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Jurassic igneous-related metallogeny of southwestern North America Mark D. Barton*, James D. Girardi, Douglas C. Kreiner, Eric Seedorff, and Lukas Zurcher Department of Geosciences and Institute for Mineral Resources, University of Arizona, Tucson, AZ 85721 John H. Dilles Department of Geosciences, Oregon State University, Corvallis, Oregon 97331 Gordon B. Haxel U.S. Geological Survey, 2255 Gemini Drive, Flagstaff, AZ 86001 David A. Johnson Department of Geosciences and Institute for Mineral Resources, University of Arizona, Tucson, AZ 85721, now Bronco Creek Exploration, Tucson, AZ ABSTRACT Jurassic magmatism and related hydrothermal systems formed across much of southwestern North America. Hydrothermal systems are numerous and varied, although fewer major mineral deposits are known than with later magmatism. Principal types of Jurassic mineralized systems include: (1) porphyry, skarn, replacement, and vein Cu(±Au±Ag±Mo±Zn±Pb±Ag) systems; (2) IOCG (Fe oxide-Cu-Au) vein, breccia and skarn systems; (3) VMS systems; (4) granite-related W(-Au); and (5) a spectrum of advanced argillic (±Au) systems. Some districts represent hybrid or composite systems requiring multiple fluid sources. Compared to more recent periods, the Jurassic contains few epithermal and lithophile element deposits. Multiple factors contributed to the diversity and help rationalize differences with other times. These include compositions of magmas and external fluids, levels of exposure, and superimposed events. Jurassic magmatism varied from calc-alkaline and oxidized to relatively alkaline compositions and, in the Great Basin, crust-dominated, ilmenite-bearing types. IOCG occurrences require external brines generated in Jurassic arid settings, in contrast to seawater-dominated VMS deposits. Advanced argillic systems include high- and low-sulfidation styles. The scarcity of epithermal systems likely reflects erosion of the shallowest crust. Comparisons with other times (Laramide and mid-Tertiary) and places (Canadian Cordillera, central Andes, southwestern Pacific) highlight common patterns and temporal progressions. Key Words: Jurassic, hydrothermal, porphyry, IOCG, advanced argillic, metallogeny, magmatism, southwestern North America INTRODUCTION The Great Basin and surrounding regions in southwestern North America encompass one of the world’s great mineral belts yet we have few definitive answers about the origins of this metal endowment or what factors have controlled deposit distribution in time and space. What are the relative roles of crustal provenance, of magma and fluid types, mineralization processes, and preservation and exposure? This paper focuses on Jurassic igneous-related mineralization in southwestern North *E-mail: [email protected] America because it is important in some areas and it provides an instructive contrast with younger periods that are apparently more richly endowed with ore deposits. Patterns of early to middle Mesozoic magmatism and associated hydrothermal systems in the cordillera of the Americas differ in fundamental ways from later Mesozoic and, particularly, Cenozoic patterns (e.g., Barton, 1996; Sillitoe and Perello, 2005). Older episodes tend to have somewhat more mafic magmatism, be neutral to extensional in character, and have fewer and smaller porphyry and epithermal deposits but more iron-oxide-rich deposits. These observations have been interpreted differently by various authors, most commonly with an 373 374 Mark D. Barton, J. Dilles, J. Girardi, G. Haxel, D. Johnson, D. Kreiner, E. Seedorff, and L. Zurcher emphasis on magmatic compositions and correlations with tectonic setting but without any satisfying mechanistic interpretation for the differences. In an earlier synthesis, Barton (1996) suggested that the broad metallogenic patterns in southwestern North America should be evaluated in terms of provincial effects (composition of the crust and near-surface fluids, physical state of the lithosphere), process (fluid sources and drives), and a key role for preservation. Expanding on our earlier work, this paper presents a preliminary synthesis of the Jurassic metallogeny of southwestern North America. We briefly review Jurassic magmatism and its geologic framework including the tectonic and paleogeographic settings. This background forms the context for review of the types, spatial distribution and timing of hydrothermal systems. These are interpreted in terms of the nature of the ore-forming systems, including types of available fluids. Finally, we briefly compare the Jurassic with other, better mineralized epochs including the Laramide and the middle Tertiary in the western US and with analogous regions elsewhere in the Americas. Although of fundamental interest, we do not address the system-scale process controls or the regional petrotectonic controls on magmatism and related mineralization. JURASSIC MAGMATISM AND GEOLOGIC FRAMEWORK The 60 m.y. span of the Jurassic1 in southwestern North America records the first extensive development of magmatism along the Phanerozoic continental margin coupled with diverse, and still unsettled tectonic evolution in an arid environment. Magmatism developed intermittently throughout the Jurassic, spanning the truncated Paleozoic continental margin (e.g., Tosdal et al., 1989; Saleeby and Busby-Spera, 1992; Miller and Busby, 1995; Anderson et al., 2005b). Magmatic activity developed across multiple crust types including transitional, cratonal and reactivated cratonal Proterozoic crust in Arizona, eastern California and the central Great Basin, and juvenile Paleozoic or Mesozoic crust in northwestern Nevada and western and northern California. Magmatism evolved in a broadly extensional environment during the Early and Middle Jurassic changing to a mixed transpressional (north) to transtensional (south) regime in the Late Jurassic. Throughout the Jurassic, the terrestrial environment was distinctively arid. These factors affected the nature of Jurassic hydrothermal systems and associated mineral deposits. Time-Space Distribution of Jurassic Igneous Rocks Figure 1 shows a simple time-space division of Jurassic magmatism in southwestern North America as it is exposed today. In this paper, we make no attempt to account for Jurassic and younger deformation that first amalgamated Jurassic magmatic belts, shuffled them laterally along the margin in the Mesozoic, compressed them in the later part of the Mesozoic and early Tertiary, and then extended and translated them in the middle and later parts of the Cenozoic, a process that continues today in the Walker Lane and in California (e.g.,Faulds et al., 2005). Latest Triassic to Early Jurassic (ca. 205–175 Ma) arc magmatism took place in a zone which is presently no more than 200 km in width and which extends from northern Sonora across southern Arizona and then northwestward approximately along the California-Nevada border (Figure 1B; Saleeby and Busby-Spera, 1992). This zone includes sparse plutons and older parts of volcanic sections in Arizona (Riggs and Busby-Spera, 1990; Lang et al., 2001). Similar, subaerial to marine volcanic assemblages with scattered plutons occur across eastern California into western Nevada (e.g., Thomson et al., 1995; Quinn et al., 1997; Dunne et al., 1998; Sorensen et al., 1998). To the northwest, the same belt continues into northern California (Christie, 2010; Dilles and Stephens, 2010), with a few plutons as far west as the Central Belt of the northern Sierra Nevada (Day and Bickford, 2004), and as remnants in Mesozoic pendants within the central and southern Sierra Nevada batholith (Tobisch et al., 2000). Scattered gabbroic to dioritic intrusions of ~200 Ma occur in ophiolitic terranes in the western Klamath Mountains and Sierra Nevada foothills (Saleeby, 1982; Irwin, 2003). Overall, Early Jurassic magmatism appears to have been considerably less voluminous than that of the Middle and Late Jurassic, as evidenced by the scarcity of dated Early Jurassic plutons and volcanic sequences (Barton et al., 1988; Dilles and Wright, 1988). Abundant Middle to early Late Jurassic (ca. 175–155 Ma) magmatism took place across an exceptionally broad region (Figure 1C). Rocks of this age range from predominantly mafic to intermediate oceanic arc and ophiolitic domains in central and western California, through a relatively well defined intermediate zone from Oregon to Sonora that is roughly coincident with the Early Jurassic arc. In the north, igneous activity blossomed eastward across the central Great Basin into western Utah (Barton et al., 1988; Elison, 1995). Volcanic rocks are also widely preserved through the western and central parts of this region, particularly along the principal arc from northern Sonora into northern California (Tosdal et al., 1989; Christe and Hannah, 1990; Riggs et al., 1993; Dunne et al., 1998; Quinn et al., 1997; Haxel et al., 2005), eastward into central Nevada (e.g., Muffler, 1964; Dilek and Moores, 1995), and westward into the marine arc and ophiolitic terranes of 1 For the purposes of this paper, we use the time scale of Walker and Geissman (2009) for which the Triassic-Jurassic boundary is 201.6 Ma. This is 3–6 m.y. younger than other recently published time scales. This difference does not have a material effect on this synthesis apart from the fact that a number of intrusions previously termed Early Jurassic are now probably Late Triassic. Also, we chose intervals that approximate Jurassic Epochs, but are not precise because they simplify describing the key geologic patterns. Jurassic igneous-related metallogeny 375 Figure 1. Framework and distribution of Jurassic igneous rocks. Note that age intervals do not correspond exactly to the Jurassic Epochs although they are informally used in this manner (see footnote in text). A. Geologic framework for Jurassic magmatism including age of crust, some major structural elements, and distribution of crosscutting Cretaceous batholiths. B. Locus of 205–175 Ma intrusions and volcanic rocks and selected regions that are mentioned in text. C. Locus of 175–155 Ma intrusions and volcanic rocks and selected regions that are regions mentioned in text. D. Locus of 155–140 Ma intrusions and volcanic rocks and selected regions that are regions mentioned in text. central and western California (Saleeby, 1982; Harper, 1984; Hopson et al., 1981, 2008; Dickinson, 2008a). Age-correlative ash beds, with similar mineral make-up to the plutons, are widespread in the sedimentary sequences of the Colorado Plateau (Riggs and Blakey, 1993; Kowallis et al., 2001). Based on measured exposure areas, Middle Jurassic plutons are much more voluminous than either Early or Late Jurassic and represent substantially higher magmatic fluxes than for the other periods (Barton et al., 1988). Late Jurassic to earliest Cretaceous magmatism (ca. 155–140 Ma) retracted to a relatively narrow belt that passes from the Arizona-Sonora border region across southern California and northward along through eastern and central California into the Klamath Mountains (Figure 1D; Irwin, 2003; Barth et al., 2008; Haxel et al., 2008a). Volcanic rocks in this age range are sparse. This suite is compositionally diverse, with alkalic intrusions in Arizona (Tosdal et al., 1989; Haxel et al., 2008a) and multiple intrusion compositions in California, notably represented by the Independence dike swarm and related rocks (Chen and Moore, 1979; Glazner et al., 2008; R.F. Hopson et al., 2008) and tonalites and quartz diorites of the northern Sierra and Klamath Mountains (Irwin, 2003). 376 Mark D. Barton, J. Dilles, J. Girardi, G. Haxel, D. Johnson, D. Kreiner, E. Seedorff, and L. Zurcher Compositional Character of Jurassic Igneous Rocks Jurassic igneous compositions span basaltic to rhyolitic, alkaline to strongly peraluminous; as a group, they are compositionally expanded compared to later Mesozoic magmatism (e.g., Barton et al., 1988; Saleeby and Busby-Spera, 1992). In many cases, the rocks have been substantially modified by post-magmatic processes that obscure original compositions (e.g., Fox and Miller, 1990; Battles and Barton, 1995; Haxel et al., 2008b; see Hydrothermal Alteration, below). Figure 2 shows modal and selected accessory mineral data and total alkali-silica (TAS) plots for representative intrusive suites of Early, Middle and Late Jurassic age. Unlike Cretaceous magmatism in the western U.S., there appears to be little overall secular variation in Jurassic igneous compositions (Barton, 1996). Most Jurassic igneous rocks are subalkaline, oxidized (magnetite ± titanite bearing) hornblende-biotite granitoids or their volcanic equivalents. These include the majority of plutons of Early to Middle Jurassic age in Arizona and southern California, which are predominantly biotite-hornblende magnetite-titanite-bearing granodiorite and granite (e.g., Tosdal et al., 1989; Haxel et al., 2008b). To the north, similar broadly granodioritic compositions predominate in eastern California and western Nevada (e.g., John et al., 1994) and across the central Great Basin (du Bray, 2007). In the northern Sierra Nevada and the Klamath Mountains, many plutons are metaluminous, relatively mafic compared to those to the south, and exhibit both calc-alkaline and tholeiitic affinities. These areas are dominated by tonalite-granodiorites, diorites and, in ophiolitic complexes, gabbros (e.g., Hopson et al., 1981, 2008; Harper, 1984; Beard and Day, 1987). The original locations and modes of amalgamation of these western terranes remains controversial (Dickinson, 2008b). At the other end of the compositional spectrum, two-mica granites in a number of areas form discrete plutons or constitute late phases in composite weakly peraluminous plutons. They are nearly all Middle Jurassic. These strongly peraluminous compositions are most common in the Great Basin (e.g., Ruby Mountains, Kistler et al., 1981; Snake Range, Lee et al., 1981) yet they occur as far west as the Klamath Mountains (e.g., the Slinkard pluton, Barnes et al., 1986a). Although most Jurassic granites and granodiorites are relatively oxidized, the two-mica granites and a minority of the metaluminous plutons are relatively reduced; they may contain abundant ilmenite with or without some magnetite but have little or no titanite (e.g., Lee et al., 1981; Nutt et al., 2000). Mildly to locally strongly alkaline compositions are fairly widespread in the Jurassic. In Arizona and Sonora, the ~150 Ma Ko Vaya suite includes alkaline perthite granites and quartz syenite (Tosdal et al., 1989; Haxel et al., 2008a). Elsewhere along the arc, most alkaline intrusions are Early to Middle Jurassic and are relatively silica poor—mainly monzonites, quartz monzonites, and syenites. These other alkalic suites are best de- veloped in eastern and northern California (Miller, 1978; Sylvester et al., 1978; Fox and Miller, 1990) but extend locally into the eastern Klamath Mountains (e.g., Ironside Mountain batholith, Barnes et al., 2006) and sporadically across the central Great Basin (e.g., Hoisch and Miller, 1990). Rare examples have silica undersaturated phases (e.g., Joshua Flat pluton, Inyo Mountains; Miller, 1978). The corresponding volcanic record presents a greater challenge because of widespread alteration (see below), however immobile elements and some major element data indicate that high-K, moderately alkaline volcanic rocks may have been widespread along the arc (reviewed by Christe and Hannah, 1990). Although Jurassic igneous systems have not received as much attention as Cretaceous and Cenozoic igneous systems, a number of representative areas are well studied. These include plutons in southern Arizona (Haxel et al., 2008a,b) , the Mojave Desert (e.g. Fox and Miller, 1990; Young et al., 1992; Mayo et al., 1998), the Inyo Mountains (Miller, 1978; Sylvester et al., 1978), the Yerington district (Dilles, 1987), the Snake Range (Lee et al., 1981; Lee and Christensen, 1983), the Smartville Complex (e.g., Beard and Day, 1987, 1988), and the Klamath Mountains (e.g., Barnes, 1986a,b, 2006). Some of these plutons are remarkably diverse with broadly consanguineous quartz diorites to peraluminous granites (e.g., Slinkard pluton, Barnes et al., 1986a; Snake Creek pluton, Lee and Christensen, 1983). Jurassic Tectonic and Paleogeographic Framework The tectonics and paleogeography of the Jurassic are of interest here primarily because of their role in the governing structural settings, depositional environments, paleoelevation and probability and type of non-magmatic fluid sources—all of which influence ore-forming systems. Jurassic tectonics have been extensively reviewed elsewhere (e.g., Saleeby and Busby-Spera, 1992; Dickinson, 2008a; papers in Miller and Busby, 1995, and in Anderson et al., 2005a). Broadly viewed, the key features of the Jurassic are: (1) It was generally low standing and mainly though not entirely extensional. Extensional basins, mainly tectonic but partly volcanic, accumulated thick sequences of volcanic, clastic and locally lacustrine to marine (in the north) sedimentary rocks including evaporites. (2) Magmatism occurred in a largely continuous arc extending from reactivated craton onto the Paleozoic margin of cratonal North America, with the important exception of older ocean crust-floored terranes in northern and western California. These oceanic terranes were either forearc/intra-arc assemblages associated with east-facing subduction along the North American margin or exotic arcs/back-arc basins developed above west-facing subduction zones and amalgamated to North America in the later half of the Jurassic. (3) There was a transition over time from mildly extensional within-arc tectonism in the Early Jurassic to variably transpressive and transtensional tectonism in the Middle to Late Jurassic. This transition led to deformation and shortening in northern California (broadly Jurassic igneous-related metallogeny 377 Figure 2. Total alkali versus silica (TAS) plots and QAPF plots with mineralogical data for Jurassic intrusive and volcanic rocks. Areas noted on diagrams are mentioned in the text and/or shown on Figures 1 or 3. A. Klamath Mountains and Sierra Nevada foothills. B. Sierra Nevada and northwestern Nevada. C. Mojave (SE California) and southern Arizona-northern Sonora. D. Back-arc plutons of the central Great Basin. 378 Mark D. Barton, J. Dilles, J. Girardi, G. Haxel, D. Johnson, D. Kreiner, E. Seedorff, and L. Zurcher speaking the “Nevadan orogeny” but see discussion in Dickinson, 2008b) and, locally, in parts of the western and central Great Basin. During this period to the south there was continued normal and strike-slip faulting that corresponded to opening of the Gulf of Mexico that has been related by some to the postulated Mojave-Sonora megashear (Anderson et al., 2005a). Sedimentary paleoclimate evidence, including abundant evaporites and eolian sandstones, show that, during the Jurassic southwestern North America was arid to hyper-arid to arid reflecting its northward drift through the horse latitudes (May et al., 1989; Busby et al., 2005; cf. Figure 3C). Marine volcanic sequences are well preserved in the western and central Sierra Nevada, the western Klamath Mountains, and the Coast Range ophiolites (Saleeby, 1982; Hopson et al., 1981; Irwin, 1981). Transitional, restricted marine to terrestrial sequences are preserved in western Nevada and eastern California (Dunne et al., 1988; Sorensen et al., 1998; Garside, 1998). In some of these areas, limestone-shale sequences interfinger with Jurassic volcanic rocks, sedimentary gypsum, and terrestrial clastic rocks including local eolian sandstones (e.g., Dilles et al., 2000; Barton and Johnson, 2000; Proffett and Dilles, 2008). Farther south and east, terrestrial sedimentary sequences, including eolian sands and local evaporites, interfinger with epiclastic and volcanic rocks where this part of the arc is interpreted to be broadly extensional in the Middle Jurassic and with back-arc basins that may reflect strike-slip tectonics of the later part of the Jurassic (e.g., Saleeby and Busby-Spera, 1992; Busby et al., 2005). Complicating the Jurassic record are younger tectonic events that have shuffled, deformed, buried and uplifted various parts of the arc. These include intrusion, metamorphism and foundering by Cretaceous batholiths (e.g., Hanson et al., 1993; Tobisch et al., 2001), strike-slip faulting in the Cretaceous (e.g. Schweikert and Lahren, 1990; Wyld and Wright, 2001) and Cenozoic (e.g., Powell, 1993), shortening and metamorphism in the Cretaceous and early Tertiary (e.g., Tosdal et al., 1989), and normal faulting and crustal extension in the Eocene to present (e.g., Dickinson, 2002). JURASSIC HYDROTHERMAL SYSTEMS AND MINERAL DEPOSITS Hydrothermal alteration, typically accompanied by some type of mineralization, is associated with most Jurassic igneous complexes in the southwestern United States. Thousands of mineralized occurrences are spatially associated with Jurassic intrusive centers. In a systematic review of 1600 Mesozoic intrusive centers in the western United States, Barton et al. (1988) found that nearly one-half (129 of 280) of all Jurassic plutons had reported evidence of mineralization. Figure 3 shows the distribution of some of the most important or illustrative examples. Even where mineral deposits are not reported, most Jurassic plutons exhibit some evidence of metasomatism. Alteration is even more extensive—nearly ubiquitous—in Jurassic volcanic Figure 3. Distribution of Jurassic hydrothermal systems. (Same base maps as Figure 1.) A. Magmatic hydrothermal systems including porphyry Cu, Cu skarn, polymetallic Zn-Pb-Ag skarn/vein/replacement, high-sulfidation advanced argillic Cu-Au systems, and granite-related W-Au systems. B. Marine hydrothermal Cu-Zn(±Pb) VMS deposits and related high-sulfidation occurrences, plus terrestrial hydrothermal Fe(-Cu) skarns, Fe(-Cu) igneous-hosted vein, replacement and breccia deposits, and low-sulfidation advanced argillic hydrothermal occurrences. C. Indicators of (near-)surface fluids and distribution of intense Na(Ca), regionally extensive (low-T) K, and seafloor-type Na(±Ca) alteration. The distribution of eolian sands is generalized based on occurrences from the Colorado Plateau west and south into the arc. Jurassic igneous-related metallogeny rocks, where it can be hydrothermal or diagenetic (e.g., Schiffman et al., 1991; Sorensen et al., 1998; Johnson, 2000; Haxel et al., 2008a). Much of this alteration bears, at best, only a general link to deposits. Regional patterns in Jurassic alteration types are shown in Figure 3. Types of hydrothermal systems and related mineral deposits Jurassic igneous-related hydrothermal systems, including but not restricted to mineralized systems, can be categorized in several ways. First and most familiar is classification by deposit style and metal as summarized in Table 1. These classes include various common intrusion, skarn, replacement, breccia and vein types. Table 1 also categorizes Jurassic igneous-related systems into families based on the principal sources of fluids as inferred from multiple lines of geological and geochemical evidence (Figure 3; Barton, 1996). These groups are: (1) magmatic-hydrothermal, where the key fluids are derived from hydrous magmas with/or without external fluids—these include granite-related, porphyry, skarn, and some lower temperature deposit 379 types; (2) terrestrial-hydrothermal, where surficial or basinal fluids are essential, although other fluids are often present and can, in some cases, contribute key components—these include many hydrothermal iron oxide-rich systems (“IOCGs”) as well as a number of epithermal precious metal deposits; and (3) marine-hydrothermal, where fluids are dominated by seawater but may have magmatic aqueous fluids—these include volcanic-hosted massive sulfides, but also epigenetic deposits. Many districts have mixed sources and thus are hybrids of possible end-members. Nonetheless, this fluid-based classification helps rationalize the distribution of deposits and other types of alteration in time and space in the Jurassic and aids comparison with other episodes and regions. Porphyry-skarn(-replacement) Cu(-Mo-Au) systems Porphyry Cu(-Mo-Au) and related copper skarn/replacement systems are scattered along the Jurassic arc and spread into the central Great Basin (Figure 3A). A handful of areas are known to contain economically significant deposits; the best known are Yerington, Nevada (168–169 Ma), and Bisbee (Warren), Arizona (201 Ma). In these two districts, as in other Table 1. PRINCIPAL TYPES OF MINERAL DEPOSITS ASSOCIATED WITH JURASSIC MAGMATISM IN THE GREAT BASIN AND SURROUNDING REGIONS. Deposit Type Key Characteristics Examples Cu(-Mo-Au), Au-W, Zn-Pb-Ag(-Au) – magmatic hydrothermal or hybrid Cu(-Mo-Au) porphyry Intense K-silicate to sulfide-rich sericitic alteration; ± high level advanced argillic Cu skarn/replacement Garnet-pyroxene to actinolite-chalcopyrite skarns and sulfide-rich, replacement Zn, Pb, Fe (± Cu) sulfides ± scheelite in replacement and/or pyroxene-garnet skarn Ag-bearing galena-sphalerite-quartz veins with hydrolytic alteration Quartz-Au/W veins, local skarns, and As-Bi bearing sulfide-silica replacement Aluminum silicate minerals associated with alunite, pyrite ± chalcopyrite, bornite Zn-Pb-Ag(-Cu-W-Au) skarn/replacement Ag(-Pb-Zn) vein Au-W granite/skarn/ replacement High sulfidation advanced argillic Yerington, NV; Ann Mason, NV; Bisbee, AZ; Lights Creek, CA; Royston, NV Yerington, NV; Contact, NV; Dolly Varden, NV; Bisbee, AZ Cerro Gordo, CA; Darwin, CA; Cortez, NV; Courtland-Gleeson, AZ Candelaria, NV Bald Mountain, NV; Gold Hill, UT; Osceola, NV Alunite Hill, NV; North Keystone, CA (VMS link?) Terrestrial hydrothermal – Fe(-P), Fe(-Cu), Cu(-Fe) (“IOCG”) Fe(-Cu) skarn Fe(-Cu/-P) igneoushosted Magnetite(-hematite)-rich pyroxene-garnet-actinolite-chalcopyrite skarns; intrusions have Na(Ca) alteration and endoskarn Magnetite(-apatite-actinolite) with intense Na(Ca) alteration to magnetite-hematite-Cu sulfide with Na and acid alteration Low sulfidation advanced argillic Aluminum silicate minerals (kyanite, where metamorphosed) ± hematite / magnetite ± dumortierite; little or no Fe sulfide Pumpkin Hollow, NV; Eagle Mountain, CA; Hall, CA Calico, NV; Quijotoa, AZ; Humboldt, NV; Cortez Mountains, NV; Bessemer, CA; Palen, CA; Lights Creek, CA Palen Mountains, CA; Dome Rock Mountains, AZ; American Girl-Vitrifax Hill, CA Marine hydrothermal – Cu(-Zn), Cu-Zn(-Pb) Cu(-Zn) Cyprus type Stratabound and stockwork Cu(-Zn) with ophiolites Cu-Zn(-Pb) Kuroko, Noranda type Cu(-Zn-Co) Besshi type Stratabound and stockwork Cu-Zn(±Pb) with marine arc Turner-Albright, CA-OR; Copperopolis, CA Foothill Copper Belt, CA Stratabound and stockwork Cu(-Zn) in clastic section with mafic sills Green Mountain, CA 380 Mark D. Barton, J. Dilles, J. Girardi, G. Haxel, D. Johnson, D. Kreiner, E. Seedorff, and L. Zurcher well-mineralized areas, multiple types of mineralization are typically present (Table 2). Many other intrusive centers contain mineralization and alteration types that may represent parts of variable exposed and/or eroded porphyry centers. For example the weakly mineralized Royston, Nevada, occurrence may represent only the fringes of a better mineralized ~200 Ma system (Seedorff, 1991a). Other plutons, lacking skarns or high level features, appear to be the roots of mineralized systems. Examples include the Austin pluton in central Nevada (which hosts polymetallic silver veins) and the Santa Rita pluton in the Inyo Mountains. Both plutons contain copper-bearing quartz veins with vein-controlled and disseminated K-silicate alteration (Barton, Seedorff, and Kreiner, unpubl. data). Copper skarns and replacement deposits are well developed in carbonate rocks in Jurassic porphyry districts. Among the best developed are those at Bisbee, where high-grade mantos surround advanced argillic alteration in the upper part of a composite porphyry center (Einaudi, 1982a). In other places, garnet-rich copper skarns and breccia pipes occur adjacent to plutons with relatively weak copper-bearing K-silicate and/or sericitic alteration (e.g., ~158 Ma, Contact, Nevada, LaPointe et al., 1991; ~159 Ma, Dolly Varden, Nevada, Atkinson et al., 1982). Whether these also represent deeper or lateral parts of better mineralized centers is uncertain. As a further complication, some districts are clearly composite in nature, i.e., they contain both porphyry-style mineralization with high-temperature K-silicate and acid alteration styles and IOCG-style sodic(-calcic) alteration and iron oxide-rich, sulfide-poor mineralization. These districts can be either hybrids that formed in the same overall thermal event with mixed fluid sources (e.g., Yerington, Table 2) or composite systems where temporally and compositionally distinct systems are superimposed (e.g., Lights Creek, Table 2). As is the case globally (Seedorff et al., 2005), porphyry copper and copper skarn mineralization occurs with oxidized (magnetite-titanite-bearing), intermediate to felsic, hornblende-biotite-bearing metaluminous to weakly peraluminous granitoids. Although copper and gold occurrences are known with the alkaline intrusions along the arc in eastern California and elsewhere, none of these silica-poor intrusive complexes in this region are known to contain porphyry copper mineralization, such as those found in the broadly coeval Jurassic terranes of British Columbia (Lang et al., 1995). Granite-skarn(-replacement) Au-W(-base metal) systems Gold-tungsten-base metal mineralization is spatially associated with a number of Jurassic granitic plutons in the central Great Basin (Figure 2D, Figure 3A). These include deposits in the Gold Hill district, Utah (~158 Ma, Nolan, 1935; El Shatoury and Whelan, 1970), the Osceola and nearby districts in the Snake Range, Nevada (~160 Ma, Hose and Blake, 1976; Lee et al., 1981), and the Bald Mountain district in Nevada (159 Ma, Nutt et al., 2007). In these areas gold is associated with quartz veins and silicification, commonly with sericitic (/greisen), skarn or jasperoidal alteration. Tungsten occurs as scheelite in skarns or, more rarely, as wolframite in granite-hosted quartz veins. Sulfide contents are relatively low, however these areas all have added lead, zinc, bismuth, and arsenic. The intrusions are felsic, generally biotite quartz monzonites to granodiorites with sparse hornblende and, rarely, magmatic muscovite. Magmatic oxides are sparse to absent, consistent with relatively reduced magmas, although late(?) titanite is present at Bald Mountain (Nutt et al., 2007). The character of these deposits combined with their associated Fe-Ti-oxide-poor quartz monzonites to granites suggests parallels with granite-related gold(-tungsten) systems worldwide (Thompson et al., 1999; Nutt et al., 2007). Polymetallic Pb-Zn-Ag(-Au) systems Polymetallic silver-base metal vein, replacement and skarn deposits are widespread across the Great Basin and in adjoining areas where carbonate rocks are present, particularly along the Cordilleran miogeocline, but also in the Paleozoic cover sequences in Arizona (Figure 3A; Titley, 1993). Within this region, most base-metal replacement systems are Cretaceous or Tertiary; however, scattered deposits across the Great Basin, from western Utah into eastern California, form a Jurassic lead-zinc province (Albers, 1981; Albino, 1995). Among the better known examples are Cortez (Mill Canyon) in Nevada (158 Ma; Stewart and McKee, 1977), Cerro Gordo, Darwin, Goodsprings and nearby districts in eastern California and southernmost Nevada (155–175; 190–200(?) Ma; Newberry et al., 1991; Church et al., 2005), and the Courtland-Gleeson district in Arizona (196±5 Ma; Gilluly et al., 1956; Lang et al., 2001). Associated intrusions commonly contain sparse quartz veins and weak potassic or sericitic alteration; yet only the Courtland-Gleeson district is known to have associated porphyry copper mineralization. Although Jurassic base-metal skarn and replacement deposits are relatively common, there are few compelling examples of vein, stockwork or disseminated, lower-temperature style precious metal deposits. Numerous small, ill-described precious metal deposits are associated with Jurassic plutons and volcanic complexes. The only convincing example of a low-sulfidation-type precious metal deposit is the 192 Ma Candelaria Ag(-Pb-Zn-Mn) deposit in western Nevada (Page, 1959; Thomson et al., 1995). Even at Candelaria, the style differs from ordinary epithermal environments in that the thin sulfosalt-bearing quartz-dolomite(-sphalerite-galena ±arsenopyrite±chalcopyrite) veins are associated with well-developed quartz-sericite-pyrite(-tourmaline) alteration. Similar Ag-rich quartz veins occur at Austin. Small silver-base metal occurrences near Jurassic igneous centers are widespread along the arc (e.g., du Bray et al., 2007) Iron oxide-rich ±P, ±Cu(-Au-Ag) (~ IOCG) systems Hydrothermal systems with voluminous hypogene magnetite and hematite and varying amounts of accessory Cu, Au, Jurassic igneous-related metallogeny 381 Table 2. SELECTED JURASSIC IGNEOUS-RELATED MINERAL DISTRICTS. Deposits Types Geology Hydrothermal features References Bisbee District, AZ – Cu(-Mo) porphyry, Cu(-Au-Zn) replacement & skarn / 201 Ma Bisbee, AZ Porphyry Cu(-Au-Mo) Bisbee, AZ Cu(-Au-Zn-Mn) replacement ± skarn Jurassic granitic porphyries and breccias intrude carbonate-clastic section above schists Jurassic granitic porphyries and breccias intrude carbonate-clastic section above schists Deep K-silicate to quartz-sericite-pyrite; shallow intense pyrite-pyrophyllite-quartz-sericite Qz monzodiorite to granite batholith intruding coeval volcanic rocks; porphyry dikes generated from granite late in history Granite porphyry dikes intrude Jur-Tri mixed carbonate-volcanic sequence K-silicate to sericitic alteration with Cu(-Mo) associated with granite porphyry dikes; episodic coeval Na(Ca) alteration Proffett and Dilles (1995), Dilles et al., (2000), Carten (1986) Andradite(-diopside) to actinolite-chalcopyrite skarns postdate abundant skarnoid, endoskarn Quartz-alunite-pyrophyllite-sericite over quartz-sericite-pyrite Magnetite-hematite-chalcopyrite with chlorite-actinolite-quartz in veins or garnet-pyroxene skarn Magnesian magnetite(-pyrite) skarn; magnetite-apatite-actinolite with intense Na(Ca) in intrusion Einaudi (1977), Harris and Einauidi (1982) Bryant (1966), Einaudi (1982a), Lang et al. (2000) Bryant (1966), Einaudi (1982a) Mainly pyrite-chalcopyrite-bornite replacement with minor skarn and distal Zn-Pb sulfide ± hematite Yerington District, NV – Cu(-Mo) porphyry, Cu skarn, Fe(-Cu) skarn, replacement, epithermal / 168–169 Ma Yerington, Ann Mason Cu(-Mo) porphyry Ludwig, Mason Valley Cu skarn Alunite Hill Pumpkin Hollow/ Buckskin High sulfidation advanced argillic Fe oxide(-Cu-Au) skarn / vein Minnesota / Easter Fe oxide skarn/ replacement Andesite-dacite volcanic rocks above coeval batholith Mixed Jur-Tri carbonate-volcanic package with qz monzodiorite sill at batholith contact Mixed Jur-Tri carbonate-volcanic sequence and subjacent qz monzodiorite Lipske and Dilles, (2000) Proffett and Dilles (1995), Dilles et al. (2000) Dilles et al. (2000) Lights Creek District, Northern, CA – composite IOCG and porphyry Cu / 148, 178 Ma Moonlight Valley, Engels and Superior IOCG-like veins and breccias Early Jurassic quartz monzonite (178 Ma) intrudes Early Jurassic volcanic rocks Moonlight Creek Cu porphyry Granite porphyry dikes (148 Ma) intrude Middle Jurassic volcanic and volcaniclastic rocks Chalcopyrite-bornite-magnetite(± sphalerite) veins with associated Na(-Ca) ± potassic alteration, coeval apatite-actinolite veins Quartz-tourmaline(-chalcopyrite-pyrite) veins with strong sericitic alteration Storey (1978), Dilles and Stephenson (2010) Storey (1978), Dilles and Stephenson (2010) Bald Mountain District, NV – W(-Pb-Zn-Au) skarn, Au(-Bi) replacement, vein / 159 Ma Bald Mountain, NV Au replacement with W(-Zn-Bi) skarn Miogeoclinal carbonte-clastic rocks intruded by Jurassic quartz monzonite to granodiorite W(-Pb-Zn-Au) skarn at lower levels with peripheral base metal and Au(-Bi)-jasperoid veins Nutt et al., 2007 Darwin, CA – Zn-Pb(-Ag-W) skarn and replacement / 155 (?) Ma Darwin, CA Pb-Zn(-Ag-W) skarn and replacement Deformed Paleozoic carbonate rocks intruded by Jurassic alkalic and calc-alkalic plutons Ag-bearing sphalerite-galena-garnet-pyroxene skarn to garnet-sulfide vein to shallow sphalerite-argentiferous-galena replacement Newberry et al. (1991) Candelaria District, NV – Ag(-Pb-Zn) veins, stockwork / 192 Ma Candelaria (continued) Ag(±Pb-Zn) veins Paleozoic to Triassic marine units overlain by ophiolitic mélange, intruded by Jurassic calc alkaline rhyolites and dacites Quartz stockworks, local tabular bodies with Ag minerals and trace Pb, Zn, Sb and As; sericite ± tourmaline ± pyrite alteration Thomson et al. (1995) 382 Mark D. Barton, J. Dilles, J. Girardi, G. Haxel, D. Johnson, D. Kreiner, E. Seedorff, and L. Zurcher Table 2 (continued). SELECTED JURASSIC IGNEOUS-RELATED MINERAL DISTRICTS. Deposits Types Geology Hydrothermal features References Humboldt Mafic Complex, NV – Igneous-hosted Fe-oxide(-P-Cu±Au), Ni-Co-Ag-U / 165-170 Ma Mineral Basin Fe oxide(-P±Cu) Tilted basalt—gabbro/diorite complex with dike swarms marking intrusive centers Boyer / White Rock Cu-Fe sulfides—Fe oxide Basalt to basaltic andesite and volcaniclastic sequence Magnetite-apatite-actinolite breccias, veins and replacements in zoned, intense Na(Ca) alteration Hematite(±magnetite)bornite-chalcopyrite with intense Na alteration, locally syngenetic FeOx Dilek and Moores (1995), Johnson and Barton (2000) Johnson (2000) Eagle-Palen Trend, CA – Fe(-Cu-Au) skarn, Fe(-P) breccias replacement, vein / ~165 (155-175) Ma Eagle Mountain Fe(±Cu-Au) magneisan skarn Jurassic composite granodioritic stock in carbonate-bearing metasedimentary rocks Palen Mountains Fe oxide (-P-Cu), advanced argillic Fe oxide Basaltic to andesitic Jurassic volcanic and volcaniclastic rocks intruded by Jurassic diorite Magnetite-pyrite(-apatite)-actinolite in marble, local Cu±Au; Na(Ca) ± K alteration in intrusion Magnetite-apatite-actinolite±Cu in Na(Ca) alteration zone upwards to pyrophyllite-hematite-quartz±Cu Dubois and Brummet (1968), Mayo et al. (1998) Stone et al. (1985), Fackler-Adams et al. (1997) Foothill Copper Belt, CA – Cu(-Zn) volcanogenic massive sulfides, Cu(-Au) / 160-165 Ma Penn, Green Mountain, Copperopolis Kuroko, Noranda, Besshi variants North Keystone and others Intense acid, advanced argillic Hosted in andesite-dacite and overlying mafic-clastic section; up to amphibolite facies overprint In andesite-dacite sequence; up to amphibolite facies overprints REE, and other metals are widespread in the Jurassic with several hundred occurrences and a few dozen significant deposits (Figure 3B; Barton et al., 2000; Johnson and Barton, 2000). In addition to the iron oxides and associated metals, these areas also have the other features that are characteristic of IOCG systems worldwide, including voluminous sodic, sodic(-calcic), and potassic (at shallow levels) alteration, skarn (in carbonate host rocks), a diversity of structural styles (breccias, veins, replacements, stratabound), a distinctive set of abundant associated minerals (e.g., actinolite, apatite), and a paucity of sulfides (Hitzman et al., 1992; Barton and Johnson, 1996; Williams et al., 2005). The most common deposit associations include: (1) magnetite(-apatite-actinolite±minor sulfides) hosted in intrusive or volcanic rocks, (2) magnetite-rich skarns with or without appreciable copper hosted in carbonate host rocks, and (3) various kinds of hematite-magnetite(-chalcopyrite±bornite) deposits in igneous or carbonate rocks. The carbonate-hosted varieties comprise a subset of iron skarns (Einaudi et al., 1981) and traditionally have been classified that way. The largest IOCG districts, each of which contains multiple deposits, are along the main trend of the arc, and—notably—all are inboard of the Jurassic shoreline (cf. Figures 3B, 3C). Scattered IOCG prospects in western Baja California (Figure 3B; e.g., San Fernando; Lopez et al., 2005) may belong to a separate belt of Late Jurassic or Early Cretaceous age. Iron has been the principal commodity produced from Jurassic IOCG systems, Massive sulfide lenses (pyrite- or pyrrhotite-rich) above sericite-chlorite-altered stringer zones Andalusite-quartz(-bornite-chalcopyrite-pyrite), distal jasper Heyl (1948), Kemp (1982), Mattinen and Bennett (1986) Clark and Lydon (1962) though many of these districts also contain Cu(-Au) mineralization (Figure 3B). Several dozen districts have produced iron and at least 10 districts have had some significant Cu ± Au production. The largest Cu resources are at Lights Creek (Superior-Engels-Moonlight Valley), Pumpkin Hollow in the Yerington district, the Calico Hills, and San Fernando (Figure 3B), however spotty Cu-Au(-REE±U) mineralization occurs all along the Jurassic trend and is not restricted to a particular area. Most occurrences have seen little modern exploration. The larger districts commonly contain multiple deposit types within the same complex and, in some cases, broadly overlap in space and time with other types of mineralization. Table 2 summarizes four districts with IOCG mineralization; each has multiple deposits, and they show the internal and between-district variability that is typical of IOCGs: 1. The Humboldt Mafic Complex is associated with a gabbro-diorite and basalt-andesite complex and varies from deep magnetite-apatite-actinolite bodies with intense scapolitic alteration to shallow hematite-rich, sulfide-bearing mineralization with albite-chlorite-carbonate alteration (Johnson and Barton, 2000). Geological association with contemporaneous evaporites, geochemical data, petrology, and mass balance considerations show that evaporitic brines dominated, perhaps were the sole contributors to, the hydrothermal system (Johnson and Barton, 2000). Jurassic igneous-related metallogeny 2. The Yerington district, as noted above, has several porphyry copper centers and classic copper skarns; beyond that, it is an excellent example of a hybrid system Contemporaneous with, but distal to the porphyry mineralization, hydrothermal flow in the Yerington batholith created extensive sodic(-calcic) alteration zones that are directly linked to deep magnetite-apatite-actinolite veins that zone upwards into shallow chlorite-hematite-sericite-quartz(-chalcopyrite) or outwards into magnetite-actinolite(-chalcopyrite) skarns (Dilles et al., 2000). Geological and geochemical data demonstrate that the IOCG systems formed by circulation of non-magmatic brines from the host Mesozoic sedimentary sections (e.g., Dilles et al., 1995, 2000). 3. The Plumas County Copper Belt, including the Lights Creek (Superior-Engels) district is a variant on this theme. Early Jurassic IOCG mineralization is overprinted 30 m.y. later by porphyry style alteration (178 and 148 Ma, respectively; Dilles and Stephens, 2010), creating a composite system with key parts separated in time; Lights Creek thus contrasts with the hybrid system at Yerington all parts of which formed roughly concurrently. Unlike Humboldt and Yerington, sources of fluids have yet to be confirmed although both of magmatic and non-magmatic fluid sources are probable (Dilles and Stephens, 2010). 4. The Eagle Mountain-Palen Mountain trend (~165–170 Ma; Stone et al., 1985) has aspects reflecting many other deposits in southeastern California and southern Arizona (e.g., Lamey, 1948; Hall et al., 1988). Pluton-associated magnetite-actinolite skarns with minor gold and copper at Eagle Mountain (DuBois and Brummett, 1968) line up along inferred regional structures with higher level volcanic-hosted magnetite-apatite-actinolite, Cu-Au prospects, and extensive sulfide-poor advanced argillic alteration typified by pyrophyllite + specular hematite + quartz in the southern Palen Mountains. This style of sulfide-poor advanced argillic alteration is a common associate of IOCG systems (see below) and, like other aspects of their geochemistry, stands in contrast to other types of Jurassic hydrothermal systems. Collectively, the IOCG systems share many geochemical characteristics and mineralogical themes, yet they occur in many settings and with the whole gamut of igneous compositions along the Jurassic arc. Comparison of areas with and without obvious porphyry style mineralization, geochemical considerations, and regional patterns in alteration and paleowater sources all point to an essential role for non-magmatic, evaporitic (e.g., basinal or surface-derived) fluids as proposed by Barton and Johnson (1996). Advanced argillic (high and low sulfidation types) systems Zones of advanced argillic alteration, characterized by 383 hypogene kaolinite, pyrophyllite, kyanite or andalusite and accessory minerals such as rutile, zunyite and dumortierite (Meyer and Hemley, 1967), are developed in several distinctive modes in the Jurassic (Figure 3A,B; Kreiner and Barton, in prep.). The most familiar are a handful of porphyry-related occurrences including Yerington (Lipske and Dilles, 2000; Dilles and Einaudi, 1992) and Bisbee (Bryant, 1966; Einaudi, 1982b). In these examples, intense pyrite-rich pyrophyllite-alunite-bearing assemblages form in the upper levels of porphyry Cu(-Mo) systems. Although in some cases advanced argillic alteration extends laterally for a kilometer or along favorable strata or structures; all examples of these sulfide-rich assemblages are clearly centered on shallow intrusive complexes. At neither Bisbee nor Yerington does the advanced argillic alteration carry abundant Au or Cu, although these metals are present in small amounts. A second, more widespread group consists of sulfide-poor to absent, hematite or magnetite-stable, quartz-pyrophyllite(-kaolinite) alteration. This association occurs along the main Jurassic trend from southern Arizona into eastern California and western Nevada (Figure 3B). Some of these systems can be quite extensive and, unlike the high-sulfidation examples, the low-sulfidation variety is commonly structurally controlled. In the Palen Mountains, for example, pyrophyllite-quartz(-specular hematite) alteration extends several kilometers along a major structural trend, in the upper part of the middle Jurassic volcanic pile, stratigraphically overlying IOCG-style mineralization and intense sodic(-calcic) alteration near subjacent plutons (Stone et al., 1985). In the vicinity of the Colorado River, regionally metamorphosed Jurassic volcanic and, locally, intrusive rocks contain kyanite-quartz occurrences (Reynolds et al., 1988; Haxel et al., 2002) which are commonly associated with gold, copper and iron oxide occurrences, and with sodic-calcic alteration. The geochemical similarities and proximity to unmetamorphosed variants argues that these kyanite-bearing examples are metamorphosed, low-sulfide, advanced argillic systems. Others have proposed that they reflect hydrothermal formation under deeper, mesothermal conditions (e.g., Owens and Hodder, 1994). A third group of aluminum silicate (typically andalusite)-quartz-rich altered rocks is found in the western foothills of the Sierra Nevada (e.g., Clark and Lydon, 1962) and along the western side of the Peninsular Ranges batholith in southern California (Jahns and Lance, 1950). The sulfide-rich, copper-bearing occurrences in the Foothill Belt may represent the intensely acid-dominated parts of marine arc-related hydrothermal systems (e.g., Hannington et al., 1999; Resing et al., 2007). In contrast, the southern California pyrophyllite/kyanite / andalusite ± dumortierite occurrences appear to be relatively sulfide poor and are not obviously associated with massive sulfides; these occurrences may represent the northward extend of the Alisitos terrane of Baja California Norte which contains Late Jurassic or Early Cretaceous IOCG deposits. 384 Mark D. Barton, J. Dilles, J. Girardi, G. Haxel, D. Johnson, D. Kreiner, E. Seedorff, and L. Zurcher Volcanic-hosted massive sulfide (VMS) systems Scattered Cu-Zn-dominated volcanic-hosted massive sulfide deposits occur in the Middle to early Late Jurassic marine terranes of the Sierra Nevada foothills, the Klamath Mountains, and the disrupted ophiolitic terranes of the Coast Ranges (Albers, 1981; Figure 3B). Most deposits are quite small containing at most a few million tons of ore with grades of 3–10% combined Zn+Cu±Pb. The largest group constitutes the Foothill Copper Belt (Heyl, 1948) where metamorphosed and deformed Kuroko- and Noranda-like deposits are associated with the felsic, transitionally calc-alkaline portions of the Smartville complex (Kemp, 1982). In the earlier basaltic to andesitic volcanic parts of these composite sequences, there are a few Cu(-Au) rich deposits such as Copperopolis (Clark and Lydon, 1962). Besshi-style (Co-bearing pyrrhotite-dominated) mineralization at Green Mountain in the southern end of this terrane occurs in slightly younger carbonaceous epiclastic rocks (Mariposa Formation) intruded by mafic sills (Mattinen and Bennett,1986). VMS deposits in the Coast Range and Josephine ophiolites are rare. The Turner-Albright deposit (Kuhns and Baitis, 1987; Zierenberg et al., 1988) in the Josephine ophiolite is one of the few of economic interest. The paucity of deposits may reflect the lack of through-going structures to focus fluid flow and discharge (Schiffman et al., 1991; Harper, 1999). Although the igneous rocks of this region have many similarities to those inboard in northeastern California and northwestern Nevada, there is no evidence in the west for Jurassic IOCG mineralization or IOCG-like intense sodic(-calcic) and potassic alteration. Regionally extensive alkali-exchange alteration Widespread, intense alkali-exchange alteration is another prominent facet of Jurassic hydrothermal systems (Figures 3C). This process directly bears on the nature and origin of some Jurassic mineral deposits and can obfuscate the earlier features in others. Three types of alkali-exchange alteration predominate: (1) intense sodic(-calcic) alteration associated mainly with and proximal to plutons, (2) sodic alteration that is ubiquitous in marine volcanic sections and cogenetic hypabyssal rocks, typically in more distal positions, and also occurs in plutons, and (3) widely distributed, intense potassic alteration in continental volcanic rocks (e.g., Meyer and Hemley, 1967; Chapin and Lindley, 1986; Barton et al., 1991; Alt, 1999; Rougvie and Sorensen, 2002; Seedorff et al., 2005). These types are the most voluminous metasomatic types in the upper crust (Johnson, 2000), yet they are less well known than other types probably because they rarely host ores. Sodic(-calcic), sodic, and (less common) calcic or endoskarn alteration types manifest themselves through development of secondary plagioclase (Ab100 to Ab70) and/or scapolite plus related (Ca-)Mg minerals (chlorite to actinolite to diopside to grossular—in order from sodic to calcic) and, typically, removal of many transition metals and potassium (e.g., Dilles and Einaudi, 1992; Dilles et al., 1995). Intense, voluminous potassic alteration is widespread, though less well known, and is characterized by conversion of igneous feldspars and groundmass to secondary K-feldspar and conversion of mafic minerals to chlorite, hematite and clays (Chapin and Lindley, 1986; Barton and Johnson, 2000). This type of K-alteration typically reddens rocks which can also lose most of their Na, Ca and base metals. As illustrated in Figures 4 and 5, whole-rock compositions change markedly from their fresh equivalents. Alkali ratios and, typically, ferric-ferrous rations are modified, commonly much more than in most other types of alteration (e.g., in porphyry-related potassic alteration, Figure 5D). In many older studies, the presence or extent of alteration was not recognized, and alkali-altered rocks commonly were interpreted as having distinctive or unusual igneous compositions. Sodic and sodic(-calcic) alteration is reported (or can be inferred from published data) for dozens of intrusive systems and some volcanic terranes throughout the duration and extent of the Jurassic (Battles and Barton, 1995; Dilles et al., 1995). At the district scale, sodic(-calcic) is well described in Nevada (Carten, 1986; Dilles and Einaudi, 1992; Johnson, 2000; Johnson and Barton, 2000) and in the Mojave (Fox and Miller, 1990). Similar, but typically less intense, sodic alteration assemblages are seen in the marine arc and ophiolite sequences of central and northwestern California (Figure 3C; e.g., Harper et al., 1988). Stratabound potassic alteration is widely developed along the subaerial part of the arc, for example in the Peavine sequence (Garside, 1998), Yerington (Dilles et al., 2000), Cortez Mountains (Muffler, 1964), Owens Valley region including the Inyo Mountains and Sierran pendants (Sorensen et al., 1998), and Arizona (Haxel et al., 2008a). Interestingly, stratabound sodic alteration is seemingly uncommon along the main arc; it becomes abundant only to the north and west where magmatism overlapped with transitional marine sequences, for instance in the central Sierra Nevada and in northwestern Nevada (Figure 3C). Voluminous alkali-exchange alteration requires reaction of volcanic rocks and shallow intrusions with moderately to strongly saline surface-related waters; magmatic fluids (if even present) are neither sufficiently voluminous and would generate substantially different assemblages and patterns than are observed (Barton and Johnson, 2000). Seawater is an obvious candidate for involvement, and is well known for leading to such changes (e.g., Alt, 1999). However, saline surface or basinal waters are compelling candidates in other areas. Such fluids were abundant in the Jurassic, and should have been readily accessed via plumbing systems consisting of extensional faults and variably porous, penecontemporaneous supracrustal rocks. Seawater was available to the west and north, and Jurassic marine and continental evaporites were (and still are) widespread in Nevada (Speed, 1974; Proffett and Dilles, 2008) extending eastward to the Colorado Plateau (Turner and Fishman, 1991). Well-documented Jurassic aridity (e.g., Peterson, 1988) would also have contributed to formation of saline surface and Jurassic igneous-related metallogeny 385 Figure 4. Na2O vs. K2O and TAS diagrams showing effect of alkali-exchange alteration for selected areas of Jurassic hydrothermal in the southwestern United States (from Barton and Johnson, 1997, compiled from multiple sources). A) and B) Three Middle Jurassic IOCG-bearing districts from Nevada: a mafic example (Humboldt), a felsic example (Cortez Mountains), and a hybrid example (intrusion-hosted alteration from the Yerington district, including porphyry copper related potassic alteration). C) and D) Similar data from a number of other Jurassic areas from the Great Basin and southeastern California, some of which are closely linked to iron deposits, but others are not. near-surface fluids. Aridity might have had a considerable impact on other aspects such as the nature of Jurassic volcanism (Busby et al., 2005). Correlations with mineralization are clear—sodic and lesser sodic(-calcic) alteration is clearly related to seafloor hydrothermal activity and VMS mineralization in northwestern California (Figure 3B). The spatial and temporal link of sodic styles of alteration with iron deposits is equally compelling and has been long recognized (e.g., Lindgren, 1913; Einaudi et al., 1981; Williams et al., 2005). The diverse character of associated igneous rocks, the affinity to evaporitic environments, and geochemical (mass balance and solution chemistry) arguments that first led Barton and Johnson (1996, 2000) to suggest that the IOCG family in this part of the world, indeed globally, is fundamentally the product of thermally circulated non-magmatic brines and not mainly the product of magmatic-hydrothermal fluids. Distribution in time and space—correlations With the exception of marine arc and ophiolite-related VMS systems of central and northwestern California, most known Jurassic mineralization in the western US lies in eastern California, northern Nevada, and Arizona. A few porphyry and IOCG occurrences are found in southern California and Baja California (Barton et al., 2000; Staude and Barton, 2001). These include the porphyry Cu deposit at El Arco in Baja California Norte (165 Ma, Valencia et al., 2006) and iron oxide-rich deposits with copper, gold and extensive alkali-rich alteration along the west coast of Baja California (Late Jurassic or Early Cretaceous, Barton et al., 2000; Lopez et al., 2005). The location of these areas relative to North America in Jurassic time is problematic; they may have formed in arcs some distance removed from the continent (e.g., Dickinson and Lawton, 2001; Valencia et al., 2006). 386 Mark D. Barton, J. Dilles, J. Girardi, G. Haxel, D. Johnson, D. Kreiner, E. Seedorff, and L. Zurcher Overall, there are two broad episodes of mineralization, one at ~190–210 Ma and the other at ~155-170 Ma, which correspond to ill-defined pulses of magmatism (Figure 6). Like intrusions of the same age, Early Jurassic mineralization (porphyry Cu, Zn-Pb-Ag, and IOCG) is fairly sparse (Figure 3B). It occurs mainly along the arc from southern Arizona (e.g., Bisbee, ~200 Ma, Courtland-Gleeson, ~190 Ma, Lang et al., 2001) through eastern California and western Nevada (e.g., Candelaria, 192 Ma, Thomson et al., 1995; Royston ~200 Ma, Seedorff, 1991a; possibly the Jackson Mountains, ~190–196 Ma, Quinn et al., 1997), and into northern California (Lights Creek, 178 Ma; Dilles and Stephens, 2010). Curiously, few deposits are known to be between 190 and 170 Ma. Intrusions in this age range are seemingly less common than others which may explain part of the pattern. As noted later the same apparent gap is present in British Columbia. The second episode, to which most porphyry Cu and IOCG systems and all the known VMS and W-Au systems belong, correlates with the mid-Jurassic magmatic flare-up that extends from the oceanic arcs and ophiolites in the west to the scattered back-arc granitoids of the north-central Great Basin (Figures 3, 6). All the porphyry and most of the base metals systems are located, possibly because they are best preserved there, in the Great Basin with only scant evidence for porphyry-style mineralization of this age in the Sonora, Arizona, or California. In contrast, IOCG systems extend throughout the entire magmatic belt inboard of the Jurassic shoreline. Many of the southern occurrences are magnetite-dominated which might reflect a preponderance of deeper exposures; on the other hand, low-sulfidation advanced argillic occurrences are also widespread in this region perhaps negating a depth of exposure argument. Figure 5. K2O/Na2O vs. Fe2O3/FeO illustrating the large changes with surface- / basin-derived saline fluids which are commonly related to areas hosting IOCG systems (data compiled from multiple sources by Barton and Johnson). A) Compositions for the Humboldt (mafic) and Cortez Mountains (felsic) districts in Nevada, both of which have IOCG mineralization of the same age and in the same rock suites. B) Similar data from other areas along the main Jurassic arc (cf. Figure 3B,C). C) Comparison of results from the Mesoproterozoic IOCG-bearing terrane of SE Missouri, illustrative of such areas worldwide (e.g., Williams et al., 2005). D) Data from plutonic rocks in the Yerington district illustrating the small changes in porphyry-related (high-T) potassic alteration compared to the low-T stratabound variety (intense, low-T K alteration is present in the volcanic section at Yerington). Jurassic igneous-related metallogeny 387 Levels of exposure Figure 6. Synopsis of timing of hydrothermal systems and generalized patterns in related features during the Jurassic. The maxima in deposits observed at 200 Ma and 160 Ma closely match the timing observed in British Columbia. See text for source of information and discussion. Late Jurassic–Early Cretaceous magmatism was relatively sparse and, with only a few exceptions of little economic importance (e.g., Lights Creek porphyry, 148 Ma), apparently lacks well mineralized igneous centers. In many areas, magmatism is represented by dike swarms or small intrusions, characteristics that belie the large energy and material supplies necessary to make large deposits. In northern California, where Late Jurassic magmatism appears to be best developed, many plutons of this age appear to be relatively deeply eroded with the exception of the NE Sierra Nevada (Christe, 2010; Dilles and Stephens, 2010). The same may be true in southern California where a Late Jurassic plutonic belt apparently preserves only granitoids (Barth et al., 2008). The relative abundance of Jurassic volcanic rocks throughout this region and related evidence, based on petrography and the styles of hydrothermal systems, led Barton et al. (1988) to infer that a considerable fraction of the Jurassic igneous province was preserved at relatively high levels, the upper 4–8 km). Although shallow crustal levels are inferred in most regions, some barometric data indicates that much deeper levels can be exposed locally. For example, barometry on some plutons in the Klamath Mountains (Barnes et al., 1986b) yields mid-crustal pressures (3–8 kb, equivalent to 10–25 km paleodepths). In many areas, the preserved Jurassic superstructure is relatively sparse, though it is often comparable in outcrop area to that of the plutons. The rocks that are preserved consist of sequences that may represent the thickest parts of the original volcano-sedimentary piles; in many cases they accumulated in tectonically induced syn-volcanic depressions or in caldera fill (Dunne et al., 1998; Riggs et al., 1993; Schermer and Busby, 1994). In many areas, Tertiary extension and block rotation have exposed multiple levels through the Jurassic crust. The classic example of this process is the Yerington district where ~6 km of upper Jurassic crust are preserved from the volcanic carapace well into the underlying batholith (Proffett, 1977; Dilles and Proffett, 1995). The Royston porphyry Cu system (Figure 3B; Seedorff, 1991a) is another example of a tilted system. In addition to Tertiary extension, contractional events in the Mesozoic and early Tertiary deformed and rotated some systems. One of the best documented examples of this type of structural tilting and erosion is in the Klamath Mountains where Mesozoic shortening led to the exposure of multi-kilometer depth intervals through the Wooley Creek and Slinkard plutons (Barnes et al., 1986b). Beyond tilting, the complex syn- and post-Jurassic tectonic history likely had additional effects. Contractional orogenic events in the Middle(?) to Late Jurassic including the Nevadan (Late Jurassic), Sevier (Cretaceous), Laramide (Late Cretaceous-Paleocene), and even Neogene (restraining bends along the strike-slip margin) played multiple roles. Undoubtedly, crustal shortening uplifted and exposed to erosion the upper parts of many Jurassic centers. In contrast, in other places burial by reverse faulting or by foundering (as is the case for screens within the Cretaceous batholiths) contributed to high-level preservation. Jurassic and Early Cretaceous extension in the southern California and Arizona helped also preserve high levels (e.g., Busby et al., 2005) and similarly low base levels—evidenced by original near-sea level elevations—in other parts of the Jurassic arc such as northwestern Nevada and northern California no doubt aided preservation of upper Jurassic crust. COMPARISON WITH OTHER TIMES AND REGIONS Here we briefly compare the Jurassic metallogeny of southwestern North America with other times and other regions. 388 Mark D. Barton, J. Dilles, J. Girardi, G. Haxel, D. Johnson, D. Kreiner, E. Seedorff, and L. Zurcher These patterns are then considered in terms of possible metallogenic controls—process, province, and preservation. Comparison with other periods in SW North America Jurassic metallogeny differs appreciably from other periods in the latter half of the Phanerozoic (Barton et al., 1988, 1995; Barton, 1996). Compared with other times, alkaline igneous compositions are more abundant, IOCG and VMS deposits and alkali-rich alteration are more common, economically significant porphyry deposits are sparse, and precious metal (mainly epithermal) deposits are rare. Pre-Jurassic (mainly Permo-Triassic) Igneous-related Paleozoic to Triassic deposits are rare. A few middle to late Paleozoic VMS systems occur in the eastern Klamath Mountains and locally in northern Nevada. Local IOCG-style mineralization and sodic alteration of probable Permian and Triassic age occurs in the eastern Klamaths (Pit River) and sparsely along the scattered Permo-Triassic arc (Battles and Barton, 1995) extending across the Southwest and into east-central Mexico. A few tungsten skarn deposits are co-located with Triassic granites in eastern California. Porphyry and epithermal systems older than latest Triassic seem to be absent, with a very few exceptions, such as the poorly dated but likely Triassic (~230 Ma) Cu-Au deposits of the Pine Grove district, Nevada (Pricehouse and Dilles, 1995). Cretaceous—Laramide (~145–50 Ma) Magmatism during the Cretaceous (145–65 Ma) and Laramide (~80–50 Ma) in the southwestern North America is almost entirely subalkaline and shows consistent secular variation in composition and distribution in space (e.g., Barton, 1996). The main Cretaceous batholithic belts (Figure 1) are rather deeply eroded, and contemporaneous volcanic rocks are rarely preserved (e.g., Barton et al., 1988). Even though igneous compositions are compatible with those observed in economically productive belts elsewhere, mineral deposits are rare. The principal types within the main arc are tungsten skarns formed where the felsic granitoids intrude carbonate-bearing host rocks. In contrast, large magmatic-hydrothermal deposits are widespread in the interior region, mainly in the Great Basin east of the main belt. These deposits formed with the subalkaline metaluminous to peraluminous felsic granitoids of the Cretaceous back arc in the Great Basin and in the Laramide province of the southern Basin and Range. Porphyry Cu(-Mo±Au) deposits are widespread with biotite(-hornblende) granodiorites and granites; lithophile element-rich systems occur with the strongly peraluminous granites. Volcanic rocks are sporadically preserved, but epithermal and IOCG-like systems are rare. Mid- to late Cenozoic (~50–0 Ma) The middle and late parts of the Cenozoic have considerable metallogenic and magmatic diversity and resemble aspects of the Jurassic: notably the association of a wide spectrum of igneous compositions, mostly extensional tectonics, and evidence for the influence of distinctive surface conditions on hydrothermal fluids (Barton, 1996). Tertiary porphyry, skarn, and base metal systems of multiple types occur with the majority of intrusive centers in the Great Basin (Seedorff, 1991b) and northern Mexico (Staude and Barton, 2001). Their characteristics vary systematically with the compositions of genetically related igneous rocks. Similarly, but in contrast to the Jurassic, numerous vein and stratabound precious metal deposits of a variety of types occur throughout the region. Many of these are clearly associated with coeval magmatism with which they show correlations in metal contents, alteration types, and other parameters (e.g., John, 2001). Conversely, other types of deposits are less clearly related to magmatic activity and may reflect circulation of fluids from non-magmatic sources during Cenozoic extension. Carlin-type gold deposits are one example (Ilchik and Barton, 1997; Cline et al., 2005). Modern low-salinity, gold-bearing geothermal systems are being driven by crustal extension in the absence of magmatism in northwestern Nevada (Coolbaugh et al., 2005). Cenozoic IOCG-style mineralization is widespread and locally active, as in the modern hypersaline Salton Sea geothermal system (Barton et al., 2000). Structurally controlled IOCG deposits in the southern Basin and Range are inferred to be caused by circulation of basinal (evaporitic) brines during extension, perhaps aided by magmatic thermal input (Wilkins et al., 1986). Broadly coeval, low-temperature K metasomatism is abundant throughout the same region (e.g., Chapin and Lindley, 1986; Rougvie and Sorensen, 2002). Comparison with other circum-Pacific regions Canadian Cordillera The amalgamated terranes of the Canadian Cordillera share many features in both timing and styles of magmatism with the southern Cordillera in United States and Mexico, and they have broad parallels, but also some major differences with the Andes (e.g., Coney and Evenchick, 1994). Middle Mesozoic rocks of the Canadian Cordillera contain a range of igneous compositions quite similar to the Jurassic farther south. Plutons range from strongly alkaline including rare silica-undersaturated compositions to ordinary oxidized (magnetite-titanite-bearing) calc-alkaline intrusions. Porphyry and skarn Cu(-Au) mineralization is associated with both types. Likewise, iron skarns (Meinert, 1984) and other IOCG-like deposits are widespread, particularly in southern British Columbia. VMS deposits are relatively uncommon, though some of the early Mesozoic systems that are present have exceptional size or grade (e.g., Windy Craggy, Peter and Scott, 1999; Eskay Creek, Roth et al., 1999). Perhaps most remarkable, given the postulated exotic nature of these terranes is that the timing of the Triassic to Jurassic magmatism and porphyry Cu(-Au-Mo) mineralization closely matches that in the western United States—200±10 Ma and Jurassic igneous-related metallogeny 389 165±5 Ma (Figure 6; McMillan et al. 1995; Lang et al., 2001). The reason for this coincidence remains uncertain. Does it reflect a far-field effect that influenced disparate arcs across the Jurassic northern Pacific, or does it demonstrate a more closely shared history tied to the evolving margin of North America? Subsequently in British Columbia, just as farther south, contractional tectonics led to substantial crustal thickening and abundant generation of later, Laramide (Late Cretaceous to Eocene) Cu(-Mo±Au) deposits. Central Andes The coastal cordillera of northern Chile and southern Peru provide another interesting comparison. In this case, voluminous magmatism that begins in the Jurassic and extends into the Early Cretaceous is linked to small porphyry Cu systems and widespread IOCG mineralization in Chile, changing northward in Peru to IOCG systems and then VMS deposits (Vidal et al., 1990; Sillitoe, 2003; Maksaev et al., 2010). In their continental-scale comparisons, Coney and Evenchick (1994) note fundamental similarities in the tectonic histories of the South and North American margins that are offset in time in relation to opening of the central and southern Atlantic Ocean basins (Figure 7). The parallels in metallogeny are similarly striking: older episodes are characterized by abundant IOCG and sparse porphyry systems, which change northward in both cases to marine settings with VMS deposits. Later orogenesis in both regions leads to the principal episodes of porphyry formation—Late Cretaceous to Paleocene in southwestern North America (Barton, 1996) and Eocene-Oligocene in the central Andes (Sillitoe and Perello, 2005). Both regions also have far more abundant epithermal systems during these younger periods although IOCG occurrences remain common in both (Figure 7; Barton, 2009). Southwest Pacific Like the North American Jurassic, the southwestern Pacific Cenozoic comprises an evolving collage of arcs built across terranes ranging from oceanic crust to continental basement. Diverse subalkaline to alkaline magmatism developed intermittently in these arcs and is linked to a wide variety of mineral deposits (e.g., Garwin et al., 2005). Porphyry Cu(-Au-Mo) systems are widespread with both alkaline and subalkaline intrusive complexes, much like in western North America from British Columbia to northwestern Mexico. Where the volcanic superstructure is preserved, some of the calc-alkaline centers have high-sulfidation epithermal deposits; in other areas adularia-sericite and even alkaline epithermal systems are preserved (Jensen and Barton, 2000; Garwin et al., 2005). Such high-level deposits are not known in the Jurassic of the western US and are problematically preserved in Canada. Numerous modern and recent VMS systems occur in the western Pacific along the submarine portions of arcs and in back arc basins. Preservation of some of these during later accretionary events seems plausible; if so, it could generate relationships not unlike Figure 7. Comparison of tectonic evolution and temporal distribution of porphyry and IOCG systems in the cordillera of North and South America. Tectonic base modified from Coney and Evenchick (1994). those seen in the Foothill Belt of the Sierra Nevada and the western Klamath Mountains. The tropical latitudes, and corresponding abundance of rainfall, make highly saline groundwaters scarce; thus the contrast between abundant IOCG-style systems in the western US and the more abundant adularia-sericite deposits of the southwestern Pacific may reflect a climatic control on near-surface fluids and, in turn, on hydrothermal geochemistry. Metallogenic Controls for the Jurassic of Southwestern North America The distribution and characteristics of Jurassic hydrothermal systems, and their comparison and contrast with other periods and regions are logically considered in terms of the variety of processes involved, crustal and surface-related provincial effects, and the filter imposed by preservation and exposure (Barton, 1996). Process controls Jurassic magmatic-hydrothermal systems correlate well with the composition of associated intrusions, as is observed globally in porphyry systems (e.g., Seedorff et al., 2005). This correlation is founded on the chemical controls that magmas exert on the compositions of exsolved fluids. Many Jurassic magmas may have been relatively dry, as evidenced by their mineralogy (lacking early crystallizing hornblende, cf. Figure 2) and their fairly mafic bulk compositions as compared to younger intrusions. These features contrast with the relatively hornblende-rich, felsic intrusions of later periods. Igneous compositions also influence the composition of fluids circulated through them following crystallization, with or without contributions from magmatic fluids, leading to differences in the nature of associated mineral deposits. This igneous control (mediated by other materials present) is evident for a wide range of deposit types including VMS (Franklin et al., 2005), epithermal 390 Mark D. Barton, J. Dilles, J. Girardi, G. Haxel, D. Johnson, D. Kreiner, E. Seedorff, and L. Zurcher (Jensen and Barton, 2000; John, 2001), and IOCG systems (Barton and Johnson, 1996, 2000). For the Jurassic, there are systematic differences in marine and terrestrial hydrothermal systems depending on location and nature of associated rocks (Figure 3B). Copper-dominated VMS systems (Cyprus and Besshi types) occur in the basalt-rich settings, whereas Zn-Cu±Pb VMS deposits (Kuroko type) including local advanced argillic assemblages are linked to the tonalite-granodiorite-bearing Smartville arc. The IOCGs also exhibit systematic differences that reflect felsic (high REE, U; hematite-rich) and mafic (low REE; magnetite rich) host rocks (Johnson, 2000). These patterns show up in IOCG systems globally (Barton and Johnson, 1996; Williams et al., 2005). Physical factors, such as thermal structure and state of stress, profoundly affect magmatic compositions and the nature of permeability and hence fluid flow. Thermal models for Jurassic and other magmatism have been presented elsewhere to rationalize the patterns observed (Barton, 1990, 1996; Elison, 1995). Apart from their importance in accentuating permeability through faulting and basin development, and in preservation (below), extensional tectonics should have impacted the size and diversity of Jurassic (and Cenozoic) magma bodies. Jurassic chambers might have been relatively small compared with those of compressional settings, leading to less fluid and perhaps less effective focusing for generation of large magmatic hydrothermal systems. The extensional environments correlate with greater magmatic diversity perhaps because of an ineffective density filter due to thinning crust. Similar reasoning might account for the somewhat small size of most oceanic / primitive arc porphyry systems elsewhere, as in parts of the southwestern Pacific and British Columbia. Finally, fluid salinity and availability of sulfur for trapping metals help rationalize other differences. In the case of porphyry systems, built-in traps—cooling of sulfur-bearing, metal-rich fluids—account for the large proportion of mineralized (not necessarily economic) magmatic-hydrothermal systems. VMS systems are similar in containing elevated contents of both metals and reduced sulfur. Conversely, IOCG systems have high-salinities but are sulfur-deficient and thus, they can precipitate iron oxides on cooling but require an additional sulfur source for removing chalcophile metals from solution. A high salinity setting would have the additional effect of suppressing the formation of typical adularia-sericite-type epithermal systems, which are characterized by the low salinities of the surface waters that dominate their formation. These factors likely all contribute to the observed Jurassic patterns and some of the differences with other areas (e.g., SW Pacific) and times (e.g., Laramide) that are noted above. Provincial controls Provincial controls have received considerable attention in understanding the sources and differences in magmatism and with regard to metallogenic productivity (e.g., Titley, 1987). Provincial characteristics strongly influence, but do not fully govern, the nature of magmas. The principal effect is that of diluting a subcrustal magma with materials from the overlying crustal column. This is best seen in the number of igneous compositional factors that influence the nature of hydrothermal fluids discussed in the preceding section. In the Jurassic, mafic compositions to the northwest, predominantly granitic compositions in the Great Basin, and varied compositions (but not strongly peraluminous) along the main arc in the central and southern areas surely reflect in part the composition of the underlying crust—mafic to the northwest, sediment-rich along the miogeocline, and crystalline Proterozoic (mainly metaigneous) to the south. The apparent restriction of Au-W systems to areas with relatively reduced granitoids in the Great Basin is another logical consequence of this regional variability. Regional variations in crustal composition and thickness of Jurassic crust also contribute to freeboard and to the chemical reactivity of available host rocks. Thinning continental crust and the transition to oceanic crust to the northwest contributes to the observed marine overlap. There is a close correspondence of Zn-Pb-Ag deposits with the carbonate-rich sedimentary rocks of the miogeocline (Figure 3A). The effect on metal contents and other aspects of VMS and IOCG systems has already been described. Surface fluids and ground waters are a final, and critical, provincial aspect of Jurassic metallogeny. As is apparent from Figure 3C, Jurassic surface fluids were atypically saline compared to those of other times (Laramide, much of the Cenozoic) and climates (tropical or temperate). The role of seawater speaks for itself; on the other hand, saline ground waters in arcs have received relatively little attention as a provincial control. The consequences of high salinity fluids have already been described—creation of IOCG mineralization, extensive and intense alkali exchange alteration, suppression of adularia-sericite-type epithermal systems – all features that are abundant in the Jurassic, but less common or absent in other times and places. Preservation and exposure As has been long recognized, the exposure and preservation of mineralized systems profoundly affects our understanding of metallogenic fertility and its underlying controls. The weight of evidence for the Jurassic is that a considerable fraction of high level crust has been preserved, but only locally does this include the very highest levels near igneous centers. As discussed above, conventional epithermal systems are sparse, perhaps because erosion has removed the top kilometer or more in most areas or because of chemical factors reflecting the nature of ground waters. Nevertheless, there are areas, in the Great Basin, and the Sierran foothills, and in southern California and Arizona where high level rocks are at least locally preserved. Many of these are in areas that were possibly affected by later Jurassic contractional tectonics which may have helped depress or bury them (e.g., Schermer et al., 2001). Some may have been pre- Jurassic igneous-related metallogeny served in the footwall of reverse / thrust systems or in their local foreland basins, as is clearly the case along the Colorado River where Jurassic volcanic and sedimentary rocks and enclosed hydrothermal systems have been taken up in the Maria fold and thrust belt (Figures 1A, 3B). The pre-Miocene tilting that preserved the epithermal systems on the west side of the Yerington district (Dilles and Proffett, 1995) could be any of several younger tectonic episodes. Conversely, relatively deep exposures in the Klamath Mountains, the western Mojave, and parts of the east-central Great Basin might reflect erosion following Jurassic and/or younger uplift in these areas. Syn-magmatic or slightly younger extension undoubtedly contributed to preservation (e.g., Busby, 1988; Dilles and Proffet, 1995). For example at Bisbee the principal ores are preserved in the down-dropped hanging wall of the Late Jurassic-Early Cretaceous Dividend Fault (Bryant and Metz, 1966). Younger Mesozoic and Cenozoic materials cover many of these areas providing exploration opportunities. Some possible economic implications Although by no means as productive as later Mesozoic and Cenozoic magmatic episodes, Jurassic magmatism southwestern North America produced dozens of hydrothermal systems that have been economically important. Moreover, much of the Jurassic arc is covered by younger materials. A newly compiled digital map of the entire region shows that Jurassic rocks make up about 7% of exposures in the overall domain of Jurassic magmatism. Given that 80% of the same region consists of younger materials, of which later intrusions (mainly Cretaceous) comprise about 20%, the implication is that only about one-quarter of the Jurassic of the same region is exposed; three-quarters is under younger cover. Furthermore, tectonic and sedimentary burial tend to cover higher-level rocks, and hence more prospective rocks. Thus, there may be considerable potential for known deposit types under Late Jurassic and younger cover. For example, the Foothill Copper Belt almost surely continues under younger materials to the north and south along its trend. One might ask if there are environments in this domain that might host economic deposits similar to those in Canada of the same age (e.g., Eskay Creek; Roth et al., 1999)? Although conventional epithermal systems are sparse in Jurassic, some areas preserve the Jurassic paleosurface. Evidence includes the presence of a number of iron-oxide advance argillic systems; reasonable places to look for preserved epithermal systems may still be found. If conventional low-salinity epithermal precious metal systems are present—in our view IOCGs represent a high-salinity equivalent—an additional challenges will be reading through the variable overprinting of metamorphism and deformation in volcanic sections. Quartz veins might be recrystallized and alteration minerals modified. Furthermore, if such systems formed with the Jurassic alkaline magmatism, the hydrothermal products might be cryptic be- 391 cause of the scarcity of quartz, rarity of acid alteration, and fine-grained gold in such systems (Jensen and Barton, 2000), and the similarity of associated K alteration (± pyrite and hematite) to features that accompany regionally extensive stratabound K metasomatism. Apart from iron exploration, there has been little evaluation of the possibility of higher grade (Cu-Au-rich) IOCG systems. Only the Pumpkin Hollow deposit in the Yerington district (Dilles et al., 2000) has seen much recent exploration. A number of the iron districts contain large magnetite resources; however, given the large number of copper prospects and small mines there is clearly potential for other Cu (± Au), and even U- or REE-bearing deposits. The comparison with Chile suggests that those areas nearer the Jurassic shoreline might be better candidates because of the possibility of H2S and/or hydrocarbon-bearing fluids to trap Cu (Barton, 2009). The evidence suggests, without obvious evidence to understand why, that the most productive times were 200±10 Ma and 160 ± 10 Ma—remarkably like the temporal patterns seen in the northern Cordillera. Conversely, much of the Early Jurassic and most of the Late Jurassic appear to be metal-poor, perhaps because of lower magmatic fluxes overall, or deeper levels of exposure in areas where these ages predominate such as northern Sierra Nevada and Klamath Mountains. All of these issues and opportunities deserve closer scrutiny. SUMMARY The igneous-related Jurassic metallogeny in southwestern North America reflects the combined influences of distinctive tectonic, magmatic, and surficial conditions. Broadly extensional tectonism was accompanied by diverse magmatic compositions, including mildly alkaline varieties. Early and Late Jurassic magmatism was restricted to a belt, which is presently 100–200 km wide, that obliquely crosses the Paleozoic continental margin. Middle Jurassic magmatism encompassed a much broader area in the north; it extended on the west from central and northern California, where it was relatively mafic, and eastward across much of the Great Basin into western Utah, where it comprises a compositionally varied family of roughly coeval igneous centers. Controls on Jurassic metallogeny include tectonic, magmatic and hydrothermal processe, the sources of materials for the magmas and external fluids which reflect provincial crustal and surface controls, and a complex history of exposure and preservation. Relatively mafic magmatism and abundant seawater in the northwest led to predominantly VMS systems, whereas more felsic intrusions in the main part and eastern expansion of the arc generated magmatic hydrothermal systems of porphyry Cu(-base metal) and W-Au families. Analogously, the abundant alkali-rich alteration in most areas and the ample independent evidence for highly saline surface and basinal waters is consistent with the abundance of IOCG-family deposits through much of the region. Sparse evidence for preservation of 392 Mark D. Barton, J. Dilles, J. Girardi, G. Haxel, D. Johnson, D. Kreiner, E. Seedorff, and L. Zurcher epithermal deposits may reflect erosion of the uppermost kilometer of the Jurassic crust. Alternatively, these types of deposits may be precluded by the unusually saline terrestrial fluids and/or the common occurrence of significantly alkaline magmas. The Jurassic differs markedly from younger periods in southwestern North America reflecting these differences in magmatism, fluid sources, and likelihood of preservation. The relatively sparse known metal endowment compared to the Cretaceous, Laramide, and middle Tetiary reflects these factors, yet three-fourths or so of the Jurassic is covered by younger sedimentary and volcanic rocks thus much remains to be discovered. Beyond the region, interesting parallels exist between southwestern North America, the Canadian Cordillera, the central Andes, and the southwestern Pacific. Each of these latter regions exhibits considerable similarities but also some marked differences with southwestern North America. Comparison of these regions still has much to teach us. ACKNOWLEDGMENTS This work reflects a synopsis of long-standing and ongoing work by the authors. The University of Arizona efforts have been supported most recently by grants from the USGS MRERP (08HQGR0060), the NSF (EAR08-38157, EAR98-15032), and the Science Foundation Arizona-Institute for Mineral Resources. We thank Steve Koehler for his helpful review and Roger Steininger for help in the manuscript submission. Finally, we gratefully acknowledge the interest, knowledge, and help from many colleagues, too numerous to name individually, in industry, academia and government who have contributed to our work in the Great Basin and surrounding regions. REFERENCES CITED Albers, J.P., 1981, A lithologic-tectonic framework for the metallogenic provinces of California: Economic Geology, v. 76, p. 765–790. 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