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Transcript
Jurassic to Holocene tectonics, magmatism, and metallogeny
of northwestern Mexico
John-Mark G. Staude*
Mark D. Barton
Center for Mineral Resources, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
ABSTRACT
The present metallic distribution in
northwestern Mexico is the culmination of
superposed magmatism, tectonism, erosion,
and burial over more than 150 m.y. Detailed palinspastic reconstructions of preextensional configurations—the first study of
its kind for this region—clarify the interplay among these features on the present
distribution and character of mineralized
geologic systems. This new synthesis goes
beyond previous metallogenic investigations of northwestern Mexico by separating
events into specific timing and structural
relationships, and by restoring the geology
to its preextensional configuration. Metallogenic factors such as enrichment, preservation, and erosion play major roles in the
present distributions and for the first time
are related to the overall metallogenic
framework of northwestern Mexico. The
analysis concludes that modern metallogenic patterns are the result of the complex superposition and subsequent redistribution
of geologic systems in a way that is related
directly to the regional history, rather than
simply metallic belts or interpreted angle of
a subducting slab.
Three main extensional events in the Oligocene–Holocene have been restored, and
the palinspastic distributions have been analyzed. Reconstructions reveal the following: (1) Mineralization events, igneous centers, and sedimentary sequences are
continuous across the Gulf of California
and other areas with large amounts of extension. (2) Middle Tertiary gold-silver
mineralization in Baja California may be
the western part of the Sierra Madre Oc*Present address: BHP Billiton., Avenida America Vespucio Sur 100, 8th floor, Las Condes, Santiago, Chile; e-mail: John-Mark.G.Staude@
BHPBilliton.com.
cidental metallogenic province, thus expanding the previously recognized extent of
this province’s mineralization. (3) Late
Cretaceous–early Tertiary porphyry copper deposits and intrusive centers form a
narrower belt than previously noted and
are traceable for over 400 km, with parts
of the belt buried beneath the younger Sierra Madre Occidental volcanic fields. (4)
Interpreted alignments of older geologic
features, including lineaments of ore deposits, are displaced in the reconstructions. (5)
Sedimentary-rock–hosted gold deposits and
low-angle-detachment gold systems are
closely related and occur around core
complexes.
By using structurally restored time slices,
it becomes clear that older deposit types
tend to be those formed at greater depths
and more proximal to intrusions, whereas
younger deposits formed at shallower
depths are less eroded and are more commonly volcanic-rock hosted. These characteristics express themselves in the regional
distribution of deposit types. Second, mineralization of widely differing ages is spatially superposed, commonly associated
with coeval magmatic and tectonic events.
The structural and magmatic events together with paleodistribution of ore deposits define a new framework to interpret the metallogenic history of northwestern Mexico.
Keywords: metallogeny, Mexico, ore deposits, reconstruction, Sonora, tectonics.
INTRODUCTION
Although there has been considerable interest in the tectonic evolution of Mexico (e.g.,
Sedlock et al., 1993), the region has received
comparatively little attention in syntheses of
the metallogenic formation (Gonzalez-Reyna,
1956; Salas, 1975; Clark et al., 1982). Nu-
merous published and unpublished geologic
studies in the past two decades provide the
opportunity for a new synthesis and interpretation of Mexican metallogeny in the framework of the current tectonic thinking. This paper presents a time-space synthesis for these
data in northwestern Mexico (Staude, 1995),
generated as part of an integrated project on
Mexican mineral deposits and geology involving collaboration among the University of Arizona, the U.S. Geological Survey, and mining
companies. The results show that mineralization is not individual mineral belts, but rather
a dynamic interplay of magmatism, tectonism,
erosion, and preservation.
This study focuses on a 0.5 3 106 km2 area
in northwestern Mexico that has complex geology and many mineral deposits (Fig. 1).
Prominent mineral deposits in this region include the major porphyry copper deposits of
Cananea and La Caridad, Sonora; large, volcanic-rock–hosted precious-metal deposits
throughout the Sierra Madre Occidental; and
a number of other igneous-related and basinrelated deposit types (e.g., Salas, 1975, 1994;
Wisser, 1966). The large area permits broad
regional comparisons to address the concepts
of metal belts, preservation, and superposition
of mineralizing events. Likewise, the complex
magmatic and deformational history of this region since the Early Jurassic allows evaluation
of metallogenic controls after appropriate reconstructions are made of distinctive time slices. Well-defined reconstructions are possible
with the many new geochronometric, structural, and petrologic data generated in the past
decade (e.g., Henry, 1989; Pérez-Segura and
Jacques-Ayala, 1991; Aguirre-Dı́az and McDowell, 1993; Aranda-Gómez et al., 1997;
McDowell et al., 1997; Stewart et al., 1998;
Henry and Aranda-Gómez, 2001). We synthesize the characteristics, distribution, and timing of mineralization and tectonism that are
based on new age, geologic, and palinspastic
GSA Bulletin; October 2001; v. 113; no. 10; p. 1357–1374; 12 figures.
For permission to copy, contact editing@geosociety.org
q 2001 Geological Society of America
1357
STAUDE and BARTON
Figure 1. Location map of study area with larger symbols corresponding to deposits referred to in the text. Symbol shape and shading
indicate deposit type and mineralization age with sources for districts in Staude (1995). Larger symbols are labeled districts and are
referred to in text.
compilations and new field work. We use the
results to interpret metallogenic controls. The
controls can be compared to other regions, and
the approach illustrates some of the complexities of mineralization in a long-lived continental margin with multiple magmatic arcs.
GEOLOGIC FRAMEWORK
Northwest Mexico consists of translated
and accreted terranes along the southwestern
margin of the North American craton, which
have been intruded and covered by coeval and
younger igneous rocks since the middle Mesozoic. Mesozoic and early Tertiary compres-
1358
sional tectonism was followed by several
styles of extensional tectonism beginning in
the middle Tertiary. These events generated
distinctive lithologic sequences and expose diverse crustal levels across the region. Although terranes have been defined on basement (pre-Jurassic) stratigraphy (Campa and
Coney, 1983; Sedlock et al., 1993), an alternative division simply considers the combination of crustal structure, type, and level.
Three distinctive geologic domains (western,
central, and eastern), compared to the nine terranes defined by Campa and Coney (1983),
provide a simple basis for comparison across
the region (Fig. 2). The domains are based on
magmatic and structural style rather than on
basement and stratigraphic sequence. Mesozoic and Cenozoic magmatic and tectonic features (which overlap the stratigraphic assemblages of terrane terminology) help define
these domains but typically cut terrane boundaries. Because mineralization commonly is the
product of magmatism and tectonism, the domains help unify metallogenic observations.
Lithologic Framework
The western domain (Fig. 2) consists of accreted marine sedimentary and volcanic rocks
and minor mafic intrusions (Sedlock et al.,
Geological Society of America Bulletin, October 2001
TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO
Figure 2. Geologic domains of northwestern Mexico with the schematic stratigraphic columns for each domain. These domains cross
terrane boundaries and define packages based largely on distinctions in Cenozoic superimposed effects. SMO—Sierra Madre Occidental.
1993; Morán-Zenteno, 1994) cut by numerous
pre-Cenozoic thrust faults and younger strikeslip shear zones. The western half of Baja California contains various accreted units ranging
from continental-margin to ophiolitic rocks,
most of which have been intruded by multiple
plutonic events and partly covered by Cenozoic volcanic rocks and conglomerates (Fig.
2; Abbott and Gastil, 1979). Jurassic–Cretaceous clastic sedimentary rocks and volcanic
rocks of the Alisitos Formation form an interior belt over much of western Baja California. Rocks of the Alisitos Formation underwent metamorphism during accretion and
subsequent contact metamorphism by the Peninsular Ranges batholith (Gastil et al., 1975).
No pre-Mesozoic units are reported in this domain, and all but the widespread Neogene
conglomerate and bimodal volcanic rocks are
metamorphosed.
The central domain (Fig. 2) contains Cretaceous tonalitic and dioritic batholithic rocks
intruding Paleozoic to middle Mesozoic carbonate, clastic, and minor volcanic sequences
of eugeoclinal affinity (Gastil et al., 1981).
This domain exhibits Mesozoic shortening
and later normal faulting. Highly deformed
Paleozoic sedimentary rocks extend throughout the region, the oldest recognized being Ordovician (Poole et al., 1991; Ortega-Gutierrez
et al., 1992). Continent-derived volcano-sedimentary rocks of the Late Triassic–Jurassic
Barranca Group overlie Paleozoic rocks in
central Sonora, whereas to the north, Jurassic
pyroclastic flows and volcanogenic sediments
were deposited in basins that parallel the Jurassic continental margin (Tosdal et al., 1989;
Riggs and Blakey, 1993). Widespread but
poorly dated mixed andesitic and clastic sedimentary sequences have been assigned to
both the Late Cretaceous and early Tertiary,
largely on the basis of stratigraphic correlations and crosscutting relationships. These sequences are intruded and metamorphosed by
Cretaceous (in the west) to early Tertiary (in
the east) metaluminous granitoids (Damon et
al., 1983b; Gastil, 1993). Abundant middle
Tertiary calc-alkaline volcanic rocks are overlain by Neogene coarse clastic sequences and
basalts.
The eastern domain (Fig. 2) contains exposed Proterozoic basement overlain by miogeoclinal sedimentary rocks, which are intruded and overlain by Mesozoic and Cenozoic
igneous rocks and continental sedimentary
rocks (Roldán-Quintana and Clark, 1992;
Gonzalez-León and Lawton, 1995). Laramide
compression was followed by highly variable
amounts of Cenozoic extension. This domain
is the southwestern margin of the North
American craton (Stewart, 1988). Eocambrian
and lower Paleozoic miogeoclinal sedimentary rocks are overlain by middle Paleozoic carbonate reef and platform deposits. Upper Pa-
leozoic rocks record the transition from
marine to subaerial environments. Local Triassic and Jurassic volcano-sedimentary basins
resemble those of the central domain. Volcanic-rock–free Lower Cretaceous sequences record the transition from the vast carbonate
platform sequences of northeastern Mexico to
clastic nearshore facies in central Sonora.
Younger limestone conglomerates, thin discontinuous sandstone beds, and volcanogenic
materials record transition to continental conditions and impingement of the late Mesozoic
arc. Locally, Laramide volcanic rocks are
common and are overlain by abundant Oligocene–Miocene calc-alkaline to bimodal volcanic rocks (Ortega-Gutierrez et al., 1992;
McDowell and Roldán-Quintana, 1993; NietoSamaniego et al., 1999) and minor upper Cenozoic syntectonic clastic rocks (McDowell et
al., 1997).
Magmatic and Tectonic Framework
Broadly continuous magmatism and tectonism over the past 150 m.y. have been documented across northwestern Mexico (Damon
et al., 1981; Pérez-Segura, 1985; Sedlock et
al., 1993). Although earlier work demonstrates
that overall patterns in Mexico parallel patterns in the southwestern United States (e.g.,
Dickinson, 1981), the characteristics of petrogenetic and tectonic events in northwestern
Geological Society of America Bulletin, October 2001
1359
STAUDE and BARTON
Some of these structures host mineralization.
Later events reactivated the Jurassic structures
and used them as conduits for hydrothermal
fluid flow, as at the San Francisco mine in
Sonora (Pérez-Segura et al., 1996).
Figure 3. Temporal distribution of igneous and deformational events in northwestern Mexico, denoted as the numbers of outcrops per mountain range. (A) Abundance of exposed
plutonic and volcanic rocks. Younger rocks are more extrusive-dominated, and magmatism has been active in different parts of the region for most of the past 110 m.y. (B)
Relative abundance of deformational features. Son—Sonora, Sin—Sinaloa, BC—Baja California, Chih—Chihuahua, Dgo—Durango.
Mexico have not been well resolved. On the
basis of previous studies and our new work
(Staude, 1995), we divide the history into five
distinguishable petrogenetic and tectonic episodes (Figs. 3 and 4; these occurred during the
Late Jurassic–Early Cretaceous, Cretaceous,
Late Cretaceous–early Tertiary, middle Tertiary, and late Tertiary. Distinguishing features
include magmatic compositions, tectonic
styles, and levels of exposure.
Jurassic–Early Cretaceous (ca. 150–120
Ma)
The Jurassic–Early Cretaceous episode
marks the beginning of magmatism that has
associated metallic mineralization in northwestern Mexico. Sparse plutonic and widespread volcanic rocks occur in all three geologic domains; they are concentrated along the
coast in Baja California and across the interior
of Sonora, extending southeastward into Sinaloa and Durango (Fig. 4A; Dickinson, 1981;
Stewart et al., 1986). The deposition of volcanic rocks correlates in time with the translational tectonics of the Jurassic, which
evolved into the Mojave-Sonora megashear
(Dickinson, 1981; Anderson et al., 1982). The
1360
coastal belt consists of Jurassic–Early Cretaceous mafic to intermediate-composition volcanic rocks, sparse intrusions, and ophiolitic
suites (Sedlock et al., 1993). These rocks are
typically metamorphosed to greenschist facies
and deformed. The interior belt, which is solely Jurassic in age, contains intermediate to felsic calc-alkaline to alkaline volcano-plutonic
complexes that are interpreted to have formed
in an extensional arc setting (Busby-Spera,
1988; Saleeby et al., 1992; Tosdal et al.,
1989).
Jurassic–Early Cretaceous structure is complex and obscured by superposed events. Both
thrust and strike-slip structures in northwestern Mexico have been identified as Jurassic
(Anderson et al., 1980; de Cserna, 1989).
Widely distributed left-lateral, northwest-striking shear zones in Sonora appear to be part of
the megashear, which likely ceased activity
prior to 150 Ma (Anderson and Silver, 1979).
Deformation in the Jurassic Parral Formation
of Chihuahua and Durango could be the
southeastern extension of this shearing. Thrust
and strike-slip faults in Sinaloa and Baja California juxtapose Mesozoic and Paleozoic
rocks and are cut by Cretaceous batholiths.
Cretaceous (120–80 Ma)
Cretaceous magmatism formed the batholithic terranes of Baja California and Sinaloa.
The calcic to calc-alkaline Peninsular Ranges
batholith in Baja California formed from 120
to 90 Ma (Walawander et al., 1990), with
equivalents in Sinaloa (Henry and Fredrikson,
1987). Exposure levels of the batholith shallow southward, reaching subvolcanic levels
near lat 288N, where numerous pendants contain volcanic rocks (e.g., at El Arco, Baja California Norte; Fig. 4B). The west-to-east increase in silica in the intrusive suites is
paralleled by a temporal progression to more
felsic compositions during the Cretaceous.
Oxygen, Pb, and Sr isotope ratios indicate
greater crustal contents of younger and farther-east–emplaced magmas in Baja California
and Sonora (Taylor, 1986). Isotope ages and
Pb isotope contours match across pre-Miocene
restorations of the Gulf of California (Silver
et al., 1993).
Cretaceous thrusts verge both to the east
and west (King, 1939; Drewes, 1978; RoldánQuintana and Gonzalez-León, 1979; JacquesAyala et al., 1990). They both cut and locally
are cut by Cretaceous intrusions, yet the volcanic rocks commonly show extensional faults
that apparently formed synchronously (e.g., El
Arco; Barthelmy, 1979). Cretaceous brittle
and ductile faults host precious and base-metal
mineralization. Rangin (1986) attributed these
structures to the accretion of Baja California
to mainland North America.
Late Cretaceous–Early Tertiary (80–40
Ma)
Late Cretaceous–early Tertiary (80–40 Ma,
‘‘Laramide’’) igneous rocks parallel the inferred subducting arc along a south-southeast
trend through most of the eastern half of the
study area, extending from southern Arizona
and New Mexico into Durango and Sinaloa.
This calc-alkaline granodioritic to granitic
batholithic belt intrudes voluminous, coeval
volcano-sedimentary rocks in western Mexico
(Henry, 1975; Damon, 1978; Roldán-Quintana, 1991; Cochemé and Demant, 1991; Gonzalez-Léon et al., 2000). Outcrop patterns and
radiometric dates illustrate the general distribution of Laramide magmatism, even though
the volcanic sequences are poorly characterized because of pervasive hydrothermal alteration and stratigraphic complexity (Fig. 4C).
Geological Society of America Bulletin, October 2001
TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO
These rocks appear discontinuously through
the Oligocene–Miocene Sierra Madre Occidental volcanic province, emerging in central
Chihuahua (McDowell and Mauger, 1994). Sr
isotope ratios in igneous rocks increase from
values of ,0.706 in western (older) rocks to
.0.708 in eastern (younger) rocks (Damon et
al., 1983a; Mead et al., 1988; Roldán-Quintana, 1991). Eocene intrusions have 87Sr/86Sr
ratios as high as 0.714 in two-mica granites
(Mead et al., 1988). Andesitic volcanic rocks
generally have associated porphyritic intrusions that commonly localize mineralization
(e.g., Bockoven, 1980).
Laramide thrusting is prominent in the
northeastern part of the region, although extension and basin-bounding structures of probable Laramide age occur throughout the region (Drewes, 1978; de Cserna, 1989).
Transport directions and amounts of crustal
shortening in Sonora vary greatly (JacquesAyala et al., 1990). In some areas in northern
Sonora, 20%–40% shortening can be documented (Merriam and Eells, 1978), but insufficient data exist to quantify the shortening
across the Laramide orogen. Thrusting in Sonora apparently ended prior to early Tertiary
plutonism, as Laramide plutons in this area
intrude but apparently are not cut by thrust
faults (Staude, 1995).
Middle Tertiary (40–20 Ma)
Calc-alkaline volcanism and associated
hypabyssal intrusions of the Sierra Madre Occidental and nearby areas formed through
much of western Mexico from the late Eocene
through the late Oligocene (McDowell and
Clabaugh, 1979). These are the southern continuation of the better-studied Oligocene volcanic fields of the southern United States
(Christiansen and Lipman, 1972; McIntosh et
al., 1992; Spencer et al., 1995). Compared to
Laramide rocks, the mid-Tertiary rocks are
more felsic and less altered. They form a 0.5–
2-km-thick veneer across large parts of the
Laramide arc.
The transition from mainly andesitic, hydrothermally altered rocks to dominantly dacitic and rhyolitic, less altered rocks provides a
useful demarcation between the older and
younger suites (Wisser, 1966; McDowell and
Clabaugh, 1981). The age of this transition remains to be well defined in most areas, although it is mostly Eocene. From central Sonora to the east, there appears to be little if
any hiatus in magmatism during the transition,
whereas in the west, a temporal break is well
established (Fig. 4D; Clark et al., 1982). Geochemical data indicate as much or more crustal component in the mid-Tertiary magmas as
Figure 4. Time slices showing locations of
isotope ages and correlative magmatic
units. Ages compiled from .100 sources
are reconstructed to their pre-Miocene extension in Figure 10. Outcrop locations
modified from Ortega et al. (1992) and age
sources.
in earlier periods (e.g., eNd of22.3 to25.2;
87
Sr/86Sri of .0.709 in western Chihuahua; Albrecht, 1990). A regionally extensive gravity
low along the eastern margin of Sonora into
western Durango roughly corresponds with
the axis of mid-Tertiary volcanism and likely
reflects an underlying coeval granitic batholith
(Aiken et al., 1988).
The mid-Tertiary marks the beginning of
orogenic collapse in northwestern Mexico.
Major extension with concomitant core-complex formation began in the middle Oligocene
Geological Society of America Bulletin, October 2001
1361
STAUDE and BARTON
(Nourse et al., 1994). Across northern and
western Sonora, Tertiary extension unroofed
rocks formed at mid-crustal levels (Figs. 5 and
6). Extension of lower magnitude continued
south and east into central Mexico (Henry et
al., 1991). The zone of major extension fades
into the more coherent, but still disrupted, Sierra Madre Occidental structural province
(Gans, 1997; Stewart et al., 1998). Faults
within the Sierra Madre Occidental structural
province localize some volcanic centers (Staude, 1995). Certain normal faults predate Oligocene ignimbrites and create a structurally
disrupted topography beneath the ignimbrite
tuffs, such as at Santa Ana (Gans, 1997) and
Mulatos (Staude, 2001) in Sonora. The main
extension, however, took place farther west,
where there are large half grabens and major,
basin-bounding faults (McDowell et al.,
1997). Where most pervasive, the postignimbrite faulting tilted mid-Tertiary volcano-sedimentary sections by .308. Later faults typically trend northwest, are high angle, and
commonly host middle Tertiary mineralization
(Drier, 1984; Staude, 1994).
Figure 5. Cross sections from Jurassic to present, showing major fault systems during
each period at the latitude of Hermosillo, Sonora. Section not restored so as to more clearly
represent superimposed features. Structures are generalized and represent major fault
sets. A—Away, T—Toward, SMO—Sierra Madre Occidental.
Late Cenozoic (20 Ma–Holocene)
Magmatism became more heterogeneous
and dispersed beginning in the early Miocene.
Early phases of this volcanism were felsic
tuffs and minor basaltic lavas that contain
strong crustal signatures. Volcanism evolved
in the Miocene to bimodal volcanism associated with normal faulting, ultimately devel-
Figure 6. Map A. Location of core-complex, basin and range, and Gulf of California structural features. Directions of extension and
broad time spans are given for each style on inset Map B. The three structural styles are largely superimposed and must be unraveled
in order to restore the geology to its preextensional configuration. Map sources include Davis (1980), DeJong et al. (1988), JacquesAyala et al. (1990), Nourse (1989), Staude (1993a), Stewart and Roldán-Quintana (1994), and Staude (unpublished mapping).
1362
Geological Society of America Bulletin, October 2001
TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO
oping the mafic-dominated, rift-type volcanism associated with generation of the Gulf of
California. Basaltic andesites and high-K basaltic centers around the Gulf of California are
variably tilted and were locally erupted synchronously with active high-angle extension
and sedimentation (McDowell et al., 1997).
Volcanism occurred throughout northwestern
Mexico in the early Miocene, but became concentrated around the Gulf of California in the
late Miocene (Fig. 4E). These younger lavas
have either alkaline or MORB-like characteristics (MORB is mid-oceanic-ridge basalt) and
occur in areas that are still tectonically active
(Donnelly, 1974; Neally and Sheridan, 1989).
The younger lavas include the modern basaltic
centers around the margins of the Gulf of California and sparse centers throughout northern
Mexico (Sawlan, 1991; Fig. 4E).
Northwest-striking, high-angle normal and
strike-slip faults characterized crustal attenuation during the late Cenozoic (Stock and
Hodges, 1989; Staude, 1993a; Lee et al.,
1996). The faulting was synchronous and
postdated the southwest-directed core-complex extension. This post–core-complex faulting can be divided into two groups: Miocene–
Pliocene high-angle extensional faults (King,
1939; Henry and Aranda-Gómez, 1992) and
late Miocene–Holocene normal and strike-slip
(transtensional) faults associated with rifting
(Moore and Buffington, 1968; Atwater, 1970;
Lonsdale, 1989; Stock and Hodges, 1989; Atwater and Stock, 1998). High-angle normal
faults are widely distributed. They trend north
in the north (INEGI, 1981; Stewart et al.,
1998) and north-northwest in the south (Henry
and Fredrikson, 1987). Younger strike-slip and
normal faulting is oblique (north-northwest
trending) to the gulf and accommodates both
extension and translation (Lonsdale, 1989).
Tectonic Reconstruction
Figure 7 shows a three-step palinspastic reconstruction for northwestern Mexico based
on the information we have outlined. First, the
Gulf of California was closed by restoring the
likely protogulf configuration (Stock and
Hodges, 1989). This step involves both moving the Baja California Peninsula ;350 km
southeastward along the San Andreas fault
system and restoring the east-northeast extension relative to mainland Mexico.
Second, ;10% extension attributed to highangle normal faulting was removed block-byblock across the states of Sonora and Sinaloa.
This step was based on tilt-direction maps
showing fault orientation, block rotations, and
areas of higher degrees of extension (Stewart
Figure 7. Three-step reconstruction to the . 26 Ma (late Oligocene) preextension geography
of northwestern Mexico. Dark lines plot younger position; dashed line indicates older, restored position. Shading delineates area of greatest extension between Pacific Ocean and
Sierra Madre Occidental structural province. Circle is schematic restored strain ellipse.
and Roldán-Quintana, 1994; Stewart et al.,
1998; Staude, 1995). Minor extension, probably ,5%, occurred throughout westernmost
Chihuahua and Durango; these areas are here
treated as a fixed block east of the extended
domains. Strike-slip motion in these areas
may have been significant during the time of
Basin and Range extension in areas to the
west, but few faults have been identified with
substantial strike-slip translation.
The third step involved restoration of the
core complexes and related extension. This
step is the most difficult to carry out because
few piercing points exist across the complexes
and because of the compartmentalized nature
of this extension (Nourse, 1992). This restoration results in 50% lineation-parallel shortening in the core-complex region of central
and northern Sonora, diminishing to nothing
on the margins of the mid-Tertiary extensional
regime to the south and east (cf. Fig. 6). Previous studies of extension in the southwest
Cordillera give comparable estimates for high
extension (Davis et al., 1981; Wust, 1986;
Dickinson, 1991; Richard, 1994), although no
detailed reconstruction for Mexico has been
published previously.
Although simplified in nature, this model
enables an improved evaluation of the Paleogene distribution of rocks and mineral deposits. Ideally, one would like to carry the reconstruction farther back in time; however, as
noted earlier, the lack of constraints on Laramide and older structures precludes meaningful quantitative reconstructions of earlier deformation. These earlier deformation events
are schematically illustrated in the time-space
reconstruction that we describe next.
MINERALIZATION
Metallic mineralization is abundant in
northwestern Mexico, with .2500 known
Figure 8. Histogram of deposit types plotting the number of Au, Cu, W, Pb-Zn districts versus time period. Compiled data
are available in the U.S. Geological Survey,
Mineral Resource Data System, computer
databank. Mineralization associated with
SZ—shear zone, Ign—igneous, Adul—adularia-quartz epithermal, Sed—sedimentary, Acid—acid sulfate epithermal, Repl—
replacement, Porp—porphyry.
metal occurrences distributed throughout most
of the region. The types include deposits associated with magmatic (intrusion- and extrusion-related), metamorphic, and sedimentary
rocks that formed over much of the past ;150
m.y. The main metals and the associated deposit types with their ages of formation are
summarized in Figure 8. Abundances for each
deposit type are tabulated from a computerized database we developed for this study
Geological Society of America Bulletin, October 2001
1363
STAUDE and BARTON
adularia-sericite Ag-Au, and high-silica rhyolite volcanic centers with F 6 Mo. Low-sulfide Au-bearing quartz veins are the main
metamorphic type. Metallic ores associated
with sedimentary rocks include stratiform and
strata-bound Cu deposits. Pb-Zn, F, and U deposits related to diagenetic processes occur to
the east of the Sierra Madre Occidental and
are not a focus of this paper; yet they do relate
to the overall metallogenic evolution as they
formed to the east, possibly driven in part by
the thermal and tectonic events to the west
(Salas, 1975; Kesler, 1997).
Nonmetallic mineral deposits, although not
discussed in detail in this study, make up a
significant number of deposits in the region,
and many appear to be related to synchronous
magmatic and tectonic events. Mesozoic and
Cenozoic intrusions generated wollastonite
and garnet skarns (Pérez-Segura, 1985). Late
Cretaceous–early Tertiary intrusions raised the
geothermal gradient in central Sonora, resulting in the formation of the numerous graphite
deposits in carbonaceous sedimentary units.
Evaporite-bearing basins developed during the
middle Cenozoic extension and contain borate, zeolite, and halite beds (Aiken and Kistler, 1992). Volcanism around the western
edge of the Gulf of California during the late
Tertiary formed sulfur, perlite, and opal
deposits.
Age, Characteristics, and Distribution of
Mineralization
Figure 9. Present distribution of major metal districts and their related stratigraphy
for major time intervals. Lithologic distributions are compiled from Gastil et al.
(1975), Gastil and Krummenacher (1977),
Bockoven (1980), Anderson and Silver
(1979), INEGI (1981), Swanson and McDowell (1985), Cochemé (1985), Rangin
(1986), Henry and Fredrikson (1987), de
Cserna (1989), Lonsdale (1989), Staude
(1991), Ortega-Gutierrez et al. (1992), and
Roldán-Quintana and Clark (1992).
(available through the U.S. Geological Survey, Mineral Resource Data System, and
Leonard, 1989). Intrusion-related deposit
types include Cu 6 (Mo, W), Fe-W-Cu and
1364
Ag-Zn-Pb skarns, W greisen deposits, Au-Ag
veins, and Ag-Pb-Zn replacement ores. Volcanic deposit types include epithermal Au-Ag(Pb-Zn-Cu) veins, advanced argillic Au-Cu,
Although relatively few metallic deposits
have been dated, it is possible to estimate the
age of nearly 500 of the .1000 districts for
which geologic data have been compiled
(Leonard, 1989; Staude, 1995). Few, if any,
metallic deposits of pre-Jurassic age are
known in northwestern Mexico; thus, we begin with the Late Jurassic.
Jurassic–Early Cretaceous
The four main deposit types of Jurassic–
Early Cretaceous age are porphyry Cu, shearzone Au-bearing quartz veins, skarn, and volcanogenic Fe deposits (Fig. 9A). Early
Cretaceous porphyry Cu mineralization is associated with intermediate-composition intrusive centers in central Baja California in the
San Fernando–El Rosario areas. Early to Middle Jurassic porphyry Cu mineralization is associated with felsic magmatism in northern
Sonora, extending into Arizona (e.g., Bisbee).
Mesothermal Au-bearing quartz veins occur in
thrust and strike-slip fault zones and are common in Jurassic clastic and metavolcanic rocks
along a northwest trend paralleling the Juras-
Geological Society of America Bulletin, October 2001
TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO
sic arc (Fig. 9A). The shear-zone–hosted veins
may be related to motion along the ArizonaSonora megashear (Silberman et al., 1988) or
to forearc deformation related to arc accretion.
Ages of deposits are uncertain because few
dates on veins have been published for either
the mainland or peninsular localities (Menchaca, 1985).
Late Jurassic–Early Cretaceous marine, arcrelated volcanic and sedimentary rocks host
Fe-Cu (6Au) deposits along the western margin of the Sierra Madre Occidental in Sinaloa
(continuing south into Nayarit) and along
western and central parts of the Baja Peninsula. These hydrothermal Fe deposits and Fe
(6Cu, Au) skarns are penecontemporaneous
with mafic and intermediate-composition
magmatic centers (Zurcher, 1994). Hydrothermal magnetite-hematite vein occurrences cut
Jurassic–Early Cretaceous basaltic flows in
Baja California, particularly between Santa
Rosalı́a and El Rosario (Menchaca, 1985).
These deposits are typically metamorphosed
to lower greenschist grade by the Cretaceous
Baja California batholith. Cu staining and turquoise are found in at least four of the Fe districts. These deposits appear to belong to a
middle Mesozoic belt of Fe-oxide (6Cu, Au)
occurrences along the Cordillera (Barton and
Johnson, 1996). Ni, Co, and Cr deposits hosted in accreted marine sedimentary and volcanic-intrusive (ophiolitic) rocks on the Vizcaino Peninsula and other parts of
westernmost Baja California are associated
with mafic igneous units, are strongly serpentinized, and are highly disrupted by multiple
stages of postmineralization faulting. Associated intrusions have Early Cretaceous U-Pb
ages (Sedlock et al., 1993).
Cretaceous
During the mid-Cretaceous, economically
significant Cu (6Au) porphyries, W skarns,
and mesothermal Au-bearing quartz veins
formed in the western half of the region (Fig.
9A). These deposits form belts that parallel
the Cretaceous arc. Few locations other than
intrusive centers have been directly dated, and
the ages of the remaining deposits are inferred
from field relationships. The most abundant
types of mineralization are W skarn and greisen deposits, followed by Au-bearing quartz
veins and porphyry Cu deposits (Fig. 8).
Tungsten skarns become increasingly common
northward from central Baja California. They
are most common in Paleozoic carbonate
rocks intruded by tonalitic and granodioritic
intrusions (Weise, 1945; Menchaca, 1985).
Tungsten most commonly occurs in scheelite,
both in skarn and quartz veinlets with little
additional associated mineralization. The deposits are small, rarely exceeding 2 3 106
tonnes of ore, and were mostly mined during
World War II (Fries and Schmitter, 1945).
The best-known middle Cretaceous porphyry Cu (6Au) deposit is at El Arco in central
Baja California (Barthelmy, 1979). Others occur in Sinaloa and northern Baja California
and correlate with 120–80 Ma magmatic
events (cf. Fig. 4B). The Cu porphyry systems
have lower initial 87Sr/86Sr values, are more
dioritic, and have higher Au/Cu ratios than
their younger Laramide counterparts. Cretaceous porphyries are preserved in areas that
have been less deeply eroded, particularly in
central Baja California, where volcanic rocks
of similar age surround the porphyry deposits
(Echavarri-Pérez and Rangin, 1978). In northern Baja California, Cretaceous rocks are
more deeply eroded. Equigranular, coarsegrained, intermediate-composition batholithic
complexes contain W skarn and greisen mineralization rather than porphyry centers.
Two other deposit types that likely formed
during the middle Cretaceous are intrusionhosted bonanza Au-Ag quartz veins and mesothermal Au-bearing quartz veins in shear
zones cutting greenschist-facies rocks (Mother
Lode–type deposits). Intrusion-hosted quartz
veins in Baja California are as long as several
kilometers and locally blossom into .50-mwide stockworks (Menchaca, 1985). In Sonora
and Sinaloa, similar veins occur, but their ages
are difficult to determine because of plutonic
overprinting. Mesothermal Au-Ag quartz
veins are common along the western margin
and within roof pendants of the Peninsular
Ranges batholith. The veins form anastomosing stockworks of quartz, carbonate, and chlorite with small, high-grade bonanza Au shoots
(Wisser, 1954).
Laramide
Diverse and abundant mineral deposits
formed during the Laramide (Late Cretaceous–early Tertiary) in northwestern Mexico.
Cu (6Mo) porphyries and skarns, W and PbZn skarns, and Au-Ag quartz veins are the
most economically significant. These deposits
are east of the middle Cretaceous ones, found
mainly in Sinaloa and Sonora. Mineralization
is related to calc-alkaline magmatism and was
emplaced in volcanic to fairly deep plutonic
environments with a range of styles of hydrothermal alteration, brecciation, and deposition
(e.g., Bushnell, 1988; Wilkerson et al., 1988;
Barton et al., 1995).
Porphyry Cu (6Mo) deposits are widespread in areas containing Laramide volcanic
rocks; hydrothermal alteration commonly ex-
tends many kilometers away from known deposits (Sillitoe, 1976). These form part of the
world-class porphyry Cu (6Mo) province that
extends from northwestern Arizona through
Sonora into Sinaloa and Durango (Fig. 9B).
Within this belt, the porphyry Cu–related intrusions are older to the south and west. Cu
and Pb-Zn skarns are widespread where calcareous host rocks are present (Pérez-Segura,
1985; Consejo de Recursos Minerales, 1992,
1993; Valencia-Moreno, 1998). In western Sonora, Cu skarn occurrences are common, as
are equigranular intrusions and regionally
metamorphosed rocks, whereas porphyrystyle mineralization and volcanic rocks are uncommon. Cu-bearing porphyry-style mineralization is associated with intrusive centers
older than 60 Ma in central Chihuahua (McDowell and Mauger, 1994) and northern Durango (Aguirre-Dı́az and McDowell, 1991).
These occurrences and others in deep canyons
within the Sierra Madre Occidental (Barton et
al., 1995) suggest that the Laramide porphyry
province extends beneath much of the younger
Sierra Madre Occidental ignimbrite sequence.
Tungsten and Mo become more important
commodities in more felsic intrusive systems,
which overlap with, but in general are younger
than, the Cu-rich centers. Transitional Mo-W
(6Cu) porphyry, breccia-pipe, and skarn deposits (e.g., Cumobabi and Santa Ana, Sonora)
occur with metaluminous quartz-feldspar porphyries of early Tertiary age. Tungsten-dominated deposits are best developed in central Sonora and extend southeastward into
northwestern Durango. The largest tungsten
districts are in the largest intrusive massif of
Sonora, the Aconchi batholith (e.g., El Jaralito
district). Most W-rich systems are associated
with peraluminous granitoids (Roldán-Quintana, 1991), unlike the Cretaceous tungsten districts of Baja California and transitional Mo-W
(6Cu) deposits that are associated with metaluminous granitoids. Tungsten deposits young
from west to east and change from granodiorite-associated skarns in the west to granite-associated greisen deposits in the east (Weise,
1945; Mead et al., 1988). This control may be
the effect of level of exposure, because deeper
carbonate rocks are exposed in the west, and
younger, shallower clastic sedimentary and volcanic rocks are preserved in the east.
Numerous crustiform, low-sulfidation AuAg quartz veins are widespread throughout the
central and eastern parts of the study area.
Many of these are inferred to be Laramide on
the basis of truncation of veins and alteration
zones prior to mid-Tertiary volcanic units
(Staude, 1995). Precise ages are available for
only a few deposits (e.g., Tayoltita; Henry,
Geological Society of America Bulletin, October 2001
1365
STAUDE and BARTON
1975; Clarke and Titley, 1988). Some of the
larger districts are plotted in Figure 9B to indicate their general distribution. These veins are
hosted in granodiorite intrusions, metamorphosed sedimentary rocks, and andesitic-dacitic
flows and tuffs. They have Ag/Au ratios of
.100 (commonly .1000), and minor Zn, Pb,
and Cu sulfide minerals (Wisser, 1966; Clark et
al., 1979). They can exceed several kilometers
in length and commonly show systematic zonation, in some cases around intrusive centers,
such as at Alamos and Batopilas (Loucks and
Petersen, 1988; Wilkerson et al., 1988).
Shear zones hosting Au mineralization,
such as at La Choya and Quitovac in western
Sonora, have white mica that crystallized during Laramide time (80–50 Ma) (Durgin and
Terán, 1996; Alex Iriondo, 1999, personal
commun.). These shear-zone deposits are
complex; Jurassic and older host rocks are
covered by extensionally faulted Miocene volcanic rocks. The precise age of mineralization
is poorly determined.
Middle Tertiary
Like the Laramide, the late Eocene–Oligocene was a major period of metallic mineralization in western Mexico, with the formation
of diverse types and a large number of ore
deposits, most of which are associated with
volcanic rocks of the Sierra Madre Occidental.
The principal types are low-sulfidation Ag-Au
(6Pb-Zn-Cu) veins, high-sulfidation Au-(Cu)
deposits, and high-temperature carbonatehosted deposits (Fig. 9C). Other probable midTertiary mineralization includes sedimentaryrock–hosted and low-angle-fault-zone Au
deposits. These two may be directly related to
mid-Tertiary extension (Fig. 9D).
Of the .800 middle Tertiary volcanicrock–hosted epithermal precious-metal occurrences known in northwestern Mexico (Fig.
9C; Leonard, 1989; Orris et al., 1993; Staude,
1993b), the majority are quartz 6 calcite veins
with chlorite 1 adularia 1 sericite alteration
halos. These deposits, such as at Ocampo
(Knowling, 1977) and Maguarichic (Staude,
1995), are Ag dominated with spotty Au and
base-metal pockets. Advanced argillic Au
(6Cu) districts, such as Mulatos (Staude,
2001), are scarcer but number in the tens. Disseminated (Moris), hot-spring (Pinos Altos),
and polymetallic vein districts (Uruachic) are
also present. Most districts lack precise dates
but can be assigned an Oligocene age by regional correlation to basal rhyolitic ignimbrites that host mineralization or underlie orehosting volcanic rocks (Swanson and
McDowell, 1985; Wark et al., 1990). Mafic
dikes and flows, regionally dated at late Oli-
1366
gocene to early Miocene, cut or cap these systems (Bockoven, 1980; Duex, 1983; Cameron
et al., 1989).
Carbonate- and volcanic-rock–hosted mineralization on the eastern flank of the Sierra
Madre Occidental is best known for large AgPb-Zn deposits and also includes many Hg,
As, Sb, Mn, Sn, and U occurrences. Sparse
geochronometry and regional correlations indicate that many carbonate-hosted deposits are
Oligocene (Megaw et al., 1988). However, the
fact that similar systems formed during the
Laramide (Piedras Verdes [Chihuahua], La
Reforma, Oposura-Moctezuma) demonstrates
that repeated skarn- and replacement-style
mineralization occurred in areas where magmatism overlaps the carbonate-rich eastern domain (Figs. 2 and 9C).
Sedimentary-rock–hosted and low-angleshear-zone Au deposits found to the west of
the Sierra Madre Occidental volcanic province
may be broadly coeval with volcanic-rock–
hosted deposits in the Sierra Madre Occidental
but are not necessarily directly related to
magmatism. K-Ar ages on postmineralization
basaltic dikes and prealteration two-mica
granites restrict Au mineralization in the Santa Teresa district to 36–28 Ma (Bennett and
Atkinson, 1993), which is consistent with evidence from other districts where mineralization is associated with low-angle (extensional?) faults and is cut by high-angle
(Neogene) faults (Fig. 9D). Deposit characteristics include strong stratigraphic control by
favorable beds and jasperoidal Au-Hg-As
mineralization (Bennett, 1993), which resembles that of Carlin-like deposits (Vikre et al.,
1997). Deposit distribution corresponds with
that of major extension areas containing abundant sedimentary rocks (Fig. 9D). A less
clearly defined group of Au-bearing quartz deposits occurs in low- and high-angle shear
zones west of the sedimentary-rock–hosted
deposits (e.g., La Herradura, Chinate). These
have conflicting data indicating both Laramide
and mid-Tertiary ages (Leonard, 1989; Alex
Iriondo, 1999, personal commun.).
Late Tertiary–Holocene
Sedimentary-rock–hosted stratiform Cu deposits and hot-spring Au deposits are associated with opening of the Gulf of California
(Fig. 9E). The Boleo district contains Cu-CoAg and manganese oxide deposits in upper
Miocene clastic rocks and tuffs related to early
stages of Gulf of California opening (Wilson
and Rocha, 1955; Schmidt, 1975; Ochoa-Landı́n, 1998). Sedimentary-rock–hosted Cu prospects extend several hundred kilometers along
the western edge of the gulf (Staude, 1992).
Over a dozen hot-spring Au occurrences have
been identified around the gulf. Most deposits
lie along the western edge of the gulf, where
transform faults and fault splays extend onto
land along the eastern parts of the Baja California peninsula (Staude, 1992). The hotspring deposits are hosted in units as old as
Miocene (Santa Lucı́a) and as young as Holocene beach sands (Puertocitos).
TIME-SPACE DISTRIBUTION OF
MAGMATISM AND MINERALIZATION
Preextension Events
The geology of northwestern Mexico is
complex, with superposed magmatic and metallogenic events that have in many cases been
translated from their location of formation by
later events. Previous metallogenic summaries, based on collaboration among the University of Arizona, the U.S. Geological Survey, and mining companies, have defined belts
according to present-day configurations (Salas, 1975). Clark et al. (1982) accounted for
some of the translation associated with the
most recent Gulf of California rifting but did
not take into account earlier extension or displacements inland of the gulf. By using the
tectonic reconstruction already presented, the
metallogeny of northwestern Mexico can be
reevaluated, starting with the Late Jurassic.
The preextension restoration allows tracing
of the outcrops of Jurassic volcanic and volcano-sedimentary rocks with sparse intrusive
centers from southern Arizona through northern Sonora and southeastward into Mexico.
This restoration of the Jurassic arc shows a
trend similar to those presented by Dickinson
(1981) and Tosdal et al. (1989). The three
main groups of Jurassic rocks (Fig. 10A) form
a belt of volcanic and minor intrusive rocks
with scattered clastic sequences. The units of
eastern Baja California and Sonora appear to
join with Jurassic rocks of the Parral Formation in southern Chihuahua and thus link the
geology beneath the Sierra Madre Occidental.
Mineral deposits of possible Jurassic age restore to closer proximity, and similar deposits
become grouped once extension is taken into
account.
By using this same preextension reconstruction, the Cretaceous intrusive rocks in western
Sonora can be traced to the restored Baja Peninsula; the Cretaceous W deposits can be
linked between northern Baja California and
Sonora; Mesozoic accretionary sedimentary
rocks can be restored between central Baja
California and southern Sonora; and the porphyry Cu district of El Arco correlates with
Geological Society of America Bulletin, October 2001
TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO
Figure 10. Restored positions of mineralization and major lithologic units restored to the appropriate preextensional configuration.
Geological Society of America Bulletin, October 2001
1367
STAUDE and BARTON
Figure 10. (Continued.)
Cu-bearing parts of the Sinaloa batholith (Fig.
10B). When the abundance of Cretaceous intrusive outcrop area is contoured, one can see
how the batholith restores across the gulf and
1368
continues northward into the reconstructed position of southern California. The abundance
of Cretaceous intrusions appears to decrease
to the east. Also to the east, the intrusions may
continue the younging trend that has been
documented between Ensenada and Mexicali
(Ortega-Rivera et al., 1994); however, more
data are needed. To the west along the Pacific
Geological Society of America Bulletin, October 2001
TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO
coastal edge of Baja California, accretion of
forearc sedimentary rocks and ophiolites contributed to the metallogeny during Cretaceous
time, although their precise locations during
the Cretaceous is uncertain (Hagstrum et al.,
1985). The Cr, Co, and Ni deposits formed
with mafic magmatism during Late Jurassic
and Early Cretaceous time and were accreted
to Baja California prior to the Cenozoic (Abbott and Gastil, 1979). Since Oligocene time,
the western part of Baja California has not undergone tremendous extension; however,
translation along faults parallel to the San Andreas and Agua Blanca fault systems has
caused right-lateral movement, for which the
reconstruction accounts.
For the Late Cretaceous–early Tertiary, the
reconstruction proves useful for interpreting the
batholiths in Sonora and Sinaloa and their relationship to rocks of similar age in southern
Arizona. Once restored, mineralized systems
form a narrow belt trending southeast along the
western edge of Mexico rather than a wide
bulge as one sees today in the Sonora-Arizona
region. The bulge narrows by as much as 50%
once postmineralization extension is removed,
and the width becomes more typical of other
porphyry Cu belts throughout the world (e.g.,
central Andes [Sillitoe, 1988], British Columbia [McMillan et al., 1995]). By contouring the
extent of 80–40 Ma intrusive centers and the
restored present-day outcrop areas (Fig. 10C),
the abundance of plutonic rocks is found to be
high beneath the Sierra Madre Occidental.
Without the cover of the volcanic rocks, both
undated Laramide and younger, the abundance
of Laramide intrusions would be larger. Porphyry systems, exposed in windows through
the younger ignimbrites of the Sierra Madre
Occidental, aid in tracing the batholith and suggest a high probability for other deposits beneath the younger volcanic rocks. Study of present lineaments and position of porphyry Cu
deposits do not take into account the tripartite
deformation. Once the deformation is restored,
many of the lineaments no longer appear as
prevalent. Lineaments, if they do exist, become
rotated from their present orientations, and if
lineaments are used for interpretations, they
need to be corrected for the substantial displacements of the past 30 m.y. Some deposits,
such as Cananea (Wodzicki, 1995) are moderately tilted, whereas others are strongly extended and dissected as at Piedras Verdes, Sonora
(Drier and Braun, 1995).
Outlines of regions with porphyry Cu, AuAg, W, and Pb-Zn deposits indicate that the
mineral deposits follow the same south-southeast trend as the magmatic arc (Fig. 10C).
Moreover, mineralization clusters in areas with
superposed metal associations—not as separate
metallic belts. The clusters correspond with
many of the exposed Laramide igneous rocks
(Fig. 10D). This finding could be quite significant, because it suggests that much of the Laramide is mineralized. Moreover, if more Laramide rock areas are discovered, they might
have substantial mineralization. Exposure plays
a vital role in controlling the present-day appearance of the metal distribution, and it is possible that prior to erosion and Cenozoic volcanic superposition, there could have been an
even larger extent of exposed rocks and mineralization. Looking at the present-day abundance of deposits in southern Arizona and
northern Sonora, there are potentially hundreds
of deposits that were covered by younger volcanic rocks of the Sierra Madre Occidental
from central Sonora to southern Durango. Laramide igneous rocks crop out in most canyons
that expose pre–middle Tertiary rocks throughout the Sierra Madre Occidental. These outcrops help define the contours of the abundance
of magmatic rocks.
Loops in Figure 10C delineate Cu districts
that are the most widely distributed (they are
outlined by the light-weight dashes). Tungsten
deposits are inside the area defined by the outline enclosing the Cu districts. Tungsten deposits are recognized by our study much farther east and over a larger area than in
previous metallogenic summaries for Mexico
(Clark et al., 1982). In the reconstruction, PbZn, W, Au-Ag, and Cu districts occur in the
same areas, showing their superposition prior
to extension.
Late Eocene–Oligocene mineralization and
magmatism can be restored to approximate
their distribution prior to extension, although
some of this mineralization is likely related to
the onset of extension and may have formed
as extension was underway. Au deposits are
most abundant in the Sierra Madre Occidental
volcanic province along the Sinaloa-Durango
and Sonora-Chihuahua state borders; however,
a few Au-Ag deposits of Oligocene age are
found farther west in northern Sonora. The Au
(6Cu) districts within the Sierra Madre Occidental are at present recognized in only a
few areas; thus, their extent of distribution is
preliminarily drawn as a smaller region within
the larger Au-Ag districts of the Sierra Madre
Occidental (Fig. 10E). Carbonate-hosted PbZn-Ag deposits occur to the east and along the
western edge of the Sierra Madre Occidental.
The full extent of volcanism during the midTertiary ignimbrite period is not known; yet,
by restoring the deformation and using the
currently exposed and dated outcrops, it is
possible to estimate the extent of Oligocene
volcanism (Fig. 10E) and contour the restored
geography on the basis of the thicknesses and
extents of tuffs older than 27 Ma. Radiometric
dating in Baja California indicates that the
earliest middle Tertiary volcanism began at ca.
30 Ma (Gastil et al., 1975), and the volume
increased as time progressed. In Baja and in
westernmost Sonora, the few locations of rhyolitic volcanism link these regions to the more
coherent and extensive Sierra Madre Occidental volcanic province to the east.
Synextension and Postextension Events
Two major periods of mineralization in
northwestern Mexico may be restored approximately to their synextension configurations.
The ca. 27–20 Ma extensional faulting widened Sonora (Gans, 1997), possibly forming
sedimentary-rock–hosted Au systems in the
east and shear-zone Au deposits in the west
(Fig. 10E). The mid-Tertiary, post–core-complex reconstruction mainly affects the northern
and central parts of Sonora; the dotted line
surrounding much of Sonora in Figure 10F reconstructs the approximate boundary of .308
Tertiary tilting and areas of low-angle (,258)
normal faults (modified from Stewart and Roldán-Quintana, 1994). During the core-complex extensional event, northern Sonora may
have extended as much as 30%, and some areas had .100% extension (Nourse, 1989;
Nourse et al., 1994); these are restored to their
pre–Basin and Range extension position in
black. The region of low-angle-shear-zone Au
deposits is within the area of middle Tertiary
extension; many deposits occur around and to
the west of restored core complexes. The Jajoba district may have been faulted off the top
of the Magdalena core complex. Sedimentaryrock–hosted Au districts restore to the eastern
edge of core complexes but reside within the
area extended during middle Tertiary time.
One hypothesis for this association is that the
deposits are related to extensional phenomena,
as has been suggested for middle Tertiary sedimentary-rock–hosted mineralization in central Nevada (Seedorff, 1991). The Sonoran deposits have had far less study than
sedimentary-rock–hosted deposits in central
Nevada, and interpretations of the genesis for
Sonoran deposits are speculative.
The late Tertiary reconstruction shows the
distribution of volcanic-rock– and sedimentaryrock–hosted hot-spring Au districts and the Boleo stratiform Cu district (Fig. 10G). These deposits are associated with the rift opening of
the Gulf of California and restore to the general
boundary of the protogulf region. The reconstruction reveals that the region is encompassed
Geological Society of America Bulletin, October 2001
1369
STAUDE and BARTON
Figure 11. Time-space synthesis of magmatism, deformation, and mineralization across
Baja California through Sonora to western Chihuahua at latitude of Hermosillo. The
present 500 km width has changed due to older compression and younger extension,
creating an hourglass shape. Right diagram summarizes mineralization with font sizes
corresponding to the relative abundance of metallic ore-deposit types. The position of the
Gulf of California is marked by the dashed black line. Chih—Chihuahua.
by late Tertiary mafic igneous rocks and the
generalized location of the restored, pre–gulfopening geography (modified from Atwater,
1970; Lonsdale, 1989; Stock and Hodges,
1989). The reconstruction indicates that opening of the gulf during the past 6 m.y. yielded
greater extension in the southern part of the
gulf, which necessitates more closure to restore
than points farther north (cf. Henry and Aranda-Gómez, 2001). Rift-related Miocene Cu
mineralization restores to the latitude of Culiacán, Sinaloa (Schmidt, 1975; Guilbert and Damon, 1977). Hot-spring Au districts ring the
Gulf of California, with a larger abundance of
deposits on its western margin. This asymmetry may be due to the proximity of spreading
and rift centers along the eastern coast of northern Baja California over the past 12 m.y., in
addition to preservation and covering by younger Pliocene and Quaternary clastic sedimentary units around the gulf (Staude, 1993a).
The three-step reconstruction can be summarized in a time-space model linking tectonism, magmatism, and mineralization, thereby
providing a basis for interpreting the distribution of mineralization through time.
Synthesis
Figure 11 summarizes the temporal and
spatial relationships of magmatism, deforma-
1370
tion, and mineralization in northwestern Mexico. Although the observations summarized
are not exhaustive, general patterns are clear
and provide the basis for broad interpretation.
Overall, magmatism was largely continuous
while systematically changing location and
composition with time. One or more ill-defined Jurassic arcs in the eastern and western
domains were superseded in the Cretaceous by
a well-developed coastal batholith that was
built mainly across the continental-margin–accreted metasedimentary package of the western domain. Gradual and then more rapid eastward migration of magmatism began in the
late Mesozoic through the mid-Tertiary (Fig.
11A). These changes correlate with compressional deformation, indicated in Figure 11 by
the shortening of the width from Pacific Ocean
to the Chihuahua-Sonora border. Subsequent
retreat of magmatism toward the coast in the
early Cenozoic was followed by the change
from subduction-related to rift-related magmatism during the Miocene; these magmatic
patterns correlate with a shift to neutral, then
transtensional tectonics, shown in Figure 11
by the width’s expanding and by the prevalence of volcanic rocks, unlike the older period with more intrusion-dominated rock
types. In general, it is in the rift periods that
volcanic rock types are preserved, whereas in
the compressional periods, one finds intrusions. The deposit types plotted in Figure 11B
show this association. There is an overall increase in the ratio of abundance of volcanic
rocks to plutonic rocks beginning in the Cretaceous, which appears to be a preservation
phenomenon in that the older rocks are more
deeply eroded and thus expose more deeply
formed deposit types. An additional association is that in single areas, the magmatic compositions vary over the 10–25 m.y. periods of
activity; crustal (felsic) contents increase during compressional events (Cretaceous–Oligocene), whereas compositions are heterogeneous or have increasing mantle (mafic)
content during extensional and rifting events
(Oligocene–Holocene; cf. Barton, 1996).
Trends in the diverse styles of mineralization parallel tectonic and magmatic events
(Fig. 11B). The abundance of known deposits
increases with time through the Paleogene and
then declines in the Neogene. Intrusion-associated deposits such as skarn and porphyry
systems are most common in the older rocks,
whereas epithermal systems are most common
in the mid-Tertiary (cf. Fig. 8). Compositional
variations in igneous-associated deposits correlate with variations in magmas (Barton et
al., 1995), whereas deposits of questionable or
non–igneous-related origin correlate with particular types of tectonism (e.g., shear-zone and
sedimentary-rock–hosted Au deposits in
northern Sonora with core complexes). Eastwest variations in deposit types at a given age
are common, such as Cu skarns to the west
and Cu porphyries to the east during early Tertiary time. Although deposit types peak in
abundance during particular periods, few commodities are restricted to single metallogenic
episodes (Figs. 8 and 11). For example, Au
occurs in economic deposits spanning 150
m.y. Deposit types cross time and space
boundaries and commonly correlate with exposed igneous rocks. The volcanic-rock–hosted deposits are in either young volcanic rocks
or volcanic rocks that are preserved in extended areas, such as the Jurassic arc of northcentral Sonora. Batholith-hosted deposits such
as W skarn deposits occur in older igneous
rocks (Baja California) or in the deeper zones
of extended terranes (east-central Sonora).
Ore-deposit types found in deeper environments are generally older, such as skarn and
greisen deposits, whereas shallower environments have younger deposit types like hotspring Au and sandstone-hosted Cu deposits.
METALLOGENIC CONTROLS
Metallogenic patterns in northwestern Mexico were influenced by spatial, temporal, or
Geological Society of America Bulletin, October 2001
TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO
process-related factors. No single set of factors can (or should) explain these variations;
rather, they reflect a combination of influences
(cf. Barton, 1996). Regional and temporal differences in the composition, thickness, thermal structure, and state of stress of the crust
affect the source of materials, nature of material transport, and depositional environments. Nonmagmatic fluids also reflect climate and tectonic regime. Finally, observed
patterns reflect exposure levels; preservation
or exhumation become key for interpreting
process and provincial patterns.
Spatial Controls
The provinciality of metal assemblages
(Fig. 11) has been interpreted as reflecting differences in crustal composition (e.g., Titley,
1991), but could also be influenced by differences in host rocks, available fluids, or shifting locus of magmatism. The eastward increase in Pb and Ag relative to Cu and Au
thus may reflect the increasingly felsic, Pbrich, Cu-poor crust with the transition from
crust formed in the western-domain continental volcanic margin to crust formed in the eastern-domain Precambrian basement and platform carbonates (Fig. 9). Alternatively, more
abundant carbonate rocks to the east may have
enhanced this trend by providing favorable
traps for Pb (and Ag). Metal and alteration
suites correlate with igneous compositions,
which in turn reflect crustal composition. Cu
contents of intrusion-hosted mineralization decline from west to east, whereas Pb and Ag
contents of carbonate-hosted systems increase.
A further regional pattern might reflect differences in surface-derived fluids. Rifting in the
gulf may circulate seawater or evaporitic
brines, leading to preferential transport of Cu
1 Fe 6 Mn, whereas dilute meteoric fluids in
continental basins may be more suited for precious-metal transport alone.
Temporal Controls
Temporal variations in the physical state of
the crust, in magmas, and in surface fluids influenced metallogenic patterns. Compressional
and extensional tectonic regimes produced
differences in the locus and character of magmatism and mineral deposits (Fig. 11). Compressional regimes during the Cretaceous and
Laramide produced systematic increases in
crustal contents of magmas through coupling
of magmatic evolution with thickening and
warming crust (Barton, 1996). Thus, metallogenic characteristics follow magmatic patterns
in time as well as in space. During neutral to
Figure 12. Estimated exposure depths for igneous rocks and principal deposit types in a crosssectional composite of northwestern Mexico. (A) Typical surface-exposed emplacement depths,
(B) environment for preserved mineralization, (C) types of deposits presently exposed.
extensional tectonics in the Jurassic and middle to late Cenozoic, heterogeneous magmatism, commonly bimodal, is typical. Deposits
not only reflect magmatic variability during
these times, but also may result from fluid circulation directly related to extension and basin
formation (cf. Seedorff, 1991; Ilchik and Barton, 1997). In contrast to compression, extension creates widespread permeability. Saline
waters generated from marine incursions during rifting or in closed basins may account for
some of the Fe-Cu-Mn-Au mineralization and
perhaps some base-metal mineralization not
directly related to magmatism.
Preservation and Exposure
The distribution of deposits through time in
northwestern Mexico reflects erosion and cover
(Fig. 12). Progressive exposure of deeper parts
of the crust by uplift and erosion or by tectonic
denudation generates a progression from older,
deeper exposures to younger, shallower depositional environments. The pattern of older batholithic to younger volcanic-rock–dominated terranes from west to east corresponds to
shallowing exposures. The complementary metallogenic signatures from weakly mineralized
deep plutonic through porphyritic to epithermal
environments reflect preservation and erosion—
not simply process and spatial controls. Differences in exposure levels help rationalize regional
metal-zoning patterns, which need not reflect
purely spatial or process controls. Uplift and/or
erosion expose deeper geologic areas, typically
uncovering magmatic sources and root zones to
hydrothermal systems. Crustal extension lowers
base levels, preserving near-surface environments. Burial, for example by younger volcanic
rocks, also helps preserve shallow levels. Both
extension and burial can be used to rationalize
the distribution of porphyry Cu occurrences in
northwestern Mexico (Barton et al., 1995). Prolonged weathering favors oxidation and supergene enrichment. These factors often govern the
economics of Cu and Au deposits and thus their
apparent distribution.
In conclusion, northwestern Mexico exhibits
superimposed metallogenic suites that correlate
with progressive magmatic and tectonic evolution in the past 150 m.y. across a complex continental margin. Metallogenic associations overlap and do not form restricted metallic belts.
Palinspastic reconstruction back through midTertiary extension aids evaluation of the timespace history of mineralization. The reconstruct-
Geological Society of America Bulletin, October 2001
1371
STAUDE and BARTON
ed distributions show that metallogenic zones
are thinner and typically superposed through
time. Metallogenic patterns cannot be rationalized in terms of single parameters; rather, they
reflect a combination of crustal and igneous
compositions, tectonic style, and preservation.
This type of reconstruction and preservation
study may be applicable to reevaluating magmatic and metallic belts worldwide.
ACKNOWLEDGMENTS
This paper has evolved from . 10 years of field
work with numerous contributing field visits, discussions, and reviews of manuscript versions from Eric
Seedorff, Fred McDowell, Jaime Roldán-Quintana,
Ricardo Amaya, John Stewart, Norman Page, Wojteck Wodzicki, Jose Perello, Suzanne Baldwin,
DeVerle Harris, Spencer Titley, and Robert Ilchik.
Peter Coney, Ricardo Torres, Pedro Restrepo, and
Joaquin Ruiz assisted in stratigraphic analysis. Lukas
Zurcher, Lance Miller, Karen Bolm, and Wodzicki
aided in acquisition of data on mineralization. Steve
Barney, Noelle Sanders, and Ricardo Torres collaborated in compiling radiometric age data. Mike Gutierrez and Giovanna Gamboa provided computer
drafting support. Ryan Houser, Frank Mazdab, Porfirio Padilla, Felipe Rendon, and David Simpson
joined in field work. Carl Kuehn, Peter Megaw, Jorge
Aranda, and Luca Ferrari supplied key references. Financial sponsorship came from Kennecott, Placer
Dome International, the United States Geological
Survey, and the University of Arizona Consortium
for Mexico geologic studies. Chris Henry and Jaime
Urrutia Fucugauchi reviewed the submitted manuscript, improving its final form.
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