Geology of the Los Pelambres Porphyry Copper Deposit, Chile

ECONOMIC

GEOLOGY AI•TD

BULLETIN

VOL.

OF

68

THE

SOCIETY

THE

OF

ECONOMIC

GEOLOGISTS

JANUARY--FEBRUARY, 1973

No.

1

Geologyof the Los PelambresPorphyryCopper Deposit, Chile RICHARD H. SILLITOE Abstract

The Los ?elambresporphyry copperdeposit,locatedin the Andean Cordillera of CoquimboProvince, Chile, is related to an Upper Miocene tonalite stock which was passively emplaced into Upper Jurassic-Middle Cretaceous andesitic volcanic and sedimentarystrata which constitutea north-south-trendingmonocline. A zonal pattern of hydrothermal alteration and mineralization is centered on the stock, and is representedby a core of potassiumsilicate alteration, surroundedby a roughly annular zone of feldspar-destructivealteration, which grades outward to propylitic alteration. The potassiumsilicate zone is characterizedby hydrothermal biotite, K-feldspar, quartz, anhydrite and sulfides; these sulfides, dominantly chalcopyrite,bornire, pyrite and lesser quantitiesof molybdenite,comprisethe potentially economicmineralization. A nucleuswithin the potassiumsilicate zone carries abundant bornite,and magnetiteis widespreadin two areas,one of which possesses large vuglike masseslined with coarse quartz, anhydrite, magnetite, tourmaline and sulfides. The feldspar-destructivealteration is largely of the sericitic and argillic types in the marginal parts of the stock, and mainly silicification in the adjoining volcanic-sedimentary formations. Pyrite is the most common sulfide in rocks which have undergonefeldspar-destructive alteration. Two varieties of hydrothermal breccia, largely confinedto the zone of feldspardestructivealteration, have been distinguished. The more commonvariety is a hydrothermal intrusion breccia which is characterizedby the presenceof rounded fragments and interstitial rock flour, and which gradeslaterally into in situ shatter breccia. The secondvariety is a hydrothermalreplacementbreccia, in xvhicha jigsaw-like array of unmovedfragments is cementedby tourmaline, sulfidesand somequartz. Supergenealteration is little developedat the Los Pelambresdeposit,becauseof its relatively recent age of emplacementand the effect of intenseglacial erosion. A shallow zone of partial leachingis developed,in which oxidized copperminerals are uncommon. Minor supergeneenrichmentis presentfrom surfaceto the base of anhydrite leaching;belowthis anhydritefront supergeneeffectsare absent. In the light of recent studies, Los Pelambres would seem to be a typical and relativelysimpleexampleof a porphyrycopperdeposit,with a genesisthat is reasonably explicablein terms of an orthomagmaticmodel. The large area occupiedby potassium silicate alteration, the paucity of hydrothermal brecciation and the dominantly phaneritictexture of the host intrusive suggestthat erosionhas exposeda fairly deep level of an upright columnof porphyrycopper-typemineralization. Introduction

Los Pelambres is located in the Andean Cordillera

IN view of the scarcityof publishedinformationon of CoquimboProvince,adjoiningthe frontier with South Americanporphyrycopperdeposits,a pre- Argentinaat lat 31ø42.5'Sand long 70ø30'W (Fig. liminaryaccountis presentedhere of the geological 1). Accessfrom the Panamericanhighway via and hydrothermal alteration characteristicsof the Salamanca(Fig. 1) is largelyon a dirt road, except recentlyinvestigated Los Pelambres depositin Chile. for thefinal22 km whichare completed on muleback.

2

RICHARD H. SILLITOE

Significant mineralization in thisimmediate region Juan Province,Argentina(United Nations,1970) of the Andesis not widespread, and is restrictedto (Fig. 1). The zone of alteration and mineralization at Los the Cu-W-bearingbreccia pipes worked at the Llamucomine (Sillitoeand Sawkins,1971) and the Pelambresoccupiesan area of 6 x 2.5 kin, and Cerro 3Aercedario porphyrycopperprospectin San ranges in altitude from 2,900 to 4,450 m above sea

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FIG.1. Location of theLosPelambres deposit withrespect to: A. othermajorporphyry copper deposits in Chile,B,

,the principal towns, roads andcopper deposits ofcentral Chileandadjoining Arger•t. ina:

THE LOS PEL•tMBRES PORPHYRY COPPER DEPOSIT

3

level. 'Vegetation is essentiallyabsent from the to SSW-trendingstructures,the mostimportantof precipitous cliffs and steeptalus slopeswhichchar- which is shownin Figure 2. acterize the area, and is restricted to the floor of the Two K-Ar ages of 9.74-----0.16and 9.96-----0.18 Los Pelambresvalley (Fig. 2). Los Pelambresex- m.y. (Quirt et al., 1971; Sillitoe, Quirt, Clark, periencesa cold, semi-aridclimate,with plentiful Farrar, and Neumann,in prep.) obtainedfor hydrosunshineand strongwinds; summersare dry but thermal biotite from two localities within the Los snowstorms in winter render the area inaccessible Pelambresstockdemonstrate an Upper Mioceneage from May throughAugust. for alterationand mineralization,and probablyalso From 1914, when the strikingcolorationzoneat for associatedstock-emplacement.Uplift of the Los Pelambreswas investigated by William Braden Permo-Triassic core of the anticlinorinm is also (the initial operator of both the E1 Teniente and thoughtto have occurredat aboutthis time (ViPotrerillosporphyrycopperdeposits in Chile), little cente,1972). development was undertaken until 1969, when

In the Pleistocene,the Los Pelambresarea under-

drillingwas commenced by a United Nationsmin- went severeglacial erosionresultingin the excavaeral explorationprogram. tion of the U-shapedLos Pelambresvalley which This description of the geologyand alterationat transectsthe mineralizedstock(Fig. 2). The northLosPelambres is basedon surface mapping at a scale easttributarystreamat the headof the Los Pelamof 1: 10,000and the examinationof over 4,000 m of bres valley is incisedinto a glaciallystriatedroche drill core,aidedby the studyof thin and polished mouton•e, which is the site of some of the highest sections,and X-ray analysis,of representativegradeore encountered to date. Pleistocene andmore samples.

recenttalus,includingrock glaciers,and alluviumon the valleyfloorcoversome70 percentof the mapped GeologicalSetting

The host rocks of the mineralized Los Pelambres

stockare chieflysubaerialandesiticlavas, tuffs and agglomerates. They are assignable,on the basisof

area ('Fig. 2). The presence of "stonestripes"on the higherslopesdemonstrates the continuingoperation of freeze-thaw

action. Intrusive

Rocks

their stratigraphic positionimmediatelyoverlyinga regionally prominent horizonof gypsumof Oxfordian The roughly oval Los Pelambresstockpossesses age (Vicente, 1972), to the Upper Jurassic maximum dimensions of 4.5 x 2.4 km and is oriented (Kimmeridgian) Rio Damas Formationof Klohn in a north-southdirection (Fig. 2). Where ob(1960). In theextremesouthwest of thearearepre- servable,the contactsof the stock are sharp and sentedin Figure 2, well-bedded,continentallutites, steep,althoughon a large scalethey are rendered arenitesand andesiticvolcanics conformably overlie irregularby dikesand sills protrudinginto the adthe Rio Damas'Formation,and probablybelongto joining wall rocks. No structuraldisturbanceseems the ColimapuFormation(Klohn, 1960) of Middle to have been inducedby intrusion, which may be Cretaceousage. considered to havebeena passiveprocess.Evidence The volcanic-sedimentary formationsin the Los for the operationof assimilationis locallypresent, Pelambresarea strike approximatelynorth-north- but this processwas obviouslynot an important westand dip westward;dips steepenfrom 20ø near mechanismof stockintrusionat the presentlevel of to the frontierwith Argentinato 50-80ø immediately exposure. west and southof the stock(Figs. 2 and 3) to 20Extensive hydrothermalalteration of the stock 30ø farther west, indicatinga monoclinalstructure. makes recognitionof the original rock type someThis interpretation is confirmed by morecomplete ex- what difficult.Nevertheless,microscopic examination posures a few km north of Los Pelambres. The of numeroussamples,especiallythose from the monoclinemay be considered as a subsidiaryflexure southwestsector of the stock, shows the original on the westernflank of a north-south-trending anti- stockto be a tonalite,with local transitionstoward clinorium,thecoreof whichis represented by Permo- a tonaliteporphyry. Only one phaseof intrusion Triassic rhyolites in Argentina, a few km east of has been recognizedwith certaintyto date. The rock, Los Pelambres(J. C. Vicente,pers.commun.,1971). tonaliteis a phaneritic,hypidiomorphic-granular Foldingis probablyMiddle to Upper Cretaceous in consistingof approximately40-50% plagioclase age (Vicente, 1972). The monoclinemay per- (andesine),commonlyzoned,and 15-20% quartz, hapsbe a surfacereflectionof a major fault in depth, accompanied by lesseramountsof biotiteand hornand seems to have controlled the location of the Los blende,and accessory apatite,zirconand sphene.As Pelambresstock. Faulting in the immediateLos far as canbe determined,magmaticK-feldsparreprePelambres area is limited to a series of minor SW-

sentsconsiderably lessthan 10% of the rock.

4

RICHARD H. SILLITOE

SECT/ON LINE

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ALTERATION

Hydrothermal breccias (outcrop on[y] :•

Potassium Silicate •'• Fault observed/inferred •.I•"• --]TonaLitc porphyry • Sericitic, Ar9iJLic /"• Rock contact •UPPER TonaLitc • SiUcification /"•, ALteration contact •_ observed / inferred JURASSICobserved/inferred

MIDDLE CRETACEOUS J• Propytitic • Strike,dip vJ•-•1 VoLcanic -sedimentary formations •:'"•' ktaõnetite-Hch area G) Siteofdrill-hoLe

1 ROCK OUtCROP D TALUS, ALLUVIUM

FIG. 2. Geologyand hydrothermalalterationat Los Pelambres. The inset showsthe distributionof rock outcropsand talus and alluvium.The approximateouter limit of the area with abundantbornite is basedon data taken from drill cores,

The positions of 13 adits,mainlyin the eastandsoutheast of thearea,arenotshown,

THE LOS PEL.4MBRES PORPHYRY

COPPER DEPOSIT

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Fro. 3. A generalizedsectionthroughthe Los Pelambresdeposit,alongline A-A' in Figure 2. Legendand symbolsare

the sameas thosein Figure 2. The positionof the bornite-richnucleusis basedon data taken from the drill holesmarked and from holes both north and south of the line of section.

An irregulardikeof pale-colored tonaliteporphyry alterationtypesis presentat Los Pelambres(Figs. 2 outcropssoutheastof the principal stock in vicinity of the Argentinian frontier (Fig. Porphyry copper-typemineralization at the Pach6nprospect,on the Argentinianside of

the 2). E1 the

and 3). A core of potassiumsilicatealteration is surroundedby a roughly annular zone of feldspardestructive alteration, comprising sericitic and argillicalterationand silicification,and averaging500 frontier, seems to be associatedwith the south- m in width, which passesoutward into propylitic eastward continuation of this dike. The dike is alteration. The concentric alteration pattern is consideredas an upward- and outward-projecting clearlycenteredon the tonalitestock(Figs. 2 and 3). apophysisof the main stock, to which it is believed Ignoring the dike and its associatedalteration,it can to be connectedat no great depth. Evidencesup- be appreciatedthat the potassiumsilicatealteration portingthis suggestion is providedby the elongate is confinedto the stockand its enclosedroof pendarea of potassiumsilicate-alteredandesiticvolcanics ants. The propyliticfaciesis essentiallyconfinedto enclosing a restrictedareaof tonaliteporphyryand the surrounding volcanic-sedimentaryformations, associated igneousintrusionbrecciawhichlies along and the feldspar-destructive alteration occursin an the projectedstrikeof the dike (Fig. 2). The ton- intermediatezone comprisingthe outermostparts of alite porphyry possesses phenocrystsof andesine, the stock and its immediately adjacent wall rocks. quartz and biotite in an aplitic groundmassdom- The southeastwardextension of potassiumsilicate inatedby plagioclase and quartz. alterationassociatedwith the tonalite porphyry dike Andesiticroof pendantsand xenolith swarmsare and its postulated underground continuationto the relativelycommonin the stock,evenin depth. The northwest grades outward into propylitic alteration partial assimilation of andesiticvolcanicsoccurring without an interveningzone of feldspar-destructive as roof pendantsor on the contactsof the stockhas alteration (Fig. 2). resultedin areas of dark, porphyriticrock which The salientfeaturesof eachof the alterationtypes may be recognized by an abundance of biotitein the will now be brieflyconsidered:

groundmass and by a great variabilityin composition and texture; all gradationsfrom tonalite to andesiteare present. Hydrothermal Alteration and Mineralization

Potassium silicate alteration

The economicallymost importanttype of alteration at Los Pelambresis the potassiumsilicatetype with whichvirtually all the significantcopper-molybdenurn mineralization

encountered

to

date is as-

Potassiumsilicate,sericitic,argillicand propylitic alterationtypes and silicificationhave been recog- sociated. Four hundredand thirty million tons of

nized at Los Pelambres;the definitionof the altera- ore assaying0.8% Cu and0.035% Mo havebeenouttion types follows current practice (Meyer and lined largely in the northwesternsectorof the poHemley, 1967; Lowell and Guilbert, 1970). An tassiumsilicate-alteredcore,the only area which has exceptionallywell-developed zonaldistributionof the been systematically drilled up to the presenttime

6

RICHARD H. SILLITOE

(Fig. 2) (United Nations, 1971). The delimitation of the area of potassiumsilicatealteration at Los Pelambresprovided an effectivemeansof predictingthe distributionof copper-molybdenum mineralization. The zoneof potassiumsilicatealteration was subsequently foundto be closelycoincidentwith the area which yieldedanomalouscopperand molyb-

Sulfidesin the potassiumsilicatezone are dominantly chalcopyriteand bornitc,with lesserquantities of pyrite and subordinatemolybdenite. Pyrite is uncommonin the inner part of the potassium silicatecore,exceptas a component of superimposed sericiticalteration(seelater), but increases in amount toward the contactwith feldspar-destructive altera-

denurn values in both rock and the fine fraction

tion.

of

Sulfides occur in veinlets and on short hair-

line partings,and in additionas disseminations, a in the center The potassiumsilicate-alteredtonalitc, easilyntis- modeof occurrencewhichpredominates taken as unaltered on casualexamination,is chiefly of the potassiumsilicatecore. Informationderived characterizedby the presenceof K-feldspar,locally from drill core indicatesthe presenceof a nucleus microperthitic,whichmay compriseup to 30% of the characterizedby abundantbornitc within the potastonalite(Figs. 2 and 3). Bornitc altered rock. Hornblende grains are altered to siumsilicate-altered absentnearthe bordersof the potassium aggregatesof pale brown biotite flakes. Irregular is essentially patches(on the scaleof a thin section)of an aplitic silicate zone. This bornitc-rich nucleus does not groundmass,dominatedby quartz and K-feldspar, outcropin its northernpart ('Fig. 3) but extendsto with subordinate biotite, anhydrite and sulfides, a depthof at least435 m. Bornitclocallycomprises developedin placesat the expenseof the hypidio- up to 60% of the sulfidein this inner core,and ore morphic-granularmagmatictexture. This ground- gradesrange from 1.0 to 1.5% Cu. Patchesof mass has eaten into plagioclasegrains, and when bornite occur within grains of chalcopyrite,and chalcocite (or djurleite) well developedgivesa porphyriticaspectto the rock. minoramountsof hypogene phaseare intergrownwith the Locally, the potassiumsilicate-alteredtonalitc is so and a digenite-type enriched in postmagmaticquartz that it may be bornite. classifiedas silicificationwithin the feldspar-stable An elongatearea in the northeastof the potassium environment. Scarce tourmaline, as veinlets and silicatezoneandthe areaof potassium silicate-altered rosettes,as well as minor sericite,calcite,apatite and andesitc marginal,and to the northwestof, the tonaof chlorite (intergrown with biotite) are also com- litc porphyrydike bothcarry notablequantities magnetite(Fig. 2), accompanied by minor specuponentsof this alterationtype. Roof pendantsof andesitcwithin the potassium laritc, as clots, veinlets and disseminations. Within the northwesternmagnetite-richarea irsilicatezonehave had their originaltexture destroyed and are now composedof fine-grainedmosaicsof regular, vuglike bodies,severalmeters across,are quartz, K-feldspar, plagioclase(albite-oligoclase), linedwith crustsof prismaticquartzcrystals,smoky biotite,anhydrite,sulfides,and minor serfcite,rutile, or amethystincin parts, overlain by anhydrite, magnetiteand apatite. Original, magmaticplagio- largely hydratedto gypsum (Fig. 4), with some clasephenocrysts seemto be alteredto plagioclase of pyrite, chalcopyrite,molybdenite,magnetiteand a more sodiccomposition(commonlyalbite), often tourmaline. as outgrowthsin optical continuitywith the phenocrysts,and K-feldspar. Biotite is especiallyabund- Sericiticandar#illic alteration ant in theseandesiticroof pendants. As a generalization,feldspar-destructive alteraA stockworkof short,irregular veinletscontaining tion of tonalitc has resulted in sericitic or argillic one or more of quartz,chalcopyrite, bornitc,pyrite, alteration,whereasthe productsof feldspar-destrucformations molybdenite, biotite,anhydrite,magnetite,K-feldspar tive alteration of volcanic-sedimentary and tourmaline pervades the potassium silicate- are best classifiedas silicification,despitethe local of sericite. Sericiticand argillicalteration altered tonalRe and its enclosed roof pendants. presence Alteration envelopesare absentfrom theseveinlets. havenot beenseparatedon Figure 2 sincethey are Quartz veinlets,with or withoutK-feldspar,biotite, transitional and intimately associated. alteraanhydrite or magnetite commonlycarry sulfides. In partsof the haloof feldspar-destructive Crystallineanhydriteas a filling to veinletspossessestion where sericiticalterationis pervasive,as for a characteristiclavendercolor. Quartz-biotite vein- instancein the northwest sector of the area, the tonato a mosaicof quartzand sericite lets are generally earlier than veinlets carrying litc is transformed

talus (Maranzana,1972).

quartz alone. Uncommon bornite-chalcopyrite-grainsaveraging0.05 mm in diameter;magmatic quartz-anhydriteveinlets with selvagesof sericite, quartz grains remain and some have undergone veinletsandrosettes of chlorite,calcite,epidoteand anhydriteare tentatively marginalcorrosion.Patches, and chloriteand epidote ascribedto a late stageof potassiumsilicatealtera- tourmalineare widespread tion. were observedlocally. The abundance of gypsum

THE LOS PELAMBRES PORPHYRY COPPER DEPOSIT

7

in andesiticrocks, and are tentativelyattributedto west of the area suggeststhe existenceof anhydrite pre-alterationthermaleffectsof a regionaltype. in depth. The sericitic-alteredtonalite carrieslarge Hydrothermal Breccias amounts of disseminatedand veinlet pyrite, accompaniedby subordinatechalcopyrite. The chalcoHydrothermal brecciasare quantitativelyunimpyrite contentis too low to. attain ore grade, except portant at Los Pelambresand are mainly locatedin perhapslocallyin a very narrow fringe at the con- the southernpart of the area (Fig. 2). Two tact with the potassiumsilicatealteration. Molyb- varietiesof brecciawere distinguished. deniteoccursin the sericiticzone,usuallyin veinlets, The more widespreadvariety of breccia,and for and specularitewas observedparticularly in the reasonsof scalethe only one markedon Figure 2, northeast and northwest of the area. may be classifiedas hydrothermalintrusionbreccia, Argillic alterationis characterized by the partial or utilizing the terminologyof Bryner (1968). The completereplacementof plagioclasegrains by fine bodies of breccia do not exceed 200 m in maximum aggregatesof kaolinitc and sericite,and by the re- diameter,appearto be nearlyvertical,and consistof tention of the magmatic texture. Argillic-altered angular to completelyroundedfragmentsof the host areas are criss-crossed by quartz-sericiteveinlets, rock, either tonalite or andesitic volcanics; exotic which increase in number as sericitic alteration is fragmentshave not beenrecognized. All gradations approached.As in the sericiticalteration,pyrite is seem to exist in these breccia bodies between the abundant. in situ shatteredrock (sometimestermed shatteror In the potassiumsilicatezone,veinletsand patches cracklebreccia) so commonin the zone of feldsparof sericitic alteration are superimposedon earlier destructivealteration,and pebblebrecciasand dikes (at any one place) feldspar-stable alteration,and composedof rounded fragments cementedby a are notably more abundantas the sericiticzone is matrix of powdered rock. The fragments in the neared. Quartz-sericite-anhydrite-pyrite veinlets breccia bodies are generally silicified or sericitic boundedby quartz-sericite envelopes are widespread; altered,althoughin the two northernmost('Fig. 2) someveinletscarry apatite,and one occurrence of many of the larger fragmentsstill retain potassium sphalerite was noted. The occurrenceof these silicatealteration. Limonite,probablyderivedfrom veinletsconfirmsanhydriteas a component of serici- pyrite, and tourmalineare presentbetweenfragments tic alteration(Meyer andHemley,1967). in somebodies. A spatial associationof severalof the hydrothermal intrusion breccias in andesitic Silicification at surface in the sericitic alteration zone in the north-

volcanics with

small dikes of tonalite

has been ob-

Silicificationwas producedat Los Pelambresby served. the feldspar-destructive alteration of volcanic-sedi- The secondvariety of brecciarecognizedat Los mentaryformationsandmaybe gradational to serici- Pelambresis conjecturedto be a product of in situ tic, argillicor propyliticalterationtypes.Microscopic replacementof rock, and is perhaps akin to the examination of silicified rock typically reveals a chemical breccias recently describedby Sawkins mosaic of fine-grained quartz, with subordinate (1969). It is commonestin the northwestof the amounts of sericite, kaolinite, chlorite and plagio- area in sericitic and silicified zones but also occurs in clase (albite, at least in parts). The occasional the eastern sector of the potassiumsilicate-altered presenceof biotitepresumablyindicatesa transition tonalitc. Hydrothermalreplacement brecciatypically to potassiumsilicatealteration. iVyriteis the only consistsof a jigsaw-likeassemblage of angularfragsignificantsulfide. ments tightly cementedby tourmaline,pyrite and some quartz. The breccia generally exists as irPropylitic alteration regular, veinlike masses,and less intense replaceA broadhaloof propyliticalterationrepresentsthe ment is representedby a network of tourmaline outermosteffectsof hydrothermalalteration at Los veinlets in the rock. iVelambres. Chlorite is the diagnosticmineral, as-

sociatedwith epidote,calcite and pyrite. In restrictedparts of the propyliticzone,magnetite(Fig. 2) and speculariteabound. At a locality in the northeastof the area, propylitic-altered andesitesare cut by a 10-cm-wideveinletof freshpyrite, gypsum and crystallinenative sulfur,apparentlyof hypogene origin. In the marginalpartsof the area represented in Figure 2, nodulesof epidoteincreasein abundance

SupergeneAlteration The process of supergenealteration has been

relativelyunimportantat Los iVelambres compared with manyporphyrycopperdepositsin the western Americas.

Oxidation,represented by partial leachingof sulfides, normally extendsdownward to 40-70 m, althoughdepthsof as little as 6 m on the floor of the

8

RICH,4RD H. S1LL1TOE

still remains. In someof the large massesof gypsum found in the magnetite-rich tonalite (see above), remanent cores of unaltered, lavender-coloredanhy-

drite have beenpreservedbecauseof the insulating effectof the enclosing gypsum(Fig. 4). At a depth belowsurfacerangingfrom 140 to 250 m a remarkablyabruptcontactoccursbetween"tight" anhydritebearingrockandthe porousrockfree from anhydrite which extendsto surface;this anhydritefront is the downwardlimit of phreaticcirculation. Minor supergenesulfideenrichmentis observable from surfaceto the anhydrite front, beneathwhich supergeneeffectsare apparentlyentirely absent.Enrichmenttakes the form of thin films of coppersulfides,mainly chalcociteor djurleite, on chalcopyrite and bornite grains; it is nearly absentfrom pyrite grains. The superimposition of oxidation on the coppersulfidefilmsof enrichmentorigin hasresulted in the formationof flamesand patchesof normaland blaubleibender covellite (Sillitoe and Clark, 1969). The paucity of supergeneenrichment at Los Pelambresmay be attributedto two causes:First, the deposit was emplacedin the Upper Miocene, when the major zones of enrichmentin Chile had alreadybeenformed (Sillitoe et al., 1968). Second much of the probablyminor amountsof enriched ore of post-Mioceneage was eroded during the Pleistoceneglaciation. Leaching of sulfidesand anhydrite from the ore body in post-Pleistocene times has producedthe Fro. 4. Lavender-coloredanhydrite partially hydrated to •vhite gypsum, from vuglike masses in the northeast sector limonite and gypsumwhich locally cementgravels and alluvium in the Los Pelambresvalley. A reof the potassium silicate-altered tonalite. Note the inheritanceof anhydrite cleavageby the replacive gypsum. strictedoutcropof talus in the southeastof the area is cementedby chrysocolla,presumablyprecipitated Los Pelambresvalley and as great as 100 m have from solutionsderived from the oxidation of nearby been recorded. Completeleachingmay locally ex- potassiumsilicate-alteredandesites. tend downward from surface for a few meters, but Discussion sulfidesare normallyvisiblein outcrop. Only minor amountsof oxidizedcoppermineralsare presentin In the light of recent compilationsof patterns of the zone of partial oxidation;malachiteis the most alteration-mineralization zoningin porphyryore decommonmineral, althoughazurite, chrysocolla,chal- posits (Lowell and Guilbert, 1970; Rose, 1970), it canthite,brochantiteand pseudomalachite have been canbe concludedthat Los Pelambresis botha typical identified,mainly in the easternsectorof the potas- and a very simpleporphyrycopperdeposit. sium silicate zone. Limonite is common at surface, The extreme relief in the region of Los Pelambres a jarosite componentbeing widespreadover pyrite- gives rise to the exposureof alteration and minrich, feldspar-destructive alterationtypes. Leaching eralization over a vertical interval of 1,450 m, and has not usually been sufficientlyintenseto remove sulfide mineralizationin the main stock, excluding all molybdenitefrom outcrop,althoughpartial oxida- the lateral dike, is known over a vertical distanceof tion has resulted in the formation of coatings of 1,100 m. It can also be appreciatedthat the zone ferrimolybdite. Partial martitization of magnetite of feldspar-destructive alteration is situatednot only may also be attributed to the effects of supergene peripheralto but also in part at a higher level than oxidation. the potassiumsilicate-altered core. The exhumingof An almostcompleteleachingof anhydritehas oc- this core was probablylargely due to incisionof the curred in the surfacezone (Fig. 3), exceptfor iso- U-shaped Los Pelambresvalley in the Pleistocene. lated areas where gypsum producedby hydration These relations are in accord with the considerable

THE LOS PEL.4MBRES PORPHYRY

COPPER DEPOSIT

9

lateralmixingwith verticalextension,perhapsof the order of 3,000 m, posurein thisarea,sominimizing of porphyry copper deposits,as determinedfrom ground waters. studiesat San Manuel-Kalamazoo, Arizona (Lowell The abundanceof quartz in both feldspar-stable and Guilbert, 1970). At Los Pelambres,the large area occupiedby the zoneof potassiumsilicatealteration,the dominantly phaneritic texture of the intrusive rocks, and the subordinater61e of hydrothermal brecciasall suggest that erosionhas exposeda relatively deep level

and -destructivealteration types suggeststhat the activityof silicain solutionwashigh, and its precipi-

thekindenvisaged by LowellandGuilbert(1970).

(crackle) brecciationof the rock, and to bodiesof hydrothermalintrusion breccia marking points of more forcefulreleaseof fluidsin the southernpart of

tation, most marked in the zone of silicification,was probablydue to cooling. Expulsionof hydrothermalfluidsthroughthe zone of feldspar-destructive alteration,and their probable of a vertical,columnarporphyrycoppersystem,oœ mixing with meteoricwaters,gave rise to a shatter

The orthomagmaticmodel of porphyry copper genesissupportedby Lowell and Guilbert (1970) seemsto offer a plausibleexplanationfor the origin of the Los Pelambresdeposit. According to this type of model, a body of magma of tonalitic composition becamesaturatedwith aqueousfluids as it intrudedtowardthe surfacezone,ascentceasingwith the commencement of crystallizationof an outer shell. Continuedaccumulation of fluidsnear the top of the partially liquid core of the stockeventuallyresulted in their expulsion,and the concomitantsolidification of all remainingmagma,apparentlyrelativelyslowly judging by the dominantlyphaneritictexture of the tonalite.

The ruglike bodiesof coarsequartz,anhydriteand other minerals in the magnetite-richpart of the potassium silicate-altered tonalite are apparently similar to the pegmatitemasses,also in areas enrichedin magnetite,at Ajo, Arizona (Gilluly, 1946). They may representincipientpegmatiticcrystallization from aqueousfluids which accumulatednear the roof of the Los Pelambresstockprior to their release,in agreementwith the abovepostulatethat the expulsionof fluidswas slow. The formationof potassiumsilicatealterationwas, with the exceptionof that associated with the tonalite porphyry dike, confinedto the core of the Los Pelambres stock, and probably commencedonce aqueousfluids had cooledbelow the solidustemperature of the magma. Toward the margins of the stock,potassiumsilicatealterationgaveway abruptly to feldspar-destructive alteration types. As sug-

the area.

Acknowledgments

Initial reconnaissance mappingat Los Pelambres was undertakenunder the auspicesof the Instituto de Investigaciones GeolGgicas of Chilein the company of Sr. I•I. NeumannJ., whoseassistance is gratefully acknowledged. Most of the work was completed whilethe writer wasengagedby the United Nations, New York as a technical consultant with the Chile-28

Project (United Nations-ENAMI). Thanks are due to Mr. J. Carman, Technical Adviserof United Nations,New York for permission to publishthis paper; to Mr. D. P. Robertson,Project Manager, and Sr. L. Kaiser, 'Mr. F. Maranzana, Mr. J. R. Mowat, Mr. D. G. W. Norris, Project personnel,for their assistance and support;to Mr. J. C. Vicente for discussions; and to Norris for readingthe manuscript. The paper was completedwith the supportof a Shell postdoctoralresearchfellowshipat the Royal Schoolof Mines, Imperial College,London. DEPARTMENT OF MINING

GEOLOGY

ROYALSCHOOLOFMINES, IMPERIAL COLLEGE PRXNCE CONSORT ROAD

LONDON,S.W.7, ENGLAND

June18, July 25,.1972 REFERENCES

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