26Models of Origin for Carlin.docx

26Models of Origin for Carlin.docx

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26ModelsofOriginforCarlin

26ModelsofOriginforCarlinGoldDeposits,Nevada,USA

生词1：

trap捕获

density密度，比重

lithostatic净岩压的

gradient梯度

overpressured超压的

laterally从侧向

passive被动的

hydrostatic净水压的

veinlet小脉

burial埋藏

documented有文件证明的

excess过度

circulate循环

meteoric大气降水的

juxtaposition并列

regime政权制度，体制，状态

throttling扼杀

abrupt突然的

expansion膨胀

paleodepth古深度

stratiform层状的

permeability渗透性，渗透率

bioclastic生物碎屑的

horizon水平，层位

hydrologic水文的

sustainable可以忍受的

brecciation角砾岩化

strength强度

postulate假定，要求，基本条件

Thissectionincorporatespreviousgeologic,geochemical,fluidinclusion,andstableisotopewiththoseofthepresentstudyintoamodelfortheformationoftheCarlingolddepositinadeepgeologicsettingoffluidmixingbetweenhydrostaticandnormallypressuredfluids.

Depthoforeformation

Atrappingpressureof800±1400barsisrequiredbythehigh-densityCO2-richfluidinclusionspresentinMGOstagequartzsamples,giventhetemperaturesindicatedbycoexistingH2O-richinclusions.Typicallithostaticpressuregradientsof250bars/kmrequiredepthsofatleast3.2±1.8kmtoreachthispressure.Atamorelikelygradientof80to85percentoflithostatic,whichisatypicalgradientforformationofverticalfracturesbyoverpressuredfluidsinlaterallystressedrocks（Gretener,1981,EngelderandLacazette,1990）,thedepthwouldbeabout3.8±1.9km.Attheotherextreme,passivehydrostaticloadconditionsinvolvingopencommunicationwiththesurface（typicalofhotspringsandmeteoricwatercirculation）wouldrequireminimumformationdepthsof8±4km.Depthsof6kmormoreappearGeologicallyunreasonablegiventhefactthatMCstageveinletorientationandfluidsinclusiondatadefinemaximumdepthsofonlyabout6.1kmunderconditionsoflithostaticload（Kueth,1989）,andsignificantburialisnotdocumentedinnorth-centralNevadaafterHCstagetime（EarlyCretaceous）.Similarly,the0to1.0-kbarpressuresreportedbyHoistraetal.（1987）atJerrittCanyon,Nevada,forgas-rich（eventIA）fluidsresponsibleforgoldmineralizationdefinedepthsof5to10kmunderhydrostaticconditions.Pressuregradientsappreciablyinexcessofhydrostaticarerequiredtoaccountforthe1-to5-kmdepthsofmineralizationproposedforJerrittCanyon（Hofstraatal.,1988）.

FormationoftheCarlindepositbyacirculatingmeteoricwaterat150℃to220℃underconditionsapproachinglithostaticseemsunlikely,becausealithostaticallypressuredfluidcouldnotcirculatedirectlyfromthesurfaceandbecauseofthelikelyincreaseoftheδ18Ovalueofthewaterduringevenashortresidenceat3.9±1.9kmand150℃to220℃.Therefore,juxtapositionofahydrostaticfluidwithanear-lithostatic（overpressured）hydrothermalsystemisindicated.

Flowofthedeepfluidfromtheoverpressuredtonormallypressuredregimewouldhaveinvolvedthrottlingofthefluid,definedbyabruptpressuredecreaseandfluidexpansion,asdiscussedbyBartonandToulmin（1961）,ToulminandClark（1967）,andSimsandBarton（1962）.Flowisfocusedthroughthethrottlingzone,andactivemixingofthetwofluidsisexpectedonthelow-pressureside.

Underthismodel,thedeephotoverpressuredCO2-richfluidwouldascendalongfaultstructurestothesiteofthepresentorebodyatpaleodepthsof3.8±1.9km.OnreachingtheRobertsMountainFormation,itwouldencounteranextensivezoneofstratiformpermeabilitydefinedbythebioclastichorizon.Thiszoneispostulatedtohavecontainedfluidatnear-hydrostaticpressurewithgoodhydrologicconnectiontothesurface.Apressuredropofabout500barsissustainablebyrockstrengthatthisdepth（T.Engelder）.Someofthebrecciationandfaultingmayreflecttherapidpressuretransitionandtheaccompanyingrapidflow.

生词2

isotopic同位素的

episode幕

salinity盐度

countryrock围岩

jasperoid碧玉状的

Paleozoic古生代

silicate硅酸盐

Jurassic侏罗系

exotic外来的

analogue类似物

consumption

molalities摩尔浓度

permissive许可的

volatilization挥发作用

elevated提高的

retention保持力

channel通道

outcrop露头

footwall下盘

FLuidcharacteristics

Fluidinclusionandisotopicresultsalsoindicatethepresenceoftwofluidsduringthegoldoreepisode（Table4）:

（l）agas-rich（primarilyCO2andH2S）,moderate-salinityfluidwhichwas18Oenriched（6.5-10‰,at225℃）asaresultofconsiderableoxygenisotopeexchangewithsurroundingcountryrocks,butisapparentlydominantlyofmeteoricoriginbasedonitsδDvaluesof-87to-165permil,and

（2）averylowsalinity,relativelygas-poorfluidwithbothδDandδ18Ovaluestypicalofunevolvedmeteoricwater.Thesetwofluidsarerequiredtoaccountforthewiderangeofδ18OvaluesinMGOandLGOstagejasperoidandquartzandcalciteveins,aswellasthevariablefluidinclusioncharacteristicsofthesesamefeatures.

PossibleoriginsofthehighCO2contentofMGOstagefluidsare

（1）tappingoflow-tomoderate-grademetamorphicfluidsbydeep-seatedstructures,

（2）contactmetamorphismofcarbonateslowerinthePaleozoicsectiontoformcalc-silicatesadjacenttoburiedintrusions,or（3）directmagmaticcontributionfromasyetunidentifiedintrusionsatdepth（i.e.,nottheJurassicintrusiverocksexposedwithinabout10kmofCarlin）.

AnexoticsourcefortheCO2isproposedbecausetherelativelyhighgascontents（5-10mole%）farexceedanycontemporarygeothermalanalogueandrequireveryhighconfiningpressuresnotavailableinanear-surfacemeteoricwater.TheacquisitionofCO2attheseconcentrationsbyordinarywater-rockinteractionswouldrequireconsumptionofsimilarmolalitiesofacidoroxidizingcomponents（i.e.CaCO3+2H+=Ca2++CO2+H2O;CO~+O2=CO2）.Suchhighmolalitiesofacidoroxidizingspeciesarefarhigherthanthoserecordedinnormalgeothermalsystems.Therefore,thermalbreakdownofcarbonates,oranigneoussource,seemsnecessarytofurnishtheCO2.TheCisotopedataforCO2-richfluidinclusionsarealsoinconsistentwithadominantlyorganicoriginoftheCO2butarepermissiveoftheothersources.AnyoftheabovesourcesofCO2mightalsofurnishelevatedlevelsofH2Sbypyritebreakdownaccompanyingcarbonatebreakdownorbymagmaticvolatilization.

TheseconddistinctivecharacteristicofMGOstagefluidsistheirhighδ18OH20values,whichrequiresignificantisotopicexchangewithsedimentaryrocksathightemperaturesiftheinitialwaterismeteoric.BecausetheCO2-richfluidswouldbecarbonateandsilicatedestructive,theoriginalmeteoricfluidwasevidentlyheatedandexchanged18OwithsedimentarymaterialsprioracquiringtheCO2.TheretentionofacarbonatedestructivecharacterdespitepassagethroughunderlyingcarbonatesbeforereachingtheRobertsMountainFormationseemstorequireflowalongrestrictedchannelsborderedbysilicifiedcountryrocks,assuggestedbythelargeoutcropsofjasperoidseeninsurfaceexposuresoffootwallrocks.

InclusionsofLGOstagefluidsincalciteandbariteveinscontainingAs±Sb±Hg±Tlphaseshavefillingtemperaturesof150℃to220℃,　butrequireanunknownpressurecorrection.Iforedepositionoccurredinathrottlebetweenshallowhydrostaticanddeeperoverpressuredenvironmentsat3.8±1.9km,thenforanapproximatepressurecorrectionof10℃/km,thetrappingtemperatureforlow-temperatureLGOstageinclusionsmightbeaslowas17℃.

Possiblegeologicsettings

生词3

illustrat图示

diagram图解

seal封闭（构造）

diagenetically成岩期

cemente胶结

boundary边界

compaction压实，压缩

petroleum石油

leakage泄漏

episodic分幕式的

Ordovician奥陶系

Silurian志留系

Cambrian寒武系

argillaceous泥质的

section剖面

bitumen沥青

porosity孔隙度

gaswindow生气窗

anticline背斜

emanation放出

obscure模糊的

soluble可溶的

scavenge清扫

channel引导

jogs漫步，轻推

immiscibility不混溶性

ThepossibledeepmixingenvironmentisillustratedinFigure12.ThisdiagramshowsamagmaticorskarnsourcefortheCO2andH2S,oranalternativesourceofmetamorphicfluidsfromthemiddleordeepcrust（Cameron1988）.Mixingwithmeteoricfluidsisinterpretedtooccuratapressuresealseparatingtheoverpressuredandnormallypressuredenvironments.

Overpressuring（"geopressuring"）tovaluesexceedinghydrostaticisreportedtooccurinmostdeepsedimentarybasins（Hunt,1990,Powly,1990）.Typicallythechangefromoverpressuredtonormallypressuredoccursatadepthofabout3,000m,atasealzonewithathicknessofafewhundredmetersorless.Thesealzonemaybearelativelyimpermeablestratigraphicunitsuchasashaleorevaporite,oritmaybeazoneofdiageneticallycementedsandstonethatcutsacrossstratigraphicunits.Inadditiontosubhorizontalorstratigraphicboundaries,theoverpressuredzonemaybelaterallylimitedbysteeplydippingsealedfaultzones.Awidevarietyofcausesissuggestedforoverpressuring,includingasedimentationrateexceedingtherateatwhichfluidcanescapetoallowcompaction,tectoniccompression,thermalexpansionofporefluids,andvolumeincreaseongenerationofgasoroil.Evidencefrompetroleumfieldsindicatesthatthesealcommonlyundergoesepisodicleakagecausedbypressuresexceedingthefracturestrengthoftheseal.Duringtheseleakageperiods,oilandgasgeneratedbelowthesealleakintoreservoirsabovetheseal,presumablyaccompaniedbywaterinsomecases.

TheformationoftheCarlindepositinadeepsedimentarybasinbytheinvolvementoftwofluidswithmarkedlydifferentpressures,withthehigherpressurefluidinferredtobeoverpressuredrelativetohydrostatic,isstronglysuggestivethatthedepositwasformedatorjustaboveapressureseal.Beneaththissealliesatleast1kmofOrdovicianthroughLowerSiluriancarbonateandquartzite（Radtke,1985）,plusprobablyanother2kmofCambrianrocksofsimilarlithology（Robertsetal.,1958）.Thus,inth