2009 International Nuclear Atlantic Conference - INAC 2009 Rio de Janeiro,RJ, Brazil, September27 to October 2, 2009 Associação Brasileira de Energia Nuclear - ABEN ISBN: 978-85-99141-03-8

URANIUM MINERALIZATION AT LAGOA REAL, BA-BRAZIL: THE ROLE OF FLUIDS IN ITS GENESIS

Sônia Pinto Prates, José Marques Correia Neves and Kazuo Fuzikawa

Centro de Desenvolvimento da Tecnologia Nuclear (CDTN/CNEN-MG) Av. Antonio Carlos 6627 – Campus UFMG - Pampulha 30270-901 Belo Horizonte, MG [email protected]

ABSTRACT

The Lagoa Real uranium province is situated in the central-south of Bahia state – Brazil and it is presently by far the most important and best known uranium occurrence in Brazil. Nowadays 34 anomalies are known in a 30 Km long and 5 km wide area. An open pit mine was open in Cachoeira Mine, in the north portion of the area, and it is the only uranium mine in operation in Brazil and even in South America as well. The uranium mineralization in the Lagoa Real uranium province occurs in metamorphic rocks named albitites, due to their albite content (over 70%). Uraninite is the main uranium mineral, followed by pechblende, uranophane, torbernite and other uranyl minerals. Uraninite occurs as tiny round and irregular crystals (20 a 30 μm) included or associated to mafic minerals, mainly pyroxene and garnet, and also to amphibole and biotite and sometimes to albite. Some secondary minerals such as, for instance, uranophane, torbernite and tyuyamunite are also found. The main albitites minerals from the Cachoeira mine (plagioclase, garnet, biotite, pyroxene, amphibole and titanite) were studied by means of Infrared Spectroscopy Techniques. Good results were obtained from small quantities of material (around 2 mg) and allowed the minerals identification, and also to know their composition (from the peak position) and to detect the presence of water molecules, which indicates an aqueous phase during the uranium formation, probably rich in Fluorine.

1. INTRODUCTION

The Lagoa Real uranium province is situated in the south-central of Bahia State – Brazil, close to the town of Caetité (Fig.1). The uranium mineralization was found in 1977 by Indústrias Nucleares Brasileiras – NUCLEBRÁS. It is the main uranium occurrence in Brazil and nowadays 34 mineralized areas are known along a 30 Km long and 5 Km wide area.

The Cachoeira Mine is an open pin mine located in the north part of the province, actually the only uranium mine in operation in South America. According to Departamento Nacional da Produção Mineral – DNPM the uranium production in 2006 was estimated in 400 ton of U3O8 [1].

The uranium mineralization in the Lagoa Real uranium province occurs in metamorphic rocks named albitites, due to their albite content (over 70%) [2].

Uraninite is the main uranium mineral, followed by pechblende, uranophane and torbernite. It occurs as tiny, sometimes round and irregular crystals (20 a 30 μm) associated and frequently enclosed in mafic minerals, mainly pyroxene and garnet, and also to amphibole and biotite. Sometimes the uraninite crystals are associated to hematite and plagioclase (albite type).

In order to contribute to the understanding of the uranium mineralization, the main minerals from the albitites rocks (plagioclase, garnet, biotite, pyroxene, amphibole and titanite) were studied by means of Infra-Red Spectroscopy technique (FTIR).

The results showed the presence of an aqueous phase, probably rich in fluorine.

2. GEOLOGICAL SETTING OF THE LAGOA REAL URANIUM BODIES

A geological map of the Lagoa Real uranium province is shown in Fig.1 [3].

Figure 1. Localization and Geologic Setting of the Lagoa Real uranium bodies.

The uranium mineralization in the Lagoa Real uranium province is related to Proterozoic granites and gneissic rocks, the Lagoa Real Igneous Metamorphic Complex and São Timóteo granite, located in the south-central part of São Francisco Craton.

The uranium mineralization in the Lagoa Real uranium province is associated to rocks named albitites due to their albite content (over 70% in volume). The albitites occur as discontinuous tabular and lenses like bodies enclosed in the Lagoa Real Complex, constituting an arcuate zone of 34 anomalies. Most bodies trend N40E to N30W and dip 30o to 90o to the southwest or northwest, with the exception of the northernmost deposits which dip to the east and those

INAC 2009, Rio de Janeiro, RJ, Brazil. situated in the central part of the region which are almost vertical. Each albitite body may vary up to 3 km in length, averaging 10 m in width (max. 30 m). Mineralization extends up to 850 m below the surface as shown by drill cores. Each albitite body contains one or more mineralized levels, which may be interrupted in places. The contacts between mineralized levels with microcline-gneisses (host rocks) is transitional or eventually abrupt.

Besides feldspars, albitites may contain other minerals such as pyroxene (aegirine-augite), garnet (andradite), amphibole (hastingsite) and biotite. Accessories minerals are titanite, zircon, apatite, magnetite and hematite.

3. METHODOLOGY

In order to study the minerals and consequently the uraniferous mineralization of Lagoa Real, the following steps were undertaken:

(a) Field work for geological survey and samples collecting from the Cachoeira mine (20 samples). The samples were selected according to the presence of mineralization measured by a SRAT SPP2 cintilometer.

(b) Preparation of polished thin sections and subsequent study by polarizing microscope (8 samples) in order to observe mainly the minerals present and their relationship. It was used a Leica DMRXP microscope from the Fluid Inclusion and Metalogenesis Laboratory at CDTN.

(c) Mineral separation: during mineral separation the rocks were first milled to 60# and then sifted by 150#, 270#, 325#, 400# and 500# sieves. Only 4 samples were chosen for this step according to their features, and mainly time and costs. The minerals of interest (except plagioclase) concentrate mainly in the heavy fraction. The heavy fraction was obtained by means of heavy liquids (Bromoform) and Franz Isodynamic Separator. During heavy liquids separation, the minerals were separated according to their density while the Franz equipment separates the minerals according to their magnetic properties. Some minerals fractions were very pure, but others sometimes had to be separated manually by stereomicroscope before FTIR analysis.

(d) The obtained mineral fractions (around 2 to 10 mg each) were mixed do KBr in order to obtain thin discs that were analyzed by FTIR at CDTN laboratories. The KBr must be extremely dry because humidity may cause errors in the analysis.

The equipment used for FTIR analysis was a BOMEN MB102, with a DTGS detector in the range 4000-400cm-1. The software used was BRGM/32.

4. RESULTS AND DISCUSSIONS

The silicates amphibole, biotite, garnet, pyroxene, titanite and plagioclase were studied during this work. They all that have SiO4 tetrahedron in their structures, besides some cations as for instance Ca, Mn, Mg, Fe, Al, Li. Only amphibole and biotite have water in their structural formula while the others are anhydrous [4].

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Fig. 2 shows the infrared spectra obtained for the studied minerals.

In silicates the band related to the tetrahedron is situated on the region 900-1100 cm-1. All the six spectra show important stirring vibration in this region (Fig.2).

The band on the region 3500 cm-1 is related to the vibrational stirring of the various hidroxyl groups. A band between 1000 and 1100 cm-1 reflects the assymetrical Si-O-Si bondings from –1 the SiO4 tetrahedron. A band around 1630 cm is typical from the deformation of the adsorved water molecules.

Amphibole and biotite are minerals that have water in their structure and it is possible to observe the bands related to the hidroxyl groups and deformation of the adsorved water molecules, which are 3428-3430 cm-1 and 1620-1650 cm-1, respectively (Fig.2 A and B). A -1 single band in 1000 cm related to SiO4 tetrahedron vibration is observed in biotite (Fig. 2 B). This can be explained by the mineral layer structure.

The other minerals (plagioclase, garnet, pyroxene and titanite) do not have water in the structure [4].

The spectra obtained from these minerals show, however, very clear bands around 3450 cm-1 and 1620-1640 cm-1 indicating the presence of some water in their structure (Fig. 2 C, D, E and F).

In the garnet spectrum (Fig. 2 C) a band around 2340 cm-1 is also observed. It is well known from literature that the band in this region is related to CO2 [5, 6]. This compound is probably included in fluid inclusions frequently observed in this mineral [7, 8].

Plagioclase is also an anhydrous mineral but bands in 3445 cm-1 and 1630 cm-1, both related to the presence of hidroxyl vibrations are clear observed (Fig.2 D ).

Bands in 3430 cm-1 and 1620 cm-1 are observed in titanite spectrum (Fig.2 E). Besides the presence of water molecules in this mineral, some chemical analysis performed in titanite indicated the presence of Fluorine [9]. This element is probably associated to the fluids indicated by the spectra.

In pyroxene spectrum the bands relative to the presence of water are clearly marked in region 3440 cm-1 and 1630 cm-1 (Fig.2 F).

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Amphibole biotite 80 80 A B 75 70 70 1650 3430 60 65 3428 1620 60 50

55

40 Transmittance

transmittance 50

30 45

40 20

35 4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500 -1 wavenumber (cm ) wavenumber (cm-1)

plagioclase C Garnet D 60

50 3445 1630 1648 40

2340 30

transmittance Transmittance 3455 20

10

4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500 -1 -1 wavenumber (cm ) wavenumber (cm )

pyroxene Titanite E F 75 75

70 70 1630 1630 65

65 60 3430 3440

60 55

transmittance transmittance

50 55

45 50 4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500 wavenumber (cm-1) wavenumber (cm-1)

Figure 2. Infrared spectra obtained from the studied minerals.

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5. CONCLUSIONS

All the spectra indicated the presence of OH absorption region, showing the occurrence of water in the structures of minerals nominally anhydrous, such as garnet, titanite and pyroxene. The presence of OH-, besides the presence of F and V, as shown by chemical analysis [3, 9], indicates that the fluid involved in the PURL rocks metamorphic process had these constituents which help to form complex anions with uranyl, which transport the uranium that later was immobilized as uraninite.

The analytical technique allowed reliable results from small quantities of material (maximum 10 mg).

ACKNOWLEDGMENTS

Work supported by the Minas Gerais State FAPEMIG (Fundação de Amparo a Pesquisa do Estado de Minas Gerais). Also thanks to CDTN for its unrestricted support and to Indústrias Nucleares do Brasil (INB), especially the geologist Evando Carele de Matos, for field work and sampling support.

REFERENCES

1. Departamento Nacional da Produção Mineral, DNPM – Anuário Mineral Brasileiro (2005, 2005a). 2. E. Geisel Sobrinho, C. Raposo, J. V. Alves, W. de Brito, T. G. Vasconcelos, “O Distrito Uranífero de Lagoa Real, Bahia,” Congresso Brasileiro de Geologia, Balneário de Camboriú, Vol. 3, pp. 1499-1512 (1980). 3. A. O. Chaves, M. Tubrett, F. J. Rios, L. A. R. Oliveira, J. V. Alves,K. Fuzikawa, J. M. Correia Neves, E. C. de Matos, A. M. D. V. Chaves, S. P. Prates, “U-pb Ages Related to Uranium Mineralization of Lagoa Real, Bahia – Brazil. Tectonic Implications,” Revista de Geologia, Vol. 20, nº 2, pp.141-156 (2007). 4. H. Strunz, E. H. Nickel, Strunz Mineralogical Tables. Chemical Structural Mineral Classification System, 9ª Edição, Stuttgart, Schweizerbat (2001). 5. V. C. Farmer, The infrared spectra of minerals. The Mineralogical Society of America, London, England (1974). 6. C. Hirschmugl, “An Introduction to Infrared Spectroscopy for Geochemistry and Remote Sensing”. Infrared Spectroscopy in Geochemistry, Exploration Geochemistry, and Remote Sensing, Vol. 33, pp. 1-16 (2004). 7. J. V. Alves, K. Fuzikawa, “O estudo de inclusões fluidas da Jazida Uranífera da Cachoeira, Caetité, BA – resultados preliminares”. Anais do Congresso Brasileiro de Geologia, Rio de Janeiro, Vol. 1, pp. 1503-1517 (1984). 8. K. Fuzikawa, J. V. Alves, P. Maruéjol, M. Cuney, C. Kostolanyl, B. Poty, “The Lagoa Real uranium province: some petrographic aspects and fluid inclusions studies”, Geochimica Brasiliensis, Vol. 2, pp. 109-118 (1989). 9. S. P. Prates, “Significado Metalogenético da Mineralogia dos Albititos da Jazida Cachoeira (Província Uranífera de Lagoa Real)”, Dissertação (Mestrado em Ciência e Tecnologia das Radiações, Minerais e Materiais), Centro de Desenvolvimento da Tecnologia Nuclear, Belo Horizonte (2008).

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