Special Paper: Major Gold Deposits and Belts of the North and

Transcription

©2008 Society of Economic Geologists, Inc.
Economic Geology, v. 103, pp. 663–687
Special Paper:
Major Gold Deposits and Belts of the North and South American Cordillera:
Distribution, Tectonomagmatic Settings, and Metallogenic Considerations
RICHARD H. SILLITOE
27 West Hill Park, Highgate Village, London N6 6ND, England
Abstract
A compilation of economically viable gold concentrations containing ≥10 Moz in the North and South
American Cordillera reveals the existence of 22 discrete belts in addition to five major isolated deposits, most
formed over the last 150 m.y. The gold concentrations are attributed to eight widely recognized deposit types,
of which porphyry, sediment-hosted, and high-sulfidation epithermal are economically the most important.
Individual gold belts are typically several tens to hundreds of kilometers long, dominated by single deposit
types, and metallogenically active for relatively brief periods (<5–20 m.y.).
Many of the gold belts and major isolated deposits were generated under extensional or transtensional tectonic conditions in either arc or back-arc settings. Nevertheless, the two main high-sulfidation epithermal gold
belts were generated in thickening or already-thickened crust during low-angle subduction. Eight other gold
belts or districts also accompanied compression or transpression, with two of them, the main orogenic gold
belts, occupying fore-arc sites. There is a strong suggestion that the preeminent Cordilleran gold concentrations formed either during or immediately following prolonged contraction. The major gold deposits and belts
occur along the craton edge as well as in adjoining accreted terranes, but almost all are of postaccretionary timing. Many of the gold belts and isolated deposits were localized by crustal-scale faults or lineaments, which may
be either parallel or transverse to the Cordilleran margin.
The gold concentrations accumulated during active subduction, commonly in close spatial and temporal
association with intermediate to felsic, medium- to high-K calc-alkaline igneous rocks. By contrast, the lowsulfidation epithermal gold deposits accompany bimodal volcanic pulses of calc-alkaline, tholeiitic, or alkaline
affinity. However, a gold-alkaline rock association is uncommon. A genetic link between gold mineralization
and coeval magmatism is widely accepted for most of the deposit types, the exceptions being the sedimenthosted and orogenic gold deposits.
Notwithstanding the small cumulative extent of the gold concentrations relative to the entire Cordilleran
margin, there is a marked tendency for two or more belts or isolated deposits of different ages and genetic types
to occur in close proximity within relatively restricted arc (including fore- and back-arc) segments. In the case
of the western United States, for example, six belts and four isolated major deposits make up a particularly
prominent cluster. If fortuity is discounted, this clustering or pairing of gold concentrations must imply a predisposition of certain arc segments to gold mineralization. An analogous situation is evident for other metals,
particularly copper and tin. The reason for the recurrent generation of major deposits and belts dominated by
one or more metals remains uncertain, although heterogeneously distributed metal preconcentrations, favorable redox conditions, or other parameters somewhere above the subducted slab, between the mantle wedge
and upper crust, are widely contemplated possibilities. Elucidation of the reason(s) for this metallogenic
inheritance at the scale of limited arc segments poses an important and challenging series of research questions
as well as being critical to the planning of potentially successful greenfield exploration programs.
Introduction
GOLD DEPOSITS occur at numerous localities along the full
length of the North and South American Cordillera, from
Alaska in the north to Patagonia in the south. The Cordillera
has a long and colorful gold-mining history dating back to the
16th century, since when the cumulative production plus current reserves of the yellow metal total >1,000 Moz (30,000
metric tons), ~20 percent of the known global total. Today, six
of the top 20 gold-producing countries of the world are located there. Cordilleran gold is contained in a spectrum of
widely recognized and well-documented ore deposit types
(e.g., Robert et al., 2007; Fig. 1; Table 1) generated in a variety of tectonomagmatic settings, a situation in marked contrast to the majority of tin or copper resources, which in the
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0361-0128/08/3747/663-25
western Americas are hosted almost exclusively by single deposit types (e.g., Turneaure, 1971; Sillitoe and Perelló, 2005).
Most of the major Cordilleran gold deposits and belts are
Mesozoic or Cenozoic in age (Table 2), although it should be
remarked that the major Homestake deposit in South Dakota,
not considered further herein, is Paleoproterozoic and part of
the cratonic basement to the North American Cordillera
(Slaughter, 1968; Fig. 2).
This analysis incorporates all gold belts and isolated deposits in the western Americas that are estimated to contain
≥10 Moz (>~300 t) of gold (Fig. 2; Table 2), an approach that
probably takes account of at least three-quarters of the region’s gold endowment. Only gold concentrations with
demonstrated economic potential or a reasonable chance of
commercial production are considered. By-product gold in
base metal and silver deposits is ignored, although gold-rich
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RICHARD H. SILLITOE
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
Sediment-hosted
Au
V
V
V
V
V
Dike
V
V
V
V
V
1
V
V
V
V
Volcanic
rocks
V
V
V
V
V
V
V
V
V
km
0
Low-sulfidation
epithermal Au
Intermediate-sulfidation
epithermal Au
V
High-sulfidation
epithermal Au
Placer Au
V
Volcanic
rocks
Limestone
Pluton-related
Au (±skarn)
Porphyry Cu-Au/Au
(±skarn)
Carbonaceous +
calcareous siltstone
Limestone
Porphyry
intrusion
5
Turbidite
Greenstone
Inferred
felsic
intrusion
Oxidized or
reduced felsic
equigranular
intrusion
10
Orogenic lode Au
Granitoid
Shear zone
FIG. 1. Schematic geologic settings and interrelationships of the principal gold deposit types in the North and South American Cordillera, inspired by Robert et al. (2007). The approximate depth scale is logarithmic. Selected deposit characteristics
are summarized in Table 1. Note that placer gold is most commonly derived by erosion of orogenic and pluton-related
deposits.
porphyry copper deposits are included if either the value of
the contained gold roughly equals or exceeds that of the copper, or the gold endowment is metallogenically outstanding
(e.g., Bingham Canyon, Utah; Pebble, Alaska; Fig. 2). The individual gold belts are defined by either single or several ore
deposit types, one of which tends to predominate, and were
generated during relatively restricted metallogenic epochs,
most of which have been reasonably well defined by isotopic
dating (Table 2).
The paper summarizes the distribution and tectonomagmatic settings of the principal gold concentrations in the
western Americas (including the Caribbean region) and,
thereby, highlights the preeminent tectonic and associated
igneous activity that controlled and influenced gold deposit
formation here over the last 150 m.y. or so. Tectonomagmatic
aspects are emphasized over deposit-scale physicochemical
conditions and mechanisms throughout this discussion. The
paper is organized by gold deposit type, although skarn deposits are not considered further in view of their relatively
minor contribution (Table 1) and typical close association
with other intrusion-related deposit types. A prefatory section provides a brief summary of gold deposit types and their
relative economic importance in the American Cordillera. A
concluding synthesis of the tectonomagmatic settings for
gold and comparisons with the main Cordilleran copper and
tin belts provides the basis for comment on broader metallogenic issues, particularly the concepts of provinciality and
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inheritance. Finally, some suggestions for future research are
offered.
Gold Deposit Types
The western American Cordillera contains most of the
world’s economically important gold deposit types (Fig. 1;
Table 1). The gold deposits are attributable to five broad categories: epithermal deposits in shallow volcanic environments; porphyry copper-gold or gold-only deposits in the subvolcanic environment; sediment-hosted (or Carlin-type)
deposits in nonmetamorphosed, carbonate-rich sedimentary
sequences; pluton-related deposits in deeper, but still epizonal intrusive environments; and orogenic deposits in metamorphic rocks, commonly assignable to greenschist facies.
Some investigators currently subdivide the epithermal
deposits of both vein and disseminated styles into high-, intermediate-, and low-sulfidation types, as defined by mineralogic criteria (Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003; Table 1). The porphyry deposits are typically
large and always in the form of stockworks, whereas the sediment-hosted deposits are predominantly disseminated in
style and controlled by a combination of faults and receptive
stratigraphy. The pluton-related deposits range from veins
and vein swarms to stockworks, but include other styles
(Thompson et al., 1999; Lang and Baker, 2001; Fig. 1). The
pluton-related deposits are divisible into two distinct classes
depending on the redox state (e.g., Ishihara, 1977) of their
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SPECIAL PAPER: MAJOR GOLD BELTS OF THE NORTH AND SOUTH AMERICAN CORDILLERA
665
TABLE 1. Key Features of Main Gold Deposit Types in the American Cordillera
Contribution
to total
Cordilleran
gold endowment (%)1
Principal
mineralization
style(s)
Characteristic
accompanying
elements
Typical
host rocks
Typical proximal
alteration type(s)
Ore fluid
Reference
16
Stockwork,
disseminated,
veins, breccias
Cu, As, Ag
Andesitic to
dacitic volcanic
rocks + basement
Advanced argillic
Mixed magmaticmeteoric
Simmons
et al. (2005)
Intermediatesulfidation
epithermal Au
6
Veins,
stockwork
Ag, Zn, Pb, Cu,
As, Sb, Mn
Andesitic
to dacitic
volcanic rocks
Intermediate
argillic ± adularia
Simmons
et al. (2005)
Low-sulfidation
epithermal Au
6
Veins,
disseminated
As, Sb, Zn, Pb
Felsic and basaltic
volcanic rocks
Intermediate
Simmons
et al. (2005)
Alkalic lowsulfidation
epithermal Au
3
Disseminated,
veins
Te, V, F
Alkaline
volcanic rocks
Intermediate
Magmatic
Simmons
et al. (2005)
Mo
Quartz diorite
to granodiorite
porphyry stocks
+ wall rocks
Potassic, intermediate argillic,
sericitic
Magmatic
Sillitoe (2000)
Potassic, intermediate argillic
Magmatic
Vila and
Sillitoe (1991)
Gold deposit
type
High-sulfidation
epithermal Au
Porphyry
Cu-Au
20
Stockwork +
disseminated
2
Stockwork
Cu, Mo
Diorite to quartz
diorite stocks +
wall rocks
Skarn Au
4
Irregular to
strata-bound
replacements
As, Bi, Te or
Cu, Zn, Pb
Carbonate rocks
Calc-silicate
Magmatic
Meinert
et al. (2005)
Reduced plutonrelated Au
5
Sheeted veins,
stockworks
As, Bi, Te,
W, Mo
Felsic plutons +
wall rocks
Alkali feldspar,
sericitic
Magmatic
Thompson
et al. (1999)
Oxidized plutonrelated Au
4
Sheeted veins,
stockworks
Zn, Pb, Cu, Mo
Felsic plutons +
wall rocks
Alkali feldspar,
sericitic
Magmatic
Sillitoe (1991)
As, Sb, Hg, Tl
Impure
carbonate rocks
Decalcification,
silicification
Mixed magmaticmeteoric, meteoric, Cline
or metamorphic
et al. (2005)
As, Te
Greenschist-facies
metavolcanosedimentary rocks
Sericite
(Cr mica)carbonate
Metamorphic
and/or deepseated magmatic
Porphyry Au
Sedimenthosted Au
Orogenic Au
1 Based
21
Disseminated
13
Veins,
stockworks
Goldfarb
et al. (2005)
on production and/or reserves plus resources (measured + indicated categories), as listed in Table 2
genetically related plutons: deposits related to relatively reduced, magnetite-poor (and commonly ilmenite-bearing) intrusions contain minor amounts of lithophile elements but no
appreciable base metals (Thompson et al., 1999), whereas
those with oxidized, magnetite-bearing intrusions contain
modest amounts of base metals (Table 1). The orogenic
deposits are typically in the form of quartz veins and vein arrays. Clearly, depth of erosion is a cogent control on the distribution of outcropping gold deposit types. For example,
many epithermal deposits are removed once erosional depths
exceed 1 km (Sillitoe, 1993; Fig. 1) whereas orogenic gold deposits are only exposed once erosion has reached paleodepths
ranging from 3 to 15 km (Goldfarb et al., 2001).
The perceived role of magmatism as a source of the goldbearing fluids responsible for the different deposit types is far
from straightforward. Porphyry and pluton-related gold deposits are generally accepted to be products of dominantly
magmatic fluids derived from subjacent parental intrusions,
and a similar—albeit generally more distant—magmatic connection is now widely accepted for epithermal deposits (e.g.,
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Sillitoe and Hedenquist, 2003; Table 1). However, continued
discussion surrounds the origin of sediment-hosted gold deposits, with heated meteoric, metamorphic, and diluted magmatic fluids preferred by different investigators (see Muntean
et al., 2004). Orogenic gold deposits are widely considered to
lack a close genetic association with intrusions (Goldfarb et
al., 2001), although both deeply sourced metamorphic and
magmatic fluids remain viable alternatives (Table 1). It should
be cautioned, however, that distinction between pluton-related and orogenic gold deposits is commonly not straightforward because both typically formed from dilute, CO2-rich fluids as a consequence of their relatively deep settings (Sillitoe
and Thompson, 1998; Groves et al., 2003). A case in point is
the Pataz-Parcoy belt in northern Peru (Fig. 2), which is assigned herein to the oxidized pluton-related category because
of the close association of the quartz veins with porphyry
dikes, but considered as orogenic by others (Haeberlin et al.,
2004). Of course, if pluton-related and orogenic gold deposits
were both products of magmatic fluids, the distinction would
become largely arbitrary anyway.
665
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RICHARD H. SILLITOE
TABLE 2. Summary Characteristics of Principal Gold Belts and Isolated Deposits in the American Cordillera1
Gold belt/
deposit
Au content
(Moz)2
Preeminent
deposit(s)
Main (subsidiary)
Au deposit type(s)
Kuskokwim
belt, Alaska
23
Donlin
Creek
Pebble, Alaska
82
FairbanksTombstone belt,
Alaska-Yukon
Territory
Klondike
district, Yukon
Territory
Mineralization
age, Ma
Regional
tectonic setting
Related
magmatic rocks
Synthetic
reference(s)
Reduced
pluton-related
71–65
Back-arc
transpression
Reduced calcalkaline stocks
and dikes
Hart et al. (2002),
Goldfarb
et al. (2004)
Pebble
Porphyry Cu-Au
~90
Uncertain, possible compression
Calc-alkaline
stocks and sills
Bouley
et al. (1995)
29
Fort Knox,
Pogo
Reduced
pluton-related
104; 93-91
Back-arc
weak extension
Reduced calcalkaline stocks
and plutons
McCoy et al.
(1997), Hart
et al. (2002)
13
Klondike
placers
Orogenic
Early-Middle
Jurassic (?)
(<178)
Localized extension following
compression
None
Rushton et al.
(1993), MacKenzie
et al. (2007)
Calc-alkaline
plutons in nearby
batholith
Goldfarb et al.
(1988, 1997), Miller et al. (2000)
Juneau belt,
Alaska
12
Kensington
Orogenic
56–53
Forearc
transpression
Independence
trend, Nevada
14
Jerritt Canyon
Sediment-hosted
~42–36
Low-magnitude
extension
Calc-alkaline
dikes
Cline et al. (2005)
Getchell trend,
Nevada
35
Twin Creeks
Sediment-hosted
~42–36
Low-magnitude
extension
Calc-alkaline
dikes
Cline et al. (2005)
~42–36
Low-magnitude
extension
Calc-alkaline
felsic dikes
Cline et al.
(2005), Ressel and
Henry (2006)
Calc-alkaline
felsic stocks
+ dikes
Cline et al. (2005)
Carlin trend,
Nevada
Battle MountainEureka trend,
Nevada
105
Goldstrike
Sediment-hosted
57
Pipeline,
Cortez Hills
Sedimenthosted (skarn)
~42–36
Low-magnitude
extension
Midas
Low-sulfidation
epithermal
16–14
Backarc extension
Tholeiitic basaltrhyolite volcanics
John (2001)
Bingham
Canyon
Porphyry Cu-Au
(skarn, sediment
hosted)
39–37
Initiation of
extension or
termination of
compression
High-K calcalkaline stocks
and volcanics
Babcock
et al. (1995)
Round
Mountain
Low-sulfidation
epithermal
26
Extension
Calc-alkaline
volcanics
Henry
et al. (1996)
Comstock
Lode
(low-sulfidation,
high-sulfidation)
epithermal
21–4
Transtension
Calc-alkaline
volcanics and
stocks
John (2001)
135–115
Forearc
transpression
Calc-alkaline
plutons in
nearby batholith
Böhlke and
Kistler (1986),
Marsh et al. (2007)
31.6–28.4
Transition from
compression
to backarc
(Rio Grande
rift) extension
Bimodal alkaline
volcanics and
minor intrusions
Kelley et al.
(1998), Rampe
et al. (2005)
Extension
Calc-alkaline
volcanics and
stocks
Albinson et al.
(2001), Camprubí
et al. (2003)
Extension (?)
Inferred calcalkaline stock
Kesler et al.
(1981), Sillitoe
et al. (2006)
Extension
Oxidized calcalkaline plutons
González (2001),
AngloGold
Ashanti (2007)
Northern Nevada
rift, Nevada
11
Bingham
district, Utah
55
Round Mountain,
Nevada
14
Walker Lane,
Nevada
Sierra Foothills
belt, California
Cripple Creek,
Colorado
Sierra Madre
Occidental,
Mexico
Pueblo Viejo,
Dominican
Republic
Segovia belt,
Colombia
47
100
Mother Lode
Orogenic
30
Cripple
Creek
Alkaline lowsulfidation
epithermal
22
Tayoltita,
El Oro
epithermal
Pueblo Viejo
High-sulfidation
epithermal
Frontino
Oxidized
pluton-related
24
24
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36–27
~77–62
~160
666
667
TABLE 2. (Cont.)
Gold belt/
deposit
Middle Cauca
belt, Colombia
Chocó belt,
Colombia
Zamora belt,
Ecuador
Pataz-Parcoy
belt, Peru
CajamarcaHuaraz belt,
Peru
El IndioMaricunga belt,
Chile
Farallón
Negro district,
Argentina
Patagonian
province,
Argentina-Chile
Au content
(Moz)2
16
Preeminent
deposit(s)
Main (subsidiary)
Au deposit type(s)
Mineralization
age, Ma
Colosa,
Marmato
Porphyry Au
(intermediatesulfidation
epithermal,
porphyry Cu-Au)
8–6
55–43
(porphyry CuAu prospects)
Regional
tectonic setting
Related
magmatic rocks
Synthetic
reference(s)
Transpression
Calc-alkaline
volcanics and
stocks
Cediel
et al. (2003)
Probably extension
Calc-alkaline
stocks
Sillitoe et al.
(1982), AngloGold Ashanti
(2007)
Transtension
Calc-alkaline
volcanics, stocks,
and plutons
Fontboté
et al. (2004)
Compression
Oxidized calcalkaline plutons
and dikes
Haeberlin
et al. (2004)
Episodic
compression
Medium- to
high-K calcalkaline volcanics
and stocks
Noble and McKee
(1999), Longo
and Teal (2005)
Kay et al. (1994),
Bissig et al.
(2001, 2003)
10
Placers
Uncertain
23
Fruta del
Norte,
Nambija
epithermal (skarn)
Pataz
Oxidized
pluton-related
Yanacocha
High-sulfidation
epithermal
(porphyry Cu-Au)
17–8.4
Pascua-Lama,
El Indio
High-sulfidation
epithermal
(porphyry Au)
El Indio:
~10–6.2
Maricunga:
23–21; 12–10
Postcompression
relaxation
Medium- to
high-K calcalkaline volcanics
and stocks
19
Bajo de la
Alumbrera
Porphyry Cu-Au
(intermediatesufidation
epithermal)
8–4.9
Initiation of
compression
High-K calcalkaline stocks
and volcanics
Sasso and
Clark (1998)
15
Cerro
Vanguardia
Low-sulfidation
epithermal
Back-arc extension
Calc-alkaline
rhyolite-basalt
volcanics
Schalamuk
et al. (1997)
>10
86
81
~155–146
314–312
~160–150
1 Gold
2
belts and isolated deposits listed from north to south (Fig. 2)
Production and/or reserves plus resources (measured + indicated categories)
Epithermal, porphyry, and sediment-hosted deposits account for three-quarters of Cordilleran gold endowment
(Table 1). Sediment-hosted deposits clearly dominate in western North America, whereas high-sulfidation epithermal
deposits are preeminent in the Andes, including the
Caribbean region (Fig. 2; Table 2). These types are followed
in importance by orogenic and pluton-related deposits if appreciable quantities of derivative placer gold are included
(Fig. 2). Individually, the low- and intermediate-sulfidation
epithermal as well as the skarn-type deposits make only modest contributions to the total Cordilleran gold inventory.
Tectonomagmatic Settings
High-sulfidation epithermal gold deposits
The Cajamarca-Huaraz belt in northern Peru and El
Indio-Maricunga belt in northern Chile, each ~400 km long,
and the Pueblo Viejo district in the Dominican Republic are
the three main concentrations of large high-sulfidation epithermal gold deposits in the American Cordillera (Fig. 2;
Table 2), although smaller examples occur in the Walker
Lane, Nevada (Goldfield, Paradise Peak; Fig. 3) and Sierra
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Madre Occidental, Mexico gold belts, and elsewhere. The
large, high-sulfidation deposits are hosted by either voluminous, long-lived volcanic complexes of medium-K andesiticdacitic-rhyolitic composition (e.g., Yanacocha, northern Peru;
Longo and Teal, 2005) or basement rocks unaffected by appreciable contemporaneous volcanism (e.g., Pascua-Lama,
northern Chile-Argentina: Bissig et al., 2001; Pueblo Viejo,
Dominican Republic: Sillitoe et al., 2006).
Both the Cajamarca-Huaraz and El Indio-Maricunga belts
occur within the two late Miocene to Recent, amagmatic, flatslab segments of the central Andes defined by Barazangi and
Isacks (1976). However, the temporal and, hence, genetic relationships between the major high-sulfidation gold deposits
and the slab flattening and eventual cessation of magmatism
are not everywhere the same.
In the Cajamarca-Huaraz belt, the oldest major high-sulfidation gold deposits (e.g., Alto Chicama, Pierina), along with
the porphyry copper-gold deposits at Minas Conga and Cerro
Corona (Fig. 4), were formed from ~17 to 14 Ma (Noble and
McKee, 1999; Gustafson et al., 2004) during a noncompressive interval between the Quechua I (19.5–17 Ma) and
Quechua II (~9 Ma) tectonic pulses (Noble and McKee,
667
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RICHARD H. SILLITOE
Kuskokwim belt
RPR 20(3)
Fairbanks-Tombstone belt
RPR 20(9)
Independence trend
SEDH 14
Klondike
ORO (13)
60°
Carlin trend
SEDH 105
Pebble
PCu 82
Juneau belt
ORO 12
Getchell trend
SEDH 35
Homestake
ORO 40
Bingham district
PCu, skarn,sedh55
North Nevada rift
LS 11
Battle Mountain-Eureka
trend SEDH, skarn 57
Sierra Foothills belt
ORO 35(65)
Cripple Creek
LS 30
Walker Lane
LS, IS,hs, sedh 47
(NB Abbreviations for major deposit types
in upper case)
15°
57(5) Contained Au (placer Au),
in million oz
Segovia belt
OPR 13(11)
Sierra Madre Occidental
LS, IS22
0°
Chocó belt
(10)
Middle Cauca belt
PAu, is, pCu 15(1)
Zamora belt
IS, skarn 22(1)
15°
Pataz-Parcoy belt
OPR >10
Cajamarca-Huaraz belt
HS, pCu, ls 79(7)
30°
El Indio-Maricunga belt
HS, pAu81
1000 km
30°
Pueblo Viejo
HS 24
Round Mountain
LS 14
Pacific Ocean
45°
Farallón
Negro district
PCu, is19
Patagonian province
LS 15
SEDH Sediment hosted
ORO Orogenic
RPR Reduced pluton related
OPR Oxidized pluton related
PCu Porphyry Cu-Au
PAu Porphyry Au
SKARNSkarn
HS
High-sulfidation epithermal
IS
Intermediate- sulfidation epithermal
LS
Low-sulfidation epithermal
45°
Miocene
Oligocene
Eocene
Late Cretaceous
Early Cretaceous
Jurassic
Carboniferous
45°
60°
75°
90°
105°
120°
135°
150°
175°
Proterozoic
FIG. 2. Principal gold belts and isolated major deposits in the North and South American Cordillera. The box for each belt
shows the gold deposit type(s) (abbreviated names in black), the gold content expressed as million ounces (in red), and the
general age (color of box). Data from multiple sources, including Table 2 and references therein.
1999). In contrast, development of the giant Yanacocha highsulfidation gold district spanned the 13.6 to 8.2 Ma interval
(Longo and Teal, 2005), temporally closer to the final cessation of magmatism in the Peruvian flat-slab segment at ~4 Ma
(Noble and McKee, 1999). A change at Yanacocha from dominantly andesitic volcanism to emplacement of small volumes
of highly oxidized, dacitic to rhyolitic magma at 9.8 Ma
(Longo and Teal, 2005) may chart an advanced stage of slab
flattening, contraction, and crustal thickening.
The major high-sulfidation gold deposits in the El IndioMaricunga belt (Pascua-Lama, Veladero, and El IndioTambo) formed from ~10 to 6.2 Ma (Jannas et al., 1999; Bissig et al., 2001; Charchaflié et al., 2007), after andesitic
volcanism in the segment had ceased (Kay et al., 1999) and
compressive tectonism had migrated eastward (Bissig et al.,
2003; Charchaflié et al., 2007). The gold mineralization accompanied volumetrically minor silicic magmatism (cf.
Yanacocha district) in tectonically thickened crust at high elevations during flat-slab subduction (Kay and Mpodozis,
2001, 2002; Bissig et al., 2003). However, the upper crust
may have been weakly extended in places at the time of gold
0361-0128/98/000/000-00 $6.00
mineralization as a result of gravitational instability consequent upon the high elevation (e.g., Veladero: Charchaflié
et al., 2007).
Recently presented alteration and lithogeochemical evidence suggests that the Pueblo Viejo high-sulfidation gold deposits in the Dominican Republic were generated as part of a
regionally extensive Late Cretaceous to Paleogene volcanoplutonic arc (e.g., Lebrón and Perfit, 1994) rather than
being genetically related to the hosting bimodal volcanic succession of island-arc tholeiite composition and Early Cretaceous age (Sillitoe et al., 2006). The tectonic regime that prevailed in the Late Cretaceous to Paleogene arc, defined by
both calc-alkaline intrusive and extrusive rocks, remains to be
defined, although extensional conditions are a distinct possibility given the inferred existence of a broadly contemporaneous back-arc basin (Mann et al., 1991).
Cross-arc zones of Miocene tectonomagmatic activity, up to
several tens of kilometers wide, appear to have localized the
Alto Chicama and Yanacocha high-sulfidation gold deposits in
the Cajamarca-Huaraz belt. In the case of Yanacocha, Quiroz
(1997), followed by Turner (1999) and Longo and Teal
668
669
120°
112°
Idaho
Oregon
Nevada
40°
INDEPENDENCE
TREND 14
120°
National
Twin Creeks
40°
Gold Quarry
UINTA AXIS
Mule Canyon
Reno
Sri =
< 0.706
Comstock
Lode
Sri =
> 0.706
Paradise
Peak
Mother
Lode
district
Buckhorn
CARLIN
TREND
105
55
Metallogenic Belt Age
Goldfield
Early Cretaceous
Oligocene (26 Ma)
NORTHERN
NEVADA RIFT
10
ROUND
MOUNTAIN
14
Bingham
Mercur
Eocene
Miocene (20 – 4 Ma)
BATTLE MNT EUREKA TREND
57
Miocene (16 – 14 Ma)
Bodie
SIERRA
FOOTHILLS BELT
35 (65)
Arizona
Barneys Canyon
PipelineCortez Hills
Grass
Valley
Salt Lake City
Post / Betze
Carlin
Copper
Canyon
district
Alleghany
California
Utah
32°
Jerritt Canyon
Getchell
GETCHELL
TREND
35
Pacific
Ocean
Midas Ivanhoe
Sleeper
GREAT
BASIN
112°
250 km
WALKER LANE
47
Deposit Type
Porphyry Cu-Au
Skarn
Sediment-hosted Au
Orogenic Au
Low-sulfidation Au
Intermediate-sulfidation Au-Ag
High-sulfidation Au ± Ag
FIG. 3. Major gold belts and isolated major deposits in the Great Basin and surrounding regions of the western United
States, showing their ages and gold contents (numbers beneath belt names). The number in parenthesis is placer gold content. Several important deposits in each belt, assigned to deposit types, are also marked. Data from multiple sources, including Table 2 and references therein. Dot-dash line shows the initial 87Sr/86Sr = 0.706 isopleth (Sri) for Meso-Cenozoic plutonic rocks (after Kistler, 1991), which is inferred to mark the western craton margin. Inset marks the position of the Great
Basin physiographic province, mentioned in the text
(2005), defined the 200-km-long, northeast-striking, Chicama-Yanacocha structural corridor, which encompasses the
25-km-long alignment of volcanic and hydrothermal centers
at Yanacocha as well as nearby high-sulfidation gold and porphyry copper-gold deposits (Fig. 4a). An apparently similar,
but northwest-striking basement feature is present in the El
Indio part of the El Indio-Maricunga belt, with the 6-km-long
Pascua-Veladero trend coincident with its northern margin
(Bissig et al., 2001; Charchaflié et al., 2007; Fig. 4b). A comparable transverse structure was also recently proposed as a
localizing influence on the Goldfield high-sulfidation gold deposit in the Walker Lane (Berger et al., 2005).
Low-sulfidation epithermal gold deposits
Two of the less important gold belts in the western Americas, the Patagonian province in southern Argentina and contiguous Chile and the Northern Nevada rift in Nevada, are
dominated by low-sulfidation epithermal deposits. The
Walker Lane in western Nevada and Sierra Madre Occidental belt in Mexico also contain several additional examples
(Fig. 2; Table 2). The Patagonian province and Walker Lane
cover extensive areas, ~200,000 km2 in the case of the former,
which encompasses the easternmost foothills of the Southern
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Andes (Esquel deposit) besides the extra-Andean Somuncura
and Deseado massifs (Cerro Vanguardia deposit; Fig. 5a). In
contrast, the Northern Nevada rift is >500 km long and 4 to 7
km wide, but is paralleled westward by other similar coincident gravity and magnetic trends (Ponce and Glen, 2003; Fig.
5c). The most important concentrations of low-sulfidation
epithermal gold in the western Americas, however, are the
isolated Round Mountain, Nevada (Fig. 3) and Cripple
Creek, Colorado (Fig. 2) deposits, which are of disseminated
and subordinate vein type and vein and subordinate disseminated type, respectively, rather than exclusively vein systems
as in the Patagonian province, Northern Nevada rift, Walker
Lane, and Sierra Madre Occidental (Table 2).
The two low-sulfidation epithermal-dominated belts, the
Patagonian province and Northern Nevada rift, possess a
number of tectonomagmatic similarities. Compositionally bimodal, rhyolite and basalt-basaltic andesite volcanic suites
characterize both belts, but rhyolite dominates the former
province whereas basalt is volumetrically preeminent in the
latter (Kay et al., 1989; Pankhurst et al., 1998; John, 2001).
The silicic component in Patagonia constitutes oxidized, peraluminous, medium- to high-K calc-alkaline ignimbrites,
defining an ash-flow caldera province (Kay et al., 1989;
669
670
RICHARD H. SILLITOE
78° 30’ W
70° 00’ W
Pascua-Lama
Tantahuatay
Cerro Corona
Veladero
Yanacocha district
29° 30’ S
Sipan
Minas Conga
6° 00’ S
El Galeno
Río del Medio
El Indio
0
10
0
km
a
b
Fault
10
km
El Tambo
Deposit type
Reverse fault
High-sulfidation epithermal Au±Ag
Transverse fault / lineament
Intermediate-sulfidation epithermal Au
Structural corridor limits
Porphyry Cu-Au
FIG. 4. Examples of arc-transverse structures and their relationships to gold and copper-gold deposits in the two main
high-sulfidation epithermal gold belts of the American Cordillera. The arcs trend roughly perpendicular to the transverse
structures. a. Northeast-striking Chicama-Yanacocha structural zone in the Cajamarca-Huaraz belt, northern Peru (after
Quiroz, 1997, and Longo and Teal, 2005). b. Northwest-striking structural zone in the El Indio part of the El Indio-Maricunga belt, northern Chile (after Bissig et al., 2001 and Charchaflié et al., 2007).
Pankhurst et al., 1998). In contrast, the rhyolites of the
Northern Nevada rift comprise volumetrically restricted
domes and dikes that are chemically reduced and plot in the
tholeiitic field (John, 2001). The silicic volcanic rocks are
inferred to be products of lower-crustal anatexis in response
to mafic underplating (Pankhurst and Rapela, 1995; John,
2001).
In concert with the bimodal volcanic association, both the
gold belts were generated during extension in back-arc settings while subduction-related arc magmatism was active farther west. The Chon Aike rhyolites accumulated in a series of
regionally continuous, north-northwest–striking half grabens,
which underwent reactivation during the low-sulfidation epithermal gold mineralization and accompanying final-stage
(<160 Ma) volcanism in the western parts of the province
(Pankhurst et al., 2000; Ramos, 2002). The Northern Nevada
rift and similar parallel structures farther west, all underpinned by mafic dike swarms (Zoback et al., 1994; Ponce and
Glen, 2003), were the sites of low-sulfidation epithermal gold
mineralization during a short interval (16–14 Ma) at the end
of the early rift-related volcanism (John, 2001).
The Chon Aike rhyolite province and Northern Nevada rift
are both products of mantle plume activity: the DiscoveryShona-Bouvet group of plumes for the former and the inception of the Yellowstone hotspot for the latter (Riley et al.,
0361-0128/98/000/000-00 $6.00
2001; Zoback et al., 1994; Glen and Ponce, 2002; Fig. 5b, c).
The Patagonian gold province is located at the paleo-Pacific
margin of Gondwana and—along with the partially timeequivalent Karoo and Ferrar flood basalt provinces (Fig.
5b)—formed during rifting preparatory to South Atlantic
Ocean opening and breakup of western Gondwana
(Pankhurst et al., 1998, 2000; Riley et al., 2001). In contrast,
the Northern Nevada rift aborted before any separation of
the North American plate could take place.
The low-sulfidation epithermal gold deposits in the Walker
Lane (e.g., Bodie) along with the Round Mountain deposit
just to the east (Fig. 3) are associated with rhyolitic centers,
including a major ash-flow caldera at Round Mountain
(Henry et al., 1996). The Walker Lane is a dextral transtensional belt (Stewart, 1988), whereas extension prevailed at 26
Ma when the Round Mountain deposit was formed and in
the Sierra Madre Occidental during the Oligocene (El Oro
low-sulfidation epithermal deposit). Cripple Creek, located
along the eastern margin of the North American Cordillera
(Fig. 2), has a very different magmatic affiliation, being part
of a diatreme complex related to bimodal phonolite-alkaline
lamprophyre rocks (Kelley et al., 1998). The alkaline magmatic activity may be related to small degrees of mantle
melting triggered by the transition, between 40 and 35 Ma
(Chapin and Cather, 1994), from Laramide compression to
670
671
a
70°
74°
66°
62 °
40°
Esquel
44°
48°
Cas
cad
es
48°
Gastre
fault
zone
Cerro Bayo
High
Yellowstone
hot spot
40 °
Midas
rra
Sie
500 km
South margin
of Farallon
Plate
East Antarctica
FERRAR
South
America
CHON
AIKE
20 Ma
da
va
Ne
10 Ma
KAROO
National
Sleeper
Au deposit
Africa
Columbia
River
basalts
McDermitt
caldera 16.5 Ma
Chon Aike
volcanic
province
Pacific
Ocean
c
Pacific
Ocean
Cerro Vanguardia
Manantial Espejo
52 °
112°
120°
128°
Atlantic
Ocean
Nevada
Ivanhoe
Mule Canyon
Buckhorn
Northern
Nevada
rift axis
500 km
New
Zealand
Antarctic
Peninsula
2000 km
b
FIG. 5. Geologic comparison of the Patagonian and Northern Nevada rift low-sulfidation epithermal gold belts and their
relationship to hot spots. a. Outcropping Chon Aike Group volcanic rocks (from Riley et al., 2001) and main gold deposits in
the Patagonian province. b. Large Igneous Provinces (Chon Aike rhyolites and Karoo and Ferrar basalts) related to the Discovery-Shona-Bouvet plume group (from Riley et al., 2001). c. Northern Nevada rift and similar magnetic feature farther
west (after Ponce and Glen, 2003; heavy dashed lines), main low-sulfidation epithermal gold deposits, present and approximate initial (16.5 Ma caldera) positions of the Yellowstone hot spot, hot spot-related Columbia River Basalt Group, and western graben of Snake River Plain (hachured lines). East-west lines along the Pacific coast show approximate southern margin
of the Farallon plate that was being subducted beneath North America at the ages indicated (from John et al., 2000).
the extensional regime that culminated in opening of the Rio
Grande rift (Kelley et al., 1998; Rampe et al., 2005).
Intermediate-sulfidation epithermal gold deposits
Intermediate-sulfidation epithermal deposits, mostly of
vein type, make important contributions to the 600-kmlong Walker Lane in Nevada (Comstock Lode; Fig. 3), the
750-km-long Sierra Madre Occidental belt in Mexico (Tayoltita), the 100-km-long Zamora belt in southern Ecuador
(Fruta del Norte), and the 300-km-long Middle Cauca belt in
Colombia (Marmato; Fig. 6). All these deposits are closely associated with medium- to high-K calc-alkaline volcanism,
which ranges from andesitic in the Comstock Lode area (Castor et al., 2005) to voluminous rhyolitic ignimbrites in the
Sierra Madre Occidental (McDowell and Clabaugh, 1979;
Ferrari et al., 2007).
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As noted above, the Walker Lane developed under dextral
transtensional conditions (Stewart, 1988) and the Sierra
Madre Occidental belt is part of an extensional arc, the southern continuation of the Basin and Range province of the western United States (Henry and Aranda-Gomez, 1992; Staude
and Barton, 2001; Ferrari et al., 2007). Extension in the Sierra
Madre Occidental culminated in opening of the Gulf of California (Ferrari et al., 2007). Although not well studied, the
Late Jurassic Zamora belt may have been extensional (Table
2) prior to the onset of Late Cretaceous collision-related compression (Pratt et al., 2005). Generation of the Fruta del
Norte deposit, the main product of this event, overlapped
with subaerial graben formation, coarse clastic sedimentation,
and nearby dacitic ash-flow eruption (Stewart and Leary,
2007). The Middle Cauca belt lies along the Cauca-Romeral
fault system, the site of a collisional suture, and appears to
671
672
RICHARD H. SILLITOE
75°W
0°
SEGOVIA
BELT
13 (11)
PANAMA
ANTIOQUIA
BATHOLITH
3 (2)
CHOCO BELT
(10)
VENEZUELA
Segovia
Medellín
Angostura
CALIFORNIA
DISTRICT
7
Marmato
MIDDLE CAUCA BELT
15 (1)
5°S
Bogotá
Colosa
Cali
Western
craton
edge
Metallogenic Belt Age
Jurassic
Late CretaceousEarly Tertiary
Eocene (?)
COLOMBIA
Miocene
10°S
Deposit Type
Porphyry Au
ECUADOR
Intermediate-sulfidation
250 km
75°W
Oxidized pluton related
FIG. 6. Principal gold belts and districts in the northern Colombian Andes,
showing their ages and gold contents (numbers beneath belt names). The
numbers in parentheses are placer gold contents. Note that the Antioquia
batholith and California districts, estimated to contain <10 Moz Au, are not
discussed in the text. The approximate limit between continental crust to the
east and oceanic accreted terranes to the west is also shown. Data from multiple sources, including Table 2 and references therein; and AngloGold
Ashanti (2007).
have been subjected to dextral transpression during the
Miocene (Rossetti and Colombo, 1999; Cediel et al., 2003)—
a prominent exception to the extensional or transtensional
settings of the other major intermediate-sulfidation epithermal gold deposits.
Porphyry copper-gold deposits
The three preeminent porphyry copper-gold deposits,
Bingham Canyon in the Bingham district of Utah, Bajo de la
Alumbrera in the Farallón Negro district of northwestern Argentina, and Pebble in Alaska, dominate the major, isolated
gold concentrations rather than helping to define belts (Fig.
2). However, relatively small porphyry copper-gold deposits
also contribute to the gold inventory of the CajamarcaHuaraz belt in northern Peru (Figs. 2, 4a).
0361-0128/98/000/000-00 $6.00
The Bingham and Farallón Negro districts occupy a superficially similar position at the landward extremity of the
American Cordillera (Fig. 2). Both are centered on high-K
calc-alkaline porphyry stocks that have partially preserved coeval volcanic edifices (Waite et al., 1997; Sasso and Clark,
1998). Waite et al. (1997) also hypothesized a primitive mafic
alkaline contribution to the Bingham magmas because of
their unusual enrichment in chromium, nickel, and barium.
In both districts, as well as at Pebble, the magmatic rocks
have typical supra-subduction zone petrochemistry.
Bajo de la Alumbrera and other porphyry copper-gold deposits and prospects in the Farallón Negro district were emplaced above a rapidly shallowing subduction zone during the
initiation of contractional tectonism, crustal shortening, and
surface uplift. The resulting high-angle reverse faults led to
formation of the basement-cored blocks that characterize the
northernmost Sierras Pampeanas structural province (Ramos
et al., 2002). Uplift was initiated some time between 7.6 and
6.0 Ma and is still ongoing (Ramos et al., 2002) while porphyry copper-gold deposit generation at Bajo de la Alumbrera
spanned the 8- to 6-m.y. interval (Harris et al., 2004, 2008).
The Bingham district occupies a broadly similar position in
the context of low-angle Laramide subduction, but rather
than being linked to slab shallowing, like the Farallón Negro
district, it presaged slab steepening. Presnell (1997) related
the magmatism and ore formation to the initial onset of postLaramide extension, although end-stage contraction, basement uplift, and basin sedimentation may still have been active (Dickinson et al., 1988).
Both districts lie along fundamental transverse tectonomagmatic discontinuities: the Farallón Negro district on an
east-northeast–striking feature, the Tucumán transfer zone,
marking the southern limit of the Arequipa-Antofalla craton
(Sasso and Clark, 1998) where it intersects a northwest-oriented, trans-Andean lineament (Chernicoff et al., 2002); and
the Bingham district on the east-northeast–striking Uinta axis
(Fig. 3), the shallow manifestation of an Archean-Proterozoic
suture zone, at its confluence with other regional-scale structural elements (Billingsley and Locke, 1941; Ericksen, 1976;
Presnell, 1997). Kutina (1991) considered the Uinta axis to be
part of a mantle-penetrating structure that transgresses the
entire Cordilleran margin.
The plate-tectonic setting of the mid-Cretaceous Pebble
deposit is much less certain, although both Goldfarb (1997)
and Young et al. (1997) inferred a continental-margin arc setting, possibly during northward subduction beneath the previously accreted Wrangellia superterrane. Crustal shortening,
thickening, and uplift may have followed Wrangellia accretion
(Nokleberg et al., 2005). The structural setting of Pebble remains to be defined, although it does lie near the major,
northeast-striking Lake Clark fault zone. The small porphyry
copper-gold deposits in the Cajamarca-Huaraz belt, which
are broadly contemporaneous with the earliest high-sulfidation gold deposits (Noble and McKee, 1999; Longo and Teal,
2005), appear to coincide with a noncompressive interval (see
above).
Porphyry gold deposits
Significant porphyry gold deposits, distinguished by much
lower (e.g., <~0.25 %) copper contents than those defining
672
the porphyry copper-gold category dealt with above, are confined to the Maricunga portion of the El Indio-Maricunga
belt in northern Chile and the Middle Cauca belt in Colombia (Fig. 2). The Maricunga belt hosts porphyry gold deposits
of two discrete ages (~24–21 and ~14–11 Ma; Sillitoe et al.,
1991; McKee et al., 1994), both of which are closely associated with fine-grained diorite to quartz diorite porphyry
stocks emplaced into andesitic stratovolcanoes. These magmatic products have a medium- to high-K calc-alkaline affinity (Kay et al., 1994; Mpodozis et al., 1995). Porphyry gold
formation in the Middle Cauca belt (Fig. 6) took place in association with porphyries of similar composition (R. H. Sillitoe, personal observations, 2007).
The trace element characteristics of magmatic rocks in the
Maricunga belt suggest that during the earlier porphyry gold
event (Refugio deposit) a relatively thick crust was subjected
to regional extension as a consequence of the steep subduction angle (Kay et al., 1999; Kay and Mpodozis, 2001, 2002).
Slab flattening, contractional tectonism, crustal thickening,
and magmatic quiescence followed from 20 to 18 Ma, but
stress relaxation and mild extension were reimposed during
the later porphyry gold event responsible for the Marte,
Lobo, and Cerro Casale deposits (Kay and Mpodozis, 2001,
2002). At least the northern part of the Maricunga belt coincides with the transverse tectonomagmatic discontinuity that
influenced the localization of the Farallón Negro copper-gold
district farther east (Sillitoe et al., 1991; Sasso and Clark,
1998; see above).
The tectonic setting of the Middle Cauca belt (Fig. 6) at the
time of porphyry gold mineralization in the late Miocene (R.
Padilla, pers. commun., 2007) is less precisely defined, although dextral transpression linked to accretion of the Baudó
allochthonous oceanic terrane farther west in northwestern
Colombia certainly characterized much of the Miocene
(Cediel et al., 2003; see above).
Sediment-hosted gold deposits
The Carlin, Cortez, Independence, and Battle MountainEureka gold trends in northern Nevada are defined by the
only major sediment-hosted gold deposits in the American
Cordillera, although small deposits of this type are also part
of the Bingham district, Utah, farther east (Fig. 3). The Carlin, Cortez, and Independence trends have lengths of up to
about 130 km but widths of only 5 km, whereas the Battle
Mountain-Eureka trend is longer (320 km) as well as also
containing important skarn gold deposits (e.g., Copper
Canyon district).
Only small volumes of coeval igneous rocks, chiefly minor
felsic dikes of high-K calc-alkaline affinity, accompany the
sediment-hosted gold deposits (Hofstra et al., 1999; Ressel
and Henry, 2006). The presence of these dikes in combination with spatially related magnetic anomalies points to the
existence of an extensive subsurface plutonic complex beneath the Carlin trend (Ressel and Henry, 2006). Areally extensive volcanic fields accumulated at broadly the same time
in nearby areas as part of the mid-Tertiary southward migration of the volcanic front (Ressel and Henry, 2006).
There is general consensus that the sediment-hosted gold
trends of Nevada developed during a relatively short, late
Eocene interval (~42–36 Ma) marked by the initiation of
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673
low-magnitude extension (Hofstra and Cline, 2000; Cline et
al., 2005), but prior to the large-scale crustal attenuation that
characterized the Great Basin during the Oligocene and
Miocene. The late Eocene extension gave rise to several faultbounded, fluviolacustrine basins as well as to reactivation of
suitably oriented high- and low-angle faults in underlying Paleozoic rocks (Cline et al., 2005). At least some of the gold
trends may have acted as accommodation zones across which
extension histories differed (Tosdal and Nutt, 1999; Muntean
et al., 2001). The contemporaneous extension and volcanism
may have been triggered by processes linked to removal of
the shallowly inclined Farallon plate from the base of the
North American lithosphere, its position during the preceding Laramide orogeny (Sonder and Jones, 1999); however,
any volcanic products along the sediment-hosted gold trends
have been lost to erosion.
Notwithstanding the incipiently extended crust during the
late Eocene, the aftermath of a complex history of much older
deformation and intrusive activity had profound effects on
sediment-hosted gold deposit localization. Particularly important ore-localizing sites are provided by the following: receptive carbonate rock types juxtaposed with overlying, relatively
impermeable siliciclastic units along low-angle, reverse faults,
especially the Roberts Mountain thrust; structural culminations formed by inversion of high-angle faults during Paleozoic contractional tectonism; and Mesozoic plutons that
caused perturbation of the late Eocene stress field and helped
to focus fluid upflow (Cline et al., 2005 and references
therein).
The gold trends of northern Nevada are located near the
western margin of the North American craton (Fig. 3) and are
inferred to reflect the basement fault fabric (Roberts, 1960).
The northwest and northeast gold trends may follow fundamental, crustal-scale faults that controlled Neoproterozoic
rifting to form a passive continental margin as well as influencing subsequent sedimentation, contractional deformation,
magmatism, and hydrothermal activity (Tosdal et al., 2000;
Crafford and Grauch, 2002; Grauch et al., 2003).
Orogenic gold deposits
The Sierra Foothills belt in California, including the Mother
Lode and Grass Valley districts plus a major contribution from
derivative placer deposits of Eocene and younger age (Lindgren, 1911; Garside et al., 2005), is the premier orogenic gold
concentration in the American Cordillera (Fig. 2; Table 2). It
dwarfs the second- and third-largest orogenic gold concentrations, the Juneau belt in southeastern Alaska and Klondike district in the Yukon Territory, the latter dominated by Plio-Pleistocene placers derived from numerous, apparently small
orogenic gold veins (Rushton et al., 1993; Knight et al., 1999;
Fig. 2; Table 2). The primary gold deposits comprise quartz
veins and veinlet swarms, the largest of which in the Sierra
Foothills belt were exploited downdip for >1 km (Knopf,
1929). The overall geologic settings of the Sierra Foothills and
Juneau belts, 270 and 200 km long, respectively, but both no
more than 5 km wide, have many similarities, notwithstanding
their appreciable age difference (see below). The Klondike
placer district occupies an area of approximately 1,200 km2.
The Sierra Foothills and Juneau belts both occur on the
western, fore-arc sides of Andean-type magmatic arcs defined
673
674
RICHARD H. SILLITOE
by the major Sierra Nevada and Coast batholiths, respectively (Fig. 7). The deposits reveal intimate relationships to
major fault zones, but no clear-cut connections to individual
intrusions. Nonetheless, both belts were generated during
emplacement of the flanking calc-alkaline batholiths. New
40Ar/39Ar ages for gold mineralization in the Sierra Foothills
belt suggest formation between ~135 and 115 Ma (Marsh et
al., 2007). Emplacement of the western parts of the Sierra
Nevada batholith, immediately east of the Sierra Foothills
belt (Fig. 7a), overlaps with this Early Cretaceous interval
(Bateman, 1992). Moreover, just south of the gold belt, the
batholith transgresses the Sierra Foothills terrane, which
continues only as a series of contained roof pendants (e.g.,
Wolf and Saleeby, 1995; Fig. 7a), and subjacent plutons remain a distinct possibility farther north where erosion level is
shallower. Gold mineralization in the Juneau belt is dated at
56 to 53 Ma, essentially coeval with Early Eocene (55 ± 2
Ma) Coast batholith emplacement (Miller et al., 1994; Goldfarb et al., 1997). Gold introduction in the Klondike district,
probably in the Early to Middle Jurassic (Table 2), was not
accompanied by any known intrusive activity (MacKenzie et
al., 2007).
The primary orogenic gold mineralization of the Sierra
Foothills, Juneau, and Klondike is hosted by lithotectonic terranes that underwent deformation and greenschist facies
metamorphism during their accretion to the North American
craton. The accretion events clearly predated the gold mineralization, by several tens of million years in the case of both
the Sierra Foothills and Juneau belts (e.g., Dickinson, 2004,
2006). The gold deposits are hosted by steep, relatively minor
brittle faults and shears related to regionally extensive reverse
fault zones that display complex and incompletely understood
ductile and brittle histories (e.g., Paterson and Wainger, 1991;
Wolf and Saleeby, 1995). These structures are exemplified by
the Melones fault zone (Fig. 7a), which contains abundant
serpentinized ultramafic bodies and acts as a fundamental
control to the Mother Lode and other deposits in the Sierra
Foothills belt (Knopf, 1929; Landefeld and Silberman, 1987;
Böhlke and Kistler, 1986; Fig. 7a). These major fault zones
appear to be crustal-scale, terrane-bounding features active
FIG. 7. Geologic comparison of the Sierra Foothills, California, and Juneau, Alaska, orogenic gold belts and their flanking
batholiths, taken from Böhlke and Kistler (1986) and Goldfarb et al. (1997). The gold deposits lie along terrane-bounding
fault zones, abbreviated as follows: BMFZ, Bear Mountains fault zone; MFZ, Melones fault zone; FFZ, Fanshaw fault zone;
SFZ, Sumdum fault zone.
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674
during mid-Jurassic island-arc accretion responsible for the
Nevadan orogeny (e.g., Schweickert and Cowan, 1975; Paterson and Wainger, 1991; Gehrels, 2000; Hildenbrand et al.,
2000); however, fault initiation may have been as part of the
California-Coahuila sinistral transform of Permo-Triassic age
(Dickinson, 2006).
Miller et al. (1994, 2000) and Goldfarb et al. (1997) proposed that generation of the Juneau gold belt immediately
followed the transition from orthogonal to oblique subduction
of the Kula oceanic plate, an event that initiated dextral transpression along the controlling fault zones as well as concomitant uplift. A broadly similar scenario may be envisioned for
the origin of the Sierra Foothills gold belt, which is coeval
with reinvigorated eastward subduction of the Farallon
plate—following the Nevadan collisional orogeny—and
batholithic arc construction (Ernst et al., 2008). At that time,
the major crustal-scale, gold-localizing faults may have been
reactivated in a dextral transpressive regime (Glazner, 1991).
Accretion of the Yukon-Tanana terrane, host to the Klondike
placer district, is an Early Jurassic event (e.g., Plafker and
Berg, 1994), with orogenic vein formation taking place during
localized extension that concluded the collision-induced
thrust stacking (MacKenzie et al., 2007; Table 2).
Reduced pluton-related gold deposits
Two temporally distinct belts of pluton-related gold deposits in the Yukon Territory and Alaska are commonly assigned to the Tintina gold province (Flanigan et al., 2000;
Hart et al., 2002; Nokleberg et al., 2005). The older is the
470-km-long Tombstone belt, Yukon and its extension, dextrally displaced ~430 km to the northwest, in the Fairbanks
district, Alaska, and the younger is the 550-km-long Kuskokwim belt still farther west in Alaska (Fig. 2). Both belts are
characterized by varied styles of gold mineralization, which
range from sheeted quartz veins in the apical parts of plutons
to distal disseminated, vein, and skarn-type mineralization
within their hornfels aureoles (Newberry et al., 1995; McCoy
et al., 1997; Thompson et al., 1999). Emplacement depth,
ranging from 2 to 10 km, is an important control on exposed
deposit style (Thompson et al., 1999; Lang and Baker, 2001;
Hart et al., 2002).
The deposits are related to small (<5 km across), felsic intrusions, which are parts of areally more extensive plutonic
suites. The intrusive rocks are metaluminous to locally peraluminous, calc-alkaline, isotopically evolved, and I-type
(Newberry et al., 1995; McCoy et al., 1997; Aleinikoff et al.,
2000; Hart et al., 2004); all of them are moderately reduced,
broadly spanning the boundary between Ishihara’s (1977)
magnetite and ilmenite series (Thompson et al., 1999;
Thompson and Newberry, 2000). Those in the Tombstone
belt are both magnetite and ilmenite poor (Hart et al., 2004).
High Sri (>0.709) and δ18O values suggest an appreciable
crustal contribution to the magmas (Hart et al., 2002). Although the intrusive rocks in the Fairbanks-Tombstone belt
span the ~110 to 88 Ma interval (Mortensen et al., 2000), the
most important gold mineralization appears to cluster at 92 ±
1 Ma (McCoy et al., 1997), except for the Pogo deposit, at
104.2 ± 1.1 Ma (Selby et al., 2002). The Kuskokwim belt is
appreciably younger, ~71 to 65 Ma (Hart et al., 2002).
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Most investigators assign both parts of the Tintina gold
province and its contained plutonic rocks to a subduction-related arc on the basis of overall petrochemical characteristics
(Newberry et al., 1995; Bundtzen and Miller, 1997; McCoy
et al., 1997; Aleinikoff et al., 2000; Flanigan et al., 2000). Arc
construction postdated Early Jurassic to mid-Cretaceous terrane accretion, contractional deformation, and metamorphism in southern Alaska. The Tombstone belt was generated during north- to northwest-directed, low-magnitude
extension (Rubin et al., 1995; Rhys et al., 2003; Hart et al.,
2004), whereas dextral transpression on the orogen-parallel
Iditarod-Nixon Fork fault was coincident with gold mineralization in the Kuskokwim belt (Bundtzen and Miller, 1997;
Miller et al., 2002; Goldfarb et al., 2004).
Reconstruction of the Fairbanks-Tombstone belt, by
restoring the ~430 km of Cenozoic dextral offset on the
Tintina fault, results in a broad, >400-km-wide magmatic arc
(Flanigan et al., 2000) suggestive of relatively low-angle subduction (Plafker and Berg, 1994); moreover, the FairbanksTombstone gold belt is confined to the landward periphery of
the arc. Indeed, both the Fairbanks-Tombstone and Kuskokwim belts may be considered to occupy back-arc settings
(Thompson et al., 1999; Fig. 2). Notwithstanding the inferred
existence of low-angle subduction, contractional tectonism is
not reported.
The relatively reduced nature of the host intrusions and, as
a direct consequence, the auriferous fluids themselves are
atypical of western American arc terranes and remain poorly
understood. Nevertheless, it is tempting to suggest that either
assimilation of material from the reduced, siliciclastic hostrock sequences—Neoproterozoic and Paleozoic in the Fairbanks-Tombstone belt and Late Cretaceous in the Kuskokwim belt (McCoy et al., 1997; Thompson et al., 1999;
Thompson and Newberry, 2000)—or contamination by reduced fluids expelled from them played an influential chemical role (Thompson et al., 1999; Thompson and Newberry,
2000; Hart et al., 2002, 2004).
Oxidized pluton-related gold deposits
The principal gold deposits related to oxidized plutonic
rocks are located in the northern Central Cordillera of
Colombia where they define the Segovia belt (Fig. 2; Table
2). The Segovia belt (Fig. 6), about 300 km long and up to 75
km wide, is dominated by quartz veins and vein swarms containing minor amounts of zinc, lead, and copper sulfides, although half of the gold production has come from derivative
placers. Substantially smaller is the 160-km-long Pataz-Parcoy belt in northern Peru, where the auriferous, sulfide-rich
quartz veins also contain subsidiary quantities of zinc and
lead.
Both belts occur within and around composite, calc-alkaline, metaluminous batholiths of I-type, magnetite series, and
subduction origin (González, 2001; Haeberlin et al., 2004).
Compositionally, these equigranular plutons are similar to the
porphyry stocks that host Cordilleran porphyry copper-gold
and gold deposits (see above). The Segovia batholith, considered to be of Late Jurassic age, is dominated by hornblende
diorite and tonalite intrusions, whereas the Carboniferous
(Mississippian) Pataz-Parcoy batholith comprises mainly
tonalite and granodiorite (Haeberlin et al., 2004). The
675
676
RICHARD H. SILLITOE
batholiths were emplaced into greenschist- to amphibolitegrade metasedimentary sequences, which accumulated along
the edge of the Amazon craton during the Neoproterozoic
and early Paleozoic (Cediel et al., 2003; Haeberlin et al.,
2004).
The Segovia batholith was emplaced under extensional tectonic conditions as part of the late Paleozoic through Jurassic
“Bolivar aulacogen” rifting and continental sedimentation
event in northwestern South America, which was linked to
opening of the Caribbean basin (Cediel et al., 2003). Postcollisional crustal thickening and uplift characterized the PatazParcoy belt during batholith emplacement and the subsequent gold mineralization, which took place along a major,
north-northwest–striking crustal lineament under conditions
of east-directed crustal shortening (Haeberlin et al., 2004).
HS Au
a
P Cu-Au RPR Au
b
strike-slip
fault
Tectonomagmatic Synthesis
SEDH Au
Tectonic associations
The major gold deposits and belts in the western Americas
occur both along the craton margin and in some of the many
accreted terranes. However, all the major gold deposits and
belts were generated after their host terranes were accreted
to the craton edge, a situation that contrasts with the preaccretionary timing of some other Cordilleran mineral deposits
(e.g., volcanogenic massive sulfides; McMillan, 1991; Mortensen et al., 2008). Nonetheless, the placer deposits that
constitute the Chocó belt in the northern Colombian Andes
(Fig. 6; Table 2) are likely to have been derived from preaccretionary gold concentrations, although not necessarily from
the porphyry copper-gold prospects that are the only significant deposit type currently known in the belt (Sillitoe et al.,
1982; Table 2). The craton-hosted gold deposits and belts
were formed independently of whether tectonic erosion, as in
the central Andes (Rutland, 1971), or accretion, as in much of
the North American Cordillera and northern Andes (Coney
et al., 1980; Cediel et al., 2003), dominated at the continental
edge at the time of mineralization.
Many of the major Cordilleran gold deposits and belts
were generated within active, subduction-related magmatic
arcs, although back-arc and fore-arc environments are also
mineralized in places: the low-sulfidation epithermal and reduced pluton-related gold deposits typically occur in back
arcs, whereas the orogenic gold deposits are confined to fore
arcs (Figs. 2, 8). Approximately two-thirds of the major gold
deposits and belts occupy a variety of extensional or
transtensional tectonic environments. Broadly contractional
settings, consequent upon shallow subduction, appear to be
less common at the times of major gold deposit formation,
although they host the largest high-sulfidation epithermal
gold deposits in the Cajamarca-Huaraz and El Indio-Maricunga belts, the porphyry copper-gold deposits of the Farallón Negro district, and, rather less certainly, the Bingham
Canyon and Pebble porphyry copper-gold deposits. The
Middle Cauca belt in Colombia, Kuskokwim belt in westcentral Alaska, and Sierra Foothills and Juneau orogenic
gold belts were most likely generated during transpression,
induced either by oblique subduction or, in the case of the
Middle Cauca belt, oblique terrane docking. Notwithstanding the prevalence of extension at the times of major gold
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P Cu-Au
c
crustal-scale
fault
LS, IS,
HS Au
LS Au
d
rift
mantle
plume
strike-slip
fault
ORO Au
e
batholith
terrane-bounding
fault
FIG. 8. Cartoon sections showing generation of selected gold belts at different times and places along the American Cordillera, all during active, eastdirected subduction. a. Cajamarca-Huaraz high-sulfidation (HS) belt during
the Miocene. b. Pebble porphyry (P) copper-gold deposit and Kuskokwim reduced pluton-related (RPR) gold belt in west-central Alaska during the Late
Cretaceous. c. Sediment-hosted (SEDH) gold trends of Nevada and porphyry (P) copper-gold, skarn, and sediment-hosted deposits of the Bingham
district, western United States during the Eocene. d. Walker Lane low-, intermediate-, and high-sulfidation (LS, IS, HS) and Northern Nevada rift lowsulfidation (LS) gold belts, western United States during the mid-Miocene.
e. Sierra Foothills orogenic (ORO) gold belt in California during the Early
Cretaceous. See Figures 2 and 3 for belt and deposit locations.
deposit and belt formation, the largest Cordilleran gold concentrations (Fig. 2) either coincide with (CajamarcaHuaraz, El Indio-Maricunga, and Sierra Foothills belts) or
conclude or immediately follow (Bingham district and Carlin and Battle Mountain-Eureka trends) long-standing contractional episodes. This observation may suggest that the
consequent crustal thickening and uplift may somehow have
676
been conducive to the formation of different types of major
gold concentrations.
Tectonic relaxation, extension, or transtension following contractional episodes responsible for crustal thickening accompanied the generation of several of the gold belts and isolated
deposits. This extension was due to slab steepening following
Laramide low-angle subduction in the case of the Bingham
Canyon and Cripple Creek deposits and the sediment-hosted
gold trends of Nevada, whereas tectonic relaxation consequent
upon eastward migration of crustal compression prevailed in
the El Indio part of the El Indio-Maricunga belt (Charchaflié
et al., 2007) and during generation of the younger Maricunga
porphyry gold deposits in the El Indio-Maricunga belt (Kay et
al., 1994). Exactly the opposite situation, the transition from
extension to contraction, prevailed when the CajamarcaHuaraz belt and Farallón Negro district were formed. In contrast, extensional collapse of overthickened crust produced by
terrane accretion may have occurred in the reduced plutonrelated Tombstone-Fairbanks belt of the Yukon and adjoining
Alaska.
These and some other porphyry, epithermal, sedimenthosted, and orogenic gold deposits were generated during
magmatic pulses that accompanied marked changes in tectonic regime, a scenario that has been considered to favor
major gold deposit generation elsewhere in the circum-Pacific region (Solomon, 1990; Richards, 1995; Sillitoe, 1997;
Barley et al., 2002; Mungall, 2002; Garwin et al., 2005). The
time gaps between major contractional episodes and the goldrelated tectonic regimes may be minimal (e.g., Bingham district, sediment-hosted gold trends of Nevada) or amount to a
few million (e.g., El Indio belt, Klondike) or even several tens
of million years (e.g., Sierra Foothills and Juneau belts). However, other major gold deposits in the western Americas are
not so readily correlated with tectonic change at the convergent margin, as exemplified by the epithermal deposits of the
Walker Lane and at Round Mountain in Nevada and Pueblo
Viejo in the Dominican Republic. Elsewhere in the western
Americas, the extension that was synchronous with gold mineralization was not immediately preceded by a compressive
regime, as observed in the smaller gold concentrations of the
Segovia belt of Colombia, the Patagonian province, and the
Northern Nevada rift.
Notwithstanding the fact that all gold mineralization is at
least partly structurally controlled, the positions of some of
the deposits appear to have been influenced by fundamental
arc-parallel or arc-transverse fault zones or lineaments. The
linkage between reactivation of terrane-bounding, brittleductile fault zones parallel to active arcs and the Sierra
Foothills and Juneau orogenic gold belts is widely appreciated (Goldfarb et al., 2005), as are the arc-parallel transtensional (e.g., Walker Lane) and, locally, transpressional faults
(Kuskokwim belt, Middle Cauca belt) that were active during
gold mineralization elsewhere. Arc-transverse features are
considered important in the Cajamarca-Huaraz high-sulfidation epithermal belt in northern Peru (Quiroz, 1997) and
elsewhere, as well as in the localization of the isolated porphyry copper-gold deposits in the Bingham and Farallón
Negro districts. Deep-seated fault zones inherited from a late
Neoproterozoic rifting event are proposed as first-order controls on the sediment-hosted gold trends of Nevada (Tosdal et
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677
al., 2000). Elsewhere, however, major gold concentrations,
such as the isolated Pueblo Viejo and Cripple Creek epithermal deposits and the Fairbanks-Tombstone pluton-related
belt, lack any recognized controlling structures of continental-margin scale.
Magmatic associations
This review shows that a broad petrochemical spectrum of
intrusive and/or volcanic rocks is temporally and spatially related to the major gold deposits and belts in the American
Cordillera. The most common magmatic rocks accompanying
the gold mineralization events are undoubtedly parts of oxidized, magnetite-series, medium- to high-K calc-alkaline
suites. Porphyry copper-gold and gold, high- and intermediate-sulfidation epithermal, and oxidized pluton-related deposits are everywhere genetically related to such suites. In
sharp contrast, the fractionated, calc-alkaline intrusions genetically linked to the reduced pluton-related gold deposits
are themselves moderately reduced and span the ilmenitemagnetite series transition (Thompson et al., 1999). The lowsulfidation epithermal gold belts are alone in accompanying
compositionally bimodal volcanism, which is calc-alkaline in
Patagonia (Pankhurst et al., 1998) but tholeiitic in the Northern Nevada rift (John, 2001). Alkaline rocks are not abundant
in the Cordilleran arc and back-arc environments, and the
only major gold concentration with this affiliation is the isolated Cripple Creek low-sulfidation epithermal deposit in
Colorado, where compositional bimodality is also evident
(Kelley et al., 1998).
Sediment-hosted and orogenic gold deposits have less
clear-cut magmatic connections, as noted above, although the
premier belts of both deposit types do coincide spatially and
temporally with well-defined magmatic arcs. In the case of
the sediment-hosted deposits in Nevada, minor felsic dikes
were intruded during the gold mineralization and, on the
basis of the broadly coincident magnetic anomalies, appear to
be upward offshoots of an underlying plutonic complex (Ressel and Henry, 2006). In contrast, the Sierra Foothills and
Juneau orogenic gold belts lie along the western margins of
major calc-alkaline batholiths, which may have transcrustal
thicknesses as great as 35 km (Ducea, 2001). The intrusive
rocks coeval with the sediment-hosted and orogenic gold
belts would offer a ready source for any magmatic contributions to the respective ore-forming fluids or, alternatively,
may simply have provided the thermal impetus required to either circulate or liberate nonmagmatic fluids (e.g., Jia et al.,
2003; Ressel and Henry, 2006).
Comparison with copper and tin belts
Porphyry deposits containing co- or by-product gold
and/or molybdenum completely dominate the copper inventory of the western Americas (e.g., Sillitoe and Perelló, 2005;
Perelló, 2006). Like the gold deposits dealt with in this review, porphyry copper deposits are emplaced in a variety of
tectonic settings, which range from extensional to contractional. Furthermore, the three preeminent copper belts, in
northern Chile-southern Peru (42–31 Ma), central Chile
(11–4 Ma), and southwestern North America (Laramide:
74–52 Ma; Fig. 9), were generated during intervals of regional compression, crustal thickening, and rapid surface
677
678
RICHARD H. SILLITOE
60°
Pebble
45°
Butte
Southwestern
North America
Bingham
30°
15°
Gold
Copper
0°
15°
Northern Chilesouthern Peru
30°
Central Chile
Farallón
Negro
district
1000 km
45°
60°
75°
90°
105°
120°
135°
150°
175°
45°
FIG. 9. Major copper and gold belts and isolated major deposits in the North and South American Cordillera. Note the
mutual exclusivity of the principal gold and copper concentrations, although the porphyry copper-gold deposits obviously
contain major quantities of both metals. Gold belts and isolated major deposits identified in Figures 2 and 3.
uplift and exhumation (Sillitoe, 1998; Cooke et al., 2005; Sillitoe and Perelló, 2005; Perelló, 2006). These compressional
events may be related to low-angle subduction of topographically prominent, buoyant features such as aseismic ridges on
the downgoing oceanic plates in conjunction with subduction
erosion of the fore arcs (e.g., Henderson and Gordon, 1984;
Stern, 1991; Kay et al., 2005), although gravitational delamination of the lithospheric mantle has been proposed recently
as a cause of the Laramide slab flattening (Wells and Hoisch,
2008). Additional outstanding copper concentrations occur
in the isolated porphyry copper-gold deposits at Bingham
Canyon and Pebble (Figs. 2, 9; see above), as well as the isolated, ~65 Ma (Lund et al., 2002) Butte porphyry coppermolybdenum deposit in Montana, also emplaced during
Laramide compression (e.g., Kalakay et al., 2001; Fig. 9). Nevertheless, a far greater percentage (>90%) of the copper than
of the gold (<50%; Table 2) is confined to contractional settings. In common with gold deposits, arc-transverse structures
are also considered important for porphyry copper localization by some investigators, particularly in the central Andes
and southwestern North America (e.g., Richards, 2000).
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The magnetite-series, medium- to high-K calc-alkaline
magmas responsible for copper deposit formation in the three
premier belts (e.g., Lang and Titley, 1998; Sillitoe and
Perelló, 2005, and references therein) appear to have been
largely confined to mid- to upper-crustal chambers because
the synmineralization contraction inhibited widespread arc
volcanism (e.g., Mpodozis and Ramos, 1990). The extreme
size of the giant deposits that make major contributions to
these three copper belts may be directly related to the large
size of the parent magma chambers, which may be reasonably
assumed to fractionate efficiently and evolve larger volumes
of magmatic fluid than do their smaller counterparts (Sillitoe,
1998; Richards, 2003). However, such reasoning cannot be
applied to all the major gold deposits and belts, except perhaps for the principal high-sulfidation epithermal gold concentrations, because, as noted above, many of them formed
during extension.
Nearly all Cordilleran tin resides in the Bolivian tin-silver
belt, which occupies a unique behind-arc position in the Andean Cordillera, extending from southernmost Peru through
the Eastern Cordillera of Bolivia to northwesternmost
678
Argentina (Ahlfeld, 1967; Turneaure, 1971; Fig. 10). The
intrusion-related veins were generated during three discrete
epochs: 225 to 180 and 26 to 22 Ma in the northern part of
the belt, and 20 to 12 Ma in the southern part (Grant et al.,
1979; Clark et al., 1990; Sempere et al., 2002; Fig. 10). The
Late Triassic-Early Jurassic epoch was a product of crustal
melting induced by shoshonitic mafic melts generated in a
back-arc rift environment (Clark et al., 1990; Sempere et al.,
2002). In contrast, the Miocene epochs took place under neutral to weakly extensional conditions in response to slab steepening (Clark et al., 1990), detachment of subcontinental
lithospheric mantle, and inflow of hot asthenosphere; these
events in the Eastern Cordillera of Bolivia took place some 5
m.y. after the cessation of amagmatic crustal compression and
thickening (Hoke and Lamb, 2007).
The tin mineralization is related to emplacement of peraluminous, ilmenite-series, and locally S-type magmas (Sugaki et
al., 1988; Clark et al., 1990; Lehmann et al., 1990) bearing
some similarities to those linked to the reduced plutonrelated gold (and nearby major tungsten) deposits in the back
arc of Alaska and the Yukon Territory (see above). In common
with the Alaskan and Yukon back-arc gold belts, a thick prism
of reduced, siliciclastic marine sedimentary rocks of early
Paleozoic age hosts the Bolivian tin-silver belt and seems
likely to have influenced the redox state of the crustally derived magmas, which were generated as a result of the deep
225-187 Ma
26-22 Ma
Arequipa
La Paz
Pacific
Ocean
21-12 Ma
20°
66-52 Ma
Antofagasta
Salta
43-32 Ma
8-5 Ma
135-110 Ma
500
0
Cu belts
km
30°
70°
Sn-Ag belt
FIG. 10. Principal copper and tin belts of the central Andes, with formational age ranges shown. The copper belts coincide with linear magmatic
arcs, which migrated progressively eastward with time, whereas the composite tin belt occupies an essentially fixed, behind-arc position. Taken from
Grant et al. (1979), Clark et al. (1990), and Sillitoe and Perelló (2005).
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679
emplacement of mafic mantle melts (Lehmann et al., 1990;
Redwood and Rice, 1997; Hoke and Lamb, 2007).
A noteworthy aspect of the distribution of the preeminent
copper and tin concentrations in the North and South American Cordillera is their clear spatial separation from the major
gold belts (Fig. 9). Although the isolated giant porphyry copper-gold deposits at Bingham Canyon, Pebble, and Bajo de la
Alumbrera combine major quantities of the two metals, this
mutual exclusivity suggests that the fundamental controls on
copper and gold belts are different. This mutually exclusive
pattern of major gold and copper concentrations is further
emphasized by the spatial separation of the major porphyry
copper and high-sulfidation epithermal gold belts in the central Andes (Fig. 9), notwithstanding the well-documented fact
that the latter deposit type typically forms above the former
as parts of single hydrothermal systems (Fig. 1; Sillitoe, 2000).
Difference in erosion level alone does not seem to adequately
explain this spatial separation of major gold and copper belts
because the known porphyry copper and copper-gold deposits in the Cajamarca-Huaraz belt, including those beneath
the Yanacocha high-sulfidation gold deposits (Gustafson et
al., 2004; e.g., Fig. 4a), are all of low to moderate grade and
probably relatively small.
Metallogenic Considerations
The 22 Phanerozoic gold belts and five isolated giant gold
deposits defined herein together occupy only a small proportion of the American Cordillera, perhaps <5 percent of its
area (Fig. 2). This situation mirrors that for most metals,
whereby the majority of the resources are concentrated in a
few giant deposits and highly endowed belts (e.g., Singer,
1995). Each Cordilleran gold belt was typically generated in
<5 to 20 m.y. (Table 2; see above), with individual giant deposits having lifespans of <0.5 to >5 m.y. (e.g., Henry et al.,
1996; Harris et al., 2004, 2008; Longo and Teal, 2005). The
central Andean copper and tin belts (Fig. 10) are analogous in
being constructed by one or more metallogenic epochs of
comparable duration (11–20 m.y.).
All the major gold deposits and belts formed during subduction-related magmatic activity (Fig. 8), with each gold deposit type tending to occupy a reasonably well-defined
tectonomagmatic niche rather than occurring randomly.
However, when taken together, the major gold deposits and
belts span a spectrum of tectonic settings and magma compositions, although the largest concentrations formed either
during or immediately after major contractional episodes in
association with calc-alkaline magmatism. Fundamental arcparallel and arc-transverse faults and lineaments or abrupt
changes in tectonomagmatic regime at the Cordilleran margin seem to have acted as localizers of some, but by no means
all, of the major deposits and belts. Another possibility, optimization of ore-forming physicochemical processes at the deposit scale, can be invoked for at least some of the isolated,
major deposits, but would fail to satisfactorily explain the origin of the several deposits needed to define the individual
gold belts. Therefore, although the spatial and temporal restriction of the major belts and isolated deposits can be ascribed to specific tectonomagmatic events, the overall distribution of the premier Cordilleran gold concentrations at the
orogen scale remains enigmatic, because apparently similar
679
680
RICHARD H. SILLITOE
events elsewhere resulted in only relatively small or no gold
deposits.
Perusal of Figure 2 (and Fig. 8) reveals that at least six
Cordilleran arc (including fore- and back-arc) segments contain at least two gold belts or major isolated deposits, typically
comprising different deposit types, formed at different times
under different tectonomagmatic conditions. The Great
Basin and contiguous areas of the western United States and
the northern Andes of Colombia are perhaps the most striking examples. The Great Basin and environs comprise a
cluster of six relatively short belts (Carlin, Getchell, and Independence sediment-hosted gold trends, Battle MountainEureka sediment-hosted and skarn gold trend, Walker Lane
epithermal gold belt, and Sierra Foothills orogenic gold belt),
the Bingham district (Bingham Canyon porphyry copper-gold
and nearby sediment-hosted gold deposits), and the isolated
Round Mountain epithermal gold and Cripple Creek alkaline
epithermal gold deposits. To these gold concentrations may
be added the Paleoproterozoic Homestake deposit (Fig. 2)
and spatially associated, but subordinate Tertiary gold mineralization (Paterson et al., 1988). In the Colombian Andes, the
three main belts are defined by pluton-related, porphyry and
epithermal, and placer gold deposits, respectively, to which
may be added two less important gold districts of completely
different age to the three main belts (Fig. 6). The pairing or
clustering of gold belts and isolated major deposits might be
dismissed as fortuitous or, alternatively, ascribed to some fundamental predisposition of the particular orogen segments
concerned to upper-crustal gold concentration. The close
proximity of both the major copper and the major tin belts in
the central Andes provides closely analogous situations (Fig.
10). Explanations involving differences in erosion level and
preservation potential (see above) do not convincingly account for the observed proximity of the metal-enriched and
poorly mineralized arc segments.
One of the more widely debated hypotheses for recurrent
metallic mineralization within relatively restricted orogen
segments is inheritance from preexisting metal concentrations (e.g., Noble, 1970; Routhier, 1976). The site of any
such available metal preconcentration is commonly assumed
to be in the middle to upper crust (e.g., Titley, 1987; Hodgson, 1993; Fig. 11), a concept inherent in the proposal that
much of the gold in the sediment-hosted trends of the Great
Basin came from subeconomic sedimentary-exhalative mineralization in some of the late Paleozoic eugeoclinal host
rocks (Emsbo et al., 1999, 2006). Keith et al. (1991), however, preferred the notion that crustal redox state, rather
than crustal metal preconcentration, is the ultimate metallogenic control, with reduced crust like that beneath much of
the Great Basin being conducive to repeated gold mineralization; by contrast, oxidized crust, as beneath southwestern
North America, results in a dominance of copper mineralization. The likely role of carbonaceous sedimentary sequences in the development of reduced magmas was alluded to above for the Alaskan and Yukon pluton-related
gold belts and Bolivian tin-silver belt.
Au deposit
Sea level
Oceanic crust
Crust
Partial
melting
of mantle
wedge
Hypothetical Au
preconcentrations
Upper crust
Lower crust
Mafic underplate
Man
tl
flow e
Subcontinental
lithospheric
mantle
Asthenosphere
Slab
dehydration
Mantle wedge
FIG. 11. Cartoon section of a convergent margin to show possible sites of gold (or other metal) concentrations that may
be tapped during the magmatism or transcrustal fluid flow responsible for upper-crustal mineralization. Alternatively, another chemical parameter, such as redox state, may influence metal availability. See text for further discussion.
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680
The lower crust or underlying subcontinental lithospheric
mantle, including underplated mafic igneous rock produced
by mantle melting and ultramafic to mafic restites or cumulates resulting from mantle-plume or lower-crustal melting,
may offer a viable alternative site for either gold preconcentration or influential redox conditions (e.g., Noble, 1970; Fig.
11). Lower- or subcrustal influences would be eminently feasible given the ubiquitous petrochemical evidence for mantle
contributions to ore-related, arc and back-arc magmatism
(e.g., Kelley et al., 1998; Riley et al., 2001; Richards, 2003;
Hart et al., 2004; Kay et al., 2005; Hoke and Lamb, 2007) and
geophysical evidence for penetration of some gold-localizing
fault zones and lineaments to deep crustal levels (e.g.,
Hildenbrand et al., 2000; Grauch et al., 2003). Furthermore,
the gold contents of porphyry copper deposits are completely
independent of upper crustal structure and composition and,
hence, most likely subcrustally imposed (e.g., Sillitoe and
Perelló, 2005). Core et al. (2006) furnished evidence for a localized, copper-enriched magma source, in either the deep
crust or subcontinental lithospheric mantle (Fig. 11), for the
Bingham Canyon porphyry copper-gold deposit, and further
suggested that similar, but areally more extensive copper preconcentrations may have been tapped to form premier copper belts elsewhere. A tin-enriched segment of behind-arc
crust (± upper mantle), extending eastward to the Rondônian
tin province of contiguous Brazil (e.g., Dardenne and
Schobbenhaus, 2001), has also been invoked beneath the Bolivian tin-silver belt (Schuiling, 1967; Halls and Schneider,
1988). However, the concept was refuted by Lehmann et al.
(1990), who related the tin concentration to the reduced nature of the magmatism, a factor favoring tin (Sn2+) partition
into the magmatic aqueous phase (Ishihara, 1978). Nevertheless, acceptance of the latter hypothesis does not necessarily preclude the former when the trivial amounts of tin
throughout the rest of the American Cordillera are taken into
account.
In both the Great Basin and contiguous regions of the western United States and the northern Andes of Colombia, the
gold deposits and belts are situated both at the craton edge
and in the adjoining accreted terranes of oceanic affinity
(Sierra Foothills belt and Middle Cauca and Chocó belts, respectively; Figs. 3, 6). Similarly, the paired Kuskokwim gold
belt and giant Pebble porphyry copper-gold deposit in Alaska
(Fig. 2) lie within different accreted terranes. Therefore, a
lower-crustal or subcontinental lithospheric mantle gold preconcentration may not offer an adequate explanation for the
recurrence of gold mineralization events unless the accreted
terranes were originally rifted from the craton near their sites
of eventual accretion (Colpron et al., 2007) and/or overthrust
the continental margin along shallow-level décollements
(Snyder et al., 2002). Kutina’s (1991) concept of ore deposit
localization by mantle-rooted, latitudinal discontinuities cutting across both the craton and accreted terranes fails to adequately explain the gold belts, most of which are oriented parallel, not transverse, to the Cordilleran margin. Alternative
explanations might include preexisting metal concentrations
in the mantle wedge (e.g., McInnes et al., 1999; Fig. 11) or
highly oxidized conditions, imposed by slab-derived aqueous
fluids, conducive to efficient stripping of gold and copper
from residual sulfides that are broken down in the mantle
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681
wedge (e.g., Mungall, 2002). Furthermore, gold might be preferentially stripped under more oxidized mantle-wedge conditions than those that favor copper removal (Richards, 2005).
Although the gold and associated chalcophile and siderophile
metals in Cordilleran ore deposits are ultimately derived from
the dehydration of subducted oceanic crust (e.g., Sillitoe,
1972; Richards, 2003, 2005; Fig. 11), anomalously metal-rich
slab material cannot be convincingly invoked to explain highly
gold endowed Cordilleran segments because such material
would have had to have been intermittently subducted for
120 to 150 m.y. to explain both the Great Basin and Colombian Andes provinces and even longer elsewhere (northern
Peru: ~300 m.y.). Similarly, central Andean copper belts were
formed recurrently over a time span of 130 m.y. (Fig. 10).
Nevertheless, protracted subduction of oceanic lithosphere,
perhaps coupled with episodic erosion of the leading edge of
the overthrust plate (e.g., Stern, 1991), may play a key role in
the progressive enrichment of metals in as well as above the
mantle wedge.
The complex and varied Precambrian and Phanerozoic histories of the American Cordillera and subjacent subcontinental lithospheric mantle and mantle wedge undoubtedly resulted in profound chemical heterogeneity. Notwithstanding
the effects of episodic and relatively localized delamination,
and related processes (Menzies et al., 2007), the subcontinental lithospheric mantle and overlying lower crust seem
likely to be the longest-lived and, hence, potentially most heterogeneous parts of the crust-mantle system, particularly beneath Archean cratons. Some of this chemical heterogeneity,
in particular with regard to the highly siderophile elements
like gold, may even date from formation of the Earth’s core
some 4.5 billion years ago (Marty, 2008). Some aspect of the
resulting chemical provinciality, whether it is in metal content, redox state, or some other parameter, seems the most
likely reason for the predisposition of certain arc segments to
repetitive epochs of gold mineralization. A redox control, imposed in the mantle wedge (e.g., Richards, 2005), crust (Keith
et al., 1991), or possibly somewhere in the subcontinental
lithospheric mantle where oxidation state varies markedly
(e.g., Haggerty, 1990), might explain the mutual exclusivity of
major gold and copper belts documented above (Fig. 9), as
well as perhaps permitting the local co-concentration of the
two metals under intermediate redox conditions. However,
the effective upper-crustal concentration of gold in association with both oxidized and reduced magmas (see above) seriously complicates comprehension of mechanisms that might
be involved. Metal acquisition or redox imposition could be
accomplished by interaction of either magmas or, in the case
of amagmatic gold belts, deeply derived fluids (cf. Hodgson,
1993) with the lithosphere, particularly at times of stasis in
subepizonal magma or fluid reservoirs. These subepizonal
magmas or fluids would, of course, have their own intrinsic
metal contents prior to any supplementation from external
sources.
General Conclusions
Consideration of Cordilleran gold belts along with the central Andean copper and tin belts strongly suggests that certain
upper-crustal segments are subjected to repeated concentration of individual metals or metal suites, whereas adjoining
681
682
RICHARD H. SILLITOE
segments are relatively poorly mineralized. The same metallogenic provinciality is observed for gold and other metals
worldwide in orogens as old as Archean (e.g., Robert et al.,
2005). The processes involved appear to be inherently independent of tectonic and magmatic settings, which commonly
differ between closely spaced metallogenic belts within restricted arc segments. Whether or not mantle or crustal metal
preconcentrations or other factors, such as specific redox conditions, are called upon, some fundamental predisposition of
certain orogen segments with regard to particular metals or
metal suites seems inescapable.
This conclusion, if accepted, has profound implications for
exploration because diligent search beyond the favorable orogen segments is unlikely to be rewarded by major gold discoveries. So the gold explorer may either focus attention on
orogen segments containing two or more belts or isolated
giant deposits, and therefore with proven metallogenic credentials but almost certainly well explored, or try to define
new parts of the orogen predisposed to exceptional gold endowment. The latter course of action implies that the reason
for the predisposition, perhaps the most critical as well as
intractable metallogenic research topic, can be successfully
deciphered.
Future Directions
Classification of the gold deposit types present in the
American Cordillera (and elsewhere), their defining alteration assemblages and mineralogy, the physicochemical characteristics of the fluids involved, and the timing of their formation are reasonably well documented. Nevertheless, this
review underscores many aspects of Cordilleran gold metallogeny that require additional investigation, with improved
understanding of genetic controls having a direct effect on exploration at the regional scale.
Further clarification of the regional tectonic settings of
many of the major gold deposits and belts in the Cordillera
would help to ascertain whether or not specific deposit types
occupy unique upper-crustal tectonic niches or whether they
may form across a spectrum of tectonic settings. Several of
the more fundamental questions relate to the economically
preeminent high-sulfidation epithermal, sediment-hosted,
and orogenic categories: The largest high-sulfidation epithermal gold deposits are restricted to the two central Andean arc
segments presently characterized by flat-slab subduction
(Fig. 2). Is this fortuitous and unrelated to the modern slab
configuration or does decreasing subduction angle somehow
favor major high-sulfidation deposit formation, perhaps by allowing direct crustal input of slab-derived auriferous fluids
(Bissig et al., 2003) or, as in the case of major porphyry copper deposits, by inducing crustal compression and generation
of large epizonal magma chambers capable of voluminous
fluid discharge (Sillitoe, 1998)? Major sediment-hosted deposits are largely responsible for definition of the four closely
spaced gold trends in northern Nevada (Fig. 3), but such deposits appear unimportant elsewhere in the American
Cordillera. Is this a reflection of the role played by the deeply
penetrating structures inherited from Neoproterozoic rifting
(Tosdal et al., 2000) or are other processes involved? The
Sierra Foothills belt is the only world-class orogenic gold concentration in the American Cordillera, which is somewhat
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surprising if such deposits are one of the hallmarks of accretionary tectonic settings (Goldfarb et al., 2001). Is the apparent absence of orogenic gold deposits from the Late Cretaceous and Tertiary collisional collage of the northern Andes of
Colombia and Ecuador (Cediel et al., 2003) due to insufficient erosion to allow deposit exhumation (cf. Goldfarb et
al., 2001) or to some other factor(s), such as absence of suitable deeply tapping fault zones and/or major associated
batholiths?
Additional detailed petrochemical studies of well-dated
magmatic suites associated with the major Cordilleran gold
deposits and belts, like those carried out in the El Indio-Maricunga belt (Kay et al., 1994, 1999; Kay and Mpodozis, 2001),
Patagonian province (Pankhurst et al., 1998; Riley et al.,
2001), and Bingham district (Waite et al., 1997), would assist
with assessment of any direct links between specific gold deposit types and specialized magma chemistries or evolutionary paths. In this regard, Richards and Kerrich (2007) show,
contrary to recent proposals, that adakitic (high Sr/Y and
La/Yb) magmatism is unlikely to be a requirement for major
porphyry copper ± gold deposit genesis. Injection of mafic
magma into more felsic epizonal chambers has been proposed as an important step in the development of major porphyry copper-gold deposits (e.g., Waite et al., 1997; Hattori
and Keith, 2001; Halter et al., 2004) and perhaps even the intrusive rocks of the Carlin sediment-hosted gold trend
(Ryskamp et al., 2008). Is similar magma mixing involved in
the formation of other major Cordilleran gold deposits and
belts? If so, it is important to systematically collect the supporting petrologic and petrochemical evidence, with a view to
defining possible exploration criteria. Cordilleran gold belts
accompany both oxidized and reduced magma suites, although the latter are restricted to the northern Cordillera of
Alaska and the Yukon Territory. Is generation of reduced
magmas an exclusively upper-crustal process or can deeper
crustal or mantle redox controls, such as those proposed by
Keith (1991) and Richards (2005), have an influence too? And
why are the reduced magmatic rocks of the Bolivian tin-silver
belt so poor in gold mineralization? As described above, the
largest Cordilleran sediment-hosted and orogenic gold belts
are likely underlain by concealed plutons emplaced at the
time of mineralization. Is the gold derived from nonmagmatic
sources, as currently preferred by most investigators, or could
it have been liberated during cooling of these deep intrusions? An approach using lead isotopes may be applicable, as
employed recently to link the low-sulfidation epithermal gold
of the Northern Nevada rift to mafic volcanic products of the
Yellowstone mantle plume (Kamenov et al., 2007).
Although these and numerous other tectonomagmatic
problems need our attention, the most fundamental questions of all relate to the concepts of metallogenic inheritance
and provinciality as explanations for the metal distribution
patterns highlighted in this review and elsewhere, as a fortuitous association seems an unsatisfactory explanation, particularly to the avid explorationist. Perhaps the best longterm means of addressing this topic would be to routinely
acquire analytical data for gold, copper, tin, and associated
metals as well as information to determine the associated
redox conditions while conducting integrated tectonic, geochemical, and geophysical studies of the lower crust and
682
upper mantle, like that recently synthesized by Karlstrom et
al. (2005) for the Rocky Mountains region of the western
United States. The analytical data could be obtained from
outcropping rocks in deeply eroded, high-grade metamorphic terranes as well as from xenoliths of both upper mantle
and lower crustal provenance. Such analytical data are
needed for both well-endowed and poorly mineralized belts
in order to properly define the nature and environment of any
metal preconcentrations. Particular attention might be paid
to high-grade terranes containing Precambrian mineralization in proximity to Phanerozoic belts defined by the same
metal(s), as exemplified by the Paleoproterozoic Homestake
gold deposit in the context of the major Meso-Cenozoic gold
deposits and belts in the western United States (Fig. 2).
These are just a few of the basic questions, and possible approaches, which investigators may consider when identifying
research topics of merit.
Acknowledgments
The manuscript benefited from constructive reviews by Jeff
Hedenquist, Jay Hodgson, Pepe Perelló, Jeremy Richards,
and François Robert, all of whom are absolved of any direct
responsibility for the opinions expressed.
May 6, May 12, 2008
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