Jurassic to Holocene Tectonics, Magmatism, And Metallogeny Mexico

For permission to copy, contact [email protected] 2001 Geological Society of America 1357

GSA Bulletin; October 2001; v. 113; no. 10; p. 13571374; 12 figures.

Jurassic to Holocene tectonics, magmatism, and metallogenyof northwestern Mexico

John-Mark G. Staude*Mark D. BartonCenter for Mineral Resources, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

ABSTRACT

The present metallic distribution innorthwestern Mexico is the culmination ofsuperposed magmatism, tectonism, erosion,and burial over more than 150 m.y. De-tailed palinspastic reconstructions of preex-tensional configurationsthe first study ofits kind for this regionclarify the inter-play among these features on the presentdistribution and character of mineralizedgeologic systems. This new synthesis goesbeyond previous metallogenic investiga-tions of northwestern Mexico by separatingevents into specific timing and structuralrelationships, and by restoring the geologyto its preextensional configuration. Metal-logenic factors such as enrichment, preser-vation, and erosion play major roles in thepresent distributions and for the first timeare related to the overall metallogenicframework of northwestern Mexico. Theanalysis concludes that modern metallogen-ic patterns are the result of the complex su-perposition and subsequent redistributionof geologic systems in a way that is relateddirectly to the regional history, rather thansimply metallic belts or interpreted angle ofa subducting slab.

Three main extensional events in the Ol-igoceneHolocene have been restored, andthe palinspastic distributions have been an-alyzed. Reconstructions reveal the follow-ing: (1) Mineralization events, igneous cen-ters, and sedimentary sequences arecontinuous across the Gulf of Californiaand other areas with large amounts of ex-tension. (2) Middle Tertiary gold-silvermineralization in Baja California may bethe western part of the Sierra Madre Oc-

*Present address: BHP Billiton., Avenida Amer-ica Vespucio Sur 100, 8th floor, Las Condes, San-tiago, Chile; e-mail: [email protected].

cidental metallogenic province, thus ex-panding the previously recognized extent ofthis provinces mineralization. (3) LateCretaceousearly Tertiary porphyry cop-per deposits and intrusive centers form anarrower belt than previously noted andare traceable for over 400 km, with partsof the belt buried beneath the younger Si-erra Madre Occidental volcanic fields. (4)Interpreted alignments of older geologicfeatures, including lineaments of ore depos-its, are displaced in the reconstructions. (5)Sedimentary-rockhosted gold deposits andlow-angle-detachment gold systems areclosely related and occur around corecomplexes.

By using structurally restored time slices,it becomes clear that older deposit typestend to be those formed at greater depthsand more proximal to intrusions, whereasyounger deposits formed at shallowerdepths are less eroded and are more com-monly volcanic-rock hosted. These charac-teristics express themselves in the regionaldistribution of deposit types. Second, min-eralization of widely differing ages is spa-tially superposed, commonly associatedwith coeval magmatic and tectonic events.The structural and magmatic events togeth-er with paleodistribution of ore deposits de-fine a new framework to interpret the me-tallogenic history of northwestern Mexico.

Keywords: metallogeny, Mexico, ore depos-its, reconstruction, Sonora, tectonics.

INTRODUCTION

Although there has been considerable inter-est in the tectonic evolution of Mexico (e.g.,Sedlock et al., 1993), the region has receivedcomparatively little attention in syntheses ofthe metallogenic formation (Gonzalez-Reyna,1956; Salas, 1975; Clark et al., 1982). Nu-

merous published and unpublished geologicstudies in the past two decades provide theopportunity for a new synthesis and interpre-tation of Mexican metallogeny in the frame-work of the current tectonic thinking. This pa-per presents a time-space synthesis for thesedata in northwestern Mexico (Staude, 1995),generated as part of an integrated project onMexican mineral deposits and geology involv-ing collaboration among the University of Ar-izona, the U.S. Geological Survey, and miningcompanies. The results show that mineraliza-tion is not individual mineral belts, but rathera dynamic interplay of magmatism, tectonism,erosion, and preservation.

This study focuses on a 0.5 3 106 km2 areain northwestern Mexico that has complex ge-ology and many mineral deposits (Fig. 1).Prominent mineral deposits in this region in-clude the major porphyry copper deposits ofCananea and La Caridad, Sonora; large, vol-canic-rockhosted precious-metal depositsthroughout the Sierra Madre Occidental; anda number of other igneous-related and basin-related deposit types (e.g., Salas, 1975, 1994;Wisser, 1966). The large area permits broadregional comparisons to address the conceptsof metal belts, preservation, and superpositionof mineralizing events. Likewise, the complexmagmatic and deformational history of this re-gion since the Early Jurassic allows evaluationof metallogenic controls after appropriate re-constructions are made of distinctive time slic-es. Well-defined reconstructions are possiblewith the many new geochronometric, structur-al, and petrologic data generated in the pastdecade (e.g., Henry, 1989; Perez-Segura andJacques-Ayala, 1991; Aguirre-Daz and Mc-Dowell, 1993; Aranda-Gomez et al., 1997;McDowell et al., 1997; Stewart et al., 1998;Henry and Aranda-Gomez, 2001). We synthe-size the characteristics, distribution, and tim-ing of mineralization and tectonism that arebased on new age, geologic, and palinspastic

1358 Geological Society of America Bulletin, October 2001

STAUDE and BARTON

Figure 1. Location map of study area with larger symbols corresponding to deposits referred to in the text. Symbol shape and shadingindicate deposit type and mineralization age with sources for districts in Staude (1995). Larger symbols are labeled districts and arereferred to in text.

compilations and new field work. We use theresults to interpret metallogenic controls. Thecontrols can be compared to other regions, andthe approach illustrates some of the complex-ities of mineralization in a long-lived conti-nental margin with multiple magmatic arcs.

GEOLOGIC FRAMEWORK

Northwest Mexico consists of translatedand accreted terranes along the southwesternmargin of the North American craton, whichhave been intruded and covered by coeval andyounger igneous rocks since the middle Me-sozoic. Mesozoic and early Tertiary compres-

sional tectonism was followed by severalstyles of extensional tectonism beginning inthe middle Tertiary. These events generateddistinctive lithologic sequences and expose di-verse crustal levels across the region. Al-though terranes have been defined on base-ment (pre-Jurassic) stratigraphy (Campa andConey, 1983; Sedlock et al., 1993), an alter-native division simply considers the combi-nation of crustal structure, type, and level.Three distinctive geologic domains (western,central, and eastern), compared to the nine ter-ranes defined by Campa and Coney (1983),provide a simple basis for comparison acrossthe region (Fig. 2). The domains are based on

magmatic and structural style rather than onbasement and stratigraphic sequence. Meso-zoic and Cenozoic magmatic and tectonic fea-tures (which overlap the stratigraphic assem-blages of terrane terminology) help definethese domains but typically cut terrane bound-aries. Because mineralization commonly is theproduct of magmatism and tectonism, the do-mains help unify metallogenic observations.

Lithologic Framework

The western domain (Fig. 2) consists of ac-creted marine sedimentary and volcanic rocksand minor mafic intrusions (Sedlock et al.,

Geological Society of America Bulletin, October 2001 1359

TECTONICS AND METALLOGENY OF NORTHWESTERN MEXICO

Figure 2. Geologic domains of northwestern Mexico with the schematic stratigraphic columns for each domain. These domains crossterrane boundaries and define packages based largely on distinctions in Cenozoic superimposed effects. SMOSierra Madre Occidental.

1993; Moran-Zenteno, 1994) cut by numerouspre-Cenozoic thrust faults and younger strike-slip shear zones. The western half of Baja Cal-ifornia contains various accreted units rangingfrom continental-margin to ophiolitic rocks,most of which have been intruded by multipleplutonic events and partly covered by Ceno-zoic volcanic rocks and conglomerates (Fig.2; Abbott and Gastil, 1979). JurassicCreta-ceous clastic sedimentary rocks and volcanicrocks of the Alisitos Formation form an inte-rior belt over much of western Baja Califor-nia. Rocks of the Alisitos Formation under-went metamorphism during accretion andsubsequent contact metamorphism by the Pen-insular Ranges batholith (Gastil et al., 1975).No pre-Mesozoic units are reported in this do-main, and all but the widespread Neogeneconglomerate and bimodal volcanic rocks aremetamorphosed.

The central domain (Fig. 2) contains Cre-taceous tonalitic and dioritic batholithic rocksintruding Paleozoic to middle Mesozoic car-bonate, clastic, and minor volcanic sequencesof eugeoclinal affinity (Gastil et al., 1981).This domain exhibits Mesozoic shorteningand later normal faulting. Highly deformedPaleozoic sedimentary rocks extend through-out the region, the oldest recognized being Or-dovician (Poole et al., 1991; Ortega-Gutierrezet al., 1992). Continent-derived volcano-sedi-mentary rocks of the Late TriassicJurassic

Barranca Group overlie Paleozoic rocks incentral Sonora, whereas to the north, Jurassicpyroclastic flows and volcanogenic sedimentswere deposited in basins that parallel the Ju-rassic continental margin (Tosdal et al., 1989;Riggs and Blakey, 1993). Widespread butpoorly dated mixed andesitic and clastic sed-imentary sequences have been assigned toboth the Late Cretaceous and early Tertiary,largely on the basis of stratigraphic correla-tions and crosscutting relationships. These se-quences are intruded and metamorphosed byCretaceous (in the west) to early Tertiary (inthe east) metaluminous granitoids (Damon etal., 1983b; Gastil, 1993). Abundant middleTertiary calc-alkaline volcanic rocks are over-lain by Neogene coarse clastic sequences andbasalts.

The eastern domain (Fig. 2) contains ex-posed Proterozoic basement overlain by mio-geoclinal sedimentary rocks, which are intrud-ed and overlain by Mesozoic and Cenozoicigneous rocks and continental sedimentaryrocks (Roldan-Quintana and Clark, 1992;Gonzalez-Leon and Lawton, 1995). Laramidecompression was followed by highly variableamounts of Cenozoic extension. This domainis the southwestern margin of the NorthAmerican craton (Stewart, 1988). Eocambrianand lower Paleozoic miogeoclinal sedimenta-ry rocks are overlain by middle Paleozoic car-bonate reef and platform deposits. Upper Pa-

leozoic rocks record the transition frommarine to subaerial environments. Local Tri-assic and Jurassic volcano-sedimentary basinsresemble those of the central domain. Volca-nic-rockfree Lower Cretaceous sequences re-cord the transition from the vast carbonateplatform sequences of northeastern Mexico toclastic nearshore facies in central Sonora.Younger limestone conglomerates, thin dis-continuous sandstone beds, and volcanogenicmaterials record transition to continental con-ditions and impingement of the late Mesozoicarc. Locally, Laramide volcanic rocks arecommon and are overlain by abundant Oli-goceneMiocene calc-alkaline to bimodal vol-canic rocks (Ortega-Gutierrez et al., 1992;McDowell and Roldan-Quintana, 1993; Nieto-Samaniego et al., 1999) and minor upper Ce-nozoic syntectonic clastic rocks (McDowell etal., 1997).

Magmatic and Tectonic Framework

Broadly continuous magmatism and tecto-nism over the past 150 m.y. have been docu-mented across northwestern Mexico (Damonet al., 1981; Perez-Segura, 1985; Sedlock etal., 1993). Although earlier work demonstratesthat overall patterns in Mexico parallel pat-terns in the southwestern United States (e.g.,Dickinson, 1981), the characteristics of pet-rogenetic and tectonic events in northwestern


STAUDE and BARTON

Figure 3. Temporal distribution of igneous and deformational events in northwestern Mex-ico, denoted as the numbers of outcrops per mountain range. (A) Abundance of exposedplutonic and volcanic rocks. Younger rocks are more extrusive-dominated, and magma-tism has been active in different parts of the region for most of the past 110 m.y. (B)Relative abundance of deformational features. SonSonora, SinSinaloa, BCBaja Cal-ifornia, ChihChihuahua, DgoDurango.

Mexico have not been well resolved. On thebasis of previous studies and our new work(Staude, 1995), we divide the history into fivedistinguishable petrogenetic and tectonic epi-sodes (Figs. 3 and 4; these occurred during theLate JurassicEarly Cretaceous, Cretaceous,Late Cretaceousearly Tertiary, middle Tertia-ry, and late Tertiary. Distinguishing featuresinclude magmatic compositions, tectonicstyles, and levels of exposure.

JurassicEarly Cretaceous (ca. 150120Ma)

The JurassicEarly Cretaceous episodemarks the beginning of magmatism that hasassociated metallic mineralization in north-western Mexico. Sparse plutonic and wide-spread volcanic rocks occur in all three geo-logic domains; they are concentrated along thecoast in Baja California and across the interiorof Sonora, extending southeastward into Sin-aloa and Durango (Fig. 4A; Dickinson, 1981;Stewart et al., 1986). The deposition of vol-canic rocks correlates in time with the trans-lational tectonics of the Jurassic, whichevolved into the Mojave-Sonora megashear(Dickinson, 1981; Anderson et al., 1982). The

coastal belt consists of JurassicEarly Creta-ceous mafic to intermediate-composition vol-canic rocks, sparse intrusions, and ophioliticsuites (Sedlock et al., 1993). These rocks aretypically metamorphosed to greenschist faciesand deformed. The interior belt, which is sole-ly Jurassic in age, contains intermediate to fel-sic calc-alkaline to alkaline volcano-plutoniccomplexes that are interpreted to have formedin an extensional arc setting (Busby-Spera,1988; Saleeby et al., 1992; Tosdal et al.,1989).

JurassicEarly Cretaceous structure is com-plex and obscured by superposed events. Boththrust and strike-slip structures in northwest-ern Mexico have been identified as Jurassic(Anderson et al., 1980; de Cserna, 1989).Widely distributed left-lateral, northwest-strik-ing shear zones in Sonora appear to be part ofthe megashear, which likely ceased activityprior to 150 Ma (Anderson and Silver, 1979).Deformation in the Jurassic Parral Formationof Chihuahua and Durango could be thesoutheastern extension of this shearing. Thrustand strike-slip faults in Sinaloa and Baja Cal-ifornia juxtapose Mesozoic and Paleozoicrocks and are cut by Cretaceous batholiths.

Some of these structures host mineralization.Later events reactivated the Jurassic structuresand used them as conduits for hydrothermalfluid flow, as at the San Francisco mine inSonora (Perez-Segura et al., 1996).

Cretaceous (12080 Ma)Cretaceous magmatism formed the batho-

lithic terranes of Baja California and Sinaloa.The calcic to calc-alkaline Peninsular Rangesbatholith in Baja California formed from 120to 90 Ma (Walawander et al., 1990), withequivalents in Sinaloa (Henry and Fredrikson,1987). Exposure levels of the batholith shal-low southward, reaching subvolcanic levelsnear lat 288N, 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 isparalleled by a temporal progression to morefelsic compositions during the Cretaceous.Oxygen, Pb, and Sr isotope ratios indicategreater crustal contents of younger and far-ther-eastemplaced magmas in Baja Californiaand Sonora (Taylor, 1986). Isotope ages andPb isotope contours match across pre-Miocenerestorations of the Gulf of California (Silveret al., 1993).

Cretaceous thrusts verge both to the eastand west (King, 1939; Drewes, 1978; Roldan-Quintana and Gonzalez-Leon, 1979; Jacques-Ayala et al., 1990). They both cut and locallyare cut by Cretaceous intrusions, yet the vol-canic rocks commonly show extensional faultsthat apparently formed synchronously (e.g., ElArco; Barthelmy, 1979). Cretaceous brittleand ductile faults host precious and base-metalmineralization. Rangin (1986) attributed thesestructures to the accretion of Baja Californiato mainland North America.

Late CretaceousEarly Tertiary (8040Ma)

Late Cretaceousearly Tertiary (8040 Ma,Laramide) igneous rocks parallel the in-ferred subducting arc along a south-southeasttrend through most of the eastern half of thestudy area, extending from southern Arizonaand New Mexico into Durango and Sinaloa.This calc-alkaline granodioritic to graniticbatholithic belt intrudes voluminous, coevalvolcano-sedimentary rocks in western Mexico(Henry, 1975; Damon, 1978; Roldan-Quin-tana, 1991; Cocheme and Demant, 1991; Gon-zalez-Leon et al., 2000). Outcrop patterns andradiometric dates illustrate the general distri-bution of Laramide magmatism, even thoughthe volcanic sequences are poorly character-ized because of pervasive hydrothermal alter-ation and stratigraphic complexity (Fig. 4C).



Figure 4. Time slices showing locations ofisotope ages and correlative magmaticunits. Ages compiled from .100 sourcesare reconstructed to their pre-Miocene ex-tension in Figure 10. Outcrop locationsmodified from Ortega et al. (1992) and agesources.

These rocks appear discontinuously throughthe OligoceneMiocene Sierra Madre Occi-dental volcanic province, emerging in centralChihuahua (McDowell and Mauger, 1994). Srisotope ratios in igneous rocks increase fromvalues of ,0.706 in western (older) rocks to.0.708 in eastern (younger) rocks (Damon etal., 1983a; Mead et al., 1988; Roldan-Quin-tana, 1991). Eocene intrusions have 87Sr/86Srratios as high as 0.714 in two-mica granites(Mead et al., 1988). Andesitic volcanic rocksgenerally have associated porphyritic intru-sions that commonly localize mineralization(e.g., Bockoven, 1980).

Laramide thrusting is prominent in thenortheastern 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 crustalshortening in Sonora vary greatly (Jacques-Ayala et al., 1990). In some areas in northernSonora, 20%40% shortening can be docu-mented (Merriam and Eells, 1978), but insuf-ficient data exist to quantify the shorteningacross the Laramide orogen. Thrusting in So-nora apparently ended prior to early Tertiaryplutonism, as Laramide plutons in this areaintrude but apparently are not cut by thrustfaults (Staude, 1995).

Middle Tertiary (4020 Ma)Calc-alkaline volcanism and associated

hypabyssal intrusions of the Sierra Madre Oc-cidental and nearby areas formed throughmuch of western Mexico from the late Eocenethrough the late Oligocene (McDowell andClabaugh, 1979). These are the southern con-tinuation of the better-studied Oligocene vol-canic fields of the southern United States(Christiansen and Lipman, 1972; McIntosh etal., 1992; Spencer et al., 1995). Compared toLaramide rocks, the mid-Tertiary rocks aremore felsic and less altered. They form a 0.52-km-thick veneer across large parts of theLaramide arc.

The transition from mainly andesitic, hy-drothermally altered rocks to dominantly dac-itic and rhyolitic, less altered rocks provides auseful demarcation between the older andyounger suites (Wisser, 1966; McDowell andClabaugh, 1981). The age of this transition re-mains to be well defined in most areas, al-though it is mostly Eocene. From central So-nora to the east, there appears to be little ifany hiatus in magmatism during the transition,whereas in the west, a temporal break is wellestablished (Fig. 4D; Clark et al., 1982). Geo-chemical data indicate as much or more crust-al component in the mid-Tertiary magmas as

in earlier periods (e.g., eNd of22.3 to25.2;87Sr/86Sri of .0.709 in western Chihuahua; Al-brecht, 1990). A regionally extensive gravitylow along the eastern margin of Sonora intowestern Durango roughly corresponds withthe axis of mid-Tertiary volcanism and likely

reflects an underlying coeval granitic batholith(Aiken et al., 1988).

The mid-Tertiary marks the beginning oforogenic collapse in northwestern Mexico.Major extension with concomitant core-com-plex formation began in the middle Oligocene


STAUDE and BARTON

Figure 5. Cross sections from Jurassic to present, showing major fault systems duringeach period at the latitude of Hermosillo, Sonora. Section not restored so as to more clearlyrepresent superimposed features. Structures are generalized and represent major faultsets. AAway, TToward, SMOSierra Madre Occidental.

Figure 6. Map A. Location of core-complex, basin and range, and Gulf of California structural features. Directions of extension andbroad time spans are given for each style on inset Map B. The three structural styles are largely superimposed and must be unraveledin order to restore the geology to its preextensional configuration. Map sources include Davis (1980), DeJong et al. (1988), Jacques-Ayala et al. (1990), Nourse (1989), Staude (1993a), Stewart and Roldan-Quintana (1994), and Staude (unpublished mapping).

(Nourse et al., 1994). Across northern andwestern Sonora, Tertiary extension unroofedrocks formed at mid-crustal levels (Figs. 5 and6). Extension of lower magnitude continuedsouth and east into central Mexico (Henry etal., 1991). The zone of major extension fadesinto the more coherent, but still disrupted, Si-erra Madre Occidental structural province(Gans, 1997; Stewart et al., 1998). Faultswithin the Sierra Madre Occidental structuralprovince localize some volcanic centers (Stau-de, 1995). Certain normal faults predate Oli-gocene ignimbrites and create a structurallydisrupted topography beneath the ignimbritetuffs, such as at Santa Ana (Gans, 1997) andMulatos (Staude, 2001) in Sonora. The mainextension, 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 .308. Later faults typi-cally trend northwest, are high angle, andcommonly host middle Tertiary mineralization(Drier, 1984; Staude, 1994).

Late Cenozoic (20 MaHolocene)Magmatism became more heterogeneous

and dispersed beginning in the early Miocene.Early phases of this volcanism were felsictuffs and minor basaltic lavas that containstrong crustal signatures. Volcanism evolvedin the Miocene to bimodal volcanism associ-ated with normal faulting, ultimately devel-



Figure 7. Three-step reconstruction to the . 26 Ma (late Oligocene) preextension geographyof northwestern Mexico. Dark lines plot younger position; dashed line indicates older, re-stored position. Shading delineates area of greatest extension between Pacific Ocean andSierra Madre Occidental structural province. Circle is schematic restored strain ellipse.

Figure 8. Histogram of deposit types plot-ting the number of Au, Cu, W, Pb-Zn dis-tricts versus time period. Compiled dataare available in the U.S. Geological Survey,Mineral Resource Data System, computerdatabank. Mineralization associated withSZshear zone, Ignigneous, Adulad-ularia-quartz epithermal, Sedsedimenta-ry, Acidacid sulfate epithermal, Replreplacement, Porpporphyry.

oping the mafic-dominated, rift-type volca-nism associated with generation of the Gulf ofCalifornia. Basaltic andesites and high-K ba-saltic centers around the Gulf of California arevariably tilted and were locally erupted syn-chronously with active high-angle extensionand sedimentation (McDowell et al., 1997).Volcanism occurred throughout northwesternMexico in the early Miocene, but became con-centrated around the Gulf of California in thelate Miocene (Fig. 4E). These younger lavashave either alkaline or MORB-like character-istics (MORB is mid-oceanic-ridge basalt) andoccur in areas that are still tectonically active(Donnelly, 1974; Neally and Sheridan, 1989).The younger lavas include the modern basalticcenters around the margins of the Gulf of Cal-ifornia and sparse centers throughout northernMexico (Sawlan, 1991; Fig. 4E).

Northwest-striking, high-angle normal andstrike-slip faults characterized crustal attenu-ation during the late Cenozoic (Stock andHodges, 1989; Staude, 1993a; Lee et al.,1996). The faulting was synchronous andpostdated the southwest-directed core-com-plex extension. This postcore-complex fault-ing can be divided into two groups: MiocenePliocene high-angle extensional faults (King,1939; Henry and Aranda-Gomez, 1992) andlate MioceneHolocene normal and strike-slip(transtensional) faults associated with rifting(Moore and Buffington, 1968; Atwater, 1970;Lonsdale, 1989; Stock and Hodges, 1989; At-water and Stock, 1998). High-angle normalfaults are widely distributed. They trend northin the north (INEGI, 1981; Stewart et al.,1998) and north-northwest in the south (Henryand Fredrikson, 1987). Younger strike-slip andnormal faulting is oblique (north-northwesttrending) to the gulf and accommodates bothextension and translation (Lonsdale, 1989).

Tectonic Reconstruction

Figure 7 shows a three-step palinspastic re-construction for northwestern Mexico basedon the information we have outlined. First, theGulf of California was closed by restoring thelikely protogulf configuration (Stock andHodges, 1989). This step involves both mov-ing the Baja California Peninsula ;350 kmsoutheastward along the San Andreas faultsystem and restoring the east-northeast exten-sion relative to mainland Mexico.

Second, ;10% extension attributed to high-angle normal faulting was removed block-by-block across the states of Sonora and Sinaloa.This step was based on tilt-direction mapsshowing fault orientation, block rotations, andareas of higher degrees of extension (Stewart

and Roldan-Quintana, 1994; Stewart et al.,1998; Staude, 1995). Minor extension, prob-ably ,5%, occurred throughout westernmostChihuahua and Durango; these areas are heretreated as a fixed block east of the extendeddomains. Strike-slip motion in these areasmay have been significant during the time ofBasin and Range extension in areas to thewest, but few faults have been identified withsubstantial strike-slip translation.

The third step involved restoration of thecore complexes and related extension. Thisstep is the most difficult to carry out becausefew piercing points exist across the complexesand because of the compartmentalized natureof this extension (Nourse, 1992). This resto-ration results in 50% lineation-parallel short-ening in the core-complex region of centraland northern Sonora, diminishing to nothingon the margins of the mid-Tertiary extensionalregime to the south and east (cf. Fig. 6). Pre-vious studies of extension in the southwestCordillera give comparable estimates for highextension (Davis et al., 1981; Wust, 1986;Dickinson, 1991; Richard, 1994), although nodetailed reconstruction for Mexico has beenpublished previously.

Although simplified in nature, this modelenables an improved evaluation of the Paleo-gene distribution of rocks and mineral depos-its. Ideally, one would like to carry the recon-struction farther back in time; however, asnoted earlier, the lack of constraints on Lar-amide and older structures precludes meaning-ful quantitative reconstructions of earlier de-formation. These earlier deformation eventsare schematically illustrated in the time-spacereconstruction that we describe next.

MINERALIZATION

Metallic mineralization is abundant innorthwestern Mexico, with .2500 known

metal occurrences distributed throughout mostof the region. The types include deposits as-sociated with magmatic (intrusion- and extru-sion-related), metamorphic, and sedimentaryrocks that formed over much of the past ;150m.y. The main metals and the associated de-posit types with their ages of formation aresummarized in Figure 8. Abundances for eachdeposit type are tabulated from a computer-ized database we developed for this study


STAUDE and BARTON

Figure 9. Present distribution of major met-al districts and their related stratigraphyfor major time intervals. Lithologic distri-butions are compiled from Gastil et al.(1975), Gastil and Krummenacher (1977),Bockoven (1980), Anderson and Silver(1979), INEGI (1981), Swanson and Mc-Dowell (1985), Cocheme (1985), Rangin(1986), Henry and Fredrikson (1987), deCserna (1989), Lonsdale (1989), Staude(1991), Ortega-Gutierrez et al. (1992), andRoldan-Quintana and Clark (1992).

(available through the U.S. Geological Sur-vey, Mineral Resource Data System, andLeonard, 1989). Intrusion-related deposittypes include Cu 6 (Mo, W), Fe-W-Cu and

Ag-Zn-Pb skarns, W greisen deposits, Au-Agveins, and Ag-Pb-Zn replacement ores. Vol-canic deposit types include epithermal Au-Ag-(Pb-Zn-Cu) veins, advanced argillic Au-Cu,

adularia-sericite Ag-Au, and high-silica rhyo-lite volcanic centers with F 6 Mo. Low-sul-fide Au-bearing quartz veins are the mainmetamorphic type. Metallic ores associatedwith sedimentary rocks include stratiform andstrata-bound Cu deposits. Pb-Zn, F, and U de-posits related to diagenetic processes occur tothe east of the Sierra Madre Occidental andare not a focus of this paper; yet they do relateto the overall metallogenic evolution as theyformed to the east, possibly driven in part bythe thermal and tectonic events to the west(Salas, 1975; Kesler, 1997).

Nonmetallic mineral deposits, although notdiscussed in detail in this study, make up asignificant number of deposits in the region,and many appear to be related to synchronousmagmatic and tectonic events. Mesozoic andCenozoic intrusions generated wollastoniteand garnet skarns (Perez-Segura, 1985). LateCretaceousearly Tertiary intrusions raised thegeothermal gradient in central Sonora, result-ing in the formation of the numerous graphitedeposits in carbonaceous sedimentary units.Evaporite-bearing basins developed during themiddle Cenozoic extension and contain bo-rate, zeolite, and halite beds (Aiken and Kis-tler, 1992). Volcanism around the westernedge of the Gulf of California during the lateTertiary formed sulfur, perlite, and opaldeposits.

Age, Characteristics, and Distribution ofMineralization

Although relatively few metallic depositshave been dated, it is possible to estimate theage of nearly 500 of the .1000 districts forwhich geologic data have been compiled(Leonard, 1989; Staude, 1995). Few, if any,metallic deposits of pre-Jurassic age areknown in northwestern Mexico; thus, we be-gin with the Late Jurassic.

JurassicEarly CretaceousThe four main deposit types of Jurassic

Early Cretaceous age are porphyry Cu, shear-zone Au-bearing quartz veins, skarn, and vol-canogenic Fe deposits (Fig. 9A). EarlyCretaceous porphyry Cu mineralization is as-sociated with intermediate-composition intru-sive centers in central Baja California in theSan FernandoEl Rosario areas. Early to Mid-dle Jurassic porphyry Cu mineralization is as-sociated with felsic magmatism in northernSonora, extending into Arizona (e.g., Bisbee).Mesothermal Au-bearing quartz veins occur inthrust and strike-slip fault zones and are com-mon in Jurassic clastic and metavolcanic rocksalong a northwest trend paralleling the Juras-



sic arc (Fig. 9A). The shear-zonehosted veinsmay be related to motion along the Arizona-Sonora megashear (Silberman et al., 1988) orto forearc deformation related to arc accretion.Ages of deposits are uncertain because fewdates on veins have been published for eitherthe mainland or peninsular localities (Men-chaca, 1985).

Late JurassicEarly Cretaceous marine, arc-related volcanic and sedimentary rocks hostFe-Cu (6Au) deposits along the western mar-gin of the Sierra Madre Occidental in Sinaloa(continuing south into Nayarit) and alongwestern and central parts of the Baja Penin-sula. These hydrothermal Fe deposits and Fe(6Cu, Au) skarns are penecontemporaneouswith mafic and intermediate-compositionmagmatic centers (Zurcher, 1994). Hydrother-mal magnetite-hematite vein occurrences cutJurassicEarly Cretaceous basaltic flows inBaja California, particularly between SantaRosala and El Rosario (Menchaca, 1985).These deposits are typically metamorphosedto lower greenschist grade by the CretaceousBaja California batholith. Cu staining and tur-quoise are found in at least four of the Fe dis-tricts. These deposits appear to belong to amiddle Mesozoic belt of Fe-oxide (6Cu, Au)occurrences along the Cordillera (Barton andJohnson, 1996). Ni, Co, and Cr deposits host-ed in accreted marine sedimentary and vol-canic-intrusive (ophiolitic) rocks on the Viz-caino Peninsula and other parts ofwesternmost Baja California are associatedwith mafic igneous units, are strongly serpen-tinized, and are highly disrupted by multiplestages of postmineralization faulting. Associ-ated intrusions have Early Cretaceous U-Pbages (Sedlock et al., 1993).

CretaceousDuring the mid-Cretaceous, economically

significant Cu (6Au) porphyries, W skarns,and mesothermal Au-bearing quartz veinsformed in the western half of the region (Fig.9A). These deposits form belts that parallelthe Cretaceous arc. Few locations other thanintrusive centers have been directly dated, andthe ages of the remaining deposits are inferredfrom field relationships. The most abundanttypes of mineralization are W skarn and grei-sen deposits, followed by Au-bearing quartzveins and porphyry Cu deposits (Fig. 8).Tungsten skarns become increasingly commonnorthward from central Baja California. Theyare most common in Paleozoic carbonaterocks intruded by tonalitic and granodioriticintrusions (Weise, 1945; Menchaca, 1985).Tungsten most commonly occurs in scheelite,both in skarn and quartz veinlets with little

additional associated mineralization. The de-posits are small, rarely exceeding 2 3 106tonnes of ore, and were mostly mined duringWorld War II (Fries and Schmitter, 1945).

The best-known middle Cretaceous porphy-ry Cu (6Au) deposit is at El Arco in centralBaja California (Barthelmy, 1979). Others oc-cur in Sinaloa and northern Baja Californiaand correlate with 12080 Ma magmaticevents (cf. Fig. 4B). The Cu porphyry systemshave lower initial 87Sr/86Sr values, are moredioritic, and have higher Au/Cu ratios thantheir younger Laramide counterparts. Creta-ceous porphyries are preserved in areas thathave been less deeply eroded, particularly incentral Baja California, where volcanic rocksof similar age surround the porphyry deposits(Echavarri-Perez and Rangin, 1978). In north-ern Baja California, Cretaceous rocks aremore deeply eroded. Equigranular, coarse-grained, intermediate-composition batholithiccomplexes contain W skarn and greisen min-eralization rather than porphyry centers.

Two other deposit types that likely formedduring the middle Cretaceous are intrusion-hosted bonanza Au-Ag quartz veins and me-sothermal Au-bearing quartz veins in shearzones cutting greenschist-facies rocks (MotherLodetype deposits). Intrusion-hosted quartzveins in Baja California are as long as severalkilometers and locally blossom into .50-m-wide stockworks (Menchaca, 1985). In Sonoraand Sinaloa, similar veins occur, but their agesare difficult to determine because of plutonicoverprinting. Mesothermal Au-Ag quartzveins are common along the western marginand within roof pendants of the PeninsularRanges batholith. The veins form anastomos-ing stockworks of quartz, carbonate, and chlo-rite with small, high-grade bonanza Au shoots(Wisser, 1954).

LaramideDiverse and abundant mineral deposits

formed during the Laramide (Late Creta-ceousearly Tertiary) in northwestern Mexico.Cu (6Mo) porphyries and skarns, W and Pb-Zn skarns, and Au-Ag quartz veins are themost economically significant. These depositsare east of the middle Cretaceous ones, foundmainly in Sinaloa and Sonora. Mineralizationis related to calc-alkaline magmatism and wasemplaced in volcanic to fairly deep plutonicenvironments with a range of styles of hydro-thermal alteration, brecciation, and deposition(e.g., Bushnell, 1988; Wilkerson et al., 1988;Barton et al., 1995).

Porphyry Cu (6Mo) deposits are wide-spread in areas containing Laramide volcanicrocks; hydrothermal alteration commonly ex-

tends many kilometers away from known de-posits (Sillitoe, 1976). These form part of theworld-class porphyry Cu (6Mo) province thatextends from northwestern Arizona throughSonora into Sinaloa and Durango (Fig. 9B).Within this belt, the porphyry Curelated in-trusions are older to the south and west. Cuand Pb-Zn skarns are widespread where cal-careous host rocks are present (Perez-Segura,1985; Consejo de Recursos Minerales, 1992,1993; Valencia-Moreno, 1998). In western So-nora, Cu skarn occurrences are common, asare equigranular intrusions and regionallymetamorphosed rocks, whereas porphyry-style mineralization and volcanic rocks are un-common. Cu-bearing porphyry-style mineral-ization is associated with intrusive centersolder than 60 Ma in central Chihuahua (Mc-Dowell and Mauger, 1994) and northern Du-rango (Aguirre-Daz and McDowell, 1991).These occurrences and others in deep canyonswithin the Sierra Madre Occidental (Barton etal., 1995) suggest that the Laramide porphyryprovince extends beneath much of the youngerSierra Madre Occidental ignimbrite sequence.

Tungsten and Mo become more importantcommodities in more felsic intrusive systems,which overlap with, but in general are youngerthan, the Cu-rich centers. Transitional Mo-W(6Cu) porphyry, breccia-pipe, and skarn de-posits (e.g., Cumobabi and Santa Ana, Sonora)occur with metaluminous quartz-feldspar por-phyries of early Tertiary age. Tungsten-domi-nated deposits are best developed in central So-nora and extend southeastward intonorthwestern Durango. The largest tungstendistricts are in the largest intrusive massif ofSonora, the Aconchi batholith (e.g., El Jaralitodistrict). Most W-rich systems are associatedwith peraluminous granitoids (Roldan-Quin-tana, 1991), unlike the Cretaceous tungsten dis-tricts of Baja California and transitional Mo-W(6Cu) deposits that are associated with metal-uminous granitoids. Tungsten deposits youngfrom west to east and change from granodio-rite-associated skarns in the west to granite-as-sociated greisen deposits in the east (Weise,1945; Mead et al., 1988). This control may bethe effect of level of exposure, because deepercarbonate rocks are exposed in the west, andyounger, shallower clastic sedimentary and vol-canic rocks are preserved in the east.

Numerous crustiform, low-sulfidation Au-Ag quartz veins are widespread throughout thecentral and eastern parts of the study area.Many of these are inferred to be Laramide onthe basis of truncation of veins and alterationzones prior to mid-Tertiary volcanic units(Staude, 1995). Precise ages are available foronly a few deposits (e.g., Tayoltita; Henry,


STAUDE and BARTON

1975; Clarke and Titley, 1988). Some of thelarger districts are plotted in Figure 9B to in-dicate their general distribution. These veins arehosted in granodiorite intrusions, metamor-phosed sedimentary rocks, and andesitic-daciticflows and tuffs. They have Ag/Au ratios of.100 (commonly .1000), and minor Zn, Pb,and Cu sulfide minerals (Wisser, 1966; Clark etal., 1979). They can exceed several kilometersin length and commonly show systematic zo-nation, in some cases around intrusive centers,such as at Alamos and Batopilas (Loucks andPetersen, 1988; Wilkerson et al., 1988).

Shear zones hosting Au mineralization,such as at La Choya and Quitovac in westernSonora, have white mica that crystallized dur-ing Laramide time (8050 Ma) (Durgin andTeran, 1996; Alex Iriondo, 1999, personalcommun.). These shear-zone deposits arecomplex; Jurassic and older host rocks arecovered by extensionally faulted Miocene vol-canic rocks. The precise age of mineralizationis poorly determined.

Middle TertiaryLike the Laramide, the late EoceneOligo-

cene was a major period of metallic mineral-ization in western Mexico, with the formationof diverse types and a large number of oredeposits, most of which are associated withvolcanic 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 carbonate-hosted deposits (Fig. 9C). Other probable mid-Tertiary mineralization includes sedimentary-rockhosted and low-angle-fault-zone Audeposits. These two may be directly related tomid-Tertiary extension (Fig. 9D).

Of the .800 middle Tertiary volcanic-rockhosted epithermal precious-metal occur-rences known in northwestern Mexico (Fig.9C; Leonard, 1989; Orris et al., 1993; Staude,1993b), the majority are quartz 6 calcite veinswith chlorite 1 adularia 1 sericite alterationhalos. These deposits, such as at Ocampo(Knowling, 1977) and Maguarichic (Staude,1995), are Ag dominated with spotty Au andbase-metal pockets. Advanced argillic Au(6Cu) districts, such as Mulatos (Staude,2001), are scarcer but number in the tens. Dis-seminated (Moris), hot-spring (Pinos Altos),and polymetallic vein districts (Uruachic) arealso present. Most districts lack precise datesbut can be assigned an Oligocene age by re-gional correlation to basal rhyolitic ignim-brites that host mineralization or underlie ore-hosting volcanic rocks (Swanson andMcDowell, 1985; Wark et al., 1990). Maficdikes and flows, regionally dated at late Oli-

gocene to early Miocene, cut or cap these sys-tems (Bockoven, 1980; Duex, 1983; Cameronet al., 1989).

Carbonate- and volcanic-rockhosted min-eralization on the eastern flank of the SierraMadre Occidental is best known for large Ag-Pb-Zn deposits and also includes many Hg,As, Sb, Mn, Sn, and U occurrences. Sparsegeochronometry and regional correlations in-dicate that many carbonate-hosted deposits areOligocene (Megaw et al., 1988). However, thefact that similar systems formed during theLaramide (Piedras Verdes [Chihuahua], LaReforma, Oposura-Moctezuma) demonstratesthat repeated skarn- and replacement-stylemineralization occurred in areas where mag-matism overlaps the carbonate-rich eastern do-main (Figs. 2 and 9C).

Sedimentary-rockhosted and low-angle-shear-zone Au deposits found to the west ofthe Sierra Madre Occidental volcanic provincemay be broadly coeval with volcanic-rockhosted deposits in the Sierra Madre Occidentalbut are not necessarily directly related tomagmatism. K-Ar ages on postmineralizationbasaltic dikes and prealteration two-micagranites restrict Au mineralization in the San-ta Teresa district to 3628 Ma (Bennett andAtkinson, 1993), which is consistent with ev-idence from other districts where minerali-zation is associated with low-angle (exten-sional?) faults and is cut by high-angle(Neogene) faults (Fig. 9D). Deposit character-istics include strong stratigraphic control byfavorable beds and jasperoidal Au-Hg-Asmineralization (Bennett, 1993), which resem-bles that of Carlin-like deposits (Vikre et al.,1997). Deposit distribution corresponds withthat of major extension areas containing abun-dant sedimentary rocks (Fig. 9D). A lessclearly defined group of Au-bearing quartz de-posits occurs in low- and high-angle shearzones west of the sedimentary-rockhosteddeposits (e.g., La Herradura, Chinate). Thesehave conflicting data indicating both Laramideand mid-Tertiary ages (Leonard, 1989; AlexIriondo, 1999, personal commun.).

Late TertiaryHoloceneSedimentary-rockhosted stratiform Cu de-

posits and hot-spring Au deposits are associ-ated with opening of the Gulf of California(Fig. 9E). The Boleo district contains Cu-Co-Ag and manganese oxide deposits in upperMiocene clastic rocks and tuffs related to earlystages of Gulf of California opening (Wilsonand Rocha, 1955; Schmidt, 1975; Ochoa-Lan-dn, 1998). Sedimentary-rockhosted Cu pros-pects extend several hundred kilometers alongthe western edge of the gulf (Staude, 1992).

Over a dozen hot-spring Au occurrences havebeen identified around the gulf. Most depositslie along the western edge of the gulf, wheretransform faults and fault splays extend ontoland along the eastern parts of the Baja Cali-fornia peninsula (Staude, 1992). The hot-spring deposits are hosted in units as old asMiocene (Santa Luca) and as young as Ho-locene beach sands (Puertocitos).

TIME-SPACE DISTRIBUTION OFMAGMATISM AND MINERALIZATION

Preextension Events

The geology of northwestern Mexico iscomplex, with superposed magmatic and me-tallogenic events that have in many cases beentranslated from their location of formation bylater events. Previous metallogenic summa-ries, based on collaboration among the Uni-versity of Arizona, the U.S. Geological Sur-vey, and mining companies, have defined beltsaccording to present-day configurations (Sa-las, 1975). Clark et al. (1982) accounted forsome of the translation associated with themost recent Gulf of California rifting but didnot take into account earlier extension or dis-placements inland of the gulf. By using thetectonic reconstruction already presented, themetallogeny of northwestern Mexico can bereevaluated, starting with the Late Jurassic.

The preextension restoration allows tracingof the outcrops of Jurassic volcanic and vol-cano-sedimentary rocks with sparse intrusivecenters from southern Arizona through north-ern Sonora and southeastward into Mexico.This restoration of the Jurassic arc shows atrend similar to those presented by Dickinson(1981) and Tosdal et al. (1989). The threemain groups of Jurassic rocks (Fig. 10A) forma belt of volcanic and minor intrusive rockswith scattered clastic sequences. The units ofeastern Baja California and Sonora appear tojoin with Jurassic rocks of the Parral Forma-tion in southern Chihuahua and thus link thegeology beneath the Sierra Madre Occidental.Mineral deposits of possible Jurassic age re-store to closer proximity, and similar depositsbecome grouped once extension is taken intoaccount.

By using this same preextension reconstruc-tion, the Cretaceous intrusive rocks in westernSonora can be traced to the restored Baja Pen-insula; the Cretaceous W deposits can belinked between northern Baja California andSonora; Mesozoic accretionary sedimentaryrocks can be restored between central BajaCalifornia and southern Sonora; and the por-phyry Cu district of El Arco correlates with



Figure 10. Restored positions of mineralization and major lithologic units restored to the appropriate preextensional configuration.


STAUDE and BARTON

Figure 10. (Continued.)

Cu-bearing parts of the Sinaloa batholith (Fig.10B). When the abundance of Cretaceous in-trusive outcrop area is contoured, one can seehow the batholith restores across the gulf and

continues northward into the reconstructed po-sition of southern California. The abundanceof Cretaceous intrusions appears to decreaseto the east. Also to the east, the intrusions may

continue the younging trend that has beendocumented between Ensenada and Mexicali(Ortega-Rivera et al., 1994); however, moredata are needed. To the west along the Pacific



coastal edge of Baja California, accretion offorearc sedimentary rocks and ophiolites con-tributed to the metallogeny during Cretaceoustime, although their precise locations duringthe Cretaceous is uncertain (Hagstrum et al.,1985). The Cr, Co, and Ni deposits formedwith mafic magmatism during Late Jurassicand Early Cretaceous time and were accretedto Baja California prior to the Cenozoic (Ab-bott and Gastil, 1979). Since Oligocene time,the western part of Baja California has not un-dergone tremendous extension; however,translation along faults parallel to the San An-dreas and Agua Blanca fault systems hascaused right-lateral movement, for which thereconstruction accounts.

For the Late Cretaceousearly Tertiary, thereconstruction proves useful for interpreting thebatholiths in Sonora and Sinaloa and their re-lationship to rocks of similar age in southernArizona. Once restored, mineralized systemsform a narrow belt trending southeast along thewestern edge of Mexico rather than a widebulge as one sees today in the Sonora-Arizonaregion. The bulge narrows by as much as 50%once postmineralization extension is removed,and the width becomes more typical of otherporphyry Cu belts throughout the world (e.g.,central Andes [Sillitoe, 1988], British Colum-bia [McMillan et al., 1995]). By contouring theextent of 8040 Ma intrusive centers and therestored present-day outcrop areas (Fig. 10C),the abundance of plutonic rocks is found to behigh beneath the Sierra Madre Occidental.Without the cover of the volcanic rocks, bothundated Laramide and younger, the abundanceof Laramide intrusions would be larger. Por-phyry systems, exposed in windows throughthe younger ignimbrites of the Sierra MadreOccidental, aid in tracing the batholith and sug-gest a high probability for other deposits be-neath the younger volcanic rocks. Study of pre-sent lineaments and position of porphyry Cudeposits do not take into account the tripartitedeformation. Once the deformation is restored,many of the lineaments no longer appear asprevalent. Lineaments, if they do exist, becomerotated from their present orientations, and iflineaments are used for interpretations, theyneed to be corrected for the substantial dis-placements of the past 30 m.y. Some deposits,such as Cananea (Wodzicki, 1995) are moder-ately tilted, whereas others are strongly extend-ed and dissected as at Piedras Verdes, Sonora(Drier and Braun, 1995).

Outlines of regions with porphyry Cu, Au-Ag, W, and Pb-Zn deposits indicate that themineral deposits follow the same south-south-east trend as the magmatic arc (Fig. 10C).Moreover, mineralization clusters in areas with

superposed metal associationsnot as separatemetallic belts. The clusters correspond withmany of the exposed Laramide igneous rocks(Fig. 10D). This finding could be quite signif-icant, because it suggests that much of the Lar-amide is mineralized. Moreover, if more Lar-amide rock areas are discovered, they mighthave substantial mineralization. Exposure playsa vital role in controlling the present-day ap-pearance of the metal distribution, and it is pos-sible that prior to erosion and Cenozoic vol-canic superposition, there could have been aneven larger extent of exposed rocks and min-eralization. Looking at the present-day abun-dance of deposits in southern Arizona andnorthern Sonora, there are potentially hundredsof deposits that were covered by younger vol-canic rocks of the Sierra Madre Occidentalfrom central Sonora to southern Durango. Lar-amide igneous rocks crop out in most canyonsthat expose premiddle Tertiary rocks through-out the Sierra Madre Occidental. These out-crops help define the contours of the abundanceof magmatic rocks.

Loops in Figure 10C delineate Cu districtsthat are the most widely distributed (they areoutlined by the light-weight dashes). Tungstendeposits are inside the area defined by the out-line enclosing the Cu districts. Tungsten de-posits are recognized by our study much far-ther east and over a larger area than inprevious metallogenic summaries for Mexico(Clark et al., 1982). In the reconstruction, Pb-Zn, W, Au-Ag, and Cu districts occur in thesame areas, showing their superposition priorto extension.

Late EoceneOligocene mineralization andmagmatism can be restored to approximatetheir distribution prior to extension, althoughsome of this mineralization is likely related tothe onset of extension and may have formedas extension was underway. Au deposits aremost abundant in the Sierra Madre Occidentalvolcanic province along the Sinaloa-Durangoand Sonora-Chihuahua state borders; however,a few Au-Ag deposits of Oligocene age arefound farther west in northern Sonora. The Au(6Cu) districts within the Sierra Madre Oc-cidental are at present recognized in only afew areas; thus, their extent of distribution ispreliminarily drawn as a smaller region withinthe larger Au-Ag districts of the Sierra MadreOccidental (Fig. 10E). Carbonate-hosted Pb-Zn-Ag deposits occur to the east and along thewestern edge of the Sierra Madre Occidental.The full extent of volcanism during the mid-Tertiary ignimbrite period is not known; yet,by restoring the deformation and using thecurrently exposed and dated outcrops, it ispossible to estimate the extent of Oligocene

volcanism (Fig. 10E) and contour the restoredgeography on the basis of the thicknesses andextents of tuffs older than 27 Ma. Radiometricdating in Baja California indicates that theearliest middle Tertiary volcanism began at ca.30 Ma (Gastil et al., 1975), and the volumeincreased as time progressed. In Baja and inwesternmost Sonora, the few locations of rhy-olitic volcanism link these regions to the morecoherent and extensive Sierra Madre Occiden-tal volcanic province to the east.

Synextension and Postextension Events

Two major periods of mineralization innorthwestern Mexico may be restored approx-imately to their synextension configurations.The ca. 2720 Ma extensional faulting wid-ened Sonora (Gans, 1997), possibly formingsedimentary-rockhosted Au systems in theeast and shear-zone Au deposits in the west(Fig. 10E). The mid-Tertiary, postcore-com-plex reconstruction mainly affects the northernand central parts of Sonora; the dotted linesurrounding much of Sonora in Figure 10F re-constructs the approximate boundary of .308Tertiary tilting and areas of low-angle (,258)normal faults (modified from Stewart and Rol-dan-Quintana, 1994). During the core-com-plex extensional event, northern Sonora mayhave extended as much as 30%, and some ar-eas had .100% extension (Nourse, 1989;Nourse et al., 1994); these are restored to theirpreBasin and Range extension position inblack. The region of low-angle-shear-zone Audeposits is within the area of middle Tertiaryextension; many deposits occur around and tothe west of restored core complexes. The Ja-joba district may have been faulted off the topof the Magdalena core complex. Sedimentary-rockhosted Au districts restore to the easternedge of core complexes but reside within thearea extended during middle Tertiary time.One hypothesis for this association is that thedeposits are related to extensional phenomena,as has been suggested for middle Tertiary sed-imentary-rockhosted mineralization in cen-tral Nevada (Seedorff, 1991). The Sonoran de-posits have had far less study thansedimentary-rockhosted deposits in centralNevada, and interpretations of the genesis forSonoran deposits are speculative.

The late Tertiary reconstruction shows thedistribution of volcanic-rock and sedimentary-rockhosted hot-spring Au districts and the Bo-leo stratiform Cu district (Fig. 10G). These de-posits are associated with the rift opening ofthe Gulf of California and restore to the generalboundary of the protogulf region. The recon-struction reveals that the region is encompassed


STAUDE and BARTON

Figure 11. Time-space synthesis of magmatism, deformation, and mineralization acrossBaja California through Sonora to western Chihuahua at latitude of Hermosillo. Thepresent 500 km width has changed due to older compression and younger extension,creating an hourglass shape. Right diagram summarizes mineralization with font sizescorresponding to the relative abundance of metallic ore-deposit types. The position of theGulf of California is marked by the dashed black line. ChihChihuahua.

by late Tertiary mafic igneous rocks and thegeneralized location of the restored, pregulf-opening geography (modified from Atwater,1970; Lonsdale, 1989; Stock and Hodges,1989). The reconstruction indicates that open-ing of the gulf during the past 6 m.y. yieldedgreater extension in the southern part of thegulf, which necessitates more closure to restorethan points farther north (cf. Henry and Aran-da-Gomez, 2001). Rift-related Miocene Cumineralization restores to the latitude of Culia-can, Sinaloa (Schmidt, 1975; Guilbert and Da-mon, 1977). Hot-spring Au districts ring theGulf of California, with a larger abundance ofdeposits on its western margin. This asymme-try may be due to the proximity of spreadingand rift centers along the eastern coast of north-ern Baja California over the past 12 m.y., inaddition to preservation and covering by youn-ger Pliocene and Quaternary clastic sedimen-tary units around the gulf (Staude, 1993a).

The three-step reconstruction can be sum-marized in a time-space model linking tecto-nism, magmatism, and mineralization, therebyproviding a basis for interpreting the distri-bution of mineralization through time.

Synthesis

Figure 11 summarizes the temporal andspatial relationships of magmatism, deforma-

tion, and mineralization in northwestern Mex-ico. Although the observations summarizedare not exhaustive, general patterns are clearand provide the basis for broad interpretation.Overall, magmatism was largely continuouswhile systematically changing location andcomposition with time. One or more ill-de-fined Jurassic arcs in the eastern and westerndomains were superseded in the Cretaceous bya well-developed coastal batholith that wasbuilt mainly across the continental-marginac-creted metasedimentary package of the west-ern domain. Gradual and then more rapid east-ward migration of magmatism began in thelate Mesozoic through the mid-Tertiary (Fig.11A). These changes correlate with compres-sional deformation, indicated in Figure 11 bythe shortening of the width from Pacific Oceanto the Chihuahua-Sonora border. Subsequentretreat of magmatism toward the coast in theearly Cenozoic was followed by the changefrom subduction-related to rift-related mag-matism during the Miocene; these magmaticpatterns correlate with a shift to neutral, thentranstensional tectonics, shown in Figure 11by the widths expanding and by the preva-lence of volcanic rocks, unlike the older pe-riod with more intrusion-dominated rocktypes. In general, it is in the rift periods that

volcanic rock types are preserved, whereas inthe compressional periods, one finds intru-sions. The deposit types plotted in Figure 11Bshow this association. There is an overall in-crease in the ratio of abundance of volcanicrocks to plutonic rocks beginning in the Cre-taceous, which appears to be a preservationphenomenon in that the older rocks are moredeeply eroded and thus expose more deeplyformed deposit types. An additional associa-tion is that in single areas, the magmatic com-positions vary over the 1025 m.y. periods ofactivity; crustal (felsic) contents increase dur-ing compressional events (CretaceousOligo-cene), whereas compositions are heteroge-neous or have increasing mantle (mafic)content during extensional and rifting events(OligoceneHolocene; cf. Barton, 1996).

Trends in the diverse styles of mineraliza-tion parallel tectonic and magmatic events(Fig. 11B). The abundance of known depositsincreases with time through the Paleogene andthen declines in the Neogene. Intrusion-asso-ciated deposits such as skarn and porphyrysystems are most common in the older rocks,whereas epithermal systems are most commonin the mid-Tertiary (cf. Fig. 8). Compositionalvariations in igneous-associated deposits cor-relate with variations in magmas (Barton etal., 1995), whereas deposits of questionable ornonigneous-related origin correlate with par-ticular types of tectonism (e.g., shear-zone andsedimentary-rockhosted Au deposits innorthern Sonora with core complexes). East-west variations in deposit types at a given ageare common, such as Cu skarns to the westand Cu porphyries to the east during early Ter-tiary time. Although deposit types peak inabundance during particular periods, few com-modities are restricted to single metallogenicepisodes (Figs. 8 and 11). For example, Auoccurs in economic deposits spanning 150m.y. Deposit types cross time and spaceboundaries and commonly correlate with ex-posed igneous rocks. The volcanic-rockhost-ed deposits are in either young volcanic rocksor volcanic rocks that are preserved in ex-tended areas, such as the Jurassic arc of north-central Sonora. Batholith-hosted deposits suchas W skarn deposits occur in older igneousrocks (Baja California) or in the deeper zonesof extended terranes (east-central Sonora).Ore-deposit types found in deeper environ-ments are generally older, such as skarn andgreisen deposits, whereas shallower environ-ments have younger deposit types like hot-spring Au and sandstone-hosted Cu deposits.

METALLOGENIC CONTROLS

Metallogenic patterns in northwestern Mex-ico were influenced by spatial, temporal, or



Figure 12. Estimated exposure depths for igneous rocks and principal deposit types in a cross-sectional composite of northwestern Mexico. (A) Typical surface-exposed emplacement depths,(B) environment for preserved mineralization, (C) types of deposits presently exposed.

process-related factors. No single set of fac-tors can (or should) explain these variations;rather, they reflect a combination of influences(cf. Barton, 1996). Regional and temporal dif-ferences in the composition, thickness, ther-mal structure, and state of stress of the crustaffect the source of materials, nature of ma-terial transport, and depositional environ-ments. Nonmagmatic fluids also reflect cli-mate and tectonic regime. Finally, observedpatterns reflect exposure levels; preservationor exhumation become key for interpretingprocess and provincial patterns.

Spatial Controls

The provinciality of metal assemblages(Fig. 11) has been interpreted as reflecting dif-ferences in crustal composition (e.g., Titley,1991), but could also be influenced by differ-ences in host rocks, available fluids, or shift-ing locus of magmatism. The eastward in-crease in Pb and Ag relative to Cu and Authus may reflect the increasingly felsic, Pb-rich, Cu-poor crust with the transition fromcrust formed in the western-domain continen-tal volcanic margin to crust formed in the east-ern-domain Precambrian basement and plat-form carbonates (Fig. 9). Alternatively, moreabundant carbonate rocks to the east may haveenhanced this trend by providing favorabletraps for Pb (and Ag). Metal and alterationsuites correlate with igneous compositions,which in turn reflect crustal composition. Cucontents of intrusion-hosted mineralization de-cline from west to east, whereas Pb and Agcontents of carbonate-hosted systems increase.A further regional pattern might reflect differ-ences in surface-derived fluids. Rifting in thegulf may circulate seawater or evaporiticbrines, leading to preferential transport of Cu1 Fe 6 Mn, whereas dilute meteoric fluids incontinental basins may be more suited for pre-cious-metal transport alone.

Temporal Controls

Temporal variations in the physical state ofthe crust, in magmas, and in surface fluids in-fluenced metallogenic patterns. Compressionaland extensional tectonic regimes produceddifferences in the locus and character of mag-matism and mineral deposits (Fig. 11). Com-pressional regimes during the Cretaceous andLaramide produced systematic increases incrustal contents of magmas through couplingof magmatic evolution with thickening andwarming crust (Barton, 1996). Thus, metallo-genic characteristics follow magmatic patternsin time as well as in space. During neutral to

extensional tectonics in the Jurassic and mid-dle to late Cenozoic, heterogeneous magma-tism, commonly bimodal, is typical. Depositsnot only reflect magmatic variability duringthese times, but also may result from fluid cir-culation directly related to extension and basinformation (cf. Seedorff, 1991; Ilchik and Bar-ton, 1997). In contrast to compression, exten-sion creates widespread permeability. Salinewaters generated from marine incursions dur-ing rifting or in closed basins may account forsome of the Fe-Cu-Mn-Au mineralization andperhaps some base-metal mineralization notdirectly related to magmatism.

Preservation and Exposure

The distribution of deposits through time innorthwestern Mexico reflects erosion and cover(Fig. 12). Progressive exposure of deeper partsof the crust by uplift and erosion or by tectonicdenudation generates a progression from older,deeper exposures to younger, shallower deposi-tional environments. The pattern of older bath-olithic to younger volcanic-rockdominated ter-ranes from west to east corresponds toshallowing exposures. The complementary me-tallogenic signatures from weakly mineralized

deep plutonic through porphyritic to epithermalenvironments reflect preservation and erosionnot simply process and spatial controls. Differ-ences in exposure levels help rationalize regionalmetal-zoning patterns, which need not reflectpurely spatial or process controls. Uplift and/orerosion expose deeper geologic areas, typicallyuncovering magmatic sources and root zones tohydrothermal systems. Crustal extension lowersbase levels, preserving near-surface environ-ments. Burial, for example by younger volcanicrocks, also helps preserve shallow levels. Bothextension and burial can be used to rationalizethe distribution of porphyry Cu occurrences innorthwestern Mexico (Barton et al., 1995). Pro-longed weathering favors oxidation and super-gene enrichment. These factors often govern theeconomics of Cu and Au deposits and thus theirapparent distribution.

In conclusion, northwestern Mexico exhibitssuperimposed metallogenic suites that correlatewith progressive magmatic and tectonic evolu-tion in the past 150 m.y. across a complex con-tinental margin. Metallogenic associations over-lap and do not form restricted metallic belts.Palinspastic reconstruction back through mid-Tertiary extension aids evaluation of the time-space history of mineralization. The reconstruct-


STAUDE and BARTON

ed distributions show that metallogenic zonesare thinner and typically superposed throughtime. Metallogenic patterns cannot be rational-ized in terms of single parameters; rather, theyreflect a combination of crustal and igneouscompositions, tectonic style, and preservation.This type of reconstruction and preservationstudy may be applicable to reevaluating mag-matic and metallic belts worldwide.

ACKNOWLEDGMENTS

This paper has evolved from . 10 years of fieldwork with numerous contributing field visits, discus-sions, and reviews of manuscript versions from EricSeedorff, Fred McDowell, Jaime Roldan-Quintana,Ricardo Amaya, John Stewart, Norman Page, Wo-jteck Wodzicki, Jose Perello, Suzanne Baldwin,DeVerle Harris, Spencer Titley, and Robert Ilchik.Peter Coney, Ricardo Torres, Pedro Restrepo, andJoaquin Ruiz assisted in stratigraphic analysis. LukasZurcher, Lance Miller, Karen Bolm, and Wodzickiaided in acquisition of data on mineralization. SteveBarney, Noelle Sanders, and Ricardo Torres collab-orated in compiling radiometric age data. Mike Gu-tierrez and Giovanna Gamboa provided computerdrafting support. Ryan Houser, Frank Mazdab, Por-firio Padilla, Felipe Rendon, and David Simpsonjoined in field work. Carl Kuehn, Peter Megaw, JorgeAranda, and Luca Ferrari supplied key references. Fi-nancial sponsorship came from Kennecott, PlacerDome International, the United States GeologicalSurvey, and the University of Arizona Consortiumfor Mexico geologic studies. Chris Henry and JaimeUrrutia Fucugauchi reviewed the submitted manu-script, improving its final form.

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