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Lagoa Real - Caetite, Cachoeira, Engenho, Gameleira I, Quebradas, Aranjeras, Brejal, Rabicha, Modesto, Monse Nhor Bastos |
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Bahia, Brazil |
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U
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Super Porphyry Cu and Au
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The Lagoa Real uranium district is located to the east of the town of Caetité in southern Bahia, Brazil. The district covers an area of 1200 km2, in which 38 anomalous areas have been defined, including 17 identified 'ore deposits'. Of these, 7 orebodies have been exploited from the Cachoeira open-pit mine. In 2022, approval was being sought for open-pit extraction of a new deposit at the Engenho mine which is 3 km SE of Cachoeira. Other significant deposits, and their location relative to Cachoeira include: Gameleira I, ~2 km SW; Quebradas, ~5 km SSE; Aranjeras, ~9 km SSE; Brejal (Anomaly 5), ~10 km SSE; Rabicha, ~12.5 km SSE; Anomaly 10 ~14 km SSE; Modesto (Anomaly 7) ~16 km south; Monse Nhor Bastos (Anomaly 2) ~26.5 km south. Operations (to 2022) have been centered on mining at Cachoeira and an adjacent uranium processing unit located ~30 km NE of the town of Caetité (Lobato et al., 2015; Wilde, 2021; Indústrias Nucleares do Brasil [INB] website, viewed June, 2022).
(#Location: 13° 50' 10"S, 42° 16' 43"W).
Indústrias Nucleares do Brasil (INB) commenced uranium production in the district in 1999. Since then, it has produced an average of 300 tonnes of uranium concentrate per annum from the 7 orebodies at the Cachoeira mine (Anomaly 13), for a total of ~3750 tonnes of U3O8 over a cumulative 16 years. The known deposits of the district are quoted as comprising a total resource of ~112 000 tonnes of U3O8 (Brito et al., 1984; Matos et al., 2003; Matos and Villegas, 2010). The Indústrias Nucleares do Brasil website (viewed June 2022) quote 'reserves' at the Caetité unit as 99 100 t of U3O8.
The Lagoa Real district is located in the Paramirim river valley, in the northern part of the Ediacaran to Cambrian Araçuaí Orogen where it forms a tongue like encroachment into the São Francisco Craton along the long-lived, late Palaeo- to early Neoproterozoic, NW-SE aligned Paramirim Aulacogen (Pedrosa Soares and Wiedeman, 2000; Pedrosa Soares et al., 2001, 2007, 2008; Alkmim et al., 2006). The Paramirim Valley largely corresponds to the Paramirim Block, a basement 'horst' in the core of the Aulacogen which exposes older Palaeo- to Mesoarchaean basement of the Gavião Block. This older block separates the Northern Espinhaço and the wider Chapada Diamantina plateau that are morphotectonic domains to the west and east respectively. The sequence within these domains represents the preserved rift-facies clastic sedimentary and volcanic sequences of the Paramirim Aulacogen, which comprises the late Palaeo- to late Mesoproterozoic Espinhaço Supergroup, that is, in turn, unconformably overlain by the Neoproterozoic São Francisco Supergroup shelf sequence that includes significant carbonates (Schobbenhaus, 1996; Misi and Veizer, 1996; Danderfer Filho and Dardenne, 2002). Both of these Supergroups are related to the evolution of the complex, polyphase Espinhaço and temporally overlapping Macaúbas basins that extend across the São Francisco Craton and the rift basin that preceded the tectonism of the Araçuaí Orogen, respectively (Schobbenhaus, 1996; Alkmim, 2012). Extensional events are indicated by Meso- and Neoproterozoic mafic dyke swarms that intrude the Espinhaço Supergroup. These dykes represent two main temporal groups: i). Group I, characterised by U-Pb ages of 1492 ±16 Ma (Loureiro et al., 2008), 1496 ±3.2 Ma (Guimarães et al., 2005), and ~1514 Ma (Babinski et al., 1999); and ii). Group II, which has U-Pb ages of 934 ±14 Ma (Loureiro et al., 2008) and 854 ±23 Ma (Danderfer Filho et al., 2009). Phanerozoic sedimentary rocks also cover large areas of the Paramirim valley.
The Paramirim Block basement in the northern tongue of the Araçuaí Orogen is made up of terranes containing granitic-gneiss and granulite of Archaean and early Palaeoproterozoic age. Crustal formation is related to at least three Archaean plutonic events, the oldest of which comprises 3.4 to 3.2 Ga tonalite-trondjemite-granite plutons (Martin et al., 1991; Nutman and Cordani, 1993; Bastos Leal et al., 1998; Santos Pinto et al., 1998, 2012). These were followed by 3.2 to 3.1 Ga granitic-granodioritic rocks that may have been responsible for the reworking of the Palaeoarchaean rocks (Bastos Leal, 1998; Bastos Leal et al., 1998). The youngest Archaean plutonic suite was from 2.9 to 2.7 Ga (Brito Neves et al., 1979; Costa et al., 1985; Cordani et al., 1985, 1992; Santos Pinto, 1996). Neosomes from migmatitic gneisses dated at 2650 ±100 Ma (isochronic Rb-Sr) are interpreted to represent the age of migmatisation during the Jequié tectonothermal event. These rocks are intruded by 2300 to 2050 Ma Rhyacian and 2050 to 1800 Ma Orosirian calc-alkaline granitoids (Bastos Leal et al., 1998; Guimarães et al., 2005; Leal et al., 2005). These rocks have been variously affected by tectonometamorphism during the i). Neoarchaean 2.8 to 2.6 Ga Jequié event; ii). Paleoproterozoic ~2.1 to 1.8 Ga in the Orosirian; and iii). the Neoproterozoic, Brasiliano event at ~0.6 to 0.5 Ga (e.g., Inda and Barbosa, 1978; Brito Neves et al., 1980, 1999; Barbosa and Dominguez, 1996; Cordani et al., 1992; Barbosa and Sabaté, 2002; Corrêa Gomes and Oliveira, 2002; Cruz and Alkmim, 2006; Danderfer Filho et al., 2009).
The granitic-gneiss of the Lagoa Real Complex, within which the Lagoa Real uranium mineralisation is located, forms a central core to the Paramirim Block. It is flanked by the regional Archaean granulite and amphibolite facies orthogneiss and migmatites described above. These older orthogneisses are deformed coarse-grained augen gneisses that have been incipiently foliated. To the east, these gneisses are overlain by the Lower Palaeoproterozoic Ibitira-Brumado volcano-sedimentary suite composed of amphibolite, banded iron formation, gneiss, metachert, marble and schist. To the west and east of the Lagoa Real Complex, all of these rocks are overlain by the Late Palaeoproterozoic to Mesozoic Espinhaço Supergroup successions of the Northern Espinhaço and Chapada Diamantina domains respectively. The Lagoa Real Complex has a width of 12 to 18 km, splitting into narrower strands to the north and south, and has a strike length of >100 km. It is predominantly composed of plagioclase-microcline orthogneiss, which in the western and southern sections of the complex have more heavily sheared with extensive zones that are finely foliated (Lobato and Fyfe, 1990). It grades from rocks which contains the same basic mineralogy as the enclosing regional gneisses, grading to those which exhibit pyroxene and/or garnet replacing original hornblende along grain margins and cleavages in the more strongly foliated varieties. In both extremities, however, the plagioclase is either albite or oligoclase, K feldspar is granoblastic microcline, and fine albite rims very commonly surround the original plagioclase, representing incipient albitisation (Lobato and Fyfe, 1990). Lobato et al. (2015) map the orthogneisses of the Lagoa Real Complex as 'albitised gneisses', and as such may represent metasomatic altered Archaean protoliths. The Lagoa Real Complex has a 'flattened' reverse 'S' shape, with its strike curving from NNW-SSE in the south → NNE-SSW in the centre → NNW-SSE in the north. The bulk of the uranium anomalies and deposits lie within the finely foliated section of the complex.
These gneisses of the Lagoa Real Complex are cut by several intrusions of the less extensive, porphyritic São Timóteo granitoid. These latter granitoids are composed of coarse-grained albitised syenogranite, alkali-feldspar granite and syenite. They have been metamorphosed to gneiss and locally to protomylonite, augen mylonite and ultramylonite of the same composition, and are collectively known as albitised gneisses (Lobato and Fyfe, 1990; Cruz and Alkmim, 2006; Cruz et al., 2007, 2008). The granitoid lithofacies that is most abundant in these albitised gneisses is a potassic, hastingsite-biotite monzo-to syenogranite that is iron-rich, with a meta-aluminous and high-K calc-alkaline affinity (Teixeira, 2000), which is interpreted to be of an intracontinental origin (Maruéjol et al., 1987). Dating of the albitised São Timóteo granite has returned a wide range of ages, with the best estimate being that from a U-Pb zircon with an age of 1724 ±5 Ma (Turpin et al., 1988). Other confirmatory ages include 1710 ±45 Ma and 1706 ±107 Ma by Rb-Sr and Pb-Pb respectively (Cordani et al., 1992), whilst magmatic titanite grains of the São Timóteo granitoid yielded an Pb-Pb age of 1743 ±28 Ma (Cruz et al., 2007).
The uranium mineralisation at Lagoa Real is intimately associated with discontinuous tabular, lenticular albite (±oligoclase) rich bodies ('albitites') exposed over much of the ~100 km long extent of the Lagoa Real granitic-gneiss complex, but more densely concentrated in its reverse 'S' shaped arcuate centreal section. More than 180 such albitites have been mapped in both the finely foliated and less foliated plagioclase-microcline gneisses, although none are apparently found in the São Timóteo granitoid. Most of these albititic bodies trend at between ~330 and ~40° and dip at 30°WSW, vertical or WNW following the reverse 'S' shape of the complex. The northern-most lenses dip to the east while those in the central part of the district are almost vertical. Individual albite-rich bodies may be up to 1 km in length, up to a maximum of 30 m in width, but averaging 6 to 7 m, and persist to up to 850 m depth below the surface. However, at some of the key deposits, such as a Cachoeira, albitisation is shown to occur over widths of >100 m, (de Oliveira et al., 1985) although this may be due to structural juxtaposition. The strike of these albitites predominantly parallels the foliation, and each may contain one or more discontinuous mineralised lens, each of which generally has a sharp contact with the enclosing rocks. West vergent reverse faulting and incipient to intense folding producing isoclinal structures is common, accompanied by boudinage and shearing along the foliation. Brecciated albitites have also been observed locally. It has been suggested that the mineralised lenses are located on the limbs of a large isoclinal fold, with the best development of mineralisation at the intersection of two directions of foliation, while the plunge of ore shoots parallels the direction of mineral lineation (Lobato and Fyfe, 1990).
These albite (±oligoclase) rich rocks/albitites are fine-to medium-grained, with a weak to pronounced foliation (Lobato and Fyfe, 1990; Cruz et al., 2007). Where well sheared, they locally contain uranium mineralisation, mainly as uraninite, strongly concentrated in the most oxidised zones mainly in garnet-hedenbergite-rich rocks. The uraninite crystals are rounded and very fine, from 5 to 25 µm across, and are predominantly associated with andradite, but are also found as inclusions within and bordering aegirine-augite, albite-oligoclase, hastingsitic-hornblende, biotite, calcite and martitised magnetite within the albite rich hosts (Lobato and Fyfe, 1990). Calcium-rich oligoclasites may also host uranium mineralisation, with associated epidote, hedenbergite, diopside, grossular and also martitised magnetite (Cruz, 2004). Geochemical data indicate that the albititic rocks, with and without uraniferous mineralisation, are not the product of sodic syenitic magmatism. On the basis of field relationships, and petrographical and geochemical evidence it has been suggested that the albite (±oligoclase) rich rocks, with and without uraniferous mineralisation, are the result of sodic metasomatism and shearing of granitoid protoliths that belong to the São Timóteo granite (Marques, Cabral and Rios, 2021).
NOTE: It is unclear from the literature whether these albitites are i). metasomatic alteration zones nucleated on lenses of São Timóteo granite intruded into the plagioclase-microcline orthogneiss of the Lagoa Real Complex, as inferred by Marques, Cabral and Rios (2021), or ii). are structurally controlled alteration zones within the latter orthogneiss alone. The second option is implied by Lobato and Fyfe (1990) in the passage "U-bearing albitites are formed upon metasomatism of amphibolite facies orthogneisses along ductile shear zones. The original augen, porphyroblastic textures are preserved outside of the alteration zones and become progressively recrystallised with grain size reduction in the albitite zones." Alteration in the central core of these albitites is apparently so intense that all original texture has been obliterated.
It is observed that all uranium-rich zones in the Lagoa Real district are associated with such metasomatised rocks (e.g., Lobato and Fyfe, 1990). The metasomatism principally involved Na and Ca enrichment and depletion of Si, K, Rb, and Ba under oxidising conditions (Lobato and Fyfe, 1990). From these observations, Lobato and Fyfe (1990) concluded the shearing, metasomatism and mineralisation must have been coeval. However, microstructural criteria observed by Cruz et al. (2007) suggested that Na-Ca alteration pre-dates the activation of those shear zones that host uranium mineralisation. It has also been noted that flame perthite in K feldspar of the alkaline São Timóteo granitoid, as well as albite/oligoclase veins and agglomerates, which are all found distal to mineralisation, are progressively recrystallised and grade into albitites proximal to shear zones (Cruz, 2004; Cruz et al., 2008).
Lobato and Fyfe (1990) found that uranium mineralisation was developed in association with hot, ~500 to 550°C, oxidising fluids with oxygen isotope values somewhat similar for both non-mineralised, from -0.8 to +7.3‰ and mineralised albite (±oligoclase) rich rocks ranging from -3.7 to +2.6‰. Fluid inclusion studies of albite, late-stage quartz and calcite (Fuzikawa et al., 1988) from these altered granitoids indicate the presence of aqueous and aquo-carbonic fluids and imply a late-stage brine may have existed with oscillating salinities of up to 20% NaCl equivalent, whilst Souza (2009) and Oliveira (2010) obtained salinity values of between 9 to 18% NaCl equivalent for the aqueous fluid. Oliveira et al. (2012) undertook fluid inclusion studies coupled with LAICMPS analyses, focussed on pyroxenes (both metamorphic and metasomatic), garnet and plagioclase of albitites, from three of the most important deposits at Lagoa Real. This work showed that early-stage aqueous fluids in equilibrium with mafic minerals were saline, containing 12 to 16% NaCl equivalent, with Na, Mg, U, Rb, Ba, Sr, Pb, K, Ca, Fe, Cu, Zn and Li, as well as some Mn, As and Sb. However, plagioclase precipitated from a more diluted aqueous fluid had 0.5 to 6.4% NaCl equivalent, with up to 11%, contained Na, Mg, K, Ca, Fe, Cu, Zn, As, Sr, Ba, Pb. Aquo-carbonic fluids have only been detected in vein quartz crosscutting mineralisation. It has been variously postulated that the mineralisation was derived from; i). the Palaeoproterozoic São Timóteo granitoid which contains uranium-bearing accessory minerals (e.g., Maruéjol et al., 1987), Turpin et al., 1988) and Maruéjol,1989) producing a metasomatic deposit similar to Elkon in Siberia; or alternatively ii). from the Espinhaço Supergroup rocks, now eroded from above the Lagoa Real granitic-gneiss complex (Lobato, 1985), expelled during the Neoproterozoic Brasiliano orogenesis. This would represnt an unconformity deposit such as in the Athabasca Basin in Canada or Alligator Rivers Province in Australia, e.g., Ranger.
U-Pb dating of uraninite grains extracted from the Lagoa Real albitised gneisses yielded a concordant age of ~820 Ma (Stein et al., 1980), interpreted to represent the time of the metasomatic alteration caused by alkaline fluids generated during a basin-forming event. Zircon from the heavy minerals from the albitite were dated (U-Pb, TIMS; Turpin et al., 1988) and showed extreme variations, although the albitite zircon grains align along a discordia line with an upper intercept age of 1504 ±12 Ma. This is consistent with an 1520 ±20 Ma Rb-Sr whole-rock isochron age reported by
Cordani et al. (1992) for variably albitised augen orthogneisses from localities near the uranium mineralisation. These authors interpreted the unusual behavior of the albitite zircon grains to be the result of the relative mobility of U, Pb and U238 series products. Other dating of acid-soluble fractions of whole-rock albitite samples, heavy mineral concentrates and uraninite concentrates by Turpin et al. (1988) yielded upper and lower intercept ages of 1397 ±9 and 480 Ma, interpreted by that author to be the ages of uranium mineralisation and reworking of the uranium mineralisation, respectively. However, these ages are not considered robust (Lobato et al., 2015).
The most recent source geological information used to prepare this decription was dated: 2015.
This description is a summary from published sources, the chief of which are listed below.
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Caetite mine and processing
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Selected References:
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Amorim, L.E.D., Rios, F.J., Freitas, M.E., Cutts, K., Geraldes, M.C., Diniz, A.C. and de Matos, E.C., 2021 - Zircon U-Pb geochronology of Paleoproterozoic Statherian intraplate A-Type magmatic associations of the Lagoa Real Uranium Province, Sao Francisco Craton (Bahia, Brazil): in J. of South American Earth Sciences v.109, 22p. doi.org/10.1016/j.jsames.2021.103245.
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Chaves, A.O., 2013 - New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil): in Central European Journal of Geoscience v.5, pp. 354-373.
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DeFerreira, T.H., Oliveira, L.A.R., Amorim, L.A.D., Pedrosa, T.A. and Rios, F.J., 2021 - Rare earth element (REE)-enriched granitic pegmatite pockets of Lagoa Real Uranium Province, Brazil: in Geochemistry (Chemie der Erde) v.81, 19p. doi.org/10.1016/j.chemer.2021.125810.
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dos Santos C.M., Rios, F.J., Amorim, L.E.D. and Palmeiri, H.E., 2017 - Caracterization of titanite generations from Gameleira - I deposit (U - Anomaly 35) Lagoa Real Uranium Province (LRUP), Bahia State: in 2017 International Nuclear Atlantic Conference - INAC 2017, Belo Horizonte, MG, Brazil, October 22-27, 2017, Associacao Brasileira De Energia Nuclear - ABEN Proceedings, 12p.
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Lobato, L.M. and Fyfe, W.S., 1990 - Metamorphism, Metasomatism, and Mineralization at Lagoa Real, Bahia, Brazil: in Econ. Geol. v.85, pp. 968-989.
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Lobato, L.M., Pimentel, M.M., Cruz, S.C.P., Machado, N., Noce, C.M. and Alkmim, F.F., 2015 - U-Pb geochronology of the Lagoa Real uranium district, Brazil: Implications for the age of the uranium mineralization: in J. of South American Earth Sciences v.58, pp. 129-140.
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Marques, C., Cabral, A.R. and Rios, F.J., 2021 - Whole-rock chemistry of the Gameleira I uranium deposit, Lagoa Real, Brazil: in Geochemistry (Chemie der Erde) v.81, 20p. doi.org/10.1016/j.chemer.2020.125677.
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Pimentel, M.M., Machado, N. and Lobato, L.M., 1994 - U-Pb geochronology of granitic and gneissic rocks from Lagoa Real, Bahia State, Brazil: Implications for the age of the uranium mineralization: in 7th Congreso Geologico Chileno, 1994, Universidad de Concepcion, Departamento de Ciencias de la Terra, Actas Volumen II pp. 1523-1526.
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Prates, S.P., Neves, J.M.C, and Fuzikawa, K., 2009 - Uranium Mineralization at Lagoa Real, BA-Brazil; The role of fluids in its genesis,: in 2009 International Nuclear Atlantic Conference - INAC 2009, Rio de Janeiro, RJ, Brazil, September 27 to October 2, 2009, Associacao Brasileira De Energia Nuclear - ABEN, Proceedings, 6p.
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Santos, H.S, Sardinha, A.F., Cabral, M.D. and Guimaraes, S.N.P., 2015 - Radiometric and magnetometric study of the Lagoa Real uranium Province, Brazil: in International Journal of Engineering Sciences & Emerging Technologies, v.8, pp. 30-44.
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Villegas, R.A.S. and Gomiero, L.A. 2010 - Uranium Production in Caetite, Brazil: in XV Congreso Peruano de Geologia. Resumenes Extendidos, Cusco. Sociedad Geologica del Peru, Publicacion Especial, No 9, pp. 416-419.
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Wilde, A., 2013 - Towards a Model for Albitite-Type Uranium: in Minerals (MDPI) v.3, 13p. doi:10.3390/min3010036.
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Wilde, A., 2020 - Shear-Hosted Uranium Deposits: A Review: in Minerals (MDPI) v.10, 20p. doi:10.3390/min10110954.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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