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ABSTRACT cidental metallogenic province, thus ex- merous published and unpublished geologic panding the previously recognized extent of studies in the past two decades provide the The present metallic distribution in this province's mineralization. (3) Late opportunity for a new synthesis and interpre- northwestern Mexico is the culmination of Cretaceous±early Tertiary porphyry cop- tation of Mexican metallogeny in the frame- superposed magmatism, tectonism, erosion, per deposits and intrusive centers form a work of the current tectonic thinking. This pa- and burial over more than 150 m.y. De- narrower belt than previously noted and per presents a time-space synthesis for these tailed palinspastic reconstructions of preex- are traceable for over 400 km, with parts data in northwestern Mexico (Staude, 1995), tensional con®gurationsÐthe ®rst study of of the belt buried beneath the younger Si- generated as part of an integrated project on its kind for this regionÐclarify the inter- erra Madre Occidental volcanic ®elds. (4) Mexican mineral deposits and geology involv- play among these features on the present Interpreted alignments of older geologic ing collaboration among the University of Ar- distribution and character of mineralized features, including lineaments of ore depos- izona, the U.S. Geological Survey, and mining geologic systems. This new synthesis goes its, are displaced in the reconstructions. (5) companies. The results show that mineraliza- beyond previous metallogenic investiga- Sedimentary-rock±hosted gold deposits and tion is not individual mineral belts, but rather tions of northwestern Mexico by separating low-angle-detachment gold systems are a dynamic interplay of magmatism, tectonism, events into speci®c timing and structural closely related and occur around core erosion, and preservation. relationships, and by restoring the geology complexes. This study focuses on a 0.5 ϫ 106 km2 area to its preextensional con®guration. Metal- By using structurally restored time slices, in northwestern Mexico that has complex ge- logenic factors such as enrichment, preser- it becomes clear that older deposit types ology and many mineral deposits (Fig. 1). vation, and erosion play major roles in the tend to be those formed at greater depths Prominent mineral deposits in this region in- present distributions and for the ®rst time and more proximal to intrusions, whereas clude the major porphyry copper deposits of are related to the overall metallogenic younger deposits formed at shallower Cananea and La Caridad, Sonora; large, vol- framework of northwestern Mexico. The depths are less eroded and are more com- canic-rock±hosted precious-metal deposits analysis concludes that modern metallogen- monly volcanic-rock hosted. These charac- throughout the Sierra Madre Occidental; and ic patterns are the result of the complex su- teristics express themselves in the regional a number of other igneous-related and basin- perposition and subsequent redistribution distribution of deposit types. Second, min- related deposit types (e.g., Salas, 1975, 1994; of geologic systems in a way that is related eralization of widely differing ages is spa- Wisser, 1966). The large area permits broad directly to the regional history, rather than tially superposed, commonly associated regional comparisons to address the concepts simply metallic belts or interpreted angle of with coeval magmatic and tectonic events. of metal belts, preservation, and superposition a subducting slab. The structural and magmatic events togeth- of mineralizing events. Likewise, the complex Three main extensional events in the Ol- er with paleodistribution of ore deposits de- magmatic and deformational history of this re- igocene±Holocene have been restored, and ®ne a new framework to interpret the me- gion since the Early Jurassic allows evaluation the palinspastic distributions have been an- tallogenic history of northwestern Mexico. of metallogenic controls after appropriate re- alyzed. Reconstructions reveal the follow- constructions are made of distinctive time slic- ing: (1) Mineralization events, igneous cen- Keywords: metallogeny, Mexico, ore depos- es. Well-de®ned reconstructions are possible ters, and sedimentary sequences are its, reconstruction, Sonora, tectonics. with the many new geochronometric, structur- continuous across the Gulf of California al, and petrologic data generated in the past and other areas with large amounts of ex- INTRODUCTION decade (e.g., Henry, 1989; PeÂrez-Segura and tension. (2) Middle Tertiary gold-silver Jacques-Ayala, 1991; Aguirre-DõÂaz and Mc- mineralization in Baja California may be Although there has been considerable inter- Dowell, 1993; Aranda-GoÂmez et al., 1997; the western part of the Sierra Madre Oc- est in the tectonic evolution of Mexico (e.g., McDowell et al., 1997; Stewart et al., 1998; Sedlock et al., 1993), the region has received Henry and Aranda-GoÂmez, 2001). We synthe- *Present address: BHP Billiton., Avenida Amer- ica Vespucio Sur 100, 8th ¯oor, Las Condes, San- comparatively little attention in syntheses of size the characteristics, distribution, and tim- tiago, Chile; e-mail: John-Mark.G.Staude@ the metallogenic formation (Gonzalez-Reyna, ing of mineralization and tectonism that are BHPBilliton.com. 1956; Salas, 1975; Clark et al., 1982). Nu- based on new age, geologic, and palinspastic

GSA Bulletin; October 2001; v. 113; no. 10; p. 1357±1374; 12 ®gures.

For permission to copy, contact [email protected] ᭧ 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 ®eld work. We use the sional tectonism was followed by several magmatic and structural style rather than on results to interpret metallogenic controls. The styles of extensional tectonism beginning in basement and stratigraphic sequence. Meso- controls can be compared to other regions, and the middle Tertiary. These events generated zoic and Cenozoic magmatic and tectonic fea- the approach illustrates some of the complex- distinctive lithologic sequences and expose di- tures (which overlap the stratigraphic assem- ities of mineralization in a long-lived conti- verse crustal levels across the region. Al- blages of terrane terminology) help de®ne nental margin with multiple magmatic arcs. though terranes have been de®ned on base- these domains but typically cut terrane bound- ment (pre-Jurassic) stratigraphy (Campa and aries. Because mineralization commonly is the GEOLOGIC FRAMEWORK Coney, 1983; Sedlock et al., 1993), an alter- product of magmatism and tectonism, the do- native division simply considers the combi- mains help unify metallogenic observations. Northwest Mexico consists of translated nation of crustal structure, type, and level. and accreted terranes along the southwestern Three distinctive geologic domains (western, Lithologic Framework margin of the North American craton, which central, and eastern), compared to the nine ter- have been intruded and covered by coeval and ranes de®ned by Campa and Coney (1983), The western domain (Fig. 2) consists of ac- younger igneous rocks since the middle Me- provide a simple basis for comparison across creted marine sedimentary and volcanic rocks sozoic. Mesozoic and early Tertiary compres- the region (Fig. 2). The domains are based on and minor ma®c intrusions (Sedlock et al.,

1358 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 de®ne packages based largely on distinctions in Cenozoic superimposed effects. SMOÐSierra Madre Occidental.

1993; MoraÂn-Zenteno, 1994) cut by numerous Barranca Group overlie Paleozoic rocks in leozoic rocks record the transition from pre-Cenozoic thrust faults and younger strike- central Sonora, whereas to the north, Jurassic marine to subaerial environments. Local Tri- slip shear zones. The western half of Baja Cal- pyroclastic ¯ows and volcanogenic sediments assic and Jurassic volcano-sedimentary basins ifornia contains various accreted units ranging were deposited in basins that parallel the Ju- resemble those of the central domain. Volca- from continental-margin to ophiolitic rocks, rassic continental margin (Tosdal et al., 1989; nic-rock±free Lower Cretaceous sequences re- most of which have been intruded by multiple Riggs and Blakey, 1993). Widespread but cord the transition from the vast carbonate plutonic events and partly covered by Ceno- poorly dated mixed andesitic and clastic sed- platform sequences of northeastern Mexico to zoic volcanic rocks and conglomerates (Fig. imentary sequences have been assigned to clastic nearshore facies in central Sonora. 2; Abbott and Gastil, 1979). Jurassic±Creta- both the Late Cretaceous and early Tertiary, Younger limestone conglomerates, thin dis- ceous clastic sedimentary rocks and volcanic largely on the basis of stratigraphic correla- continuous sandstone beds, and volcanogenic rocks of the Alisitos Formation form an inte- tions and crosscutting relationships. These se- materials record transition to continental con- rior belt over much of western Baja Califor- quences are intruded and metamorphosed by ditions and impingement of the late Mesozoic nia. Rocks of the Alisitos Formation under- Cretaceous (in the west) to early Tertiary (in arc. Locally, Laramide volcanic rocks are went metamorphism during accretion and the east) metaluminous granitoids (Damon et common and are overlain by abundant Oli- subsequent contact metamorphism by the Pen- al., 1983b; Gastil, 1993). Abundant middle gocene±Miocene calc-alkaline to bimodal vol- insular Ranges batholith (Gastil et al., 1975). Tertiary calc-alkaline volcanic rocks are over- canic rocks (Ortega-Gutierrez et al., 1992; No pre-Mesozoic units are reported in this do- lain by Neogene coarse clastic sequences and McDowell and RoldaÂn-Quintana, 1993; Nieto- main, and all but the widespread Neogene basalts. Samaniego et al., 1999) and minor upper Ce- conglomerate and bimodal volcanic rocks are The eastern domain (Fig. 2) contains ex- nozoic syntectonic clastic rocks (McDowell et metamorphosed. posed Proterozoic basement overlain by mio- al., 1997). The central domain (Fig. 2) contains Cre- geoclinal sedimentary rocks, which are intrud- taceous tonalitic and dioritic batholithic rocks ed and overlain by Mesozoic and Cenozoic Magmatic and Tectonic Framework intruding Paleozoic to middle Mesozoic car- igneous rocks and continental sedimentary bonate, clastic, and minor volcanic sequences rocks (RoldaÂn-Quintana and Clark, 1992; Broadly continuous magmatism and tecto- of eugeoclinal af®nity (Gastil et al., 1981). Gonzalez-LeoÂn and Lawton, 1995). Laramide nism over the past 150 m.y. have been docu- This domain exhibits Mesozoic shortening compression was followed by highly variable mented across northwestern Mexico (Damon and later normal faulting. Highly deformed amounts of Cenozoic extension. This domain et al., 1981; PeÂrez-Segura, 1985; Sedlock et Paleozoic sedimentary rocks extend through- is the southwestern margin of the North al., 1993). Although earlier work demonstrates out the region, the oldest recognized being Or- American craton (Stewart, 1988). Eocambrian that overall patterns in Mexico parallel pat- dovician (Poole et al., 1991; Ortega-Gutierrez and lower Paleozoic miogeoclinal sedimenta- terns in the southwestern United States (e.g., et al., 1992). Continent-derived volcano-sedi- ry rocks are overlain by middle Paleozoic car- Dickinson, 1981), the characteristics of pet- mentary rocks of the Late Triassic±Jurassic bonate reef and platform deposits. Upper Pa- rogenetic 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 ¯uid ¯ow, as at the San Francisco mine in Sonora (PeÂrez-Segura et al., 1996).

Cretaceous (120±80 Ma) Cretaceous magmatism formed the batho- lithic 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 shal- low southward, reaching subvolcanic levels near lat 28ЊN, where numerous pendants con- tain volcanic rocks (e.g., at El Arco, Baja Cal- ifornia Norte; Fig. 4B). The west-to-east in- crease 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 far- ther-east±emplaced magmas in Baja California and Sonora (Taylor, 1986). Isotope ages and Pb isotope contours match across pre-Miocene Figure 3. Temporal distribution of igneous and deformational events in northwestern Mex- restorations of the Gulf of California (Silver ico, denoted as the numbers of outcrops per mountain range. (A) Abundance of exposed et al., 1993). plutonic and volcanic rocks. Younger rocks are more extrusive-dominated, and magma- Cretaceous thrusts verge both to the east tism has been active in different parts of the region for most of the past 110 m.y. (B) and west (King, 1939; Drewes, 1978; RoldaÂn- Relative abundance of deformational features. SonÐSonora, SinÐSinaloa, BCÐBaja Cal- Quintana and Gonzalez-LeoÂn, 1979; Jacques- ifornia, ChihÐChihuahua, DgoÐDurango. Ayala et al., 1990). They both cut and locally are cut by Cretaceous intrusions, yet the vol- Mexico have not been well resolved. On the coastal belt consists of Jurassic±Early Creta- canic rocks commonly show extensional faults basis of previous studies and our new work ceous ma®c to intermediate-composition vol- that apparently formed synchronously (e.g., El (Staude, 1995), we divide the history into ®ve canic rocks, sparse intrusions, and ophiolitic Arco; Barthelmy, 1979). Cretaceous brittle distinguishable petrogenetic and tectonic epi- suites (Sedlock et al., 1993). These rocks are and ductile faults host precious and base-metal sodes (Figs. 3 and 4; these occurred during the typically metamorphosed to greenschist facies mineralization. Rangin (1986) attributed these Late Jurassic±Early Cretaceous, Cretaceous, and deformed. The interior belt, which is sole- structures to the accretion of Baja California Late Cretaceous±early Tertiary, middle Tertia- ly Jurassic in age, contains intermediate to fel- to mainland North America. ry, and late Tertiary. Distinguishing features sic calc-alkaline to alkaline volcano-plutonic include magmatic compositions, tectonic complexes that are interpreted to have formed Late Cretaceous±Early Tertiary (80±40 styles, and levels of exposure. in an extensional arc setting (Busby-Spera, Ma) 1988; Saleeby et al., 1992; Tosdal et al., Late Cretaceous±early Tertiary (80±40 Ma, Jurassic±Early Cretaceous (ca. 150±120 1989). ``Laramide'') igneous rocks parallel the in- Ma) Jurassic±Early Cretaceous structure is com- ferred subducting arc along a south-southeast The Jurassic±Early Cretaceous episode plex and obscured by superposed events. Both trend through most of the eastern half of the marks the beginning of magmatism that has thrust and strike-slip structures in northwest- study area, extending from southern Arizona associated metallic mineralization in north- ern Mexico have been identi®ed as Jurassic and New Mexico into Durango and Sinaloa. western Mexico. Sparse plutonic and wide- (Anderson et al., 1980; de Cserna, 1989). This calc-alkaline granodioritic to granitic spread volcanic rocks occur in all three geo- Widely distributed left-lateral, northwest-strik- batholithic belt intrudes voluminous, coeval logic domains; they are concentrated along the ing shear zones in Sonora appear to be part of volcano-sedimentary rocks in western Mexico coast in Baja California and across the interior the megashear, which likely ceased activity (Henry, 1975; Damon, 1978; RoldaÂn-Quin- of Sonora, extending southeastward into Sin- prior to 150 Ma (Anderson and Silver, 1979). tana, 1991; Cocheme and Demant, 1991; Gon- aloa and Durango (Fig. 4A; Dickinson, 1981; Deformation in the Jurassic Parral Formation zalez-LeÂon et al., 2000). Outcrop patterns and Stewart et al., 1986). The deposition of vol- of Chihuahua and Durango could be the radiometric dates illustrate the general distri- canic rocks correlates in time with the trans- southeastern extension of this shearing. Thrust bution of Laramide magmatism, even though lational tectonics of the Jurassic, which and strike-slip faults in Sinaloa and Baja Cal- the volcanic sequences are poorly character- evolved into the Mojave-Sonora megashear ifornia juxtapose Mesozoic and Paleozoic ized because of pervasive hydrothermal alter- (Dickinson, 1981; Anderson et al., 1982). The rocks and are cut by Cretaceous batholiths. ation and stratigraphic complexity (Fig. 4C).

1360 Geological Society of America Bulletin, October 2001 TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO

These rocks appear discontinuously through the Oligocene±Miocene Sierra Madre Occi- dental volcanic province, emerging in central Chihuahua (McDowell and Mauger, 1994). Sr isotope ratios in igneous rocks increase from values of Ͻ0.706 in western (older) rocks to Ͼ0.708 in eastern (younger) rocks (Damon et al., 1983a; Mead et al., 1988; RoldaÂn-Quin- tana, 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 intru- sions that commonly localize mineralization (e.g., Bockoven, 1980). Laramide thrusting is prominent in the northeastern part of the region, although ex- tension and basin-bounding structures of prob- able Laramide age occur throughout the re- gion (Drewes, 1978; de Cserna, 1989). Transport directions and amounts of crustal shortening in Sonora vary greatly (Jacques- Ayala et al., 1990). In some areas in northern Sonora, 20%±40% shortening can be docu- mented (Merriam and Eells, 1978), but insuf- ®cient data exist to quantify the shortening across the Laramide orogen. Thrusting in So- nora 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 Oc- cidental 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 con- tinuation of the better-studied Oligocene vol- canic ®elds of the southern United States Figure 4. Time slices showing locations of (Christiansen and Lipman, 1972; McIntosh et isotope ages and correlative magmatic al., 1992; Spencer et al., 1995). Compared to units. Ages compiled from Ͼ100 sources Laramide rocks, the mid-Tertiary rocks are are reconstructed to their pre-Miocene ex- more felsic and less altered. They form a 0.5± tension in Figure 10. Outcrop locations 2-km-thick veneer across large parts of the modi®ed from Ortega et al. (1992) and age Laramide arc. sources. The transition from mainly andesitic, hy- drothermally altered rocks to dominantly dac- itic 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 re- mains to be well de®ned in most areas, al- though it is mostly Eocene. From central So- nora to the east, there appears to be little if in earlier periods (e.g., ⑀Nd ofϪ2.3 toϪ5.2; re¯ects an underlying coeval granitic batholith 87 86 any hiatus in magmatism during the transition, Sr/ Sri of Ͼ0.709 in western Chihuahua; Al- (Aiken et al., 1988). whereas in the west, a temporal break is well brecht, 1990). A regionally extensive gravity The mid-Tertiary marks the beginning of established (Fig. 4D; Clark et al., 1982). Geo- low along the eastern margin of Sonora into orogenic collapse in northwestern Mexico. chemical data indicate as much or more crust- western Durango roughly corresponds with Major extension with concomitant core-com- al component in the mid-Tertiary magmas as the axis of mid-Tertiary volcanism and likely plex 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, Si- erra Madre Occidental structural province (Gans, 1997; Stewart et al., 1998). Faults within the Sierra Madre Occidental structural province localize some volcanic centers (Stau- de, 1995). Certain normal faults predate Oli- gocene 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 postignim- brite faulting tilted mid-Tertiary volcano-sed- imentary sections by Ͼ30Њ. Later faults typi- cally trend northwest, are high angle, and commonly host middle Tertiary mineralization (Drier, 1984; Staude, 1994).

Late Cenozoic (20 Ma±Holocene) Figure 5. Cross sections from Jurassic to present, showing major fault systems during Magmatism became more heterogeneous each period at the latitude of Hermosillo, Sonora. Section not restored so as to more clearly and dispersed beginning in the early Miocene. represent superimposed features. Structures are generalized and represent major fault Early phases of this volcanism were felsic sets. AÐAway, TÐToward, SMOÐSierra Madre Occidental. tuffs and minor basaltic lavas that contain strong crustal signatures. Volcanism evolved in the Miocene to bimodal volcanism associ- ated 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 con®guration. Map sources include Davis (1980), DeJong et al. (1988), Jacques- Ayala et al. (1990), Nourse (1989), Staude (1993a), Stewart and RoldaÂn-Quintana (1994), and Staude (unpublished mapping).

1362 Geological Society of America Bulletin, October 2001 TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO oping the ma®c-dominated, rift-type volca- nism associated with generation of the Gulf of California. Basaltic andesites and high-K ba- saltic centers around the Gulf of California are variably tilted and were locally erupted syn- chronously with active high-angle extension and sedimentation (McDowell et al., 1997). Volcanism occurred throughout northwestern Mexico in the early Miocene, but became con- centrated around the Gulf of California in the late Miocene (Fig. 4E). These younger lavas have either alkaline or MORB-like character- Figure 7. Three-step reconstruction to the Ͼ 26 Ma (late Oligocene) preextension geography istics (MORB is mid-oceanic-ridge basalt) and of northwestern Mexico. Dark lines plot younger position; dashed line indicates older, re- occur in areas that are still tectonically active stored position. Shading delineates area of greatest extension between Paci®c Ocean and (Donnelly, 1974; Neally and Sheridan, 1989). Sierra Madre Occidental structural province. Circle is schematic restored strain ellipse. The younger lavas include the modern basaltic centers around the margins of the Gulf of Cal- ifornia and sparse centers throughout northern and RoldaÂn-Quintana, 1994; Stewart et al., Mexico (Sawlan, 1991; Fig. 4E). 1998; Staude, 1995). Minor extension, prob- Northwest-striking, high-angle normal and ably Ͻ5%, occurred throughout westernmost strike-slip faults characterized crustal attenu- Chihuahua and Durango; these areas are here ation during the late Cenozoic (Stock and treated as a ®xed block east of the extended Hodges, 1989; Staude, 1993a; Lee et al., domains. Strike-slip motion in these areas 1996). The faulting was synchronous and may have been signi®cant during the time of postdated the southwest-directed core-com- Basin and Range extension in areas to the plex extension. This post±core-complex fault- west, but few faults have been identi®ed with ing can be divided into two groups: Miocene± substantial strike-slip translation. Pliocene high-angle extensional faults (King, The third step involved restoration of the 1939; Henry and Aranda-GoÂmez, 1992) and core complexes and related extension. This late Miocene±Holocene normal and strike-slip step is the most dif®cult to carry out because (transtensional) faults associated with rifting few piercing points exist across the complexes (Moore and Buf®ngton, 1968; Atwater, 1970; and because of the compartmentalized nature Lonsdale, 1989; Stock and Hodges, 1989; At- of this extension (Nourse, 1992). This resto- water and Stock, 1998). High-angle normal ration results in 50% lineation-parallel short- faults are widely distributed. They trend north ening in the core-complex region of central in the north (INEGI, 1981; Stewart et al., and northern Sonora, diminishing to nothing 1998) and north-northwest in the south (Henry on the margins of the mid-Tertiary extensional and Fredrikson, 1987). Younger strike-slip and regime to the south and east (cf. Fig. 6). Pre- normal faulting is oblique (north-northwest vious studies of extension in the southwest Figure 8. Histogram of deposit types plot- trending) to the gulf and accommodates both Cordillera give comparable estimates for high ting the number of Au, Cu, W, Pb-Zn dis- extension and translation (Lonsdale, 1989). extension (Davis et al., 1981; Wust, 1986; tricts versus time period. Compiled data Dickinson, 1991; Richard, 1994), although no are available in the U.S. Geological Survey, Tectonic Reconstruction detailed reconstruction for Mexico has been Mineral Resource Data System, computer published previously. databank. Mineralization associated with Figure 7 shows a three-step palinspastic re- Although simpli®ed in nature, this model SZÐshear zone, IgnÐigneous, AdulÐad- construction for northwestern Mexico based enables an improved evaluation of the Paleo- ularia-quartz epithermal, SedÐsedimenta- on the information we have outlined. First, the gene distribution of rocks and mineral depos- ry, AcidÐacid sulfate epithermal, ReplÐ Gulf of California was closed by restoring the its. Ideally, one would like to carry the recon- replacement, PorpÐporphyry. likely protogulf con®guration (Stock and struction farther back in time; however, as Hodges, 1989). This step involves both mov- noted earlier, the lack of constraints on Lar- ing the Baja California Peninsula ϳ350 km amide and older structures precludes meaning- metal occurrences distributed throughout most southeastward along the San Andreas fault ful quantitative reconstructions of earlier de- of the region. The types include deposits as- system and restoring the east-northeast exten- formation. These earlier deformation events sociated with magmatic (intrusion- and extru- sion relative to mainland Mexico. are schematically illustrated in the time-space sion-related), metamorphic, and sedimentary Second, ϳ10% extension attributed to high- reconstruction that we describe next. rocks that formed over much of the past ϳ150 angle normal faulting was removed block-by- m.y. The main metals and the associated de- block across the states of Sonora and Sinaloa. MINERALIZATION posit types with their ages of formation are This step was based on tilt-direction maps summarized in Figure 8. Abundances for each showing fault orientation, block rotations, and Metallic mineralization is abundant in deposit type are tabulated from a computer- areas of higher degrees of extension (Stewart northwestern Mexico, with Ͼ2500 known ized database we developed for this study

Geological Society of America Bulletin, October 2001 1363 STAUDE and BARTON

adularia-sericite Ag-Au, and high-silica rhyo- lite volcanic centers with F Ϯ Mo. Low-sul- ®de 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 de- posits 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 signi®cant 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 (PeÂrez-Segura, 1985). Late Cretaceous±early Tertiary intrusions raised the geothermal gradient in central Sonora, result- ing in the formation of the numerous graphite deposits in carbonaceous sedimentary units. Evaporite-bearing basins developed during the middle Cenozoic extension and contain bo- rate, zeolite, and halite beds (Aiken and Kis- tler, 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

Although relatively few metallic deposits have been dated, it is possible to estimate the age of nearly 500 of the Ͼ1000 districts for Figure 9. Present distribution of major met- which geologic data have been compiled al districts and their related stratigraphy (Leonard, 1989; Staude, 1995). Few, if any, for major time intervals. Lithologic distri- metallic deposits of pre-Jurassic age are butions are compiled from Gastil et al. known in northwestern Mexico; thus, we be- (1975), Gastil and Krummenacher (1977), gin with the Late Jurassic. Bockoven (1980), Anderson and Silver (1979), INEGI (1981), Swanson and Mc- Jurassic±Early Cretaceous Dowell (1985), Cocheme (1985), Rangin The four main deposit types of Jurassic± (1986), Henry and Fredrikson (1987), de Early Cretaceous age are porphyry Cu, shear- Cserna (1989), Lonsdale (1989), Staude zone Au-bearing quartz veins, skarn, and vol- (1991), Ortega-Gutierrez et al. (1992), and canogenic Fe deposits (Fig. 9A). Early RoldaÂn-Quintana and Clark (1992). Cretaceous porphyry Cu mineralization is as- sociated with intermediate-composition intru- sive centers in central Baja California in the San Fernando±El Rosario areas. Early to Mid- dle Jurassic porphyry Cu mineralization is as- sociated with felsic magmatism in northern Sonora, extending into Arizona (e.g., Bisbee). (available through the U.S. Geological Sur- Ag-Zn-Pb skarns, W greisen deposits, Au-Ag Mesothermal Au-bearing quartz veins occur in vey, Mineral Resource Data System, and veins, and Ag-Pb-Zn replacement ores. Vol- thrust and strike-slip fault zones and are com- Leonard, 1989). Intrusion-related deposit canic deposit types include epithermal Au-Ag- mon in Jurassic clastic and metavolcanic rocks types include Cu Ϯ (Mo, W), Fe-W-Cu and (Pb-Zn-Cu) veins, advanced argillic Au-Cu, along a northwest trend paralleling the Juras-

1364 Geological Society of America Bulletin, October 2001 TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO sic arc (Fig. 9A). The shear-zone±hosted veins additional associated mineralization. The de- tends many kilometers away from known de- may be related to motion along the Arizona- posits are small, rarely exceeding 2 ϫ 106 posits (Sillitoe, 1976). These form part of the Sonora megashear (Silberman et al., 1988) or tonnes of ore, and were mostly mined during world-class porphyry Cu (ϮMo) province that to forearc deformation related to arc accretion. World War II (Fries and Schmitter, 1945). extends from northwestern Arizona through Ages of deposits are uncertain because few The best-known middle Cretaceous porphy- Sonora into Sinaloa and Durango (Fig. 9B). dates on veins have been published for either ry Cu (ϮAu) deposit is at El Arco in central Within this belt, the porphyry Cu±related in- the mainland or peninsular localities (Men- Baja California (Barthelmy, 1979). Others oc- trusions are older to the south and west. Cu chaca, 1985). cur in Sinaloa and northern Baja California and Pb-Zn skarns are widespread where cal- Late Jurassic±Early Cretaceous marine, arc- and correlate with 120±80 Ma magmatic careous host rocks are present (PeÂrez-Segura, related volcanic and sedimentary rocks host events (cf. Fig. 4B). The Cu porphyry systems 1985; Consejo de Recursos Minerales, 1992, Fe-Cu (ϮAu) deposits along the western mar- have lower initial 87Sr/86Sr values, are more 1993; Valencia-Moreno, 1998). In western So- gin of the Sierra Madre Occidental in Sinaloa dioritic, and have higher Au/Cu ratios than nora, Cu skarn occurrences are common, as (continuing south into Nayarit) and along their younger Laramide counterparts. Creta- are equigranular intrusions and regionally western and central parts of the Baja Penin- ceous porphyries are preserved in areas that metamorphosed rocks, whereas porphyry- sula. These hydrothermal Fe deposits and Fe have been less deeply eroded, particularly in style mineralization and volcanic rocks are un- (ϮCu, Au) skarns are penecontemporaneous central Baja California, where volcanic rocks common. Cu-bearing porphyry-style mineral- with ma®c and intermediate-composition of similar age surround the porphyry deposits ization is associated with intrusive centers magmatic centers (Zurcher, 1994). Hydrother- (Echavarri-PeÂrez and Rangin, 1978). In north- older than 60 Ma in central Chihuahua (Mc- mal magnetite-hematite vein occurrences cut ern Baja California, Cretaceous rocks are Dowell and Mauger, 1994) and northern Du- Jurassic±Early Cretaceous basaltic ¯ows in more deeply eroded. Equigranular, coarse- rango (Aguirre-DõÂaz and McDowell, 1991). Baja California, particularly between Santa grained, intermediate-composition batholithic These occurrences and others in deep canyons RosalõÂa and El Rosario (Menchaca, 1985). complexes contain W skarn and greisen min- within the Sierra Madre Occidental (Barton et These deposits are typically metamorphosed eralization rather than porphyry centers. al., 1995) suggest that the Laramide porphyry to lower greenschist grade by the Cretaceous Two other deposit types that likely formed province extends beneath much of the younger Baja California batholith. Cu staining and tur- during the middle Cretaceous are intrusion- Sierra Madre Occidental ignimbrite sequence. quoise are found in at least four of the Fe dis- hosted bonanza Au-Ag quartz veins and me- Tungsten and Mo become more important tricts. These deposits appear to belong to a sothermal Au-bearing quartz veins in shear commodities in more felsic intrusive systems, middle Mesozoic belt of Fe-oxide (ϮCu, Au) zones cutting greenschist-facies rocks (Mother which overlap with, but in general are younger occurrences along the Cordillera (Barton and Lode±type deposits). Intrusion-hosted quartz than, the Cu-rich centers. Transitional Mo-W Johnson, 1996). Ni, Co, and Cr deposits host- veins in Baja California are as long as several (ϮCu) porphyry, breccia-pipe, and skarn de- ed in accreted marine sedimentary and vol- kilometers and locally blossom into Ͼ50-m- posits (e.g., Cumobabi and Santa Ana, Sonora) canic-intrusive (ophiolitic) rocks on the Viz- wide stockworks (Menchaca, 1985). In Sonora occur with metaluminous quartz-feldspar por- caino Peninsula and other parts of and Sinaloa, similar veins occur, but their ages phyries of early Tertiary age. Tungsten-domi- westernmost Baja California are associated are dif®cult to determine because of plutonic nated deposits are best developed in central So- with ma®c igneous units, are strongly serpen- overprinting. Mesothermal Au-Ag quartz nora and extend southeastward into tinized, and are highly disrupted by multiple veins are common along the western margin northwestern Durango. The largest tungsten stages of postmineralization faulting. Associ- and within roof pendants of the Peninsular districts are in the largest intrusive massif of ated intrusions have Early Cretaceous U-Pb Ranges batholith. The veins form anastomos- Sonora, the Aconchi batholith (e.g., El Jaralito ages (Sedlock et al., 1993). ing stockworks of quartz, carbonate, and chlo- district). Most W-rich systems are associated rite with small, high-grade bonanza Au shoots with peraluminous granitoids (RoldaÂn-Quin- Cretaceous (Wisser, 1954). tana, 1991), unlike the Cretaceous tungsten dis- During the mid-Cretaceous, economically tricts of Baja California and transitional Mo-W signi®cant Cu (ϮAu) porphyries, W skarns, Laramide (ϮCu) deposits that are associated with metal- and mesothermal Au-bearing quartz veins Diverse and abundant mineral deposits uminous granitoids. Tungsten deposits young formed in the western half of the region (Fig. formed during the Laramide (Late Creta- from west to east and change from granodio- 9A). These deposits form belts that parallel ceous±early Tertiary) in northwestern Mexico. rite-associated skarns in the west to granite-as- the Cretaceous arc. Few locations other than Cu (ϮMo) porphyries and skarns, W and Pb- sociated greisen deposits in the east (Weise, intrusive centers have been directly dated, and Zn skarns, and Au-Ag quartz veins are the 1945; Mead et al., 1988). This control may be the ages of the remaining deposits are inferred most economically signi®cant. These deposits the effect of level of exposure, because deeper from ®eld relationships. The most abundant are east of the middle Cretaceous ones, found carbonate rocks are exposed in the west, and types of mineralization are W skarn and grei- mainly in Sinaloa and Sonora. Mineralization younger, shallower clastic sedimentary and vol- sen deposits, followed by Au-bearing quartz is related to calc-alkaline magmatism and was canic rocks are preserved in the east. veins and porphyry Cu deposits (Fig. 8). emplaced in volcanic to fairly deep plutonic Numerous crustiform, low-sul®dation Au- Tungsten skarns become increasingly common environments with a range of styles of hydro- Ag quartz veins are widespread throughout the northward from central Baja California. They thermal alteration, brecciation, and deposition central and eastern parts of the study area. are most common in Paleozoic carbonate (e.g., Bushnell, 1988; Wilkerson et al., 1988; Many of these are inferred to be Laramide on rocks intruded by tonalitic and granodioritic Barton et al., 1995). the basis of truncation of veins and alteration intrusions (Weise, 1945; Menchaca, 1985). Porphyry Cu (ϮMo) deposits are wide- zones prior to mid-Tertiary volcanic units Tungsten most commonly occurs in scheelite, spread in areas containing Laramide volcanic (Staude, 1995). Precise ages are available for both in skarn and quartz veinlets with little rocks; hydrothermal alteration commonly ex- 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 gocene to early Miocene, cut or cap these sys- Over a dozen hot-spring Au occurrences have larger districts are plotted in Figure 9B to in- tems (Bockoven, 1980; Duex, 1983; Cameron been identi®ed around the gulf. Most deposits dicate their general distribution. These veins are et al., 1989). lie along the western edge of the gulf, where hosted in granodiorite intrusions, metamor- Carbonate- and volcanic-rock±hosted min- transform faults and fault splays extend onto phosed sedimentary rocks, and andesitic-dacitic eralization on the eastern ¯ank of the Sierra land along the eastern parts of the Baja Cali- ¯ows and tuffs. They have Ag/Au ratios of Madre Occidental is best known for large Ag- fornia peninsula (Staude, 1992). The hot- Ͼ100 (commonly Ͼ1000), and minor Zn, Pb, Pb-Zn deposits and also includes many Hg, spring deposits are hosted in units as old as and Cu sul®de minerals (Wisser, 1966; Clark et As, Sb, Mn, Sn, and U occurrences. Sparse Miocene (Santa LucõÂa) and as young as Ho- al., 1979). They can exceed several kilometers geochronometry and regional correlations in- locene beach sands (Puertocitos). in length and commonly show systematic zo- dicate that many carbonate-hosted deposits are nation, in some cases around intrusive centers, Oligocene (Megaw et al., 1988). However, the TIME-SPACE DISTRIBUTION OF such as at Alamos and Batopilas (Loucks and fact that similar systems formed during the MAGMATISM AND MINERALIZATION Petersen, 1988; Wilkerson et al., 1988). Laramide (Piedras Verdes [Chihuahua], La Shear zones hosting Au mineralization, Reforma, Oposura-Moctezuma) demonstrates Preextension Events such as at La Choya and Quitovac in western that repeated skarn- and replacement-style Sonora, have white mica that crystallized dur- mineralization occurred in areas where mag- The geology of northwestern Mexico is ing Laramide time (80±50 Ma) (Durgin and matism overlaps the carbonate-rich eastern do- complex, with superposed magmatic and me- TeraÂn, 1996; Alex Iriondo, 1999, personal main (Figs. 2 and 9C). tallogenic events that have in many cases been commun.). These shear-zone deposits are Sedimentary-rock±hosted and low-angle- translated from their location of formation by complex; Jurassic and older host rocks are shear-zone Au deposits found to the west of later events. Previous metallogenic summa- covered by extensionally faulted Miocene vol- the Sierra Madre Occidental volcanic province ries, based on collaboration among the Uni- canic rocks. The precise age of mineralization may be broadly coeval with volcanic-rock± versity of Arizona, the U.S. Geological Sur- is poorly determined. hosted deposits in the Sierra Madre Occidental vey, and mining companies, have de®ned belts but are not necessarily directly related to according to present-day con®gurations (Sa- Middle Tertiary magmatism. K-Ar ages on postmineralization las, 1975). Clark et al. (1982) accounted for Like the Laramide, the late Eocene±Oligo- basaltic dikes and prealteration two-mica some of the translation associated with the cene was a major period of metallic mineral- granites restrict Au mineralization in the San- most recent Gulf of California rifting but did ization in western Mexico, with the formation ta Teresa district to 36±28 Ma (Bennett and not take into account earlier extension or dis- of diverse types and a large number of ore Atkinson, 1993), which is consistent with ev- placements inland of the gulf. By using the deposits, most of which are associated with idence from other districts where minerali- tectonic reconstruction already presented, the volcanic rocks of the Sierra Madre Occidental. zation is associated with low-angle (exten- metallogeny of northwestern Mexico can be The principal types are low-sul®dation Ag-Au sional?) faults and is cut by high-angle reevaluated, starting with the Late Jurassic. (ϮPb-Zn-Cu) veins, high-sul®dation Au-(Cu) (Neogene) faults (Fig. 9D). Deposit character- The preextension restoration allows tracing deposits, and high-temperature carbonate- istics include strong stratigraphic control by of the outcrops of Jurassic volcanic and vol- hosted deposits (Fig. 9C). Other probable mid- favorable beds and jasperoidal Au-Hg-As cano-sedimentary rocks with sparse intrusive Tertiary mineralization includes sedimentary- mineralization (Bennett, 1993), which resem- centers from southern Arizona through north- rock±hosted and low-angle-fault-zone Au bles that of Carlin-like deposits (Vikre et al., ern Sonora and southeastward into Mexico. deposits. These two may be directly related to 1997). Deposit distribution corresponds with This restoration of the Jurassic arc shows a mid-Tertiary extension (Fig. 9D). that of major extension areas containing abun- trend similar to those presented by Dickinson Of the Ͼ800 middle Tertiary volcanic- dant sedimentary rocks (Fig. 9D). A less (1981) and Tosdal et al. (1989). The three rock±hosted epithermal precious-metal occur- clearly de®ned group of Au-bearing quartz de- main groups of Jurassic rocks (Fig. 10A) form rences known in northwestern Mexico (Fig. posits occurs in low- and high-angle shear a belt of volcanic and minor intrusive rocks 9C; Leonard, 1989; Orris et al., 1993; Staude, zones west of the sedimentary-rock±hosted with scattered clastic sequences. The units of 1993b), the majority are quartz Ϯ calcite veins deposits (e.g., La Herradura, Chinate). These eastern Baja California and Sonora appear to with chlorite ϩ adularia ϩ sericite alteration have con¯icting data indicating both Laramide join with Jurassic rocks of the Parral Forma- halos. These deposits, such as at Ocampo and mid-Tertiary ages (Leonard, 1989; Alex tion in southern Chihuahua and thus link the (Knowling, 1977) and Maguarichic (Staude, Iriondo, 1999, personal commun.). geology beneath the Sierra Madre Occidental. 1995), are Ag dominated with spotty Au and Mineral deposits of possible Jurassic age re- base-metal pockets. Advanced argillic Au Late Tertiary±Holocene store to closer proximity, and similar deposits (ϮCu) districts, such as Mulatos (Staude, Sedimentary-rock±hosted stratiform Cu de- become grouped once extension is taken into 2001), are scarcer but number in the tens. Dis- posits and hot-spring Au deposits are associ- account. seminated (Moris), hot-spring (Pinos Altos), ated with opening of the Gulf of California By using this same preextension reconstruc- and polymetallic vein districts (Uruachic) are (Fig. 9E). The Boleo district contains Cu-Co- tion, the Cretaceous intrusive rocks in western also present. Most districts lack precise dates Ag and manganese oxide deposits in upper Sonora can be traced to the restored Baja Pen- but can be assigned an Oligocene age by re- Miocene clastic rocks and tuffs related to early insula; the Cretaceous W deposits can be gional correlation to basal rhyolitic ignim- stages of Gulf of California opening (Wilson linked between northern Baja California and brites that host mineralization or underlie ore- and Rocha, 1955; Schmidt, 1975; Ochoa-Lan- Sonora; Mesozoic accretionary sedimentary hosting volcanic rocks (Swanson and dõÂn, 1998). Sedimentary-rock±hosted Cu pros- rocks can be restored between central Baja McDowell, 1985; Wark et al., 1990). Ma®c pects extend several hundred kilometers along California and southern Sonora; and the por- dikes and ¯ows, regionally dated at late Oli- the western edge of the gulf (Staude, 1992). phyry Cu district of El Arco correlates with

1366 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 con®guration.

Geological Society of America Bulletin, October 2001 1367 STAUDE and BARTON

Figure 10. (Continued.)

Cu-bearing parts of the Sinaloa batholith (Fig. continues northward into the reconstructed po- continue the younging trend that has been 10B). When the abundance of Cretaceous in- sition of southern California. The abundance documented between Ensenada and Mexicali trusive outcrop area is contoured, one can see of Cretaceous intrusions appears to decrease (Ortega-Rivera et al., 1994); however, more how the batholith restores across the gulf and to the east. Also to the east, the intrusions may data are needed. To the west along the Paci®c

1368 Geological Society of America Bulletin, October 2001 TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO coastal edge of Baja California, accretion of superposed metal associationsÐnot as separate volcanism (Fig. 10E) and contour the restored forearc sedimentary rocks and ophiolites con- metallic belts. The clusters correspond with geography on the basis of the thicknesses and tributed to the metallogeny during Cretaceous many of the exposed Laramide igneous rocks extents of tuffs older than 27 Ma. Radiometric time, although their precise locations during (Fig. 10D). This ®nding could be quite signif- dating in Baja California indicates that the the Cretaceous is uncertain (Hagstrum et al., icant, because it suggests that much of the Lar- earliest middle Tertiary volcanism began at ca. 1985). The Cr, Co, and Ni deposits formed amide is mineralized. Moreover, if more Lar- 30 Ma (Gastil et al., 1975), and the volume with ma®c magmatism during Late Jurassic amide rock areas are discovered, they might increased as time progressed. In Baja and in and Early Cretaceous time and were accreted have substantial mineralization. Exposure plays westernmost Sonora, the few locations of rhy- to Baja California prior to the Cenozoic (Ab- a vital role in controlling the present-day ap- olitic volcanism link these regions to the more bott and Gastil, 1979). Since Oligocene time, pearance of the metal distribution, and it is pos- coherent and extensive Sierra Madre Occiden- the western part of Baja California has not un- sible that prior to erosion and Cenozoic vol- tal volcanic province to the east. dergone tremendous extension; however, canic superposition, there could have been an translation along faults parallel to the San An- even larger extent of exposed rocks and min- Synextension and Postextension Events dreas and Agua Blanca fault systems has eralization. Looking at the present-day abun- caused right-lateral movement, for which the dance of deposits in southern Arizona and Two major periods of mineralization in reconstruction accounts. northern Sonora, there are potentially hundreds northwestern Mexico may be restored approx- For the Late Cretaceous±early Tertiary, the of deposits that were covered by younger vol- imately to their synextension con®gurations. reconstruction proves useful for interpreting the canic rocks of the Sierra Madre Occidental The ca. 27±20 Ma extensional faulting wid- batholiths in Sonora and Sinaloa and their re- from central Sonora to southern Durango. Lar- ened Sonora (Gans, 1997), possibly forming lationship to rocks of similar age in southern amide igneous rocks crop out in most canyons sedimentary-rock±hosted Au systems in the Arizona. Once restored, mineralized systems that expose pre±middle Tertiary rocks through- east and shear-zone Au deposits in the west form a narrow belt trending southeast along the out the Sierra Madre Occidental. These out- (Fig. 10E). The mid-Tertiary, post±core-com- western edge of Mexico rather than a wide crops help de®ne the contours of the abundance plex reconstruction mainly affects the northern bulge as one sees today in the Sonora-Arizona of magmatic rocks. and central parts of Sonora; the dotted line region. The bulge narrows by as much as 50% Loops in Figure 10C delineate Cu districts surrounding much of Sonora in Figure 10F re- once postmineralization extension is removed, that are the most widely distributed (they are constructs the approximate boundary of Ͼ30Њ and the width becomes more typical of other outlined by the light-weight dashes). Tungsten Tertiary tilting and areas of low-angle (Ͻ25Њ) porphyry Cu belts throughout the world (e.g., deposits are inside the area de®ned by the out- normal faults (modi®ed from Stewart and Rol- central Andes [Sillitoe, 1988], British Colum- line enclosing the Cu districts. Tungsten de- daÂn-Quintana, 1994). During the core-com- bia [McMillan et al., 1995]). By contouring the posits are recognized by our study much far- plex extensional event, northern Sonora may extent of 80±40 Ma intrusive centers and the ther east and over a larger area than in have extended as much as 30%, and some ar- restored present-day outcrop areas (Fig. 10C), previous metallogenic summaries for Mexico eas had Ͼ100% extension (Nourse, 1989; the abundance of plutonic rocks is found to be (Clark et al., 1982). In the reconstruction, Pb- Nourse et al., 1994); these are restored to their high beneath the Sierra Madre Occidental. Zn, W, Au-Ag, and Cu districts occur in the pre±Basin and Range extension position in Without the cover of the volcanic rocks, both same areas, showing their superposition prior black. The region of low-angle-shear-zone Au undated Laramide and younger, the abundance to extension. deposits is within the area of middle Tertiary of Laramide intrusions would be larger. Por- Late Eocene±Oligocene mineralization and extension; many deposits occur around and to phyry systems, exposed in windows through magmatism can be restored to approximate the west of restored core complexes. The Ja- the younger ignimbrites of the Sierra Madre their distribution prior to extension, although joba district may have been faulted off the top Occidental, aid in tracing the batholith and sug- some of this mineralization is likely related to of the Magdalena core complex. Sedimentary- gest a high probability for other deposits be- the onset of extension and may have formed rock±hosted Au districts restore to the eastern neath the younger volcanic rocks. Study of pre- as extension was underway. Au deposits are edge of core complexes but reside within the sent lineaments and position of porphyry Cu most abundant in the Sierra Madre Occidental area extended during middle Tertiary time. deposits do not take into account the tripartite volcanic province along the Sinaloa-Durango One hypothesis for this association is that the deformation. Once the deformation is restored, and Sonora-Chihuahua state borders; however, deposits are related to extensional phenomena, many of the lineaments no longer appear as a few Au-Ag deposits of Oligocene age are as has been suggested for middle Tertiary sed- prevalent. Lineaments, if they do exist, become found farther west in northern Sonora. The Au imentary-rock±hosted mineralization in cen- rotated from their present orientations, and if (ϮCu) districts within the Sierra Madre Oc- tral Nevada (Seedorff, 1991). The Sonoran de- lineaments are used for interpretations, they cidental are at present recognized in only a posits have had far less study than need to be corrected for the substantial dis- few areas; thus, their extent of distribution is sedimentary-rock±hosted deposits in central placements of the past 30 m.y. Some deposits, preliminarily drawn as a smaller region within Nevada, and interpretations of the genesis for such as Cananea (Wodzicki, 1995) are moder- the larger Au-Ag districts of the Sierra Madre Sonoran deposits are speculative. ately tilted, whereas others are strongly extend- Occidental (Fig. 10E). Carbonate-hosted Pb- The late Tertiary reconstruction shows the ed and dissected as at Piedras Verdes, Sonora Zn-Ag deposits occur to the east and along the distribution of volcanic-rock± and sedimentary- (Drier and Braun, 1995). western edge of the Sierra Madre Occidental. rock±hosted hot-spring Au districts and the Bo- Outlines of regions with porphyry Cu, Au- The full extent of volcanism during the mid- leo stratiform Cu district (Fig. 10G). These de- Ag, W, and Pb-Zn deposits indicate that the Tertiary ignimbrite period is not known; yet, posits are associated with the rift opening of mineral deposits follow the same south-south- by restoring the deformation and using the the Gulf of California and restore to the general east trend as the magmatic arc (Fig. 10C). currently exposed and dated outcrops, it is boundary of the protogulf region. The recon- Moreover, mineralization clusters in areas with possible to estimate the extent of Oligocene struction reveals that the region is encompassed

Geological Society of America Bulletin, October 2001 1369 STAUDE and BARTON

volcanic rock types are preserved, whereas in the compressional periods, one ®nds intru- sions. The deposit types plotted in Figure 11B show this association. There is an overall in- crease in the ratio of abundance of volcanic rocks to plutonic rocks beginning in the Cre- taceous, 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 associa- tion is that in single areas, the magmatic com- positions vary over the 10±25 m.y. periods of activity; crustal (felsic) contents increase dur- ing compressional events (Cretaceous±Oligo- cene), whereas compositions are heteroge- neous or have increasing mantle (ma®c) content during extensional and rifting events (Oligocene±Holocene; cf. Barton, 1996). Trends in the diverse styles of mineraliza- tion 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-asso- Figure 11. Time-space synthesis of magmatism, deformation, and mineralization across ciated deposits such as skarn and porphyry Baja California through Sonora to western Chihuahua at latitude of Hermosillo. The systems are most common in the older rocks, present 500 km width has changed due to older compression and younger extension, whereas epithermal systems are most common in the mid-Tertiary (cf. Fig. 8). Compositional creating an hourglass shape. Right diagram summarizes mineralization with font sizes variations in igneous-associated deposits cor- corresponding to the relative abundance of metallic ore-deposit types. The position of the relate with variations in magmas (Barton et Gulf of California is marked by the dashed black line. ChihÐChihuahua. al., 1995), whereas deposits of questionable or non±igneous-related origin correlate with par- by late Tertiary ma®c igneous rocks and the tion, and mineralization in northwestern Mex- ticular types of tectonism (e.g., shear-zone and generalized location of the restored, pre±gulf- ico. Although the observations summarized sedimentary-rock±hosted Au deposits in opening geography (modi®ed from Atwater, are not exhaustive, general patterns are clear northern Sonora with core complexes). East- 1970; Lonsdale, 1989; Stock and Hodges, and provide the basis for broad interpretation. west variations in deposit types at a given age 1989). The reconstruction indicates that open- Overall, magmatism was largely continuous are common, such as Cu skarns to the west ing of the gulf during the past 6 m.y. yielded while systematically changing location and and Cu porphyries to the east during early Ter- greater extension in the southern part of the composition with time. One or more ill-de- tiary time. Although deposit types peak in gulf, which necessitates more closure to restore ®ned Jurassic arcs in the eastern and western abundance during particular periods, few com- than points farther north (cf. Henry and Aran- domains were superseded in the Cretaceous by modities are restricted to single metallogenic da-GoÂmez, 2001). Rift-related Miocene Cu a well-developed coastal batholith that was episodes (Figs. 8 and 11). For example, Au mineralization restores to the latitude of Culia- occurs in economic deposits spanning 150 built mainly across the continental-margin±ac- caÂn, Sinaloa (Schmidt, 1975; Guilbert and Da- m.y. Deposit types cross time and space creted metasedimentary package of the west- mon, 1977). Hot-spring Au districts ring the boundaries and commonly correlate with ex- Gulf of California, with a larger abundance of ern domain. Gradual and then more rapid east- posed igneous rocks. The volcanic-rock±host- deposits on its western margin. This asymme- ward migration of magmatism began in the ed deposits are in either young volcanic rocks try may be due to the proximity of spreading late Mesozoic through the mid-Tertiary (Fig. or volcanic rocks that are preserved in ex- and rift centers along the eastern coast of north- 11A). These changes correlate with compres- tended areas, such as the Jurassic arc of north- ern Baja California over the past 12 m.y., in sional deformation, indicated in Figure 11 by central Sonora. Batholith-hosted deposits such addition to preservation and covering by youn- the shortening of the width from Paci®c Ocean as W skarn deposits occur in older igneous ger Pliocene and Quaternary clastic sedimen- to the Chihuahua-Sonora border. Subsequent rocks (Baja California) or in the deeper zones tary units around the gulf (Staude, 1993a). retreat of magmatism toward the coast in the of extended terranes (east-central Sonora). The three-step reconstruction can be sum- early Cenozoic was followed by the change Ore-deposit types found in deeper environ- marized in a time-space model linking tecto- from subduction-related to rift-related mag- ments are generally older, such as skarn and nism, magmatism, and mineralization, thereby matism during the Miocene; these magmatic greisen deposits, whereas shallower environ- providing a basis for interpreting the distri- patterns correlate with a shift to neutral, then ments have younger deposit types like hot- bution of mineralization through time. transtensional tectonics, shown in Figure 11 spring Au and sandstone-hosted Cu deposits. by the width's expanding and by the preva- Synthesis METALLOGENIC CONTROLS lence of volcanic rocks, unlike the older pe- Figure 11 summarizes the temporal and riod with more intrusion-dominated rock Metallogenic patterns in northwestern Mex- spatial relationships of magmatism, deforma- types. In general, it is in the rift periods that ico were in¯uenced by spatial, temporal, or

1370 Geological Society of America Bulletin, October 2001 TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO process-related factors. No single set of fac- tors can (or should) explain these variations; rather, they re¯ect a combination of in¯uences (cf. Barton, 1996). Regional and temporal dif- ferences in the composition, thickness, ther- mal structure, and state of stress of the crust affect the source of materials, nature of ma- terial transport, and depositional environ- ments. Nonmagmatic ¯uids also re¯ect cli- mate and tectonic regime. Finally, observed patterns re¯ect 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 re¯ecting dif- ferences in crustal composition (e.g., Titley, 1991), but could also be in¯uenced by differ- ences in host rocks, available ¯uids, or shift- ing locus of magmatism. The eastward in- crease in Pb and Ag relative to Cu and Au thus may re¯ect the increasingly felsic, Pb- rich, Cu-poor crust with the transition from crust formed in the western-domain continen- tal volcanic margin to crust formed in the east- ern-domain Precambrian basement and plat- Figure 12. Estimated exposure depths for igneous rocks and principal deposit types in a cross- form carbonates (Fig. 9). Alternatively, more sectional composite of northwestern Mexico. (A) Typical surface-exposed emplacement depths, abundant carbonate rocks to the east may have (B) environment for preserved mineralization, (C) types of deposits presently exposed. enhanced this trend by providing favorable traps for Pb (and Ag). Metal and alteration suites correlate with igneous compositions, extensional tectonics in the Jurassic and mid- deep plutonic through porphyritic to epithermal which in turn re¯ect crustal composition. Cu dle to late Cenozoic, heterogeneous magma- environments re¯ect preservation and erosionÐ contents of intrusion-hosted mineralization de- tism, commonly bimodal, is typical. Deposits not simply process and spatial controls. Differ- cline from west to east, whereas Pb and Ag not only re¯ect magmatic variability during ences in exposure levels help rationalize regional contents of carbonate-hosted systems increase. these times, but also may result from ¯uid cir- metal-zoning patterns, which need not re¯ect A further regional pattern might re¯ect differ- culation directly related to extension and basin purely spatial or process controls. Uplift and/or ences in surface-derived ¯uids. Rifting in the formation (cf. Seedorff, 1991; Ilchik and Bar- erosion expose deeper geologic areas, typically gulf may circulate seawater or evaporitic ton, 1997). In contrast to compression, exten- uncovering magmatic sources and root zones to brines, leading to preferential transport of Cu sion creates widespread permeability. Saline hydrothermal systems. Crustal extension lowers ϩ Fe Ϯ Mn, whereas dilute meteoric ¯uids in waters generated from marine incursions dur- base levels, preserving near-surface environ- continental basins may be more suited for pre- ing rifting or in closed basins may account for ments. Burial, for example by younger volcanic cious-metal transport alone. some of the Fe-Cu-Mn-Au mineralization and rocks, also helps preserve shallow levels. Both perhaps some base-metal mineralization not extension and burial can be used to rationalize Temporal Controls directly related to magmatism. the distribution of porphyry Cu occurrences in northwestern Mexico (Barton et al., 1995). Pro- Temporal variations in the physical state of Preservation and Exposure longed weathering favors oxidation and super- the crust, in magmas, and in surface ¯uids in- gene enrichment. These factors often govern the ¯uenced metallogenic patterns. Compressional The distribution of deposits through time in economics of Cu and Au deposits and thus their and extensional tectonic regimes produced northwestern Mexico re¯ects erosion and cover apparent distribution. differences in the locus and character of mag- (Fig. 12). Progressive exposure of deeper parts In conclusion, northwestern Mexico exhibits matism and mineral deposits (Fig. 11). Com- of the crust by uplift and erosion or by tectonic superimposed metallogenic suites that correlate pressional regimes during the Cretaceous and denudation generates a progression from older, with progressive magmatic and tectonic evolu- Laramide produced systematic increases in deeper exposures to younger, shallower deposi- tion in the past 150 m.y. across a complex con- crustal contents of magmas through coupling tional environments. The pattern of older bath- tinental margin. Metallogenic associations over- of magmatic evolution with thickening and olithic to younger volcanic-rock±dominated ter- lap and do not form restricted metallic belts. warming crust (Barton, 1996). Thus, metallo- ranes from west to east corresponds to Palinspastic reconstruction back through mid- genic characteristics follow magmatic patterns shallowing exposures. The complementary me- Tertiary extension aids evaluation of the time- in time as well as in space. During neutral to tallogenic signatures from weakly mineralized space history of mineralization. The reconstruct-

Geological Society of America Bulletin, October 2001 1371 STAUDE and BARTON

ed distributions show that metallogenic zones Society of America, Guidebook for Field Trip 7, An- canism and plate-tectonic evolution of the western nual Meeting, p. 59±68. United States: II. Late Cenozoic: Royal Society of are thinner and typically superposed through Anderson, T.H., Silver, L.T., and Salas, G.A., 1980, Distri- London Philosophical Transactions, Series A, v. 271, time. Metallogenic patterns cannot be rational- bution and U-Pb isotope ages of some lineated plu- p. 249±284. ized in terms of single parameters; rather, they tons, northwestern Mexico, in Crittenden, M.D., Co- Clark, K.F., Damon, P.E., Shutter, S.R., and Sha®qullah, M., ney, P.J., and Davis, G.H., eds., Cordilleran 1979, Magmatismo en el norte de Mexico en relacioÂn a re¯ect a combination of crustal and igneous metamorphic core complexes: Geological Society of los yacimientos metalõÂferous: AsociacioÂn de Ingenieros de compositions, tectonic style, and preservation. America Memoir 153, p. 269±283. Minas, Metalurgistas y GeoÂlogos de Mexico, ConvencioÂn Anderson, T.H., Bajek, D.T., Chepega, J.R., Ichikawa, Nacional XIII, Acapulco, Memoria, p. 8±57. This type of reconstruction and preservation K.M., Rodriguez, J.L., Stephens, W.E., and Silver, Clark, K.F., Foster, C.T., and Damon, P.E., 1982, Cenozoic study may be applicable to reevaluating mag- L.T., 1982, Crystalline thrust sheets near the Moja- mineral deposits and subduction-related magmatic matic and metallic belts worldwide. ve-Sonora megashear, Sonora, Mexico [abs.]: Geo- arcs in Mexico: Geological Society of America Bul- logical Society of America Abstracts with Programs, letin, v. 93, p. 533±544. v. 16, p. 430. Clarke, M., and Titley, S.R., 1988, Hydrothermal evolution ACKNOWLEDGMENTS Aranda-GoÂmez, J.J., Henry, C.D., Luhr, J.F., and McDow- in the formation of silver-gold veins in the Tayoltita ell, F.W., 1997, Cenozoic volcanism and tectonics in mine, San Dimas district, Mexico: Economic Geology, NW MexicoÐA transect across the Sierra Madre Oc- v. 83, p. 1830±1840. This paper has evolved from Ͼ 10 years of ®eld cidental volcanic ®eld and observations on extension CochemeÂ, J.J., 1985, Le magmatisme Cenozoique dans le work with numerous contributing ®eld visits, discus- related magmatism in the southern Basin and Range nord-ouest du Mexique: Cartographic de la regioÂn de sions, and reviews of manuscript versions from Eric and Gulf of California tectonic provinces, in Aguirre- Yecora-Maicoba-Mulatos illustration magmatique de Seedorff, Fred McDowell, Jaime RoldaÂn-Quintana, DõÂaz, G.J., Aranda-GoÂmez, J.J., Carrasco-Nunez, G., la ®n d'un regime en subduction et du passage a un Ricardo Amaya, John Stewart, Norman Page, Wo- and Ferrari, L., eds., Magmatism and tectonics in the regime distensif [Doctoral Thesis]: Universite de jteck Wodzicki, Jose Perello, Suzanne Baldwin, central and northwestern MexicoÐA selection of the Droit, France, 280 p. DeVerle Harris, Spencer Titley, and Robert Ilchik. 1997 IAVECI General Assembly excursions: Univer- CochemeÂ, J.J., and Demant, A., 1991, Geology of the Ye- Peter Coney, Ricardo Torres, Pedro Restrepo, and sidad Nacional Autonoma de Mexico, Mexico, D.F., cora area, northern Sierra Madre Occidental, Mexico, Excursion 11, p. 41±84. in PeÂrez-Segura, E., and Jacques-Ayala, C., eds., Stud- Joaquin Ruiz assisted in stratigraphic analysis. Lukas Atwater, T., 1970, Implications of plate tectonics for the ies in Sonoran geology: Boulder, Colorado, Geologi- Zurcher, Lance Miller, Karen Bolm, and Wodzicki Cenozoic tectonic evolution of western North Amer- cal Society of America Special Paper 254, p. 81±94. aided in acquisition of data on mineralization. Steve ica: Geological Society of America Bulletin, v. 81, Consejo de Recursos Minerales, 1992, MonografõÂa geolo- Barney, Noelle Sanders, and Ricardo Torres collab- p. 3513±3536. gico-minero del Estado de Sinaloa: Consejo de Re- orated in compiling radiometric age data. Mike Gu- Atwater, T., and Stock, J., 1998, Paci®c±North America cursos Minerales, Secretaria de EnergõÂa, Minas e In- tierrez and Giovanna Gamboa provided computer plate tectonics of the Neogene southwestern United dustria Paraestatal Subsecretaria de Minas e Industria drafting support. Ryan Houser, Frank Mazdab, Por- StatesÐAn update: International Geology Review, Basica, Pachuca, Hgo., Mexico, 159 p. ®rio Padilla, Felipe Rendon, and David Simpson v. 40, p. 375±402. Consejo de Recursos Minerales, 1993, MonografõÂa Geolo- Barthelmy, D.A., 1979, Regional geology of the El Arco gico-Minero del Estado de Sonora: Cardenas-Vargas, joined in ®eld work. 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1374 Geological Society of America Bulletin, October 2001