1097-1136 Metallogenic Provinces in an Evolving Geodynamic Framework.pdf

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@ 5Societyof EconomicGeologists,nc.

Economic Geology lOOthAnniversary Volume

pp. 1097-1136

Metallogenic Provinces n n Evolving Geodynamic Framework

ROBERT KERRICH, t

Departnwnt of Geological Sciences University of Saskatchewan 114 Science Place Saskatoon Saskatchewan Canada S7N 5E2

RICHARD J. GOLDFARB,

U.S.GeologicalSurvey Box25046 MS964,DenverFederalCenter Denver Colorado80225 0046and

Departnwntof GeologicalSciences Universityof Colorado 2200ColoradoAve. CampusBox399,Boulder Colorado80309

AND JEREMY P. RICHARDS

Departnwnt of Earth and Atmospheric Sciences University of Alberta Edmonton Alberta Canada T6G 2E3

Abstract

Thermal decay of Earth resulted in decreased mantle-plume intensity and temperature and consequently a

gradual reduction of abundant komatiitic basalt ocean plateaus at -2.6 Ga. In the Neoarchean, ocean crust was

-11 km thick at spreading centers, and abundant bimodal arc basalt-dacite magmatic edifices were constructed

at convergent margins. Neoarchean greenstone belt orogenesis stemmed from multiple terrane accretion in

Cordilleran-style external orogens with multiple sutures, where oceanic plateaus captured arcs by jamming

subduction zones, and plateau crust melted to generate high thorium tonalite-trondhjemite-granodiorite suites.

Archean cratons have a distinctive -250- to 350-km-thick continental lithospheric mantle keel with buoyant re-

fractory properties, resulting from coupling of the buoyant residue of deep plume melting to imbricated

plateau-arc crust. In contrast, Proterozoic and younger continental lithospheric mantle is 240-km

depth, mostly pre-2.7 Ga. They were entrained in kimberlitic to lamproitic melts related to superplume events

at 480, 280, and -100 Ma. Preservation of resulting mineral provinces stems from their location on stable

Archean continental lithospheric mantle.

Decreased plume activity after 2.6 Ga caused sea level to fall, leading to the first extensive passive-margin

sequences, including deposition of phosphorites, iron formations, and hydrocarbons, during dispersal of

Kenorland from 2.4 to 2.2 Ga. Deposits of Cr-Ni-Cu-PGE were generated where plumes impinged on failed

rifts at the transition from thick Archean to thinner Proterozoic continental lithospheric mantle, e.g., the Great

Dyke, Zimbabwe, and later at Norilsk, Russia. Paleoproterozoic orogenic belts, for example, the Trans-Hudson

orogen in North America and the Barramundi orogen in Australia, welded together the new continent of Co-

lumbia. Foreland basins associated with these orogens, containing reductants graphitic schists in the base-

ment, led to the formation of unconformity U deposits, with multiple stages of mineralization generated from

diagenetic brines for as much as 600 m.y.after sedimentation. Plume dispersal of Columbia at 1.6 to 1.4 Ga led

to SEDEX Pb-Zn deposits in intracontinental rifts of North America and Australia, extensive belts of Rapakivi

A-type granites on all continents, with associated Sn veins, and Fe oxide-Cu-Au-REE deposits. All were con-

trolled by rifts at the transition from thick to thin continental lithospheric mantle. Plume impingement on Ro-

dinia at -1 Ga formed extensive belts of anorogenic anorthosites and Rapakivi granites in Laurentia and

Baltica, the former hosting Fe-Ti-V deposits. Sedimentary rock-hosted Cu deposits formed in intracontinental

basins from plume dispersal of Rodinia at -800 Ma.

Iron formations and mantle plumes have common time series: Algoman type occur from 3.8 Ga to 40 Ma,

granular iron formations precipitated on the passive margins of Kenorland at -2.4 Ga, Superior-type formed

on the passive margins of Laurentia, and Rapitan iron formations were created in rifts during latter stages of

dispersal of Rodinia at -700 Ma. Accordingly,such deposits are not proxies for the activity of atmospheric 02.

Rich Tertiary placer deposits of Ti-Zr-Hf, located on the passive margins ofAustralia and Southern Africa, re-

flect multiple cannibalistic cycles from orogens that welded Rodinia and Pangea.

Orogenic Au deposits formed during Cordilleran-type orogens characterized by clockwise pressure-temper-

ature-time paths from -2.7 Gato the Tertiary; Au-As-W and Hg-Sb deposits reflect the same ore fluids at pro-

gressively shallower levels of terrane sutures. The MVT-type Pb-Zn deposits formed in foreland basins, with

t Corresponding author: [email protected]

1097


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98

KERRICH ET AL.

Phanerozoic Pb-Zn SEDEX ores localized in rifted passive continental margins containing evaporites at low

latitudes. Porphyry Cu and epithermal Au-Ag deposits occur in both intraoceanic and continental margin arcs;

ore fluids were related to slab dehydration, peridotite fusion, and hybridization with upper-plate crust. De-

posits exposedtoday are largely

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1102 KERRICH ET AL.

negatively buoyant (Fig. 2A). For a hotter Archean upper

mantle, greater degrees ofmelting occurred at spreading cen-

ters (Bickle, 1986). According to calculations of Abbott et al.

(1994a), Neoarchean basaltic oceanic crust was -11 km thick,

with a commensurately thicker mantle lithosphere residue

depleted in incompatible elements from basalt extraction.

Consequently, Archean ocean lithosphere would have sub-

ducted at shallower angles from thermal and buoyancy con-

siderations. There has been a secular decrease in the temper-

ature of mantle plumes; accordingly, ocean plateau crust has

alsobecome thinner through time (Fig. 3).

The continental lithosphere has a 30- to 80-km-thick crustal

sector in Archean and younger eons. Continental lithospheric

mantle is 250 to 350 km thick under Archean continental

crust but -150 km thick for Proterozoic and -100 km for

35

30

:0 25

CI

;6 to

8 X 106km2in area (Artemieva and Mooney, 2001), and with

implications for diamond potential. Younger plumes were less

frequent and cooler, so they did not generate refractory

residues (Fig. 2; White, 1988; Jordan, 1988; Pollack, 1997

Herzberg, 1999; Artemieva and Mooney, 2001). For example

the continental lithospheric mantle is 190 to 240 km thick in

the diamondiferous Magan and Anabar cratons but thins to

150 to 180km for the Proterozoic Olenek province (Griffinet

al., 1999). From studies of xenolith suites, there is a secular

trend from highly depleted harzburgites in Archean conti-

nentallithospheric mantle, through intermediate depletion in

the Proterozoic, to mildly depleted lherzolites in the Phaner-

zoic. Archean continental lithospheric mantle has a density o

3.36 glcm whereas Proterozoic continental lithospheric

mantle is 3.38

glcm

marginally less dense than ambient as

thenosphere (Griffin et al., 2003).

Archean supracrustal terranes are dominated by bimoda

volcanic arc sequences and postvolcanic tonalite-trond-

hjemite-granodiorite batholiths, whereas Archean continenta

lithospheric mantle is refractory harzburgite, with the com

position of the residue of plume melting. This apparent para-

doxmay be resolved ifmigrating arcs captured ocean plateaus

erupted from mantle plumes. Buoyant plateaus jam subduc-

tion zones, generating composite arc-plume crust, and the

buoyant residue of plume melting couples to the base of the

crust (Wymanand Kerrich,2002 Prior to capture and cou

pling of plume residue, subduction caused metasomatism o

peridotitic subarc lithosphere. During subsequent exten

sional events, and/or plume impingement, metasomatized do

mains melted to generate the voluminous noritic magma

characteristic of Neoarchean to Proterozoic layered igneous

complexes in or near Archean cratons (Fig. 2B; Hall and

Hughes, 1980). Those magmas are integral to formation o

Ni-Cu-PGE and Fe-Ti-V deposits. Proterozoic and younger

plumes were not hot enough to generate refractory residue

consequently, Proterozoic and younger continental lithos

pheric mantle is thinner, denser, and less refractory, such tha

crustal terranes are more readily reworked during subsequent

orogenies (Figs. 1, 2).

During collisional orogens in the Proterozoic and Phanero

zoic, both crust and continental lithospheric mantle thicken

and part of the latter may delaminate; hot asthenosphere then

flows under thinned lithosphere, creating elevated orogens

as in the Tibetan plateau (Houseman and Molnar, 1997)

During lithosphere thickening under compression, radioac

tive heat weakens the crust, and decoupling of lower crus

and continental lithospheric mantle may occur at the base o

the upper felsic crust (Meissner and Mooney, 1998). High

temperature-low-pressure metamorphism and extensiona

collapse with escape tectonics ensue, in conjunction with

5

5

5


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MET LLOGENIC PROVINCES IN N EVOLVING GEODYN MIC FR MEWORK

1103

asthenospheric and crustal magmatism. Delaminated conti-

nentallithospheric mantle has been imaged by teleseismic to-

mography beneath the Alpine-Himalayan orogen Schott and

Schmeling, 1998 . Delamination is in progress beneath the

Basin and Range province and Tibetan plateau, is interpreted

to have occurred beneath the Puna plateau of northwestern

Argentina Kay and Kay, 1993 , and characterized the late

stages in the development of the Variscan and Grenvillian

continent-continent orogens Windley, 1995 .

The low-velocity zone is the thermal boundary layer be-

tween torsionally rigid lithospheric plates and the convecting

asthenosphere; low S wave velocities result from domains of

partially melted lherzolite, conferring low strength. This zone

is 100 to 200 km thick below ridges where thermal gradients

are high, thinner below normal continental lithosphere, and is

thin to absent beneath Archean continental lithospheric man-

tle where thermal gradients are low Fig. 2 A, B; Keary and

Vine, 1996 .

Characteristics of plate boundaries

Divergent plate boundaries As oceanic plates separate at

ridges due to far-field extensional forces, decompressional melt-

ing of asthenospheric mantle generates mafic magmas that ac-

crete to the edges of plates to form new crust Keary and Vine,

1996 . Upwelling of asthenospheric upper mantle beneath

ridges is passive, in response to plate separation.

a simplified

cross section, the oceanic lithosphere is composed oflower ul-

tramafic mantle mantle tectonites, dunites, lherzolites, and

harzburgites at the base, and mafic crustal rocks gabbros,

sheeted dike complex, and basalts at the top, bounded by the

oceanic Moho. The thickness of the lithosphere increases from

zero at ridges to 70 to 100 km at an age of -70 m.y., then main-

tains approximately unifonn thickness, as plates move away

from spreading centers. Commensurately, the depth of the

ocean floor increases with the age of oceanic lithosphere, due to

thennal cooling of the lithosphere associated with thickening

and subsidence Fig. 2A; Parsons and Sclater, 1977 .

Convergent plate boundaries At convergent margins, the

plate with higher density sinks beneath the lighter plate,

forming a subduction zone, and the leading edge of the over-

riding plate becomes a paired fore arc and magmatic arc.

Where two oceanic plates converge, the older and denser

oceanic plate generally sinks beneath the younger and lighter

one, generating oceanic island arcs, such as the Marianas and

the south Sandwich arcs. Given its higher density, oceanic

lithosphere subducts underneath continental lithosphere to

form a continental magmatic arc, such as the Andean, Suma-

tran, and Japanese arcs.

Convergent margins generally feature the following tec-

tonic elements: 1 a deep marine trench seaward of the fore

arc; 2 a subduction-accretion complex located between the

underriding plate and the fore-arc basin; 3 a fore-arc basin

between the arc axis and the subduction-accretion complex;

4 a magmatic arc; and 5 an inboard foreland basin-thrust

belt, which undergoes subsidence and sedimentation due to

tectonic loading, tectonic imbrication, and later compression-

driven uplift Fig. 4 . Porphyry Cu deposits form in oceanic

and continental arcs, and most preserved volcanic rock-asso-

ciated massive sulfide deposits form in oceanic arcs or

oceanic or continental back arcs.

Based on relative plate motions, magmatic arcs are divided

into extensional, neutral, and compressional Dewey, 1980;

Sengor, 1990 . Extensional arcs, such as the Marianas, are

characterized by dominantly mafic volcanism, back-arc basin

opening, an ophiolitic fore-arc basement, deep trenches, and

steeply dipping Wadati-Benioff zones. Given its thermally

weak nature, arc lithosphere generally undergoes extension to

form an intra-arc basin or an intra-arc spreading center; the

Lesser Antilles and Taupo arcs are examples of initial stages,

whereas the Lau basin has evolved into a back arc.

Compressional arcs, such as the Central Andes, lie on con-

tinentallithosphere, and are characterized by mainly inter-

mediate to felsic magmatism, back-arc thrusting, continental

fore-arc basement, shallow trenches, and shallow Benioff

zones. Neutral arcs such as the Central American, Sumatran,

and Alaska Range-Aleutian arcs have characteristics interme-

diate between extensional and compressional arcs and usually

have large subduction-accretion complexes and orogen-paral-

lel strike-slip faults Windley, 1995 .

Arc magmatism varies along and across strike. All arc mag-

mas are characterized by variably light rare earth element

REE and lithophile element Cs, Rb, Ba, K, and Pb earth

element enriched patterns and depletions in Nb, Ta, P, and Ti

Pearce, 1982; Saunders et aI., 1991; Keleman et al., 2004 .

Tholeiitic magmatism is dominant between the fore-arc basin

and arc axis; calc-alkaline magmatism occurs mainly in the

central region of the arc, whereas late alkaline igneous rocks

tend to occur between the arc axis and back-arc region the

K-h relationship; see Wilson, 1989, for a review . The com-

position of continental crust requires that mafic cumulates

founder under arc crust Rudnick and Gao, 2004 , with space

conservation accommodated by inflowing asthenosphere. Re-

gional metamorphism varies from subgreenschist to eclogite

facies Fyfe et aI., 1978 , and the occurrence of adjacent high-

temperature and/or low-pressure greenschist and high-

pressure and/or low-temperature blueschist metamorphic

belts is unique to convergent plate boundaries Ernst, 1975 .

The uppermost section of subducting oceanic lithosphere is

prevalently marine turbidites but may include pelagic sedi-

ments, oceanic islands, seamounts, and carbonate platforms.

These are commonly scraped off, deformed, metamorphosed,

and accreted to the base of the overriding plate to form a sub-

duction-accretion complex. Complex interaction between

overriding and subducting plates results in thrusting, folding,

and melange formation within the subduction-accretion com-

plex, with late transpression and associated strike-slip fault-

ing.

arcs characterized by strong coupling between the

overriding and subducting plates, attrition of the fore arc oc-

curs by subduction-erosion von Huene et al., 2004 . Trench

turbidites have a catchment in the upper levels of subduction-

accretion complexes. Plate movement is driven by the nega-

tive buoyancy of subducting slabs, not by mantle convection

Conrad and Lithgow-Bertelloni, 2002 . Stem 2002 has re-

cently reviewed processes in subduction zones.

Transform plate boundaries

Transform, or conservative,

boundaries accommodate the motion from divergent- to con-

vergent-plate boundaries and accommodate translation be-

tween ridge sectors spreading at different rates, as required

by plate motion on a spherical surface Wilson, 1965 . Trans-

form-plate boundaries separating continental lithospheric


8/40

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FIG. 4. A. Life span-geodynamic relationships of sedimentary basins. Modified f rom Woodcock (2004) . Abbreviations: BA

=

back are, FA = fore are, FL = foreland, IA = intra-arc, 0 = oceanic, PM = passive margin, R = continental margin rift, RA

= retro-arc, 55 = strike slip, T = trench, T5 = trench slope. (A) after Kyser et al. (2000), (B), (C), and (D) modified from Ross

(2000), (E) a compos ite from miscel laneous sources and R. Kerrich.


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MET LLOGENIC PROVINCES IN AN EVOLVING GEODYN MIC FR MEWORK

n05

blocks are termed transcurrent or continental strike-slip

faults. Examples are the San Andreas fault zone of California,

the North Anatolian strike-slip fault zone in Turkey, and the

Tintina and Denali fault zones of western Canada and Alaska.

Transtensional regions are characterized by normal faulting,

pull-apart basins, and dominantly basaltic volcanism, whereas

transpressional regions feature thrusting, folding, and uplift,

in addition to strike-slip faulting in both cases Christie-Blick

and Biddle, 1985; Sylvester, 1988).

Supeifamilies of orogens

Cordilleran orogens

Sengor and Natal in 1996a) classified

orogenic belts into two superfamilies, Cordilleran and conti-

nent-continent. This insight has profound implications for

metallogeny. Cordilleran-type orogens, also referred to as

Turkic or transpressional Sengor and Natal in, 1996a), exter-

nal Murphy and Nance, 1992), or accretionary Windley,

1995), represent continental growth via the process of terrane

accretion. These sutured tectonostratigraphic terranes are

fragments of juvenile arcs and ocean plateaus, plus marine

sedimentary rocks, tectonically assembled in accretionary

prisms; there is typically little addition or reworking of older

continental crust Ben-Avraham et aI., 1981). Collision of ter-

ranes occurs dominantly in an oblique manner, with the par-

titioned compressional component responsible for much of

the orogeny. Cordilleran-type orogens are characterized by

both extensive lateral and vertical accretion above a subduct-

ing slab. Where subduction-erosion hinders terrane collision

and thus lateral accretion, Andean-type orogens dominate.

These possess the arc-related porphyry and epithermal de-

posits that also characterize Cordilleran-type orogens but lack

most other, more deeply formed deposit types that are com-

mon throughout the blocks of allochthonous juvenile crust

within such orogens, as described below.

Multiple sutures at terrane boundaries are inherent to

long-lived terrane collison. Sutures commonly serve as sites

for ensuing economic mineralization. Seaward growth of con-

tinental margins, with such sutures defining progressive ter-

rane accretion, tends to be a long-lived process of perhaps

-300 to 400 m.y.; examples include the Cordilleran orogen,

370 Ma to present; Altaid orogen, 610 to 250 Ma; and Pan-

African orogen, 900 to 630 Ma Burchfiel et al., 1992; Sengor

and Natal in, 1996a). Depending on the degree of obliquity to

each terrane collision, these sutures behave as thrust and/or

strike-slip faults. In many cases, lateral displacement of ter-

ranes becomes relatively more common late during orogene-

sis or even subsequent to all collision. Such transform conti-

nental margins concentrate juvenile crust, and likely

associated mineral deposits, in restricted regions of an evolv-

ing orogen Patchett and Chase, 2002). The terranes of the

Altaid orogen underwent thousands of kilometers of left lat-

eral and right lateral movements during the final stages of Pa-

leozoic tectonism in central Asia Sengor and Natal in,

1996a). Major shifts from compressional to more translational

regional stress regimes appear conducive to seismic events

and extensive episodes of fluid flow Sibson et al., 1988; Ker-

rich and Wyman, 1990) and maybe important controls on the

development of large orogenic gold provinces in Cordilleran

orogenic belts e.g., Goldfarb et al., 1991, 2005). Given far-

field compressional regimes superimposed on more localized

transtensional to transpressional zones late during orogenesis,

Cordilleran orogens generally undergo significant oroclinal

bending e.g., Alaskaand the Altaids; Yakubchuk et al., 2002,

2005). These strike-slip regimes also cause the highly dis-

membered nature of ophiolite sequences within most oro-

gens and thus a discontinuous distribution to many preaccre-

tionary VMS and chromite ores.

Cordilleran-style orogens may show a similarly wide

>1,000 km) pattern of subduction-related magmatism, as in

the Altaids and mainland Alaska. By contrast, continent-con-

tinent orogens feature narrow magmatic arcs. In Cordillean

orogens, the ages of the igneous rocks young toward the

ocean, as arc magmatism migrates episodically in that direc-

tion as the continental margin is built outward Sengor and

Natal in, 1996a). Agesof orogenic gold deposits tend to follow

the same approximate spatial and/or temporal pattern Gold-

farb et al., 1997). Most igneous rocks are juvenile in the oro-

gens; there are limited examples of remelted crust seaward of

the craton edges Windley, 1995).

Lithological units in Phanerozoic orogens are dominated by

deep marine turbidite sequences and lesser basalts and

cherts, as these are the dominant rocks being accreted off the

top of subducting oceanic slabs and comprising the growing

prism defining the arc-trench gap Fig. 4E). Addition of new

crust to a craton margin is also common in a spreading back-

arc regime, where foreland or retroarc basins may evolve in a

region of extension between the continental arc and craton

edge. These units may have a higher volume of fine-grained

terrigenous and biogenic material than units in the fore are,

which are more likely dominated by the clastic products of

deep-sea turbiditic currents. Importantly, the pelitic sedi-

mentary rocks and related mafic volcanic and volcaniclastic

sequences commonly contain volatile-rich mineral phases

such as phengite, biotite, lawsonite, chlorite, dolomite, mag-

nesite, and pyrite, all potential contributors of H2O, CO2,and

S to fluid phases produced during later thermal and de-

volatilization events e.g., Fyfe et al., 1978). Furthermore,

marine pyrite maycontain trace amounts of gold that can also

be mobilized during subsequent heating of the marine rocks.

During the last decade, Cordilleran-style orogens, as prod-

ucts of a present -day style of plate tectonics, have become

widely accepted as having developed far back into the Pre-

cambrian Sengor and Natal in, 1996a; deWit, 1998).Archean

and Paleoproterozoic terranes are dominated by greenstones

and tonalites, with minor turbidites; these linear belts also

likely formed via processes of accretion at convergent plate

boundaries Kusky and Kidd, 1992; Kusky and Polat, 1999;

Foley et al., 2002). Structural style and metamorphic regimes

in many Precambrian greenstone belts, or composite

tectonostratigraphic terranes, resemble those in Phanerozoic

Cordilleran-style orogens. For example, accretionary assem-

bly of the -2.7 Ga Superior province and Yilgarn cratons

Sengor and Natal in, 2004), including the involvement of sig

nificant volumes of plume-derived oceanic plateau crust

Stein and Hofmann, 1994; Polat et al., 1999), was character-

ized by terrane accretion and batholith emplacement that mi-

grated in a seaward direction Kerrich et al.,

2000

Ophiolites, long appreciated as footprints of Cordilleran-

style tectonics, are now widely recognized in Precambrian en-

vironments, with a particularly high abundance of tholeiitic


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1106

KERRICH ET AL.

pillow basalts in many cratons de Wit, 2004). Higher geot-

herms in the Archean are reflected by the widespread high-

grade gneissic basement rocks, which, with refractory conti-

nental lithospheric mantle, have preserved the mid-crustal

Cordilleran-like greenstone belts for billions of years. Simi-

larity in the geologic evolution of Precambrian and Phanero-

zoic Cordilleran-style continental margins is reflected in a

similar metallogenic record being preserved in metamor-

phosed rocks of all such orogens, regardless of geologic age

Goldfarb et al., 2001).

Continent continent orogens:

A second type of orogen is

termed continent-continent collisional or Tethyan. It is typi-

cally marked by the closure of an ocean basin, a single well-

defined Z- or C-shaped suture zone containing ophiolites be-

tween blocks of continental crust, a magmatic arc on the

active margin, and deformation of passive margin sequences.

Collision is orthogonal to oblique, with an exceptional amount

of crustal thickening, and reworking of the older crustal

blocks Windley, 1995; Sengor and Natal in, 1996a). This tec-

tonism includes metamorphism, widespread partial melting

of the lower crust during lithosphere thickening, delamina-

tion, and commonly underplating by mafic magmas. Depend-

ing on the structural complexity, these orogens may show

abundant, high-level overthrusting exemplified by the Alpine

type or limited thrusting of allochthonous blocks as in the Hi-

malayan type Sengor, 1990; Sengor and Natal in, 1996a).

Mantle plumes

Pirajno 2000 givesa comprehensivetreatment of mantle

plumes and ore deposits upon which this section draws ex-

tensively. Jets of anomalously hot mantle are ejected from

thermal boundary layers, most likely the core-mantle bound-

ary at 2,900 km, which advect through the mantle by thermal

buoyancy on timescales of only 10to 50 m.y.The plume head

is 500 to 1,000 km in diameter, whereas the tail, which feeds

the head, is -100 km in diameter. At the top of the upper

mantle, ambient temperature is

1 280C

the plume head

1 480C and the tail -1, 700C. Plumes conductively heat

ambient mantle, which is entrained into the plume head. On

impinging upon normal lithosphere at -150-km depth, the

plume head flattens to 1,000 to 2,000 km while undergoing

extensive decompressional melting White, 1992). Anom-

alously hot plumes, with high buoyancy-driven flux, advect

basalts through continental lithosphere to erupt as continen-

tal flood basalts. Basaltic liquids from cooler plumes, or from

adiabatically decompressed asthenosphere under thinned

continental crust, pond at the Moho density filter Herzberg

et al., 1983);here they fractionate to form anorogenic gabbro-

anorthosite complexes that may host Fe-Ti-V deposits

Cawthornet al.,

2005

and also fuse refractorylowercrust

into A-type granites with which Fe oxide-Cu-Au-REE

provinces are associated Fig. 2B; Windley, 1995; Williams et

al., 2005).

Crucial to the understanding of magmatic Ni-Cu Arndt et

al., 2005; Barnes and Lightfoot, 2005) and chromite deposits

Cawthorn et al., 2005), as well as deposits associated with

anorogenic magmatism, is that plumes do not melt by de-

compression at -250 km beneath Archean continentallithos-

pheric mantle but rather penetrate laterally as dikes. These

include the 2596 Ma Great Dyke and 2200 Ma Matachewan

swarm. Alternatively, the plumes spread laterally under the

normal continental lithospheric mantle Fig. 2B). Plume ac-

tivity,particularly of superplumes, is episodic, with maxima at

-3.8,3.4,3.0,2.7,2.4, 1.9, and 1.7 Ga, with one at -250 Ma

and another superplume in the Cretaceous Fig. 3A; Larson,

1991;Ernst and Buchan,2001;Abbottand Isley,2002 .

Mantle plumes occur in three broad varieties, as discussed

below.

Long lived hotspots with low magma flux:

These plumes

generate ocean islands, such as the Emperor-Hawaii chain on

oceanic lithosphere, or hotspot tracks on continents, e.g., the

Columbia River-Yellowstonetrack spanning 45 Ma to the pre-

sent Schissel and Smail, 2001).

Short lived plumes that generate flood basalt provinces:

Plumes that erupt through oceanic lithosphere form oceanic

plateaus, including Kerguelen, Ontong-Java, and Iceland, or

continental flood basalts. For the Siberian and Deccan conti-

nental flood basalts, 1 to 3 X 106km3of flows erupted during

d m.y; tholeiitic basalts predominate, with minor alkali

basalts and picrites. The Tertiary North Atlantic igneous

province, which includes continental flood basalts on Green-

land, a volcanic passive margin on eastern North America,

and the Iceland plume, collectively represent a transition

from continental flood basalts to an ocean plateau as North

America and Scandinavia rifted apart. The three elements of

superplumes, continental flood basalts, giant dike swarms,

and mafic intrusive complexes, are collectively referred to as

large igneous provinces Coffin and Eldholm, 1994; Saunders

et al., 1997; Eldholm and Coffin, 2000). All three elements

are present in the 1267 Ma Mackenzie giant dike swarm,

Coppermine CFB, and the Muskox intrusive complex of

northern Canada Ernst and Buchan, 2004). Giant dike

swarms may represent a failed triple junction and, therefore,

point toward a paleo-ocean Fahrig, 1987). The secular distri-

bution of iron formations, from 3.8 Ga to 40 Ma, is controlled

by mantle plumes.

Superswells or mantle upwellings:

These features have di-

ameters of -10,000 km and spawn hotspots. There are two

known, one centered on the South Pacific and another below

Africa, both with dynamic topography McNutt, 1998). The

African superswell was responsible for rifting of Gondwana

from Laurasia.

Plumes and ore deposits: All three expressions of mantle

plumes have a role in mineral provinces, from diamond fields

Gurney et al.,2005 to Ni-Cu-PGEdeposits Pirajno,2000).

Hotspot plumes are approximately fixed with respect to the

mantle, so ocean island chains provide a reference frame for

hotspots that constrains plate motions Norton, 2000). Mantle

plumes and lithospheric plate motions are not strongly

coupled. However, where a plume erupts proximal to a spread-

ing center it may capture the ridge, as with the Iceland

plume-Mid-Atlantic Ridge. Plumes may interact with conver-

gent margins, such as impingement of the mid-Cretaceous

Marie Byrd Land plume with the Phoenix plate subducting be-

neath Antarctica Weaver et al., 1994). Present-day examples

include the Samoan plume proximal to the Tonga trench and

interaction of the Yellowstone plume with the Farallon plate

Schissel and Smail,2001). Plume-ocean ridge and plume-con-

vergent margin interactions cause some of the largest known

structural and geochemical anomalies Ito et al., 2003).


11/40


1107

Ocean plateaus with >30-km thickness of basaltic crust,

erupted from anomalously hot mantle plumes, resist subduc-

tion, and cause collisional orogenesis when they jam up

against a subduction zone Cloos, 1993). The Solomon-New

Ireland arc has migrated to capture the 120 to 90 Ma Ontong-

Java ocean plateau, which is being jammed against the sub-

duction zone; this is where the Lihir Au deposit has formed

MacInnes et al., 1999). Formation of the giant 2.7 Ga Kidd

Creek VMS deposit followed capture of the Abitibi arc by an

ocean plateau Wyman et aI., 1999)

There is compelling evidence for the influence of mantle

plumes on conditions of surface geology, the hydrosphere, at-

mosphere, and biosphere Larson, 1991; Coffin and Eldholm,

1994; Kerr, 1998). Isley and Abbott 1999) and Condie et ai.

2001) demonstrated a coincidence in timing of mantle

plumes, deposition of iron formations and black shales, and

the chemical index of alteration. Ocean plateaus that erupted

from plumes formed thick crust that displaced oceans across

continents and caused flooding of continental shelves; the

plumes also resulted in the discharge of Fe-rich hydrothermal

fluids and the release of CO2 and other gases that generated

greenhouse conditions, causing intense silicate weathering

Kerr, 1998).

Sedimentary basins

The geodynamic setting of sedimentary basins, and their

lifespan and fate, have been summarized by Ross 2000) and

Woodcock 2004). This discussion deals only with foreland,

intracontinental, passive margin, and oceanic basins, drawing

mainly on these summaries Fig. 4).

Foreland basins develop as a consequence of tectonic loading

at convergent margins. A classic profile involves a foredeep axis

proximal to an orogen, a continental ramp or outer slope, and a

peripheral bulge. Lithosphere elastic thickness determines

basin characteristics; transitions from narrow, deep-water flysch

sequences to wide, marine or fluvial molasses facies reflect

propagation of the load from elastically thin lithosphere at a sea-

ward position to thicker continental lithosphere. Proterozoic

unconformity U deposits and Phanerozoic Mississippi Valley-

type MVT) Pb-Zn deposits accumulated in foreland basins that

evolved to intracratonic basins Fig. 4B, D).

The pattern of stratigraphic onlap so-called steershead

geometry) of intracratonic and passive margin sequences is

consistent with extension being driven by far-field forces, in

which differential tensile strength causes mantle lithosphere

to extend over a wider area than the crust Fig. 4B,C; White

and McKenzie, 1988). The Williston as well as Michigan and

Illinois basins developed inboard of the Cordilleran and Ap-

palachian orogens, respectively, but the cause of this relation-

ship is not clear Ross, 2000). According to Pysklywec and

Mitrovica 2000), some intracratonic basins stem from dy-

namic topography generated by foundering of subducted

lithosphere. Sublithospheric loading generates flexural wave-

lengths one order of magnitude longer than surface loads, ac-

counting for both the relative dimensions and lifespans of in-

tracontinental versus foreland basins cf. Woodcock, 2004).

Proterozoic sedimentary-hosted SEDEX Pb-Zn deposits de-

veloped in intracontinental rifts Leach et al., 2005a,b).

Passive-margin sequences that develop as intracontinental

rifts evolve into ocean basins. A typical sequence is rifting of

continental lithosphere followed by sedimentation, magma-

tism linked to thinned continental lithosphere, and evolu-

tion to ocean lithosphere. The Atlantic margin, with its con-

tinental shelf, continental slope, and rise, is a typical

example. The sedimentary wedge may be deposited at nor-

mal, oblique, or transform continental margins. Transfer

faults accommodate differential extension rates and patterns

of sedimentation. Subsidence initiates by lithospheric thin-

ning from far-field forces and then evolves by thermal con-

traction and sediment loading. Basins driven mainly by ther-

mal subsidence are characterized by concave-up subsidence

patterns, as documented for aging oceanic lithosphere,

whereas foreland basins have concave-down subsidence pat-

terns Fig. 4C; Ross, 2000).

Phosphorites and iron formations accumulated on passive

margins from -2.4 Ga. Rifted passive-margin clastic sedi-

mentary sequences, formed at low latitudes, are favorable

hosts for Phanerozoic Pb-Zn ores. The deposits are generated

by metal-rich brines that evolved in adjacent carbonate units

and basement Leach et al., 2005a,b). Placer deposits of Ti-

Zr-Hf are preserved in Teriary and younger passive margin

sequences Freeman and Donaldson, 2004).

Where extension is focused within a continent, as in the

Basin and Range province, a continental back-arc basin may

develop. The Bathurst and Iberian pyrite VMS provinces are

examples of continental back-arc basins that closed; sill-sedi-

ment complexes in the Gulf of Cortez may be a present-day

analog Boulter, 1993).

The supercontinent and or superevent cycle

The concept of the supercontinent cycle emerged in the

late 1980s from recognition that the continental masses as-

semble and disaggregate in a cyclic pattern on a timescale of

200 to 500 m.y. Fig. 5; Hoffman, 1988; Murphy and Nance,

1992; Rogers, 1996; Rogers and Santosh, 2004 All of the

present continents formed a single landmass, Pangea, that

broke up -180 Ma. Previous supercontinents were Kenorland

at -2.7 to 2.2 Ga, Columbia at -1.7 to 1.4 Ga, and Rodinia at

-1.0 at 0.6 Ga Fig. 5; Condie, 2004; Zhao et al., 2004

A consensus has emerged that rifting of continents and dis-

persal of supercontinents is generally triggered by a mantle

plume, in keeping with Ziegler s 1993) estimates of tractional

forces for plumes that impinge on continents White, 1992;

Duncan and Turcotte, 1994; Carlson, 1997). Sill-sediment

complexes of the Mesoproterozoic Sullivan Pb-Zn deposit and

Neoproterozoic basalt sequences associated with the Central

African Cu province are expressions of mantle plumes that dis-

persed the supercontinents Columbia and Rodinia, respec-

tively. Condie 1998, 2004) envisaged superevent cycles at 2.7,

1.9, and 1.2 Ga in which graveyards of subducted oceanic

lithosphere, stored at the 670-km D boundary, avalanched to

the core-mantle boundary, thus ejecting plumes from that

boundary and causing plume bombardment under the lithos-

phere Fig. 5). Larson 1991) associated the increased rate of

ocean crust formation at ridges and plateaus in the Pacific

Ocean with a superplume ejected from the core-mantle

boundary, coinciding with cessation of magnetic field reversals

at 41 Ma for a contrary view see Anderson, 1994).

Murphy and Nance 1992) recognized two principal styles

of supercontinent aggregation, which they termed internal


12/40

1108

KERRICH AL.

Kenorland

Columbia Rodinia

Grenville

Gondwana

PonAfrican

Pangea

Cordilleran/Alpine

A

Superior Dharwar Yilgarn TransHudson

-

ME

~

. JuvenileCrust

. Collision Orogens

. SuperplumeEvent

0-

0

.......

~

Q

E

:I

~

2.6

. .

1.3 1.1 0.9

Age Go

Superplume Events

0.7

0.5

0.3

.

0.1

.

B

Cyprus-type

VMS

Abitibi-type

Kuroko

Gold Veins

Gold ond Uranium Conglomerates

Porphyry

Deposits

Porphyry Molys

Porphyry Coppers

Uranium in Weathered Profile

Anorogenic

Intrusions

Kiruna-type

Olympic Dam-type

Ilmenite-anorthosite

Supercontinent Cycles

Ga 3.0 2.0 1.5 1.0

c

Kenorland Columbia Rodinia Gondwana

E.Gondwana

rw. ondw n~ Gondwana

/~

(Atlantica and /

other plates inAfrica) ~.

~ laurasia

/

Kazakhstan, N. China

S. China, and other

plates that formed Asia

Pangea

/ /

\

t

.Yilgarn N. India, E.Aust.

rc Ica

/

E.Antarctica Nena

/

4

Baltica

/

tlantica

FIG. 5. A. Secular distribution of collisional orogens and juvenile crust, with supercontinents (modified from Condie,

1997; Columbia after Zhao et aI.,2004). B. Secular distribution ofmineral deposits, modified from Meyer (1988). C. Super-

continent cycle,modified from Rodgers (1996).


13/40

MET LLOGENI PROVIN ES INAN EVOLVING GEODYN MI FR MEWORK

1109

and external. Internal aggregation corresponds to continent-

continent collision, for exmple, the Alpine-Himalayan, Ap-

palachian, and Grenville orogenic belts. External aggregation

corresponds to Cordilleran-style tectonics, where allochtho-

nous tectonostratigraphic terranes are transpressively ac-

creted to a continental margin. Neoarchean magmatic-accre-

tionary events in the Superior and Slave provinces of Canada,

Finland, southern Africa, India, and Western Australia likely

correspond to an early external supercontinent aggregation

that was associated with development of orogenic gold

provinces Kerrich and Wyman, 1994 . Internal cycles involve

internal oceans between continents. The North and South At-

lantic Oceans have opened and closed two or three times, as

North America-South America and Europe-Africa diverged

and then closed in Wilson cycles. The Pacific Ocean is an ex-

ternal ocean outboard of the external Cordilleran orogen.

Supercontinents may assemble in two configurations. In-

troversion involves breakup, opening then closing of interior

oceans, and reassembly. In extroversion, following supercon-

tinent dispersal, exterior margins of continental fragments ro-

tate and collide during reassembly. Combinations of the

processes may occur. The Paleozoic Appalachian-Caledonian-

Variscan orogen is an example of supercontinent introversion.

In contrast, during the Neoproterozoic East African and

Brasiliano orogens, the exterior ocean surrounding Rodinia,

which broke up at -7S0 Ma, was consumed dming the amal-

gamation of Gondwana, representing extroversion Murphy

and Nance, 2003 .

Metallogenic provinces in a supercontinent cycle framework

In an important synthesis for economic geology, Barley and

Groves 1992 showed that the temporal distribution of sev-

eral major classes of metallic mineral deposits can be related

to the cyclic aggregation and breakup of the continents in the

supercontinent cycle. Metal deposits related to continental

rifting sedimentary rock-hosted Cu and Pb would form

mainly during initiation of supercontinent fragmentation,

whereas deposits related to convergent tectonics porphyry

Cu, VMS, orogenic Au predominate during periods of sub-

duction and supercontinent aggregation Fig. S .

Superimposed on this -SOO-m.y. cycle are variations aris-

ing from preservation, thermal decay, and subtleties of tec-

tonic style. The scarcity of porphyry Cu and epithermal Au

deposits in rocks older than 200 Ma is widely considered to

be the consequence of their low preservation potential in

rapidly eroded magmatic arcs and collisional mountain

belts. Preservation potential is considered to be higher in

external Cordilleran style than internal continent-conti-

nent mountain belts Barley and Groves, 1992 . The

change in style of base metal-bearing VMS deposits, from

Archean Abitibi type to the Phanerozoic Kuroko and Cyprus

types, may reflect differences in style of subduction, nature

of the mantle wedge, and composition of arc magmas, and

these differences in turn stem from decreasing thermal gra-

dients. Archean crust is resistant to reworking in younger

orogenic events due to its thick, refractory continental

lithospheric mantle. This characteristic accounts for preser-

vation of the prodigiously rich orogenic gold provinces of

Neoarchean greenstone terranes Cordilleran-type accre-

tion , VMS back-arc camps of the Superior province, and

komatiite-associated Ni deposits Figs. 1,2,3, S; Kerrich et

al., 2000; Groves et aI., 200S .

The abundance ofVMS deposits in the Superior province,

particularly when compared to the sparseness of similar de-

posits in Neoarchean counterpart terranes of India, southern

Africa, and Western Australia, might be considered contra-

dictory to such a unified framework. However, volcanic rocks

in the Yilgarn craton of similar age to those of the Superior

province were generally erupted through continental crust

and, therefore, do not correspond to the more primitive

oceanic arc settings represented by the 2.7 Ga VMS-hosting

terranes in Canada Wymanet al., 1999 .

In summary, the empirical association of mineral deposit

classes with specific stages of the supercontinent cycle sup-

ports the precept that mineral deposits are products of par-

ticular geodynamic settings Fig. S .

Archean Geodynamics and Greenstone Terranes

Neoarchean greenstone-granitoid terranes show both differ-

ences from and similarities to Proterozoic and Phanerozoic

Cordilleran-type orogenic belts that formed by terrane accre-

tion at convergent margins Burke et al., 1976; Sleep and

Windley, 1982; Card and Ciesielski, 1986; Friend et al., 1988;

Sengor, 1990; Sleep, 1992; Windley, 1995; Polat et al., 1999 .

Komatiitic liquids stem from melting in anomalously hot man-

tle plumes. Their eruption temperature of 1,650C contrasts

with -1,200C for basalts. Komatiites are ubiquitous in

Archean greenstone terranes but are rare in Proterozoic or

Phanerozoic counterparts Arndt, 1994 .Together with basalts,

they represent intraoceanic plateaus or continental flood

basalts. Given higher mantle temperatures in Archean plumes,

plateau crust would have been thicker, -30 to SOkm Fig. 3

and thus not able to be subducted; rather, such crust was im-

bricated where plateaus jammed against convergent margins

Bickle, 1986;Abbott et aI., 1994a;Wyman et al., 1999 .

At Archean convergent margins, bimodal arc magmatism

involved slab dehydration and wedge melting, generating arc

basaltic liquids as in the Phanerozoic Pearce and Peate,

1995;Wyman, 2003 . However, given their high thorium con-

tents, trondhjemite-tonalite-granite TTG batholiths likely

formed as melts of enriched, gamet-amphibolite facies,

plateau basalt crust subcreted beneath the convergent mar-

gin, rather than depleted MORB-like crust Foley et aI.,

2002 . The TTG suite is characterized by a secular increase of

Mg number and Ni from 4 to 2 Ga, conferring evidence of the

involvement of a progressively thicker mantle wedge as sub-

duction steepened Martin and Moyen, 2002 . Models of the

thermal structure of the mantle predict a transition from flat

to steep subduction at -2.S Ga, in keeping with the distribu-

tion of TIG in Archean terranes and the transition in sedi-

mentary rock REE patterns at this time Abbott et al., 1994a;

Taylor and McLennan, 1995 . Given smaller plates, and a

commensurately longer global ridge system in the Archean

Hargraves, 1986 , ridge subduction would have been more

frequent, accounting for high heat flow in convergent mar-

gins, which was responsible for the abundant TIG Polat and

Kenich, 2004 .

Similarities between Neoarchean greenstone terranes and

Phanerozoic convergent margins include accretionary tecton-

ics, melanges, subduction-accretion complexes, ophiolites,


14/40

K RRI H T

AL.

and Cenozoic-type arc associations. The Superior province

was assembled by diachronous accretion in a Cordilleran-type

orogen from 2.74 to 2.65 Ga Card and Ciesielski, 1986;Card,

1990; Thurston et al., 1991; Percival et al., 1994; Calvert and

Ludden, 1999 . A few small melange occurrences have been

documented in Archean terranes Kusky, 1991;Wang et al.,

1996; Polat and Kerrich, 1999 , with melanges indicating the

presence of subduction-accretion complexes. Precambrian

ophiolites, reviewed by Kusky 2004 , indicate paleo-conver-

gent margins. Boninites have been recorded from several

Archean volcanic rock sequences, aswell as an association of

adakites, high Mg andesite, and Nb-enriched basalts, typical

of Cenozoic arcs that are linked to shallowsubduction of rel-

atively hot oceanic lithosphere Kerrich et al., 1998; Hollings,

2002; Polat et al., 2003 .

Neoarchean greenstone belts are now generally considered

to be Cordilleran-style collages of oceanic arc and plateau ter-

ranes, in which orogenesis was induced by plateaus jamming

against arcs. The composite arc-plateau crust was stabilized

by the residue of plume melting, coupled to the composite

crust as continental lithospheric mantle Wyman and Kerrich,

2002

At Archean convergent margins, shallow subduction

angles, -11-km-thick oceanic crust of which only the top -7

km was occasionally obducted and relatively high thermal

gradients, can explain the absence of blueschist-eclogite asso-

ciations and rare ophiolites that generally lack a mantle sec-

tion Figs.2E, 3; cf. Mooreset al.,2000

Metallogeny of Intraoceanic Arcs

Podiform r

Podiform bodies of spinel are an important resource of

chromium. Most of the deposits are in Caledonian or younger

suprasubduction zone ophiolites. Notable are the -500, -460,

and -370 Ma ophiolites of northwestern China, obducted

during accretion of arc terranes along composite sutures be-

tween the Kazakhstan, Siberian, and Tarim blocks; Ap-

palachian ophiolites; Hercynian ophiolites of Eurasia;

Tethyan Mesozoic ophiolites, including those in Turkey,

Oman, and Cyprus; and Mesozoic-Cenozoic ophiolites in ac-

creted terranes of the North American Cordillera. Rare pod-

iform chromitite bodies have been reported from a 3.0 Ga

ophiolite in the Ukraine, and the 2.5 Ga Zunhua ophiolite of

the North China craton Thayer, 1976;Duke, 1996a; Zhou et

al., 2001; Polat et aI., 2004 .

Podiform bodies are dominated by Cr-rich spinels en-

veloped by dunite in harzburgite of the mantle section, or the

crust-mantle transition, of oceanic lithosphere from intrao-

ceanic arcs. Podiform morphology reflects mantle flowpaths.

A current model for development of chromitite bodies in-

volves generation initially of hydrous basaltic melts in the

peridotitic mantle wedge from dehydration of the subducting

slab. Hydrous melts depolymerize, enhancing the octahedral

site preference for ci3+ Subsequent reaction of melt WIth

peridotite in an open system induces polymerization accom-

panied by precipitation of Cr spinel at -7-km depth and 0.2

GPa Fig. 2C; Edwards et al., 2000 .

Podiform chromite deposits reflect obduction of intrao-

ceanic arc crust-upper mantle sections in both continent-con-

tinent Appalachian, Tethyan and Cordilleran-type orogens

Figs. 1, 2C . Sparsity of these deposits in Precambrian ter-

ranes reflects the same process responsible for the absence of

blueschists and eclogites, or of complete ophiolite sections,

given that the upper basaltic sections of thicker oceanic

lithosphere were obducted Fig. 2E; Moores, 2002; Polat et

aI., 2004 .

VMS deposits

VMS deposits Franklin et al., 2005 form in oceanic

spreading centers, arcs, and rifts Hannington et al., 2005 ,

but mid-ocean-ridge crust is rarely preserved in the geologic

record due to the likelihood that oceanic lithosphere will be

subducted Cloos, 1993 . Many VMS deposits formed at

convergent margins under extensional conditions, specifically

in back arcs, where thinned and fractured lithosphere,

upwelling asthenosphere, and high-temperature magmas

generate long-lived high heat flow and enhanced hydraulic

conductivity Figs. 2C, 4E . Back-arc lithosphere is more

readily obductible, being young and hot. The fact that all

VMS deposits are associated with some mafic magmatism sig-

nifies a functional relationship to thermal anomalies in the

upper mantle Barrie and Hannington, 1999 . A lack of sig-

nificant VMS deposits in the Mesoproterozoic and Neopro-

terozoic Hutchinson, 1981; Meyer, 1981, 1988 reflects the

drift stage in dispersal of first Columbia and then Grenville

orogens that stitched together Rodinia. These orogens now

expose deep erosional levels, which is ultimately due to de-

lamination of mantle lithosphere Fig. 5 .

Based on rock associations, and therefore tectonic setting,

Barrie and Hannington 1999 and Franklin et al. 2005

classified VMS deposits into five groups. Mafic and bimodal

siliciclastic rock-associated deposits are mainly restricted to

the Phanerozoic. The former consists of tholeiitic with minor

boninitic rocks and includes ocean-ridge deposits that were

obducted as part of ophiolite fragments, exemplified by

Tethyan ores of Cyprus and Turkey. The geodynamic setting

is a suprasubduction zone, and such magma-ore associations

extend to the Paleoproterozoic Flin Flon VMS province

Wyman, 1999 . The latter, characterized by large tonnages

with high Pb but low Cu contents, formed in a continental

arc or back-arc setting; VMS ores of the Bathurst and Iber-

ian Pyrite Belt provinces are prominent examples of this

group.

The other three groups ofVMS deposits have broader sec-

ular distributions. Bimodal-mafic and bimodal-felsic group

deposits occur in oceanic terranes back to the Neoarchean of

some cratons. The former represent primitive oceanic arcs or

back arcs; examples include Noranda and Matagami, Quebec,

some ores of Flin Flon, Saskatchewan, and Manitoba, and

Jerome, Arizona. The latter represents precipitation of VMS

deposits in mature arcs, such as the Mt. Read district, Tasma-

nia. A mafic volcanic-volcaniclastic rock and turbidite associ-

ation with VMS formation occurred from the Mesoprotero-

zoic through the Phanerozoic. These deposits developed in

sediment-rich oceanic rifts, notably Windy Craggy, British

Columbia, or in propagating continental rifts, exemplified by

the Besshi district of Japan. The Middle Valleyand Escanaba

trough, and the Sea of Cortez, are present-day metal-rich

analogs to these two environments in the final group, respec-

tively Barrie and Hannington, 1999 .


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MET LLOGENI PROVIN ES IN N EVOLVING GEODYN MI FR MEWORK

Intraoceanic and Continental Margin Arc

Porphyry- Epithermal Systems

Porphyry Cu-Mo-Au hereafter referred to as porphyry Cu)

and related epithermal Au-Ag deposits are predominantly, but

not exclusively, a Phanerozoic occurrence Seedorff et al.,

2005; Simmons et al., 2005). The majority of both deposit types

occur in Mesozoic and Cenozoic subduction-related subvol-

canic plutonic complexes and related volcano-sedimentary se-

quences, but this may be in part a function of the low preser-

vation potential of shallow-level crustal sequences within active

convergent plate margins. Rapid uplift and erosion, tectonic

erosion, and collision either with oceanic terranes such as is-

land arcs, seamounts, or plateaus, or with continental masses)

commonly result in destruction of supracrustal sequences in

both oceanic and continental volcanic arcs. Nevertheless, de-

posits of both types do occur in older terranes, but with in-

creasing rarity back to the Mesoarchean, to the point that Pre-

cambrian occurrences in Australia, Canada, India, and

Scandinavia are noted as exceptions; the earliest known de-

posits are -3.3 Ga in age Barley, 1982). The characteristics of

Precambrian deposits are little different from those of their

Phanerozoic counterparts GaaI and Isohanni, 1979; Barley,

1982; Roth et al., 1991; Fraser, 1993; Sikka and Nehru, 1997;

Stein et al., 2004), suggesting that similar tectonomagmatic

processes were involved in their formation.

Porphyry u deposits

Porphyry Cu deposits show one of the clearest relationships

to specific plate tectonic processes of any ore deposit type

Fig. 6; Sillitoe, 1972; Burnham, 1981). The relationship to

subduction of oceanic crust relates primarily to the large flux

of water and other volatiles from the slab into the overlying

asthenospheric mantle wedge. As recently reviewed by

Richards 2003; see also Candela and Piccoli, 2005), these

volatiles metasomatize the mantle wedge and reduce its melt-

ing point, such that hydrous basaltic magmas are produced by

partial melting in the highest temperature regions. These

melts are the ultimate sources of more evolved magmas that

are emplaced into the overlying crust and which may gener-

ate porphyry and related epithermal deposits.

Subduction represents the return flowof materials into the

mantle to compensate for the creation of new oceanic lithos-

phere at mid-ocean ridges. But processes of sea-floor meta-

morphism, resulting in hydration and introduction of other

sea water-derived elements, such as S, CI, and alkalis ex-

changed for Ca), mean that the return flow ismodified from

the original MORB composition. Upon return into the man-

tle, these same water-soluble elements are released during

prograde dehydration reactions, whereby minerals such as

serpentine, amphibole, chlorite, zoisite, and lawsonite Fig.6;

Tatsumi, 1986; Schmidt and Poli, 1998; Winter, 2001;

Forneris and Holloway,2 3

are converted to progressively

more anhydrous blueschist- and eclogite-facies assemblages.

Additional components may be added by subduction of sea-

floor sediment and tectonic erosion of upper plate rocks e.g.,

de Hoog et aI., 2001).

Basaltic crust of the downgoing slab may partially melt

where the lithosphere is young


16/40

1112

KERRICH ET L

A

,

,

, ,

, ,

,

,

,

,

,

14OQC

,

Transpression High Crustal

Permeability at Releasing ends

Composite volcanoes develop

above shallow magma chambers;

high potential for PCD formation

FIG. 6. A. Normal subduction configuration beneath a continental arc from Richards, 2003; modified from Winter, 2001 .

Slab dehydration leads to hydration of the overlying asthenospheric mantle wedge and partial melting in the hotter central

regions of the wedge. Hydrous basaltic melts pool at the base of the crust due to density contrasts, where they fractionate,

release heat, and interact with crustal materials to generate more evolved, less dense andesitic magmas by melting, assimi-

lation, storage, and homogenization-MASH process of Hildreth and Moorbath, 1988 , which can then rise to upper crustal

levels. It is these evolved magmas that are directly associated with porphyry Cu deposit formation. B. Oblique convergence

leads to the generation of structurally permeable transpressional sites along trench-linked strike-slip faults, up which magma

may ascend from lower crustal MASH zones. Rapid, voluminous emplacement of magmas in the upper crust is regarded here

to be a prerequisite for the subsequent formation of large porphyry Cu deposits by magmatic-hydrothermal fluid exsolution.

crust. If the rate and volume of supply of magma is limited,

then so too will be the flux of heat, metals, and other ore-

forming components Fig. 6 . This constraint implies that the

largest porphyry systems will be associated with long-lived

and voluminous arc magmatic suites.

Tosdal and Richards 2001 and Richards 2003 reviewed

structural controls on the emplacement of porphyry magmas

in the upper crust and argued that tectonic stresses acting on

a regional architecture of translithospheric structures may in-

fluence the location of magma ascent by providing relatively

permeable pathways. Optimal sites are extensional structural

domains formed at jogs and stepovers in large strike-slip fault

systems deforming under mildly oblique compressional stress

Fig. 6B . Although magma ascent can occur in the absence

of such structures, their existence may act to focus magma

flux, thus enhancing subsequent ore-forming potential.


17/40


spatial relationship of ore deposits to such structural nodes,

often recognized in regional exploration as lineament inter-

sections, has been noted in many porphyry and related ep-

ithermal districts e.g., Corbett and Leach, 1998; Sasso and

Clark, 1998; Padilla Garza et al., 2001; Richards et aI., 2001;

Chemicoff et aI., 2002; Sapiie and Cloos, 2004 .

Other models for porphyry Cu formation have invoked the

direct involvement of slab melts adakites; Sajona and Maury,

1998; Oyarzun et aI., 2001 or the role of crustal thickening

and shallowing of subduction angle in affecting magma gen-

eration and composition Kay et aI., 1999 . However, al-

though these processes may be important locally,they do not

seem to be universally applicable, and a more general rela-

tionship to subduction magmatism is implied. Variations

among the porphyry suite may arise from the wide variety of

possible tectonic configurations in subduction zones, and spe-

cific events or combinations of events may cumulatively act to

maximize or reduce porphyry-forming potential. Notably,

differences in porphyry systems in oceanic versus continental

arcs occur mainly in subtle details and not in overall

processes. Oceanic arc systems tend to be associated with

somewhat more mafic dioritic plutonic rocks, whereas con-

tinental arc systems are typically associated with more felsic

systems Hollister, 1975;Kesler et al., 1975 . There is a com-

mon tendency for oceanic systems also to be somewhat more

Au versus Mo rich in continental systems, although manyex-

ceptions exist. Both of these variations may relate to the de-

gree of fractionation and crustal interaction experienced by

the primary magmas oceanic systems representing more

primitive systems and continental porphyries being more

fractionated loss of Au and contaminated with crustal com-

ponents higher Mo; Farmer and DePaolo, 1984; Blevin and

Chappell, 1992 .

Epithermal u g deposits

Historically, an understanding of the relationship between

shallow-level epithermal Au-Agdeposits and subvolcanic por-

phyry systems was slower to develop than the overall rela-

tionship to convergent plate margins. This was primarily due

to problems of preservation and exposure level, which meant

that where near-surface deposits were preserved, erosion had

not penetrated deeply enough to reveal underlying mag-

matic-hydrothermal systems. Conversely,where porphyry de-

posits were exposed, overlying epithermal deposits had al-

ready been removed. Consequently, near-surface advanced

argillic alteration, characteristic of high-sulfidation-type ep-

ithermal deposits, was not included in the classic model of

porphyry alteration and mineralization zoning of Lowell and

Guilbert 1970 . Nevertheless, Sillitoe 1973 made an early

connection between porphyry formation and surficial vol-

canic and fumarolic activity,and later studies, such as those of

the adjacent Far Southeast porphyry and Lepanto high-sul-

fidation epithermal deposits by Arribas et al. 1995 and

Hedenquist et aI. 1998 , clearly demonstrated a connection

between these distinct ore-forming environments. As such,

the tectonic controls on high-sulfidation epithermal mineral-

ization are closely related to those affecting porphyry de-

posits. However, economic deposits of both types need not

form together, because local details of fluid evolution, trans-

port, and deposition processes may favor ore deposition in

one or the other environment but not necessarily both envi-

ronments. For example, Bissig et al. 2002 recently proposed

that regional uplift and erosion history was critical in control-

ling the development of mineralized epithermal systems in

the EI Indio-Pascua belt Chile and Argentina , which are as-

sociated only with apparently barren plutons. Thus, drilling

beneath a known epithermal deposit will not necessarily re-

veal an economic porphyry deposit, although evidence of a

high-temperature magmatic hydrothermal system is likely to

be encountered.

Unlike high-sulfidation systems, low-sulfidation epithermal

deposits do not show a clear, exclusive relationship to sub-

duction zone magmatism, and many deposits are generated

by thermal anomalies caused by crustal extension, such a

epithermal Au-Ag deposits in the Basin and Range district,

Nevada Berger and Bonham, 1990; John, 2001; Simmons

et al., 2005 . In this respect, the involvement of specific

magmatic components both volatiles and metals in low-sul-

fidation epithermal systems is less clear, and the key input fo

such systems may simply be a heat source of any origin. By

contrast, intermediate-sulfidation epithermal systems are

commonly found in porphyry districts, and either a direct o

distal association with magmatism has been proposed in many

instances e.g., Rye, 1993; Hedenquist et aI., 1996; Hayba,

1997; Faure et al., 2002 . A common structural control on

most epithermal-type deposits is extensional faulting and

brecciation, either generated regionally by tectonic stress

fields as in the case of the Basin and Range or locally by

forces involved with magma emplacement crustal doming

or by elevated fluid pressure hydraulic fracturing . The latter

tectonic condition is commonly generated in association with

porphyry formation but not exclusively so.

Metallogeny of Cordilleran Orogens

Metallogenic context

In contrast to the shallow crustal regions that characterize

continental magmatic arcs, as described above, much of an

evolved orogen exposes rocks that were deformed and meta

morphosed at deeper crustal levels. Crustal rocks that woul

have hosted porphyry and related epithermal mineral de

posits are typically unroofed and eroded in fore- and back-ar

regions. The exposed middle crustal rocks in these regions ar

dominated, in contrast, by mineral deposits that reflec

deeper hydrothermal processes that are active in convergen

to transform continental margins. These processes form

mainly orogenic Au deposits, with commonly related As,W

Sb, and Hg resources. In addition, preaccretionary minera

deposits, such as podiform Cr and VMS deposits that wer

described above, may also be present and hosted within th

same blocks of accreted juvenile crust Fig. lA .

High heat flowand intense fluid regimes are important tec

tonic features inherent to most Cordilleran orogens. The gen

eration of Barrovian P-T conditions is typical for progressiv

accretion of a broad zone of radiogenic juvenile materia

scraped off a downgoing slab, where clockwise P-T-time tra

jectories generate deeper and later metamorphism. Unde

these heat-flow conditions, peak metamorphism at mid-crusta

levels greenschist facies predates peak metamorphism i

the deeper crust, such that fluids generated by dehydration


18/40

1114

KERRI H ET AL.

reactions in deeper crust advect to the mid-crust where they

overprint the peak-metamorphic assemblage McCuaig and

Kerrich, 1998).Within about 15m.y. of accretion, large areas

of the mid-crust willbegin to experience a significant rise in

geotherms e.g., Jamieson et aI., 1998). The causes of the

thermal episode are complex; increased radioactive heat pro-

duction of accreted material isthe most commonly cited trig-

ger, but shear heating, massive fluid flow, crustal thickening,

ridge subduction, or slab rollback are all processes that may

add heat into the growing continental margin. Rapid uplift

and/or continued outboard subduction typically yields a pat-

tern of inverted isotherms, such that more highly metamor-

phosed rocks are thrust above lower grade rocks Peacock,

1987).

Fluid reservoirs are present both in the subducted slab, as

noted above, and in the accreted sedimentary and volcanic

rock sequences. As described above, slab devolatilization

releases volatiles into the overlying mantle wedge. The pro-

grading accreted juvenile crust represents a significant sec-

ond reservoir, with voluminous fluid release across various

metamorphic isograds Fyfe et al., 1978; Powell et al., 1991).

Estimates for progressive metamorphism of an average pelite

are that about 5 vol percent of the rock will be lost to the fluid

phase at metamorphic reaction boundaries e.g., Walther and

Orville, 1982). Fluids released at greenschist- and amphibo-

lite-facies conditions typically consist of H2O, CO2, CH4, and

N2 Mullis, 1979), aswell as relative enrichments of H2S from

desulfidation reactions Ferry, 1981), therefore explaining the

dominance of c-o- H-N-S fluids in Cordilleran orogens. Spe-

cific volatile composition of these fluids generated during

metamorphism will depend on the composition of the juve-

nile rocks, particularly on the clay, carbonate, and organic

matter content Yardley,1997). Numerous studies see sum-

mary by Goldfarb et al., 2005) also indicate a progressive mo-

bilization of As, Au, B, Hg, Sb, and W in such fluids with in-

creasing degree of metamorphism. Concentration of these

species in metamorphic fluids may determine, to a large part,

Formerly Active

Magmatic Arc.

300 C

epth in rust

Subgreenschist

10

km- --+--+---

Greenschist

00 C

SOO C

600 C

mineral resource potential within Cordilleran orogens. Silica

metasomatism in both the mantle wedge and overlyingcrust

is commonplace Manning, 1997) and, as a result, there is a

consistent association of epigenetic ore deposits in metamor-

phic environments with large quartz vein systems Fig.7).

Orogenic u

The fore-arc regions of Cordilleran orogens inherently are

characterized bywidespread orogenic gold deposits. The type

Cordilleran orogen of western North America, which is still

evolving, may have begun to form anywhere from 400 to 200

m.y.ago, depending on how an orogen is defined. Subsequent

to Rodinian rifting in the Neoproterozoic, the Pacific margin

of North America was the passive margin site of sedimenta-

tion through the Middle Devonian Dickinson, 2004). Bythe

Late Devonian, convergent tectonism began along the mar-

gin, with the Late Devonian to Early Mississippian Antler or

Ellesmerian in the far north) and Late Permian to Early Tri-

assic Sonoma allochthons of oceanic rocks being thrust over

the miogeoclinal shelf edge Burchfiel et al., 1992). Such ob-

duction of oceanic rocks was not associated with any type of

subduction zone geodynamics, continental arc development,

or metamorphism, and this low-temperature tectonism also

lacked any associated ore deposit formation of significance.

Cordilleran orogenesis essentially began with the accretion of

more than 200 terranes along the seaward side of the former

passive margin post-Early Triassic Fig.lA; Coney et al., 1980;

Monger et aI., 1982). The exact time of initiation of simulta-

neous slab subduction and terrane accretion, and thus the

best estimate of the start of orogenesis, could be any time be-

tween -240 and 70 Ma. Moores et al. 1999) noted that there

is a lack of evidence for such terrane collision along much of

the margin prior to the younger part of this age range.

With the onset of subduction-accretion and the deeper and

later style of metamorphism, economically significant oro-

genic Au deposits have formed within mainly greenschist fa-

cies rocks of the Cordilleran orogen for probably the last 170

FIG. 7. Cordilleran-type orogens are recognized for the widespread distribution of orogenic gold deposits in metamor-

phosed juvenile rocks on either side of the magmatic arc. Ore-forming fluids in the fore arc may be derived from prograde

metamorphism of accreted material above a subducting slab and from the slab itself; where slab fluids are released into the

mantle wedge, mantle-derived melts may carry some of the fluid into the accreted oceanic rocks. The metalliferous fluids

are focused along major crustal shear zones in the fore are, which previously may have been sites of terrane suturing.


19/40

MET LLOGENI PROVIN ES IN AN EVOLVING GEODYN MI FR MEWORK

m.y. Fig. 5; Goldfarb et aI., 2001). The youngest such ores

are the -50 Ma gold deposits on Chichagof Island, southeast-

ern Alaska. However, it is likely that younger orogenic gold

deposits have formed at depth within the fore arc of the oro-

gen since the middle Eocene, but these mid-crustal ore-host-

ing regimes have yet to be uplifted and exposed at the surface

Fig. 7; Goldfarb et aI., 2000).

The most significant lode deposits are associated with ter-

rane-bounding fault systems. Where no such major conduits

occur within a deeper and later thermal sequence, veins are

smaller and more widely distributed, and world-classeconomic

gold lodes are unlikely to have formed e.g., Chugach Moun-

tains/Kenai peninsula, Nome, Klondike). Both exotic oceanic

blocks, such as hosts for the Mother Lode and Juneau gold

belt, and terranes of pericratonic miogeoclinalstrata, including

the Fairbanks and Klondike districts in the Yukon-Tananater-

rane, all of which were translated along the North American

margin, are equally likely to host orogenic gold deposits.

The oldest gold lodes in the North American Cordillera are

those of Middle and Late Jurassic in the Canadian sector and

Late Jurassic to Early Cretaceous in California Goldfarb et

al., 1998). In Alaska, both gold and arcs young seaward, from

-100 Ma in the north and interior to -60 to 50 Ma along the

present-day active margin. Typically, the orogenic gold

provinces occur at geologic and structurally favorable loca-

tions in terranes of the fore arc, such as within the Juneau

gold belt and Sierra foothills. However, where arcs are rela-

tively diffuse, rather than occurring as distinct Andean-style

batholiths, important lodes occur within an evolving arc in-

cluding hosts to deposits of the Klamath Mountains and Fair-

banks districts). Where well-defined batholiths have already

been crystallized and are in the process of regional uplift,

competent margins to these igneous masses may also host

orogenic gold deposits e.g., Willow Creek). In addition to a

number of small orogenic gold deposits in the Cordilleran

back-arc regions e.g., Polaris-Taku, northern British Colum-

bia; Humboldt Range, Nevada), the world-class Late Creta-

ceous Bridge River deposit in southern British Columbia in-

dicates important orogenic gold ore formation, as well as

subduction-related plutonism, may also continue landward

into oceanic terranes inboard of an evolving continental mar-

gin arc. The thermal profile of a Cordilleran orogen, rather

than simply a geographic location in a growing margin, ap-

parently controls fluid evolution and ore genesis in the

oceanic rocks McCuaig and Kerrich, 1998). Indeed, a similar

arc to back-arc position characterizes many of the Late Juras-

sic-Cretaceous orogenic gold deposits in the deformed terri-

geneous rocks to the west of the Siberian craton in eastern

Russia Fridovsky and Prokopiev, 2002).

The Altaid orogen presents a similar Au-rich Cordilleran-

type orogen composed of Vendian through Jurassic units

accreted to the margins of the Siberian craton Sengor and

Natal in, 1996b). Inclusion of the Baikalides and Uralides,

both containing important Paleozoic orogenic gold provinces,

remains controversial Sengor, 1993). Tectonism and defor-

mation span the entire duration of the Paleozoic. Giant Early

Permian orogenic gold deposits e.g., Muruntau, Zarmitan,

Kumtor, Sawyaerdun) continue along the length of the oro-

gen in what is probably one of the outermost accreted ter-

ranes Yakubchuket al., 2002 In a pattern similarto that

observed in Alaska, older orogenic gold provinces reflect ear-

lier subduction closer to the craton margin. Giant deposits

such as Olympiada and Zun-Kholba formed in Proterozoic

terranes along the southwestern side of the craton in the lat-

est Neoproterozoic and early Paleozoic, followed by ores in

more seaward regions of Kazakhstan and the Urals in the

mid-Paleozoic, and then the Permian ores developed along

the edge of the closing Paleo-Tethyan Ocean Herrington et

al.,2005;Yakubchuket al.,

2005

Significant characteristics of the Altaid orogen Yakubchuk

et al., 2005) illustrate other broad tectonic controls on oro-

genic gold in Cordilleran orogens. First, the immense gold re-

source at the Sukhoi Log deposit, probably of mid-Paleozoic

age Goldfarb et al., 2001), is hosted by carbonaceous and

pyrite-rich flysch in a retroarc location within complexly de-

formed Neoproterozoic pericratonic Baikal terranes Bulga-

tov and Gordiyenko, 1999). The thermal event associated

with emplacement of the immense Angara-Vitim batholith

Yarmolyuket al., 1998) correlates with the major period of

orogenic gold deposit formation within 100 km of the craton.

Thus, there are clearly significant exceptions to the general

observation that orogenic gold ores in a Cordilleran orogen

will always be younger in an oceanward direction. Second,

with the exception of this Baikal region, large gold placers,

such as those that dominate the circum-Pacific goldfields, are

absent. Perhaps this reflects the fact that continent -continent

collision closed the Altaid orogen and has, at least temporar-

ily, formed a Paleozoic craton. This preserved paleo-

Cordilleran margin has thus not been susceptible to rework-

ing and erosion of significant amounts of its contained lode

gold systems. Further support for such a concept isthat much

of the interior of the Altaid orogen still contains numerous

Paleozoic porphyry and epithermal deposits Yakubchuk et

aI., 2002, 2005), whereas such shallow crustal levels have

been already removed by uplift and erosion from many of the

circum-Pacific Cordilleran terranes.

The Paleozoic Tasman orogen of eastern Australia, which

includes the gold-rich Thomson, Hodgkinson-Broken River,

and, particularly, Lachlan fold belts, may also be considered

an accretionary orogen but with important differences from

the more classic Cordilleran-type orogens of western North

America and the Altaids. Rather than a series of accreted ter-

ranes, much of the more deformed and metamorphosed sec-

tors of the orogen reflect a single, quartz-rich turbidite fan

system shed off the Delamerian-Ross highlands in the earliest

Paleozoic. Ordovician-Silurian orogenesis was dominated by

shortening and folding, as is typical of Cordilleran orogens,

but these were thin-skinned tectonic events and lacked any

major uplift of basement blocks Coney, 1992; Goldfarb et aI.,

1998). This difference in crustal response may be indicative

of subduction and/or accretion in association with a large fan

system, rather than a series of terranes, along a continental

margin Gray and Foster, 2000). The extensive ores of the

Victorian goldfields formed during Late Ordovician deforma-

tion, metamorphism, and subduction in the western province

of the Lachlan fold belt -440 Ma: Bierlein et aI., 2001); how-

ever, no magmatic arc developed during subduction beneath

the deforming turbidite wedge Fergusson, 2003).

Thrust-fault development and uplift of the Victorian ore

host rocks began at -455 Ma, with perhaps slab rollback -15


20/40

KERRI H ET AL.

my later, providing the main thermal event related to gold

formation (Squire and Miller, 2003). Therefore, the Tasman

orogen scenario suggests that deformation, heating, and uplift

of juvenile material along a margin may be essential to fluid

production, fluid migration, and related lode gold formation,

regardless of the presence of associated magmatism or an

abundance ofwell-defined terrane-bounding fault zones.

The Otago schist belt of the South Island of New Zealand

may be more like a classic Cordilleran orogen, or at least a

part of such an orogen, but alsowithout any magmatic arc ac-

tivity recognized in the gold-hosting terranes. This Permian-

Cretaceous accretionary wedge contains a number of terranes

that likely amalgamated and were simultaneously deformed

and metamorphosed in Late Jurassic-Early Cretaceous (Mor-

timer, 1993;Gray and Foster, 2004). Orogenic gold formation

occurred in the schists during this deformation and uplift,

probably at -150 to 130 Ma (Craw, 2002). This time is ap-

proximately mid-way through the 150-m.y.-Iong episode of

terrane translation along the margin of East Gondwana

(Pickard et al., 2000), such that hydrothermal activity oc-

curred within the activelydeforming rocks as they were partly

between their original location off the northeastern coast of

Australia and present South Island location.

Another variation on a Cordilleran-style margin might be

east-central Asia, where terranes that now form the Japanese

islands and southeastern Russiawere at one time immediately

seaward of the eastern margin of China and have since un-

dergone significant Jurassic(?)-Cretacous strike-slip transla-

tion (Sengor and Natal in, 1996b; Charvet et al., 1999). The

resulting Cenozoic configuration, subsequent to northward

migration of the entire subduction and/or accretion complex,

includes rocks of the North China craton now located imme-

diately along the Pacific margin. These Precambrian rocks, as

well

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