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
Jurassic magmatism and related hydrothermal systems formed across much of southwestern North America. Hydrothermal systems are numerous and varied, al- though fewer major mineral deposits are known than with later magmatism. Princi- pal 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 com- posite 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 expo- sure, 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 Ju- rassic 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, south- western Pacific) highlight common patterns and temporal progressions.
Key Words: Jurassic, hydrothermal, porphyry, IOCG, advanced argillic, metallogeny, magmatism, southwestern North America
INTRODUCTION America because it is important in some areas and it provides an instructive contrast with younger periods that are apparently The Great Basin and surrounding regions in southwestern more richly endowed with ore deposits. North America encompass one of the world’s great mineral Patterns of early to middle Mesozoic magmatism and asso- belts yet we have few definitive answers about the origins of this ciated hydrothermal systems in the cordillera of the Americas metal endowment or what factors have controlled deposit distri- differ in fundamental ways from later Mesozoic and, particu- bution in time and space. What are the relative roles of crustal larly, Cenozoic patterns (e.g., Barton, 1996; Sillitoe and Perello, provenance, of magma and fluid types, mineralization pro- 2005). Older episodes tend to have somewhat more mafic cesses, and preservation and exposure? This paper focuses on magmatism, be neutral to extensional in character, and have Jurassic igneous-related mineralization in southwestern North fewer and smaller porphyry and epithermal deposits but more iron-oxide-rich deposits. These observations have been inter- *E-mail: [email protected] preted 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 tec- magmatism in southwestern North America as it is exposed to- tonic setting but without any satisfying mechanistic interpreta- day. In this paper, we make no attempt to account for Jurassic and tion for the differences. In an earlier synthesis, Barton (1996) younger deformation that first amalgamated Jurassic magmatic suggested that the broad metallogenic patterns in southwestern belts, shuffled them laterally along the margin in the Mesozoic, North America should be evaluated in terms of provincial ef- compressed them in the later part of the Mesozoic and early Ter- fects (composition of the crust and near-surface fluids, physical tiary, and then extended and translated them in the middle and state of the lithosphere), process (fluid sources and drives), and later parts of the Cenozoic, a process that continues today in the a key role for preservation. Walker Lane and in California (e.g.,Faulds et al., 2005). Expanding on our earlier work, this paper presents a pre- Latest Triassic to Early Jurassic (ca. 205–175 Ma) arc liminary synthesis of the Jurassic metallogeny of southwestern magmatism took place in a zone which is presently no more than North America. We briefly review Jurassic magmatism and its 200 km in width and which extends from northern Sonora geologic framework including the tectonic and across southern Arizona and then northwestward approxi- paleogeographic settings. This background forms the context mately along the California-Nevada border (Figure 1B; Saleeby for review of the types, spatial distribution and timing of hydro- and Busby-Spera, 1992). This zone includes sparse plutons and thermal systems. These are interpreted in terms of the nature of older parts of volcanic sections in Arizona (Riggs and the ore-forming systems, including types of available fluids. Fi- Busby-Spera, 1990; Lang et al., 2001). Similar, subaerial to ma- nally, we briefly compare the Jurassic with other, better miner- rine volcanic assemblages with scattered plutons occur across alized epochs including the Laramide and the middle Tertiary in eastern California into western Nevada (e.g., Thomson et al., the western US and with analogous regions elsewhere in the 1995; Quinn et al., 1997; Dunne et al., 1998; Sorensen et al., Americas. Although of fundamental interest, we do not address 1998). To the northwest, the same belt continues into northern the system-scale process controls or the regional petrotectonic California (Christie, 2010; Dilles and Stephens, 2010), with a controls on magmatism and related mineralization. few plutons as far west as the Central Belt of the northern Sierra Nevada (Day and Bickford, 2004), and as remnants in Mesozoic JURASSIC MAGMATISM AND GEOLOGIC pendants within the central and southern Sierra Nevada FRAMEWORK batholith (Tobisch et al., 2000). Scattered gabbroic to dioritic intrusions of ~200 Ma occur in ophiolitic terranes in the western The 60 m.y. span of the Jurassic1 in southwestern North Klamath Mountains and Sierra Nevada foothills (Saleeby, America records the first extensive development of magmatism 1982; Irwin, 2003). Overall, Early Jurassic magmatism appears along the Phanerozoic continental margin coupled with diverse, to have been considerably less voluminous than that of the Mid- and still unsettled tectonic evolution in an arid environment. dle and Late Jurassic, as evidenced by the scarcity of dated Early Magmatism developed intermittently throughout the Jurassic, Jurassic plutons and volcanic sequences (Barton et al., 1988; spanning the truncated Paleozoic continental margin (e.g., Tosdal Dilles and Wright, 1988). et al., 1989; Saleeby and Busby-Spera, 1992; Miller and Busby, Abundant Middle to early Late Jurassic (ca. 175–155 Ma) 1995; Anderson et al., 2005b). Magmatic activity developed magmatism took place across an exceptionally broad region across multiple crust types including transitional, cratonal and re- (Figure 1C). Rocks of this age range from predominantly activated cratonal Proterozoic crust in Arizona, eastern Califor- mafic to intermediate oceanic arc and ophiolitic domains in nia and the central Great Basin, and juvenile Paleozoic or Meso- central and western California, through a relatively well de- zoic crust in northwestern Nevada and western and northern fined intermediate zone from Oregon to Sonora that is roughly California. Magmatism evolved in a broadly extensional environ- coincident with the Early Jurassic arc. In the north, igneous ac- ment during the Early and Middle Jurassic changing to a mixed tivity blossomed eastward across the central Great Basin into transpressional (north) to transtensional (south) regime in the western Utah (Barton et al., 1988; Elison, 1995). Volcanic Late Jurassic. Throughout the Jurassic, the terrestrial environ- rocks are also widely preserved through the western and cen- ment was distinctively arid. These factors affected the nature of tral parts of this region, particularly along the principal arc Jurassic hydrothermal systems and associated mineral deposits. from northern Sonora into northern California (Tosdal et al., 1989; Christe and Hannah, 1990; Riggs et al., 1993; Dunne et Time-Space Distribution of Jurassic Igneous Rocks al., 1998; Quinn et al., 1997; Haxel et al., 2005), eastward into central Nevada (e.g., Muffler, 1964; Dilek and Moores, 1995), Figure 1 shows a simple time-space division of Jurassic and westward into the marine arc and ophiolitic terranes of
1For 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 sim- plify 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 infor- mally used in this manner (see footnote in text). A. Geologic framework for Jurassic magmatism including age of crust, some major structural elements, and distri- bution 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 se- lected regions that are regions mentioned in text. central and western California (Saleeby, 1982; Harper, 1984; from the Arizona-Sonora border region across southern Califor- Hopson et al., 1981, 2008; Dickinson, 2008a). Age-correlative nia and northward along through eastern and central California ash beds, with similar mineral make-up to the plutons, are into the Klamath Mountains (Figure 1D; Irwin, 2003; Barth et widespread in the sedimentary sequences of the Colorado Pla- al., 2008; Haxel et al., 2008a). Volcanic rocks in this age range teau (Riggs and Blakey, 1993; Kowallis et al., 2001). Based on are sparse. This suite is compositionally diverse, with alkalic in- measured exposure areas, Middle Jurassic plutons are much trusions in Arizona (Tosdal et al., 1989; Haxel et al., 2008a) and more voluminous than either Early or Late Jurassic and repre- multiple intrusion compositions in California, notably repre- sent substantially higher magmatic fluxes than for the other sented by the Independence dike swarm and related rocks (Chen periods (Barton et al., 1988). and Moore, 1979; Glazner et al., 2008; R.F. Hopson et al., 2008) Late Jurassic to earliest Cretaceous magmatism (ca. and tonalites and quartz diorites of the northern Sierra and 155–140 Ma) retracted to a relatively narrow belt that passes 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 veloped in eastern and northern California (Miller, 1978; Sylvester et al., 1978; Fox and Miller, 1990) but extend locally Jurassic igneous compositions span basaltic to rhyolitic, al- into the eastern Klamath Mountains (e.g., Ironside Mountain kaline to strongly peraluminous; as a group, they are batholith, Barnes et al., 2006) and sporadically across the central compositionally expanded compared to later Mesozoic Great Basin (e.g., Hoisch and Miller, 1990). Rare examples have magmatism (e.g., Barton et al., 1988; Saleeby and Busby-Spera, silica undersaturated phases (e.g., Joshua Flat pluton, Inyo 1992). In many cases, the rocks have been substantially modi- Mountains; Miller, 1978). The corresponding volcanic record fied by post-magmatic processes that obscure original composi- presents a greater challenge because of widespread alteration tions (e.g., Fox and Miller, 1990; Battles and Barton, 1995; (see below), however immobile elements and some major ele- Haxel et al., 2008b; see Hydrothermal Alteration, below). Fig- ment data indicate that high-K, moderately alkaline volcanic ure 2 shows modal and selected accessory mineral data and total rocks may have been widespread along the arc (reviewed by alkali-silica (TAS) plots for representative intrusive suites of Christe and Hannah, 1990). Early, Middle and Late Jurassic age. Unlike Cretaceous Although Jurassic igneous systems have not received as magmatism in the western U.S., there appears to be little overall much attention as Cretaceous and Cenozoic igneous systems, a secular variation in Jurassic igneous compositions (Barton, number of representative areas are well studied. These include 1996). plutons in southern Arizona (Haxel et al., 2008a,b) , the Mojave Most Jurassic igneous rocks are subalkaline, oxidized Desert (e.g. Fox and Miller, 1990; Young et al., 1992; Mayo et (magnetite ± titanite bearing) hornblende-biotite granitoids or al., 1998), the Inyo Mountains (Miller, 1978; Sylvester et al., their volcanic equivalents. These include the majority of plutons 1978), the Yerington district (Dilles, 1987), the Snake Range of Early to Middle Jurassic age in Arizona and southern Califor- (Lee et al., 1981; Lee and Christensen, 1983), the Smartville nia, which are predominantly biotite-hornblende magnetite-ti- Complex (e.g., Beard and Day, 1987, 1988), and the Klamath tanite-bearing granodiorite and granite (e.g., Tosdal et al., 1989; Mountains (e.g., Barnes, 1986a,b, 2006). Some of these plutons Haxel et al., 2008b). To the north, similar broadly granodioritic are remarkably diverse with broadly consanguineous quartz compositions predominate in eastern California and western diorites to peraluminous granites (e.g., Slinkard pluton, Barnes Nevada (e.g., John et al., 1994) and across the central Great Ba- et al., 1986a; Snake Creek pluton, Lee and Christensen, 1983). sin (du Bray, 2007). In the northern Sierra Nevada and the Klamath Mountains, many plutons are metaluminous, relatively Jurassic Tectonic and Paleogeographic Framework mafic compared to those to the south, and exhibit both calc-al- kaline and tholeiitic affinities. These areas are dominated by The tectonics and paleogeography of the Jurassic are of in- tonalite-granodiorites, diorites and, in ophiolitic complexes, terest here primarily because of their role in the governing struc- gabbros (e.g., Hopson et al., 1981, 2008; Harper, 1984; Beard tural settings, depositional environments, paleoelevation and and Day, 1987). The original locations and modes of amalgam- probability and type of non-magmatic fluid sources—all of ation of these western terranes remains controversial which influence ore-forming systems. Jurassic tectonics have (Dickinson, 2008b). been extensively reviewed elsewhere (e.g., Saleeby and At the other end of the compositional spectrum, two-mica Busby-Spera, 1992; Dickinson, 2008a; papers in Miller and granites in a number of areas form discrete plutons or constitute Busby, 1995, and in Anderson et al., 2005a). Broadly viewed, late phases in composite weakly peraluminous plutons. They the key features of the Jurassic are: (1) It was generally low are nearly all Middle Jurassic. These strongly peraluminous standing and mainly though not entirely extensional. compositions are most common in the Great Basin (e.g., Ruby Extensional basins, mainly tectonic but partly volcanic, accu- Mountains, Kistler et al., 1981; Snake Range, Lee et al., 1981) mulated thick sequences of volcanic, clastic and locally lacus- yet they occur as far west as the Klamath Mountains (e.g., the trine to marine (in the north) sedimentary rocks including Slinkard pluton, Barnes et al., 1986a). Although most Jurassic evaporites. (2) Magmatism occurred in a largely continuous arc granites and granodiorites are relatively oxidized, the two-mica extending from reactivated craton onto the Paleozoic margin of granites and a minority of the metaluminous plutons are rela- cratonal North America, with the important exception of older tively reduced; they may contain abundant ilmenite with or ocean crust-floored terranes in northern and western California. without some magnetite but have little or no titanite (e.g., Lee et These oceanic terranes were either forearc/intra-arc assem- al., 1981; Nutt et al., 2000). blages associated with east-facing subduction along the North Mildly to locally strongly alkaline compositions are fairly American margin or exotic arcs/back-arc basins developed widespread in the Jurassic. In Arizona and Sonora, the ~150 Ma above west-facing subduction zones and amalgamated to North Ko Vaya suite includes alkaline perthite granites and quartz America in the later half of the Jurassic. (3) There was a transi- syenite (Tosdal et al., 1989; Haxel et al., 2008a). Elsewhere along tion over time from mildly extensional within-arc tectonism in the arc, most alkaline intrusions are Early to Middle Jurassic and the Early Jurassic to variably transpressive and transtensional are relatively silica poor—mainly monzonites, quartz tectonism in the Middle to Late Jurassic. This transition led to monzonites, and syenites. These other alkalic suites are best de- 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 men- tioned 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 cen- tral Great Basin. During this period to the south there was con- tinued 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 re- flecting its northward drift through the horse latitudes (May et al., 1989; Busby et al., 2005; cf. Figure 3C). Marine volcanic se- quences are well preserved in the western and central Sierra Ne- vada, the western Klamath Mountains, and the Coast Range ophiolites (Saleeby, 1982; Hopson et al., 1981; Irwin, 1981). Transitional, restricted marine to terrestrial sequences are pre- served in western Nevada and eastern California (Dunne et al., 1988; Sorensen et al., 1998; Garside, 1998). In some of these ar- eas, limestone-shale sequences interfinger with Jurassic volca- nic rocks, sedimentary gypsum, and terrestrial clastic rocks in- cluding 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 Ce- nozoic (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 Figure 3. Distribution of Jurassic hydrothermal systems. (Same base maps as Figure 1.) A. Magmatic hydrothermal systems including porphyry Cu, Cu intrusive centers. In a systematic review of 1600 Mesozoic in- skarn, polymetallic Zn-Pb-Ag skarn/vein/replacement, high-sulfidation ad- trusive centers in the western United States, Barton et al. (1988) vanced argillic Cu-Au systems, and granite-related W-Au systems. B. Marine found that nearly one-half (129 of 280) of all Jurassic plutons hydrothermal Cu-Zn(±Pb) VMS deposits and related high-sulfidation occur- had reported evidence of mineralization. Figure 3 shows the dis- rences, plus terrestrial hydrothermal Fe(-Cu) skarns, Fe(-Cu) igneous-hosted vein, replacement and breccia deposits, and low-sulfidation advanced argillic tribution of some of the most important or illustrative examples. hydrothermal occurrences. C. Indicators of (near-)surface fluids and distribu- Even where mineral deposits are not reported, most Jurassic tion of intense Na(Ca), regionally extensive (low-T) K, and seafloor-type plutons exhibit some evidence of metasomatism. Alteration is Na(±Ca) alteration. The distribution of eolian sands is generalized based on oc- even more extensive—nearly ubiquitous—in Jurassic volcanic currences from the Colorado Plateau west and south into the arc. Jurassic igneous-related metallogeny 379 rocks, where it can be hydrothermal or diagenetic (e.g., types; (2) terrestrial-hydrothermal, where surficial or basinal Schiffman et al., 1991; Sorensen et al., 1998; Johnson, 2000; fluids are essential, although other fluids are often present and Haxel et al., 2008a). Much of this alteration bears, at best, only a can, in some cases, contribute key components—these include general link to deposits. Regional patterns in Jurassic alteration many hydrothermal iron oxide-rich systems (“IOCGs”) as well types are shown in Figure 3. as a number of epithermal precious metal deposits; and (3) ma- rine-hydrothermal, where fluids are dominated by seawater but Types of hydrothermal systems and related mineral may have magmatic aqueous fluids—these include volca- deposits nic-hosted massive sulfides, but also epigenetic deposits. Many districts have mixed sources and thus are hybrids of possible Jurassic igneous-related hydrothermal systems, including end-members. Nonetheless, this fluid-based classification but not restricted to mineralized systems, can be categorized in helps rationalize the distribution of deposits and other types of several ways. First and most familiar is classification by deposit alteration in time and space in the Jurassic and aids comparison style and metal as summarized in Table 1. These classes include with other episodes and regions. various common intrusion, skarn, replacement, breccia and vein types. Table 1 also categorizes Jurassic igneous-related systems Porphyry-skarn(-replacement) Cu(-Mo-Au) systems into families based on the principal sources of fluids as inferred Porphyry Cu(-Mo-Au) and related copper skarn/replace- from multiple lines of geological and geochemical evidence ment systems are scattered along the Jurassic arc and spread into (Figure 3; Barton, 1996). These groups are: (1) magmatic-hy- the central Great Basin (Figure 3A). A handful of areas are drothermal, where the key fluids are derived from hydrous mag- known to contain economically significant deposits; the best mas with/or without external fluids—these include granite-re- known are Yerington, Nevada (168–169 Ma), and Bisbee (War- lated, porphyry, skarn, and some lower temperature deposit ren), 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 ad- Yerington, NV; Ann Mason, NV; vanced argillic Bisbee, AZ; Lights Creek, CA; Royston, NV Cu skarn/replacement Garnet-pyroxene to actinolite-chalcopyrite skarns and sulfide-rich, re- Yerington, NV; Contact, NV; Dolly Var- placement den, NV; Bisbee, AZ Zn-Pb-Ag(-Cu-W-Au) Zn, Pb, Fe (± Cu) sulfides ± scheelite in replacement and/or Cerro Gordo, CA; Darwin, CA; skarn/replacement pyroxene-garnet skarn Cortez, NV; Courtland-Gleeson, AZ Ag(-Pb-Zn) vein Ag-bearing galena-sphalerite-quartz veins with hydrolytic alteration Candelaria, NV Au-W granite/skarn/ Quartz-Au/W veins, local skarns, and As-Bi bearing sulfide-silica re- Bald Mountain, NV; Gold Hill, UT; replacement placement Osceola, NV High sulfidation Aluminum silicate minerals associated with alunite, pyrite ± chalco- Alunite Hill, NV; North Keystone, CA advanced argillic pyrite, bornite (VMS link?) Terrestrial hydrothermal – Fe(-P), Fe(-Cu), Cu(-Fe) (“IOCG”) Fe(-Cu) skarn Magnetite(-hematite)-rich pyroxene-garnet-actinolite-chalcopyrite Pumpkin Hollow, NV; Eagle Moun- skarns; intrusions have Na(Ca) alteration and endoskarn tain, CA; Hall, CA Fe(-Cu/-P) igneous- Magnetite(-apatite-actinolite) with intense Na(Ca) alteration to mag- Calico, NV; Quijotoa, AZ; Humboldt, hosted netite-hematite-Cu sulfide with Na and acid alteration NV; Cortez Mountains, NV; Besse- mer, CA; Palen, CA; Lights Creek, CA Low sulfidation ad- Aluminum silicate minerals (kyanite, where metamorphosed) ± he- Palen Mountains, CA; Dome Rock vanced argillic matite / magnetite ± dumortierite; little or no Fe sulfide 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 Turner-Albright, CA-OR; Copperopolis, CA Cu-Zn(-Pb) Kuroko, Stratabound and stockwork Cu-Zn(±Pb) with marine arc Foothill Copper Belt, CA Noranda type Cu(-Zn-Co) Besshi Stratabound and stockwork Cu(-Zn) in clastic section with mafic sills Green Mountain, CA type 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 skarn or jasperoidal alteration. Tungsten occurs as scheelite in typically present (Table 2). Many other intrusive centers contain skarns or, more rarely, as wolframite in granite-hosted quartz mineralization and alteration types that may represent parts of veins. Sulfide contents are relatively low, however these areas variable exposed and/or eroded porphyry centers. For example all have added lead, zinc, bismuth, and arsenic. The intrusions the weakly mineralized Royston, Nevada, occurrence may rep- are felsic, generally biotite quartz monzonites to granodiorites resent only the fringes of a better mineralized ~200 Ma system with sparse hornblende and, rarely, magmatic muscovite. Mag- (Seedorff, 1991a). Other plutons, lacking skarns or high level matic oxides are sparse to absent, consistent with relatively re- features, appear to be the roots of mineralized systems. Exam- duced magmas, although late(?) titanite is present at Bald ples include the Austin pluton in central Nevada (which hosts Mountain (Nutt et al., 2007). The character of these deposits polymetallic silver veins) and the Santa Rita pluton in the Inyo combined with their associated Fe-Ti-oxide-poor quartz Mountains. Both plutons contain copper-bearing quartz veins monzonites to granites suggests parallels with granite-related with vein-controlled and disseminated K-silicate alteration gold(-tungsten) systems worldwide (Thompson et al., 1999; (Barton, Seedorff, and Kreiner, unpubl. data). Nutt et al., 2007). Copper skarns and replacement deposits are well devel- oped in carbonate rocks in Jurassic porphyry districts. Among Polymetallic Pb-Zn-Ag(-Au) systems the best developed are those at Bisbee, where high-grade Polymetallic silver-base metal vein, replacement and skarn mantos surround advanced argillic alteration in the upper part of deposits are widespread across the Great Basin and in adjoining a composite porphyry center (Einaudi, 1982a). In other places, areas where carbonate rocks are present, particularly along the garnet-rich copper skarns and breccia pipes occur adjacent to Cordilleran miogeocline, but also in the Paleozoic cover se- plutons with relatively weak copper-bearing K-silicate and/or quences in Arizona (Figure 3A; Titley, 1993). Within this re- sericitic alteration (e.g., ~158 Ma, Contact, Nevada, LaPointe et gion, most base-metal replacement systems are Cretaceous or al., 1991; ~159 Ma, Dolly Varden, Nevada, Atkinson et al., Tertiary; however, scattered deposits across the Great Basin, 1982). Whether these also represent deeper or lateral parts of from western Utah into eastern California, form a Jurassic better mineralized centers is uncertain. As a further complica- lead-zinc province (Albers, 1981; Albino, 1995). Among the tion, some districts are clearly composite in nature, i.e., they better known examples are Cortez (Mill Canyon) in Nevada contain both porphyry-style mineralization with high-tempera- (158 Ma; Stewart and McKee, 1977), Cerro Gordo, Darwin, ture K-silicate and acid alteration styles and IOCG-style Goodsprings and nearby districts in eastern California and sodic(-calcic) alteration and iron oxide-rich, sulfide-poor min- southernmost Nevada (155–175; 190–200(?) Ma; Newberry et eralization. These districts can be either hybrids that formed in al., 1991; Church et al., 2005), and the Courtland-Gleeson dis- the same overall thermal event with mixed fluid sources (e.g., trict in Arizona (196±5 Ma; Gilluly et al., 1956; Lang et al., Yerington, Table 2) or composite systems where temporally and 2001). Associated intrusions commonly contain sparse quartz compositionally distinct systems are superimposed (e.g., Lights veins and weak potassic or sericitic alteration; yet only the Creek, Table 2). Courtland-Gleeson district is known to have associated As is the case globally (Seedorff et al., 2005), porphyry cop- porphyry copper mineralization. per and copper skarn mineralization occurs with oxidized (mag- Although Jurassic base-metal skarn and replacement de- netite-titanite-bearing), intermediate to felsic, hornblende-bio- posits are relatively common, there are few compelling exam- tite-bearing metaluminous to weakly peraluminous granitoids. ples of vein, stockwork or disseminated, lower-temperature Although copper and gold occurrences are known with the alka- style precious metal deposits. Numerous small, ill-described line intrusions along the arc in eastern California and elsewhere, precious metal deposits are associated with Jurassic plutons and none of these silica-poor intrusive complexes in this region are volcanic complexes. The only convincing example of a known to contain porphyry copper mineralization, such as those low-sulfidation-type precious metal deposit is the 192 Ma found in the broadly coeval Jurassic terranes of British Columbia Candelaria Ag(-Pb-Zn-Mn) deposit in western Nevada (Page, (Lang et al., 1995). 1959; Thomson et al., 1995). Even at Candelaria, the style dif- fers from ordinary epithermal environments in that the thin Granite-skarn(-replacement) Au-W(-base metal) systems sulfosalt-bearing quartz-dolomite(-sphalerite-galena ±arseno- Gold-tungsten-base metal mineralization is spatially asso- pyrite±chalcopyrite) veins are associated with well-developed ciated with a number of Jurassic granitic plutons in the central quartz-sericite-pyrite(-tourmaline) alteration. Similar Ag-rich Great Basin (Figure 2D, Figure 3A). These include deposits in quartz veins occur at Austin. Small silver-base metal occur- the Gold Hill district, Utah (~158 Ma, Nolan, 1935; El Shatoury rences near Jurassic igneous centers are widespread along the and Whelan, 1970), the Osceola and nearby districts in the arc (e.g., du Bray et al., 2007) Snake Range, Nevada (~160 Ma, Hose and Blake, 1976; Lee et al., 1981), and the Bald Mountain district in Nevada (159 Ma, Iron oxide-rich ±P, ±Cu(-Au-Ag) (~ IOCG) systems Nutt et al., 2007). In these areas gold is associated with quartz Hydrothermal systems with voluminous hypogene magne- veins and silicification, commonly with sericitic (/greisen), tite 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) Jurassic granitic porphyries and Deep K-silicate to quartz-seri- Bryant (1966), breccias intrude carbon- cite-pyrite; shallow intense py- Einaudi (1982a), ate-clastic section above rite-pyrophyllite-quartz-sericite Lang et al. schists (2000) Bisbee, AZ Cu(-Au-Zn-Mn) re- Jurassic granitic porphyries and Mainly pyrite-chalcopyrite-bornite Bryant (1966), placement ± breccias intrude carbon- replacement with minor skarn Einaudi (1982a) skarn ate-clastic section above schists and distal Zn-Pb sulfide ± he- matite Yerington District, NV – Cu(-Mo) porphyry, Cu skarn, Fe(-Cu) skarn, replacement, epithermal / 168–169 Ma Yerington, Ann Cu(-Mo) porphyry Qz monzodiorite to granite K-silicate to sericitic alteration with Proffett and Dilles Mason batholith intruding coeval vol- Cu(-Mo) associated with granite (1995), Dilles et canic rocks; porphyry dikes porphyry dikes; episodic coeval al., (2000), generated from granite late in Na(Ca) alteration Carten (1986) history Ludwig, Mason Cu skarn Granite porphyry dikes intrude Andradite(-diopside) to Einaudi (1977), Valley Jur-Tri mixed carbonate-volca- actinolite-chalcopyrite skarns Harris and nic sequence postdate abundant skarnoid, Einauidi (1982) endoskarn Alunite Hill High sulfidation Andesite-dacite volcanic rocks Quartz-alunite-pyrophyllite-sericite Lipske and Dilles, advanced argillic above coeval batholith over quartz-sericite-pyrite (2000) Pumpkin Hollow/ Fe oxide(-Cu-Au) Mixed Jur-Tri carbonate-volcanic Magnetite-hematite-chalcopyrite Proffett and Dilles Buckskin skarn / vein package with qz monzodiorite with chlorite-actinolite-quartz in (1995), Dilles et sill at batholith contact veins or garnet-pyroxene skarn al. (2000) Minnesota / Fe oxide skarn/ Mixed Jur-Tri carbonate-volcanic Magnesian magnetite(-pyrite) Dilles et al. (2000) Easter replacement sequence and subjacent qz skarn; magnetite-apatite-actinolite monzodiorite with intense Na(Ca) in intrusion Lights Creek District, Northern, CA – composite IOCG and porphyry Cu / 148, 178 Ma Moonlight Valley, IOCG-like veins and Early Jurassic quartz monzonite Chalcopyrite-bornite-magnetite(± Storey (1978), Engels and breccias (178 Ma) intrudes Early Juras- sphalerite) veins with associ- Dilles and Superior sicvolcanicrocks ated Na(-Ca) ± potassic alter- Stephenson ation, coeval apatite-actinolite (2010) veins Moonlight Creek Cu porphyry Granite porphyry dikes (148 Ma) Quartz-tourmaline(-chalcopy- Storey (1978), intrude Middle Jurassic volcanic rite-pyrite) veins with strong Dilles and and volcaniclastic rocks sericitic alteration Stephenson (2010) Bald Mountain District, NV – W(-Pb-Zn-Au) skarn, Au(-Bi) replacement, vein / 159 Ma Bald Mountain, Au replacement with Miogeoclinal carbonte-clastic W(-Pb-Zn-Au) skarn at lower levels Nutt et al., 2007 NV W(-Zn-Bi) skarn rocks intruded by Jurassic quartz with peripheral base metal and monzonite to granodiorite Au(-Bi)-jasperoid veins Darwin, CA – Zn-Pb(-Ag-W) skarn and replacement / 155 (?) Ma Darwin, CA Pb-Zn(-Ag-W) skarn Deformed Paleozoic carbonate Ag-bearing sphalerite-galena-gar- Newberry et al. and replacement rocks intruded by Jurassic net-pyroxene skarn to garnet-sul- (1991) alkalic and calc-alkalic plutons fide vein to shallow sphalerite-ar- gentiferous-galena replacement Candelaria District, NV – Ag(-Pb-Zn) veins, stockwork / 192 Ma Candelaria Ag(±Pb-Zn) veins Paleozoic to Triassic marine units Quartz stockworks, local tabular Thomson et al. overlain by ophiolitic mélange, bodies with Ag minerals and trace (1995) intruded by Jurassic calc alka- Pb, Zn, Sb and As; sericite ± tour- line rhyolites and dacites maline ± pyrite alteration (continued) 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 Magnetite-apatite-actinolite brec- Dilek and Moores complex with dike swarms cias, veins and replacements (1995), Johnson marking intrusive centers in zoned, intense Na(Ca) al- and Barton teration (2000) Boyer / White Cu-Fe sulfides—Fe ox- Basalt to basaltic andesite and Hematite(±magnetite)- Johnson (2000) Rock ide volcaniclastic sequence bornite-chalcopyrite with intense Na alteration, locally syngenetic FeOx Eagle-Palen Trend, CA – Fe(-Cu-Au) skarn, Fe(-P) breccias replacement, vein / ~165 (155-175) Ma Eagle Mountain Fe(±Cu-Au) Jurassic composite granodioritic Magnetite-pyrite(-apa- Dubois and magneisan skarn stock in carbonate-bearing tite)-actinolite in marble, local Brummet (1968), metasedimentary rocks Cu±Au; Na(Ca) ± K alter- Mayo et al. ationinintrusion (1998) Palen Moun- Fe oxide (-P-Cu), ad- Basaltic to andesitic Jurassic Magnetite-apatite-actinolite±Cu Stone et al. (1985), tains vanced argillic Fe volcanic and volcaniclastic in Na(Ca) alteration zone up- Fackler-Adams oxide rocks intruded by Jurassic wards to pyrophyllite-hema- et al. (1997) diorite tite-quartz±Cu Foothill Copper Belt, CA – Cu(-Zn) volcanogenic massive sulfides, Cu(-Au) / 160-165 Ma Penn, Green Kuroko, Noranda, Hosted in andesite-dacite and Massive sulfide lenses (pyrite- or Heyl (1948), Kemp Mountain, Besshi variants overlying mafic-clastic sec- pyrrhotite-rich) above seri- (1982), Mattinen Copperopolis tion; up to amphibolite facies cite-chlorite-altered stringer and Bennett overprint zones (1986) North Keystone Intense acid, ad- In andesite-dacite sequence; up Andalusite-quartz(-bornite-chal- Clark and Lydon and others vanced argillic to amphibolite facies overprints copyrite-pyrite), distal jasper (1962)
REE, and other metals are widespread in the Jurassic with sev- though many of these districts also contain Cu(-Au) mineraliza- eral hundred occurrences and a few dozen significant deposits tion (Figure 3B). Several dozen districts have produced iron and (Figure 3B; Barton et al., 2000; Johnson and Barton, 2000). In at least 10 districts have had some significant Cu ± Au produc- addition to the iron oxides and associated metals, these areas tion. The largest Cu resources are at Lights Creek (Supe- also have the other features that are characteristic of IOCG sys- rior-Engels-Moonlight Valley), Pumpkin Hollow in the tems worldwide, including voluminous sodic, sodic(-calcic), Yerington district, the Calico Hills, and San Fernando (Figure and potassic (at shallow levels) alteration, skarn (in carbonate 3B), however spotty Cu-Au(-REE±U) mineralization occurs all host rocks), a diversity of structural styles (breccias, veins, re- along the Jurassic trend and is not restricted to a particular area. placements, stratabound), a distinctive set of abundant associ- Most occurrences have seen little modern exploration. ated minerals (e.g., actinolite, apatite), and a paucity of sulfides The larger districts commonly contain multiple deposit (Hitzman et al., 1992; Barton and Johnson, 1996; Williams et types within the same complex and, in some cases, broadly al., 2005). The most common deposit associations include: (1) overlap in space and time with other types of mineralization. Ta- magnetite(-apatite-actinolite±minor sulfides) hosted in intru- ble 2 summarizes four districts with IOCG mineralization; each sive or volcanic rocks, (2) magnetite-rich skarns with or without has multiple deposits, and they show the internal and be- appreciable copper hosted in carbonate host rocks, and (3) vari- tween-district variability that is typical of IOCGs: ous kinds of hematite-magnetite(-chalcopyrite±bornite) depos- its in igneous or carbonate rocks. The carbonate-hosted 1. The Humboldt Mafic Complex is associated with a gab- varieties comprise a subset of iron skarns (Einaudi et al., 1981) bro-diorite and basalt-andesite complex and varies from and traditionally have been classified that way. deep magnetite-apatite-actinolite bodies with intense The largest IOCG districts, each of which contains multiple scapolitic alteration to shallow hematite-rich, sulfide-bear- deposits, are along the main trend of the arc, and—notably—all ing mineralization with albite-chlorite-carbonate alteration are inboard of the Jurassic shoreline (cf. Figures 3B, 3C). Scat- (Johnson and Barton, 2000). Geological association with tered IOCG prospects in western Baja California (Figure 3B; contemporaneous evaporites, geochemical data, petrology, e.g., San Fernando; Lopez et al., 2005) may belong to a separate and mass balance considerations show that evaporitic brines belt of Late Jurassic or Early Cretaceous age. Iron has been the dominated, perhaps were the sole contributors to, the hydro- principal commodity produced from Jurassic IOCG systems, thermal system (Johnson and Barton, 2000). Jurassic igneous-related metallogeny 383
2. The Yerington district, as noted above, has several por- hypogene kaolinite, pyrophyllite, kyanite or andalusite and ac- phyry copper centers and classic copper skarns; beyond cessory minerals such as rutile, zunyite and dumortierite that, it is an excellent example of a hybrid system Contem- (Meyer and Hemley, 1967), are developed in several distinctive poraneous with, but distal to the porphyry mineralization, modes in the Jurassic (Figure 3A,B; Kreiner and Barton, in hydrothermal flow in the Yerington batholith created ex- prep.). The most familiar are a handful of porphyry-related oc- tensive sodic(-calcic) alteration zones that are directly currences including Yerington (Lipske and Dilles, 2000; Dilles linked to deep magnetite-apatite-actinolite veins that zone and Einaudi, 1992) and Bisbee (Bryant, 1966; Einaudi, 1982b). upwards into shallow chlorite-hematite-seri- In these examples, intense pyrite-rich pyrophyllite-alu- cite-quartz(-chalcopyrite) or outwards into magne- nite-bearing assemblages form in the upper levels of porphyry tite-actinolite(-chalcopyrite) skarns (Dilles et al., 2000). Cu(-Mo) systems. Although in some cases advanced argillic al- Geological and geochemical data demonstrate that the teration extends laterally for a kilometer or along favorable IOCG systems formed by circulation of non-magmatic strata or structures; all examples of these sulfide-rich assem- brines from the host Mesozoic sedimentary sections (e.g., blages are clearly centered on shallow intrusive complexes. At Dilles et al., 1995, 2000). neither Bisbee nor Yerington does the advanced argillic alter- 3. The Plumas County Copper Belt, including the Lights ation carry abundant Au or Cu, although these metals are Creek (Superior-Engels) district is a variant on this present in small amounts. theme. Early Jurassic IOCG mineralization is overprinted A second, more widespread group consists of sul- 30 m.y. later by porphyry style alteration (178 and 148 fide-poor to absent, hematite or magnetite-stable, Ma, respectively; Dilles and Stephens, 2010), creating a quartz-pyrophyllite(-kaolinite) alteration. This association composite system with key parts separated in time; occurs along the main Jurassic trend from southern Arizona Lights Creek thus contrasts with the hybrid system at into eastern California and western Nevada (Figure 3B). Some Yerington all parts of which formed roughly concur- of these systems can be quite extensive and, unlike the rently. Unlike Humboldt and Yerington, sources of fluids high-sulfidation examples, the low-sulfidation variety is com- have yet to be confirmed although both of magmatic and monly structurally controlled. In the Palen Mountains, for ex- non-magmatic fluid sources are probable (Dilles and ample, pyrophyllite-quartz(-specular hematite) alteration ex- Stephens, 2010). tends several kilometers along a major structural trend, in the 4. The Eagle Mountain-Palen Mountain trend (~165–170 upper part of the middle Jurassic volcanic pile, Ma; Stone et al., 1985) has aspects reflecting many other stratigraphically overlying IOCG-style mineralization and in- deposits in southeastern California and southern Arizona tense sodic(-calcic) alteration near subjacent plutons (Stone et (e.g., Lamey, 1948; Hall et al., 1988). Pluton-associated al., 1985). In the vicinity of the Colorado River, regionally magnetite-actinolite skarns with minor gold and copper at metamorphosed Jurassic volcanic and, locally, intrusive rocks Eagle Mountain (DuBois and Brummett, 1968) line up contain kyanite-quartz occurrences (Reynolds et al., 1988; along inferred regional structures with higher level volca- Haxel et al., 2002) which are commonly associated with gold, nic-hosted magnetite-apatite-actinolite, Cu-Au prospects, copper and iron oxide occurrences, and with sodic-calcic alter- and extensive sulfide-poor advanced argillic alteration typ- ation. The geochemical similarities and proximity to ified by pyrophyllite + specular hematite + quartz in the unmetamorphosed variants argues that these kyanite-bearing southern Palen Mountains. This style of sulfide-poor ad- examples are metamorphosed, low-sulfide, advanced argillic vanced argillic alteration is a common associate of IOCG systems. Others have proposed that they reflect hydrothermal systems (see below) and, like other aspects of their geo- formation under deeper, mesothermal conditions (e.g., Owens chemistry, stands in contrast to other types of Jurassic hy- and Hodder, 1994). drothermal systems. A third group of aluminum silicate (typically andalu- site)-quartz-rich altered rocks is found in the western foothills Collectively, the IOCG systems share many geochemical of the Sierra Nevada (e.g., Clark and Lydon, 1962) and along the characteristics and mineralogical themes, yet they occur in western side of the Peninsular Ranges batholith in southern Cal- many settings and with the whole gamut of igneous composi- ifornia (Jahns and Lance, 1950). The sulfide-rich, copper-bear- tions along the Jurassic arc. Comparison of areas with and with- ing occurrences in the Foothill Belt may represent the intensely out obvious porphyry style mineralization, geochemical con- acid-dominated parts of marine arc-related hydrothermal sys- siderations, and regional patterns in alteration and paleowater tems (e.g., Hannington et al., 1999; Resing et al., 2007). In con- sources all point to an essential role for non-magmatic, trast, the southern California pyrophyllite/kyanite / andalusite ± evaporitic (e.g., basinal or surface-derived) fluids as proposed dumortierite occurrences appear to be relatively sulfide poor by Barton and Johnson (1996). and are not obviously associated with massive sulfides; these occurrences may represent the northward extend of the Alisitos Advanced argillic (high and low sulfidation types) systems terrane of Baja California Norte which contains Late Jurassic or Zones of advanced argillic alteration, characterized by 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 voluminous potassic alteration is widespread, though less well Scattered Cu-Zn-dominated volcanic-hosted massive sul- known, and is characterized by conversion of igneous feld- fide deposits occur in the Middle to early Late Jurassic marine spars and groundmass to secondary K-feldspar and conversion terranes of the Sierra Nevada foothills, the Klamath Mountains, of mafic minerals to chlorite, hematite and clays (Chapin and and the disrupted ophiolitic terranes of the Coast Ranges Lindley, 1986; Barton and Johnson, 2000). This type of K-al- (Albers, 1981; Figure 3B). Most deposits are quite small con- teration typically reddens rocks which can also lose most of taining at most a few million tons of ore with grades of 3–10% their Na, Ca and base metals. As illustrated in Figures 4 and 5, combined Zn+Cu±Pb. The largest group constitutes the Foothill whole-rock compositions change markedly from their fresh Copper Belt (Heyl, 1948) where metamorphosed and deformed equivalents. Alkali ratios and, typically, ferric-ferrous rations Kuroko- and Noranda-like deposits are associated with the fel- are modified, commonly much more than in most other types sic, transitionally calc-alkaline portions of the Smartville com- of alteration (e.g., in porphyry-related potassic alteration, Fig- plex (Kemp, 1982). In the earlier basaltic to andesitic volcanic ure 5D). In many older studies, the presence or extent of alter- parts of these composite sequences, there are a few Cu(-Au) rich ation was not recognized, and alkali-altered rocks commonly deposits such as Copperopolis (Clark and Lydon, 1962). were interpreted as having distinctive or unusual igneous com- Besshi-style (Co-bearing pyrrhotite-dominated) mineralization positions. at Green Mountain in the southern end of this terrane occurs in Sodic and sodic(-calcic) alteration is reported (or can be in- slightly younger carbonaceous epiclastic rocks (Mariposa For- ferred from published data) for dozens of intrusive systems and mation) intruded by mafic sills (Mattinen and Bennett,1986). some volcanic terranes throughout the duration and extent of the VMS deposits in the Coast Range and Josephine ophiolites are Jurassic (Battles and Barton, 1995; Dilles et al., 1995). At the rare. The Turner-Albright deposit (Kuhns and Baitis, 1987; district scale, sodic(-calcic) is well described in Nevada Zierenberg et al., 1988) in the Josephine ophiolite is one of the (Carten, 1986; Dilles and Einaudi, 1992; Johnson, 2000; John- few of economic interest. The paucity of deposits may reflect son and Barton, 2000) and in the Mojave (Fox and Miller, 1990). the lack of through-going structures to focus fluid flow and dis- Similar, but typically less intense, sodic alteration assemblages charge (Schiffman et al., 1991; Harper, 1999). Although the ig- are seen in the marine arc and ophiolite sequences of central and neous rocks of this region have many similarities to those in- northwestern California (Figure 3C; e.g., Harper et al., 1988). board in northeastern California and northwestern Nevada, Stratabound potassic alteration is widely developed along the there is no evidence in the west for Jurassic IOCG subaerial part of the arc, for example in the Peavine sequence mineralization or IOCG-like intense sodic(-calcic) and potassic (Garside, 1998), Yerington (Dilles et al., 2000), Cortez Moun- alteration. tains (Muffler, 1964), Owens Valley region including the Inyo Mountains and Sierran pendants (Sorensen et al., 1998), and Regionally extensive alkali-exchange alteration Arizona (Haxel et al., 2008a). Interestingly, stratabound sodic Widespread, intense alkali-exchange alteration is another alteration is seemingly uncommon along the main arc; it be- prominent facet of Jurassic hydrothermal systems (Figures 3C). comes abundant only to the north and west where magmatism This process directly bears on the nature and origin of some Ju- overlapped with transitional marine sequences, for instance in rassic mineral deposits and can obfuscate the earlier features in the central Sierra Nevada and in northwestern Nevada (Figure others. Three types of alkali-exchange alteration predominate: 3C). (1) intense sodic(-calcic) alteration associated mainly with and Voluminous alkali-exchange alteration requires reaction of proximal to plutons, (2) sodic alteration that is ubiquitous in ma- volcanic rocks and shallow intrusions with moderately to rine volcanic sections and cogenetic hypabyssal rocks, typically strongly saline surface-related waters; magmatic fluids (if even in more distal positions, and also occurs in plutons, and (3) present) are neither sufficiently voluminous and would generate widely distributed, intense potassic alteration in continental substantially different assemblages and patterns than are ob- volcanic rocks (e.g., Meyer and Hemley, 1967; Chapin and served (Barton and Johnson, 2000). Seawater is an obvious can- Lindley, 1986; Barton et al., 1991; Alt, 1999; Rougvie and didate for involvement, and is well known for leading to such Sorensen, 2002; Seedorff et al., 2005). These types are the most changes (e.g., Alt, 1999). However, saline surface or basinal voluminous metasomatic types in the upper crust (Johnson, waters are compelling candidates in other areas. Such fluids 2000), yet they are less well known than other types probably were abundant in the Jurassic, and should have been readily ac- because they rarely host ores. cessed via plumbing systems consisting of extensional faults Sodic(-calcic), sodic, and (less common) calcic or and variably porous, penecontemporaneous supracrustal rocks. endoskarn alteration types manifest themselves through devel- Seawater was available to the west and north, and Jurassic ma- opment of secondary plagioclase (Ab100 to Ab70) and/or rine and continental evaporites were (and still are) widespread scapolite plus related (Ca-)Mg minerals (chlorite to actinolite in Nevada (Speed, 1974; Proffett and Dilles, 2008) extending to diopside to grossular—in order from sodic to calcic) and, eastward to the Colorado Plateau (Turner and Fishman, 1991). typically, removal of many transition metals and potassium Well-documented Jurassic aridity (e.g., Peterson, 1988) would (e.g., Dilles and Einaudi, 1992; Dilles et al., 1995). Intense, 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 por- phyry 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 Distribution in time and space—correlations impact on other aspects such as the nature of Jurassic volcanism (Busby et al., 2005). With the exception of marine arc and ophiolite-related Correlations with mineralization are clear—sodic and VMS systems of central and northwestern California, most lesser sodic(-calcic) alteration is clearly related to seafloor hy- known Jurassic mineralization in the western US lies in eastern drothermal activity and VMS mineralization in northwestern California, northern Nevada, and Arizona. A few porphyry and California (Figure 3B). The spatial and temporal link of sodic IOCG occurrences are found in southern California and Baja styles of alteration with iron deposits is equally compelling and California (Barton et al., 2000; Staude and Barton, 2001). These has been long recognized (e.g., Lindgren, 1913; Einaudi et al., include the porphyry Cu deposit at El Arco in Baja California 1981; Williams et al., 2005). The diverse character of associated Norte (165 Ma, Valenciaet al., 2006) and iron oxide-rich depos- igneous rocks, the affinity to evaporitic environments, and geo- its with copper, gold and extensive alkali-rich alteration along chemical (mass balance and solution chemistry) arguments that the west coast of Baja California (Late Jurassic or Early Creta- first led Barton and Johnson (1996, 2000) to suggest that the ceous, Barton et al., 2000; Lopez et al., 2005). The location of IOCG family in this part of the world, indeed globally, is funda- these areas relative to North America in Jurassic time is prob- mentally the product of thermally circulated non-magmatic lematic; they may have formed in arcs some distance removed brines and not mainly the product of magmatic-hydrothermal from the continent (e.g., Dickinson and Lawton, 2001; Valencia fluids. 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, The second episode, to which most porphyry Cu and IOCG one at ~190–210 Ma and the other at ~155-170 Ma, which cor- systems and all the known VMS and W-Au systems belong, cor- respond to ill-defined pulses of magmatism (Figure 6). Like in- relates with the mid-Jurassic magmatic flare-up that extends trusions of the same age, Early Jurassic mineralization (por- from the oceanic arcs and ophiolites in the west to the scattered phyry Cu, Zn-Pb-Ag, and IOCG) is fairly sparse (Figure 3B). It back-arc granitoids of the north-central Great Basin (Figures 3, occurs mainly along the arc from southern Arizona (e.g., 6). All the porphyry and most of the base metals systems are lo- Bisbee, ~200 Ma, Courtland-Gleeson, ~190 Ma, Lang et al., cated, possibly because they are best preserved there, in the 2001) through eastern California and western Nevada (e.g., Great Basin with only scant evidence for porphyry-style miner- Candelaria, 192 Ma, Thomson et al., 1995; Royston ~200 Ma, alization of this age in the Sonora, Arizona, or California. In Seedorff, 1991a; possibly the Jackson Mountains, ~190–196 contrast, IOCG systems extend throughout the entire magmatic Ma, Quinn et al., 1997), and into northern California (Lights belt inboard of the Jurassic shoreline. Many of the southern oc- Creek, 178 Ma; Dilles and Stephens, 2010). Curiously, few de- currences are magnetite-dominated which might reflect a pre- posits are known to be between 190 and 170 Ma. Intrusions in ponderance of deeper exposures; on the other hand, this age range are seemingly less common than others which low-sulfidation advanced argillic occurrences are also wide- may explain part of the pattern. As noted later the same apparent spread in this region perhaps negating a depth of exposure gap is present in British Columbia. argument.
Figure 5. K2O/Na2Ovs.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 Ju- rassic arc (cf. Figure 3B,C). C) Comparison of results from the Mesoproterozoic IOCG-bearing terrane of SE Mis- souri, 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
The relative abundance of Jurassic volcanic rocks through- out 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 prov- ince 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-sed- imentary 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 addi- tion 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 short- ening 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 Figure 6. Synopsis of timing of hydrothermal systems and generalized patterns (Late Jurassic), Sevier (Cretaceous), Laramide (Late Creta- in related features during the Jurassic. The maxima in deposits observed at 200 ceous-Paleocene), and even Neogene (restraining bends along Ma and 160 Ma closely match the timing observed in British Columbia. See text the strike-slip margin) played multiple roles. Undoubtedly, for source of information and discussion. crustal shortening uplifted and exposed to erosion the upper parts of many Jurassic centers. In contrast, in other places burial Late Jurassic–Early Cretaceous magmatism was relatively by reverse faulting or by foundering (as is the case for screens sparse and, with only a few exceptions of little economic impor- within the Cretaceous batholiths) contributed to high-level pres- tance (e.g., Lights Creek porphyry, 148 Ma), apparently lacks ervation. Jurassic and Early Cretaceous extension in the south- well mineralized igneous centers. In many areas, magmatism is ern California and Arizona helped also preserve high levels represented by dike swarms or small intrusions, characteristics (e.g., Busby et al., 2005) and similarly low base levels—evi- that belie the large energy and material supplies necessary to denced by original near-sea level elevations—in other parts of make large deposits. In northern California, where Late Jurassic the Jurassic arc such as northwestern Nevada and northern magmatism appears to be best developed, many plutons of this California no doubt aided preservation of upper Jurassic crust. age appear to be relatively deeply eroded with the exception of the NE Sierra Nevada (Christe, 2010; Dilles and Stephens, COMPARISON WITH OTHER TIMES AND REGIONS 2010). The same may be true in southern California where a Late Jurassic plutonic belt apparently preserves only granitoids Here we briefly compare the Jurassic metallogeny of south- (Barth et al., 2008). western 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 metallo- of the Jurassic: notably the association of a wide spectrum of ig- genic controls—process, province, and preservation. neous compositions, mostly extensional tectonics, and evidence for the influence of distinctive surface conditions on hydrother- Comparison with other periods in SW North America mal fluids (Barton, 1996). Tertiary porphyry, skarn, and base metal systems of multiple types occur with the majority of intru- Jurassic metallogeny differs appreciably from other peri- sive centers in the Great Basin (Seedorff, 1991b) and northern ods in the latter half of the Phanerozoic (Barton et al., 1988, Mexico (Staude and Barton, 2001). Their characteristics vary 1995; Barton, 1996). Compared with other times, alkaline igne- systematically with the compositions of genetically related ig- ous compositions are more abundant, IOCG and VMS deposits neous rocks. Similarly, but in contrast to the Jurassic, numerous and alkali-rich alteration are more common, economically sig- vein and stratabound precious metal deposits of a variety of nificant porphyry deposits are sparse, and precious metal types occur throughout the region. Many of these are clearly as- (mainly epithermal) deposits are rare. sociated with coeval magmatism with which they show correlations in metal contents, alteration types, and other Pre-Jurassic (mainly Permo-Triassic) parameters (e.g., John, 2001). Igneous-related Paleozoic to Triassic deposits are rare. A Conversely, other types of deposits are less clearly related few middle to late Paleozoic VMS systems occur in the eastern to magmatic activity and may reflect circulation of fluids from Klamath Mountains and locally in northern Nevada. Local non-magmatic sources during Cenozoic extension. Carlin-type IOCG-style mineralization and sodic alteration of probable gold deposits are one example (Ilchik and Barton, 1997; Cline et Permian and Triassic age occurs in the eastern Klamaths (Pit al., 2005). Modern low-salinity, gold-bearing geothermal sys- River) and sparsely along the scattered Permo-Triassic arc (Bat- tems are being driven by crustal extension in the absence of tles and Barton, 1995) extending across the Southwest and into magmatism in northwestern Nevada (Coolbaugh et al., 2005). east-central Mexico. A few tungsten skarn deposits are co-lo- Cenozoic IOCG-style mineralization is widespread and locally cated with Triassic granites in eastern California. Porphyry and active, as in the modern hypersaline Salton Sea geothermal sys- epithermal systems older than latest Triassic seem to be absent, tem (Barton et al., 2000). Structurally controlled IOCG deposits with a very few exceptions, such as the poorly dated but likely in the southern Basin and Range are inferred to be caused by cir- Triassic (~230 Ma) Cu-Au deposits of the Pine Grove district, culation of basinal (evaporitic) brines during extension, perhaps Nevada (Pricehouse and Dilles, 1995). aided by magmatic thermal input (Wilkins et al., 1986). Broadly coeval, low-temperature K metasomatism is abundant through- Cretaceous—Laramide (~145–50 Ma) out the same region (e.g., Chapin and Lindley, 1986; Rougvie Magmatism during the Cretaceous (145–65 Ma) and Lara- and Sorensen, 2002). mide (~80–50 Ma) in the southwestern North America is almost entirely subalkaline and shows consistent secular variation in Comparison with other circum-Pacific regions composition and distribution in space (e.g., Barton, 1996). The main Cretaceous batholithic belts (Figure 1) are rather deeply Canadian Cordillera eroded, and contemporaneous volcanic rocks are rarely pre- The amalgamated terranes of the Canadian Cordillera share served (e.g., Barton et al., 1988). Even though igneous compo- many features in both timing and styles of magmatism with the sitions are compatible with those observed in economically pro- southern Cordillera in United States and Mexico, and they have ductive belts elsewhere, mineral deposits are rare. The principal broad parallels, but also some major differences with the Andes types within the main arc are tungsten skarns formed where the (e.g., Coney and Evenchick, 1994). Middle Mesozoic rocks of felsic granitoids intrude carbonate-bearing host rocks. the Canadian Cordillera contain a range of igneous composi- In contrast, large magmatic-hydrothermal deposits are tions quite similar to the Jurassic farther south. Plutons range widespread in the interior region, mainly in the Great Basin east from strongly alkaline including rare silica-undersaturated of the main belt. These deposits formed with the subalkaline compositions to ordinary oxidized (magnetite-titanite-bearing) metaluminous to peraluminous felsic granitoids of the Creta- calc-alkaline intrusions. Porphyry and skarn Cu(-Au) mineral- ceous back arc in the Great Basin and in the Laramide province ization is associated with both types. Likewise, iron skarns of the southern Basin and Range. Porphyry Cu(-Mo±Au) de- (Meinert, 1984) and other IOCG-like deposits are widespread, posits are widespread with biotite(-hornblende) granodiorites particularly in southern British Columbia. VMS deposits are and granites; lithophile element-rich systems occur with the relatively uncommon, though some of the early Mesozoic sys- strongly peraluminous granites. Volcanic rocks are sporadically tems that are present have exceptional size or grade (e.g., Windy preserved, but epithermal and IOCG-like systems are rare. Craggy, Peter and Scott, 1999; Eskay Creek, Roth et al., 1999). Perhaps most remarkable, given the postulated exotic nature of Mid- to late Cenozoic (~50–0 Ma) these terranes is that the timing of the Triassic to Jurassic The middle and late parts of the Cenozoic have consider- magmatism and porphyry Cu(-Au-Mo) mineralization closely able metallogenic and magmatic diversity and resemble aspects 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 re- flect 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, volumi- nous magmatism that begins in the Jurassic and extends into the Early Cretaceous is linked to small porphyry Cu systems and Figure 7. Comparison of tectonic evolution and temporal distribution of por- widespread IOCG mineralization in Chile, changing northward phyry and IOCG systems in the cordillera of North and South America. Tec- in Peru to IOCG systems and then VMS deposits (Vidal et al., tonic base modified from Coney and Evenchick (1994). 1990; Sillitoe, 2003; Maksaev et al., 2010). In their continen- tal-scale comparisons, Coney and Evenchick (1994) note fun- those seen in the Foothill Belt of the Sierra Nevada and the west- damental similarities in the tectonic histories of the South and ern Klamath Mountains. The tropical latitudes, and correspond- North American margins that are offset in time in relation to ing abundance of rainfall, make highly saline groundwaters opening of the central and southern Atlantic Ocean basins (Fig- scarce; thus the contrast between abundant IOCG-style systems ure 7). The parallels in metallogeny are similarly striking: older in the western US and the more abundant adularia-sericite de- episodes are characterized by abundant IOCG and sparse por- posits of the southwestern Pacific may reflect a climatic control phyry systems, which change northward in both cases to marine on near-surface fluids and, in turn, on hydrothermal settings with VMS deposits. Later orogenesis in both regions geochemistry. leads to the principal episodes of porphyry formation—Late Cretaceous to Paleocene in southwestern North America Metallogenic Controls for the Jurassic of Southwestern (Barton, 1996) and Eocene-Oligocene in the central Andes North America (Sillitoe and Perello, 2005). Both regions also have far more abundant epithermal systems during these younger periods The distribution and characteristics of Jurassic hydrother- although IOCG occurrences remain common in both (Figure 7; mal systems, and their comparison and contrast with other peri- Barton, 2009). ods and regions are logically considered in terms of the variety of processes involved, crustal and surface-related provincial ef- Southwest Pacific fects, and the filter imposed by preservation and exposure Like the North American Jurassic, the southwestern Pacific (Barton, 1996). Cenozoic comprises an evolving collage of arcs built across ter- ranes ranging from oceanic crust to continental basement. Di- Process controls verse subalkaline to alkaline magmatism developed intermit- Jurassic magmatic-hydrothermal systems correlate well tently in these arcs and is linked to a wide variety of mineral with the composition of associated intrusions, as is observed deposits (e.g., Garwin et al., 2005). Porphyry Cu(-Au-Mo) sys- globally in porphyry systems (e.g., Seedorff et al., 2005). This tems are widespread with both alkaline and subalkaline intru- correlation is founded on the chemical controls that magmas ex- sive complexes, much like in western North America from Brit- ert on the compositions of exsolved fluids. Many Jurassic mag- ish Columbia to northwestern Mexico. Where the volcanic mas may have been relatively dry, as evidenced by their miner- superstructure is preserved, some of the calc-alkaline centers alogy (lacking early crystallizing hornblende, cf. Figure 2) and have high-sulfidation epithermal deposits; in other areas adu- their fairly mafic bulk compositions as compared to younger in- laria-sericite and even alkaline epithermal systems are pre- trusions. These features contrast with the relatively served (Jensen and Barton, 2000; Garwin et al., 2005). Such hornblende-rich, felsic intrusions of later periods. Igneous com- high-level deposits are not known in the Jurassic of the western positions also influence the composition of fluids circulated US and are problematically preserved in Canada. Numerous through them following crystallization, with or without contri- modern and recent VMS systems occur in the western Pacific butions from magmatic fluids, leading to differences in the na- along the submarine portions of arcs and in back arc basins. ture of associated mineral deposits. This igneous control (medi- Preservation of some of these during later accretionary events ated by other materials present) is evident for a wide range of seems plausible; if so, it could generate relationships not unlike 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 Provincial characteristics strongly influence, but do not fully (Barton and Johnson, 1996, 2000). govern, the nature of magmas. The principal effect is that of di- For the Jurassic, there are systematic differences in marine luting a subcrustal magma with materials from the overlying and terrestrial hydrothermal systems depending on location and crustal column. This is best seen in the number of igneous nature of associated rocks (Figure 3B). Copper-dominated compositional factors that influence the nature of hydrothermal VMS systems (Cyprus and Besshi types) occur in the ba- fluids discussed in the preceding section. In the Jurassic, mafic salt-rich settings, whereas Zn-Cu±Pb VMS deposits (Kuroko compositions to the northwest, predominantly granitic compo- type) including local advanced argillic assemblages are linked sitions in the Great Basin, and varied compositions (but not to the tonalite-granodiorite-bearing Smartville arc. The IOCGs strongly peraluminous) along the main arc in the central and also exhibit systematic differences that reflect felsic (high REE, southern areas surely reflect in part the composition of the un- U; hematite-rich) and mafic (low REE; magnetite rich) host derlying crust—mafic to the northwest, sediment-rich along the rocks (Johnson, 2000). These patterns show up in IOCG sys- miogeocline, and crystalline Proterozoic (mainly metaigneous) tems globally (Barton and Johnson, 1996; Williams et al., to the south. The apparent restriction of Au-W systems to areas 2005). with relatively reduced granitoids in the Great Basin is another Physical factors, such as thermal structure and state of logical consequence of this regional variability. stress, profoundly affect magmatic compositions and the nature Regional variations in crustal composition and thickness of of permeability and hence fluid flow. Thermal models for Juras- Jurassic crust also contribute to freeboard and to the chemical sic and other magmatism have been presented elsewhere to ra- reactivity of available host rocks. Thinning continental crust tionalize the patterns observed (Barton, 1990, 1996; Elison, and the transition to oceanic crust to the northwest contributes to 1995). Apart from their importance in accentuating permeabil- the observed marine overlap. There is a close correspondence of ity through faulting and basin development, and in preservation Zn-Pb-Ag deposits with the carbonate-rich sedimentary rocks (below), extensional tectonics should have impacted the size of the miogeocline (Figure 3A). The effect on metal contents and diversity of Jurassic (and Cenozoic) magma bodies. Juras- and other aspects of VMS and IOCG systems has already been sic chambers might have been relatively small compared with described. those of compressional settings, leading to less fluid and per- Surface fluids and ground waters are a final, and critical, haps less effective focusing for generation of large magmatic provincial aspect of Jurassic metallogeny. As is apparent from hydrothermal systems. The extensional environments correlate Figure 3C, Jurassic surface fluids were atypically saline com- with greater magmatic diversity perhaps because of an ineffec- pared to those of other times (Laramide, much of the Cenozoic) tive density filter due to thinning crust. Similar reasoning might and climates (tropical or temperate). The role of seawater account for the somewhat small size of most oceanic / primitive speaks for itself; on the other hand, saline ground waters in arcs arc porphyry systems elsewhere, as in parts of the southwestern have received relatively little attention as a provincial control. Pacific and British Columbia. The consequences of high salinity fluids have already been de- Finally, fluid salinity and availability of sulfur for trapping scribed—creation of IOCG mineralization, extensive and in- metals help rationalize other differences. In the case of por- tense alkali exchange alteration, suppression of adularia-seri- phyry systems, built-in traps—cooling of sulfur-bearing, cite-type epithermal systems – all features that are abundant in metal-rich fluids—account for the large proportion of mineral- the Jurassic, but less common or absent in other times and ized (not necessarily economic) magmatic-hydrothermal sys- places. tems. VMS systems are similar in containing elevated contents of both metals and reduced sulfur. Conversely, IOCG systems Preservation and exposure have high-salinities but are sulfur-deficient and thus, they can As has been long recognized, the exposure and preserva- precipitate iron oxides on cooling but require an additional sul- tion of mineralized systems profoundly affects our understand- fur source for removing chalcophile metals from solution. A ing of metallogenic fertility and its underlying controls. The high salinity setting would have the additional effect of sup- weight of evidence for the Jurassic is that a considerable frac- pressing the formation of typical adularia-sericite-type epither- tion of high level crust has been preserved, but only locally does mal systems, which are characterized by the low salinities of the this include the very highest levels near igneous centers. As dis- surface waters that dominate their formation. These factors cussed above, conventional epithermal systems are sparse, per- likely all contribute to the observed Jurassic patterns and some haps because erosion has removed the top kilometer or more in of the differences with other areas (e.g., SW Pacific) and times most areas or because of chemical factors reflecting the nature (e.g., Laramide) that are noted above. of ground waters. Nevertheless, there are areas, in the Great Ba- sin, and the Sierran foothills, and in southern California and Ar- Provincial controls izona where high level rocks are at least locally preserved. Many Provincial controls have received considerable attention in of these are in areas that were possibly affected by later Jurassic understanding the sources and differences in magmatism and contractional tectonics which may have helped depress or bury with regard to metallogenic productivity (e.g., Titley, 1987). them (e.g., Schermer et al., 2001). Some may have been pre- Jurassic igneous-related metallogeny 391 served in the footwall of reverse / thrust systems or in their local cause of the scarcity of quartz, rarity of acid alteration, and foreland basins, as is clearly the case along the Colorado River fine-grained gold in such systems (Jensen and Barton, 2000), where Jurassic volcanic and sedimentary rocks and enclosed and the similarity of associated K alteration (± pyrite and hydrothermal systems have been taken up in the Maria fold and hematite) to features that accompany regionally extensive thrust belt (Figures 1A, 3B). The pre-Miocene tilting that pre- stratabound K metasomatism. served the epithermal systems on the west side of the Yerington Apart from iron exploration, there has been little evaluation district (Dilles and Proffett, 1995) could be any of several youn- of the possibility of higher grade (Cu-Au-rich) IOCG systems. ger tectonic episodes. Conversely, relatively deep exposures in Only the Pumpkin Hollow deposit in the Yerington district the Klamath Mountains, the western Mojave, and parts of the (Dilles et al., 2000) has seen much recent exploration. A number east-central Great Basin might reflect erosion following of the iron districts contain large magnetite resources; however, Jurassic and/or younger uplift in these areas. given the large number of copper prospects and small mines Syn-magmatic or slightly younger extension undoubtedly there is clearly potential for other Cu (± Au), and even U- or contributed to preservation (e.g., Busby, 1988; Dilles and REE-bearing deposits. The comparison with Chile suggests that Proffet, 1995). For example at Bisbee the principal ores are pre- those areas nearer the Jurassic shoreline might be better candi- served in the down-dropped hanging wall of the Late Juras- dates because of the possibility of H2S and/or hydrocar- sic-Early Cretaceous Dividend Fault (Bryant and Metz, 1966). bon-bearing fluids to trap Cu (Barton, 2009). Younger Mesozoic and Cenozoic materials cover many of these The evidence suggests, without obvious evidence to under- areas providing exploration opportunities. stand why, that the most productive times were 200±10 Ma and 160 ± 10 Ma—remarkably like the temporal patterns seen in the Some possible economic implications northern Cordillera. Conversely, much of the Early Jurassic and most of the Late Jurassic appear to be metal-poor, perhaps be- Although by no means as productive as later Mesozoic cause of lower magmatic fluxes overall, or deeper levels of ex- and Cenozoic magmatic episodes, Jurassic magmatism south- posure in areas where these ages predominate such as northern western North America produced dozens of hydrothermal sys- Sierra Nevada and Klamath Mountains. All of these issues and tems that have been economically important. Moreover, much opportunities deserve closer scrutiny. of the Jurassic arc is covered by younger materials. A newly compiled digital map of the entire region shows that Jurassic SUMMARY rocks make up about 7% of exposures in the overall domain of Jurassic magmatism. Given that 80% of the same region con- The igneous-related Jurassic metallogeny in southwestern sists of younger materials, of which later intrusions (mainly North America reflects the combined influences of distinctive Cretaceous) comprise about 20%, the implication is that only tectonic, magmatic, and surficial conditions. Broadly about one-quarter of the Jurassic of the same region is ex- extensional tectonism was accompanied by diverse magmatic posed; three-quarters is under younger cover. Furthermore, compositions, including mildly alkaline varieties. Early and tectonic and sedimentary burial tend to cover higher-level Late Jurassic magmatism was restricted to a belt, which is pres- rocks, and hence more prospective rocks. Thus, there may be ently 100–200 km wide, that obliquely crosses the Paleozoic considerable potential for known deposit types under Late Ju- continental margin. Middle Jurassic magmatism encompassed a rassic and younger cover. For example, the Foothill Copper much broader area in the north; it extended on the west from Belt almost surely continues under younger materials to the central and northern California, where it was relatively mafic, north and south along its trend. One might ask if there are envi- and eastward across much of the Great Basin into western Utah, ronments in this domain that might host economic deposits where it comprises a compositionally varied family of roughly similar to those in Canada of the same age (e.g., Eskay Creek; coeval igneous centers. Roth et al., 1999)? Controls on Jurassic metallogeny include tectonic, mag- Although conventional epithermal systems are sparse in Ju- matic and hydrothermal processe, the sources of materials for rassic, some areas preserve the Jurassic paleosurface. Evidence the magmas and external fluids which reflect provincial crustal includes the presence of a number of iron-oxide advance argillic and surface controls, and a complex history of exposure and systems; reasonable places to look for preserved epithermal preservation. Relatively mafic magmatism and abundant seawa- systems may still be found. If conventional low-salinity epither- ter in the northwest led to predominantly VMS systems, mal precious metal systems are present—in our view IOCGs whereas more felsic intrusions in the main part and eastern ex- represent a high-salinity equivalent—an additional challenges pansion of the arc generated magmatic hydrothermal systems of will be reading through the variable overprinting of metamor- porphyry Cu(-base metal) and W-Au families. Analogously, the phism and deformation in volcanic sections. Quartz veins might abundant alkali-rich alteration in most areas and the ample inde- be recrystallized and alteration minerals modified. Further- pendent evidence for highly saline surface and basinal waters is more, if such systems formed with the Jurassic alkaline consistent with the abundance of IOCG-family deposits magmatism, the hydrothermal products might be cryptic be- 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 kilo- Barnes, C.G., Allen, C.M., and Saleeby, J.B., 1986a, Open and closed system meter of the Jurassic crust. Alternatively, these types of deposits characteristics of a tilted plutonic system, Klamath Mountains, Califor- nia: Journal of Geophysical Research, v. 91, p. 6073–6090. may be precluded by the unusually saline terrestrial fluids Barnes, C.G., Rice, J.M., and Gribble, R.F., 1986b, Tilted plutons in the and/or the common occurrence of significantly alkaline Klamath Mountains of California and Oregon: Journal of Geophysical magmas. Research, v. 91, 6059–6071. The Jurassic differs markedly from younger periods in Barnes, C.G., Mars, E.V.,Swapp, S., and Frost, C.D., 2006, Petrology and geo- southwestern North America reflecting these differences in chemistry of the Middle Jurassic Ironside Mountain Batholith; evolu- tion of potassic magmas in a primitive arc setting: Geological Society of magmatism, fluid sources, and likelihood of preservation. The America Special Paper, v. 410, p. 199–221. relatively sparse known metal endowment compared to the Cre- Barth, A.P., Wooden, J.L., Howard, K.A., and Richards, J.L., 2008, Late Juras- taceous, Laramide, and middle Tetiary reflects these factors, yet sic plutonism in the southwest U. S. Cordillera: Geological Society of three-fourths or so of the Jurassic is covered by younger sedi- America Special Paper, v. 438, p. 379–396. mentary and volcanic rocks thus much remains to be discov- Barton, M.D., 1990, Cretaceous magmatism, metamorphism, and metallogeny ered. Beyond the region, interesting parallels exist between in the east-central Great Basin: Geological Society of America Memoir 174, p. 283–302. southwestern North America, the Canadian Cordillera, the cen- Barton, M.D., 1996, Granitic magmatism and metallogeny of southwestern tral Andes, and the southwestern Pacific. Each of these latter re- North America: Transactions of the Royal Society of Edinburgh—Earth gions exhibits considerable similarities but also some marked Sciences, v. 87, p, 261–280. differences with southwestern North America. Comparison of Barton, M. D., 2009, IOCG deposits: A Cordilleran perspective: in P. J. Wil- these regions still has much to teach us. liams, Smart Science for Exploration and Mining, Proceedings of the 10th Biennial Meeting of the Society for Geology Applied to Mineral Deposits, EGRU, James Cook University, Townsville, Australia, p. 5–7. ACKNOWLEDGMENTS Barton, M.D., and Johnson, D.A., 1996, Evaporitic source model for igne- ous-related Fe oxide-(REE-Cu- Au-U) mineralization: Geology, v. 24, This work reflects a synopsis of long-standing and ongoing p. 259–262. work by the authors. The University of Arizona efforts have Barton, M.D., and Johnson, D.A., 1997, A comparison of Fe-ox- been supported most recently by grants from the USGS MRERP ide(-Cu-Au-REE-U-Co-Ag) mineralization: Geological Society of (08HQGR0060), the NSF (EAR08-38157, EAR98-15032), and America Abstracts with Programs, v. 30(7), p. 51. the Science Foundation Arizona-Institute for Mineral Re- Barton, M.D., and Johnson, D.A., 2000, Alternative brine sources for Fe-ox- ide(-Cu-Au) systems: Implications for hydrothermal alteration and sources. We thank Steve Koehler for his helpful review and metals, in Porter, T. M., ed., Hydrothermal iron oxide copper-gold and Roger Steininger for help in the manuscript submission. Finally, related deposits: a global perspective: Australian Mineral Foundation, we gratefully acknowledge the interest, knowledge, and help Glenside, South Australia, p. 43–60. from many colleagues, too numerous to name individually, in Barton, M.D., Ilchik, R.P., and Marikos, M.A., 1991, Metasomatism, in industry, academia and government who have contributed to our Kerrick, D.M., Contact Metamorphism: Reviews in Mineralogy v. 26, p. 321–350. work in the Great Basin and surrounding regions. 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