26Models of Origin for Carlin文档格式.docx

26Models of Origin for Carlin文档格式.docx

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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