petrology of the yerington batholith nevada - evidence for evolution of porphyry copper ore fluids

7/27/2019 Petrology of the Yerington Batholith Nevada - Evidence for Evolution of Porphyry Copper Ore Fluids

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EconomicGeologyVol. 82, 1987, pp. 1750-1789

Petrologyof the YeringtonBatholith,Nevada:Evidence orEvolutionof Porphyry Copper Ore FluidsJOHNH. DILLES*

Departmentof Applied Earth Sciences, tanfordUniversity,Stanford,California 94305Abstract

The JurassicYerington batholith, western Nevada, is a compositepluton that containsseveralcentersof porphyrycoppermineralization nd s exposedn structural ross ectionat paleodepths anging rom 0 to 8 km. Within these exposureshe McLeod Hill quartzmonzodiorite,Bear quartz monzonite,and Luhr Hill granite orm successiventrusions hatare in turn volumetricallysmaller (75, 19, and 6 vol %, respectively),more deeplyemplaced topsat


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YERINGTONBATHOLITH,NV: PETROLOGY 1751Thesedatasuggest n orthomagmatic odel n whichbothhigh-and ow-density alineaqueousluids ormedduringexsolution f water from the crystallizing uhr Hill granite;and from whichsalts,Cu, Fe, and S strongly artitioned nto the high-densityore" fluid.Low-densityluidsmayhaverisen rom the magma sa vaporplume.Mineralization c-curredwhen fluid overpressuresaused racturingof wall rock and upward ntrusionofgraniteporphyrydikesandhigh-densityalineore fluids.Both he o2pathduringsubsoli-duscooling nd he deposition f mostSasehaleopyrite-pyriten oresuggesthatreduction

of magmatie to sulfidemayhavebeen a mechanismor oxidation f magmatieerromag-nesian silicate and Fe-Ti oxide minerals.

IntroductionTHE problem of the relationshipbetween graniticplutons and porphyry copper deposits has longbeen an issueof debate.Currently, two theoriesarepopular: the orthomagmatic heory that ore fluidsand metals are derived directly from the crystalliz-ing pluton (e.g., Gustafson nd Hunt, 1975; Burn-ham, 1979) and the remobilization meteoricwater)theory that the plutonsmerely serveasheat enginesto drive hydrothermal convective cells which leachmetals rom the surroundingwall rocks (e.g., Nor-ton, 1982).This paper addresses ome unresolvedquestionsin the orthomagmaticheory on the basisof a pet-rologic and geochemical study of the Yeringtonbatholith in the area of the Ann-Masonporphyrycopperdeposit.Chemicaland sotopic ompositionsare used to constrain he origin and differentiationmechanisms f the magmas,and the petrography,mineralogy,and phasecompositions re used o es-timate temperature,water content, and oxygen u-gacityduring crystallization nd subsolidusoolingof the batholith.The emphasiss placedon the evo-lution of the magmaticaqueous luids, which arepotential ore fluids for porphyry copper minerali-zation, during crystallization of the batholith. Asubsequent aper will address he evolution of thehydrothermalsystem o lower temperatures y doc-umentation of alteration and mineralization charac-teristics of the Ann-Mason porphyry copper de-posit, which contains approximately 495 milliontonsof '--0.4 percent Cu ore (Einaudi,1982).The Yeringtonbatholith s a large, composite u-rassicpluton that hosts hree porphyry copper de-posits, ncluding he Ann-Masondeposit,and is as-sociatedwith severalcopper-ironskarns.These de-posits contain a total of '--6 million tons Cu(Einaudi, 1982). The Yerington district s well stud-ied by detailed geologic mapping (Proffett andDilles, 1984), age dating (Dilles and Wright, inpress),and detailed studiesof mineralization Ein-audi, 1977; Harris and Einaudi, 1982; Carten,1986). Importantly, Tertiary tilting associatedwithnormal faulting (Proffett, 1977) has exposed hebatholithandore depositsn cross ection nd acili-tated study.

The area around he Ann-Masonporphyry copperdepositwaschosen ecauset offers he mostcon-tinuous unfaulted)exposures f the batholith andbecause gneouseventscan be directly related toalteration and mineralization events. Concurrentgeologic and alteration-mineralization mappingwere conductedat a scaleof 1:4,800. Petrographicstudies f 140 thin andpolished hin sections ereconducted on the freshest rocks; ten of them wereselected or analysis y electron microprobe.Thepetrography s summarizedn AppendixA and theanalytical echniquesor electronmicroprobe nal-yses are given in Appendix B. Selected sampleswere also analyzed for major and trace elements(21), strontium sotopecomposition14), and oxy-gen sotopecomposition8).Geologic Setting

The Yeringtonmining district ies in westernNe-vada within the volcanic-arc errane of the earlyMesozoicmarine province (Speed, 1978) (Fig. 1).The district s thought o be underlainby late Pre-cambrian or Paleozoic oceanic crust which is over-lain by Paleozoiccontinentalmargin-arcvolcanicrocksand volcaniclastic edimentary ocksaswell asminor continent-derived terrigenous sedimentaryrocks Stewart,1972, 1980; Speed,1979). Theserocks ie west of the edge of inferred Precambriancratonmarked by the 0.7060 initial strontium so-pleth for Mesozoicgneousocks Fig. 1).The summary f the Mesozoicgeology hat fol-lows is taken from field relations of Proffett andDilles (1984) andJ. M. Proffettet al. (in prep.) andthe U-Pb zircon geochronology by Dilles andWright (in press).The oldest ocksexposedn thedistrict are a 1,300-m sequence of Middle(?) orearly Late Triassic, ntermediate omposition, alc-alkaline volcanic rocks of the McConnell Canyonvolcanics. hey are overlainby a 1,800-m sequenceof Late Triassic to Middle(?) Jurassiccarbonate,volcaniclastic, uffaceous,and argillaceoussedi-mentary rocks, including minor gypsum andquartzite. n the Middle Jurassic, major event ofvolcanism lutonism,metamorphism, nd deforma-tion occurred n the Yeringtondistrictbetween 170


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1752 JOHN H. DILLES

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120 118 116FIG. 1. Tectonic setting and location map. The Yeringtonbatholith ies within a belt of Middle Jurassic rc rocks,probablyrelated to subduction to the west in California (Schweick-ert, 1978; Dilles and Wright, in press).Early Mesozoic MZ) arcterrane Speed,1978), Luning-Fencemakerhrustbelt (Oldow,1983),and nitialstrontiumsoplethsSri) Kistler ndPeterman,1973; Kistler, 1983) are alsoshown.

and 165 m.y. This event s part of a belt of Andean-type arcmagmatismhat developed long he conti-nentalmargin n responseo subductionectonicsothe west (Fig. 1). In the Yeringtondistrict,magma-tism was nitiated by eruptionof the volcanic ocksof Artesia Lake, closely ollowed by emplacementof the cogeneticYeringtonbatholith at 169 to 168m.y. Latite volcanic ocksof FulstoneSpringwereeruptedat 167 m.y. and ie disconformablytop heArtesia volcanics. The Fulstone volcanics were in-truded by quartz monzodiorite ikesand the largeShamrockbatholith granite (Jsg)at 165 to 166 m.y.Plutons of the Early CretaceousSierra Nevadabatholith were emplacedpredominantlywest ofYerington Fig. 1), but a few intrude the district.Oligocene ilicic gnimbrites ndMioceneandesiticlavasunconformablyverlie Mesozoic ocks Prof-fett and Proffett, 1976). Miocene andesitesandolder rocks have been tilted 60 to 90 W duringMiocene to recent basin-and-range ormal faultingthat accommodated> 100 percent east-west exten-sion (Proffett,1977). Structuralattitudes ProffettandDilles, 1984) andpaleomagnetictudies Geiss-man et al.: 1982) of Mesozoicrocks ndicate thattheynow ip 90 W as he esult f he ateCen-

ozoicnormal aulting and a smalladditionalcompo-nent of pre-Oligocenedeformation.Thus, the Ju-rassicYerington batholith and associated re de-positsare exposed n structuralcrosssectionwithstratigraphic ops o the west.Field Relations and PetrographyGeneral eatures

The Yeringtonbatholith s a large composite lu-ton,with a planareaof approximately50 km (12X 25 km, elongateNW-SE), afterTertiary deforma-tion is removed.The morphology, olume,and tex-ture of each of the phasesof the batholith and co-genetic olcanics,sshown y district-scaletudies(Proffett and Dilles, 1984, and unpub. data), aresummarized n Table 1. The batholith is composedof three major, successive ranitic ntrusions:heMcLeod Hill quartz monzodiorite, he Bear quartzmonzonite, and the Luhr Hill porphyritic granite.The latter unit includescogeneticgranite porphyrydikes, which are closelyassociatedwith porphyrycoppermineralizationat the Ann-Masondepositand the Yerington mine (Proffett, 1979; Dilles,1984' Carten, 1986). Apparentlyconcordant -Pbzircon age datesof 169 and 168 m.y. have beenobtained n earlyquartzmonzodiorite nda weaklymineralizedgraniteporphyry,respectively Fig. 2;Dilles and Wright, in press).These datesestablishthat (1) mineralizationoccurred n the Middle Ju-rassic,2) the three main ntrusions f the Yeringtonbatholith were emplaced within 1 m.y., permis-sive evidence hat they are cogenetic,and (3) theYeringtonbatholith predatesother Middle Jurassicintrusions in the area.The three successiventrusionsare progressivelycoarser grained, more silica rich, volumetricallysmaller, and more deeply and centrally emplacedwithin the batholith (Fig. 3). The graniticrocksofthe batholith are commonlynonfoliateand rangefrom equigranular o porphyritic. These texturessuggest mplacement t shallowdepth n the "epi-zone" or upper "mesozone" Buddington, 959),in agreementwith the geologically econstructeddepthsof i to 6 km in the Ann-Mason rea andpossibly sdeep as 7 to 8 km in the Luhr Hill area(Table 1, Fig. 3). All intrusive units contain thesameessentialassemblage f minerals:plagioclase,K-feldspar,quartz, hornblende,biotite, magnetite,sphene, patite,and zircon,with trace amounts faugite and ilmenite as cores to hornblende andsphene, espectively,n both moremaficandmorerapidlycooledunits.The entire batholithand vol-canic cover rocks are found within a >l-km-deepvolcanotectonic graben bounded by high-anglefaults hought o havebeenactiveduringor shortlyafter plutonism (J. M. Proffett, pers. commun.,1982; Proffett and Dilles, 1984).


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YERINGTONBATHOLITH, NV: PETROLOGY 1753TABLE1. GeneralFeaturesof the YeringtonBatholith

Nameand Age Vol Vol4 Grain Depth Morphologyfabbreviation (m.y.) percent (km ) size (mm) (km) Texture igneous nitVolcanics f >250 1-2 0-1 Volcanic Up to 1.5 km thick,Artesia ake centered top

(Jaf) YeringtonbatholithMcLeod Hill

intrusionQuartzmonzodiorite(QMD)Gabbro (GB)

Bear intrusionQuartzmonzonite

(QM)Border granite(B)Granite seriesLuhr Hill

porphyriticgranite PG)Graniteporphyrydikes (GPD)

169 75 1000 0.5-2


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1754 JOHNH. DILLES


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YERINGTON ATHOLITH,NV: PETROLOGY 1755the volcanics nd the Yeringtonbatholith suggestthat they are cogenetic.At Ann-Mason, nly rocksin the basal150 m of Artesiavolcanics rop out, andthey consistof propyliticallyaltered augite-plagio-clase phyric basalt flows and breccias containingabundantmagnetite and sparse lmenite.Quartz monzodioriteof McLeodHill and gabbro

Fine- to medium-grained, ypidiomorphic-gran-ular biotite-hornblende quartz monzodiorite ofMcLeod Hill is the earliest and most voluminousunit of the Yeringtonbatholith Figs.2 and 3, Table1). It intrudesArtesia volcanics t paleodepthsof1 km and Triassicvolcanics t paleodepthsof 7 to8 km. The unit hasa purple-graycolor and is char-acterized by a relatively fine grain size that in-creaseswith depth, a high hornblende+ augite tobiotite ratio of '--4:1, and abundantaccessorymag-netite and sphene Fig. 5B, C, and D, Appendix A).It was emplaced as a series of dikelike bodies,shownby texturally mappableunits, igneous lowfoliationsparallel to contacts Fig. 2), and internalcontacts.Sparse,ellipsoidal, -10-cm-long,maficinclusions ave poorly preserved olcanicand plu-tonic textures and consistchiefly of plagioclase,hornblende, and biotite. Locally, comb layers ofcoarse, bladed hornblende and plagioclase andequant magnetite occur with country rock as sub-strate and with euhedral mineral terminations inquartz monzodiorite.Allotriomorphic-granular biotite-hornblendegabbrooccurs spodsand enses p to 30 by 350 mwithin quartzmonzodiorite n relativelydeep expo-sures 4-km paleodepth)at Ann-Mason nd else-where in the batholith. Gabbro has a color index of'--60, is massive r weakly layered parallel to layer-ing in quartz monzodiorite, and is interpretedpartly frompetrographicextures sa cogenetic u-mulate (Fig. 5A, Appendix A). Gabbro and quartzmonzodioriteshowmutually ntrusivecontact ela-tions, ndicating hat both behavedas luid magmascontemporaneously.Bear quartz monzonite

The Bear quartz monzonite orms a fiat-toppedintrusion at a ---1- to 2-km paleodepth into thecenter of the McLeod Hill quartz monzodiorite ndconsists f two units:a fine-grainedborder, termedborder granite,and he main,medium-grainednte-rior quartz monzonitephase Table 1, Fig. 3). Theborder phaseoccursalong he upper contactof thepluton where t is up to 300 m thick, narrowingwith

depth. Everywhereborder granitegradesdown-ward or laterally into the main interior phaseofquartz monzonite Table 1), which indicates hetwo phasesdevelopedcontemporaneously.n theAnn-Mason rea, he bordergranite ormsan rreg-ular body as well asdikes n the quartz monzodio-rite (Fig. 2). It is a fine-grained,eucocratic colorindex = 5-7; Tables 1 and 2), biotite granite with ahypidiomorphic-granular o porphyritic texture(Fig. 5F, Appendix A). Weak potassicalteration,characterized y biotitizationof hornblende ndK-feldspar imson plagioclase,s common, nd ts ori-gin is discussedater herein.Throughout the batholith, the border granitephasegradesdownward nto the interior quartzmonzonitephaseover an intervalof --100 m andshowsan increase n grain size, color index, andhornblende content and a decrease in quartz, K-feldspar, ndbiotite content.The interior phase s alight purple-gray, medium-grained,hypidiomor-phic-granular, ornblendequartz monzonite Fig.5E, AppendixA) in which grain size ncreases ithdepth rom ---1 to 2 mm at the bordergranitecon-tact to 2 to 5 mm at 300 m below the contact. It ischaracterized y a high hornblende o biotite ratioof>10:l andby a color ndex 11-13), silicavalues(Table3), andquartzcontent ntermediate etweenquartz monzodioriteand porphyritic granite (Ta-ble 2).Granite series intrusions

The youngest ntrusions onsistof hornblende-biotite granite characterized y K-feldsparmega-crystsup to 2 cm long; hey are divisible nto twomain cogenetic units: the medium-grainedpor-phyriticgraniteof Luhr Hill and a seriesof graniteporphyry dikes with ---50 percent fine-grained,aplitic groundmassAppendixA). Despite exturalvariations, ll of theserocksare mineralogically ndcompositionallyimilarand are characterized y alight gray color, a color ndexof 6 to 11, a horn-blende o biotite ratio of '--1:1, and relativelyhighquartz and silica 68 wt %) contents Tables2 and3). Locally,Luhr Hill porphyriticgranite s textur-ally gradational upward into granite porphyrydikes,althoughwhereversharpcontacts ccur heporphyry dikes ntrude the Luhr Hill granite. m-portantly,emplacement f graniteporphyrydikesat both the Yeringtonmine (Proffett,1979; Carten,1986; M. T. Einaudi et al., in prep.) and Ann-Mason(Dilles,1984) is synchronousith porphyrycoppermineralization. The granite series also contains

FIG. 2. Geologicmapof the Ann-Mason reaof the Yerington atholith.Jurassicross ectionmaybe viewedby rotatingmap80 clockwise. -Pbzircon ges re romDillesandWright in press). heAnn-Masonorphyry opper eposits ocated 1 km westof Singatse eak.


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17 5 6 JOHNH. DILLES

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YERINGTON BATHOLITH, NV: PETROLOGY 1757

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Quartz v v v _ PIg35 I0FIG. 4. Modalclassificationf the Yerington atholith n the UGS systemStreckeisen,976) basedon pointcounts f thin sectionsndstained labsTable2, J. M. Proffett,unpub.data;Carten,1981).

minorvolumes f leucogranite,plite,andpegma-tite dikes.Porphyritic raniteof Luhr Hill: The porphyriticgraniteof Luhr Hill intrudedasa deep and centralintrusion into preexisting batholith units andformeda cupola t Ann-Mason ith its apex3.6 kmbelow he lower Tertiary erosion urface, t an esti-matedJurassicalcodepth f 4.6 km (Table1, Fig.2). A fine-grained hase f porphyritic ranitewitha seriateporphyritic extureextendswestward up-ward) 1 km as a "spire" from the apex of thecupola Fig. ),probablyanalogouso the northdikeof the Nevada-Empireorphyryn the Yering-ton mine (Carten, 1986). In the spirephase,quartzand eldsparorma graphic-mmntergrowth ithminor fine-grained nterstitial quartz and K-feld-spar.Along he top of the cupola,a fine-grained,borderphase PG-t, Fig. 5H) hasa sharpcontact,sintrusive nto quartz monzodiorite,and ranges nwidth rom>20 to 4-km palcodepth. All varieties of these dikes cutboth quartz monzodioriteand porphyriticgraniteand are in turn cut by granite porphyry dikes.Granite porphyry dikes:Granite porphyry dikesform the youngestand someof the smallest ntru-sionsof the batholith (Table 1). Most dikes occurwithin one of three main dike swarms, but a fewform isolated dikes or plugs (Proffett and Dilles,1984). The Ann-Mason,Yerington mine, and Mac-Arthur-Bear-Lagomarsinogranite porphyry dikeswarmsare associatedwith porphyry copper min-eralization, which occurs at Ann-Mason and theYerington mine immediately above where theswarm emanates rom the apex of a cupola of theLuhr Hill granite (Figs. 2, 3). The Ann-Masondikeswarm containsseveral ypes of dikes that locallycrosscut ne anotherbut have similargroundmassand phenocryst mineralogiesand textures. Theserelationsand otherspresentedbelow suggest hatall types of dikes were cogeneticand were em-placed within a short time interval.Narrow (5 cm- m), crystal-poor,dark "chilled"dikes GP-c, Fig. 5J, AppendixA) were the earliestdikes and were cut by the main seriesof granite


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1758 JOHN H. DILLES

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YERINGTON ATHOLITH,NV: PETROLOGY 1759TABLE . RepresentativeModal Mineralogyof YeringtonBatholith in vol %)

QMD GPD Y-781GB QM BG PGMineral Y-751 W-7 194 167B BH-13 Y-787 Y-750 Total GroundmassPlagioclase 29.4 50.9 52.6 50.8 37.1 30.8 46.7 36.5 1.5

(33.4)aK-feldspar 15.5 15.0 17.2 31.7 30.4 25.0 31.2 24.5Quartz 7.9 9.2 17.0 18.1 28.4 19.3 21.8 17.2Hornblende 54.1 4.1 12.8 9.5 10.0 tr 3.6 3.2 0.3Biotite 7.9 6.3 4.62 2.0 0.1 4.8 2.5 5.0 1.2Opaques 4.5 3.7 2.2 1.8 2.2 1.4 0.8 0.6 0.1Sphene 3.0 1.4 1.0 1.2 0.7 tr 1.2 0.4 0.3Zircon tr tr tr tr tr tr tr 0.3Epidote 1.0 0.5 tr 2.8 0.5 0.6 0.4Augitc 0.1 9.7Sericite 0.5 0.7Chlorite tr 0.3 0.1Points 836 568 500 400 558 721 885 867

I See Table 1 for abbreviations2 Mostly hloritizeda Modalplagioclaseecalculatedy addition f alteration pidote, hlorite, ndsericiteBased npointcounts f thinsectionsnd forporphyriticocks) tained labs r outcrops;amples94 and167Bby J.M. Proffett, r.tr = trace

porphyry dikes. Chilled dikes also occur as thin(


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1760 JOHNH. DILLESTABLE3. Major and Trace Element Analyses nd CIPW Norms

Analysisno. i 2 3 4 5 6 7 8Rockname Artesia Gabbro QMD QMD QM BG PG GPDbasaltSampleno. Y-797 Y-751 Y-767 Y-776 Y-800 Y-787 Y-750 Y-781

SiO2 50.6 43.5 57.6 62.4 66.3 68.6 67.4 68.1A1203 17.3 13.4 17.3 16.4 15.2 14.9 15.6 15.6Fe203 total 9.68 15.2 6.41 4.83 3.22 2.78 2.71 2.42MgO 4.50 9.22 2.76 1.97 1.36 0.99 1.12 0.98CaO 9.23 10.5 5.88 4.67 3.57 2.44 2.92 2.84NaO 2.74 2.21 4.61 4.34 3.75 3.78 4.08 4.32KO 2.76 1.41 2.43 2.97 4.21 4.08 3.88 3.35TiO 0.74 2.39 0.96 0.73 0.55 0.40 0.42 0.37P205 0.43 0.52 0.38 0.28 0.21 0.13 0.16 0.14MnO 0.12 0.14 0.08 0.05 0.02 0.04 0.03 (0.02HO 1.1 2.1 0.9 1.0 1.0 2.5 1.4 1.1Total 99.2 100.6 99.3 99.6 99.4 100.6 99.7 99.2Ba 770 600 1,400 1,400 1,400 1,300 1,700 1,400Cr 10 27 10 10 10 10 10 10Cu 84 170 120 99 6.8 94 5.8 16Ni 11 93 23 17 12 11 11 8V 270 390 140 110 69 63 59 46Rb 53 31 86 73 109 146 91 81Sr 815 941 1,268 1,053 1,015 621 1,082 1,033Zr 101 120 + 162 169 130 + 190 141 127Y 20 21 + 16 14 11 + 19 11 10Nb 8.4 25 + 12 6.8 25 + 13 7.1 7.8K/Rb 398 384 234 338 321 232 354 343Rb/Sr 0.069 0.032 0.077 0.069 0.170 0.029 0.084 0.078Ba/Rb 13.2 19.7 14.4 19.2 12.8 7.7 18.7 16.0V/Ni 25 4.2 6.1 6.5 5.8 5.7 5.4 5.8CIPW normsQuartz 5.92 13.96 19.96 24.58 21.35 22.9Corundum 0.14 0.04Orthoclase 16.74 8.55 14.59 17.85 25.34 24.61 23.36 20.21Albite 23.79 15.05 39.64 37.34 32.31 32.65 35.17 37.20Anorthit e 27.45 23.06 19.65 16.77 12.43 11.49 13.03 13.45Nepheline 2.24Diopside 13.57 21.63 6.06 3.97 3.38 0.53Hypersthene 8.81 7.63 5.20 3.11 3.79 3.76 3.52Olivine 2.37 16.04Magnetite 4.81 7.54 3.77 2.85 1.90 1.64 1.60 1.44Ilmenite !.44 4.66 1.85 1.41 1.06 0.78 0.81 0.72Apatite 1.05 1.26 0.91 0.67 0.51 0.31 0.39 0.34Color Index 4 31.00 49.87 19.31 13.42 9.46 6.21 6.70 5.68FeO*/MgO 1.94 1.48 2.23 2.21 2.13 2.52 2.17 2.23

See Table 1 for abbreviations All iron reportedasFe20 For normcalculationshe ratio of Fe203: FeO (atomicFe) wasassumedo be 2:3 for 55 percentSiO and 1:2 for 55 per-cent SiO4 Sum of normative mafic minerals FeO* is total iron calculated as FeOMajor elementoxides eported n weightpercent, romwavelength ispersive -ray fluorescencenalyseserror a _ 1-2%) andbyCHN analyzer or water (error a ___0%) by U.S. Geological urvey,Denver, Colorado. race elements y emission pectroscopy(error a _+10%) by the U.S. Geological urvey,Menlo Park, California, xceptRb, Sr, Zr, Y, andNb analyses ithout"+" symbol,doneby energy ispersive-ray luorescenceKevexmachine)errora _+ -10%) at Stanford niversity. amples ereprepared sing1.5 to 5.0 kgof unweatherednd resh except -797 andY-787)rock or fine-andmedium-grainedamples,nd 5 kg orporphyriticrocks; amples ere broken o 1 cm with hammer, hen crushedo -300 meshwith a tungsten arbide aw crusher ndshatterbox.The CIPW normativemineralogy ascalculated sing he PETCALcomputer rogram f Bingleret al. (1976)depth (Fig. 2). Crosscuttingelationsbetween dikesindicate that the swarm was emplaced as severalmagmapulses. n the central part of the swarm, the

northwarddowndippart of the dikes ormsa knot orplug that is the center of the Ann-Masonporphyrycopper deposit.


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YERINGTONBATHOLITH, NV: PETROLOGY 1761Within the main granite porphyry dike swarm,the grainsizeof the groundmassnd he phenocrystto groundmass atio increase with palcodepth.Groundmassgrain size increases rom shallowestexposures o deep exposures n the porphyriticgranitecupola Fig. 2) from 0.02 to >0.2 mm, coin-

cidentwith a changeroma fine aplitic o a graphictexture and an increase in the phenocryst togroundmassatio from as low as 1:2 to >1:1 (Ap-pendixA). Somedikespenetrate he deepestexpo-sures 5.5 km) of porphyriticgraniteand musthavehad a deeper source.However, the rate of coarsen-ing of dike groundmassesuggestshat dikeswouldbe indistinguishablerom porphyriticgraniteat anadditional -0.5 to 1.0-kmdepth.Granite porphyry dikes are characterizedby a"-50 percentgray aplitic groundmass f quartz, al-kali feldspar, nd minorbiotite and oligoclase nd"-50 percent 1- to 5-mm phenocrysts f oligoclase,microcline"-1 cm), biotite,hornblende, mbayedquartz, and accessory phene,magnetite,apatite,andzircon Tables2 and4, Figs. 4 and 5I, AppendixA). Hornblendephenocrystsre commonly lightlyembayed nd biotitized,whichsuggestshat theywere slightly unstable with respect to the melt

when the groundmasscrystallized. In both thechilled and transitional dike phases,hornblende iscommonly ompletely eplacedby shreddyaggre-gates of biotite. Becausephenocrysts re consid-ered to be preserved rystalsn the melt, whichwassubsequentlyquenched o form the groundmass,the phenocryst o groundmassatio represents heprequenchcrystal o melt ratio.

GeochemistryMajor elements

Major element compositional ata indicate thatthe rocks of the Yerington batholith are plutoniceqivalents f high K20 orogenic arc) andesites nddacites.The Yeringtonbatholith (exceptcumulategabbro) s hypersthenenormativeand contains 5to 69 wt percent SiO2, 2.2 to 4.2 wt percent KO,and


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1762 JOHNH. DILLES

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i -

65 70

FIG. 6. Variationdiagram or majorandminorelementoxides ersus ilica.Data are fromTable 3,Dilles (1984), Carten 1981), andJ. M. ProffettandM. T. Einaudi unpub.data).Alkalic,highA1,andtholeiiticboundariesor alkalies ersus ilicaare from Kuno 1959); 55.7 is the Peacockndex wt %SiOn).Andesite-daciteields for the KO-SiO plot are from Ewart (1979). Note that oxidesvarysmoothly,except for slight scatter n Na, K, AI, and Ca, which is probably due to minor subsolidusmetasomatism.bbreviations PD = graniteporphyrydikes,PG -- Luhr Hill porphyriticgranite,BG-- border granite,and QM -- quartz monzoniteof the Bear ntrusion,QMD = quartz monzodioriteandGB -- gabbroof the McLeodHill intrusion, af-- basalt rom the volcanics f ArtesiaLake.

numberof analyzed amples f quartzmonzodioriteare petrographically resh and do not containveinsor alterationminerals, ndK20 increaseselativelysmoothly ith SiO2andshows nlyminor rregular-ity due to potassiummetasomatismFig. 6). Theigneoussuite has a Peacock ndex of 55.7, e.g., atthe boundarybetween the alkali-calcic nd calc-al-kalic fields, and hasNaO + KO values n the alka-lic field of Kuno 1959). The batholithhasa simplealkali-enrichmentrend at constant eO3total/MgO('--2.2) towardhighersilica,similar o othercalc-al-kaline volcanic suites such as the Cascades of NorthAmerica (Fig. 7). FeO3total,MgO, CaO, MnO,

TiO2, andPO5decreasewith increasing iO (Fig.6). NaO varies ittle with increasing iO above55wt percentbut issomewhat cattered wing o somealtered samples.A1203decreases lightlywith in-creasingSiO2above55 wt percent but alsoshowsslightlyscattered lot (Fig. 6). In contrast,NaO inmany ypicalcalc-alkaline olcanic ocks e.g., Cas-cades) ncreases lightlywith increased iO (Car-michaelet al., 1974). The major elementcomposi-tion of the basalt of Artesia Lake is consistent withthe interpretation hat it is a cogeneticmemberofthe Yeringtonbatholith magmaticseries Table 3,Figs. 6 and 7).


14/40

YERINGTON BATHOLITH, NV: PETROLOGY 1763F Fe203 T

AFM PlotWeight Percent Oxide

Proportions

K 0+a2y MgO^/ .......... MFIG. 7. AFM plot of analyses f the Yerington atholith.Triangle s basaltof ArtesiaLake.Data arefrom sources iven n Figure 6. Skaergaardtholeiitic) nd Cascadecalc-alkaline)rendsare fromCarmichaelt al. (1974).Trends f owK arcandshoshoniticolcanicuites re romEwart 1982).

Trace elementsThe high contentsof large ion lithophile (LIL)elements Rb and Ba in the batholith are consistentwith its high potassium ontent Table 3). The highabundance f lithophile elementsand the low con-

tents of compatible trace elements Cr, Ni, and V(Table 3) indicate hat the suite s more composi-tionally evolved than most orogeniccalc-alkalinevolcanicsuites, ncluding he Cascade uite Carmi-chael et al., 1974), which also containsporphyrycopper mineralization. However, Ti, Zr, and Nbhave ow valueswhicharecharacteristicf orogenicsuites Gill, 1978) but which are slightly higherthan values of tholeiitic basalts. Sr is abundant(600-1500 ppm) andhigher han n mostorogenicandesitesTables and4; Gill, 1981). Owing o thehigh Sr contents, he Rb/Sr ratio is low (-'0.07)relative o mostandesites nd other Triassic-Juras-sic gneousocks rom he Yerington istrict 0.08-0.33; Table 4, Fig. 8). Ba/Rb and K/Rb ratios(10-23 and -'320, respectively)are greater thanmostorogenicandesitesTaylor, 1965; Gill, 1981).Rb, Rb/Sr,Ba/Rb,andK/Rb increase lightlyandSrdecreases lightly with increasedSiO2. At similarSiO2contents he porphyry-bearingntrusions t E1Salvador,Chile, have similar Cr, Ni, and V contentsbut greater Sr, Ba/Rb, and K/Rb and lower Rb andRb/Sr (Gustafson,979) than the Yeringtonbatho-lith. Copper decreaseswith differentiation rom 62ppm n quartz monzodioriteo 10 ppm in the gran-ite series, sdiscussedelow (Fig. 9, Table 3).

Oxygenand strontium sotopesOxygen isotope analysesof fresh and alteredrocks from the Yerington batholith indicate thatprimaryvalueswere approximately/isO 6.8 permil (Solomonet al., 1983; Dilles, 1984), as calcu-

lated from sampleswith equilibrium magmatic200

t50

IOO

60Rb(ppm)

5O

[ I I

EXPLANATION-'l- SHAMROCK BATHOLITH Q7Z MONZODIORITE PPYX FULSTONE VOLCANICS APLITELEUCOGRPD mYERINGTONO(BPGGBATHOLITHQM7 GB jA 8sL OF TSS LK5D WASSUK

i I I , I I , I I

// \

IO I I I i i I I ill i iI00 300 600 I000 2000Sr (ppm)

FIG. 8. Plot of Rb versusSr for Mesozoic gneous ocks romthe Yeringtondistrict.Data are from Tables3 and 4. Cascade ndChile trendsare from Gill (1978). Abbreviations: tz -- quartz,ppy = porphyry, eucogr= leucogranite, eeFigure6 for others.


15/40

1764 JOHNH. DILLES150

I00

E

50

KEY0 GPD[] PG BG QM/ OMDA Weakly altered

I I I [ [ [ I

050 52 54 56 58 60 62Si 02 (wt%)

[ OA [Yer[ngton Mine

Ann-Mason/-- - 0 /1/Luhr,I,-.t /'-Ill/64 66 68 70FIG. 9. Plot of copperversus ilica n the Yeringtonbatholith,showing ow Cu contents n porphy-ritic granite and granite porphyry dikes. Analyses re from Table 3 and unpublished ata of R. B.Carten,J. H. Dilles, M. T. Einaudi,andJ. M. Proffett.SeeFigure 6 for abbreviations.

quartz-feldspar ractionations f 1.0 to 1.5 per miland the assumption f a quartz magma ractionationof 1.5 per mil. The value of 6.8 per mil is at the highend of the worldwide range of basalts 5.4-6.0%0;Taylor, 1968, 1980) and arc andesites 5.5-7.0%0;Gill, 1981).Ten analysesof the Yerington batholith give anaverage alculatednitial strontium sotope ompo-sition of 0.7040 that is very similar o the 0.7039value for the basaltof ArtesiaLake (Table 4) andsupportscogenesisof the two units. The batholithhas an Sr isotope composition that is similar toslightlyyoungerMiddle Jurassicgneous ocks romthe Yeringtondistrict (0.7039-0.7043) but is owerthan valuesof older Triassic gneous ocks, suchasthe Wassuk diorite (0.7052) (Table 4) and theMcCannell Canyon volcanics (0.7052-0.7057;J. M. Proffett, M. T. Einaudi, and D. E. Livingston,unpub. data). The initial Sr isotopecomposition f0.7040 is similar o other high K andesites ut is atthe high end of the range of primitive basalts(0.7020-0.7040; Taylor, 1980) and of orogenicarcandesites 0.7030-0.7040; Gill, 1981).Inferred magma origin

As discussedbelow, the Yerington batholithprobably formed by a two-stageprocess:differen-

tiation of primitive basalts o form high K andesitemagmashat crystallized nto quartz monzadiorite,and differentiation of high K andesite magma toform quartz monzonite and granites.These differ-entiation mechanisms must account for the batho-lith'sslightlyelevatedoxygen ndstrontiumsotopevalues, which suggest that primitive parentalmagmas ssimilated omecrustalmaterial Carter etal., 1978; Taylor, 1980). However, he low initial Srisotopevalue of 0.7040 precludesassimilation fanybut traceamounts f isotopically eavy radio-genic) Precambriansialic crust, consistentwithYerington'spositionwest of the limit of inferredPrecambriancraton (Fig. 1). Because he Yeringtonbatholithhasa high Sr content (--- ,000 ppm), its Srisotope composition s an insensitivemeasureofsmallor moderatedegreesof crustalassimilation.Origin of high K andesitemagmas:Rocksof inter-mediate composition such as the McLeod Hillquartz monzadiorite have been proposed o formby modification f primitive basalticmagmasrommantle sources e.g., Hildreth, 1981) by the com-bined effects of fractional crystallizationand wall-rock assimilation t crustal evels (DePaolo, 1981).A test calculation of such an assimilation-fractionalcrystallizationprocesswas made after DePaolo'smethod,and it produceda best fit when the ratio of


16/40

YERINGTONBATHOLITH,NV: PETROLOGY 1765the assimilationate to the fractional rystallizationrate was "* 1:3 and the bulk partition coefficientofSr betweencrystals xls)and melt (Dsr(xls/melt)as"*0.5,where nassumedRblxs/metl"0was sed.The test suggestshat assimilation-fractionalrys-tallization proceeded until the magmas eached"*30 wt percent of the original basaltic magma,correspondingo a ' 1' 1 ratio of assimilated ock tobasalt n the high K andesitc.This 1:1 ratio repre-sentsa maximum ratio of crustal component to ba-salt because he estimatedSr isotopecompositionsof the basalt and assimilated crust were chosen asminimum values. Basalt was arbitrarily assignedvaluesof 400 ppm Sr, 10 ppm Rb, and 87Sr/86Sr= 0.7030, similar to tholeiitic basalts from oceanicisland arcs (e.g., Lowder and Carmichael, 1970).The crustwas assumedo be predominantly rcvolcanic rocks (e.g., Burchfiel and Davis, 1972'Speed,1978) andwasassigned aluesof the TriassicWassuk diorite' 500 ppm Sr, 25 ppm Rb, and87Sr/86Sr0.7052 Table ). Thecalculationgreeswith Nd and Sr isotopestudies y Farmerand De-Paolo 1983), which ndicate hat Mesozoicmagmasin western Nevada contain 25 to 40 percent assimi-lated crustal material. The bulk partition coeffi-cient, Dsrlxs/mekof 0.5 implies hat plagioclase(withKDsr../_,,1.8;Gill,1981)made p "*25 o 30percent of the fractionatingcrystalsbecause heother fractionating phases exclude Sr and haveKDs 1.The assimilation-fractionalrystallizationmodelpresented above is consistentwith the major andtrace element concentrationsn quartz monzodior-itc. In particular, Ba, Rb, Sr, and K are 3.5 to 9 timesenriched relative to tholeiitic basalt and exceedvaluesachievableby simplecrystal ractionation.The low content of compatibleelementsFe, Ca,Mg, Mn, Cr, Ni, and V requires large degrees ofcrystal ractionation. lagioclase,ugitc,magnetite,and ilmenite (?) are the phenocrystphases n thebasaltic ava of Artesia Lake (which is also olivinenormative),and togetherwith orthopyroxene, rethe likely fractionating hases eeded o generateandesitesrom basaltparents see Gill, 1981). Re-moval of large amountsof augitc and olivine is re-quired both to increase he Fe2Oatotal/MgOatio,SiO2, and alkaliesand to reduce MgO, FegOatotal,CaO, Mn, Cr, Ni, and V. The rare earth elementpattern of Yeringtongraniteporphyry Gustafson,1979) s ypicalof mosthighK Andean aeitesGill,1981) because t has a twenty-fold enrichmentoflight/heavy are earth elements 80 times/4 timesehondrite) ndhasnoEu anomaly. he patternsug-gests hat plagioelaseraedonatedn amounts p-proximatelyequal to clinopyroxeneand/or horn-blendebecause D/xymtfor heavy are earthele-ments s 0.01 to 0.1 for plagioelase nd 0.6 to 6 for

clinopyroxenendhornblende, D(xl/Oor Eu is0.5 forplagioclasend 1 forclind)mroxenendhornblende, ndKD(xl/melt)or light rare earth ele-ments (La and Ce) is 0.2 for all three minerals(Schnetzlerand Philpotts, 1970; Drake and Weill,1975). Assimilation of Triassic igneous rocks byearly Yerington magmas s suggested y both thespaceproblem presentedby eraplacementof thebatholith (Fig. 3) and by the presenceof marieme-tavolcanic nclusionsn quartz monzodiorite.Thus,although fractional crystallizationof a basalticpar-ent was the dominant process n deriving the Yer-ingtonhigh K andesiticmagmas, rustalassimilationof older (Triassic?) are volcanicmaterialmusthaveprovided an important (up to 50%) contaminant othe melt.Differentiation rom quartz monzodiorite o gran-ite: Differentiation of the Yerington batholith fromquartz monzodiorite o granite composition ppearsto have occurredprimarily by crystal ractionationwithout significantassimilation. he initial Sr iso-tope composition f the Yeringtonbatholith s con-stant at 0.7040 despitea ten-fold increaseof Rb/Srduring differentiation (Fig. 10). These data pre-clude significantassimilationduring differentiationunless the assimilated rock also had an isotopiccompositionnear 0.7040, such as cogenetic vol-canic or plutonic rocks. The quartz monzonite andthe graniteshave sharpcontacts primarily againstearlier phasesof the batholith) and lack wall-rockinclusions at the observable levels of exposure.These facts argue against significantassimilation.Modeling of crystal fractionation by the computerprogramXLFRAC (Stormerand Nicholls, 1978) in-dicates that major and minor element variationsduring differentiation rom quartz monzodiorite ogranite can be closely fitted by fractionation ofphases hat crystallizedearly. Fractionationof 40wt percent of assemblageA (plagioclase,augitc,biotite, magnetite,sphene, ndapatite)givesa good

0 7052

o 7048

07044

0 7040

i [ ] i

WASSUK DIORITEBASALT OF ARTESIA LAKEYERINGTON BATHOLI-HLATITE TUFF OF FULSTONE SPQTZ MONZODIORtTE PORPHYRYSHAMROCK BATHOLITH

07036 0 o

o

I I I I OIi I I I I OI2 IRb/SrFIG. 10. Plot of calculated nitial strontium sotoperatioversusRb/Sr weight ratio for Mesozoic gneous ocks rom theYeringtondistrict.Data are from Table 4.


17/40

1766 JOHNH. DILLESfit (sumof residuals quared,F(R) = 0.02; Table5), and ractionation f 37 wt percentof assemblageB (plagioclase, hornblende, biotite, magnetite,sphene,and apatite) n proportionssimilar o thoseof cumulategabbrosalso gives a good fit ((R) 2-- 0.04). Both A and B are likely fractionatingas-semblages ecause,asdiscussedn the next section,clinopyroxene rystallizedearly and was replacedby hornblendeat a higher water content during so-lidificationof the quartz monzodioritemagma.Bio-tite in the assemblagesequires hat they formed atless han 850C (seebelow). Fractionationof ei-ther assemblage ives easonable its for most raceelement (Rb, Sr, Mn, V, and Ni) variationsduringdifferentiation f known and estimatedcrystal-meltpartition coefficientsare employed (e.g., assem-blage A in Table 5). However, the model cannotaccount or the high Ba or the low Zr in differen-tiates, the latter which requires that Zr also frac-tionated (Table 5). Cumulate gabbroswithin theMcLeod Hill quartz monzodioriteare good candi-dates or accumulations f crystal ractionates hatmay have produceddifferentiation o quartz mon-zonite and granite.The best evidenceof suchcrys-tal accumulation s found in exposuresof gabbroassociatedwith quartz monzodiorite dikes, whichintrude metavolcanic ocks mmediatelyunderlying

the floor(?) of the porphyritic granite at Luhr Hill atan '8-km depth. However, the volume of gabbroexposed t a 2- to 8-km palcodepths far too small,e.g., 1 percentof the exposed olumesof quartzmonzoniteand granites Table 1), to represent hevolume of crystal fractionates required by themodel. Thus, I propose hat most of the fractiona-tion occurredbelow an 8-km palcodepth.The decreaseof Cu during differentiation from62 to 10 ppm cannot be simply accounted or bycrystal fractionation, and therefore, suggests hatCu partitioned into an aqueousphase or precipi-tated as a sulfide. These abundances and this behav-ior for Cu are characteristic f calc-alkalinemagmas(e.g, Wedgepohl,1969; Eilenbergand Carr, 1981).The Cu content of the Yeringtonbatholith s nearlyconstant rom 58 to 66 wt percent SiO but dropssharply rom 66 to 68 wt percentSiO (from quartzmonzonite to granite; Fig. 9). This behavior is in-consistentwith gradual emovalby fractionationofsilicates. istribution oefficientsKt)cu,,,)e-tween silicateor oxide crystals nd melt are not wellknown, ut a bulkOcuxls/melt)0.5 for the batholithcan be estimated Table 5). Magnetite and augitc,the only phasespresentknown to concentrateCu,have low partition coefficients Ewart et al., 1973;Irving and Frey, 1978). Ion microprobestudiesof

TABLE5. Crystal FractionationModelsParent Daughter Calculated ResidualElement avgQMD avggranite daughter observed-calculated Dcrystal.melt

SiO2 60.87 68.78 68.75 0.03A12Oa 16.64 15.47 15.44 0.03FeOa 5.88 2.73 2.71 0.02MgO 2.43 1.11 1.07 0.04CaO 5.28 3.03 3.04 -0.01Na20 4.36 4.25 4.28 -0.03K20 3.04 3.84 3.90 -0.06TiO 0.89 0.42 0.40 0.02PgOs 0.36 0.16 0.10 0.06MnO 0.07 0.03 0.05 -0.02HO' (3) (4-5) (4.5)Ba 1,500 1,400 800 600Rb 75 100 114 -14Sr 1,100 1,000 918 82Zr 150 130 256 -126V 130 60 65 -5Ni 20 11 16 -5Cu 62 10 75 -65

3.640.2

2.10.311.30.122.151.350.5

i HO is the estimatedwater contentof magmaDaughtercompositionsnd residuals re calculated y leastsquaresit by the computerprogramXLFRAC (Stormer nd Nichols,1978) givenparentcomposition f average uartzmonzodioritecolumn ) anddaughter omposition f average uhr Hill graniteandgraniteporphyrydikes column3). Oxidesare in wt percent,elements n ppm. Sum of squares f residualsor major and minorelements s 0.02. Daughter s calculated y removalof 39.1 percent crystals onsisting f 61.2 percent plagioclase, 6.7 percentaugitc, 11.9 percent biotite, 5.65 percent magnetite,2.6 percent lmenite, and 1.9 percent apatite. Major elementsand MnO weremodeledusingmicroprobe hasecompositionsherein and Dilles, 1984). Trace elementswere modeledusingbulk partitioncoeffi-cients Derystal/melt)alculatedndestimatedromempirical ataof Duncan ndTaylor 1968),Philpotts ndSchnetzler1970),Ewartet al. (1973), Irving (1978), Luhr and Carmichael 1980), and magnetiteand lmenite analyses erein


18/40

YERINGTON ATHOLITH,NV: PETROLOGY 1767minerals rombarrenandmineralizedplutonsshowthat ferromagnesianmineralsand magnetite havelow Cu contents '- 7 ppm) in westernNorth Amer-ica (Hendry et al., 1985) and slightly higher Cucontents (7-42 ppm) in the southwesternPacific(Hendry et al., 1981). Both results ndicate a bulkDcu(a,/lt)


19/40

1768 JOHNH. DILLESafter hornblende, and replaced early ilmenite bythe probablecoupledreaction:

5CaMgSi2Oa q- 3FeTiO3diopside(augite) ilmenite+ SiO2 + H20 + 02 = 3CaTiSiO5q-melt q- melt q- gas = sphene+ Ca2MgsSisO22(OH)2 Fe304.+ tremolite(hornblende)magnetite2)

Reaction 2) indicates hat the activitiesof H20 andO2 ncreased uringcrystallization ndcooling.Forsuch an andesitic melt with >3 wt percent H20,water saturationmusthave occurredduring crystal-lization because he solidquartz monzodioritecon-tains only 0.5 to 1.0 wt percent H20 in hydrousminerals and fluid inclusions Table 2). Abundantfluid inclusions,endoskarn alteration, and hydro-thermalbrecciationare possible videnceof fluidsof magmatic rigin (Dilles, 1984). The hornblende-plagioclase-magnetiteomb ayersat the wall-rockcontactof quartz monzodioritesuggest ndercool-ing and variablewater pressureduring crystalliza-tion (Lofgren and Donaldson,1975). Thus, >2 wtpercentH20 was ostbetweenwater saturation ndthe solidus,estimated at 700 to 725C at 2 kb(after Piwinskii, 1968; Naney, 1983).Bear quartz monzonite

The crystallization conditions of Bear quartzmonzonite,Luhr Hill granite, and granite porphyrydikescan be inferred by comparison f the crystal-lization ordersof their minerals Fig. 11) with the 2kb T-X(uzo) xperimental hase elationsof Naney(1977, 1983) on a syntheticgranodiorite Fig. 12),which has a similar composition 67.5 wt % SiO2).In the main interior quartz monzonite phaseof theBear ntrusion, he rarity of augite ascores)and theearly crystallizationof hornblendeand sphene e-quire that reaction (2) occurred early and that themagma ad nherentlyhigherH20 andO2 activitiesthan quartz monzodiorite. The crystallization ofhornblende before biotite requires >4 wt percentH20 (Fig. 12) and indicates hat the magmawas ator near water saturation at 825 to 875C and 2 kb.The rarity of biotite also suggestshat hornblendehad a significantlyhigher liquidus temperature.During cooling,the magma ntersected he watersaturationcurve prior to solidification nd lost 3wt percent H20 during cooling to its solidus at675C (Fig. 12).In the more silicic, graniticborder phaseof theBear intrusion,a similar crystallizationsequence sinferred, but biotite replaced much hornblende atnear-solidus temperatures (as discussedbelow).This effect is consistentwith Naney's data, whichshow that hornblende is unstablewith respect to

I000 -

900 -

T(C)800

700

\ , ..H?_OSaturation

,, PIn, Opx n _Bt In

,-i ....DSolidusAfI + Af+Qz+Bt +V'1 I I I I I I ] I I I00

0 2 4 6 8 I0 12wt. % H20FIG. 12. Temperature T) - X(H=O)hasediagram or syntheticgranodiorite sampleR5 + 10M1) at 9. kb from Naney (1983)."In" lines are liquidi of plagioclasePI), orthopyroxeneOpx),hornblende Hbl), quartz (Qz), biotite (Bt), and alkali feldspar(Af); "Out" linesare low-temperature tability imitsof mineral;"L" is silicate iquid; "V" is a separateH20-rich fluid phase,which exists o the right of and below the "H20-Saturation"curve. As discussedn the text, A-B-C-D is the inferred crystalli-zation path of the granite porphyry dike magma,which has acomposition imilar o syntheticgranodiorite.

biotite below 740 ___5C (Fig. 12). The reasonwhy hornblendes stable o the solidusn the quartzmonzonite (and not in the border granite) is notclear, but it couldbe due to the higher solidus em-perature, lower K20 content, or higher CaO con-tent of the quartzmonzonite.The graphic extureofborder granitesuggestshat it was moderatelyun-dercooled and water saturatedduring crystalliza-tion (after Fenn, 1977), which s consistent ith theestimateof >3 wt percent H20 from Naney's data(Fig. 12).Luhr Hill granite and granite porphyry dikes

The porphyriticgraniteof Luhr Hill and graniteporphyry dikes are cogeneticand show a similarcrystallization istory,exceptduring he late stages.Therefore, only crystallization f dikes s reviewedhere. Plagioclase, ugite(?),magnetite,and lmenitecrystallizedearliestand were followed shortlybythe assemblage lagioclase,hornblende, biotite,magnetite, and sphene (Fig. 11), which requiresconditions of 740 to 830C and >4 wt percent


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YERINGTONBATHOLITH,NV: PETROLOGY 1769H20 at 2 kb (Fig. 12, point A). The absencen crys-talline graniteof orthopyroxene,which is not stablebelow "--780C (Fig. 12), suggestshat the magmacompletely equilibrated at


21/40

1770 JOHN H. DILLES

Leuco-G reinire Aphte (4, 2)GP (?, 42)PG (9,92)BG (6,45)QM (2,33)QMD (24,240)GB (6,60Jof (i, 28)

OPTICAL COMPOSITIONSRIMSBODIES

..... CORES(5,52) (No oi:Scumpies,No of Points)

MICROPROBE ANALYSES AVE MOST No or Co

I) 2o 31o 410 .510 610 710 810Mole percent Anorthite

FIG. ] 3. Compositionsf igneous IagiocIase f the YeringtonbathoIith.Opticaldeterminationrom59 samples ere madeby the a-normalmethod,with an estimated rror of


22/40

YERINGTONATHOLITH,V:PETROLOGY 1771


23/40

1772 JOHNH. DILLEStionsmaybe written similar o reaction 3) aboveoras he followingreaction:FeTiO3 + CaA1Si2Os + K20ilmenite + anorthite(plagioclase) melt

5SIO2 + 1/aO2 CaTiSiO5melt + gas = sphene+ 2KA1Si3Os 1/3Fe304.+ K-feldsparmagnetite4)

As shownby reaction (4), the replacementof il-menire plus ealeie plagioelaseby spheneplus K-feldspar,as observedduring differentiation, s pro-moted by increasedactivitiesof K20, SiO2, and O2in the magmaor in subsolidusmagmatieaqueousfluids.Ilmenite and hematiteshow wo stages f exsolu-tion textures. First-stageexsolutionoccursonly inquartz monzodioriteand gabbro and is character-ized by broad lamellae (0.1 mm) of hematite inilmenite hosts and of ilmenite in hematite hosts.These textures probably formed during cooling ofmagmatie ilmenite below the ilmenite-hematitesolvus,which hasa maximumat "800C (Lindsley,1976). Further exsolution t a secondstage s char-acterized by


24/40


25/40

1774 JOHNH. DILLES

ooo

0o,1oo

oo

oo

oo

oo


26/40


lidification ccurred t 700C, slightly bove heilmenite-sphene uffer, '--2 to 3 log units of fobelow the magnetite-hematite uffer. The iron-tita-nium oxidephase elations nd compositionsn allunitsof the Yerington atholith ndicate hat theyhavereequilibrated t subsolidusemperaturest


27/40

1776 JOHN . DILLES

( 00000000 II

....... .

II


28/40

YERINGTONBATHOLITH, NV: PETROLOGY 1777

0 (LTS)

_ 2000

4000,_,+_, 6000

o 8000

o I0,000

2,ooo,+_

t- 14,ooo

A

I

BIOTITEi !///,

//!/

i z2

Ii

16,000 0 4

RockSymbol Type

0 GPo[] PG

QM, QMO' GB

160.6 , , i i i , 1TANNIT SIDEOPHYL/'I TR WQssukiorite

.0.$ -"/ J. ShamrockBathollth ....----"'] xPotassicB" lteration . YERINGONOC4THOTH

IZP, KOIOulQB P-3,FinnmarkQ02- I , I 0 02 0.4 0.6 O

AiTMFIG. 15. VAriAtions[ biotitecompositionsn the Yeringtonbtholith.AnAlysesereby electronmicroprobeTAble ; Dilles,1984). A. Ti contento[ biotiteplotted versus Jeodepth. helowerTerrify erosion ur[ce s estimatedo representl-mpleodepth. Ti content decreases ith depth in biotRes[resh gneousocks s well s in potssicllyltered nd low T(lte) ltered rocks.B. XF, (tomic Fe/Fe + g) is plottedversusocthedrl AluminumA1TM)or biotiteson the bsisnionicchargeo[ gg oxygen quivalents. ieldso[ biotitesgrAnodioriteo[ FinnmArA complex (P-3) (CzAmAnseAndWones,1973), the minerJizednmumuzonedplutonKoloul (Chivs, 1981), nd the pottssic,hydrothermal iotitezone "B" Jtertion)romAnn-son (DiJJes,984), re shown[or compAriso,.iotite ymbolsre: olid= Alteredoc, open= [rash oc,hl[-6lled= wely ltered oc or shreddy iotitetexture. See Figure 6 [or brevit[ons.

tion relations,microprobe ompositionsf biotite,magnetite, and K-feldspar (Tables 6 and 8; Dilles,1984), anda correction or the K-feldspar tructuralstate (after Helgeson et al., 1978). Substantialerrors are introduced into these calculationsbynonstoichiometry f the natural A1- and Ti-bearingbiotites elative o the synthetic nnite-phlogopiteof Wonesand Eugster 1965) and alsoby the addi-tionalstability f biotitecaused y octahedral e+3,as discussed y Wones (1981) (note that although10-30% of Fe in biotite probablyoccurs sFe+3,Table 8 reports ll Fe asFe+2because e+ wasnotdetermined).Amphiboleand pyroxene

Blue-green o light green calcicamphibole ormssubhedralprismsup to 5 mm, which have blue-green ('y), green (/), and pale yellow (a) pleochro-ism of variable intensities. Amphibole containsabundantmagnetite nclusions; ometimes ontainsrelic augitc cores,which it texturally replaces;andis intergrown with plagioclaseand sphene, whichindicates t began crystallizing elatively early (Fig.11). Rarely, in structurallydeep exposures, mphi-bole contains utile needlesand may be rimmed bysmallsphenegrains,which both formed by exsolu-tion.Amphibole ranges n composition rom magne-sio-hornblende to actinolitic hornblende, with onesample (gabbro) of magnesianhastingsitichorn-blende in the classification ystemof Leake (1978)(A1203 = 4.2-10.0 wt %, TiO = 0.4-1.7 wt %,Na=O = 0.7-1.7 wt %, and K=O = 0.3-1.1 wt %;Table 8). Tetrahedral A1and octahedralTi decreasefrom values characteristicof hornblende in quartzmonzodioriteand gabbro to values near the horn-blende-actinoliteboundary n graniteporphyry andporphyriticgranite (Fig. 16). The coupledsubstitu-tion of tetrahedral A1 and Ti occurs in a ratio of7Ah1Ti, compared o a ratio of 4Ah1Ti for amphi-bolesof the Finnmarka omplex,Norway Fig. 16B;Czamanskeand Wones, 1973). The transition romhornblende o actinolite s interpreted to reflect adecrease n temperature of last equilibration,withthe lower tetrahedralA1 and Ti valuesoverlappingwith the field of hydrothermal amphiboles Fig.16). Actinolite-bearingocksprobablyequilibratedat


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1778 JOHNH. DILLES

08

0.6Na+K+AI

4

0.2

] I IPargaste

0 Tremolite0 0.1

KEY0 GPD[] PG Yermgton0 QM /3G ' 'botholithA QMD GB Altered Shornrock batholifh) Wassu dorite

i iAMPHIBOLES

hosed on 2:30xyg.-Equiv.

Actinoli t e ,'

I I I0.2 O.:3 0.4Fe/(Fe+Mg)

2.0 i i I I

I.O

0.5 -B Ol f j

HornblendeActinol/te

I0 0 05 0 IO O.15 0.20Ti

FIG. 16. Compositionsf amphibolesn the Yeringtonbatho-lith by electronmicroprobeanalysis Table 7; Dilles, 1984). A.Plot of atomic (Na + K + A1)/4 versusFe/(Fe + Mg), showingstrongcorrelation.B. Plot of tetrahedralA1 versusTi. Note thestrong oupled ubstitutionf A1TMor Ti in a ratioof "'7:1 aswellas the compositional ange of igneousamphibole rom horn-blende to actinolite.For comparison, mphibolesrom the Finn-markacomplex,Norway, are alsoplotted as + and X symbols(Czamanske nd Wones, 1973). See Figure 6 for abbreviations.

In singlegrains, here is no obviouscore-edgede-crease in Fe/(Fe + Mg), as reported by Mason(1978). In one hydrothermally altered sample, acore of actinolitic hornblende is replaced bypatchesof actinolite, similar to those reported byCzamanskeand Wones (1973) for Finnmarka and

by Chivas (1981) for the igneouscomplexat theKoloulaporphyry copper system.Very pale green augitc occursas phenocrystsnthe basalt of Artesia Lake and as sparsecrystalsrimmed by or largely replaced by amphibole nquartz monzodiorite.Augites n quartz monzodior-itc and basalt have similar Fe/(Fe + Mg) values(0.21 and 0.24, respectively),whereashornblendeenclosing he analyzed augitc in quartz monzo-diorite has a value of 0.30 (Table 7). A calcu-lated partition coefficient from these values(ln{Kp(ug/ve)(d.oy.....ho.Ue.a))0.48) is similaro anexperimentaletermined/n{K))0.4 or ona-litc at 800C and 15 kb (HuangandWyllie, 1986),suggestingquilibriumMg/Fe partitioning.Halogencontentof hydrousminerals

The fluorine and chlorine contents of biotite,hornblende, nd apatitevary systematically ithinthe Yeringtonbatholith. n general,F in thesemin-erals s high relative to the Jurassic hamrock ath-olith and increases with the silica content of theYerington batholith, whereas C1 is low relative tothe Shamrock batholith and decreases with the sil-ica content. gneousapatite variesmostconsistentlyand contains a 2.37 to 3.74 wt percent F (i.e.,60-100 mole % fluorapatite)and 0.03 to 0.35 wtpercent CI (1-7 mole % chlorapatite) n the Yer-ingtonbatholith Table 9, Fig. 17A). C1andF con-tentsof apatite n gabbroand quartz monzodioriteoverlap with halogen contents of apatite in theShamrock batholith and the basalt of Artesia Lake,whereas he lower C1 and higher F contentsof apa-tire in the granite series, he Bear quartz monzonite,and border granite overlap with the halogen con-tents of apatite n potassically ltered and mineral-ized rocks at Ann-Mason (Fig. 17A). Both biotiteand amphibolevary similarlybut have lower halo-gen contentsand more restricted ranges.Biotitecontains0.37 to 1.73 wt percent F (5-20 mole % Fbiotite) and 0.04 to 0.27 wt percent C1 0-1 mole %CI biotite) (Table 8, Fig. 17B). Amphibolescontain0.18 to 0.74 wt percent F (4-17 mole % F amphi-bole) and 0.02 to 0.11 wt percent C1 (0-1 mole %C1 amphibole) Table 7, Fig. 17C).The F, CI, and OH contents of biotite, horn-blende, and apatite appear o representa mixture ofmagmatic and lower temperature compositions.They fall within the compositionalangeof mineralsfrom mineralized and unmineralized, intermediatecomposition plutons from the Basin and Rangeprovince ParryandJacobs, 975; Jacobs ndParry,1976, 1979). Although he KD(avatit/mdt)or F is notknown, Stormerand Carmichael 1971) have notedthat apatite in volcanic rocks commonly containshigh atomicF/(F + OH) ('--0.9) and therefore thatKD s probably arge. However, Stormerand Carmi-


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YERINGTON BATHOLITH, NV: PETROLOGYTABLE . Fluorine and Chlorine Contentsof Apatite

1779

F CPRockype Sampleo. (wt%) (wt%) F/(F+ OH) CI/F3Artesiabasalt Y-797 2.93 0.39 0.84 0.071Gabbro Y-751 2.37 0.35 0.68 0.079Gabbro Y-751 2.77 0.34 0.79 0.066Quartzmonzodiorite Y-651 3.05 0.22 0.90 0.039Quartzmonzodiorite Y-754 3.45 0.08 0.97 0.012Quartzmonzodiorite Y-767 3.39 0.14 0.94 0.022Quartzmonzodiorite Y-789 3.16 0.34 0.90 0.020Quartzmonzonite Y-800 3.74 0.05 0.98 0.007Quartzmonzonite Y-800 3.40 0.05 0.93 0.008Quartzmonzonite Y-800 3.39 0.05 0.92 0.007Border ranite Y-787 3.49 0.06 0.95 0.009Porphyriticranite Y-753 3.58 0.03 0.99 0.004Granite orphyry Y-781 3.69 0.09 0.97 0.013Shamrock atholith Y-391 2.93 0.45 0.85 0.082Shamrock atholith Y-828 2.87 0.30 0.83 0.056PotassiclterationQMD) D-222-917 3.36 0.03 0.92 0.005PotassiclterationQMD) D-222-917 3.16 0.08 0.89 0.014

Partialmicroprobenalysiswithout .zOs),ecalculatedo 10 (Ca+ Mn + Fe);standardrror_+ .05 wtMolecular raction,with OH calculated y difference: + C1+ OH = 2.0Molecular ratio percent or CI andF

chael (1971) found that biotite in volcanic rockscontainsa lower F/(F + OH) ratio than apatite andis more susceptible o low-temperature F-C1-OHexchange. Yerington apatite has atomic (F/(F+ OH) = 0.9 (Table 9), similar o volcanic ocks,and therefore probably preserves magmatic com-positions, whereas Yerington biotite has a lowerF/(F + OH) than either apatite or volcanicbiotiteand therefore probably reequilibrated at submag-matic temperatures. At magmatic temperatures, C1strongly partitions into aqueous fluids relative tograniticmagma DClmcll,l/mclm,t)30-40; KilincandBurnham, 1972), whereas F tends to partitionslightly nto magma (Burnham, 1979). Therefore,the observed rend of decreasingC1/F in apatitewith differentiation can be ascribed to a trend ofevolution in increasing amounts of magmaticaqueous luids during crystallization,as suggestedby Candela (1983). However, the similar trend ofC1/F in both biotite and hornblende probably re-flects subsoliduspartitioning between aqueousfluids and minerals. Thus, both the inferred mag-matic and subsolidus trends are consistent withevolutionof greater amounts f aqueousluids andpossibly ower temperatures) n the most differen-tiated magmasof the porphyritic granite series.

Discussion and ConclusionsThe dataand arguments resentedabovestronglysupportan orthomagmaticmodel n which Cu-richaqueous luidswere derived from crystallization fthe Yerington batholith magma and causedpor-phyry copper mineralization.This model is devel-

opedbelowby including dditional onstraints ro-videdby other studies f the Yeringtondistrict,andthen it is comparedwith previous models (e.g.,Gustafsonand Hunt, 1975; Burnham, 1979).One of the central questions f the orthomagma-tic model s whether the mineralizingmagmas aveany peculiarmodesof origin or whether hey are"normal" magmas.The Yerington magmaswerehigh K andesites nd dacites hat fall within thecompositionalangeof normalcalc-alkaline agmasgeneratedn continental rc settings, sdefinedbyEwart (1979) and Gill (1981). Gustafson 1979),Burnham 1979), and Cornwall 1982) all reachedsimilarconclusionsegardingmineralizing,calc-al-kalinemagmas. he origin of this type of magma sthe fundamental uestion f andesitc enesis nd sbeyond he scopeof this paper. However, the iso-topicdata 878r/86Srinitial0.7040, So - 6.8 %0)and compositionalata for the Yeringtonbatholithare consistentwith a model ndicating hat high Kandesitccompositions, haracteristicof the earlyMcLeod Hill quartz monzodiorite intrusion, werederived (after DePaolo, 1981) by fractionalcrystal-lization of a basalticparent combinedwith lesserdegrees f assimilationf crust,whichwaspossiblycomposed rimarilyof Triassicgneous rc rocks.Owing to Tertiary normal faulting and conse-quent90 W tilting, the Yeringtonbatholith s nowexposedn structural rosssectionat palcodepthsranging rom 0 to 8 km. Within theseexposuresthe McLeod Hill quartz monzodiorite, Bear quartzmonzonite, and the Luhr Hill granite (and asso-ciated granite porphyry dikes) form successiven-


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1780 .JOHN . DILLESo t APATITE

oo6

OO4

0 I , I I i I I I i II0 i I i8 2o .F om

- ' I / AMPHIBOLES010 BIOTITE 0.0 bed 23Oxyge.eqmv

o I o, o o o.2 oL [ . . / F toms'F ' t

0 02 04 06 08 0F otoms

FIc. 1?. F andCI contentsapatiteA),biotite B),and mphboleC) n heYerin$tonatholith,showngncreasin$anddeereasn$I durin$ erentationn n succeedinsubsolidusotassicydrothermaliodteB)alteradon.nalyseseremade yelectron croproBeTables , 8, and9;Dfiles, 1984). SeeFisure 6 Forarevatons.

trusionshatare n turnvolumetricallymallerap-proximately 5, 19, and 6 vol %, respectively),more deeplyemplaeed topsat


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YERINGTON ATHOLITH, V: PETROLOGY 1781tallization have been derived from phasepetrologyand mineral compositions. y inference, he origi-nal quartz monzodiorite magmas containing au-gitc) were emplaced t 850 to 1,050C at ---2 to 3wt percent H20. However, the presenceof horn-blende and biotite during much of the crystalliza-tion sequence f quartzmonzodiorite ndall of thesequencen quartz monzoniteand granite (Fig. 11)indicates hat these magmas ontained>3 wt per-cent H20, which led to water saturation duringcrystallizationBurnham, 979; Nancy, 1983). Thegranitemagmas ontained --4 to 5 wt percentH20(Fig. 12), which is compatiblewith derivationbyfractional crystallization of quartz monzodioritecontaining 3 wt percent H20. Increasing watercontent during differentiation indicates earlierwater saturation n successive ifferentiates.Crys-tallization emperatures re estimatedat ---675 to700C and 750C at 2 and 1 kb ('--7- and 3.5-kmpalcodepth), espectively Burnham,1979; Nancy,1983). Fe-Ti oxidesand the phaseassemblage fmafic minerals ndicate that fo2 of quartz monzo-diorite was initially high (near an ilmenite-sphenefo2buffer, ---2-3 log unitsbelow the hematite-mag-netite buffer at 800C), similar to many relativelyoxidized andesitc magmas.The decreasedabun-dance of ilmenite and the decreasedFe/(Fe + Mg)of Ca amphibole from '--0.4 to '--0.3 indicate aslight increaseof fo during differentiation.How-ever, FeoTi oxide compositionsndicate hat duringcrystallization nd subsolidusoolingof each ntru-sive, o increased trikinglyby "- 2 to 3 log units oas high as the hematite-magnetite buffer at---500C (Fig. 14). These highly oxidized condi-tions are alsoreflected in the low Fe/(Fe + Mg) ofall mafic silicates, ncludingbiotite (0.34-0.41), Caamphibole (0.26-0.40), and augitc (0.20-0.24).Similar data for the Finnmarka complex, Norway,have been interpreted by Czamanskeand Wones(1973) as indicating magmatic oxidation. Otherstudies have concluded that porphyry coppermagmasare inherently more oxidized than mostcalc-alkalinemagmas e.g., Moore and Czamanske,1973; Mason, 1978; Chivas, 1981; and as discussedby Wones, 1981). The Yerington data, however,suggesthat porphyrymagmasnitially had high ox-idationstates imilar o the states f manycalc-alka-line magmas,and that very high oxidation stateswere only achievedduring crystallization nd sub-solidus ooling.Thus, the rapid oxidation s spatiallyand temporally inked closelywith the exsolutionofmagmatic queousluidsdescribed bove.This pos-sibility s also supportedby the low C1/F ratio ofapatite, which decreases rom 0.08 to 0.01 duringdifferentiationand which suggestsncreasing arti-tioningof CI into an aqueous hase Holland,1972;Candela, 1983).

Although he causeof magmatic xidations notknown, the close association f oxidationwith theevolutionof magmatic queousluidssuggestsge-netic link. Regardless f the meansby which itoccurs,oxidationof the magma equires hat themagma ehaves sanopensystem nd oses r gainscomponentso change he oxidation tate.One pos-sibility s that H2 gasescapedrom the magma sproposed y Henley and McNabb 1978) andEas-toe (1982). The highdiffusivity f H2 (103 imesH20; Burnham,1979) and the high vapor pressureof H2 gas '--1 bar at magmaticfo-T;Eastoe,1982)allow it to escape rom the magma,which causeswater n magma o dissociatey the reaction:H20 = H2 + 1/202. (7)

The O2 gas rom thisreactionwouldcausemagma-tic oxidation Burnham,1979; Eastoe, 1982).A secondmeansof magmatic xidations by sepa-ration of sulfur rom the magma.When water satu-ration occursduring magmatic rystallization, ul-fur partitionsstrongly nto the aqueousluid phase(KD(s.u,dSm,,,)4-40 at P(,o)= 2-0.5 kb;Burnham,1979). Sulfuroccurs n the magmaprimarily asHS-(Burnham,1979), but in the aqueous luid SO2 spredominant50-90%) overH2Sat magmatic -fo2conditions Ohmotoand Rye, 1979). During cool-ingof the aqueousluidalonganfobuffer, he fluidmust cross the SO2-H2S boundary and the ratioSO2/H2SmustdecreaseFig. 14) by a reaction uchas:

SO2(a,) H20(a,) H2S(,) 3/202(g). (8)The O2 from reaction 7) would oxidize he magmaor crystallinelutonby convertinge+2 to Fe+3,whereas he H2S could separate rom the magma(pluton)by flow of the aqueousluid or could eactwith metals in the fluid to precipitate sulfides.These processeswould drive reaction (8) to theright.By mass alance,emoval f reduced ulfurorsulfidesn the aqueous luid would causeoxidationof the magmatic erromagnesianilicateand Fe-Tioxide assemblage. yrite would be the stable Fesulfide t


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1782 JOHNH. DILLESmole fractionof albite in alkali feldspar;all theseeffects ncreasewith both depth of emplacementand differentiation f the batholith. n igneous m-phibole, he correlation f decreasinge/(Fe + Mg)withdecreasing1TMndTi and he overlap f com-positions ith the field of hydrothermal mphiboles(Fig. 16) suggesthat low Fe/(Fe + Mg) couldbepartly due to subsolidus queous lteration,a con-clusionalso eachedby Chivas 1981) for the Ko-loula porphyry copper complex. However, Cza-manske ndWones 1973) andMason 1978) haveinterpreted his trend to be a magmatic ffect.An orthomagmaticmodel (after Burnham, 1967,1979), in whichCu was ntroduced uringemplace-ment of granite porphyry dikes, is proposed orporphyry copper mineralization n the Yeringtondistrict Fig. 18). At the Yeringtonmine porphyrydeposit,Cu introductions directly associated iththe emplacementof individual granite porphyrydikes that were successivelyntruded (Proffett,1979; Carten, 1986; M. T. Einaudi et al., in prep.).A similar relation is inferred for the Ann-Mason de-

posit (K. L. Howard, pers. commun., 1980; Dilles,1982, 1984). These authorsdescribed he hydro-thermal alterationeffects hat overlapand postdatemagmatism.The details of the orthomagmaticmodel at themagmaticstageare as follows.Field relations ndi-cate that graniteporphyry dikesare mineralizedat'-'2- to 4-km paleDdepthmmediately bovewherethey intruded (upward) through the apicesof cu-polasof the Luhr Hill granite (Fig. 2). The dikesgradedownward nto granite,and assuminghat thedikes were emplaced upward or laterally, theirsourcewas he graniteat 3- to 6-km depth (Fig. 3).A graphic-textured order on the apexof the gran-ite cupola Fig. 2) and abundant plite andpegma-tite dikes in granite indicate water saturationoc-curred hroughout he Luhr Hill graniteas t crys-tallized. The Luhr Hill granite crystallizeddownwardand inward at Ann-Mason Fig. 2), at-tendedby successivepwardemplacement f gran-ite porphyry dikes (Fig. 18). These dikes contain'-'50 percent phenocrysts et n an aplitic-textured,

A EARLYPG CRYSTALLIZATIONA /X A A

/x COGENETIC VOLCANICS AA A A

QMD G APHiC_TXTI+ QMPGBORDER BGCRYSTALLINE,CARAPACE of PG

QMD GB

/,,"/

MAGMA fl- HIGH-DENSITYH20-SALT FLUIDINWARDCRYSTALLIZATI

PG Magma(+Xls)-"" -""UPWARD'" ISOTHERMALQM:'.,,\IFAGMARYSTALLIZAT',

S B EARLYOLATE G .EXTRUSIONS

SUCCESSIVE FRACTURINGGP DIKE INTRUSION8 UPWARD FLOW OFHIGH-DENSITYORE FLUIDS

NON-MAGMATICH2O-SALT FLUIO

GPD

PG

POTASSlCLTER'N Cu MINERALIZ'N

MAGMA HIGH-DENSITYH20- SALT-CuFLUID

LAST PG MAGMA

FIG. 18. Orthomagmatic odel or generation f porphyry opperore fluidsduringcrystallizationof the graniteseriesmagmas, sdiscussedn text.A. Very earlyduringcrystallization,eforeemplace-ment of graniteporphyrydikes.B. Early to late during crystallization. ranite porphyrydikesandmagmatic ydrothermalluidsare emplaced pward n successiveulses, ausingmultiplemineraliza-tion events.Hydrothermal lterationeventsabovegraphitecupolasummarizedrom Dilles (1984),Carten (1986), and M. T. Einaudi et al. (in prep.). See Figures3 and 6 for abbreviations.

KM.8


34/40


granitic groundmass. he phenocrystassemblagerequires hat the graniteporphyrydikeswere watersaturated nd near their solidus emperaturepriorto emplacementafter Naney, 1983), and he apliticgroundmassndicates pressurequenching (Jahnsand Burnham, 1969; Fenn, 1977), which in turnimplies apid upwardemplacement ndwater exso-lution. Granite porphyry dikes are sparseor absentin deep exposures>6 km) of Luhr Hill granite,anddikes are not associated with the older, more shal-lowly emplaced Bear quartz monzonite (Fig. 3).These actssuggesthat the 3- to 6-km-depthnter-val may be a controlling actor n the generationofgraniteporphyry dikes,as proposed y Burnham(1979).In the modelpresented ere (Fig. 18), a separateaqueousphase coexistedwith the crystallizinggranite and flowed upward along the hot, highlypermeable raniteporphyrydikesshortly fter heywere emplaced (Dilles, 1984). As proposedbyBurnham (1979), "second boiling" of aqueousfluidsseparatingrom graniteat thesepaleodepthswould createPAV energy hat couldproduce luidoverpressures>P lithostatic),which would in turnfractureoverlying ockandallowupwardemplace-mentof dikesandupward low of aqueousluids.A slight,but important,variationadvocated ereis that aqueousluidsseparatingrom the granitemagma at 3- to 6-km depth (0.9-1.8 kb), wouldfurther separate nto a low-densityaqueous luidcoexisting ith a saline,high-density queousluid.This possibility, iscussedut dismissed y Burn-ham (1979), has been proposedby Bodnar et al.(1985) on the basisof recent experimentalstudieswhich ndicate hat the two-phaseield of the sys-tem NaC1-H20 extends o >1.5 kb and >1,000C(Fig. 19). This hasalsobeen documented y coex-istinghigh-salinityluid, ow-salinityluid,andglassinclusionsn quartzphenocrystsrom the Pangunaporphyry copper deposit Eastoeand Eadington,1986). For example,a reasonable arent magma,containing 0.1 wt percent C1, upon cooling to750Cat 1.1 kb pressure ould ose -3 wt percentH20 (due to water saturation), nto which 75 wtpercentof C1would partition KilincandBurnham,1972). The resultant aqueous luid would furthersplit nto two phases:--98 wt percent ow-salinityfluidcontaining wt percentsalts nd ---2 wt per-cent high-salinity luid containing51 wt percentsalts Fig. 19). Fluid inclusions ith high salinity(40- >60 wt % salts) ndhigh homogenizationem-peratures 400-800C) are common n the por-phyry environment Roedder,1971, 1984) andcommonlycoexistwith a low-salinity,vapor-richfluid (Bodnar,1982). In the Yeringtonbatholith,igneousquartz commonlycontains bundanthigh-salinity luid nclusionscontaining alite +_sylvite

2

(kb)I

o o

RANODIORITH20SATURATbSOglDUS--ex.\'\ / Of ORIGINONEH,O-NaC:I/ -v* oc,s-600 700 800 900C

FIC. 19. P-T projectionof granodiorite nd NaC1-H20phaserelations. The water-saturated granodiorite solidus and thewater-saturatediquidus or biotite (BIOT-L) are modified romBurnham 1979) andNaney (1983). NaC1-H20relationsare fromSourirajanand Kennedy 1962) and Bodnaret al. (1985). In theregion of two NaC1-HeO luids, separatehigh- and low-salinityfluid phases oexistwhere the bulk compositions appropriate(e.g., 2-60 wt % NaC1at 800C and 1 kb). Dashed inesshowwtpercent NaC1 of the high-salinity luid. Stippling ndicates hepreferred area for a magmaticorigin of high-salinity luids romporphyrycoppermagmas, uchas he Yeringtonbatholithgranitemagmacontaining>4 wt percent HO.

+_hematite _ other salts).Hydrothermallyalteredrock at the Ann-Mason porphyry copper depositcontainsabundant halite-bearing inclusions,someof which have up to 62 wt percent saltsand homo-genizeat 500 to 550C (Dilles, 1984). Low-den-sity aqueous luids, possibly ncluding H, wereprobably ost during the rise of a "vapor plume"(Henley and McNabb, 1978), whereas igh-densityfluids emainedwith the magma, o be releasedup-ward only upon emplacement of porphyry dikes(Fig. 18). Na, K, Ca, Fe, Cu, and S would havestronglypartitioned into the Cl-rich, high-density,aqueous luid relative to either the magma or thelow-density queousluid (Holland, 1972; Candelaand Holland, 1984, 1986). Candela and Holland(1986) calculate hat up to 80 wt percent of Cucouldbe partitioned nto Cl-rich aqueousluid fromthe magma.The Cu versusSiO trend during dif-ferentiationof the Yeringtonbatholith (Fig. 9) sug-gests hat --80 wt percent of the Cu or '--50 ppmCu was extracted uniformly from the entire LuhrHill granite rom 3- to 8-km paleodepth.Extractionof 50 ppm Cu from a minimumestimated olumeof65 km3 of Luhr Hill granite Table 1) wouldyield10 million tons of Cu, more than sufficient toproduce he '-6 million tonsof Cu depositedn theYeringtondistrict (Einaudi, 1982). The concentra-tion of saltsand Cu into high-densityaqueousorefluids containingonly 3 wt percent of the wateroriginally n the granite magma educes he prob-lem inherent n previousorthomagmaticmodelsoftransportingarge volumesof dilute aqueous luids


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1784 JOHNH. DILLESfrom the crystallizingmagmaupward hrough hesmallcross ectional rea of the stockapex.The proposed orthomagmatic model can intheory provide all the Cu deposited. However,whether it is practicable o uniformly remove 50ppm Cu from >65 km3 of magma s debatable.Asshownabove, the mineralizinggranite porphyrydikesand therefore ore fluidsappear o have theirsources t '--3- to 6-km depth in the upper part ofthe Luhr Hill granite. However, Cu must also beextractedrom deeperparts 6-8 km) of the granitemagmaby evolutionand upward migrationof anaqueous luid to 6-km depthwouldhavebeen nitiallywaterun-dersaturated ue to the increase f water solubilitywith pressure.The extractionof 3 wt percentwaterduring saturationwould require that the magmacool o '--675 to 700C (Figs.12 and 19). Thus, fwater were extracted rom the lower portionsof theLuhr Hill granitewhile graniteporphyrydikeswerebeing emplaced rom the upper portions, hen themagmachambermusthave been nearly thermallyunzoned'over "-2-km vertical nterval Fig. 18).This modeldiffers rom the strong hermalzonationin Burnham'smodel (1979, fig. 3.5A, B, C). Therelativelyuniformphenocryst ssemblagef graniteporphyrydikesand lack of significantmineralogicor compositional oning in the dikes or the LuhrHill granite also suggest hat the magmas ackedmajor thermal gradients suchas are implied formany other shallowsilicic magmachambers; .g.,Smith,1979). Conversely,f graniteporphyrydikestappedonly a narrowvertical nterval at the top ofthe magmachamber, he small observedvariationsof the phenocryst o groundmassatio could indi-cate slight thermal gradients. nternal, thermallydriven convection Shaw, 1965) could have keptthe magmawell mixed and nearly isothermal.Anearly sothermalmagmawould require a minimumof heat input at its base,consistentwith the absenceof youngermaficdikes n the Luhr Hill granite,ex-cept for rare quartz monzodioriteporphyry dikesthat are "-3 m.y. younger (Dilles and Wright, inpress).The model proposedhere is that the LuhrHill granite crystallized nward under low thermalgradientsrom ts top, sides, ndbottom ?)and hatthe aqueous luids evolved during crystallizationand water saturationmigrated o the upper part ofthe magma hamberat 3- to 6-km depth.

AcknowledgmentsThis paper representspart of a Ph.D. dissertationcompletedat StanfordUniversityunder the direc-tion of M. T. Einaudi. Supportwasprovidedpartlyby a National ScienceFoundation graduate fellow-

ship, he AnacondaCompany, he Harvey S. MuddFund of the Departmentof AppliedEarth Sciences,and GeologicalSocietyof AmericaPenrosegrants.Critical reviews by and discussions ith Einaudi,A. L. Grunder, G. A. Mahood, and E. Seedorff havesubstantiallymproved his manuscript. he pre-vious work and ideas of J. M. Proffett, M. T. Ein-audi, R. B. Carten, K. L. Howard, Jr., and otherAnacondaCompanygeologists rovided he frame-work for this study. M. L. Rivers assistedwith themicroprobe nalyses. eviewsof the manuscript ytwo EconomicGeology efereeswere helpful.August 19; 1986; March 25, 1987

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17 8 6 JOHNH. DILLESHaggerty, S. E., 1976, Opaque

petrology of the yerington batholith nevada - evidence for evolution of porphyry copper ore fluids

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