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Age and Paleotectonic Setting of Volcanogenic Massive Sulfide Deposits in the Guerrero Terrane of Central Mexico:

Constraints from U-Pb Age and Pb Isotope Studies

JAMES K. MORTENSEN,†

Mineral Deposit Research Unit, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4

BRIAN V. HALL,* International Croesus Ventures Corporation, 95 Armstrong Street, Ottawa, Ontario, Canada K1A 2V6

THOMAS BISSIG, Mineral Deposit Research Unit, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4,

and Universidad Católica del Norte, Depto. Ciencias Geológicas, Avenida Angamos 610, Antofagasta, Chile

RICHARD M. FRIEDMAN, THOMAS DANIELSON,Mineral Deposit Research Unit, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4

JAMES OLIVER, Oliver Geoscience International, 4377 Karindale Road, Kamloops, British Columbia, Canada V2B 8N1

DAVID A. RHYS, KIKA V. ROSS, Panterra Geoservices, 14180 Greencrest Drive, Surrey, British Columbia, Canada V4P 1L9

AND JANET E. GABITES

Mineral Deposit Research Unit, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4

AbstractThe Guerrero terrane of central and western Mexico comprises volcanic and sedimentary strata of mainly

Mesozoic age, with geochemical signatures indicating formation in juvenile back-arc to slightly more evolvedarc environments. At least 60 volcanogenic massive sulfide (VMS) deposits have been discovered thus farwithin the Guerrero terrane. They comprise two main belts: the Coastal belt mainly comprising deposits in theZihuataneo subterrane (including the Cuale-Bramador district), and the Central belt, which includes depositsin the Teloloapan subterrane (Campo Morado, Tlanilpa-Azulaquez, Tizapa, Rey de Plata, etc.), the Guanaju-ato subterrane (El Gordo, Las Gavalinas), and the Zacatecas subterrane (San Nicolas and El Salvador deposits).The age of host rocks for the VMS mineralization in the various subterranes was poorly constrained prior tothis study. Eleven new U-Pb zircon ages for volcanic host rocks from the Campo Morado, Tlanilpa-Azulaquez,Leon-Guanajuato, and San Nicolas-El Salvador VMS districts, together with recently determined U-Pb agesfrom the Cuale area, demonstrate that VMS deposits in the Guerrero terrane range from latest Middle Juras-sic to early Early Cretaceous (Callovian to Valanginian) in age. The oldest ages were obtained from the Cualedistrict (total age range of 157.4 ± 4.1 Ma for three units) and the youngest are from the Tlanilpa-Azulaquezdistrict (total age range of 139.7 ± 2.5 Ma for two samples). Volcanogenic massive sulfide deposits in the Cen-tral belt resemble those of the bimodal-siliciclastic deposit type, whereas those in the Coastal belt are moresimilar to the bimodal-felsic type. Published geochemical and radiogenic isotope data from the various subter-ranes suggest that VMS deposits in the Coastal belt and most of those in the Central belt are hosted within ju-venile to slightly evolved arc settings. The San Nicolas and El Salvador deposits of the Zacatecas subterrane arethe only deposits that unequivocally formed in a back-arc setting. Our new U-Pb dating results, together withexisting stratigraphic, geochemical, and radiogenic isotope data from throughout the Guerrero terrane, areconsistent with the Guerrero terrane having developed as a west-facing continental margin arc that was builton mainly oceanic crust (and overlying Early Mesozoic siliciclastic fans shed from the east) along the westernmargin of nuclear Mexico. Slab rollback in the Middle Jurassic led to the development of a “Rocas Verdes-type”continental back-arc inboard of the arc, which is now preserved as the Arperos basin in the central and south-ern Guerrero terrane and the host rocks for the San Nicolas and El Salvador deposits farther to the north insoutheastern Zacatecas State.

† Corresponding author: e-mail, jmortensen@eos.ubc.ca*Current address: Laurentian University, 935 Ramsey Lake Road, Sudbury, Ontario, Canada P3E 2C6.

©2008 Society of Economic Geologists, Inc.Economic Geology, v. 103, pp. 117–140

IntroductionTHE GUERRERO terrane of central and western Mexico (Fig.1) comprises volcanic and sedimentary strata of mainly Meso-zoic age, with geochemical signatures consistent with forma-tion in back-arc and juvenile to somewhat more evolved arcsettings (e.g., Dickinson and Lawton, 2001; Keppie, 2004).Volcanogenic massive sulfide (VMS) deposits are widely

distributed throughout the Guerrero terrane and define anumber of spatially distinct clusters or districts. The exact agesof host rocks for the mineralization in the various districts have,until this point, only been loosely constrained by regional strati-graphic correlation, a limited number of fossil ages, which arein some cases contradictory, K-Ar dating studies, and a verysmall number of Ar-Ar and U-Pb age determinations. In this

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We have determined Pb isotope compositions of sulfides from VMS deposits and several other styles of min-eralization from throughout the Guerrero terrane. A compilation of these and previously published data showsthat isotopic compositions from the various VMS deposits fall in a relatively confined field that overlaps the av-erage upper crustal growth curve. This suggests that metals were mainly sourced from continentally derivedsedimentary strata that are interlayered with or immediately underlie the host volcanic section and/or from thehost volcanic rocks themselves (or subvolcanic intrusive equivalents) that had assimilated significant quantitiesof upper crustal material either at the point of origin or during subsequent ascent and eruption. Lead isotopecompositions from the Francisco I Madera deposit in Zacatecas State indicate that most of the sulfides in thisdeposit are epigenetic rather than syngenetic in origin and are likely Tertiary in age.

NuevoLeon

Zihuatanejo subterraneHeutamo subterraneTeloloapan subterraneArcelia subterrane

Zacatecas subterraneArteaga complexLas Ollas complexSan Jose de Gracia subterraneSierra Madre terrane

Leon-Guanajuato subterrane

PacificOcean

Gu

erre

ro t

erra

ne

18°

108° 106° 104°

26°

24°

22°

20°

102° 100° 98°

Colima

Guerrero

Hidalgo

Sinaloa

Coahuila

Mexico

Morelos

Chihuahua

Durango

NuevoLeon

San LuisPotosi

GuanajuatoNayarit

Jalisco

Michoacan

Puebla

Zacatecas

D

Z

AC

Gu

L

A

Zi

PV

M

Gd

SLP

MC

FIG. 1. Distribution of the Guerrero terrane and individual subterranes in central and western Mexico. Cities as follows:A = Acupulco, AC = Aguas Calientes, D = Durango, Gd = Guadalajara, Gu = Guanajuato, L = Leon, MC = Mexico City,M = Mazatlan, PV = Puerto Vallarta, SLP = San Luis Potosi, Z = Zacatecas, Zi = Zihuatanejo.

study we report 11 new U-Pb zircon ages for volcanic hostrocks from the Campo Morado, Tlanilpa-Azulaquez, SanNicolas-El Salvador, and Leon-Guanajuato districts. We alsoreport Pb isotope analyses of sulfides for VMS deposits andother types of sulfide occurrences from throughout the Guer-rero terrane. Together the results of these U-Pb dating and Pbisotope studies provide new constraints on the overall tectonicevolution of the Guerrero terrane and the paleotectonic set-ting(s) in which the various deposits formed and have implica-tions for the regionl VMS potential in the Guerrero terrane.

Regional Geology of the Guerrero TerraneAlthough the Guerrero terrane is interpreted to underlie

much of western Mexico (Fig. 1), the Mesozoic assemblagesthat define the Guerrero terrane are only exposed over lessthan 5 percent of the surface area, occurring as scattered ero-sional windows within the extensive Tertiary and Quaternaryvolcanic and sedimentary strata of the Sierra Madre Occiden-tal Province and Trans-Mexican volcanic belt. Consequentlyindividual rock units or sequences within the Guerrero ter-rane are generally not mappable between windows, and cor-relations between areas of exposure are mainly based onbroad lithostratigraphic and/or age similarities. Other expo-sures of Mesozoic strata in northwestern Mexico (Sonora andBaja California) occur outside the Guerrero terrane (asshown in Fig. 1) but have been included in the Guerrero ter-rane by some workers (e.g., Umhoefer, 2003; Wetmore andPaterson, 2003).

Most previous workers have subdivided the Guerrero ter-rane into several subterranes on the basis of differences inpredominant lithological associations, lithogeochemical andisotopic compositions, and/or inferred depositional age. Theoriginal subdivision of the southern portion of the Guerreroterrane into the Zihuatanejo, Huetamo, and Teloloapan sub-terranes (Campa and Coney, 1983) has been revised by nu-merous subsequent workers, with the most recent contribu-tion by Centeno-García et al. (2003). In the presentcontribution we employ the terrane and subterrane nomen-clature from Centeno-García et al. (2003a), which, althoughnot universally accepted, does incorporate most of the recentwork in the Guerrero terrane. As considered by Centeno-García et al. (2003a) the Guerrero terrane consists mainly offive components: the Zihuatanejo (including Huetamo),Arcelia, Teloloapan, Guanajuato, and Zacatecas subterranes(Fig. 1). The southern portion of the Guerrero terrane hasbeen thrust eastward onto strata of the adjacent Mixteco ter-rane (Fig. 1), and the northeastern portion of the Guerreroterrane is juxtaposed against continental margin strata of theSierra Madre terrane to the northeast (Fig. 1; see later dis-cussion). Contacts between the various subterranes arethought to be mainly structures of Laramide age. Thesebounding structures are interpreted to occur between the ex-posed windows of the Guerrero terrane and if exposed at allare generally of poor quality. Consequently the nature and/orage of many of these structures are, at best, poorly con-strained.

The main lithostratigraphic and geochemical characteris-tics, and inferred paleotectonic settings, of each of the sub-terranes of the Guerrero terrane are briefly summarizedbelow.

Zihuatanejo subterrane

The Zihuatanejo subterrane (Fig. 1) consists predominantlyof thick sequences of Late Jurassic and Early Cretaceous,mafic to felsic volcanic rocks with locally abundant reefallimestone and fine-grained clastic rocks. The volcano-sedi-mentary assemblages were mainly deposited in a submarineenvironment; however, subaerial facies are locally present(e.g., Centeno-García et al., 2003b; Bissig et al., 2008). Thevolcanic sequence is at least 2.4 km thick in the southwesternJalisco-Colima area and is characterized by rapid lateral facieschanges and internal unconformities (Centeno-García et al.,2003a).

Most of the mafic volcanic rocks of the Zihuatanejo subter-rane are tholeiitic to locally calc-alkaline in composition withεNdt ranging from 2 to 9, whereas felsic volcanic rocks are allcalc-alkaline with relatively homogeneous εNdt values of ap-proximately 5 (Freydier et al., 1997; Bissig et al., 2008). Thereis some notable variability in εNdt values for volcanic unitsalong the length of the Zihuatanejo subterrane. For example,lavas in the Zihuatanejo area have εNdt values in the range of7.5 to 8.4, whereas those in the Playa Azul area to the north-east give values in the range of 2 to 9, and those farther northin the Puerto Vallarta area give values around 4.8 (Centeno-García et al., 1993; Freydier et al., 1997).

The Huetamo subterrane (Fig. 1; here considered to bepart of the Zihuatanejo subterrane) consist mainly of sedi-mentary rocks that contain fossils ranging in age from Tithon-ian (latest Jurassic) to Neocomian (earliest Cretaceous). In-terlayered volcanic rocks comprise volcaniclastic units,volcanic-dominated conglomates and rare pillow basalts (seesummary by Talavera-Mendoza and Suastegui, 2000). Lim-ited geochemical data for pillow basalts suggest a dominantlyarc tholeiitic signature and juvenile εNdt values of 7.1 (Talav-era-Mendoza and Suastegui, 2000).

Volcanic and sedimentary rocks of the Zihuatanejo subter-rane unconformably overlie a deformed and metamorphosedbasement assemblage referred to as the Arteaga Complex(Centeno-García, 1994; Fig. 1). The following summary ofthe geology of the Arteaga Complex is taken from Centeno-García et al. (2003a). Volcanic rocks in the vicinity of theArteaga Complex were mainly erupted subaerially. The base-ment complex consists primarily of terrigineous siliciclasticrocks (termed the Varales lithofacies) that are interpreted tobe distal turbidites. The youngest detrital zircons within thissequence are 260 Ma (Late Permian) and the only fossils dis-covered thus far are Late Triassic; therefore, a broadlyPermo-Triassic age is assigned to the package. Locally pil-lowed mafic volcanic rocks are interlayered with the clasticunits. Geochemical signatures of most of these units are typ-ical of normal mid-ocean ridge basalts (N-MORB), althougha few samples have island-arc tholeiitic compositions. Age-corrected εNd values for the mafic volcanic rocks are juvenile(13–8.6); however εNdt values of –6.2 to –7.2 for the clasticsedimentary rocks, together with older detrital zircons, indi-cate a more evolved source for this package. A gabbro intru-sion within the Arteaga Complex has given a U-Pb zircon ageof 180 ± 6 Ma, and two post-tectonic granitoids have given U-Pb zircon and Ar-Ar biotite ages of 163 ± 3 and 159 ± 1 Ma,respectively (Centeno-García et al., 2003a).

AGE AND PALEOTECTONIC SETTING OF VMS DEPOSITS IN THE GUERRERO TERRANE, MEXICO 119

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On the basis of the abundance of felsic volcanic rocks, lo-cally subaerial eruptive environment and nature of its under-lying basement, Freydier et al. (1997) concluded that the Zi-huatanejo subterrane represents a continuous, east-facing arcassemblage built on a heterogeneous basement ranging fromcontinental to overthickened oceanic crust. Centeno-Garcíaet al. (1993) also argued that the Zihuatanejo subterrane rep-resents an evolved island arc. Although spatially separatefrom the Zihuatanejo subterrane, Centeno-García et al.(2003a) suggested that the Heutamo subterrane (Fig. 1) isvery similar to the Zihuatanejo subterrane in terms of age,general lithological makeup, and inferred basement. Talav-era-Mendoza and Suastegui (2000) and Guerrero-Sausteguiet al. (2003) suggested that the Heutamo subterrane formedin a back-arc position closely related to the Zihuatanejo arcbased on stratigraphic and sedimentological studies.

The Zihuatanejo and Heutamo subterranes have beenthrust eastward over the Arcelia subterrane to the southeastand the Guanajuato subterrane to the northeast.

Arcelia subterrane

The Arcelia subterrane (Fig. 1) consists predominantly ofpillowed mafic volcanic rocks interlayered with and overlainby black shale and chert, all of which appear to have been de-posited in relatively deep water (Ruiz and Centeno-García,2000; Centeno-García et al., 2003a). Basalts of the Arceliasubterrane are characterized by arc tholeiite compositions;however, Talavera-Mendoza and Suastegui (2000) interpretedit to be a primitive arc and back-arc assemblage that has beenthrust to the east over the Teloloapan subterrane (Elías-Her-rara et al., 2000).

Teloloapan subterrane

The Teloloapan subterrane (Fig. 1) comprises mafic vol-canic rocks stratigraphically overlain by intermediate to felsicvolcanic and volcaniclastic units with interlayered fine-grained clastic rocks (e.g., Talavera-Mendoza et al., 1995; Ta-lavera-Mendoza and Suastegui, 2000; Centeno-García et al.,2003a). The entire sequence is interpreted to have been de-posited in a submarine setting, and the presence of Early Cre-taceous radiolarian cherts in the lower part of the section sug-gests relatively deep water conditions (Talavera-Mendoza etal., 1995; Centeno-García et al., 2003a). The volcanic rocksare calc-alkaline in composition, with εNdt values between 1.6and 4.6 and are interpreted to represent an immature toevolved arc (Talavera-Mendoza et al., 1995; Talavera-Men-doza and Suastegui, 2000; Centeno-García et al., 2003a) orintra-arc basin setting (Guerrero-Saustegui et al., 2003).

The entire Teloloapan subterrane has been metamor-phosed to greenschist facies. It is internally imbricated bythrust and low angle normal faults, with Teloloapan subter-rane thrust eastward over Albian carbonates and older clasticstrata of the Mixteco terrane. Elías-Herrara et al. (2000) in-terpreted a Triassic to Early Jurassic metamorphic complex(the “Tejupilco metamorphic suite”) that underlies theTeloloapan volcano-sedimentary sequence to represent base-ment for the subterrane. A 186 Ma foliated granitoid withinthis complex, with εNdt of –5 and abundant ~1.3 Ga inheritedzircon, is evidence that this package represents a fragment ofcontinental crust (Elías-Herrara et al., 2000). The contact

between the volcano-sedimentary package and the underly-ing metamorphic complex, however, is a fault (Lewis andRhys, 2000); therefore the unconformable relationship in-ferred by Elías-Herrara et al. (2000) is equivocal.

Guanajuato subterrane

The Guanajuato subterrane (Fig. 1) comprises two distinctcomponents: a juvenile, mafic to locally felsic volcanic-sedi-mentary arc package (the “Guanajuato arc”) that has locallybeen thrust eastward over a package of deep-water sedimen-tary rocks and associated mafic volcanic rocks of the ArperosFormation that yield Late Jurassic to Early Cretaceous fossilages (Ortiz-Hernández et al., 2003). The tectonic setting ofthe Guanajuato arc is generally accepted; however, consider-able debate concerning the nature of the Arperos Formationexists. Sedimentary units in the Arperos Formation includedeep-water pelagic sedimentary rocks at the base with rela-tively juvenile εNdt values (1–9) that pass upward into fine-grained clastic rocks with more evolved Nd signatures(Lapierre et al., 1992; Freydier et al., 2000). Pillowed maficvolcanic rocks that underlie and are interlayered with the sed-imentary strata yield MORB to ocean island basalt (OIB)compositions and juvenile εNdt values of 5.8 to 10.9 (Freydieret al., 1996, 2000; Ortiz-Hernández et al., 2003). Together thesedimentary rocks and pillow basalts have been termed theArperos basin by Tardy et al. (1994) and Freydier et al. (1996,2000), who interpret the assemblage to represent a piece ofoceanic crust that was trapped between an east-facing Gua-najuato arc on the west and cratonic Mexico on the east.Elías-Herrara and Ortega-Gutiérrez (1998) disputed this in-terpretation and suggested that geochemical data support aback-arc basin setting for formation of the Arperos Forma-tion. These authors also suggested that the Arperos Forma-tion and Guanajuato arc likely formed close to one anotherand that the Guanajuato arc was probably west facing.

Zacatecas subterrane

The Zacatecas subterrane (Fig. 1) consists of two mainlithological assemblages. A basement sequence of mainlyfine-grained siliciclastic rocks termed the Zacatecas Forma-tion is interpreted to have formed in relatively deep water andyields sparse fossils of Late Triassic age (Burckhardt andScalia, 1905; Burckhardt, 1930) and evolved Nd isotope sig-natures (Centeno-García and Silva-Romo, 1997). Pillowedmafic volcanic rocks that are interlayered with, and locallyoverlie, the clastic strata in the vicinity of Zacatecas City yieldN-MORB and OIB compositions and juvenile isotopic signa-tures (εNdt 6.8–7.4), suggesting an open-ocean or possibly back-arc setting (Centeno-García and Silva-Romo, 1997; Danielson,2000; Centeno-García et al., 2003b). Correlative sedimentarystrata are also exposed in eastern Zacatecas State; however,mafic volcanic rocks are absent from this section. Silva-Romoet al. (2000) interpreted this sequence to be part of a subma-rine fan developed along the western margin of nuclear Mex-ico that was continuous with similar rocks of the Sierra Madreterrane still farther to the east. Supporting this interpretationare the very similar age distributions for detrital zircons re-covered from the Late Triassic sedimentary units both withinthe Zacetas subterrane and adjacent parts of the Sierra Madreterrane (Centeno-García et al., 2003b, 2005).

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Tectonically overlying the Triassic basement assemblage incentral and eastern Zacatecas State are fine-grained clasticrocks that are correlated with the regionally developed Chili-tos Formation of de Cserna (1976). Locally abundant mafic tofelsic volcanic rocks are interlayered with the clastic strata,with volcanic accumulations reaching at least 500 m in thick-ness. Danielson (2000) carried out detailed lithogeochemicaland isotopic investigations of the volcanic stratigraphy in east-ern Zacatecas State and determined that the main volcanicsection is tholeiitic with a few samples having transitional tocalc-alkaline compositions. Basalts have εNdt values of 7.2 to8.5, whereas felsic rocks have slightly more evolved εNdt val-ues of 0.7 to 6.8. Interbedded sedimentary rocks all yieldmuch more evolved isotopic compositions with εNdt values of–0.8 to –3.9. Danielson (2000) interpreted the volcanic se-quence to be most consistent with formation in a back-arc set-ting. An upper mafic volcanic package that overlies the mainsequence yields geochemical characteristics typical of arctholeiites, apparently reflecting the initiation of a new arc se-quence on top of the back-arc assemblage (Danielson, 2000).

Las Ollas Complex

The Las Ollas Complex (Fig. 1) is tectonically overlain byrocks of the Zihuantanejo subterrane and represents a keytectonostratigraphic component in the southern end of theGuerrero terrane. The Las Ollas Complex comprises bothsediment- and serpentinite-matrix mélange, some of whichhas experienced blueschist facies metamorphism in a subduc-tion zone (Talavera-Mendoza, 2000). Mafic volcanic rocksand associated intrusions within the sedimentary package dis-play island-arc tholeiitic affinities with juvenile isotopic signa-tures (εNdt values of 8.0; Talavera-Mendoza, 2000; Talavera-Mendoza and Suastegui, 2000; Centeno-García et al., 2003a).The sedimentary rocks are locally quartz rich with moreevolved isotopic signatures (εNdt values of –4.1; Centeno-Gar-cía et al., 2003a). Although the volcanic and sedimentary rockunits within the Las Ollas Complex have not been directlydated, the complex is interpreted to represent a subductioncomplex that formed concurrently with at least some of theJurassic to Cretaceous Guerrero arc and back-arc magmatism(Talavera-Mendoza, 2000). The position of this inferred sub-duction zone on the southwestern side of the Guerrero ter-rane led Talavera-Mendoza (2000) to conclude that the sub-duction was probably east dipping.

VMS Deposits in the Guerrero TerraneThe Guerrero terrane hosts between 60 and 150 individual

VMS deposits and/or occurrences (Miranda-Gasca, 1995,2000, 2003; Hall and Gomez-Torres, 2000c), some of the moreimportant examples are shown in Figure 2. The distribution ofthese deposits form two broadly linear trends referred toherein as the Central and Coastal belts that appear to con-verge in the southern portion of the Guerrero terrane. In partsof the Guerreo terrane, such as the Leon-Guanajuato, CampoMorado, and Cuale-Bramador districts (Fig. 1), numerousVMS deposits are spatially clustered, being confined to narrowstratigraphic intervals within the volcano-sedimentary se-quences. The vast majority of Guerrero terrane VMS depositsare hosted within arc or back-arc(?) facies of the Jurassic toCretaceous stratigraphic assemblages. Exceptions include the

Copper King deposit (Fig. 2), which occurs within the LasOllas Complex, and the Arroyo Seco (Fig. 2), which is hostedwithin the Arteaga Complex.

Most VMS deposits in the Central belt occur in strati-graphic sequences that include abundant sedimentary as wellas volcanic host rocks, and in most instances felsic volcanicrocks are in subequal proportions to, or dominate over, maficvolcanic rocks. These deposits therefore fall within the “bi-modal-siliciclastic” class of VMS deposit as defined by Barrieand Hannington (1999), and all but those in the Zacatecassubterrane would also be assigned to the “siliciclastic felsic”class of Franklin et al. (2005). The immediate host rocks fordeposits in the Zacatecas subterrane are dominantly maficvolcanic rocks with less abundant interlayered felsic volcanicand siliciclastic rocks; however, this assemblage represents arelatively small volcanic center within a regionally developed,dominantly siliciclastic rock sequence (Johnson et al., 1999,2000). Thus, although the Zacatecas deposits could be in-cluded in the “bimodal-mafic” class of Franklin et al. (2005),for simplicity we will hereafter refer to all the VMS depositsof the Central belt as belonging broadly to the bimodal-silici-clastic class. The deposits generally contain moderate levelsof copper, lead, and zinc but typically have low silver and goldcontents, another characteristic of bimodal-siliciclastic de-posits (e.g., Barrie and Hannington, 1999). Exceptions to thisgeneralization are deposits in the Campo Morado district,which are gold rich (Oliver et al., 2001). Analogous camps tothe Central belt include the Iberian Pyrite Belt and theBathurst district of eastern Canada (Hall and Gomez, 2000a,b)

The Coastal belt VMS deposits are almost exclusivelyhosted by volcanic rocks and/or volcanic-derived sedimentaryrocks of local provenance. Felsic volcanic rocks dominateover mafic volcanic rocks, with this class of deposit falling inthe “bimodal-felsic” class of Barrie and Hannington (1999;see also Franklin et al., 2005). These deposits have high zinc,lead, and silver contents for deposits in this class (e.g.,Franklin et al., 2005), with lesser amounts of copper and gold.The Cuale district has some of the highest silver to base metalratios in the world for VMS deposits (Hall and Gomez,2000c), and prior to the 1900s were mined purely for their sil-ver content (Giles and Garcia, 2000). The Kuroko deposits ofJapan are a good analogy (Berrocal and Querol-Sune, 1991;Hall and Gomez-Torres, 2000c).

The VMS deposits of the Coastal belt tend to have veryhigh ore grades but are in general relatively small, averagingabout 0.5 million tons (Mt), with the La Minita deposit beingthe largest at 6 Mt. The VMS deposits of the Central belt tendto be in the several to tens of million tons range but lower ingrade. The San Nicolas deposit in eastern Zacatecas State(Fig. 2) is the largest VMS deposit discovered thus far in theGuerrero terrane, with a preliminary mineable reserve andresouce estimate of 72 Mt, grading 1.35 percent copper, 2.27percent zinc, 0.53 g/t gold, and 30 g/t silver for the Main zonewith an additional 11.5 Mt, grading 1.62 percent copper and0.48 percent zinc for the Lower zone (Johnson et al. 1999).

Historically, the generally low silver content of the Guer-rero VMS deposits in comparison to classic silver vein de-posits of the Pachuca, Taxco, and Zacatecas districts, or thecarbonate replacement deposits (CRD) of the Santa Eulalia,

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Naica, and Charcas districts of the Central Mexican silver beltcaused early Spanish explorers to overlook many of theseVMS deposits. Cuale, because of its high silver content beganproduction in 1805 (Hall and Gomez, 2003c) and the Re-forma deposit of the Campo Morado district recorded earlyproduction from 1903 (Oliver et al., 1999). The discovery ofancient artisanal workings and tools at the Tizapa deposit sug-gest it was exploited by the native people in pre-Columbustimes. At present only the Tizapa deposit (Fig. 2) is beingmined.

Geology and U-Pb Geochronology of Selected VMS Deposits and Districts

Absolute age constraints from the main volcano-sedimen-tary sequences that host VMS deposits within the Guerreroterrane have been very limited prior to this study. Existing agedata consisted of scattered fossil ages as well as a small num-ber of K-Ar and a few Ar-Ar and U-Pb zircon ages. Most ofthe K-Ar and Ar-Ar ages are younger than the known or in-ferred depositional ages for the dated samples and appear to

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Colima

Nayarit

JaliscoGuanajuato

Zacatecas100°102°

18°

20°

22°

104°

Michoacan Mexico

Guerrero

San LuisPotosi

Z

A

GuL

A

Zi

PVGd

MC

SLP

U.S.Guerrero terrane Zihuatanejo subterrane Heutamo subterrane Teloloapan subterrane Arcelia subterrane

Zacatecas subterrane Arteaga complex Las Ollas complexSierra Madre terraneMixteco terrane

Leon-Guanajuato subterrane

VMS deposit or district

1. Cuale-Bramador2. La America3. Dios Me Ayuda4. La Minita5. Arroyo Seco6. Copper King7. Campo Morado

8. Rey de Plata9. Tlanilpa-Azulaquez10. Tizapa11. El Gordo12. Las Gavilenas13. Carmen14. San Nicolas

VMS Deposits and Districts

1

2

3

45

67

8

10

1112

13

14

9

FIG. 2. Distribution of volcanogenic massive sulfide (VMS) deposits and districts within the Guerrero terrane. City ab-breviations as in Figure 1.

have been disturbed or thermally reset, probably during theLaramide deformation, metamorphism, and associated mag-matism that has affected much of the Guerrero terrane.

Samples for U-Pb zircon dating for this study were col-lected during the course of geologic mapping and explorationwork by the various co-authors in all of the main VMS dis-tricts. In the following sections the geologic setting of VMSmineralization in each of the studied camps is briefly de-scribed with the results of U-Pb dating presented and inter-preted. Methods used for the U-Pb dating are described inthe Appendix, and analytical data are given in Table 1 and dis-played in conventional U-Pb concordia diagrams in accompa-nying figures.

The VMS deposits of the Teloloapan, Guanajuato, and Za-catecas subterranes all belong to the Central belt, whereasthe Zhijuantanejo subterrane hosts the VMS deposits of theCoastal belt.

Deposits in the Teloloapan subterrane

The largest concentration of VMS deposits within theGuerrero terrane occurs within the Teloloapan subterrane(Figs. 2, 3). From south to north, these are the deposits of theCampo Morado district, including the Rey de Plata deposit,the Tlanilpa-Azulaquez district, and finally the Tizapa-SantaRose deposits. In each case, the VMS mineralization occurs asseveral individual sulfide lenses closely associated with felsicvolcanic or volcaniclastic rocks, usually within carbonaceousclastic sedimentary rocks (e.g., Oliver et al., 1999).

The geology and VMS deposits of the Campo Morado dis-trict (Figs. 3, 4) have been described recently by Oliver et al.

(1999, 2000, 2001), Weston (2002), and Weston and Gibson(2003), and the following summary is taken from their work.Nine individual deposits have been identified within theCampo Morado district, and four known deposits have a com-bined resource of 29 Mt (Oliver et al., 2000, 2001). Strati-graphic analysis of the Campo Morado district is complicatedby thrust faulting and recumbent folding, which has led to lo-cally overturned bedding (as at the Reforma and El Rey de-posits; Fig. 4).

The lowermost stratigraphic unit is a marine hemipelagicsedimentary succession referred to as the Reforma Forma-tion. This unit is overlain by a thin (generally <400 m), dom-inantly felsic volcanic and volcaniclastic sequence named theCampo Morado Formation, which consists mainly of massiverhyolite, a variety of rhyolitic breccia, and volcaniclastic andhemipelagic sedimentary rocks. Locally this felsic volcanicpackage is underlain by or interlayered with intermediatecomposition flows and volcaniclastic rocks. The El Largo andNaranjo deposits formed at the southern terminus of a 1.2-km-long, 400-m-thick rhyolite cryptodome complex that wasemplaced into a north-south–tending graben.

Most of the sulfide mineralization in the Campo Moradodistrict is interpreted to occur at or near the upper contact ofthe felsic volcanic package with overlying, dominantly clasticsedimentary strata. Sulfide textures, deposit morphology, andstratigraphic location indicate that the Naranjo and El Largodeposits formed on the sea floor, whereas the El Largo southdeposit formed as subsea-floor replacement processes.

Four felsic igneous units within and in close proximity tothe Campo Morado district were dated using U-Pb zircon

AGE AND PALEOTECTONIC SETTING OF VMS DEPOSITS IN THE GUERRERO TERRANE, MEXICO 123

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km

0 20 40

N

Teloloapan subterraneArcelia subterraneMixteco terraneOther terranes

VMS deposit

Tertiary and Quaternaryvolcanic rocks

Cuernavaca

Tizapa

99°30'100°00'

100°

30'

19°30'

Zitacuaro Mexico City

19°00'

AzulaquezTlanilpa

Zacualpan

Santa Rosa

Teloloapan

Rey de Plata

Campo Morado

Arcelia

18°30'

Tejupilco

GuerreroState

Sultepec

Taxco

Iguala

La Suriana

El Faisan Apixtla

MexicoState

DistritoFederal

MorelosState

Mic

ho

acan

Sta

te

FIG. 3. Simplified geology and location of volcanogenic massive sulfide deposits and districts in the Teloloapan subterrane.

124 MORTENSEN ET AL.

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TABLE 1. U-Pb Analytical Data for Volcanic Rock Units from the Guerrero Terrane, Mexico

TotalSample Wt U Pb2 206Pb/204Pb common % 206Pb/238U4 207Pb/235U4 207Pb/206Pb4 206Pb/238U age 207Pb/206Pb agedescription1 (mg) (ppm) (ppm) (meas.)3 Pb (pg) 208Pb2 (± % 1σ ) (± % 1σ ) (± % 1σ ) (Ma; ± % 2σ) (Ma; ± % 2σ )

Sample 1 (13 at 77 m – Reforma deposit)A: N5, 74-105 0.007 209 13.0 806 7 12.7 0.05846(0.22) 0.6245(0.44) 0.07747(0.38) 366.3(1.6) 1133.4(15.2)B: N5, 74-105 0.005 307 7.2 561 4 14.2 0.02241(0.21) 0.1511(0.70) 0.04890(0.64) 142.9(0.6) 143.1(29.9)C: N5, 74-105 0.009 295 7.3 565 7 12.1 0.02400(0.18) 0.1678(0.81) 0.05071(0.74) 152.9(0.6) 227.8(34.2)

Sample 2 (265 at 165 m, Naranjo deposit)A: N5, 74-105 0.020 152 7.1 2501 4 8.8 0.04655(0.12) 0.4144(0.20) 0.06456(0.14) 293.3(0.7) 760.1(5.8)B: N5, 74-105 0.020 149 3.9 878 5 14.0 0.02520(0.17) 0.1777(0.40) 0.05115(0.33) 160.4(0.5) 247.8(15.0)C: N5, 74-105 0.020 182 7.7 3116 3 10.1 0.04172(0.19) 0.3580(0.29) 0.06224(0.25) 263.5(1.0) 682.2(10.5)

Sample 3 (PR-35, El Faisan prospect)A: N2,+134 0.105 134 3.2 1857 11 12.1 0.02289(0.10) 0.1547(0.26) 0.04900(0.21) 145.9(0.3) 148.1(9.7)B: N2,+134 0.072 189 4.6 1349 15 11.3 0.02396(0.11) 0.1628(0.25) 0.04926(0.17) 152.7(0.17) 160.4(7.8)C: N2,+134 0.092 218 5.2 953 30 13.7 0.02274(0.10) 0.1534(0.28) 0.04893(0.21) 144.9(0.3) 144.7(10.0)D: N2,+134 0.093 238 6.7 2012 19 11.9 0.02735(0.11) 0.1974(0.22) 0.05235(0.15) 173.9(0.4) 300.7(6.8)E: N2,+134 0.148 212 6.4 1740 33 12.8 0.02908(0.09) 0.2458(0.20) 0.06129(0.13) 184.8(0.3) 649.4(5.4)

Sample 4 (170 at 208.5 m, La Suriana deposit)A: N2,+74 0.030 158 8.5 411 40 10.1 0.05293(0.12) 0.4361(0.68) 0.05976(0.61) 332.5(0.8) 595.0(26.7)B: N2,+74 0.032 166 4.9 200 51 13.0 0.02802(0.15) 0.2311(1.51) 0.05981(1.42) 178.1(0.5) 596.7(61.4)C: N2,+74 0.036 162 3.9 2860 3 13.7 0.02287(0.13) 0.1549(0.25) 0.04912(0.20) 145.8(0.4) 153.7(9.1)

Sample 5 ( DR-117, Manto Rico deposit area)A: N10,+74 0.007 145 3.1 128 12 10.1 0.02165(0.34) 0.1458(2.78) 0.04882(2.61) 138.1(0.9) 139.4(122.3)B: N10,+74 0.013 141 7.0 472 12 13.3 0.04775(0.18) 0.3618(0.74) 0.05495(0.66) 300.7(1.1) 410.1(29.3)C: N10,+74 0.009 131 2.9 134 14 11.8 0.02192(0.46) 0.1523(2.43) 0.05037(2.18) 139.8(1.3) 212.2(101.3)D: N10,+74 0.010 325 7.9 340 14 17.7 0.02208(0.19) 0.1502(0.76) 0.04934(0.65) 140.8(0.5) 164.2(30.6)E: N10,44-74,u 0.039 214 4.7 212 55 13.3 0.02093(0.28) 0.1406(1.27) 0.04873(1.27) 133.5(0.7) 134.7(59.6)

Sample 6 (AU1-6, Aurora I deposit)A: N2,134 0.055 105 2.0 609 13 12.5 0.02178(0.17) 0.1466(0.63) 0.04881(0.56) 138.9(0.5) 138.7(26.0)B: N2,104-134 0.041 112 3.0 464 14 12.8 0.02172(0.12) 0.1462(0.66) 0.04881(0.60) 138.5(0.3) 138.6(28.0)C: N2,74-104 0.067 122 3.0 1199 10 14.6 0.02245(0.10) 0.1521(0.32) 0.04913(0.27) 143.1(0.3) 154.2(13.0)D: N2,62-74 0.078 128 3.0 1670 10 12.8 0.02580(0.25) 0.1913(0.25) 0.05376(0.19) 164.2(0.3) 360.8(8.5)

Sample AD-01A: N12,74-104 0.016 187 4.8 186 27 14.2 0.02433(0.42) 0.1686(5.56) 0.05025(5.37) 155.0(1.3) 206.7(249.0)B: N5,74-104 0.018 141 4.2 39 194 15.2 0.02749(1.47) 0.2193(7.26) 0.05787(6.58) 174.8(5.1) 524.9(281.0)C: N5,74-104 0.009 48 4.6 259 11 7.7 0.09595(0.45) 0.9155(3.15) 0.06927(2.94) 590.1(5.1) 906.9(121.0)D: N5,74-104 0.022 171 4.3 76 93 15.9 0.02320(0.91) 0.1576(3.47) 0.04926(02.97) 147.8(2.7) 160.4(139.0)E: N5,74-104 0.014 110 2.6 169 14 12.8 0.02292(0.37) 0.1548(5.16) 0.04898(4.99) 146.1(1.1) 146.8(234.0)

Sample 98-TD-153A: N1,62-74 0.080 116 3.0 1795 7 12.9 0.02282(0.21) 0.1541(0.41) 0.04897(0.35) 145.5(0.6) 146.6(16.5)C: N1,62-74 0.032 120 3.0 1248 4 13.7 0.02132(0.31) 0.1454(0.49) 0.04946(0.46) 136.0(0.8) 169.5(21.7)D: N1,62-74 0.120 144 4.0 3061 8 14.1 0.02327(0.12) 0.1574(0.21) 0.04906(0.13) 148.3(0.6) 150.8(6.3)E: N1,62-74 0.120 114 3.0 3789 5 14.3 0.02278(0.17) 0.1560(0.24) 0.04966(0.14) 145.2(0.5) 179.0(6.4)F: N1,62-74 0.100 149 4.0 1664 13 14.3 0.02270(0.12) 0.1534(0.23) 0.04902(0.15) 144.7(0.3) 148.9(7.2)

Sample 98-TD-11A: N2,104-134 0.026 127 3.0 152 36 15.6 0.02346(0.21) 0.1587(2.40) 0.04907(2.20) 149.5(0.6) 151.2(108.1)B: N2,104-134 0.026 150 4.0 146 43 15.4 0.02299(0.22) 0.1562(2.50) 0.04927(2.40) 146.5(0.6) 160.9(114.1)C: N2,104-134 0.023 138 3.0 116 47 16.0 0.02327(0.26) 0.1574(3.20) 0.04906(3.10) 148.3(0.8) 150.8(151.0)

Sample 98-TD-12B: N2,74-104 0.018 159 4.0 123 40 15.9 0.02362(0.22) 0.1599(2.80) 0.04911(2.70) 150.5(0.6) 152.9(130.5)C: N2,74-104 0.028 156 4.0 156 49 16.2 0.02502(0.19) 0.1769(2.20) 0.05129(2.10) 159.3(0.6) 253.7(97.6)D: N2,62-74 0.073 250 6.0 2979 9 14.2 0.02367(0.16) 0.1607(0.18) 0.04923(0.18) 150.8(0.5) 158.7(8.7)E: N2,62-74 0.140 237 6.0 749 66 14.2 0.02294(0.21) 0.1557(0.41) 0.04922(0.29) 146.2(0.6) 158.5(13.7)

Sample 98-TD-01A: N2,104-134 0.055 151 4.0 2344 5 14.8 0.02330(0.14) 0.1578(0.23) 0.04911(0.16) 148.5(0.4) 152.9(7.3)B: N2,104-134 0.055 138 3.0 807 14 15.0 0.02315(0.11) 0.1569(0.31) 0.04914(0.23) 147.5(0.3) 144.4(10.8)C: N2,104-134 0.040 141 3.0 1105 7 14.7 0.02309(0.13) 0.1557(0.41) 0.04893(0.36) 147.1(0.4) 144.3(17.0)D: N2,104-134 0.040 140 3.0 1623 5 15.2 0.02337(0.15) 0.1578(0.15) 0.04897(0.24) 148.9(0.4) 146.3(11.2)

1 N1, N2, N5 = nonmagnetic at n degrees side slope on Frantz magnetic separator; grain size given in microns; u = abraded2 Radiogenic Pb; corrected for blank, initial common Pb, and spike3 Corrected for spike and fractionation4 Corrected for blank Pb and U, and common Pb

methods. Sample 1 is from a large felsic dike that is closely as-sociated with the Reforma deposit. This body is discordant tostratigraphy and is interpreted to be a feeder to the main fel-sic flows that host the deposit. A small amount of zircon wasrecovered, comprising clear, colorless, stubby prisms. Threestrongly abraded fractions, comprising all but the finest of therecovered zircon grains, were analyzed (Fig. 5a; Table 1).Fraction B is concordant with a 206Pb/238U age of 142.9 ± 0.6Ma, which is interpreted to be the best estimate for the crys-tallization age of the sample. Two other fractions yield con-siderably older 207Pb/206Pb ages, and together the three analy-ses define a linear array with a calculated upper intercept ageof 1.45 Ga. The data are interpreted to reflect the presence ofa significant component of older, inherited zircon in fractionsA and C, presumably as “cryptic cores” that could not be dis-tinguished visually. Because all fractions consisted of multiplegrains, this upper intercept age can only be interpreted togive an average age for the inherited zircon component.

Sample 2 is from a massive felsic flow that is interpreted tostratigraphically underlie the Naranjo deposit (Fig. 4). Thesmall amount of zircon recovered from this sample is similarin appearance to that in sample 1. All but the finest zircongrains in the sample were moderately abraded and split intothree fractions (Table 1; Fig. 5b). Fraction C is concordantwith a 206Pb/238U age of 145.8 ± 0.4 Ma, which is interpreted

to be the best estimate for the crystallization age of the sam-ple. Fractions A and B yield older 207Pb/206Pb ages, thought tobe due to the presence of older inherited cores in some or allof the grains. Unlike the previous sample, however, the threeanalyses from sample 2 do not form a linear array. Two-pointregressions through concordant fraction C and each of thetwo discordant fractions yield calculated upper intercept agesof 0.76 and 1.68 Ga. Since these are all multigrain fractions,the two upper intercept ages represent a minimum range forthe average age of the inherited zircon components in frac-tions A and B.

Samples 3 and 4 are from the the El Faisan prospect and theLa Suriana deposit, which are located approximately 14 and 8km, respectively, south of Campo Morado (Fig. 3; Miranda-Gasca, 1995). Sample 3 is from a heterolithic felsic fragmen-tal unit from a mineralized horizon at El Faisan (Fig. 3). Thisvolcaniclastic unit contains rare transported massive sulfidefragments. Regional stratigraphic constraints suggest that themineralization occurs 0.8 to 1.2 km higher in the section thanthe main mineralized horizons within the Campo Morado dis-trict proper. Relatively abundant zircon was recovered fromthe sample; it comprises a single population of clear, palepink, stubby square prismatic grains with no evidence of in-herited cores. The zircons are euhedral and sharply faceted,with no evidence for abrasion that might reflect significant

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12

4

3

87

Pueblo Reforma

10 9

PuebloCampo Morado

2040000N

2012000N37

8000

E

3800

00E

2014000N

6 5Felsic subvolcanicintrusive rocksGuerrerito intermediate volcanic unitNaranjo sedimentary (volcanic) unitCampo Morado felsic volcanic unitReforma sedimentaryunitLa Canita felsic-inter-mediate volcanic unitSedimentary unit - age uncertain

Massive sulfide deposits and occurrences 1. Reforma 2. El Rey 3. Naranjo 4. Historic Naranjo 5. El Largo 6. San Rafael 7. Estrella de Oro 8. El Profundo 9. G-9 10. La Lucha

LEGEND

km0 1

18°12’ 18°12’

100°08’

Thrust faultDeposit area

FIG. 4. Geology of the Campo Morado district, showing location of the main volcanogenic massive sulfide deposits (mod-ified from Oliver et al., 1999, 2000).

transport. Five strongly abraded multigrain zircon fractionswere analyzed (Table 1, Fig. 5c). Fractions A and C yield con-cordant analyses, whereas fraction B is slightly discordant andfractions D and E are strongly discordant. Because of theclastic character of the sample it is uncertain whether the dis-cordance is related to the presence of older inherited zirconcores in some of the grains or admixture of a xenocrystic com-ponent of zircon that was similar in appearance to the mainigneous population, or both. Assuming that a xenocrystic zir-con component might be present, the best estimate for thecrystallization age of the sample is given by the 206Pb/238U ageof fraction C, at 144.9 ± 0.3 Ma. This fraction was relativelycoarse grained, had a low U content, and was stronglyabraded; thus postcrystallization Pb loss should be minimal.Fractions A and B are interpreted to have included a minorinherited and/or xenocrystic zircon component that was only

slightly older than the inferred depositional age, whereasfractions D and E clearly contained significantly older inher-ited and/or xenocrystic zircon components. The calculatedupper intercept ages calculated for these (pinned on analysisC) of 0.81 and 1.66 Ga suggest that the sample containedolder zircon components of both Neoproterozoic and Meso-proterozoic age.

Sample 4 is from a quartz-phyric felsic flow in the immedi-ate footwall of the La Suriana deposit (Fig. 3). A very smallamount of zircon was recovered (approx 0.1 mg total). All ofthe coarsest zircon grains were moderately abraded and splitinto three multigrain fractions (Table 1, Fig. 5d). All threeanalyses are discordant and the data forms a linear array withcalculated lower and upper intercept ages of 150.0 ± 5.2 Maand 1.10 Ga, respectively. The lower intercept age gives arough approximation of the crystallization age of the felsic

126 MORTENSEN ET AL.

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160

200

240

280

320A

BC

0.1 0.3 0.5 0.70.01

0.03

0.05

120

160

200

240

280 B

CD

EA

0.1 0.2 0.3 0.40.015

0.025

0.035

0.045

206

238

Pb/

U20

623

8P

b/U

207 235Pb/ U 207 235Pb/ U

fraction B concordantat 142.9 ± 0.6 Ma

Sample 1 (felsic dike- Reforma)

Sample 5 (Tlanilpa-Azulaquez felsic tuff)

Sample 6 (diorite)

Tlanilpa-Azulaquez

140

150

160

AB

C

D

0.14 0.180.021

0.023

0.025

0.027

to 1.45 Ga

fractions A, C & D concordantat 139.7 ± 2.5 Ma

fractions A & B concordantat 138.7 ± 0.7 Ma

to 0.49 Gato 1.12 Ga

a

130

170

210

250

A

B

C

0.1 0.2 0.3 0.40.015

0.025

0.035

0.045Sample 4 (felsic flow

- La Suriana)

to 1.10 Ga

150.0 ± 5.2 Ma3-pt. regression (MSWD=7.5)

d

160

200

240

280

A

B

C

0.1 0.2 0.3 0.4 0.50.02

0.03

0.04

0.05

fraction C concordantat 145.8 ± 0.4 Ma

to 0.76 Ga

to 1.68 Ga

b20

623

8P

b/U

fraction C concordantat 144.9 ± 0.3 Ma

140

150

160

170

A

B

C

D

E

0.14 0.20 0.260.021

0.025

0.029Sample 3 (felsic fragmental

- El Faisan)to 0.81 Ga

to 1.66 Gac

e f

Sample 2 (felsic flow- Naranjo)

FIG. 5. Conventional U-Pb concordia diagrams for volcanic and subvolcanic rock units in the Campo Morado andTlanilpa-Azulaquez volcanogenic massive sulfide districts.

unit and the upper intercept indicates an average age for anolder inherited zircon component of 1.1 Ga.

The only other available constraint on the depositional agefor VMS deposits in the Campo Morado district and vicinitycomes from an upper Valanginian to lower Hauterivian fossilfrom near the western edge of the Reforma deposit (Haggart,1997).

The new age data indicate that the deposits in the CampoMorado district proper (Reforma and Naranjo deposits)formed between 146.2 and 142.3 Ma. The fossil informationfrom near the Reforma deposit poses a problem, since theValanginian-Hauterivian boundary is currently placed at136.4 ± 2.0 Ma (Gradstein et al., 2004). This may indicate ei-ther that there are additional structural complications in thevicinity of the Reforma deposit that have not yet been recog-nized or that the presently accepted age for the Valanginian-Hauterivian boundary is incorrect. Interpreted crystallizationages are identical for VMS host rocks in the El Faisan areaand the main Campo Morado district, although regional cor-relations suggest that the El Faisan mineralized horizon isstratigraphically significantly higher than deposits in the mainCampo Morado area. Host rocks at the La Suriana deposit are

imprecisely dated; however, they also appear to be very simi-lar in age to those in the main Campo Morado district.

The Tlanilpa-Azulaquez district in the central part of theTeloloapan subterrane (Figs. 2, 3, 6) hosts at least 15 separateVMS deposits (Miranda-Gasca, 1995, 2003). Recent geologicstudies of this area by Rhys et al. (2000) have documented aregional stratigraphy comprising a lower andesite flow andvolcaniclastic unit in excess of 400 m in thickness, overlain bya 20- to 130-m-thick sequence of carbonaceous mudstoneand limestone, followed by a package of felsic tuffs and minorflows more than 250 m thick, and finally an upper andesiticsequence of flows and volcaniclastics. Volcanogenic massivesulfide mineralization is hosted by carbonaceous mudstoneand felsic tuff at the base of the felsic volcanic package.Laramide age thrust faulting, local recumbent folding, andpenetrative deformation complicate stratigraphic reconstruc-tions in the district.

Two samples from the Tlanilpa-Azulaquez district weredated. Sample 5 is from a 30-m-thick felsic tuff unit that over-lies and is in part interlayered with carbonaceous mudstoneand forms the hanging wall for syngenetic sulfide lenses at theManto Rico VMS occurrence (Fig. 6). A small amount of zir-

AGE AND PALEOTECTONIC SETTING OF VMS DEPOSITS IN THE GUERRERO TERRANE, MEXICO 127

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0 2km

Upper Volcanic Sequence Ash to lapilli tuff; andesitic flows Felsic tuffSedimentary Rocks Limestone Graywacke, mudstone, siltstoneIntrusive Rocks Medium grained diorite

Lower Volcanic Sequence Andesite and basalt flows, volcanic conglomerate, volcaniclastic rocks

VMS occurrence Fault

Los MantosManto Rico

Otates

Tlanilpa

El CapireAurora I

Aurora II

Tejocoates

AzulaquezGuadalupeSan Antonio

Cruz Blanca

Santa Rita

2059000N

2055000N

2051000N

408000E 412000E 416000E

18°33’

18°38’

99°4

8’99°5

3’

FIG. 6. Geology of the Tlanilpa-Azualaquez district, showing location of the main volcanogenic massive sulfide deposits(modified from Rhys et al., 2000).

con was recovered from the sample, consisting of euhedral,pale pink, stubby to elongate prismatic grains with no visiblecores. Five fractions were analyzed (Table 1, Fig. 5e). Four ofthe analyses were strongly abraded, and three of these (A, C,and D) give partially overlapping concordant analyses with atotal range of 206Pb/238U ages of 139.7 ± 2.5 Ma, which is in-terpreted as the crystallization age of the sample. Fraction Econsisted of finer, more elongate grains and was not abraded.The fraction gives a concordant yet slightly younger206Pb/238U age, reflecting minor postcrystallization Pb loss.Fraction B is discordant with a much older 207Pb/206Pb age. Aregression through this fraction and the cluster of concordantanalyses gives a calculated upper intercept age of 0.49 Ga, in-dicating that this fraction contained an older inherited zirconcomponent with an early Paleozoic average age.

Sample 6 is from a massive, medium-grained diorite bodyimmediately north of the Aurora I deposit southeast ofTlanilpa (Fig. 6). Dikes of this unit are observed to intrudethe lower andesitic volcanic package but do not appear to in-trude rocks of the upper andesitic sequence. Zircons fromthis sample form clear to pale pink stubby euhedral prisms,from which four fractions were analyzed. Two of these giveoverlapping concordant analyses with a total range of206Pb/238U ages of 138.8 ± 0.6 Ma, which is interpreted as thecrystallization age of the sample. Fraction C falls near con-cordia with a 206Pb/238U age of 154 Ma. Fraction D yields amuch older 207Pb/206Pb age, indicating the presence of anolder inherited zircon component. A regression line throughfractions A, B, and D gives a calculated upper intercept ageof 1.20 Ga, indicating that this fraction contained an older in-herited zircon component with Mesoproterozoic average age.

Although the diorite body was interpreted from field rela-tionships to possibly be a feeder to the lower andesite se-quence, the crystallization age is slightly younger than the fel-sic tuff unit that overlies the lower andesite. It is thereforemore probable that the diorite is a feeder to the upper an-desite package or a younger intrusion.

The Rey de Plata deposit (Figs. 2, 3; Miranda-Gasca et al.,2001) is located between the Campo Morado district in thesouthwest and the Tlanilpa-Azulaquez district to the north. Itconsists of several sulfide lenses comprising approximately 3Mt of massive sulfide (Giles and Garcia, 2000) and is hostedwithin a 400-m-thick sequence of felsic volcanic and volcani-clastic rocks with interlayered carbonaceous siliciclastic rocksand andesitic volcanic rocks that locally predominate in theupper part of the unit. The felsic unit is underlain strati-graphically by an older andesitic volcanic package and isstructurally overlain by calcareous and carbonaceous shaleand siltstone. There are presently no direct age constraints onrock units within the Rey de Plata deposit area.

The Tizapa deposit (Figs. 2, 3) is hosted by strongly folded,thrust imbricated, and penetratively deformed phyllite andschist of the Teloloapan subterrane (Lewis and Rhys, 2000).The deposit consists of multiple structurally stacked orelenses with a total mined plus indicated and inferred resourceof approximately 4.5 Mt (Giles and Garcia, 2000). The Tizapaarea comprises two main structural assemblages. The south-ern part of the area is underlain by the Tejupilco schist, an as-semblage of noncalcareous phyllitic mudstones and siltstoneswith minor intercalated metatuffaceous rocks, which is

intruded by a K-feldspar augen orthogneiss that has yielded aU-Pb zircon age of 186 Ma (Elías-Herrara et al., 2000). Thisassemblage is separated from the host units for the VMS de-posit by a north-dipping normal fault; hence, the originalstratigraphic relationship between the two assemblages is un-certain. The northern assemblage is comprised of a lowerchloritic schist unit, which is overlain by a sericite schist unitthat forms the main host to the ore, and subsequently by acarbonaceous phyllite that passes upward into limestoneswith carbonaceous clastic interbeds. Whole-rock geochemicalanalyses of the chloritic and sericitic schist units indicatebroadly andesitic and dacitic to rhyolitic compositions, re-spectively (Lewis and Rhys, 2000). There are no direct agesavailable from the northern assemblages yet; however, U-Pbzircon dating of the sericitic schist unit that hosts the mainVMS deposits is currently underway (J. K. Mortensen and E.Centeno-García, unpub. data).

Deposits in the Guanajuato subterrane

The Leon-Guanajuato district includes at least five signifi-cant VMS deposits and occurrences in the Leon-Guanajuatosubterrane (Figs. 2, 7; Hall and Gomez-Torres, 2000a, b).Mineralization is hosted by a thin sequence (<100 m thick) ofintermediate and felsic flows and volcaniclastic rocks that cor-respond to the Guanajuato arc assemblage as describedabove. This volcanic-dominated section is interpreted tostructurally overlie a package of mainly carbonaceous shales,which is correlated with the Arperos Formation. Althoughepigenetic base and precious metal deposits have been exten-sively mined for at least four centuries in the Leon-Guanaju-ato district, VMS mineralization has only been recognized re-cently and all of the known occurrences are still at anexploration stage. Mineralization at the El Gordo deposit inthe southeastern part of the district comprises up to 50 m ofpyrite-rich massive and semimassive sulfides (Hall andGomez-Torres, 2000a). Intersections of up to 9.3 m grading4.3 percent Cu with low levels of Zn, Pb, Ag, and Au havebeen reported from the El Gordo deposit (Moss and Hall,2001). At the Las Gavilanes deposit in the northwestern partof the district (Hall and Gomez-Torres, 2000b) massive and

128 MORTENSEN ET AL.

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km0 5 10

N

21°00'21°00'

101°30'

101°30'

El Gordo

La Paz

Los Gavilanes

Leon

Guanajuato

Andesite - basaltIgnimbriteConglomerateComanja graniteIntrusiveEsperanza Formation - volcanic/sedimentaryMassive sulfide deposit

Tert

iary

J-K?

J±Tr?

FIG. 7. Simplified geology of the Guanajuato subterrane, showing loca-tions of the main volcanogenic massive sulfide deposits (modified from Halland Gomez, 2000a, b).

semimassive sulfide lenses occur within a ~50-m-thick miner-alized horizon. Individual sulfide lenses in this area range upto 5.4 m and are separated by layers of felsic tuff and siliceousexhalite. Sampling of underground workings at Los Gavilanesproduced an intersection of 1.14 percent Cu with low levelsof Zn, Pb, Ag, and Au over a true thickness estimated at 31 m(Hall and Gomez-Torres, 2000b).

Sample 7 is from a rhyolite flow in the footwall of the ElGordo deposit. Zircons from the sample form clear, pale yel-low, stubby prisms. Five strongly abraded fractions were an-alyzed (Table 1; Fig. 8a). Fraction E is concordant with a206Pb/238U age of 146.1 ± 1.1 Ma, which is interpreted to bethe crystallization age of the sample. The remaining fourfractions yield slightly to much older 207Pb/206Pb ages, andtogether the five analyses define a linear array with a calcu-lated upper intercept age of 0.99 Ga. The data indicate a

Mesoproterozoic average for an inherited zircon componentthat is present in four of the five fractions.

Deposits in the Zacatecas subterrane

The San Nicolas and closely related El Salvador depositsare the only VMS deposits that have been discovered so farwithin the Zacatecas subterrane (Figs. 2, 9, 10). The syn-genetic nature of the small El Salvador deposit (~1 Mt),which had previously been mined for copper on a very smallscale, was recognized in 1996, and subsequent geophysicalsurveys and drilling led to the discovery of the much largerSan Nicolas deposit (83 Mt) in an unexposed area approxi-mately 2 km west of El Salvador. The nature and geologic set-ting of the VMS deposits has been described by Johnson et al.(1999, 2000) and Danielson (2000) and the following descrip-tion is summarized from these references. Rock units in this

AGE AND PALEOTECTONIC SETTING OF VMS DEPOSITS IN THE GUERRERO TERRANE, MEXICO 129

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200

300

400

500

AB

C

DE

0.1 0.5 0.90.01

0.05

0.09

144

148

152

AB

C

D

0.15 0.16 0.170.022

0.023

0.024

130

140

150

160

170

A

BC

0.12 0.16 0.200.018

0.022

0.026

140

150

160

170

B C

D

E

0.13 0.15 0.17 0.190.019

0.023

0.027

136

140

144

148

A

C

D

EF

0.140 0.148 0.156 0.1640.020

0.021

0.022

0.023

0.024

206

238

Pb/

U20

623

8P

b/U

206

238

Pb/

U

206

238

Pb/

U

207 235Pb/ U

207 235Pb/ U

Sample 7 (El Gordofelsic volcanic)

to 0.99 Gaa b

c d

e

Sample 8 (San Nicolasfootwall rhyolite)

Sample 9 (San Nicolas footwallrhyolite dike)

Sample 10 (El Salvadorfootwall felsic volcanic)

Sample 11 (La Virgenrhyolite flow)

fraction E concordantat 146.1 ± 1.1 Ma

fractions A, B & C concordantat 148.9 ± 1.4 Ma

fractions B & D concordantat 150.6 ± 0.7 Ma

fractions A & D concordantat 148.7 ± 0.6 Ma

fraction D concordantat 148.3 ± 0.4 Ma

FIG. 8. Conventional U-Pb concordia diagrams for volcanic and subvolcanic rock units in the Guanajuato and Zacatecassubterranes.

130 MORTENSEN ET AL.

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Zacatecas

San Luis

Potosi

San Nicolas

Zacatecas

102°00' 101°30'

22°30'

23°00'

22°00'

Quaternary coverTertiary volcanic rocksuK felsic intrusionuK marine sedimentary rockslK marine sedimentary rocksuJ - lK marine volcanic and sedimentary rocksuTr marine sedimentary rocks

km

0 25

FIG. 9. Geology of southeastern Zacatecas subterrane (modified from Johnson et al., 2000). Box shows the location of thedetailed geology of the San Nicolas-El Salvador area in Figure 10.

N

Quaternary UndividedTertiary Rhyolitic volcaniclastic rocksUpper Jurassic - Lower Cretaceous Mudstone, chert, limestone Chert Lithic wacke, lithic tuff, chert Rhyolite flows and felsic intrusions Mafic to intermediate flows and associated intrusions

0 2km

SanNicolas

ElSalvador

La Virgen area

VMS occurrence

102

00’

22 34.8’

FIG. 10. Geology of the San Nicolas and El Salvador areas, southeastern Zacatecas (modified from Johnson et al., 2000).

area are only weakly deformed and the primary depositionalcharacter of individual stratigraphic units is well preserved.The main San Nicolas deposit comprises a large keel-shapedmass of sulfides that is bounded on the southwest by a felsicflow-dome complex and on the northeast by a steeply south-west dipping, possibly synvolcanic fault. Massive sulfides arespeculated to have accumulated in a local topographic de-pression between the flow-dome complex and an active faultscarp. The deposit is underlain by a sequence of carbona-ceous mudstone and minor limestone interlayered with maficand felsic volcanic and volcaniclastic rocks. The felsic flow-dome complex along the southwest margin of the deposit isup to 300 m thick and consists of massive, locally flow-bandedrhyolite with abundant intercalations of autoclastic breccia.The immediate hanging wall of the sulfide body consists ofcarbonaceous mudstones with locally abundant mafic flowsand sills. The stratigraphic setting of the El Salvador has notbeen studied in as much detail. The main footwall assemblageat El Salvador comprises a package of intermediate to mafictuffaceous rocks that are overlain by a ~200-m-thick se-quence of felsic flows and tuffs. The sulfide horizon is hostedwithin a ~130-m-thick package of carbonaceous mudstoneswith abundant intercalated mafic flows and sills that overliesthe felsic unit.

Four rock units were dated from the San Nicolas and ElSalvador area using U-Pb zircon methods. Sample 8 is a mas-sive aphyric rhyolite from the felsic flow-dome complex thatin part underlies and in part interfingers with the massive sul-fide body at San Nicolas (Fig. 10). Zircons recovered fromthis and all other samples from the area comprise clear, col-orless, stubby prismatic grains. Five strongly abraded zirconfractions were analyzed (Table 1, Fig. 8b). Three of the analy-ses (A, D, and F) are concordant but scatter along concordia.A conservative estimate for the crystallization age of the rhy-olite sample is given by the total range of 206Pb/238U ages forthe three concordant analyses (146.5 ± 2.2 Ma); however, oneabraded fraction (C) gives a considerably younger 206Pb/238Uage, indicating that a significant amount of postcrystallizationPb loss has affected the zircons and that this was not com-pletely eliminated by abrasion. It is therefore likely that thescatter in 206Pb/238U ages of the three concordant analyses isdue to Pb loss and the best estimate for the crystallization ageof the sample is given by the oldest 206Pb/238U age of the threeconcordant analyses (for fraction D, at 148.3 ± 0.4 Ma). Frac-tion E is slightly discordant with an older 207Pb/206Pb age, pre-sumably reflecting the presence of a very minor inherited zir-con component.

Sample 9 is from a quartz-phyric felsic dike within the foot-wall package at San Nicolas. It is strongly altered, containsabundant sulfide disseminations, and is interpreted to pre-date ore deposition. Three abraded zircon fractions were an-alyzed (Table 1; Fig. 8c) and all yield concordant analyses. Acrystallization age of 148.9 ± 1.4 Ma is assigned to this unitbased on the total range of 206Pb/238U ages for the three con-cordant analyses.

Sample 10 is a quartz- and feldspar-phyric massive felsicflow from the footwall of the El Salvador deposit (Fig. 9), ap-proximately 120 m stratigraphically below the ore horizon.Four abraded zircon fractions were analyzed (Table 1; Fig.8d). Two analyses (B and D) give overlapping concordant

analyses with a total range of 206Pb/238U ages of 150.6 ± 0.7Ma, which gives the crystallization age of the sample. Frac-tion E has experienced minor postcrystallization Pb loss, andfraction C is interpreted to have contained a minor inheritedzircon component.

Sample 11 was collected from a massive aphanitic rhyoliteflow unit in the La Virgen area approximately 5 km northeastof the San Nicolas deposit (Fig. 10). Four abraded zirconfractions all yield concordant analyses with a small amount ofscatter along concordia. Assuming this scatter reflects that re-sult of postcrystallization Pb loss, the best estimate for thecrystallization age of the sample is given by the range of206Pb/238U ages for fractions A and D at 148.7 ± 0.6 Ma.

The four U-Pb ages from the San Nicolas and El Salvadorarea indicate that felsic volcanism associated with VMS de-posit formation occurred over a narrow time interval between147.9 and 151.3 Ma.

Deposits in the Zihuatanejo subterrane

The Cuale district (Fig. 2) of the Coastal belt includes 19individual VMS deposits and occurrences, all of which are rel-atively small (Hall and Gomez, 2000c; Bissig et al. 2008). Thetotal recorded historical production from the district is esti-mated to be ~2 Mt (Giles and Garcia, 2000), with theNaricero deposit being the largest at 0.8 Mt (Hall andGomez, 2000c). Styles of mineralization present includebanded massive and fragmental sulfides, with underlyingstringer and stockwork zones.

The oldest strata in the area consist of undated pelitic schisttogether with chloritic and sericitic schist units and meta-arkose that occurs in the westernmost part of the district. Thisunit is presumed to represent the stratigraphic basement tothe main volcanic-dominated section, which is termed theCuale volcanic sequence by Bissig et al. (2008). The lowerpart of Cuale volcanic sequence, forming the footwall to themain mineralized part of the section, is a >400-m-thick pack-age of quartz- and feldspar-phyric rhyolite flows, flow-domecomplexes, and associated volcaniclastic units. This package isoverlain by a diverse assemblage of aphyric or sparsely phyricrhyolitic flows and tuffs, interlayered with coarse volcaniclas-tic and epiclastic units with discontinuous lenses of carbona-ceous argillite. The presence of local welded tuff units as wellas the accretionary lapilli-bearing horizons within this se-quence led Bissig et al. (2008) to infer that at least some ofthe local eruptive activity during this time period occurred ina subaerial environment. The VMS mineralization is mainlyhosted within this upper assemblage, with both the footwallsequence and the main ore-bearing assemblage cut by largenumbers of quartz- and feldspar-phyric rhyolite dikes, sills,and small plugs, as well as less abundant and probably signif-icantly younger andesite dikes. Rocks of the Cuale volcanicsequence have calc-alkaline to tholeiitic-arc geochemical sig-natures (Bissig et al., 2008).

Three U-Pb zircon ages are reported by Bissig et al. (2008)from the Cuale volcanic sequence. A hyaloclastic rhyolite unitin the footwall package gives an age of 157.2 ± 0.5 Ma, anaphyric rhyolite flow from within the upper, ore-hosting se-quence was dated at 154.0 ± 0.9 Ma, and a rhyolite dike thatcrosscuts volcanic units in the immediate footwall of one ofthe deposits yielded an age of 155.9 ± 1.6 Ma. Collectively the

AGE AND PALEOTECTONIC SETTING OF VMS DEPOSITS IN THE GUERRERO TERRANE, MEXICO 131

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age data indicate that the volcanic sequence in the Cuale dis-trict and its contained VMS deposits are somewhat older thanthose that have been dated so far elsewhere in the Guerreroterrane.

Another cluster of nine small VMS deposits comprises theBramador district (Fig. 2), located approximately 20 km southof Cuale, in the southern part of the same erosional windowthat hosts the Cuale deposits. Stratigraphy in the Bramadordistrict is generally similar to that at Cuale (Miranda-Gasca,1995), comprising andesitic tuffs at the base, overlain by a se-quence of rhyolitic tuffs and domes, which is in turn overlainby carbonaceous shales. Volcanogenic massive sulfide bodiesoccur at the contact between the felsic volcanic sequence andthe overlying carbonaceous shales. There are at present no di-rect age constraints for rock units in the Bramador district butthey have been inferred to be similar in age to those in theCuale district (e.g., Hall and Gomez-Torres, 2000c).

Other deposits

Two other deposits that have been interpreted as possiblyVMS in character are the Copper King deposit, which ishosted within the Las Ollas Complex and the Arroyo Seco de-posit, which has been described as being hosted within theArteaga Complex (Fig. 2; Miranda-Gasca, 1995, 2001, 2003).The Copper King deposit comprises two small copper- andzinc-rich lenses and associated stockworks hosted by de-formed and metamorphosed andesitic to basaltic tuffs andflows with interlayered black slates (Miranda-Gasca, 1995).No constraints for the depositional age of the host sequenceare available, although it is assumed to have formed in theearly or middle Mesozoic. The Arroyo Seco deposit is some-what different from all of the other VMS deposits within theGuerrero terrane, being lead rich with only minor amounts ofzinc, copper, and barite, and hosted entirely within sedimen-tary rocks. Mineralization includes both banded and stratifiedsulfides and concentrations of veins and stringers, all of whichare hosted within a package of variably carbonaceous shalesand sandstones. Individual ore horizons are typically thin (upto 10 m thick) but have considerable strike length (up to 2.5km). Although Miranda-Gasca (1995) described the ArroyoSeco deposit as being hosted within the Arteaga Complex, themineralization is actually contained within the TepalcatepecFormation, which lies well above the unconformable uppercontact of the Arteaga Complex and is, in fact, part of the Zi-huantanejo subterrane. The Tepalcatepec Formation con-tains Albian to Cenomanian fossils (Centeno-García et al.,2003a), indicating that the age of formation of the ArroyoSeco deposit, if it is indeed syngenetic, is 40 to 50 m.y.younger than any of the VMS deposits within the Guerreroterrane.

Sulfide Lead Isotope StudiesThe Pb isotope composition of sulfides in mineral deposits

can provide valuable information concerning the sources ofcontained metals and in some cases the age(s) of mineralizingevents. Lead in VMS deposits may be derived from severaldistinct sources, including volcanic and volcaniclastic rocksand hypabyssal intrusions in the immediate footwall of the de-posit as well as possible magmatic contributions from deepersubvolcanic intrusions and crystalline basement rocks (e.g.,

Franklin et al., 2005; Mortensen et al., 2006). The high-tem-perature reaction zone beneath a VMS deposit or cluster ofdeposits may extend to depths in excess of 1 km (e.g.,Franklin et al., 2005), and therefore any rock units presentbeneath the site of sulfide deposition represent potentialsource rocks for metals. In the case of VMS deposits hostedin relatively thin volcanic successions, the high-temperaturereaction zone and associated metal leaching may extend welldown into the underlying basement strata as well. Thus, evenin cases where the volcanic rock units associated with VMSdeposits are clearly mantle derived, the Pb isotope signatureof sulfides in the deposits may reflect variable mixtures be-tween a typical mantle Pb of the appropriate age (from thehost volcanic rocks) and potentially much more radiogeniccomponents from either underlying basement rocks and/orinterlayered continent-derived clastic rocks. This is particu-larly the case for VMS deposits of the bimodal-siliciclasticclass (e.g., Bathurst: Hussein, 1996; Iberian Pyrite Belt: Mar-coux, 1998; Yukon-Tanana terrane: Mortensen et al., 2006),including many of the VMS deposits in the Central belt of theGuerrero terrane.

We have determined Pb isotope compositions from a totalof 31 sulfide samples from VMS deposits and occurrencesthroughout the Guerrero terrane, including the Francisco IMadero deposit. These data are compiled in Table 2 togetherwith data from Miranda-Gasca (1995), who reported analysesfrom a variety of VMS and other occurrences in the Guerreroterrane. Also included in Table 2 are Pb isotope analyses ofsulfides for VMS occurrences in the Cuale and CampoMorado districts (from Cumming et al., 1979) and the Tizapadeposit (from JICA-MMAJ, 1991, reported in Elías-Herraraet al., 2000). Most of the data are from galena; however, someof the new analyses are from other sulfide minerals such aspyrite or sphalerite. We did not measure U, Th, or Pb con-centrations for these samples and are therefore unable to cor-rect the individual analyses for radiogenic Pb producedthrough U and Th decay since the formation of the sulfides;however, the U and Th contents are likely to very low andgiven the young age of the deposits being studied, the addi-tion of radiogenic Pb is likely to have been negligible. Dataare plotted in conventional uranogenic and thorogenic Pb iso-tope diagrams in Figure 11 with reference to the average“upper crustal” growth curve of Doe and Zartman (1979). An-alytical methods employed in the study are described in theAppendix.

Several general observations can be made from the Pb iso-tope data arrays. With the exception of several analyses withquite anomalous compositions that are discussed below, mostof the analyses from VMS deposits in the Guerrero terranefall within a relatively restricted field (Fig. 11). Most of thedata are relatively radiogenic, falling on or near the averageupper crustal growth curve of Doe and Zartman (1979) in207Pb/204Pb versus 206Pb/204Pb space, and above the averageupper growth curve in 208Pb/204Pb versus 206Pb/204Pb space(Fig. 11). This indicates either that a substantial componentof the Pb in each deposit was derived from continent-derivedsediments that are interlayered with the host volcanic rocks orfrom underlying radiogenic basement, or that the igneousrocks themselves include a significant component of assimi-lated crustally derived material (as is also suggested by their

132 MORTENSEN ET AL.

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AGE AND PALEOTECTONIC SETTING OF VMS DEPOSITS IN THE GUERRERO TERRANE, MEXICO 133

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TAB

LE

2. S

ulfid

e Pb

Iso

tope

Com

posi

tions

from

the

Gue

rrer

o Te

rran

e

Dep

osit

Stat

eV

MS

dist

rict

Subt

erra

neM

iner

eal

206 P

b/20

4 Pb

1 s.

d. (

%)1

207 P

b/20

4 Pb

1 s.

d. (

%)1

207 P

b/20

4 Pb

1 s.

d. (

%)1

Ref

eren

ce2

El G

ordo

Gua

naju

ato

Leo

n-G

uana

juat

oG

uana

juat

oPy

rite

18.6

706

0.09

15.6

695

0.10

38.6

439

0.11

1R

efug

ioG

uana

juat

oL

eon-

Gua

naju

ato

Gua

naju

ato

Gal

ena

18.6

758

0.04

15.6

501

0.05

38.6

379

0.07

1C

ata

de A

gua

Gua

naju

ato

Leo

n-G

uana

juat

oG

uana

juat

oG

alen

a18

.643

00.

0415

.661

60.

0738

.625

70.

091

El G

ordo

Gua

naju

ato

Leo

n-G

uana

juat

oG

uana

juat

oG

alen

a18

.635

20.

0615

.655

30.

0838

.571

50.

101

BH

-1, S

occo

redo

raJa

lisco

Cua

leZi

huat

anej

oG

alen

a18

.684

70.

1215

.652

10.

1638

.557

30.

211

LP-

1, L

a Pr

ieta

Jalis

coC

uale

Zihu

atan

ejo

Gal

ena

18.6

846

0.04

15.6

672

0.06

38.6

738

0.09

1D

ios

Me

Ayu

daJa

lisco

Zihu

atan

ejo

Pyri

te18

.586

60.

0415

.586

30.

0738

.326

80.

091

Dio

s M

e A

yuda

Jalis

coZi

huat

anej

oPy

rite

18.5

903

0.04

15.5

901

0.07

38.3

365

0.09

1L

a Tr

osad

aJa

lisco

Bra

mad

orZi

huat

anej

oM

ixed

sul

fides

18.7

299

0.14

15.6

858

0.14

38.5

537

0.17

1L

os A

lpes

Jalis

coB

ram

ador

Zihu

atan

ejo

Gal

ena

18.6

784

0.05

15.6

589

0.07

38.6

386

0.09

1E

l Pol

icim

oJa

lisco

Bra

mad

orZi

huat

anej

oTe

trah

edri

te18

.608

70.

2015

.601

50.

2038

.476

80.

221

Las

Del

icas

Jalis

coB

ram

ador

Zihu

atan

ejo

Pyri

te18

.702

30.

0515

.707

40.

0738

.713

80.

091

Las

Del

icas

Jalis

coB

ram

ador

Zihu

atan

ejo

Spha

leri

te18

.758

80.

0515

.754

50.

0738

.949

40.

091

San

Jose

Jalis

coB

ram

ador

Zihu

atan

ejo

Pyri

te18

.694

70.

0415

.681

20.

0738

.721

60.

091

Ros

ario

Jalis

coB

ram

ador

Zihu

atan

ejo

Gal

ena

18.6

500

0.06

15.6

316

0.07

38.5

842

0.09

1Sa

n N

icol

asZa

cate

cas

Zaca

teca

sZa

cate

cas

Mix

ed s

ulfid

es18

.603

00.

013

15.6

340

0.01

238

.483

00.

014

2Sa

n N

icol

asZa

cate

cas

Zaca

teca

sZa

cate

cas

Mix

ed s

ulfid

es18

.602

00.

013

15.6

270

0.01

238

.481

00.

014

2Sa

n N

icol

asZa

cate

cas

Zaca

teca

sZa

cate

cas

Mix

ed s

ulfid

es18

.593

00.

013

15.6

370

0.01

238

.485

00.

014

2Sa

n N

icol

asZa

cate

cas

Zaca

teca

sZa

cate

cas

Pyri

te18

.612

40.

013

15.6

458

0.01

238

.483

70.

014

2Sa

n N

icol

asZa

cate

cas

Zaca

teca

sZa

cate

cas

Pyri

te18

.651

50.

006

15.6

341

0.00

538

.467

30.

008

2Sa

n N

icol

asZa

cate

cas

Zaca

teca

sZa

cate

cas

Pyri

te18

.605

70.

017

15.6

132

0.00

938

.422

10.

019

2Sa

n N

icol

asZa

cate

cas

Zaca

teca

sZa

cate

cas

Pyri

te18

.581

30.

012

15.6

117

0.00

938

.395

50.

015

2Sa

n N

icol

asZa

cate

cas

Zaca

teca

sZa

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somewhat evolved Nd isotope compositions: e.g., Freydier etal., 1997; Danielson, 2000, see discussion above), or both.Data from individual deposits or districts form loose clusterswith a few outliers. This clustering suggests that the mineral-izing process has led to a degree of homogenization of Pbfrom the various individual metal sources that contributed toeach deposit (see discussion above) and the scatter withineach cluster likely reflects variability in the relative propor-tions of Pb that were contributed from each source. The Pbisotope compositions from the Bramador district yield thegreatest range of compositions of any of the districts (Fig. 11).Analyses from the El Bramador and El Policimo depositsyield the least radiogenic compositions and plot near theanalyses from the Dios Me Ayuda deposit, which is located inthe same part of the Zihuantanejo subterrane. The Pb inthese deposits was presumably derived mainly from igneousrocks that were not significantly contaminated by crustalmaterial.

There are several anomalous samples in the data set. Twoanalyses reported by Miranda-Gasca (1995) for sulfide sam-ples from the Bramador and Tlanilpa-Azulaquez districts aremuch more radiogenic than other samples from those orother VMS districts (Fig. 11) and may reflect analytical errorin these analyses (although the anomalous compositions donot appear to result from simple 204Pb measurement error).Five samples of galena from the Francisco I Madero depositin western Zacatecas State all yield compositions that are sig-nificantly more radiogenic than data from the various VMSdeposits but fall within the broad field of compositions asso-ciated with Tertiary epigenetic deposits from throughout theGuerrero terrane (Fig. 11). This deposit is hosted by LateJurassic to Early Cretaceous sedimentary strata of the Za-catecas subterrane and, although some of the mineralizationis finely disseminated and banded and has the appearance ofpossible syngenetic origin (e.g., Miranda-Gasca, 1995), thebulk of the ore is coarser grained and appears to representskarn or manto-style mineralization. The Pb isotope data sup-ports this interpretation and indicates that the Francisco IMadero is an epigenetic deposit of probable Tertiary age.

Two Pb isotope analyses reported by Miranda-Gasca (1995)from the La Minita deposit in the Zihuatanejo subterrane inwestern Michoacón State (Fig. 2) yield relatively nonradi-ogenic compositions. This deposit is strata bound and hasbeen interpreted as a VMS deposit; however, it is unusuallybarite rich (48%), in addition to containing 4 percent zinc,0.33 percent lead, and 78 g/t silver. Furthermore, accordingto Miranda-Gasca (1995), sulfide textures in the deposit indi-cate recrystallization, replacement, and cavity filling with oneof the host rocks being a reefal limestone. Lenticular bodiesof magnetite occur deeper in the stratigraphic package, rais-ing the possibility that some of the mineralization may repre-sent a skarn. The fact that the Pb isotope composition of theore falls well outside of the main field defined for Guerreroterrane VMS deposits suggests that it may represent a differ-ent age or style of mineralization.

Miranda-Gasca (1995) reported two analyses of galenafrom the Copper King deposit in the Las Ollas Complex. Oneanalysis falls within the same field as the rest of the GuerreroVMS deposits; however, the second sample yields a very non-radiogenic composition (Fig. 11). No information is provided

134 MORTENSEN ET AL.

0361-0128/98/000/000-00 $6.00 134

Tiza

paM

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oyo

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rror

s gi

ven

at th

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ors

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ted

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ious

ly p

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hed

data

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ces

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1 =

this

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anie

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00);

3 =

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1995

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umm

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Con

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Dep

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concerning errors associated with these analyses and it istherefore uncertain whether the large discrepancy betweenthe two analyses reflects analytical error or two completelyunrelated styles of mineralization.

One sample from the Arroyo Seco deposit yields a very ra-diogenic composition, particularly in terms of the reported207Pb/204Pb ratio (Fig. 11; Miranda-Gasca, 1995). The datasupports the interpretation that this deposit is distinct in ageand/or style from the other Guerrero VMS deposits.

Discussion and Conclusions

The paleotectonic settings in which the various compo-nents of the Guerrero terrane formed and the relationships,if any, between the subterranes have been issues of consider-able debate over the past three decades (e.g., Freydier et al.,2000; Talavera-Mendoza and Suastegui, 2000). Significantvariations in geochemical and isotopic characteristics withinand between the subterranes that make up the Guerrero

AGE AND PALEOTECTONIC SETTING OF VMS DEPOSITS IN THE GUERRERO TERRANE, MEXICO 135

0361-0128/98/000/000-00 $6.00 135

38.0

38.2

38.4

38.6

38.8

39.0

39.2

39.4

15.5

15.6

15.7

15.8

15.9

16.0

18.2 18.4 18.6 18.8 19.0 19.2

208

204

Pb/

Pb

207

204

Pb/

Pb

206 204Pb/ Pb

0

0

70

70

140

140

210

210

250

250

290

290

360

360510

410

410

Francisco I Madero

Francisco I Madero

Arroyo Seco

Arroyo Seco

Copper King

Copper King

La Minita

La Minita

San Nicolas - ElSalvador

CualeBramadorTizapaCampo MoradoTlanilpa-AzulaquezLeon-GuanajuatoDios me AyudaLa MinitaRey de PlataCopper KingArroyo SecoFrancisco I Madero

Th

isst

ud

yM

-G,1

995

FIG. 11. Sulfide lead isotope compositions from volcanogenic massive sulfide and other deposits in the Guerrero terrane.Data from this study and from Miranda-Gasca (1995). The average upper crustal growth curves of Doe and Zartman (1979)are shown for reference.

terrane have been well documented by previous workers(e.g., Centeno-García et al., 1993; Talavera-Mendoza andSuastegui, 2000). In general, the Late Jurassic to Early Cre-taceous components of all of the subterranes are consistentwith having formed in either a volcanic arc or back-arc set-ting. Exceptions are the Las Ollas Complex, which is inter-preted to represent a Jurassic to Cretaceous subduction com-plex (Talavera-Mendoza, 2000), and the Arperos basin, whichis considered to be either a back-arc basin or possibly a frag-ment of oceanic crust (e.g., Freydier et al., 1996, 2000; Elías-Herrara and Ortega-Gutiérrez, 1998).

Centano-García (2003) and Centeno-García et al. (2003a)pointed out that most proposed correlations between the var-ious components of the Guerrero terrane are based on theLate Jurassic and Cretaceous portions of the individual sub-terranes. The basement on which these younger assemblageswas built is not well understood; however, where exposed inthe Zihuatanejo and Zacatecas subterranes (the ArteagaComplex and Zacatecas Formation, respectively), it consistsof Triassic siliciclastic rocks with abundant late Paleozoic de-trital zircons and evolved Nd isotope signatures, interlayeredwith mafic volcanic rocks with MORB compositions (e.g.,Centeno-García, 1994; Centeno-García et al., 2003b). Dick-inson and Lawton (2001) interpreted this basement to be aTriassic subduction complex and suggested that the associ-ated mafic volcanic rocks represent slices of oceanic crust thathave been tectonically interleaved with the sedimentarystrata. This interpretation is at odds with observations in boththe Arteaga Complex (Centeno-García et al., 2003a) and theZacatecas Formation in the Zacatecas subterrane (Centeno-García and Silva-Romo, 1997), which suggests that the maficvolcanic rocks are in depositional contact with the interlay-ered sedimentary units. The Arteaga Complex was deformedin the Early Jurassic and intruded by late Early and MiddleJurassic plutons (Centeno-García et al., 2003a). Centeno-García (2003) and Centeno-García et al. (2003a) suggestedthat the basement of the Late Jurassic-Early Cretaceous vol-cano-sedimentary assemblages in the Guerrero terrane mightcomprise different basement blocks that were amalgamatedand accreted to the western margin of nuclear Mexico priorto the deposition of the Late Jurassic-Early Cretaceous vol-cano-sedimentary units. In that case the Guerrero terranecould be considered a composite terrane, at least during itsearly history. The close similarity between age populations fordetrital zircons from Triassic siliciclastic units in the ArteagaComplex, Zacatecas Formation, and Sierra Madre terrane(Centeno-García et al., 2005), however, argues for an earlyMesozoic stratigraphic linkage between the basement for theLate Jurassic-Early Cretaceous volcano-sedimentary unitsand nuclear Mexico. This suggests that the basement of theLate Jurassic-Early Cretaceous volcano-sedimentary unitswas not composite but rather was oceanic crust that lay im-mediately adjacent to the western margin of nuclear Mexico.This implies that the Guerrero terrane mainly comprises awest-facing arc that developed along the western margin ofMexico.

Other workers (e.g., Talavera-Mendoza and Suastegui,2000) have suggested that the subterranes in the southernpart of the Guerrero terrane may represent an amalgamationof unrelated fragments of intra-oceanic arc assemblages. Still

other groups (e.g., Tardy et al., 1994; Freydier et al., 1996,1998, 2000) argued that the Late Jurassic to Early Cretaceouscomponents of the Guerrero terrane are related to one an-other but formed part of an east-facing arc that was separatedfrom nuclear Mexico by an oceanic plate now represented bystrata of the Arperos Formation. This latter view was furtherdeveloped by Dickinson and Lawton (2001), who suggestedthat the Arperos basin is a remnant of the “Mezcalara plate,”which was an oceanic plate that the authors interpreted tohave existed between nuclear Mexico and an east-facingGuerrero arc. These authors argued that a Mezcalara ocean isrequired to explain the presence of Late Triassic to MiddleJurassic magmatic arc (the “Nazas arc”) that lay east of theGuerrero terrane. However, the Arperos Formation cannotrepresent a portion of the proposed Mezcalara plate, becausethe Arperos Formation is Late Jurassic to Early Cretaceous inage on the basis of available fossils (Ortiz-Hernández et al.,2003) and is therefore coeval with formation of the spatiallyassociated Guanajuato arc.

Much of the debate concerning the nature of the Guerreroterrane stems from a paucity of reliable crystallization ages forthe various igneous assemblages. The 14 new U-Pb zirconages reported in this contribution and in Bissig et al. (2008)show that depositional ages for most volcanic and associatedintrusive rock units in the different subterranes fall within arelatively narrow interval between 147.8 ± 3.5 Ma (total agerange for samples from the San Nicolas and/or El Salvadorarea of the Zacatecas subterrane) and 139.7 ± 2.5 Ma (totalage range for samples from the Tlanilpa-Azulaquez district inthe Teloloapan subterrane). Crystallization ages from theCuale district are slightly older (total age range of 157.4 ± 4.1Ma). Arc and back-arc magmatism in the Guerrero terranewas therefore entirely latest Middle Jurassic to early EarlyCretaceous (Callovian to Valanginian) in age. Geochemicalstudies of different parts of the Guerrero terrane indicate thatat this time magmatism mainly occurred in a volcanic-arc set-ting in the Zihuatenajo, Teloloapan, and Guanajuato subter-ranes and a mixed arc and/or back-arc setting in the Arceliaand Zacatecas subterranes (see discussion above).

The Arperos Formation in the Guanajuato subterrane andfarther south represents a deep water basin (Arperos basin)that was accumulating hemipelagic sediments at the sametime that the Guanajuato arc was being constructed to thewest. The lowest stratigraphic sequence in the Arperos For-mation consists of tuffaceous shale that appears to have beenderived from oceanic igneous sources; however, sedimentaryunits become increasingly continental in composition upsec-tion (Freydier et al., 2000). Sediments that accumulated in arelatively deep water basinal setting are also present withinthe Huetamo, Arcelia, and Teloloapan subterranes (e.g., Ta-lavera-Mendoza and Suastegui, 2000).

Direct evidence for original facing direction is lacking in allof the arc and/or back-arc assemblages in the various subter-ranes. However, available geochemical and radiogenic isotopedata, together with isotopic and fossil ages and sedimentolog-ical constraints, are entirely consistent with formation of theGuerrero terrane as a continuous west-facing volcanic arc andback-arc assemblage that developed along the western mar-gin of nuclear Mexico. Arc basement mainly comprised maficvolcanic rocks interlayered with siliciclastic rocks that were

136 MORTENSEN ET AL.

0361-0128/98/000/000-00 $6.00 136

shed from the continental margin to the east. This assemblageis Late Triassic, and possibly older, and represents the upperpart of oceanic (or highly attenuated continental?) crust thatbordered nuclear Mexico to the west in the early Mesozoic.Subduction-related igneous rocks of Early and Middle Juras-sic age within both the Guerrero terrane (including plutonswithin the Arteaga Complex and the Tejupilco metamorphicsuite that forms the structurally deepest levels of the Teloloa-pan subterrane) and farther to the east (the “Nazas arc” ofDickinson and Lawton, 2001) likely resulted from an earlyphase of east-dipping subduction under this margin thatlasted until the Middle Jurassic. Slab rollback in the LateJurassic caused arc magmatism to become localized withinthe Guerrero terrane and also initiated extension in the back-arc region. We envision this back arc to have been analogousto the modern Okinawa trough, a continental back-arc basinthat is currently forming behind the coeval Ryukyu arc (e.g.,Sibuet et al., 1998; Shinjo and Kato, 2000), or the “RocasVerdes basin” which formed in the Jurassic to Cretaceous asa result of back-arc extension behind the Andean arc in thePatagonia region of South America (e.g., Dalziel, 1981; Sternand De Wit, 2003). The Okinawa trough is a relatively youngfeature and has accommodated small amounts of extension(probably <100 km; Sibuet et al., 1998). Although some ex-tensional grabens within the Okinawa trough are up to~2,200 m deep, sedimentation is dominated by thick se-quences of mainly terrigenous sediments, and dredging anddrilling of the floor of the trough suggests that it is nowhereunderlain by typical mafic back-arc crust (Shinjo and Kato,2000). The Rocas Verdes basin may be a better analogy forthe Guerrero back arc. The Rocas Verdes basin represents azone of long-lived continental back-arc extension that lasted~40 to 50 m.y. and locally evolved to include areas of newlyformed oceanic crust (now represented by ophiolitic frag-ments such as the Sarmiento and Tortuga ophiolites; Sternand De Wit, 2003). Much of the Rocas Verdes basin was sed-iment starved and anoxic and accumulated an average of1,000 to 1,200 m of pyritic black shale that was in part de-rived from erosion of uplifted portions of the newly formedmafic basin floor (Fildani and Hessler, 2005). Of key impor-tance is that the magnitude of crustal extension in both theOkinawa trough and the Rocas Verdes basin was quite vari-able along strike (Sibuet et al., 1998; Fildani and Hessler,2005), leading to patchy areas underlain by anomalouslythick accumulations of volcanic rocks and/or the develop-ment of new mafic crust in some parts of the basin, whereasother parts are underlain by thinned continental crust. TheArperos basin with its MORB- and ocean island-like maficrocks associated with deep-water pelagic sediments may beanalogous to some of these more highly extended portions ofthe Rocas Verdes back arc. Although there is compelling ev-idence that the basement to the Guerrero arc and back-arcassemblages formed along a continental margin, the actualextent of continental crust in the basement of the varioussubterranes that comprise the Guerrero terrane is still un-certain (e.g., Freydier et al., 1997; Elías-Herrara et al.,2000). A possible alternative model for the Jurassic to Creta-ceous magmatism and VMS deposit formation in the Guer-rero terrane may be the east Manus basin (e.g., Hanningtonet al., 2005), which is located behind the south-facing New

Britain arc and has developed by rifting of arc crust with lit-tle or no continental basement present.

If correct, the analogies that we have proposed between theGuerrero terrane and other continental margin arc and/orback-arc settings have important implications for VMS min-eralization in the terrane. Volcanogenic massive sulfide de-posits in the Central belt of the Guerrero terrane (Teloloa-pan, Guanajuato, and Zacatecas subterranes) fall broadlywithin the bimodal-siliclastic category, whereas those in theCoastal belt (Zihuatanejo subterrane) are more typical of thebimodal-felsic type. Bimodal-felsic–type VMS deposits aretypical of rifted volcanic-arc settings, whereas bimodal-sili-clastic–type deposits are commonly found in continentalback-arc or continental rift settings (e.g., Barrie and Han-nington, 1999; Franklin et al., 2005). However, on the basisof whole-rock geochemical compositions most of the Cen-tral belt deposits and districts appear to have formed withinvolcanic-arc rather than back-arc environments. The signif-icance of this observation is not clear. It may indicate thatthe volcanic sequences that host the Central belt VMS de-posits developed near the outer edge of the area of back-arcextension (and hence display a dominantly arc geochemicalsignature). It is notable that by far the largest single VMSdeposit in the Guerrero terrane (San Nicolas) is the only onethat clearly formed in a back-arc setting. Even in this case,however, although the immediate host rocks for the depositreflect a back-arc setting, volcanic units that directly overlieit have a volcanic-arc signature (Danielson, 2000). It is pos-sible that the apparent arc signature of the uppermost vol-canic rocks may be due to crustal contamination of back-arcmagmas. However, if the arc signatures are primary this im-plies either that the San Nicolas area lay on the immediateinner flank of the coeval arc (to explain interfingering ofback-arc and arc volcanic units) or that the overlying arc vol-canic rocks reflect a shift of the locus of arc magmatism tothe east following an episode of back-arc extension. Accord-ing to our interpretation of the paleotectonic setting of theGuerrero terrane the Arperos basin represents the mainpreserved remnant of a back-arc basin and any felsic vol-canic centers within the sequence may have potential tohost other substantial VMS deposits of the bimodal-siliclas-tic type, analogous to San Nicolas.

The nature and origin of the Copper King deposit in theLas Ollas Complex is still uncertain. It has characteristics ofa Besshi-type deposit and differs from other VMS deposits inthe Guerrero terrane in terms of its association with maficvolcanic rocks with a juvenile arc tholeiite composition andits Cu- and Zn-rich composition (Miranda-Gasca, 1995). Theage of the deposit is not well constrained, since no fossil agesor reliable isotopic ages are available and the two Pb isotopevalues that have been reported for the deposit are very dif-ferent and do not permit any credible age assignment (seeabove discussion). The deposit occurs within a structuralmélange that has been interpreted to represent a subductionmélange (Talavera-Mendoza, 2000; Centeno-García et al.,2003a). If the Las Ollas rock units are coeval with the Guer-rero arc magmatism as has been speculated by some work-ers (e.g., Talavera-Mendoza, 2000; Centeno-García et al.,2003a), the Copper King deposit and its host rocks may haveformed in a fore-arc setting. Subsequent incorporation into a

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subduction complex could have resulted from subductionerosion along the base of the west-facing Guerrero arc.

AcknowledgmentsWe thank the staff of the Pacific Centre for Isotopic and

Geochemical Research at the University of British Columbia,for assistance in producing much of the analytical data re-ported in this contribution. The paper was much improved bycritical reviews provided by J.M. Franklin, J.D. Keppie, andS.J. Piercey.

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U-Pb Geochronology

Zircon was separated from 5- to 20-kg samples using con-ventional crushing, grinding, Wilfley table, heavy liquids, andFrantz magnetic separator techniques. U-Pb analyses weredone using a modified VG-54R mass spectrometer at the Pa-cific Centre for Isotopic and Geochemical Research (PCIGR)at the University of British Columbia. The methodology forzircon grain selection, abrasion, dissolution, geochemicalpreparation, and mass spectrometry are described byMortensen et al. (1995). Zircon yield was relatively low formany of the samples, and in some cases this necessitated asmaller number of analyses of poorer quality zircon grainsthan optimum. This is typically reflected by a limited numberof analyses and lower sample/blank ratios for individual analy-ses than would have been desired. Most zircon fractions wereair abraded (Krogh, 1982) prior to dissolution to minimize theeffects of postcrystallization Pb loss. Procedural blanks for Pband U were 2 and 1 pg, respectively. U-Pb data are plotted ona conventional U-Pb concordia plot in Figures 3 and 4. Errorsattached to individual analyses were calculated using the nu-merical error propagation method of Roddick (1987). Decayconstants used are those recommended by Steiger and Jäger(1977). Compositions for initial common Pb were taken from

the model of Stacey and Kramer (1975). All errors are givenat the 2σ level.

Sulfide Pb isotope measurements

Sample preparation, geochemical separations, and isotopicmeasurements were done at the PCIGR facility at the Uni-versity of British Columbia. For trace lead sulfide samples,approximately 10 to 50 mg of handpicked sulfides were firstleached in dilute hydrochloric acid to remove surface conta-mination and then dissolved in dilute nitric acid. Samples ofgalena required no leaching and were directly dissolved in di-lute hydrochloric acid. Following ion exchange chemistry, ap-proximately 100 to 250 ng of Pb in chloride form was loadedon rhenium filaments using a phosphoric acid-silica gel emit-ter. Isotopic ratios were determined with a modified VG54Rthermal ionization mass spectrometer in peak-switchingmode on a Faraday detector. Measured ratios were correctedfor instrumental mass fractionation of 0.12%/amu-based re-peated measurements of the NBS 981 standard and the val-ues recommended by Thirwall (2000). Errors were numeri-cally propagated throughout all calculations and are reportedat the 2σ level (Table 2).

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APPENDIX

Analytical Methods