Evolution of Montviel alkaline-carbonatite complex by coupled fractional crystallization, fluid mixing

20 Pages·2015·13.69 MB·English

OREGEO-01625;NoofPages20
OreGeologyReviewsxxx(2015)xxx–xxx
ContentslistsavailableatScienceDirect
Ore Geology Reviews
journal homepage: www.elsevier.com/locate/oregeorev
Evolution of Montviel alkaline–carbonatite complex by coupled
fractional crystallization, fluid mixing and metasomatism — part I:
Petrography and geochemistry of metasomatic aegirine–augite and
biotite: Implications for REE–Nb mineralization
OlivierNadeaua,⁎,R.Stevensonb,M.Jébrakb
aDepartmentofEarthSciences,UniversityofOttawa,Ottawa,K1N6N5,Canada
bDépartementdesSciencesdelaTerreetdel'Atmosphère,UniversitéduQuébecàMontréal,Montréal,H2X3Y7,Canada
a r t i c l e i n f o a b s t r a c t
Articlehistory: Magmaticvolatilesarecriticallyimportantinthepetrogenesisofigneousrocksbuttheirinherenttransience
Received31July2015 hamperstheidentificationoftheirroleinmagmaticandmineralizationprocesses.Wepresentevidencethat
Receivedinrevisedform17September2015 magmaticvolatilesplayedacriticalroleintheformationofthe1894MaPaleoproterozoicMontvielalkaline–
Accepted21September2015 carbonatitecomplex,Canada,andtherelatedcarbonatite-hostedREE–Nbdeposit.Fieldanddrillcorerelation-
Availableonlinexxxx
shipsindicatethatlithologicalunitsofthecomplexwereemplacedinthefollowingorder:clinopyroxenites,
melteigites,ijolites,melanosyenites,leucosyenites,granites,lamprophyricsilicocarbonatites,raremagne-
Keywords:
siocarbonatites,calciocarbonatites,ferrocarbonatites,latemixedcarbonatites,kimberliticsilicocarbonatitesand
Alkalineandcarbonatitemagmas
Assimilation,fractionalcrystallizationand polygenicbreccias.Magmaticmineralswithintheseunitsweresystematicallymetasomatized.Inundersaturated
metasomatism silicaterocks,augiterecrystallizedtoaegirine–augiteandaegirine,plagioclaserecrystallizedtoalbite,andneph-
Magmatic–hydrothermalevolution elinerecrystallizedwithanalcime,cancriniteandalbite.Primarybiotitewasreplacedbysecondary,REE-rich
Mantlepulses metasomaticbiotite,particularlyalongfracturesandalterationpockets.Incarbonatites,liquidusphasesconsisted
Sm–Ndisotopes ofcalciteanddolomiteandwererecrystallizedtoferroandolomite,ankerite,siderite,barytocalcite,witheriteand
strontianite,whichareintimatelyrelatedtotheREE-bearingcarbonatesandfluorocarbonates.Biotiteiscommon
toalllithologies,rangesinREEconcentrationsfrom1.5to230ppmandyieldedsubsoliduscrystallizationtem-
peraturesrangingfrom770°Cto370°C.Sm–Ndisotopeanalysesfrombiotiteandaegirine–augiteyielda
rangeofε values(+3.4to−3.0)thatsuggestsmixingoffluidsfromthreesourcesduringthecrystallization
Nd
oftheMontvielmagmas.Theclinopyroxenitestomelteigite,ijolitesandmelanosyenitescrystallizedaugiteand
biotitewithinitialε value≥3.4andthesemineralsweremetasomatizedbya1stfluid,loweringtheirε to
Nd Nd
valuescomprisedbetween0.8and3.4.Silicocarbonatitesandcarbonatitessubsequentlycrystallizedaegirine–au-
giteandbiotitewithinitialε value≥2.6anda2ndfluidmetasomatizedthemineralstolowerɛvalues.Boththe
Nd
1standthe2ndfluidseventuallymixedwitha3rdrecrystallizingaegirine–augiteandbiotiteandlowertheirεNd
valuesdownto−3.0.Theresultspresentedhereinsuggestthatthemantlemagmasevolvedthrough4distinct
mantlepulsesbyfractionalcrystallization,mixingofdepletedmantlefluidswithcrustalfluids,andmetasoma-
tism.Someofthesilicaterocksalsoshowevidenceofassimilationofwallrockaspartoftheirpetrogeneticevo-
lution.Duringthelaststagesofitsevolutionincarbonatites,thefluidsourcetransitedfromthedepletedmantle
tothecrustandwespeculatethatthisresultedinaviolentexplosiveeruptioncreatingthediatreme-shaped,
HREE-richpolygenicbreccia.
©2015ElsevierB.V.Allrightsreserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
2. Tectonicandgeologiccontext . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
3. Analyticalmethods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
⁎ Correspondingauthor.
E-mailaddress:[email protected](O.Nadeau).
http://dx.doi.org/10.1016/j.oregeorev.2015.09.022
0169-1368/©2015ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:Nadeau,O.,etal.,EvolutionofMontvielalkaline–carbonatitecomplexbycoupledfractionalcrystallization,fluidmixing
andmetasomatism—part...,OreGeol.Rev.(2015),http://dx.doi.org/10.1016/j.oregeorev.2015.09.022
2 O.Nadeauetal./OreGeologyReviewsxxx(2015)xxx–xxx
3.1. Petrography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
3.2. X-raydiffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
3.3. Electronmicroprobe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
3.4. LaserablationICP-MS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
3.5. Thermalionizationmassspectrometry(TIMS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
4. Petrography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
4.1. Clinopyroxenites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
4.2. Melteigitesandijolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
4.3. Syenitesandgranites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
4.4. Silicocarbonatites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
4.5. Carbonatites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
4.6. Polygenicbreccias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
5. Mineralparagenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
6. Wholerockcomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
7. Biotite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
7.1. Majorelementconcentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
7.2. Crystallizationtemperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
7.3. REEconcentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
8. Clinopyroxene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
9. Tectosilicates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
10. TheSm–Ndisotopegeochemistryofbiotiteandaegirine–augite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
11. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
11.1. Metasomatism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
11.1.1. Theeffectofmetasomatism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
11.1.2. Natureandoriginofthemetasomaticfluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
11.1.3. Silicasaturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
11.2. Magmaticevolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
11.2.1. Multiplemantlepulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
11.2.2. Immiscibilityandunmixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
11.2.3. RedoxandpH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
12. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
1.Introduction thehightemperatureNaalterationcommontoIOCG-typeandaffiliated
deposits(e.g.,Corriveau,2007;Richards,2014).Potassiummetasomatism
Magmaticvolatilesarecriticallyimportantinthepetrogenesisofig- (orpotassicalteration)isusuallyassociatedwithmineralssuchasmi-
neousrocksbuttheirinherenttransiencehamperstheidentificationof crocline(adularia),biotiteandsericite,whereasNametasomatismre-
theirroleinmagmaticandmineralizationprocesses(e.g.,Bowen,1928; crystallizes minerals such as albite, riebeckite, aegirine–augite and
Hamiltonetal.,1964;HedenquistandLowenstern,1994;deMooretal., aegirine.DifferentrocksshowdistinctkindsofmetasomatismasK-Na
2013;Blacketal.,2014;Plailetal.,2014).Theroleofmagmaticvolatiles metasomatismismorecommontoijoliteswhereascarbonatitesare
inporphyrycoppersystems,forexample,isrelativelywellunderstood usuallyassociatedwithintenseKmetasomatism(Pirajno,2013),and
(e.g.,Sillitoe,1973,2010),butunderstandingthebehaviorofvolatiles bothprocessesareoftenreferredtoasfenitization.Ithasbeenshown
andmetasomaticfluidsinmantleandcrustalmagmatic–hydrothermal that Fe was introduced hydrothermally in large-scale mineralized
systemsremainsaworkinprogress. systemssuchasthatofOlympicDamdistrict,Australia(Bastrakov
Metasomatismiswidelyusedtodescribethechemicalalteration etal.,2007)andBayanObo,China(Smithetal.,2015),recrystallizing
ofsolidrocksandmineralsbyexternalfluids,butthedefinitionof mineralssuchasmagnetite,hematite,Fe-bearingcarbonates,pyrite,
metasomatism (e.g., Lindgren, 1933; Ramberg, 1952; Best and andFesilicatessuchasbiotiteandotherFe-bearingsilicates.Metaso-
Christiansen,2001;Pirajno,2013)doesnotexcludethepossibility matismhasbeendocumentedindifferenttectonicsettingssuchas
thatpartiallycrystallizedmagmasmaystartbeingmetasomatized subductionzones(e.g.,Bebout,2013),collisionalorogeny(e.g.,Touret
bytheirownfluids:autometasomatismisaspecialcaseofmetaso- andNijland,2013),rifts(e.g.,Dupuyetal.,1991)andinthemantle
matismwherebythemetasomaticagentissourcedfromthesame (e.g.,O'ReillyandGriffin,2013).
magmasthatarebeingmetasomatized(e.g.,Elsdon,1982).Inamag- TheMontvielalkalinecomplex,Canada,representsandidealset-
matic–hydrothermalcontextthisnecessitatethat(1)themagmatic tingtostudytheroleofmetasomatismintheevolutionofmagmat-
volatilephasebeexsolvedfromthemeltbutstayinthemagmaas ic–hydrothermalsystemsbecause:(1)ofthepresenceofdiverse
itcrystallizes,and(2)eithertherockorthefluidchangeitschemical lithologies, (2) each of the lithologies was metasomatized; and
compositionsotheylosetheirthermodynamicequilibrium.Theloss (3)thealkalinecomplexwasextensivelydrilledforcoresamples
ofthermodynamicequilibriumbetweenthemagmaticfluidandthe whichwerereadilyavailableforthisstudy.Thisstudypresentsthe
solidifyingrockscanbeachieved,forexamplebyfractionalcrystalli- petrographyofthelithologiesandestablishesaparageneticmineral
zationofthemagmaandmixingofthemagmaticvolatilephasewith sequencebasedonfieldworkanddrillcorerelations(Goutier,2006;
externalfluids. Nadeau et al., 2015). The surface geology, the major element
AlkalimetasomatismbyKandNahasbeendocumentedinseveral composition of bulk rocks, biotites, clinopyroxenes and tecto-
typesofmagmatic–hydrothermalsystemssuchasinthehightempera- silicates,therareearthelement(REE)concentrationinbiotiteand
tureKalterationofporphyrydeposits(e.g.,Seedorffetal.,2005)and theSm–Ndisotopicratiosofbiotiteandaegirine–augitearereported
Pleasecitethisarticleas:Nadeau,O.,etal.,EvolutionofMontvielalkaline–carbonatitecomplexbycoupledfractionalcrystallization,fluidmixing
andmetasomatism—part...,OreGeol.Rev.(2015),http://dx.doi.org/10.1016/j.oregeorev.2015.09.022
O.Nadeauetal./OreGeologyReviewsxxx(2015)xxx–xxx 3
anddiscussedwithemphasisontheroleofmetasomatisminthe carbonatite complex in the Abitibi Subprovince (David et al.,
petrogenesisoftherocksandonthesourceofthefluids.Thetrace 2006), the 1907 Ma Cargill pyroxenite–carbonatite complex
elementandSm–Ndisotopiccompositionofbulkrocksarepresent- (Sage, 1988) and the 1872 Ma Borden silicocarbonatite–car-
edinacompanionpapertitled:“EvolutionofMontvielAlkaline– bonatitecomplex(Belletal.,1987)intheKapuskasingSubprovince.
CarbonatiteComplexbyCoupledFractionalCrystallization,Fluid TheoriginofMontviel,CargillandBordenalkaline–carbonatitecom-
MixingandMetasomatism—PartII:TraceElementandSm–NdIso- plexesisunclearbutisprobablyrelatedtothesubductions,collisions
topeGeochemistryofMetasomaticRocks;ImplicationsforREE–Nb andpost-collisionrelaxationsoccurringontheoutskirtsofthecraton
mineralizations”. andtherelatedreadjustmentsalongmajorfaultzones.Montvielis
locatedwithinafaultnetworkthatextendsforover235kmbetween
2.Tectonicandgeologiccontext MatagamiandChibougamauandlessthan10kmawayfrommajor
regionalthrustsofthegreenstonebelt(Goutier,2006).
TheSubprovincesoftheSuperiorCratonofLaurentiawereas- ThePaleoproterozoicMontvielalkaline–carbonatitecomplex
sembledintoasingleArcheancratonbytheendoftheArchean intrudedfoliatedArcheantonalitesinthenortheasternpartofthe
(KerrichandCassidy,1994;Muelleretal.,1996).TheProterozoic Abitibi Subprovince at 1894 ± 4 Ma, based on U–Pb on zircons
was characterized by the collision of numerous cratons around fromtheijoliteunits(Davidetal.,2006).Achronologicalsequence
theworldbetween2000Maand1750Ma(Condieetal.,2015). ofthelithologicalunitswasestablishedbasedonfieldanddrillcore
OntheeastsideoftheSuperior,eastwardsubductionwasinitiated cross-cuttingrelationships(Goutier,2006;Nadeauetal.,2015).
at 2170 Ma and resulted in the formation of the New Quebec The alkaline system consisted of injections of clinopyroxenites,
Orogenbetween1870and1815Ma.Similarly,northoftheSuperi- melteigites, ijolites, melanosyenites, leucosyenites, granites,
or,subductionwasonsetat1980Ma,resultingintheTrans-Hudson lamprophyricsilicocarbonatites,raremagnesiocarbonatites,calcio-
orogenesisfrom1850to1800Ma(Condieetal.,2015).Although carbonatites,ferrocarbonatites,latemixedcarbonatites,kimberlitic
nomajortectonicactivityisreportedatthesetimeswithinthe silicocarbonatitesandpolygenicbreccias(Fig.1).Theintrusionis
Superior,anumberofalkalineintrusionswereemplacedwithin neither deformed nor metamorphosed. Based on underground
the Superior Craton such as the 1894 Ma Montviel alkaline– mapping,thecarbonatiteiscutbyamainNNW–SSEfaultanda
Fig.1.SurfacegeologyofMontvielalkalinecomplex.InsetmapshowsthelocationofMontvielalkalinecomplexwithintheArcheanSuperiorcraton(ingray)andNorthAmerica.Modified
afterGoutier(2006).
Pleasecitethisarticleas:Nadeau,O.,etal.,EvolutionofMontvielalkaline–carbonatitecomplexbycoupledfractionalcrystallization,fluidmixing
andmetasomatism—part...,OreGeol.Rev.(2015),http://dx.doi.org/10.1016/j.oregeorev.2015.09.022
4 O.Nadeauetal./OreGeologyReviewsxxx(2015)xxx–xxx
seriesofsecondorderNE–SWfaults,althoughthefaultsappearto 3.Analyticalmethods
haveallowedverylittledisplacement(Nadeauetal.,2015).The
carbonatite,andespeciallytheferrocarbonatiteandthepolygenic 3.1.Petrography
breccia host aREE–Nbdeposit, which consistof REE carbonates
andfluorocarbonatesassociatedwithzonesofhydrothermalBa– PolishedsectionswerepreparedandobservedusingaLeicaDMLP
Sr–P–Falteration(Nadeauetal.,2015). petrographicmicroscopeequippedwithLeicaDFC450digitalcamera
Pleasecitethisarticleas:Nadeau,O.,etal.,EvolutionofMontvielalkaline–carbonatitecomplexbycoupledfractionalcrystallization,fluidmixing
andmetasomatism—part...,OreGeol.Rev.(2015),http://dx.doi.org/10.1016/j.oregeorev.2015.09.022
O.Nadeauetal./OreGeologyReviewsxxx(2015)xxx–xxx 5
andusingaHitachiTM3000tabletopscanningelectronmicroscope weremeasuredinstaticcollectionmodeviaadoubleRefilamentas-
(SEM)equippedwithaBrukerQuantax70energydispersiveX-ray semblyonaThermoScientificTritonPlusthermalionizationmassspec-
spectrometer(EDS). trometer(TIMS).NeodymiumwasmonitoredusingaJNd1standard
havingavalueof143Nd/144Nd=0.51209±0.00005and2σerrors
3.2.X-raydiffraction were0.000009inaverage(n=19).Typicalcombinedprocedureblanks
forNdandSmwereb150pg.
PowderedbulkrocksampleswerescannedusingaSiemensD500
X-raydiffractometerequippedwithaCotubeandaSidetectorat 4.Petrography
UQAM.X-raydiffractionspectrawereidentifiedusingEvasoftware
version1.3. 4.1.Clinopyroxenites
3.3.Electronmicroprobe Inthefieldandindrillcores,clinopyroxenites,melteigites,ijolitesand
melanosyenitesareobservedasdykes,podsandsillsthatcross-cuteach
Biotite,clinopyroxene,amphiboleandavarietyoftectosilicateswere other.Forexample,clinopyroxenitecommonlyhostsdykes,podsandsills
analyzedforaselectionofmajortominorelementsusingaJEOLJXA- ofijoliteandmelanosyenitewhereasijolitesandmelanosyenitescom-
8900Lelectronmicroprobe,witha15kVand20nAbeam,intheGeo- monlyhostdykes,podsandsillsofclinopyroxenite.Thegeologicalmap
chemicalLaboratories,McGillUniversity.Thebeamdiameterwas10μm inFig.1thus,representsasimplifiedgeologyandisbasedonthework
forbiotite,pyroxeneandamphibolesand50μmfortectosilicates.Forbi- ofGoutier(2006).Clinopyroxenitesconsistofcoarseaugite,titaniteand
otite,countingtimesformostelementswere20sanddetectionlimits biotitewithpyrrhotite,chalcopyrite,sphaleriteandilmenite.Augiteis
variedbetweenabout200and500ppm.Acountingtimeof100swas systematicallymetasomatizedtoaegirine–augitealongfracturesand
usedforF,returningdetectionlimitsofabout700ppm,and200sfor grainboundariesandisalsosometimestobiotite(Figs.2a–c,3b).Acces-
ClandS,returningdetectionlimitsof85and140ppm,respectively. sorymineralsincludezircon,apatite,feldspathoidssuchasnephelineand
Forpyroxeneandamphibole,countingtimeswere20sforallelements, sodalite,analcime(azeolite),cancrinite(apseudo-zeolite),andother
returninglimitsofdetectionofabout200–500ppm,exceptforFwhich tectosilicatessuchasalbite–celsian(aNa–Ba–feldspar)(Figs.2e–gand
hadalimitofdetectionof1200–1500ppm.Fortectosilicatescounting 3a,c,d,e).Inclinopyroxenite,alltectosilicates(feldspathoids,zeolites,
times were also of 20 s, resulting in detections limits of about pseudo-zeolitesandalbite)displaysecondaryalterationorrecrystalliza-
200–500 for most elements except F (1500–2000 ppm) and Ba tiontextures(Fig.3b,c,e).Calcite,dolomiteandankeritearepresent.
(700–1000ppm).Theinstrumentwascalibratedusingasetofcommer-
cialandin-housestandardsandrelativestandarddeviations(1σ)were 4.2.Melteigitesandijolites
generallylessthan1%(e.g.,CaO=20±0.2wt.%).
Themelteigites andijolites are found in association with clino-
3.4.LaserablationICP-MS pyroxenites and melanosyenites and are always metasomatized.
Melteigites consist of coarse augite altered to aegirine–augite and
AttemptstoanalyzethesamebiotitecrystalsforREEconcentrations aegirine,alongwithsignificantamountsofcoarsebiotite,titaniteand
usingalaserablationICPMSsystemweremadeattheGeotop,UQAM. fluorapatite (Fig. 3b,d).Abundant microscopic nepheline, sodalite,
The system consists of a Photon-machines Analyte.193 G2 Pulse cancriniteandanalcimeappeartobesecondary(Figs.2f,3b,e).The
ExcimerlaserandaNuInstrumentAttom,highresolution,sectorfield ijolitesareverysimilartomelteigitesbutcontainlessclinopyroxene
ICPMS.ANBS610glassstandardwasusedasexternalstandardand andmorenephelineandanalcime.
theconcentrationsofREEwerequantifiedusingTiobtainedfromthe
electronmicroprobeastheinternalstandard. 4.3.Syenitesandgranites
3.5.Thermalionizationmassspectrometry(TIMS) Thesyenitesarecommonlyassociatedwithmelteigites–ijolitesand
clinopyroxenites and range in composition from K-feldspar clino-
Sm–NdisotopeswereanalyzedatGeotop,UQAM.Sampleweighing pyroxenitestoquartzleucosyenites.Theyresembleijolitesbutcontain
andchemistrywereperformedinaclass100(ISO5)cleanroom.Pow- greateramountsofmicroclineandalbiteandlessnepheline,cancrinite
deredsampleswereweighed(about0.1g)anda149Sm–150Ndspike andanalcime.Themelanosyenitesconsistofferropotassicmagnesio-
wasaddedtodetermineSmandNdconcentrations.Thesampleswere hastingsite,riebeckite,aegirine–augiteandmicrocline.Aegirine–augite
sequentiallydissolvedanddriedin29NHF,29NHF+15NHNO , and albite are secondary. The leucosyenites (Fig. 2h) are the only
3
AquaRegiaand6NHClacids.TheSmandNdfractionswerethenpuri- quartz-saturated rocks of the alkaline complex, along with the
fiedbycationexchangechromatographyfollowingtheprocedureof riebeckitegranitedescribedbelow,andoccuronlyintheNEendof
Roulleau and Stevenson (2013). The Sm and Nd isotopes samples thealkalinecomplex.
Fig.2.Photomicrographsofsilicaterocksshowingmineralandtexturerelations.Photographsareintransmitted,single-polarizedlightunlessotherwisestated.(a–b)Clinopyroxenite
showingmagmaticaugite(Aug)withzonesofmetasomaticaegirine–augite(Aeg–aug)andbiotite(Bt)alonggrainboundariesandfractures.Coarserbiotitealsooccursindecussatetex-
turewithaugite.Titanite(Ttn)andaugitedisplayinclusionsofapatite,ilmenite,chalcopyrite,silicateandcarbonatitemelt(SampleMV12.IJ02).(c)Biotiteclinopyroxenitewithaugite,
aegirine–augiteandbiotite,cutbydykletsofcalcite,aegirine–augiteandcancrinite(Can)(SampleMV12.IJ02).(d)Reflectedlightphotomicrographofsphalerite(Sph)withCu-richsulfide
meltinclusions(intermediatesolidsolution)alignedalongcrystallographicgrowthplanes(chalcopyritedisease),inthecarbonatitepartofaglimmeritebreccia(SampleLP12.UMBX01).
(e)Clinopyroxeniteshowingmoreorlessmetasomatizedaugite,titanite,fluorapatiteandbiotite.Biotiteappearstobehydrothermalasitoccursalonggrainboundariesandfracturezones.
ThealterationalsoincludesCa-richstrontianiteandnepheline(notshown)(SampleMV12.UM04).(f)Ijoliteconsistingofeuhedralbutmetasomatizedaegirine–augiteandfluorapatitein
amatrixofintermixednepheline(Ne)andanalcime(Anl).TheBSEviewofthenepheline–analcimemixshowsskeletalnephelineinamatrixofanalcime(Fig.3e).Otherareasofthesame
polishedsectionalsoshoweuhedralanalcime(SampleMV12.SY02).(g)Sameijoliteasinpanelf,showingaegirine–augite,biotite,calcite,titaniteandrutile(Rt).Thesmalldarkcrystalsin
theupper-rightarerutilewithamountsofNbdetectableundertheEDS.ThetitanitecrystalhasconcentrationsofZrdetectableundertheEDSandhostsinclusionsofcalcite,zirconand
aegirine–augite(SampleMV12.SY02).(h)Crossedpolarviewofthequartz-saturatedleucosyeniteshowingeuhedralplagioclase(Pl)andquartz(Qtz)withmetasomaticmicrocline(Mi)
withtartantwin,andalbite(Ab)(SampleMV12.SY04).
Pleasecitethisarticleas:Nadeau,O.,etal.,EvolutionofMontvielalkaline–carbonatitecomplexbycoupledfractionalcrystallization,fluidmixing
andmetasomatism—part...,OreGeol.Rev.(2015),http://dx.doi.org/10.1016/j.oregeorev.2015.09.022
6 O.Nadeauetal./OreGeologyReviewsxxx(2015)xxx–xxx
Theriebeckitegranitesoutcroponlyinthesouthernpartoftheintru- and relatively less microcline. Biotite and riebeckite are present in
sion and are mapped as a single unit (Fig. 1). They resemble the minoramounts.Hydrothermalalterationhasalteredplagioclasetoalbite
leucosyenitesbutcontainsignificantlymorequartzandplagioclase, andproducedriebeckitefromanunidentifiedferromagnesianmineral.
Pleasecitethisarticleas:Nadeau,O.,etal.,EvolutionofMontvielalkaline–carbonatitecomplexbycoupledfractionalcrystallization,fluidmixing
andmetasomatism—part...,OreGeol.Rev.(2015),http://dx.doi.org/10.1016/j.oregeorev.2015.09.022
O.Nadeauetal./OreGeologyReviewsxxx(2015)xxx–xxx 7
4.4.Silicocarbonatites 4.6.Polygenicbreccias
Silicocarbonatites at Montviel have compositions intermediate Thepolygenicbrecciasconsistofclastsandxenocrystsofsilico-
between ultramafic undersaturated silicate rocks (clinopyroxenites, carbonatitesandcarbonatitesinapervasivelyalteredmatrixofdolo-
melteigites,ijolitesandmelanosyenites)andcarbonatites,andcontain mite,calcite,ankerite,strontianite,barytocalcite,fluorapatite,biotite,
10–50%magmaticcarbonateminerals.Silicocarbonatitesaresubdivided aegirine–augite, and ferropotassic magnesio-hastingsite. Three-
intolamprophyric-(Figs.4band5a,b),glimmeritebreccias(Figs.4a,d, dimensionalmodelingofthebrecciafragmentssuggeststhattheshapes
e, and 5d) and (3) kimberlitic silicocarbonatites (Figs. 4c and 5c). areahybridbetweendykesandadiatreme(Nadeauetal.,2013)and
The terms ‘lamprophyric’ and ‘kimberlitic’ are introduced to enable Nadeauetal.(2015)proposedthatthebrecciaunitformedasaresult
distinction of the two types of silicocarbonatites and because the ofhigh-energyfragmentationwithsignificantdisplacementbasedon
silicocarbonatiteshavevisualsimilitudewithultramaficlamprophyres modelingofthedistributionandsizeofthefragments(Jébrak,1997)
andkimberlites.Lamprophyricsilicocarbonatitesaremostcommonand and threedimensional modeling of thedeposit based on drill core
consistofkinkedorfoldedbookletsofbiotite(Figs.4band5a)and data. This explosion was the last event of the emplacement of the
pseudomorphsofolivineinamatrixofdolomite,ankerite,calciteandpy- Montvielalkalinecomplex(Figs.6eand7h).
rite.Glimmeritebrecciasconsistofdecussate-textureglimmeritesbrecci-
atedbydykletsofcarbonates(Fig.4a).Kimberliticsilicocarbonatitesare
foundmainlyasa50mwidedykecross-cuttingthroughallsilicate 5.Mineralparagenesis
rocksandcarbonatitesandconsistofbrecciasmadeofalteredfragments
and xenocrysts of carbonates and ferromagnesian rocks in a Amineralparagenesis(Fig.8)wasestablishedbasedonrelative
hydrothermally-alteredmatrixofbiotiteandotherminerals.Theytextur- chronology of petrological observation in the field and drill cores
allyandvisuallyresemblekimberlitesbutwithhigheramountsofcar- (Goutier,2006;Nadeauetal.,2015).Thelithologicalunitsareordered
bonates,lesseramountsofK-bearingmineralsandnofresholivine. chronologically as mentioned above. A large dyke of kimberlitic
silicocarbonatitecutsthroughalllithologiesexceptthepolygenicbrec-
ciaandisthuspositionedneartheendimmediatelybeforethepolygen-
4.5.Carbonatites ic breccia. Magnesiocarbonatite is very rare (1 occurrence) but
intimatelyassociatedwithcalciocarbonatite,soitwasarbitrarilyplaced
Themagnesiocarbonatitesarerareandoccuraspodsandlenses beforecalciocarbonatite.
intermixedwithothertypesofcarbonatitesandsilicocarbonatites,and Olivinewasonlyobservedinthreeofthelithologies:aspseudo-
aredominatedbydolomiteofmagmaticorigin.CarbonatesofBa–Ca– morphsinclinopyroxenitesandkimberliticsilicocarbonatitesandasa
Mg (Ca-bearing norsethite or Ba-bearing dolomite?), barytocalcite, minorunalteredoccurrenceinthepolygenicbreccias.Augiteisonly
strontianite,fluorite,REEcarbonatesandfluorocarbonatesandalbite found in clinopyroxenites and melteigites whereas aegirine–augite
arehydrothermalandpresentinmajorconcentrations(Fig.6a). was the most common clinopyroxene in all ultramafic and mafic
Calciocarbonatitesaredominatedbycalciteandrangefrombarren silica-deficientrocks.Aegirinewasobservedinferrocarbonatites.Alka-
andunalteredcalcitecarbonatitetoREE-bearing,hydrothermallyal- lineamphibolessuchasferropotassicmagnesio-hastingsite,riebeckite
teredcalciocarbonatites.Calciocarbonatitescontainmajoramountsof andarfvedsonitewereidentifiedinmelanosyenites,ferrocarbonatites
primarycalciteanddolomitewhicharecutbyveinletsofankerite, andkimberliticsilicocarbonatites,respectively.Itisnoteworthythatbi-
strontianite,witheriteandbarytocalciteandcontainminortotracecon- otiteandfluorapatitewerepresentthroughouttheevolutionofthe
centrationsofaegirine–augite,biotite,fluorapatite(Fig.7a),bariteand complex.
REEcarbonatesandfluorocarbonates. Nepheline, sodalite, cancrinite and analcime were observed in
Ferrocarbonatitesarethemostabundanttypeofcarbonatites,cross- clinopyroxenites,melteigitesandijolites(Fig.8).Cancrinitewasalso
cutthecalciocarbonatitesandarestronglyandpervasivelyhydrother- seen in lamprophyric silicocarbonatites.Microclineand plagioclase
mallyaltered.Theyaredominatedbyhydrothermalormetasomatican- wereidentifiedinmelanosyenitesandleucosyeniteswhereasquartz
keriteandFe-dolomite,siderite,barytocalcite,strontianite,witherite, was found only in leucosyenites and granites. In the silicate rocks,
fluorite,fluorapatiteandminortotraceamountsofbiotite,aegirine, feldspathoidswerefoundtooccurintheearlyunits,microclineand
arfvedsonite, barite,REE carbonate and fluorocarbonate,monazite- plagioclasecrystallizedintheintermediateunitsandquartzcrystallized
(Ce),xenotime-(Y),loparite(REEperovskite),sphaleriteandgalena in the last silicate rocks. Metasomatic and/or hydrothermal albite
(Figs.6cand7c–f). wasobservedinallsilicaterocks.Followingthesilicatemagmas,theal-
Thehydrothermallyalteredferrocarbonatitesarecross-cutbyinjec- kalinecomplexwasinjectedwithsilicocarbonatiteandcarbonatite
tionsofmixedsilicocarbonatites,calciocarbonatitesandferrocarbonatites magmaswherenotectosilicateswasfoundexceptforalbiteinsome
gathered under the term of ‘mixed carbonatites’. These mixed silicocarbonatitesandmagnesiocarbonatitessamples.
carbonatiteswerealsohydrothermallyalteredtovariousdegreeswithal- Exceptforankeritewhichwashydrothermal,calciteanddolomite
terationproductssimilartothosedescribedabove.Theirexistenceclearly weretheonlycarbonatestocrystallizeintheprimitivesilicaterocks.
demonstratesthatcarbonatiteswereinjectedrepeatedlyandthatepi- Incontrast,strontianite,siderite,barytocalciteandwitheritewereal-
sodesofmagmatismbothprecededandfollowedepisodesofhydrother- wayshydrothermalandfoundsolelyincarbonatites.Primarysulfides
malalteration.Themixedcarbonatitesshowzonesofalterationswith consistedonpyrrhotite,chalcopyrite,sphaleriteandgalena.Mostpyrite
REEfluorocarbonates(qaqarssukite,carbocernaite),barite,biotiteand issecondaryandappearstohaveresultedfromlowtemperaturerecrys-
chloriteincalcite(Fig.7b,g). tallization,exceptforsomeprimarypyritewhichformedduringthelast
Fig.3.Backscatterelectronimageofsilicaterocksshowingmineralandtexturerelations.(a)Clinopyroxeniteconsistingofaugite(notshown)accompaniedbyfluorapatite,calcite,titanite
andfluoroapatite(Ap).Thepresenceofpyrrhotite(Po)andchalcopyrite(Cpy)(formerlymonosulfideandintermediatesolidsolutions)testifiesoftherelativelyreducingconditionsand
thehighactivityofsulfurinthemagma.Titaniteispoikiliticwithinclusionsofcarbonatitemeltandfluorapatite.Azoneofalteration(Alt)consistsofamixofunidentifiedO–Ca–Fe–Ti–Zr–
Nb(brannerite?polymignite?)andZr–O(baddeleyite?)phasesandfluorapatite(SampleMV12.UM01).(b)Melteigitemademostlyofaegirine–augite(Aeg–aug)andaugite(Aug),here
showingthesecondarynatureofthesubordinatenepheline(Ne)andcancrinite(Can)(sampleMV12.IJ01).(c)Zoneofmetasomaticreplacementinclinopyroxenite,recrystallizingan
assemblageofnepheline,sodalite(Sdl)andalbite–celsian(inset)(SampleMV12.UM04).(d)Aegirinizationandalbitizationoftitaniteinmelteigite.Titaniumwasmobile(sample
MV12.IJ01).(e)SkeletalnephelineandanalcimeinthesameijoliteasinFig.1g(SampleMV12.SY02).
Pleasecitethisarticleas:Nadeau,O.,etal.,EvolutionofMontvielalkaline–carbonatitecomplexbycoupledfractionalcrystallization,fluidmixing
andmetasomatism—part...,OreGeol.Rev.(2015),http://dx.doi.org/10.1016/j.oregeorev.2015.09.022
8 O.Nadeauetal./OreGeologyReviewsxxx(2015)xxx–xxx
Fig.4.Photomicrographsofsilicocarbonatitesshowingmineralandtexturerelations.Photographsareintransmittedlightunlessotherwisestated.(a)Carbonate-richpartofaglimmerite
breccia-typesilicocarbonatiteconsistingofferroandolomite(Fe-Dol),strontianite(Str)andqaqarssukite-(Ce)(Qaq)pseudomorphingapre-existingmineral(SampleLP12.UMBX01).(b)
Lamprophyre-likesilicocarbonatitesshowingkinkedbiotitebooklets,likeinmanylamprophyres,andferroandolomitewithmagnetiteinamatrixofdolomite,fluoriteandsecondary
hematite(SampleMV13.DYK2).(c)Crossedpolarviewofthekimberlite-likesilicocarbonatiteshowingroundedenclavesofcalciocarbonatiteandotherferromagnesian-richrocksand
mineralsinamatrixofcalcite,ferromagnesianminerals,biotite,partiallyresorbedpyriteandzircon(SampleMV12.KI01).(d)Biotite-rich(glimmerite)partoftheglimmeritebreccia-
typesilicocarbonatiteshowingacloseupviewofpyrochlore(Pycl)withmetamicthalosinbiotite.Thetranslucentmineralmayincludecalcite,dolomiteandstrontianite(Carb)(Sample
LP12.UMBX01).(e)Partiallydigestedenclaveofbiotiteclinopyroxeniteincalciocarbonatiteshowingphenocrystsofaegirine–augitealteredtobiotiteinamassofcalcite(Sample
MV12.SICACB02).
Pleasecitethisarticleas:Nadeau,O.,etal.,EvolutionofMontvielalkaline–carbonatitecomplexbycoupledfractionalcrystallization,fluidmixing
andmetasomatism—part...,OreGeol.Rev.(2015),http://dx.doi.org/10.1016/j.oregeorev.2015.09.022
O.Nadeauetal./OreGeologyReviewsxxx(2015)xxx–xxx 9
Fig.5.Backscatterelectronimageofsilicocarbonatitesshowingmineralandtexturerelations.(a)Hydrothermalized,lamprophyre-likesilicocarbonatitewithfoldedbiotitebooklets,ol-
ivinepseudomorphs(Ol)replacedwithilmenite,Fe–Mgcarbonatesandchlorite.ThematrixhostshydrothermalFe–Tioxides,biotite,apatite,monaziteandpyrite(Sample
MV12.HREE02A).(b)Closeupviewofanolivinepseudomorphinlamprophyre-likesilicocarbonatiteshowingeuhedralbutsecondarycrystalsofilmeniteandMg–Fecarbonateinamatrix
offerroandolomiteandbiotite(SampleMV12.HREE01).(c)Lamprophyre-likesilicocarbonatiteshowinghydrothermalmonazite,biotite,ilmeniteinferroandolomite(Sample
MV12.HREE01).(d)Glimmeritebreccia-typesilicocarbonatiteshowingthealteredfluorapatite–strontianitebiotititecutbycalcite,apatiteandunidentifiedREE-bearingfluorocarbonate
(SampleMV12.UMBX01).
stagesofthepetrogenesis.Primaryoxidesconsistlargelyofilmenite magnesiocarbonatitesandferrocarbonatitesfallwithintheirfields.How-
withsomemagnetite.Rareearthelement-bearingphasesincludingcar- ever,asampleoffluorite-richferrocarbonatitefallswithinthefieldof
bonatesandfluorocarbonates(seeNadeauetal.,2015forspecificmin- calciocarbonatite because of the large concentration of CaF . Mixed
2
eralnames)aswellasmonazite-(Ce)andxenotime-(Y). carbonatitesfallwithinthefieldofferrocarbonatitesbecausetheyare
dominatedbyFe-bearingcarbonateminerals(ankerite,ferroandolomite,
andsiderite).Thepolygenicbrecciahasonly8.9wt.%CO andisnottruly
2
6.Wholerockcomposition acarbonatite.Itneverthelessplotswithinthefieldofferrocarbonatites.
Majorelementanalysesofrepresentativewholerocksamplesofthe
MontvielalkalinecomplexarepresentedinTableA1andFig.9.Silicate 7.Biotite
rocks(clinopyroxenites,melteigites,ijolites,melano-andleucosyenites
andgranites)rangefromabout35.6to73.4wt.%SiO ,12.1to0.1wt.% 7.1.Majorelementconcentration
2
MgO(exceptforonewith25wt.%CO ),1.4to17.1wt.%CaO,1.2to
2
5.5 wt.% Na O, 1.0 to 6.2 WT.% K O and 1.3 to 17.8 Fe Otot. The BiotiteisubiquitousatMontvielbothinsilicateandcarbonatite
2 2 2 3
carbonatites(magnesiocarbonatites,calciocarbonatites,ferrocarbonatites rocksanddisplayshighlyvariabletexturesandhabits,i.e.,coarseto
andmixedcarbonatites)rangefrom0.3to4.9wt.%SiO ,0.2to13.6wt.% aphanitic,greentobrown,euhedraltoanhedral,cumulate,disseminat-
2
MgO,4.6to51.3WT.%CaO,0.1to1.9WT.%Na O,0.0to1.0wt.%K Oand edandvein-shaped.BiotitewasanalyzedformajorelementsandTi,Ce,
2 2
0.6to44.9wt.%Fe Otot.Silicocarbonatiteshaveintermediatecomposi- F,ClandS(TableA2).CeriumandSwerealwaysbelowthelimitofde-
2 3
tions andrangefromabout 10.8 to34.8wt.%SiO ,9.6 to16.4wt.% tectionofabout800and250ppm,respectively.Chlorinewasoftende-
2
MgO,10.8to19.7wt.%CaO,0.2to3.3wt.%Na O,1.7to6.3wt.%K O, tected with concentrations around 150–400 ppm, with a few data
2 2
and12.7to14.6wt.%Fe Otot.Thesampleofpolygenicbrecciaalsohasin- pointsataround1000ppm.Somebiotitewaschemicallyalteredand
2 3
termediatecompositionswith15.3wt.%SiO ,6.6wt.%MgO,29.1wt.% returnedlowK Oand/orhighNa OorCaOand/orhighFeO+MgO.
2 2 2
CaO,1.3wt.%Na O,2.5wt.%K Oand12.0Fe Otot. These altered crystals were removed from subsequent calculations
2 2 2 3
ThenomenclatureofcarbonatiteusedhereinfollowsthatinGoutier anddiagrams.
(2006);DesharnaisandDuplessis(2011)andNadeauetal.(2015)and AplotofTiO vsF(Fig.10a)highlights3distinctbiotitepopulations:
2
samplesareplottedontheCaO–MgO–(FeO+MnO)ternaryclassification (1) high TiO –low F (TiO 2.5–3.5 wt.%; F 0–1 wt.%) present
2 2
diagramofWoolleyandKempe(1989);Fig.9b).Thecalciocarbonatites, in clinopyroxenite, melteigite and ijolite; (2) high F and variable
Pleasecitethisarticleas:Nadeau,O.,etal.,EvolutionofMontvielalkaline–carbonatitecomplexbycoupledfractionalcrystallization,fluidmixing
andmetasomatism—part...,OreGeol.Rev.(2015),http://dx.doi.org/10.1016/j.oregeorev.2015.09.022
10 O.Nadeauetal./OreGeologyReviewsxxx(2015)xxx–xxx
Fig.6.Photomicrographsofcarbonatitesshowingmineralandtexturerelations.Photographsareintransmittedlightunlessotherwisestated.(a)Hydrothermallyalteredandmineralized
magnesiocarbonatite.DolomiteisstronglyalteredtoBa–Ca-richstrontianite,Ba-richdolomite,fluorite,pyriteandsphalerite(SampleMV12.FECB01AB).(b)Crossedpolarviewof
calciocarbonatitewithcalciteandewaldite-(Y)(Nadeauetal.,2015)(SampleMV12.HREE02B).(c)Comb-like,ankerite–strontianiteassemblageinferrocarbonatiteshowingopen
spacecrystallizationofbiotiteandBa–REEfluorocarbonate(SampleMV12.FECB02).(d)Calciocarbonatiteshowingdecussatetexturedcalcitecutbyveinletsofclearbarytocalciteand
strontianite,reddishqaqarssukite-(Ce)andewaldite-(Y),andopaquesphaleriteandgalena(SampleMV12.LATE1).(e)Polygenicbrecciadisplayingastronglyalteredmatrixmadeof
fluorapatiteandcalcite,withstrontianite,aegirine–augite,biotite,qaqarssukite-(Ce),sphaleriteandclastsofcarbonatites(SampleMV12.HREE03).
Pleasecitethisarticleas:Nadeau,O.,etal.,EvolutionofMontvielalkaline–carbonatitecomplexbycoupledfractionalcrystallization,fluidmixing
andmetasomatism—part...,OreGeol.Rev.(2015),http://dx.doi.org/10.1016/j.oregeorev.2015.09.022