The Rhyolite-Hosted Volcanogenic Massive Sulfide District ... · The Rhyolite-Hosted Volcanogenic...

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The Rhyolite-Hosted Volcanogenic Massive Sulfide District of Cuale, Guerrero Terrane, West-Central Mexico: Silver-Rich, Base Metal Mineralization

Emplaced in a Shallow Marine Continental Margin Setting *

THOMAS BISSIG,†,** JAMES K. MORTENSEN, RICHARD M. TOSDAL,Mineral Deposit Research Unit, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4

AND BRIAN V. HALL

International Croesus Ventures Corporation, 95 Armstrong Street, Ottawa, Ontario, Canada K1A 2V6

AbstractThe Cuale mining district, situated in the Cordillera Madre del Sur, ~30 km southeast of Puerto Vallarta,

Mexico, comprises a number of small, polymetallic Pb, Zn, Cu, volcanogenic massive sulfide deposits (VMS),and vein and stockwork-hosted deposits with low-sulfidation epithermal characteristics. Massive sulfide min-eralization contains high Ag ± Au grades when compared to similar deposits in western Mexico. The massivesulfide bodies are hosted by an interbedded rhyolitic volcanic and volcaniclastic succession, herein termed theCuale volcanic sequence. Regionally, the Cuale volcanic sequence is part of the Zihuatanejo subterranebelonging to the Guerrero terrane, which is interpreted to be a complex Mesozoic accreted arc terrane host-ing the majority of VMS deposits in Mexico.

The rocks of the Cuale volcanic sequence are not metamorphosed and are only weakly deformed but exhibitintense chlorite, sericite, and quartz hydrothermal alteration. Three stratigraphic units are identified within theCuale volcanic sequence: (1) a more than 400-m-thick footwall unit of quartz and plagioclase phyric rhyoliteflows, related hyaloclastites, and subordinate heterolithic volcaniclastic breccias; (2) an ore horizon character-ized by sedimentary rocks ranging from volcaniclastic conglomerates to black shales, as well as aphyric to quartzand plagioclase phyric rhyolite; and (3) late intrusions composed of quartz and plagioclase phyric rhyolite andsubordinate andesitic dikes intruding all units. The sedimentary rocks of the ore horizon are intercalated withrhyolite tuff horizons, some of which exhibit welding or accretionary lapilli that are interpreted as evidence forshallow submarine or subaerial deposition.

The rhyolites of the Cuale volcanic sequence are calc-alkaline with only limited compositional variations.However, the trace element signatures reveal a weak trend from the footwall rhyolites, which are characterizedby low Zr/Y (1–1.9) and low chondrite normalized La/Yb ratios (9 of 11 samples between 1.24 and 3.3), toslightly higher values in the ore horizon and late intrusive phase (Zr/Y = 1.6–7.6 and chondrite normalizedLa/Yb ratios between 2.25 and 15). This compositional evolution can be explained by slightly decreasingdegrees of partial melting related to a transition from amphibole-free assemblages for the footwall to amphi-bole-bearing assemblages in the residuum for the ore horizon and intrusive rhyolites. This interpretation is sup-ported by the observed volcanic stratigraphy and indicates increasing pressure at the site of melt generation asthe volcanic edifice was built.

Zircons from five samples of the Cuale volcanic sequence have been dated by the U-Pb method, using iso-tope dilution thermal ionization mass spectrometry (ID-TIMS). Although some samples show evidence of aminor inherited Pb component, an age range of 157.2 ± 0.5 to 154.0 ± 0.9 Ma has been established. Cuale rep-resents the oldest dated VMS camp in western Mexico as other VMS districts of western Mexico are between151.3 and 138 Ma.

Two styles of mineralization are recognized: volcanogenic massive sulfide mineralization (VMS) and late low-sulfidation epithermal vein and stockwork mineralization characterized by high Au (1.89 ppm) and Zn (2.35%)but low Cu (0.2%) contents. The VMS mineralization is contained in more than 15 small orebodies, whichexhibit a range of mineralization styles. Orebodies interpreted as proximal are characterized by stockwork veinnetworks with high Au and Cu contents, underlying massive pyrite bodies, which grade laterally and verticallyinto sphalerite- and galena-rich massive sulfide ore. Stockwork mineralization is locally hosted by aphyric rhy-olite. More distal orebodies, rich in Pb, Zn, and Ag, are hosted by black shale and locally exhibit laminated orbrecciated sedimentary textures in the galena-sphalerite-pyrite ore. The sedimentary textures observed insome massive sulfide orebodies suggest that the sulfide has been transported and deposited in anoxic basinsadjacent to the rhyolite domes. Some of the more significant orebodies show evidence of both distal sedimen-tary ore textures and proximal features indicated by intense quartz-sericite-pyrite alteration in the immediatefootwall.

*A digital supplement to this paper is available at <http://www.geoscienceworld.org/> or, for members and subscribers, on the SEG website,<http://www.segweb.org>.

† Corresponding author: e-mail, [email protected]**Alternate address: Universidad Católica del Norte, Departamento de Ciencias Geológicas, Avenida Angamos 610, Antofagasta, Chile.

©2008 Society of Economic Geologists, Inc.Economic Geology, v. 103, pp. 141–159

IntroductionRECENT discoveries of large and/or precious metal-rich (bi-modal-) volcanogenic massive sulfide deposits (VMS) in ac-creted arc terranes of Jurassic to Cretaceous age, such as the83-Mt San Nicolas deposit in Mexico (Fig. 1; Johnson et al.1999, 2000; Danielson, 2000) and the gold-rich Eskay Creekdeposit in British Columbia, Canada (Roth et al., 1999), haveincreased exploration interest in areas of similar geologic set-ting along the North American Cordillera. The Guerrero ter-rane, composing much of west-central Mexico, is of interestas it is interpreted as a Jurassic to Cretaceous accreted arc ter-rane (Campa and Coney, 1983; Centeno-García et al., 1993;Dickinson and Lawton, 2001) and has a long history of min-ing and exploration for VMS deposits. The known depositsrange from large to small in size (Fig. 1; Miranda-Gasca,2000). From a historical perspective, the Cuale district is oneof the more significant VMS districts of the Americas. Miningoperations were initiated in the Prieta orebody by the Span-ish in 1805 to extract silver-rich ore zones (averaging 500 g/tAg: Hall and Gomez-Torres, 2000a).

The Cuale district is situated at latitude 20° 22' N and lon-gitude 105° 7' W, ~30 km southeast of Puerto Vallarta, Jalisco,

in western Mexico (Figs. 1, 2) and ~4 km southwest of thesmall town of Cuale. So far, only 2.5 million metric tons (Mt)of ore averaging 0.83 ppm Au, 103 ppm Ag, 1.03 percent Pb,3.22 percent Zn, and 0.23 percent Cu have been producedfrom more than 18 small but high-grade orebodies. All butthe Chivas de Abajo deposit (Fig. 3) had been discovered andhigh graded prior to 1919 (Berrocal and Querol, 1991; Halland Gomez-Torres, 2000a). The silver-rich nature of the ore(drill intercepts of up to 9630 g/t Ag) was the major drivingforce for mining in the 19th century, whereas Pb, Zn, Cu, Ag,and Au were the main commodities produced in the 1980s byZimapán S.A de C. V., which bulk mined orebodies wherehigh-grade silver ore was extracted previously. Mining in thedistrict ceased in 1989, once the easily accessible reserveswere mined out; however, the area is currently being explored.

Despite the long history of mining and exploration atCuale, only limited scientific research has been carried out.Macomber (1962) produced a detailed geologic map and es-tablished a stratigraphic succession. This work was done be-fore the current genetic models for VMS deposits were ac-cepted (e.g., Hutchinson, 1965). In 1986, the JapanInternational Cooperation Agency and Metal Mining Agencyof Japan, in conjunction with the Consejo de Recursos Min-erales de Mexico, conducted a multidisciplinary study of VMSdeposits of western Jalisco state (JICA-MMAJ, 1986), whichincluded reconnaissance-scale geologic mapping. Additionalgeologic mapping of the immediate Cuale district was carriedout by Zimapán geologists during the exploration activities ofthe late 1970s to early 1980s. Diamond drilling in the districtwas initiated in the late 1950s by Eagle Picher S.A. de C.V.with Zimapán S.A. de C.V. completing approximately 500(generally short), diamond drill holes between 1978 and1989. Unfortunately, much of the drill core is lost or inacces-sible and reliable drill log records are scarce.

142 BISSIG ET AL.

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FIG. 1. Map of the Guerrero terrane of western Mexico, showing the dis-tribution of various exposures and the subterrane divisions of Centeno-Gar-cia et al. (2003), modified from Mortensen et al. (2008). The Arteaga Com-plex (Centeno-Garcia et al., 1993) and the Las Ollas Complex(Talavera-Mendoza, 2000), both part of the Zihuatanejo subterrane, are indi-cated. Black stars show volcanogenic massive sulfide (VMS) deposits andprospects hosted by the Guerrero terrane (see text and Mortensen et al.,2008, for further discussion). The gray shaded area in the inset map indicatesthe inferred extent of the Guerrero terrane Abbreviations: SMO = SierraMadre Occidental, SMS = Sierra Madre del Sur mountain belts.

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RHYOLITE-HOSTED VMS OF THE CUALE, GUERRERO TERRANE, MEXICO 143

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District-scale geologic mapping at Cuale is challenging, asthe volcanic rocks display only limited compositional varia-tions (see below), are altered and deeply weathered, and theterrain is steep and vegetated. Despite these limitations, theCuale district is well suited to the study of the volcanologicsetting at the time of ore formation, since it represents a con-tinental margin VMS district that has not undergone meta-morphism or significant later deformation, and the steep ter-rain permits geologic mapping in three dimensions.

The main aim of this study is to improve the geologic un-derstanding of the district and establish constraints on thevolcanologic setting at the time of mineralization. This paperpresents the results of new district-scale mapping (Figs. 3, 4),including areas only accessible using technical climbingequipment, supplemented by whole-rock geochemistry, andU-Pb geochronology.

Tectonostratigraphic Framework: The Guerrero TerraneThe subdivision of Mexico into tectonostratigraphic ter-

ranes was laid out by Campa and Coney (1983), subsequently

discussed and refined by Centeno-Garcia et al. (1993, 2003),Dickinson and Lawton (2001), Keppie (2004), and Centeno-Garcia (2005). The Guerrero terrane constitutes much ofwest-central Mexico (Fig. 1) and, physiographically, includesparts of the Sierra Madre Occidental and Sierra Madre delSur mountain belts. It is a composite arc terrane, divided intofive subterranes (Fig. 1) on the basis of differences in base-ment geology, stratigraphy, style, and intensity of deformation(Campa and Coney, 1983; Centeno-García et al., 1993, 2003;Centeno-García, 2005). We follow the subterrane definitionsproposed by Centeno-Garcia et al. (2003) and, for simplicity,hereafter use the term Guerrero terrane when referring tothe Guerrero Composite terrane.

The complex arc-back arc system of the Guerrero terranewas accreted to the North American continent late in theEarly Cretaceous (Dickinson and Lawton, 2001) and hoststhe majority of the VMS occurrences in Mexico (Miranda-Gasca, 2000; Mortensen et al., 2008; Fig. 1). Among themost significant are the 31-Mt Campo Morado district(Oliver et al., 2000, 2001), the 4.3-Mt Tizapa deposit (Parga

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StructuresGeology

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Rhyolitic tuff breccia /coarse hyaloclastiteOre zones: projected to cross section plane

Observed normal fault

DDH17A

geology not projected

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FIG. 4. Geologic cross sections based on the lithologic map of the Cuale mining district and limited drill core informa-tion. The approximate extents of the ore zones are projected onto the cross-section plane. Traces of the cross sections are in-dicated in Figure 3.

and Rodriguez, 1991), and the 3-Mt Rey de Plata deposit(Miranda-Gasca et al., 2001). All of these are located in thesouthern Guerrero terrane (Fig. 1) and hosted by metamor-phosed bimodal calc-alkaline volcanic rocks of Upper Jurassicto Lower Cretaceous age. The 83-Mt San Nicolas deposit inthe eastern Guerrero terrane (Johnson et al., 1999, 2000) isthe largest known VMS deposit of Mexico and is hosted by abimodal sequence of tholeiitic volcanic and volcaniclasticrocks. Massive sulfide deposits of the western Guerreo ter-rane are smaller and include the ~6-Mt barite-rich Zn-Ag de-posit of La Minita, near Colima (De la Campa, 1991), and theCuale deposits, as well as the small Bramador and Amalteadistricts 10 to 12 km to the south and southeast of Cuale(Figs. 1, 2).

The Guerrero terrane is composed of mostly submarinevolcanic and volcaniclastic rocks of Jurassic and Cretaceousage, interbedded with shallow marine limestones, and silici-clastic sedimentary rocks. These rocks were deposited uncon-formably on a Permian(?)-Triassic to Early Jurassic basementof deformed deep marine siliciclastic sequences (e.g., Campaand Coney, 1983; Centeno-Garcia et al., 1993, 2003).

Large volumes of calc-alkaline magmas intruded the Guer-rero terrane after accretion to the continent in the Late Cre-taceous to Eocene (Schaaf et al., 1995), whereas Tertiary torecent volcanic rocks emplaced in the continental arcs of theSierra Madre Occidental and Sierra Madre del Sur, as well asthe Trans-Mexican volcanic belt, cover the Mesozoic rocks(Morán-Zenteno et al., 1999). These Cretaceous to recent ig-neous rocks leave only isolated exposures of the Mesozoic andolder rocks, particularly in the northern and eastern parts ofthe Guerrero terrane.

Terrane subdivisions

Five subterranes have been indentified by Centeno-Garciaet al. (2003; Fig. 1).

The Teloloapan subterrane, host to the Campo Morado, Ti-zapa, and Rey de Plata VMS districts, occupies the southeast-ern part of the Guerrero terrane, which consists mostly ofshallow submarine, Lower Cretaceous basaltic to andesiticlavas with subordinate limestone intercalations. The base-ment is unknown. These rocks are strongly deformed andmetamorphosed to lower greenschist facies assemblages. TheTeloloapan subterrane was thrusted eastward over Lower toUpper Cretaceous platform carbonates deposited on the ad-jacent Mixteco terrane.

The Arcelia subterrane borders the Teoloapan subterraneto the northwest and is an intensely deformed sliver of basaltand ultramafic rocks intercalated with black shale and chert,to which an Upper Cretaceous age is assigned (Ramírez et al.,1991). No significant VMS mineralization has been reportedfrom this subterrane.

The Zihuatanejo subterrane is the largest subterrane, occu-pying much of the central Mexican Pacific coastal region. Itconsists of Upper Jurassic to Cretaceous shallow marine lime-stones and marine to subaerial basaltic to rhyolitic volcanicrocks, overlying a Triassic basement, which locally is intrudedby Jurassic granitoids. Some of the best studied parts of theGuerrero terrane lay in this subterrane. It is discussed inmore detail below, as it hosts the Cuale district and, in itseastern extensions, the large San Nicolas deposit.

The Guanajuato subterrane constitutes the eastern part ofthe Guerrero terrane (Centeno-García and Silva-Romo, 1997)and has similarities to the Arcelia terrane, consisting of primi-tive arc basalts and deep marine sedimentary rocks. It is, how-ever, poorly exposed and therefore not well understood. Thepotential of the Guanajuato terrane to host VMS mineraliza-tion has only recently been revealed by the discovery of VMSmineralization at the El Gordo and Los Gavilánes prospects(Hall and Gomez-Torres, 2000b, c; Mortensen et al., 2008).

The San José de Gracia subterrane represents the north-western extension of the Guerrero terrane along the Pacificcoast and occupies much of the state of Sinaloa. It is thoughtto consist of Cretaceous volcanic and volcaniclastic rocks de-posited on a Paleozoic basement. However, its stratigraphyand its contact relationships with the other parts of the Guer-rero and surrounding terranes have not been studied in de-tail. No significant massive sulfide mineralization has been re-ported from the San José de Gracia subterrane.

Geology of the Zihuatanejo SubterraneThe Zihuatanejo subterrane occupies the Pacific coastal por-

tion of Mexico from Puerto Vallarta to Zihuatanejo and extendsseveral hundred kilometers inland (Fig. 1). According to Cen-teno-García et al. (2003), it includes the Huetamo subterrane,which was considered a separate tectonostratigraphic domainby previous workers (e.g., Centeno-García et al., 1993).

The Zihuatanejo subterrane has been studied in most detailsome 300 km southeast of Cuale, near the city of Zihuatanejo(e.g., Centeno-Garcia et al., 1993; Freydier et al., 1997; Men-doza and Suastegui, 2000; Talavera-Mendoza, 2000). Thestratigraphy defined by these workers is characterized by alate Paleozoic and Triassic basement, consisting of weaklymetamorphosed, strongly deformed terrigenous clastic sedi-mentary rocks with minor basalts (Arteaga Complex: Cen-teno-Garcia et al., 1993; Fig. 1). These basement rocks are in-truded by calc-alkaline, I-type granitoid rocks to which a LateJurassic age of ~160 Ma has been assigned (Tumbiscatío andMacías intrusions: Centeno-García et al., 2003; PetróleosMexicanos (PEMEX) unpub. company reports). The ArteagaComplex is overlain by the weakly deformed Upper Jurassicto Lower Cretaceous Zihuatanejo sequence of Mendoza andSuastegui (2000), consisting of arc-related volcanic and vol-caniclastic rocks, ranging from andesitic to rhyolitic in com-position. An Early Cretaceous subduction-related mélange,spatially associated with the largely undeformed Zihuatanejosequence, is recognized as the Las Ollas complex (Talavera-Mendoza, 2000; Fig. 1). The Early Cretaceous volcanic andvolcaniclastic sequences are capped by Albian reefal lime-stones and red beds (Mendoza and Suastegui, 2000).

The geology and stratigraphy of the Cuale area (Figs. 2, 3)have been loosely correlated with the above stratigraphy on thebasis of overall stratigraphic similarities (e.g., Hall and Gomez-Torres, 2000a); however, our new observations (see below) leadto a significant refinement of this regional comparison.

Geologic Setting of the Cuale DistrictThe oldest unit at Cuale comprises pelitic schists, intercalated

with chloritic and sericitic schists and meta-arkoses and cropsout to the west of the Cuale mining district (Figs. 2, 3). Thispackage is gently folded and metamorphosed to subgreenschist


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facies assemblages. The base of the unit is not exposed, butthe overall thickness is thought to be at least 800 m (Berrocaland Querol, 1991). It is separated from the overlying volcanicrocks by an angular unconformity and is locally intruded byhypabyssal rhyolite (Macomber, 1962). Although the age ofthese pelitic schists is not directly constrained, a minimumEarly to Middle Jurassic age is given by the U-Pb dates ob-tained for the overlying Cuale volcanic sequence (see below).However, the pelitic schists may well be as old as late Paleo-zoic to Triassic and can, on the basis of similar lithology andstratigraphic context, be correlated with the Permian to Tri-assic Arteaga Complex situated some 300 km to the southeast(Centeno-Garcia et al., 1993).

The largely undeformed and unmetamorphosed volcanic andassociated volcaniclastic rocks that overlie the pelitic schists ofthe basement have been subdivided into several stratigraphicunits based on reconnaissance-scale geologic mapping (JapanInternational Cooperation Agency and Metal Mining Agency ofJapan (JICA-MMA: 1986)). Volcanic rocks in the region rangingfrom basalt to dacite were reported and assigned to the LowerCretaceous to lower Tertiary (JICA-MMA, 1986). The volcanicrocks of the Cuale mining district, herein termed the Cuale vol-canic sequence, are part of this stratigraphic succession. How-ever, the results of this study cast doubt on the age constraintsand compositional variations reported by JICA-MMA (1986).We interpret the Cuale volcanic sequence to be almost entirelycomposed of Late Jurassic rhyolite and rhyolitic volcaniclasticsedimentary rocks with an overall thickness exceeding 800 m.Cretaceous and Tertiary volcanic rocks have not been recog-nized in the Cuale mining district.

The youngest rock unit found in the study area is massive andunaltered granodiorite that has intruded the Cuale volcanic se-quence (Fig. 2). It is assigned to the Cretaceous Puerto Vallartabatholith, which, in the vicinity of the Cuale district, exhibitsweak peraluminous characteristics (Schaaf et al., 1995).

Stratigraphy of the Cuale Volcanic SequenceDue to the intense weathering (particularly on ridges) and

locally strong hydrothermal alteration, distinguishing minortextural differences among the felsic rocks of the Cuale vol-canic sequence is difficult. However, the coherent rhyoliteunits can be reasonably well defined on the basis of quartz-phenocryst content, flow banding, devitrification textures,and crosscutting relationships. A lithologic map and cross sec-tions are presented in Figures 3 and 4, respectively. The dis-trict-scale map forms the basis for the simplified stratigraphicmap (Fig. 5) and schematic stratigraphic column (Fig. 6).

The stratigraphy of the Cuale volcanic sequence has beensubdivided, in relationship to the massive sulfide mineraliza-tion and alteration, into a footwall, ore horizon, and late in-trusive phase, and is described below. The stratigraphic unitsdescribed herein have been deposited in an environment withsignificant relief, and individual lithologic horizons are gener-ally of limited lateral extent.

Footwall

The footwall stratigraphy exceeds 400 m in thickness and isdominated by quartz and feldspar phyric rhyolite flows andcryptodomes, commonly enveloped by large volumes ofmonomictic, commonly matrix-supported, volcanic breccias

interpreted as carpace or flow breccias and hyaloclastites (Fig.7). Its outcrop extent includes the topographically low-lyingareas to the south and southeast of Naricero and much of thewestern and northwestern flank of Cerro Caracol (Fig. 5). Therhyolite flows rarely are massive but normally exhibit flowbanding, spherulitic devitrification, amygdules, and litho-physae (Fig. 7). They contain 5 to 8 vol percent slightly em-bayed quartz phenocrysts that are up to 3 mm in diameterand, where not obliterated by alteration, similar amounts ofeuhedral feldspars. The outcrop extent and accessibility doesnot permit detailed mapping of lateral variations in individualflows and their geometric relationships to the brecciated rocksnor does it allow identification of individual extrusive centers.

Coarse, polymictic volcaniclastic breccias (Fig. 7) to finergrained conglomerates are interbedded with the volcanicrocks of the sequence. Clasts are all derived from the Cualerhyolitic sequence but may be from flows as well as from pre-viously deposited volcaniclastic rocks. The coarse polymicticvolcaniclastic breccia deposits are commonly matrix sup-ported, with angular clasts of up to 30 cm in diameter, and aregenerally not graded nor welded (Fig. 7). Thus, the footwallsequence is thought to have been extruded subaqueously and,at least partly, in an explosive manner (hyaloclastites). Parts ofthe volcanic rocks have been reworked into volcaniclasticbreccias and conglomerate horizons.

The overall stratigraphy of the footwall is dominated byproximal volcanic deposits, such as quartz-feldspar phyricrhyolite flows, monomictic volcanic breccias (hyaloclastites),and unstratified, coarse polymicitic volcaniclastic breccias, in-terpreted as debris-flow deposits. Stratified volcaniclasticsedimentary rocks are relatively subordinate, and fine-grained silt and mudstone are absent.

Ore horizon

Quartz and feldspar phyric to aphyric rhyolite forms mas-sive bodies with no internal structure and units with flowbanding, spherulites developed in banded arrays, and rarehorizons or lenses of hyaloclastite. This series of textures andstructures is interpreted as representing flow and/or domecomplexes using the criteria of McPhie et al. (1993).

These flow and/or dome complexes crop out in the CerroCaracol and Coloradita area, whereas the central portion ofthe district (Figs. 3–5) is dominated by volcaniclastic sedi-mentary rocks ranging from conglomerate to sandstones, in-terbedded with rhyolitic tuffs and lenses of black argillite.The general absence of fine-grained sedimentary rocks atCerro Caracol and Coloradita, combined with the relativelyabundant rhyolite flows, suggests that these areas may repre-sent paleotopographic highs bounding central sedimentarybasins. Syndepositional tectonic activity at the westernboundary of these central basins is inferred from soft-sedi-ment deformation, such as east- southeast-vergent slumpfolding and locally reworked shale to siltstone deposits (Fig.8A). At one location (coord. UTM zone 13: 488.004/2252.992), a coarse breccia with rhyolite boulders exceeding1 m in diameter was deposited directly over shale and silt-stone, which themselves overlie massive quartz-feldsparphyric rhyolite similar to the boulders (Fig. 8B). Presumably,the boulder breccia represents mass wasting related to syn-depositional normal faulting.

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Cerro CantónCerro Cantón

Cerro CaracolCerro Caracol

1km

22550002255000

4855

0048

5500

ColoraditaColoraditaChivos-Chivos-

SocorredoraSocorredora

La Prieta/RubíLa Prieta/Rubí

CorazónCorazón

San NicolasSan Nicolas

-de Arriba-de Arriba-de Abajo-de Abajo

Triassic

Foot wall

Rhyolite flows and domes


Shale to sandstoneRhyolitic pyroclastic/volcaniclastic rocks

Rhyolitic pyroclastic/volcaniclastic rocks

Ore horizonCretaceous

Upper Jurassic (CVS)

Granodiorite

Intrusive rhyoliteAndesite dikes

Late Intrusive units

Slate

Jesús-MariaJesús-Maria

Minas de Oro/GrandezaMinas de Oro/Grandeza

CerroDescumbridoraCerroDescumbridora

La Prieta Fault

La Prieta Fault

Paso Caracol Fault

Paso Caracol Fault

RefugioRefugio

Normal Reverse

Major Faults (inferred)

Abandoned mine Mountain

Stratigraphic map of theCuale Mining DistrictN

NariceroNaricero

0.5

FIG. 5. Stratigraphic map of the Cuale mining district, based on the lithologic map of Figure 3. Locations mentioned inthe text are given. Coordinates are given as UTM zone 13, NAD83.

Unconformity

Unconformity

Tria

ssic

Mid

dle

Jura

ssic

(Cal

lov i

an)

Upp

erJu

rass

ic(O

xfor

d ian

)C

ret.

Slate, chlorite-sericite schist


Pyroclastic andhyaloclastitic rhyolite

Footwall (U/Pb age 157.2 +/- 0.5 Ma)


Volcaniclastic sandstonesto conglomerates

Shale to siltstone

Ore horizon (U/Pb age ca. 152-155 Ma)

Granodiorite

Massive sulfide/stockwork mineralization

Puerto Vallarta Batholith

Rhyolite tuffs

Rhyolite intrusions and domes

Epithermal veins/stockwork

Andesite dikes

Intrusive units (U/Pb age ca. 152-155 Ma)

FIG. 6. Schematic stratigraphic column for the Cuale volcanic sequence, as interpreted from mapping carried out in thisstudy (see Figs. 3–5). Mineralization events are indicated in their respective time-stratigraphic position.

The rhyolites of the ore horizon differ from those of thefootwall sequence in that some flows contain only minor(<<5%) and small (<1 mm) quartz phenocrysts. These aphyricrhyolites are intercalated with quartz and feldspar phyric rhy-olites similar to those in the footwall. The ore horizon is char-acterized by relatively abundant fine grained sedimentary

rocks interbedded with rhyolitic tuffs and volcaniclastic con-glomerates, whereas rhyolite flows and hyaloclastic brecciasare volumetrically subordinate compared to the footwall. Incontrast to the footwall sequence, the tuffaceous rocks of theore horizon include welded horizons (Fig. 8C) and, immedi-ately below Mina Jesús María (Fig. 3), tuffaceous strata con-taining accretionary lapilli (Fig. 8D). Although welded rocksmay also be deposited at considerable water depth (e.g., Koke-laar and Busby, 1992), the presence of both welding and ac-cretionary lapilli (e.g., Brown et al., 2002) is taken as an indi-cation of shallow water depths at the time of deposition.

The late intrusive phase

The footwall and ore horizon sequences are intruded bydikes, sills, and small hypabyssal bodies of quartz and feldsparphyric rhyolites. In contrast to the flows of the footwall andore horizon, the intrusive rhyolites are more massive and lackspherulitic devitrification textures, lithophysae, and amyg-dules, although flow banding is common in some dikes andnear the summit of Cerro Cantón. The intrusive rhyolites areslightly less intensely altered compared to the footwall andore horizon sequence. Moderate quartz-sericite ± pyrite ±chlorite alteration dominates and chloritized mafic phe-nocrysts (most likely biotite pseudomorphs) are preserved lo-cally. Columnar jointing in some dikes indicates intrusion intocold wall rock (Fig. 9). The dikes have a variable but pre-dominantly northwesterly strike. This late phase of rhyoliticmagmatism probably consists of several discrete intrusivepulses but cannot be subdivided further due to the limitedpetrographic variability.

The latest phase of magmatism of the Cuale volcanic se-quence is represented by four subvertical, west-northwest–striking andesitic dikes cutting the intrusive rhyolites (Figs. 3,5). The andesites are fine grained and have a plagioclase-pi-lotaxitic texture in a fine-grained matrix, which is weakly al-tered to chlorite-rich assemblages. Their age is uncertain butbracketed by the intrusive rhyolite (Late Jurassic; see below)and the granodiorite of the Puerto Vallarta batholith (LateCretaceous).

GeochronologyA total of five samples of rhyolitic flows, tuffs, and high-

level intrusions, distributed over the entire Cuale volcanic se-quence, were dated using conventional U-Pb zircon isotopedilution thermal ionization mass spectrometry (ID-TIMS) atthe Pacific Centre for Isotopic and Geochemical Research(PCIGR) facility at the University of British Columbia. Zir-cons were generally small and inclusion rich which, in somecases, resulted in relatively imprecise age determinations.The methodology for zircon grain selection, abrasion, dissolu-tion, geochemical preparation, and mass spectrometry are de-scribed by Mortensen et al. (1995). All 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. Analytical data are listedin Table 1 and are shown on conventional U-Pb concordiaplots in Figure 10. Errors attached to individual analyses werecalculated using the numerical error propagation method ofRoddick (1987). Decay constants used are those recom-mended by Steiger and Jäger (1977) and compositions for ini-

148 BISSIG ET AL.

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A

B

C

20 cm

rr

rr

FIG. 7. Field photographs of typical outcrops from the footwall and lower-most ore horizon of the Cuale volcanic sequence. All samples shown are in-tensely chloritized. A. Vesicular rhyolite flow from the immediate footwall ofthe San Nicolas deposit. Sample TB-389A, dated at 152.5 ± 1.5 Ma, was takenfrom this outcrop (pencil for scale; see text for discussion). B. Coarse, polymic-tic matrix-supported volcaniclastic breccia from the south flank of Cerro Cara-col (coord. 486.677/2251.881, hammer handle with 10-cm marks for scale). C.Matrix-supported monomictic breccia, interpreted to be of hyaloclastic originsince many fragments have darker and finer grained rims (r). Near south-westridge of Cerro Caracol (coord. 486.493/2251.760, pencil for scale).

tial common Pb were taken from the model of Stacey andKramer (1975). All errors are given at the 2σ level. Results forthe five samples are discussed briefly below, sample locationsare given within parentheses as coordinates UTM zone 13,NAD83.

Sample TB-368A (486.680/2253.326): This sample was ahyaloclastic, quartz and plagioclase phyric rhyolite from thefootwall to the VMS bodies, north of Cerro Caracol. Zirconrecovered from this sample comprised pale-brown, stubby,square prisms with abundant clear bubble- and rod-shapedinclusions. Four strongly abraded fractions were analyzed(Table 1). Two fractions (A and D, Fig. 10A) yielded overlap-ping concordant analyses with a total range of 206Pb/238U agesof 157.2 ± 0.5 Ma, which is considered to give the crystalliza-tion age of the sample. Two other fractions are slightly dis-cordant with older 207Pb/206Pb ages. The data form a shortarray with a poorly defined calculated upper intercept age of~1.62 Ga. The data are interpreted to indicate the presenceof a minor inherited zircon component in the sample with anaverage age of ~1.62 Ga.


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A B

D

5 cm

C

FIG. 8. Field photographs of typical outcrops from the ore horizon of the Cuale volcanic sequence. A and B show evidencefor synvolcanic faulting. C and D show features indicating probable subaerial or shallow-water deposition of volcanic products.A. Siltstones and shale of the ore horizon featuring slump folding (Naricero, coord. 488.006/2252.379; hammer for scale, han-dle ~50 cm). B. Coarse boulder breccia (b) deposited directly on laminated siltstones (s). The inset shows detail of the con-tact relationship. Note the siltstone beds deformed due to the impact of the boulder (coord. 488.004/2252.992, hammer, ~50cm, for scale). C. Welded rhyolite lapilli tuff. Aproximate plane of welding is indicated by the dashed white line (coord.487.330/2252.311). D. Accretionary lapilli from Jesús-Maria (coord. 489.335/2252.625, pencil for scale).

FIG. 9. Field photograph of a rhyolite dike (between white dashed lines)intruding massive outcrops of volcaniclastic deposits (coord. 486.493/2251.760, person for scale). The dike features columnar jointing perpendic-ular to the dike walls. Note also the color contrast between the intensely chlo-rite-pyrite-quartz altered wall rock and the only moderately quartz-sericite ±chlorite altered dike.

Sample TB-369A (488.569/2253.978): This sample wasfrom an aphyric rhyolite flow from the footwall of the La Pri-eta fault, collected in the Coloradita open pit, at approxi-mately the ore horizon in this part of the Cuale district. Zir-cons were pale-pink stubby prisms with morphologies similarto those in the previous sample. Four abraded fractions wereanalyzed, and the results also indicate the presence of a minorolder inherited zircon component (Table 1, Fig. 10B). Thebest estimate for the crystallization age of the sample is givenby fraction A with a 206Pb/238U age of 154.0 ± 0.9 Ma. Theother three fractions are either imprecise due to the presenceof high levels of common Pb (from abundant inclusions) orfall slightly to the right of concordia, indicating the presenceof inherited zircon.

Sample TB-389A (487.699/2252.037): This sample wasfrom a vesicular, quartz and feldspar phyric rhyolite flowfrom the immediate footwall of the San Nicolas deposit. Theflow belongs to the lowermost ore horizon. Only a smallamount of relatively fine grained zircon was recovered. Fivefractions of lightly abraded zircon were analyzed (Table 1,Fig. 10C-D). A more substantial amount of inherited zircon ispresent in this sample as indicated by the much older207Pb/206Pb age of 1190 Ma for fraction C. Two fractions (Dand E) give overlapping concordant analyses with a totalrange of 206Pb/238U ages of 152.5 ± 1.5 Ma. This volcanic unit

is intruded by a quartz-plagioclase phyric rhyolite dike, whichyields a slightly older crystallization age (sample TB241b; seediscussion below). The data are interpreted to indicate thatthese two fractions have experienced postcrystallization Pb-loss effects that were not completely removed by the lightabrasion. The 152.5 ± 1.5 Ma age does provide a minimumcrystallization age for the sample. The other three fractionscontain older inherited zircon components, and a calculatedupper intercept age of 1.59 Ga gives the average age of the in-herited component.

Sample TB-241B (488.122/2253.341): This sample wasfrom a quartz-plagioclase phyric rhyolite dike that intrudesthe rhyolite flow (sample TB389A) in the immediate footwallof the San Nicolas deposit. Zircons from the sample form col-orless elongate prisms. Four strongly abraded fractions wereanalyzed (Table 1, Fig. 10E). All yielded concordant analyseswith a total range of 206Pb/238U ages of 155.9 ± 1.6 Ma, whichis considered to give the crystallization age of the sample.

Sample TB-57C (488.527/2254.049): This sample is from aquartz and feldspar phyric crystal tuff from the hanging wallof the La Prieta fault taken in the Coloradita open pit. Threestrongly abraded fractions were analyzed (Table 1, Fig. 10F)and yielded concordant analyses with a total range of 206Pb/238U ages of 159.2 ± 2.2 Ma. This is inconsistent with the otherage determinations discussed above and the significance of

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TABLE 1. U-Pb Analytical Data

206Pb/ TotalSample/ Wt U Pb2 204Pb3 common % 206Pb/238U4 207Pb/235U4 207Pb/206Pb4 206Pb/238U age 207Pb/206Pb agedescription1 (mg) (ppm) (ppm) (meas.) Pb (pg) 208Pb2 (± % 1σ ) (± % 1σ ) (± % 1σ ) (Ma; ± % 2σ) (Ma; ± % 2σ )

Sample TB-368AA: N2, 104-134 0.008 1278 32.0 3090 5 11.0 0.02465(0.13) 0.1672(0.25) 0.04921(0.19) 157.0(0.4) 158.0(8.9)B: N2, 104-134 0.014 1381 34.8 2395 12 11.5 0.02472(0.11) 0.1680(0.21) 0.04931(0.14) 157.4(0.3) 162.4(6.6)C: N2, 104-134 0.007 697 17.8 2224 3 12.2 0.02485(0.16) 0.1703(0.36) 0.04971(0.30) 158.2(0.5) 181.2(14.2)D: N2, 104-134 0.008 1506 38.0 4233 4 11.7 0.02465(0.16) 0.1675(0.26) 0.04930(0.21) 157.0(0.5) 162.3(9.9)

Sample TB-369AA: N2,104-134 0.017 1073 26.4 404 70 11.5 0.02418(0.30) 0.1635(0.83) 0.04905(0.67) 154.0(0.9) 150.1(31.2)B: N2,104-134 0.014 680 18.2 63 327 15.0 0.02515(1.21) 0.1807(4.00) 0.05212(3.28) 160.1(3.8) 290.5(149)D: N2,104-134 0.011 920 23.7 275 59 14.0 0.02452(0.24) 0.1679(0.79) 0.04968(0.63) 156.1(0.7) 179.9(29.6)E: N2,104-134 0.008 854 23.0 36 592 15.7 0.02514(3.03) 0.1765(10.1) 0.05092(8.33) 160.0(9.6) 237.2(387)

Sample TB-389AA: N2,74-104 0.007 551 16.0 923 7 11.6 0.02834(0.89) 0.2089(4.20) 0.05347(3.85) 180.1(3.2) 348.7(175)B: N2,74-104 0.005 622 16.5 2097 2 16.3 0.02461(0.20) 0.1701(0.50) 0.05011(0.45) 156.8(0.6) 200.2(20.8)C: N2,74-104 0.004 571 34.9 3354 2 14.8 0.05614(0.21) 0.6171(0.29) 0.07972(0.22) 352.1(1.4) 1190.2(8.8)D: N2,74-104 0.006 444 10.9 728 5 11.9 0.02392(0.54) 0.1602(4.02) 0.04859(3.76) 152.4(1.6) 128.1(177)E: N2,74-104 0.007 390 9.6 824 5 12/1 0.02388(0.40) 0.1620(1.90) 0.04920(1.74) 152.1(1.2) 157.3(81.0)

Sample TB-241BA: N2,+74 0.033 160 4.0 160 56 11.8 0.02442(0.41) 0.1660(1.36) 0.04930(1.12) 155.6(1.3) 162.2(52.1)D: N2,+74 0.038 290 7.4 2020 8 13.5 0.02454(0.11) 0.1664(0.27) 0.04918(0.20) 156.3(0.4) 156.6(9.4)E: N2,+74 0.045 250 6.5 1758 10 13.9 0.02462(0.10) 0.1668(0.56) 0.04914(0.54) 156.8(0.3) 154.4925.3)F: N2,+74 0.046 203 5.3 2134 7 15.4 0.02457(0.32) 0.1668(0.69) 0.04925(0.61) 156.5(1.0) 160.0(28.5)

Sample TB-57CA: N5,104-134 0.011 348 9.4 92 69 16.4 0.02500(0.74) 0.1705(2.74) 0.04949(2.32) 159.1(2.3) 171.0(108)C: N5,104-134 0.018 342 9.2 455 22 16.4 0.02489(0.17) 0.1704(0.58) 0.04965(0.50) 158.5(0.6) 178.7(22.7)E: N5,104-134 0.025 205 5.5 143 63 16.2 0.02483(0.38) 0.1691(2.22) 0.04939(2.05) 158.1(1.2) 166.4(95.8)

1 N2, N5 = nonmagnetic at n degrees side slope on Frantz magnetic separator; grain size given in microns2 Radiogenic Pb; corrected for blank, initial common Pb, and spike3 Corrected for spike and fractionation4 Corrected for blank Pb and U, and common Pb

the results is not certain. Fraction C falls slightly to the rightof concordia, indicating the presence of a minor older inher-ited zircon component, and the other two fractions are tooimprecise to evaluate whether a minor inherited componentwas also present in these analyses. In view of the stratigraphicposition of this sample in the hanging wall of the ore and the

relatively robust crystallization ages that we have obtainedfrom stratigraphically lower units, we tentatively concludethat all three zircon fractions contained minor inherited zir-con components or represent reworked zircons. An unequiv-ocal crystallization age, therefore, cannot be assigned to thesample from our data.


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156

160

A BC

D

0.165 0.1710.024

0.025

0.026

140

150

160

170

180

A

B

D E

0.125 0.165 0.2050.0205

0.0245

0.0285

152

156

160

AD

E F

0.160 0.166 0.1720.0235

0.0245

0.0255

150

160

AC

E

0.15 0.17 0.190.022

0.024

0.026

to 1.62 Ga

TB-368A: Footwall rhyolite, 157.2 +/- 0.5 Ma

144

150

156

162

168

B

DE

0.14 0.16 0.180.022

0.024

0.026

TB-389A (expanded):152.5 +/- 1.5 Ma

150

200

250

300

350

AB

C

D+E

0.1 0.50.015

0.035

0.055

TB-389A: lowermostore horizon rhyolite

TB-241B: Intrusive rhyolite (dyke):155.9 +/- 1.6 Ma

TB-57C: ore horizon rhyolite tuff:159.2 +/- 2.2 Ma (Inheritedzircon component)

206

238

Pb/

U

206

238

Pb/

U20

623

8P

b/U

206

238

Pb/

U

206

238

Pb/

U

206

238

Pb/

U

207 235Pb/ U207 235Pb/ U

to 1.59 Ga

A B

C D

E F

TB-369A: Aphyric rhyolite,ore horizon, 154.0 +/- 0.9 Ma

FIG. 10. U-Pb concordia diagrams for felsic volcanic and high-level intrusive rocks in the Cuale district.

Characteristics of the Sulfide Deposits at CualeEleven orebodies, ranging from 20,000 to 783,000 metric

tons (t) of polymetallic ore (Table 2) along with a number ofsmaller occurrences were in production in the 1980s (Halland Gomez-Torres, 2000a). Most of this ore is mined out andthe following descriptions are summarized from Berrocal andQuerol (1991) and Hall and Gomez-Torres (2000a). The styleof mineralization and the relative abundance of the metalsvary significantly between the orebodies.

The Naricero, Socorredora, La Prieta-Rubí, San Nicolas,and Refugio deposits (Table 2), which were the source ofabout half of the mined ore in the district, are hosted by orspatially associated with black argillites. The massive sulfideore, compared to the other deposits of the district, is charac-terized by high average silver grades up to several hundredsof parts per million. The silver is contained within fine-grainedgalena and sphalerite that is locally brecciated (e.g., Naricero)or laminated (e.g., Socorredora). As the Pb-Ag–rich orebodieslocally exhibit sedimentary textures (lamination) we interpretthem to be in a distal setting, possibly transported into anoxicbasins adjacent to proximal sulfide mounds.

The Coloradita, Chivos de Arriba, and Chivos de Abajo de-posits are considered to be proximal to hydrothermal upflowzones. These deposits contain up to 3 g/t Au and 0.4 to 1.5percent Cu but are relatively low in Ag and Pb compared tothe more distal deposits. The precious and base metals arepartly hosted by pyrite- and chalcopyrite-rich stockworkzones that are hydrothermally altered. The stockwork zonesare in turn overlain by massive sulfide bodies (mostly pyritewith chalcopyrite that grade laterally and vertically into spha-lerite- and galena-rich zones). Host rock for the Coloraditadeposit is aphyric rhyolite.

Minas de Oro/Grandeza consists of stockwork mineraliza-tion and gold grades of 1.9 ppm, but only 0.2 wt percent Cu,which distinguishes it from other gold-rich orebodies of thedistrict where Cu is more abundant. The described ore min-eralogy at Minas de Oro/Grandeza (Macias and Solis, 1985) isessentially sphalerite, pyrite, and lesser amounts of galenaand freibergite [(Ag,Cu,Fe)12(Sb,As)4S13], whereas gangue ismainly composed of quartz and calcite. Limited fluid inclu-sion data on liquid-rich inclusions in quartz and sphalerite

(Macias and Solis, 1985) support the interpretation of Minasde Oro/Grandeza being similar to low-sulfidation epithermaldeposits (i.e., homogenization temperatures between 220°and 265°C, salinities around 5 wt % NaCl equiv, together withthe estimated average density of 0.86 g/cm3) and havingformed at 400- to 700-m depth below surface.

An approximately 4-m-wide quartz vein with epithermaltextural characteristics (open-space filling and local colloformtexture) is present on the summit of Cerro Descumbridora.This vein was mined historically (no grade and tonnage dataavailable) and crops out northwest of and along strike fromthe stockwork mineralization at Minas de Oro/Grandeza.Both the vein and the stockwork mineralization postdate themain VMS deposits of Cuale since they are hosted by thepost-VMS intrusive rhyolite. Further evidence for a late min-eralization event with epithermal characteristics comes fromquartz-carbonate ± adularia vein material found in the float ofthe northwestern ridge of Cerro Cantón, as well as fromstream-sediment geochemistry near Santa Rita (a smallprospect ~3 km north of Cerro Cantón, outside the study areaand not shown in Fig. 2) where anomalous arsenic and mer-cury concentrations have been reported (International Croe-sus Ventures Corp. unpub. reports).

Hydrothermal AlterationModerate to intense hydrothermal alteration is ubiquitous

and affects all rocks of the Cuale volcanic sequence. The foot-wall alteration associated with the deposits considered proxi-mal is characterized by intense quartz-sericite-pyrite alter-ation, laterally grading into more chlorite-rich assemblages.However, the alteration in the footwall of Socorredora andNaricero also exhibits moderate to intense quartz-sericite-pyrite alteration, which indicates that part of the ore of thesedeposits may have been contributed by a low-temperaturehydrothermal fluid and not all mineralization is the result oftransport and redeposition in anoxic basins. The blackargillite which host part of the ore is locally slightly carbona-ceous or cut by calcite veinlets, but it is unclear whether ornot this carbonate is related to hydrothermal alteration.

The late intrusive rhyolites are affected by less intense al-teration (Fig. 9). The rocks are moderately silicified with

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Table 2. The Ore Deposits of the Cuale District

Mine Metric tons Au (g/t) Ag (g/t) Pb (%) Zn (%) Cu (%)

Naricero 782,544 0.34 157 1.05 2.85 0.06Mina de Oro/Grandeza 756,661 1.89 22 1.41 2.35 0.2Socorredora 200,492 0.13 187 1.89 6.93 0.16Coloradita 170,055 0.66 85 1.99 6.51 0.37Las Talpas 141,425 0.34 24 0.65 1.91 0.24La Prieta-Rubí 113,335 0.73 226 3.8 9.24 0.32Chivos de Abajo 85,771 1.08 179 1.48 4.71 1.54San Nicolas 79,965 0.19 121 1.57 3.18 0.13Jesus Maria 46,751 0.06 109 1.85 3.31 0.09Refugio 34,569 0.14 156 0.89 1.95 0.1Chivos de Arriba 23,588 2.79 70 0.85 2.18 0.74

Eight other small deposits 39,199

Total 2,474,355 0.83 103 1.03 3.22 0.23

modest amounts of sericite. Disseminated pyrite is absent ormuch less abundant than in the lower parts of the stratigra-phy. Ferromagnesian minerals (most likely biotite) are pre-served as chloritized relicts only in the rhyolites of the late in-trusive phase.

Structural Evolution of the Cuale DistrictThe major control on the erosional landforms at Cuale is

the type and degree of alteration rather than fault traces, andair-photo interpretation is therefore of limited use for struc-tural interpretations. The following is a summary based onlimited mapping.

Synvolcanic faulting (D1) and subsequent erosion is evi-denced by coarse volcaniclastic rocks commonly found on thesouthern and eastern flanks of Cerro Caracol. Slump folds(Fig. 8A) and boulders deposited directly on fine-grained sed-iment (Fig. 8B) may be taken as further evidence for defor-mation during the deposition of the Cuale volcanic sequence.Five out of six measurements of fold axis in slump folds withinfine-grained sedimentary rocks plunge at shallow angles tothe north or south, an orientation consistent with slumpinginduced by steepening relief due to displacements alongnorth-south–striking normal faults. Synvolcanic structureshave not been observed directly, but two roughly north-south–striking cleavage zones have been recognized at SanNicolas and 200 m northwest of Jesús-Maria. These zonesmay represent zones of structural weaknesses that controlledfluid flow.

The late andesite dikes and many rhyolite dikes cutting allthe previous units within the volcanic pile have an east-west tosoutheast-northwest strike direction (Fig. 3). This predominantdike orientation, together with the inferred north-south–strik-ing structures described above, is consistent with emplacementof the rhyolite in tension fractures associated with sinistraltranstensional rifting along north-south– to north-northwest–south-southeast–striking faults. From this interpretation of theUpper Jurassic tectonic environment, Cuale is in agreementwith the Upper Jurassic subduction geometry suggested byDickinson and Lawton (2001) as well as the parallel UpperJurassic sinistral transtensional arc that has been documentedfor northwestern Mexico (Busby et al., 2005).

A northeast-dipping district-scale fault, the Paso Caracolfault (Fig. 3), has been inferred from limited outcrop expo-sure at Paso Caracol as well as from map interpretation alongstrike to the northwest and southeast. Although no unequivo-cal kinematic indicators have been recognized, we interpretthe offset to be reverse (D2) since smaller scale northwest-southeast–striking reverse faults have been observedthroughout the district. Bedding planes of fine-grained sedi-mentary units of the ore horizon, where not affected bysynsedimentary slump folding, have dips varying from 0° to35° at variable strikes. It is thus unclear whether contractionaldeformation resulted in gentle folding in the district orwhether the variation in bedding plane dips may be attributedsolely to the intrusion of the late rhyolite. D2 deformationpredates the later La Prieta fault as the latter appears to ter-minate at a structure parallel to the Paso Caracol fault north-east of Socorredora (Figs. 3, 5).

Late normal faults are important as they locally separatethe Cuale volcanic sequence from older units, but normal

faults with offsets of typically less than 30 m are observedthroughout the district. However, in the case of the La Prietafault (Figs. 3, 5) a minimum normal offset of 200 m has beeninferred from drill log records. The fault crops out in the Col-oradita open pit as a ~30-m-thick fault zone filled with gougeand brecciated rocks. The La Prieta fault dips ~45° to thenorth-northwest and steepens at depth (Macomber, 1962). Anormal offset is further suggested by parallel smaller scalefractures in the vicinity of the fault trace which have a demon-strably normal offset. The massive sulfide mineralization atLa Prieta, Coloradita, Chivos de Arriba, and Chivos de Abajoterminates at the La Prieta fault and thus predates normal slipon the fault.

Whole-Rock GeochemistryFifty samples of rhyolite from the Cuale volcanic sequence

have been analyzed for major and trace elements using in-ductively coupled plasma atomic emission spectroscopy (ICP-AES) for major elements and inductively coupled plasmamass spectroscopy (ICP-MS) for trace elements. Lithiummetaborate fusion was applied to ensure complete dissolutionof the lithophile elements. The rocks were submitted as asingle batch and analyzed using standard exploration gradeanalytical packages at ALS-Chemex laboratories in NorthVancouver, British Columbia, Canada. The complete data setis available as a digital supplement to this paper at <http://www.geoscienceworld. org/> or, for members and sub-scribers, on the SEG website, <http://www.segweb.org>.

All analyzed samples have experienced considerable hy-drothermal alteration, precluding the use of mobile elementsfor tectonic discrimination purposes. Thus, major elementswere used only to characterize the alteration by means of al-teration indices. The Cuale samples, taken over a verticalrange of 800 m from throughout the map area shown in Fig-ure 3 (see digital data supplement for sample coordinates),have been plotted in the alteration box plot of Large et al.(2001; Fig. 11). Most samples of rhyolite from the footwalland ore horizon fall within the range typical for intense chlo-rite-pyrite ± sericite alteration. In contrast, the samples ofintrusive rhyolite occupy a range from unaltered to sericite-altered rhyolite but do not extend as far into the chlorite-pyrite–altered field as the stratigraphically lower units. Thelate mafic dikes all plot within or close to the field for the leastaltered andesite. Thus, the intensity of alteration clearly dis-tinguishes between the footwall and ore horizon versus thelate, post-VMS mineralization intrusive rhyolites.

With the exception of three late-stage andesite dikes, allrocks have rhyolitic compositions when using the Zr/TiO2 ver-sus Nb/Y diagram of Winchester and Floyd (1977; Fig. 12A).The Y versus Nb diagram of Pearce et al. (1984; Fig. 12B)shows the Cuale samples straddling the triple point between“volcanic arc,” “ocean ridge,” and “anomalous ridge” fields.The Cuale volcanic rocks plot within a very narrow composi-tional range consistent with a rifted-arc environment, butthese widely used diagrams fail to discriminate between thestratigraphic units of the district. Incompatible trace ele-ments such as Zr, Y, and REE show sufficient variation to dis-tinguish footwall rhyolite from the rhyolite of the ore horizonand late intrusive rhyolite (Fig. 13). The chondrite normalized(La/Yb)n versus Ybn tectonic discrimination diagram of Hart


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et al. (2004; Fig. 13A), originally proposed by Lesher et al.(1986), shows that the Cuale volcanic rocks straddle FI andFII fields, indicating calc-alkaline compositions (Fig. 13A).The footwall rhyolites, with two exceptions, plot at relativelylow (La/Yb)n values, whereas the rhyolite of the ore horizonand late intrusive rhyolite have somewhat higher ratios. Asimilar relationship can be observed when using the La ver-sus Yb discrimination diagram proposed by Barrett andMacLean (1999; Fig. 13B); the footwall rhyolites generallyoccupy a field transitional between calc-alkaline and tholeiiticrocks, whereas the rhyolite of the ore horizon and late intru-sive rhyolite have overlapping but generally more calc-alka-line compositions (Fig. 13B).

The footwall rhyolites have low Zr contents (56–109 ppm),whereas Zr ranges from 58.5 to 195 ppm in the rhyolite of theore horizon and hanging-wall rhyolite (Fig. 13C). Late an-desite dikes have the highest Zr concentrations (176–230ppm) within the Cuale volcanic sequence. Titanium concen-trations in the footwall rhyolite are low (0.05–0.08 wt % TiO2)and are lower than in the stratigraphically higher rhyolites(0.03–0.2 wt % TiO2). The andesite dikes contain from 1.31 to1.84 wt percent TiO2.

Discussion

Stratigraphy

The footwall sequence is characterized by large volumes ofrhyolite flows and hyaloclastite deposits with subordinate vol-caniclastic sedimentary rocks, all of which are interpreted to

have been deposited in a subaqueous setting. The ore hori-zon, in contrast, is dominated by volcaniclastic sedimentaryrocks ranging from conglomerates to sandstones with interca-lated tuffs. Black shale to siltstone horizons within this se-quence are the main host for Pb-Zn-Ag–rich massive sulfidemineralization. Rhyolite flows and domes, commonly aphyricto weakly quartz-phyric, crop out at Coloradita and at CerroCaracol and are separated by the central domain of fine-grained volcaniclastic sedimentary rocks (Figs. 3, 14). Thedistribution of volcanic and volcanic-sedimentary rocks sug-gests that Coloradita and Cerro Caracol may have repre-sented topographic highs bordering a central sedimentarybasin (Fig. 14). A general decrease in water depth during theevolution of the Cuale volcanic sequence is indicated by someof the tuff layers within the ore horizon that have welded oraccretionary lapilli. Tuffites, locally welded, are common on

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10 30 50 70 900

20

40

60

80

100

CC

PI

AI

Least altered andesite/basalt

Least alteredrhyolite

ep, cc dol ank py, chl

Ksp

ser

ab

AndesiteQuartz-plagioclase phyric rhyolite (late intrusive phase)Aphyric rhyolite (ore-horizon)

Quartz-plagioclase phyric rhyolite (footwall)

AlterationDiagenetic

FIG. 11. Alteration box plot for samples from the Cuale district, afterLarge et al. (2001). AI = 100 (K2O + MgO)/(K2O + MgO + Na2O + CaO).CCPI = 100 (MgO + FeO)/(MgO + FeO + Na2O + K2O). Alteration miner-als are indicated within the diagram as follows: ab = albite, ank = ankerite, cc= calcite, chl = chlorite, dol = dolomite, ep = epidote, Ksp = K-feldspar, py= pyrite, ser = sericite. Most samples of rhyolite from the the footwall and theore horizon rhyolite fall within the range typical for intense chlorite-pyrite ±sericite alteration, whereas the intrusive rhyolite varies from unaltered tosericite altered, but does not extend into the field for chlorite-pyrite alter-ation. The late andesite dikes all fall within or close to the box of least alteredandesites.

.01 .1 1 10

.01

.1

1

Zr/

TiO

2

Nb/Y

Basalt

Andesite

Rhyodacite/Dacite

AlkBasalt

TrachyAnd

Trachyte

Phonolite

Foidite

Andesite

Quartz-plagioclase phyric rhyolite (late intrusive phase)Aphyric rhyolite (ore-horizon)Quartz-plagioclase phyric rhyolite (footwall)

A

1 10 100 10001

10

100

1000

Nb

(ppm

)

Y (ppm)

syn-collisionalvolanic arc

within plate

ocean ridge

B

Anomalous ridge

Andesite/Basalt

Com/Pant

Rhyolite

FIG 12. Discrimination diagrams for rhyolite and andesite of the Cualevolcanic sequence. A. Zr/TiO2 vs. Nb/Y diagram after Winchester and Floyd(1977), showing the limited range in composition for the Cuale rhyolites andthe clearly distiguishable andesites. B. Nb vs. Y tectonic discrimination dia-gram after Pearce et al. (1984). The rocks from Cuale occupy a limited com-positional range straddling the border between the “volcanic arc,” “oceanridge,” and “anomalous ridge” fields.

the southern flanks of Cerro Cantón and lie in the hangingwall of the La Prieta fault, but at a topographic level of <200m above the mineralization at Coloradita. These tuffitestherefore correspond to the higher portions of the ore hori-zon stratigraphy and suggest that the area may have becomeemergent during the waning stages of volcanic activity.

No significant input of continent-derived siliciclastic sedi-ment is apparent within the Cuale volcano-sedimentary se-quence, indicating a position that was isolated from majorcontinental sediment sources in the Late Jurassic. However, aminor older inherited zircon component is present in severalof the rhyolite samples (TB-368A, Tb-369A, TB-389A, TB-57C), indicating that older rock units, or sediments derivedfrom them, are present in the subsurface. A continental sedi-ment influx, if present, may also have been obscured by rapideruption rates for the Cuale volcanic sequence, an interpreta-tion which is in agreement with the U-Pb age determinations.

The lack of carbonate sedimentary rocks that would normallybe expected in a shallow marine setting may be interpreted asfurther indication for such a rapid eruptive cycle representedin the footwall and at the ore horizon.

Whole-rock geochemistry

The Cuale rhyolite sequence is remarkably homogeneousin chemistry. Apart from the late andesitic dikes, which maynot be directly related to the rhyolites as indicated by theirhigher Zr concentrations, only subtle geochemical variationover a vertical stratigraphic interval of more than 800 m isobserved. Similar to the gold-rich Eskay Creek deposit (Bar-rett and Sherlock, 1996), Ti and Zr concentrations in the rhy-olite are low at Cuale. However, Zr, Ti, and with LREE areslightly enriched in rhyolite at the ore horizon and in late in-trusive rhyolite compared to the footwall. This small changeis interpreted as an evolution from transitional tholeiitic to


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10 30 50 70 900

100

200

300Zr/Y = 7 4.5

2

Calc alkaline Transitional

Tholeiitic

2 3 4 5 6 7 80

50

100

La(p

pm)

Yb (ppm)

Calc-alkaline

Transitional

Tholeiitic

La/Yb = 6

3

1

Quartz-plagioclase phyric rhyolite(late intrusive phase)

Aphyric rhyolite (ore-horizon)

Quartz-plagioclase phyric rhyolite (footwall)

(La/

Yb)

n

Ybn

FI

FII

FIIIaFIIIb

FIV

0.1

1

10

100

0 50 100 150 200

Y (ppm)

Zr

(ppm

)

FIG. 13. Trace element ratios used to discriminate between different rhyolites from Cuale. A. (La/Yb)n vs. Ybn diagramwith field subdivisions from Lesher et al. (1986), modified by Hart et al. (2004). Chondrite normalization values are fromNakamura (1974). The Cuale rocks are restricted to the fields FI and FII. Interpretations from Hart et al. (2004) and refer-ences therein are as follows: FI = alkaline-calc-alkaline dacites and rhyolites barren of VMS mineralization, FII = calc-alka-line dacites and rhyolites associated to VMS deposits emplaced in rifted continental arc or continental back-arc environmentsinvolving rifted continental crust, FIIIa and FIIIb = felsic rocks of tholeiitic compositions largely associated with ArcheanVMS deposits, FIV = felsic rocks of tholeiitic compositions associated with VMS mineralization emplaced in intra-oceanic is-land arcs. B. La vs. Yb diagram showing the generally less calc-alkaline character for the footwall rhyolites compared to thosefrom the ore horizon. Subdivisions from Barrett and MacLean (1999). C. Zr vs. Y diagram adapted from Barrett andMacLean (1994). Note the generally low Zr content of the Cuale rhyolites.

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EW

6 km

Footwall phase

Ore horizon phaseCerroCaracolCerroCaracol

Coloradita

Late intrusive phase

Quartz-plagioclase phyric rhyolite flowsand domes

Aphyric rhyolite domes

Slate (greenschist facies)

Black shale to fine sandstones

Volcaniclastic sandstones to conglomerates

Rhyolitic pyroclastic and hyaloclastic rocks

0 km

Late Jurassic (Oxfordian)

Triassic

Massive sulfide

Sulfide stringer zone, stockwork mineralization

Sea level

Sea level

Sea level

Quartz plagioclase phyric rhyolite intrusionsand domesLate epithermal mineralization

Cerro DescumbridoraCerro Cantón

FIG. 14. Schematic E-W cross section through the Cuale district in the Late Jurassic. The volcanic sequence evolves froma submarine to a Late Jurassic, partly subaerial setting. Stage 1 (footwall) is characterized by subaqueous eruption of rhyo-lite flows, cryptodomes, and hyaloclastites. Stage 2 (ore horizon) is characterized by emplacement of rhyolite domes andproximal pyroclastic and hyaloclastic rocks near Cerro Caracol and Coloradita. Some of the volcanism was subaerial at thistime. A central basinal domain is characterized by reworked volcaniclastic facies and black shales. Massive sulfide mineral-ization occurs as sulfide mound and underlying stockwork mineralization associated with aphyric rhyolite domes, as well asin the central basinal domain, associated with black shale. Stage 3 (late intrusive phase) is characterized by emplacement ofshallow intrusions, dikes, and domes of quartz-feldspar phyric rhyolite. Stockwork and vein mineralization with epithermalcharacteristics cuts the late intrusive rhyolite unit.

calc-alkaline to calc-alkaline compositions. The overall geo-chemical compositions are consistent with an intra-oceanic is-land-arc setting (Barrett and MacLean, 1999) or a periconti-nental arc setting, the latter interpretation being favoredbecause of the presence of an inherited Pb component in thezircon U-Pb data.

Low Zr/Y and (La/Yb)n ratios have been related to large de-grees of melting and high zircon saturation temperatures, in-dicative of high magma temperatures (Watson and Harrison,1983; Barrie, 1995) or, alternatively, melting at low pressures(Hart et al., 2004). Mineralization in other Phanerozoic mas-sive sulfide districts has been shown to be associated with high-temperature rhyolites which have undergone little assimilationand fractional crystallization (Lentz, 1998; Piercey et al., 2001).In these cases, the high zircon saturation temperatures (Barrie,1995; Piercey et al., 2001) typically distinguish VMS-bearingfelsic sequences from barren rhyolites. In the Cuale volcanicsequence a slight overall increase of Zr/Y and (La/Yb)n in theore horizon and late intrusive rhyolites, compared to the foot-wall rhyolite, is observed. This evolution may indicate slightlydecreasing magma temperatures. However, the observed traceelement patterns may also be the effect of lower degrees ofmelting related to changes in the residual mineralogy from pla-gioclase + clinopyroxene- to hornblende + clinopyroxene-bear-ing assemblages (e.g., Hart et al., 2004), which would be in-dicative for increasing pressure at the site of magmageneration. The pressure increase is in agreement with the ob-served stratigraphy, which indicates a buildup of a volcanic ed-ifice that becomes emergent in the late stages; therefore, we in-terpret the data as reflecting variations in the degree of meltingand increasing pressure at the site of melting rather thanchanging magma temperatures. VMS ore was emplaced imme-diately following this weak geochemical transition, whereas thelate mineralization with low-sulfidation epithermal characteris-tics is associated with more typical calc-alkaline rocks.

The late andesite dikes have relatively high Zr concentra-tions, which indicate a different magma source and may indi-cate more involvement of continental crust or higher degreesof partial melting. These dikes clearly postdate the rhyolitesequence and may have been emplaced during accretion ofthe Guerrero terrane to the North America craton.

While small changes in magma chemistry commonly coin-cide with VMS mineralization (e.g., Barrett and MacLean,1999), the transition from a deep to shallow marine setting,along with the late epithermal mineralization observed atCuale is unusual. VMS mineralization in most deposits of sim-ilar settings (e.g., Kuroko: Ohmoto, 1983, 1996; KutchoCreek: Barrett et al., 1996; see also Lentz, 1998) occurredduring extensional tectonic events and is commonly overlainby deep marine sedimentary sequences. However, a shallowmarine ore-forming environment also has been postulated forEskay Creek (Barrett and Sherlock, 1996) and may be an im-portant factor that led to the silver-rich mineralization atCuale. The unusual evolution of Cuale indicates that the min-eralization was not emplaced in a typical evolving back-arcenvironment but rather suggests a setting in an epicontinen-tal transtensional volcanic arc undergoing rifting simultane-ous to uplift. It is likely that the epithermal mineral potentialof Cuale and surrounding areas has been underappreciated(e.g., Sillitoe and Hedenquist, 2003).

Regional context of the Cuale stratigraphy and VMS mineralization in the Guerrero terrane

Previous studies loosely correlated the Cuale volcanic se-quence with the Early Cretaceous volcanic sequences in thesouthern Zihuatanejo subterrane (JICA-MMA, 1986). How-ever, rhyolites from Cuale are now known to be Late Jurassicand are only slightly younger than the Tumbiscatío and Ma-cias granitoid intrusions of the southern Zihuatanjeo subter-rane. These granitoids may represent part of the same arc asthe Cuale rhyolites, which predates the Lower Cretaceous arcpreviously recognized. The Late Jurassic arc, judging fromlimited inherited Pb in the dated zircons, may have been un-derlain by vestiges of continental crust. Early Cretaceousrocks similar to those of the southern Zihuatanejo subterranehave not been recognized in the immediate Cuale area butare likely to be present regionally around Cuale (JICA-MMA,1986).

The folded and weakly metamorphosed pelitic schists un-derlying Cuale are similar to the Varales Formation of theArteaga Complex that forms the basement in the southern Zi-huatanejo subterrane. The age of the latter is poorly con-strained but considered late Paleozoic to Triassic (Centeno-García et al., 1993), a reasonable estimate for the rocks foundat Cuale, as well.

Volcanogenic massive sulfide mineralization in the Guer-rero terrane is generally hosted by latest Jurassic to EarlyCretaceous arc and back-arc sequences (Mortensen et al.,2003, 2008). Cuale is an exception because it formed in theLate Jurassic, ~5 to 16 m.y. before the other VMS districtsof the Guerrero terrane (Mortensen et al., 2003, 2008).Thus, the new data on Cuale not only extend the previouslyestablished age range for VMS mineralization but also pro-vide important constraints on the tectonic evolution of west-ern Mexico.

ConclusionsThe Cuale mining district contains silver-rich VMS miner-

alization emplaced in a shallow marine setting, as well as lateAu-Zn (-Pb, Ag) stockwork mineralization with low-sulfida-tion epithermal characteristics. The ore deposits are allhosted by rhyolitic rocks (Cuale volcanic sequence) with tran-sitional tholeiitic to calc-alkaline to calc-alkaline affinities.High field strength element contents of the rhyolites at Cualeare consistent with those observed in rhyolites associated withother Phanerozoic VMS districts. Increasing La/Yb and Zr/Yratios during eruption of the Cuale volcanic sequence arethought to be the result of a change in residual mineralogyfrom amphibole-free to amphibole-bearing assemblages aspressure at the melting site increased. This is in agreementwith the observed stratigraphy which indicates buildup of amore than 800-m-thick volcanic edifice that became emer-gent in the late stages.

The age of the Cuale volcanic sequence is 157.2 ± 0.5 to154.0 ± 0.9 Ma, which corresponds to the Upper Jurassic.Cuale, thus, represents the oldest known VMS deposit of theGuerrero terrane. Based on geologic mapping, whole-rockgeochemistry, geochronology (inherited Pb in zircons), and re-gional comparisons we propose that the Cuale district was em-placed in a transtensional pericontinental arc environment.


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AcknowledgmentsThis study represents MDRU contribution 170 and was

made possible thanks to financial support from the NaturalSciences and Engineering Research Council of Canada andInternational Croesus Ventures Corp. The senior authorwould further like to thank the Swiss National Science Foun-dation for a stipend. Antonio Orozco, Don José Castellón, andDoña Lydia Castellón from Cuale are thanked for their hos-pitality and assistance. Michelle Robinson, José Vargas, andDavid Smithson all provided valuable field assistance, whilediscussions with Ken Hickey and José Cembrano were valu-able during data interpretation. A. Toma helped with thedrafting of figures. Guest editor S. Piercey, Philipps C.Thurston, Joseph Whalen, and one anonymous Economic Ge-ology reviewer are thanked for their constructive reviews andcomments.

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