porphyry copper mineralization immediately above the apices of granite cupolas in the batholith. .... batholith and ore deposits in cross section and facili-.
EconomicGeology Vol. 82, 1987, pp. 1750-1789
Petrologyof the YeringtonBatholith,Nevada:Evidencefor Evolutionof Porphyry Copper Ore Fluids JOHNH. DILLES* Departmentof Applied Earth Sciences, StanfordUniversity,Stanford,California 94305 Abstract
The JurassicYerington batholith, western Nevada, is a compositepluton that contains severalcentersof porphyrycoppermineralizationandis exposedin structuralcrosssection at paleodepthsrangingfrom 0 to 8 km. Within these exposuresthe McLeod Hill quartz monzodiorite,Bear quartz monzonite,andLuhr Hill graniteform successive intrusionsthat are in turn volumetricallysmaller (•75, 19, and 6 vol %, respectively),more deeply emplaced(topsat 2 wt percentH20 waslostbetweenwater saturationand the solidus,estimated at •700 ø to 725øC at 2 kb (after Piwinskii, 1968; Naney, 1983). Bear quartz monzonite
The crystallization conditions of Bear quartz monzonite,Luhr Hill granite, and granite porphyry dikescan be inferred by comparisonof the crystallization ordersof their minerals(Fig. 11) with the 2
, •
•,
•..H?_O - Saturation
•,,• PIIn OpxIn Bt
T(øC)
_
In
800
,-
i
....
700
•DSolidus XAf PI + Af+Qz+Bt
600
+V
•'1 I I I I I I ] I I I 0
2
4
6
8
I0
12
wt. % H20 FIG. 12. Temperature(T) - X(H=O) phasediagramfor synthetic granodiorite(sampleR5 + 10M1) at 9. kb from Naney (1983). "In" lines are liquidi of plagioclase(PI), orthopyroxene(Opx), hornblende(Hbl), quartz (Qz), biotite (Bt), and alkali feldspar (Af); "Out" linesare low-temperaturestabilitylimitsof mineral; "L" is silicateliquid; "V" is a separateH20-rich fluid phase, which existsto the right of and below the "H20-Saturation" curve.As discussed in the text, A-B-C-D is the inferred crystallization path of the granite porphyry dike magma,which has a compositionsimilarto syntheticgranodiorite.
kb T-X(uzo)experimentalphaserelationsof Naney (1977, 1983) on a syntheticgranodiorite(Fig. 12), which has a similar composition(67.5 wt % SiO2). In the main interior quartz monzonitephaseof the Bearintrusion,the rarity of augite(ascores)andthe early crystallizationof hornblendeand sphenerequire that reaction (2) occurredearly and that the magmahadinherentlyhigherH20 andO2 activities than quartz monzodiorite. The crystallization of hornblende before biotite requires >4 wt percent H20 (Fig. 12) and indicatesthat the magmawas at
biotite below •740 ø ___ 15øC (Fig. 12). The reason why hornblendeis stableto the solidusin the quartz monzonite (and not in the border granite) is not clear, but it couldbe due to the higher solidustemperature, lower K20 content, or higher CaO content of the quartzmonzonite.The graphictextureof
border granitesuggests that it wasmoderatelyundercooled and water saturatedduring crystallization (after Fenn, 1977), whichis consistentwith the or near water saturation at 825 ø to 875øC and 2 kb. estimateof >3 wt percent H20 from Naney's data The rarity of biotite also suggeststhat hornblende (Fig. 12). had a significantlyhigher liquidus temperature. During cooling,the magmaintersectedthe water Luhr Hill granite and granite porphyry dikes The porphyriticgraniteof Luhr Hill and granite saturationcurve prior to solidificationand lost •3 wt percent H20 during cooling to its solidus at porphyry dikes are cogeneticand show a similar crystallizationhistory,exceptduringthe late stages. •675øC (Fig. 12). In the more silicic,graniticborder phaseof the Therefore, only crystallizationof dikesis reviewed augite(?),magnetite,andilmenite Bear intrusion,a similarcrystallizationsequenceis here. Plagioclase, inferred, but biotite replaced much hornblende at crystallizedearliestand were followed shortlyby near-solidus temperatures (as discussedbelow). the assemblageplagioclase,hornblende,biotite, This effect is consistentwith Naney's data, which magnetite, and sphene (Fig. 11), which requires show that hornblende is unstablewith respect to conditionsof 740 ø to 830øC and >4 wt percent
YERINGTONBATHOLITH,NV: PETROLOGY
H20 at 2 kb (Fig. 12, point A). The absencein crystalline graniteof orthopyroxene,which is not stable below "--780øC (Fig. 12), suggeststhat the magma completely equilibrated at 6 km) of Luhr Hill granite,and
1783
•RANODIORIT
H20 -SATURATb •
SOglDUS--ex.\
2
'\
/ Of ORIGIN
ONE H•,O-NaC:I (kb) I
•/•
dikes are not associated with the older, more shalo
••
-v* •oc,s,•-•
o
lowly emplaced Bear quartz monzonite (Fig. 3). 600 700 800 900 •øC Thesefactssuggest that the 3- to 6-km-depthinterval may be a controllingfactorin the generationof FIC. 19. P-T projectionof granodioriteand NaC1-H20phase graniteporphyrydikes,as proposedby Burnham relations. The water-saturated granodiorite solidus and the (1979). water-saturatedliquidusfor biotite (BIOT-L) are modifiedfrom In the modelpresentedhere (Fig. 18), a separate Burnham(1979) andNaney (1983). NaC1-H20relationsare from Sourirajanand Kennedy(1962) and Bodnaret al. (1985). In the aqueousphase coexistedwith the crystallizing region of two NaC1-HeOfluids, separatehigh- and low-salinity granite and flowed upward alongthe hot, highly fluid phasescoexistwhere the bulk compositionis appropriate permeablegraniteporphyrydikesshortlyafterthey (e.g., 2-60 wt % NaC1at 800øC and 1 kb). Dashedlinesshowwt were emplaced (Dilles, 1984). As proposedby percent NaC1 of the high-salinityfluid. Stippling indicatesthe Burnham (1979), "second boiling" of aqueous preferred area for a magmaticorigin of high-salinityfluidsfrom coppermagmas,suchasthe Yeringtonbatholithgranite fluidsseparatingfrom graniteat thesepaleodepths porphyry magmacontaining>4 wt percent H•O. would createPAV energythat couldproducefluid overpressures (>P lithostatic),which would in turn
fractureoverlyingrockandallowupwardemplace+_hematite _ other salts).Hydrothermallyaltered mentof dikesandupwardflow of aqueousfluids. A slight,but important,variationadvocatedhere
rock at the Ann-Mason porphyry copper deposit
is that aqueousfluidsseparatingfrom the granite containsabundant halite-bearing inclusions,some magmaat 3- to 6-km depth (0.9-1.8 kb), would further separateinto a low-densityaqueousfluid coexistingwith a saline,high-densityaqueousfluid. This possibility,discussed but dismissed by Burnham (1979), has been proposedby Bodnar et al. (1985) on the basisof recent experimentalstudies
whichindicatethat the two-phase field of the system NaC1-H20 extendsto >1.5 kb and >1,000øC
(Fig. 19). This hasalsobeen documentedby coexistinghigh-salinity fluid,low-salinityfluid,andglass inclusions in quartzphenocrysts from the Panguna porphyry copper deposit(Eastoeand Eadington, 1986). For example,a reasonableparent magma, containing 0.1 wt percent C1, upon cooling to 750øCat 1.1 kb pressurewouldlose'-•3 wt percent H20 (due to water saturation),into which 75 wt percentof C1wouldpartition(KilincandBurnham, 1972). The resultant aqueousfluid would further
splitinto two phases:'--98 wt percentlow-salinity fluidcontaining 4 wt percentsaltsand ---2 wt percent high-salinityfluid containing51 wt percent salts(Fig. 19). Fluid inclusionswith high salinity (40- >60 wt % salts)andhighhomogenization temperatures(400ø-800øC) are commonin the porphyry environment(Roedder,1971, 1984) and commonlycoexistwith a low-salinity,vapor-rich fluid (Bodnar,1982). In the Yeringtonbatholith,
of which have up to 62 wt percent saltsand homogenizeat 500ø to 550øC (Dilles, 1984). Low-density aqueousfluids, possibly including H•, were probablylost during the rise of a "vapor plume" (HenleyandMcNabb, 1978), whereashigh-density fluidsremainedwith the magma,to be releasedupward only upon emplacementof porphyry dikes (Fig. 18). Na, K, Ca, Fe, Cu, and S would have stronglypartitioned into the Cl-rich, high-density, aqueousfluid relative to either the magma or the low-densityaqueousfluid (Holland, 1972; Candela and Holland, 1984, 1986). Candela and Holland (1986) calculatethat up to 80 wt percent of Cu couldbe partitionedinto Cl-rich aqueousfluidfrom the magma.The Cu versusSiO• trend during differentiationof the Yeringtonbatholith (Fig. 9) suggeststhat --•80 wt percent of the Cu or '--50 ppm Cu was extracted uniformly from the entire Luhr Hill granitefrom 3- to 8-km paleodepth.Extraction of 50 ppm Cu from a minimumestimatedvolumeof
65 km3 of Luhr Hill granite(Table 1) wouldyield •10
million tons of Cu, more than sufficient to
producethe '-•6 milliontonsof Cu depositedin the Yeringtondistrict (Einaudi, 1982). The concentration of saltsand Cu into high-densityaqueousore fluids containingonly 3 wt percent of the water originallyin the granite magmareducesthe probmodelsof igneousquartz commonlycontainsabundanthigh- lem inherentin previousorthomagmatic salinityfluidinclusions (containing halite +_sylvite transportinglarge volumesof dilute aqueousfluids
1784
JOHNH. DILLES
from the crystallizingmagmaupwardthroughthe smallcrosssectionalarea of the stockapex. The proposed orthomagmatic model can in theory provide all the Cu deposited. However, whether it is practicableto uniformly remove 50
ship,the AnacondaCompany,the Harvey S. Mudd Fund of the Departmentof AppliedEarth Sciences, and GeologicalSocietyof AmericaPenrosegrants. Critical reviews by and discussions with Einaudi, A. L. Grunder, G. A. Mahood, and E. Seedorff have
ppm Cu from >65 km3 of magmais debatable.As substantiallyimprovedthis manuscript.The preshownabove, the mineralizinggranite porphyry vious work and ideas of J. M. Proffett, M. T. Eindikesand therefore ore fluidsappearto have their sourcesat '--3- to 6-km depth in the upper part of the Luhr Hill granite. However, Cu must also be extractedfromdeeperparts(6-8 km) of the granite magmaby evolutionand upward migrationof an aqueousfluid to 6-km depthwouldhavebeeninitiallywaterundersaturateddue to the increaseof water solubility
audi, R. B. Carten, K. L. Howard, Jr., and other AnacondaCompanygeologists providedthe framework for this study. M. L. Rivers assistedwith the microprobeanalyses. Reviewsof the manuscript by two EconomicGeologyrefereeswere helpful. August 19; 1986; March 25, 1987 REFERENCES
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--
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Holloway,J. R., andBurnham,C. W., 1972, Meltingrelationsof Norton,D., 1982, Fluid andheat transportphenomenatypicalof copper-bearingpluton environments:Southeastern Arizona,in basaltwithequilibriumwaterpressures lessthantotalpressure: Jour.Petrology,v. 13, p. 1-29. Titley, S. R., ed., Advancesin geologyof the porphyrycopper deposits,southwesternNorth America:Tucson,Univ. Arizona Huang, W.-L., and Wyllie, P. J., 1986, Phaserelationships of gabbro-tonalite-granite-water at 15 kbarswith applicationto Press,p. 59-72. differentiation and anatexis:Am. Mineralogist, v. 71, p. Ohmoto,H., 1986, Stableisotopegeochemistryof ore deposits: 301-316.
Rev. Mineralogy, v. 19, p. 491-559.
Huebner,J. S.,andSato,M., 1970,The oxygenfugacity-tempera- Ohmoto,H. andRye,R. O., 1979, Isotopesof sulfurandcarbonin ture relationships of manganese oxideand nickel-nickeloxide Barnes,H. L., ed., Geochemistryof hydrothermalore deposits: buffers:Am. Mineralogist,v. 55, p. 934-952. New York, Wiley Intersci., p. 509-567. Irving, A. J., 1978, A review of experimentalstudiesof crystal- Oldow, J. S., 1983, Tectonicimplicationsof a late Mesozoicfold liquid trace element partitioning:Geochim. et Cosmochim. and thrust belt in northwesternNevada: Geology, v. 11, p. Acta, v. 42, p. 743-770.
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Osiecki,R. A., 1981, Textural developmentof pegmatite,aplite, betweengarnetmegacrysts and hostvolcanicliquidsof kimandassociated rocktypesin the Mason-Milfordgranite:Unpub. berlitic to rhyolitic composition:Geochim. et Cosmochim. Ph.D. thesis,StanfordUniv., 158 p. Acta, v. 42, p. 771-787. Parry, W. T., and Jacobs,D.C., 1975, Fluorine and chlorinein biotite from basinand rangeplutons:ECON.GEOL.,v. 70, p. Jacobs, D.C., andParry,W. T., 1976, A comparison of the geo554-558. chemistryof biotite from somebasinandrangestocks:ECON. GEOL., v. 71, p. 1029-1035. Philpotts,J. A., and Schnetzler,C. C., 1970, Phenocryst-matrix partitioncoefficients for K, Rb, Sr, andBa, with applications to -1979, Geochemistryof biotite in the SantaRita porphyry copperdeposit,New Mexico:ECON.GEOL.,v. 74, p. 860-887. anorthositeand basaltgenesis:Geochim.et Cosmochim.Acta, Jahns,R. H., andBurnham,C. W., 1969, Experimentalstudiesof v. 34, p. 307-322. pegmatitegenesis: I, A modelfor the derivationandcrystalli- Piwinskii, A. J., 1968, Experimental studies of igneous rock series,central SierraNevadabatholith,California:Jour.Geolzationof graniticpegmatites: ECON.GEOL.,v. 64, p. 843-864. ogy, v. 76, p. 548-570. Kilinc,I. A., andBurnham,C. W., 1972, Partitioningof chloride betweena silicatemeltandcoexisting aqueous phasefrom2 to Price, J. G., 1977, Geologicalhistoryof alterationand minerali8 kilobars:ECON.GEOL.,v. 67, p. 231-235. zation at the Yerington porphyry copper deposit, Nevada: Kistler,R. W., 1983, Isotopegeochemistry of plutonsin the Unpub. Ph.D. thesis,Univ. California-Berkeley,168 p. northernGreat Basin:GeothermalResearchCouncil Spec. Proffett,J. M., 1977, Cenozoicgeologyof the Yeringtondistrict, Rept. 13, p. 3-8.
Nevada,andimplicationsfor the natureandoriginof basinand rangefaulting:Geol. Soc.AmericaBull., v. 88, p. 247-266. Na andinitial 87Sr/86Sr in Mesozoicgraniticrocksin central -1979, Ore depositsof the western United States:A sumCalifornia:Geol. Soc.AmericaBull., v. 84, p. 3489-3512. mary: NevadaBur. Mines and GeologyRept. 33, p. 13-32.
Kistler,R. W., andPeterman,Z. E., 1973, Variationsin Sr, Rb, K,
YERINGTONBATHOLITH,NV: PETROLOGY Proffett,J. M., andDilles,J. H., 1984, Geologicmapof the Yeringtondistrict,Nevada:NevadaBur. Minesand Geology,map
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Proffett,J. M., Jr., andProffett,B. H., 1976, Stratigraphyof the Stewart,J. H., 1972, Initial depositsin the Cordillerangeosyncline:Evidenceof a late Precambrian(•850 m.y.) continental Tertiary ashflowtuffs in the Yeringtondistrict, Nevada:NevadaBur. Mines and GeologyRept. 27, 28 p. separation:Geol. Soc.AmericaBull., v. 83, p. 1345-1360. 1980, Geologyof Nevada:NevadaBur. MinesandGeology Roberts,J. L., 1976, Titaniumsolubilityin syntheticphlogopite -Spec.Pub. 4, 136 p. solidsolutions:Chem. Geology,v. 17, p. 213-227. onestimates of Robie,R. A., Hemingway,B. S., andFisher,J. R., 1978, Thermo- Stormer,J. C., 1983, The effectsof recalculation temperatureandoxygenfugacityfromanalyses ofmulticompodynamicpropertiesof mineralsand related substances at nent iron-titanium oxides: Am. Mineralogist, v. 68, p. 298.15Kand1 bar (105pascals) pressure andhighertempera586-594. tures:U.S. Geol. SurveyBull., v. 1452, 456 p. Robinson,A., and Kistler, R. W., 1986, Maps showingisotopic Stormer,J. C., andCarmichael,I. S.E., 1971, Fluorine-hydroxyl exchangein apatiteandbiotite:A potentialigneousgeotherdatingin the Walker Lake 1ø X 2 ø quadrangle,Californiaand mometer:Contrib. MineralogyPetrology,v. 31, p. 121-131. Nevada:U.S. Geol. SurveyMisc. Field StudiesMap MF-1382N, scale 1:250,000. Stormer,J. C., andNicholls,J., 1978,XLFRAC,a programfor the interactivetestingof magmaticdifferentiationmodels:ComRoedder,E., 1971, Fluid inclusionstudieson the porphyry-type puter Geosci.,v. 4, p. 143-159. ore depositsat Bingham,Utah, Butte, Montana,and Climax, Streckeisen,A. L., 1976, To eachplutonic rock its proper name: Colorado:ECON. GEOL., v. 66, p. 98-120. Earth-Sci.Rev., v. 12, p. 1-33. -1984, Fluid inclusions: Rev. Mineralogy,v. 12, 644 p. Schnetzler,C. C., andPhilpotts,J.A., 1970, Partitioncoefficients Swanson,S. E., 1977, Relationof nucleationand crystal-growth rate to the developmentof granitictextures:Am. Mineralogist, of rare earth elementsbetween igneousmatrix material and v. 62, p. 966-978. rock-formingmineralphenocrysts-II: Geochim.et Cosmochim. Swanson,S. E., and Fenn, P.M., 1986, Quartz crystallizationin Acta, v. 34, p. 331-340. igneousrocks:Am. Mineralogist,v. 71, p. 331-342. Schweickert, R. A., 1978, TriassicandJurassic paleogeography of the SierraNevadaandadjacentregions,Californiaandwestern Taylor, H. P., Jr., 1968, The oxygenisotopegeochemistryof igneousrocks:Contr. MineralogyPetrology,v. 19, p. 1-71. Nevada, in Howell, D. G., and McDougall, K. A., eds., Meso1980, The effectsof assimilation of countryrocksby magmas zoic paleogeographyof the western United States,Pacific -on •sO/•60andS7Sr/SaSr systematics of igneousrocks:Earth CoastPaleogeography Symposium2: Soc.Econ. PaleontoloPlanet. Sci. Letters, v. 47, p. 243-254. gistsMineralogists, PacificSec.,1978, Sacramento, California, Taylor, S. R., 1965, The applicationof trace element data to p. 361-384. problemsin petrology: PhysicsChemistry Earth, v. 6, p. Shaw,H. R., 1965, Commentson viscousity,crystalsettling,and 325-363.
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APPENDIX
1969, Trace elementchemistryof andesitesand associated calc-alkalinerocks:OregonDept. Mineral IndustriesBull., v. 65, p. 43-63.
Tuttle, O. F., and Bowen, N. L., 1958, Origin of granite in the light of experimental studies in the system NaA1Si3OsKA1Si3Os-SiO2-H20: Geol. Soc.AmericaMem., v. 74, 153 p. Wedepohl,K. H., 1969, Compositionandabundanceof common
igneousrocks,in Wedepohl,K. H., ed., Handbookof geochemistry,vol. 1: Berlin-Heidelberg-New York, Springer-Verlag, p. 227-249. Williams, H., Turner, F. J., and Gilbert, C. M., 1955, Petrography: SanFrancisco,W. H. FreemanCo., 406 p. Winkler, H. G. F., 1976, Petrogenesis of metamorphicrocks,4th ed.: New York, Springer-Verlag,334 p. Wones,D. R., 1981, Mafic silicatesasindicatorsof intensivevariablesin graniticmagmas:Mining Geology,v. 31, p. 191-212. Wones,D. R., and Eugster,H. P., 1965, Stabilityof biotite:Experiment,theory,andapplication:Am. Mineralogist,v. 50, p. 1228-1272.
A
PetrographicDescriptionsof Units of the YeringtonBatholith Biotite-hornblendequartz monzodiorite(QMD) of McLeod Hill is fine to medium grained (0.5-2 mm), increasingwith depth and distancefrom wallrockcontacts,andis commonlygrayto purple-gray, with a color index of 15 to œœ(Fig. 3, Table œ).It is characterized by tabular plagioclase, prismatic hornblende,and/or anhedralaugite, euhedralbiotite, equant opaqueoxides,and accessorysphene,
euhedral apatite, and zircon enclosedby intergrownmicroclineand quartz.Augite occursonly with hornblendecoronasand is only abundantin structurally shallow, fine grained biotite-hornblende quartz monzodiorite(compareFig. 5C and D). Hornblende containsabundantinclusionsof magnetite and minor sphene and ilmenite. The hornblende (+ augite) to biotite ratio is high
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("-4:1), magnetite and sphene are abundant,and ilmenite forms sparse isolated grains sometimes rimmed by discontinuoussphene.Biotite includes apatite_ zirconandis stablewith augite (Fig. 5D). Spheneformsanhedralgrainsintergrownwith and overgrowingmagnetite,hornblende,ilmenite, and locally biotite, showingthat it is paragenetically late. In shallowexposuresplagioclaseformseuhedral normallyzoned(tn4o-17)tabletswith fine-scale oscillatoryzoningand sparse,resorbed,calciccores (An65-5o).K-feldsparand quartz poikiloblastically encloseplagioclaseand maficminerals.In deep exposures, subhedral plagioclase is less zoned (An35_17), doesnot containcalciccores,and is intergrownwith K-feldsparand quartz. Biotite-hornblende gabbro (GB) is allotriomorphic andfine to mediumgrained,with a colorindex of '-'60. It is composedof hornblende,zoned plagioclase(An42_22 with coresascalcicasAn72),biotite, magnetite,sphene,apatite,ilmenite,andtraces of augite(Table2, Fig. 3). Exceptfor prismaticapatite, all grainsare subhedralto anhedraland subequant, showing no interlocking textures. The hornblendeto plagioclaseratio variesfrom >2:1 to 1:1. Most biotite occursas a replacementof hornblende along cleavage,but someoccursas poikilobiastic grainsenclosingplagioclase,augite, minor hornblende, and opaques.Sphene forms isolated subhedralgrainsbut alsocommonlyformssmaller
