Origin of stratiform sediment-hosted manganese car

ChemicalGeology,99(1992)139-164139ElsevierSciencePublishersB.V.,AmsterdamOriginofstratiformsediment-hostedmanganesecarbonateoredeposits:ExamplesfromMolango,Mexico,andTaoJiang,ChinaPatrickM.OkitaandWayneC.ShanksIIIu.s.GeologicalSurvey,MS-954,NationalCenter,Reston,VA22092,USA(ReceivedJanuary15,1991;revisedandacceptedFebruary6,1992)ABSTRACTOkita,P.M.andShanksIII,W.C.,1992.Originofstratiformsediment-hostedmanganesecarbonateoredeposits:Exam-plesfromMolango,Mexico,andTaoJiang,China.In:P.A.Meyers,L.M.PrattandB.Nagy(Guest-Editors),Geochem-istryofMetalliferousBlackShales.Chem.Geol.,99:139-164.CarbonateandsulfidemineralsfromtheMolango,Mexico,andTaoJiang,China,Mndepositsdisplaysimilaranddis-tinctive~34Sandc~3Cpatternsinintervalsofmanganesecarbonatemineralization.6~3C-valuesforMn-bearingcarbonaterangefrom-17.8to+0.5%o(PDB),withthemostnegativevaluesoccurringinhigh-gradeorezonesthatarecomposedpredominantlyofrhodochrosite.Incontrast,calcitefrombelow,withinandaboveMn-carbonatezonesatMolangohas~3C~0%0(PDB).Markedlynegative6~3CdataindicatethatalargeproportionofthecarboninMn-carbonateswasderivedfromorganicmatteroxidation.DiageneticreactionsusingMnO2andSO4z-tooxidizesedimentaryorganicmatterweretheprinciplecausesofsuch~2Cenrichment.Pyritecontentandsulfide~34S-valuesalsoshowdistinctivevariations.Inunmineralizedrocks,verynegative~34S-values(avg.-21%0CDT)andabundantpyritecontentsuggestthatpyriteformedfromdiagenetic,bacteriogenicsulfatereduction.Incontrast,Mn-bearinghorizonstypicallycontainonlytraceamountsofpyrite(e.g.,0.5wt%S)with~34S-values34S-enriched,insomecasestonearlythevalueforcontemporaneousseawater.34S-enrichedpyritefromtheMn-carbonateintervalsindicatessulfideprecipitationinanenvironmentthatunderwentextensiveSO42-reduction,andwaslargelyaclosedsystemwithregardtoexchangeofsulfateanddissolvedsulfidewithnormalseawater.Theoccasionaloccurrenceof34S-depletedpyritewithinMn-carbonatezonesdominatedby34S-enrichedpyriteisevidencethatclosed-systemconditionswereintermittentandlimitedtolocalporewatersanddidnotinvolveentiresedimentarybasins.Mn-carbonateprecipitationmayhaveoccludedporosityinthesurficialsediments,thusestablishinganeffectivebarriertoSOl-exchangewithoverlyingseawater.SimilarisotopicandmineralogiccharacteristicsfromboththeMolangoandTaoJiangdeposits,widelyseparatedingeologictimeandspace,suggesttheywereformedsimilarlybyMnO2precipitationatthemarginsofdysaerobictoanoxicmarinebasins.Mn-carbonateformedpredominantlybyearly-diageneticreductionofMn-oxidesviaoxidationoforganicmatterinnear-surfacesediments.InadditiontoMnCO3precipitation,organicmatteroxidationreactionsresultedinoxidationofFeStoFe-oxidessuchasmagnetite,maghemiteandhematite.ThelatterprocessexplainsanomalouslylowpyritecontentandabundantFe-oxidemineralsinorezonesdominatedbyrhodochrosite.Correspondenceto:PatrickM.Okita,BHP-UtahInternationalInc.,200FairbrookDrive,Suite204,Herndon,VA22070-5200,USA.Presentaddress.BHP-UtahInternationalInc.,200FairbrookDrive,Suite204,Herndon,VA22070-5200,USA.0009-2541/92/$05.00©1992ElsevierSciencePublishersB.V.Allrightsreserved.140P.M.OKITAANDW.C.SHANKS1111.IntroductionManganesecarbonatesoccurasimportantauthigenicordiageneticmineralsinavarietyofhydrothermal,volcano-sedimentary,chem-ical-sedimentaryandmetamorphicsettings.AuthigenicMn-carbonateisdocumentedasaminor,butcommon,constituentofmodernsedimentsfromanoxiclacustrineenviron-ments(Shterenbergetal.,1966;RobbinsandCallender,1975),estuaries(Holdrenetal.,2975),andbrackishandmarinebasins(Man-heim,1961,1982;CalvertandPrice,1970;Suess,1979;PedersenandPrice,1982;Ham-ilton-TaylorandPrice,1983;JacobsenandPostma,1989).However,modemexamplesofMnCO3accumulatinginpotentiallyeconomicconcentrationsarenotknown.Incontrast,large,economicmineraldepositsofMn-car-bonateinancientmarinesedimentaryrocksarecommonandexhibitanumberoffeatures(finegrainsize,micro-lamination,faciesdistribu-tion)suggestiveofprecipitationaschemicalsediments.Sediment-hostedMnmineraliza-tionoftenisattributedtodepositionalanddia-geneticprocessesinoxygen-deficientsedimen-taryenvironments(ForceandCannon,1988).PrecipitationofeconomicconcentrationsofMninsedimentarymarinesequenceshasbeeninterpretedasaproductofreduction-oxida-tion(redox)processeswhereboundariesbe-tweenoxygen-richandoxygen-poorwatermassesintersectmarginsofanenclosingbasin(Forceetal.,1983;FrakesandBolton,1984).OnemodelfortheoriginoflargeMndepositsproposesmixingofMn-ladenfreshwaterrun-offwithseawatertoproduceflocculatesofMnthataretrappedbyaveileffectandconcen-tratedbythesweepingbroomofdiurnalandeustaticsea-levelfluctuation(FrakesandBol-ton,1984).Forceetal.(1983)suggestthatMnmineralizationoccursinamanneranalogoustotheformationofabath-tubring.Thatis,Mnisstrandedattheimpingementoftheoxic-anoxicboundaryinawatercolumnwithabasinmargin.Acommonhypothesisex-pressedintheseandotherbasin-marginredoxmodelsisthatcontrastingreduction-oxida-tionconditionsintheaqueousenvironmentarenecessarytoconcentrateandbindMninare-strictedarea.Atmoderatelyoxidizingtore-ducingEhconditionsMnissolubleandmo-bileasMn2+,butre