Jurassic to Holocene tectonics, magmatism, and metallogenyof 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; Pe´rez-Segura and Jacques-Ayala, 1991; Aguirre-Dı´az and McDowell, 1993; Aranda-Go´mez et al., 1997; McDowell et al., 1997; Stewart et al., 1998; Henry and Aranda-Go´mez, 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
[email protected] 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; Mora´n-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 (Rolda´n-Quintana and Clark, 1992; Gonzalez-Leo´n 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 Rolda´n-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; Pe´rez-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 (Pe´rez-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; Rolda´nQuintana and Gonzalez-Leo´n, 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; Rolda´n-Quintana, 1991; Cocheme´ and Demant, 1991; Gonzalez-Le´on 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; Rolda´n-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 Rolda´n-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-Go´mez, 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 Rolda´n-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 (Pe´rez-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), Cocheme´ (1985), Rangin (1986), Henry and Fredrikson (1987), de Cserna (1989), Lonsdale (1989), Staude (1991), Ortega-Gutierrez et al. (1992), and Rolda´n-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 Rosalı´a 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-Pe´rez 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 (Pe´rez-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-Dı´az 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 (Rolda´n-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 Tera´n, 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-Landı´n, 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 Lucı´a) 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 Rolda´n-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-Go´mez, 2001). Rift-related Miocene Cu mineralization restores to the latitude of Culiaca´n, 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 Rolda´n-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. 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MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 2, 1999 REVISED MANUSCRIPT RECEIVED MARCH 21, 2000 MANUSCRIPT ACCEPTED APRIL 18, 2000 Printed in the USA Geological Society of America Bulletin, October 2001