homog-enization of the solution, Re and Os were separatedusing a distillation technique (Ngler and Frei 1997),in which Os is collected into cold HBr, dried down, andfurther purified by microdistillation technique (Birck etal. 1997). Re was purified using AG1-X8 anion ex-change column chemistry. The blank correction for mo-lybdenite was insignificant.

Re and Os were later loaded on Ni and Pt filaments,respectively, with Ba salts to enhance ionization. Measure-ments were made as negative oxides using NTIMS (Creaseret al. 1991; Vlkening et al. 1991).

40Ar/39Ar dating

Biotite and plagioclase were handpicked from crushedsamples of tonalite porphyries from Toki, Quetena, andGenoveva using a low-power binocular microscope. Theselected minerals were later irradiated at a 5-MW poolreactor of Herald type in La Reina nuclear reactor,operated by the Comisin Chilena de Energa Nuclear,Santiago, Chile. Samples were irradiated with Fish Can-yon sanidine as monitor (28.030.18 Ma, Renne et al.1994). The samples studied in this paper were irradiatedfor a period of 24 h.

After irradiation monitors were analyzed by totalfusion, and J factors were calculated for each monitorgrain. The samples were analyzed at the 40Ar/39Ar

laboratory at the Servicio Nacional de Geologa y Min-era, Santiago, Chile, using a CO2 laser wherein incre-mental heating steps were achieved by successiveincreases in the power of the laser. Following each threeheating steps a line blank was analyzed.

The noble gases were separated from the other evolvedgases by the use of a cold trap at 133 C (cold finger) and aST101 getter operated at 2.2 A. Once purified, the noblegases were introduced into a high-resolution MAP 21550mass spectrometer in electron multiplier mode. The isotopes36Ar, 37Ar, 38Ar, 39Ar, and 40Ar were analyzed and the 36/40, 37/40, 38/40, and 39/40 ratios were calculated for timezero (moment of introduction of the gas into the spectrom-eter). Spectrometer bias was corrected using periodic anal-yses of air samples, from which a correction factor(discrimination factor) was calculated.

Nine drill core samples from the Chuquicamata depositwere also dated by the 40Ar/39Ar technique at StanfordUniversity, California, USA. Detailed descriptions of sam-ple preparation, analytical procedure, and data reduction areprovided by Marsh et al. (1997).

UPb dating

A sample was collected in the Quetena prospect from a drillhole at a depth of 500 m. The sample was crushed andmilled. Heavy mineral concentrates of the

2008). The reported ages for zircon grains are based entirelyon the 206Pb/238U ratios because errors of the 207Pb/235U and206Pb/207Pb ratios are significantly greater. The larger errorsare the result of the low intensity (commonly

concentrations were found in the Toki prospect. The fourToki samples yielded the same age within error(37.9 Ma). Samples from Quetena and Genoveva alsoshow similar ages (38 Ma). Miranda has the youngestages of the cluster from 36.50.2 to 36.00.2 Ma,whereas Opache has ages ranging from 37.60.2 to36.40.2 Ma. Three new ReOs molybdenite agesreported here for the Chuquicamata deposit are the samewithin error (32 Ma) and consistent with previous mo-lybdenite ages reported by Mathur et al. (2001). Onesample (CHDD-7202-1) is slightly older with an age ofca. 33 Ma.

40Ar/39Ar ages

Three samples were analyzed by step heating using aCO2 laser. Age spectra diagrams and correlation plotsare shown in Table 4 and Fig. 5. 40Ar/39Ar spectra areconstructed using Isoplot (Ludwig 2004). Plateau agesare considered here when three or more consecutivesteps release more than 50 % of the 39Ar and theirrespective errors overlap at the 1-sigma level. SampleAD-1159 is biotite from the tonalite porphyry in Que-tena. A total of nine heating steps were performed forthis sample. Initial heating steps have increasing radio-genic yields with decreasing apparent ages suggestingexcess argon. The last five steps yielded a plateau age(Fig. 5a) with 52 % of the 39Ar of 36.20.8 Ma (2-sigma). The initial 40Ar/36Ar ratio from the inverseisochron plot (Fig. 5b) shows a value of 56657, wellabove the atmospheric value (295.5), confirming the pres-ence of excess argon; the inverse isochron yielded an age of35.40.8 Ma.

Sample AD-1012 is a biotite from a tonalite porphryin the Genoveva area. The sample yielded a seven-stepage spectrum with a plateau age of 36.10.8 Ma (2-sigma) with 75 % 39Ar (Fig. 5c). Overall, the agespectrum shows a pattern of decreasing apparent agesover the initial heating steps associated with increasingradiogenic yields, indicating the presence of excess ar-gon as also shown from the inverse isochron analysis inwhich the initial 40Ar/36Ar ratio is 563110, and anisochron age of 35.40.9 Ma (2-sigma; Fig. 5d). Theage spectrum of plagioclase sample AD-1123 from Tokiyielded a plateau age of 36.11.0 Ma (2-sigma) andshows the characteristic U-shape pattern that indicatesexcess argon (Fig. 5e). The inverse isochron initial40Ar/36Ar ratio of 307.14.7 confirms the presence ofexcess argon above the atmospheric value (Fig. 5f). Theinverse isochron yielded an age of 34.41.3 Ma.

Nine drill core samples (four muscovites, three K-feldspars, and two biotites) from the deeper section of theChuquicamata deposit were also dated by the 40Ar/39Artechnique. Samples descriptions are shown in Table 2,and the results are summarized in Table 4 and Fig. 6. Forthe Chuquicamata samples, the plateau ages are consistentand within error of the isochron ages (Table 4 and Fig. 6).The overall age range is between 32 and 30 Ma.

UPb zircon age

Nineteen zircon grains were dated from a tonalite porphyrysample in Quetena. Results are shown in Table 5 where eachline represents a spot analysis. All ages in Table 5 arereported with uncertainties at the 1-sigma level and includeonly the measurement error.

Table 4 40Ar/39Ar step-heating data for the Toki samples and Chuquicamata deposit

Sample name Mineral dated Plateau age (Ma) MSWD 39Ar in plateau (%) Isochron age (Ma) MSWD

AD-1159 Biotite 36.20.4 0.26 51.7 35.40.8 1.0

AD-1012 Biotite 36.10.4 0.97 75.1 35.40.9 1.3

AD-1123 Plagioclase 36.10.5 1.70 50.3 34.41.3 2.2

CHDD-7239 Biotite 31.90.5 0.30 98.9 31.90.4 3.0

CHDD-7229 Muscovite 31.10.7 0.27 100 31.10.4 2.4

CHDD-7229 Muscovite 30.80.4 0.56 69.7 30.90.3 5.2

CHDD-7237 K-feldspar 30.60.4 1.18 67.2 30.50.3 11.0

CHDD-7237 K-feldspar 30.70.6 0.42 75.9 30.50.3 1.4

CHDD-7215 Muscovite 31.70.7 0.54 100 31.60.5 5.6

CHDD-6602 K-feldspar 30.60.3 0.60 64.5 30.40.3 4.4

CHDD-6602 Biotite 31.80.4 0.72 100 31.80.2 5.7

CHDD-6602 Muscovite 30.60.4 0.45 100 30.50.2 3.0

Errors in ages are reported at the 2 sigma level

MSWD mean standard weighted deviation

Miner Deposita (2013) 48:629651 641

JCABRERAHighlight

JCABRERAHighlight

Zircons are clear pinkish to colorless and are doublyterminated prisms. Zircons have U concentrations that range

between 106 and 629 ppm, and U/Th ratios from 0.7 to 2.3.These zircons yielded a weighted average 206Pb/238U age of

Fig. 5 Plots of apparent 40Ar/39Ar age and isotope correlation diagrams for samples from the Toki cluster. See text for discussion

642 Miner Deposita (2013) 48:629651

38.60.7 Ma (2-sigma, n=19, MSWD=0.34; Fig. 7). Noolder component was detected in the analyzed zircons.

Discussion

Geochronology of the Toki cluster and Chuquicamata deposit

Four molybdenite samples from the Toki deposit representingdifferent vein types (Table 2) were dated using ReOs

systematics. All four veinlets yielded consistent ages of38 Ma and are similar to the ReOs molybdenite ages ofGenoveva and Quetena (Table 3 and Fig. 8). These resultsindicate that molybdenite mineralization occurred almost si-multaneously in these three porphyry copper deposits of thecluster.

The ReOs ages from Toki are slightly younger than aUPb zircon age of the tonalite host rock (38.80.5 Ma)reported by Rivera and Pardo (2004). A previous KAr ageon secondary biotite of 37.31.3 Ma, dated in Toki (Rivera

Fig. 6 Plots of apparent 40Ar/39Ar age and isotope correlation diagrams for samples from the Chuquicamata deposit

Miner Deposita (2013) 48:629651 643

et al. 2006), overlaps within error with the UPb age andwith these new ReOs ages, but more recent 40Ar/39Ardating of a sericitic halo from a late pyritequartz vein inToki yielded a much younger age of 34.520.20 Ma (Riveraand Pardo 2004). The plagioclase inverse isochron ageobtained in this study (34.41.3 Ma, 2-sigma) is similar tothe sericite age reported by Rivera and Pardo (2004), but itis younger than the molybdenite ages (Fig. 4). The

plagioclase age may represent thermal resetting by a youn-ger hydrothermal event represented by the 40Ar/39Ar sericiteage, particularly since sample AD-1123 has sericitic alter-ation and plagioclase has a low closure temperature for Ar.

In Quetena, two ReOs molybdenite ages of two differ-ent samples hosted within the mineralized tonalite porphyrybut with different associated alteration (Table 2) are identi-cal (37.90.2 Ma), and younger than a molybdenite within

Fig. 6 (continued)

644 Miner Deposita (2013) 48:629651

the granodiorite stock (sample AD-1185; 38.40.2 Ma).These results show that two mineralization events are prob-ably present in Quetena, and that these occurred within avery short time interval of less than 1 Ma. The UPb zirconage obtained for the tonalite porphyry in Quetena (38.60.7 Ma) overlaps within error with the UPb age of thetonalite porphyry dated at Toki. This suggests that the em-placement of the tonalite porphyries that are responsible for

the mineralization occurred shortly after the intrusion of theAntena granodiorite stock. The biotite inverse isochron ageof 35.40.8 Ma (2-sigma) is younger than the tonaliteporphyry crystallization age and the molybdenite minerali-zation ages. The significant difference between the UPbage and the 40Ar/39Ar age indicates that the latter might bedisturbed or might represent thermal resetting by a youngerhydrothermal event.

Fig. 6 (continued)

Miner Deposita (2013) 48:629651 645

Table5

Laser

ablationICPMSmulticollector

U-Pbzircon

data

Analysis

U206P

b/U/Th

207P

b*/

206P

b*/

Error

206P

b*/

207P

b*/

206P

b*/

Bestage

(ppm

)204P

b235U

(%)

238U

(%)

corr.

238U

(Ma)

235U

(Ma)

207P

b*(M

a)(M

a)(M

a)

TOKI-1

268

3,811

1.4

0.03685

8.9

0.00592

5.9

0.66

38.0

2.2

36.7

3.2

47.7

163.1

38.0

2.2

TOKI-2

196

3,431

1.8

0.03725

12.0

0.00601

5.0

0.42

38.6

1.9

37.1

4.4

58.6

266.0

38.6

1.9

TOKI-3

215

3,735

2.2

0.04011

7.5

0.00620

3.8

0.51

39.9

1.5

39.9

2.9

44.6

154.9

39.9

1.5

TOKI-4

262

3,163

0.9

0.04661

7.3

0.00614

4.5

0.61

39.5

1.8

46.3

3.3

414.7

129.7

39.5

1.8

TOKI-5

229

3,612

1.2

0.03771

12.2

0.00607

3.3

0.27

39.0

1.3

37.6

4.5

52.8

286.9

39.0

1.3

TOKI-6

133

2,225

0.8

0.05600

18.8

0.00614

7.7

0.41

39.4

3.0

55.3

10.1

812.7

360.5

39.4

3.0

TOKI-7

143

2,086

1.6

0.04577

9.3

0.00616

4.1

0.45

39.6

1.6

45.4

4.1

366.6

186.8

39.6

1.6

TOKI-8

166

2,331

1.3

0.04205

13.8

0.00594

2.2

0.16

38.2

0.8

41.8

5.6

255.3

314.0

38.2

0.8

TOKI-9

106

2,012

1.5

0.04447

19.5

0.00626

7.1

0.36

40.2

2.8

44.2

8.4

264.1

420.0

40.2

2.8

TOKI-10

239

2,296

1.8

0.07114

19.9

0.00620

5.6

0.28

39.9

2.2

69.8

13.4

1273.8

374.9

39.9

2.2

TOKI-11

274

4,325

1.5

0.04313

5.0

0.00610

2.9

0.57

39.2

1.1

42.9

2.1

253.6

94.9

39.2

1.1

TOKI-13

281

2,804

1.0

0.03692

6.6

0.00601

3.9

0.60

38.6

1.5

36.8

2.4

80.3

128.9

38.6

1.5

TOKI-14

136

1,377

1.1

0.03764

10.7

0.00622

5.0

0.47

40.0

2.0

37.5

3.9

115.8

233.1

40.0

2.0

TOKI-15

153

1,346

1.4

0.04784

19.6

0.00616

3.5

0.18

39.6

1.4

47.4

9.1

464.6

431.7

39.6

1.4

TOKI-16

145

889

1.2

0.07406

8.5

0.00620

4.3

0.51

39.9

1.7

72.5

6.0

1351.1

141.7

39.9

1.7

TOKI-17

107

1,506

1.5

0.05872

15.4

0.00620

6.9

0.45

39.9

2.7

57.9

8.7

887.9

285.8

39.9

2.7

TOKI-18

240

3,752

1.7

0.03651

10.2

0.00581

2.8

0.27

37.3

1.0

36.4

3.7

24.8

238.4

37.3

1.0

TOKI-19

187

2,863

1.6

0.04148

8.5

0.00586

3.8

0.45

37.7

1.4

41.3

3.4

255.8

174.7

37.7

1.4

TOKI-20

629

8,738

2.3

0.03908

4.7

0.00583

2.0

0.41

37.5

0.7

38.9

1.8

127.6

101.4

37.5

0.7

Alluncertaintiesarereported

atthe1-sigm

alevel,andincludeonly

measurementerrors.Systematicerrors

would

increase

ageuncertaintiesby

12%.U

concentrationandU/Tharecalibrated

relativetoNISTSRM

610andareaccurateto2

0%.C

ommon

Pbcorrectionisfrom

204Pb,with

compositioninterpretedfrom

StaceyandKramers(1975)

anduncertaintiesof

1.0for206Pb/204Pb,

0.3for207Pb/204Pb,

and2.0for208Pb/204Pb.

U/Pband

206Pb/207Pbfractionationiscalibratedrelativeto

fragmentsof

alargeSriLanka

zircon

of5644Ma(2-sigma).Udecayconstantsand

compositionas

follows:

238U=9.8485

10

10,235U=1.5512510

10,238U/235U=137.88

646 Miner Deposita (2013) 48:629651

In Genoveva, the two molybdenite samples dated yieldedReOs ages of 38 Ma (Table 3), similar to those deter-mined for Toki and Quetena. The 40Ar/39Ar spectrum showsevidence for excess argon, as is also shown in the inverseisochron plot. The sample also yielded an inverse isochronage which is identical to the biotite 40Ar/39Ar age of Que-tena (35 Ma) and within error of the plagioclase age fromToki.

Three different molybdenite samples were dated fromMiranda, all three ages are between 35.8 and 36.7 Ma,and indicate that mineralization at Miranda occurredalmost 2 Ma after mineralization in Toki, Quetena, andGenoveva.

Three samples were also dated from Opache. SampleAD-396 was analyzed in duplicate, and both results areconsistent within error yielding an age of 37.6 Ma. ThisReOs age overlaps within error with UPb zircon ages of37.31.0 and 37.70.6 Ma reported by Campbell et al.(2006). The other two samples yielded ages of 36.4 and36.8 Ma. This shows that mineralization in Opache occurredin at least two pulses within a period of ca. 1 Ma, similar towhat is observed for Quetena (Fig. 8).

Based on the UPb ages, we can conclude that theemplacement of the tonalite porphyries, which are relatedto the CuMo mineralization, occurred shortly after theintrusion of the Antena granodiorite stock. The ReOs mo-lybdenite ages show that this type of mineralization oc-curred as a single pulse at 38 Ma in Toki and Genoveva.The new 40Ar/39Ar ages on biotite from Quetena and Gen-oveva, and plagioclase from Toki are significantly youngerthan the molybdenite mineralization ages. Hence, we inferthat these 40Ar/39Ar ages do not constrain the timing of

alteration associated with the molybdenite mineralizationbut reflect a possible later thermal event.

Previous 40Ar/39Ar ages reported by Reynolds et al.(1998) for the Chuquicamata deposit show a range ofages from 33 to 34 Ma for K-feldspars and co-exitingbiotites from the potassic zone. On the other hand, thequartzsericite alteration was dated at 31.10.3 Ma onsericite. Reynolds et al. (1998) argued that this youngerquartzsericite alteration is probably related to a youngerpulse of porphyry intrusion not exposed at the currentlevel of the mine pit. The new 40Ar/39Ar data presentedhere from the deeper levels of the deposit show biotiteplateau ages of 32 Ma, and K-feldspar and muscoviteages in the range 3031 Ma. Both K-feldspar and biotitesamples dated in this study are associated with the deeppotassic alteration zone and might be related to theyounger porphyry intrusion as proposed by Reynolds etal. (1998), that was probably emplaced at ca. 32 Ma.Additionally, most of the ReOs molybdenite ages are32 Ma (Table 4), with the exception of one sample withan age of 33 Ma, and might also be related to thisyounger (32 Ma) intrusion.

Re concentrations

Some authors have suggested that Re concentration inmolybdenites can be used to provide insights on the originof the deposit (e.g., Mao et al. 1999; Stein et al. 2001). Maoet al. (1999) stated that molybdenites derived from a mantlesource have higher Re content than those associated to I- orS-type granite-related deposits. On the other hand, Stein etal. (2001) proposed that deposits derived from mantle

Fig. 7 UPb weighted averageplot for the tonalite porphyry atQuetena

Miner Deposita (2013) 48:629651 647

underplating or metasomatism, or from melting of maficor ultramafic rocks have molybdenites with higher Recontents than molybdenites associated with deposits de-rived from the crust. The molybdenite samples analyzedin this study provide an opportunity to test these hy-potheses. Figure 9 plots total Re content versus age andillustrates that there is no direct relation between ageand Re concentration. The five prospects studied here

are spatially very close (Figs. 1 and 2) and appear tohave formed within a very short timeframe (

and Rosario deposits) and Quebrada Blanca were formedduring the interval between 33 and 35 Ma (Masterman etal. 2004). Several other porphyry CuMo deposits innorthern Chile were emplaced during the interval 31 to38 Ma (Camus 2003 and references therein), but only afew deposits are large enough that merit exploitation.Regardless, the number of porphyry deposits formed inthe period between 30 and 40 Ma is very large (>25),and the number increases almost on a yearly basis.

There is still no consensus regarding the time neces-sary to form a large deposit. Numerical models showthat single intrusions are able to provide heat for thehydrothermal system only for a few tens of thousandsof years. Field and petrographic observations indicatethat porphyry copper systems are usually formed bythe superimposition of several magmatichydrothermalevents, some of which provide Cu and/or Mo mineral-ization whereas others are relatively barren. In thisrespect, it is necessary to remember that mineralizationages are usually determined by dating molybdenitesusing the ReOs method, but that the main commodityof interest (i.e., Cu) is usually introduced before themain Mo-rich event. Due to this limitation, the overalltiming of mineralization is biased towards molybdenitemineralization, and hence, the longevity of a minerali-zation system could be underestimated.

In the few deposits where there is a large geochro-nological database, it appears that large deposits are theresult of multiple superimposed events over a shortperiod (2 Ma; e.g., El Teniente, Chile Maksaev et al.2004; Cannell et al. 2005, Rio Blanco, Deckart et al.2005). Other deposits such as Chuquicamata and Escon-dida also appear to have been formed by the superpo-sition of several mineralization events, but over a largertime span (34 Ma). Until new geochronological data

become available and new more precise methods andinstrumentation are developed, the relation between sizeand timing/number of hydrothermalmineralizationevents remains uncertain.

Conclusions

The Toki cluster comprises five porphyry copper prospects(Toki, Quetena, Genoveva, Miranda, and Opache). Miner-alization is associated with tonalite porphyries of the For-tuna granodiorite complex that were intruded at 38 Ma.Precise ReOs ages indicate that molybdenite mineraliza-tion occurred almost simultaneously in three of the fiveprospects at 38 Ma. Miranda and Opache are slightlyyounger with ages of 3736 Ma. 40Ar/39Ar spectra forthree primary minerals show evidence of excess argon andhave similar inverse isochron ages of 35 Ma that mayreflect a late hydrothermal sericitic event that occurredaround that time. Mineralization in the Toki cluster is largesynchronous with mineralization at the large Escondidadistrict.

In Chuquicamata, four new ReOs molybdenite agesrange between 33 and 32 Ma and nine new 40Ar/39Arages range between 32 and 30 Ma. These data agreewell with previously published ages from Chuquicamata.The new geochronological information coupled withprevious dating of the Chuquicamata district suggeststhat CuMo mineralization in the area was episodicand occurred over a period of several million years.

Acknowledgments This contribution is the result of a collaborativestudy between industry and academia. We thank Exploraciones Min-eras Andinas S.A. and the Gerencia Corporativa de Exploraciones ofCodelco-Chile for funding this research and permission to publish. Wealso thank Roberto Freraut and the Subgerencia de Geologa y

Fig. 9 Plot of total Re (partsper million) in molybdenitesfrom the Chuquicamata districtversus age. See text fordiscussion

Miner Deposita (2013) 48:629651 649

Geotecnia, Divisin Codelco Norte for permission to publish data fromthe Chuquicamata deposit. We gratefully acknowledge John Dilles,Andrew Tomlinson, and Jos Perell for their critical and constructivecomments which helped to improve the manuscript.

References

Ambrus J (1977) Geology of the El Abra porphyry copper deposit,Chile. Econ Geol 72:10621085

Ballard JR, Palin JM, Williams IS, Campbell IH, Faunes A (2001) Twoages of porphyry intrusion resolved for the super-giant Chuqui-camata copper deposit of northern Chile by ELA-ICP-MS andSHRIMP. Geology 29:383386

Barra F, Ruiz J, Mathur R, Titley S (2003) A ReOs study on sulfideminerals from the Bagdad porphyry CuMo deposit, northernArizona, USA. Miner Deposita 38:585596

Barra F, Ruiz J, Valencia VA, Ochoa-Landin L, Chesley JT, Zurcher L(2005) Laramide porphyry Cu-Mo mineralization in northernMexico: Age constraints from Re-Os geochronology in molybde-nite. Econ Geol 100:16051616

Behn G, Camus F, Carrasco P, Ware H (2001) Aeromagnetic signatureof porphyry copper systems in northern Chile and its geologicimplications. Econ Geol 96:239248

Berzina AN, Sotnikov VI, Economou-Eliopoulos M, Eliopoulos DG(2005) Distribution of rhenium in molybdenite from porphyryCuMo and MoCu deposits of Russia (Siberia) and Mongolia.Ore Geol Rev 26:91113

Birck JL, RoyBarman M, Capmas F (1997) ReOs measurements at thefemtomole level in natural samples. Geost Newsletter 20:1927

Boric R, Diaz F, Maksaev V (1990) Geologa y yacimientos metal-feros de la Regin de Antofagasta. Servicio Nacional de Geo-loga y Minera, Boletn No. 40, 246 p, 2 maps

Boric R, Diaz J, Becerra H, Zentilli M (2009) Geology of the MinistroHales Mine (MMH), Chuquicamata District, Chile. Actas XIICongreso Geolgico Chileno, Santiago (electronic version)

Campbell IH, Ballard JR, Palin JM, Allen C, Faunes A (2006) UPbzircon geochronology of granitic rocks from the Chuquicamata-ElAbra porphyry copper belt of Northern Chile: excimer laserablation ICP-MS analysis. Econ Geol 101:13271344

Campos E, Wijbrans J, Andriessen PAM (2009) New thermochrono-logic constraints on the evolution of the Zaldivar porphyry copperdeposits, Northern Chile. Miner Deposita 44:329342

Camus F (2003) Geologa de los sistemas porfricos en losAndes de Chile. Servicio Nacional de Geologa yMinera, Chile,p 267

Camus F, Dilles JH (2001) A special issue devoted to porphyry copperdeposits of northern Chile: preface. Econ Geol 96:233237

Cannell J, Cooke DR, Walshe JL, Stein H (2005) Geology, minerali-zation, alteration and structural evolution of the El Teniente por-phyry CuMo deposit. Econ Geol 100:9791003

Codelco Memoria Anual (2011) http://www.codelco.com/prontus_codelco/site/artic/20120411/asocfile/20120411124635/memoria_codelco_abr13_1.pdf

Creaser RA, Papanastassiou DA, Wasserburg GJ (1991) Negativethermal ion mass spectrometer of Os, Re and Ir. Geochim Cos-mochim Acta 55:397401

Cuadra P, Rojas G (2001) Oxide mineralization at the Radomiro Tomicporphyry copper deposit, Northern Chile. Econ Geol 96:387400

Damon PE, Shafiqullah M, Clark KF (1983) Geochronology of theporphyry copper deposits and related mineralization of Mexico.Can J Earth Sci 20:10521071

Deckart K, Clark AH, Aguilar C, Vargas R, Bertens A, Mortensen JK,Fanning M (2005) Magmatic and hydrothermal chronology of thegiant Ro Blanco porphyry copper deposit, central Chile:

implications of an integrated UPb and 40Ar/39Ar database. EconGeol 100:905934

Dilles JH, Tomlinson AJ, Martin MW, Blanco N (1997) El Abra andFortuna complexes: a porphyry copper batholith sinistrally dis-placed by the Falla Oeste. Actas VIII Congreso Geolgico Chi-leno, Antofagasta 3:18831887

Gehrels GE, Valencia VA, Ruiz J (2008) Enhanced precision, accuracy,efficiency, and spatial resolution of UPb ages by laser ablation-multicollector-inductively coupled plasma-mass spectrometry. Geo-chem Geophys Geosyst 9:Q03017. doi:10.1029/2007GC001805

Jensen PW (1998) A structural and geochemical study of the Sierritaporphyry copper system, Pima County, Arizona. MSc thesis,University of Arizona

Ludwig KR (2004) Users manual for Isoplot/Ex V. 3.20. A geochro-nological toolkit for Microsoft Excel. Berkeley GeochronologyCenter Special Publication, Berkeley, p 43

Maksaev V, Munizaga F, McWilliams M, Fanning M, Mathur R, RuizJ, Zentelli M (2004) New chronology for El Teniente, ChileanAndes, from UPb, 40Ar/39Ar, ReOs, and fission track dating. In:Sillitoe RH, Perello J, Vidal CE (eds) Andean metallogeny: newdiscoveries, concepts and updates. Society of Economic Geolo-gists, Boulder, pp 1554, Special Publication 11

Mao J, Zhang Z, Zuoheng Z, Du A (1999) ReOs isotopic dating ofmolybdenites in the Xiaoliugou W (Mo) deposit in the northernQilian mountains and its geological significance. Geochim Cos-mochim Acta 63:18151818

Marinovich N, Lahsen A (1984) Hoja Calama, Regin de Antofagasta,Carta Geolgica de Chile. Servicio Nacional de Geologa y Min-era, Chile, p 140

Marsh TM, Eunaudi MT, McWilliams M (1997) 40Ar/39Ar geochro-nology of CuAu and AuAg mineralization in the Potrerillosdistrict, Chile. Econ Geol 92:784806

Masterman GJ, Cooke DR, Berry RF, Clark AH, Archibald DA,Mathur R, Walshe JL, Duran M (2004) 40Ar/39Ar and ReOsgeochronology of porphyry coppermolybdenum deposits andrelated coppersilver veins in the Collahuasi district, NorthernChile. Econ Geol 99:673690

Mathur R, Ruiz J, Munizaga F (2001) Insights into Andean metallo-genesis form the perspective of ReOs analyses of sulfides. IIISSAGI International Conference volume 3, Pucn, Chile

May G, Hartley AJ, Chong G, Stuart F, Turner P, Kape SJ (2005)Eocene to Pleistocene lithoestratigraphy, chronostratigraphy andtectono-sedimentary evolution of the Calama Basin, northernChile. Rev Geol Chile 32:3358

Munizaga F, Maksaev V, Fanning CM, Giglio S, Yaxley G, TassinariCCG (2008) Late PaleozoicEarly Triassic mamgmatism on thewestern margin of Gondwana: Collahuasi area, Northern Chile.Gond Res 13:407427

Ngler TF, Frei R (1997) Plug in plug osmium distillation. Sch MineralPetrogr Mitt 77:123127

Olson SF (1989) The stratigraphic and structural setting of the Potrerillosporphyry copper district, northern Chile. Rev Geol Chile 16:329

Ossandn G, Freraut R, Gustafson LB, Lindsay DD, Zentilli M (2001)Geology of the Chuquicamata mine: a progress report. Econ Geol96:249270

Padilla-Garza RA, Titley SR, Pimentel F (2001) Geology of theEscondida porphyry copper deposit, Antofagasta region, Chile.Econ Geol 96:307324

Padilla-Garza RA, Titley SR, Eastoe CJ (2004) Hypogene evolution ofthe Escondida porphyry copper deposit, Chile. In: Sillitoe RH,Perello J, Vidal CE (eds) Andean metallogeny: new discoveries,concepts and updates. Society of Economic Geologists, Boulder,pp 141165, Special Publication 11

Perell J, Urzua F, Cabello J, Ortiz F (1996) Clustered, gold-bearingOligocene porphyry copper and associated epithermal minerali-zation at La Fortuna, Vallenar Region, northern Chile. In: Camus

650 Miner Deposita (2013) 48:629651

F, Sillitoe RW, Petersen R (eds) Andean copper deposits: newdiscoveries, mineralization, styles, and metallogeny. Society ofEconomic Geologists, Boulder, pp 8190, Special Publication 5

Proffett JM, Dilles JH (2007) SHRIMP-RG ion microprobe UPb agedeterminations of intrusive rock units northeast of the Chuquica-mata mine, Chile. Codelco internal report, p. 11.

Renne PR, Deino AL, Walter RC, Turrin BD, Swisher CC, Becker TA,Curtis GH, Sharp WD, Jaouni A (1994) Intercalibration of astro-nomical and radioisotopic time. Geology 22:783786

Reynolds P, Ravenhurst C, Zentilli M, Lindsay D (1998) High-precision40Ar/39Ar dating of two consecutive hydrothermalevents in the Chuquicamata porphyry copper system, Chile.Chem Geol 148:4560

Richards JP, Noble SR, Pringle MS (1999) A revised late Eocene agefor porphyry Cu magmatism in the Escondida area, northernChile. Econ Geol 94:12311247

Richards JP, Boyce AJ, Pringle MS (2001) Geologic evolution of theEscondida area, northern Chile: a model for special and temporallocalization of porphyry Cu mineralization. Econ Geol 96:271305

Rivera SL, Pardo R (2004) Discovery and geology of the Toki por-phyry copper deposit, Chuquicamata District, northern Chile. In:Sillitoe RH, Perello J, Vidal CE (eds) Andean metallogeny: newdiscoveries, concepts and updates. Society of Economic Geolo-gists, Boulder, pp 1554, Special Publication 11

Rivera SL, Pardo R, Alcota H, Fontecilla C, Kovacic P, Pizarro J, RojoJ (2006) Resea de la exploracion y geologia del cluster deporfidos de cobre Toki, distrito Chuquicamata. Actas XI Con-greso Geolgico Chileno, Antofagasta (electronic version)

Romero B, Kojima S, Wong C, Barra F, Vliz W, Ruiz J (2011)Molybdenite mineralization and ReOs geochronology of theEscondida and Escondida Norte porphyry deposits, NorthernChile. Resour Geol 61:91100

Selby D, Creaser RA (2001) ReOs geochronology and systematics inmolybdenite from the Endako porphyry molybdenum deposit,British Columbia, Canada. Econ Geol 96:197204

Shirey S, Walker R (1995) Carius tube digestion for low-blankrhenium-osmium analysis. Anal Chem 67:21362141

Sillitoe RH (1988) Epochs of intrusion-related copper mineralization inthe Andes. JS Am Earth Sci 1:89108

Sillitoe RH (2004) Musings on future exploration targets and strategiesin the Andes. In: Sillitoe RH, Perell J, Vidal CE (eds) Andeanmetallogeny: new discoveries, concepts, and updates. Society ofEconomic Geologists, Boulder, pp 114, Special Publication 11

Sillitoe RH, Perell J (2005) Andean Copper Province: tectonomagmaticsettings, deposit types, metallogeny, exploration, and discovery. In:Hedenquist JW, Thompson JFH, Goldfarb RJ, Richards JP (eds)Economic Geology One Hundredth Anniversary Volume 19052005, Society of Economic Geologists Inc., pp. 845890.

Stacey JS, Kramers JD (1975) Approximation of terrestrial lead isotopeevolution by a two-stage model. Earth Planet Sci Lett 26:207221

Stein HJ, Markey RJ, Morgan JW, Hannah JL, Scherstn A (2001) Theremarkable ReOs chronometer in molybdenite: how and why itworks. Terra Nova 13:479486

Titley SR (1982) Geologic setting of porphyry copper deposits, south-eastern Arizona. In: Titley SR (ed) Advances in the geology of theporphyry copper deposits in the southwestern North America.University of Arizona Press, Tucson, Arizona, pp 3758

Tomlinson AJ, Blanco N, Maksaev V, Dilles JH, Grunder A, LadinoM (2001) Geologa de la Precordillera Andina de QuebradaBlanca-Chuquicamata, Regiones I y II (20302230S). Ser-vicio Nacional de Geologa y Minera (SERNAGEOMIN),Santiago, Chile, Informe Registrado IR-01-20, 20 mapas escala1:50.000, 444 pp.

Tomlinson AJ, Blanco N (2008) Geologa de la franja El Abra-Chuquicamata, II Regin (21452230S). Servicio Nacionalde Geologa y Minera (SERNAGEOMIN), Santiago, Chile,Informe Registrado IR-08-35, 5 mapas escala 1:50.000.

Vlkening J, Walczyk T, Heumann KG (1991) Osmium isotope ratiodeterminations by negative thermal ionization mass spectrometry.Inter J Mass Spectr Ion Proc 105:147159

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c.126_2012_Article_452.pdfTiming...AbstractIntroductionGeologic settingGeology of the Toki clusterLithological unitsStructuresAlterationMineralization

Sample preparation and analytical methodsMolybdenite ReOs dating40Ar/39Ar datingUPb dating

ResultsReOs molybdenite ages40Ar/39Ar agesUPb zircon age

DiscussionGeochronology of the Toki cluster and Chuquicamata depositRe concentrationsTiming of porphyry CuMo deposits in northern Chile

ConclusionsReferences

Timing and Formation of Porphyry Cu–Mo Mineralization in the Chuquicamata District

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