©2008 Society of Economic Geologists, Inc.
Economic Geology, v. 103, pp. 141–159
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
Abstract
The 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 mineralization contains high Ag ± Au grades when compared to similar deposits in western Mexico. The massive
sulfide bodies are hosted by an interbedded rhyolitic volcanic and volcaniclastic succession, herein termed the
Cuale volcanic sequence. Regionally, the Cuale volcanic sequence is part of the Zihuatanejo subterrane
belonging to the Guerrero terrane, which is interpreted to be a complex Mesozoic accreted arc terrane hosting the majority of VMS deposits in Mexico.
The rocks of the Cuale volcanic sequence are not metamorphosed and are only weakly deformed but exhibit
intense chlorite, sericite, and quartz hydrothermal alteration. Three stratigraphic units are identified within the
Cuale volcanic sequence: (1) a more than 400-m-thick footwall unit of quartz and plagioclase phyric rhyolite
flows, related hyaloclastites, and subordinate heterolithic volcaniclastic breccias; (2) an ore horizon characterized by sedimentary rocks ranging from volcaniclastic conglomerates to black shales, as well as aphyric to quartz
and plagioclase phyric rhyolite; and (3) late intrusions composed of quartz and plagioclase phyric rhyolite and
subordinate andesitic dikes intruding all units. The sedimentary rocks of the ore horizon are intercalated with
rhyolite tuff horizons, some of which exhibit welding or accretionary lapilli that are interpreted as evidence for
shallow 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 characterized
by low Zr/Y (1–1.9) and low chondrite normalized La/Yb ratios (9 of 11 samples between 1.24 and 3.3), to
slightly higher values in the ore horizon and late intrusive phase (Zr/Y = 1.6–7.6 and chondrite normalized
La/Yb ratios between 2.25 and 15). This compositional evolution can be explained by slightly decreasing
degrees of partial melting related to a transition from amphibole-free assemblages for the footwall to amphibole-bearing assemblages in the residuum for the ore horizon and intrusive rhyolites. This interpretation is supported by the observed volcanic stratigraphy and indicates increasing pressure at the site of melt generation as
the volcanic edifice was built.
Zircons from five samples of the Cuale volcanic sequence have been dated by the U-Pb method, using isotope dilution thermal ionization mass spectrometry (ID-TIMS). Although some samples show evidence of a
minor inherited Pb component, an age range of 157.2 ± 0.5 to 154.0 ± 0.9 Ma has been established. Cuale represents the oldest dated VMS camp in western Mexico as other VMS districts of western Mexico are between
151.3 and 138 Ma.
Two styles of mineralization are recognized: volcanogenic massive sulfide mineralization (VMS) and late lowsulfidation 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, which
exhibit a range of mineralization styles. Orebodies interpreted as proximal are characterized by stockwork vein
networks with high Au and Cu contents, underlying massive pyrite bodies, which grade laterally and vertically
into sphalerite- and galena-rich massive sulfide ore. Stockwork mineralization is locally hosted by aphyric rhyolite. More distal orebodies, rich in Pb, Zn, and Ag, are hosted by black shale and locally exhibit laminated or
brecciated sedimentary textures in the galena-sphalerite-pyrite ore. The sedimentary textures observed in
some massive sulfide orebodies suggest that the sulfide has been transported and deposited in anoxic basins
adjacent to the rhyolite domes. Some of the more significant orebodies show evidence of both distal sedimentary ore textures and proximal features indicated by intense quartz-sericite-pyrite alteration in the immediate
footwall.
*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.
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BISSIG ET AL.
Introduction
RECENT discoveries of large and/or precious metal-rich (bimodal-) volcanogenic massive sulfide deposits (VMS) in accreted arc terranes of Jurassic to Cretaceous age, such as the
83-Mt San Nicolas deposit in Mexico (Fig. 1; Johnson et al.
1999, 2000; Danielson, 2000) and the gold-rich Eskay Creek
deposit in British Columbia, Canada (Roth et al., 1999), have
increased exploration interest in areas of similar geologic setting along the North American Cordillera. The Guerrero terrane, composing much of west-central Mexico, is of interest
as it is interpreted as a Jurassic to Cretaceous accreted arc terrane (Campa and Coney, 1983; Centeno-García et al., 1993;
Dickinson and Lawton, 2001) and has a long history of mining and exploration for VMS deposits. The known deposits
range from large to small in size (Fig. 1; Miranda-Gasca,
2000). From a historical perspective, the Cuale district is one
of the more significant VMS districts of the Americas. Mining
operations were initiated in the Prieta orebody by the Spanish in 1805 to extract silver-rich ore zones (averaging 500 g/t
Ag: Hall and Gomez-Torres, 2000a).
The Cuale district is situated at latitude 20° 22' N and longitude 105° 7' W, ~30 km southeast of Puerto Vallarta, Jalisco,
102°
SM
O
Mexico D.F.
SM
S
Zacatecas
100°
San Luis de Potosí
14
Mascota
22°
Guadalajara
104°
13
12
11
Puerto Vallarta
León
Guadalajara
Guanajuato
in western Mexico (Figs. 1, 2) and ~4 km southwest of the
small 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 produced
from more than 18 small but high-grade orebodies. All but
the Chivas de Abajo deposit (Fig. 3) had been discovered and
high graded prior to 1919 (Berrocal and Querol, 1991; Hall
and Gomez-Torres, 2000a). The silver-rich nature of the ore
(drill intercepts of up to 9630 g/t Ag) was the major driving
force for mining in the 19th century, whereas Pb, Zn, Cu, Ag,
and Au were the main commodities produced in the 1980s by
Zimapán S.A de C. V., which bulk mined orebodies where
high-grade silver ore was extracted previously. Mining in the
district ceased in 1989, once the easily accessible reserves
were mined out; however, the area is currently being explored.
Despite the long history of mining and exploration at
Cuale, only limited scientific research has been carried out.
Macomber (1962) produced a detailed geologic map and established a stratigraphic succession. This work was done before the current genetic models for VMS deposits were accepted (e.g., Hutchinson, 1965). In 1986, the Japan
International Cooperation Agency and Metal Mining Agency
of Japan, in conjunction with the Consejo de Recursos Minerales de Mexico, conducted a multidisciplinary study of VMS
deposits of western Jalisco state (JICA-MMAJ, 1986), which
included reconnaissance-scale geologic mapping. Additional
geologic mapping of the immediate Cuale district was carried
out by Zimapán geologists during the exploration activities of
the late 1970s to early 1980s. Diamond drilling in the district
was 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 and
1989. Unfortunately, much of the drill core is lost or inaccessible and reliable drill log records are scarce.
1
20°
2
3
Mexico DF
10
4
5
10
20km
8
Puerto Vallarta
18°
Upper Cretaceous to
recent intrusions and
volcanic cover
Guerrero terrane
Zihuatanejo subterrane
Arteaga complex
Las Ollas complex
Teloloapan subterrane
Arcelia subterrane
Guanajuato subterrane
Sierra Madre Terrane
Mixteco Terrane
N
9
7
Zihuatanejo
Mascota
6
20°30’
VMS deposit or district
1. Cuale
2. Bramador
3. Dios Me Ayuda
4. La Minita
5. Arroyo Seco
6. Copper King
7. Campo Morado
A
Talpa de Allende
8. Rey de Plata
9. Tlanilpa-Azulaquez
10. Tizapa
11. El Gordo
12. Los Gavilanas
13. Carmen
14. San Nicolas
El Tuito Cuale District
Bramador District
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Amaltea district
105°00’
105°30’
FIG. 1. Map of the Guerrero terrane of western Mexico, showing the distribution of various exposures and the subterrane divisions of Centeno-Garcia et al. (2003), modified from Mortensen et al. (2008). The Arteaga Complex (Centeno-Garcia et al., 1993) and the Las Ollas Complex
(Talavera-Mendoza, 2000), both part of the Zihuatanejo subterrane, are indicated. Black stars show volcanogenic massive sulfide (VMS) deposits and
prospects hosted by the Guerrero terrane (see text and Mortensen et al.,
2008, for further discussion). The gray shaded area in the inset map indicates
the inferred extent of the Guerrero terrane Abbreviations: SMO = Sierra
Madre Occidental, SMS = Sierra Madre del Sur mountain belts.
Cuale (town)
Alluvium
(Quaternary and Tertiary)
Puerto Vallarta Batholith
(Late Cretaceous)
Volcanic and volcaniclastic rocks
(~Late Jurassic to Early Cretaceous)
Pelitic schists (~Triassic)
FIG 2. Geologic map of the Cuale region. Only limited regional geologic
information is available and has been taken from 1:100,000 scale maps of the
Instituto Nacional de Geografía e Informatica (INEGI) Mexico and maps
from the report of the Japan International Cooperation Agency-Metal Mining Agency of Japan (JICA-MMA, 1986).
142
2252000 mN
2253000 mN
A
2254000 mN
486000 mE
143
10
16
16
20
50
ault
18
30
40
24
30
32
20
16
18
B
20
Mina
Refugio
Mina La Paz
U/Pb: 157.2 +/- 0.5 Ma
62
17
col F
Cara
26
Paso
Cerro Caracol
82
20
40
10
10
22
23
Mina San Nicolas
Mina Naricero
12
24
A'
Camp
20
20
12
40
10
18
Mina
Patrocinio
Accretionary lapilli
30
Mina
Jesús Maria
Mina Coloradita
Mina
Corazón
Mill and
flotation
plant
40
26
L
La Prieta-Rubi
Mina Chivos de Arriba
U/Pb:
Dike: 155.9 +/- 1.6 Ma
Rhyolite flow: 152.5 +/- 1.5 Ma
70
30
24
32
34
15
24
Mina
Socorredora
36
B'
U/Pb:
Hanging
wall: 159.2 +/- 2.2 Ma
30
Foot wall: 154.0 +/- 0.9 Ma
Mina
Chivos
de Abajo
28
10
lt
au
aF
t
rie
aP
0
0.25
0.5km
Minas de Oro
Grandeza
Cerro
Descumbridora
FIG. 3. Lithologic map of the Cuale mining district (this study). The traces of the cross sections (Fig. 4) are indicated. The approximate extents of the ore zones, projected to the surface are indicated. Coordinates are given as Universal Transverse Mercator (UTM) zone 13, North American Datum 83 (NAD83).
54
12
30
487000 mE
Cerro
Cantón
489000 mE
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488000 mE
N
Geology
Ore zones: approximate surface
projection
Upper Jurassic (Oxfordian)
Cuale Volcanic Sequence
Fine-grained andesite (dikes)
Quartz and plagioclase porphyritic
rhyolite (No lithophysae or
spherulites, rare flow banding)
Black shale, siltstone, sandstone,
conglomerate, coarse
volcaniclastic breccia
Rhyolitic tuff breccia /coarse
hyaloclastite
Rhyolitic ash to lapilli tuff
+/- hyaloclastite
Aphyric to minor quartz
porphyritic rhyolite
Quartz and plagioclase
porphyritic rhyolite (Lithophysae,
spherulites and flow banding
common)
Pre-Middle Jurassic
Slate, pelitic schists
(lower greenschist facies)
Structures
Fault, probable normal offset
(confirmed and inferred)
Reverse Fault, probable
reverse offset
(confirmed and inferred)
Other inferred fault traces
Cleavage zones
28
Bedding with dip
Horizontal bedding
RHYOLITE-HOSTED VMS OF THE CUALE, GUERRERO TERRANE, MEXICO
143
144
BISSIG ET AL.
District-scale geologic mapping at Cuale is challenging, as
the volcanic rocks display only limited compositional variations (see below), are altered and deeply weathered, and the
terrain is steep and vegetated. Despite these limitations, the
Cuale district is well suited to the study of the volcanologic
setting at the time of ore formation, since it represents a continental margin VMS district that has not undergone metamorphism or significant later deformation, and the steep terrain permits geologic mapping in three dimensions.
The main aim of this study is to improve the geologic understanding of the district and establish constraints on the
volcanologic setting at the time of mineralization. This paper
presents the results of new district-scale mapping (Figs. 3, 4),
including areas only accessible using technical climbing
equipment, supplemented by whole-rock geochemistry, and
U-Pb geochronology.
Tectonostratigraphic Framework: The Guerrero Terrane
The subdivision of Mexico into tectonostratigraphic terranes was laid out by Campa and Coney (1983), subsequently
A
discussed and refined by Centeno-Garcia et al. (1993, 2003),
Dickinson and Lawton (2001), Keppie (2004), and CentenoGarcia (2005). The Guerrero terrane constitutes much of
west-central Mexico (Fig. 1) and, physiographically, includes
parts of the Sierra Madre Occidental and Sierra Madre del
Sur mountain belts. It is a composite arc terrane, divided into
five subterranes (Fig. 1) on the basis of differences in basement geology, stratigraphy, style, and intensity of deformation
(Campa and Coney, 1983; Centeno-García et al., 1993, 2003;
Centeno-García, 2005). We follow the subterrane definitions
proposed by Centeno-Garcia et al. (2003) and, for simplicity,
hereafter use the term Guerrero terrane when referring to
the Guerrero Composite terrane.
The complex arc-back arc system of the Guerrero terrane
was accreted to the North American continent late in the
Early Cretaceous (Dickinson and Lawton, 2001) and hosts
the majority of the VMS occurrences in Mexico (MirandaGasca, 2000; Mortensen et al., 2008; Fig. 1). Among the
most significant are the 31-Mt Campo Morado district
(Oliver et al., 2000, 2001), the 4.3-Mt Tizapa deposit (Parga
Profile A: No vertical exaggeration
A'
Cerro Caracol
2300
2200
2100
geology
not projected
2000
Refugio
DDH552
1900
Naricero
DDH36
1800
1700
1600
B
Profile B: No vertical exaggeration
B'
2180
Coloradita
2100
2000
Naricero
DDH26B
1900
Corazón
DDH17A
0
1800
500 meters
La Prieta Fault
1730
Geology
Structures
Upper Jurassic (Oxfordian)
Fine-grained andesite (dikes)
Rhyolitic ash to lapilli tuff +/- hyaloclastite
Observed normal fault
Quartz and plagioclase porphyritic rhyolite
(No lithophysae or spherulites, rare flow banding)
Aphyric to minor quartz porphyritic rhyolite
Inferred normal fault
Black shale, siltstone, sandstone,
conglomerate, coarse volcaniclastic breccia
Quartz and plagioclase porphyritic rhyolite
(Lithophysae, spherulites and flow banding common)
Rhyolitic tuff breccia /coarse hyaloclastite
Ore zones:
projected to cross section plane
DDH17A
Diamond drill hole,
logged and considered for cross sections
FIG. 4. Geologic cross sections based on the lithologic map of the Cuale mining district and limited drill core information. The approximate extents of the ore zones are projected onto the cross-section plane. Traces of the cross sections are indicated in Figure 3.
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144
RHYOLITE-HOSTED VMS OF THE CUALE, GUERRERO TERRANE, MEXICO
and Rodriguez, 1991), and the 3-Mt Rey de Plata deposit
(Miranda-Gasca et al., 2001). All of these are located in the
southern Guerrero terrane (Fig. 1) and hosted by metamorphosed bimodal calc-alkaline volcanic rocks of Upper Jurassic
to Lower Cretaceous age. The 83-Mt San Nicolas deposit in
the eastern Guerrero terrane (Johnson et al., 1999, 2000) is
the largest known VMS deposit of Mexico and is hosted by a
bimodal sequence of tholeiitic volcanic and volcaniclastic
rocks. Massive sulfide deposits of the western Guerreo terrane are smaller and include the ~6-Mt barite-rich Zn-Ag deposit of La Minita, near Colima (De la Campa, 1991), and the
Cuale deposits, as well as the small Bramador and Amaltea
districts 10 to 12 km to the south and southeast of Cuale
(Figs. 1, 2).
The Guerrero terrane is composed of mostly submarine
volcanic and volcaniclastic rocks of Jurassic and Cretaceous
age, interbedded with shallow marine limestones, and siliciclastic sedimentary rocks. These rocks were deposited unconformably on a Permian(?)-Triassic to Early Jurassic basement
of deformed deep marine siliciclastic sequences (e.g., Campa
and Coney, 1983; Centeno-Garcia et al., 1993, 2003).
Large volumes of calc-alkaline magmas intruded the Guerrero terrane after accretion to the continent in the Late Cretaceous to Eocene (Schaaf et al., 1995), whereas Tertiary to
recent volcanic rocks emplaced in the continental arcs of the
Sierra Madre Occidental and Sierra Madre del Sur, as well as
the Trans-Mexican volcanic belt, cover the Mesozoic rocks
(Morán-Zenteno et al., 1999). These Cretaceous to recent igneous rocks leave only isolated exposures of the Mesozoic and
older rocks, particularly in the northern and eastern parts of
the Guerrero terrane.
Terrane subdivisions
Five subterranes have been indentified by Centeno-Garcia
et al. (2003; Fig. 1).
The Teloloapan subterrane, host to the Campo Morado, Tizapa, and Rey de Plata VMS districts, occupies the southeastern part of the Guerrero terrane, which consists mostly of
shallow submarine, Lower Cretaceous basaltic to andesitic
lavas with subordinate limestone intercalations. The basement is unknown. These rocks are strongly deformed and
metamorphosed to lower greenschist facies assemblages. The
Teloloapan subterrane was thrusted eastward over Lower to
Upper Cretaceous platform carbonates deposited on the adjacent Mixteco terrane.
The Arcelia subterrane borders the Teoloapan subterrane
to the northwest and is an intensely deformed sliver of basalt
and 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 reported
from this subterrane.
The Zihuatanejo subterrane is the largest subterrane, occupying much of the central Mexican Pacific coastal region. It
consists of Upper Jurassic to Cretaceous shallow marine limestones and marine to subaerial basaltic to rhyolitic volcanic
rocks, overlying a Triassic basement, which locally is intruded
by Jurassic granitoids. Some of the best studied parts of the
Guerrero terrane lay in this subterrane. It is discussed in
more detail below, as it hosts the Cuale district and, in its
eastern extensions, the large San Nicolas deposit.
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145
The Guanajuato subterrane constitutes the eastern part of
the Guerrero terrane (Centeno-García and Silva-Romo, 1997)
and has similarities to the Arcelia terrane, consisting of primitive arc basalts and deep marine sedimentary rocks. It is, however, poorly exposed and therefore not well understood. The
potential of the Guanajuato terrane to host VMS mineralization has only recently been revealed by the discovery of VMS
mineralization 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 northwestern extension of the Guerrero terrane along the Pacific
coast and occupies much of the state of Sinaloa. It is thought
to consist of Cretaceous volcanic and volcaniclastic rocks deposited on a Paleozoic basement. However, its stratigraphy
and its contact relationships with the other parts of the Guerrero and surrounding terranes have not been studied in detail. No significant massive sulfide mineralization has been reported from the San José de Gracia subterrane.
Geology of the Zihuatanejo Subterrane
The Zihuatanejo subterrane occupies the Pacific coastal portion of Mexico from Puerto Vallarta to Zihuatanejo and extends
several hundred kilometers inland (Fig. 1). According to Centeno-García et al. (2003), it includes the Huetamo subterrane,
which was considered a separate tectonostratigraphic domain
by previous workers (e.g., Centeno-García et al., 1993).
The Zihuatanejo subterrane has been studied in most detail
some 300 km southeast of Cuale, near the city of Zihuatanejo
(e.g., Centeno-Garcia et al., 1993; Freydier et al., 1997; Mendoza and Suastegui, 2000; Talavera-Mendoza, 2000). The
stratigraphy defined by these workers is characterized by a
late Paleozoic and Triassic basement, consisting of weakly
metamorphosed, strongly deformed terrigenous clastic sedimentary rocks with minor basalts (Arteaga Complex: Centeno-Garcia et al., 1993; Fig. 1). These basement rocks are intruded by calc-alkaline, I-type granitoid rocks to which a Late
Jurassic age of ~160 Ma has been assigned (Tumbiscatío and
Macías intrusions: Centeno-García et al., 2003; Petróleos
Mexicanos (PEMEX) unpub. company reports). The Arteaga
Complex is overlain by the weakly deformed Upper Jurassic
to Lower Cretaceous Zihuatanejo sequence of Mendoza and
Suastegui (2000), consisting of arc-related volcanic and volcaniclastic rocks, ranging from andesitic to rhyolitic in composition. An Early Cretaceous subduction-related mélange,
spatially associated with the largely undeformed Zihuatanejo
sequence, is recognized as the Las Ollas complex (TalaveraMendoza, 2000; Fig. 1). The Early Cretaceous volcanic and
volcaniclastic sequences are capped by Albian reefal limestones 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 the
basis of overall stratigraphic similarities (e.g., Hall and GomezTorres, 2000a); however, our new observations (see below) lead
to a significant refinement of this regional comparison.
Geologic Setting of the Cuale District
The oldest unit at Cuale comprises pelitic schists, intercalated
with chloritic and sericitic schists and meta-arkoses and crops
out to the west of the Cuale mining district (Figs. 2, 3). This
package is gently folded and metamorphosed to subgreenschist
145
146
BISSIG ET AL.
facies assemblages. The base of the unit is not exposed, but
the overall thickness is thought to be at least 800 m (Berrocal
and Querol, 1991). It is separated from the overlying volcanic
rocks by an angular unconformity and is locally intruded by
hypabyssal rhyolite (Macomber, 1962). Although the age of
these pelitic schists is not directly constrained, a minimum
Early to Middle Jurassic age is given by the U-Pb dates obtained for the overlying Cuale volcanic sequence (see below).
However, the pelitic schists may well be as old as late Paleozoic to Triassic and can, on the basis of similar lithology and
stratigraphic context, be correlated with the Permian to Triassic Arteaga Complex situated some 300 km to the southeast
(Centeno-Garcia et al., 1993).
The largely undeformed and unmetamorphosed volcanic and
associated volcaniclastic rocks that overlie the pelitic schists of
the basement have been subdivided into several stratigraphic
units based on reconnaissance-scale geologic mapping (Japan
International Cooperation Agency and Metal Mining Agency of
Japan (JICA-MMA: 1986)). Volcanic rocks in the region ranging
from basalt to dacite were reported and assigned to the Lower
Cretaceous to lower Tertiary (JICA-MMA, 1986). The volcanic
rocks of the Cuale mining district, herein termed the Cuale volcanic sequence, are part of this stratigraphic succession. However, the results of this study cast doubt on the age constraints
and compositional variations reported by JICA-MMA (1986).
We interpret the Cuale volcanic sequence to be almost entirely
composed of Late Jurassic rhyolite and rhyolitic volcaniclastic
sedimentary rocks with an overall thickness exceeding 800 m.
Cretaceous and Tertiary volcanic rocks have not been recognized in the Cuale mining district.
The youngest rock unit found in the study area is massive and
unaltered granodiorite that has intruded the Cuale volcanic sequence (Fig. 2). It is assigned to the Cretaceous Puerto Vallarta
batholith, which, in the vicinity of the Cuale district, exhibits
weak peraluminous characteristics (Schaaf et al., 1995).
Stratigraphy of the Cuale Volcanic Sequence
Due to the intense weathering (particularly on ridges) and
locally strong hydrothermal alteration, distinguishing minor
textural differences among the felsic rocks of the Cuale volcanic sequence is difficult. However, the coherent rhyolite
units can be reasonably well defined on the basis of quartzphenocryst content, flow banding, devitrification textures,
and crosscutting relationships. A lithologic map and cross sections are presented in Figures 3 and 4, respectively. The district-scale map forms the basis for the simplified stratigraphic
map (Fig. 5) and schematic stratigraphic column (Fig. 6).
The stratigraphy of the Cuale volcanic sequence has been
subdivided, in relationship to the massive sulfide mineralization and alteration, into a footwall, ore horizon, and late intrusive phase, and is described below. The stratigraphic units
described herein have been deposited in an environment with
significant relief, and individual lithologic horizons are generally of limited lateral extent.
Footwall
The footwall stratigraphy exceeds 400 m in thickness and is
dominated by quartz and feldspar phyric rhyolite flows and
cryptodomes, commonly enveloped by large volumes of
monomictic, commonly matrix-supported, volcanic breccias
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interpreted as carpace or flow breccias and hyaloclastites (Fig.
7). Its outcrop extent includes the topographically low-lying
areas to the south and southeast of Naricero and much of the
western and northwestern flank of Cerro Caracol (Fig. 5). The
rhyolite flows rarely are massive but normally exhibit flow
banding, spherulitic devitrification, amygdules, and lithophysae (Fig. 7). They contain 5 to 8 vol percent slightly embayed quartz phenocrysts that are up to 3 mm in diameter
and, where not obliterated by alteration, similar amounts of
euhedral feldspars. The outcrop extent and accessibility does
not permit detailed mapping of lateral variations in individual
flows and their geometric relationships to the brecciated rocks
nor does it allow identification of individual extrusive centers.
Coarse, polymictic volcaniclastic breccias (Fig. 7) to finer
grained conglomerates are interbedded with the volcanic
rocks of the sequence. Clasts are all derived from the Cuale
rhyolitic sequence but may be from flows as well as from previously deposited volcaniclastic rocks. The coarse polymictic
volcaniclastic breccia deposits are commonly matrix supported, with angular clasts of up to 30 cm in diameter, and are
generally not graded nor welded (Fig. 7). Thus, the footwall
sequence is thought to have been extruded subaqueously and,
at least partly, in an explosive manner (hyaloclastites). Parts of
the volcanic rocks have been reworked into volcaniclastic
breccias and conglomerate horizons.
The overall stratigraphy of the footwall is dominated by
proximal volcanic deposits, such as quartz-feldspar phyric
rhyolite flows, monomictic volcanic breccias (hyaloclastites),
and unstratified, coarse polymicitic volcaniclastic breccias, interpreted as debris-flow deposits. Stratified volcaniclastic
sedimentary rocks are relatively subordinate, and finegrained silt and mudstone are absent.
Ore horizon
Quartz and feldspar phyric to aphyric rhyolite forms massive bodies with no internal structure and units with flow
banding, spherulites developed in banded arrays, and rare
horizons or lenses of hyaloclastite. This series of textures and
structures is interpreted as representing flow and/or dome
complexes using the criteria of McPhie et al. (1993).
These flow and/or dome complexes crop out in the Cerro
Caracol and Coloradita area, whereas the central portion of
the district (Figs. 3–5) is dominated by volcaniclastic sedimentary rocks ranging from conglomerate to sandstones, interbedded with rhyolitic tuffs and lenses of black argillite.
The general absence of fine-grained sedimentary rocks at
Cerro Caracol and Coloradita, combined with the relatively
abundant rhyolite flows, suggests that these areas may represent paleotopographic highs bounding central sedimentary
basins. Syndepositional tectonic activity at the western
boundary of these central basins is inferred from soft-sediment deformation, such as east- southeast-vergent slump
folding 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 exceeding
1 m in diameter was deposited directly over shale and siltstone, which themselves overlie massive quartz-feldspar
phyric rhyolite similar to the boulders (Fig. 8B). Presumably,
the boulder breccia represents mass wasting related to syndepositional normal faulting.
146
147
RHYOLITE-HOSTED VMS OF THE CUALE, GUERRERO TERRANE, MEXICO
485500
2255000
Stratigraphic map of the
Cuale Mining District
N
ult
Fa
rieta
La P
Cerro Cantón
La Prieta/Rubí
Pa
so
Ca
rac
ol
Fa
ul t
Coloradita
Chivos-de Arriba
-de Abajo
Socorredora
Corazón
Jesús-Maria
Cerro Caracol
Refugio
Naricero
Cerro
Descumbridora
San Nicolas
Minas de Oro/
Grandeza
0.5
1km
Ore horizon
Shale to sandstone
Rhyolitic pyroclastic/volcaniclastic rocks
Rhyolite flows and domes
Upper Jurassic (CVS)
Foot wall
Late Intrusive units
Rhyolitic pyroclastic/volcaniclastic rocks
Andesite dikes
Rhyolite flows and domes
Intrusive rhyolite
Cretaceous
Granodiorite
Triassic
Slate
Major Faults (inferred)
Normal
Reverse
Abandoned mine
Mountain
Upper Jurassic
(Oxfordian)
Cret.
FIG. 5. Stratigraphic map of the Cuale mining district, based on the lithologic map of Figure 3. Locations mentioned in
the text are given. Coordinates are given as UTM zone 13, NAD83.
Puerto Vallarta Batholith
y
Unconformit
Granodiorite
Intrusive units (U/Pb age ca. 152-155 Ma)
Epithermal veins/stockwork
Andesite dikes
Rhyolite intrusions and domes
Triassic
Middle Jurassic (Callovian)
Ore horizon (U/Pb age ca. 152-155 Ma)
Massive sulfide/
stockwork mineralization
Rhyolite tuffs
Shale to siltstone
Volcaniclastic sandstones
to conglomerates
Rhyolite flows and domes
Footwall (U/Pb age 157.2 +/- 0.5 Ma)
Pyroclastic and
hyaloclastitic rhyolite
on
Unc formi
ty
Rhyolite flows and domes
Slate, chlorite-sericite schist
FIG. 6. Schematic stratigraphic column for the Cuale volcanic sequence, as interpreted from mapping carried out in this
study (see Figs. 3–5). Mineralization events are indicated in their respective time-stratigraphic position.
0361-0128/98/000/000-00 $6.00
147
148
BISSIG ET AL.
rocks interbedded with rhyolitic tuffs and volcaniclastic conglomerates, whereas rhyolite flows and hyaloclastic breccias
are volumetrically subordinate compared to the footwall. In
contrast to the footwall sequence, the tuffaceous rocks of the
ore horizon include welded horizons (Fig. 8C) and, immediately below Mina Jesús María (Fig. 3), tuffaceous strata containing accretionary lapilli (Fig. 8D). Although welded rocks
may also be deposited at considerable water depth (e.g., Kokelaar and Busby, 1992), the presence of both welding and accretionary lapilli (e.g., Brown et al., 2002) is taken as an indication of shallow water depths at the time of deposition.
A
The late intrusive phase
The footwall and ore horizon sequences are intruded by
dikes, sills, and small hypabyssal bodies of quartz and feldspar
phyric rhyolites. In contrast to the flows of the footwall and
ore horizon, the intrusive rhyolites are more massive and lack
spherulitic devitrification textures, lithophysae, and amygdules, although flow banding is common in some dikes and
near the summit of Cerro Cantón. The intrusive rhyolites are
slightly less intensely altered compared to the footwall and
ore horizon sequence. Moderate quartz-sericite ± pyrite ±
chlorite alteration dominates and chloritized mafic phenocrysts (most likely biotite pseudomorphs) are preserved locally. Columnar jointing in some dikes indicates intrusion into
cold wall rock (Fig. 9). The dikes have a variable but predominantly northwesterly strike. This late phase of rhyolitic
magmatism probably consists of several discrete intrusive
pulses but cannot be subdivided further due to the limited
petrographic variability.
The latest phase of magmatism of the Cuale volcanic sequence 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-pilotaxitic texture in a fine-grained matrix, which is weakly altered to chlorite-rich assemblages. Their age is uncertain but
bracketed by the intrusive rhyolite (Late Jurassic; see below)
and the granodiorite of the Puerto Vallarta batholith (Late
Cretaceous).
20 cm
B
C
r
r
FIG. 7. Field photographs of typical outcrops from the footwall and lowermost ore horizon of the Cuale volcanic sequence. All samples shown are intensely chloritized. A. Vesicular rhyolite flow from the immediate footwall of
the San Nicolas deposit. Sample TB-389A, dated at 152.5 ± 1.5 Ma, was taken
from this outcrop (pencil for scale; see text for discussion). B. Coarse, polymictic matrix-supported volcaniclastic breccia from the south flank of Cerro Caracol (coord. 486.677/2251.881, hammer handle with 10-cm marks for scale). C.
Matrix-supported monomictic breccia, interpreted to be of hyaloclastic origin
since many fragments have darker and finer grained rims (r). Near south-west
ridge of Cerro Caracol (coord. 486.493/2251.760, pencil for scale).
The rhyolites of the ore horizon differ from those of the
footwall sequence in that some flows contain only minor
(<<5%) and small (<1 mm) quartz phenocrysts. These aphyric
rhyolites are intercalated with quartz and feldspar phyric rhyolites similar to those in the footwall. The ore horizon is characterized by relatively abundant fine grained sedimentary
0361-0128/98/000/000-00 $6.00
Geochronology
A total of five samples of rhyolitic flows, tuffs, and highlevel intrusions, distributed over the entire Cuale volcanic sequence, were dated using conventional U-Pb zircon isotope
dilution thermal ionization mass spectrometry (ID-TIMS) at
the Pacific Centre for Isotopic and Geochemical Research
(PCIGR) facility at the University of British Columbia. Zircons were generally small and inclusion rich which, in some
cases, resulted in relatively imprecise age determinations.
The methodology for zircon grain selection, abrasion, dissolution, geochemical preparation, and mass spectrometry are described by Mortensen et al. (1995). All zircon fractions were
air abraded (Krogh, 1982) prior to dissolution to minimize the
effects of postcrystallization Pb loss. Procedural blanks for Pb
and U were 2 and 1 pg, respectively. Analytical data are listed
in Table 1 and are shown on conventional U-Pb concordia
plots in Figure 10. Errors attached to individual analyses were
calculated using the numerical error propagation method of
Roddick (1987). Decay constants used are those recommended by Steiger and Jäger (1977) and compositions for ini-
148
RHYOLITE-HOSTED VMS OF THE CUALE, GUERRERO TERRANE, MEXICO
149
B
A
C
D
5 cm
FIG. 8. Field photographs of typical outcrops from the ore horizon of the Cuale volcanic sequence. A and B show evidence
for 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, handle ~50 cm). B. Coarse boulder breccia (b) deposited directly on laminated siltstones (s). The inset shows detail of the contact relationship. Note the siltstone beds deformed due to the impact of the boulder (coord. 488.004/2252.992, hammer, ~50
cm, 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 perpendicular to the dike walls. Note also the color contrast between the intensely chlorite-pyrite-quartz altered wall rock and the only moderately quartz-sericite ±
chlorite altered dike.
0361-0128/98/000/000-00 $6.00
tial common Pb were taken from the model of Stacey and
Kramer (1975). All errors are given at the 2σ level. Results for
the five samples are discussed briefly below, sample locations
are given within parentheses as coordinates UTM zone 13,
NAD83.
Sample TB-368A (486.680/2253.326): This sample was a
hyaloclastic, quartz and plagioclase phyric rhyolite from the
footwall to the VMS bodies, north of Cerro Caracol. Zircon
recovered from this sample comprised pale-brown, stubby,
square prisms with abundant clear bubble- and rod-shaped
inclusions. Four strongly abraded fractions were analyzed
(Table 1). Two fractions (A and D, Fig. 10A) yielded overlapping concordant analyses with a total range of 206Pb/238U ages
of 157.2 ± 0.5 Ma, which is considered to give the crystallization age of the sample. Two other fractions are slightly discordant with older 207Pb/206Pb ages. The data form a short
array with a poorly defined calculated upper intercept age of
~1.62 Ga. The data are interpreted to indicate the presence
of a minor inherited zircon component in the sample with an
average age of ~1.62 Ga.
149
150
BISSIG ET AL.
TABLE 1. U-Pb Analytical Data
206Pb/
204Pb3
U
Pb2
(ppm) (ppm) (meas.)
Total
common
Pb (pg)
Sample/
description1
Wt
(mg)
%
206Pb/238U4
207Pb/235U4
207Pb/206Pb4
208Pb2
(± % 1σ )
(± % 1σ )
(± % 1σ )
A: N2, 104-134
B: N2, 104-134
C: N2, 104-134
D: N2, 104-134
0.008
0.014
0.007
0.008
1278
1381
697
1506
32.0
34.8
17.8
38.0
3090
2395
2224
4233
5
12
3
4
A: N2,104-134
B: N2,104-134
D: N2,104-134
E: N2,104-134
0.017
0.014
0.011
0.008
1073
680
920
854
26.4
18.2
23.7
23.0
404
63
275
36
A: N2,74-104
B: N2,74-104
C: N2,74-104
D: N2,74-104
E: N2,74-104
0.007
0.005
0.004
0.006
0.007
551
622
571
444
390
16.0
16.5
34.9
10.9
9.6
A: N2,+74
D: N2,+74
E: N2,+74
F: N2,+74
0.033
0.038
0.045
0.046
160
290
250
203
A: N5,104-134
C: N5,104-134
E: N5,104-134
0.011
0.018
0.025
348
342
205
206Pb/238U
age 207Pb/206Pb age
(Ma; ± % 2σ) (Ma; ± % 2σ )
Sample TB-368A
11.0
0.02465(0.13)
11.5
0.02472(0.11)
12.2
0.02485(0.16)
11.7
0.02465(0.16)
0.1672(0.25)
0.1680(0.21)
0.1703(0.36)
0.1675(0.26)
0.04921(0.19)
0.04931(0.14)
0.04971(0.30)
0.04930(0.21)
157.0(0.4)
157.4(0.3)
158.2(0.5)
157.0(0.5)
158.0(8.9)
162.4(6.6)
181.2(14.2)
162.3(9.9)
70
327
59
592
Sample TB-369A
11.5
0.02418(0.30)
15.0
0.02515(1.21)
14.0
0.02452(0.24)
15.7
0.02514(3.03)
0.1635(0.83)
0.1807(4.00)
0.1679(0.79)
0.1765(10.1)
0.04905(0.67)
0.05212(3.28)
0.04968(0.63)
0.05092(8.33)
154.0(0.9)
160.1(3.8)
156.1(0.7)
160.0(9.6)
150.1(31.2)
290.5(149)
179.9(29.6)
237.2(387)
923
2097
3354
728
824
7
2
2
5
5
Sample TB-389A
11.6
0.02834(0.89)
16.3
0.02461(0.20)
14.8
0.05614(0.21)
11.9
0.02392(0.54)
12/1
0.02388(0.40)
0.2089(4.20)
0.1701(0.50)
0.6171(0.29)
0.1602(4.02)
0.1620(1.90)
0.05347(3.85)
0.05011(0.45)
0.07972(0.22)
0.04859(3.76)
0.04920(1.74)
180.1(3.2)
156.8(0.6)
352.1(1.4)
152.4(1.6)
152.1(1.2)
348.7(175)
200.2(20.8)
1190.2(8.8)
128.1(177)
157.3(81.0)
4.0
7.4
6.5
5.3
160
2020
1758
2134
56
8
10
7
Sample TB-241B
11.8
0.02442(0.41)
13.5
0.02454(0.11)
13.9
0.02462(0.10)
15.4
0.02457(0.32)
0.1660(1.36)
0.1664(0.27)
0.1668(0.56)
0.1668(0.69)
0.04930(1.12)
0.04918(0.20)
0.04914(0.54)
0.04925(0.61)
155.6(1.3)
156.3(0.4)
156.8(0.3)
156.5(1.0)
162.2(52.1)
156.6(9.4)
154.4925.3)
160.0(28.5)
9.4
9.2
5.5
92
455
143
69
22
63
Sample TB-57C
16.4
0.02500(0.74)
16.4
0.02489(0.17)
16.2
0.02483(0.38)
0.1705(2.74)
0.1704(0.58)
0.1691(2.22)
0.04949(2.32)
0.04965(0.50)
0.04939(2.05)
159.1(2.3)
158.5(0.6)
158.1(1.2)
171.0(108)
178.7(22.7)
166.4(95.8)
1 N2,
N5 = nonmagnetic at n degrees side slope on Frantz magnetic separator; grain size given in microns
Pb; corrected for blank, initial common Pb, and spike
3 Corrected for spike and fractionation
4 Corrected for blank Pb and U, and common Pb
2 Radiogenic
Sample TB-369A (488.569/2253.978): This sample was
from an aphyric rhyolite flow from the footwall of the La Prieta fault, collected in the Coloradita open pit, at approximately the ore horizon in this part of the Cuale district. Zircons were pale-pink stubby prisms with morphologies similar
to those in the previous sample. Four abraded fractions were
analyzed, and the results also indicate the presence of a minor
older inherited zircon component (Table 1, Fig. 10B). The
best estimate for the crystallization age of the sample is given
by fraction A with a 206Pb/238U age of 154.0 ± 0.9 Ma. The
other three fractions are either imprecise due to the presence
of high levels of common Pb (from abundant inclusions) or
fall slightly to the right of concordia, indicating the presence
of inherited zircon.
Sample TB-389A (487.699/2252.037): This sample was
from a vesicular, quartz and feldspar phyric rhyolite flow
from the immediate footwall of the San Nicolas deposit. The
flow belongs to the lowermost ore horizon. Only a small
amount of relatively fine grained zircon was recovered. Five
fractions of lightly abraded zircon were analyzed (Table 1,
Fig. 10C-D). A more substantial amount of inherited zircon is
present in this sample as indicated by the much older
207Pb/206Pb age of 1190 Ma for fraction C. Two fractions (D
and E) give overlapping concordant analyses with a total
range of 206Pb/238U ages of 152.5 ± 1.5 Ma. This volcanic unit
0361-0128/98/000/000-00 $6.00
is intruded by a quartz-plagioclase phyric rhyolite dike, which
yields a slightly older crystallization age (sample TB241b; see
discussion below). The data are interpreted to indicate that
these two fractions have experienced postcrystallization Pbloss effects that were not completely removed by the light
abrasion. The 152.5 ± 1.5 Ma age does provide a minimum
crystallization age for the sample. The other three fractions
contain older inherited zircon components, and a calculated
upper intercept age of 1.59 Ga gives the average age of the inherited component.
Sample TB-241B (488.122/2253.341): This sample was
from a quartz-plagioclase phyric rhyolite dike that intrudes
the rhyolite flow (sample TB389A) in the immediate footwall
of the San Nicolas deposit. Zircons from the sample form colorless elongate prisms. Four strongly abraded fractions were
analyzed (Table 1, Fig. 10E). All yielded concordant analyses
with a total range of 206Pb/238U ages of 155.9 ± 1.6 Ma, which
is considered to give the crystallization age of the sample.
Sample TB-57C (488.527/2254.049): This sample is from a
quartz and feldspar phyric crystal tuff from the hanging wall
of the La Prieta fault taken in the Coloradita open pit. Three
strongly 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 other
age determinations discussed above and the significance of
150
151
RHYOLITE-HOSTED VMS OF THE CUALE, GUERRERO TERRANE, MEXICO
A
TB-368A: Footwall rhyolite, 157.2 +/- 0.5 Ma
Aphyric rhyolite, 180
B oreTB-369A:
horizon, 154.0 +/- 0.9 Ma
238
Pb/ U
0.0285
to 1.62 Ga
160
170
160
206
206
Pb/ 238U
0.026
0.025
0.0245
A
A
150
C
B
D
E
B
D
156
140
0.024
0.165
C
0.0205
0.125
0.171
TB-389A: lowermost
ore horizon rhyolite
D
to 1.59 Ga
350
0.055
162
156
B
206
250
206
168
0.026
300
0.035
0.205
TB-389A (expanded):
152.5 +/- 1.5 Ma
Pb/ 238U
Pb/ 238U
C
0.165
E
0.024
200
D
150
B
150
D+E
A
144
0.022
0.14
0.5
E
TB-241B: Intrusive rhyolite (dyke):
155.9 +/- 1.6 Ma
0.026
F
Pb/ 238U
160
206
F
E
156
D
0.0245
0.16
0.18
TB-57C: ore horizon rhyolite tuff:
159.2 +/- 2.2 Ma (Inherited
zircon component)
160
A
C
E
206
Pb/ 238U
0.015
0.1
0.0255
A
0.024
150
152
0.0235
0.160
0.166
207
0.022
0.15
0.172
235
0.17
207
Pb/ U
0.19
235
Pb/ U
FIG. 10. U-Pb concordia diagrams for felsic volcanic and high-level intrusive rocks in the Cuale district.
the results is not certain. Fraction C falls slightly to the right
of concordia, indicating the presence of a minor older inherited zircon component, and the other two fractions are too
imprecise to evaluate whether a minor inherited component
was also present in these analyses. In view of the stratigraphic
position of this sample in the hanging wall of the ore and the
0361-0128/98/000/000-00 $6.00
relatively robust crystallization ages that we have obtained
from stratigraphically lower units, we tentatively conclude
that all three zircon fractions contained minor inherited zircon components or represent reworked zircons. An unequivocal crystallization age, therefore, cannot be assigned to the
sample from our data.
151
152
BISSIG ET AL.
Characteristics of the Sulfide Deposits at Cuale
Eleven orebodies, ranging from 20,000 to 783,000 metric
tons (t) of polymetallic ore (Table 2) along with a number of
smaller occurrences were in production in the 1980s (Hall
and Gomez-Torres, 2000a). Most of this ore is mined out and
the following descriptions are summarized from Berrocal and
Querol (1991) and Hall and Gomez-Torres (2000a). The style
of mineralization and the relative abundance of the metals
vary significantly between the orebodies.
The Naricero, Socorredora, La Prieta-Rubí, San Nicolas,
and Refugio deposits (Table 2), which were the source of
about half of the mined ore in the district, are hosted by or
spatially associated with black argillites. The massive sulfide
ore, compared to the other deposits of the district, is characterized by high average silver grades up to several hundreds
of parts per million. The silver is contained within fine-grained
galena and sphalerite that is locally brecciated (e.g., Naricero)
or laminated (e.g., Socorredora). As the Pb-Ag–rich orebodies
locally exhibit sedimentary textures (lamination) we interpret
them to be in a distal setting, possibly transported into anoxic
basins adjacent to proximal sulfide mounds.
The Coloradita, Chivos de Arriba, and Chivos de Abajo deposits are considered to be proximal to hydrothermal upflow
zones. These deposits contain up to 3 g/t Au and 0.4 to 1.5
percent Cu but are relatively low in Ag and Pb compared to
the more distal deposits. The precious and base metals are
partly hosted by pyrite- and chalcopyrite-rich stockwork
zones that are hydrothermally altered. The stockwork zones
are in turn overlain by massive sulfide bodies (mostly pyrite
with chalcopyrite that grade laterally and vertically into sphalerite- and galena-rich zones). Host rock for the Coloradita
deposit is aphyric rhyolite.
Minas de Oro/Grandeza consists of stockwork mineralization and gold grades of 1.9 ppm, but only 0.2 wt percent Cu,
which distinguishes it from other gold-rich orebodies of the
district where Cu is more abundant. The described ore mineralogy at Minas de Oro/Grandeza (Macias and Solis, 1985) is
essentially sphalerite, pyrite, and lesser amounts of galena
and freibergite [(Ag,Cu,Fe)12(Sb,As)4S13], whereas gangue is
mainly composed of quartz and calcite. Limited fluid inclusion data on liquid-rich inclusions in quartz and sphalerite
(Macias and Solis, 1985) support the interpretation of Minas
de Oro/Grandeza being similar to low-sulfidation epithermal
deposits (i.e., homogenization temperatures between 220°
and 265°C, salinities around 5 wt % NaCl equiv, together with
the estimated average density of 0.86 g/cm3) and having
formed at 400- to 700-m depth below surface.
An approximately 4-m-wide quartz vein with epithermal
textural characteristics (open-space filling and local colloform
texture) is present on the summit of Cerro Descumbridora.
This vein was mined historically (no grade and tonnage data
available) and crops out northwest of and along strike from
the stockwork mineralization at Minas de Oro/Grandeza.
Both the vein and the stockwork mineralization postdate the
main VMS deposits of Cuale since they are hosted by the
post-VMS intrusive rhyolite. Further evidence for a late mineralization event with epithermal characteristics comes from
quartz-carbonate ± adularia vein material found in the float of
the northwestern ridge of Cerro Cantón, as well as from
stream-sediment geochemistry near Santa Rita (a small
prospect ~3 km north of Cerro Cantón, outside the study area
and not shown in Fig. 2) where anomalous arsenic and mercury concentrations have been reported (International Croesus Ventures Corp. unpub. reports).
Hydrothermal Alteration
Moderate to intense hydrothermal alteration is ubiquitous
and affects all rocks of the Cuale volcanic sequence. The footwall alteration associated with the deposits considered proximal is characterized by intense quartz-sericite-pyrite alteration, laterally grading into more chlorite-rich assemblages.
However, the alteration in the footwall of Socorredora and
Naricero also exhibits moderate to intense quartz-sericitepyrite alteration, which indicates that part of the ore of these
deposits may have been contributed by a low-temperature
hydrothermal fluid and not all mineralization is the result of
transport and redeposition in anoxic basins. The black
argillite which host part of the ore is locally slightly carbonaceous or cut by calcite veinlets, but it is unclear whether or
not this carbonate is related to hydrothermal alteration.
The late intrusive rhyolites are affected by less intense alteration (Fig. 9). The rocks are moderately silicified with
Table 2. The Ore Deposits of the Cuale District
Mine
Naricero
Mina de Oro/Grandeza
Socorredora
Coloradita
Las Talpas
La Prieta-Rubí
Chivos de Abajo
San Nicolas
Jesus Maria
Refugio
Chivos de Arriba
Eight other small deposits
Total
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Metric tons
Au (g/t)
Ag (g/t)
Pb (%)
Zn (%)
Cu (%)
782,544
756,661
200,492
170,055
141,425
113,335
85,771
79,965
46,751
34,569
23,588
0.34
1.89
0.13
0.66
0.34
0.73
1.08
0.19
0.06
0.14
2.79
157
22
187
85
24
226
179
121
109
156
70
1.05
1.41
1.89
1.99
0.65
3.8
1.48
1.57
1.85
0.89
0.85
2.85
2.35
6.93
6.51
1.91
9.24
4.71
3.18
3.31
1.95
2.18
0.06
0.2
0.16
0.37
0.24
0.32
1.54
0.13
0.09
0.1
0.74
0.83
103
1.03
3.22
0.23
39,199
2,474,355
152
RHYOLITE-HOSTED VMS OF THE CUALE, GUERRERO TERRANE, MEXICO
modest amounts of sericite. Disseminated pyrite is absent or
much less abundant than in the lower parts of the stratigraphy. Ferromagnesian minerals (most likely biotite) are preserved as chloritized relicts only in the rhyolites of the late intrusive phase.
Structural Evolution of the Cuale District
The major control on the erosional landforms at Cuale is
the type and degree of alteration rather than fault traces, and
air-photo interpretation is therefore of limited use for structural interpretations. The following is a summary based on
limited mapping.
Synvolcanic faulting (D1) and subsequent erosion is evidenced by coarse volcaniclastic rocks commonly found on the
southern and eastern flanks of Cerro Caracol. Slump folds
(Fig. 8A) and boulders deposited directly on fine-grained sediment (Fig. 8B) may be taken as further evidence for deformation during the deposition of the Cuale volcanic sequence.
Five out of six measurements of fold axis in slump folds within
fine-grained sedimentary rocks plunge at shallow angles to
the north or south, an orientation consistent with slumping
induced by steepening relief due to displacements along
north-south–striking normal faults. Synvolcanic structures
have not been observed directly, but two roughly northsouth–striking cleavage zones have been recognized at San
Nicolas and 200 m northwest of Jesús-Maria. These zones
may represent zones of structural weaknesses that controlled
fluid flow.
The late andesite dikes and many rhyolite dikes cutting all
the previous units within the volcanic pile have an east-west to
southeast-northwest strike direction (Fig. 3). This predominant
dike orientation, together with the inferred north-south–striking structures described above, is consistent with emplacement
of the rhyolite in tension fractures associated with sinistral
transtensional rifting along north-south– to north-northwest–
south-southeast–striking faults. From this interpretation of the
Upper Jurassic tectonic environment, Cuale is in agreement
with the Upper Jurassic subduction geometry suggested by
Dickinson and Lawton (2001) as well as the parallel Upper
Jurassic sinistral transtensional arc that has been documented
for northwestern Mexico (Busby et al., 2005).
A northeast-dipping district-scale fault, the Paso Caracol
fault (Fig. 3), has been inferred from limited outcrop exposure at Paso Caracol as well as from map interpretation along
strike to the northwest and southeast. Although no unequivocal kinematic indicators have been recognized, we interpret
the offset to be reverse (D2) since smaller scale northwestsoutheast–striking reverse faults have been observed
throughout the district. Bedding planes of fine-grained sedimentary units of the ore horizon, where not affected by
synsedimentary slump folding, have dips varying from 0° to
35° at variable strikes. It is thus unclear whether contractional
deformation resulted in gentle folding in the district or
whether the variation in bedding plane dips may be attributed
solely to the intrusion of the late rhyolite. D2 deformation
predates the later La Prieta fault as the latter appears to terminate at a structure parallel to the Paso Caracol fault northeast of Socorredora (Figs. 3, 5).
Late normal faults are important as they locally separate
the Cuale volcanic sequence from older units, but normal
0361-0128/98/000/000-00 $6.00
153
faults with offsets of typically less than 30 m are observed
throughout the district. However, in the case of the La Prieta
fault (Figs. 3, 5) a minimum normal offset of 200 m has been
inferred from drill log records. The fault crops out in the Coloradita open pit as a ~30-m-thick fault zone filled with gouge
and brecciated rocks. The La Prieta fault dips ~45° to the
north-northwest and steepens at depth (Macomber, 1962). A
normal offset is further suggested by parallel smaller scale
fractures in the vicinity of the fault trace which have a demonstrably normal offset. The massive sulfide mineralization at
La Prieta, Coloradita, Chivos de Arriba, and Chivos de Abajo
terminates at the La Prieta fault and thus predates normal slip
on the fault.
Whole-Rock Geochemistry
Fifty samples of rhyolite from the Cuale volcanic sequence
have been analyzed for major and trace elements using inductively coupled plasma atomic emission spectroscopy (ICPAES) for major elements and inductively coupled plasma
mass spectroscopy (ICP-MS) for trace elements. Lithium
metaborate fusion was applied to ensure complete dissolution
of the lithophile elements. The rocks were submitted as a
single batch and analyzed using standard exploration grade
analytical packages at ALS-Chemex laboratories in North
Vancouver, British Columbia, Canada. The complete data set
is available as a digital supplement to this paper at <http://
www.geoscienceworld. org/> or, for members and subscribers, on the SEG website, <http://www.segweb.org>.
All analyzed samples have experienced considerable hydrothermal alteration, precluding the use of mobile elements
for tectonic discrimination purposes. Thus, major elements
were used only to characterize the alteration by means of alteration indices. The Cuale samples, taken over a vertical
range of 800 m from throughout the map area shown in Figure 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 footwall
and ore horizon fall within the range typical for intense chlorite-pyrite ± sericite alteration. In contrast, the samples of
intrusive rhyolite occupy a range from unaltered to sericitealtered rhyolite but do not extend as far into the chloritepyrite–altered field as the stratigraphically lower units. The
late mafic dikes all plot within or close to the field for the least
altered andesite. Thus, the intensity of alteration clearly distinguishes between the footwall and ore horizon versus the
late, post-VMS mineralization intrusive rhyolites.
With the exception of three late-stage andesite dikes, all
rocks have rhyolitic compositions when using the Zr/TiO2 versus 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 compositional range consistent with a rifted-arc environment, but
these widely used diagrams fail to discriminate between the
stratigraphic units of the district. Incompatible trace elements such as Zr, Y, and REE show sufficient variation to distinguish footwall rhyolite from the rhyolite of the ore horizon
and late intrusive rhyolite (Fig. 13). The chondrite normalized
(La/Yb)n versus Ybn tectonic discrimination diagram of Hart
153
154
ep, cc
py, chl
ank
A
1
Com/Pant
Least altered andesite/basalt
80
CCPI
dol
Alt
Dia erat
ion
ge
ne
tic
60
40
Least altered
rhyolite
ser
Rhyodacite/Dacite
TrachyAnd
Andesite
Foidite
Andesite/Basalt
.01
20
Basalt
0
Phonolite
Trachyte
Rhyolite
.1
Zr/TiO2
100
BISSIG ET AL.
AlkBasalt
Ksp
ab 10
30
50
70
90
.01
AI
Andesite
Quartz-plagioclase phyric rhyolite (late intrusive phase)
Aphyric rhyolite (ore-horizon)
Quartz-plagioclase phyric rhyolite (footwall)
.1
Nb/Y 1
10
1000
B
within plate
FIG. 11. Alteration box plot for samples from the Cuale district, after
Large et al. (2001). AI = 100 (K2O + MgO)/(K2O + MgO + Na2O + CaO).
CCPI = 100 (MgO + FeO)/(MgO + FeO + Na2O + K2O). Alteration minerals 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 the
ore horizon rhyolite fall within the range typical for intense chlorite-pyrite ±
sericite alteration, whereas the intrusive rhyolite varies from unaltered to
sericite altered, but does not extend into the field for chlorite-pyrite alteration. The late andesite dikes all fall within or close to the box of least altered
andesites.
Nb (ppm)
100
syn-collisional
volanic arc
An
o
lo
ma
us
rid
ge
10
ocean ridge
1
1
et al. (2004; Fig. 13A), originally proposed by Lesher et al.
(1986), shows that the Cuale volcanic rocks straddle FI and
FII fields, indicating calc-alkaline compositions (Fig. 13A).
The footwall rhyolites, with two exceptions, plot at relatively
low (La/Yb)n values, whereas the rhyolite of the ore horizon
and late intrusive rhyolite have somewhat higher ratios. A
similar relationship can be observed when using the La versus Yb discrimination diagram proposed by Barrett and
MacLean (1999; Fig. 13B); the footwall rhyolites generally
occupy a field transitional between calc-alkaline and tholeiitic
rocks, whereas the rhyolite of the ore horizon and late intrusive rhyolite have overlapping but generally more calc-alkaline 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 the
ore horizon and hanging-wall rhyolite (Fig. 13C). Late andesite dikes have the highest Zr concentrations (176–230
ppm) within the Cuale volcanic sequence. Titanium concentrations 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 to
1.84 wt percent TiO2.
Discussion
Stratigraphy
The footwall sequence is characterized by large volumes of
rhyolite flows and hyaloclastite deposits with subordinate volcaniclastic sedimentary rocks, all of which are interpreted to
0361-0128/98/000/000-00 $6.00
10
Y (ppm) 100
1000
Andesite
Quartz-plagioclase phyric rhyolite (late intrusive phase)
Aphyric rhyolite (ore-horizon)
Quartz-plagioclase phyric rhyolite (footwall)
FIG 12. Discrimination diagrams for rhyolite and andesite of the Cuale
volcanic sequence. A. Zr/TiO2 vs. Nb/Y diagram after Winchester and Floyd
(1977), showing the limited range in composition for the Cuale rhyolites and
the clearly distiguishable andesites. B. Nb vs. Y tectonic discrimination diagram after Pearce et al. (1984). The rocks from Cuale occupy a limited compositional range straddling the border between the “volcanic arc,” “ocean
ridge,” and “anomalous ridge” fields.
have been deposited in a subaqueous setting. The ore horizon, in contrast, is dominated by volcaniclastic sedimentary
rocks ranging from conglomerates to sandstones with intercalated tuffs. Black shale to siltstone horizons within this sequence are the main host for Pb-Zn-Ag–rich massive sulfide
mineralization. Rhyolite flows and domes, commonly aphyric
to weakly quartz-phyric, crop out at Coloradita and at Cerro
Caracol and are separated by the central domain of finegrained volcaniclastic sedimentary rocks (Figs. 3, 14). The
distribution of volcanic and volcanic-sedimentary rocks suggests that Coloradita and Cerro Caracol may have represented topographic highs bordering a central sedimentary
basin (Fig. 14). A general decrease in water depth during the
evolution of the Cuale volcanic sequence is indicated by some
of the tuff layers within the ore horizon that have welded or
accretionary lapilli. Tuffites, locally welded, are common on
154
155
RHYOLITE-HOSTED VMS OF THE CUALE, GUERRERO TERRANE, MEXICO
100
La (ppm)
(La/Yb)n
100
FI
10
50
Calc-alkaline
1
La/Yb
=6
FII
FIIIb
FIIIa
Transitional
3
FIV
Tholeiitic
0.1
0
300
50
Ybn 100
Zr/Y = 7
Calc alkaline
150
0
200
2
3
4
5
Yb (ppm)
6
1
7
8
4.5
Quartz-plagioclase phyric rhyolite
(late intrusive phase)
Transitional
200
Zr (ppm)
Tholeiitic
Aphyric rhyolite (ore-horizon)
2
Quartz-plagioclase phyric rhyolite (footwall)
100
0
10
30
50
70
90
Y (ppm)
FIG. 13. Trace element ratios used to discriminate between different rhyolites from Cuale. A. (La/Yb)n vs. Ybn diagram
with field subdivisions from Lesher et al. (1986), modified by Hart et al. (2004). Chondrite normalization values are from
Nakamura (1974). The Cuale rocks are restricted to the fields FI and FII. Interpretations from Hart et al. (2004) and references therein are as follows: FI = alkaline-calc-alkaline dacites and rhyolites barren of VMS mineralization, FII = calc-alkaline dacites and rhyolites associated to VMS deposits emplaced in rifted continental arc or continental back-arc environments
involving rifted continental crust, FIIIa and FIIIb = felsic rocks of tholeiitic compositions largely associated with Archean
VMS deposits, FIV = felsic rocks of tholeiitic compositions associated with VMS mineralization emplaced in intra-oceanic island arcs. B. La vs. Yb diagram showing the generally less calc-alkaline character for the footwall rhyolites compared to those
from the ore horizon. Subdivisions from Barrett and MacLean (1999). C. Zr vs. Y diagram adapted from Barrett and
MacLean (1994). Note the generally low Zr content of the Cuale rhyolites.
the southern flanks of Cerro Cantón and lie in the hanging
wall of the La Prieta fault, but at a topographic level of <200
m above the mineralization at Coloradita. These tuffites
therefore correspond to the higher portions of the ore horizon stratigraphy and suggest that the area may have become
emergent during the waning stages of volcanic activity.
No significant input of continent-derived siliciclastic sediment is apparent within the Cuale volcano-sedimentary sequence, indicating a position that was isolated from major
continental sediment sources in the Late Jurassic. However, a
minor older inherited zircon component is present in several
of the rhyolite samples (TB-368A, Tb-369A, TB-389A, TB57C), indicating that older rock units, or sediments derived
from them, are present in the subsurface. A continental sediment influx, if present, may also have been obscured by rapid
eruption rates for the Cuale volcanic sequence, an interpretation which is in agreement with the U-Pb age determinations.
0361-0128/98/000/000-00 $6.00
The lack of carbonate sedimentary rocks that would normally
be expected in a shallow marine setting may be interpreted as
further indication for such a rapid eruptive cycle represented
in the footwall and at the ore horizon.
Whole-rock geochemistry
The Cuale rhyolite sequence is remarkably homogeneous
in chemistry. Apart from the late andesitic dikes, which may
not be directly related to the rhyolites as indicated by their
higher Zr concentrations, only subtle geochemical variation
over a vertical stratigraphic interval of more than 800 m is
observed. Similar to the gold-rich Eskay Creek deposit (Barrett and Sherlock, 1996), Ti and Zr concentrations in the rhyolite are low at Cuale. However, Zr, Ti, and with LREE are
slightly enriched in rhyolite at the ore horizon and in late intrusive rhyolite compared to the footwall. This small change
is interpreted as an evolution from transitional tholeiitic to
155
156
BISSIG ET AL.
Cerro Descumbridora
Cerro Cantón
Late intrusive phase
Sea level
Ore horizon phase
Cerro
Caracol
Coloradita
Sea level
Sea level
Footwall phase
W
E
6 km
0 km
Late Jurassic (Oxfordian)
Aphyric rhyolite domes
Quartz plagioclase phyric rhyolite intrusions
and domes
Late epithermal mineralization
Rhyolitic pyroclastic and hyaloclastic rocks
Quartz-plagioclase phyric rhyolite flows
and domes
Massive sulfide
Sulfide stringer zone, stockwork mineralization
Triassic
Black shale to fine sandstones
Volcaniclastic sandstones to conglomerates
Slate (greenschist facies)
FIG. 14. Schematic E-W cross section through the Cuale district in the Late Jurassic. The volcanic sequence evolves from
a submarine to a Late Jurassic, partly subaerial setting. Stage 1 (footwall) is characterized by subaqueous eruption of rhyolite flows, cryptodomes, and hyaloclastites. Stage 2 (ore horizon) is characterized by emplacement of rhyolite domes and
proximal pyroclastic and hyaloclastic rocks near Cerro Caracol and Coloradita. Some of the volcanism was subaerial at this
time. A central basinal domain is characterized by reworked volcaniclastic facies and black shales. Massive sulfide mineralization occurs as sulfide mound and underlying stockwork mineralization associated with aphyric rhyolite domes, as well as
in the central basinal domain, associated with black shale. Stage 3 (late intrusive phase) is characterized by emplacement of
shallow intrusions, dikes, and domes of quartz-feldspar phyric rhyolite. Stockwork and vein mineralization with epithermal
characteristics cuts the late intrusive rhyolite unit.
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RHYOLITE-HOSTED VMS OF THE CUALE, GUERRERO TERRANE, MEXICO
calc-alkaline to calc-alkaline compositions. The overall geochemical compositions are consistent with an intra-oceanic island-arc setting (Barrett and MacLean, 1999) or a pericontinental arc setting, the latter interpretation being favored
because of the presence of an inherited Pb component in the
zircon U-Pb data.
Low Zr/Y and (La/Yb)n ratios have been related to large degrees of melting and high zircon saturation temperatures, indicative of high magma temperatures (Watson and Harrison,
1983; Barrie, 1995) or, alternatively, melting at low pressures
(Hart et al., 2004). Mineralization in other Phanerozoic massive sulfide districts has been shown to be associated with hightemperature rhyolites which have undergone little assimilation
and 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-bearing
felsic sequences from barren rhyolites. In the Cuale volcanic
sequence a slight overall increase of Zr/Y and (La/Yb)n in the
ore horizon and late intrusive rhyolites, compared to the footwall rhyolite, is observed. This evolution may indicate slightly
decreasing magma temperatures. However, the observed trace
element patterns may also be the effect of lower degrees of
melting related to changes in the residual mineralogy from plagioclase + clinopyroxene- to hornblende + clinopyroxene-bearing assemblages (e.g., Hart et al., 2004), which would be indicative for increasing pressure at the site of magma
generation. The pressure increase is in agreement with the observed stratigraphy, which indicates a buildup of a volcanic edifice that becomes emergent in the late stages; therefore, we interpret the data as reflecting variations in the degree of melting
and increasing pressure at the site of melting rather than
changing magma temperatures. VMS ore was emplaced immediately following this weak geochemical transition, whereas the
late mineralization with low-sulfidation epithermal characteristics is associated with more typical calc-alkaline rocks.
The late andesite dikes have relatively high Zr concentrations, which indicate a different magma source and may indicate more involvement of continental crust or higher degrees
of partial melting. These dikes clearly postdate the rhyolite
sequence and may have been emplaced during accretion of
the Guerrero terrane to the North America craton.
While small changes in magma chemistry commonly coincide 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 at
Cuale is unusual. VMS mineralization in most deposits of similar settings (e.g., Kuroko: Ohmoto, 1983, 1996; Kutcho
Creek: Barrett et al., 1996; see also Lentz, 1998) occurred
during extensional tectonic events and is commonly overlain
by deep marine sedimentary sequences. However, a shallow
marine ore-forming environment also has been postulated for
Eskay Creek (Barrett and Sherlock, 1996) and may be an important factor that led to the silver-rich mineralization at
Cuale. The unusual evolution of Cuale indicates that the mineralization was not emplaced in a typical evolving back-arc
environment but rather suggests a setting in an epicontinental transtensional volcanic arc undergoing rifting simultaneous to uplift. It is likely that the epithermal mineral potential
of Cuale and surrounding areas has been underappreciated
(e.g., Sillitoe and Hedenquist, 2003).
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157
Regional context of the Cuale stratigraphy and
VMS mineralization in the Guerrero terrane
Previous studies loosely correlated the Cuale volcanic sequence with the Early Cretaceous volcanic sequences in the
southern Zihuatanejo subterrane (JICA-MMA, 1986). However, rhyolites from Cuale are now known to be Late Jurassic
and are only slightly younger than the Tumbiscatío and Macias granitoid intrusions of the southern Zihuatanjeo subterrane. These granitoids may represent part of the same arc as
the Cuale rhyolites, which predates the Lower Cretaceous arc
previously recognized. The Late Jurassic arc, judging from
limited inherited Pb in the dated zircons, may have been underlain by vestiges of continental crust. Early Cretaceous
rocks similar to those of the southern Zihuatanejo subterrane
have not been recognized in the immediate Cuale area but
are likely to be present regionally around Cuale (JICA-MMA,
1986).
The folded and weakly metamorphosed pelitic schists underlying Cuale are similar to the Varales Formation of the
Arteaga Complex that forms the basement in the southern Zihuatanejo subterrane. The age of the latter is poorly constrained but considered late Paleozoic to Triassic (CentenoGarcía et al., 1993), a reasonable estimate for the rocks found
at Cuale, as well.
Volcanogenic massive sulfide mineralization in the Guerrero terrane is generally hosted by latest Jurassic to Early
Cretaceous arc and back-arc sequences (Mortensen et al.,
2003, 2008). Cuale is an exception because it formed in the
Late Jurassic, ~5 to 16 m.y. before the other VMS districts
of the Guerrero terrane (Mortensen et al., 2003, 2008).
Thus, the new data on Cuale not only extend the previously
established age range for VMS mineralization but also provide important constraints on the tectonic evolution of western Mexico.
Conclusions
The Cuale mining district contains silver-rich VMS mineralization emplaced in a shallow marine setting, as well as late
Au-Zn (-Pb, Ag) stockwork mineralization with low-sulfidation epithermal characteristics. The ore deposits are all
hosted by rhyolitic rocks (Cuale volcanic sequence) with transitional tholeiitic to calc-alkaline to calc-alkaline affinities.
High field strength element contents of the rhyolites at Cuale
are consistent with those observed in rhyolites associated with
other Phanerozoic VMS districts. Increasing La/Yb and Zr/Y
ratios during eruption of the Cuale volcanic sequence are
thought to be the result of a change in residual mineralogy
from amphibole-free to amphibole-bearing assemblages as
pressure at the melting site increased. This is in agreement
with the observed stratigraphy which indicates buildup of a
more than 800-m-thick volcanic edifice that became emergent in the late stages.
The age of the Cuale volcanic sequence is 157.2 ± 0.5 to
154.0 ± 0.9 Ma, which corresponds to the Upper Jurassic.
Cuale, thus, represents the oldest known VMS deposit of the
Guerrero terrane. Based on geologic mapping, whole-rock
geochemistry, geochronology (inherited Pb in zircons), and regional comparisons we propose that the Cuale district was emplaced in a transtensional pericontinental arc environment.
157
158
BISSIG ET AL.
Acknowledgments
This study represents MDRU contribution 170 and was
made possible thanks to financial support from the Natural
Sciences and Engineering Research Council of Canada and
International Croesus Ventures Corp. The senior author
would further like to thank the Swiss National Science Foundation for a stipend. Antonio Orozco, Don José Castellón, and
Doña Lydia Castellón from Cuale are thanked for their hospitality and assistance. Michelle Robinson, José Vargas, and
David Smithson all provided valuable field assistance, while
discussions with Ken Hickey and José Cembrano were valuable during data interpretation. A. Toma helped with the
drafting of figures. Guest editor S. Piercey, Philipps C.
Thurston, Joseph Whalen, and one anonymous Economic Geology reviewer are thanked for their constructive reviews and
comments.
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