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New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

Open Geosciences

October 2013
5(3)

DOI:10.2478/s13533-012-0134-7

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CC BY-NC-ND 3.0

Authors:

Alexandre de Oliveira Chaves

Federal University of Minas Gerais

New evidence supported by petrography (including mineral chemistry), lithogeochemistry, U-Pb geochronology by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), and physicochemical study of fluid and melt inclusions by LA-ICP-MS and microthermometry, point to an orogenic setting of Lagoa Real (Bahia-Brazil) involving uraniferous mineralization. Unlike the previous models in which uraniferous albitites represent Na-metasomatised 1.75 Ga anorogenic granitic rocks, it is understood here that they correspond to metamorphosed sodium-rich and quartz-free 1.9 Ga late-orogenic syenitic rocks (Na-metasyenites). These syenitic rocks are rich not only in albite, but also in U-rich titanite (source of uranium). The interpretation of geochemical data points to a petrogenetic connection between alkali-diorite (local amphibolite protolith) and sodic syenite by fractional crystallization through a transalkaline series. This magmatic differentiation occurred either before or during shear processes, which in turn led to albitite and amphibolite formation. The metamorphic reactions, which include intense recrystallization of magmatic minerals, led uraninite to precipitate at 1.87 Ga under Oxidation/Reduction control. A second population of uraninites was also generated by the reactivation of shear zones during the 0.6 Ga Brasiliano Orogeny. The geotectonic implications include the importance of the Orosirian event in the Paramirim Block during paleoproterozoic Săo Francisco Craton edification and the influence of the Brasiliano event in the Paramirim Block during the West-Gondwana assembly processes. The regional microcline-gneiss, whose protolith is a 2.0 Ga syn-collisional potassic granite, represents the albitite host rock. The microcilne-gneiss has no petrogenetic association to the syenite (albitite protolith) in magmatic evolutionary terms.

Geological map of the Lagoa Real uraniferous albitites, Bahia (BA-Brazil). Modified from [11] and [1]. Crosssection presents UO 2 contents in ppm for some mineralized levels of the anomaly 3, which represents one of 34 mineralized albitite with high uranium content of the Lagoa Real Uraniferous Province. In addition to surrounding microcline-gneisses, samples of amphibolites and mineralized albitites from anomalies 1, 3, 7, 9, and 13 (An1, An3, An7, An9, and An13) have been investigated in this work.

…

Sharp contacts between uraniferous albitite bodies and microcline-gneiss from Cachoeira Mine (anomaly 13).

…

Photomicrographs of the metamorphosed syenites (planepolarized light and crossed nicols). 1-Igneous texture and antiperthites show the magmatic stage, corresponding to region 1 of the schematically displayed foliation anisotropy in the right side of the figure. 2-Recrystallization of a large albite crystal, associated with recrystallized iron-rich augite suggests the initial stages of the metamorphic recrystallization (region 2 in the scheme). 3-Well developed granoblastic textures indicate the final stages of the metamorphic recrystallization (region 3 with strongest deformation in the scheme). Metamorphic hastingsite appears in region 3 (Ab-Magmatic albite, Aug-Magmatic augite, Mc-Magmatic microcline, AbR-Albite recrystallized during metamorphism, AugR-Augite recrystallized during metamorphism, Hst-hastingsite).

…

U-Pb discordia anchored to 0 Ma to zircons of microclinegneisses. Analysed crystal zones are shown. Error ellipses are 2σ .

…

+10

U-Pb discordia to zircons of metamorphosed syenites (uraniferous albitites) from anomalies 3, 7, and 13. Analysed crystal zones are shown. Error ellipses are 2σ .

…

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Cent. Eur. J. Geosci. • 5(3) • 2013 • 354-373

DOI: 10.2478/s13533-012-0134-7

Central European Journal of Geosciences

New geological model of the Lagoa Real uraniferous

albitites from Bahia (Brazil)

Research Article

Alexandre de Oliveira Chaves∗

Institute of Geosciences - Minas Gerais Federal University (IGC-UFMG) - Brazil

Received 24 April 2013; accepted 8 July 2013

Abstract: New evidence supported by petrography (including mineral chemistry), lithogeochemistry, U-Pb geochronology

by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), and physicochemical study of

ﬂuid and melt inclusions by LA-ICP-MS and microthermometry, point to an orogenic setting of Lagoa Real (Bahia-

Brazil) involving uraniferous mineralization . Unlike the previous models in which uraniferous albitites represent

Na-metasomatised 1.75 Ga anorogenic granitic rocks, it is understood here that they correspond to metamor-

phosed sodium-rich and quartz-free 1.9 Ga late-orogenic syenitic rocks (Na-metasyenites). These syenitic rocks

are rich not only in albite, but also in U-rich titanite (source of uranium). The interpretation of geochemical data

points to a petrogenetic connection between alkali-diorite (local amphibolite protolith) and sodic syenite by frac-

tional crystallization through a transalkaline series. This magmatic diﬀerentiation occurred either before or during

shear processes, which in turn led to albitite and amphibolite formation. The metamorphic reactions, which include

intense recrystallization of magmatic minerals, led uraninite to precipitate at 1.87 Ga under Oxidation/Reduction

control. A second population of uraninites was also generated by the reactivation of shear zones during the 0.6 Ga

Brasiliano Orogeny. The geotectonic implications include the importance of the Orosirian event in the Paramirim

Block during paleoproterozoic S˘

ao Francisco Craton ediﬁcation and the inﬂuence of the Brasiliano event in the

Paramirim Block during the West-Gondwana assembly processes. The regional microcline-gneiss, whose pro-

tolith is a 2.0 Ga syn-collisional potassic granite, represents the albitite host rock. The microcilne-gneiss has no

petrogenetic association to the syenite (albitite protolith) in magmatic evolutionary terms.

Keywords: Lagoa Real • uraniferous albitites • late-orogenic syenite • LA-ICP-MS • U-Pb

©Versita sp. z o.o.

1. Introduction

Currently,thereisonlyoneactiveuraniummineinLatin

AmericalocatedatLagoaRealdistrict,StateofBahia–

Brazil(Figure1),whichisfoundinthecentral areaof

theSăoFranciscoCraton. TheLagoaRealUraniferous

Provinceanditssurroundingshavebeenthesubject of

∗E-mail:alex2010@ufmg.br

geologicalandaerogeophysicalsurveys[1–3]andofvari-

ousstudiesoftheoriginandcontrolofuraniumdeposits,

includingthoseof[4–12].Someofthesestudiesdisagree

abouttheageoftheLagoaRealuraniferousmineraliza-

tion,butusuallyitsgenesisisattributedtouraniumand

sodium-bearinghydrothermalﬂuids,whichmetasomatised

the1.75GaanorogenicSăoTimóteogranite(graniteage

by[7,10,13])toyieldU-richalbitites.

[14]show? thatmanyuraniumrichprovincesarerelated

toevolvedfelsicigneousrocksintrudedintothecrust,not

onlyanorogenically,butalsoduringtheﬁnalstages of

354

Alexandre de Oliveira Chaves

orogenesis. Accordingto[15],duringthelateorogenic

stages,ductileshearfaultzonescontrolledthesiteofem-

placementoffelsicmagmaticprovinces.Pressurerelease

causedbythefaultingcanproduceapartialmeltinginthe

deepzonesofthethickened orogeniclithosphere. Fur-

thermore,thepartialmeltingofthelithosphericmantle

abovethesubductedslabisalsosupportedbythede-

hydrationofthe latter. Theinteraction between ﬂuids

generatedduringthisdehydrationandoverlaying man-

tlematerialwouldberesponsibleforthetraceandrare

earthelements,thorium,anduraniumenrichmentinmag-

mas[16].Whensubmittedtofractionalcrystallizationpro-

cesses, such magmaseventually evolveto U-richfelsic

lithotypes.

New evidence presented here, supported by petrogra-

phy(includingmineralchemistry),lithogeochemistry,LA-

ICP-MSU-Pbgeochronology,andphysicochemicalstudy

of ﬂuid and melt inclusions by LA-ICP-MS and mi-

crothermometry, show an orogenic setting older than

1.75 Ga involving uraniferous mineralization of Lagoa

Real. Thisraisesthequestion: aretheuraniferousal-

bititesNa-metasomatisedgraniticrocksoraretheymeta-

morphosedsodium-rich and quartz-freemagmatic rocks

(Na-metasyenites)?

Therefore,theaimofthepresentstudyistoproposeanew

geologicalmodelabletoexplaintherelationshipbetween

magmatism,shearing,subsequentmetamorphicreactions

andUmineralizationofLagoaRealaswellasthetectonic

implicationsunderanewscenario.

2. Geological setting of the Lagoa

Real Uraniferous Province

TheLagoaRealregionislocatedinthecentral-southern

partofSăoFranciscoCraton(Figure1). Thebasement

of this region is formed by Archean/Palaeoproterozoic

granulitic, migmatitic, and gneissic rocks, which be-

long to the Paramirim and Gaviăo blocks [17]. The

Ibitira-Brumadovolcanosedimentaryunitisfoundinthe

areaandcomprisesamphibolites,bandedironformations,

gneisses, metacherts, marbles, and schists. [18] inter-

pretedthisunitasaLowerPalaeoproterozoicgreenstone

belt.ThePalaeoproterozoicLagoaRealGranitic-Gneissic

Complex covers an area larger than 2,000 km2of the

ParamirimBlockandincludesgranitoidbodies,gneisses,

albititesandamphibolites.[8] attributed the genesisof

theuranium-bearingalbititestometamorphismandpro-

gressivedeformationofthe1.75GaanorogenicSăoTimó-

teoGranitealongshearzones,whereaepisyenitization

processtookplace under theinﬂuence of uraniumand

sodium-richhydrothermalﬂuids.

AnotherimportantgeologicalunitintheregionistheEs-

pinhaçoSupergroup(notshowninFigure1),comprising

sandstones,conglomerates,siltstones,shales,quartzites

and schists overlaying a sequence of 1.7 Ga rhyolites

andrhyodacites. Thissupergroupisrelatedtoabasin

developedduringUpperPalaeoproterozoicriftingevent.

TertiaryandQuaternaryalluvialsedimentscompletethe

geologicalsettingofthisregion.Accordingto[19]and[20]

thegeologicalandtectoniccontextoftheLagoaRealre-

gionispartoftheevolutionof the SăoFranciscoCra-

tonandofsuccessivegeologicalcycles: Jequié(Archean

- with orogenicevent around2.7 Ga), Transamazonian

(Palaeoproterozoic–withOrosirianorogenic event be-

tween2.05and1.8Ga),andBrasiliano(transitionNeo-

proterozoic/Phanerozoic-withorogeniceventaround0.54

+/-0.1Ga). Duringthelattercycle,Archeangneissic

basementoverthrusted metasediments of theEspinhaço

SupergroupandthereforeN-Sregionalthrustfaultsare

foundinParamirimBlock[21].

Figure 1. Geological map of the Lagoa Real uraniferous albitites,

Bahia (BA-Brazil). Modiﬁed from [11] and [1]. Cross-

section presents UO2contents in ppm for some mineral-

ized levels of the anomaly 3, which represents one of 34

mineralized albitite with high uranium content of the Lagoa

Real Uraniferous Province. In addition to surrounding

microcline-gneisses, samples of amphibolites and miner-

alized albitites from anomalies 1, 3, 7, 9, and 13 (An1,

An3, An7, An9, and An13) have been investigated in this

work.

UraniummineralizationatLagoaRealisfoundasﬁnely

disseminated (micro- to milimeter size) uraninite asso-

ciatedwithdiscontinuoustabularbodiesofalbititeslo-

catedalongshearzones[1,4,7,9,13,22]. Mostbodies

trendN40EtoN30Wanddip30°to90°tothesouth-

westornorthwest,withtheexceptionofthenorthernmost 355

New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

deposits,whichdiptotheeast,andthosesituatedinthe

centralpartoftheregion,whicharealmostvertical.Each

bodyhasmaximumlengthof3kmandaveragewidthof

10m(max. 30m). Mineralizationextendsupto850m

belowthesurfaceasshownbydrillcores.Bodiescontain

oneormoremineralizedlevels,whichmaybeinterrupted

inplaces. Thecontactsbetweenmineralizedlevelswith

hostgneissicrocksareabrupt(Figure2).Accordingto[1],

amphibolitesoftenoccuralongtabularbodiesofalbitites

withthesamestructuraltrendsandarealsoattainedby

shearzones.

Figure 2. Sharp contacts between uraniferous albitite bodies and

microcline-gneiss from Cachoeira Mine (anomaly 13).

OrereservesattheLagoaRealUraniferousProvinceare

presentlyreasonablywellestimatedat94,000tonsofUO2

and6,700tonsofUO2ofinferredreserves(CPRM/CBPM,

2003).Figure1insetshowsarepresentativealbititever-

ticalsection,whichpresentsUO2contentsinppmtosome

mineralizedlevelsoftheanomaly3. Thesecontentsare

similartothemainuraniumanomaliesoftheprovince(be-

tween1000and5000ppm).

3. Methodology

InordertounderstandthegenesisoftheLagoaRealuran-

iferousalbitites,thefollowingstepswereundertaken:

1.ageologicalsurveyandsamplecollectingfromCa-

choeiraMinepit(anomaly13)anddrill-corerocks

fromanomalies1,3,7,and9oftheLagoaReal

UraniferousProvince(Fig.1);

2.preparationofpolishedthinsectionsintheSample

LaboratoryoftheDevelopmentCenterofNuclear

Technology(CDTN)forpetrographic,microanalyt-

ical,andgeochronologicstudies;

3.interpretationofthephysicalandchemicalproper-

tiesoftheﬂuidandmeltinclusions.

The petrography of several rock types from the Lagoa

RealregionwascarriedoutattheFluidInclusionsand

MetallogenesisLaboratory(LIF)ofCDTN.ALeicaDMR-

XPmicroscopewasused. Microanalysesofthemineral

phaseswerecarriedoutatthePhysicsDepartmentofMi-

nasGeraisFederalUniversity(UFMG-Brazil)onaJeol-

JXA-8900RLWD/EDElectronMicroprobe.Quantitative

WDSmeasurementshavebeendoneatanalyticalcon-

ditionsof15Kvand20nA,witha5µmelectronbeam

diameter,byusingSmithsonianmicrobeamstandardsand

x-raylinesdescribedin[48]. Mössbauerspectroscopyof

57FeinstalledatCDTNprovidedsupportmeasurestothe

qualitativestudyoftheIronoxidationstateinisolated

mineralphases. Thesemeasurementswereconductedat

roomtemperature,atmosphericpressureandwithoutex-

ternalmagnetic ﬁeld intransmission geometry usinga

conventionalWisselspectrometeranda57Co/Rhsource.

Spectrawereﬁttedbytheleastsquaremethod.Abbrevia-

tionsusedfornamesofrock-formingmineralsarefrom[38].

For geochronological purposes, Pb/U isotope ratios in

uraniniteandzirconcrystalsofLagoaRealalbititeswere

determinedbyLA-ICP-MStechnique(LaserAblationIn-

ductivelyCoupledPlasmaMassSpectrometry, reported

by[23])usingzircon91500anduraniniteTSAstandards.

ThecoupledLaserAblation(Cetac/Geolas-Pro-operat-

ingwavelength193nm,energydensity40J/cm2withspot

sizeof20micrometers)andICP-MS(Thermo/Element2-

sensitivity1x109cps/ppmIn,massresolution600,8,000,

20,000FWHM,magneticscanspeedm/z7->240to7

<150ms,signalstabilitybetterthan2%over1hour)in-

strumentsusedinthisstudyareinstalledattheMemorial

UniversityofNewfoundland,St. John’s- Canada. LA-

ICP-MSanalyseswereperformed withthesameafore-

mentionedpolishedthinsections. CommonPbhasbeen

correctedafter[24]methodandU-Pbdiagramshavebeen

madewiththeIsoplotsoftware[25].

Inordertounderstandthephysicalandchemicalproper-

tiesoftheﬂuidandmeltinclusionsassociatedwiththe

LagoaRealalbititeminerals,thefollowinginitial steps

wereundertaken:(1)mappingofﬂuidandmeltinclusions

insomemineralphasesinFluidInclusionsandMetal-

logenesisLaboratory(LIF)ofCDTN.ALeicaDMR-XP

microscopewasused.(2)microthermometricstudieswere

carriedoutinLIFbyusingChaixmeca heating/freezing

systemstageadaptedtoLeicaDMR-XPmicroscope.The

equipment was previously calibrated with conventional

standardsandnaturalﬂuidinclusions. Thedataarere-

producibleto±0.2°Cforthefreezingrunsand±3°Cfor

theheatingruns. Fluidinclusionswereanalyzedafter

freezingthesamplesdownto-160°Candheatingthem

uptoroomtemperature.Homogenizationtemperaturesof

ﬂuidandmeltinclusionswerenotmeasuredbutthelatter

onesdidnotmeltduringheatingupto450°C?.(3)analy-

sesofthechemicalcontentsofﬂuidandmeltinclusionsin

356

Alexandre de Oliveira Chaves

somemineralsoftheparagenesisassociatedtotheuran-

iferousmineralizationofLagoaRealwereperformedby

usingtheLA-ICP-MStechniquewithstandardNIST610

GlassReferenceMaterial.

TobeawareofpetrologicalevolutionoftheLagoaReal

UraniferousProvince,23representativesamples(ﬁveam-

phibolites,nineuraniferousalbitites,andninemicrocline-

gneisses)fromCachoeiraMinepit(anomaly13)anddrill-

corerocksfromanomalies1,3,7,and9werecomminuted

tolithogeochemistrystudiesbyusingaringmill. ?Total

abundancesofthemajoroxidesandsometraceelements

of2gofrepresentativesamplepowderwerefusedina

metaborate/tetraboratemixture,dissolvedindilutenitric

acidand analyzed atSGS Laboratories byInductively

Coupled Plasma Optical Emission Spectroscopy (major

oxides)andInductivelyCouplePlasmaMassSpectrome-

try(traceelementsZrandTh;Uwasnotanalyzedbecause

itisassumedtohavebeenmobileduringthemetamorphic

events). Thedetectionlimitsaregenerallyaround0.01%

formajoroxidesand1ppmfortraceelements. Thepre-

cisionofanalysisisusuallyinthe1-2%RSD(relative

standarddeviation)range.Lossonignition(LOI)wasde-

terminedfromtheweightdiﬀerencebeforeandaftertem-

peringthepowdersat1000°Cfor60minutes.

4. Petrography and mineral chem-

istry

Thirtyrepresentativepolishedthinsectionsoftheamphi-

bolites,albititesandmicrocline-gneissesofLagoaReal

Granitic-GneissicComplexwereinvestigated. Thisinves-

tigationledtoabetterunderstandingofthetexturesand

mineralparagenesisrelatedtoeachtypeaswellasof

themetamorphicreactionsinthealbitites.Aplateofrep-

resentativephotomicrographsisgiveninthediscussions

andconclusionssection. Electron microprobe analyses

werecarriedouttodeterminethecompositionofthemin-

eralphases. Thefactthatsomeoftheanalysesyielded

totalsbelow100%indicatescontentsofOH,water and

Fe3+ notdetectablebythemicroprobe.

Amphibolites – Exhibit a predominantly nematoblastic

texture,markedbypreferredorientationofmetamorphic

pargasitecrystals,associatedwithpolygonalizedoligo-

clase. Thesetwomineralsare responsiblefor75%of the

rockvolume.Taramitewasalsofoundsubstitutingparga-

siteandcorrespondsto3%oftherockvolume.Thecontent

ofilmeniteandtitaniteisnoticeableinthisrock.Together,

theyrepresentalmost15%ofthetotalvolume. Allanite-

(Ce),zircon,calciteandﬂuor-apatitecompletetheminer-

alogy.Table1showsthemicroanalysesofmineralphases

oftheamphibolites.

Albitites–Thetermalbititerepresentstwodistinctpetro-

graphictypesinthiswork.Botharerichinalbite,asthe

nameindicates,andarecloselyrelatedtoductileshear

zones. Theﬁrstoneisametamorphosedsyenitewithout

quartzbutwithassociateduraniferousmineralization.The

second one is a U-free metamorphosed quartz-syenite.

Themineralogyisnearlythesameforbothpetrographic

types.

Micropetrographic studies indicated anisotropy in the

metamorphicfoliation.Thereareportionsoftherockthat

keepthetextureandmineralogyofthemagmaticstage,

including antiperthites. Other ones mix magmatic and

metamorphictexturesandmanyothershaveexclusively

granoblastictexture(Figure3).

Accessory minerals from magmatic portions are

dark brown U-rich titanite [formula between

(Ca0.82Fe+2

0.10Pb0.08)(Ti0.57U0.40Al0.01V0.01Th0.01)(Si0.94Al0.06)

O4.40(OH,F)0.60and(Ca0.93Fe+2

0.05Pb0.02)(Ti0.82U0.07Al0.05V0.05

Th0.01)SiO4.65(OH,F)0.35 –titanitecrystalswithhighura-

niumconcentrationshavebeenreportedby[26]and[10]

andcanbeunderstoodbythereplacementbetweenTi4+

andU4+, whichhavesimilarionicradius],allanite-(Ce)

withUandTh,magnetite, ﬂuor-apatite, zircon, ﬂuorite,

andapophyllite. Magmaticcalciteissometimespresent,

whichcanbefoundbetweenundeformedaugitecrystals.

Table2showsthemineralmicroanalysesofthemagmatic

stage. Table 3shows the chemical analyses of the

recrystallized mineral phases and of the newly formed

phases: oligoclase, aegirine-augite, microcline, calcite,

titanite, allanite-(Ce), ﬂuor-apatite, zircon, ﬂuorite,

andradite, hastingsite, epidote, biotite, hematite, and

uraninite. Additionally, uraninite was found not only

scatteredthroughalbititebut also inside recrystallized

augite,hastingsite,andradite,calcite,biotiteandepidote.

Microcline-gneisses–Theyvaryfrom“augen”gneisses

tolesscoarsetypesandrepresentthehostrocksofthe

mineralizedalbitites.Microclinepredominates(35to50%

of the rockvolume), followedby oligoclaseand quartz

(20to30%each). Hastingsiteandbiotitealsoappear.

Together,theyform20%oftherockvolume,bothalong

withaegirine-augite. Theopaquemineralismagnetite.

Titanite,ﬂuor-apatite,zircon,andallanite-(Ce)appearas

accessoryminerals. Themicroanalysesoftheseminerals

arepresentedinTable4.

5. U-Pb geochronology by LA-ICP-

Pb/UisotopicratiosobtainedbyLA-ICP-MS,whichhave

showntobereliable? ingeochronologicalstudies[31], 357

New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

Table 1. Representative chemical analyses of amphibolite minerals obtained by electron microprobe. Fe2+ and Fe3+ proportions deﬁned by

Mössbauer Spectroscopy. Ion calculation according to [45]. Amphibole names according to [46].

MineralName Pargasite Oligoclase Titanite Ilmenite Taramite Allanite-(Ce) Zircon Fluor-Apatite Calcite

SiO243.58 60.17 29.20 1.44 43.12 35.57 31.88 0.00 0.00

TiO20.74 0.00 38.56 51.42 0.00 0.00 0.00 0.00 0.00

Al2O311.91 24.32 0.00 0.00 20.85 21.43 0.19 0.00 0.00

FeO 16.75 0.00 0.00 44.35 10.41 11.93 0.00 0.00 0.00

Fe2O30.79 0.00 0.00 0.00 5.81 0.00 0.00 0.00 0.00

V2O30.00 0.00 0.00 0.00 0.89 0.00 0.00 0.00 0.00

MnO 0.33 0.00 0.00 0.00 0.19 0.41 0.00 0.00 0.00

MgO 9.27 0.00 0.00 0.00 6.36 0.00 0.00 0.00 0.00

CaO 11.98 6.25 27.74 1.73 2.48 20.01 0.37 54.51 58.25

Na2O 1.59 8.88 0.00 0.00 6.10 0.00 0.00 0.00 0.00

K2O 0.97 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

P2O50.00 0.00 0.00 0.00 0.00 0.00 0.00 41.55 0.00

F 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.25 0.00

ZrO20.00 0.00 0.00 0.00 0.00 0.00 67.09 0.00 0.00

UO20.00 0.00 0.00 0.00 0.00 0.00 0.32 0.00 0.00

PbO 0.00 0.00 0.55 0.00 0.00 0.48 0.00 0.00 0.00

ThO20.00 0.00 0.00 0.00 0.00 0.31 0.00 0.00 0.00

Ce2O30.00 0.00 0.00 0.00 0.00 4.89 0.00 0.00 0.00

La2O30.00 0.00 0.00 0.00 0.00 3.39 0.00 0.00 0.00

Nd2O30.00 0.00 0.00 0.00 0.00 0.60 0.00 0.00 0.00

CO20.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 41.75

Total 97.91 99.62 96.05 98.94 96.21 99.02 99.85 98.31 100.00

Oxygens 23 32 20 6 23 12 16 13 6

Si6.53 Si10.78 Si3.99 Si0.07 Si6.49 Si2.93 Si3.92 Ca5.07 Ca2.12

Al1.47 Al5.13 Ti3.97 Ti1.95 Al1.51 Al2.08 Al0.03 P3.17 C1.94

Al0.64 Ca1.20 Ca4.06 Fe21.87 Al2.19 Fe20.82 Ca0.05 F1.28 -

Fe30.11 Na3.08 Pb0.02 Ca0.09 Fe30.87 Mn0.03 Zr4.02 - -

Ti0.08 K0.00 - - V0.11 Ca1.76 U0.01 - -

I Mg2.07 - - - Mg1.43 Pb0.01 - - -

O Fe22.08 - - - Fe20.40 Th0.01 - - -

N Mn0.02 - - - Mn0.01 Ce0.15 - - -

S Fe20.01 Ab75 - - Fe20.78 La0.10 - - -

Mn0.02 An25 - - Mn0.01 Nd0.02 - - -

Ca1.93 Or0 - - Ca0.40 - - - -

Na0.04 - - - Na0.81 - - - -

Na0.42 - - - Na0.97 - - - -

K0.19 - - - - - - - -

allowedtheagedeterminationofmagmaticandmetamor-

phiceventsthatresultedintheformationoftheuraninite

oftheLagoaRealGranitic-GneissicComplex.

Rimandcoreareasoftwozirconcrystalsfrommicrocline-

gneisses,hostrocksofuraniferousmetamorphosedsyen-

itesfromradioactiveanomaly13(CachoeiraMine),pro-

ducedtheU-PbdiscordiaofFigure4,anchoredto0Ma

(probablerecentPbloss).Inthisﬁgure,oneﬁndstheval-

uesofPb/Uratiosofeachzircon. Theageof2,009+/-

78Macorrespondingtotheupperinterceptisinterpreted

asbeingthemagmaticcrystallizationofgranitoids,which

represent the parent rocks of the microcline-gneisses.

Palaeoproterozoicagesaround2.0Gahavebeenfound

forthemagmatismassociatedwiththeOrosirianOrogen-

358

Alexandre de Oliveira Chaves

Table 2. Representative chemical analyses of former magmatic minerals in the metamorphosed syenites, obtained by electron microprobe. Fe2+and Fe3+ proportions were measured by Mössbauer

Spectroscopy. Ion calculation according to [45]. Two uraniferous titanite represent the observed range of UO2content.

MineralName Albite Iron-richaugite Microcline Calcite Magnetite U-richTitanite U-richTitanite Allanite-(Ce) Fluor-Apatite Zircon Fluorite Apophyllite

SiO269.19 52.78 64.01 0.00 0.00 19.34 27.67 34.53 0.00 33.88 0.00 51.45

TiO20.00 0.10 0.00 0.00 0.00 16.04 29.46 0.00 0.00 0.00 0.00 0.00

Al2O319.30 0.89 18.41 0.00 0.00 2.02 1.26 13.40 0.00 0.00 0.00 0.00

FeO 0.00 10.02 0.00 0.00 30.11 1.84 1.76 12.09 0.00 0.00 0.00 0.00

Fe2O30.00 3.10 0.00 0.00 66.70 0.00 0.00 3.40 0.00 0.00 0.00 0.00

V2O30.00 0.65 0.00 0.00 2.23 0.05 1.53 0.00 0.00 0.00 0.00 0.00

MnO 0.00 0.35 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

MgO 0.00 9.29 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

CaO 0.26 20.83 0.06 56.34 0.00 15.79 23.80 13.23 52.49 0.00 52.39 24.78

Na2O 11.29 2.07 0.77 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

K2O 0.00 0.00 15.85 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5.09

P2O50.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 40.97 0.00 0.00 0.00

F 0.00 0.00 0.00 0.00 0.00 0.20 0.59 0.25 2.22 0.00 46.94 1.13

ZrO20.00 Q1,68 0.00 0.00 0.00 0.00 0.00 0.00 0.00 65.98 0.00 0.00

UO20.00 J0,30 0.00 0.00 0.00 38,14(UO2Max) 8,71(UO2Min) 1.30 0.00 0.12 0.00 0.00

PbO 0.00 Wo45 0.00 0.00 0.00 3.51 2.63 0.51 0.00 0.08 0.00 0.00

ThO2 0.00 En30 0.00 0.00 0.00 0.69 0.07 0.68 0.00 0.00 0.00 0.00

Ce2O30.00 Fs25 0.00 0.00 0.00 0.00 0.00 10.39 2.26 0.00 0.00 0.00

La2O30.00 WEF85 0.00 0.00 0.00 0.00 0.00 3.71 0.91 0.00 0.00 0.00

Nd2O30.00 Jd4 0.00 0.00 0.00 0.00 0.00 2.64 0.63 0.00 0.00 0.00

CO20.00 Ae11 0.00 43.66 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 100.04 100.08 99.10 100.00 99.04 97.62 97.48 96.13 99.48 100.06 99.33 82,45(+H2O)

Oxygens 32 6 32 6 4 20 20 12 13 16 - 20

Si12,05 Si1,99 Si11,94 Ca2,01 Fe20,98 Si3,75 Si4,09 Si3,05 Ca5,04 Si4,10 Ca0,95 Si7,76

Al3,96 Al0,01 Al4,05 C1,99 Fe31,95 Al0,46 Al0,22 Al1,40 Ce0,07 Zr3,90 F2,05 Ca4,00

Ca0,05 Al0,04 Ca0,01 - V0,07 V0,01 V0,18 Fe30,25 La0,03 U0,00 - K0,98

I Na3,81 Ti0,01 Na0,28 - - Ti2,34 Ti3,28 Fe20,90 Nd0,02 Pb0,00 - F1,08

O K0,00 V0,02 K3,77 - - U1,65 U0,29 U0,03 P3,11 - - -

N - Fe30,10 - - - Ca3,28 Ca3,77 Ca1,25 F1,26 - - -

S - Fe20,31 - - - Fe20,30 Fe20,22 Pb0.01 - - - -

Ab99 Mg0,52 Ab07 - - Pb0,18 Pb0,10 Th0.01 - - - -

An01 Mn0,01 An00 - - Th0,03 Th0,00 Ce0,34 - - - -

Or00 Ca0,84 Or93 - - - - La0,12 - - - -

- Na0,15 - - - - - Nd0,08 - - - -

359

New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

Table 3. Representative chemical analyses of metamorphic minerals from metamorphosed syenites obtained by electron microprobe. Fe2+and Fe3+ proportions deﬁned by Mössbauer Spectroscopy. Ion

calculation according to [45]. Amphibole naming according to [46].

MineralNameOligoclaseAegirine-Augite Microcline Calcite Titanite Allanite-(Ce) Fluor-Apatite Zircon FluoriteAndraditeHastingsite Epidote Biotite Uraninite Hematite

SiO265.21 52.04 65.24 0.00 30.51 38.35 1.09 34.47 0.00 37.12 41.11 37.91 38.89 1.75 0.00

TiO20.00 0.00 0.00 0.00 34.28 0.00 0.00 0.00 0.00 0.40 0.45 0.00 0.68 0.00 0.00

Al2O320.49 2.55 18.83 0.00 1.05 16.22 0.00 0.00 0.00 3.39 11.38 20.78 12.95 0.00 0.00

FeO 0.00 6.02 0.00 0.00 1.82 12.66 0.00 0.00 0.00 0.00 9.51 0.00 13.32 0.00 0.00

Fe2O30.00 9.04 0.00 0.00 0.00 2.78 0.00 0.00 0.00 23.04 10.10 16.30 3.33 0.00 98.32

V2O30.00 0.00 0.00 0.00 1.73 0.00 0.00 0.00 0.00 1.88 0.00 0.00 0.22 0.00 0.00

MnO 0.00 0.13 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.01 0.48 0.00 0.19 0.00 0.00

MgO 0.00 7.75 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 9.12 0.00 14.86 0.00 0.00

CaO 2.63 18.10 0.35 56.53 28.42 16.75 55.28 0.00 51.27 32.78 10.41 23.35 0.00 3.98 0.00

Na2O 10.59 4.12 1.12 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.15 0.00 0.00 0.00 0.00

K2O 0.00 0.00 14.93 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.35 0.00 11.04 0.00 0.00

P2O50.00 0.00 0.00 0.00 0.00 0.00 39.93 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

F 0.00 0.00 0.00 0.00 0.47 0.16 2.24 0.00 48.68 0.00 0.00 0.00 1.54 0.00 0.00

ZrO20.00 Q1,35 0.00 0.00 0.00 0.00 0.00 63.24 0.00 0.00 0.00 0.00 0.00 0.00 0.00

UO20.00 J0,60 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 85.60 0.00

PbO 0.00 Wo42 0.00 0.00 0.00 0.43 0.00 0.00 0.00 Alm0 0.00 0.40 0.00 6.40 0.00

ThO20.00 En28 0.00 0.00 0.00 0.50 0.00 0.00 0.00 And76 0.00 0.00 0.00 0.00 0.00

Ce2O30.00 Fs30 0.00 0.00 0.00 4.56 0.72 0.00 0.00 Gros15 0.00 0.00 0.00 0.00 0.00

La2O30.00 WEF69 0.00 0.00 0.00 1.61 0.21 0.00 0.00 Pyr0 0.00 0.00 0.00 0.00 0.00

Nd2O30.00 Jd6 0.00 0.00 0.00 1.42 0.09 0.00 0.00 Spes2 0.00 0.00 0.00 0.00 0.00

CO20.00 Ae25 0.00 43.47 0.00 0.00 0.00 0.00 0.00 Gold7 0.00 0.00 0.00 0.00 0.00

Total 98.92 99.75 100.47 100.00 98.28 95.44 99.56 97.71 99.95 99.62 98.06 98.74 97.02 97.73 98.32

Oxygens 32 6 32 6 20 12 13 16 - 12 23 12 24 2 3

Si11,61 Si1,95 Si11,95 Ca2,03 Si4,11 Si3,07 Ca4,85 Si4,22 Ca0,98 Si3,09 Si6,37 Si2,91 Si6,16 Si0,07 Fe32,00

Al4,30 Al0,05 Al4,05 C1,99 Al0,17 Al1,73 Ce0,02 Zr3,78 F2,01 Al0,00 Al1,63 Al1,88 Ti0,08 U0,80 -

Ca0,50 Al0,06 Ca0,07 - V0,18 Fe30,20 La0,01 - - AlIV0,33 Al0,45 Fe30,94 Al2,42 Ca0,18 -

Na3,66 Ti0,00 Na0,40 - Ti3,47 Fe20,90 Nd0,00 - - Fe31,44 Fe31,22 Ca1,92 V0,03 Pb0,07 -

I K0,00 V0,00 K3,49 - U1,65 U0,00 P2,77 - - Ti0,03 Ti0,05 Pb0,01 Fe30,41 - -

O - Fe30,28 - - Ca4,10 Ca1,63 F1,16 - - V0,12 Mg2,11 - Fe21,80 - -

N - Fe20,19 - - Fe20,20 Pb0.01 - - - Mn0,07 Fe21,14 - Mn0,03 - -

S Ab90 Mg0,43 Ab10 - - Th0.01 - - - Ca2,92 Mn0,03 - Mg3,51 - -

An10 Mn0,01 An02 - - Ce0,14 - - - - Fe20,10 - K2,23 - -

Or00 Ca0,73 Or88 - - La0,05 - - - - Mn0,03 - F1,54 - -

- Na0,30 - - - Nd0,04 - - - - Ca1,73 - - - -

- - - - - - - - - - Na0,14 - - - -

- - - - - - - - - - Na0,80 - - - -

- - - - - - - - - - K0,47 - - - -

360

Alexandre de Oliveira Chaves

Figure 3. Photomicrographs of the metamorphosed syenites (plane-

polarized light and crossed nicols). 1 - Igneous texture and

antiperthites show the magmatic stage, corresponding to

region 1 of the schematically displayed foliation anisotropy

in the right side of the ﬁgure. 2 - Recrystallization of a

large albite crystal, associated with recrystallized iron-rich

augite suggests the initial stages of the metamorphic re-

crystallization (region 2 in the scheme). 3 - Well devel-

oped granoblastic textures indicate the ﬁnal stages of the

metamorphic recrystallization (region 3 with strongest de-

formation in the scheme). Metamorphic hastingsite ap-

pears in region 3 (Ab – Magmatic albite, Aug – Magmatic

augite, Mc – Magmatic microcline, AbR – Albite recrystal-

lized during metamorphism, AugR – Augite recrystallized

during metamorphism, Hst – hastingsite).

esisinseveralregionsoftheSăoFranciscoCraton[32,33].

ThePb/Uratios of rimandcore zones ofthree zircon

crystalsfrommetamorphosedsyenitesofthreediﬀerent

radioactiveanomalies(3,7,and13)producedtheU-Pb

discordiaofFigure5,whichalsoshowsthevaluesofthe

Pb/Uratiosofeachzirconanditscrystalzone. Zircon

datashow how thegrainshave lost variedamounts of

leadwithtime,i.e.,theclassicdiscordiafromoriginalage

toclosure. Theage of 1,904+/- 44 Ma, correspond-

ingtotheupper intercept, canbeinterpretedeitheras

magmaticcrystallizationand/orasinﬂuenceofOrosirian

metamorphism.Theageof483+/-100Ma,correspond-

ingtothelowerintercept,isinterpretedasimprintofthe

BrasilianometamorphismonthezirconU-Pbsystemdur-

ingthereactivationoftheshearzoneswherethemeta-

morphosedsyenitesarefound.[21]and[12]showedthe

resultoftheBrasilianoeventontheLagoaRealGranitic-

GneissicComplex.

Assuming? thattheuraninitegrainswereformedduring

metamorphicevents,thePb/Uisotopicratiostothesemin-

eralswerealsodetermined. Andradite-relateduraninite

andepidote-relateduraninitewereanalysed. 207Pb/235U

is around 0.7 for the andradite-related uraninite and

around0.3fortheepidote-relateduraninitegrains. The

Figure 4. U-Pb discordia anchored to 0 Ma to zircons of microcline-

gneisses. Analysed crystal zones are shown. Error el-

lipses are 2σ.

U-Pbdiscordiaofthesetwopopulationsofuraninitean-

choredto0MaaregiveninFigure6alongwiththevalues

ofPb/Uratiosofeachgrainanalysed.

Figure 5. U-Pb discordia to zircons of metamorphosed syenites

(uraniferous albitites) from anomalies 3, 7, and 13. Anal-

ysed crystal zones are shown. Error ellipses are 2σ.

TheﬁrsturaninitepopulationshowsaU-Pbsystemstart-

ingduringa1,868+/-69Mametamorphicepisode.This

Palaeoproterozoicagecouldbeattributedtothepeakof

themetamorphismthataccompaniedthedevelopmentof

theshear zones created during theﬁnal stages of the 361

New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

Table 4. Representative chemical analyses of minerals found in the microcline-gneisses, obtained by electron microprobe. Fe2+ and Fe3+ pro-

portions deﬁned by Mössbauer Spectroscopy. Ion calculation according to [45]. Amphibole name according to [46].

Mineral Microcline Oligoclase Quartz Hastingsite Biotite Aegirine-Augite Magnetite Titanite Allanite-(Ce) Fluor-Apatite Zircon

Name

SiO263.92 65.62 99.03 38.34 34.13 50.87 0.00 30.87 38.15 0.00 32.04

TiO20.00 0.00 0.00 0.00 2.03 0.00 0.00 30.04 0.00 0.00 0.00

Al2O318.75 21.31 0.00 13.13 15.55 1.45 0.00 6.17 15.44 0.00 0.00

FeO 0.00 0.00 0.00 25.11 28.04 13.30 29.82 0.00 17.41 0.00 0.00

Fe2O30.00 0.00 0.00 5.51 4.56 10.04 69.50 1.66 0.00 0.00 0.00

V2O30.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

MnO 0.00 0.00 0.00 0.86 0.79 0.32 0.00 0.00 0.00 0.00 0.00

MgO 0.00 0.00 0.00 1.77 4.23 3.65 0.00 0.00 0.00 0.00 0.00

CaO 0.00 2.16 0.00 10.79 0.00 15.77 0.00 29.01 14.80 55.01 0.00

Na2O 0.26 10.69 0.00 1.36 0.00 4.38 0.00 0.00 0.00 0.00 0.00

K2O 16.04 0.20 0.00 2.20 8.81 0.00 0.00 0.00 0.00 0.00 0.00

P2O50.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 42.06 0.00

F 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.04 0.00

ZrO20.00 0.00 0.00 0.00 0.00 Q1,30 0.00 0.00 0.00 0.00 66.57

UO20.00 0.00 0.00 0.00 0.00 J0,66 0.00 0.00 0.00 0.00 0.00

PbO 0.00 0.00 0.00 0.00 0.00 Wo40 0.00 0.00 0.00 0.00 0.00

ThO20.00 0.00 0.00 0.00 0.00 En13 0.00 0.00 0.24 0.00 0.00

Ce2O30.00 0.00 0.00 0.00 0.00 Fs47 0.00 0.00 6.87 0.36 0.00

La2O30.00 0.00 0.00 0.00 0.00 WEF67 0.00 0.00 3.86 0.21 0.00

Nd2O30.00 0.00 0.00 0.00 0.00 Jd4 0.00 0.00 0.00 0.00 0.00

CO20.00 0.00 0.00 0.00 0.00 Ae30 0.00 0.00 0.00 0.00 0.00

Total 98.97 99.98 99.03 99.07 98.14 99.78 99.32 97.75 96.77 99.68 98.61

Oxygens 32 32 2 23 24 6 4 20 12 13 16

Si11,93 Si11,55 Si1,00 Si6,06 Si5,66 Si1,97 Fe20,97 Si4,09 Si3,27 Ca4,77 Si3,97

Al4,12 Al4,42 - Al1,94 Ti0,25 Al0,03 Fe32,02 Al0,97 Al1,56 Ce0,01 Zr4,03

Ca0,00 Ca0,41 - Al0,50 Al3,04 Al0,04 - Fe30,17 Fe30,00 La0,01 -

Na0,09 Na3,65 - Fe30,76 V0,0 Ti0,00 - Ti3,00 Fe21,25 Nd0,00 -

K3,82 K0,05 - Ti0,00 Fe30,62 V0,00 - Ca4,10 U0,00 P2,88 -

- - - Mg0,42 Fe23,90 Fe30,32 - - Ca1,36 F1,04 -

I - - - Fe23,26 Mn0,11 Fe20,43 - - Pb0,00 - -

O Ab02 Ab89 - Mn0,06 Mg1,05 Mg0,21 - - Th0,01 - -

N An00 An10 - Fe20,02 K1,86 Mn0,01 - - Ce0,22 - -

S Or98 Or01 - Mn0,06 - Ca0,66 - - La0,12 - -

- - - Ca1,83 - Na0,33 - - Nd0,0 - -

- - - Na0,09 - - - - - - -

- - - Na0,33 - - - - - - -

- - - K0,44 - - - - - - -

OrosirianOrogenesis,eithersimultaneouslyorimmedi-

atelyafterthecrystallizationoftheuraniferoussyenites

(consideringageerrors).Theabsenceofagedatarelated

totheBrasilianoeventisprobablycausedbyrecentPb

loss.

Withintheageerrors(605+/-170Ma),uraninitegrains

ofthesecondpopulationseemtohavebeencrystallized

duringtheBrasilianometamorphicevent,whichiswell

recordedbythelowerinterceptofthezirconU-Pbdis-

cordiaofuraniferousmetasyenites at 483 +/-100Ma

(Figure5).

Itisobvioustointerpretthetwometamorphicfoliations

with planar surfaces oblique to each other within the

microcline-gneissesoftheLagoaRealComplex[1]asbe-

362

Alexandre de Oliveira Chaves

ingofOrosirianandofBrasilianoorigin.

Figure 6. U-Pb discordias anchored to 0 Ma of two populations

of uraninites of metamorphosed syenites (uraniferous al-

bitites). Error ellipses are 2σfor the older ones and 1σfor

the younger ones.

6. Fluid and melt inclusions by LA-

ICP-MS and microthermometry

Chemicalcontentofﬂuidandmeltinclusionsinsomemin-

eralsoftheparagenesisrelatedtotheuraniferousminer-

alizationofLagoaRealwerequalitatively analysed by

LA-ICP-MS(LaserAblationInductivelyCoupledPlasma

MassSpectrometry).Thistechniquehasproventobeex-

tremelyeﬀectiveinchemicalstudiesofﬂuidandmeltin-

clusionsinminerals[34].Thegraphicinterpretationofthe

ICP-MSsignalswasdoneasfollows:backgroundmeans

standardsignalintensity,whichincreaseswhenthelaser

ablatedthehostmineral.Thenextchangeinsignalinten-

sitymeansthatthelaserablatedameltorﬂuidinclusion.

Thepresenceofiron-richaugite inthemagmaticstage

ofmetamorphosedsyenites(uraniferousalbitites)iscon-

ﬁrmedbyitsmeltinclusionsaswellasbyzonedstruc-

turesfoundinsomeaugitecrystals.Meltinclusionscon-

tainapalebrownmonophasesolid.TheICP-MSsignals

showthatmeltinclusionsarericherinNa,AlandTithan

augiteitself(Figure7).Themagmacontainedtheseele-

mentswhenaugitecrystallizedandtheywereconsumed

byalbite(NaandAl)anduraniferoustitanite(Ti)dur-

ingsyenitecrystallizationprocesses.Themeltinclusions

alsocontainNb,Rb,andBa,whichareincompatiblein

themainsilicatemineralsoftherock.Itisinterestingto

notethatthecontentofCaandSrissmallerinmeltin-

clusionsthaninaugiteitselfbecausetheseelementsare

compatiblewiththe augite structure. Some radiogenic

leadinaugitereveals thepresenceofUinthesyenite

magma.

In order to get some ideas about the magmatic ﬂuids,

theprimarythree-phaseﬂuid inclusionsinaugitecrys-

talsofthemagmaticstageofthemetamorphosedurani-

feroussyenite(uraniferousalbitite)wereanalysed(solid

crystallinephase-S,vapourphase-V,andaqueousphase-

L).ThediagraminFigure8showsthattheprimaryﬂuid

inclusioninmagmaticiron-richaugiteofthealbititecon-

tainsNa,Rb,andBa(RbandBaareincompatibletothe

syenitemineralsandremainedintheﬂuidphase). Mi-

crothermometricstudiespointtoaverylowinitialmelt-

ingtemperature,between-69.7°Cand-62.6°C(Table5),

whichisprobablycausedbythepresenceofRbandBain

thiscomplexsalinesystem,certainlyofmagmaticorigin.

Fluorine(inferredfromthepresenceofﬂuoriteinrock),

rubidiumandbariumlowerthestabilityoftheﬂuidphase

downtoveryloweutecticmeltingtemperatures.

Fluidinclusionsingarnetandrecrystallizedapatitewere

alsoanalysedinordertodeterminetheﬂuidswithinthe

metamorphosedsyenitesandandraditewithuraninitein-

clusions. Theﬂuid inclusions ingarnet normallyhave

eitherfourphases(L,V,andtwoS–dark-orangeand

colorless)orthreephases(L,V,andeithercolorlessSor

dark-orangeS)andrecrystallizedapatitecontainseither

twophases(V-L)oronephase(L)inclusions. Itisworth

pointingoutthatapparentsolid(remnantmelt?) inclu-

sionswereidentiﬁedinrecrystallizedapatite.

Theformationofandraditefromiron-richaugiteduring

sheareventscontemporarywiththemetamorphosedsyen-

itesiscertiﬁedbytheICP-MSsignalspresentedinFig-

ure9.TheelementsSi,Ca,Ti,V,Fe,Na,Mg,Al,andSr

thatcanenterthestructureofthemagmaticaugitewere

foundingarnet,withtheexceptionofMgandNa,which 363

New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

Figure 7. ICP-MS signals of magmatic iron-rich augite from syenite

(uraniferous albitite) and of its pale brown solid monopha-

sic melt (m) inclusions (photomicrograph inset; px =augite

pyroxene). Logarithmic intensity scale.

Figure 8. ICP-MS signals of magmatic iron-rich augite from syenite

(uraniferous albitite) and of its primary three-phase ﬂuid

inclusion (photomicrograph inset). Logarithmic intensity

scale.

preferentiallywentintotheﬂuidphase.BesidesMgand

Na,Rb,Ba,U(235Uand238U)andassociatedradiogenic

Pbwerealsofound.

Uranium released from magmatic titanite during the

1.9Ga metamorphismwas recordedin theﬂuid inclu-

sionsinandradite,whichprobablyformedsimultaneously

withtherecrystallizedtitanite. Thisalsocausedthedis-

seminateduraniniteinsideandradite. Furthermore,the

dark-orangecrystalinsidetheﬂuidinclusionsofandra-

dite(Figure9)probablycontainsradiogenicPb.

Thematerialreleasedduringoneofthelaserablationsof

recrystallizedapatiteproducedtheICP-MSsignalspre-

sentedinFigure10. Thelaserablatedtheapatite,two

Figure 9. ICP-MS signals of andradite garnet of the metamorphic

stage, which generated metamorphosed syenites (uranif-

erous albitites) and two of its ﬂuid inclusions. One repre-

sentative four-phase ﬂuid inclusion (2 solid phases – one

of them is dark-orange and the other one is colorless, 1

liquid phase, and 1 vapour phase, see photomicrograph

inset) in andradite is shown in photo. Logarithmic inten-

sity scale.

ﬂuidinclusions,andaprobableremnantsolidinclusion.

P,Ca,Sr,V,andtherareearthelements(La,Ce, Nd,

Sm)resultfromapatite.Therareearthelementsandtho-

riumarethemainconstituentsofthesolidinclusion.The

contentoftheﬂuidinclusionsinrecrystallizedapatiteis

thesameasthoseofthegarnetﬂuidinclusions.Together

withinitialmeltingtemperaturesbetween-53.7°Cand-

49.5°C(Table5)thissuggeststhatandraditecrystallized

togetherwithrecrystallizedapatiteinthesamemetamor-

phicevent.

Figure 10. ICP-MS signals of recrystallized apatite of the metamor-

phic stage, which generated metamorphosed syenites

(uraniferous albitites). The ICP-MS signals of two of

its ﬂuid inclusions and of a solid inclusion are also pre-

sented. Monophase and two-phase ﬂuid inclusions in

recrystallized apatite from a metamorphosed syenite be-

fore (left) and after (right) laser ablation are shown (pho-

tomicrograph inset). Logarithmic intensity scale.

364

Alexandre de Oliveira Chaves

Table 5. Microthermometric data of ﬂuid inclusions (FI) in augite, andradite and apatite from Lagoa Real metamorphosed syenite (uraniferous

albitite).

FI AUGITE ANDRADITE APATITE

Primaryﬂuidinclusions

(yieldedduring

magmaticstage) Secondaryﬂuidinclusions

(yieldedduring

metamorphicstage) Fluidinclusionsyielded

during

metamorphicstage Fluidinclusionsyielded

during

metamorphicstage

Initial

ice-melting

temperature

(°C) Final

ice-melting

temperature

(°C) Initial

ice-melting

temperature

(°C) Final

ice-melting

temperature

(°C) Initial

ice-melting

temperature

(°C) Final

ice-melting

temperature

(°C) Initial

ice-melting

temperature

(°C) Final

ice-melting

temperature

(°C)

1 -64.0 -12.0 -52.5 -12.2 -53.0 -12.0 -49.5 -9.0

2 -65.4 -11.5 -52.3 -15.0 -52.1 -11.6 -51.4 -8.4

3 -63.8 -11.7 -52.7 -11.5 -52.0 -11.9 -51.7 -10.7

4 -62.6 -11.4 -55.0 -11.0 -52.1 -11.6 -50.0 -10.1

5 -64.4 -11.1 -50.9 -11.2 -53.5 -9.3 -49.9 -11.6

6 -65.5 -13.1 -52.2 -11.5 -53.1 -11.7 -52.4 -9.5

7 -64.2 -12.1 -52.0 -12.0 -51.7 -13.3 -51.0 -8.9

8 -66.2 -12.1 -55.0 -11.2 -52.5 -13.0 -53.7 -9.2

9 -69.7 -11.4 -54.9 -13.0 -51.6 -9.6 -50.2 -10.0

7. Lithogeochemistry

Contentsofmajor(weight%)andtraceelements(ppm)of

thethreediﬀerentrocktypes arepresentedinTable6.

CIPWnormativemineralcontentwascalculatedbythe

Minpetsoftware[35]. Althoughtheanalysedrocksare

metamorphic,CIPWdataarepresentedinTable6inorder

todrawsomeconclusionsabouttheigneousprotoliths.

NotethatthehigherLOIofsamplesAb1,Ab2andAb18

isduetothepresenceofuranophane(hydratedmineral).

A/XversusB/XPearcediagrams[36]areusefultoolsto

provewhethersupposedlymobileelementssuch as Na

andKfromalbititesandmicrocline-gneisswere immo-

bileduringmetamorphism. InthediagramsXrepresents

Zr(immobilenormalizingelement),BsilicaandAalka-

lis(Na2OandK2O).Irregularpatternsandlineartrends

indicateelementmobilizationorimmobilityduringmeta-

morphismrespectively. Thelineartrends in Figure 11

show that both Na and K were not mobilized during

metamorphism of albitites and microcline-gneiss. An-

otherway to test elementmobilization is thechemical

indexofalteration(CIA[47]),whichiscalculatedasCIA

=Al2O3/(Al2O3+CaO*+Na2O+K2O)]*100;theelemental

abundancesareexpressedasmolarproportions,andCaO*

representstheCaOcontentofthesilicatefraction. Only

calcite-free, low CaO albititesamplesAb23, Ab30 and

Ab31havebeenusedforCIAcalculations, resultingin

valuesaround47,whichisintherangeofigneousrocks

(45-55).CIAvaluesofallMgn#samplesinTable6range

between45and55.

Wehaveshownthatthereisnoimportantmobilizationof

Figure 11. Linear trends for Na and K in the Pearce diagrams [36]

suggest no representative alkalis mobilization during

metamorphism of albitites (circles) and microcline-

gneiss (triangle) during metamorphism.

alkalis. Thereforethemetamorphicrockscanbetreated

astheirigneousprotoliths. Inthetotalalkaliversussil-

icadiagram(TAS,Figure12[37]),amphiboliteprotolith

isanalkali-dioriteandalbititeprotolithis an alkaline

rockrangingfromsyenodioritetosyenite. Furthermore,

inS1,S2,S3,andTﬁeldsfromFigure12,rocksbelong-

ingtotransalkalisuiteof[39]mustbeclassiﬁedas“sodic”

ifNa2O–2.0>K2Oor“potassic”ifNa2O–2.0<K2O

after[40]. Threeofﬁvealkali-dioritesamplesaresodic

aswellasalloftheninesyenodiorite(albititeprotolith)

samples. TheMicrocline-gneissprotolith, however,isa

potassicsubalkalinesyenogranite.

Harker diagrams with silica versus major oxides (Fig-

ure 13) and silica versus trace elements (Figure 14)

showthepetrogeneticrelationbetweenalkali-dioriteand

syenitic rocks. Increasing silica content is accompa-

nied by decreasingamountsof Ti, Fe, Mg, Ca, P, and

increasing of Al, Na (hence albite, not K-feldspar, is 365

New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

Table 6. Chemical analyses results of major oxides (weight %; total iron expressed as FeOT)and trace elements Th and Zr (ppm) of amphibolites

(samples amp#), albitites (samples alb#), and microcline-gneisses (samples mgn#). CIPW normative mineral contents are also shown.

Sample SiO2TiO2Al2O3FeOTMnO MgO CaO Na2O K2O P2O5LOI Total Zr Th

Amp2 45.98 2.07 14.66 14.94 0.20 6.15 8.65 4.23 0.81 0.17 0.50 98.36 180 0

Amp4 46.70 2.73 13.99 13.80 0.22 5.65 8.60 3.64 1.92 0.28 0.55 98.08 204 2

Amp1 47.33 2.09 13.77 13.77 0.20 5.89 10.17 2.80 1.35 0.16 0.60 98.13 140 1

Amp5 48.71 2.38 12.94 14.50 0.24 5.31 9.00 2.61 0.32 0.22 2.64 98.87 196 1

Amp6 48.32 1.92 14.63 11.42 0.15 7.29 8.90 4.33 1.54 0.22 0.83 99.55 143 1

Alb18 55.12 0.58 16.19 6.25 0.13 1.22 6.33 8.04 0.64 0.15 5.60 100.25 940 18

Alb2 55.98 0.58 16.26 6.45 0.15 1.90 5.48 7.90 1.03 0.00 5.90 101.63 862 17

Alb1 57.34 0.44 17.13 6.23 0.11 0.77 5.20 8.96 0.46 0.01 4.57 101.22 809 16

Alb8 59.06 0.73 15.24 6.75 0.11 0.57 7.93 7.89 0.20 0.02 0.55 99.05 1389 19

Alb11 59.72 0.55 16.25 5.72 0.14 0.34 7.17 7.65 0.20 0.00 0.31 98.05 990 32

Alb6 61.33 0.47 16.59 6.52 0.11 0.21 6.04 8.35 0.27 0.00 0.39 100.28 982 36

Alb31 62.88 0.58 18.35 5.50 0.03 0.69 2.90 9.11 0.49 0.00 0.26 100.79 1570 21

Alb30 64.49 0.35 17.78 4.65 0.06 0.00 3.01 8.38 0.67 0.00 0.37 99.76 1500 21

Alb23 65.24 0.35 16.79 4.54 0.11 0.18 2.63 9.26 0.24 0.00 0.31 99.65 1590 24

Mgn42 67.40 0.53 14.89 4.34 0.06 0.00 2.15 3.56 5.29 0.00 0.54 98.76 490 7

Mgn23 68.18 0.32 14.63 4.29 0.06 0.27 2.31 3.45 5.25 0.04 0.15 98.95 649 9

Mgn30 68.54 0.32 14.23 5.20 0.07 0.15 1.70 4.03 5.16 0.00 0.20 99.60 555 29

Mgn64 69.60 0.41 13.78 4.90 0.07 0.15 1.96 3.58 5.23 0.00 0.45 100.13 570 14

Mgn52 70.09 0.23 13.79 4.08 0.04 0.00 1.26 4.30 4.38 0.01 0.34 98.52 568 25

Mgn68 72.31 0.24 12.65 3.01 0.05 0.00 1.28 2.62 6.72 0.00 0.87 99.75 474 37

Mgn58 72.50 0.19 13.43 2.65 0.04 0.21 0.85 3.19 6.24 0.00 0.46 99.76 570 80

Mgn62 73.20 0.12 12.46 2.01 0.04 0.00 0.85 2.87 6.20 0.00 0.57 98.32 647 20

Mgn13 73.58 0.22 12.46 2.77 0.02 0.00 0.82 3.02 5.90 0.00 0.34 99.13 516 35

CIPWNorm Q Or Ab An Ne C Ac DiWo DiEn DiFsHyEn HyFs OlFoOlFa Mt He Ilm Total

amp1 0.00 8.20 19.4521.55 2.63 0.000.00 12.64 5.09 7.67 0.00 0.00 7.02 11.67 0.00 0.00 4.08100.00

amp2 0.00 4.90 19.3019.03 9.37 0.000.00 10.40 4.08 6.45 0.00 0.00 8.16 14.27 0.00 0.00 4.03100.00

amp4 0.00 11.68 17.2516.59 7.80 0.00 0.00 11.40 4.60 6.90 0.00 0.00 6.95 11.52 0.00 0.005.33 100.00

amp5 0.00 14.00 20.6617.04 1.00 0.00 0.00 11.91 4.43 7.71 0.00 0.00 6.38 12.26 0.00 0.004.61 100.00

amp6 0.00 9.25 18.7416.15 9.98 0.000.00 11.98 5.90 5.84 0.00 0.00 8.82 9.64 0.00 0.00 3.70100.00

alb1 0.00 2.82 59.35 5.33 10.300.000.00 8.92 1.45 8.23 0.00 0.00 0.38 2.35 0.00 0.00 0.87 100.00

alb11 0.53 1.21 66.15 9.61 0.00 0.00 0.00 9.86 0.87 10.07 0.00 0.00 0.00 0.00 0.00 0.001.07 99.36

alb18 0.00 4.01 52.93 6.54 10.280.000.00 11.15 2.62 9.23 0.00 0.00 0.43 1.66 0.00 0.00 1.17 100.00

alb2 0.00 6.36 53.25 6.11 8.94 0.00 0.00 9.31 2.89 6.78 0.00 0.00 1.45 3.76 0.00 0.00 1.15 100.00

alb23 3.11 1.43 78.78 3.55 0.00 0.00 0.00 4.00 0.24 4.24 0.21 3.77 0.00 0.00 0.00 0.00 0.67100.00

alb30 4.48 3.99 71.26 8.96 0.00 0.00 0.00 2.54 0.00 2.89 0.00 5.22 0.00 0.00 0.00 0.00 0.67100.00

alb31 0.00 2.88 76.59 7.67 0.00 0.00 0.00 2.77 0.48 2.53 0.01 0.03 0.86 5.08 0.00 0.00 1.10100.00

alb6 0.00 1.60 67.61 6.98 1.65 0.00 0.00 9.61 0.48 10.31 0.00 0.00 0.04 0.84 0.000.000.89100.00

alb8 0.00 1.20 63.34 5.65 2.37 0.00 0.00 11.83 1.45 11.55 0.00 0.00 0.00 0.00 0.000.001.41 98.80

mgn13 30.13 35.33 25.84 3.01 0.00 0.00 0.00 0.47 0.00 0.53 0.00 4.28 0.00 0.00 0.00 0.00 0.42100.00

mgn23 20.11 31.44 29.52 8.99 0.00 0.00 0.00 1.09 0.10 1.11 0.58 6.43 0.00 0.00 0.00 0.00 0.62100.00

mgn30 18.11 30.71 34.27 5.49 0.00 0.00 0.00 1.25 0.06 1.35 0.32 7.84 0.00 0.00 0.00 0.00 0.61100.00

mgn42 19.30 31.86 30.63 9.14 0.00 0.00 0.00 0.72 0.00 0.82 0.00 6.50 0.00 0.00 0.00 0.00 1.03100.00

mgn52 23.00 26.39 37.02 5.45 0.00 0.00 0.00 0.38 0.00 0.44 0.00 6.87 0.00 0.00 0.00 0.00 0.45100.00

mgn58 26.11 37.17 27.15 3.87 0.00 0.00 0.00 0.16 0.02 0.16 0.51 4.50 0.00 0.00 0.00 0.00 0.36100.00

mgn62 30.35 37.52 24.81 2.82 0.00 0.00 0.00 0.63 0.00 0.71 0.00 2.93 0.00 0.00 0.00 0.00 0.23100.00

mgn64 21.39 31.03 30.35 6.06 0.00 0.00 0.00 1.55 0.07 1.66 0.30 6.80 0.00 0.00 0.00 0.00 0.78100.00

mgn68 27.31 40.20 22.39 2.89 0.00 0.00 0.00 1.48 0.00 1.68 0.00 3.59 0.00 0.00 0.00 0.00 0.46100.00

366

Alexandre de Oliveira Chaves

Figure 12. Total alkali-silica – TAS – diagram [37,40]. Dashed line

after [41] separates alkaline and subalkaline rocks. Am-

phibolite (cross) protolith is a sodic alkali-diorite (S1) and

albitite (circle) protolith is a sodic alkaline rock ranging

from syenodiorite (S3) to syenite (T). Microcline-gneiss

(triangle) protolith is a potassic syenogranite (T and R),

however, classiﬁed as subalkaline.

formed in sodicsyenite, thealbitite protolith), Zr, and

Th(immobile\incompatibleHFSelements)contents. In

otherwords,reasonabletrendssuggestdiﬀerentiationof

analkali-dioriticbasicmagmaby fractionalcrystalliza-

tiontoanintermediatesyeniticmagmaeitherbeforeor

during metamorphism along shear zones. Both alkali-

diorite and syenite belong to the same transalkaline

series(also named transalkali suite -Figure 12; [39]),

whichis silica-saturated and characterizedby absence

ofmodalnephelineandpresenceofnormativenepheline

(see CIPW norm, samplesamp#and somealb#-Ta-

ble6). Quartz-syenites(non-uraniferousquartz-albitite

protolith)arelithostructurallyrelatedtouraniferousal-

bitites(seecross-sectionoftheFigure1)and certainly

representthelast magmatic evolutionarystepof syen-

itesin the transalkalineseries. Although the absence

ofnormativecorundumandacmiteinsyeniteandgran-

iteallowsustoclassifythemasmetaluminous rocksin

terms of aluminasaturation, Harkerdiagrams withsil-

ica versus major and minor elements reveal trends for

Ti,Fe,Mg, CaandPbutAl,Na, K,Zr,andTh(Fig-

ures13and14)suggestthatthereisnopetrogeneticrela-

tionbetweenalbitites(metamorphosedsodicsyenites)and

microcline-gneiss(metamorphosedgranites). Nepheline-

normativeandsodictransalkalineseries(orsuite)mag-

masdonotevolvetohighquartz-normativesubalkaline

potassic graniticrocks. Furthermore, abrupt geological

contactsinFigure2indicatethatsodicsyeniticmagma

intrudedinpreviouslycrystallizedpotassicgranite. This

ﬁeldfeatureisconﬁrmedbyR1versusR2diagram[42],

afterwhichthegeotectonicsettingsduringgraniticand

syeniticintrusionsweresyn-collisionalandlate-orogenic,

respectively.Inagreementwiththesesettings,U-Pbages

byLA-ICP-MSat2,009+/-78Matomicrocline-gneiss

(olderpotassicgranite)representsyn-collisionalepisode

oftheOrosirianOrogeny,andat1,904+/-44Matouran-

iferous albitites (younger sodic syenite) represent next

late-orogenicshearing.

Figure 13. Binary (Harker) diagrams with silica versus major oxides.

Cross =amphibolite (protolith =alkali-diorite), circle =

uraniferous albitites (protolith =syenitic rocks), triangle

=microcline-gneiss (protolith =syenogranite). Curves

illustrate the trend between alkali-diorite and syenite.

Figure 14. Binary (Harker) diagrams with silica versus trace el-

ements Zr and Th. Cross =amphibolite (protolith =

alkali-diorite), circle =uraniferous albitites (protolith =

syenitic rocks), triangle =microcline-gneiss (protolith =

syenogranite). Curves illustratethe trend between alkali-

diorite and syenite.

367

New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

Figure 15. R1 [4Si-11(Na+K)-2(Fe+Ti)] versus R2 [6Ca+2Mg+Al]

multicationic diagram [42] of the geotectonic setting dis-

crimination of granitoid rocks, after which syenites (al-

bitite protolith; circles) belong to late-orogenic setting

and potassic granites (microcline-gneiss protolith; trian-

gles) belong to syn-collisional setting. Some circles were

omitted because they plot outside the ﬁeld of the dia-

gram.

8. Discussions and conclusions

Theinitialreason why uraniferousalbitites are classi-

ﬁedassyenitesinthispaperandnothydrothermalal-

bititesaspreviouslysuggestedby[8]and[9]wasfound

duringmicropetrographicstudies,whichrevealedoriginal

magmatictexture(inaddition,antiperthitesdemonstrated

moresodicthanpotassiccompositionofthefeldsparsbe-

foreexsolution). Mixedmagmaticandmetamorphictex-

turesandfrequentexclusivelygranoblastictexturewere

generated during shearing. Therefore, the transforma-

tionofthemagmaticmineralsduringmetamorphismup

tocompleterecrystallizationisevident(Figure 3). Be-

sidestherecrystallizedminerals,newmineralsalsore-

sultedfromthemetamorphicreactions.Furthermore,there

isnoquartzpreservedintheU-bearingsyeniteandfea-

turesresemblingsilicadissolutionwerenotfound. This

contradictssodicmetasomatismoftheSăoTimóteoGran-

itetogeneratealbitites.Albite,iron-richaugitewithmelt

inclusions,andsomemicroclinearefoundinpartsthat

preservedthemagmaticstage,supportingclassiﬁcationof

theserocksassodicsyenites.

Inaﬁrsthigh-gradeamphibolitefaciesmetamorphicstage

not only hastingsite, but also andradite resulting from

iron-rich augite transformation appeared (Figure 16A).

Simultaneoustotherecrystallizationofiron-richaugite,

albite, microcline (+/- calcite) the accessory minerals

formed. During recrystallization, iron-rich augite be-

camemoresodic-richaegirine-augiteandalbitebecame

slightlymorecalcicoligoclase. Theassociationbetween

oligoclaseandandraditerevealsthehighpressuremeta-

morphismcommontoductileshearzones[27].Magnetite

wasreplacedbyhematite,consistentwithoxidizingcon-

ditionsduringmetamorphism(e.g. garnetcontainsonly

Fe3+).

Figure 16. Plate of representative photomicrographs. A: Formation

of andradite edge (garnet-Adr) from magmatic iron-rich

augite (Aug) in the presence of albite (Ab). B: U-rich

magmatic dark brown titanite (TtnU) that released ura-

nium to form uraninite (black - Urn). Augite (Aug) and

albite (Ab) also appear in picture. C: U-rich magmatic

titanite (TtnU - dark) that released uranium during meta-

morphism to form uraninite (in black - Urn). Beside it,

the recrystallized and fractured uranium-free titanite (Ttn

- light) but with uraninite in its fractures also appears.

D: Uraninite (in black, metamictic - Urn) inside andradite

(Adr - on the left). Aug =augite, Ab =albite. E: Zir-

con (Zrn – white) and uraninite (in black, metamictic -

Urn) inside andradite (Adr). F: Channels (CH) that con-

tain uraninite (in black - Urn) inside recrystallized augite

(Aug). Uraninite precipitated in the channels from a so-

lution containing U6+that reacted with the Fe2+of augite.

Ab =albite. G: Recrystallized calcite (Cal) with uraninite

(Urn) inside augite (Aug; under crossed nichols). Ab =

albite. H: Uraninite (black - Urn) inside epidote (Ep).

Uraninite,whoseuraniumderivesessentiallyfromU-rich

magmatictitanite(Figure16B),wasalsoformedduring

thisprocess.Itisusuallylocatednearthelightcoloured

recrystallizedtitanite,insideandradite,hastingsite,and

recrystallizedaugiteandcalcite(Figure16C,DandE).

368

Alexandre de Oliveira Chaves

DuringshearingofU-richtitaniteandallanite,aqueous

ﬂuidsreleaseduraniumintheformofU+4 andthemore

mobileoxidizedform(uranylions–UO+2

2),leadingtothe

formationofuraninite.

Thesuggestedchemicalmechanismoftheprecipitation

ofuraniniteinmetamorphosedsyeniteswithor without

calciteisdescribednext:

-STEP1: TheU4+ ionsreleasedfromU-richtitanite

andallaniteduringthesheareventstogetherwithOH−

ionsreleasedfromthe partialhydrolysisofalbite,form

uraniniteinanon-Oxidation/Reductionprocess

U4++4OH−→UO2+2H2O[orU(OH)4](step1)

-STEP2: Inmetamorphosed syenites without calcite,

uraniniteinteractedcompletelyorpartiallywiththefree

oxygencirculatingthroughtheaqueousﬂuidsduringthe

shearingprocess.U4+ oxidizedtoaqueousuranylhydrox-

idecomplexes(withU6+),whicharestableundertemper-

atureandpressureconditionsoftheshearprocess[28].

2UO2+2H2O+O2→2UO2+

2+4OH−

(step2withoutcalcite–Oxidation/Reduction)

Inmetamorphosedsyeniteswithcalcite,calciumcarbonate

hydrolyzedandformeduranyltricarbonatecomplex,which

isverystableinthealkalineaqueousenvironmentgen-

erated.[29]showthatrelativeabundancesoftheuranyl

tricarbonatecomplexinsolutionincreasewithincreasing

temperature,underrelativelyoxidizingandslightlyalka-

lineconditions.

2UO2+2H2O+O2+6CaCO3→

2[UO2(CO3)3]4−+4OH−+6Ca2+

(step2withcalcite–Oxidation/Reduction)

Theaqueousalkalineenvironmentcertainlyfacilitatedthe

dissolutionofsilicafromsilicatesoftherock,andeventu-

allyuranylhydroxisilicatecomplexeswereformed,which

alsohelpedinthemobilizationofuranium.

Magnetitealsointeractedwithfreeoxygenandbecame

hematite.Theincreaseinthepartialpressureoffreeoxy-

genprobablyfavoredhematitebythereaction

4Fe3O4+O2↔6Fe2O3

-STEP3:Althoughuraniumbecameextremelymobilein

theformofuranyltricarbonate,theFe2+ ofthemagmatic

augiteledtothereductionofU6+ toU4+ anduraninite

toprecipitate. Theprecipitateduraninitewasretained

insidetherecrystallizedaugiteandcalciteaswellasin-

sidethesimultaneouslyformedandradite. Inthinsec-

tions,wecanclearlynoticechannelsorsurfacescontain-

inguraninite,whichprecipitatedwhentheU6+ containing

ﬂuidpassedthroughtheaugiteandreactedwithitsFe2+

(Figure16F).Uraninitealsoco-precipitatedrecrystallized

calcitefromauranyl-tricarbonatecontainingﬂuid,after

reactionwithFe+2 oftheaugite(Figure16G):

3Ca2++[UO2(CO3)3]4−+2Fe2+→UO2+2Fe3++

3CaCO3

(step3withcalcite–Oxidation/Reduction)

Inmetamorphosedsyeniteswithoutcalcite,thefollowing

reactionissuggestedforprecipitationofuraninite:

UO2+

2+2Fe2+→UO2+2Fe3+ (step3withoutcalcite

–Oxidation/Reduction)

SimilarOxidation/Reductionprocesseshavealreadybeen

experimentallydescribedby[30]forhydrothermalcondi-

tions.[9]previouslysuggestedthaturaniniteprecipitation

inLagoaRealwascontrolledbythereductionofanuran-

iferousﬂuidphase,viaprogressiveoxidationofmaﬁcmin-

erals.

Epidoteandbiotiteappearedduringanewmetamorphic

stage.Theypartiallyreplacedthemineralsformedduring

theinitialmetamorphism.Thisparagenesisindicatesare-

equilibriumestablishedundernewtemperatureandpres-

sureconditions,lessintensethantheoneswhichformed

garnetduringtheinitialmetamorphism. Itisinteresting

tonotethaturaninitecrystalsarealsofoundinsideepi-

dote(Figure16H)andbiotitesuggestingasimilarpre-

cipitationreactionasthatdescribedinsteps2and3.Bi-

otitecontainsbothFe2+ andFe3+ whileinepidoteonly

Fe3+ occurs. Uraniniteprecipitationinsidetheseminer-

als,eventuallywithinvolvementofcalcite,wouldhaveoc-

curredunderthesenewmetamorphicconditions,between

greenschistandamphibolitefacies.

Thegenerationofmagmasinsubductionzonesisthought

tobethemostimportantmechanismtothegrowthofcon-

tinentalcrustsincetheProterozoic. Mostofthesemag-

masderivefromthemeltingofthemantlewedgeabove

thesubductedslab driven by itsdehydration. Thein-

teractionbetweenﬂuidsgeneratedduringthisdehydra-

tionandoverlayingmantlematerialwouldberesponsi-

bleforthetraceandrareearth elements, thorium,and

uranium enrichment in magmas [16]. During the late

Orosirianorogenicstages,ductileshearfaultzonesprob-

ablycontrolledthesiteofemplacementofalkali-diorite

andsyenitestudiedhere. TheLagoaRealsyenitescer-

tainlyhaveformedbycrystalfractionationofferromagne-

siansilicatesandcalcicplagioclasefromdioriticparent

magmas,currentlyrepresentedbycloselyassociatedlo-

calamphibolites.Therefore,thesyeniticrockshavelower

abundancesofCa,MgandFethandiorites,butgenerally 369

New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

higherabundancesofNa,whichissupportedbythepre-

dominantoccurrence of sodic plagioclasetogether with

augite. Thisexplanationdiﬀersfromthesodicmetaso-

matismproposedby[8,9]and[12]generatinganageof

attaining1,750MafortheSăoTimóteoGranite.Accord-

ingto[8,9]and[12],augitesfrommetamorphosedsyen-

ites(uraniferousalbitites)would have derived fromthe

dehydrationofamphiboles during metamorphismofthe

1,750MaSăoTimóteoGranite. Accordingtothisstudy,

however,augitesareofmagmaticoriginaround1,900Ma

ago.

Meltinclusionsfoundin magmatic augite are richerin

Na,AlandTithantheaugitehost. Theseelementsbe-

longedtothemagmawhenaugitecrystallizedandwere

furtherincorporatedbyalbite(NaandAl)anduranifer-

oustitanite(Ti)duringsyenitecrystallizationprocesses.

ThereissomeradiogenicPbintheaugitestructure,which

revealsthepresenceofUinthesyenitemagma.Primary

ﬂuidinclusions in magmatic iron-richaugite of the al-

bititesuggestasodium-richoriginalmagma,makingsodic

metasomatismobsolete. Therefore,amagmaticmodelfor

theLagoaRealuraniferousalbititesisreasonable;how-

ever,metasomaticalterationofmicroclinegneisscannot

betotallyexcluded: ThesharpcontactsinFig.2might

becausedbyﬂuidinﬁltration.TrendsbetweenSiO2and

majorandminorelementsarepartlycontradictoryandage

dataofthesetworocktypesoverlapwithinerrorrange.

Accessorymineralslikezircon,titanite,allanite,andap-

atiteaccompanyuraniferoussyenites.Theypreferentially

incorporateU,Thandrareearthelements,incompatible

tothestructureofthemajorsilicatesintheserocks. Due

totheresemblanceoftheionicpotentialsofUandTi,ti-

taniteisthemostprobableprimaryuranium-bearingmin-

eral(ﬁgures16BandC).Uranium wasreleasedduring

themetamorphicepisodestoformuraniniteinanalmost

closedsystem. Inthisway,thedataofthepresentwork

deviatefromthemodelsuggestedby[8].[8]proposedthat

desiliciﬁcationanduranium-richﬂuidsderivedfromac-

cessorymineralsofthequartz-richSăoTimóteoGranite

generatedtheuraniferousalbititesbymetasomaticpro-

cesses.

PartsofthegeneticmodeloftheUraniferousProvince

proposedby[4],whichassociatesuraniumto“polycyclic

diapiricprocesses”,isinagreementwiththepresentwork.

However,accordingtothisauthor,thediapirismoccurred

duringtheBrasilianoevent,whichdoesnotconcurwith

theOrosiriangeochronologicaldataofthemagmatismde-

scribedhere.

[14]pointedoutthatmanyimportanturaniferousprovinces

intheworldareultimatelyrelatedtoevolvedfelsicig-

neousrocksintrudedatshallowlevelsofthecrust,ei-

theranorogenicallyorduringtheﬁnalstagesoforogene-

sis.Thepresentinvestigationssuggestthaturaniumfrom

LagoaRealalbititesisrelatedtothesyeniticmagmatism

belongingtomaﬁc/felsic association linked to theﬁnal

stagesoftheOrosirianOrogenyintheParamirimBlock

around1,900Ma.Theductileshearingthataﬀected?the

uraniferoussyenites(albitites)formeduraniniteinthese

rocksnotonlyduringtheOrosirianmetamorphism,when

theserocksbecamemetasyenitesduetointenserecrys-

tallizationofitsminerals,butalsointhelaterBrasiliano

metamorphism.

Therearetwogeotectonicimplicationsresultingfromthe

presentstudy. Theﬁrstoneisthe conﬁrmation of the

OrosirianorogeneticeventintheParamirimBlock that

culminatedinthetectonicstructuringoftheSăoFran-

cisco/Congo Craton in the Palaeoproterozoic, probably

throughcollisionbetweenWestSăoFranciscoandEast

SăoFrancisco/Congocontinentalmasses.TheN-Strend-

ingsuturesinParamirimBlock,visibleintheGeologic

MapofBahia[43],werepresumablyreactivatedfromthe

PalaeoproterozoicorogenyaftertheOrosirianevent.The

secondimplicationistheconﬁrmationofthe Brasiliano

eventintheregion as proposed by[21]and[12]. The

reactivationofOrosirianshearzonesandmetamorphism

inParamirimBlockduringProterozoic/Phanerozoictran-

sitionwouldhavebeenpromotedbycontinentalaggre-

gationprocesses,which led to the appearance of West

Gondwana.

Accordingtothelithogeochemicaldatathemagmaticcom-

positioncanberecognizedinallstudiedsamples. This

observation implies surprisingly isochemical processes

duringthemetamorphism,whichcreatedtheLagoaReal

albitites. Thisisquitediﬀerentfromthepreviousmeta-

somaticmodels. Thedata pointtowardsapetrogenetic

associationbetweenalkali-diorite(amphiboliteprotolith)

andsodicsyenite(albititeprotolith)byfractionalcrys-

tallization through transalkaline series developed in a

Palaeoproterozoiclate-orogenictectonicscenario. This

magmaticdiﬀerentiationoccurredeitherbeforeorduring

shearing,whichinturnledtothealbititeandamphibo-

liteformation. Themicrocline-gneiss,whoseprotolithis

asyn-collisionalpotassicgranite,representsthealbitite

hostrockandisapparentlynotpetrogeneticallyassoci-

atedtothelate-orogenicsodicsyenite(albititeprotolith).

Areviewstudyofmaﬁcandfelsicmagmasinwithin-plate

regimesworlwide[44]identiﬁednotonlythatlate-topos-

torogenicigneousassociationsyieldedlesspotassicand

moresodiccompositions,butalsothattheigneoussuites,

comprisingmaﬁcandfelsicrocks,rangefromalkali-calcic

metaluminoustoalkaline,whicharepreciselythechar-

acteristicsoftheLagoaReal alkali-diorite(amphibolite

protolith)andsodicsyenite(albititeprotolith). Further-

more,[44]proposedthatsuchigneousassociationsevolve

370

Alexandre de Oliveira Chaves

progressively into more markedly alkaline within-plate

suites,suggestingthatthe 1.75Gaanorogenicalkaline

SăoTimóteoGraniterepresentstheclosingwithin-plate

stageoftheaforementionedtectonicscenario.

Acknowledgments

ThanksgotoCNPq for the author’sPost-Doctoralre-

search support, to the Development Center of Nuclear

Technology(CDTN-CNEN),to theMemorialUniversity

ofNewfoundland(Canada),wheregeochronologicalstud-

iesbyLA-ICP-MSwerecarriedout,andtotheBrazilian

NuclearIndustries(INB)forﬁeldworkandsamplingsup-

port.

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Dating hydrothermal processes related to the formation of the Lagoa Real uranium deposits using in situ U–Pb dating of andradite and titanite

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Alexandre de Oliveira Chaves

A começar pela idade, há inúmeras semelhanças entre as suítes magmáticas alcalinas Lagoa Real sódica (1,90 Ga, Bahia) e Gouveana potássica (1,95 Ga, Minas Gerais). Ambas apresentam desde lamprófiros e monzonitos até sienitos e quartzo-sienitos metaluminosos, pertencentes à série magmática alcalina saturada em sílica. Sugere-se que a fusão parcial do manto litosférico, que havia sido metassomatizado por fluidos derivados de uma placa subductada antes da colisão, teria inicialmente gerado um magma lamprofírico. A cristalização fracionada deste magma teria levado ao surgimento de magmas monzoníticos que, por sua vez, evoluíram para os sieníticos e quartzo-sieníticos. Ambas suítes alcalinas fazem parte de um domínio estrutural alinhado na direção N-S, com mais de 1.000 km de comprimento, dentro do qual ocorrem outras associações alcalinas, como a sienítica de 2,0 Ga na borda sul do Cráton São Francisco (Minas Gerais), o batólito Guanambi (2,05 Ga, Bahia) e seus termos lamprofíricos, monzoníticos e sieníticos, de gênese associada à dos lamprófiros e sienitos da Suíte Paciência (norte de Minas Gerais) e, ainda, o complexo sienítico-lamprofírico-carbonatítico Angico dos Dias de 2,0 Ga (norte da Bahia). Aparentemente, todas estas associações representam os fragmentos de uma província alcalina orosiriana pós-colisional Minas-Bahia, de idade entre 1,90 e 2,05 Ga.

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Rare-earth-element (REE) bearing minerals have been recognized for the first time within rare-element granitic pegmatite pockets from Lagoa Real Uranium Province (LRUP), Bahia state, NE Brazil. The Province is one of the chief U deposits in the world and the largest in South America. Notwithstanding its economic and strategic importance, there are gaps in the geological knowledge about other mineral resources which could occur in the area. One of them, is the lack of petrographic and chemical data sets on its REE content, especially in pegmatite pockets which occur in the contact between Juazeirinho (1755 ± 6Ma) and São Timóteo (1741 ± 4Ma) granites. In this paper, it is reported a petrographic description and geochemical characterization of allanite, bastnäsite, monazite, xenotime, cheralite, and parisite recognized in three spongy mineral assemblages within unzoned granitic pegmatite pockets from Lagoa Real Uranium Province. The REE-bearing minerals present moderate-to-high REE content with significant amounts of Ce, La, Y, and critical REE such as Nd, Er, and Yb, totalling to up to 49wt%. Petrographical and geochemical evidence arguably indicate that the REE-enrichment in granitic pegmatite pockets of Lagoa Real Uranium Province is a result of late-magmatic and hydrothermal events involving F⁻, OH⁻, CO3²⁻ and PO4³⁻ REE-bearing fluids. The report and characterization of these recently recognised minerals indicate that the Lagoa Real Uranium Province, might hold important REE mineralization as U by-product.

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Feb 2021
J S AM EARTH SCI

The Lagoa Real Complex, in the Lagoa Real Uranium Province, is a manifestation of extensive intraplate A-type magmatism that occurred in the São Francisco Craton during the Paleoproterozoic. U–Pb dating of zircon by LA-MC-ICP-MS shows that granites were emplaced during a Statherian, rift-related, 1.76–1.72 Ga magmatic episode, coeval with emplacement of the A-type Borrachudos suite and felsic metavolcanic rocks from the southeastern, central and northern Espinhaço Igneous Province of the São Francisco Craton. The Lagoa Real Complex involves at least three co-genetic pulses, starting with the initial emplacement of the alkali-feldspar-bearing Lagoa do Barro granite, which crystallized at 1762 ± 9 Ma. This was followed by emplacement of the Juazeirinho granite at 1755 ± 6 Ma, and finally the intrusion of the São Timóteo granite, which represents the main magmatic peak of the Lagoa Real Uranium Province, at 1741 ± 4 Ma. Although the São Timóteo granite has some characteristics that suggest it may be a rapakivi granite, there is no classical cogenetic association with basic rocks. This study confirms that the charnockites of the Lagoa Real Complex have an age of 2047 ± 5 Ma. They are therefore not part of the evolution of the 1.76–1.74 Ga magmatic episode. Also, rocks within the Lagoa Real Uranium Province that were previously mapped as leucodiorites are, in fact, albite-gneiss with ages of 1743–1740 Ma, which originated through the gneissification processes that affected the São Timóteo granite and can also be classified as albitites. The highly negative values of ɛNd(0) from −27 to −33 and high initial Sr ratio are compatible with the Lu–Hf isotopic data of previous studies. These results suggest the Lagoa Real Complex has a crustal derivation with minimum mantle involvement. The formation of the Lagoa Real Complex granites is inferred to be a result of fractional crystallization. All studied granites have a similar geochemical signature in terms of trace elements including REEs, which indicates that the granites come from the same magmatic source and are related to rifting and the initial stages of the Espinhaço Supergroup evolution. The source of the Lagoa Real Complex would be reworked Archean crustal material, probably in the mid to deep crust. Finally, the studied granites, although predominantly metaluminous, have U contents (maximum 12 ppm) below the world average for A-type granites (20 ppm). The fractionation values of Th in relation to U, and the absence of metamictization of the minerals (mostly allanite) that potentially host uranium confirms this observation. Therefore, the granites of the Lagoa Real Complex, despite being the protolith of bodies that host U mineralization, should not be considered as the principal source of U in the U mineralization found in the Lagoa Real albitites. However, the provenance of an important part of the uranium could be associated with Statherian meta-rhyolites and meta-rhyodacites, contemporaneous with the São Timóteo granite, and occurring to the west of the Lagoa Real Complex.

On the classification of amphiboles

Article

Full-text available

Feb 2006

Major issues involved in the classification of the amphiboles are examined: (1) the role of (OH), Li and Fe3+, (2) the formal definition of a root name, (3) irreducible charge-arrangements and distinct species, (4) the use of prefixes, (5) the principal chemical variables used in a classification procedure, and (6) the use of the dominant-constituent principle. The current IMA-approved classification scheme is based on the A, B and T groups of cations in the amphibole formula: AB2C5T8O22 W2. We argue here that classification should be based on the A, B and C groups of cations as (i) it is in these groups of cations that the maximum variation in chemical composition occurs, and (ii) as a result of (i), the scheme is more in accord with the IMA-sanctioned dominant-constituent principle, which governs the recognition (and approval) of distinct mineral species. Two new classifications are presented here; one is based on the A, B and C groups of cations, and another on the dominant-constituent principle. These two schemes were produced to illustrate (i) the problems inherent in the classification of a group of minerals as complicated as the amphiboles, and (ii) the sometimes disparate needs of crystallographer, mineralogist, petrologist and geochemist. Scheme 1 conserves current formulae and names as much as possible, whereas scheme 2 minimizes the number of formulae and names as much as possible. The differences between the current classification and the two schemes presented here are discussed, and we highlight the problems associated with each scheme.

Classification of the Amphiboles

Article

Full-text available

Jun 2007

Major issues involved in the classifi cation of the amphiboles are examined: (1) the role of (OH), Li and Fe3+, (2) the formal defi nition of a root name, (3) irreducible charge-arrangements and distinct species, (4) the use of prefi xes, (5) the principal chemical variables used in a classifi cation procedure, and (6) the use of the dominant-constituent principle. The current IMA- approved classifi cation scheme is based on the A, B and T groups of cations in the amphibole formula: AB2C5T8O22W2. We argue here that classifi cation should be based on the A, B and C groups of cations as (i) it is in these groups of cations that the maximum variation in chemical composition occurs, and (ii) as a result of (i), the scheme is more in accord with the IMA-sanctioned domi- nant-constituent principle, which governs the recognition (and approval) of distinct mineral species. Two new classifi cations are presented here; one is based on the A, B and C groups of cations, and another on the dominant-constituent principle. These two schemes were produced to illustrate (i) the problems inherent in the classifi cation of a group of minerals as complicated as the amphiboles, and (ii) the sometimes disparate needs of crystallographer, mineralogist, petrologist and geochemist. Scheme 1 conserves current formulae and names as much as possible, whereas scheme 2 minimizes the number of formulae and names as much as possible. The differences between the current classifi cation and the two schemes presented here are discussed, and we highlight the problems associated with each scheme.

A classification of igneous rocks and glossary of terms: Recommendations of the IUGS subcommission on the systematics of igneous rocks

Book

Jan 2002

O CRÁTON DO SÃO FRANCISCO

Article

Sep 1977

ALMEIDA F. F. M. DE

3. Systematics and Paragenesis of Uranium Minerals: Mineralogy, Geochemistry, and the Environment

Chapter

Dec 1999

6. Uranium Ore Deposits— Products of the Radioactive Earth: Mineralogy, Geochemistry, and the Environment

Chapter

Dec 1999

Isoplot 3.00: A Geochronological Toolkit for Microsoft Excel, Berkeley

Article

Jan 2003

K.R. Ludwig

Rock forming minerals

Article

Jan 2004

Systematics and paragenesis of uranium minerals. In: uranium: mineralogy, geochemistry and the environment

Article

Jan 1999

Approximately five percent of all known minerals contain U as an essential structural constituent. Uranium minerals display a remarkable structural and chemical diversity. The chemical diversity, especially at the Earth's surface, results from different chemical conditions under which U minerals are formed. U minerals are therefore excellent indicators of geochemical environments, which are closely related to geochemical element cycles. The oxidation and dissolution of U minerals contributes U to geochemical fluids, both hydrothermal and meteoric. Under reducing conditions, U transport is likely to be measured in fractions of a centimeter, although F and CI complexes can stabilize U(IV) in solution (Keppler and Wyllie 1990). Where conditions are sufficiently oxidizing to stabilize the uranyl ion, UO/+, and its complexes, U can migrate many kilometers from its source in altered rocks, until changes in solution chemistry lead to precipitation of U minerals. Where oxidized U contacts more reducing conditions, U can be reduced to form uraninite, coffinite, or brannerite. The precipitation of U(VI) minerals can occur in a wide variety of environments, resulting in an impressive variety of uranyl minerals. Because uraninite dissolution can be rapid in oxidizing, aqueous environments, the oxidative dissolution of uraninite caused by weathering commonly leads to the development of a complex array of uranyl minerals in close association with uraninite. This chapter focuses on U-mineral paragenesis and chemistries (see Burns 1999 in this volume for a review of U-mineral structures). Some detailed descriptions are provided for U minerals reported since the review by Smith (1984). Minerals containing reduced U are discussed first, followed by uranyl minerals, in which U occurs as U6+. Minerals are further divided chemically according to the major anionic component (e.g. silicate, phosphate, etc.), with some chemical groups listed together because of structural similarities. Tables list minerals in alphabetical order within each chemical group. Mineral names, formulae, and references are provided, together with comments pertaining to recent work reported for these minerals.

An introduc-tion to the rock forming minerals

Article

Jan 1992

New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

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