Download A Plate Tectonic Model for the Origin of Porphyry Copper Deposits

Economic Geology
Vol. 67, 1972, pp. 184-197
A Plate TectonicModel for the Origin
PorphyryCopper Deposits
RICHARD H. SILLITOE
Abstract
The theory of lithosphereplate tectonics,embodyingthe conceptsof sea-floorspreading, transform faulting, and underthrusting at continental margins and island arcs, is
employedas a basis for an actualistic, though speculative,model for the origin and
space-timedistribution of porphyry copper and porphyry molybdenumdeposits.
Porphyry ore deposits,occurring in the western Americas, southwest Pacific and
Alpide orogenicbelts, are thought to constitutea normal facet of calc-alkalinemagmatism. Chemical and isotopicdata cited are consistentwith the generation of the componentsof calc-alkalineigneousrocks and porphyry ore depositsby partial melting of
oceaniccrustal rocks on underlying subductionzonesat the elongatecompressirejuncturesbetweenlithosphericplates.
It is proposedthat the metalscontainedin porphyry ore depositswere derived from
the mantleat divergentplate junctures,the oceanrises,as associates
of basicmagmatism,
and transported laterally to subductionzones as componentsof basaltic-gabbroic
oceaniccrust and small amountsof suprajacentpelagic sediments;evidencesupporting
the presenceof significantamountsof metals in the oceaniccrust is listed.
It is suggestedthat the temporaland spatial distributionof porphyry ore depositsis
dependenton two principal factors, namely the erosion level of an intrusive-volcanic
chain, and the time and locationof magmageneration,and the availability of metals,on
an underlyingsubductionzone. The erosionfactor is beli.evedto offer an explanation
for the paucityof porphyryore depositsin pre-Mesozoicorogenicbelts, and for the
relative abundanceof exposedporphyry depositsof Upper Cretaceous-Paleogene
age in
post-Paleozoic
orogens. Provinceswith a high concentrationof porphyrycopperdeposits,suchas southernPeru-northernChile and the southwest
United States,may be
interpretedas regionsbeneathwhich anomalously
copper-richoceaniccrust was subductedat the time of porphyrycopperemplacement;
one possibleexplanationfor the
episodicformationof volumesof copper-richoceaniccrust is the presenceof a hetero-
geneous
distributionof metalsin the low velocityzoneof the uppermantle. Porphyry
ore depositsseemto have formedduring a seriesof relativelyshort, discretepulses,
perhapscorrelablewith changesin the relative rates and directionsof motion of
lithospheric
plates. In someregions,suchas Chile,porphyryore deposits
are arranged
in parallel,linearbelts,whichmaybe explicablein termsof shiftingloci of magmaand
includedmetal generationon a subductionzone, and which seemto be largely indepen-
dent of controlby tectoniclineamentintersections.The time intervalsduring which
the formationof porphyrydeposits
tookplaceare shownto be broadlycoincident
with
periodsof lithosphere
plate convergence,
and porphyrydepositsmay still be forming
abovecurrentlyactivesubduction
zones.
A numberof potentialregionsfor the discoveryof porphyryore depositsare suggested,andthe importance
to exploration
of analyzingorogenicbeltsin termsof plate
tectonicsis emphasized.
into six large and severalsmallerlithosphericplates
Tn•. sea-floorspreadinghypothesis
of Dietz (1961) (Fig. 2), which accreteat oceanrises by uprise of
andHess(1962) hasrecentlybeenfurtherdeveloped subcrustalbasic magrnas,slide past one another
by Isacks, Oliver and Sykes (1968), Le Pichon along transform (horizontal shear) faults, and are
(1968), McKenzieand Parker (1967) and Morgan destroyedat trench systemsby descent into the
(1968), resultingin the theoryof lithosphereplate asthenospheredown inclined subduction(Benioff)
tectonics
(the newglobaltectonics),
theonlyexisting zones(Fig. 3).
The conceptsof the new global tectonicshave
globaltectonicmodelcompatiblewith muchrecently
accumulatedgeologicaland geophysicaldata. The renderedthe stabilistgeosynclinaltheory of orogeny
model considers the Earth's surface to be divided
outmoded;it is now realizedthat a generalizedmodel
Introduction
184
MODEL
FOR ORIGIN
OF PORPHYRY
COPPER
DEPOSITS
185
for geosynclinal
development,
commencing
with the posits,from which molybdenumis the principal
accumulationof a thick volcanicand sedimentary metalrecovered,
aresimilarto thoseof theporphyry
pile, and followedby plutonismand deformation
and coppers.Hypogeneore gradein the porphyrycopfinally by epeirogenicuplift and volcanism(e.g., persrarelyexceeds
1% Cu, and is commonly
below
Beloussov,
1962) doesnot satisfactorily
explainthe 0.5% Cu.
evolutionof mostorogens(Coney,1970). In terms
The hostintrusionsof porphyrycopperdeposits,
of the theoryof platetectonics,
geosynclines
may be andtheirvarioustypesof count-ry
rocks,maybothbe
equatedwith oceansand continentalmargins, and ore-bearing,and are characterized
by widespread,
oceansand island arcs. Mountain systemsare gen- zonally-arranged
hydrothermal
alteration,commonly
erated as a consequence
of the underthrustingof of potassic,phyllic, argillic and propylitic types
oceaniclithosphere
beneathan adjacentplate at con- (Meyer and Hemley, 1967; Lowell and Guilbert,
tinental margins (-e.g., the cordilleran system of 1970), and by hydrothermalbrecciation.
The close association of intrusion and mineralizaNorth and South America), or at island arcs (e.g.,
Japan). In some instances,eventual continent-con- tion in porphyrycopperand molybdenum
deposits
tinent collision(e.g., the Himalayas) or continent- is emphasized
by K-Ar dating which has demon-
islandarc collision(e.g., New Guinea) are involved
in orogenicdevelopment(Dewey and Bird, 1970;
Dewey and Horsfield, 1970). It shouldbe stressed,
however,that eachorogenicbelt displaysan essentially unique sequentialhistory, even though certain
stratedthe two processes
to be temporallyindistinguishable
in severalinstances(e.g., Livingston,
Mauger and Damon, 1968; Moore, Lanphereand
Obradovich,1968; Laughlin,Rehrig and Mauger,
1969). In at least some deposits,part of the
sequencesof eventsare more commonthan others.
mineralization
mayin factbe syngenetic
with respect
intrusiverock (e.g., Ely, Nevada;
Currently acceptedconceptsof the metallogenesis to the associated
of post-Precambrianorogenic belts are based on Fournier, 1967). The intimatetemporalassociation
lendssupportto the
stabilist geotectonicdoctrine (Bilibin, 1968; Mc- of intrusionand mineralization
model of porphyry coppergenesis
Cartney and Potter, 1962; McCartney, 1964), and orthomagrnatic
supportthe association
of distincttypesof mineraliz- (Nielsen, 1968; Lowell and Guilbert, 1970). Acation with each stage of geosynclinaldevelopment. cordingto this model, a felsic magma,becoming
Porphyrycopperdeposits,for instance,are considered water-saturated as it intrudes towards the surface
to typify the post-orogenic,
late tectonicstage (Mc- zone, undergoescrystallizationof its outer parts,
brecciated
by the releaseof
Cartney and Potter, 1962). In view of the recent whichare subsequently
advancesin geotectonictheory, it would therefore accumulatedfluids,which alsoproducethe alteration
seemopportuneto reexaminethe metallogenesis
of and mineralization. Meteoric waters are involved in
orogenicbelts in terms of the new globaltectonics. the formationof the outer zonesof hydrothermal
This paperoutlinesa platetectonicmodelto account alteration and mineralization.
for the genesisand distributionin both spaceand
time of one important class of mineralization,the
The Origin of Calc-Alkaline Igneous Rocks
porphyrycopperand porphyrymolybdenum
deposits.
and Porphyry Copper Deposits
Someaspectsof the modelwere presentedelsewhere
l/l/orldDistributionoI Porphyry Ore Deposits
as an abstract (Sillitoe, 1970). It shouldperhaps
be stressedthat the proposedmodelis of a speculative Figure 1 showsthe locationof the majorityof exnature, and does not purport to embody rigorous ploitedporphyrycopperand molybdenumdeposits,
proofsof its validity.
and of manyimportantprospects,
whichin Figure 2
are relatedto Mesozoic-Cenozoic
orogenicbeltsand
currently active lithosphericplate boundaries. It
Geologicaland Genetic Characteristicsof
can be appreciatedthat the majority of the world's
Porphyry Copper Deposits
porphyry depositsare locatedin the circum-Pacific
Over one half of the world'scopperproductionis orogenicbelts and in the central portion of the
currently derived from porphyry copper deposits, Alpide orogenicbelt. The western Americas belt,
large tonnage (commonlyexceeding500 m tons), containingmost of the known porphyrycopperdelow grade, roughly equidimensional
depositsof dis- posits,extendsfrom westernArgentina and central
seminatedand stockwork-veinlet,pyrite-chalcopyrite and northernChile,throughPeru, Ecuador,Panama,
mineralization,carrying at least trace amounts of Mexico, the western United States (Arizona, New
molybdenum,gold and silver. They are spatially Mexico, Nevada,Utah, Colorado,Idaho, Washingandgeneticallyrelatedto passively-emplaced
hypabys- ton and Montana), to British Columbia, the Yukon
sal felsicstocks,commonlyporphyries. The geologi- andAlaska. Markedconcentrations
of deposits
occur
cal characteristics
of the porphyrymolybdenum
de- in Sonora-Arizona-New Mexico and in Brit.i•sh.
186
RICHARD
H. SILLITOE
ignimbrites.Thesevolcanicrocksare commonlyobserved to be intruded or underlain by extensive
ß P•d•PHYRY COPPER
ALPIDE BELT
ß PO•YRy
IIOL'Y'B•E#UM
•POSITS
batholiths
ra•AllNl
tlld
CASINO,
KE#lIaNOl•T•l½•
e•
•4$TMICT.W•ST
P&#ISTIN
WESTERN
AMERICAS
and smaller intrusions
volcanic nature
BELT
L.O•ADO
ß
'
ø ß NEV MCX•O-•NOnA
PUERTO RiCO
SOUTHWEST
PACIFIC BELT
of a similar
com-
position. Hamilton and Myers (1967) and Hamilton (1969a, b) have convincinglydemonstrated
the
consanguinity
of the calc-alkalinevolcanicsuite and
the spatiallyrelated felsic plutonicrocks,the latter
interpretedas the roots of major eruptive chains.
The locationof porphyry copperorebodiesin the
cupolasof plutonsof intermediatecompositionwas
emphasized
many years ago (Emmons,1927), and
subsequent
work tendsto confirmthe high-level,subof their
environment
of formation.
Some porphyry copperdepositsmay have been emplacedat very shallowdepths,perhapsat less than
1,500 m (Fournier, 1968). This is emphasizedby
the occurrenceat Bingham, Utah of a porphyrycopper stock and nearby penecontemporaneous,
comagmaticvolcanic rocks (Moore, Lanphere and
Obradovich,1968).
It is suggested
that the accumulation
of copperand
molybdenumin high-levelfelsicstockswas a normal
part of calc-alkalinemagrnatismin post-Paleozoic
orogenicbelts.
P•
The Origin ol Calc-AlkalineIgneousRocks
Many workers (e.g., Benioff, 1954; Coats, 1962;
Dickinson and Hatherton, 1967; Dickinson, 1968;
Ringwood,1969) agreethat the magmaswhich have
given rise to calc-atkalinevolcanicrocks--and by
analogytheirplutonicequivalents
(Hamilton,1969a)
zoneswhichunderFro. 1. Porphyry copper and molybdenum deposits and --were generatedon subduction
prospectsin the western Americas, southwest Pacific and lay the eruptivechains. The magmaswere probably
Alpide belts.
generatedby partial fusion consequentupon frictional heatingof subducted,water-saturatedoceanic
Columbia. Depositsin the DominicanRepublicand lithospherewhich originally was generatedat ocean
PuertoRico may be considered
as an offshootof the rises, and transportedlaterally into ocean trenches
western Americas belt. Two other belts, for which
(Fig. 3). Restrictedvolumesof fused ocean-floor
publishedinformationon the porphyry depositsis sediments
(layer 1) andthe lowestmeltingfractions
scant,are locatedin the Taiwan, Philippines,Borneo, of layers2 and 3 of the oceaniccrustseemto be the
West Irian, Papua-NewGuineaand SolomonIslands mostlikely sourcematerialsfor calc-alkaline
magnaas
region(the southwest
Pacificbelt), andin the South (Oxburgh and Turcotte,1970). Severalchemical
Banat district of Romania,Yugoslavia,central Bul- and isotopicparameters
determinedfor calc-alkaline
garia,Armenia,Iran andWest Pakistan(the Alpide volcanicand intrusive rocks are now cited in support
belt). The only well-authenticated
occurrences
of of an originby partialmeltingon a subduction
zone.
porphyrydepositsoutsideof thesepost-Paleozoic Recent volcanic rocks in the circum-Pacific belt
orogenic
beltsare thosein Uzbekstanand Kazakh- showa systematic
increasein their potashto silica
stan, USSR.
ratios landwards from the trench (Dickinson and
Hatherton, 1967; Dickinson, 1968).; these ratios
Relationships
betweenCalc-AlkalineIgneousRocks correlatewith the depthfrom the site of eruptionto
andPorphyryCopperDeposits
the Benioff zone dippingbeneaththe island arc or
The post-Paleozoic
historyof the continental
mar- continentalmargin, suggestingan origin on the
ginsandislandarcswhereporphyrycopperdeposits Benioff zone, and the absenceof widespreadcrustal
are locatedwas characterizedby widespreadcalc- contamination.A comparablelandwardincreasein
for post-Paleozoic
alkalinevolcanismwhichgaverise to basalts,ande- potashhasalsobeendemonstrated
sites,dacites,rhyolitesand, in someparts, felsic volcanic and intrusive rocks in parts of western
MODEL FOR ORIGIN OF PORPHYRY COPPER DEPOSITS
EuL••sian
Plate
ß..'.;.'.
187
..:::
ß
ß
ß
ß
•
•/'•
//
Southeast
Asian
Plate
American
Plate
fine Plate
ß
ß
Caribbean
Plate
/
Pacific Plate
:
.r'
ß
•.•.East Pacific
,
Plate
o.
IndianI•ate
Nricon Plate
,,
.J
ß
ß
ß
ß
;
t'"'\--.•....x.
....v
•
'"
............
Antarctic
Plate
Accreting
platemargins
Active transform faults
Consuming
platemargins
Platemargins
of indeterminate
nature
"'."'i•Mesozoic-Cenozoic
mountain
belts
Regions
with
porphyry
copper
and
molybdenum
6el)OSits
(Plate boundaries
tokenfrom Deweyand Bird, 1970)
Fro.
2.
The western Americas. southwestPacific and Alpide porphyry belts in relation to Mesozoic-Cenozoic
orogenic belts and accreting and consumingplate boundaries
North America (Moore, 1959, 1962; Moore, Grantz
and Blake, 1963; Batemanand Dodge,1970).
Initial Sr87/Sr8øratios (0.703-0.706) obtainedfor
andesiticvolcanics(Ewart and Stipp, 1968; Pushkar,
1968; Peterman,Carmichaeland Smith, 1970), and
those (0.705-0.709) obtainedfor felsic rocks from
the British Columbia, Sierra Nevada and Boulder
and calc-alkalinemagrnatism,summarizedin the
preceeding
sections,the components
of porphyrycopper stocks,including the containedmetals, are
likewisepostulated
to possess
an originby partial
meltingof oceaniccruston a subductionzone. Initial
strontiumisotoperatiosin the range0.706-0.708obtained for severalporphyry-copper
stocksin the
batholiths (Fairbairn, Hurley and Pinson, 1964; southwest United States and for the stock associated
Hurley et al., 1965; Doe et al., 1968) are incom- with theQuesta,New Mexicoporphyrymolybdenum
patiblewith an originby partial meltingor wholesale deposit (Moorbath, Hurley and Fairbairn, 1967;
assimilationof continentalcrust, but would seem to Laughlin,Rehrig and Mauger, 1969) supportthis
be in accordwith a derivationby partial meltingof contention.A deep,homogenized,
probablymantleoceanic crust on a subduction zone. The trace elesourcefor sulfidesulfur in porphyry•opper and
ment content of andesires is also consistent with a
molybdenumdepositsin the southwestUnited States
Benloft-zone
.origin (Taylor, 1969; Taylor et al., is suggestedby •S3• values close to the meteoric
1969).
standard(Field, 1966; Jensen, 1967; Laughlin,
Rehrig and Mauger, 1969).
The Origin of Porphyry Copper and Molybdenum
In summation,therefore, porphyry copper and
Deposits
molybdenumdepositsare consideredto be confined
In view of the closetemporaland spatialrelation- to orogenic belts characterizedby calc-alkaline
ship betweenthe genesisof porphyryore deposits magrnatism,and resulting from plates of oceanic
188
RICHARD
H. SILLITOE
Oceanicsediments
(layert) withmetal-rich horizon
at theirbase
Porp•lyrycopperdeposit
Copperconcentration
in oceaniccrust
Volcanic chain
/
Bat halitl•s
Old continental crust
TREN'CH
Oce'-
/
/
OCEAN
RIS
r
Basalt
encI
gabbro
'iS,;;e ai;metal-rlc
•' ' h
qa-ers
2and
31
Basaltic
/
magma
on [layers
,ang
3•
MOHO
Calco alkaline
magma
IOO
-
-----e,,--LOW velocity zone
150
200
To 700 Km.
•'m. 3.
Schematic repœesentation
of the genesis of porphyry copper depositsin the context of plate tectonics.
lithosphereunderthrustingadjacent lithosphericextracted from oceaniccrust during partial melting
plates,in somecaseswith ensuingcontinental
col- as it sank into the mantle on a subduction zone.
lision-the compressive
type of elongatecontactbetweentwo lithosphericplates.
Collectionof metalsby saline fluids expelledfrom
oceaniccrust during subductionmay also be opera-
tive. Initially, the metalswere largelyderivedfrom
the mantle at the oceanrise systemand carried to
th marginsof oceanbasinsas components
of layers
The sourceof metalsin post-magmatic
sulfideore 1, 2 and 3 of the oceaniccrust (Fig. 3). The wedge
deposits
haslongbeena topicof contention.For of mantle above a Benioff zone may also act as a
theporphyry
copper
deposits
in particular,
a con- sourcefor basalticmagmas(Oxburgh and Turcotte,
tinentalcrustalprovenance
of copperby extraction 1970), and minor quantitiesof copperand molybfrom andesiticvolcanics(Ney, 1966), geosynclinal denum. The metalsreleasedduring partial melting
sediments
(UnitedNations,1970),or shalehorizons ascendedas componentsof calc-alkalinemagmas
(Jensen,
1971),duringigneous
intrusion,
hasbeen (Fig. 3), and were ultimatelyconcentrated
in chloinferred. However, consideringthe wide spectrum ride-richfluid phasesassociated
with the roof-zones
of host rock typesof porphyrycopperdeposits,of certain intrusions. The fluid phasewas released
namelycalc-alkaline
volcanics,
intrusiveand meta- upwardsduringconsolidation
of the magmato give
morphic
rocks,andmarinesediments,
partiallycal- rise to the typical upright cylindersof porphyry
careous,
andtakingintoaccount
the probable
absence copperandmolybdenum
mineralization.
of certain of these sourcerock types from beneath
A corollaryof this premiseis that regionspossesssome
porphyry
copper
provinces,
theprovision
ofthe ing highconcentrations
of porphyrycopperdeposits,
The Sourceof Metals in Porphyry Ore Deposits
metalsby a specific
rock type in the continentalsuch as the southwest United States and southern
crust seemsimprobable.Furthermore,the only Peru-northernChile, and consideredas copper-rich
source of metals available to all porphyry copper
metallogenetic
provinces(e.g., Turneaure,1955),
deposits,
whichtakesinto consideration
the thin are not regionsof the Earth where the subjacent
crust and almosttotal absenceof sialic crust beneath
continentalcrust or upper mantle are enrichedin
someporphyry
copperprovinces
(e.g.,the Solomon copper,but regionsbeneathwhichanomalously
cop-
Islands;Coleman,
1966),wouldseemto betheupper
mantle.
per-richoceaniccrust,includingpelagicsediments,
An important
aspect
of the modeloutlinedabove hasbeensubducted.Variation in the amountof copin a zoneof subduction,
andtherefore
for the originof porphyryore deposits,
is the im- perconsumed
potentially
available
for
mineralization,
may depend
plication
thata largepercentage
of themetals
were
MODEL FOR ORIGIN OF PORPHYRY COPPERDEPOSITS
189
pillow lavas,which,accordingto Gass(1968), would
representa part of layer 2 of the oceaniccrust. The
closegeneticrelation betweenthe basalticvolcanics
and the copperdeposits(Vokes, 1966; Hutchinson
and Searle, 1970) is evidencethat concentrations
of
x
mantle,perhapsin the low velocityzone (Fig. 3); copperare present in the oceaniclithosphere.
5. Manganesenoduleson the ocean floors have
chemicalheterogeneity
of the mantle has recently
beendemonstrated(Petermanand Hedge, 1971).
coppercontentsas high as 2.5%. Analysesof nodules
from the Pacific Ocean showedfrom 0.03 to 1.6%
EvidenceBearingon the Derivationof Metals from Cu (United Nations, 1970a).
Oceanic Crust
6. Normal pelagicclays (layer 1) possessmetal
contentsin excessof most sedimentaryrocks, and
The followingpointsare thoughtto provideevi- averageabundances
of 323 ppm Cu and 18 ppm Mo
dencefavoringthe oceaniccrustas a sourcefor the havebeenrecordedfor Pacificclays (Cronan, 1969).
copperand molybdenumcontentsof porphyry ore
7. Basemetals,includingcopperand molybdenum,
deposits:
of probablemantlederivationare concentratedon the
1. Recent workers (Cann, 1968; Oxburgh and
East Pacific Rise (Bostr6m and Peterson, 1966,
Turcotte,1968; Thayer, 1969; Christensen,
1970) 1969), and on the rest of the ocean rise system
haveproposed
that the third layer of the oceanic (Bostr6m et al., 1969). Layer 1 sedimentson the
crusthasa gabbroic
composition,
perhapswith dole- flanks of the East Pacific Rise (and presumably
rite dike complexes
in its lower part (Dewey and
away from the rise beneatha blanket of normal
Bird,1970).It is overlain
bybasalts
of layer2. Basic ocean-floor sediments) have average copper and
igneous
rockshaveaveragecopperandmolybdenummolybdenumcontentsof 990 ppm and 100 ppm,
contentsof the order of 100 ppm and 1.5 ppm,
respectively.
on the rate of sea-floorspreadingand hencethe
volumeof oceaniccrustenteringthe subduction
zone,
or the intensityof volcanism
andmetalproduction
at
the oceanrise, or, morefundamentally,may reflectan
inhomogeneous
distributionof thismetalin the upper
respectively,
five timesthat of graniticrocksin the
8. The floor of the Red Sea, also a locus for the
caseof theformerelement(TurekianandWedepohl,
generation
of new oceaniccrust (e.g., Vine, 1966),
1961;Vinogradov,
1962). Oceanic
basalts(layer2)
a seriesof metal-richbrine poolsand sediin the Atlantic and Pacific Oceansaverage77 ppm possesses
ments
(Miller
et al., 1966;Degensand Ross,1969),
Cu (Engel,EngelandHavens,1965).
possiblyalsochargeddirectlyfrom the mantle.
2. Sulfidephasescontaining
up to approximately
9. The early PlioceneBoldo beddedcopperde10 percentcopperare presentin vesiclesand as
posit
in Baja California(Wilson and Rocha,1955)
globulesin pillowbasaltsfrom layer 2 on the ocean
may
possibly represent another, albeit somewhat
rises(Moore and Calk, 1971).
older, example of copperwhich has risen directly
3. Sulfidegrainsof a similarnatureto thoseoc- from the mantle, in this case related to the north-
curringin ocean-floor
basalts(but with up to 25 westwardrotation of Baja Californiaaway from the
percentcopper) have been discovered
in recent rest of the North American continent, along segHawaiianbasaltflows (Desborough,
Andersonand
ments of the East Pacific Rise mutually offset by
Wright, 1969), and in basalticoozeswhichflowed transform faulting, a processinitiated in the late
into a drill hole in the crust of Alae lava lake, Hawaii
Mioceneto middle Pliocene (Moore and Buffing-
(Skinnerand Peck,1969). The Hawaiianislands ton, 1968; Larsen, Menard and Smith, 1968). The
are mantle-fed volcanoesin the center of the Pacific
lithospheric
plate (Menard, 1969).
high manganeseand iron oxidecontentsof the Bol•o
ores (Wilson and Rocha, 1955) providea further
4. Large alpine-type
mafic-ultramafic
(ophiolite) similarity to the metal concentrationsof the East
complexes
suchas thosein Cyprus,Turkey,Papua Pacific Rise and the Red Sea.
andelsewhere
are thoughtto represent
fragments
of
It is also conceivable
that the metals (6 ppm Cu;
surficialrinds of ancientocean-floorplates (Dietz, White, 1968) in the brines of the Salton Sea geo1963;Thayer,1969;Bird andDewey,1970). Sup- thermal system,lying just north of the Gulf of
portfor sucha beliefstems
fromrecords
of similar California,and possiblyunderlainby the East Pacific
rock typeson the mid-Atlanticand Indian Ocean Rise, have a comparablemantle source, although
ridges(Bonatti,1968;EngelandFisher,1969).The their derivation by low-grade metamorphismof
MesozoicTroodosophiolitecomplexof Cyprusis clasticsediments
has beenproposed(Skinner et al.,
thoughtto correspond
to layers2 and 3, formed 1967).
beneathan oceanrise in the Tethys Ocean (Gass,
x This suggeststhe possibility that other massive pyritic
1968). The well-known
cupriferous
pyritedepositssulfidedepositsassociatedwith basalticpillow lavas may have
of Cyprusareinterbedded
in a succession
of basaltic formed on the crests and flanks of ocean rise systems.
190
RICHARD
H. $ILLITOE
TABLE1. Agesof PorphyryCopperand MolybdenumDeposits
Nameof PorphyryDeposits
or Regions
Age"of PorphyryDeposits
Sourceof Data
British Columbia
Mostporphyrycopperand molybdenum
deposits Upper Triassic-MiddleJurassic White, Harakal and Carter (1968);
Upper Jurassic
Brown (1969)
Late Lower Cretaceous
Vancouver Island
Western United States and Sonora
Most porphyry copperdeposits
Bisbee, Arizona
Ely and Yerington,Nevada
Bingham, Utah
Front Rangeporphyrymolybdenum
deposits
Upper Paleocene-UpperEocene
Lower Eocene-Lower Oligocene
Creaseyand Kistler (1962);
McDowell and Kulp (1967);
Lower Cretaceous
Moorbath, Hurley and Fairbairn
(1967); Livingston, Mauger and
Lower Oligocene
Damon (1968); Moore, Lanphere
Upper Oligocene-LowerMiocene
and Obradovich (1968); Tweto
(1968); Wallace et al. (1968);
Laughlin, Rehrig and Mauger
(1969)
Upper Cretaceous-Paleocene
Middle Jurassic
Panama
Botija
Ecuador
Chaucha
Peru
Southern
Peru
Michiquillay
Argentina
Farel16n Negro, Catamarca
Carson (1969)
Lower Oligocene
Feren[i6 (1970)
Upper Miocene
M/iller-Kahle and Damon (1970)
Paleocene
? Lower Miocene
Laughlin, Damon and Watson
(1968); Stewart and Snelling (in
prep.)
United Nations (1970)
Llambias (1970)
Tertiary (S)
Upper Miocene-Pliocene(S)
Chile
Upper Cretaceous
Paleocene
Sillitoe, Quirt, Clark, Farrar and
Neumann (in prep.)
Upper Eocene-Oligocene
Upper Miocene-Pliocene
BougainvilleIsland
? Pliocene(S)'
Macnamara (1968)
Taiwan
Chemei
Miocene or later
Po and Lee (1970)
Philippines
Tertiary (S)
Upper Paleocene
Bryner (1969)
? post-Oligocene
(S)
Schmidt (1968)
Upper Oligocene-Miocene
(S)
Bazin and H/ibner (1969)
Upper Eocene
Lower Oligocene
Bagdasaryan,Gukasyan and Karamyan (1969)
Panguna
Atlas
West Pakistan
Chagaidistrict
Iran
Kerman region
Armenia
Lower Miocene
aTimescaleaccording
to Hadand,SmithandWilcock(1964).
(S) Stratigraphicestimate.
The Distribution of Ages of Porphyry
Ore Deposits
Published
agesfor the maingroupsof porphyry
the quantityof metalsincorporatedin magmas,on a
subductionzone. It is, of course, recognizedthat
additionalvariables,suchas the quantityof chloride-
fluidpresentduringthefinalstagesof consolidacopper
andmolybdenum
deposits
aresummarized
in rich
tion of an intrusion,are of considerableimportance.
Table1. It is apparent
thatdeposits
wereemplaced
at intervals
throughout
the Mesozoic
andCenozoic, Evidence has been advanced to show that mewitha particularly
largenumber
of deposits
of late chanismsimplicit in the new globaltectonicswere
Cretaceous-Paleogene
age.
operativein pre-Mesozoic
times (Bird and Dewey,
with the exceptionof UzbekIt is suggested
thattwoprincipal
factors
control 1970). Nevertheless,
stan
and
Kazakhstan,
porphyry
ore depositshavenot
thespace-time
distribution
ofporphyry
oredeposits.
described
fromthe olderorogenic
Theseare: 1. the levelof exposure--largely
depend- yetbeendefinitely
is tentativelyattributed
entonerosion
rate--ofa plutonic-volcanic
chain;and belts. This apparentabsence
2. the time and locationof magmageneration,
and to the effects of Mesozoic-Cenozoicerosion, which
MODEL FOR ORIGIN OF PORPHYRY
COPPER DEPOSITS
191
has been sufficientto remove the upper parts of presently available suggestthat the periods of
batholiths,the loci of porphyrydeposits. However, porphyrycopperformationin Chile were separated
it is predictedthat porphyry depositswill be en- by quiescentintervalswith durationsof about 15-25
counteredin parts of certain pre-Mesozoicorogens m.y. Similar pulse-likeigneousintrusionhas also
where erosionhas been less severe.
8 The apparent beenreportedfrom parts of westernNorth America,
predominanceof late Cretaceous-Paleogene
ages of pulseslastingapproximately10-15 m.y. and being
porphyrydeposits(Table 1) mightalsobe dependent separatedby 30 m.y. intervals(Damon and Mauger,
on the erosionfactor,wherebymany early Mesozoic 1966; Evernden and Kistler, 1970; Gabrielse and
depositshave been eroded away, and some post- Reesor, 1964). Such magmaticperiodicitymay be
Paleogenedepositshave yet to be exhumedfrom correlablewith changesin the thermal regime on
beneaththeir volcaniccover. Supportfor this pro- subductionzonesinducedby variationsin the relative
posal is derived from conditionsin Chile north of spreadingrate or motionpattern of the plates. In
latitude 30øS, where the erosionlevel becomespro- this context,changesin the relative motion of plates
gressivelydeeperfrom the recentvolcanicchain on every 10-20 m.y. in the northeastPacific, deduced
the Andeancrestwestwardsto the Jurassicintrusions by Francheteau,Sclaterand Menard (1970), might
on the Pacificlittoral (Sillitoe and Sawkins,1971). be significant.
The eastwardmigration of the loci of intrusion
The majority of the exposedporphyry copper depositsare Paleogenein age; Jurassicdeposits,if they and porphyry copperemplacementevident in northever existed,havebeenlost by erosion,and deposits ern Chile, which perhapsreflectsa parallel trend in
yet to be exposedmay exist in the recent volcanic the positionof magmagenerationon the underlying
chain. However, the secondfactor, discussedbelow, subduction
zone,whetheror not causedby a change
may also be an importantcontributary,or even the in its position or inclination relative to the condominant, causeof the relative abundanceof Paleo- tinental margin, is less well definedin the western
genedepositsin northernChile.
United States. In the latter region,Gilluly (1963)
an overalldecrease
in the ageof MesozoicIf the importanceof the erosionfactor has been recognized
correctly evaluated,then porphyry depositsin re- Cenozoic intrusion landwards from the continental
gionswith a high erosionrate, suchas the south- margin, but subsequentprograms of radiometric
west Pacificbelt characterized
by a tropicalclimate, dating have shown many exceptionsto this gencouldbe expectedto yield a predominance
of particu- eralization. A broadly comparablepattern of eastlarly youngages;more radiometricdating is needed ward youngingis apparentfrom the porphyry deposits. The belt of mid-Tertiary porphyry molyin order to test this proposal.
Explanationsof many featuresof the distribution bdenumdepositsin the Front Range lies east of the
porphyry
of porphyry copperand molybdenumdepositscan main clusterof late Cretaceous-Paleocene
be attemptedin termsof the secondfactor,the lateral copper deposits in Sonora-Arizona-New Mexico
and secularpattern of magma generation,and the (Fig. 1), and the Lower Cretaceousporphyrydeavailabilityof copperandmolybdenum,
on subduction positsat Ely and Yerington, Nevada are locatedin
zones. In northern Chile, discrete post-Paleozoic the westernpart of this porphyry province. The
intrusiveepisodes
are manifested
by a seriesof north- great concentrationof late Cretaceous-Paleocene
south-trendingbelts of batholithsand stocks.The porphyry copper depositsin the southwestUnited
ages of these belts decreasefrom Lower Jurassic Statesis visualizedas being due to the subductionof
copper-richoceaniccrust. This
nearthe coastto late Tertiary in the AndeanCordil- areasof exceptionally
lera (Ruiz et al., 1965; Farrar et al., 1970). The contentionis supportedby the occurrencein the
ages of porphyry copperdepositsin this region sameprovinceof unmineralized(with the exception
possessan analogousspace-timedistribution,al- of Bingham, Utah) mid-Tertiary stocks, which I
thoughJurassicdeposits
are as yet unknown. Thus consider to have been intruded at a time when lesser
the possibilityarises that each discretepulse of amountsof copperwere availableon the subjacent
magmageneration
had the potentialto give rise to subductionzone. Theories invoking the extraction
porphyryore deposits;the extendedtime interval of copperfrom the continentalcrust or uppermantle
fail to account for the concentration within a limited
and episodicityof porphyrydepositgenesis,particularly in westernNorth America,as reflectedby the time period of most of these porphyry copper deposits;furthermore,post-Paleozoic
stocksof all ages
agesin Table 1, supportthis conclusion.The data in the provincewould be expectedto be similarly
with porphyrycopperdeposits.Continuing
a Pre-Mesozoic porphyry copper-typedepositshave been endowed
reported from northwest of St. John, New Brunswick the sameline of argument,large amountsof moly(Ruitenberg,Shafiquallahand Tupper, 1970), and from eastcentral Queensland,(Cornelius, 1969), but no particulars bdenum,and only minorcopper,are thoughtto have
of the occurrenceswere given.
been available on a subduction zone vertically
192
RICHARD
H. $ILLITOE
Tertiary times (Freeland
beneaththe Front Range in mid-Tertiary time. If in mid-Cretaceous-early
the locusof magmagenerationon a subduction
zone and Dietz, 1971).
doesnot migrate systematically
with time, then no IVestern North America
clear pattern of porphyry copperages is to be expected,a situationwhichmay explainthe apparently
It is now generallyaccepted(Vine, 1966; Yeats,
randomdistributionof agesin British Columbia.
1968; Hamilton, 1969b; Page, 1970) that thrusting
of the East Pacific ocean floor beneath the American
Porphyry CopperDeposits/Plate Tectonics
Interrelationships
Details of the world distribution of the evolving
systemof oceanrises and trencheswhich existed
during Mesozoicand early Cenozoictimes are not
yet well known. Nevertheless,evidencederived
from both continents and ocean basins has enabled
plate in westernNorth America took placeat times
duringthe Mesozoicand early and middleTertiary.
Underthrusting terminated in the western United
Statesin the Miocene (Vine, 1966; Atwater, 1970)
by the overrunningof the East Pacific Rise by the
trench system. Porphyry copperand molybdenum
depositsin the westernUnited Statesrange in age
from middle Jurassicto Lower Miocene (Table 1),
in excellentagreementwith the time of plate convergence. In view of the absenceof an active sub-
the establishment
of someaspectsof plate tectonics
at this time. The distributionof porphyrydeposits
in selectedregionswill now be related in general
duction zone off this coast, it is concluded that
terms to the plate tectonicmodel:
porphyrycopperformationis not currently active in
North America, north of the tip of Baja California,
IVestern South America
except in the Alaska Peninsulawhere a trench system is still active,and possiblylandwardof the Juan
netic anomalypatternsin the PacificOceanbasin de Fuca plate. It is noteworthythat calc-alkaline
(Heirtzler et al., 1968; Morgan, Vogt and Falls, volcanism, typical of convergent plate junctures,
1969) and the paleontological
study of sediment ceasedin the westernUnited Statessouthof Oregon
cores in the South Atlantic Ocean basin (Maxwell in the Miocene (Christiansenand Lipman, 1970;
et al., 1970) hasdemonstrated
the convergence
of the Lipman, Prostka and Christiansen,1970).
American and East Pacific plates along the western
The characterof Mesozoic-middleCenozoicplate
Evidencederived from the interpretationof mag-
marginof SouthAmerica(Fig. 2) sinceat leastthe
interactions off British Columbia cannot be elucidated
late Cretaceous. This interval of active underthrust-
from studiesof magneticanomalypatterns(Atwater,
ing embraces
the periodof formationof porphyry 1970), but it is noteworthythat calc-alkalinevolcancopperdeposits
in Chile,Argentina,Peru and Ecua- ism in British Columbia terminated in the Eocene
dor. In western South America, porphyry copper (Souther, 1970), at aboutthe sametime as the de-
deposits
maywellbe formingstillbeneath
theactive dine in the formationof porphyry copperdeposits•
volcanic chains, for underthrustingis continuing excludingthoseon VancouverIsland.
(e.g.,PlafkerandSavage,1970).
The small Juan de Fuca plate, sandwichedbetweenNorth Americaand the northeastPacific (Fig.
Central America and the Caribbean
2), hasdescended,
andmay still be descending,
along
The Panama trench is sediment-filled,and under- a trench systemparalleling the coast of Oregon,
thrusting
is inactive(Le Pichon,1968). The trench Washingtonand VancouverIsland (Morgan, 1968;
was abandonedin the Miocenewhen the pattern of Tobin and Sykes, 1968) giving rise to calc-alkaline
sea-floorspreadingin the North Pacificchanged magmatismin the Cascades. The Lower Eocene(Vine, 1966;Le Pichon,1968),but waspart of a Lower Oligoceneporphyry depositson Vancouver
continuous
trenchsystemborderingthe westof the Island (Carson, 1969) do not fit well into the overAmerican continent intermittently during the all space-timedistributionpattern of porphyry deMesozoicand early and middle Cenozoic,during positsin British Columbia,and might be ascribed
which time the Cerro Petaquilla and Botija to earlier activity in the vicinity of this localized
porphyry
copper
deposits
of Panama
wereemplaced.compressivesystem, as might apparently young
In PuertoRico,porphyrycopperformationseems porphyrycoppersin Washington. Extrapolationof
to havebeenassociated
with a phaseof igneousin- platemotionsbackinto the early Cenozoic(Atwater,
trusionof Eoceneage (Mattson,1965). Although 1970) has shownthat subductionof the Juan de
present-day
eastwardmovement
of the American Fuca plate was precededby more rapid underplatenearlyparallels
thePuertoRicotrench(Chase thrustingof the Farallonplate,at a trenchwhichdid
and Bunce, 1969; Molnar and Sykes, 1969), re- not extend further north than Vancouver Island.
constructions
of platemotionsin thisregionindicate However, her model approximatelypredicts the
of subductionin the Upper Eocene,
that underthrusting
normalto the trenchprevailed commencement
MODEL
FOR ORIGIN
OF PORPHYRY
COPPER DEPOSITS
193
mid-way through the interval of formation of the
porphyry deposits.
Miocene times, they would be related to a northward dipping subductionzone, which becameex-
LineamentIntersectionsand Porphyry CopperDe-
tinct during the Miocene,by the collisionof its
overlyingislandarc (Bismarckarc) with the Australiancontinent(DeweyandBird, 1970).
positsin Western .dmerica
The locations of several southwest North American
porphyry copper depositshave been attributed to
major orogen- and fault-zone intersections(Bill-
The .dlpideBelt
The Alpide belt, in termsof the new globaltecingsley and Locke, 1941; Mayo, 1958; Schmitt, tonics,is one of the leastknownand mostcomplex
1966). More specifically,the locationsof several of the compressive
plate boundaries.On a global
porphyry copperdeposits(e.g., Ajo, Pima-Mission scale,the compressire
forcesin the Alpide belt have
and Silver Bell) have been consideredto have been been attributed to relative movements between the
influencedby elementsof the west-northwesttrend- African and Eurasian plates related to sea-floor
ing Texas lineament,particularlyby its intersection spreadingin the Central and North Atlantic Oceans
with the Wasatch-Jeromeorogen (Mayo, 1958; (Hs/i, 1971; Smith, 1971). Lithospherewas conSchmitt, 1966; Guilbert and Sumner, 1968; Wertz, sumedalongthe northernandnortheastern
edgesof
1970). Schmitt (1966) and Guilbert and Sumner the Arabianplateat the Zagrosthrustzonein Iran
(1968) have interpretedthe Texas lineamentas a and West Pakistanand its westerlycontinuationin
continental
manifestation
of now-extinct
transform
faults in the North Pacificbasin. Although several
porphyry copper depositsoutside of the southwest
United States (e.g., Chuquicamata,Chile; Taylor,
1935) lie adjacent to important faults, none have
been describedas being locatedby major structural
intersections. It is suggestedthat the control of
porphyry copper emplacementby extinct transform
faults and major structural intersectionsis not universally applicable,and is subordinateto a fundamental dependenceon elongate zones of plate convergence. In Chile, for example, the linear, longitudinal array of porphyrycopperdeposits(Fig. 1)
providesstrongsupportfor a subduction-zone
origin,
and no indicationof control by structural intersections is evident (Sillitoe, unpublished). In the
southwestUnited States, the less regular, disperse
pattern of porphyry copper deposits may be explained in terms of partial fusion and consequent
magma and metal generationover a greater downdip extension of the underlying subductionzone;
this situation might be expectedif the subduction
zone were flat-dippingand imbricateas invokedby
Lipman, Prostkaand Christiansen(1971). It is not
denied,however,that lineamentsmay haveinfluenced
locallythe uprise of magmaand includedmetals.
The SouthwestPacificBelt
In view of the youngages(Table 1) suggested
for
the porphyry copper depositsin Bougainvilleand
Taiwan, it seemsprobablethat their formation is
linked to Benioff zonesoccupyingpositionsclosely
similar to thosecurrently active (Fig. 2). If the
porphyrycopperdepositsin West Irian and PapuaNew Guinea prove to be post-Miocenein age, then
they would seem to be related to the southward
underthrustingof the Pacificplate (Fig. 2). On the
other hand, if the depositswere formed in pre-
Turkey (DeweyandBird, 1970). Porphyrycopper
depositsin Iran and West Pakistan, north of the
Zagros zone, were emplacedwhile subductionwas
active. The porphyrydepositsin Romania,Yugoslaviaand Bulgaria appearto be relatedto a Meso-
zoic-Tertiarysubduction
zone which, accordingto
Dewey and Bird (1970; Fig. 14), is marked by
ophiolitecomplexes,and extendedwestwardsfrom
the southernshoreof the Black Sea. It might be
conjectured
that all the porphyryore depositsin the
Alpide belt were generatedduring phasesof sub-
ductionrelatedto theclosure
of thewesternTethyanIndian Ocean.
In the caseof orogenicbeltsin whichthe collision
of continents with island arcs or with other con-
tinentshas contributedto their development,
as in
the Alpine-Mediterranean system, calc-alkaline
igneousrocksand associated
ore depositsmay have
beenconcealed
by overthrustslicesor by flyschdepositsduring or after collision.
Concluding Remarks
In termsof the plate tectonicmodeloutlinedabove
for the genesisof porphyryore deposits,several
suggestions
for explorationmay be made. A consideration of the distribution of Mesozoic-Cenozoic
subductionzones(Fig. 2) indicatesseveralareasas
potentialporphyryprovinces,in additionto the orogenic belts of western America and their southward
continuation
into the AntarcticPeninsula.Probably
the most obviousof theseregionsare Japan• and
New Zealandwhereporphyrydepositshavenot yet
beendiscovereddespiteextensiveexploration.A recentcompilationof agesof magmatismin islandarcs
(Mitchell and Bell, 1970) showsthat Upper Cretaceous-Eocene
or mid-Tertiary periodsof volcanism
accompanied
by intrusionare represented,in addition to in islandarcs where porphyrycopperde-
194,
RICHARD H. SILLITOE
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ROYALSCFIOOLOF MINES,
IMPERIAL COLLEGE,
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