Gemmell Et Al-1992

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Economic eologyVol. 87, 1992,pp. 2085-2112

Genesis f the AguilarZinc-Lead-Silver eposit,Argentina:ContactMetasomatic s. Sedimentary xhalativej. BRUCE EMMELL,* ALFZANTOP,Department f Earth Sciences,artmouthCollege,Hanover,NewHampshire 3755

AND LAWRENCE D. MEINERTDepartment f Geology,WashingtontateUniversity, ullman,Washington9164-2812

AbstractLocatedn the Eastern ordillera f northwesternrgentina,heAguilarmineandseveralsmaller rospectsre alignednorth-southn a 30- by 5-kmdistrict.The Zn-Pb-Ag ulfide resare hostedn intenselyoldedand aultedLowerOrdovician uartzites ndhornfelses,eartheir contactwith the Cretaceous guilargranite.The orebodies re stratigraphicallyon-formablewith the siliciclasticocksand parallel to the contactmetamorphic alo of thegranite, n the pyroxene-hornblendend albite-epidote ornfels acies.The oresareassociatedith sedimentsormed n localdepressionsf a tectonically ctive,shallow-marine environment and consist of strata-bound and stratiform sulfide lenses andlayersseveral undreds f meters ongandwide, and ensof meters hick.A distinctive restratigraphys recognizable: isseminatedndstockwork ulfides verlainby breccia-hostedsulfides ndbanded o massive ulfides. ead-rich issure einsand quartzveinscut thesetypesof strata-boundulfides; arite s abundantn oneof the prospects.he oresare madeup of fine- o medium-grainedntergrowthsf sulfides ndsulfosaltsn quartzite ndcalc-sili-categangue. he sulfides re intensely ecrystallized nd annealedn the Aguilardeposit,which s ocated n the pyroxene-hornblendeetamorphic alo. n the Esperanza rospect,located n the lower gradealbite-epidote alo, he sulfides re finer grained, ess ecrystal-lized, not annealed, nd containabundant amboidalpyrite and texturesof soft-sedimentdeformation.The calc-silicatessemblagen the Aguilardeposits characterizedy subcalcic arnetswith significant roportions f spessartineSp26_7sole%), almandineA15_23),rossularite

(Grs_52),ndandradite Ado_14).yroxenes re manganeseich and ron poor,with an end-member range of hedenbergite Hdlo_55mole %), johannsenite Jos-3s), nd diopside(Di2o_so).ate-stage karnmineralsncludecalcium-rich ustamite, ubcalcicctinolite, hlo-rite, andvesuvianite ith up to 20 wt percent are earthelements.Sulfur sotope alues ange rom10.8 to 26.5 per mil forsulfides, nd rom32.4 to 34.0 permil for barite.The strongly ositive 34S alues or sulfides ndbariteare consistent ithfractionation f sulfide ndsulfate romLower Ordovician eawater. ead sotope atios ngalenasromall Aguilaroreshavemeans f 26Pb/24pb18.04, 27pb/24pb15.64, and2Spb/24pb38.03. Potassiumeldsparrom he Cretaceousguilargranite s far moreradiogenic, aving atiosof 6Pb/4pb 19.28, 7pb/4pb 15.67, and Spb/4pb:39.00. These sotope atios uggestnearlyPaleozoic, rustal ourceor the ead n the ores,unrelated o the lead n the granite.The overallgeologic etting ndgeometry f the Aguilarores, he distinctive re stratig-raphy, he mineral ompositionnd extures,he calc-silicatessemblage,nd he stable ndradiogenicsotope vidence uggesthat he sulfidesn theAguilardistrict ormed sexhala-tive accumulationsn a Lower Ordovician, hallow-marineedimentary asin.The presentdistribution f the oresand their metamorphicextures ndicateoverprinting y contactmetamorphismuring he emplacement f the Cretaceous guilargranite.

IntroductionTHEAguilardeposit,Argentina's rincipalZn-Pb-Agproducer, is located in the Eastern Cordillera ofnorthwestern rgentina Fig. 1). The deposit ndsev-

Present ddress:entre orOreDeposit ndExplorationtud-ies,University f Tasmania, .P.O.Box252C, Hobart,Tasmania,Australia 7001.

eral other basemetal prospects re alignednorth-south,along he eastern dgeof a districtwhichmea-sures pproximately0 by 5 km (Figs.2 and 3).Large-scale xploitation f the Aguilardepositbe-gan in 1936. Cumulativeproductionsince hen hasbeenover 25 million onsof ore yielding2.2 milliontonsof Zn-Pbconcentrates. urrentproductions ap-proximately ,000 tonsper day at an averagegrade0361-0125/92/1409/2055-2553.00 2085

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2086 GEMMELL,ZANTOP,AND MEINERT

80* 70'./' A UIRUVILCA BRAZIL PASTOUENO--10' , -..-' CERROEPASCO {

CASAPALCA\ 'JULCA"I_ PERU BOLIVIA',/ PACIFIC e . POTO81 7 -. OCEAN CHOCAYA

ICHILE /

EL ALVADOR POTRERILLOS Gu-(Mo-Au) / FARALLON Cu-Pb-Zn-Ag / NEGRO Sn-(W-Ag-Bi) ( FAMATmAQ L INDIO--30* Au-Ag _/ ARGENTINA/

LOS PELAMBRES PARAMILLOS

0 km 500 f IL TENIENTEFIG. . Location f theAguilar istrict ithin he rameworkf theprincipal ineral epositsf theSouthAmericanAndes modified rom Sillitoe,1976).

of 6.2 percentZn, 5.5 percentPb, and 60 to 110g/tonAg (Sureda ndMartin,1990).The highest ut-cropsof ore are at an elevationof 4,750 m; the mineworkings xtenddownward or over 720 m. The min-ing historyand currentminingmethods re outlinedin Edwards 1986).Traditionally onsidered magmatic ydrothermalreplacement f favorablesedimentaryocks n the

contact aureole of the Aguilar granite (Spencer,1950; Ahlfeld, 1955; Whiting, 1959; Einaudiet al.,1981), the deposithas more recentlybeen reinter-preted as a sedimentary xhalativemassive ulfidewith metamorphic verprint,an idea first broachedby Brown 1962), andmore ecentlyandmorespecif-icallyby Brodtkorbet al. (1978), SuredaandAmstutz(1981), Brodtkorband Brodtkorb 1984), Suredaet

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AGUILARZn-PboAu EPOSIT,ARGENTINA:GENESIS 2087\+ + + I o+ + + + + + + + ++ + .+ + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + +

+ ++++++++++++++++++++++++++++++4++++++++++ +++++++++++ + +++++++ + + +

_

FAULT MINEFIG.2. Mapof the Aguilar istrict howinghe regional eology nd ocationsf theAguilar epositand associatedrospects. ge of geologic nits,Q = Quaternary,K = Cretaceous, = Ordovician.

al. (1986), Zantop and Gemmell (1987), and Suredaand Martin (1990).The structuraland textural complexities f theAguilar ocks, rought boutby repeated pisodes fintense olding, aulting,and igneous ctivity,havemadean unequivocalnterpretation f the oresdiffi-cult. The field work and he mineralogic, eochemi-cal, and sotopic tudies resented ere were under-

taken o providea wide spectrum f criteria or thegenetic nterpretation f the Aguilar deposit,at atimewhennewstrategiesor explorationndexploi-tationof the districtwerebeingdeveloped.Three months of field work included observationof the geologic, tructural, ndmineralogicalharac-teristics f the Aguilardeposit nd otherbasemetalprospectsn the district, o allowa reconstructionfINTRUSIVE PHASES and CONTACT METAMORPHIC AUREOLES

:::::::::::::::: : :::: ::: ...,.-:i i i i hi:i::: :: :::: :: ..::::::::::::::::::::::::::::::::::::: N:::::,:.:.:.:.::::::::: ::::::::: :::::::: :.:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::].: ....:.............i.:.;::OBlENh.&.;;;;;;h::: ....................::::::::::.:......

' ":':" ' '"""::::::: ' "":'.'.':':;:::.:.:.)?;h. + + V * + + + + + + + + +' +' + + + + + +::':j:::::::.':::: :: :: ::;:: - -. . + .:::::::...:::: ...........::;:::::I + ' ' ' ' ' ' ' ' ' + + + +' + %:.-:"'::::::...::.:::::::::'$ / IV ' ' ' ' ' ' ' ' ' -'.:::.:::%:}::::: ::.:.

'"'::::h::::: N+ '..: ...... . .. ......':::::::::::::::::::::::: :::::::::::::::::::: + + + + + + + + +. + p + + ..p ..+ + + + + + + + + + + + + + + + + + .. =======================::::::::::::::

:: ::::::+ + + + + + + + + + + + + 'L. ;... + . .+ + + + + + ..d + + + .::{::::"::::::::{:::::::::::: } ::::::::::::h:::::: ::::/+ + + + + + + + + + + + ,.. 4 + + + + + + $:::..:::::::::::::::::::: :::: f:::.::: :.-:::::::::

.:. ::::::::::::::::::::::::::::::::::::::::::::::: II BIOTITE RANITE ????? .... PYROXENE-HORNFELS ::];]:':: :; .... III RAPAKIVI & PORPHYRITIC GRANITE MINE AGUILARNTRUSIVE IVMONZONITEDIORITE PROSPECTV ALKALI GRANITE

FIG. 3. Intrusivephases nd contactmetamorphic ureoles f the Aguilargranite modified romBrodtkorbet al., 1978).

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2088 GEMMELL, ZANTOP, AND MEINERT

their formation and geologichistory. The deposi-tionalenvironment f the host ockswasgivenpartic-ular attention,aswere the relationships etweenoreand host rock, and between ore and metamorphichalo in which the ore is located. Sampleswere se-lected over the full vertical and horizontal extent ofthe Aguilardeposit ndalong he surface utcrops fthe other mineralprospects.The geologic ackground f the Aguilardistricthasbeen described n the recent iterature (Brodtkorbetal., 1978; Sureda and Amstutz, 1981; Martin, 1987;SuredaandMartin, 1990). We will present he essen-tial geologic nformationas needed for an under-standingand interpretation of the overall geologicrelationships nd will emphasize he resultsof ourown petrographic,ore microscopic,geochemical,and sotopicnvestigationss hey relate o the Agui-lar depositand prospects.

RegionalGeologyThe AguilarRange s an uplifted ault block n theCordilleraOriental,at its westernboundarywith thehigh volcanicplateau of the Puna. The range isbounded o the eastand westby steeplydipping e-verse aultsandconsists ainlyof foldedand aultedlower Paleozoicsedimentary ockswhich were in-truded by granodioritic o graniticplutons n EarlyCretaceousime (Fig. 2).The basement f the AguilarRangeconsistsf Pre-cambrian raywackes, andstones,nd shales,whichare variablymetamorphosedp to greenschistacies.This basement s unconformably verlainby Cam-brian, shallow-marineiliciclasticocksandby a se-quence of transgressive, arly Ordovician sand-stones, hales,minorconglomerates,nd imestones.Shallow-marine ocks of this Ordoviciansequencehost the deposits f the Aguilar district. At strati-graphically igher evelsand to the west and north-west of the Aguilararea, Ordovicianvolcanic ocksaccumulatedn the deeperpartsof the sedimentarybasin.In addition o the mine and prospects f the Agui-lar district, he Ordovician ockshost a variety ofother ore assemblagesf as yet poorly definedge-netic associationSuredaet al., 1986), among hemAu-Cu-Feveinswhichcut the rocksdepositedn the

deep partsof the Ordovicianbasinabout 100 km tothe westof Aguilar,and Pb-Zn-Ba eins n rocksof aproximal helf acies bout100 to 300 km to the east,northeast, nd north of Aguilar.No late Paleozoicor early Mesozoic ocksare ex-posed n the Aguilar Range.Two phases f gentle,open oldingoccurred uring hat time interval,oneprobably n Late Ordovician nd he other n mid- tolatePaleozoicime (Coiraet al., 1982). Vigorous eo-logic activityresumedwith the Andean ectoniccy-cle n the Late Jurassico Early Cretaceous. he east-

west compressions nd extensions ssociatedwiththis cycleare responsibleor the north-south truc-tural alignmentof geologic eatures n the SierraMadre Oriental. Felsic intrusions, mong hem theAguilargranite, nvaded he Paleozoic edimentarysequence arly in the Cretaceous arallel to thepreestablishedtructuralgrain. The SaltaGroup, athicksequence f continental edimentaryocks,de-posited o the east of the Aguilar Range rom lateMesozoic until late Eocene time. The Andean calc-al-kalinemagmatismndvolcanism hich ormedmostof the largebaseand preciousmetal deposits f theAndeanmetallogenic rovincecommencedn Oligo-cene time, in a belt that passeso the west of theAguilar egion Fig. 1).The AguilarRange ehaved sa rigidandcoherentblockduring he late Cenozoic plift, deformation,and aultingof the Andes ndwasuplifted o itspres-ent positionalongwest-dippinghrust faultswhichsteepen oward he surface.Geologyof the Aguilar District

A north-south-trending ajor syncline n lowerOrdoviciansedimentary ocks runs through thecenterof the AguilarRange Fig. 2). The two adja-cent anticlines to the east and west have been in-truded, domed,and further folded by the Aguilargraniteand he Abra Laite granite.The ore depositandprospectsf theAguilardistrict rehosted y themetamorphosedrdovician edimentarynitson heeasternimb of the syncline, long he western lankof the Aguilargranite.Siliciclastic, shallow-marinesedimentary rocksdominate he Ordoviciansequencen the Aguilararea (Suredaand Martin, 1990). Lowermost s theDespensa ormation,a section350 m thick of gray-greengraywackes, rkosic andstones,nd siltstonesdeposited n a shallow-marine o deltaic environ-ment.The PadriocFormation, ost o the Aguilarde-posit,conformably verlies he Despensa ormation.It consists f 560 m of white to light gray, fine- tocoarse-grainedrkosicand calcareous andstonesn-terbedded with black shales. Minor limestone unitsare the protoliths o someof the skarnassemblagesnow found n the contactmetamorphic alo of theAguilar granite. The proportion of interbeddedshales ncreases pward and basinward, .e., to thenorth and northwest. he shalesn the hangingwallof the ore are colloquially eferred o as he "Aguilarshale," and the shalesdepositedbasinwardof theAguilardeposit,ormerlyconsidered separate nit,are now interpretedas aciesequivalents f the Pad-riocFormation Sureda ndMartin, 1990). Theyhostthe stratiformoresof the Esperanza rospectnorthof the Aguilardeposit.Crossbedding,ipple marks, occasional hannelconglomerates,nd fossilsn the PadriocFormation

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AGUILAR Zn-Pb-Au DEPOSIT, ARGENTINA: GENESIS 2 0 8 9

indicate hallow-marine,idal, n partdeltaic, eposi-tion. Abundantntraformationalreccias ndslumpstructuresn the host ocks nd n the ore suggestheexistencef a steep-sidedepositionalasin, ossiblybounded y active aults,n whichslopenstabilitycausedperiodicbasinward ollapseof sediments.The predominancef shales ver quartzitesn theupper ayersof the Padrioc ormation uggestshatthe basinbecameprogressivelyeeperand moreprotected rom clastic nflux with time.The Padrioc ormations overlain y a series fnonmineralized edimentaryunits. The CardonalFormationconsists f 340 m of greenish-grayobrown sandstones, iltstones, nd shaleswith occa-sionalchannelconglomerates.he Acoite Forma-tion, seenonly n tectonic ontactwith the underly-ingrocks f the AguilarRange,s the uppermost r-dovicianunit. It is made up of over 3,000 m ofgreenish-grayhales, raywackes,nd arkosic and-stones, nterbeddedwith occasionalmpure lime-stones ndconglomerates.retaceousedimentaryrocksof the SaltaGroup,andmore ecentalluvium,are uxtaposedwith the Ordovician ockseastof theAguilarRange.The Aguilarand Abra Laite graniticcomplexes,which have intruded he Ordovician edimentaryrocksn theeast ndwestof heAguilarRange, trikenorth-southndmaycoalescet depth o forma argebatholith. -Ardating f the Aguilar ranite y Lin-aresandLaTorre 1975) ndicates Cretaceousge(118 ___5 Ma). The Aguilargranites ocally utbyyoungerbasaltic ikes.

The Aguilargranite s surrounded y a metamor-phic halo that extendsup to 1.5 km into thesurroundingedimentaryocks Fig.3). From he n-trusivecontact utward,Brodtkorb t al. (1978) dis-tinguishedhree metamorphic ureoles: yroxenehornfels, ornblende ornfels, nd albite-epidotehornfels. he Aguilardeposit nd he Oriental,Pir-ita, andEsperanza rospectsre ocatedn theseaur-eoles. n the immediate icinityof the Aguilarde-posit, he contactmetamorphismas ransformedheLower Ordovician edimentsnto quartzites, orn-felses,and calc-silicateskarns.The complex tructure f the Aguilardistrict e-flects he regionalnorth-south rientation f mostgeologic lements. he ore deposits,ocated n theintenselyolded nd aulted asternimbof hemajorsyncline,recutby undeformedikes f theAguilargranite, an indication hat much of the complexminordeformationround nd n the minepredatesmagmatism. he time of deformationcannotbe de-terminedprecisely ecause f the lackof late Paleo-zoicandMesozoicocksn thearea.Some f he ightfoldsn the quartzite-skarnrepackage aybe re-lated o west-east-thrustingrior o ntrusivectivity(Lea,1952). ntrusion f the Aguilar ndAbraLaite

graniteshas produceddome structures nd a ten-sionalset of fractureswith generalnorth-northwesttrends n the ore and he surroundingocks Brown,1962; Martin, 1987).Once the graniteswere emplaced, he Aguilarblockbehaved sa rigid structural nit andmanyofthe faultspreviouslyhoughtof asnormal aults e-lated o the Andean rogeny, nd o the formation fthe ore (Spencer,1950), have lately been reinter-pretedon the basis f new geophysicalnd ieldevi-dence: he Aguilar East Branch,which borders heAguilargraniteon he eastside,appearso be a thrustfault dipping to the west; and the Aguilar WestBranch ault, formerlyconsidered major normalfault of essentialmportanceo the formationof theore, is now interpretedas a part of the Canedo e-verse ault(Fig. 4). While these aultsappear o pre-date the intrusionof the granite,setsof northeast-striking aults postdate he intrusions.One majornormal ault, the No. 8 fault, displaces ll rocksandother faults in the mine.

Ore Deposits f the AguilarDistrictThe AguilarZn-Pb-Agmine and he Oriental,Pir-ita, andEsperanza rospectsrealigned outh-north,parallel o the regional tructure,he localstratig-raphy,and he ntrusive ontact f the Aguilar ran-ite with the PadriocFormation. he Aguilar,Orien-tal, andPiritadeposits re ocatednear he contactnthe highergrade,pyroxene ndhornblende ornfelsfacies nd he Esperanzare n the owergrade, l-bite-epidoteacies Fig. 3). The ore in the Aguilardeposits predominantlyostedn quartzitic ayers,and n the Esperanzarospectn finergrained orn-felses.Stratigraphicontroland stratiform exturesarecharacteristichroughoutnd he overall ompo-sitions f different xposuresre similar: phaleriteandgalena s hemainoreminerals,ccompaniedyabundantyrite-pyrrhotite, inorchalcopyrite,ndvery minor silver sulfosalts nd sulfides.Stratiformbarite s abundantn the Esperanzarospect.

AguilardepositTen strata-bound n-Pb-Ag ore lensesare con-

tained n the Aguilarquartzitememberof the Pad-rioc Formation Fig. 4), a thick sequence f quartz-ites interbedded with minor hornfelses. Garnets arecommonas disseminated rainsand in thin, mono-mineralic andswhich highlightsedimentaryex-tures. n the vicinityof the ore, he quartzites oftenthinly bedded (as opposed o the more massivequartzite utsidehe deposit), ery inegrained, nddistinctlyhickerstratigraphicallyhan quartzitenthe surroundingreas.D. Sangsterpers.commun.)has entativelynterpretedhe hinlybedded uartz-ite as recrystallized xhalative hert. The garnets

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2090 GEMMELL, ANTOP, NDMEINERT

Do

..... - _,, ,,,r-,.._' .' --t...,.......... - . '. .. . . .. ...,.f. . '.'. "" "-'""............ ........,. ...,.: . ...., ........., (--.. -,,,, .-.-.: ..:/,,,''' + + + + + .' '+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

+.......+ + + + + + + + + + + + + + + + +. + + + + + + + + + + + + + + + + + + + . .. .+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + . ......+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + + + + +++*+'*'++++++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + +++++ + . . + + + + + + + + + + + + + + + + + + ++++++++++++ + + + + + + + + + + + + + + + + + + + + + + .+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +'+.,+ ..+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + .+ +..+. ++++OREBODIES SCALE + +

+ - METACONGLOMERATE - HORNFELSi. _QUARTZITE * - GRANITE - FAULT"

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AGUILARn-Pb-AuEPOSIT,RGENTINA:ENESIS 2091

3250 N CROSS SECTIONAGUILAR MINE

.....

LEVEL 11

'- .';';'. LEVEL2

LEL 14

LEGEND- OREODY

......... ] - HORNFELB - QRANITE

0 50m -'"-", - FAULTAFTERCOMPANIA INERA GLAR MAP1984FIG. . Crossectionn he entralortionf heAguilareposithowinghe trata-boundatureftheorebodiesnd heir elationo hehost edimentarynits nd raniteontact.er a CompafiiaMineraAguilarmap 1984).

may epresenthemetamorphicquivalentsf ron-andmanganese-richedimentaryocks.Thesetting f heorebodiessshownn Figs. , 5,and 6. The total mineralized rea, imited by theAguilar est ndCanedoaultso hewest nd ytheAguilarraniteo heeast, as verallimensionsofapproximately,000malongtrike,t east ,000m downdip,ndasmuch s200 m acrosstratig-

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9.0 9 9. GEMMELL, ZANTOP, AND MEINERT

raphy. Individual ore lensesare roughly enticular,strike north-south, ip steeply o the west, and aregenerally everalhundreds f meters ong andwideand tens of meters thick. The orebodies occur at ornear he contact f the quartzitewith overlying,mas-sive hornfels, and within tens to hundredsof metersfrom he intrusion. he upper evel orebodiesA-up-per, A-uppernorth, A-north,B, C, and B2B3) havelargely been mined out and current productioncomesrom he deeperorebodiesA-lower,C-lower,C-18, and K). Orebody C-18 is open to depth andrepresentsuturereserves.There are five major types of ore in the Aguilardistrict: sulfidesdisseminatedn quartzite, stock-works,sulfide illings n quartzitebreccias,massivestratiformsulfides,and late fracture fillings.Eventhoughmasked y the structural omplexities f thedistrict, a distinctiveore stratigraphy an be recon-structed (P. Dicindio, pers. commun.;Sureda andMartin, 1990): the disseminatedulfides nd stock-work ore are overlainby brecciaore, which n turn isoverlain by massiveand banded ore. Sulfide-richquartzveinscut thisverticalsequenceFig. 7).Quartziteoccursn the footwallof mostorebodiesandsulfidesirstappear sdisseminationsf very finegrainedsphalerite ndgalena,minorpyrite andpyr-rhotite, and trace amountsof pyrargyrite n thequartzite.The proportionof sulfidesncreases p-ward, o the point of formingdiscrete ulfidebands.Stockworkmineralizations developed n the upperpartsof the disseminatedre. Here, the quartzite sshattered ndcrisscrossedy fine, anastomosingul-fide veinsand veinlets Fig. 8A). The stockworksgalenarich and containssignificant uantitiesofsphalerite, yrite, andpyrrhotite.Disseminatedand stockworkore grade upwardinto brecciaore. There are several ypesof breccia,all composed f quartzite ragments ut with differ-ent matrices.Most abundant s a matrix-supportedquartzitebrecciawith sulfidematrix (Fig. 8B). Thequartzite ragments ary in size rom i cm to morethan i m and are generallyangularand of irregularshape.Someof the quartzite ragments howdistinctcrossbeddingighlighted y layersof disseminatedsulfides r garnet.Fine-grained phalerite ndgalenawith minoramounts fpyrite, pyrrhotite,andchalco-pyrite makeup the matrix.Lessabundants a matrix-supported recciawith a calc-silicatematrix. n thiscase, he matrix s dominated y diopside ndgarnet,with subordinateamountsof wollastonite, hodonite,bustamite, uartz,andcalcite.Sulfides re containedin the matrix as disseminations,lots,and stringers.In a few locations, he calc-silicatebreccia hasa strik-inggreenmatrixdominated y epidote,diopside, c-tinolite, and chlorite. A finer grainedbrecciawithsubroundeduartziteclastsanging n size rom 1 to

HORNFELS(unmlnrallzed)

BANDED ORE

HANGINGWALL

. , '

FISSURE VEIN ORE

SRECCIA ORE

STOCKWORK ORE

DISSEMINA TED ORE

QUARTZITE(unmlnarallz ed:

' FOORANITEFIG.7. Schematicepresentationf ore stratigraphyn the ndi-vidualore lenses f the Aguilardeposit.5 cm s associated ith thin-beddedpyrite-sphaleriteore in orebodies C and C-18.Banded, hinlybedded o massive ulfides verliethe breccias n conformable,gradational o sharpcontactFig. 8C). The banded ulfides ccurnear hehanging-wall ontactof the orebodies nd containfine-grained pyrite and sphalerite with lesseramounts f galena,pyrrhotite,and are chalcopyritein a matrixof fine-grained uartzite.They generallyare thin bedded

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AGUILAR Zn-Pb-AuDEPOSIT, ARGEHTINA: GEHESIS 2093

Fc. 8. Types f ore n theAguilar rebodies.. Disseminatedulfidesn footwall uartziteutbystockworkalena-sphaleriteorebody , 12 level,2650 N). B. Breccia ith fragmentsf quartzite(white) ementedygalenand phaleritedark)ntergrownithwollastoniteorebody, 7 level, 360N).C. Conformableransitionrom uartzitereccia ithasulfide atrixlowereft) o inelyaminatedpyrite light)-sphaleritedark) anded re photographaken mabovehebrecciahownnB (orebodyA, 7 level,3360N). D. Massive,hinly aminatedrecomposedf pyrite light)-sphaleritedark) ore-bodyC-18,19 evel, 550N). E. Bandedreconsistingfcontortedarnetiferousuartzitewhite) ndsphaleritedark) orebody , 9 level, 680N). F. Galena-richissureein center)n footwall uartzitewithminordisseminationsf galena ndsphaleriteorebody , 14 evel,3050 N).

(Fig. 8E). The minor olding,brecciation, ndbou-dinage f themorecompetent,iliciousayersn esscompetentulfideayers, resentn weakly swellasin intenselymetamorphosedre, are suggestivefsoft-sediment eformationduring sedimentary rearly diagenetic rocesses.The open pit and the uppermostundergroundworkings f orebodyA expose large ensof massive

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AGUILAR n-Pb-Au EPOSIT, RGEHTINA: EHESIS 2095

FIG. 10. Surface utcrops f the Esperanzarospect. . Ore stratigraphy.= disseminatedalena-sphalerite-bariten quartzite atrix;I -- fragmentsfquartzitend arbonateementedysulfidesndbarite;II -- bandedyrite-sphaleritedark ands)lternatingithshalelight ands).halesh) verliesorealong onformableontactjust elow ammer). . Soft-sedimenteformationuggestedychaoticfoldingn banded hale-sulfidessemblageracketedy fiat-lying tratash)below ndabove. nife s9m ong.

Mineral Composition f the Aguilarand EsperanzaOresThe oresof the Aguilardistrictare composed ffine- o medium-grainedntergrowths f sulfides ndsulfosaltsn quartziteandcalc-silicate angue.Ubiq-uitousandmostabundant re sphalerite, alena,py-rite, pyrrhotite in grains ndas nclusions),ndchal-copyrite mostly s nclusionsut also sseparatemin-eral grains).Minor phases, omeof economicnterest

becauseof their silver content, nclude a seriesofsul-fosalts, ulfides, nd nativemetals: etrahedrite,pyr-argyrite, polybasite,boulangerite,bournonite,ar-gentitc, molybdenite,gudmundite,breithauptite,and identified by Brodtkorb et al. (1978), traceamountsof stannite, native gold, silver, and anti-mony.Accessory inerals f geochemicalather haneconomic nterest nclude arsenopyrite,marcasite,mackinawitc,ubanitc, ematite,magnetite, laban-dite, and wolframite.The gangue s dominated yquartz,calcite,pyroxenes, nd garnets,with minoramountsof rhodonite, bustamite,wollastonite, actin-olite, vesuvianite, pidote,scapolite, ndchlorite. nthe Esperanza rospect, arite sanabundant anguemineral.There are no distinct correlations between orecompositionnd extural ype of ore, or distance fore to the intrusion.Rather, there seems o be overalluniformityof composition ithin individualore-bodies, egardless f textural ype, and significantdifferencesn compositionetweenorebodies. hebestgeneralizationspplyingo the districtmaybethat sphalerite ndgalena re abundanthroughoutand that pyrite, pyrrhotite, and chalcopyrite remuch ess bundant ndmore rregularly istributed.All othermineralphases re minorand rregularlydistributed.

Sphaleritendgalenarom heAguilar epositndfrom heEsperanzarospect ereanalyzedy elec-tron microprobe,o comparehe mineral omposi-tionsof the different re ypes n the two placesTa-ble 1). The sphaleriteselectedromorebody intheAguilar epositave istinctlyigher e andMnconcentrationshan those rom the Esperanza ros-pect 9.2vs.5.5%Fe;5.5 vs.1.2%Mn).Fe andMnconcentrationseem o be highestn the lower,dis-seminated nd stockwork re, an exception eingsphaleriten the calc-silicateatrix,which howedanunusuallyighMn concentrationf 5.95 percent.No such e or Mn trends rerecognizablen the oresfrom he Esperanzarospect. he Cd andCu con-tentsof sphaleritesre very ow n all typesof orefrombothplacesavg0.2%Cd, 0.03%Cu). Galenahas ery owsilver oncentrationsnboth heAguilardeposit nd he Esperanzarospect0.7%Ag, Agui-lar; 0.3% Ag, Esperanza). ostof the silver s con-tained n silver-bearingulfosalts,speciallyn tetra-hedrite,which verages7wt percent g (Baum ndGilbert, 1986).Mineral Texturesand ParageneticSequenceTextures ndparageneticelationshipsf sulfides,sulfosalts,ndgangueminerals ive mportant luesabout the mode of formation and about the tectonicandmetamorphicistory f the ores n the Aguilardistrict. Strata-bounddisseminations, re breccias,andstratiformulfideshowingvidence f soft-sedi-mentdeformationppearoreflect rimary, edimen-tary, and diagenetic rocesses.uperimposednthese are structures and textures not related to theemplacementf the ore but rather o later tectonicstress, uchas olds, aults,and ractures ormedbe-

fore and after emplacement f the Aguilargranite,

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z.

1 c::

Fit;. l l. Samplesf banded ndcontorted re rom heEsper-anzaprospectA) andAguilardeposit B). White and ight gray,uniform ands re quartzite ndshale;ightgrayanddarkgray,granular ands re galena ndsphaleriteich.Pyrite s the abun-dant ronsulfiden A (Esperanza)ndpyrrhotiten B (Aguilar).

and o metamorphic verprint,mainly ecrystalliza-tion andannealingelated o the granitic ntrusion.In contrast o the overallmineralogic niformity,there are distinct extural characteristics, ome den-tifying ypesof ore in differentorebodies ndothersrelating o the distanceof ore to the intrusion.Thedisseminatedre consists ainlyof isolated, ine- tomedium-grained, ubhedral o anhedralsphaleriteand galenadistributedalong grain boundaries nquartzite.The sphalerites rich in inclusions f pyr-rhotite and chalcopyrite. yrite and pyrrhotiteareirregularlydistributed, nd noninclusion halcopy-rite is rare. Tetrahedrite,pyrargyrite,polybasite,

boulangerite, nd bournoniteare intergrownwithgalena,pyrrhotite,and chalcopyrite. reithauptiteand gudmundite re mostabundant n the dissemi-natedore; heycommonlyccurwith,andmayocca-sionallyeplace, yrrhotite.Parageneticallyate ar-gentitereplaces yrargyriteand tetrahedrite.Gar-nets n the disseminatedre commonlyontaininyinclusionsf pyrite. Stockwork re in andabovedis-seminated ore contains abundant, inclusion-richsphaleritend alena, ariablemountsf pyrrho-tite, but very ittle pyrite,chalcopyrite,r sulfosalts.Brecciaore with a sulfidematrixcontains phaler-ite, galena, pyrite, and minor pyrrhotite as fine-grainedmassesntergrownwith needle-shaped ol-lastoniteand actinolite.Sphaleriteand galenare-place fractured,euhedralpyrite and arsenopyrite.The brecciaore with predominant alc-silicatema-tfix is much lower grade and contains ery finegrained, disseminated ulfidesand sulfosalts ndsmall clots of sulfidesdistributed hroughout hecalc-silicatematrix.Sphalerite, alena, ndpyrite areintergrownwith pyrrhotiteand chalcopyrite. phal-erite is rich in inclusions f chalcopyrite, yrrhotite,mackinawite, ndalabandite. ulfosalt hases ppearto be mostcommonly ssociated ith galenaor pyr-rhotite. Medium-grained,subhedral o anhedral,earlypyriteandarsenopyrite rains recorroded ndreplacedby a mixtureof sulfides nd calc-silicategangue.These extural characteristicslso apply othegreen, pidote-diopside-chlorite-bearingalc-sili-cate breccia.In the bandedpyrite-sphalerite re, pyrite s pres-ent as large (100-600 ttm), fractured, corrodedgrainswith abundant angue nclusions nd as tiny(10-100 ttm) euhedral grains without inclusions.Sphalerite ontains bundant halcopyritenclusionsin some ess ecrystallized rainsand none n other,more ntensely ecrystallized rains. ead andsilver-bearingsulfosaltsre intimately ntergrownwith ga-lena. Galena-rich and sphalerite-rich ayers, bothcontaining bundantquartz, alternatewith quartz-rich layers.Fissure veins in the strata-bound ore are domi-nantly composed f galena,but they also containabundant phalerite nd pyrite. The sulfides re in-tergrownwith quartzandminorcalc-silicate ineralsin disrupted nddeformedextures. yrrhotite svir-tually absent n the fissure einsand pyrite is com-monlyassociated ith marcasite.Latequartzveins hatcut acrossll typesof oreandhost ockcontainminoramounts f sulfides, yrmek-itic intergrowths f chalcopyrite-sphaleritendchal-copyrite-tetrahedrite,ndhuebnerite.Recrystalliza-tion texturesare lesspronouncedhan in the othertypesof ore, but deformationextures re common,pointing o overprinting y postdepositionalectonicprocesses.

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2098 GEMMELL, ZANTOP, AND MEINERT

AP

FIG. 12. Mineral textures n the Aguilardepositand Esperanza rospect. he black material n allpolished ection hotomicrographss quartzitegangue.AoAnnealed yrite (PY)with typical120 grainboundariesnd nclusion-freephaleriteSP) n banded yrite-sphaleritere (orebody , 7 level,3360N; scalebar = 100 #m). B. Pyrrhotite nclusionsPO) alignedalong ecrystallized rainboundariesnsphaleriteSP) n bandedpyrite-sphaleritere (orebodyA, 7 level, 3360 N; scalebar = 50 #m). C.ArsenopyriterystalsAP)partially eplaced y pyrrhotite PO)andgalena GN) n massiveulfide re.SphaleriteSP)hostswogenerationsf chalcopyritenclusionsorebody , openpit;scale ar = 50 #m).D. Disseminatedre with annealed,nclusion-freephaleriteSP) n galena GN). Smallgrains f pyrite(PY)arepartially eplaced y galena.Notecurved leavage lanesn galena,ndicating eformationfterrecrystallizationorebodyC, 13 level, 2570 N; scalebar = 200 #m). EoSphalerite iddledwith twogenerationsf chalcopyritenclusions,ssociatedith galena GN) in massiveulfide re (orebody ,open pit; scalebar = 50 #m). F. Native lead (Pb) intergrownwith galena GN) and sphalerite SP) nfissure ein (orebodyK, 14 level, 3050 N; scale ar = 25 #m). G. Banded ulfidesrom the Esperanzaprospect. yrite white)andsphaleritegray)with a gangue f quartz black) ndbarite.Note ayeringand wopyritehabits: mall-grained,oundedramboidsnd arger,sub- o euhedral rystals. cale ar =200 #m.H. Closeup f framboidalyrite PY)andan nclusion-freerainofsphaleriteSP), rom hesamesamplepictured n G. Scalebar = 10 #m.

setting and compositionbut lower metamorphicgrade han heAguilardeposit, ontainsine-grained,simple, nterlockingmixturesof pyrite, sphalerite,galena, arite,minorchalcopyrite,ndpyrrhotite nthe disseminatedndbrecciaores.Textures f high-grade ecrystallizationndannealing re absent. hebanded ore consists f thin, monomineralicbandsoffine-grained yrite andsphalerite ith disseminatedbarite (Fig. 12G). Pyrite bands requentlycontaintiny (5-10 txm) pherical yrite aggregatesnd ram-bolds Fig. 12H), andsmall 10-30 txm) ubhedraloeuhedral pyrite grains associated ith sphalerite,

texturessuggestivef pyrite precipitationn a sedi-mentaryenvironment Craig and Vaughan,1981).Sphalerite ontains nly iny inclusionsf chalcopyr-ite and pyrrhotite.SkarnMineralogy

The majorskarnmineralsn the Aguilardeposit,exposedmainly n the brecciawith a calc-silicatema-trix, are garnet, pyroxene, pyroxenoid, actinolite,chlorite,vesuvianite,pidote,scapolite,ourmaline,quartz,andcalcite.Brodtkorb t al. (1978) estimatedthat metamorphismookplaceat pressures f 1 to 2

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AGUILARZn-Pb-Au EPOSIT,ARGEIqTINA: EIqESIS 2099

PY

FIe. 12. (Cont.)

,,GN

kbars correspondingo a depthof 4-8 km) and em-peratures f 350 to 650C.Calc-silicate ineralsrom hree ransectsampledoutward rom he Aguilargranite Fig. 13) were ana-lyzedby electronmicroprobeo check or composi-tional oning. ll minerals rehigh n manganesendiron, as s typical or Zn-Pb skarns Zharikov,1970;Burt, 1972, 1977; Einaudi et al., 1981). However,the overall chemicalcomposition f the mineralphasess different rom hat n typicalZn-Pbskarns.Garnetsn the Aguilaroreshavehighproportionsof spessartinendalmandine ndare subcalcicTa-ble 2). Data rom23 analyses,xpressedsmoleper-cent almandineAI: FeaAl.SiaO.),pessartineSp:MnaAl.SiaO., pyrope Py: MgaAl.SiaO.),rossu-larite (Gr: CaaAl.SiaO.), nd andradite (Ad:CaaFe.SiaO., cover the rangesof AI_.PY0-a,Grs_., and Ad0_4. n contrast,zinc skarnsusually ontain ndraditic arnetsFig. 14A;Einaudiet al., 1981). Skarns ith a similar ubcalcicarnetcomponentre reported rom egionalmetamorphicterranesand deeper W and Sn skarns,n which ex-treme differentiation and low oxidation statesof asso-ciatedplutons roduce ubstantial anganesendferrousronenrichmentsNewberry, 982).

Pyroxenes 25 analyses) re moderately manga-nese ich and ron poor (Table 3). Expressed s theend membershedenbergite Hd: CaFeSb. a), jo-hannsenite (Jo: CaMnSb. a), and diopside (Di:CaMgSb. a), pyroxeneshave end-membercompo-nents in the ranges Hd0_, Jo15_35,nd Db.0_s0.More than 90 percentof the pyroxenes nalyzed nthisstudyhavea diopside omponent reater han50percent.This s muchhigher hanthat of pyroxenesin mostzinc skarns,which are generally ohannseni-tic-hedenbergiticwith less than 40 mole percentdiopside Fig. 14B; Meinert, 1983, 1987).Pyroxenoids16 analyses),xpressedy thecompo-nents wollastonite Wo: CaSiOa), rhodonite (Rd:MnSiOa), nd errosiliteFs:FeSiOa), remainly ep-resented by calcium-rich bustamite with end-member ariations fWo40_,Rd36_0,ndFs_0Ta-ble 4). Such compositionsepresentubiquitousfminorphasesn mostskarns.High manganese,ubcalcic ctinolite 11 analyses,Table 5) is a local alterationproductof pyroxene.Garnet s altered o manganese-richhlorite, ourma-line, andvesuvianite.he vesuvianites deepgreen,strongly oned, ndcontains p to 20 wt percent areearth elements. Cerium is most abundant, followed

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2100 GEMMELL, ANTOP, NDMEINERTw

Section 3250 NorthE

Level 4

Samplenumber.lohannsenite22] 5 arnetyroxenehedenbergite almandine

27 8

50 m

::iii:' Level

18Level 12

Fro.13. East-westrossectionhroughheAguilar epositllustratingompositionsf garnet ndpyroxeneampleselativeo heAguilarranitendnferredluidlow athsarrows)xtendingutwardfrom the granite.

by lanthanum, eodymium,ndpraseodymiumTa-ble 6). Only two otheroccurrencesf REE enrich-ment n vesuvianite avebeenreported n the litera-ture: in the Enisei Mountains,USSR (Orlov andMart'yanov,1961), and in San Benito,California(Murdoch nd ngram,1966;Fitzgerald t al., 1987).However, REE-enriched esuvianite nd allanitcoc-cur in severalgold skarnsn Montana Meinert, un-pub. data).

IsotopeGeochemistryThe geologic nd mineralogicalnvestigationsweresupplementedy measurementsf sulfur, ar-bon,oxygen,nd ead sotopesrom heAguilar e-positandEsperanza rospect.Sulfur sotopes

A total of 57 sulfur sotopedeterminations eremadeof galena, phalerite,yrrhotite, yrite,chal-

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AGUILARZn-Pb-Au EPOSIT,ARGENTINA:GENESIS 2101TAI 2. Representative lectronMicroprobeAnalyses f Garnet

Sampleno. A3-3 A60-2 A60-2 A41-5 A41-5 A43a-2 A45-3Location Rim Core Rim Core Rim Rim CoreSiO2 37.3 38.2 38.2 38.1 37.4 37.1 38.0TiO2 0.4 0.3 0.7 0.2 0.6 0.4 0.3A1203 20.8 21.2 21.1 21.2 20.7 21.2 20.8FeO 10.2 3.8 3.4 3.2 4.2 4.8 4.0MnO 29.8 23.2 26.5 24.7 33.9 33.2 24.0MgO 0.4 0.4 0.5 0.4 0.6 0.6 0.3CaO 2.4 13.4 10.0 12.8 3.6 3.6 13.3NazO 0.0 0.1 0.0 0.0 0.0 0.0 0.0K20 0.0 0.0 0.0 0.0 0.0 0.0 0.0Total 101.3 100.6 100.3 100.6 100.9 100.8 100.6

Cationsbasedon 24 oxygensSi 6.014 6.025 6.053 6.020 6.012 5.971 6.021Ti 0.048 0.031 0.078 0.019 0.077 0.044 0.032A1 3.951 3.947 3.944 3.951 3.931 4.016 3.888Fe 1.373 0.504 0.454 0.422 0.562 0.642 0.529Mn 4.063 3.099 3.556 3.313 4.616 4.534 3.215Mg 0.098 0.094 0.116 0.090 0.134 0.154 0.064Ca 0.413 2.260 1.698 2.168 0.612 0.618 2.254Na 0.003 0.021 0.000 0.006 0.000 0.000 0.000K 0.000 0.000 0.000 0.000 0.000 0.000 0.000Sum 15.964 15.981 15.897 15.989 15.945 15.978 16.003Mole percentSpessartine 68.4 52.2 61.1 55.5 78.1 76.2 53.9Almandine 23.0 8.2 7.8 6.6 9.3 10.8 7.3Pyrope 1.7 1.6 2.0 1.5 2.3 2.6 1.1Grossular 6.7 37.7 29.2 35.7 10.1 10.4 35.4Andradite 0.2 0.4 0.0 0.7 0.3 0.0 2.3

Total iron as FeO

copyrite,molybdenite, ndbarite from the Aguilardeposit nd he EsperanzarospectTable7). Sulfidesampleswere cornbustedwith excessCuO in vacuoto produceSO2 Fritz et al., 1974). Analyses ereobtained sing Nuclide6" 60 sectornstrument tIndianaUniversity. aritewasanalyzed y KruegerGeochronLaboratoriesof Cambridge,Massachu-setts.Sampleswere dissolvedn a boiling mixtureof HC1, H2PO3, and HI to produce HS, whichwas reacted with a cadmium acetate solution toproduceSO (KruegerGeochron aboratories,writ.commun.).Strata-bound re in the Aguilar district hasstrongly ositiveb34S alues hat range rom 10.8 to26.5 per mil for sulfidesnd32.4 to 34.0 per mil forbarite Fig. 15A). n the Aguilardeposit, 34Saluesof sulfidesange roma meanof 20.2 per mil for ga-lena o 21.2 per mil for pyrrhotite nd21.9 per milforsphalerite.hese alues re n general greementwith values ivenby Linares 1968), who analyzedsphalerite nd galenaseparatesrom differentore-bodieswithin he depositwithoutmaking istinc-

tions between different stylesof mineralization sdone in this study (Fig. 15B). Molybdenite rom agranitiedike ntruding he Aguilardeposithasa 634Svalueof 8.2 per mil.There is little variation n the range of sulfur so-tope compositionsn differentorebodies f the Agui-lar depositand ittle difference n the rangesshownby the Aguilar depositcompared o the Esperanzaand Oriental prospects. here are, however, varia-tions n 634S alues n differentstylesof mineraliza-tion (Fig. 16A andB). In the Aguilardeposit, here san increasen the 634S aluesof about5 per mil forboth sphalerite nd galena rom the disseminatedothe breeeia sulfide to the banded ore. The breeeiaswith a eale-silieate matrix and the fissure veins havelowerandmore rregular 34Salues, ossiblyhe re-sultof modification y metasomatierocesses.n theEsperanzaprospect, he 64S rend in the differentore types s reversed,decreasing y 2 to 3 per milfrom the disseminated to the breeeia and banded ore.Neither trend is diagnosticor sedimentary r meta-somatieormation f the deposits ndboth rends p-

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210 2 GEMMELL, ZANTOP, AND MEINERT

Pyral

^)

Typicalgarnet rom Zn skams

Gr Ad

B)Jo

,icalcomposition ffrom distalportions f Zn skams

Di HdFI(. 14. Compositionsf garnetsA) andpyroxenesB) fromthe Aguilardeposit xpresseds moleproportionsf pyralspite

(Pyral: Mg, Fe, Mn)3AI2Si302),rossulariteGr: Ca3AI2SiO),andradite Ad, CaFeSiO2) or garnetsand johannsenite Jo:CaMnSiO6), iopside Di: CaMgSi206), nd hedenbergiteHd:CaFeSiO6)or pyroxenes.ypicalgarnet ndpyroxene omposi-tions or zinc skarns, s reportedby Meinert (1983), shown orcomparison.pear to be common n sedimentary xhalativede-posits Gustarson nd Williams, 1981). Systematicupwardncreasesf ba4Saluesn galena ndsphaler-itc havebeen noted,e.g., at Lady Loretta,H. Y. C.,Australia, y Taylor 1987) andat the Anvildistrictofthe Selwynbasin n Canada y Shanks t al. (1987);upwarddecreases ave been described, .g., fromSullivan, anada, y Campbell t al. (1978).Carbonand oxygensotopes

Carbon ioxidewas iberated romcalcite y reac-tion with 100 percentHaPO (McCrea,1950). Car-bonandoxygensotope nalyses ereperformed na FinniganMAT Delta E mass atio spectrometer tDartmouthCollege. Stable sotope esultsare ex-pressedn PDB for carbon nd SMOW for oxygen.Sample eproducibilitys typically_+0.2 er mil andanalytical ncertaintyess han_+0.1 er mil.

Eleven calcites rom different ypes of ore in theAguilardeposit ndone rom heEsperanzarospectwere analyzed Table8). Two populationsfand lsO appear o be present Fig. 17). Onepopula-tion ncludesmost ypesof ore and he sampleromthe Esperanza rospect 3C = -15.1 to -3.1%0,sO = 2.0-5.2%0).The other population haracter-izes the brecciaswith a calc-silicatematrix (3C ---4.1 to -7.1%0, bsO= -0.2 to +2.1%0).Lead sotopes

Samplesf galenarom heAguilar,Esperanza,ndOrientaloresand potassiumeldspar rom he Agui-lar granitewere analyzedor their ead sotope om-positiono gain nsightselatingo thesource ndageof the ore metals.Galena ragmentsweighing20 mg or lesswerehandpicked nd dissolvedn 6 N HC1.A small rac-tion of the lead was purified usinga 120-ml anionexchangeolumn ndHC1eluant.Mass pectrometrywasdoneon a variablemulticollector innigan261 atthe University f California,SantaBarbara.Sampleswere loadedontoa Re filamentusingSi gel-H3PO4.The data were corrected for mass ractionationby0.12 _+0.03 percent per amu; this correctionwasmonitored y multipleanalysesf NBS981. The un-certaintyn the corrections n all cases reater hanthe counting tatisticsn ndividual amples.Lead sotope alues re isted n Table 9. There sno significantariationn isotopic aluesof lead ngalena etween eposits,rebodiesn theAguilar e-posit,or typesof mineralizationFig. 18).The rangesand averages f isotope alues rom all galenas re26Pb/24pb18.013 o 18.079,mean 8.040; 7Pb/24pb 15.615 to 15.678, mean 15.640; and2SPb/4pb = 37.941 to 38.150, mean 38.025. Potassiumfeldsparrom heAguilargranite asa highlyanoma-lous,radiogenicead isotopesignature, s shown nFigure19AandB: 6Pb/4pb- 19.280, 7Pb/4pb= 15.669, and SPb/4pb39.000. Thus, he leadin the various trata-boundeposits ithin he Agui-lar district svirtually dentical ut different rom helead n the Aguilargranite.

Genetic nterpretationof the AguilarDepositGeologicelationshipsThe alignment f the Aguilardeposit ndrelatedprospects arallel o stratigraphic, tructural, ndmagmatic rientationsn theAguilardistrict rovidesa uniqueopportunityo establishriteria or the dif-ferentiation etweenmagmatic ydrothermal karndeposits nd sedimentary xhalativedeposits ver-printed by contactmetamorphism. ajor featureswhich he Aguilardeposit as n common ithZn-Pbskarns Einaudiet al., 1981) are spatialassociationwith a felsicstock,occurrence f the ore alongstruc-

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AGUILARZn-Pb-Au EPOSIT, RGENTINA: ENESIS 2103Tai 3. RepresentativelectronMicroprobeAnalyses f Pyroxene

Sample o. A22 A79 A60 A41 A43a-1 A45 A20SiO 53.4 52.8 52.7 52.6 52.6 52.4 52.4TiO 0.0 0.0 0.0 0.0 0.0 0.0 0.0AIO 3 0.2 0.2 0.2 0.2 0.2 0.2 0.1FeO 1 3.1 5.8 5.0 5.7 7.4 7.0 5.7MnO 5.1 6.0 7.6 8.8 4.6 6.4 7.4MgO 13.3 11.6 10.9 10.2 12.0 10.9 10.5CaO 24.5 23.4 23.5 22.7 22.8 22.9 24.0NaaO 0.0 0.0 0.0 0.1 0.0 0.0 0.1KO 0.0 0.0 0.0 0.0 0.0 0.0 0.0Total 99.7 99.8 99.9 100.3 99.6 99.8 100.2

Cationsbasedon sixoxygensSi 2.001 2.003 2.003 2.007 1.997 2.002 1.99Ti 0.000 0.001 0.001 0.000 0.000 0.000 0.001A1 0.007 0.007 0.010 0.009 0.009 0.008 0.00Fe 0.097 0.183 0.160 0.182 0.235 0.225 0.18Mn 0.163 0.192 0.244 0.285 0.147 0.205 0.23cMg 0.742 0.656 0.616 0.579 0.682 0.619 0.59Ca 0.983 0.951 0.958 0.926 0.927 0.935 0.97 cNa 0.001 0.001 0.000 0.000 0.000 0.000 0.001K 0.000 0.000 0.000 0.000 0.000 0.000 0.00(Sum 3.995 3.993 3.991 3.988 3.998 3.994 4.00(

Mole percentHedenbergite 9.7 17.7 15.7 17.4 22.1 21.4 17.8Diopside 74.0 63.6 60.4 55.3 64.1 59.0 58.7Johannsenite 16.2 18.6 23.9 27.3 13.8 19.6 23.6Total iron as FeO

tural or lithological ontacts istal o the stock,Mn-and Fe-rich calc-silicate angueminerals,pyroxeneas the dominant alc-silicatemineral,a high pyrox-ene to garnetratio, and the presenceof a late, hy-drousmineralassemblage.Even hough hesecriteriaapply o the Aguilardis-trict, the large-scale tructural nd stratigraphic et-ting and the small-scaleithologic,mineralogic, ex-tural, and isotopic videncegathered n this studysuggesthat he originalmineralizationormed n Or-dovician ime by sedimentary xhalativeprocessesand hat thiswas ollowedby contactmetamorphismandminormetasomatismuring he emplacement fthe Aguilargranite n Cretaceousime. On the basisof the descriptionsnd datagiven n the precedingsections, the different lines of evidence that led tothis conclusion are discussed below.The sulfideorebodiesn the Aguilar district aresheetor lenslikewith lateraldimensions any imestheir stratigraphichickness. hey are stratigraphi-callycontrolled, trikingly otbe carbonateayers, swould e ypicalorskarn eposits,ut ather yme-dium- o fine-grainediliciclasticocks. he geome-try of the orebodies onformso the distributionofsulfide accumulationsn first-order,second-order,

and hird-ordersedimentary asins ftenrecognizedin sedimentaryexhalative sulfide accumulations(Large,1983; Shanks t al., 1987). The environmentof depositionn the large Cambro-Ordovieianasinranged rom platform o shallowmarine o deepma-rine westward nd epresentshe eastern eaches f adeep-marine,irst-orderbasin Suredaet al., 1986;Martin, 1987; Suredaand Martin, 1990). The de-posits n the Aguilardistrict,alignedalonga north-south linear trend over a distance of 10 km within thesamestratigraphicnterval, appear to reflect a re-strieted, ault-controlled econdary asin. ntrafor-mational breeeias and textures of soft-sediment defor-mation n both he rocks nd he oressuggesthat hefaultsbounding hisbasinwere activeduringdeposi-tion andmayhaveactedaschannelwaysor the min-eralizing luids. ndividually, he Aguilardeposit ndthe other metalliferous rospectsmay indicate hepresence f third-order asins r they maymark heloci of metal supply.The intensedeformationandmetamorphism f the rocks n the district precludedetailed nterpretationsn this regard.The setting of sulfides n sandstones,iltstones,shales,and breeeiasconforms o the association om-monly nterpretedas sedimentary xhalative, s de-

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2104 GEMMELL, ZANTOP, AND MEINERTTI 4. RepresentativelectronMicroprobeAnalysesof Pyroxenoids

Sample o. A22 A-3-6 A-2 A-1 A-4SiOTiOAI03FeO MnOMgOCaONaOKOTotal

50.4 49.7 49.6 49.3 37.70.0 0.0 0.0 0.0 0.20.0 0.0 0.0 0.0 21.71.8 3.9 3.6 3.5 4.9

10.0 21.7 23.4 28.4 34.20.4 0.7 0.9 1.3 0.6

37.0 24.7 23.5 18.5 3.10.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0

99.6 100.8 101.0 101.1 102.3

SiTiA1FeMnMgCaNaKSum

Cations asedon 18 oxygens6.004 6.020 6.013 6.021 4.4870.001 0.001 0.000 0.001 0.0200.002 0.002 0.002 0.004 3.0430.179 0.393 0.361 0.355 0.4881.011 2.226 2.403 2.939 3.4470.074 0.134 0.155 0.236 0.0984.720 3.199 3.048 2.420 0.3890.007 0.004 0.008 0.001 0.0000.000 0.000 0.000 0.000 0.000

11.998 11.979 11.990 11.977 11.972Mole percentWollastonite 78.9 53.7 51.1 40.7 8.8Rd 16.9 37.4 40.3 49.4 78.0Fs 4.2 8.9 8.6 9.9 13.2

fidesmaybe moreproximalhan hoseat the Esper-anzaprospect,whichconsistsf only one smallorelens,a point urtherelaborated elow Fig.20).Mineralcompositionnd textures

On a smaller cale, t becomesncreasingly ifficultto separate he effectsof sedimentary eposition,metamorphic verprint,and metasomatism.n theEsperanza rospect,he characteristicsf sedimen-tary exhalativemineralization re well preserved.The mineral ompositionssimple: yrite,sphalerite,galena,and minor chalcopyrite,with barite as anabundantganguemineral. The sulfidesare finegrained, orm monomineralic r mixed aminaesev-eral millimeters o centimeters hick, and showabun-dant evidence of soft-sediment deformation. Thepresenceof framboidalpyrite within sulfidebandsfurthersupportssedimentaryxhalativenterpreta-tion.

In the Aguilardeposit ndOrientalprospect, igh-gradecontactmetamorphismas esulted n perva-sive recrystallization, nnealing, ormationof newminerals pyrrhotitebecomes major component)and n intergrowths f sulfides ith calc-silicate in-erals.Someprocessesostdatehe formation f bothdepositionalndmetamorphicextures:racturing fbrittle pyrite andarsenopyrite ith consequentar-Total iron as FeO TAI 5. RepresentativelectronMicroprobeAnalysesof Amphibole

scribedby Large (1983): "autochthonous"itholo-giesconsisting f fine-grained lastics nd carbon-ates, and "allochthonous"ithologies onsisting fconglomerates,ntraformationalreccias, ndcoarse-grained lastic edimentaryocks.The genetic ela-tionshipof sedimentary recciaso the formation fexhalative assiveulfide epositsasbeennoted nmany nstances nd s well illustrated, .g., in WinnandBailes 1987). In the Aguilardistrict,mostof thequartzites osting ulfides re of clastic erivation, sshown y sedimentaryextures uch s ipplemarksand crossbedding.s suggestedy Sangsterpers.commun.), ome f the quartzites irectlyassociatedwith ore maybe the metamorphosedquivalents fhydrothermal, cryptocrystallinechert depositedwith the sulfides. lso of exhalative riginmay besomeof the calcium-rich,ron, and manganese-richlayers,The vertical and lateral zonation of disseminated,stockwork, reccia,and bandedore in the Aguilarmine is unlike zonations in skarns but conforms tothoseof sedimentary xhalative eposits. istrict-wide, hemultipleore enses ndwell-developedrestratigraphyt the Aguilarminesuggesthat he sul-

Sample A60-1 A41-5 A43a-1 A-4 A-5no.

SiO2 55.8 54.2 53.9 54.8 53.0TiO2 0.0 0.1 0.1 0.0 0.0AlzO3 0.4 0.5 1.4 0.4 0.7FeO 8.9 12.3 10.7 12.0 17.6MnO 4.1 5.7 3.6 5.1 6.6MgO 16.7 13.3 15.0 14.1 10.6CaO 12.0 11.0 11.6 11.3 9.1NazO 0.0 0.2 0.5 0.1 0.1KO 0.0 0.1 0.3 0.1 0.1Total 97.9 97.3 97.0 97.9 97.8

Cations asedon 23 oxygensSi 7.967 7.971 7.861 7.980 7.951Ti 0.002 0.006 0.009 0.004 0.004AI 0.072 0.092 0.246 0.070 0.122Fe 1.066 1.508 1.304 1.464 2.211Mn 0.493 0.709 0.445 0.624 0.841Mg 3.549 2.925 3.255 3.066 2.372Ca 1.844 1.741 1.819 1.756 1.467Na 0.003 0.051 0.136 0.033 0.031K 0.000 0.000 0.000 0.000 0.000Sum 14.996 15.003 15.075 14.998 14.999

Total iron as FeO

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A GUILAR Zn-Pb-Au DEPOSIT, ARGENTINA: GENESIS 2107

A

15'

o,

B

15

' 10

Z5

0

[] Barite thsstudyJ iunljslhd;iudy

o o,i oj 04 0404 co co co+ + + + +34S (%0)

[] Gramte dike [] Esperanza-arM [] Esperanza-usjjj Oriental {--I [] Ore edKD r M [ [] OreBedCJill r Jill Ill [] Ore Bed B2B3{[-]l{-JJJJJJ}ll] OreedB

t .................... I. . .Fl. .Iq+ + + + +

34S (%)FIG. 15. Distribution f 34S aluesn the Aguilardistrict romthisstudy ndLinares 1968). A. Range f sulfur sotope aluesorall sulfides and barites. B. Distribution of sulfide and barite 34Svalues n differentorebodies f the Aguilardeposit nd rom heOrientalandEsperanza rospects. bbreviations:ar = barite,Sus= sulfides.

tive zonation Fig. 20) or it could reflect metamor-phic andmetasomatic ffects elated to the intrusionof the Aguilargranite.Skarnmineralogy

Overall, metasomatic eposits howa characteris-tic zonationof major silicateand sulfidemineralsaway roman ntrusion,ypicallygarnet + magnetiteandchalcopyrite) pyroxene + sphalerite) wol-lastonite (+ bustamite) - marble (Einaudi et al.,1981). The quartziteswhichhost he ore andcalc-sili-cates n the Aguilardepositwouldnot be favorablefor contact metasomatic eactions, and we found noconsistentoningpatterns or either the sulfides rthe calc-silicate hasesFig. 13).The garnetsn the Aguilardeposit re fairlycoarsegrained,but individualcrystalsare not as stronglyzonedas mightbe expected n metasomatickarns,except or occasional anganesenrichment f theirrim relative to their core. The absence of bulk com-positional onationof garnetswith respect o dis-tance o the Aguilargranite uggestshat he owcal-

AFissureBanded

Breccia

Disseminated]4 ]

AGUILARIn

In

2'0 2'2 2'4 2'6 2b34S

ESPERANZA

tial replacement y galena, phalerite, nd chalco-pyrite; the developmentof deformation wins inpyrrhotite ndchalcopyrite;nd hebending f ga-lenaandsphalerite,s ndicatedy anisotropyn ga-lena and curvedcleavage lanes n both minerals.These eatures re argely bsentrom he sulfidesntheEsperanzarospect, hich s farther way romthe granite, n the lower grademetamorphicalo,and ess ectonically tressed.Somecompositionalharacteristicsf the Aguilaroresmay be related as much o depositional s tometamorphiceffects.The presenceof semimetal-bearing ulfide ndsulfosalthases, hichare com-mon n the Aguilar eposit ut not n theEsperanzaprospect,may reflectprimarysedimentaryxhala-

Banded

Breccia

Disseminated

[]

5345 Galena [] Sphalerite

FIG.16. Stratigraphicariation faverage34Saluesn sphaler-ite andgalenan the Aguilar epositA) andEsperanzarospect

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2108 GEMMELL, ZANTOP, AND MEINERTTABLE. CalciteCarbonand Oxygen sotopeData (%o)

xaC /i'aOOrebody Ore type (PDB) (SMOW)AguilarA Fissure vein -12.6 2.0

A Massive sulfide -15.1 3.9A Breccia--calc-silicate matrix -4.1 2.1A Breccia--calc-silicate matrix -6.0 -0.2C Breccia--calc-silicate matrix -5.8 2.0C Breccia--calc-silicate matrix -5.9 1.9J Breccia--calc-silicate atrix -6.2 0.9J Disseminated -13.8 2.8K Fissure vein -3.1 5.2K Breccia--calc-silicate matrix -7.1 1.2K Stockwork -6.8 3.7

Esperanza Disseminated -9.5 4.9

-15

-20

-25

+ Fissure Banded Breccia skarn)0 Stockwork Disseminated[] Esperanzadissem.)

i0 1

+

I i I i2 3 4 5180 (%0)

FIG. 17. Distribution of 5'3C and 5'so values n calcites romthe Aguilardeposit ndEsperanzarospect.

cium and high manganesend ron contents re in-herited rom the protolith nd not introduced ymetasomaticrocesses.he relativelyuniformman-ganese ontentof the Aguilarpyroxenes nd theirvariableMg/Fe ratiosdiffer from trendsobservedntypicalzinc skarns,which end to showa concomitantincrease f iron with manganeseue to metasomatic

zoning. eitherarepyroxenesoticeablyonedwithrespecto the Aguilargranite, smightbe expectedin skarnsormedby metasomaticeplacementMein-ert, 1987).Althoughhebulkof thesulfidesn typicalmetaso-matic skarns s associatedwith alteration of an earliergarnet-pyroxene-pyroxenoidkarn Einaudi t al.,TaLE . Lead sotopeData

Orebody Ore ype 26pb/24Pb 2?Pb/4pb Spb/PbAguilarA Disseminated 18.037 15.630 38.004

A Breccia--sulfide matrix 18.042 15.642 38.017A Quartzvein 18.013 15.617 37.952A Fissure vein 18.045 15.638 38.025C Stockwork 18.054 15.660 38.098C Disseminated 18.061 15.665 38.109C Breccia--sulfide matrix 18.014 15.615 37.941C Breccia--calcosilicate matrix 18.016 15.617 37.951C Bandedpyrite-sphalerite 18.020 15.620 37.951C Bandedpyrite-sphalerite 18.042 15.650 38.058C Quartzvein 18.023 15.616 37.953C Disseminated sulfide in dike 18.038 15.624 37.970C Sulfidepod n dike 18.046 15.641 38.026K Stockwork 18.046 15.635 38.011K Breccia--sulfide matrix 18.029 15.628 37.986K Breccia--calc-silicate matrix 18.048 15.657 38.084K Breccia--calc-silicate matrix 18.038 15.632 38.005K Bandedpyrite-sphalerite 18.048 15.639 38.020K Fissure vein 18.057 15.666 38.115Mean 18.038 15.636 38.015Esperanza

OrientalGranite

Disseminated 18.079 15.674 38.091Disseminated 18.046 15.656 38.079Breccia--sulfide matrix 18.028 15.631 37.991Banded yrit -sphalerit 18.065 15.678 38.150Banded yrite-sphalerite 18.042 15.647 38.046Massive sulfide 18.047 15.652 37.984Mean 18.047 15.652 38.057Potassiumeldspar 19.280 15.669 39.000

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AGUILAR Zn-Pb-AuDEPOSIT, ARGENTINA: GENESIS 2109

A15.8

15.6154

15.2 '170

1.0 b.y

118.0

Agudarsulhde+ Agudar ranile

--x- S & K (1975), , i i i i

19.02O6pb/2O4pb

B39.0

38.0

37.0

36.017.0

+ 0.0.y.,5Aguilarulhde ,0 y + Aguilarramle--x- S & K (1975)18.0 19 0

2O6pb/2O4pbFIc. 18. Lead isotopedata for the Aguilar district. Growthcurve s fromStacey ndKramers 1975).

1981), there is no texturalevidence or this relation-shipat the Aguilardeposit.The epidote-chlorite-ac-tinolite breccia matrix present n one part of themine,whichmost loselyesembleshe common sso-ciationof a late, hydrousmetasomatichasewith sul-fides, ormedduringa late metamorphic vent hatoverprintedpreviously ormed calc-silicatephasesbut appearso be unrelated o the sulfideemplace-ment.REE-enriched esuvianite nd the locally abun-dant tourmaline,scapolite, eldsparand barite areatypicalminerals n mostzinc skarns, nd heir pres-enceat Aguilarmay reflectpremetamorphicompo-sitions f the protolith.Isotope eochemistry

The highlypositive 348values n the Aguilarsul-fidesandsulfates re similar o those rom ype vol-canogenic nd sedimentary xhalative epositsFig.19) andcanbe explained y reductionof sulfur romEarly Ordovician eawater ulfate,estimated t 27 to29 per mil (Claypool t al., 1980).The widerangeofsulfide 34Salues t Aguilar s typical or sedimen-tary exhalative epositsLarge,1983; Taylor, 1987)andcouldbe due o differencesn the degreeof re-ductionof seawater ulfate, o biogenic eduction, rto mixingof seawater-derivedulfidewith minorex-halativesulfide.The sulfur sotopevaluesare notconsistentwith values eported for zinc-leadskarndeposits,which generally ange rom -5 to +5 permil (Taylor, 1987) and have a fairly narrowspread.

34S -15 -10 - 5 0 5 10 15 20 25 30 35 %,sAGUILAR G BPo

PRammels Cberg G BPSMeggen G B

Silver -35.8 p-33mines G BPMcArthurRiver G

PLady SLoretta G B Po

Mt. Isa PSGPoSullivan S

FIG.19. Range f 834Salues f sulfidesndbarite n theAguilar istrictn comparisonith hose fselectedediment-hostedassiveead-zinculfide epositsmodifiedromLarge, 983).Abbreviations:B = barite,C = chalcopyrite,= galena, = pyrite,Po= pyrrhotite, = sphalerite.

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2110 GEMMELL, ZANTOP,AND MEINERT

AGUILAR ESPERANZA

__ LAMINATEDREDISSEMINATIONS OREOLUTIONMASSIVE.

RUBBLY ORBRECClATED ORE

FIG.20. Schematiciagramf characteristiceaturesf sediment-hostedtratiformulfide epositsand roposedocationf heAguilar ine ndEsperanzarospectn this cheme.

The comparativelyow (34Salueof molybdenite8per mil) associatedith one of the graniticdikes nthe Aguilar deposit, n contrast,may reflect an ad-mixtureof magmatic ulfurprovided y the intrusionof the Aguilargranite.Given he above nterpretationhat the sulfides resedimentary xhalative, he similarityof sulfur so-tope values n the AguilarandEsperanza res n spiteof their location n different contact metamorphicaureolesndicateshat metamorphismasnot signifi-cantlychangedheir overallsulfur sotope haracter.However,sulfur sotope ractionationsmongdiffer-ent ore minerals anbe usedasgeothermometersfwe assumemetamorphiceequilibration f sulfur so-topes n mineralphases. he expected rderof 634Senrichmentamong coprecipitating ulfide phasesgivenby Bachinski1969) andKajiwaraet al. (1969)is 634S yrite > 64S phalerite 64Spyrrhotite>64S halcopyrite 64Sgalena. his orderwasob-served n mostsulfidepairs rom he Aguilardeposit.The isotopicemperaturesTable7), calculated singequationsromOhmoto ndRye (1979) for the sphal-erite-galena nd he pyrrhotite-galena ineralpairs,range from 276 to 503C with a mean of about380C. Some eversedor unreasonablyigh values,whichmayreflectnonequilibrium r point o the un-certainties nherent in this approach,were not in-cluded n these calculations. he averageof thesesulfur isotope emperatureestimates s consistentwith the temperature f pyroxene- ndhornblende-hornfels-gradecontact metamorphism Turner,1981). Two mineralpair temperatures alculatedorthe Esperanzaprospect give values of 210 and445C. The 210C temperature is too low for re-

equilibrationof the sulfidesduring albite-epidotehornfelsmetamorphismndmayreflectdepositionalconditions,which for sedimentary xhalativede-posits ypically range from less han 100 to over220C (Taylor,1987). The 445C emperatures oohigh for albite-epidote ornfelsmetamorphic rade.The lack of evidence for equilibrium conditionsamong ulfidephasesn the analyzed amplesmakesthese emperature nterpretations ery tentative.The carbonand oxygenvalues or mostorebodieswithouta calc-silicatematrix n the Aguilarmine andEsperanzaprospect Fig. 17) may be interpreted asreflecting hoseof the original,biogenic arbonandoxygen n the ore-forming nvironment. he popula-tion of high613C nd ow 6sOvaluesrom he brec-ciaswith a calc-silicatematrix in the Aguilar minemaybe interpretedas he resultof interactionof theoriginal ock with a water-rich,CO2-poor luid dur-ing metamorphism.The presence of wollastonite,which forms n a water-rich,CO2-poorenvironmentat high metamorphic emperatures,supports hispoint.Lead isotopevalues n the Aguilar deposits lotnear he growthcurveof Stacey ndKramers 1975)on the 7Pb/4pb 6Pb/4pbnd 2SPb/4pb6Pb/4pbiagramsFig. 18). Theseplots ndicatethat the lead was derived from a moderately adio-genic, upper crustalsourceand bearsno relation tothe highly adiogenicead n the Aguilargranite. helead isotope regressionine for samples rom theAguilar district ntersects he Staceyand Kramers(1975) two-stage rowthcurveat a leadmodelageofapproximately 80 Ma. This is in good agreementwith the Ordovicianage of the sedimentary ocks

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AGUILAR Zn-Pb-Au DEPOSIT, ARGENTINA: GENESIS 2111

that host he oresand endscredence o the hypothe-sis that the Aguilar orebodies ormed at the sametime as he enclosing edimentaryocks.Conclusions

On the basis f stratigraphic,tructural, omposi-tional,and sotopic vidence,he oresof the Aguilardistrictare interpretedas Ordovician,sedimentaryexhalative ulfide ccumulationsFig. 20). The large-scale tructures,mall-scaleextures,mineralcompo-sition,and isotopegeochemistry f the Aguilarde-positandEsperanza rospect oint o a sequence fevents hat spans he time from sedimentation-dia-genesisof sedimentarymaterials hrough meta-morphismand postmetamorphic eformation: a)sedimentation nd eraplacement f disseminated,stockwork, reccia,and bandedore, includingsoft-sediment eformation ndbrecciation;b) preintru-sion deformation, as reflected in folds and faultswhich affect host rocks and strata-bound and strati-form orebodies like; (c) high-grademetamorphismof the Aguilardeposit,esultingn contactmetamor-phichalos, he formation f calc-silicates,ndperva-sive recrystallization nd annealingof the ore, andlower ntensitymetamorphic verprintof the oresofthe Esperanza rospect; d) deposition f nonrecrys-tallized sphalerite,galena,chalcopyrite,molybden-itc, and fluorite n the graniticdikes,and of wolfra-mite n crosscuttinguartzveins elated o the ntru-sionof the Aguilargranite;and (e) postmetamorphicfracturing nd olding elated o Andeandeformationof the Aguilarblock.The sedimentary ndmineralo-gic zonationsn the districtsuggesthat the Aguilardeposit epresents roximal, nd he Esperanza ros-pect, distalmineralization.

AcknowledgmentsWe gratefullyacknowledgehe full cooperationgivenus at every stageof our projectby M. RossettiandM. J. Edwards f the E1AguilarCompany, ndT. N. Walthier, R. E. Beane, and P. E. Gerdemann,thenof the St.JoeMineralCorporation. he researchwas undedby the St.JoeCorporation ndwe appre-ciate he permissiono publishour results.We wishto emphasizehe valuable nsightsnto the geologic

characteristicsf the districtgivenby thosewhohaveworked n it for manyyears,specifically . A. Dicin-dio,J.L. Martin,J.R. Husman, ndJ.Duff.We appre-ciate the help given us in the collectionof data:T.JohnsonndE. Ripleyat IndianaUniversity ssistedwith the sulfur sotopemeasurements; . P. Cham-berlain at DartmouthCollegewith the carbonandoxygenanalyses; . Barreiroat DartmouthCollegewith the lead sotope xtractionsndwith the analy-ses n G. Tilton's aboratory t the Universityof Cali-fornia,SantaBarbara.Microprobe nalyses f sulfideminerals erecompleted t HarvardUniversitywith

the help of D. Lange,and microprobe nalyses fsilicateminerals t Washington tateUniversity.Wethank wo Economic eologyeferees or their criti-caland horougheviews f earlierdraftsof thisman-uscript.March26, 1991;June 0, 1992

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