Sillitoe - Porphyry Copper

EconomicGeology VoL 68, 1973, pp. 799-815

The TopsandBottomsof PorphyryCopperDeposits RICHARD

H. SILLITOE

Abstract

Although it is now widely acceptedthat porphyry copperdepositsconsistof zonally arranged shells of alteration and mineralization centered on high-level, calc-alkaline stocks,the nature of their uneconomicupward and downward extensionsremains undocumented. This paper attempts to characterize these upward and downward extensionsand to integrate the resulting conceptsinto a hypotheticalmodel for complete porphyry coppersystems. Examples from Chile, Argentina, and elsewhereare used to aid in the substantiationof the model. Programs of exploration for porphyry ore depositscan clearly benefitfrom the applicationof a modelof this sort. A typical porphyry copper-bearingstock is inferred to grade downward into stockwork mineralization and potassiumsilicate alteration in a phaneritic intrusive, which in turn is transitional downward to an essentially unaltered pluton of considerably larger dimensionsthan the stock. Porphyry copperdepositsare normally locatedin the basementbeneatha comagmaticvolcanic pile, which is transectedby a column of hydrothermal alteration representingthe upper parts of the porphyry copper system. This alteration consistsof propylitic and argillic types with localizedpatchesof silicification and advanced argillic alteration. The volcanic pile is thought to constitute a stratovolcanowhich possesses large native sulfur depositsand small quantitiesof basemetals, particularly copper, in sublimatesat high-temperaturefumaroles in the vicinity of its central vent; these surficial deposits are consideredas the effluent products of active porphyrycoppersystems. The availableevidencefavors the emplacement of the tops of typical porphyrycopper depositsat depthsof 1.5-3 km beneaththe summitsof stratovolcanoes and suggeststhat entire porphyry copper systemspossessvertical extensionsas great as 8 km. At Chuquicamata,Chile, a major high-anglefault may have cut the porphyry copper deposit,and subsequent erosionhas removedthe portion of the depositthat was situated in the upthrown block. The hydrothermal alteration pattern in the remaining part of the ore body is incompleteand terminatesabruptly against the fault. The unaltered, phaneriticgranodiorite,containingminor veins and pegmatiticbodies,in the upthrown block is interpretedas the root zone of the Chuquicamataporphyry coppersystem. The low•ermost,rMneralizedpart of a porphyry systemis believedto be exposedat Los Loros, Chile. There a zone of molybdenum-rich and copper-poorpotassiumsilicate alteration carrying abundantK-feldspar occupiesan area in the interior of a relatively large plutonof phaneriticgranite. At Faral16n Negro, northwest Argentina, several small porphyry copper deposits pierce the infrastructureof a temporallyrelated, andesiticstratovolcano. This unusual locus of the depositsabove the subvolcanicbasementenablesit to be determinedthat porphyry copper emplacementwas a late event in the constructionof the stratovolcano, succeededonly by the formation of minor rhyolite intrusives and "epithermal" veins. Extensive zones of pyritic alteration including widespreadsilicification, in which intrusive rocksare virtually absent,are visible in the centersof erodedstratovolcanoes, as at Cerro Marquez in northern Chile. Such zones are interpreted as the columns of alterationspanningthe vertical interval betweenporphyry copperdepositsand the vent areas of uneroded volcanoes. At Cerro Queva in northwest Argentina, leadsilver mineralizationassociated with advancedargillic alterationis locatedin an alteration zonebeneaththe summitregionsof a stratovolcano. It may be concludedthat during the final stagesof constructionof stratovolcanoes, fumarolic and hot-spring activity are the surficial manifestationsof the effiux of metalbearing magmatic fluids from magma chambersduring retrograde boiling, the interaction of these fluids with the groundwatersystemand the consequentformation of

alteration and mineralization. The proposedmodel implies that porphyry copper systemseffectivelyspanthe boundarybetweenthe plutonicand volcanicenvironments. 799

800

RiCH/IRD H. SILLiTOE Introduction

their high level of emplacementin the continental crust, and the large volumesof mineralizing fluids A •R•.ATdeal of attentionhas recently been focused and high temperaturesinvolvedin their generation,it on patterns of lateral and vertical zoning of alterais evidentthat entire porphyry coppersystemsmust tion and mineralizationin porphyry ore deposits (Lowell and Gullbert, 1970; Rose, 1970; James, extend downward and especiallyupward for very considerabledistancesbeyond the parts which are 1971). This work has led to the generalacceptance likely to be of economicinterest. Even at Kalamaof many porphyrydepositsas upright cylindersconzoo,Arizona (Lowell, 1968), wherethe mineralized sistingof coaxiallydistributedzonesof alterationand a mineralization centered on felsic stocks, commonly bodyhasbeentectonicallydisturbedand possesses near-horizontal attitude, the complete porphyry porphyries. The silicate and sulfide zoning com-

prisesa core of potassiumsilicatealterationenveloped successively by zonesof sericitic,argillic, and propyliticalteration. This typical pattern of alteration and mineralizationhas been widely recognized during studiesof porphyry copperdepositsin the Andes,and is particularlywell exemplifiedby the Los Pelambresdeposit(Fig. 2) in Chile (Sillitoe, 1973).

Knowledgeof the characterand distributionof

coppersystemis not observable.Thereforeit seems necessaryto combineinformationfrom many areas in orderto attemptto construct a modelof a porphyry system.

In the first sectionof this paper, a preliminary speculativemodel for an idealizedporphyry copper systemis advanced,althoughit is not claimedthat every porphyry-typedevelopmentnecessarilycomplieswith all its features.Future studiesof porphyry

depositsin various parts of the world will undoubtedlybe able to clarify or modifysomeof the fined to the economically exploitableportionsof moreenigmatic aspects of the model. In the second porphyryore deposits,whereasthe nature of the section of thepaperdescriptions aregivenof localities uneconomicextensionsof these deposits,both up- in Chile,Argentinaand elsewherethat are thought ward and downward,remainsundocumented.When to be typicalof variouslevelsin the upwardand oneconsiders the hugetonnageof porphyrydeposits, downwardextensionsof porphyrycopperdeposits.

alteration-mineralizationis, however, largely com-

PORPHYRY STOCK PHANERITIC

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Fro.1. Idealized cross section of a typical, simple porphyry copper deposit showing itsposition at theboundary between

plutonie andvolcanic environments. Vertical andhorizontal dimensions aremeant tobeonlyapproximate.

TOPS AND BOTTOMS OF PORPHYRY COPPER DEPOSITS

This sectiondoesnot purport,however,to vindicate every aspectof the proposedmodel. It is dear that a hypotheticalmodel of this sort

801

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will be of considerable value for the discrimination

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of zonesof hydrothermalalterationin the searchfor porphyryore deposits.

TICNAMAR•j •.

ProposedModel for a Porphyry CopperSystem

• 'CE•RRO MARQUEZ •

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The proposedmodel acceptsthe premisethat economic concentrations of copperand molybdenum in a typicalporphyrycoppersystemoccurin a subvolcanicenvironmentassociated with small, high-

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level stocks,and emphasizesthe closeassociation

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with subaerial calc-alkaline volcanism.It is proposedthat commonlya porphyrycopper-bearing stock gradesdownwardinto a pluton of larger

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dimensions that may possess stockworkmineraliza-

tion in its upperparts,but that is essentially unmineralized, exceptperhapsfor veinsor pegmatite bodies,at deeperlevels. It is furtherproposed that a porphyrycopperdepositis overlainby a columnof

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pyritic alteration which transectsa calc-alkalinevol-

canicpile, commonlysurmounted by an andesitic stratovolcano with native sulfur deposits. This

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modelis schematized in Figure 1.

A surveyof the literatureon porphyrycopperdepositsin thecircum-Pacific andAlpideorogenic belts revealsthat most of the depositswere emplaced withinmucholderand genetically unrelatedformations. In Chile,for example, deposits are normally muchyoungerthan their hostrocks(exceptperhapsfor Ticnamarand Mocha;Fig. 2), whichare commonly andesitic volcanics of Cretaceous or Jurassicage. It seems,therefore,that the normallocation

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ß LOCALITIES DESCRIBED IN THE TEXT

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FIG. 2. Positions of localities in Chile and Argentina that are referred

for theeconomic portions of porphyry oredeposits is

to in the text.

in rock formations that underlie the coeval volcanic

pile. Hence, overlyingvolcanicformationswould (Fig. 1).

In many instances,therefore, porphyry

be expectednormallyto have been completelystocksmay be likened to cupola-likeprojectionson erodedfromthevicinityof porphyrycoppersystems the upper surfacesof larger plutons,a conceptexby the timethat Cu-Momineralization is exposed.poundedlong ago by Emmons(1927). • Stockwork mineralizedmay extenddownward The instructiveexamplescited belowof economic and disseminated depositswith which calc-alkalinevolcanicsare still

from the stockinto the upperparts of the subjacent

spatiallyassociated may thereforebe considered as pluton, but it is thought probablethat it soondies somewhatatypical.

out with depth. Pegmatiticbodiesin the underlying

Manyporphyry copper deposits aregenetically re- plutonsmay result from trapping of aqueousfluidlated to porphyritic-texturedstocksthat are com- rich magma fractions,geneticallyrelated to fluids monlythe hostsfor a significant proportion of the which give rise to porphyry-typemineralizationat

Cu-Mo mineralization.In depth a transitionis higherlevels. considered to occurfroma porphyritic to a phaneritic Despite known exceptions,a typical porphyry rockof similarcomposition.Sucha transition,from daciteporphyryto quartzdiorite,hasbeenobserved

•A variation of this model (Fig. 1) is the case where porphyry copper-bearingstocks do not representthe apical in drillholeswhichtransect theEl Teniente deposit portions of extensive plutons but were intruded into the solid upper parts of probablygeneticallyrelated, but (Fig. 2) and hasbeendocumented by Portigliati already slightly earlier, plutons. This case is included in the sub(1971). Theposition of thistexturalchange is also hood cupola model of Sales (1954). Examples include envisaged as the approximate point at whichthe El Abra, Chile (Fig. 2) (Sillitoe and Neumann, unpub.)

the Highland• yalley del•o•sit• in BritishColumbia small stockstartsto increasein diameterdownward and (White, 1957). ' '

802

RICHARD H. SILLITOE

copperdepositpossesses a central economicsection data, someof which are includedin the succeeding characterizedby concentricshellsof potassiumsili- section. The columnof potentiallyeconomicporcate, sericitic,argillic, and propylitic alteration,as phyry-typeCu-Mo mineralizationmay extenddownnoted previously(Fig. 1). In the deeperparts of ward from its apex for some3 km if the situation depositspotassiumsilicatealterationtendsto be the at San Manuel-Kalamazoo,Arizona (Lowell and preponderantalterationtype, and in the basalparts Gullbert, 1970), is typical. Further supportfor a of depositsmay gradeinto a modifieddeeppotassium vertical extent of this magnitudecomesfrom obsersilicatealterationtype in whichbiotiteis lesscommon vations at Los Pelambres(Sillitoe, 1973) and E1 and which consists of the assemblagequartz-K- Teniente (Howell and Molloy, 1960) (Fig. 2) feldspar-sericite-chlorite (LowellandGuilbert,1970). which show that alteration and mineralization have In an upwarddirectionin a typicaldepositsericitic vertical extents of at least 1.45 and 1.3 km, reand argillic alteration take on an increasingim- spectively. At Santa Rita, New Mexico, Nielsen portanceat the expenseof potassiumsilicatealtera- (1968) suggestedthat the porphyrycopperdeposit tion. At this point, near to the upper limit of was emplacedbeneatha cover of not greater than economichypogenemineralization,intrusive bodies 0.5 km of older rocks. It is here contended,howare likely to be smaller and less regular, and large ever, that this figure merely representsthe depth areas are likely to be occupiedby hydrothermal beneaththe top of the subvolcanicbasement,and in this context it has been used in the construction of breccias('Fig. 1). Upward in manysystems, but perhapsnot in all, a Figure 1.2 Since stratovolcanoes in the Andes have comagrnaticvolcanicsuperstructureis encountered, heightsof 1,000-2,500m abovethe underlyingbasein which alteration tends to possessa less regular ment,the apicesof the columnsof Cu-Mo mineralizadistributionand to consistof propyliticand argillic tion, commonlyassociatedwith sericiticalteration, alteration, with areas of intense silicificationand were most probablyformed at approximately1.5-3 advancedargillic alteration; these two alteration km beneath the summits of stratovolcanoes. This typesperhapsreflectareaspreferredby ascending figure might be somewhatreducedif a depositwas hydrothermalfluids. This proposalconformswith eraplacedeccentricallywith respectto the principal that of Hemley et al. (1969) who showedthat volcaniccone,or if the magma from which the cone sericiticalteration is likely to pass upward into was constructed was less viscous than the andesire advancedargillic alteration. Pyrite is ubiquitous consideredhere, so reducingthe height of the cone. and other sulfides,with the possibleexceptionof This estimate is in accord with evidence derived mamasite,are uncommon. Intrusive rocks are rare from studiesof fluid inclusionsthat is interpreted to in these overlying volcanicsand are restricted to showthat potassiumsilicatealterationat Bingham, dikelike bodies,althoughhydrothermalbrecciasmay Utah, took place beneath a cover 4.3 km thick still be widespread. (Roedder, 1971). Therefore, from the available "Epithermal"copper,lead,zinc,andprecious metal evidence, it would seem reasonableto estimate that veins and replacements are considerednot only to a complete,unerodedporphyrycoppersystemcould represent thefringeproducts of mineralization in the have a vertical extent of 4-6 km, and perhapsnearer propylitic-alteredparts of an economicdeposit 8 km if the upper part of the underlyingphaneritic (LowellandGullbert,1970), but alsoto accomvany intrusiveis included(Fig. 1). advanced argillicalteration,silicification, or propylitic Evidence Bearing on the Tops and Bottoms alteration in the supradjacentvolcanic ediface of Porphyry CopperDeposits (Fig. 1). It is inferred that the tops of porphyry copper Regionalgeneralizations systemsare characterized by depositsof native sulIn Chile and adjacentArgentina,longitudinalbelts fur, perhapsaccompanied by pyriteor marcasite, and of post-Paleozoicbatholiths, stocks,and porphyry while the systemis still active,by high-temperature copper depositsyoung eastward from the Pacific fumaroles;thesemay be consideredas the effluent coastand are geometricallycomparable with beltsof

productsof porphyrycopperformationin depth.

The stratovolcano abovea pomhyry copperdeposit need not be a simplecone (Fig. 1), but may be multiplein characterand includethe development of domesand collapsecalderas,perhapsresurgent. In Figure 1 an attempthasbeenmadeto quantify the vertical dimensionsof a porphyry copper system, althoughthe depthsgiven shouldbe treated

Pliocene and more recent andesitic volcanoes in the

high Andes (Sillitoe, 1972a). Porphyrycopperdepositsand prospects are uncommon in the batholithic 2 Failure to recognize that many porphyry devositswere originally overlain by penecontemporaneous volcanic piles may provide an explanation for the very shallow depths of formation that have been proposed for some porphyry copver depositslocated in regions, such as the southwestern

United States, where the volcanicshave been largely lost

onlyas approximations basedon currentlyavailable by erosion.

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Fro. 3. The West Fissure and hydrothermalalteration pattern at Chuquicamata,Chile, taken mainly from Taylor (1935). The unshadedarea betweenthe sericiticand propylitic alteration zones probably cor.responds largely to argillic alteration: it was termed "normal rock" by Taylor and is describedas having less intense sericitic alteration and retaining its magmatictexture.

belt adjacentto the coastwheremostdeposits are

'Furthermore, the distributions and aerial extents

assumed to havebeenremovedby erosion.Economic of Chileanporphyrycopperdeposits andprospects in

deposits are considered to havebeenexhumed only

the belts of stocks and of alteration zones associated

locally from beneath the belt of recent volcanoes. with stratovolcanoes in the recent volcanic belt bear

North-southbelts of stocksof intermediateage, a strongmutual resemblance. situated between the batholiths in the west and the

volcanoes in the east, where erosionlevelsmay Bottomof a porphyrycoppersystem generally be said to be intermediate between the Chuquicamata, Chile: At Chuquicamata (Fig. 2), easternand westernextremes,hostthe majority of the bottompart of a porphyrycoppersystemis betheporphyrycopperdeposits, includingall the major lievedto havebeenexposedas a resultof high-angle ore bodies. faulting, the present ore body representingthe

804

RICHARD H. SILLITOE

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