Ore Geology Reviews 41 (2011) 112–121

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The Catalão I niobium deposit, central Brazil: Resources, geology andpyrochlore chemistry

Pedro Filipe de Oliveira Cordeiro a,⁎, José Affonso Brod a,b, Matheus Palmieri a,c,Claudinei Gouveia de Oliveira a, Elisa Soares Rocha Barbosa a,b, Roberto Ventura Santos a,José Carlos Gaspar a, Luis Carlos Assis c

a Universidade de Brasília, Campus Darcy Ribeiro ICC Central, Instituto de Geociências, Brasília-DF, 70910-900 Brazilb Universidade Federal de Goiás, Campus Samambaia, Instituto de Estudos Sócio-Ambientais, Universidade Federal de Goiás, Goiânia-GO, 74001-970 Brazilc Anglo American Brazil LTDA, Avenida Interlândia 502, Setor Santa Genoveva, Goiânia-GO, 74672-360 Brazil

⁎ Corresponding author. Tel.: +55 61 38779639.E-mail address: cordeiropfo@gmail.com (P.F.O. Cord

0169-1368/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.oregeorev.2011.06.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 March 2010Received in revised form 23 June 2011Accepted 24 June 2011Available online 22 July 2011

Keywords:Catalão ICarbonatitePhoscoriteNelsonitePyrochlore

The Catalão I alkaline–carbonatite–phoscorite complex contains both fresh rock and residual (weathering-related) niobium mineralization. The fresh rock niobium deposit consists of two plug-shaped orebodiesnamed Mine II and East Area, respectively emplaced in carbonatite and phlogopitite. Together, theseorebodies contain 29 Mt at 1.22 wt.% Nb2O5 (measured and indicated). In closer detail, the orebodies consistof dike swarms of pyrochlore-bearing, olivine-free phoscorite-series rocks (nelsonite) that can be eitherapatite-rich (P2 unit) or magnetite-rich (P3 unit). Dolomite carbonatite (DC) is intimately related withnelsonite. Natropyrochlore and calciopyrochlore are the most abundant niobium phases in the fresh rockdeposit. Pyrochlore supergroup chemistry shows a compositional trend from Ca–Na dominant pyrochlorestoward Ba-enriched kenopyrochlore in fresh rock and the dominance of Ba-rich kenopyrochlore in theresidual deposit. Carbonates associated with Ba-, Sr-enriched pyrochlore show higher δ18OSMOW thanexpected for carbonates crystallizing from mantle-derived magmas. We interpret both the δ18OSMOW andpyrochlore chemistry variations from the original composition as evidence of interaction with low-temperature fluids which, albeit not responsible for the mineralization, modified its magmatic isotopicfeatures. The origin of the Catalão I niobium deposit is related to carbonatite magmatism but the process thatgenerated such niobium-rich rocks is still undetermined and might be related to crystal accumulation and/oremplacement of a phosphate–iron-oxide magma.

eiro).

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Brazil is the largest niobium producer in the World due to miningof residual deposits overlying the Araxá and Catalão I and IIcarbonatite complexes. These deposits represent more than 85% ofthe world's niobium supply. Although these complexes have beenmined for more than 30 years, data from the Araxá niobium deposit isvirtually unavailable and information on the Catalão I (Cordeiro et al.,2010, 2011) and Catalão II (Palmieri, 2011) deposits was publishedonly recently. Not only general information is restricted but geneticinterpretation of these niobium deposits is limited to “weathering ofcarbonatite related rocks” (Carvalho and Bressan, 1981; Gierth andBaecker, 1986).

Cordeiro et al. (2010, 2011) studied the primary fresh ore anddetermined that pyrochlore occursmostly in apatite- andmagnetite-richrocks that crosscut previous phoscorites and phlogopitites. According to

the classification of Yegorov (1993) for olivine-poor member of thephoscorite series these unusual rocks are named nelsonite. At Catalão I,nelsonites are intimately associatedwith dolomite carbonatites and formtwo main swarms of densely-packed thin dikes near the center of thecomplex (East Area and Mine II orebodies). The direct relationshipbetween phoscorite-series rocks and niobium mineralization in freshrock has also been suggested in the Catalão II (Palmieri, 2011) and Araxániobium deposits (Nasraoui and Waerenborgh, 2001).

Although it is only the second largest niobium deposit in Brazil,Catalão I is the best understood. Mining of the Catalão I residual depositstarted in 1976with a reserve of 19 Mt at 1.08 wt.%Nb2O5 (Hirano et al.,1990; Rodrigues and Lima, 1984) and was discontinued in 2001 with aremaining residual reserve of 9.65 Mt at 1.19 wt.% Nb2O5 (our data)while mining focused on the Boa Vista mine in Catalão II. Recentmodeling of the fresh rock deposit indicate a unpublished resource of21.8 Mt at 1.22 wt.% Nb2O5 for the East Area orebody and 7.2 Mt at1.23 wt.% Nb2O5 for the Mine II, adding up to a total reserve ofapproximately 29 Mt at 1.22 wt.% Nb2O5 for the Catalão I complex.

In this paper we studied drill core samples from the fresh rockCatalão I deposit collected between depths of 100 and 500 m. Our

Fig. 1. Grade-tonnage data showing metal grades (wt.% Nb) for carbonatite-relatedniobium deposits (please refer to Table 1 for references). Stars are deposits inproduction and circles represent resources.

113P.F.O. Cordeiro et al. / Ore Geology Reviews 41 (2011) 112–121

main aim is to describe the deposit and provide information onpyrochlore chemistry in order to establish the main crystal chemistryfeatures and substitutions. We also compare Catalão I pyrochlorechemical composition with that of Lueshe (Nasraoui and Bilal, 2000),Oka (Gold et al., 1986; Zurevinski andMitchell, 2004) and Sokli (Lee etal., 2006) to contribute for a broader model of pyrochlore evolution incarbonatite complexes. Finally, we address some points of significancefor the bearing of magmatic processes in the origin of a phoscorite-related niobium deposit.

2. Niobium deposits

Most commercial niobium is from carbonatite-related sources, butminor production comes as a byproduct of tantalum and tin mining inpegmatites. In Table 1 we compiled and updated data from Woolleyand Kjarsgaard (2008) on the World's niobium reserves. Whenpossible, we report only measured, indicated and historical reserves,hence several resources listed in Table 1 are smaller compared towhatis found in the literature.

There are several carbonatite related niobium deposits worldwide,comprising residual and/or fresh rock resources, but only the BoaVista (Catalão II), CBMM (Araxá) and Niobec (Saint Honoré) depositsare currently in production (Fig. 1). The number of untapped niobiumdeposits in Africa and the general lack of information on the Brazilianunderground resources is noteworthy. Detailed information onBrazilian carbonatite-related deposits is given by Biondi (2005), butan equivalent study of African niobium deposits is still to be made.

3. The Alto Paranaíba Igneous Province (APIP)

The APIP is a NW trending province of Late-Cretaceous alkalineigneous rocks intruding Neoproterozoic rocks of the Brasília Belt,between theNEborder of the Paleozoic Paraná Basin and the SWborder

Table 1Comparison of carbonatite-related niobium deposits (adapted from Woolley and Kjarsgaar

Complex Country Status Style Association Resources

St-Honoré Canada Active mine Primary Nb+REE Measured andAraxá Brazil Active mine Residual Nb+Fe+P

Catalão II Brazil Active mine Residual Nb Probable reser

Lueshe Congo Past producer Residual NbSukulu Uganda Past producer Residual P+NbOka Canada Past producer Primary Nb Measured, Ind

and historicalCatalão I Brazil Past producer

and resourceResidual Nb+Fe+P

Catalão I Brazil Resource Primary Nb+Fe+P Measured andAraxá Brazil Resource Primary Nb+Fe+PTapira Brazil Resource Residual NbBonga Angola Resource Primary Nb

Bingo Congo Resource Residual Nb+PMrima Kenya Resource Residual Nb+REE

Ondurakorume Namibia Resource Primary P+Nb+REE

Mbeya(Panda Hill)

Tanzania Resource Primary Nb+P

Aley Canada Resource Primary N+P+REEBone Creek(Fir)

Canada Resource Primary Ta+Nb Indicated

Argor Canada Resource Primary Nb+P+Zr

Martison Lake Canada Resource Residual P+Nb Measured and

NemegosendaLake

Canada Resource Primary Nb Inferred

Seis Lagos Brazil Resource Residual Nb Measured and

of the Archean São Francisco Craton. The province origin is attributed tothe initial impact of theTrindadeMantle PlumebeneathCentral Brazil atca. 85 Ma. According toGibson et al. (1995) and Thompson et al. (1998),thinningof the lithosphereunder theBrasília Belt allowedmantleplumeheat to penetrate by conduction and advection causing melting ofreadily fusible, K-rich parts of the lithospheric mantle.

Xenoliths of perovskite-rich pyroxenite (bebedourite) and pyroxe-nite in APIP kamafugite lavas and pyroclastics are analogous toultramafic rocks occurring in the carbonatite complexes, thus providingevidence of the intimate association between kamafugites and

d, 2008).

Reserve Mt Nb2O5% Nb% Main references

indicated 32 0.56 0.39 www.iamgold.com (Resources 2009)462 2.48 1.73 Rodrigues and Lima (1984),

Hirano et al. (1990)ve 3.4 1.67 1.17 http://www.cbmm.com.br/ (conference

paper by Guimarães andWeiss)30 1.34 0.94 Deans (1966)

230 0.25 0.17 Deans (1966); van Straaten (2002)icatedreserves

37.5 0.53 0.37 http://www.niocan.com/(Technical Report February 10 2010)

19 1.08 0.76 Rodrigues and Lima (1984), Hirano et al.(1990)

indicated 29 1.22 0.85 This paper940 1.6 1.12 Issa Filho et al. (1984)166 0.73 0.51 Melo (1997)824 0.48 0.34 Pena (1989); Kamitani and Hirano

(1991)13 3.3 2.31 Woolley (2001)75 0.7 0.49 Deans (1966); Notholt et al. (1990);

Pell (1966); Woolley (2001)8 0.3 0.21 Verwoerd (1967, 1986); Woolley

(2001)125 0.3 0.21 Deans (1966); Woolley (2001);

van Straaten (2002)20 0.7 0.49 Richardson and Birkett (1996)23.1 1.14 0.80 www.commerceresources.com

(Technical Report June 20 2007)62.5 0.52 0.36 Stockford (1972); Woolley (1987);

Sage (1988)indicated 62.2 0.34 0.24 Woolley (1987); www.sedar.com

(Technical Report May 31 2007)49.9 0.43 0.30 www.sarissaresources.com

(Technical Report July 2009)indicated 239 2.47 1.73 Justo and Souza (1986)

114 P.F.O. Cordeiro et al. / Ore Geology Reviews 41 (2011) 112–121

carbonatites in the APIP (Brod, 1999; Brod et al., 2000, 2001). Thoseauthors argued in favor of a common subcontinental lithosphericmantle origin for kamafugites and the parental magma of APIPcomplexes (phlogopite picrite). The temporal and spatial associationbetween these alkaline rocks defines a kamafugitic–carbonatiticassociation in the APIP, similar to those occurring in Italy (Stoppa etal., 1997; Stoppa and Cundari, 1995) and China (Yang and Woolley,2006).

APIP carbonatite complexes also host phosphate (Araxá, Catalão Iand II, Tapira and Salitre), titanium (Serra Negra, Salitre, Tapira andCatalão I), and rare earth (Catalão I) deposits, as well as occurrences ofvermiculite, copper, barite and magnetite. Thus, the APIP is of greateconomic interest and can provide key information for mineralexploration of carbonatite-related deposits.

Fig. 3. Combination of an Ikonos image showing the roughly circular Mine II open pitand a 3-D model of the Mine II and East Area orebodies.

4. The Catalão I Carbonatite Complex

The Catalão I Complex (Fig. 2) is located in Central Brazil at 18°08′S, 47°48′W, near the cities of Catalão and Ouvidor. The complex hasintruded quartzites and schists of the Late Proterozoic Araxá Group asa vertical pipe with a diameter of ~6 km at surface, creating a dome-like structure. The age of the intrusion is reported by Sonoki and Garda(1988) as 85±6.9 Ma (K–Ar, phlogopite). The complex can be dividedinto an outer zone dominated by phlogopitite and an inner zonecomposed mostly of dolomite carbonatites and phoscorite-seriesrocks.

The outer zone comprises phlogopitites and rare dunites, pyroxe-nites and bebedourites (perovskite-rich pyroxenites). Phlogopitite isinterpreted as the result of interaction of former ultramafic rocks withcarbonatite fluids (Brod et al., 2001). Ultramafic relicts withinphlogopitite, which sometimes retain the original mineral assemblageunaffected by fluid alteration, are a very strong evidence for phlogopi-tization. The dominance of phlogopitite over other rock types in theouter zone attests to the extremely intense carbohydrothermalalteration that occurred in the complex.

Fig. 2. Geological sketch of the Catalão I Complex. The fresh rock niobium deAdapted from Brod et al. (2004).

The inner zone is composed of magnetite–apatite-rich rocks andcarbonatite. The Catalão I fresh rock deposit is intimately related tothese rocks and can be divided into Mine II and East Area orebodies(Fig. 3).Mine II is a roughly oval, pipe-like body, 200 m long and 100 mwide, hosted mainly by dolomite carbonatite. East Area is an L-shapedorebody, 400 m long, 200 m wide hosted by phlogopitite. Bothorebodies are open at depth and deep drilling confirmed theirextension at until a depth of at least 800 m.

Fig. 4 shows the general pipe-like geometry of East Area and MineII orebodies. Despite their shape, the orebodies do not representsingle, homogeneous pyrochlore-bearing magnetite–apatite rocks,but rather dike swarms up to 2 m wide and plugs up to 10 m wide.

The main Nb-mineral within the orebodies is pyrochlore. Asidefrom pyrochlore modal content, ore grades are also controlled by

posit occurs in the center of the complex comprising the nelsonite unit.

Fig. 4. Schematic model of the fresh rock niobium ore, where apatite nelsonite P2, magnetite nelsonite P3, and dolomite carbonatite DC crosscut phlogopitite. The detail shows thecommon textural feature of DC pockets.

115P.F.O. Cordeiro et al. / Ore Geology Reviews 41 (2011) 112–121

frequency and width of nelsonite dikes and can be largely diluted bybarren wallrocks (Fig. 5A, phlogopitite). Because of their dike-like,plug-like and vein-like shape, these terms are used in a generallydescriptive sense.

The occurrence of magnetite–apatite rich rocks, named phoscorite,in carbonatite complexes was reported by several authors (Krasnovaet al., 2004 and references therein) and due to their rarity,nomenclature remains problematic. A discussion on phoscorite seriesrocks is provided in Krasnova et al. (2004). We favor an adaptedversion of the nomenclature of Yegorov (1993) as used by Cordeiro etal. (2010, 2011). Therefore, phoscorite is an olivine-, phlogopite-,apatite- and magnetite-bearing rock and nelsonite is a phlogopite-,apatite- and magnetite-bearing rock.

In Catalão I, the phoscorite-series can be divided into two stages,according to mineral chemistry and modal mineralogy. Early-stagephoscorites are grouped under the P1 unit (Fig. 5B). Their main

Fig. 5. Main rock types of the Catalão I Nb-deposit. A. Phlogopitite with relicts of theoriginal ultramafic rock cut by a magnetite nelsonite dike (P3) with dolomitecarbonatite (DC) pockets. B. Coarse-grained phoscorite (P1), cut by P3 dikes with DCpockets. C. Equigranular apatite nelsonite (P2) with DC pockets.

characteristics are a) breccia structure; b) emplacement as small plugsand dikes; c) no obvious direct relationship with carbonatite; d) olivineoccurs as altered to minute tetra-ferriphlogopite, indicating interactionwith carbohydrothermal fluids; and e) pyrochlore is rare (although thisstage is an important source of apatite for the Catalão I residualphosphate deposit). Late-stage P2 and P3 units (Fig. 5B and C) arenelsonites and represent the bulk of the fresh rock niobium mineral-ization. Nelsonites can be distinguished from early-stage phoscorites bya) emplacement as dikes and small plugs; b) occurrence of internalpockets of dolomite carbonatite; c) no visible evidence of carbohy-drothermal alteration; d) absence of olivine; e) abundant pyrochlore,reaching up to 50 vol.% in some samples.

Dolomite carbonatite is abundant, but up to 15 mwide plugs and upto 2 m wide dikes of calcite carbonatite occur. Phoscorite is intenselycrosscutbydolomite carbonatitedikes,whichoriginates the breccia-likeaspect of these rocks. Widespread alteration of olivine crystals withinphoscorite to tetra-ferriphlogopite suggests that the inner zonewas alsoaffected by carbohydrothermal fluids. Nelsonite, on the other hand,shows no sign of metasomatic alteration, indicating that its emplace-ment occurred later, after the widespread alteration event.

Carbonatites, particularly dolomite carbonatites, dikes and plugsare widespread in Catalão I and are especially abundant within P1.One particular set of dolomite carbonatite, designated here DC, isintimately related to P2 and P3 and may occur within them ascentimetric to metric pockets as well as dikes and plugs. DC can beeasily discriminated from earlier generations of dolomite carbonatitesby the absence of olivine and presence of pyrochlore and ilmenite.

4.1. Primary ore

Primary (fresh) rock ore in Catalão I is represented by nelsonitedikes, but subordinate DC dikes with more than 1% modal pyrochloreoccur. P2 nelsonite is apatite-rich and its essential silicate phases aretetra-ferriphlogopite crystals with phlogopite cores. Apatite isprismatic, frequently zoned with cores surrounded by a fluidinclusions-rich rim. Magnetite is interstitial and may contain verythin (ca. b0.01 mm) ilmenite lamellae.

P3 is magnetite-rich (apatite/magnetite b0.8 vol.%) and itsessential silicate phase is tetra-ferriphlogopite. In contrast to P2,aluminous phlogopite cores are virtually absent. Apatite is prismaticto rounded, but also occurs as aggregates of anhedral crystals, usuallyassociated with massive anhedral magnetite clusters. Magnetiteforms interstitial masses and may reach up to 71 vol.%.

Dolomite carbonatite (DC) occurs as pockets within nelsonites andalso as independent dikes and veins. Although other dolomitecarbonatite phases occur in the complex, the variety geneticallyrelated to nelsonites crosscuts all rock types. DC dikes are believed to

Fig. 6. Textural characteristics of nelsonitespyrochlore. A. P2nelsonitewith subhedral, brown toorangepyrochlore. B. P3nelsonitewith anhedral to subhedral brown toorangepyrochlore.C. Sector zoning in pyrochlore from DC. D. P2 aggregates within DC, crossed polars. (Mag =magnetite; Apt = apatite; TFP = tetra-ferriphlogopite; Carb = carbonate; Pcl = piroclore).

116 P.F.O. Cordeiro et al. / Ore Geology Reviews 41 (2011) 112–121

represent the product of extraction of carbonatite from the nelsonitecrystallizing assemblage (Cordeiro, 2009). The pyrochlore content inDC varies, but it is hardly more than 5 vol.%.

Pyrochlore from P2 and P3 nelsonites are texturally similar andgenerally fine-grained. They occur as anhedral to subhedral brownishor yellowish crystals often showing optical zoning (Fig. 6A, B). DCpyrochlore is often euhedral to subhedral and may occur as inclusionsin ilmenite, together with betafite and columbite, and in magnetite(Cordeiro, 2009). It is medium- to fine-grained, often optically zoned(Fig. 6C). Aggregates of pyrochlore and apatite occur within DC(Fig. 6D).

Fig. 7. Triangular Nb–Ti–Ta pyrochlore classification scheme (Atencio et al., 2010;Hogarth, 1977, 1989) showing fresh rock pyrochlore as black circles. Outlines forpyrochlore compositions from the Catalão I residual deposit (square pattern, Fava,2001), Oka (gray, Gold et al., 1986; Zurevinski and Mitchell, 2004), Sokli (solid blackoutline; Lee et al., 2004, 2006) and Salitre (dotted black outline, Barbosa, 2009) areshown for comparison. BET = betafite, PCL = piroclore, MCL = microlite.

5. Pyrochlore chemistry

Pyrochlore composition was determined by WDS using a CAMECASX-50 electron microprobe at the University of Brasília. The analyticalconditions were beam diameter 2 μm, 20 kV, 20 nA and two minutecount times. Detection limits varied between 0.01 and 0.05 wt.%,except Nb and Ta (0.07%) and La, Ce and Y (0.2%).

The pyrochlore general formula is A2−mB2X6−wY1−n·pH2O (Atencioet al., 2010; Lumpkin and Ewing, 1995). The A site is occupied by largeanions such asAs, Ba, Bi, Ca, Cs, K,Mg,Mn, Na, Pb, REE, Sb, Sr, Th, U andY.The B site comprises smaller and highly charged cations such as Nb, Ta,Ti, Zr, Fe3+, Al and Si (Zurevinski and Mitchell, 2004) and rarely W+5

(Caprilli et al., 2006). The Y and X anions can be O, OH and F. Vacanciesare common in the A and Y sites. In this paper, pyrochlore has beencalculated to produce a total of 2 cations in the B site (Wall et al., 1996).

Pyrochlore classification is originally described by Hogarth (1977)but an up to date CNMNC-IMA-approved nomenclaturewas publishedby Atencio et al. (2010). The new nomenclature is composed of twoprefixes and a root name based on the content of Y, A and B sites. The Ysite content (cation, anion, H2O or vacancy) determines the first prefixand the A site content refers to the second prefix. The dominantelement in the B site determines the root name: pyrochlore (Nb),microlite (Ta), roméite (Sb), betafite (Ti) and elsmoreite (W).

The abundance of Nb over other B site elements classifies Catalão Ipyrochlore supergroup minerals within the pyrochlore group (Fig. 7).Pyrochlore representative compositions are shown in Table 2. Datapublished by Fava (2001) indicates that more than 95% of all Catalão Ifresh rock pyrochlore exceeds 0.5 apfu and therefore should have theprefix fluor. However, we haven't analyzed fluorine and Atencio et al.(2010) suggest prefixes should be droped in face of lack of data to avoidmisclassification. Therefore the first prefix won't be used in this paper.

Table 2Representative compositions of pyrochlore group minerals from the Catalão I primary niobium deposit (b.d. = below detection limit; calcio = calciopyrochlore; keno =kenopyrochlore; natro = natropyrochlore).

Sample 178-2 C

192B-2

192B-8

056-2 178-1 183-03

339-3 C

157B-06

157B-2

230B-2B

230B-2B

149-1 093-3

056-1

183-05

304B-2

170-6

170-4 230A-2

170-2

Type calcio calcio calcio calcio calcio calcio calcio calcio calcio calcio calcio calcio keno keno keno keno keno natro natro natroUnit P2 P2 P2 DC P2 P3 P2 P3 P3 P3 P3 DC P3 DC P3 P3 DC DC P2 DCNb2O5 62.66 59.26 55.76 59.93 61.68 55.58 55.26 64.26 63.14 63.39 63.39 68.71 59.99 50.10 62.17 52.26 63.96 72.56 63.76 52.85Ta2O5 0.15 b.d b.d. b.d. 0.33 0.70 0.16 b.d. b.d. b.d. b.d. 0.28 0.81 0.77 0.57 0.80 0.81 1.61 0.37 0.92SiO2 b.d. b.d. 0.16 0.12 0.61 0.57 b.d. b.d. 0.04 b.d. b.d. b.d. 1.20 2.93 b.d. 0.61 1.10 b.d. b.d. b.d.TiO2 3.52 4.64 5.59 6.15 3.15 4.16 3.67 3.91 4.71 4.35 4.35 2.04 3.16 5.27 4.87 2.37 1.26 0.78 4.20 17.35ZrO2 0.17 2.05 0.90 2.13 0.13 3.95 0.26 1.78 1.65 0.53 0.53 0.53 b.d. 2.44 0.32 3.20 0.75 b.d. 0.94 0.09UO2 0.36 b.d. b.d. 1.02 0.59 2.35 0.19 0.14 b.d. b.d. b.d. b.d. 0.77 1.01 1.17 3.72 0.12 b.d. 0.82 b.d.ThO2 1.08 3.39 2.13 2.04 1.09 2.69 1.44 1.12 1.13 2.40 2.40 0.22 0.41 2.15 4.66 4.94 0.74 0.19 1.72 b.d.La2O3 1.21 0.39 0.75 0.62 1.14 0.62 0.87 0.95 0.68 0.71 0.71 1.63 1.30 0.32 1.13 0.42 0.92 0.37 0.96 0.35Ce2O3 4.20 2.90 2.85 2.37 3.42 2.92 3.09 2.47 2.00 2.72 2.72 3.26 3.37 2.91 4.09 3.04 3.54 0.73 2.68 0.24Y2O3 0.44 0.57 0.49 0.55 0.32 0.26 0.34 0.45 0.46 0.53 0.53 0.57 0.26 0.40 0.48 0.20 0.68 0.39 0.55 0.20FeO 0.14 0.50 0.70 0.40 0.94 1.93 0.31 0.86 0.46 0.20 0.20 0.18 0.69 4.47 0.40 1.49 0.77 0.18 0.16 4.22MnO b.d. b.d. b.d. b.d. b.d. 0.36 b.d. b.d. b.d. b.d. b.d. 2.00 0.11 b.d. b.d. 0.08 b.d. b.d. b.d. 0.90CaO 9.02 14.31 14.46 16.14 7.46 8.53 14.50 13.11 15.74 11.86 11.86 9.87 5.11 2.80 8.51 3.34 0.12 11.32 12.05 10.98BaO 0.35 b.d. 0.24 b.d. 4.89 3.67 0.13 b.d. b.d. b.d. b.d. b.d. 11.03 14.61 2.81 12.24 15.20 0.18 b.d. b.d.SrO 2.78 0.69 1.06 1.17 3.95 3.48 2.41 2.29 1.59 2.35 2.35 2.50 4.65 2.23 2.03 3.56 0.75 4.61 2.08 3.01Na2O 5.59 4.23 3.83 4.71 4.09 2.52 2.96 5.94 6.09 5.93 5.93 7.31 0.34 0.77 1.16 0.38 1.29 7.83 6.46 7.75Total 91.67 92.97 89.07 97.41 93.87 94.28 85.59 97.32 97.81 95.04 95.04 97.17 93.25 93.18 94.43 92.65 92.01 100.84 96.78 98.92

Structural formulae calculated based on ∑ B-site elements = 2Nb 1.82 1.71 1.68 1.65 1.80 1.62 1.79 1.77 1.73 1.78 1.78 1.88 1.75 1.46 1.75 1.70 1.83 1.94 1.77 1.28Ta b.d. b.d. b.d. b.d. 0.01 0.01 b.d. b.d. b.d. b.d. b.d. 0.01 0.01 0.01 0.01 0.02 0.01 0.03 0.01 0.01Si b.d. b.d. 0.01 0.01 0.04 0.04 b.d. b.d. b.d. b.d. b.d. b.d. 0.08 0.19 b.d. 0.04 0.07 b.d. b.d. b.d.Ti 0.17 0.22 0.28 0.28 0.15 0.20 0.20 0.18 0.22 0.20 0.20 0.09 0.15 0.26 0.23 0.13 0.06 0.04 0.19 0.70Zr 0.01 0.06 0.03 0.06 b.d. 0.13 0.01 0.05 0.05 0.02 0.02 0.02 b.d. 0.08 0.01 0.11 0.02 b.d. 0.03 b.d.∑ Bsite

2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00

U 0.01 b.d. b.d. 0.01 0.01 0.03 b.d. b.d. b.d. b.d. b.d. b.d. 0.01 0.01 0.02 0.06 b.d. b.d. 0.01 b.d.Th 0.02 0.05 0.03 0.03 0.02 0.04 0.02 0.02 0.02 0.03 0.03 b.d. 0.01 0.03 0.07 0.08 0.01 b.d. 0.02 b.d.La 0.03 0.01 0.02 0.01 0.03 0.01 0.02 0.02 0.02 0.02 0.02 0.04 0.03 0.01 0.03 0.01 0.02 0.01 0.02 0.01Ce 0.10 0.07 0.07 0.05 0.08 0.07 0.08 0.06 0.04 0.06 0.06 0.07 0.08 0.07 0.09 0.08 0.08 0.02 0.06 b.d.Y 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.01 0.01 0.02 0.01 0.02 0.01 0.02 0.01Fe2 0.01 0.03 0.04 0.02 0.05 0.10 0.02 0.04 0.02 0.01 0.01 0.01 0.04 0.24 0.02 0.09 0.04 0.01 0.01 0.19Mn b.d. b.d. b.d. b.d. b.d. 0.02 b.d. b.d. b.d. b.d. b.d. b.d. 0.01 b.d. b.d. 0.01 b.d. b.d. b.d. 0.04Ca 0.62 0.98 1.03 1.05 0.52 0.59 1.11 0.86 1.02 0.79 0.79 0.64 0.35 0.19 0.57 0.26 0.01 0.72 0.79 0.63Ba 0.01 b.d. 0.01 b.d. 0.12 0.09 b.d. b.d. b.d. b.d. b.d. b.d. 0.28 0.37 0.07 0.35 0.38 b.d. b.d. b.d.Sr 0.10 0.03 0.04 0.04 0.15 0.13 0.10 0.08 0.06 0.08 0.08 0.09 0.17 0.08 0.07 0.15 0.03 0.16 0.07 0.09Na 0.70 0.52 0.50 0.55 0.51 0.32 0.41 0.70 0.72 0.71 0.71 0.86 0.04 0.10 0.14 0.05 0.16 0.90 0.77 0.81∑ Asite

1.60 1.70 1.75 1.79 1.50 1.42 1.79 1.79 1.91 1.73 1.73 1.73 1.03 1.12 1.09 1.14 0.75 1.83 1.78 1.78

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Nb2O5 content varies from 50 to 70 wt.%. The average TiO2 contentranges from 3 to 5 wt.%, butmay reach up to 17 wt.% in natropyrochloreinclusions in ilmenite from DC. Most analysis show low Ta2O5, rangingfrom b1 wt.% to a maximum of 2 wt.% in one grain from P2 and inpyrochlore crystals within DC ilmenite. ZrO2 and SiO2 reach up to 5 and3 wt.% respectively.

Pyrochlore from fresh rock nelsonite has Ca and Na as the main Asite elements, ranging up to 19 and 8 wt.%, respectively. Thereforecalciopyrochlore dominates in the Catalão I Nb deposit followed bynatropyrochlore. Ba is one of the most common substitutes for bothNa and Ca in this site and BaO content reaches 18 wt.% whereas SrOmay reach 7 wt.%. The sum of the analyzed rare earth (La+Ce+Y)oxides varies from 3.5 to 6 wt.%. ThO2 is up to 6 wt.% but its averagecontent is b2 wt.%. UO2 is up to 4 wt.%, averaging b1 wt.%. FeO mayreach 3 wt.%, and MnO is always below 1 wt.%. Several analysesindicate the occurrence of kenopyrochlore (zero-valent-dominantpyrochlore) in the Catalão I Nb fresh rock deposit but according todata from Fava (2001) they are dominant in the residual ore.

Several studies tried to explain the evolution of pyrochlorecomposition throughout magmatic evolution (Chakhmouradian andWilliams, 2004; Hogarth et al., 2000; Knudsen, 1989). During the earlystages of carbonatite magmatism Nb and Ta are probably transportedas phosphate and fluorine complexes, which might explain thecommoncorrelation between the occurrence of apatite andpyrochlore

(Hogarth et al., 2000; Knudsen, 1989). Knudsen (1989) argued thatduring the carbonatitic magmatism Nb is more soluble than Ta, whichcould explain the occurrence of Ta-rich pyrochlore in primitivemagmas and Nb-rich, Ta-poor pyrochlore in more evolved, late stageones. Hogarth et al. (2000) concluded that the normal path ofevolution of pyrochlore in carbonatites is one of progressiveenrichment in Na, Ca and Nb and depletion in Ta, Th, REE, Ti and U. Amore detailed evolution scheme is proposed by Chakhmouradian andWilliams (2004) where Th-enriched Ca–Na dominant pyrochloreevolves toward Ba–Sr-rich compositions in calcite–dolomite carbona-tites from Kola carbonatite complexes.

Pyrochlore from the Catalão I fresh rock deposit seems to fit wellinto the proposed scheme, but early Ta–Th–U enriched pyrochlorephases are not present. Pyrochlore analyses of several stages of Sokliphoscorites (Lee et al., 2004; 2006) confirm the trend of early Ta–U–Thpyrochlore toward evolved Ca–Na compositions. Thus, Ca–Na pyro-chlore in P2 and P3 nelsonites and related DC dolomite carbonatitescan be interpreted as belonging to amore evolved phase, similar to thelate-stage D5 dolomite carbonatite phase in the Sokli complex.

5.1. Chemical evolution of pyrochlore

The range of P2 and P3 pyrochlore chemical compositions overlapswidely and we could not find an applicable chemical criterion to

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discriminate pyrochlores from the twounits. According toCordeiro et al.(2010) other minerals from P2 and P3, such as phlogopite and apatite,aren't discernible from each other. This suggests only minor chemicaldifferences in the magmas that produced the two units. Therefore, thecompositional spread seen in our datamight be related to factors suchasa) zoning (Chakhmouradian and Mitchell, 2002; Hogarth et al., 2000);b) hydrothermal alteration (Chakhmouradian and Mitchell, 1998;Geisler et al., 2004); c) weathering (Lumpkin and Ewing, 1995; Wallet al., 1996). Chakhmouradian and Zaitsev (1999) point out that severaltypes of pyrochlore may be found in the same complex or even withinthe same facies. Hence, in order to address the compositional variationwe need a classification criterion other than lithology.

Lumpkin and Ewing (1995) argued that A site large cations such asK, Ba and Sr can be useful in the identification of pyrochlore chemicalvariation because their occurrence is related to the host rockalteration. Accordingly, we adopted a division based on the variationof the A site content, allowing us to discriminate between threepyrochlore groups (Fig. 8): a) calciopyrochlore; b) natropyrochlore;and c) kenopyrochlore.

The occurrence of vacancy in pyrochlore from the fresh rock depositis an important feature of its evolution. Vacancy, sometimes accompa-nied by Ba-enrichment, was attributed to alteration at Sokli (Lee et al.,2006), to hydrothermal overprint at Oka (Zurevinski and Mitchell,2004) and Lueshe (Nasraoui and Bilal, 2000) and to both oscillatoryzoning and alteration in the Bingo carbonatite (Williams et al., 1997).

The trend from calciopyrochlore and natropyrochlore towardkenopyrochlore illustrated in Fig. 8 is related to the exchange of Ba forCa+Na, and consequent vacancy, in the A site. Similar trends can befound in Lueshe (Nasraoui and Bilal, 2000) and Bingo (Williams et al.,1997) pyrochlore, described as product of weathering, and in Kolacarbonatites pyrochlore (Chakhmouradian and Williams, 2004), asderived from supergene or low-temperature hydrothermal alteration.

An additional trend from calciopyrochlore toward natropyrochlorecan be considered. Despite considerable scatter, calciopyrochlore,natropyrochlore and the fields of Oka and Salitre fresh rock pyrochloreshow an overall alignment to the 1:1 line in a Na vs. Ca diagram. Thistrend is even more marked in crystal core composition from theCatalão I weathered pyrochlore deposit (Fava, 2001). Taking intoaccount that most natropyrochlore are inclusions in ilmenites fromthe last stage of magmatic evolution in the deposit (DC unit) and that

Fig. 8. Ternary plots of Ca, Na and A site vacancy. Compositional pyrochlore fields of otherWilliams et al. (1997).

ilmenite is one of the last minerals to crystallize, natropyrochloreformed at such stage would represent the most evolved pyrochlorecomposition. Therefore, we interpret that the negative correlation ofCa and Na represent the evolution from earlier calciopyrochloretoward a late stage natropyrochlore.

5.2. Comparison with pyrochlore from the residual deposit

Pyrochlore chemistry from the Catalão I fresh rock and residualdeposits shows no clear differences in the B site, but some importantsubstitutions occur the A site. Fava (2001) described the mineralogicalcharacteristics of pyrochlore from the residual deposit developed overCatalão I nelsonites and carbonatites and concluded that weatheringinduced substitutions in the A site and originated bariopyrochlore,renamed here as “Ba-enriched” kenopyrochlore according to thenomenclature of Atencio et al. (2010). On the other hand, Catalão I freshrock and residual Ba-enriched kenopyrochlore are different from eachother. Fresh rock crystals show a negative Sr–Ca correlation that leadstoward Sr-enriched kenopyrochlore and the same correlation occurs inthe residual deposit pyrochlore crystal cores (Fava, 2001). However, themajority of pyrochlore in the residual deposit is Ba-enriched kenopyro-chlore that lack a negative Sr–Ca correlation. These features suggest thatdifferent processes originated fresh rock kenopyrochlore and the residualdeposit kenopyrochlore.

We suggest that the chemical shift from calciopyrochlore andnatropyrochlore toward kenopyrochlore in the Catalão I fresh rockdeposit is due to interactionwith hydrothermal fluids that also carriedSr. Later weathering-related fluids originated the residual deposit Ba-enriched kenopyrochlore by depleting pyrochlore from Ca and Na.With weathering progression even Ba is eventually leached frompyrochlore leading to its destruction and consequent formation ofsecondary Nb-enriched minerals in the soil (Wall et al., 1999).

6. Carbon and oxygen isotopes

Carbon and oxygen isotopes from DC pockets dolomite (Table 3)were analyzed to establish a correlation with the pyrochlore chemistry(Fig. 9). Carbonates were extracted from pockets with a manualtungsten-carbide drill to avoid interference from different carbonategenerations or contaminationwith external sources.Oxygenand carbon

deposits are shown for comparison. Data sources as in Fig. 7, plus the Bingo field from

Table 3Representative analysis of carbon and oxygen isotopes of carbonates from pyrochlore-bearing DC pockets.

Sample 056 056E 93 093G1 178G1 178G2 192G1

Type Carbonatite Carbonatite DC pocket in P2 DC pocket in P2 DC pocket in P2 DC pocket in P2 DC pocket in P2δ13CPDB −5.53 −5.86 −5.38 −5.53 −5.85 −5.16 −6.14δ18OSMOW 11.80 20.23 19.99 10.42 15.92 11.06 8.59

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isotope data were obtained reacting carbonate samples with 100%H3PO4 at 72 °C, using a Gas Bench II System connected to a Delta VAdvantage gas-source mass spectrometer at the University of Brasília.Results are expressed in delta notation, relative to the PDB (carbon) andSMOW (oxygen) standards.

Data from Table 3 show that gray fresh dolomite (093 G1, 056) hasa mantle-like carbon- and oxygen-isotope signature, interpreted asmagmatic, whereas white brittle dolomite in the same samples hashigher δ18OSMOW values than gray calcite and is interpreted as affectedby low temperature H2O-rich fluids probably of meteoric origin. Thesame fluids are likely to have altered pyrochlore, leaching Ca and Naand leaving vacancy while partially replacing them with Ba. Thishypothesis is supported by comparison between calciopyrochlore ornatropyrochlore cores with Ba-enriched kenopyrochlore rims, sug-gesting fluid interaction. This would be consistent with generalalteration models (Wall et al., 1999).

7. Genetic implications

The dike-like emplacement of nelsonites and its relationship withDC pockets is an important feature of the Catalão I Nb deposit,suggesting a magmatic origin for the Nb-ore. Cordeiro et al. (2011)showed that DC pockets within nelsonites have mantle-like C- and O-isotope signatures and suggested Rayleigh fractionation and mag-matic degassing as important processes for the evolution of suchrocks. The authors' results have also shown that metasomatism andweathering played a role in the carbon and oxygen isotopic variationsof DC carbonates, albeit unrelated to the formation of pyrochlore.Accordingly, we interpret the occurrence of pyrochlore in nelsonitesand dolomite carbonatite as an igneous process.

Fig. 9. Comparison between pyrochlore composition and carbon–oxygen isotope signatures056 have systematically higher A site vacancies than corresponding cores. In sample 056, theand 178 show only a slight Ba-enrichment and little vacancy. Carbon and oxygen isotopes showhile less altered samples preserve the original composition. Stable isotope fields are from

The genesis of late-stage phoscorite-series rocks, and therefore ofthe Catalão I fresh rock niobium deposit, is still a matter ofcontroversy. Krasnova et al. (2004) argues in favor of AFC and/orliquid immiscibility in generating phoscorites. Lee et al. (2004)described chemical discrepancies between Sokli carbonatites andrelated phoscorites as evidence for immiscibility that generated both acarbonatite and a phoscorite melt. However, the authors point outthat experimental evidence for such process is still lacking.

The best evidence we could find for the occurrence of phoscoritemelts is given by Panina and Motorina (2008). They studied meltinclusions from the Krestovskii carbonatite complex, in theMaimecha–Kotui province, Russia, and suggested a carbonatite immiscibility eventthat originated alkali-rich phosphate melts. Evidence of Fe–P–Ti-richmelts exists in carbonatite-unrelated settings such as the andesiticAntauta subvolcanic complex in Peru (Clark and Kontak, 2004). Theauthors describe Fe-rich melt inclusions that are interpreted as derivedfrom nelsonite-like magma, indicating that such unusual magmas mayindeed occur naturally.

Formation of cumulates is another possible mechanism in thegeneration of apatite–magnetite rich rocks. Mitchell (2005) arguesthat potential niobium ore rocks in carbonatites do not representliquid compositions nor reflect the Nb content of the parental magma.Based on melt inclusion data, Veksler et al. (1998) argue that crystalfractionation resulted in the formation of calcite carbonatites, whichevolved to forsterite–apatite–magnetite–phlogopite carbonatiteswith subordinate phoscorite cumulates and dolomite carbonatites.According to Downes et al. (2005) the Kola Alkaline Provincephoscorite–carbonatite rocks series are the result of complexdifferentiation of an extremely phosphorous and iron enrichedcarbonate–silicate melt. They also favor the formation of cumulatesas a mean of generating phoscorites.

of carbonates within the same pocket. Note that pyrochlore rims from samples 093 andcore is calciopyrochlore and the rim is Ba-enriched kenopyrochlore, while samples 192Bw that samples with kenopyrochlore rims have wider variation in the δ18OSMOW contentCordeiro et al. (2011).

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At this stage we are unable to determine which of the describedmechanisms was involved in the formation of the Catalão I fresh rockniobium deposit. Despite the occurrence of phosphate melts withincarbonatite complexes, crystal accumulation is likely to be involved inthe generation of nelsonites. DC pockets within nelsonite dikes aresometimes interconnected and low viscosity carbonatite lava couldflow in the open space. Apatite, pyrochlore, magnetite and phlogopite(i.e. nelsonite) could crystallize in situ until all open spaces were filledand flow would stop. Such mechanism could explain both theoccurrence of DC pockets and the related pyrochlore-bearingmagnetite–apatite-rich rocks.

Whereas magmatic controls were vital in the formation of thefresh rock niobium deposit, weathering played an important role in itsenrichment, consequently forming the residual deposit. All rockswithin the Catalão I Complex are easily weathered compared to thecountry-rocks (fenites and quartzites). The dome-like structureprevents erosion and allows the establishment of very thick soilcover over the alkaline rocks. On average, soil depth is 80 m, butreaches at least 150 m over phoscorite-series rocks. A similar patternoccurs at Seblyavr in the Kola Alkaline Province, Russia (Balaganskayaet al., 2007) where a weathering crust up to 200 m deep is developedover phoscorite-series rocks in the intrusion core. We believe that theabundance of fractured easily-weathered carbonatites, either as dikescutting early-stage phoscorites or as DC pockets within nelsonites,contributed to the development of such deep soils and the generationof the residual deposit.

8. Conclusions

1) The pipe-like niobium orebodies at Catalão I consist of dikeswarms of late-stage phoscorite-series rocks (nelsonites) that cutprevious phlogopitite and carbonatite.

2) Weathering of such rocks originated the residual deposit, whereleaching of carbonates induced a residual concentration ofpyrochlore and other weathering-resistant phases.

3) Catalão I phocorite-series rocks can be divided into phoscorites (P1),and the niobiumores apatite nelsonite (P2) andmagnetite nelsonite(P3). The mineralization can be classified as Nb (+Fe+P) on thegrounds of high modal content of apatite and magnetite. Dolomitecarbonatites (DC) associated with nelsonites are a subordinatesource of pyrochlore but their grades are comparatively low, hardlyabove 0.3 wt.% Nb2O5.

4) The dominance of calciopyrochlore over other Nb-bearing phasesin the fresh rock deposit and its chemical variability areindependent of lithology. This indicates that pyrochlore formationchemical conditions were similar in P2, P3, and DC.

5) Substitution of Na–Ca for Ba in the fresh rock pyrochlore structure,leading to the formation of Ba-enriched kenopyrochlore, and thehigh δ18OSMOW signature of the associated carbonates suggestinteraction with hydrothermal fluids. These fluids affectednelsonites but had no role in the formation of the fresh rockniobium deposit itself.

6) We could not uniquely constrain the nelsonite formation process inCatalão I although it is clear that they are genetically related tocarbonatite magmatism. Possible alternatives for the formation andevolution of these rocks are: (a) crystallization from a phoscoritemagma (Lee et al., 2004, 2006); (b) crystal accumulation from acarbonatite magma (Veksler et al., 1998); and (c) crystal accumu-lation from a carbonated-silicate magma (Downes et al., 2005).

Acknowledgments

We are indebted to Anton Chakhmouradian, Nigel Cook and twoanonymous reviewers for their helpful review of the originalmanuscript. This work was supported by the Brazilian Council forResearch and Technological Development (CNPQ), through grants to

the first author, JAB and ESRB, as well as byMineração Catalão and theAnglo American Brazil Exploration Division. The University of Brasíliais gratefully acknowledged for fieldwork support and access tolaboratory facilities.

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