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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. 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