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Journal of South American Earth Sciences 58 (2015) 129e140

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Journal of South American Earth Sciences

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UePb geochronology of the Lagoa Real uranium district, Brazil:Implications for the age of the uranium mineralization

Lydia Maria Lobato a, *, M�arcio Martins Pimentel b, Simone C.P. Cruz c, Nuno Machado d, 1,Carlos Maurício Noce a, 1, Fernando Flecha Alkmim e

a Instituto de Geociencias-CPMTC, Universidade Federal de Minas Gerais, Av. Antonio Carlos 6627, Campus Pampulha, 30130-009 Belo Horizonte, MinasGerais, Brazilb Instituto de Geociencias, Universidade de Brasília, Campus Darcy Ribeiro ICC, Ala Central, 70910-900, Brasília, Distrito Federal, Brazilc Instituto de Geociencias, Universidade Federal da Bahia, Av. Caetano Moura, S/N, Federaç~ao, 40000-000 Salvador, Bahia, Brazild Centre de Recherche en G�eochimie et en G�eochronologie, GEOTOP, Universit�e du Qu�ebec a Montr�eal, CP 8888 Succ. A, Montr�eal, Quebec H3C 3P8, Canadae Departamento de Geologia, Universidade Federal de Ouro Preto, Morro do Cruzeiro, Bauxita, 45400-000 Ouro Preto, Minas Gerais, Brazil

a r t i c l e i n f o

Article history:Received 13 February 2014Accepted 18 December 2014Available online 25 December 2014

Keywords:Lagoa Real uranium depositMetasomatic alterationUePb mineralization ageMetallogenetic implications

* Corresponding author. Tel.: þ55 31 96120791, þ534095410.

E-mail address: llobato.ufmg@gmail.com (L.M. Lob1 In memoriam.

http://dx.doi.org/10.1016/j.jsames.2014.12.0050895-9811/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

The Lagoa Real uranium district in Bahia, northeastern Brazil, is the most important uranium province inthe country and presently produces this metal in an open-pit mine operated by Indústrias Nucleares doBrasil. Uranium-rich zones are associated with plagioclase (dominantly albite ± oligoclase) -rich rocks,albitites and metasomatized granitic-gneisses, distributed along NNW/SSE striking shear zones. We haveused the ID-TIMS UePb method to date zircon and titanite grains from the S~ao Tim�oteo granitoid, andalbite-rich rocks from the Lagoa Real district in order to assess the age of granite emplacement, defor-mation/metamorphism and uranium mineralization. The isotopic data support the following sequence ofevents (i) 1746 ± 5 Ma e emplacement of the S~ao Tim�oteo granitoid (UePb zircon age) in an extensionalsetting, coeval with the beginning of the sedimentation of the Espinhaço Supergroup; (ii) 956 ± 59 Mahydrothermal alteration of the S~ao Tim�oteo granitoid and emplacement of the uraniummineralization (UePb titanite age on an albite-rich sample); (iii) 480 Ma metamorphism, remobilization and Pb loss (UePbtitanite age for the gneiss sample), during the nucleation of shear zones related to the collision betweenthe S~ao Francisco-Congo and Amazonia paleoplates. The 956 ± 59-Ma mineralization age is apparentlyassociated with the evolution of the Macaúbas-Santo Onofre rift. This age bracket may bear an importantexploration implication, and should be included in the diverse age scenario of uranium depositsworldwide.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

The Lagoa Real granitic-gneiss complex in Bahia State, north-eastern Brazil, hosts several uranium-enriched zones and com-prises the most important uranium province in Brazil (Figs. 1 and2). A series of orebodies are known from thirty eight anomaliesdistributed over an area of 1200 km2, roughly along three semi-arched lineaments that cover an approximate extension of 35 km.The orebodies contain a total reserve of ca. 112,000 metric tons ofU3O8 (Brito et al., 1984; Matos et al., 2003; Matos and Villegas,

5 31 34095443; fax: þ55 31

ato).

2010). Uranium production initiated in 1999 by Indústrias Nucle-ares do Brasil (INB), and 300 tons per year of uranium concentrateare produced from seven orebodies exploited at the northernCachoeira open-pit mine (http://www.inb.gov.br; Matos andVillegas, 2010). Roughly 3.700 tons of U3O8 have already beenproduced by INB.

The uranium deposits are associated with rocks dominated byplagioclase (albite ± oligoclase) and albitites, which are the productof extensive, high-temperature metasomatic alteration of granitic-gneissic rocks, mainly in the form of sodium enrichment and silicadepletion, accompanied by oxidation of the original Feþ2-dominantphases (e.g., Lobato and Fyfe, 1990). Lithotypes dominated bycalcium-rich oligoclasites host some orebodies along the lineamentsuggesting NaeCa metasomatic alteration (Raposo and Matos,1982; Raposo et al., 1984; Cruz, 2004).

Fig. 1. Major geological domains of the S~ao Francisco Craton (Alkmim et al., 1993), showing the location of the Lagoa Real uranium district (inset), Bahia State, northeastern Brazil.CD ¼ Chapada Diamantina; ES: Western Espinhaço range.

L.M. Lobato et al. / Journal of South American Earth Sciences 58 (2015) 129e140130

The genesis and age of the uraniummineralization has been thesubject of investigation and debate by various authors (e.g., Turpinet al., 1988; Cordani et al., 1992; Pimentel et al., 1994). Previousgeochronological studies using RbeSr, KeAr, SmeNd and UePbmethods suggested a polycyclic history for the geological evolutionof the Lagoa Real uranium district, with events at ca. 1.7 Ga,1.3e1.5 Ga and 0.48 Ga (Turpin et al., 1988; Cordani et al., 1992). Theformation of the uranium mineralization was assigned to the1.3e1.5 Ga tectonic event, or Espinhaço collisional event of Turpinet al. (1988) and Cordani et al. (1992), based on UePb analyses ofheavy mineral concentrates and RbeSr whole-rock isochrons.

We carried out a UePb geochronological investigation on zirconand titanite grains from granitic rocks and albitites from Lagoa Real,aiming to assess more precisely the crystallization age of the hostrocks and subsequent recrystallization episodes, as well as the ageof the uranium-enrichment event. Special emphasis is given to thetitanite analyses. Due to their relatively low blocking temperatureof ca. 600 �C, compared to zircon, titanite can provide good ageestimates for metamorphic recrystallization or high-temperaturemetasomatic alteration processes (Tilton and Grünenfelder, 1968;Mattinson, 1978). The UePb data were originally presented byPimentel et al. (1994) in the form of an extended abstract. However,further comprehensive knowledge of the regional geology of thedeposit area was only better unraveled later on by the studies ofArcanjo et al. (2000), Cruz (2004) and Cruz et al. (2007, 2008),allowing a more robust interpretation of the geochronological data,which is put forward in the present contribution.

According to Cuney (2010), uranium deposits evolved throughfour major time periods. Of these, the third, from 2.2 to 0.45 Ga,records increasing atmospheric oxygen and encompasses uraniumdeposits related to sodium metasomatism, which are recorded tohave formed mostly between 1.8 and 1.4 Ga, and less importantlyduring the so-called Pan-African or Brasiliano event (~500 Ma;Cuney, 2010). The age of the Lagoa Real mineralization, as discussedbelow, departs from these, and may help constrain further theunderstanding of uranium deposition through time.

2. Geological setting

The Lagoa Real deposit area is located in the Paramirim rivervalley, in the northern part of the Ediacaran-Cambrian AraçuaíOrogen (Pedrosa Soares and Wiedeman, 2000; Pedrosa Soareset al., 2001, 2007, 2008; Alkmim et al., 2006) (Figs. 1 and 2). Thebasement of the northern portion of the Araçuaí Orogen is made upof terranes containing granitic-gneiss and granulite of Archean andPaleoproterozoic ages, as well as metavolcano-sedimentary belts(Cordani and Brito Neves, 1982; Bastos Leal et al., 1998; Silva andCunha, 1999; Barbosa and Sabat�e, 2002).

The Paramirim valley separates two morphotectonic domainsunderlain by Proterozoic sedimentary/metasedimentary units,namely the Espinhaço range (ES, Fig. 1), on the west, and the largeplateau of the Chapada Diamantina, on the east (CD, Fig. 1). TheProterozoic covers comprise the Paleo-to MesoproterozoicEspinhaço Supergroup (Figs. 1 and 2) and the Neoproterozoic S~ao

Fig. 2. Regional geological map of central Bahia state, Brazil, showing the main units discussed in the text and the location of the Lagoa Real district (modified after Cruz et al., 2008).ES ¼ Western Espinhaço range; CD ¼ Chapada Diamantina; RP ¼ Rio Preto.

L.M. Lobato et al. / Journal of South American Earth Sciences 58 (2015) 129e140 131

Francisco Supergroup (Schobbenhaus, 1996; Misi and Veizer, 1996;Danderfer Filho and Dardenne, 2002). Both these Supergroups arerelated to the evolution of the complex, polyphase Espinhaço-Macaúbas basin (Schobbenhaus, 1996; Alkmim, 2012). Periods ofextensional tectonics are indicated by Meso- and Neoproterozoicmafic dikes that intrude the Espinhaço Supergroup rocks. The dikescomprise two main age groups: (i) group I, characterized by UePbages of 1492 ± 16 Ma (Loureiro et al., 2008) and 1496 ± 3.2 Ma(Guimar~aes et al., 2005), which are similar to ages obtained byBabinski et al. (1999) at ca. 1514 Ma; (ii) group II, which has UePbages of 934 ± 14 Ma (Loureiro et al., 2008) and 854 ± 23 Ma(Danderfer Filho et al., 2009). Phanerozoic sedimentary rocks alsocover large areas of the Paramirim valley (Fig. 1).

The basement rocks of the northern portion of the Araçuaí Oro-gen have been affected by tectonometamorphism of various pe-riods: (a) Late Archean age (Jequi�e event), (b) Paleoproterozoic (ca.2.1 to 1.8Ga), (c) Espinhaço age (1.3e1.0Ga), and (d)Neoproterozoic,Brasiliano event (0.6e0.5 Ga) (e.g., Inda and Barbosa, 1978; BritoNeves et al., 1980, 1999; Barbosa and Dominguez, 1996; Cordaniet al., 1992; Barbosa and Sabat�e, 2002; Correa Gomes and Oliveira,2002; Cruz and Alkmim, 2006; Danderfer Filho et al., 2009). Sincethe significance of the Espinhaço Event is under debate (e.g.,Chemale et al., 2010), it is not discussed in detail in this paper.

According to traditional views, the Espinhaço Eventwas responsiblefor the deformation and metamorphism of the Espinhaço Super-group and their sialic basement between ca. 1.4 and 1.1 Ga (Cordaniet al., 1992). Other authors, however, demonstrated that the Espin-haço rocks and the Neoproterozoic cover units (S~ao Francisco Su-pergroup) of the northern S~ao Francisco Craton (Almeida, 1977)underwent the same deformational history and, consequently, theEspinhaço rocks did not experience deformation and regionalmetamorphism prior to ca. 600e500 Ma (Caby and Arthaud, 1987;Danderfer Filho, 1990, 2000; Uhlein, 1991; Trompette et al., 1992;Alkmim et al., 2006; Cruz and Alkmim, 2006). Furthermore, Cruzand Alkmim (2006) and Cruz et al. (2007) postulated that tectonicelements affecting both the uranium-bearing Lagoa Real granitic-gneiss complex and the Neoproterozoic S~ao Francisco Supergoupare of the same generation.

The Lagoa Real granitic-gneiss complex is enclosed within thegranitic-gneissic basement, east of the Espinhaço Range andwest ofthe Chapada Diamantina (Figs. 1 and 2). The basement gneissesexposed to the north and east of Lagoa Real consist of meta-granitoids and migmatites of late Archean and Paleoproterozoicages (Jardim de S�a, 1978; Bastos Leal et al., 1998).

The most conspicuous geological feature of the Lagoa Real areais the presence of several intrusions of porphyritic rocks known

Fig. 3. Geological sketch map of the Lagoa Real district in the Paramirim river valley (inset of Fig. 2). Towns: C ¼ Caetit�e; P ¼ Paramirim; IB ¼ Ibiassuce; ST¼ S~ao Tim�oteo (after Cruzet al., 2008).

L.M. Lobato et al. / Journal of South American Earth Sciences 58 (2015) 129e140132

L.M. Lobato et al. / Journal of South American Earth Sciences 58 (2015) 129e140 133

generically as the S~ao Tim�oteo granitoid (Figs. 3 and 4A). Theycomprise coarse-grained albitized syenogranite, alkali-feldspargranite and syenite. These are locally converted into proto-mylonites, augen mylonites and ultramylonites of the samecomposition, exhibiting gneissic structure, and are here collectivelynamed albitized gneisses (Fig. 4B; Lobato and Fyfe, 1990; Cruz andAlkmim, 2006; Cruz et al., 2007, 2008). The most abundant facies ofthe S~ao Tim�oteo granitoid corresponds to a potassic, hastingsite-biotite monzo-to syenogranite of metaluminous and high-K calc-alkaline affinity (Teixeira, 2000). Maru�ejol et al. (1987) indicate thatthese are iron-rich, metaluminous, subalkaline plutonic rocks ofintracontinental environment.

3. Uranium mineralization

In association with the gneissic rocks of the Lagoa Real granitic-gneiss complex or district, there is a significant number of lenticularalbite(±oligoclase)-rich bodies exposed along a ca. 100-km long,arcuate belt roughly in the central portion of the district (Fig. 3).Shear zone-hosted albite(±oligoclase)-rich rocks and albitites maylocally contain uranium mineralization (Fig. 4C). They are fine-tomedium-grained rocks showing weak to pronounced foliation(Lobato and Fyfe, 1990; Cruz et al., 2007). Where uraninite-bearing,these rocks may contain more than one mineralized zone. Rounduraninite crystals are very fine (5e25 mm), chiefly associated withandradite, but also found included and bordering aegirine-augite,albite-oligoclase, hastingsitic hornblende, biotite (Fig. 4D), calcite,and martitized magnetite (Lobato and Fyfe, 1990). Calcium-richoligoclasites may also host uranium ore; these contain epidote,hedenbergite, diopside, grossular and also martitized magnetite(Cruz, 2004). Further detailed mineralogical studies are found inPrates (2008).

Field evidence indicates that the plagioclase(albite ±oligoclase)-rich rocks have been formed by metasomatism andshearing of the S~ao Tim�oteo granitoid (e.g., Lobato and Fyfe, 1990).These authors point out that all uranium-rich zones in Lagoa Realare associated with metasomatized rocks. Therefore, shearing,metasomatism (sodium enrichment and silica depletion), and

Fig. 4. (A) Albitized, coarse-grained, alkaline S~ao Tim�oteo granitoid. (B) Sheared Lagoa Remartitized magnetite. (D) Photomicrograph of albitite displaying hastingsite (Hst), biotite (

mineralization must have been coeval. However, later studies byCruz et al. (2007) based on microstructural criteria suggested thatNaeCa alteration pre-dates the nucleation of the shear zones thathost uranium mineralization.

Other lines of evidence include the presence of flame perthitehosted in K-feldspar of the alkaline S~aoTim�oteo granitoid, as well asalbite/oligoclase veins and agglomerates, both features found indistal zones to mineralization. Close to shear zones, these plagio-clase agglomerates are progressively recrystallized giving place tothe albitites (Cruz, 2004; Cruz et al., 2008).

Sodiummetasomatism and uraniummineralization, collectivelyreferred to as albitite-type (e.g., Wilde, 2013), metasomatic (Cuneyand Kyser, 2008), or albitite-hosted (Polito et al., 2007) uraniumdeposits are widely distributed in the world, and no single geneticmodel is consistent with the examples cited in the literature (e.g.,Cuney and Kyser, 2008; Wilde, 2013). For the Lagoa Real uraniumdeposits, no consensus exists regarding their genesis, despite thevarious studies in the area (e.g., Geisel Sobrinho et al., 1980; Lobato,1985; Maru�ejol et al., 1987; Fuzikawa et al., 1988; Maru�ejol, 1989;Lobato and Fyfe, 1990; Cruz, 2004; Chaves et al., 2007; Souza,2009; Oliveira, 2010; Oliveira et al., 2012; Chaves, 2013), and athorough discussion of the subject is beyond the scope of thispaper.

According to Lobato and Fyfe (1990), uranium mineralizationdeveloped in association with hot (~500e550 �C), oxidizing fluidsthat had oxygen isotope values somewhat similar for both non-mineralized, from �0.8 to þ7.3 per mil, and mineralizedplagioclase(albite ± oligoclase)-rich rocks, ranging from �3.7to þ2.6 per mil.

Fluid inclusion studies were primarily undertaken by Fuzikawaet al. these authors do not propose (1988) on albite, late-stagequartz and calcite, with indication of aqueous and aquo-carbonicfluids. The authors also imply that a late-stage brine may haveexisted with oscillating salinities up to 20 percent NaCl equivalent.More recently, Souza (2009) and Oliveira (2010) obtained salinityvalues that range from 9 to 18 percent NaCl equivalent for theaqueous fluid. The latest fluid inclusion studies coupled with LA-ICMPS analyses (Oliveira et al., 2012) focus on data obtained on

al albitized gneiss. (C) Mineralized albitite containing hedenbergite, hastingsite andBt) and titanite (Ttn).

L.M. Lobato et al. / Journal of South American Earth Sciences 58 (2015) 129e140134

pyroxenes (both metamorphic and metasomatic), garnet andplagioclase of albitites, from three of the most important depositsat Lagoa Real. According to these authors, the early-stage aqueousfluids in equilibriumwith mafic minerals were saline (12e16% NaClequivalent), and contained Na, Mg, U, Rb, Ba, Sr, Pb, K, Ca, Fe, Cu, Znand Li, as well as some Mn, As and Sb. On the other hand, plagio-clase precipitated from a more diluted aqueous fluid (0.5e6.4%NaCl equivalent; up to 11% as stated in Fuzikawa et al., 1988),containing Na, Mg, K, Ca, Fe, Cu, Zn, As, Sr, Ba, Pb. Aquo-carbonicfluids are only detected in vein quartz crosscutting mineraliza-tion. Despite all these information, these authors have not come toa consensus as to the nature of the fluids involved.

Based primarily on field work, Geisel Sobrinho et al. (1980) andStein et al. (1980) anticipated that the Lagoa Real uranium derivedfrom magmatic fluids, without indicating their precise source.Studies by Maru�ejol et al. (1987), Turpin et al. (1988) and Maru�ejol(1989) suggest that the mineralizing fluids were derived from thePaleoproterozoic S~ao Tim�oteo granitoid, which contains uranium-bearing accessory minerals, as the mineralizing agent.

Additionally, on the basis of 87Sr/86Sr isotope ratios in the rangeof 0.704e0.723 obtained for the albitites, Lobato and Fyfe (1990)discarded the granitic-gneiss (87Sr/86Sr isotope ratios between0.738 and 0.814) terrane as the main source of the mineralizingfluids. Alternatively, the authors suggest that the fluids that mayhave been responsible for the uraniummineralizationwere derivedfrom the Espinhaço Supergroup rocks, which are adjacent to theLagoa Real granitic-gneiss complex (Lobato, 1985). Such fluidswould be analogous to oxidizing (Hanor, 1979), saline, modern-formation brines, characterized by low d18O values (Taylor, 1979).Due to the lack of precise geochronological data, Lobato et al. (1983)and Lobato (1985) suggested that the Espinhaço fluids expelledduring the Brasiliano orogenesis would have been responsible forthe Lagoa Real uranium mineralization. In this context, the fluids ofthis orogenetic cycle would have had been metamorphic, lacking,therefore, the characteristics of saline brines (see also Cuney andKyser, 2008).

4. Previous geochronological studies in the Lagoa Realuranium district

The main geological units exposed in the Lagoa Real uraniumdistrict have been the subject of geochronological studies usingmultichronometric approaches (UePb, PbePb, RbeSr, SmeNd andKeAr methods) in order to constrain the timing of graniteemplacement, metamorphism, hydrothermal alteration and ura-niummineralization (Turpin et al., 1988; Cordani et al., 1992). These

Table 1Previous geochronological data for the Lagoa Real District. 1: Turpin et al. (1988); 2: Cor

Rock unit Material

Basement gneiss and migmatite Whole rockBasement gneiss BiotiteMigmatite AmphiboleGranite ZirconGranite Whole rockGranite Whole rockGranite BiotiteLineated granite Whole rockGneiss Whole rockOrthogneiss Whole rockOrthogneiss Whole rockGneiss AmphiboleAlbitite Acid-soluble fractions of whole-rock,

heavy mineral, and uraninite concentratesAlbitized orthogneiss Whole rock

studies produced a wealth of isotopic and geochronological data,which are summarized in Table 1.

4.1. Basement gneisses

Geochronological UePb, RbeSr and SmeNd data for the Para-mirim valley basement rocks suggest crust formation to have beenrelated to at least three Archean plutonic events. The earliest eventdates back to the Paleoarchean at around 3.4e3.2 Ga (Martin et al.,1991; Nutman and Cordani, 1993; Bastos Leal et al., 1998; SantosPinto et al., 1998, 2012), and is represented by tonalite-trondjemite-granite plutons. A 3.2e3.1 Ga plutonic event, repre-sented by granitic-granodioritic rocks, was probably responsible forthe reworking of rocks from the Paleoarchean event (Bastos Leal,1998; Bastos Leal et al., 1998). The youngest plutonic event wasdated at 2.9e2.7 Ga by the RbeSr and PbePb systematics (BritoNeves et al., 1979; Costa et al., 1985; Cordani et al., 1985, 1992;Santos Pinto, 1996).

Samples that represent neosome of migmatitic gneissesexposed to the northwest and north of Lagoa Real were dated by aRbeSr whole-rock. An isochronic RbeSr age of 2650 ± 100 Ma wasobtained with a high initial 87Sr/86Sr ratio of ca. 0.712 (Cordaniet al., 1992). This late Archean date was interpreted as the age ofmigmatization, related to the Jequi�e tectonothermal event. Rhya-cian (2300e2050 Ma) and Orosirian (2050e1800 Ma) calc-alkalinegranitic rocks intrude these gneisses (Bastos Leal et al., 1998;Guimar~aes et al., 2005; Leal et al., 2005). Two KeAr analyses inbiotite from a gneiss and amphiboles from a migmatite revealedages of 538 and 491 Ma, respectively, indicating that cooling fol-lowed the Neoproterozoic Brasiliano event (Cordani et al., 1992).

4.2. S~ao Tim�oteo albitized granite

Samples of the S~ao Tim�oteo albitized granite (Fig. 4A) have beenpreviously investigated by the UePb, PbePb, RbeSr, SmeNd andKeAr methods (Turpin et al., 1988; Cordani et al., 1992; data inTable 1).

Turpin et al. (1988) reported a zircon UePb age of 1724 ± 5 Ma,and interpreted it as the best estimate for the age of emplacementof the S~ao Tim�oteo granitoid. Zircon grains from deformed graniteand gneissic samples lie on the same discordia, and theserocks were considered to be the recrystallized facies of theS~ao Tim�oteo granitoid. An additional discordia line was tracedbased on analyses of morphologically different zircon grains indi-cating upper and lower intercept ages of ca. 1726 Ma and 524 Ma,respectively. The latter age suggests a period of Pb loss during theCambrian that may be related to shear zones truncating the S~ao

dani et al. (1992).

Method Age (Ma) and isotopic Parameters Reference

RbeSr 2650 ± 100 (i. r. ¼ 0.712) 2KeAr 538 2KeAr 491 2UePb 1724 ± 5 1PbePb 1706 ± 107 2RbeSr 1710 ± 45 (i. r. ¼ 0.7135) 2KeAr 573 2RbeSr 1280 ± 20 (i. r. ¼ 0.775) 2RbeSr 1629 ± 30 (i. r. ¼ 0.7104) 1RbeSr 1000 ± 60 (i. r. ¼ 0.722) 2RbeSr 1220 ± 130 (i. r. ¼ 0.723) 2KeAr 503 2UePb 1397 ± 9 1

RbeSr 1520 ± 20 (i. r. ¼ 0.7221) 2

L.M. Lobato et al. / Journal of South American Earth Sciences 58 (2015) 129e140 135

Tim�oteo granitoid; these shear zones are associated with thedeformational history of the northern portion of the Araçuaí Oro-gen, and result from the collision between the S~ao Francisco-Congoand Amazonia paleoplates during the Ediacaran (Cruz and Alkmim(2006; Alkmim et al., 2006). According to Turpin et al. (1988), thegneiss formation took place during a late stage of the Brasilianoorogenic event, at ca. 480 Ma.

Cordani et al. (1992) also analyzed samples of the S~ao Tim�oteogranitoid. They report the historical whole-rock RbeSr and PbePbisochron ages of 1710 ± 45 Ma and 1706 ± 107 Ma, respectively, inagreement, within errors, with the UePb magmatic age of Turpinet al. (1988). However, RbeSr analyses on samples collected fromthe gneissic facies of the S~ao Tim�oteo granitoid yield isochron agesat ca 1.6, 1.2 and 1.0 Ga (Turpin et al., 1988; Cordani et al., 1992),probably reflecting the disturbance of the isotopic system sincethese ages have no geological significance. The high initial 87Sr/86Srratio (0.7135) and the model m1 value of 8.4 are compatible with animportant crustal component in the parental magma. This is alsosupported by the SmeNd isotopic data of Turpin et al. (1988),which give TDM model ages between ca. 2.2 and 3.0 Ga, and by thehigh d18O values reported by Lobato et al. (1983) in quartz samplesfrom the Lagoa Real granite. A considerably younger RbeSr age(1629 ± 30 Ma) was interpreted as a disturbance of the RbeSrisotopic system during the Brasiliano tectonothermal event (Turpinet al., 1988).

Magmatic titanite grains of the S~ao Tim�oteo granitoid yielded anPbePb age of 1743 ± 28 Ma (Cruz et al., 2007). Amphibole andbiotite KeAr cooling ages for samples of the S~ao Tim�oteo granitoidare 503 Ma and 573 Ma, respectively, which indicate a strongBrasiliano event overprint in the area (Cordani et al., 1992).

4.3. Albitized gneisses and albitites

The UePb data for uraninite grains extracted from the LagoaReal albitized gneisses yielded a concordant age of ca. 820Ma (Steinet al., 1980), interpreted to represent the time of the metasomaticalteration caused by alkaline fluids generated during a basin-forming event.

Heavy minerals separated from albitite samples were dated bythe UePb TIMS method (Turpin et al., 1988). The zircon fractionsstudied show extremely positive as well as negative discordances(sample 95 of their work is �20% discordant). These authorsinterpreted the unusual behavior of the albitite zircon grains as theresult of relative mobility of U, Pb and 238U series products.Nevertheless, the albitite zircon grains align along a discordia linewith an upper intercept age of 1504 ± 12Ma, which is in agreementwith an 1520 ± 20 Ma RbeSr whole-rock isochron age reported byCordani et al. (1992) for variably albitized augen orthogneisses fromlocalities near the uranium mineralization.

A discordia with seven analytical points for acid-soluble frac-tions of whole-rock albitite samples, heavy mineral concentratesand uraninite concentrates is also reported by Turpin et al. (1988).The upper and lower intercept ages of 1397 ± 9 Ma and 480 Mawere interpreted as the ages of uranium mineralization andreworking of the uranium deposit, respectively. However, theseages are not robust considering that distinct U-bearing mineralphases, with different closure temperatures, were analyzedtogether.

5. Analytical procedures

For the present work, heavy mineral fractions were obtainedfrom ca. 20 kg samples by panning at the laboratories of Vale(previously Rio Doce Geologia e Mineraç~ao S.A.), Belo Horizonte,Brazil. These fractions were further processed by sieving, magnetic

separation using a Frantz isodynamic separator, and conventionalheavy liquid separation at the GEOTOP, Universit�e du Qu�ebec aMontr�eal. A combination of the following techniques was appliedto reduce discordance: (i) hand-picking of the most transparent,least-altered and crack-free zircon grains from the least-magneticfractions, and (ii) air abrasion following the procedure of Krogh(1982).

Despite severe abrasion, many zircon fractions in this study arestill discordant, reflecting the difficulty in obtaining closed UePbsystem behavior. Titanite fractions were not abraded. The selectedzircon and titanite grains were homogeneous in size, color andshape.

Zircon dissolution and chemical extraction of U and Pb followedthe general procedures described by Krogh (1973). A205Pbe233Ue235U mixed spike was used. For titanite, the chemicalprocedures outlined by Corfu and Stott (1986) were followed. An-alyses were performed on a VG Sector 54 mass spectrometer at theCentre de Recherche en G�eochimie et en G�eochronologie (GEOTOP)of the Universit�e du Qu�ebec a Montr�eal, Canada. Errors at the 1-sigma level are generally around 0.5% and 0.1%, respectively for theU/Pb and 207Pb/206Pb ratios. In samples with higher amounts ofinitial common lead, errors in UePb and 207Pb/206Pb ratios are ca.1.0% and 0.3%, respectively. Regression lines were calculated usingthe method described by Ludwig (2008) and quoted errors repre-sent the 95% confidence level.

6. Results

6.1. S~ao Tim�oteo granitoid (sample HBU-872)

Sample HBU-872 is a coarse-grained, isotropic granite, exhibit-ing an incipient and poorly defined foliation. Other than perthitic K-feldspar, plagioclase and weakly recrystallized quartz, this granitealso contains hastingsite, iron biotite, and traces of almandine,zircon and opaque minerals.

Three zircon fractions from this intrusion were analyzed, andthe results are shown in Table 2. Although core-overgrowth re-lationships were not observed in the fractions investigated, weanalyzed the tips of large (200e300 mm), colorless, good qualityprismatic crystals (fractions HBU-872.1 and HBU-872.2; Fig. 5) toensure that no inherited Pb was included. Fraction HBU-872.3comprised only whole crystals. Analyses of fractions HBU-872.1and HBU-872.2 produced nearly identical results. They are bothca. 1.0% discordant and yielded 207Pb/206Pb ages of 1747 and1745Ma, respectively (Fig. 5). Fraction HBU-872.3 has amuch largercommon lead content and required a larger correction, whichresulted in a larger error ellipse. Nevertheless, this fraction indi-cated a similar 207Pb/206Pb age (1746Ma), being ca. 0.7% discordant.The three fraction analyses are collinear, defining a discordia linewith an upper intercept age of 1746 ± 5 Ma (Fig. 5) considered asthe best estimate for the crystallization age of the S~ao Tim�oteogranitoid.

6.2. Mineralized albitite (sample HBU-871)

Sample HBU-871 is a mineralized andradite albitite. It is fine-tomedium-grained, granoblastic to polygonal granoblastic, exhibitingalbite, andradite, diopside (aegirine-augite), uraninite, plus tracesof titanite, zircon, martitized magnetite and apatite. Relicts ofhastingsitic amphibole (after pyroxene), biotite (after garnet), car-bonate and epidote are rare. Uraninite forms inclusions in garnetand pyroxene.

Non-magnetic zircon grains in the albitite sample are morpho-logically identical to those of the S~ao Tim�oteo granitoid. Thisobservation agrees with the interpretation of Turpin et al. (1988)

Table

2UePb

isotop

icresu

ltsforthepresentpap

er.T

he1sigm

auncertainties

arein

bracke

tsan

dreferto

thelast

digitsof

theratio.

Sample

Con

centrations

Isotop

icratios

Age

s(M

a)

Numbe

rMineral

Weigh

t(m

g)Uranium

(ppm)

Rad

ioge

nic

Pb(ppm)

Com

mon

Pb(pg)

206Pb

/204Pb

208Pb

/206Pb

206Pb

/238U

%1s

207Pb

/235U

%1s

207Pb

/206Pb

%1s

206Pb

/238U

207Pb

/235U

207Pb

/206Pb

[1]

[2]

[2]

[3]

[4]

[5]

[5]

[5]

[5]

S~ aoTim� oteo

Granitoid

(HBU

-872

)1

Zrn

0.03

818

562

2849

030.15

780.30

750.42

4.53

10.45

0.10

688

0.25

1728

1737

1747

2Zrn

0.04

515

652

2165

270.15

680.30

760.5

4.52

90.54

0.10

677

0.15

1729

1736

1745

3Zrn

0.06

134

4648

533

70.18

480.30

870.9

4.54

51.1

0.10

680

0.23

1734

1739

1746

Albitite(H

BU-871

)6

Zrn

0.02

911

840

3021

720.19

550.30

620.4

4.49

70.43

0.10

651

0.25

1722

1730

1740

7Zrn

0.01

518

161

2818

920.16

460.30

520.56

4.49

00.67

0.10

672

0.26

1717

1729

1744

1Sp

n0.25

533

3.2

472

133

0.05

150.10

234

1.1

0.90

81.3

0.06

432

0.32

628

656

752

2Sp

n0.12

742

175

662

993

0.01

350.19

210.51

1.92

40.58

0.07

263

0.23

1133

1089

1004

3Sp

n0.30

152

2479

362

10.01

610.48

680.6

5.23

00.65

0.07

792

0.2

2557

1858

1145

5Sp

n0.11

911

418

374

350

0.19

600.14

462

0.59

1.40

70.67

0.07

055

0.21

871

892

945

[1]Zrn:zircon

;Sp

n:titanite.

[2]Con

centrationsarekn

ownto

2%forsample

weigh

tsof

0.4mg,

10%e20

%forsample

weigh

tsbe

low

0.02

0mg.

[3]To

talco

mmon

Pbpresentco

rrectedforco

mmon

Pbin

spike.

[4]Mea

suredratioco

rrectedforfraction

ationon

ly.

[5]Ratiosco

rrectedforsp

ike,

fraction

ation,b

lankan

dinitialc

ommon

Pb.

L.M. Lobato et al. / Journal of South American Earth Sciences 58 (2015) 129e140136

that the albitite zircon grains are relict crystals from the originalgranite. Two zircon fractions were analyzed and the results areincluded in Table 2. Fractions HBU-871.6 and HBU-871.7 are slightlymore discordant (1.2% and 1.8%, respectively; Fig. 6) than those ofthe S~ao Tim�oteo granitoid zircon grains (Fig. 5), but show207Pb/206Pb ages of 1741 and 1744 Ma, very similar to the crystal-lization age of the S~ao Tim�oteo granitoid. When zircon fractions forthe granite and the albitite are plotted together on a concordiadiagram, they define a discordia line with an upper intercept of1744 ± 4 Ma (Fig. 7), in agreement with the crystallization ageobtained from the granite zircon grains alone. This corroboratesfield evidence that the albite-oligoclase-rich rocks and albititesrepresent deformed and metasomatized portions of the S~aoTim�oteo granitoid.

Two distinct titanite populations with contrasting color char-acteristics (colorless and dark brown) are found in the albititesample. Fractions HBU-871.1 and HBU-871.3 are made of colorlessto very pale brown, good quality titanites, whereas fractions HBU-871.2 and HBU-871.5 represent the dark brown population. Thelatter revealed higher U contents (114 and 421 ppm, respectively).The average U content of titanite grains in fraction HBU-871.5 ishigh, and somewhat similar to that of zircon grains (134e185 ppm)from the same rock sample.

The four titanite fractions were variably discordant (from 17% to�150%; Fig. 8), and there is no relationship between the degree ofdiscordance and the color or U contents of the different mineralfractions. Reverse discordance in U-bearing minerals is explainedby U-loss or more likely by an excess 206Pb. It is suggested that thetitanites went through a complex history of Pb and U loss after theircrystallization. Alternatively, one could imagine that this wascaused by minute U-poor, Pb-rich mineral inclusions in titanite.Nevertheless, the four analytical points form a discordia line with a4% probability of fit, which is here understood to be a good fit, if oneconsiders the very large spread of the analytical points. The dis-cordia defines an upper intercept age of 956 ± 59 Ma and a lowerintercept age of 384 ± 100 Ma (Fig. 8).

The high U content of some of the titanite grains is unusual inthis kind of mineral and suggests that they are coeval with theuranium mineralization. The upper intercept age of 956 ± 59 Ma(Fig. 8) is, therefore, the best estimate for the age of the hydro-thermal activity that gave origin to the uranium mineralization.

7. Discussion and conclusions

The age of 1746 ± 5Ma for the S~aoTim�oteo granitoid is similar tothe UePb zircon crystallization ages, in the range of 1711 Ma and1770 Ma of acid volcanic rocks from the basal portion of theEspinhaço Supergroup dated in several areas of Bahia and MinasGerais states (Brito-Neves et al., 1979; Machado et al., 1989; Dossinet al., 1993; Schobbenhaus et al., 1994; Babinski et al., 1999).

The 1746 ± 5 Ma S~ao Tim�oteo magmatic event is roughly coevalwith this rift-type acid magmatism (1711 Ma and 1770 Ma) thatoccurs both in Minas Gerais state and further to the west in thestate of Goi�as, represented by rhyolites at the base of the Espinhaço(Minas Gerais) and Araí (Goi�as) Groups (e.g., Fuck et al., 1988;Trompette et al., 1992). It is associated with A-type plutonic rocksinMinas Gerais, known as the Borrachudos suite (Dorr and Barbosa,1963), comprising a series of sub-alkaline granitic bodies that cutthe basement rocks.

Chemale et al. (1998) and Fernandes et al. (1997) presentedUePb zircon ages for two the Borrachudos plutons at 1670 ± 32 Maand 1777 ± 30 Ma, respectively, whilst a much more precise UePbSHRIMP age of 1740 ± 8 Ma is reported by Silva et al. (2002). InGoi�as, these granites have been dated between at ca. 1769 and1770 Ma (Pimentel et al., 1991). The emplacement of the S~ao

Fig. 5. Concordia diagram for zircon grains of the S~ao Tim�oteo granitoid (sample HBU-872).

Fig. 7. Concordia diagrams for zircon grains of the S~ao Tim�oteo granitoid (sample HBU-872; 1, 2 and 3) and albitite (sample HBU-871; 6 and 7). Numbers 1 to 3, 6 and 7 referto those in Table 2.

L.M. Lobato et al. / Journal of South American Earth Sciences 58 (2015) 129e140 137

Tim�oteo granitoid is, therefore, part of a regional extensional eventduring which the deposition of the Espinhaço Supergroup tookplace in Minas Gerais and Bahia states, as well as the Araí Group inGoi�as state.

Our study failed to recognize a Mesoproterozoic event of hy-drothermal alteration and uraniummineralization, as suggested byTurpin et al. (1988) and Cordani et al. (1992). Our data indicate thatthe processes above occurred at the beginning of the Neo-proterozoic. The discordia line with the upper intercept age of ca.1397 Ma presented by Turpin et al. (1988) was defined by analysesof acid-soluble fractions derived from heavy mineral concentratesand albitite whole-rock samples. As observed in our study, theseheavy mineral concentrates may contain minerals which are ca.1742Ma old (e.g. relict zircon grains from the original granitic rock),as well as minerals crystallized or reset at 956 ± 59 Ma (e.g. titanitegrains). The age of 1397 Ma is intermediate between ca. 1742 Maand 956 ± 59 Ma, and it is here suggested to represent a mixed age,obtained by mixing U and Pb extracted from minerals with

Fig. 6. Concordia diagram for zircon grains of mineralized albitites (sample HBU-871).

different ages. This age range of Turpin et al. (1988) has beeninferred as the Lagoa Real mineralizing event in a contribution byCuney et al. (2012).

Cordani et al. (1992) based their conclusions onRbeSrwhole-rockisochrons on albitized gneisses and albitites. They report severalisochrons displaying ages of ca. 1520 Ma, 1280 Ma, 1220 Ma and1000 Ma. We suggest that these isochron ages could alternatively beinterpreted as the result of partial re-homogenization of older rocks,due to the younger (Neoproterozoic) tectonothermal event.

Age determinations performed by Babinski et al. (1999) andDanderfer Filho et al. (2009) on basic rocks exposed in the ChapadaDiamantina and northern Espinhaço range point toward a1.51e1.57 Ga magmatic event, probably associated with a renewedrifting episode in the Espinhaço basin.

The significance of the 956 ± 59 Ma hydrothermal event is stillnot completely understood. Results obtained by Loureiro et al.(2009) and Danderfer Filho et al. (2009) in the regions of theChapada Diamantina and northern Espinhaço, respectively, indi-cate the existence of a Tonian (1.0e850 Ma) mafic magmatism. Asingle outcrop belonging to the upper part of the Chapada Dia-mantina metasedimentary pile yielded an age of about 960 Ma,while the lower part furnished an age of 920 Ma (Cordani et al.,1992); these latter rocks discordantly overlie the S~ao FranciscoSupergroup. In other areas of the S~ao Francisco Craton, an exten-sional event marked by the emplacement of mafic dikes and sillshas been dated at ca. 1.0 Ga (Machado et al., 1989; Renne et al.,1990), and is interpreted as the initial stages of extension associ-ated with the formation of the Neoproterozoic Macaúbas basin(e.g., Schobbenhaus, 1996), which covers large areas of the interiorand of the eastern part of the S~ao Francisco Craton. The Salto daDivisa granitic rocks from the northern Craton regionwere dated bySilva et al. (2008), and their ages fall in the interval between 875and 850 Ma. These rocks are interpreted as a manifestation of theMacaúbas rifting event (Pedrosa Soares et al., 2008; Danderfer Filhoet al., 2009).

The titanite age at 956 ± 59 Ma falls, therefore, into the timeinterval related to the rifting stage of the precursor basin of theAraçuaí-West Congo orogen. This stage started at the onset of theTonian and lasted until ca. 875Ma, with the intrusion of the Salto daDivisa anorogenic plutonic suite in northeast Minas Gerais and

Fig. 8. Concordia diagram for titanite of albitites. Note that (B) is a detail of the lower part of (A).

L.M. Lobato et al. / Journal of South American Earth Sciences 58 (2015) 129e140138

southeast Bahia states (Silva et al., 2008). However, thick rift-related volcano-sedimentary units older than 912 Ma are onlyfound in the African side, suggesting an asymmetrical rift with thethermalemagmatic axis slowly migrating westward (PedrosaSoares et al., 2008).

As mentioned earlier, Lobato (1985) and Lobato and Fyfe (1990)pointed out that their data suggest the Lagoa Real uraniummineralization derived from the interaction with basinal brines(see also Cuney, 2009), a model that is yet to be fully established.The 956 ± 59 Ma mineralization age presented in this paper helpscorroborate this assumption, since this time period represents amajor rifting stage. This age bracket may stand as an importantexploration implication, and should be included in the diverse agescenarios proposed by Cuney (2010).

The importance of the Brasiliano tectonothermal event in theregion has been demonstrated by KeAr data obtained by variousauthors. For instance, Jardim de S�a et al. (1976) indicate an age of492 ± 25 Ma for a saussuritized gabbro intrusion in the ChapadaDiamantina sequence. Bastos Leal (1998) dated biotites of theOrosirian Cacul�e, Espírito Santo and Iguatemi granites, some35e50 km east of the Lagoa Real complex, at 551 ± 6 Ma, 490 ± 12Ma and 483 ± 5 Ma, respectively. The KeAr ages by Cordani et al.(1992) on amphibole (503 Ma) and biotite (573 Ma) of the LagoaReal Granite are further indicative of the Neoproterozoic overprint.

The AreAr age of 497 Ma on white mica of sheared gneisses inthe Paramirim valley region (Guimar~aes et al., 2005) also point to aNeoproterozoic reworking event. Moreover, Cruz (2004), Cruz andAlkmim (2006) and Cruz et al. (2007, 2008) have demonstratedthat the deformation indicators in the Lagoa Real granitic-gneissdistrict are related to shear zones that also truncate the Protero-zoic Espinhaço and S~ao Francisco Supergroups, constraining the ageof these structural features to the Neoproterozoic.

In summary, the data presented in this study support thefollowing sequence of events for the geological evolution of theLagoa Real uranium district:

(i) 1746 ± 5 Ma e emplacement of the S~ao Tim�oteo granitoid(UePb zircon age);

(ii) 956 ± 59 Ma e hydrothermal alteration, albitite formationand age of the uraniummineralization (UePb titanite age forthe albitite sample);

(iii) Cambrian age of 480 Ma e final recrystallization, remobili-zation and Pb loss at the final stages of the Brasiliano tecto-nothermal event.

Acknowledgments

The authors are grateful for the financial assistance provided bythe Canadian International Development Agency (CIDA), whichpermitted the necessary analytical procedures. We wish to thankcolleagues at Indústrias Nucleares do Brasil, especially E. Carele deMatos, and at Centro de Desenvolvimento da Tecnologia Nuclear,particularly Lucília Ramos and Kazuo Fuzikawa. LML, MMP, SCPSand FFA acknowledge grants from the Conselho Nacional deDesenvolvimento Científico e Tecnol�ogico (CNPq). Research fundsfrom CNPq were also fundamental for the present investigation.

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