Download Jurassic igneous-related metallogeny of southwestern North America

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