Hydrothermal rare earth elements mineralization in the Barra do Itapirapuã carbonatite, southern Brazil: behaviour of selected trace elements and stable isotopes (C, O)

Chemical Geology 155 Ž1999. 91–113

Hydrothermal rare earth elements mineralization in the Barra do Itapirapua˜ carbonatite, southern Brazil: behaviour of selected trace elements and stable isotopes žC, O / F.R.D. Andrade

a,b,)

b b , P. Moller , V. Luders , P. Dulski b, H.A. Gilg ¨ ¨

c

a

c

Departamento de Petrologia e Metalogenia, UniÕersidade Estadual Paulista ‘ Julio ´ de Mesquita’, AÕenida 24A, 1515, 13509-900 Rio Claro, (SP), Brazil b GeoForschungsZentrum Potsdam, PB 4.3 Lagerstattenbildung, Telegrafenberg, 14473 Potsdam, Germany ¨ Lehrstuhl fur Lichtenbergstr. 4, 85747 Garching, Germany ¨ Angewandte Mineralogie und Geochemie, Technische UniÕersitat ¨ Munchen, ¨ Received 24 September 1997; accepted 3 April 1998

Abstract The Barra do Itapirapua˜ carbonatite is located in southern Brazil and belongs to the Cretaceous Ponta Grossa alkaline–carbonatitic province related to the opening of the South Atlantic. The carbonatite complex is emplaced in Proterozoic granites and is mainly composed of plutonic magnesio- to ferrocarbonatite, with smaller amounts of subvolcanic magnesiocarbonatite. Hydrothermal alteration of the carbonatite has led to the formation of quartz, apatite, fluorite, rare earth fluorocarbonates, barite and sulfides in variable proportions. Trace element data, d13C and d18 O are presented here, with the aim of better understanding the geochemical nature of hydrothermal alteration related to rare earth elements ŽREE. mineralization. The non-overprinted plutonic carbonatite shows the lowest REE contents, and its primitive carbon and oxygen stable isotopic composition places it in the field of primary igneous carbonatites. Two types of hydrothermally overprinted plutonic carbonatites can be distinguished based on secondary minerals and geochemical composition. Type I contains mainly quartz, rare earth fluorocarbonates and apatite as hydrothermal secondary minerals, and has steep chondrite normalized REE patterns, with S REEqY of up to 3 wt.% Ži.e., two orders of magnitude higher than in fresh plutonic samples.. In contrast, the Type II overprint contains apatite, fluorite and barite as dominant hydrothermal minerals, and is characterized by heavy REE enrichment relative to the fresh samples, with flat chondrite normalized REE patterns. Carbon and oxygen stable isotope ratios of Types I and II are elevated Ž d18 O q8 to q12‰; d13 C y6 to y2‰. relative to the fresh samples. Hydrothermally overprinted carbonatites exposed to weathering show even higher d18 O values Ž d18 O 13 to 25‰. but no additional REE enrichment. The subvolcanic carbonatite has anomalously high d13C of up to q1‰, which suggests crustal contamination through interaction with carbonate-bearing metasediments. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Carbonatite; Trace elements; Stable isotopes; Hydrothermal alteration; REE mineralization

)

Corresponding author. Fax: q49-331-288-1436; E-mail: [email protected]

0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 1 4 3 - 0

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F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

1. Introduction Carbonatites are an economically important source of rare earth elements ŽREE., niobium, phosphate and fluorite Že.g., Moller, 1989; Mariano, 1989.. ¨ Several examples of hydrothermal REE mineralization in carbonatites are known, including Karonge, Angola Žvan Wambeke, 1977. and Tundulu and Kangankunde, Malawi ŽNgwenya, 1994; Wall and Mariano, 1996.. The largest known REE deposit, the sediment-hosted bastnaesite deposit of Bayan Obo in China, is thought to be derived from hydrothermal fluids released by a nearby carbonatite ŽZhou et al., 1980; Nakai et al., 1989; Zhongxin et al., 1992; Le Bas et al., 1997; Campbell and Henderson, 1997.. The Mt. Pass carbonatite, USA, hosts a very large REE deposit and represents a unique case of magmatic bastnaesite ŽMariano, 1989.; however, at least part of the bastnaesite–fluorite assemblage is related

to late stage hydrothermal activity ŽOlson et al., 1954.. If climatic and tectonic conditions are suitable, supergene processes can lead to further REE enrichment, observed for example in Araxa´ and Catalao ˜ in Brazil ŽIssa Filho et al., 1984; Morteani and Preinfalk, 1996., Mt. Weld in Australia ŽLottermoser, 1990. and Tomtor in Russia ŽKravchenko et al., 1993.. The present work focuses on the Barra do Itapirapua˜ carbonatite, southern Brazil ŽFig. 1., particularly on the geochemical aspects of the associated REE mineralization. This carbonatite massif provides the possibility for comparing of fresh plutonic rocks with rocks overprinted by different styles of hydrothermal alteration. Samples of overprinted carbonatite contain highly variable proportions of secondary minerals; the most abundant being quartz, apatite, fluorite, rare earth fluorocarbonates Žbastnaesite, synchysite, parisite. and barite ŽSouza and

Fig. 1. Location of alkaline–carbonatite Žtriangles. and alkaline complexes Žcircles. of the Ponta Grossa province in southern Brazil Ž1—Barra do Itapirapua; ˜ 2—Itapirapua; ˜ 3—Mato Preto; 4—Banhadao; ˜ 5—Barra do Teixeira; 6—Sete Quedas; 7—Tunas; 8—Jacupiranga; 9—Juquia; ´ 10—Cananeia; ´ modified after Ulbrich and Gomes, 1981., and carbonatite-related ore deposits Žafter Rodrigues and Lima, 1984..

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

Oliveira, 1984.. Samples from the neighbouring carbonatite of Mato Preto and of carbonate-bearing ´ metasediments of the Agua Clara Formation were included in the present study in order to test the possibility of crustal contamination.

2. Analytical techniques Major elements were determined by X-ray fluorescence ŽSiemens SRS303AS. on fused glass discs prepared from a mixture of rock powder, Li-tetraborate and Li-metaborate. CO 2 and S were measured in a multiphase analyser ŽLeco RC 412. and F by ion specific electrode analysis. Trace elements ŽRb, Sr, Y, Zr, Cs, Ba, REE, Hf, Pb, Th, U. were analysed by ICP-MS ŽPerkin Elmer Elan 5000. in solutions obtained by mixed digestion ŽHFrHClO4 . under pressure in sealed teflon vessels, following the procedure outlined by Dulski Ž1994.. The overall precision and accuracy for the trace elements is better than "10%. For details see Cotten et al. Ž1995., Nath et al. Ž1997. and Bau et al. Ž1996.. Due to large variations in the REE contents of the samples, up to several orders of magnitude, dilution factors of 5000, 50 000 and 500 000 were used, resulting in different detection limits relative to the solid sample. Mineralogical compositions were determined on whole rock powder samples by X-ray diffraction ŽSiemens D5000., supplemented by petrographic observation using optical and electron microscopy ŽZeiss DSM962.. Stable carbon and oxygen isotope compositions of whole rock samples were measured at the Technische Universitat on a Finnigan MAT 251 mass ¨ Munchen ¨ spectrometer calibrated with NBS-18 and NBS-19 standards. Extraction of CO 2 from carbonates using anhydrous phosphoric acid followed the method of McCrea Ž1950.. The phosphoric acid fractionation factors Ž a . used are 1.01025 for calcite at 258C ŽFriedman and O’Neil, 1977., and 1.01057 for ankerite at 508C ŽRosenbaum and Sheppard, 1986.. Analytical error is "0.1‰ for both carbon and oxygen, and the results are expressed conventionally as per mil Ž‰. variation relative to standard mean ocean water ŽSMOW. and Pee Dee belemnite ŽPDB., respectively.

93

3. Description of the Barra do Itapirapua˜ carbonatite 3.1. Geological setting The Cretaceous alkaline magmatism in southern Brazil Ž67 to 130 Ma: Amaral et al., 1967; Ulbrich and Gomes, 1981. is spatially related and partially contemporaneous with the Parana´ continental flood basalts Ž128 to 138 Ma; Turner et al., 1994.. Together, these associations represent a major magmatic event which has been attributed to hot spot activity ŽHerz, 1977; Toyoda et al., 1994. related to the opening of the South Atlantic ŽUlbrich and Gomes, 1981; Almeida, 1983; Gomes et al., 1990; Morbidelli et al., 1995.. The Barra do Itapirapua˜ carbonatite is located 300 km southwest of Sao ˜ Paulo city. It is one of several alkaline–carbonatitic complexes of the Ponta Grossa province present along the Ribeira Valley ŽFig. 1.. The Barra do Itapirapua˜ carbonatite was emplaced in a granite of the Late Proterozoic Tres batholith ˆ Corregos ´ ŽLapido-Loureiro and Tavares, 1983.. 7 km south-

Fig. 2. Regional schematic geological locations of alkaline– carbonatite complexes ŽBI—Barra do Itapirapua; ˜ MP—Mato Preto; IT—Itapirapua˜ . in the upper Ribeira Valley, southern Brazil Žmodified after Ronchi et al., 1993.. Bold lines indicate regional tectonic lineaments Ž1—Morro Agudo; 2—Cerro Azul..

94

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

east of the Barra do Itapirapua˜ carbonatite lies the neighbouring Mato Preto carbonatite, which was emplaced along the contact between the Tres ˆ Corregos ´ batholith and its country rocks, the metasediments of ´ the Agua Clara Formation ŽFig. 2.. The Barra do Itapirapua˜ carbonatite consists of a massif with a total outcrop area of around 2 km2 , the outline of which is strongly controlled by tectonic lineaments ŽFig. 3.. At Barra do Itapirapua˜ the outcrops consist mainly of weathered carbonatites, partially silicified magmatic breccias and fenitized granites. Most of the analysed samples were collected from drill cores in the northern part of the carbonatite complex, but surface samples were also included. The Barra do Itapirapua˜ carbonatite lacks the typical association with alkaline silicate rocks ŽGomes et al., 1990; Ruberti et al., 1997.. The presence of abundant magmatic breccias inside the carbonatite massif and the existence of subvolcanic carbonatite suggest a shallow level of erosion which is consistent with the fact that the Upper Ribeira Valley was protected against deep erosion by the Phanerozoic sediments of the Parana´ Basin until the Tertiary ŽRonchi et al., 1995.. A Rb–Sr isochron age

of 129 " 19 Ma ŽRuberti et al., 1997. places the Barra do Itapirapua˜ carbonatite at the early stage of alkaline magmatism in the region, broadly synchronous with the complexes of Jacupiranga ŽRb–Sr age s 131 " 3 Ma; Roden et al., 1985. and Juquia´ ŽK–Ar age s 127 Ma; Amaral et al., 1967.. Ore deposits directly related to the alkaline magmatism include the phosphate deposit of the Jacupiranga complex ŽOliveira and Trescases, 1985; Santos and Clayton, 1995., the phosphate and iron deposits of the Juquia´ complex ŽBorn, 1971; Walter et al., 1995., and the fluorite–barite–galena-REE deposit of the Mato Preto carbonatite ŽJenkins, 1987; Santos, 1988.. Some pre-Cretaceous fluorite deposits also present along the Ribeira Valley have been recrystallized and silicified by hydrothermal fluids related to the alkaline magmatism ŽRonchi et al., 1993, 1995, 1996.. 3.2. Petrographic oÕerÕiew A summary of petrographic features of the analysed rock types is given in Table 1. The terms ‘primary’ and ‘secondary’ in the following text are used to distinguish minerals formed by magmatic

Fig. 3. Simplified geological map of the Barra do Itapirapua˜ carbonatite, southern Brazil.

Table 1 Petrographic characteristics of the analysed rock types Rock type

Comment)

n

Mineral composition

Colour

Barra do Itapirapua˜ carbonatite

plutonic carbonatite

fresha

5

ankerite, pyrochlore

white to light grey

weakly overprinteda

4

white to light grey

overprinted Type I a

12

overprinted Type II a

9

weatheredb

4

subvolcanic carbonatite

weakly overprinteda

5

banded carbonate rock plutonic carbonatite

weatheredb freshc

1 4

ankerite, pyrochlore, and small amount Ž -1 vol.%. of secondary quartz, apatite, fluorite and RE-fluorocarbonates ankerite, relicts of pyrochlore, and secondary quartz, apatite, RE-fluorocarbonates, fluorite, barite, pyrite, galena, esphalerite ankerite, relicts of pyrochlore, and secondary apatite, quartz, fluorite, barite, pyrite, galena, sphalerite and RE-fluorocarbonates relicts of ankerite, and secondary calcite, goethite, fluorite, quartz, apatite, barite, RE-fluorocarbonates ankerite, and small amounts of quartz, apatite, RE-fluorocarbonates, fluorite richterite, dolomite, calcite, phlogopite calcite, apatite, fluorite, pyrite

light grey white to light grey

metasediment

weatheredb

5

dolomite, calcite, tremolite, diopside, sericite, quartz

grey

Mato Preto carbonatite

´ Agua Clara Formation

) Provenance of the samples. a Drill core. b Outcrop. c Open pit mine. n: Number of samples. Minerals are listed in order of their relative abundance.

brownish to yellowish

yellowish

brown

light grey

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

Lithological unit

95

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F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

crystallization from those derived from percolating late- to post-magmatic fluids. Primary minerals comprise ankerite with no evidence of recrystallization, and idiomorphic pyrochlore present as inclusions in ankerite. Secondary minerals are present in widespread corrosion cavities, and along fractures and cleavages of primary minerals. The corrosion cavities are formed by leaching of primary minerals, and cross-cut the primary igneous textures. They show no regularity in form, and range in size from mm to dm. Magmatic activity at Barra do Itapirapua˜ comprises a plutonic and a late subvolcanic event, represented by medium- to coarse-grained Ž2 to 10 mm. and fine-grained Ž- 1 mm. ankerite carbonatite, respectively. The contact between the plutonic and the subvolcanic carbonatites is sharp, and fragments of the earlier coarse carbonates are engulfed and partially assimilated by the subvolcanic matrix. The fresh plutonic carbonatite is white to light grey in colour, and contains small amounts of pyrochlore. Magmatic vesicles enclosed by idiomorphic primary ankerite indicate exsolution of volatiles during magmatic crystallization. Local grain size reduction in carbonates indicates minor brittle deformation, perhaps related to the tectonism that controlled the shape of the carbonatite complex. The subvolcanic carbonatite shows preferred orientation of prismatic carbonates in an aphanitic matrix, similar to the magmatic flow textures described by Keller Ž1989.. Small amounts of secondary minerals present in the subvolcanic carbonatites include quartz, violet-coloured fluorite and apatite, which occupy cavities - 1 mm in diameter. Additionally, the subvolcanic carbonatite contains fragments of apatite and idiomorphic smoky quartz, both identical to the apatite and early quartz present in the hydrothermally overprinted plutonic carbonatites Žsee description below.. The size, zoning patterns and fragmented nature of these minerals, along with the

lack of large corrosion cavities in the massive subvolcanic carbonatite implies that they are xenocrystic in nature. It suggests that the subvolcanic carbonatite was emplaced after the hydrothermal crystallization of idiomorphic quartz and apatite in the plutonic carbonatite. This is a very important point, since it suggests that the subvolcanic carbonatite was emplaced after the onset of the hydrothermal system that overprinted the plutonic carbonatite. Close to the contacts with the Barra do Itapirapua˜ carbonatite, the granitic wall rocks have experienced brecciation and Na-fenitization. Mineralogical features of these fenitized granites are partial substitution of quartz by carbonate, aegerine and sodian tremolite, together with epidotization of oligoclase, phlogopitization of biotite and hornblende, and formation of secondary albite. Based on the secondary mineral assemblage, four groups of overprinted plutonic carbonatites have been identified: Ža. weakly overprinted; Žb. strongly overprinted ŽType I.; Žc. strongly overprinted ŽType II.; and Žd. weathered. Weakly overprinted plutonic carbonatites are macroscopically similar to the fresh samples but contain up to 1 vol.% of dispersed secondary quartz, fluorite, apatite and rare earth fluorocarbonates. Strongly hydrothermally overprinted Type I and Type II carbonatites have slightly different mineralogical compositions. 3.2.1. Type I oÕerprinted carbonatites These contain secondary quartz, apatite, rare earth flurocarbonates, fluorite, barite and sulphides Žpyrite, galena, esphalerite.. Quartz, the most abundant hydrothermal mineral, is present in two varieties. The early quartz is smoky, idiomorphic, up to 5 cm in size and contains inclusions of apatite and fluorocarbonates along growth surfaces, whereas the late quartz is present in mosaic aggregates. Grains of these aggregates are colourless, xenomorphic, - 1 mm in size, and lack mineral inclusions. The two

Fig. 4. SEM images of hydrothermally overprinted carbonatite from the Barra do Itapirapua˜ carbonatite, southern Brazil. ŽA. Type I overprinted plutonic carbonatite, where ankerite Ž ak . is corroded and partially substituted by an assemblage of quartz Ž qz ., apatite Ž ap ., and bastnaesite Ž bs ., which locally forms pseudomorphs after ankerite Žlower left corner.; ŽB. Type II overprinted plutonic carbonatite, where ankerite Ž ak . is corroded and partially substituted by bastnaesite Ž bs . and barite Ž ba.; ŽC. Type II overprinted plutonic carbonatite, where corroded ankerite Ž ak . is overgrown by bastnaesite Ž bs . and barite Ž ba.; ŽD. weakly overprinted plutonic carbonatite, where the borders of the ankerite crystals Ž ak . are enriched in Mn and Fe Ž ak )., and the intersticial space is filled with quartz Ž qz . and bastnaesite Ž bs ..

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

97

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F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

types of quartz are frequently associated, whereby the early generation of quartz is overgrown by the fine grained mosaic. Fibrous aggregates of fluorocarbonates occur as inclusions in quartz, and locally form pseudomorphs after ankerite along the contacts between primary and secondary domains ŽFig. 4a.. X-ray diffractometry revealed the existence of bastnaesite, synchysite and parisite among the rare earth fluorocarbonates. Apatite occurs as radial aggregates of prismatic crystals, with abundant channel-like inclusions parallel to the c axis. Barite occurs as colourless xenomorphic crystals lacking mineral inclusions. Fluorite is violet, has grain sizes of - 1 mm and its fracture-related occurrence indicates a late crystallization. Pyrite, galena and sphalerite are idio- to hypidiomorphic, locally corroded, and overgrown by aggregates of fluorocarbonates. The secondary origin of sulphides is indicated by their association with quartz and other hydrothermal minerals. 3.2.2. Type II oÕerprinted carbonatites These have higher abundances of apatite, barite and fluorite than in type I ŽFig. 4b.. They contain minor amounts of rare earth fluorocarbonates ŽFig. 4c. and fine grained quartz, but lack the idiomorphic hydrothermal quartz present in the Type I overprinted carbonatites. Textural features of primary and secondary minerals are similar to those described for Type I. In both Types I and II, the X-ray diffractometric peaks of ankerite deviate slightly toward kutnohorite ŽCaMnŽCO 3 . 2 . in composition, reflecting zonations parallel to the crystal surface ŽFig. 4d. and speckled ankerite–kutnohorite crystals. Pyrochlore is present as corroded relicts of originally idiomorphic crystals. Samples of weathered carbonatite display similar hydrothermal alteration assemblages to the overprinted carbonatites from the drill cores, as indicated by the presence of fluorite, quartz, apatite, barite and rare earth fluorocarbonates. In addition, they contain small amounts of goethite and calcite. Goethite is disseminated inside and along the surface of ankerite crystals, and is more abundant in the weathered samples than in those from drill cores. Calcite is colourless in thin section, and encloses relicts of ankerite. A banded carbonate-bearing rock consisting of richterite ŽŽNa,K. 2 ŽMg,Mn,Ca. 6 Si 8 O 22 ŽOH. 2 ., dolo-

mite, calcite and phlogopite was collected from within the Barra do Itapirapua˜ carbonatite, close to its margin. This rock has a mineralogical composition different to that of the carbonatite and its trace element signature is suggestive of a non-carbonatitic origin.

4. Results 4.1. Major elements Major element analyses for the various rock types are presented in Table 2. According to the classification proposed by Woolley and Kempe Ž1989., the fresh plutonic carbonatite ranges from magnesio- to ferrocarbonatites in composition, whilst the subvolcanic carbonatite has lower Fe contents and is typically a magnesiocarbonatite ŽFig. 5.. Some of the scatter shown in the CaO–MgO– ŽFeOtotal q MnO. diagram is probably due to secondary enrichment in SiO 2 , Al 2 O 3 , P2 O5 , F and S caused by hydrothermal alteration. Overprinted plutonic samples of Type I plot in the field of fresh plutonic carbonatites because the abundant secondary quartz does not affect the proportions of CaO, MgO, and ŽFeOtotal q MnO.. Overprinted samples of Type II plot closer to the CaO apex due to high abundances of secondary fluorite and apatite. Samples from the weathered carbonatite plot in the field of calciocarbonatites due to the presence of secondary calcite and fluorite. The banded carbonate rock plots outside the fields of the fresh and overprinted carbonatites. 4.2. Trace elements Trace element abundances are presented in Table 2. The REE provide the most sensitive indicator for hydrothermal overprinting in the Barra do Itapirapua˜ carbonatite and are described in terms of total REE content Ž S REEqY . and fractionation of chondrite normalized patterns ŽŽCerYb. CN . ŽFig. 6.. The REE are subdivided into light REE ŽLREE: La, Ce, Pr, Nd, Sm, Eu. and heavy REE ŽHREE: Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.. Samples of fresh plutonic carbonatite have the lowest REE content and show weakly fractionated patterns Ž S REEqY 240–660 ppm; ŽCerYb. CN 20–40.. The weakly overprinted carbon-

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

atite samples show an enrichment in REE, mostly restricted to the LREE Ž S REEqY 1500–7800 ppm; ŽCerYb. CN 60–240.. Type I overprinted carbonatites have the highest REE contents, and display a

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narrow range of steep, highly fractionated patterns Ž S REEqY 7000–30 000 ppm; ŽCerYb. CN 100–550.. Type II overprinted carbonatites exhibit minor enrichment in LREE and a greater range of HREE

Table 2 Major and trace element analyses for the main rock types Rock type

Fresh plutonic carbonatite

Sample number

2r16.20

2r31.00

5r13.00

4r10.30

Weakly overprinted plutonic carbonatite 1r34.90

1r18.80

1r37.40

1r33.45

1r57.60

SiO 2 Ž%. TiO 2 Al 2 O 3 Fe 2 O 3 FeO MnO MgO CaO Na 2 O K 2O P2 O5 H 2O q CO 2 F S BaO ŽXRF. Total O5F O5S Total corrected

0.69 - 0.01 0.01 0.82 8.23 2.17 14.38 29.04 0.04 0.01 0.02 0.02 44.81 0.06 0.04 0.04 100.37 0.02 0.02 100.33

1.02 - 0.01 0.05 1.02 5.75 1.61 15.81 29.15 0.02 0.02 0.02 0.01 45.28 0.45 0.01 0.10 100.32 0.19 0.01 100.13

0.49 0.03 0.01 0.56 5.96 1.65 16.15 29.12 0.01 0.02 0.02 – 45.25 0.27 0.01 0.03 99.57 0.11 0.01 99.45

0.69 0.03 0.02 1.47 14.01 3.95 9.30 28.85 0.05 0.02 0.02 0.01 43.20 0.07 0.10 0.08 101.87 0.03 0.05 101.79

0.52 - 0.01 0.03 1.10 8.51 2.96 13.45 29.15 0.09 0.03 0.04 0.01 44.70 0.05 0.02 0.06 100.72 0.02 0.01 100.69

0.53 - 0.01 0.01 1.09 7.19 2.28 14.39 29.20 0.09 0.02 0.28 0.04 44.12 0.23 0.07 0.06 99.60 0.10 0.03 99.47

0.22 0.02 0.04 1.38 10.00 3.07 12.15 28.99 0.07 0.02 0.15 0.02 43.79 0.09 0.12 0.27 100.40 0.04 0.06 100.30

0.44 0.14 0.04 1.21 8.09 2.43 13.77 28.83 0.09 0.02 0.04 0.02 44.30 0.06 0.04 0.20 99.71 0.03 0.02 99.67

0.66 - 0.01 0.05 1.20 7.81 2.74 13.43 28.46 0.07 0.02 0.04 0.04 44.15 0.12 0.56 0.61 99.96 0.05 0.28 99.63

Rb Žppm. Sr Y Zr Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Pb Th U

0.3 5940 15.3 3.2 0.1 263 52.5 97.8 10.7 36.5 6.90 2.49 7.33 1.02 4.26 0.64 1.49 0.22 1.43 0.25 - 0.5 8.94 45.4 0.45

-3 8140 24.9 - 13 - 0.4 643 67.8 126 13.5 44.2 8.55 3.25 11.00 1.65 7.30 1.07 2.26 0.29 1.57 0.22 - 0.5 74.0 82.5 0.3

-3 9620 12.3 29.8 - 0.4 179 68.1 127 13.9 43.8 7.24 1.87 4.53 0.64 3.04 0.50 1.13 0.18 1.06 0.17 0.23 - 10 46.7 - 0.3

0.56 2770 12.8 5.9 0.05 495 90.9 184 22.6 81.0 12.8 3.09 6.50 0.67 3.12 0.52 1.32 0.21 1.44 0.23 - 0.5 40.6 36.6 0.25

0.42 4770 17.7 10.4 0.11 428 203.0 303 29.4 83.2 9.40 2.45 5.86 0.69 3.55 0.65 1.86 0.27 2.06 0.35 - 0.5 20.5 21.9 0.38

-3 7370 35.8 - 13 - 0.4 394 574 668 54.8 135 14.6 4.12 9.67 1.25 6.85 1.44 3.34 0.51 2.96 0.50 - 0.5 38.0 59.5 1.58

0.51 5340 48.0 18.6 0.06 1840 813 1296 136 433 56.2 13.4 26.9 2.67 11.7 1.94 4.37 0.56 3.26 0.55 0.38 31.2 119 2.04

0.33 6150 33.6 42.5 0.39 1410 1100 1340 116 325 36.5 8.8 18.1 1.87 7.97 1.32 3.21 0.46 3.00 0.49 0.64 317 102 4.43

0.55 5110 52.8 13.9 0.05 407 2580 3770 354 891 65.7 13.4 25.7 2.79 11.8 2.06 4.77 0.66 4.23 0.82 - 0.5 354 87.3 3.07

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

100 Table 2 Žcontinued. Rock type

Oveprinted plutonic carbonatite Type I

Sample number 2r40.30 2r40.90B 2r40.90A 3r59.10 3r35.00 1r54.70A 3r55.30 3r67.40A 3r54.00A 3r53.25 3r51.10 3r39.30 SiO 2 Ž%. TiO 2 Al 2 O 3 Fe 2 O 3 FeO MnO MgO CaO Na 2 O K 2O P2 O5 H 2O q CO 2 F S BaOŽXRF. Total O5F O5S Total corrected

2.27 0.08 0.20 2.85 12.97 3.31 8.60 28.09 0.06 0.03 0.14 0.26 41.66 0.20 0.30 0.30 101.31 0.08 0.15 101.08

5.22 - 0.01 0.03 10.71 3.19 3.53 10.28 26.42 0.01 0.01 0.04 1.42 36.51 0.31 0.34 2.16 100.17 0.13 0.17 99.87

0.71 0.03 0.06 0.42 14.17 3.86 7.87 28.35 0.07 0.01 0.75 0.17 41.29 0.23 0.36 2.05 100.39 0.10 0.18 100.11

2.67 - 0.01 0.21 0.94 8.45 2.44 11.78 27.71 0.07 0.02 0.85 0.15 40.74 0.24 0.30 1.91 98.47 0.10 0.15 98.22

32.31 - 0.01 0.07 1.05 8.35 2.08 5.82 18.69 0.02 0.01 0.15 0.05 27.76 0.23 0.38 1.56 98.53 0.10 0.19 98.25

3.81 0.02 0.23 0.98 9.00 2.58 11.69 27.63 0.03 0.02 0.17 0.14 41.59 0.26 0.05 0.34 98.54 0.11 0.03 98.40

2.67 - 0.01 0.05 0.75 6.93 1.96 13.05 26.99 0.09 0.02 0.05 0.04 41.55 0.27 0.58 2.44 97.43 0.11 0.29 97.03

4.88 - 0.01 0.08 0.34 7.67 2.03 11.99 26.14 0.08 0.02 0.09 0.08 40.43 0.30 0.47 2.55 97.15 0.13 0.24 96.79

2.67 - 0.01 0.06 0.76 6.67 1.77 12.49 27.23 0.10 0.02 0.91 0.01 40.42 0.37 0.61 2.68 96.78 0.16 0.31 96.32

63.43 - 0.01 0.08 0.89 1.99 0.52 3.87 8.75 0.01 0.01 0.05 0.07 13.02 0.33 0.42 2.33 95.78 0.14 0.21 95.42

12.97 - 0.01 0.08 0.19 8.23 1.91 9.55 23.46 0.06 0.02 0.25 0.09 35.69 0.38 0.58 2.73 96.19 0.16 0.29 95.74

5.08 0.05 0.20 – 9.81 2.09 10.20 25.06 0.08 0.02 0.21 0.19 38.91 0.49 0.67 3.98 97.03 0.20 0.33 96.49

Rb Žppm. Sr Y Zr Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

0.79 2250 113 72.4 0.04 1930 1650 3200 385 1310 189 50.9 125 13.2 45.3 5.39 8.68 0.93 4.67

0.21 3690 118 4.70 0.03 13400 2170 4170 511 1740 253 64.3 147 15.4 54.3 6.02 8.34 0.75 3.63

0.40 2440 331 5.40 0.05 11100 2220 4720 636 2350 339 83.5 191 22.7 99.4 14.5 27.4 2.77 13.0

-3 4790 180 - 13 - 0.4 11900 3330 4910 534 1710 215 51.7 103 10.7 44.9 7.07 14.9 1.59 7.50

-3 1450 211 22.1 - 0.4 8970 3860 6520 772 2540 315 74.1 143 14.0 58.2 8.96 15.1 1.27 5.72

0.24 3750 111 88.3 0.05 2060 4970 6740 662 1820 194 46.1 91.4 8.35 31.1 4.58 9.21 1.14 6.69

-3 6080 105 16.0 - 0.4 15300 6630 7470 642 1610 163 40.3 81.2 7.25 28.9 4.34 7.64 0.66 3.75

-3 5160 178 43.7 - 0.4 11300 6610 8460 848 2450 288 69.7 147 14.1 52.5 7.58 13.4 1.34 6.90

-3 5970 191 10.1 - 0.4 16800 7600 8880 800 2080 217 56.0 123 12.7 50.8 7.96 15.0 1.62 7.94

-3 1420 141 103 - 0.4 17200 8670 9550 811 2000 190 45.5 83.9 8.71 36.6 5.69 10.3 1.03 4.67

-3 3860 148 - 13 - 0.4 15900 8890 10600 1010 2760 306 68.8 131 11.8 42.8 6.37 11.5 1.06 5.26

-3 4830 255 47.8 - 0.4 13100 10000 12800 1350 3940 530 138 303 28.0 96.6 11.8 18.3 1.72 7.97

Lu Hf Pb Th U

0.88 - 0.5 85.6 680 2.25

0.95 - 0.5 36.9 744 0.16

2.06 - 0.5 113 1050 3.33

1.28 - 0.5 25.4 469 3.71

1.10 - 0.5 121 760 6.55

1.24 - 0.5 75.3 503 4.83

0.86 - 0.5 34.90 356 3.80

1.29 - 0.5 - 10 921 5.11

1.57 - 0.5 69.0 456 3.78

1.06 - 0.5 171 522 3.55

1.26 - 0.5 2160 669 10.8

1.77 - 0.5 77.3 1290 5.53

Rock type

Oveprinted plutonic carbonatite Type II

Sample number 5r32.50 3r64.90

5r43.00

1r12.90 5r40.00 5r8.50

5r55.70 3r69.50

5r11.00

SiO 2 Ž%. TiO 2 Al 2 O 3

2.94 0.07 0.37

11.45 0.04 0.33

2.17 0.01 0.50

0.84 0.04 0.26

0.94 0.13 0.02

0.51 0.05 0.13

2.85 0.05 0.15

1.45 0.13 0.45

0.51 - 0.01 0.17

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

101

Table 2 Žcontinued. Rock type

Oveprinted plutonic carbonatite Type I

Sample number

5r32.50

3r64.90

5r43.00

1r12.90

5r40.00

5r8.50

5r55.70

3r69.50

5r11.00

Fe 2 O 3 FeO MnO MgO CaO Na 2 O K 2O P2 O5 H 2O q CO 2 F S BaO ŽXRF. Total O5F O5S Total corrected

1.23 14.99 3.17 8.31 28.92 0.04 0.01 0.23 0.05 41.44 0.25 0.22 1.42 101.38 0.11 0.11 101.16

2.41 13.41 3.35 8.71 28.92 0.04 0.02 0.29 0.02 42.50 0.09 0.14 0.70 101.29 0.04 0.07 101.18

1.82 10.82 2.81 10.57 27.52 0.05 0.05 0.47 0.17 40.85 0.36 0.85 0.44 100.16 0.15 0.42 99.58

0.95 6.18 1.62 9.86 26.14 0.14 0.02 2.34 0.29 32.29 1.50 0.86 5.07 99.09 0.63 0.43 98.03

2.55 8.71 1.84 6.67 26.48 0.18 0.01 3.94 0.51 31.46 0.70 1.42 8.88 96.40 0.29 0.71 95.40

0.74 9.66 2.16 10.47 30.69 0.10 0.01 4.82 0.21 38.00 0.45 0.09 0.32 99.75 0.19 0.04 99.52

0.84 13.24 1.85 7.49 30.65 0.21 0.03 4.61 0.26 36.45 0.53 0.13 0.33 99.30 0.22 0.06 99.01

– 7.83 1.30 4.83 39.35 0.52 0.01 19.89 0.08 21.34 1.59 0.17 0.09 97.68 0.67 0.09 96.93

0.90 4.38 0.87 3.84 41.85 0.59 0.01 25.44 0.20 15.51 1.93 0.10 0.19 96.94 0.81 0.05 96.08

Rb Žppm. Sr Y Zr Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Pb Th U

-3 1320 61.1 17.2 - 0.4 8400 367 656 73.9 255 38.9 11.40 29.8 3.50 15.5 2.44 5.13 0.69 4.18 0.74 - 0.5 600 167 0.97

-3 2260 97.9 41.1 - 0.4 4210 1330 1800 175 522 69.4 17.0 38.1 4.42 21.2 3.54 7.65 0.99 4.71 0.72 - 0.5 54.9 196 3.22

-3 4890 150 115 - 0.4 2570 1260 2380 295 1060 160 37.9 80.8 8.84 39.7 6.22 12.2 1.54 7.79 1.05 - 0.5 59.4 418 2.43

0.43 6360 311 18.4 0.14 15400 1150 1880 208 701 116 34.3 91.2 12.8 66.6 11.7 27.1 3.32 17.2 2.39 - 0.5 198 260 4.46

-3 2910 882 42.4 - 0.4 12000 6110 9590 1150 3880 616 172 419 51.3 237 36.3 71.2 7.60 35.9 5.00 - 0.5 40.6 2200 3.14

-3 5720 715 60.0 - 0.4 2010 963 1890 239 902 170 54.4 158 25.7 149 27.3 65.5 8.27 42.0 5.30 - 0.5 94.9 582 5.09

-3 3850 1010 - 13 - 0.4 2150 1880 3540 454 1770 349 105 278 39.0 216 38.9 90.1 11.3 57.0 7.62 - 0.5 1220 1030 10.1

-3 7020 1850 - 13 - 0.4 649 1550 2900 397 1680 481 174 521 78.0 416 73.2 161 17.6 78.3 9.51 - 0.5 3570 956 12.5

-3 7360 3140 34.6 - 0.4 1280 1190 2540 356 1540 400 143 451 85.8 574 116 299 38.2 202 25.3 - 0.5 190 1510 16.7

Rock type

Weathered plutonic carbonatite

Sample number

2E

10F

1E

SiO 2 Ž%. TiO 2 Al 2 O 3 Fe 2 O 3

0.47 - 0.01 - 0.5 3.39

7.05 0.01 - 0.5 6.34

14.00 0.07 - 0.5 0.50

Subvolcanic carbonatite

Banded carbonate rock

1F

3r54.00B 1r30.50A 3r67.40B 1r30.50B 3r37.90

7B

6.26 0.06 - 0.5 3.76

1.46 0.51 0.22 0.01

28.92 0.15 1.32 1.78

0.82 0.09 0.29 –

0.62 0.35 0.94 –

4.31 0.50 0.91 0.70

1.77 0.48 1.70 0.03

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

102 Table 2 Žcontinued. Rock type

Weathered plutonic carbonatite

Subvolcanic carbonatite

Sample number

2E

10F

1E

1F

3r54.00B

1r30.50A

3r67.40B

1r30.50B

3r37.90

7B

FeO MnO MgO CaO Na 2 O K 2O P2 O5 H 2O q CO 2 F S BaO ŽXRF. Total O5F O5S Total corrected

– 2.15 0.19 51.82 0.02 0.03 0.11 0.77 39.70 0.07 0.04 0.60 99.37 0.03 0.02 99.32

– 1.27 0.17 46.69 0.05 0.05 1.04 0.84 35.36 0.27 0.05 0.45 99.64 0.11 0.03 99.50

– 0.52 0.22 35.88 0.17 0.03 0.59 0.77 7.22 13.52 3.40 19.21 96.09 5.69 1.70 88.70

– 1.29 0.92 50.92 0.05 0.03 0.47 1.06 13.92 22.56 0.18 0.94 102.42 9.50 0.09 92.83

7.63 0.94 15.06 28.99 0.09 0.01 0.59 0.20 43.13 0.22 0.74 0.25 100.05 0.09 0.37 99.59

7.77 1.05 15.29 29.47 0.02 0.02 1.16 0.32 43.31 0.14 0.43 0.21 100.39 0.06 0.22 100.11

6.58 0.61 15.55 29.54 0.03 0.01 2.36 0.55 42.76 0.23 0.50 0.07 100.70 0.10 0.25 100.36

4.40 0.69 14.98 28.70 0.03 0.01 2.51 0.60 39.83 0.07 0.08 0.28 98.61 0.03 0.04 98.54

6.06 0.61 14.73 28.40 0.04 0.01 2.57 0.81 41.66 0.24 0.12 0.17 99.39 0.10 0.06 99.23

– 0.17 18.50 21.14 2.78 1.87 0.02 1.71 21.19 0.49 - 0.02 0.07 100.11 0.21 0.00 99.90

Rb Žppm. Sr Y Zr Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Pb Th U

-3 1650 157 3.30 - 0.4 4350 674 1330 164 571 83.4 22.6 53.0 6.49 32.6 5.86 16.0 2.34 15.8 2.50 - 0.5 63.2 49.9 0.65

0.70 1680 173 - 13 0.02 3380 4670 7440 725 2050 193 42.4 91.1 8.95 36.9 6.79 17.9 2.47 17.3 3.01 - 0.5 80.7 92.1 2.79

0.30 4480 296 15.7 0.04 11600 2760 4660 471 1460 196 51.2 117 12.2 51.3 8.84 24.4 3.78 27.5 4.91 - 0.5 181 472 7.24

0.30 3880 396 19.0 0.07 6550 2640 4770 497 1580 210 56.4 131 14.3 64.5 11.6 32.7 4.92 35.9 5.98 - 0.5 859 477 15.90

-3 1150 43.5 88.4 - 0.4 1730 734 923 78.4 208 24.2 6.79 18.0 2.10 10.9 1.92 4.71 0.64 3.04 0.53 - 0.5 606 64.5 7.90

0.22 1950 122 72.3 0.49 1410 739 1270 141 481 68.6 17.7 39.1 4.53 23.2 4.32 11.2 1.58 8.64 1.17 - 0.5 751 202 30.2

-3 4610 148 179 - 0.4 517 602 937 99.1 324 48.5 13.1 32.6 4.68 26.1 5.42 14.4 2.12 12.3 1.69 - 0.5 712 174 21.4

0.29 4430 150 72.4 1.98 2010 541 957 113 424 64.5 16.1 38.6 4.81 25.7 5.12 15.0 2.26 13.0 1.67 - 0.5 423 115 13.1

-3 7960 199 14.8 - 0.4 1190 619 1090 116 361 51.3 14.6 34.4 5.01 33.4 7.66 24.2 3.83 23.2 3.20 - 0.5 71.8 129 5.72

67.4 1140 26.8 62.1 3.49 633 32.9 55.4 5.87 19.3 4.10 1.33 4.41 0.74 4.41 0.87 2.49 0.37 2.32 0.35 1.10 5.27 17.4 0.76

contents Ž S REEqY 1500–11 000 ppm; ŽCerYb. CN 3–100.. Samples of weathered carbonatite are enriched in REE, with a flattening of the patterns similar to those from the late hydrothermal stage Ž S REEqY 3000–15 000 ppm; ŽCerYb. CN 22–116.. The subvolcanic carbonatite samples show a marked

Banded carbonate rock

HREE enrichment, which leads to a high degree of REE fractionation Ž S REEqY 2000–3000; ŽCerYb. CN 12–82.. Relative to the fresh plutonic carbonatite, all other rock types are enriched in REE. Like the REE, the trace elements, with the exception of Sr, are enriched in the subvolcanic and

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

103

Fig. 5. Ternary plot for CaO–MgOŽFeO q MnO. showing carbonatite classification after Woolley and Kempe Ž1989.. Hydrothermally overprinted and weathered carbonatites deviate from the original magmatic compositional field due to leaching of ankerite and formation of Ca-bearing secondary minerals Žfluorite, apatite, calcite..

overprinted plutonic carbonatites ŽFig. 7.. Mineralogical heterogeneity of the samples and the very fine grain size of most of the secondary minerals causes significant trace element scatter in the spider diagrams. Cs, Rb, Zr and Hf occur in concentrations close to or below the detection limit, and cannot be unequivocally interpreted. P is particularly enriched in the samples of the subvolcanic and overprinted Type II plutonic carbonatites. Samples of the Type I and II overprinted carbonatite have similar trace element signatures, although Type II samples show significantly higher P and HREE values. The subvolcanic carbonatite is characterized by enrichment in Cs, U, Pb, P, Zr, Ti and HREE. The weathered carbonatite shows enrichment in P and in HREE similar to that observed in the overprinted Type II samples.

The sample of banded carbonate rock Žshown in the same plot as the weathered carbonatite. has a different composition from all of the other samples. It is depleted relative to the carbonatite in almost every trace element, except those of a typical crustal nature, such as Cs and Rb. 4.3. Carbon and oxygen stable isotopes Data for the carbon and oxygen stable isotope analyses are given in Table 3 and presented graphically in Fig. 8. Samples of the fresh and weakly overprinted plutonic carbonatite Ž d18 O q6.7 to q9.4‰; d13 C y6.4 to y5.4‰. plot in the field of primary igneous carbonatites ŽTaylor et al., 1967; Keller and Hoefs, 1995.. The subvolcanic carbonatite has d13 C of around q1‰ which is uncom-

104

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

Fig. 6. REE patterns for the Barra do Itapirapua˜ carbonatite Žchondrite composition after Anders and Grevesse, 1989..

monly heavy for carbonatites in general, accompanied by a slight enrichment in d18 O to q13.7‰. relative to the fresh plutonic

and is Žq11.7 carbon-

atite. Type I and II overprinted plutonic samples show elevated carbon and oxygen isotope ratios relative to the fresh samples and their ranges overlap

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

105

Fig. 7. Spider diagrams for the Barra do Itapirapua˜ carbonatite Žprimitive mantle composition after Hofmann, 1988..

Ž d18 O q8.1 to q12.5‰; d13 C y5.7 to y2.2‰.. For similar values of d13 C, the Type II samples display slightly heavier d18 O. Weathered carbon-

atites exhibit carbon isotope compositions Ž d13 C y5.5 to y2.9‰. similar to those of the hydrothermally overprinted carbonatites but have much heav-

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

106

Table 3 Stable isotope analyses for carbon and oxygen Rock type

Comment

Sample number

d 18 O SM OW ‰ ankerite

d 13 C PDB ‰ ankerite

d 18 O SMOW ‰ calcite

d 13 C PDB ‰ calcite

Barra do Itapirapua˜ plutonic carbonatite

fresh

2r16.20 2r31.00 4r10.30 5r13.00 1r34.90 1r18.80 1r33.45 1r37.40 1r57.60 1r54.70A 2r40.30 2r40.90A 2r40.90B 3r35.00 3r39.30 3r51.10 3r53.25 3r54.00A 3r55.30 3r59.10 3r67.40A 1r12.90 3r64.90 3r69.50 5r11.00 5r32.50 5r40.00 5r43.00 5r55.70 1E 1F 2E 10F 1r30.50A 3r37.90 3r54.00B 3r67.40B 7B

8.2 8.7 8.3 8.4 8.5 9.0 9.4 6.7 7.9 9.2 8.5 8.3 15.6 9.4 8.1 9.6 12.5 9.2 8.8 8.8 9.0 8.8 9.0 10.1 9.8 11.7 10.1 8.7 8.7 23.8 22.3 24.0 14.0 13.5 13.6 11.7 13.7 13.3

y6.0 y5.8 y5.9 y5.7 y6.1 y5.8 y5.4 y6.4 y6.1 y5.1 y5.2 y5.7 y4.9 y4.6 y5.2 y4.5 y2.2 y4.2 y5.2 y5.3 y4.9 y5.6 y5.4 y4.5 y4.6 y3.8 y2.7 y5.6 y5.3 y3.4 y2.9 y5.5 y2.9 1.1 1.1 1.1 1.5 y5.5

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – 24.6 14.6 – – – – 13.8

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – y5.4 y2.9 – – – – y5.6

MP1 MP2 MP3 MP4 AD19

8.0 13.4 13.5 8.2 16.3

y7.9 y1.2 y1.2 y6.4 y2.0

– – – – –

–

AD21 LM 25 LM 33 LM 46

17.4 17.8 14.8 14.9

y0.6 y1.3 0.0 0.8

– – – –

– – – –

weakly overprinted

overprinted Type I

overprinted Type II

weathered

Subvolcanic carbonatite

weakly overprinted

Banded carbonate rock Mato Preto plutonic carbonatite

weathered

Metasediments

weathered

´ of the Agua Clara Formation

fresh

SMOW—standard mean ocean water; PDB—Pee Dee belemnite.

– – –

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

107

d13 C value of y5.5‰, which is similar to the primary carbonatite, but has an oxygen isotopic composition Ž d18 O q13.3‰. that is somewhat heavier. Four samples of calcite carbonatite from the neighbouring Mato Preto complex and five samples ´ of carbonate-bearing metasediments from the Agua Clara Formation were also analysed for carbon and oxygen stable isotopes. Two of the Mato Preto carbonatites plot in the field of primary igneous carbonatites Ž d18 O q8‰; d13 C y6.5‰. and two are isotopically enriched Ž d18 O q13.5‰; d13 C y1.2‰.. The range for Mato Preto is similar to that for Barra do Itapirapua. ˜ The five carbonate-bearing metamor´ phic rocks from the Agua Clara Formation have heavier carbon and oxygen isotope compositions Ž d18 O q14.8 to q17.8‰; d13 C y1.2 to q0.8‰. than most of the carbonatite samples.

5. Discussion 5.1. Temporal eÕolution of the Barra do Itapirapua˜ carbonatite

Fig. 8. Oxygen and carbon stable isotope compositions for the Barra do Itapirapua˜ carbonatite and neighbouring carbonatebearing rocks ŽPIC—field of primary igneous carbonatite, after Taylor et al., 1967; Keller and Hoefs, 1995..

ier oxygen isotopic ratios Ž d18 O from q14 to q24‰.. The calcite present in samples of the weathered carbonatite is slightly enriched in 18 O compared with the coexisting ankerite Ž Dcalcite – ankerite s 0.5‰., indicating isotopic disequilibrium between these minerals. The banded carbonate rock has a

The Barra do Itapirapua˜ carbonatite has a complex geological history, comprising two magmatic stages and several different styles of post-magmatic alteration. There is no doubt that the emplacement of the plutonic carbonatites preceded that of the subvolcanic carbonatites. The magnesio- to ferrocarbonatites composition of the Barra do Itapirapua˜ complex represents a highly evolved magma, which has higher REE contents than normal calciocarbonatites ŽWoolley and Kempe, 1989; Wall et al., 1997.. The subvolcanic carbonatite has a higher Mg content than its plutonic counterpart which does not conform to the normal differentiation series, calciocarbonatite– magnesiocarbonatite–ferrocarbonatite, proposed by Woolley and Kempe Ž1989.. However, under conditions of low oxygen fugacitiy, Fe-rich melts may evolve towards Mg-rich composition along a carbonatite differentiation trend ŽLe Bas, 1989.. There are two possible explanations for the relationship between the Type I and Type II overprints: Ži. they represent different episodes of alteration or Žii. they represent different expressions of the same alteration. Whichever applies, differences in the min-

108

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

eral and chemical composition of the Type I and Type II overprints reflect changes in the temperature and composition of the hydrothermal solution. The mineral assemblage of the Type I overprinted carbonatite contains both smoky idiomorphic Žearly. and fine grained mosaic Žlate. quartz, whereas the Type II assemblage contains only small amounts of the mosaic quartz and lacks the idiomorphic crystals. This suggests that Type I predates Type II. Furthermore, the Type II overprint has lower LREE but higher HREE contents, which may indicate low-temperature remobilization of the REE. At low temperatures, HREE complexation is more effective than LREE complexation ŽBau and Moller, 1992.. The ¨ slight tendency toward a heavier oxygen isotope composition for the Type II overprinted carbonatites also provides evidence for lower temperatures prevailing during this hydrothermal event. Weakly overprinted plutonic carbonatite contains such small amounts of secondary minerals that no correlation between Type I and II is possible based on petrographic criteria alone. Nevertheless, the REE enrichment in these rocks is mainly restricted to the LREE which is similar to observed for Type I. The weakly overprinted samples may therefore be intermediate between the fresh plutonic and Type I overprinted carbonatite samples. For the weathered plutonic carbonatites, superposition of hydrothermal alteration makes it difficult to assess the effects of weathering on the trace element chemistry. With regard to the temporal relationship between magmatic and post-magmatic events, the plutonic carbonatites had already crystallized before hydrothermal overprinting. This is indicated by corrosion cavities which sharply cross cut the intercrystalline boundaries of ankerite in the coarse-grained plutonic carbonatite. On the other hand, the relationship between the emplacement of the subvolcanic carbonatite and hydrothermal alteration is less clear. The subvolcanic carbonatite contains xenocrysts of idiomorphic quartz and apatite which are identical to those present as hydrothermal minerals in Type I overprinted plutonic carbonatites. This indicates that emplacement of the subvolcanic carbonatite occurred after the onset of Type I hydrothermal overprinting. Additionally, the subvolcanic carbonatite was weakly affected by an hydrothermal overprint, with crystal-

lization of minor amounts of secondary apatite, fluorite, quartz, and enrichment in HREE and P relative to the fresh plutonic carbonatites. This suggests that the subvolcanic carbonatite may have been affected by Type II overprinting. If this is the case, the temporal evolution of the Barra do Itapirapua˜ carbonatite is as follows: early magmatism Žplutonic., early hydrothermal alteration ŽType I., late magmatism Žsubvolcanic., late hydrothermal alteration ŽType II., and finally weathering. 5.2. Stable isotopes The carbon and oxygen stable isotope compositions of carbonatites are often used as indicators for post-magmatic processes, since their fractionation is an inverse function of temperature Že.g., Hoefs, 1980.. In general, d18 O and d13 C correlate positively in carbonatites Že.g., Deines, 1989.; this is also observed for samples from Barra do Itapirapua. ˜ The fresh plutonic carbonatite plots in the range of primary igneous carbonatites ŽTaylor et al., 1967; Keller and Hoefs, 1995.. The subvolcanic carbonatite, however, has higher d13 C values of around q1‰, which are unusual for carbonatites and particularly unusual for late stage carbonatites. Volcanic or subvolcanic late stage carbonatites are often depleted in 13 C relative to the earlier magmatic rocks ŽReid and Cooper, 1992; Hubberten et al., 1988; Knudsen and Buchardt, 1991. due to partitioning of 13 C into CO 2 during degassing ŽMattey et al., 1990.. Ruberti et al. Ž1997. attributed the fractionation toward heavy carbon composition in the Barra do Itapirapua˜ carbonatite to medium- to low-temperature Ž350–808C. hydrothermal alteration. However, this does not explain the high d13 C values shown by the subvolcanic carbonatite which lacks evidence of strong post-magmatic alteration, and has well preserved magmatic flow textures. Crustal contamination may explain the anomalous carbon isotope composition content of the subvolcanic carbonatite. The absence of evidence for contamination in the coexisting fresh plutonic carbonatite may be a function of volume Ži.e., the larger volume of carbonatite magma that formed the plutonic carbonatite was able to buffer the crustal contamination more readily than the smaller amounts of melt that formed the subvolcanic carbonatite.. The

F.R.D. Andrade et al.r Chemical Geology 155 (1999) 91–113

presumed contaminant are carbonate-bearing ´ metasediments of the Proterozoic Agua Clara Forma´ tion. Although no direct contact between the Agua Clara Formation and the Barra do Itapirapua˜ carbonatite is observed in outcrop ŽFig. 2., interaction may have occurred at deeper crustal levels. Geophysical studies indicate that the contact between the metamorphic sequence and the granitic batholith dips to the west, towards Barra do Itapirapua˜ ŽP.C. Soares, 1996, personal communication.. Interaction with metasediments is further supported by the presence of the banded carbonate rock in the Barra do Itapirapua˜ plutonic carbonatite. Its trace element signature is remarkably different from that of the carbonatites suggesting that it is a xenolith of non-magmatic origin. The d13 C value of this sample, however, is similar to that of the primary carbonatite, indicating that isotopic equilibrium with the surrounding carbonatites was attained with respect to carbon. The heavier oxygen isotope composition might be due to interaction with meteoric waters as this sample was collected from outcrop rather than a drill core. A similar shift in d13 C is observed in the Mato Preto carbonatite which is emplaced along the contact ´ between the Agua Clara metamorphic rocks and the Tres granite batholith. Santos and Clayton ˆ Corregos ´ Ž1995. argued that crustal contamination by the Proterozoic carbonate-bearing metamorphic sequence was the cause of the high d13 C values of the Mato Preto carbonatite. The carbonatite samples collected at surface show a strong enrichment in 18 O due to recrystallization of carbonates under the influence of meteoric water at temperatures below 1008C ŽHubberten et al., 1988; Reid and Cooper, 1992; Simonetti et al., 1995.. 5.3. REE mineralization The REE-apatite–fluorite–barite mineralization in the Barra do Itapirapua˜ carbonatite is clearly of secondary origin, since these minerals occur in corrosion cavities of the primary ankerite carbonatite. A supergene origin for the REE mineralization can be ruled out, as it lacks the typical features of such mineralization, such as vertical zonation of alteration profiles ŽLottermoser, 1990; Morteani and Preinfalk, 1996., presence of monazite or rhabdophane as main

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REE minerals ŽMariano, 1989. and Ce anomalies in chondrite normalized REE patterns due to oxidation of Ce 3q to Ce 4q under oxidising conditions ŽBraun et al., 1990.. Furthermore, the fractionation to heavier d18 O in the weathered samples is not accompanied by an additional REE-enrichment. An in situ mechanism of solution and reprecipitation is not likely to be the main ore-forming process, as a concentration factor of up to one hundred times is required to achieve the REE content of the hydrothermally overprinted samples. Hence, an input of REE by residual magmatic fluids from the carbonatite must to be considered. REE are enriched in late stage magmatic volatiles of carbonatites, and may be effectively transported by high-temperature saline solutions ŽBanks et al., 1994.. Type I overprinted plutonic carbonatite represents the peak of the REE mineralization at Barra do Itapirapua, ˜ and contains quartz as the most abundant hydrothermal mineral. Silicification is a common feature of carbonatite-related REE ore deposits Že.g., van Wambeke, 1977; Wall and Mariano, 1996.. In the present case, the most plausible source of SiO 2 is the assimilation of the granitic wall rocks. The centimeter-sized idiomorphic crystals of smoky quartz imply available free space for crystal growth Ži.e., the cavities must have opened prior to crystallization of the hydrothermal minerals.. However, the occurrence of bastnaesite in ankerite pseudomorphs along the contact between primary and secondary domains is indicative of concomitant leaching-precipitation. It indicates that corrosion started before the main hydrothermal event, and remained active during the subsequent alteration. In order to reconcile REE enrichment and silicification, an alteration system should be considered, in which both carbonatite and crustal components participate. It is reasonable to assume that REE, phosphate and sulphur were concentrated in the residual magmatic volatiles; with SiO 2 originating from granite assimilation. The presence of nahcolite ŽNaHCO 3 . daughter minerals in H 2 O–CO 2-bearing fluid inclusions in early quartz ŽAndrade and Luders, in prep.., and the development ¨ of Na-fenitization in the granitic wall-rocks suggests the involvement of magmatic-hydrothermal fluids. Participation of fluids from the country rocks during the hydrothermal alteration is also likely to have occurred, since permeability would have been high

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in a tectonically controlled area such as the Barra do Itapirapua. ˜ Mixing between carbonatite-derived volatiles and groundwater has been described for other carbonatite-related hydrothermal systems ŽSimonetti and Bell, 1994, 1995.. Fluoride and sulphate may originate from outside the carbonatite, as fluorite and barite crystallized late in the hydrothermal mineral assemblage. Several fluorite deposits occur in the Ribeira Valley; some of which are related to carbonatites whilst others are of crustal origin ŽRonchi et al., 1993, 1996.. In the Amba Dongar carbonatite, India, a decrease in salinity and homogenization temperatures of fluid inclusions in early to late fluorites indicates that the fluorite was formed by interaction of ground water and carbonatite-derived fluids ŽSimonetti and Bell, 1995.. At Barra do Itapirapua, ˜ fluid inclusions of fluorite show low homogenization temperatures Ž1208C., low-salinity and low CO 2 content compared to the early hydrothermal quartz ŽAndrade and Luders, in prep.., suggesting fluids of upper crustal ¨ origin ŽSimonetti and Bell, 1995.. The following mechanisms of hydrothermal REE mineral formation suggested by Giere´ Ž1996. probably worked contemporaneously in the Barra do Itapirapua˜ carbonatite: Ž1. changes in pressure and temperature; Ž2. mixing of REE-bearing fluids with crustal fluids of different chemical composition; and Ž3. crystallization of gangue minerals. Pressure decrease would have occurred when the hydrothermal fluids penetrated the secondary corrosion cavities. Concomitant loss of volatiles, such as H 2 S and CO 2 , and rise in pH would induce the breakdown of REE complexes in solution ŽGiere, ´ 1990.. Additionally, reaction of hydrothermal fluids with magmatic carbonates would lead to an increase in pH. REE have higher solubilities in low pH waters ŽMichard, 1989., hence an elevated pH would favour precipitation of hydrothermal REE minerals. Decreasing fluid temperature by the mixing of magmatic aqueous solutions with cooler ground waters would destabilize aqueous REE complexes Že.g., Wood, 1990., and reduce the solubility of apatite ŽAyers and Watson, 1991. and fluorite ŽRichardson and Holland, 1979.. Thus the crystallization of fluorite and apatite would reduce the activities of the ligands Fy and PO43y and further restrict the solubility of the REE in the hydrothermal solution.

y become more Fluids with HCOy 3 and H 2 PO4 acidic by precipitating hydrogen free salts. For example:

5Ca2qq 3H 2 PO4y q Fyq 6Cly ™ Ca 5 Ž PO4 . 3F q 6HCl. Apatite crystallization in corroded pre-existing carbonates would therefore enhance the corrosion and permeability due to carbonate dissolution.

6. Conclusions The Barra do Itapirapua˜ carbonatite underwent hydrothermal overprinting which gave rise to REE mineralization. The secondary nature of the mineralization is indicated by the crystallization of RE-fluorocarbonates and gangue minerals in corrosion cavities. The petrographic features and geochemical composition of the different rock types suggest a five stages temporal evolution: Ž1. early magmatic Žplutonic.; Ž2. early hydrothermal ŽType I overprint.; Ž3. late magmatic Žsubvolcanic.; Ž4. late hydrothermal ŽType II overprint.; and Ž4. supergene Žweathering.. The presence of hydrothermal minerals as xenocrysts in the late carbonatites provides evidence for superposition of magmatic and hydrothermal events. The REE mineralization in the Barra do Itapirapua˜ carbonatite resulted from hydrothermal overprinting of the primary carbonatite. Two styles of hydrothermal overprint ŽType I and Type II. have been recognized based on distinct REE compositions and different secondary minerals assemblages. An early hydrothermal stage ŽType I. represents the peak of REE mineralization, whereby RE-fluorocarbonates crystallized along with quartz and apatite. A later hydrothermal stage Žtype II. is characterized by LREE depletion and HREE enrichment relative to the early hydrothermal stage, and is likely to have occurred at lower temperatures than Type I. Interaction between the carbonatite and carbonate-bearing metasediments is indicated by the presence of a banded carbonate rock Žxenolith. inside the carbonatite, and a trend toward anomalous high d13 C values in the late subvolcanic carbonatite. Similar contamination by crustal carbonates is observed in the neighbouring Mato Preto carbonatite.

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Weathering is superimposed on both the magmatic and hydrothermal stages and is characterized by further enrichment in 18 O due to precipitation of calcite in equilibrium with meteoric water at temperatures below 1008C. Due to high erosion rates and steep relief in the area, laterites containing significant concentrations of ore minerals are absent. Acknowledgements The authors wish to thank the Minerais do Parana´ for allowing drill cores of the carbonatite to be sampled. Additional samples where provided by E. Ruberti ŽMato Preto carbonatite., and T. Cava and ´ V. Maniesi ŽAgua Clara metasediments.. A.C. Artur gave invaluable geological and logistical support during field work. Analytical support was kindly provided by C. Wiesenberg and B. Zander ŽICP-MS., ŽXRD.. G. R. Naumann ŽXRF. and C. Gunther ¨ Berger is thanked for preparation of thin sections. W. Irber and M. Bau are thanked for helpful comments on an earlier version of this paper. J. Lindsay carefully reviewed and improved the english version of the text, and her contribution is gratefully acknowledged. The final version of this paper was greatly improved by comments from K. Bell and from an anonymous referee. F.R.D.A. is grateful to the Brazilian agency CNPq for financial support Žprocess no. 29.0051-94.0.. References Almeida, F.F.M., 1983. Relac¸oes das rochas alcalinas ˜ tectonicas ˆ mesozoicas da regiao ´ ˜ meridional da plataforma sul-americana. Rev. Bras. Geoc. 13, 139–158. Amaral, G., Bushee, J., Cordani, U.G., Kawashita, K., Reynolds, J.H., 1967. Potassium-argon ages of alkaline rocks from southern Brazil. Geochim. Cosmochim. Acta 31, 117–142. Anders, E., Grevesse, N., 1989. Abundances of the elements: meteoritic and solar. Geochim. Cosmochim. Acta 53, 197–214. Andrade, F.R.D., Luders, V., in prep. Fluid inclusions related to ¨ hydrothermal REE mineralization in the Barra do Itapirapua˜ carbonatite, southern Brazil. Ayers, J.C., Watson, E.B., 1991. Solubility of apatite, monazite, zircon, and rutile in supercritical aqueous fluids with implications for subduction zone geochemistry. Phil. Trans. R. Soc. London ŽA. 335, 365–375. Bau, M., Moller, P.M., 1992. REE fractionation in metamor¨ phogenic hydrothermal calcite, magnesite and siderite. Mineral. Petrol. 45, 231–246.

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