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Journal of South American Earth Sciences 22 (2006) 39–51

Re–Os and U–Pb geochronology of the El Arco porphyrycopper deposit, Baja California Mexico: Implications for the

Jurassic tectonic setting

V.A. Valencia a,*, F. Barra a,b, B. Weber c, J. Ruiz a, G. Gehrels a, J. Chesley a,M. Lopez-Martinez c

a Department of Geosciences, University of Arizona, Tucson AZ, 85721, United Statesb Instituto GEA, Universidad de Concepcion, Chile

c Departamento de Geologia, Centro de Investigacion Cientifica y de Educacion Superior de Ensenada,

Km 107 Carr. Tijuana - Ensenada. Ensenada, B.C. 22800, Mexico

Received 1 July 2004; accepted 1 February 2006

Abstract

New geochronologic data from the El Arco porphyry copper deposit, Baja California, Mexico, yield a Middle Jurassic crystallizationU–Pb age of 164.7 ± 6.5 Ma for zircons from a granodiorite porphyry that hosts the Cu–Au–(Mo) mineralization, and four Re–Osmolybdenite ages yield a weighted average age of 164.1 ± 0.4 Ma. These ages disagree with previous K–Ar age determinations that sug-gest a correlation of the intrusive rocks of El Arco–Calmalli area with the eastern Peninsular Ranges batholith of Cretaceous age.Instead, the volcanic–plutonic rocks of the El Arco–Calmalli area are more properly associated with the San Andres–Cedros volca-nic–plutonic complex (�166 Ma). El Arco–Calmalli block likely was part of the intraoceanic arc system described in the Vizcaino–Ced-ros region and accreted to the continent during the Cretaceous (98–110 Ma). The presence of a continental Jurassic magmatic arc in themainland and the occurrence of El Arco–Calmalli (San Andres–Cedros) volcanic–plutonic complex indicate two Jurassic magmatic arcsat this latitude (continental and oceanic).� 2006 Elsevier Ltd. All rights reserved.

Keywords: U–Pb; Re–Os; El Arco; Baja California; Jurassic magmatism; Porphyry copper deposits

1. Introduction

During the Mesozoic, multiple orogenies affected thewestern area of the North American continent, resultingin a complex mountain belt (Umhoefer, 2003). As a conse-quence of one of these orogenies, the Laramide, a series ofporphyry copper deposits were emplaced in southwestNorth America, making this region one of the most impor-tant porphyry copper provinces in the world (Fig. 1a) withmore than 50 known deposits (Titley and Anthony, 1989).

Although most of these deposits are Laramide in age(i.e., 50–70 Ma), older porphyry copper mineralization

0895-9811/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jsames.2006.08.005

* Corresponding author. Tel.: +1 520 621 2365; fax: +1 520 621 2672.E-mail address: victorv@geo.arizona.edu (V.A. Valencia).

from Early to Late Jurassic has been recognized in Nevada(�165 Ma, Yerington district Dilles and Wright, 1988) andArizona (�175–200 Ma, Bisbee and Courtland-Gleesonareas, Lang et al., 2001). These deposits are associated witha continental Jurassic magmatic arc (Tosdal et al., 1989).Early Cretaceous porphyry copper mineralization has alsobeen described in Baja California (El Arco deposit, Bart-helmy, 1975; Echavarri-Perez and Rangin, 1978; Coolb-augh et al., 1995) and Michoacan (La Verde-Inguarandeposit Coochey and Eckman, 1978; Sawkins, 1979;Damon et al., 1981; Solano-Rico, 1995). El Arco and LaVerde have been interpreted as associated with the Creta-ceous coastal batholith (locally known in Baja Californiaas the Peninsular Range batholith) or the accretion of theGuerrero Terrane (Campa and Coney, 1983). The Bisbee

Fig. 1. (a) Geographic overview of northwestern Mexico, showing the location of El Arco porphyry copper deposit (big star). Circles represent thelocations of other porphyry copper deposits from mainland Mexico and southern Arizona (after Damon et al., 1983; Titley and Anthony, 1989). Dashedlines indicate boundaries between the following terranes: (1) Seri, (2) Yuma, and (3) Cochimi (Sedlock et al., 1993). Small stars CI = Cedros Island,IN = Inguaran. (b) Simplified geological map of central Baja California modified after Martin-Barajas and Delgado-Argote (1996) showing pre-Cenozoicrocks and El Arco location.

40 V.A. Valencia et al. / Journal of South American Earth Sciences 22 (2006) 39–51

and El Arco deposits have copper and important gold con-tents, whereas the La Verde-Inguaran deposit has tungstenas a byproduct (Coochey and Eckman, 1978; Osoria et al.,1991).

El Arco porphyry Cu–Au deposit is of particular inter-est because of its tectonic formation environment (oceanicisland arc) and its unique hydrothermal and magmaticcharacteristics compared with other classic porphyry cop-per deposits of mainland Mexico and the US southwest.Furthermore, different age determinations of El Arco rockshave been used in studies and interpretations of the Meso-zoic tectonic evolution of northwest Mexico and Baja Cal-ifornia (i.e., Gastil et al., 1975, 1981; Rangin, 1978b; Gastiland Miller, 1984; Radelli, 1989; Gastil and Girty, 1993;Schmidt et al., 2002; Wetmore et al., 2003). However, theage of El Arco porphyry copper deposit is poorly con-strained, with K–Ar dates ranging from 93 to 138 Ma.Recently, Lopez-Martinez et al. (2002) reported a40Ar–39Ar isochron age of 137 ± 1.4 Ma on single pyritegrains. Prior to the Re–Os and U–Pb results reportedherein, this 40Ar–39Ar age has been taken as the most

probable age of formation of El Arco porphyry copperdeposit.

We present new geochronological data from two robustisotopic methods, Re–Os in molybdenite and U–Pb in zir-cons, to elucidate the mineralization and crystallizationages of El Arco porphyry copper deposit.

2. Geological background

2.1. Regional geology

The Pacific margin of Baja California is part of the largemosaic of accreted tectonostratigraphic terranes that makeup much of the North America Cordillera and Circum-Pa-cific Rim. Westward growth of the North American Cratonoccurred as fragments of ocean basins, volcanic arcs, andsubduction complexes were accreted onto the Pacificmargin of North America (Bonini, 1994). The geology ofthe Baja California peninsula is characterized by threemain geological units (Fig. 1a). The first unit is the preba-tholitic basement that includes Franciscan Formation

V.A. Valencia et al. / Journal of South American Earth Sciences 22 (2006) 39–51 41

equivalents of the Vizcaino–Cedros region, a Mesozoic vol-canic and volcaniclastic belt (Choyal and Alisitos forma-tions), a Triassic–Jurassic shale-sandstone belt, and aPaleozoic metasedimentary belt. The second unit is theMesozoic Peninsular Range batholith, and the thirdincludes sedimentary and volcanic rocks of Tertiary agethat overlie the prebatholitic and batholitic units (Gastilet al., 1975).

El Arco–Calmalli area is located in central Baja Califor-nia, approximately 40 km east of the city of GuerreroNegro (Fig. 1b). El Arco porphyry deposit is located tec-tonostratigraphically in the Alisitos terrane of Campaand Coney (1983) or the Santa Ana terrane of Coneyand Campa (1987) and the Yuma terrane of Sedlocket al. (1993). This terrane with different names is character-ized by a Lower Cretaceous volcanosedimentary sequencethat crops out discontinuously from Punta Chinos to ElArco–Calmalli, with a length of 500 km and width of50 km on average (Almazan-Vazquez, 1988). The Vizcaino(Campa and Coney, 1983) or Cochimi (Sedlock et al.,1993) terrane is located west of El Arco–Calmalli areaand comprises oceanic island arc rocks, ophiolites, melangeand blueschist of Triassic–Jurassic age, and Jurassic–Creta-ceous forearc basin sediments (Rangin, 1978a; Campa andConey, 1983; Sedlock et al., 1993; Kimbrough et al., 2001).The boundary between these terranes is not exposed, but agravimetric anomaly in the area has been interpreted bySedlock et al. (1993) as the possible boundary (Fig. 1b).

El Arco–Calmalli area has a prebatholitic basement(Gastil et al., 1975) composed of antigorite-bearing ser-pentinite with blocks of olivine–pyroxene peridotite, pyrox-enite, and amphibolite, overlain tectonically by hornblendediorites, gabbros, and possibly pillow lavas (Radelli, 1989)that are strongly deformed (Fig. 2a). These units are over-lain by foliated metavolcanic and metasedimentary rocksthat include metavolcanic agglomerates, metagraywackes,thin-bedded lenticular marble, meta-andesitic flows, andbreccias (Barthelmy, 1975; Farias-Garcıa, 1978). The upperpart of the sequence is composed of massive volcanic rocksaffected by greenschist facies metamorphism and hosts thegranodioritic porphyry intrusions responsible for the Cu–Au mineralization at El Arco (Farias-Garcıa, 1978). Onthe basis of K–Ar cooling ages of 107–93 Ma for theseintrusions (Barthelmy, 1975; Farias-Garcıa, 1978) and gen-eral stratigraphic similarities, the metavolcanic and metase-dimentary rocks have been correlated with the CretaceousAlisitos Formation (Barthelmy, 1975, 1979; Rangin, 1978a;Echavarri-Perez and Rangin, 1978; Farias-Garcıa, 1978;Coolbaugh et al., 1995).

2.2. Deposit geology

The Cu–Au–(Mo) mineralization at El Arco qualifies asa large porphyry deposit (Mutschler et al., 1999), withreserves of 600 million tons at a grade of 0.60% Cu and0.2 g/ton Au (Coolbaugh et al., 1995). The porphyrycopper deposit is hosted in hornblende andesite flows that

are intruded by a porphyry described as a granodiorite(Coolbaugh et al., 1995) or monzodiorite (Echavarri-Perezand Rangin, 1978). This stock is directly related to the Cu–Au mineralization. According to field geology and petro-graphic studies, mineralization seems to be the product ofa single intrusive event (Echavarri-Perez and Rangin,1978). Barren diabase dikes cut the andesites and granodi-orite porphyry units. All rocks have been affected by latergreenschist facies regional metamorphism, characterizedby chlorite–epidote–calcite–quartz (Gastil et al., 1975).Local amphibolite facies metamorphism is also present(Echavarri-Perez and Rangin, 1978).

Detailed geological and petrographic descriptions fromEl Arco–Calmalli area have been reported by Barthelmy(1975, 1979), Farias-Garcıa (1978), and Echavarri-Perezand Rangin (1978). These studies indicate that El Arco dif-fers significantly from other porphyry copper deposits inmainland Mexico and the southwestern United States.These differences include the tectonic formation setting(island arc), the nature of the intrusion, and the lack ofquartz–sericite (phyllic) hydrothermal alteration. All thesecharacteristics suggest that El Arco corresponds to a dio-rite-type porphyry model (Barthelmy, 1979) rather than aclassic type (Lowell and Guilbert, 1970; Titley and Beane,1981; McMillan and Panteleyev, 1988).

Hydrothermal alteration patterns show that a propylit-ic assemblage is more intensely developed in the andesiticrocks at the periphery of the deposit. This alteration ischaracterized by epidote, chlorite, and calcite with minorsphene, rutile, and apatite occurring in veinlets or as selec-tive replacements of primary minerals (Echavarri-Perezand Rangin, 1978; Coolbaugh et al., 1995). The potassicassemblage (K-feldspar, biotite, quartz, and anhydrite) iscentered at the core of the stock and observed as veinletsand K-feldpar replacing the groundmass (Fig. 2b). Sodic–calcic alteration is also present in the stock and at thecontact with the andesite. This alteration is representedby oligoclase veins (An 26) associated with quartz, epi-dote, calcite, and sphene (Echavarri-Perez and Rangin,1978). This type of alteration (sodic–calcic) has also beenrecognized in Yerington, Nevada (Jurassic porphyry Cudeposit) and interpreted to result from the flow of brinesheated by the stock and therefore exchanging alkalis withthe wallrocks to produce a strong sodium and calciummetasomatism (Dilles et al., 2000). Sericitic and argillicalteration are absent in El Arco (Echavarri-Perez andRangin, 1978). Hydrothermal alteration decreases withdistance from the intrusion until it is indistinguishablefrom the regional metamorphism.

Copper mineralization is present in veins and as dissem-inations of chalcopyrite with subordinate bornite–molyb-denite–native gold; it is restricted to the inner zone ofpotassic alteration, where magnetite is widespread. In thepotassic core, the chalcopyrite:pyrite ratio is 1:1.4, but inthe outer propylitic zone, where pyrite dominates overchalcopyrite, it reaches a ratio of 1:14 with 9 wt% pyrite(Echavarri-Perez and Rangin, 1978).

Fig. 2. (a) Simplified geologic map of El Arco–Calmalli area, modified from Barthelmy (1975) and Echavarri-Perez and Rangin (1978). (b) Hydrothermalalteration map from El Arco porphyry copper deposit with sample locations. Modified from Barthelmy (1975), Echavarri-Perez and Rangin (1978), andCoolbaugh et al. (1995).

42 V.A. Valencia et al. / Journal of South American Earth Sciences 22 (2006) 39–51

2.3. Analytical procedures

2.3.1. U–Pb systematics

A hornblende–biotite-bearing granodiorite porphyrysample was collected from an old shaft entrance of El Arcomine for U–Pb zircon analysis. The sample location isshown in Fig. 2b. Approximately 15 kg of rock werecrushed and milled. Heavy mineral concentrates were sep-arated from the sieve fraction smaller than 350 lm. Zirconswere concentrated using di-iodomethane heavy liquid andmagnetic techniques. Inclusion-free zircons were hand-picked under a binocular microscope from the nonmagnet-

ic fraction. Approximately 50 zircons were mounted inepoxy and polished for U–Pb analysis.

The single zircons crystals were analyzed with a Micro-mass Isoprobe multicollector ICP-MS equipped with nineFaraday collectors, an axial Daly detector, and four ion-counting channels (Dickinson and Gehrels, 2003; Valenciaet al., 2005). The Isoprobe was linked with an ArF Excimerlaser with an emission wavelength of 193 nm, used for laserablation. The analyses were conducted on 50 lm diameterspots with an output energy of approximately 32 mJ and arepetition rate of 9 Hz. Each analysis consisted of one 30-sintegration on the backgrounds (mass peaks with no laser

V.A. Valencia et al. / Journal of South American Earth Sciences 22 (2006) 39–51 43

firing) and twenty 1-s integrations on peaks with laser fir-ing. The depth of each ablation pit is approximately15 lm. Total measurement time was 90 s per analysis.

The collector configuration allows for the simultaneousmeasurement of 204Pb in an ion-counting channel while206Pb, 207Pb, 208Pb, 232Th, and 238U are measured with Far-aday detectors. All analyses were conducted in static mode.Interelement fractionation was monitored by analyzingfragments of SL-1, a large concordant zircon crystal fromSri Lanka (SL-1) with a known (ID-TIMS) age of564 ± 4 Ma (2r) (Gehrels, unpublished data). The reportedages for zircon grains are based on 206Pb/238U ratiosbecause errors in the 207Pb/235U and 206Pb/207Pb ratiosare significantly greater, primarily because of the low inten-sity (commonly <1 mV) of the 207Pb signal from theseyoung, low-U grains. The 206Pb/238U ratios are correctedfor common Pb according to measured 206Pb/204Pb, a com-mon Pb composition (Stacey and Kramers, 1975), anduncertainties of 1.0 on the common 206Pb/204Pb and 0.3for 207Pb/204Pb.

The weighted mean of 21 individual analyses was calcu-lated according to Ludwig (2003). The mean age consid-ered only the measurement or random errors (errors in206Pb/238U and 206Pb/204Pb of each unknown). For thissample the random error is 4.4 Ma (2r) and represents2.7%. Age of standard, calibration correction from stan-dard, composition of common Pb, and decay constantuncertainty also contributed to the error in the final agedetermination. These uncertainties are grouped and knownas systematic error. For this sample, the systematic error isapproximately 2.9%. The error of the age for the sample iscalculated by adding the two components (random or mea-surement and systematic error) quadratically; for this sam-ple, it is 3.97% (6.5 Ma). All age uncertainties are reportedat the 2r level.

2.3.2. Re–Os systematics

For Re–Os analyses, four molybdenite samples wereselected from different areas and mineral assemblages ofthe deposit (Fig. 2b). Molybdenite from EAD-131 andEAD-277 samples were separated from 20 mmquartz + molybdenite + chalcopyrite + pyrite veins. TheEAD-131 sample is located in the intrusive and EAD-277in the andesites; molybdenite from the EAD-279 samplewas collected from a 4 mm quartz + molybdenite vein inthe intrusive and molybdenite from sample EAD-275 wasseparated from a 1 cm quartz + molybdenite vein. About25–50 mg of fine-grained molybdenite was loaded in aCarius tube and dissolved with 8 ml of reverse aqua regia.While the reagents, sample, and spikes were frozen, theCarius tube was sealed and left to melt at room tempera-ture. The tube was placed in an oven and heated to240 �C overnight. The solution was processed in a two-stage distillation process for osmium separation (Naglerand Frei, 1997). Osmium was further purified using micro-distillation techniques (Birck et al., 1997) and loaded onplatinum filaments with Ba(OH)2 to enhance ionization.

After osmium separation, the remaining acid solutionwas dried and dissolved in 0.1 HNO3. Rhenium wasextracted and purified through a two-stage column usingAG1-X8 (100–200 mesh) resin and loaded on platinum fil-aments with BaSO4.

Samples were analyzed by negative thermal ion massspectrometry (N-TIMS) (Creaser et al., 1991) on a VG 54mass spectrometer. Molybdenite ages are calculated usinga 187Re decay constant of 1.666 · 10�11 year�1 (Smoliaret al., 1996). Uncertainties are calculated using error prop-agation, considering the error in the decay constant(0.31%), errors in spike calibrations (0.08% for 185Re and0.15% 190Os), and analytical and weighting errors (Barraet al., 2003; Valencia et al., 2005).

3. Results

3.1. Re–Os ages

Four analyses from four different molybdenite samplesare reported in Table 2. Sample locations are shown inFig. 2b. Total Re and 187Os concentrations vary from 67to 497 ppm and 115 to 858 ppb, respectively. All fourRe–Os ages are consistent and overlap within error. Theages range from 163.8 to 164.6 Ma and have a weightedaverage age of 164.1 ± 0.4 Ma (Fig. 3). This age is inter-preted as the time of molybdenite crystallization and hencemeasures the age of the formation of the porphyry copperdeposit at El Arco.

3.2. U–Pb ages

Twenty-one zircons from a single granodiorite porphyrysample were analyzed (Table 1). Zircons were studied opti-cally under a petrographic microscope and with SEM inback-scattered electron mode and cathodoluminescence.The zircons from El Arco are euhedral with prominent,sharp, pyramidal terminations, clear, and transparent.They display no detectable optical zoning, which is typicalof igneous morphologies (Pupin, 1980). In addition, the U/Th ratio of <3 indicates a magmatic origin (Rubatto,2002). The sample yields a 206Pb/238U weighted mean ageof 164.7 ± 6.5 Ma (n = 21 zircons, MSWD = 0.45, 2r;Fig. 4a), interpreted as the age of igneous crystallizationof El Arco stock. No inherited components are recognizedin the sample (Fig. 4b).

4. Discussion

4.1. Age of mineralization

Fig. 5 summarizes our new age and previous geochro-nology for El Arco. Previous geochronological studies ofEl Arco–Calmalli reported Aptian–Albian ages(93.4–127 Ma) on the basis of K–Ar determinations onK-feldspar and whole rock of the intrusive bodies (Bart-helmy, 1975; Farias-Garcıa, 1978). More recent studies

Table 1LA-MC-ICPMS U–Pb zircon data

Sample Concentration Isotopic ratio Apparen ges (1r)

U(ppm)

Th(ppm)

U/Th U206

Pbm/204Pbc

207Pb/235U

±% 206Pb/238U

±% Error corr 206Pb/237U

±% 206Pb/238U

Ma 207Pb/235U

± Ma 206Pb/207U

± Ma

1 101 49 2.0 225 0.155 60.43 0.0264 8.28 0.14 23.48 59.86 168.3 4.1 146.5 91.1 NA NA2 260 151 1.7 956 0.182 23.52 0.0247 4.46 0.19 18.77 23.09 157.5 7.1 169.4 42.5 340 2613 260 148 1.8 868 0.227 27.89 0.0269 3.29 0.12 16.39 27.69 171.4 5.7 207.4 62.2 639 2984 93 18 5.2 329 0.276 41.2 0.0254 5.89 0.14 12.69 40.78 161.7 9.6 247.5 109.4 1167 4045 72 29 2.5 301 0.232 53.57 0.0271 8.97 0.17 16.11 52.82 172.6 5.7 212.0 119 676 5656 112 37 3.0 464 0.163 45.47 0.0254 5.25 0.12 21.48 45.17 162.1 8.6 153.7 72.8 26 5427 114 74 1.5 425 0.138 52.32 0.0248 7.18 0.14 24.87 51.83 158.3 1.5 131.1 70.7 NA NA8 85 31 2.7 385 0.168 47.47 0.0254 6.67 0.14 20.89 47.00 162.2 1.0 157.8 78.0 92 5579 103 47 2.2 424 0.204 35.63 0.0244 5.19 0.15 16.49 35.25 155.6 8.2 188.8 71.4 627 380

10 128 70 1.8 553 0.176 38.68 0.0247 6.95 0.18 19.38 38.05 157.5 1.1 164.6 66.9 267 43611 288 189 1.5 1281 0.208 20.45 0.0264 3.66 0.18 17.88 20.12 168.3 6.2 188.5 41.5 449 22412 174 88 2 547 0.228 36.14 0.0297 6.78 0.19 18.02 35.49 189.2 3.0 208.4 80.3 431 39513 99 43 2.3 362 0.165 56.84 0.0259 8.52 0.15 21.67 56.2 165.4 4.3 155.4 91.2 NA NA14 147 61 2.4 581 0.139 48.74 0.0263 7.05 0.14 26.13 48.23 167.3 1.9 131.9 66.4 NA NA15 141 76 1.9 567 0.117 46.38 0.0249 8.07 0.17 29.55 45.67 159.1 3.0 112.0 53.5 NA NA16 181 105 1.7 699 0.140 34.45 0.0255 7.08 0.21 25.19 33.71 162.9 1.7 133.1 47.9 NA NA17 143 77 1.8 668 0.178 35.86 0.0258 10.45 0.29 20.006 34.30 164.5 7.4 166.5 62.9 194 39918 90 41 2.2 464 0.152 50.95 0.0258 10.47 0.21 23.466 49.86 164.2 7.4 143.3 75.5 NA NA19 159 82 1.9 607 0.182 33.13 0.0258 10.89 0.33 19.526 31.29 164.3 8.1 170.0 59.5 250 36020 295 225 1.3 801 0.158 32.58 0.0260 9.45 0.29 22.682 31.18 165.4 5.8 149.0 51.0 NA NA21 481 786 0.6 1077 0.157 24.59 0.0260 9.55 0.39 22.830 22.66 165.7 6.0 148.3 38.5 NA NA

Notes: U concentration and U/Th are calibrated by comparison with NBS SRM 610 and have uncertainty of approximately 25%. Isotope ratios are c ected for Pb/U fractionation by comparison withstandard zircon with an age of 564 ± 4 Ma (2r). Initial Pb composition interpreted from Stacey and Kramers (1975), with uncertainty of 1.0 for 206Pb 04Pb, 0.3 for 207Pb/204Pb, and 2.0 for 208Pb/204Pb.Decay constants: 235U = 9.8485 · 10�10, 238U = 1.55125 · 10�10, 238U/235U = 137.88. Discordance is calculated from comparison of 206Pb/238U nd 206Pb/207Pb ages. UTM sample coordinates:3103,028N, 12R264,528E.

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Table 2Re–Os ages from molybdenite

Sample Total Re 187Re (ppm) 187Os (ppb) Re–Os agea (2r)

EAD-277 (262.5 m) qtz + mo + cpy � py vein 497.1 312.5 858.2 164.6 ± 0.8EAD-131 (175.8 m) qtz + mo + py + cpy vein 430.7 270.7 739.1 163.8 ± 0.8EAD-275 qtz + mo vein 67.0 42.1 115.3 164.2 ± 0.8EAD-279 (481.2 m) qtz + mo vein 277.5 174.5 477.4 164.0 ± 0.8Weighted mean average 164.1 ± 0.4

a Uncertainties in age (2r) are calculated using error propagation and include error in the Re decay constant (0.31%), 185Re and 190Os spike calibration(0.08% and 0.15%, respectively), weighting and analytical errors. UTM coordinates: EAD-277 (3102,639N, 12R264,944E); EAD-131 (3103,306N,12R264,917E); EAD-275 (3103,069N, 12R264,167E); EAD-279 (3102,683N, 12R264,650E).

V.A. Valencia et al. / Journal of South American Earth Sciences 22 (2006) 39–51 45

using 40Ar–39Ar systematics on pyrite suggest an older ageof 138 ± 2.5 Ma, interpreted as the time when tiny, K-bear-ing inclusions were trapped by pyrite crystals (Lopez-Mar-tinez et al., 2002). These authors also report a whole-rock40Ar–39Ar age of 95.2 ± 0.8 Ma from a low-grade meta-morphosed, unmineralized diabase dike cross-cutting ElArco porphyry intrusive stock. This age was interpretedas the time of cooling after regional metamorphism(Fig. 5).

The new Re–Os and U–Pb data presented herein pro-vide strong evidence that a Middle Jurassic (Callovian)magmatic event is responsible for the porphyry intrusionand copper mineralization at El Arco. Furthermore, theconsistency of the Re–Os ages from four different samplessuggests that molybdenite mineralization occurred during ashort period of time (<1 m.y.). The previously reportedAlbian K–Ar and Ar–Ar ages (Barthelmy, 1975; Farias-Garcıa, 1978; Lopez-Martinez et al., 2002) do not reflectages of primary crystallization or mineralization at ElArco. The K–Ar ages probably reflect the influence of latermetamorphic overprinting that may have caused Ar lossand/or partial resetting of the K–Ar system, possibly asso-ciated with the accretion of the oceanic arc terrane to theNorth American continent.

The new Re–Os and U–Pb ages for El Arco porphyrycopper properly reassigns this deposit to the Middle Juras-sic period, not to the Cretaceous as previously thought(Fig. 5). The new age determination for El Arco has strongmetallogenetic implications because no Jurassic age por-

162.8

163.2

163.6

164.0

164.4

164.8

165.2

165.6

Re-

Os

age

(Ma)

Fig. 3. Weighted mean average of four Re–Os molybdenite ages. Datacollected from four vein samples. Bars show two sigma uncertainties.

phyry, epithermal, or replacement-type deposits have beenpreviously reported in Mexico (Barton, 1996). These find-ings provide new insights for porphyry copper explorationin the region.

4.2. Alisitos–El Arco correlations

The marine sedimentary sequences (finely bedded mar-ine shale and limestones), volcanic, and volcaniclastic rocksfrom El Arco–Calmalli area were previously correlatedwith the Alisitos Formation on the basis of the erroneousCretaceous age and arc-related rock types (Barthelmy,1975). The Alisitos Formation has an Albian age, asassigned by mollusk fossils (Allison, 1974) and volcanicand plutonic U–Pb ages (Carrasco et al., 1995; Johnsonet al., 1999; Wetmore et al., 2005). The new U–Pb agefor El Arco granodiorite does not support a correlationbetween the intruded rocks from El Arco–Calmalli andthe Alisitos Formation. However, the geochronologicaldata and similar stratigraphies suggest the rocks from ElArco–Calmalli could be associated with the San Andres–Cedros volcanoplutonic complex (Rangin, 1978a). Thiscomplex is a Middle–Late Jurassic arc with granitoids thatintrude Jurassic volcanic and volcaniclastic strata, which inturn overlies pillow lavas and sedimentary rocks that covera Triassic ophiolite sequence of Franciscan-like rocks(Rangin et al., 1981; Kimbrough and Moore, 2003).

More specifically, the Punta Norte arc plutonic complexlocated in the northern part of Cedros Island (Figs. 1b and6) intruded the Choyal Formation volcanic arc assemblage(Kimbrough and Moore, 2003). The stocks range in com-position from hornblende–pyroxene diorite to granodiorite(Kilmer, 1984). Intrusive bodies have intense hydrothermalalteration (albitic, potassic, and propylitic), in addition tolocal disseminated copper and gold mineralization (Kilmer,1984). The hydrothermal alteration and mineralizationstyle are similar to those described at El Arco. Further-more, geochronological data from granodiorites at PuntaNorte yield 206U/238Pb ages that range from 163.0 to167.5 Ma (Kimbrough and Moore, 2003), in agreementwith the new ages determined from El Arco porphyry cop-per deposit. However, stratigraphic and provenance studiesare required to establish a coherent correlation among ElArco–Calmalli area, Punta Norte at Cedros Island, andthe San Andres region (Fig. 6).

0.0 0.2 0.4

206

238

Pb/ U207 235

Fig. 4. 206 Pb/238 U age of granodiorite porphyry. Weight average mean and Concordia diagram showing the absence of inherited components andinterpreted crystallization age.

46 V.A. Valencia et al. / Journal of South American Earth Sciences 22 (2006) 39–51

4.3. Evidence of Jurassic magmatism

Postdating the Permo-Triassic Sonoma Orogeny, an ear-ly Mesozoic, west-facing Andean-type arc developed alongthe western margin of North America (Dilles and Wright,1988). This Jurassic magmatic arc, which extended fromCalifornia to Sonora (Anderson and Silver, 1978; Tosdalet al., 1989), was the result of the subduction of the Kulaplate under the North America plate (Brass et al., 1983).In Mexico, this arc has been identified as a NW–SE–orient-ed belt (Fig. 1) extending from NW-central Sonora state(Rangin, 1978b) to Zacatecas state (Jones et al., 1990,1995). This Jurassic arc is manifested by volcanic and vol-canoclastic rocks of andesitic to rhyolitic composition,metamorphosed to greenschist facies, and with occasional

biotite granite porphyry intrusions (Perez-Segura andEchavarri, 1981; Nourse, 2001). Intrusive ages range from165 to 175 Ma (Anderson and Silver, 1978; Stewart, 1988;Anderson et al., 2005). In addition, several intrusives ofEarly–Middle Jurassic age with U–Pb ages of 163–200 Ma have been reported in the Bisbee and Courtland-Gleeson porphyry copper districts in Arizona on the north-ern extension of the belt. This evidence suggests that atleast part of the arc remained static relative to its associatedtrench during the Early–Middle Jurassic (Lang et al., 2001)and later shifted to the Pacific coast during the Late Juras-sic (Coney and Reynolds, 1977; Damon et al., 1983). Thisbelt represents a continental arc.

I-, S-, and transitional I–S-type gneissic granites of Mid-dle Jurassic age (161–170 Ma, 206Pb/238U age in zircons)

V.A. Valencia et al. / Journal of South American Earth Sciences 22 (2006) 39–51 47

have been recognized in San Diego, southern California(Todd and Shaw, 1985; Shaw et al., 2003). The zircons havean inherited Precambrian component (Gastil and Girty,1993). North of El Arco, an orthogneiss unit in the SierraSan Pedro Martir yielded an U–Pb age of 164.4 ± 1.2 Mawith a Precambrian inherited component (Schmidt, 2000).In addition, the Agua Caliente tourmaline-bearing biotitetonalite in the same region has an age of 164 ± 2.3 Ma. Sim-ilar Jurassic–Cretaceous magmatism, deformation struc-tures, and metamorphic geobarometry have been described

Fig. 6. Generalized stratigraphic sections from Punta Norte at Cedros IslanModified after Barthelmy (1975) and Kimbrough and Moore (2003).

Fig. 5. Diagram showing geochronological ages from El Arco porphyry coppersquares are Ar–Ar ages from pyrite (older) and whole rock (younger) (Lopez-Msolid circle is U–Pb in zircons. Error bars are 2rLight gray shade represents mpyrite; cpy, chalcopyrite; K-Feld, potassic feldspar; W.R., whole rock. SupersLopez-Martinez et al. (2002).

in both areas (Schmidt and Paterson, 2002; Shaw et al.,2003). Furthermore, Jurassic magmatic island arc activityhas been recognized in Punta Norte in Cedros Island(�166 Ma). In addition to the island arc assemblages andour geochronological data, this activity indicates the pres-ence of two magmatic arcs (the oceanic Cedros–El Arcoand a continental arc from Nevada, USA, to Nazas, Mexico)that acted at the same time during the Middle Jurassic(Fig. 7). However, the position of the oceanic arc with respectto the North American continent remains unknown (Busby

d, San Andres volcano-plutonic complex, and El Arco–Calmalli region.

deposit. Solid diamonds are K–Ar ages from biotite and whole rock; solidartinez et al., 2002); solid triangles are Re–Os ages from molybdenite; andineralization duration. Abbreviations: qtz, quartz; mo, molybdenite; py,

cript indicates source: (1) Barthelmy (1975); (2) Farias-Garcia (1978); (3)

48 V.A. Valencia et al. / Journal of South American Earth Sciences 22 (2006) 39–51

et al., 1998; Schmidt et al., 2002). This continental-oceanicarc model has been proposed for the early Cretaceous forthe Baja California Peninsula but not for the Middle Jurassic(Johnson et al., 1999; Wetmore et al., 2003).

These ages, together with El Arco ages, indicate thepresence of a Jurassic intrusive belt extending along andoutboard the continental border of North America (Shawet al., 2003). However, despite the similar Jurassic age,the absence of inherited zircon components (xenocrysts)in El Arco granodiorite porphyry (n = 21 zircons) andgranitoids of Punta Norte in Isla Cedros area reflect theapparent lack of continental-derived material in the base-ment. This claim agrees with isotopic data reported byWeber and Lopez-Martinez (2002), which provides initialepsilon Nd values of >5, initial 87Sr/86Sr ratios of <0.704in whole rock, and lead isotopes in feldspar that come froma source with an average e value of 9.4. Thus, the magmasof El Arco porphyry and host rocks apparently evolvedfrom a moderately depleted mantle source reservoir with-out significant influence by the continental crust. These iso-topic data suggest that El Arco deposit was formed in anoutboard island arc terrane (Fig. 7). All these data supportthe tectonic model of an intraoceanic arc-ophiolite systemduring the Early–Middle Jurassic in Baja California (Busbyet al., 1998; Schmidt et al., 2002).

4.4. Terrane limit between El Arco–Guerrero Negro

transect?

El Arco–Calmalli area represents a Middle Jurassicoceanic arc sequence whose basement comprises ophiolit-

Unknodistan

Mid-Late Cretaceous

Middle Jurassic

El Arco porphyry copper deposit

El Arco porphyry copper deposit

Western oceanic crust

Transitional crust (Suture zone)

Eastern continental crust

Active magmatism

Extinct magmatism

Oceanic crust

Collision of exotic isa

b

?

Fig. 7. Schematic cross-section of proposed tectonic framework during the

ic rocks of Middle Jurassic age or older. These rockshave been interpreted as the roots of the mid-CretaceousAlisitos Formation, using the K–Ar mid-Cretaceous agesto assign possible correlations. Because the granodioriteat El Arco is Middle Jurassic in age, this interpretationcan be discarded; the interpretation we support insteadis that the island arc sequences in the El Arco–Calmalliarea are an extension of the Middle–Late Jurassic volca-nic–plutonic (San Andres–Cedros arc) and Late Triassicophiolites of the Vizcaino Peninsula and adjacent CedrosIsland (Fig. 6).

However, a tectonostratigraphic boundary betweenthe Alisitos and Vizcaino terranes or Cochimi andYuma terranes (Sedlock et al., 1993) has been recog-nized along the transect between El Arco and the Guer-rero Negro area (Fig. 1b). The criteria used by Campaand Coney (1983) to determine tectonostratigraphic lim-its are abrupt changes in age and/or lithologies; theyinterpret these boundaries between terranes as faults orsuspected faults. These authors highlight two main dif-ferences between the Alisitos and the Vizcaino terranes:age (Aptian–Albian versus Triassic–Jurassic) and strati-graphic sequences (partially continental affinities versusoceanic affinities). However, the boundary between thetwo terranes is not exposed, and its existence has beeninferred on the basis of reset K–Ar ages and an inter-pretation of gravity anomalies. We question the exis-tence of a tectonostratigraphic limit, because the ageand stratigraphic affinities of El Arco–Calmalli are sim-ilar to those at Cedros Island and Vizcaino Peninsula(Fig. 6).

wnce

land arc

Middle Jurassic–Late Cretaceous. Modified after Schmidt et al. (2002).

V.A. Valencia et al. / Journal of South American Earth Sciences 22 (2006) 39–51 49

5. Conclusions

The granodiorite porphyry from El Arco porphyrycopper deposit has a Middle Jurassic crystallization age(U–Pb) of 164.7 ± 6.5 Ma for the intrusive body, andquartz–molybdenite veinlets yield a mineralization age(Re–Os) of 164.1 ± 0.4 Ma.

El Arco–Calmalli area is more properly associated withthe San Andres–Cedros oceanic volcanic–plutonic complex(�166 Ma) instead of the Cretaceous Alisitos Formation.This association indicates an alloctonous, distinct origin(oceanic terrane) with respect to the Cretaceous Alisitosterrane, which was accreted to nuclear Mexico during theCretaceous (98–110 Ma). Therefore, the proposed terranelimit between the Cochimi–Yuma or Vizcaino–Alisitosterranes in the transect between Vizcaino and El Arco isnot required.

The presence of a continental Jurassic magmatic arc inmainland North America (Tosdal et al., 1989), in additionto El Arco–Calmalli and San Andres–Cedros volcanic–plu-tonic complexes, suggest the presence of two independentarcs during the middle Jurassic evolving at the same time– one continental and one oceanic – with an unknown dis-tance between arcs.

Acknowledgements

This work is the result of NSF grants EAR-9725833,EAR-9708361, EAR-9814891, and EAR-0125773 toJoaquin Ruiz; CICESE internal project 644111 to BodoWeber; and Terrones Student Research grant of the Societyof Economic Geologists to Victor A. Valencia. Valenciawas also supported by a 87199 CONACyT scholarship.The analytical work was undertaken at the University ofArizona in the W.C. Keck Laboratory. Special thanks toAlex Pullen and Mark Baker for their technical support.We are most grateful to Grupo Mexico for allowing accessto the mine. This work has benefited from the comments ofPaul Wetmore, Oscar Talavera, and Journal reviewers JohnDilles and Virginia Gillerman.

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