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Nevada,Reno,Reno,NV89557ThePatagoniaMountainsareunderlainbyaNW-elongategranodioritepluton(c.65Ma)witharelativelyconstantmodalcomposition.Areasofstronglydevelopedporphyritictextureareassociatedwithhydrothermalalterationandbasemetaldeposits.RegionaldDpatternsinmagmaticbiotitesfromthemoredeeplyerodedsouthendoftheplutonareuniformandvaryfrom-78to-83‰.TothenorthdDismoreerraticvaryingfrom-66to-93‰withmorenegativevaluesfoundneartheedgeofthepluton.Thispatternsuggeststhatmeteoricwaterenteredtheplutonalongitsouteranduppercontacts.dDofhydrothermalbiotitesvariesfrom-71to-95‰suggestingthatmixingofmagmaticandmeteoricfluidscontinuedintotheearlyhydrothermalstage.ThetexturaltransitionfromequigranulartoporphyriticgranodioriteintheFourMetalsmineareaisgradualandaccompaniedbyincreasedMgO/MgO+FeOinmagmaticbiotiteandhornblendeandincreasedmagnetite.ThisrelationshipisconsistentwithexperimentalworkofWonesandEugster(1965)andCzmanskeandWones(1970)whodemonstratedthesechangesinmineralchemistrycorrespondtoincreasedfO2andfluidsaturationinthemagma.ReleaseofaqueousfluidsduringquenchoftheporphyriticgranodioriteformedtheFourMetalscopper-bearingbrecciapipeandrelatedmetaldeposits.FandClincreaseinmagmaticbiotiteandlessconsistentlyinhornblendeasthegranodioritebecomesmoreporphyritic.F,Cl,andMgO/MgO+FeOinmagmaticbiotitearecorrelativewithhydrothermalbiotitecompositionsfromtheFourMetalsmine,indicatingagradualtransitionfromthemagmatictohydrothermalenvironment.Multiple,parallelenrichmenttrendsofF+Clinbiotitemayidentifymultiplegenerationsofaqueousfluidreleasefromprogressivelymoreporphryiticphasesoftheporphyriticgranodiorite.Graybeal(1973)foundthatthecoppercontentofbiotiteandhornblendedecreasesinthemoreporphyriticrocks,consistentwithextractionofcopperfromthesemineralsintoanaqueousfluidprogressivelyenrichedinFandCl.DistributioncoefficientssuggestthatbiotiteexchangescopperinacoexistingFandCl-enrichedaqueousfluidmorerapidlythanhornblende,butthatbothmineralsmaybeimportantsourcesofcopperinassociatedmetaldeposit
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