968-989. Metamorphism, Metasomatism, and Mineralization at Lagoa Real, Bahia, Brazil. LYDIA M. LOBATO*. Comissdo Nacional de Energia Nuclear--Centro ...
EconomicGeology Vol. 85, 1990, pp. 968-989
Metamorphism, Metasomatism,and Mineralization at Lagoa Real, Bahia, Brazil LYDIA M. LOBATO*
Comissdo Nacionalde EnergiaNuclear--Centrode Desenvolvimento da Tecnologia Nuclear, Cx. Postal 1941, Belo Horizonte, Minas Gerais, Brazil 31271 AND WILLIAM
S. FYFE
Geology Department, University of Western Ontario, London, Ontario,Canada N6A5B7 Abstract
Uraniumdepositscumulativelyin the 100,000 tonsUaO8rangeoccurwithin ductile shear zonestranseetingthe ProterozoieLagoaReal granitiecomplex,enclosedin ArchcanandProterozoiebasementgneissesof the S5.oFranciscoeraton, at the LagoaReal region of southcentralBahia,Brazil. The gneisses havebeen subjectedto amphiboliteand granulitemetamorphicfaciesmetamorphism, andthe gneissose granitesto amphibolitefacies.To the west, the gneisses overlietheProterozoieEspinhago metasedimentary sequencealonga thrustfault. Petrographyand mineralchemistryshowthat in the zonesof metasomatie alterationand mineralization,the originalK feldspar+ quartz + oligoelase+ hastingsiteassemblage is replacedby albite + aegirine-augite + andradite+ hematiteassemblages, with or withouturaninite.Uraniniteprecipitationhasoccurredasa response to water-rockinteractionviareduction of the metasomatiefluid, with mineralizingphasesbeing controlledby the marie mineral transformations. This informationalongwith oxygenisotope,whole-rockgeochemistry, and fluid inclusionstudiesindicatesthat the alterationprocessinvolvedremovalof Si, K, Rb, and Ba andadditionof Na underoxidizingconditions.¾, Pb, and Sr were introducedalongwith U via interactionwith saline,isotopicallylight fluidsunder varyingwater/rockratiosand at
temperatures of 500ø to 550øC.The 87Sr/SeSr systematics suggest thatit is unlikelythat Sr, andby extensionuranium,wasintroducedby fluidsoriginatingfrom the gneisses. Geologicconstraintsand the generalalterationpattern are consistentwith releaseof the mineralizingfluidsin responseto overloadingof the basementand graniticrocksontothe sedimentaryEspinha9ovia a thrustmechanism. Introduction
imentsaswell asin hydrothermaldeposits;this is evidenceof the tendencyof uraniumto concentrateupTHE studyof mechanismsleadingto natural uranium ward in the crust (Heier, 1979). accumulationshasbeen greatly dominatedby considIn spite of all the researchconcerningthe geoeration of moderate-to low-temperatureprocesses. chemistryof uranium(Langmuir,1978; Rogersand On the other hand,relativelylittle attentionhasbeen Adams,1978), the knowledgeaboutits behaviorin paid to the formation of uranium depositsat high metamorphic and/or metasomatieenvironmentsis temperatures(>300øC), with the exceptionof, for remarkablyrestricted.Whereas the associationof example,someunconformity,classicvein, and peg- uraniumwith K enrichmenthasbeen investigatedto matitictypes(Nashet al., 1981). The studyof hydro- some extent, the association of uranium with Na thermal uranium depositsby Rich et al. (1977) has metasomatismis a relatively new conceptin western shown that, in most cases,high uranium concentra- geologicliterature (AdamekandWilson, 1977; Yerle tions correlate well with high concentrationsof po- and Thiry, 1979; White and Martin, 1980; Kishand tassium and silica and that the uranium deposits Cuney, 1981; Maruejol, 1989). On the other hand, formedat temperaturesbelow 350øC. this association hasbeen widely documentedby RusIt is commongeologicknowledgethat high-grade siangeologists (Nikolskii,1973; Smirnov,1977; Bemetamorphicrocks are commonlydepleted in ura- levtsev, 1980; Greehishnieov,1980; Kalyaev, 1980; nium (Heier, 1973; Fyfe, 1979). Economicuranium Tugarinov, 1980; Zhukova, 1980). depositsare foundin low-grademetamorphosed sedIn the Lagoa Real region of south-centralBahia State, Brazil, granitie rocks enclosedin high-grade basementgneissesof the Paramirimcomplexshow * Present address: Universidade Federal de Minas Gerais, Departamentode Geologia,Campusda Pampulha,Belo Horizonte, localized zonesof uranium enrichment that in places Minas Gerais, Brazil 31270. attainore grade(Fig. 1). The mineralizationis located 0361-0128/90/1082/968-2253.00
968
URANIUM MINERALIZATION AT LAGOA REAL
969
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LEGENO TERTIARY
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ESPINI-gU•O SUPERGROUP
Espinho•o Ronge to the westand Chopado Diomontino to the Lost e• Logoo Real
•
Lithologic contacts
Thrust teeth on upper plateifoulh dashedfault, where approximately located Axial plane of anticline
__l---•J LOWER PROTEROZO IC Metamorphic Cnmplexes I•'•
ARCHEAN AND LC7WER PROl"EROZOIC Ba..qF. IVlENT
•:.•
INTRUSIVE GRANITES
aASC ULTBASC aOCS
Axial plane ofsincline
Uroniferous ore deposits Towns / Villages Lagoo Real uroniferous province
FIG. 1. Simplifiedgeologicmap of the LagoaReal regionandsurroundings, includingthe location of uraniferousdeposits.The insertrectangleis detailedin Figure 2. Modified after the LETOS project (Prospec/DepartamentoNacional da Produ•5o Mineral (DNPM))joint program, unpub. map, 1976).
northeastof the townCaetit•, constitutinganarcuate zones(seeBeach,1980), whichtransectthe granites. zoneof 32 anomalies(VillagaandHashizume,1982). They are associated with extensivehigh-temperature The depositso•cur within large-scaleductile shear ("•500øC) metasomaticalterationof the granites,a
970
LYDIA M. LOBATO AND WILLIAM S. FYFE
processwhich produceslong tabular bodiesof albitized rocks(Lobato et al., 1983a, b). LagoaReal is one of the largesturaniumdepositsin Brazil, with severalorebodiescomprisinga combinedore reserve on the order of 100,000 metric tons of U308 (Brito et al., 1984). The mineral associations and the ura-
Espinha•o Range lies to the west of this basement complex,constitutingmiddle Proterozoiccoverson the S5oFranciscocraton(Fig. 1). They are madeup of rocks belonging to the Espinha•o Supergroup, which extends to the south into the State of Minas
Gerais(Jardimde Sft,1978, 1981). In Bahia, the Espinha•oRange forms a roughly formationat LagoaRealprovideanimportantexample north-south narrow belt extending from Boquira of highlyeconomicaluraniumconcentrationin high- southwardto Licinio de Almeida. The Espinha•osegrade metamorphicrocksassociatedwith Na meta- quenceis characterizedby acid volcanicrocksat the somatism. base,overlainby polymicticconglomerates andsandThis study containsthe descriptionof the miner- stones,with interlayered shales,graphitic rocks,and alogic,petrographic,andgeochemicalaspectsof the orthoquartzitesat the top (L. A. Moutinho da Costa et al., 1976). metasomaticallyaltered rocksand their host assem- et al., unpub.rept., 1976; Schobbenhaus blages,with specialemphasison the south-centralore The metasedimentsin the Chapada-Espinha•oarea depositsof the LagoaReal uraniumprovince(in par- reflect a diversespectrumof platformalcontinental, ticular, anomalies03, 05, 06, 07, and09; seeFig. 2). coastal,andmarinesedimentaryfacies(Jardimde S•t, It alsois a contributionto the understandingof the 1978). sequences border historyof metasomaticand metamorphicprocesses, Lower Proterozoicsupracrustal their relation to the uranium mineralization, and their the Espinha•orangeto the west (Caetit6-Jacaraci-Lipossiblelink to the tectonicandstructuralframework cinio de Almeida metamorphiccomplex) and the of the of the basementterrane in Bahia State during late ChapadaDiamantinato the far east-southeast Proterozoic times. Lagoa Real area (Brumadometamorphiccomplex; Figs.1 and2); they comprisemetavolcanic andmetaGeologicSetting sedimentarysequencesof high amphibolite facies et al., 1976). Accordingto MascarenThe areaunderinvestigation islocatedin the south- (Schobbenhaus central part of the S5o Franciscocraton which AI- has(1973), thesemayrepresentgreenstonebelt-type meida(1977) definedasa cratonicnucleusstabilized sequences. at the end of the Tranzamazoniangeotectoniccycle The metamorphicgradeof the Espinha•orocksis (2.7-1.8 Ga), surroundedby late Proterozoic(Brazil- greenschistfacieswith sericite, chlorite, graphite, ian cycle, 700 _+0.2 Ma) belts.In the Stateof Bahia, muscovite,andbiotite.Kyaniteispresentin muscovite the S5oFranciscocratoncomprises high-grademeta- schistsof the easternportionsof the lower Espinha•o morphicbasementrockswhich are divided into var- Range(L. A. Moutinhoda Costaet al., unpub.rept., iouscomplexes(Jardimde S•tet al., 1976b) and are 1976) attestingto a higher metamorphicgrade atcoveredby a succession of Precambrian,Paleozoic, tained locally (Jardimde S•t, 1978). The Espinha•o andMezozoicmetasedimentaryandmetavolcanicse- Range,alongits easternedge,is overlainby rocksof the ParamirimComplexalonga majorthruststructure quences(Fig. 1). The LagoaReal depositsoccurin the Proterozoic (Moutinho da Costa and Inda, 1982). In particular Lagoa Real granitic complex,hostedby basement the lower and middle Espinha•ostrataare intensely rocksof the Paramirim Complex. In this latter com- deformed and folds are tight with reversalof strata plex, Archean granulite and amphibolitefaciesor- (from Boquirato Caetit6) dipping 6 ø to 15ø under thogneissesand migmatites of tonalitic to granitic the basement. Gneissesin the ParamirimComplexdefinean Rb/ compositions predominate(Jardimde S•tet al., 1976b; Jardim de S•t, 1978; Pedreira et al., 1978). Portions Sr isochronwith a dateof 2,600 Ma (IndaandBarbosa, of the S5oTim6teoGranitecropout throughoutthe 1978). Later reworkingof the basementrocksis inregionsurrounding the ore deposits(P. H. de O. Costa terpreted basedon Rb/Sr agesof 1,900 and 2,000 et al., unpub. rept., 1985) and constitutethe main Ma (Inda and Barbosa,1978), and on K/Ar agesof hostlithology,which becomesincreasinglyfoliated 1,830 _+60 and 1,140 _+35 Ma (Tftvoraet al., 1967). and finer grainedcloseto the zonesof mostintense Further reworkingof the basementin the Brazilian metasomaticalteration (Fig. 2). The S•o Tim6teo cycle is suggestedby Rb/Sr agesof 700 to 455 Ma Granite is an isotropic,porphyritic, coarse-grained (Inda andBarbosa,1978). Intrusivegranitesoutcroprock. Closer to zones of shearingit is transformed pingto the eastandnorthof the LagoaRealareayield into a gneiss,which displaystexturalcharacteristics K/Ar agesof 580 to 475 Ma. Jardimde S•tet al. (1976a) suggested that the volof blastomylonites, protomylonites,and microbreccias. Calc-silicate, mafic, charnockitic, and fine- canicrocksof the lowermostsequenceof the Espingrainedgraniticlensesoccurlocallyin thisterrane. halo (BoquiraFormation)were formedat 1,000 Ma at 750 Ma, based The ChapadaDiamantinalies to the eastand the andlater alteredor metamorphosed nium mineralization
distribution
and conditions
of
URANIUMMINERALIZATIONAT LAGOAREAL
9 71 ß
.
42•07
• zoo7 ' 50"
L EGENO
'Fil TF_RTIARY AND QUA•
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/•_BITITES
Lifholog•c confocts
•
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•
Bedding othtude
•1
Tcwns
FINELY FCX.IATED PLAGIOCLA•.
MICROCLINE-
GNEISSES
PL.•GIOCLASE - MICROCLINE GNEISSES
•
TIM•TEO GRANITE
_-• LOWER PROTEROZOIC BRUMADO • METAMORPHIC COMPLEX
Roods
iI•[•-• MIGMATITES
ORE. [DEPOSffS/URANIFEROUS ANOMALIES(AN)
FIG. 2. Simplifiedgeologicmap of the LagoaReal area showingthe distributionof the various uraniumdeposits. Sources: jointgeologic mapping, Secretaria de Minase Energia(SME)andCompanhia Bahianade PesquisaMineral (CBPM)/NUCLEBRAS,unpub.map, 1985).
972
LYDIA M. LOBATO AND WILLIAM S. FYFE
that on Rb/Sr isochrondata.In spiteof the controversy and480 Ma, respectively.The authorssuggested surroundingthe Espinhaqostratigraphyandits prob- both mineralizingand metasomaticeventsoccurred lematical correlationsto the ChapadaDiamantina at 1,400 Ma, with later reworking in the Brazilian stratigraphy(L. A. Moutinhoda Costaet al., unpub. cycle("-'480 Ma), duringthe eventwhichprobably (by shearingor metamorrept., 1976; Schobbenhaus et al., 1976), the period led to the gneissification 1.2 to 1.3 Ga is considered to include the main tec-
phisre)ofthe granite(atimespanof about1,000Ma).
tonic and metamorphicevent for the EspinhaqoSupergroup, with post-tectonicevents extending to 1,000 to 950 Ma (Jardimde S• et al., 1976a, b; Inda and Barbosa,1978; Jardimde S•, 1978). Rb/Sragedeterminationsof gneissicrockscloseto the Lagoa Real ore depositswere undertakenby U. G. Cordani(unpub.repts.,1982, 1983). Isochrons yielding agesof 2,600, 1,750, and 1,520 Ma were obtainedfor basementmigmatites,for the S5oTim6teo Granite, and for foliated microline gneisses (shearedgranite), respectively.Foliated microcline gneissesin the southernhalf of the provincealign alongan ill-definedisochronwith t = 1,000 Ma. Samples of amphibolites intercalated with microcline gneissesyield an age of 624 _ 70 Ma. U-Pb age determinationscarried out by J. A. Cooper (unpub. rept., 1979) in commonmineralized albitites indicated an age of 820 Ma for the uranium mineraliza-
Suchinterpretationis difficultto reconcilewith the generallyacceptednotionthat all LagoaRealuranium depositsoccurassociated with metasomatized sheared rocks,asshownlaterin thispaper.It isthusreasonable to assumethat theseprocesses (mineralization-shearing-metasomatism) are not separatedin time, assuggested.The geochronologic data,their discussion and the conclusions derivedfrom them, are not the object of the presentpaper,thoughthe subjectdeservesfu-
tion.
U. G. Cordani(unpub.rept., 1983) suggested that the regionalK feldspargneissesborderingthe ore depositswere metamorphosed at 1,520 Ma andlater reworked in the Brazilian cycle. The uraniferous mineralizationis probably associatedwith this last event, and the age of 1,000 Ma may representthe minimumage of metasomaticalteration. K/Ar age datingin biotitesof two foliated microclinegneisses andof onesampleof mineralizedalbitite are as follows:738 ___ 28, 677 _+24 (gneisses), 687 ___ 26 Ma (mineralizedalbitite) (R. Armstrong,writ. commun.,1985). Aspointedout by Inda andBarbosa (1978), Rb/Sr Brazilian agesof 450 to 700 Ma are commonlyfound in the Paramirimbasementrocks alongthe EspinhaqoRange,andthesehavebeen attributedto the reworkingof the basementdueto tectonicmovementsin the Braziliancycle.It is therefore reasonableto interpret the K/Ar agesin biotites as representingthe resettingof the isotopesin late Proterozoic
times.
Afterthe preparationof thismanuscript anditslater review, Turpin et al. (1988) presentedage determinationsobtainedby U/Pb, Rb/Sr,andSm/Ndsystematicsfor the LagoaReal region.Basedessentiallyon U/Pb datathey concluded thatU/Pbvaluesonzircons fromboth graniteandshearedgraniteindicateanage of 1,725 Ma for the LagoaReal graniticcomplex(in agreementwith Cordani'sRb/Srresults)andthat U/ Pb valueson heavy mineral concentratefractions (magnetite+ zircon+ uraninite),aswell asuraninite separates (zirconsyieldedincoherentresults),define a discordia with anupperandlowerinterceptat 1,400
ture detailed treatment, in order to accommodate
other geologicconstraints. Therefore,thesemorerecent resultswill not be consideredhere any further. Petrographyand Mineral Chemistry The LagoaReal ore depositsare distributedalong 33 km from northto south,formingan arcuatezone of 32 anomalies,from which 12 havebeenthe object of researchandstudyby Nuclebr•ts-Empresas NuclearesBrasileiras(Fig. 2). General descriptionsof the ore depositscan be found in Geisel Sobrinhoet al. (1980), Steinet al. (1980), RaposoandMatos(1982), Villaqa and Hashizume(1982), Brito et al. (1984), Raposoet al. (1984), andRibeiro et al. (1984). Most albititebodiestrend N 40 øE to N 30 øW anddip 35 ø to 90 ø to the southwest or northwest. Each mineral-
ized body may vary from a few centimetersto about i km in length, averaging6 to 7 m in width (max-- 30 m) with mineralizedlevelsreachingdownto at least 850 m in depth (Villaqaand Hashizume,1982). The contacts betweenalbititesandhostrocksaregenerally transitional,but sharp contactsare also commonly found.
Regionalrocks Representative microprobeanalyses of someof the minerals discussed in this and the next two sections
are given in Tables 1 and 2 (from data in Lobato, 1985). The regionalgneisses surroundingthe areaare deformedcoarse-grained augengneisses, incipientlyfoliated; they occur in metric shearzoneswithin undeformedgranite. The typical texture is porphyroclasticwith highly strainedorthoclase,plagioclase, andquartzporphyroclasts appearingin a groundmass (usually•50%) of finelyground,partlyrecrystallized material.The mineralogyincludesperthiticorthoclase (35-45%), plagioclase (maybe zonedor antiperthitic, 10-25%), hornblende(10-15%, spottedby granular sphene),biotite (5-10%, replacinghornblende),rare chlorite and calcite (after mafics),and accessoryil-
URANIUM MINERALIZATION
AT LAGOA REAL
973
menite,magnetite(bothmantiedby sphene),apatite, grained,vermiform,or medium-grained granoblastic, zircon,fluorite,andrare allaniteandsulfides.Plagio- at timesdisplayingzoning.It is a grossular-andradite claseis albite (An = 3-5 mole %) or oligoclase(An garnet,with the grossularend memberrangingfrom = 11-18%). Hornblendeispartof the pargasite-hast- 38 to 55 percent and the andradite end member ingsiteseries(Deer et al., 1976) andbestend-member rangingfrom 38 to 58 percent.When zoned,the garnamesare hastingsite andFe pargasite(Papikeet al., net displaysalmandine-and grossular-richcenters 1974), with Fetotal/Mg ionicratiosrangingfrom 15 to (52% almandine,30% grossular),whichare replaced 60. Biotitesare very rich in iron (iron biotites,Foster, by andradite-rich borders(up to 88% andradite).Py1960), with Fetotal/Mg ratiosrangingfrom 10 to 37. roxenes(5-20%) plot ashedenbergiteandferrosalite Undeformed granitesmainlycropoutin thenorth- on the diopside-hedenbergite-ferrosilite-enstatite ern andnortheastern zonesof the area(Fig. 2). They quadrilateraland may containup to 8 percentof the are blue-gray,porphyritic,and coarsegrained,with jadeite component(calculationsaccordingto Papike lowproportions of maficminerals.Amphibole-biotite et al., 1974). If zoned,pyroxenebordersare enriched graniteis the commonest, whereaspyroxenegranite in Fe203 and Na (up to 3.5%). Epidote (Ps = 28) crystals,associated with garnet, andbiotitegraniteare scarcer.Maruejol(1989) and occursasgranoblastic Maruejolet al. (1987) presentthoroughpetrographic, due to the oxidationreactionexperiencedby the latmineral,androckchemistrydatafor the S5oTim0teo ter. Epidote may also appear as a result of the amphiboledehydrationreaction(Fig. 3). granitic rocks. Blocksof typical charnockites and amphibolites Calciteand/orchloriteare rare, andif present,appear replacingmaficminerals.Accessories are magoccur locally. netite, fluorite,apatite,zircon,allanite,andrare sulTransitional rocks fides(seealsoMaruejol,1989, for detaileddescription As one approachesthe zonesof mostintensemeta- of accessoryminerals). somaticalteration,the regionalgneisses hostingthe Metasomatic rocks
albitizedrocksacquireahighlyfoliatedfabricandare fineto mediumgrained.Hereafter,theywill be re-
Once all original mineral assemblages have been modified,the resultingproducts(metasomatic rocks) are albite gneisses (albite < 50%) or albitites (albite isfine-to medium-grained granoblastic andpolygonal granoblastic; porphyroclastic texturesaresecondary.> 50%) with varyingamountsof maficmineralsand The transitional rocks have been divided into two a groupof epidote-richrocks(epidosites). The albititesare fine- to medium-grained gneissic groups:type A which containsthe samebasic minrocks showing incipient to pronounced foliation. At eralogyasthe regionalgneisses, andtype B which exhibitspyroxeneand/orgarnetreplacingoriginal the outcrop, they may exhibit an augenlikefabric, hornblende alongbordersandcleavage lines(Fig.3). similarto the host gneisses,exceptthat each augen In bothtypes,plagioclase iseitheralbite(An = 4- is now composedof fine-grainedpolygonal albite. 8%) or oligoclase (An = 10-19%) andK feldsparis They are typically polygonalgranoblastic(Fig. 3), surgranoblastic microcline.A very commonfeatureof with albite occurringin mortar concentrations, rounded by granoblastic mafic mineral patches. Their mosttransitionalrocksis the presenceof fine albite rims(An= 3-4%) surrounding theoriginalplagioclase mineralogyincludesalbite (An = 3-7%), aegirineaugite (5-20%, altering to hornblende, and more (seeTable 2). In a few type B rocks,incipientrerarely, to epidote,calcite,or prehnite), andradite(5placementof microclineby albite canbe observed. 20%, replacingaegirine-augite; it mayalterto epidote Hornblendeis hastingsite and Fe pargasitewith or calcite),hornblende(usuallylessthan 5%, but it ferred to as transitional rocks. The dominant texture
Fetotal/Mg ionicratiosrangingfrom45 to 85 (in type may be up to 15%; in few samplesit may alter to A) and3 to 29 (in type B). WhereasFe+S/Fe +2ratios biotite or chlorite, and calcite),biotite (at timesup (calculatedaccordingto Papikeet al., 1974) of am- to 15% after hornblende),calcite(occurslocally,as phibolesfromregionalgneisses andtypeA transitional a primary mineral in individualgrainsor replacing rocksvaryfrom '--0.03 to 0.20, in type B transitional marlcsin up to 15%, andalsoaslateveins),microcline rocksthesevary from --'0.08 to asmuchas0.32, due (in '•5%, usuallyinterstitialto andbeingsubstituted to its replacementby garnetand/or pyroxene.Am- by albite;in a few samplesit occursin recrystallized phibolereplacement iscommonly seenalongborders portionsin '• 15%), quartz (about3-5%, and lessas of grainsin association with magnetite, fluorite,and/ agglomerationsin •--10%, or aslate veins). Common or epidote (Fig. 3). accessories are sphene,magnetite (with up to 9% Garnet, pyroxene, amphibole, and subordinate TiO2; it mayoccurin asmuchas5%), hematite(from biotite andepidoteappearaselongatedconcentra- magnetite;Fig. 3), fluorite,allanite,zircon,apatite, tionsof fine-to medium-grained granoblastic andxe- and rare pyrite. Ca-rich albitites (albitites richer in noblastic crystals.Garnet(10-40%) canappearfine- CaO and MgO in whole rock) are foundsporadically
974
LYDIAM. LOBATOAND WILLIAMS.FYFE
URANIUMMINERALIZATION AT LAGOA REAL
975
andtheymaycontain,in additionto all mineralslisted abovefor commonalbitites,abundantcalcite(• 25%),
wollastonite(•-10% weakly altered to fibrouspectolite), and rare vesuvianite(•-5%); magnetiteand spheneareusuallyabsent.Almostpurealbitites(albite >• 90-95%) are found aswell. In albitites,pyroxeneis fine to mediumgrained, commonlyzoned.It isa memberof the diopside-hed-
enbergiteseries,in mostcases withFe+3> 0.2 (aegirine-augite,Deer et al., 1978), andcontains8 to 35 percentof the acmiteend member.If zoned,light greencoresgradeinto darkergreenborders,reflect-
ing the replacement NaFe+3 < = > Ca (Mg, Fe+2) (Fig. 4; seealsoLobatoet al., 1983b). In Ca-richalbitites, pyroxenesare diopsidic(MgO up to 12%). Andraditeis fine-grainedxenoblastic or granoblastic, replacingpyroxenealongbordersand cleavage lines.It containsup to 12 percentof the grossular end member;in Ca-rich albitites,garnet contains26 to 37 percentgrossular. When mineralized, common albitites contain ura-
ninite in very fine roundcrystals(5-25 t•m),chiefly associated with andradite but also found inside and
borderingpyroxene,albite,hornblende,biotite,calcite, and hematite(Fig. 3). In mineralizedalbitites, magnetite(presentin mostcases)is being oxidized to form hematite. In Ca-rich albitites, uraninite is al-
mostexclusivelyassociated with andradite.Pure albitites are not mineralized.
Epidositesoccurrestrictivelyin the region.They form bandsor layersvaryingfrom a few centimeters to 1 or 2 m in thickness,parallelto the mainfoliation andwedgingout at depthsof about100 m (C. Raposo, unpub.rept., 1984). They are dark green,massive,
fine-to medium-grained polygonal granoblastic rocks and their mineralogyincludesepidote (Ps = 2628, in 50-80%; it replacesor is pseudomorphous after pyroxene),oligoclase(An = 18%, in 20-30%), quartz(5-35%), aegirine-augite (2-15%), hornblende (•-1%, after pyroxene),and accessory microcline, sphene,zircon,andapatite.Epidotemaycontainup to 1.2 percentUO2 and displaysa metamictappearance;uraniniteis usuallyabsent.In epidosites,pyroxenemayalsobe zoned,andasin Ca-richalbitites, it is richer in MgO. Unmineralizedepidositesoccur as well.
Albitized hornblende-richrockscomprisea great part of the Cachoeiraore depositin the north (Fig. 2). Though one of the mosturaniferousdepositsin the whole province,it is yet not well documented. Preliminarypetrographicstudiesshowthat the rocks are highly foliated, commonlyporphyroclastic, and exhibitup to 20 percenthornblende,partiallytransformedinto aegirine-augite.However,andraditefrom the aegirine-augite is commonlylackinganduraninite is mostlyassociatedwith amphibole,pyroxene,and opaqueminerals.
976
LYDIA M. LOBATO AND WILLIAM S. FYFE TABLE
2.
RepresentativeChemical Analysesof Plagioclasein Regional,Transitional,and MetasomaticRocks Regional
Sample no. Wt percent SiO2 A1203 CaO
Na20 KzO Total
8
64.08 22.27
Transitional(types A and B) 37
67.34 19.93
Metasomatic
791
29.07
29.07
188.75
186.70
67.98 19.92
64.02 22.44
67.28 20.21
68.53 19.22
67.42 20.87
45-1
206
68.17 19.49
64.40 22.97
3.48
0.77
1.64
3.21
0.89
0.25
0.75
0.40
3.50
10.11 0.08
11.60 0.10
10.77 0.26
10.70 0.05
11.48 0.28
12.20 0.09
11.00 0.15
11.62 0.11
9.65 0.10
100.02
99.74
100.57
100.42
100.14
100.29
100.19
99.79
100.62
Number of ionson the basisof 32 oxygens Si
A1 Ca Na K
Anorthite (%) Albite (%) Orthoclase (%)
11.322
11.837
11.852
11.283
11.793
11.971
11.770
11.949
11.287
4.637 0.659 3.463 0.018
4.128 0.145 3.953 0.022
4.092 0.306 3.640 0.058
4.660 0.606 3.656 0.011
4.174 0.167 3.902 0.063
3.956 0.047 4.132 0.020
4.293 0.140 3.723 0.033
4.026 0.075 3.949 0.025
4.744 0.657 3.279 0.022
15.91 83.65 0.44
3.52 95.94 0.54
7.65 90.91 1.44
14.18 85.55 0.26
4.05 94.44 1.52
1.11 98.41 0.48
3.60 95.54 0.86
1.86 97.54 0.61
16.60 82.83 0.56
Samples: 29.07•, rimmedplagioclase, center;29.072sameas1, rim; 188.75 and 186.70, albitefromcommonalbitites;45-1, albite from Ca-rich albitite; 206, oligoclasefrom epidosite
crustalabundanceof 2.6 ppm (Dyck, 1978) but they reportedby DostalandCapedri Representativechemicalanalysesof typical re- aresimilarto gneisses (1978), which are in the rangeof i to 3 ppm. gionalgneisses, transitional andmetasomatic rocksare Type A transitional rocks representthe leastmoddisplayedin Tables3 and4. The followingdiscussion ified suite of rocks in the zone of alteration, in both is basedon a much larger numberof analysespremineralogicand chemicalterms, being principally sentedin Lobato (1985). The generalcharacteristics of the ParamirimCom- characterizedby highly shearedtextures. Though plex rocksindicatethat the complexis rathersimilar morevariable,their overallchemistryisrather similar More noticeableare to otherbasementterranesaroundthe world (Tarney to that of the regionalgneisses. the higher Na/K ratios (avg of 10 samples= 3.99), et al., 1979). The LagoaReal granitescorrespondto probably related to albite neoformation asrimsaround adamellites andgranites,with pyroxenegraniteslying original plagioclase. closeto the limit of graniteandquartzsyenitefields, Comparedto regionalgneissesand type A transipresentingmineralogical andgeochemical charactertional rocks,the chemistryof type B transitional rocks istics intermediate between calc-alkaline and alkaline is characterized by somewhat lower SiO2 and K20 associations but closerto the lastone(Maruejolet al., values, with Na/K ratios averaging 5.56 (from five 1987). The regionalgneisses to the north and south of the U depositsare mostlygraniticin composition samplesonly); Rb is substantiallylower and Sr is In typesA andB Th variesfrom 11 to 27 ppm and are rather uniform geochemically.Their Na/K higher. from a total of ten determinations. ratiosare in mostcasesaround0.5. CaO is fairly low Basedsolelyon the chemicalanalysesof albitites, and it is chiefly related to varyingplagioclasecompositionsand concentrations; it positivelycorrelates the moststrikingchemicalchangesexperiencedduralterationare a decreasein SiO• (as with Sr. The averageK/Rb ratio of the regional ing metasomatic low as 55%) and K•O contents(aslow as0.03%; Figs. gneissesis 280 (from 16 samples),which is a little 5 and 6); an increase in Na•O (from 6.2 to 10.65), higher but closeto the averageof 230 for normal higherSr,Pb,V, andU contents; uppercrustalrocks(Taylor,1964). The onlygranulite CaO,andFe•O3(total); faciesrock analyzedshowsa K/Rb ratio of 1,027, lower Rb andBacontents;andslightlyhigherNb, Zr, which is in good agreementwith Rb depletionen- and Y contents. Mass balance studiesbased on Gresens' (1967) countered in other high-gradebasementterranes givenabove, (TarneyandWindley, 1977). The averageU content work confirmedsomeof the conclusions in the regionalgneisses(17 analyses)is 3.86 ppm, showingthat, in albitites,K20 is lostin roughlythe which is about •1.4 times higher than the average sameamountthat Na•O is gained--the averageNa•O Geochemistry
URANIUM MINERALIZATION
AT LAGOA REAL
977
1
50
100
i..tm
pm
100 L
ura •
25 pm
FIG. 3. Thin and polishedsectionphotomicrographs. A. Hastingsite(am)beingreplacedby hedenbergite(pyr) plusepidote(ep) in type B transitionalrock.B. Hastingsite(am)beingreplacedby grossular-andradite (gar) in type B transitionalrock. C. Polygonalgranoblastictexture in mineralized albitite (blackdots= uraninite).D. Uraninitecrystalsassociated with an andraditeborderin mineralized albitite. E. Martitizationof magnetitein mineralizedalbitite (gray= magnetite,white = hematite).F. Uraninite(ur) crystalsassociated with hematite(hem) in mineralizedalbitite. Am = amphibole,ep = epidote, gar = garnet, pyr = pyroxene.
gain = 5.83 g, the averageK20 loss= 5.00 g (Lobato et al., 1982). A volumelossof •10 percentwasalso estimated,thusconstraining the interpretationonthe variationsin eachchemicalelementfrom "original" to "altered" assemblages. For that purpose,the min-
eral reactions discussedlater in the text must als0 be taken into consideration.
AlthoughFe2Oa(tot•), CaO,MgO,andTiO• allshow a slightgeneralincreasein commonalbitites,suchan increase can be related to the volume loss,since min-
978
LYDIA M. LOBATO AND WILLIAM S. FYFE Acmite
deformedresultingin the developmentof foliated, fine- to medium-grainedgranoblasticrocks(type A transitionalrocks).In places,this textural reorganization is accompaniedby somemineral transformationswhich include albite neoformation(albite rims and K feldsparreplacement),partial or almostcomplete replacementof originalhornblendeby grossular-andradite and/or hedenbergite-Fesalite, and sphenitizationof ilmeniteor titanomagnetite (type B transitionalrocks):
feldspar+ Na+ = albite+ K+/Ca+2,
(1)
hastingsite + O2 + SiO2= grossular-andradite + hedenbergite-Fesalite+ magnetite+ fluid,
Diopside
Hedenbergite
and
FIG. 4. Chemicalvariationofclinopyroxenes expressed asthe 3FeTiO3 + 202 + 3Ca+ + 3SiO2 end membersacmite,diopside,andhedenbergitein metasomatic = Fe304 + 3CaSiTiO5. (3) rocks,exceptfor thedashedlinewhichisfroma typeB transitional rock. Arrows indicate trends from core to rim zoned grains(data Zoningin both garnetand pyroxenesuggests that from Lobato, 1985).
oxidationof mineral phasesis continuous,accompanied by Na enrichmentin the latter. eral reactions do not account for the introduction of Thomas(1982) investigated the upperthermalstaeitheroneof theseelements; higherCaOandslightly bilityofhastingsite (NaCa2Fe4 +4Fe+3S•6A12022(OH)2) ß higher MgO valuesin Ca-richalbititesprobablyre- asa functionof temperature,fluid pressure,and oxflect the originalbulk composition. Amongthe trace ygen fugacity. His experimentsshowedthat under elements, the small increasesin Nb and Zr and less fo2 conditions controlledby the FMQ buffer,hast-
in Y may alsobe related to volume loss.The Sr increase is possiblyrelated to the CaO increase,al-
thoughthey are not perfectlycorrelative.Largedecreasesin Ba and Rb are related to the K20 decrease.
Th is practicallyunchangedcomparedwith the values obtained for transitional rocks; from 16 determina-
tionsthe Th contentin albititesrangesfrom22 to 45 ppm.V isclearlyhigherin all albitites(about10 times) andcorrelateswell with U. In regionalgneisses and transitionalrocks,V variesfrom i to 24 ppm. In unmineralizedalbitites,V varies from 7 to 344 ppm whereasin mineralizedalbitites,Ca-rich albitites,and
epidosites, V rangesfrom40 to 871 ppm.
ingsitedehydratesto producethe assemblage hedenbergite, a grossular-andradite garnet, magnetite, and plagioclaseplus fluid, in the presenceof silica. Similarresultswereobtainedfor ferro-pargasite (Gilbert, 1966). The petrographicevidencefor the mineral transformationsin type B transitionalrocksindicatesthat conditionssimilarto thosedescribedby theseauthorsprevailedin the initial stagesof metasomaticalterationat LagoaReal. Furthermore,Haggerty (1976) has shownthat at high SiO2 activity, magnetiteand sphenewill form at the expenseof ilmenitc,at oxygenfugacitiesbetweenthe FMQ and NNO
buffers.
Uranium correlates well with Na20, but it is best
In albitites,all originalfeldsparsare replacedby correlated withFe2Oa.Figure7 isa plotofFe+a/•;Fe albite,andpetrographicevidencefor the substitution versusU showingthat uraniumis moreconcentrated of microclineby albite is widespread: in rockswhere thisratio is 0.5 or higher. KA1Si3Os + Na+ = NaAISi3Os+ K+. (4) The geochemicalcharacterof epidositesalsoreflectsa distinctoriginalbulk chemistrysincethese Pyroxenein albititesis aegirine-augite,commonly are very poor in Na20 and showhighervaluesof loss zoned,indicatingcontinuous oxidationcoupledwith on ignition. Nonetheless,the trace elements follow Na enrichment.Further oxidationof pyroxeneis the samepattern asin all other albitites. characterized by itsreplacement by andradite.Ti-rieh magnetite (up to 9% TiO2) is also beingreplacedby Metamorphic Reactions spheneand/orhematite: Sequence
Close to the zone of most intense metasomatic al-
teration,the hostregionalgneisses are progressively
hedenbergite-Fesalite+ O2 + Na+ = aegirine-augite, (5)
URANIUMMINERALIZATIONAT LAGOAREAL
979
Plithostatic + tensile peraturesin the range of about 500 ø to 600øC at strength)shouldhave prevailedat least duringthe moderate pressures(Turner, 1981), estimatedat 4 initial shearingprocess(Fyfe et al., 1978). kbars or less. The characteristicsof the mineralizing fluid and 6. Oxygenisotopethermometryyields tempera- the conditionsunder which it moved are tentatively tureswhichare compatiblewith the metamorphicas- assessed by the typesof alterationdescribedsofar: semblages in the rangeof 500 ø to 550øC (Lobatoet 1. The fluidsmusthave a high Na/K ratio. Given al., 1983b;K. Fuzikawa,unpub.rept., 1985;Lobato, thatsodium 1985). The •1so isotopevaluesof the fluidphasein someof thehighsalinitydata,it ispossible equilibriumwith the mineralogyin unmineralizedal- chloridebrines may havebeen involved. 2. To accountfor a 10 percentSiO• lossandvirtual bitites range from -0.8 to +7.3 per mil, and from -3.7 to +2.6 in mineralizedalbitites(K. Fuzikawa, removalof quartz,fluidsmustbe movingup a thermal
URANIUM MINERALIZATION AT LAGOA REAL
985
Ti ___ REE oxidesand gradient(Fyfe et al., 1978). If fluidsoriginatedin any ing accessoryminerals(Nb ___ normal magmatic source, they would precipitate silicates,allanitc, zircon, and apatite, in order of desilica. ceasingU abundance)could represent a favorable uranium source.However, this interpretation is ren-
3. Fluids were oxidizing. Normal deep metamorphicfluidscontainexcessH2, andwhenmovingdown temperatures,reducerockson their path (Beachand Fyfe, 1979.).In addition,oxygenisotopedata help rule out high-temperaturemetamorphicfluids.
In the LagoaRealdistrict,thisstructuremayhave resultedfrom the updomingof the crustto the east, probablyby a thermalhighandsubsequently, the up-
A possiblemodelarisesfromthe regionalstructure of the area. A major thruststructurehaslongbeen reportedin the areaalongthe Espinha9oRange-basement border (L. A. Moutinho da Costaet al., unpub. rept., 1976; Schobbenhaus et al., 1976; Jardimde Sft, 1978; Moutinho da Costa and Inda, 1989.). This structuralfeaturehasbeeninterpretedashavingbeen generatedby the upliftingof the ParamirimComplex, posteriorto the main metamorphicevent in the Espinha9oRange. Along this segment,the basement
in a mechanismsimilarto that describedby Hyndman (1980; see also Coney, 1983). Acid plutonic rocks farther to the north datedat 815 Ma have an agesimilar to that proposedfor the uraniummineralization. The suggestionthat the metasomaticalterationmineralization at Lagoa Real is linked to a thrust mechanism wasfirst put forwardby W. S. Fyfe (unpub. rept., 1979) and later discussed in Lobato et al. (1989.,1983a,b). Thispropositiontakesinto considerationthe structuraland gravimetricevidencedoc-
rocks were thrust and folded to the west over the
umented in the literature, as mentioned above. It is,
Espinha9ostrata;there isno doubtthat the latter dips under the basement.Retrogrademetamorphismin parts of the basementto the eastand the local progrademetamorphismin rocksof the lower and middle Espinha9o(appearanceof kyanite and chloritoid) stratamay be related to the thrusting.This general structureappearsto be commonfor about 9.00 km alonga generalnorth-southtrend in Bahiaandto the southinto the Stateof MinasGerais.Recentfieldtrips
however, important to point out that attempting to model the mechanism(s) by which a thruststructure evolvedin the regionis complicatedby the poor and much controversialgeologicknowledgeat the present, andtherefore,transcends the scopeof the present
dereddifficultby theS7Sr/S6Sr results.
domed basement was detached and slid to the west,
undertaken to the north of Minas Gerais, closeto the border with State of Bahia, confirm sucha statement.
To supportthisline of evidenceare the gravimetric andmagneticstudiesreportedby Motta et al. (1981). To the eastof the Stateof Bahia (towardthe coast), depthsof the crust-mantleinterfacewere calculated at 39.km. Suchdepth increasesto about43 km at the Paramirimvalley region,where gravitydata suggest a geofracture that coincideswith the Espinha9o alignment,fromaboutnorthof OliveiradosBrejinhos (12ø 00') to a little north of Caetit• (14ø 00'). This structureconstitutesan elliptical negativeanomaly trendingN 30o-45 øW. Thickeningof the crustacross major thrust faults is a well-documented feature (FountainandSalisbury,1981). The uranium anomaliesmake up an arcuate zone parallelto this regionalstructure.Figueiredo(1980, 1981) andSighinolfiet al. (1981) havedrawnattention to the geochemistry of granulitesover wide regionsof the Stateof Bahia(i.e., lack of U, Rb depletion). The regionalgraniticgneisses surrounding the oredepositshavebeensubjectedto amphibolitefacies metamorphismand have an averageU contentof 3.8 ppm (Lobato, 1985), similarto resultsreportedby Dostal and Capedri (1978). The relatively high U contentof thesegneisses hasled to speculations that thesecouldbe the sourceof uranium(E. GeiselSobrinho,unpub.rept., 1981;Raposoet al., 1984). Maruejol et al. (1987) suggestedthat the uranium-bear-
paper.
A 12-km-thickgravity-driven thrustslicemusthave had basaltemperaturescloseto the melting temperatureof granite(Hyndman,1980). Suchtemperatures could explainthe progrademetamorphismobserved in the underthrustEspinha9o,near the contactwith the basementrocks.When thrustingoccurs,compaction of the underplatemustoccurrapidly,expelling pore water from it. High fluid pressures(similarto or greaterthan lithostaticpressures)are likely to be attained;this is a necessaryrequirementto maintain thrust motion,as first pointed out by Hubbert and Rubey(1959). Fluidsgeneratedat the baseby loading and heating would penetrate the basementby hydraulic fracture mechanism,resultingin retrograde metamorphismof the baseof the overridingplate. Risingfluidswill encounterinverted thermalgradients;the extent of retrogrademetamorphismdependson the distributionof temperatureandthe path taken by the escapingfluids (Crawford and Mark, 1989.).The underplatehigh-levelrockswould tend to be more oxidized
than the basement rocks and
cold. Salty fluids rising under the thrust into a hot overplate at high fluid pressurescould causeoxidation, silica removal, and sodium metasomatism.The
regions of fluid penetrationwould be confinedto zonesof weakness in the overthrustplate (shearzones; Fig. 8).
Oxygenisotopesystematics of theLagoaRealrocks lend supportto the thrust model.The trend toward lighter isotopicratiosin the more altered rocksand the relativeconstancy of isotopicfractionations among mineralsin all rock types(K. Fuzikawa,unpub.rept.,
986
LYDIA M. LOBATO AND WILLIAM S. FYFE
W
•.••
E
'•
•
Inverted metamorphismin regionalmetamorphic terranesassociated with thrustinghasgaineda lot of attentionin recentyears(GrahamandEngland,1976;
GNEISSIC Crawford and Mark,
BASEMENT
1982; Arita, 1983; Peacock,
1987; Duebendorfer,1988). Fluidsthat are moving up a thermal gradientcan do soby movingdown or up intorocks.Eitherprocess,if involvinga light,salty meteoric water moving from an oxidized zone and precipitatinguraniumuponreduction,couldexplain oxidation, silica removal, and Na metasomatism.In
consideringthe former case as a secondpossible model,the greatproblemisto havea mechanism for the very deep descentof suchfluidsinto a zone at FIG. 8. Schematicdiagramdisplayingthe proposedmodelfor over 500øC and '--12 km in depth. In this case,one the LagoaReal region. Dewateringof the Espinhaqosediments possiblemechanismmight involve a rising diapiric occursasa responseto load causedby thrusting.Flow is concen- pluton at even greater depth, with a fracturesystem trated alongthrust plane and in placespenetrateszonesof en- penetratingfromgreatdepth,assuggested by E. Geihancedpermeability(shearzones)leadingto metasomatism and sel Sobrinho(unpub. rept., 1981). Major problems mineralization. with this model are that at the P-T conditions envi-
sioned,rocksare plasticandimpermeable(Rutterand Brodie,1985) andit isvery difficultto producedeep by a descentmodel. 1985) is compatiblewith an early temperatureepi- conditionswhere P•uia> Plithostatie sode,followed by progradeisotopicreequilibration The nature of the fluidswhich might be released (Lobato, 1985). This processcouldbe explainedby from a sedimentarysequencelike that of the Espina thrustmechanismoccurringin a singlefastphase, hacois not known.As pointedout earlier, the metasothat maximumtemperatureswere attainedrapidly. sedimentsof the EspinhacoRange reflect a diverse OxburghandTurcotte (1974) studiedthe thermal spectrumof platformalcontinental,coastal,and maof marinepelites gradientsand regionalmetamorphismin overthrust rine sedimentaryfacies;a succession terranes,in particular,the easternAlps.In considering predominateto the westwhereasto the east,fluviala thrustsheet15 km thick with a temperatureof T at deltaicsedimentsare more abundant(Jardimde SA, the baseof the slaband a temperatureof zero at the 1978). An analogycan be made with modern forby lowfi]80values(Taytop of the underlyingrocks,they showedthat after mationbrinescharacterized thrustingthere is an initial coolingin the slabat all lor, 1979) and enrichmentin dissolvedspecies,and depths.After 4 millionyears,steadytemperaturein- which may be oxidizing(Hanor, 1979). creasesoccur at all depths. The initial temperature A final point to be mentionedis the constantasdistributionandassociated negativethermalgradients sociation of vanadium with the mineralized rocks. Its are extinguishedin lessthan a million years,andby associationwith the mineralizationagain may lend 3.3 million years,the gradientthroughthe slab is supportto a low-temperaturesedimentaryorigin of nearly linear and has abouthalf of the value of the the fluid. Vanadiumis an importantspeciesassociated initial gradient.Toward this stage,normalmetamor- with a numberof low-temperatureuraniumdeposits phic conditionsshouldbe reestablished astempera- in sedimentswhere it is in part responsiblefor urature reequilibrationis takingplace. nium precipitation(Adler, 1974). If the temperaturein LagoaReal is interpretedin Acknowledgments a similar fashion, one can conclude either that the
This paper representspart of a Ph.D. thesisdone altered rocksexposedtoday do not come from very deeplevelsof the basementslab,becausethere is no by the first authorat the Universityof WesternOnof PRONUCLEAR and evidenceof widespreadlow-temperaturealterations, tario under the sponsorship or that later reequilibrationwith highertemperatures the Commis5oNacionalde EnergiaNuclear(formerly (return to normal metamorphicthermal gradients) NuclebrAs).Complementaryfinancialsupportwas have maskedinitial lower temperatureassemblages. alsoprovidedby the Universityof Western Ontario. The authors wish to thank Comiss5o Nacional de Late-stagemineral reactions,which produceassemblagessimilarto thoseobservedin the hostgneisses EnergiaNuclear(BeloHorizonteandCaetit6regional andlower temperatureminerals,indicatethat, in the offices)for grantingpermissionandaccessto the Laandprowanningstagesof metasomatic alteration,there was goaRealarea,allowingcollectionof samples a tendencytoward reestablishment of earlier meta- vidingmaps,reports,etc. We alsoexpressour deep morphic conditions(return to normal geothermal gratitudeto allthestaffat CentrodeDesenvolvimento da TecnologiaNuclear (particularlyat the Departagradient).
URANIUMMINERALIZATION AT LAGOAREAL
mentode TecnologiaMineral) for their greatsupport -throughoutthe preparation of the thesis and the present manuscript.
The senior author is indebted to J. M. A. Forman
for his encouragementandto M. G. Ferreira, Jr., for help, discussion,and assistance at variousstagesof the investigation andmanuscriptpreparation.Thanks are due to K. Fuzikawa and A. Kishida for comments
andsuggestions. The help and supportfrom faculty, staff,and studentsat the Universityof Western Ontario is gratefullyacknowledged. REFERENCES
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