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1、©2005SocietyofEconomicGeologists,Inc.EconomicGeology,v.100,pp.115–130MineralogicalandStableIsotopeStudiesofKaolinDeposits:ShallowEpithermalSystemsofWesternSardinia,ItalyR.SIMEONE,†ViaCorvetto33,09014Carloforte,ItalyJ.H.DILLES,DepartmentofGeosciences,OregonStateUniversity,Corval
2、lis,Oregon97331G.PADALINO,DipartimentodiGeoingegneriaeTecnologieAmbientali,UniversitàdiCagliari,ItalyANDM.PALOMBAIstitutodiGeologiaAmbientaleeGeoingegneriadelC.N.R.,Sez.diCagliari,UniversitàdiCagliari,ItalyAbstractLargekaolindepositshostedbyMiocenesilicicpyroclasticrocksinnorth
3、westernSardiniarepresenthy-drothermalalterationformedwithin200moftheMiocenepaleosurface.Boilinghydrothermalfluidsascendedsteeplydippingfaultsthatareenvelopedbyalteredrock.Thebroadlystratiformkaolindepositsconstitutead-vancedargillicalterationthatwasproducedinasteam-heatedzonene
4、arthepaleosurfaceoverlyingthedeeperhydrothermalsystems.Thedeeperzonesrepresenttwodistincttypesofepithermalsystems:weaklyacidic(in-ferredlow-sulfidation)systemsatTresnuraghesandacidic(high-sulfidation)systemsatRomana.Tresnuraghesischaracterizedatdepthbychalcedony±quartz±bariteve
5、inswithina50-m-widezoneofK-feldspar-quartz-illitealterationandoverlyinglocaloccurrencesofchalcedonysinter,whichdefinethepaleo-surface.Kaolindepositsnearthepaleosurfacearecharacterizedbyzonationoutwardanddownwardfromaninnershallowzoneofkaolinite1T-opal±dickite±alunite(<20-µm-dia
6、mgrains)toanouterdeeperkaolinite1M-montmorillonite-cristobalite.Thiszonationindicatesformationbydescendingacidicfluids.ThesystemevolvedfromascendingweaklyacidicorneutralfluidsthatboiledtoproduceH2S-richvapor,whichcondensedandoxidizedwithinthenear-surfacevadosezonetoformsteam-he
7、atedacid-sulfatewatersandkaolinalter-ation.AtRomana,veinsatdepthcontainchalcedonyorquartzandminorpyriteandareenclosedinupto20-m-widezonesofkaolinite1T-quartzalteration.Nearhydrothermalventsalongthepaleosurface,chalcedonicsil-icaisenclosedwithinazoneofkaolinite1T-alunite(<50-µm-
8、diamgrains)-quartz-opal±dickite±cristobalite.Kaolinqua