Precambrian Research 119 (2002) 277 /299 www.elsevier.com/locate/precamres
Petrogenetic and mineralization processes in Paleo- to Mesoproterozoic rapakivi granites: examples from Pitinga and Goia´s, Brazil Sara L.R. Lenharo ,1, Ma´rcia A. Moura, Nilson F. Botelho Universidade de Brası´lia, IG/GMP, 70910-900 Brası´lia, DF, Brazil Received 21 May 2001; received in revised form 16 November 2001; accepted 4 May 2002
Abstract ´ gua Boa and Madeira massifs */in Amazonas (northern Brazil) are within-plate, The 1.8 Ga Pitinga granites */A shallow-level rapakivi granites associated with an extensional fracture system. They comprise an early facies of pyterlitic ´ gua Boa massif) and to wiborgitic rapakivi granite, a fine- to medium-grained biotite granite, as well as topaz granite (A albite granite (Madeira massif). The granites are usually metaluminous to peraluminous, the albite granite, however, is peralkaline. They are enriched in SiO2, K2O, Na2O, F, Rb, Th, Nb, Y, Zr and the rare-earth element (REE) and impoverished in MgO, TiO2, P2O5 and Sr, as the majority of Sn-mineralized granites. Sn contents range from /1 ppm in the rapakivi facies to /3400 ppm in the albite granite. o Nd (at 1.8 Ga) values vary from /2.2 to /0.4 and Nd model ages lie between 2.4 and 2.1 Ga. Mineralization in the Madeira massif includes disseminated magmatic cryolite, zircon, cassiterite, pyrochlore, columbite-tantalite and xenotime and massive cryolite bodies in the F-rich peralkaline albite ´ gua Boa massif, Sn mineralization is associated with greisens and episyenite. The 1.77 Ga (g1) and granite. In the A 1.58 /1.57 Ga (g2) rapakivi granites of Goia´s (central Brazil) are coeval with the Araı´ rift basin. Granites of the g1 suite are metaluminous and alkaline, while the g2 suite is metaluminous to peraluminous. Both are enriched in F, Sn, Rb, Y, Th, Nb, Ga and the REE. Primary micas range from Fe-rich biotite to zinnwaldite. The micas of the more evolved granites and the metasomatic micas are strongly enriched in F, with F/Li between 2 and 10. The initial o Nd values of both suites show a considerable range (//14 to 0) and indicate substantial compositional variation in source. Sn deposits in Goia´s are hosted mainly by greisens. Indium is concentrated in quartz /topaz rock and albitized g2d granite of the Mangabeira massif and is always related to a cassiterite-sulfide association. This quartz /topaz rock is of metasomatic origin, probably generated by a hydrothermal fluid derived from the topaz /albite granite. Mineralization in the studied deposits was essentially associated with F enrichment. In the peralkaline Madeira albite granite, ´ gua Boa extremely high F contents favored disseminated mineralization, while in the Goia´s Tin Province (GTP) and A massif greisenization was related to early fluid saturation. The GTP and Pitinga granites display tectonic, petrogenetic, geochemical, isotopic and metallogenic similarities that can be applied in search of Sn and rare-metal deposits.
Corresponding author. Tel.: /55-22-2773-6565; fax: /55-22-2773-6564 E-mail address:
[email protected] (S.L.R. Lenharo). 1 Present address: LENEP-Universidade Estadual do Norte Fluminense Rod. Amaral Peixoto, Km 163, 27925-310, Macae´, RJ, Brazil. 0301-9268/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 2 ) 0 0 1 2 6 - 2
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# 2002 Elsevier Science B.V. All rights reserved. Keywords: Brazil; Proterozoic; Within-plate magmatism; Rapakivi granite; Sn; In; Rare metals
1. Introduction The majority of primary Sn and rare-metal deposits are spatially, temporally and genetically related to plutonic rocks of granitic composition, or to their volcanic equivalents. These granitic rocks typically form composite batholiths and granites associated with Sn and rare-metal deposits are commonly the most evolved and latest intrusive phases. Sn mineralization is often associated with highly fractionated ilmenite-bearing Stype granites in collisional orogens (Lehmann, 1990; Blevin and Chappell, 1992; Pitcher, 1993; Sillitoe, 1996). Another important environment is highly evolved alkaline granites in anorogenic ring complexes (A-types; Pitcher, 1993). A-type granites generally occur in continental rift environments, but they may also be found in oceanic islands and post-orogenic settings. The Precambrian granite association with the greatest capacity to generate Sn deposits are the Proterozoic rapakivi granites that form a group of anorogenic */ post-orogenic A-type granite complexes (Haapala, 1988). Sn deposits are mainly associated with raremetal mineralization and the chemical fractionation processes that concentrate Sn also increase the rare-metal contents in granitic magmas (Pollard, 1995). Rare-metal granites were distinguished in three geochemical types by Kovalenko (1978) */ Li /F, standard and agpaitic. On the basis of geochemical signature and geological setting, the rare-metal granites and pegmatites have been classified as LCT (for Li, Ce, Ta) and NYF (for Nb, Y, F) associations (Cerny´, 1991). In Brazil, intra-cratonic Proterozoic granites are responsible for the most important Sn and raremetal mineralization, accounting for /12% of western world Sn production during the past 20 years. These granitic bodies are related to anorogenic granitoids associated with major fracturing and rifting of stable cratonic zones (Botelho and Moura, 1998; Bettencourt et al., 1999; Dall’Agnol
et al., 1999). Within-plate magmatism of rapakivi affinity is widespread in the Amazonian region, covering a time interval from /1.8 to 1.0 Ga and including world-class Sn deposits associated with the earliest (Pitinga; Lenharo, 1998; Costi et al., 2000) and latest (Rondonian Tin Province; Bettencourt et al., 1999) suites. In central Brazil, this type of magmatism gave rise to a medium-class tin province coeval with the oldest Amazonian examples. Given the economic importance of the Brazilian anorogenic granites, the aim of this paper is to compare the oldest examples of mineralized Proterozoic within-plate rapakivi granites in Brazil: the intra-cratonic 1815 /1794 Ma Pitinga granites in Amazonas and the 1770 /1580 Ma rift-related granites (e.g. Pedra Branca, Mangabeira) in Goia´s. Petrographic, geochemical and isotopic features of these A-type granites are presented, focusing on their similarities, significance and type of associated Sn and rare-metal mineralization.
2. Pitinga granites 2.1. Geologic setting The Pitinga region is located in the central Amazonian geochronologic province (Tassinari, 1996; Santos et al., 2000) or in central block of the Amazonian craton (Dall’Agnol et al., 1999). The anorogenic granites show sharp and discordant contacts with their country rocks and have been interpreted as having been emplaced at 0.5 / 2.0 kbar (Dall’Agnol et al., 1994). The Pitinga granites intruded Paleoproterozoic silicic volcanic rocks of the Uatuma˜ Supergroup /Iricoume´ Group (U /Pb age of 1962/429/33 Ma, Schobbenhaus et al., 1994; and 207Pb/206Pb age of 18889/ 3 Ma, Costi et al., 2000). These volcanic rocks show, at least in some samples, consanguinity with the less evolved rapakivi facies of the Madeira massif (Lenharo, 1998).
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Fig. 1. Geological sketch map of (a) the Pitinga area and (b) GTP. Massifs in the RPS of Goia´s are: (1) Serra do Mendes; (2) Mangabeira; (3) Mocambo; (4) Pedra Branca; (5) Sucuri; (6) Soledade. Massifs in the RTS are: (7) Serra da Mesa; (8) Serra Branca; (9) Serra Dourada. Inset shows study areas relative to South America.
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´ gua Boa and Madeira massifs are elonThe A gate, parallel to one of the dominant regional orientations, and display vertical contacts (Fig. ´ gua Boa massif 1a). The latest phase of the A (topaz granite) has a dyke-like geometry. These features suggest that the emplacement of the granitic bodies involved brittle fracturing (Clarke, 1992). The latest albite granite phase of the Madeira massif shows subhorizontal and subvertical contacts and was probably emplaced in a similar manner. ´ gua Boa and Madeira massifs are multiple The A intrusions and have been described as petrographically, geochemically and isotopically similar (Horbe et al., 1991). Recent data, however, demonstrate that although the granitic massifs belong to the same suite, they display distinct petrographic and geochemical features (Lenharo, ´ gua Boa massif comprises 1998). The /350 km2 A an early facies of medium- to coarse-grained, inequigranular, mainly pyterlitic rapakivi granite, a medium- to coarse-grained biotite granite with a fine-grained marginal porphyritic phase and a late elongate and narrow body of fine-grained porphyritic topaz granite (Fig. 1a). The contacts of pyterlitic rapakivi and biotite granites with the evolved porphyritic topaz granite are transitional (Lenharo et al., 2000a). The topaz granite displays oriented phenocrysts suggesting magmatic flow. ´ gua Boa SHRIMP U/Pb zircon data for the A massif indicate ages of 17989/10 and 18159/5 Ma for the pyterlitic rapakivi facies and topaz granite, respectively (Lenharo, 1998; Lenharo et al., 1999). The /60 km2, elongated Madeira massif is ´ gua Boa by a 1 km wide corridor separated from A of volcanic rocks (Fig. 1a). It is composed of, from border to center, an early fine- to coarse-grained, equigranular to porphyritic, mainly wiborgitic rapakivi granite; a fine- to medium-grained, equigranular and locally porphyritic biotite granite, and a fine- to coarse-grained porphyritic albite granite. The latter is found as a /2.5 km2 subcircular body in the south-central part of the massif (Fig. 1a). The albite granite is composed of a nucleus facies surrounded by a ring-shaped autometasomatically altered border facies (Costi et al., 1995, 2000). SHRIMP U/Pb zircon data for the Madeira massif indicate ages of 18109/6 Ma
and 17949/19 Ma for the biotite granite and albite granite, respectively (Lenharo, 1998; Lenharo et al., 1999). Ar /Ar in micas for the albite granite indicate that the system was closed at 17829/5.2 Ma (Lenharo, 1998). Costi et al. (2000) obtained 207 Pb/206Pb zircon ages of 18249/2 and 18189/2 Ma for the rapakivi and albite granite of the Madeira massif, respectively. 2.2. Petrography The petrographic characteristics of the granite facies from Pitinga and Goia´s are summarized in Table 1. The pyterlitic and wiborgitic rapakivi ´ gua Boa and Madeira massifs are facies in the A composed of perthitic K-feldspar, quartz, plagioclase, biotite and locally amphibole. Accessory minerals include fluorite, zircon, magnetite, ilmenite, sphalerite, galena, apatite, titanite, chlorite, sericite, carbonates and minor pyrite. Rounded phenocrysts of K-feldspar typically consist of perthite core surrounded successively by plagioclase, perthite and granophyric quartz. The biotite granite facies in both massifs are composed of perthitic K-feldspar, quartz, plagioclase and biotite. Phengite is commonly observed (up to 1 vol.%) and accessory minerals include fluorite, opaque, zircon, topaz and rarely apatite. Plagioclase grains are locally absent, suggesting a near hypersolvus character. Alteration features such as swapped rims, sericitization of the micas and plagioclase and oxidation of the feldspars are ´ gua Boa common. The biotite facies of the A massif comprises three subfacies (Lenharo, 1998): North-biotite, in which plagioclase is present only as perthite and swapped rims; South-biotite, containing plagioclase with relatively calcic cores; and Topaz-biotite, which occurs in association with the topaz granite and contains calcic and sodic plagioclase and topaz. The porphyritic topaz granite of ´ gua Boa is divided into three textural subtypes A that have many similar textural features (Lenharo et al., 2000a). The albite granite nucleus facies of the Madeira massif is composed of fine-grained groundmass and phenocrysts of quartz (diameter up to 3 mm), zoned perthitic K-feldspar and subordinate riebeckite-arfvedsonite. The quartz phenocrysts have
Table 1 Petrographic features of the granitic facies of Pitinga and GTPsa ´ gua Boa massif A
Madeira massif
Goias g1 suite
Goias g2 suite
Biotite granite
Topaz granite
Rapakivi
Biotite granite
Albite nucleus
g1a
g1b
g1c
g2a
g2b
g2c
g2d
TAG
Color
Reddish
Pink
White
Reddish
Pink
Pink
Pink
Pink to grey
White
Biotite
Primary topaz
Biotite & hornblende
Biotite
Biotite (Fe/ Mg/24), REE-bearing apatite
Biotite (Fe/ Mg/100), monazite & xenotime
Pink to grey Biotite & hornblende
Pink
Biotite & hornblende
Grey to pink Biotite (Fe/ Mg /8)a
Pink
Diagnostic mineralogy
Grey to white Albite, riebeckite & polylithionite
Biotite (Fe/ Mg/8), allanite & apatite
Biotite (Fe/ Mg/24) & monazite
Zinnwaldite & Li-siderophyllite (Fe/ Mg/100)
General texture
Medium- to coarsegrained, equigranular to seriate
Medium- to coarsegrained, seriate
Fine- to coarsegrained, seriate to porphyritic
Fine- to coarsegrained, seriate and porphyritic
Fine- to coarsegrained, porphyritic
Coarsegrained, porphyritic
Coarsegrained, porphyri- tic to equigranular
Coarsegrained, porphyritic to equigranular
Coarsegrained, porphyritic to equi-granular
Mediumto coarsegrained, porphyritic to equigranular
Medium- to coarsegrained, porphyritic to equigranular
Intergrowth
Granophyric
Rare granophyric
Granophyric and dendritic
Granophyric
Fine- to mediumgrained, equigranular to porphyritic Rare granophyric
Zinnwaldite (Fe/Mg / 500), Liphengite, primary topaz Mediumgrained, equigranular
Snowball and dendritic
Granophyric
Rare granophyric
Overgrowth
Pyterlitic rapakivi and anti-rapakivi
ASIb
Metaluminous
Pyterlitic and rare wiborgitic rapakivi Metaluminous
Metaluminous
a
Wiborgitic rapakivi and anti-rapakivi
Metaluminous to peraluminous
Peraluminous
Metaluminous
Metaluminous to peraluminous
´ gua Boa and Madeira massifs; GTP: g1 and g2 suites. , Pitinga Province: A
a
Biotite (Fe/Mg) in atoms per unit formulae.
b
Aluminum saturation index, molar Al2O3/(Na2O/K2O/CaO).
Peralkaline
Mediumgrained, porphyritic
Topaz within albite
Metaluminous
Metaluminous
Metaluminous to peraluminous
Metaluminous to peraluminous
Metaluminous to peraluminous
Peraluminous
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Rapakivi
281
282
Table 2 Whole-rock geochemical analyses of Pitinga and Goia´s granites
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Table 2 (Continued )
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Major oxides and F in wt.%, trace elements in ppm; na, not analyzed; g1a through g2d denote evolutionary sequence of Goias, TAG, topaz-albite granite. a Analyses of Sucuri and Serra Dourada massifs from Bilal et al. (1997). b Representative analyses of Serra Branca massif from Pinto-Coelho (1996).
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Table 3 Characteristic geochemical features of the granite facies from Pitinga and Goia´s Pitinga granites
Goia´s granites
Rapakivi
g1 suite
Biotite granite Evolved facies
g2 suite
Topaz granite Albite granite g1a, g1b, g2a, g2b,g2c, Topaz /albite granite g1c granitesa g2d granitesb ASIc K2O/Na2O (wt.%) SiO2 (wt.%) FeOt/(FeOt/MgO) Rb/Sr 10 000 Ga/Al (La/Yb)N (Eu/Eu)N a b c
0.92 /0.98 8.98 /9.20 69 /73 0.90 /0.96 3 /12 2.81 /3.73 2.80 /10.80 0.19 /0.35
0.95 /1.07 8.13 /8.99 73 /76 0.91 /0.98 17 /75 2.87 /5.57 2.22 /15.35 0.14 /0.22
0.98 /1.15 8.55 /8.70 75 /76 0.95 /0.96 17 /108 4.51 /5.24 1.82 /2.00 0.10 /0.20
0.85 /0.86 9.90 /10.60 68 /69 0.97 /0.99 49 /164 8.48 /8.94 0.20 /0.30 0.07 /0.09
0.88 /0.98 7.35 /8.25 68 /75 0.89 /0.98 1 /19 2.68 /4.79 4.09 /14.75 0.05 /0.49
0.90 /1.30 7.33 /8.59 67 /75 0.85 /0.99 2 /55 3.23 /5.70 4.00 /9.16 0.02 /0.19
1.20 /1.50 8.19 /8.36 73 /75 0.95 /1.00 94 /182 5.99 /12.83 1.15 /2.17 0.004 /0.01
g1a, g1b, and g1c denote distinct facies of the g1. g2a, g2b, g2c, and g2d denote distinct facies of the g2. Molar Al2O3/(Na2O/K2O/CaO).
small plagioclase and cryolite inclusions indicating contemporaneous growth of quartz, albite and cryolite. Quartz phenocrysts also contain inclusions of pale blue mica, dark blue amphibole and cassiterite. The groundmass is composed of albite, K-feldspar and minor quartz, polylithionite, riebeckite-arfvedsonite and cryolite. Further accessory minerals include zircon, cassiterite, pyrochlore, columbite, xenotime, thorite and iron oxides.
2.3. Geochemistry Geochemical data on the Pitinga (and Goia´s) samples are shown in Table 2 and analytical methods used to acquire the data are described in Appendix A. Except for the peralkaline albite ´ gua Boa and Madeira granite, the granites of the A massifs range from metaluminous to peraluminous (Table 3), are enriched in SiO2, K2O, Na2O and F and depleted in MgO, CaO, MnO, TiO2, Fe2O3 and P2O5. These compositional patterns are similar to those observed in ‘tin granites’ (Tischendorf, 1977). All facies are also enriched in Rb, U, Th, Nb, Y, Zr, Hf and Pb, and relatively depleted in Sr (Table 2). The minimum Rb content in Pitinga granites is also higher than in average A-type granite (cf. Whalen et al., 1987). Sn content varies
from /1 ppm in the rapakivi facies to more than 3400 ppm in the albite granite. The rare-earth element (REE) composition of ´ gua Boa and Madeira granites indicates the A progressive enrichment in the heavy REE (HREE) from the earliest to the latest facies (Fig. 2; Table 2). The albite granite is depleted in the light REE (LREE) relative to the HREE, indicating that the HREE behaved more incom´ gua patibly in the latest facies. All facies of the A Boa and Madeira massifs show a negative Eu anomaly that increases from the earliest to the latest facies (Fig. 2). The compositional trend from the rapakivi granite through the biotite granite to the topazbearing granite is marked by increasing SiO2 and decreasing Fe2O3, MnO, MgO, CaO, TiO2 and P2O5 (Table 2). Al2O3 displays a decreasing trend from the rapakivi to the biotite facies but slightly increases from the biotite to the topaz and albite granite. In the Madeira massif, Na2O contents decrease from the rapakivi to the biotite granite ´ gua Boa and are high in the albite granite. In the A massif, Na2O contents increase from the rapakivi granite through the biotite granite to the topaz and microgranites. Zr versus TiO2 variation (Fig. 3) displays different trends for the granitic bodies and for the different facies of each body, indicating
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285
´ gua Boa massif of Pitinga; (c) Goia´s g1 suite; (d) Fig. 2. Chondrite-normalized REE patterns for (a) Madeira massif of Pitinga; (b) A Goia´s g2 suite. For Goia´s, distinct facies (g1a, g1b, g1c, g2a, g2b, g2c and g2d) are denoted. Abbreviations: rapakivi, rapakivi facies; TAG, topaz /albite granite; biotite, biotite /granite; N-biotite, north-biotite facies; S-biotite, south-biotite facies; T-biotite, topazbiotite facies; albiteNuc, albite granite nucleus facies; albiteBor, albite granite border facies; and Topaz, topaz granite.
that each facies may have evolved by internal fractionation and that some of the facies are not directly related to each other by fractionation processes. Whole-rock Nd isotopic data for the Pitinga granites indicate o Nd (at 1.8 Ga) values between /2.2 and /0.4 (Table 4) and, in line with the geochemical data, suggest variable sources, probably both crust and mantle with predominant crustal contribution. The clearly more fractionated albite granite has a similar initial value (/0.2) as the other granites and thus does not point to a distinct source for the albite granite.
2.4. Mineralization ´ gua Boa massif is Mineralization in the A associated with cassiterite /topaz/mica /quartz greisen and cassiterite-bearing sodic episyenite (Daoud, 1988; Borges et al., 1996; Costi et al., 1996). Widespread albitization of the host granites has been considered a typical pre-greisenization process (Borges et al., 1996). Greisens and sodic episyenites are spatially associated with K-feldspar-rich zones found in all facies of the massif and are predominantly related to the topaz granite and microgranite.
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´ gua Boa and Madeira massifs. Lower plot shows the Fig. 3. TiO2 vs. Zr diagram for Pitinga indicating the evolution of the A lowermost part of the upper diagram in detail. Abbreviations: N, north-biotite facies; S, south-biotite facies; T, topaz /biotite facies.
In the Madeira massif, mineralization associated with greisen and other alteration types is minor and confined to the contact of the wiborgitic rapakivi and biotite facies and the albite granite.
The albite granite contains, however, disseminated magmatic cryolite /zircon /cassiterite /pyrochlore /columbite /tantalite /xenotime mineralization. The F- and Na-rich peralkaline albite granite
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Table 4 Nd isotopic composition of Pitinga and Goia´s granites Rock type Massif
Sample
Sm (ppm) Nd (ppm)
147
Sm/144Nda
143
Nd/144Nd (9/2 S.E.)b
o Nd (T)c TDM (Ga)d
Pitinga Rapakivi Rapakivi Biotite Biotite AlbiteN.
Madeira ´ gua Boa A ´ gua Boa A Madeira Madeira
MDRG-2 AgBG-39 AgBG-49 MDRG-39 MRDG-11
13.92 21.65 9.04 13.75 13.74
75.09 118.91 49.54 66.68 24.70
0.112 0.110 0.110 0.125 0.336
0.511603 0.511579 0.511643 0.511677 0.514272
(05) (03) (05) (08) (06)
/0.8 /0.8 /0.4 /2.2 /0.2
2.16 2.16 2.07 2.35 /
Goia´s e RPS-g1 RPS-g1 RPS-g1 RPS-g1 RPS-g1 RPS-g1 RPS-g2 RPS-g2 RPS-g2 RTS-g2 RTS-g2 RTS-g2 RTS-g2 Diorite
Mangabeira Mendes Mendes Sucuri Soledade Pedra Branca Pedra Branca Pedra Branca Mocambo Serra da Mesa Serra Branca Serra Dourada Serra da Mesa Pedra Branca
MM-1A ME-08 ME-01A SU-06A SS-1 PB-188A PB-75A PB-02 MO-06 greis 84-GD-SM2 SB-322 PE-283 SM-20 PB-152
13.17 13.43 19.57 28.73 17.06 21.35 27.97 72.40 26.87 27.57 25.51 19.19 17.73 2.55
68.75 66.44 78.79 132.3 88.62 83.12 148.8 175.5 81.86 138.0 116.1 96.26 89.21 12.19
0.116 0.122 0.150 0.131 0.116 0.155 0.114 0.249 0.198 0.121 0.133 0.120 0.120 0.126
0.511386 0.511769 0.511388 0.511675 0.511479 0.511824 0.511423 0.512639 0.512307 0.511356 0.511796 0.511761 0.511572 0.512003
(05) (06) (05) (05) (04) (03) (06) (04) (06) (04) (04) (05) (03) (10)
/6.1 0.0 /13.9 /3.9 /4.3 /6.5 /3.9 /11.9 /6.3 /7.9 /3.4 /1.4 /5.0 /3.6
2.58 2.10 / 2.50 2.43 / 2.47 / / 2.77 2.34 2.07 2.38 1.80
Abbreviations, RPS, Rio Parana˜ subprovince; RTS, Rio Tocantins subprovince; Rapakivi, pyterlitic and wiborgitic rapakivi facies; Biotite, biotite facies; AlbiteN., albite granite nucleus facies. a Estimated error on 147Sm/144Nd is better than 9/0.05% (1s ) for Pitinga samples. b Normalized to 146Nd/144Nd/0.7219, estimated error on 143Nd/144Nd is better than 9/0.00001 (1s ) for Pitinga samples. c Initial o Nd values calculated at 1.8 Ga (Pitinga), 1.77 Ga (Goia´s g1) and 1.58 Ga (Goia´s g2), using chondritic ratios of 143 Nd/144Nd/0.512638 and 147Sm/144Nd/0.1967. d Depleted mantle model age calculated according to DePaolo (1981). e Data from Pimentel et al. (1999), Pimentel and Botelho (2001).
also contains two massive cryolite bodies located approximately 150 m below the roof of the granite (Lenharo et al., 1997; Costi et al., 2000). These bodies may reflect formation of immiscible fluoride melts during crystallization of the albite granite (Lenharo et al., 2000b). The albite granite can be classified as a mixed NYF /LCT fertile granite in the sense of Cerny´ (1991).
3. Goia´s tin granites 3.1. General characteristics At least three generations of Sn-related granites have been recognized in the Goia´s state: Paleo-
proterozoic syn- to post-tectonic peraluminous granites, Paleo- to Mesoproterozoic within-plate granites and Neoproterozoic syn- to late-tectonic granites (Marini and Botelho, 1986; Pimentel et al., 1991; Botelho and Moura, 1998; Pimentel et al., 1999). The northern Goia´s state includes the Goia´s Tin Province (GTP), which is separated into the Rio Tocantins subprovince (RTS) in the west and Rio Parana˜ subprovince (RPS) in the east (Fig. 1b). The main Sn deposits are related to within-plate granites that are divided in two geochemical and geochronological suites, the 1.77 Ga g1 and the 1.58 /1.57 Ga g2 (Pimentel et al., 1991; Botelho, 1992; Rossi et al., 1992). These granite suites intruded Archaean-Paleoproterozoic granite gneisses and mylonitic rocks
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Fig. 4. Geological map of the Pedra Branca massif, GTP.
and are coeval with the development of the Araı´ rift basin. Rift sequences, represented by the Araı´ Group, surround the plutons and were, together with the granites, deformed and weakly metamorphosed during the Brasiliano-Pan-African orogeny (Alvarenga et al., 2000). According to Liverton and Botelho (2001), these granites are shallow, as evidenced by miarolitic cavities, associated volcanic rocks and existence of conglomerates in the surrounding Araı´ Group with fragments of the Snmineralized granites. The g1 suite Soledade granite and cogenetic dykes of the RPS represent the rapakivi magmatism in the GTP. The rocks are dark grey,
granophyric and porphyritic with ovoidal Kfeldspar and blue quartz phenocrysts, commonly show pyterlitic and, rarely, wiborgitic textures (Table 1). Early, weakly Sn-mineralized granites (the g1 suite) are found only in the RPS. The most important Sn deposits are related to the youngest g2 suite that is present in both subprovinces (Botelho and Moura, 1998). Granites of g1 suite are potassic and show alkaline affinity, while the g2 suite granites are metaluminous to peraluminous, have lower K/Na ratios and higher Li, Rb, Sn and Ta contents (Table 2). Both suites are enriched in F, Sn, Rb, Y, Th, Nb, Ga and REE (Table 2). REE patterns of
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Fig. 5. TiO2 vs. MgO (a) and Nb vs. Ta (b) diagrams for Goia´s tin granites indicating the evolution of the g1 and g2 suites, (a) separates different facies (g1a, g1b, g1c, g2a, g2b, g2c and g2d) of the g1 and g2 suites, (b) displays general trends.
the less evolved biotite granites are highly fractionated [(La/Yb)N /20], while the later leucogranites display nearly flat REE patterns (Fig. 2). The high concentrations of Nb, Y and F, and the high Nb/Ta and F/Li ratios show that g1 and g2 comprise a fertile NYF granite/pegmatite association (cf. Cerny´, 1991). Primary micas range from very Fe-rich biotite to zinnwaldite, with compositions close to annite in the less evolved granites. The most evolved tin granites in the g2 suite are Li-siderophyllite- to zinnwaldite-bearing granite and topaz /albite granite of the Mangabeira and Pedra Branca massifs. In the RTS, evolved g2-type
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granites are present only in the Serra Branca massif (Pinto-Coelho, 1996). In both subprovinces, these granites have the highest magmatic Sn concentrations (Table 2). The massifs display prominent greisen zones in margin of leucogranite bodies (Mangabeira) or along linear strike-slip shear zones (Pedra Branca). Sn and In deposits associated with the withinplate Goia´s granites are hosted in greisenized cupolas and fractures (Serra Branca in RTS, Pedra Branca and Mangabeira massifs in RPS), albitized zones (Serra Dourada in RTS and Sucuri massifs in RPS) and greisenized granite/mylonite country rocks (Mocambo in RPS). The main mineralization was related to fluorine-rich hydrothermal alteration that gave rise to mineral associations with quartz, fluorite, topaz, phengite, siderophylite and minor zinnwaldite. The best known examples of these deposits are found in the Pedra Branca and Mangabeira massifs that are described in more detail below. Whole-rock Nd isotopic data on the Goia´s tin granites indicate initial o Nd values between /13.9 and 0 for RPS and between /7.9 and /1.4 for RTS (Table 4). There is no substantial difference in o Nd between g1 (/13.9 to 0) and g2 (/11.9 to /1.4), indicating similar, heterogeneous, source characteristic for the two suites. The initial o Nd and TDM values (Table 4) are compatible with the idea that the Goia´s tin granites were generated by melting of a heterogeneous Archean-Paleoproterozoic crust or by different degrees of mixing between mafic and felsic (crust-derived) magmas. 3.2. Pedra Branca massif The Pedra Branca massif includes five of the lithologic facies (g1b, g1c, g2b, g2c and g2d) recognized in the GTP (Fig. 4). The g1 suite is composed of biotite granites that show substantial variation in biotite composition (2 /0.5 wt.% MgO) and in the character of REE-bearing accessory minerals */apatite and allanite are found in the early facies, monazite in the late facies. The granites of the g2 suite are biotite and zinnwaldite granites that are more aluminous than the g1 granites, and are directly related to Sn mineralization.
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Fig. 6. Geological map of the Mangabeira massif, GTP. The MGZ indicates the metasomatic aureole in the g2d granite.
3.2.1. Petrography The dominant facies in the Pedra Branca massif is a biotite granite rich in zircon, apatite and allanite (named g1b by Botelho, 1992). Further accessory minerals are ilmenite, monazite and xenotime. The greisens derived from g1b facies
are rich in chlorite and magnetite and do not host Sn mineralization. The other facies are biotite granites (g1c, g2b and g2c) or biotite/zinnwaldite granite (g2d; Table 1). The g1c facies is composed of microcline, bluish quartz and rare biotite as phenocrysts and an overall composition of 40%
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microcline, 30% quartz, 25% albite and 5% biotite. Accessory minerals are fluorite, zircon, ilmenite, magnetite, allanite, monazite and thorite. g1c is commonly greisenized and the metasomatic rocks are characterized by the association of topaz-Lisiderophyllite and quartz-Li-phengite. The g2b facies is composed of strongly perthitic microcline, quartz, plagioclase and biotite. The main accessory minerals are zircon, allanite, fluorite, apatite and xenotime. g2c is similar to g2b, except for the absence of allanite and apatite and the lower MgO contents in biotite (1.5 wt.% in g2c vs. 4.5 wt.% in g2b). The g2d facies is the most evolved granitic phase in the massif with strongly perthitic microcline, quartz, albite (An0 2) and biotite. Accessory minerals are fluorite, ilmenite, zircon, monazite and thorite. The greisenized g2d facies and greisens contain light green phengitic mica, quartz, rare topaz, niobium-tantalates and Nb-rich ilmenite. 3.2.2. Geochemistry The MgO versus TiO2 diagram (Fig. 5a) can be used to discriminate the g1 and g2 suites in Pedra Branca as trends with variable slopes are observed. For a given MgO value, the g1 suite granites are more enriched in TiO2. The behavior of Ta, Th and Nb is also specific for each suite, g1 shows large variation in Nb and small variation in Ta, g2 displays opposite trends (Fig. 5b). Also, the g1 granites are always richer in Ba than the g2 granites. The g1 granites show an increase of both LREE and HREE with increasing SiO2, and display a weak decrease of LREE in the latest facies (Fig. 2). In the g2 suite, the decrease of LREE is more pronounced. 3.2.3. Mineralization The highest Sn concentrations in the RPS are found in the greisens related to the g2d granite in the Pedra Branca massif. The g2d facies has given rise to albitized and greisenized granites and greisens with or without topaz. Two mineralized zones are recognized, a 10 km2 area hosting topazLi-siderophyllite greisen swarms at the roof of a granite cupola and a 5 km long and 100 m wide fractured zone containing topaz-Li-siderophyllite greisens.
Fig. 7. TiO2 /10 000/Zr vs. Nb/Ta diagram for Pitinga (a) and Goia´s (b). TAG denotes topaz /albite granite.
3.3. Mangabeira massif 3.3.1. Petrography The Mangabeira massif is located in the RPS (Fig. 1) and is composed of a biotite granite related to the g1 suite, and a Li-siderophyllite granite and a minor topaz /albite granite of the g2 suite. The topaz /albite granite is responsible for Sn and In mineralization in the main greisenized zone (MGZ) of the massif (Fig. 6). The MGZ is composed of several g2 suite granite facies, greisens and a quartz /topaz rock. The dominant Lisiderophyllite granite is composed of quartz (30%), microperthitic microcline (30 /35%) and albite (An0, 30 /35%), and represents the less evolved facies of the g2 suite within the MGZ (Botelho, 1992). Primary Li-siderophyllite has commonly
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been transformed to phengite. The accessory minerals are zircon, monazite, magnetite and locally ilmenite. When greisenized, the granite becomes richer in phengite (10%) and contains metasomatic anhedral topaz, monazite, fluorite and disseminated cassiterite. The phengite/quartz greisen also contains late arsenates. The topaz /albite granite is intrusive into the Lisiderophyllite granite and has caused a metasomatic aureole (Fig. 6). The granite contains 35% quartz, 20% microperthitic microcline, 20% pure albite, 5/20% magmatic topaz and 10% zinnwaldite; zircon, monazite and cassiterite are rare (Table 1). Magmatic albite contains euhedral topaz inclusions (Moura and Botelho, 2000). Two types of greisen are present, one containing quartz, topaz and up to 10% zinnwaldite and the other mainly zinnwaldite. The quartz /topaz rock is found within the topaz /albite granite as a white elongated body
composed mainly of quartz, topaz, zinnwaldite, arsenopyrite and cassiterite. Other minerals include monazite, zircon, fluorite, sphalerite, wolframite, lo¨llingite, chalcopyrite, bismuthinite, galena, stannite and tennantite. Late scorodite, malachite, covellite and In, Bi, Ba, K, Pb, U and Sn arsenates are also present. Three groups of micas can be distinguished in the MGZ; these belong to the phengite-zinnwaldite series of Foster (1960). Group A micas occur in the quartz /topaz rock and the topaz /albite granite and are rich in total FeO (/10 wt.%), F (/6.0 wt.%), Rb and Mn, and are poor in Al. Group B micas in the g2 pink granite have total FeO between 5 and 9.5 wt.%, F between 2.0 and 4.5 wt.% and low Rb, Li and Mn and high Al. Group C micas are found in some metasomatized topaz /albite granites and are characterized by intermediate contents of F, FeO, Mn, Al, Li and Rb.
Fig. 8. Nd isotopic composition of the Proterozoic granites of Pitinga and Goia´s plotted in an o Nd vs. age diagram. Evolution paths of Paleoproterozoic rocks from the Sa˜o Francisco craton (Pimentel and Botelho, 2001) and the central Amazonian geochronologic province of the Amazonian craton (Tassinari, 1996) are also shown.
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3.3.2. Geochemistry The topaz /albite granite is richer in Na2O than in K2O (molar Na/K is predominantly /1), while g2d granite of the MGZ has K2O /Na2O. The less altered samples of the topaz/albite granite have A/ CNK between 1.3 and 1.5 (Table 3) and are rich in F, Li, FeO, Al2O3, Rb Zn, Ta, Nb and Sn and poor in TiO2, MgO, P2O5, CaO, Zr, Ba and Sr. The MGZ granites, together with other evolved g2 granite facies of the province, are Ta-rich, although they still contain more Nb than Ta (Botelho and Moura, 1998; Moura and Botelho, 2000). In many variation diagrams, the topaz / albite granites and the g2d granites define linear trends that have been interpreted to reflect magmatic evolution from g2d to the topaz /albite granites (Moura, 1993). In the case of Li, F, Rb and Fe, the trends reflect increasing mica specialization toward topaz /albite granite (Moura and Botelho, 1994). The MGZ granites are enriched in REE, as are the other granites in the RPS (Fig. 2). The topaz /albite granite and g2d granites, in particular, have flat REE patterns, with strong negative Eu anomalies (Table 2).
3.3.3. Hydrothermal alteration and mineralization The topaz/albite granite and g2d granites have undergone pervasive greisenization related to the emplacement of the topaz/albite granite. This involved remobilization of several elements and resulted in economic concentrations of Sn; Al2O3, SiO2, K2O, Fe2O3, P2O5, Y, Zr, F, Zn, Li, Rb, Be, Sn and W were enriched and Na2O depleted in the process. The REE, specially the LREE, were also mobile during the greisenization, probably owing to formation of F complexes which gave rise to hydrothermal monazite. Fluid inclusion data for the quartz /topaz rock (Pontes et al., 2001) have revealed two cogenetic fluid systems: H2O /NaCl and H2O /CO2 /NaCl. Field, textural and fluid inclusion data favor a metasomatic origin for this rock, involving crystallization from a hydrothermal fluid derived from the topaz/albite granite magma at P /T conditions similar to those involved in greisen formation. The Sn mineralization is mainly hosted by two types of greisen veins: Li-phengite9/quartz and Li-
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phengite9/zinnwaldite9/topaz/quartz. Cassiterite and minor wolframite are the main ore minerals. Anomalous contents of In are restricted to the quartz /topaz rock and albitized g2d granite and are always related to a cassiterite /sulphide association. Economic concentrations (30 /4100 ppm) are found only in the quartz /topaz rock. Overall, the granites of the MGZ have less than 20 ppm In, while the greisens are richer (20 ppm in the g2d greisen and 57 ppm in the greisen related to the topaz /albite granite), suggesting enrichment of In at the magmatic stage and subsequent mobilization during greisenization. In-bearing minerals present are roquesite (CuInS2), dzhalindite (In(OH)3) and yanomamite (InAsO4 ×/ 2H2O; Botelho et al., 1994). The associated cassiterite contains 0.3 wt.% In2O3 on average.
4. Discussion The Sn- and rare-metal-mineralized granites from Pitinga and Goia´s provinces have similar Nb/Ta and TiO2/Zr signatures with a characteristic decrease in TiO2/Zr from the earlier to the evolved facies (Fig. 7; Table 2). The Pitinga albite granite has very low Ti/Zr and does not have a counterpart in the GTP. This is in line with the proposed specialized source for the albite granite. Goia´s topaz /albite granite data define a separated cluster in Fig. 7b, but the evolutionary trend suggests that this granite could represent a residual liquid evolved from g2. This is also indicated by the MgO /TiO2 and Nb /Ta correlations (Fig. 5) and REE patterns (Fig. 2). Furthermore, the relations in Figs. 3 and 7 suggest that the Pitinga topaz granite is an evolved phase of the biotite granite. The overall results from Goia´s and Pitinga suggest that the late specialized peraluminous granites may have been formed by fractionation of granite magmas with alkaline affinities. However, for the most alkaline rock in the studied provinces, the peralkaline albite granite from Pitinga, no geochemical correlation with the earlier granite phases seems to exist, and a different model involving partial melting of a specific source is suggested. Lenharo (1998) proposed that the
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albite granite could either represent a residual melt from a peralkaline magma or a magma from a very peculiar source. Fractionation processes in peralkaline magma are generally invoked to lead to the increase of F, alkalis and lithophile trace elements and high HREE/LREE ratio. The only obvious case for the origin of the albite granite through differentiation would be related to the peralkaline Europa granite located /15 km northeast of the albite granite (Fig. 1; Lenharo, 1998). The south-eastern border of the Europa granite has been dated at 18299/1 Ma (Pb /Pb zircon age; Costi et al., 2000) and was not considered a valid candidate for the initial magma of the albite granite by Costi et al. (2000). Nd isotopic composition of the Pitinga and Goia´s tin granites is illustrated in an o Nd versus age diagram in Fig. 8. Initial o Nd values for the two provinces show quite different ranges. Pitinga has relatively little variation, /2.2 to /0.4, Goia´s is much more heterogeneous, /13.9 to 0. On average, the Goia´s granites are clearly less radiogenic than the Pitinga granites. Pimentel et al. (1999) interpreted the large range in the initial o Nd values of Goia´s granites as a result of different degrees of mixing of mafic and felsic magmas. The composition of the juvenile mafic component could be provided by the dioritic intrusions with o Nd (at 1.77 Ga) of /3.9 (Table 4; Fig. 8). These diorites are probably related to the basalts of the Araı´ Group that represents the mafic member of the bimodal magmatism in the rift environment. Another interpretation for the large spread in the initial o Nd values for g1 and g2 granites would be considerable heterogeneity of the crustal (sialic) source (Pimentel and Botelho, 2001). For both models, a predominant crustal component is required. In Fig. 8, Pitinga and Goia´s granites plot within the evolution path of the Paleoproterozoic rocks from the Sa˜o Francisco craton. Pitinga granites have a considerably smaller variation in initial o Nd (close to zero values) suggesting a more important contribution from the mantle or a different and more homogeneous crustal source compared with Goia´s granites. Geochemical data of Pitinga granites suggest different sources for the majority of the facies (Figs. 3 and 7) and isotopic data
indicate that these sources are almost similar in their Nd isotopic characteristics. The TDM model ages for both granitic suites are within similar range, 2.4 /2.1 Ga for Pitinga, and 2.5 /2.1 Ga for most of the Goia´s samples. However, some TDM values for the g1 and g2 Goia´s granites are older, up to 2.77 Ga, implying, together with more negative initial o Nd values, that there was a significant contribution of older crustal material. Recent structural, geochemical and geochronologic data for syn- to post-tectonic peraluminous granites and pegmatites surrounding some of the Goia´s g1 and g2 granites indicate ages of 2.2 /2.1 Ga (Botelho et al., 1999; Sparrenberger and Tassinari, 1999). These data imply that the beginning of the within-plate magmatism in Goia´s took place at least 300 Ma after the main compressive event of the Trans-Amazonian orogeny. Thus the Goia´s rapakivi granites cannot be regarded as direct products of the Trans-Amazonian orogenic processes. Similarly, the 1.65 /1.54 Ga rapakivi granites of southeastern Fennoscandia (Finland), with petrographic and geochemical characteristics similar to the Goia´s g1 and g2 suites, are 150 /250 million year younger than the post-orogenic granitic magmatism, and a direct connection between the rapakivi granites and the Svecofennian orogeny (1.9 /1.87 Ga) has been considered improbable (Haapala and Ra¨mo¨, 1992). Two different processes of metal concentration have operated in Pitinga and Goia´s, resulting in magmatic and greisen-type mineralizing systems. In Pitinga, mineralization in the Madeira massif resulted in disseminated magmatic cryolite / zircon /cassiterite /pyrochlore /columbite /tantalite /xenotime or in massive cryolite bodies in the ´ gua Boa F-rich peralkaline albite granite. In the A massif, Sn mineralization is associated with hydrothermal processes and includes cassiterite /topaz / mica /quartz greisen and cassiterite-bearing sodic pisyenite styles. In Goia´s, Sn and In deposits are related to hydrothermal processes and are hosted mainly by greisenized cupolas, greisen veins and albitized zones in the granites. The main mineralization processes were related to F-rich hydrothermal alteration and they gave rise to mineral associations with quartz, fluorite, topaz, phengite, siderophylite and minor zinnwaldite. Despite dif-
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ferences in the mineralizing processes in Pitinga and Goia´s, the concentration of metals in both regions is essentially related to F enrichment. The data obtained for the Goia´s and Pitinga deposits accord with the experiments of Manning and Pichavant (1988) who examined the system Qz /Ab /Or /H2O with F contents between 0 and 4 wt.% and noted that increasing F leads to reduction in liquidus and solidus temperatures of granitic melts. Volatile-enriched residual granitic melts may thus persist to relatively low temperatures (below 700 8C at 1 kbar) and may become enriched in incompatible elements, especially metals of economic interest. In these systems, Sn can ´ gua Boa partition with F into the melt. In the A and Goia´s Sn and rare-metal deposits, fluorine enrichment controlled the evolution of the granites at shallow crustal levels and also enhanced the concentration of metals; the majority of the mineralization occurred as a consequence of early fluid saturation and subsequent greisenization. As a result of the early metal-bearing fluid separation, Sn enrichment in the magmatic stage was not so important: the Pitinga topaz granite has 38 ppm Sn, Goia´s g2d granite 30 ppm and Goia´s topazalbite granite 25 ppm. In the Mangabeira massif, an extremely F-rich phase separated from the topaz /albite granite and partitioned the metals, giving rise to the Sn- and In-mineralized quartz / topaz rock. In the Madeira deposit, the high F contents in the albite granite (up to 9 wt.%) favored metal concentration throughout the crystallization history of the highly evolved granitic magma. Precipitation of ore minerals occurred contemporaneously with crystallization of the albite granite and led to a magmatic disseminated mineralization. The presence of magmatic cassiterite and rare-metal minerals indicates that the melt became saturated in these metals prior to fluid saturation. Thus, fluid saturation was controlled by F and alkalis, as high F increases the solubility of H2O in the melt (cf. Holtz et al., 1993). F-saturated phases, such as topaz and cryolite, may have precipitated when the fluid segregated from the granitic melt to constitute a hydrothermal phase. In this context, magmatic cryolite and
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topaz are likely quench phases for F saturation in granitic melts (Manning, 1981). Before the forma´ gua Boa greisen systems, tion of the Goia´s and A topaz precipitated in topaz granites as a magmatic phase, lowering the vapor phase solubility and leading to subsequent exsolution of metal-bearing fluids. In the Madeira albite granite, cryolite seems to be the main indicator of F saturation.
5. Conclusions The Pitinga and Goia´s granites are typical Proterozoic rapakivi granites of Brazil. A rifting environment is inferred for the emplacement of the Goia´s granites, while the Pitinga granites probably represent within-plate magmatism associated with extensional fracture systems. The TDM model ages of both Pitinga and Goia´s rapakivi provinces are in a similar range (2.5 /2.1 Ga) and the beginning of this within-plate magmatism in Goia´s took place at least 300 Ma after the main compressive event of the Trans-Amazonian orogeny. Thus, the Goia´s rapakivi suite is not considered as a direct result of that orogenic event. The Nd isotopic data indicate a predominant Archean-Paleoproterozoic crustal source component for the Goia´s rapakivi granites, while for the Pitinga granites a source with more significant mantle component or with different younger crustal characteristics is proposed. The Goia´s and Pitinga rapakivi granites have Sn and rare-metal mineralization essentially associated with specialized F-rich system. In the GTP ´ gua Boa massif of Pitinga, the and in the A specialized metaluminous to peraluminous granites seem to have been formed by fractionation of granitic magmas with alkaline affinities. The Mangabeira massif displays the most peculiar mineralization, with high concentration of In. In Pitinga, the peralkaline albite granite of the Madeira massif displays no geochemical correlation with the earlier granite phases and a different generation model is envisaged. In the Goia´s ´ gua Boa massif, the mineralizagranites and the A tion is related to greisen systems. In the albite granite of the Madeira massif the system formed a
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disseminated polymetallic body probably owing to higher amount of F and alkalis in the magma. The characteristics of the studied provinces have important implications for exploration of Sn and rare-metal deposits both on regional and local scale. Regionally, they are related to granitic magmatism in a Paleo- to Mesoproterozoic within-plate tectonic environment. Locally, the mineralization is genetically associated with the most evolved granitic facies of rapakivi magmatism, with enrichment of F and alkalis crucial for the metal concentration and style of mineralization.
Acknowledgements The authors acknowledge financial support from FAPESP (Project 95/4732-6), CAPES and CNPq (Project 400620/95-2). Thanks are also due to Geochronological Lab of the University of Brası´lia and the IGCP-426 Committee. Special thanks go to Sylvia M. Arau´jo and Fa´bio C. Mendonc¸a for helping in manuscript organization. The authors express their thanks to the journal referees, Ilmari Haapala and Valdecir de Assis Janasi, and to the editors, Jorge S. Bettencourt and O. Tapani Ra¨mo¨, for constructive and thorough reviews of this paper.
Appendix A: Analytical methods: elemental geochemistry Geochemical data on representative samples are presented in Table 2. The complete data set (in total, 140 samples on Pitinga, 150 samples on Goia´s) is available from the senior author upon request. For Pitinga, the samples were split and pulverized in both tungsten carbide and chrome steel mills. Using tungsten carbide crush, the major elements and the majority of trace elements were analyzed by X-ray fluorescence (XRF) at the Advanced Analytical Center, James Cook University, Townsville, Australia. The measurements were carried out using a Siemens XRF sequential spectrometer, SRS303, fitted with an end-window
Rh tube. Major element analysis was carried out on fused glass discs and trace element analysis was determined on pressed powder pellets. Precision, expressed at the 2s level, is better than 1% for major elements and better than 10% for most trace elements: SiO2 1.02%, TiO2 1.91%, Al2O3 1.16%, Fe2O3 1.18%, MnO 10.0%, MgO 2.7%, CaO 1.22%, Na2O 2.15%, K2O 0.66%, P2O5 2.04%, F 10.0% (detection limit 0.02%), Rb 5.7% (3 ppm), Sr 4.2% (3 ppm), Y 4.0% (3 ppm), Zr 2.6% (3 ppm), Nb 4.6% (3 ppm), Ga 4.5% (3ppm), Sn 6.5% (10 ppm), Ba 7.1% (10 ppm). Yttrium in samples from the albite granite was also determined by ICP-AES. The remaining trace elements and REE were determined by neutron activation analyses (NAA) at Becquerel Laboratories, New South Wales, Australia. Detection limits and precision are: Ta 1.0 ppm and 9.6%, La 0.50 ppm and 4.1%, Ce 2.0 ppm and 8.0%, Sm 0.20 ppm and 3.0%, Eu 0.50 ppm and 8.0%, Tb 1.0 ppm and 8.9%, Yb 0.50 ppm and 3.7%, Lu 0.20 ppm and 4.5%. Neutron activation analysis was carried out on the splits crushed in chrome steel mill. The Goia´s were pulverized in agate mill. The major elements were analyzed by XRF [SiO2, Al2O3, Fe2O3(t), MnO, CaO, K2O, P2O5] and ICP-AES (Na2O, TiO2 and MgO) at the Geo´ cole de Mines de chemical Laboratory in the E Saint-Etienne, France, and at GeoLab-Geosol and Lageq at the University of Brası´lia, Brazil. In some samples, F was determined by specific ion electrode. The majority of trace elements (Zr, Y, Sr, Ba, Rb, Nb and Sn) were also analyzed by XRF at the same laboratories. Major and trace elements analyses were carried out by lithium borate fusion and pressed powder pellets, respectively. Ta was determined by NAA at the Pierre Su¨e Laboratory, Saclay, France. REE were analyzed by ICP-AES. Precision is better than 3% for major elements and better than 10% for most trace and RE elements. Analytical methods: Nd isotopes. Nd isotopic analyses of five samples from Pitinga granites were carried out at University of Texas, Austin. Rock powders were dissolved in Teflon pressure dissolution capsules at 210 8C using HF and HNO3. These solutions were dried and re-dissolved in the same capsules in 2.3 N HCl. REE were separated from these solutions
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using standard cation exchange techniques. Nd and Sm were separated from REE fractions on HDEHP columns. All reagents were purified by sub-boiling distillation and all chemical procedures were conducted in laminar flow HEPA filtered air. Blank contributions of about 30 pg of Nd are inconsequential compared with the hundreds of ng Nd samples. Nd and Sm were analyzed as the metal on the Finnigan MAT 261 multicollector mass spectrometer using dynamic data collection routine for Nd and a static configuration for Sm. Standards ran with each set of samples included CIT B Nd and Ames Sm. The CIT B standard averages o Nd of /14.409/0.29 2s (n/40). The o Nd values at the time of emplacement are based on the SHRIMP U/Pb in zircon age of 1800 Ma derived by Lenharo (1998). The TDM model ages were calculated according to DePaolo (1981). Nd isotopic analyses of 14 samples from Goia´s tin granites were carried out at the Geochronology Laboratory of the University of Brası´lia. The Sm and Nd analytical procedures as described by Pimentel and Botelho (2001) followed the technique of Richard et al. (1976), in which separation of the REE as a group using cation-exchange columns precedes reversed-phase chromatography for the separation of Sm and Nd using columns located with HDEHP supported on Teflon powder. Recently, the laboratory is using the RE-Spec and Ln-Spec resins for REE and Sm/Nd separation. Sm and Nd were analyzed as the metal on the Finnigan MAT 262 using static configuration and a mixed 149Sm /150Nd spike. Sm and Nd samples were loaded onto Re evaporation filaments of a double filament assembly. Uncertainties on Sm/ Nd and 143Nd/144Nd ratios are considered to be better than 9/0.05% (1s) and 9/0.00001 (1s), respectively, based on repeated analyses of international rock standards BCR-1 and BHVO-1. 143 Nd/144Nd ratios were normalized to a 146 Nd/144Nd ratio of 0.7219. Nd procedure blanks were smaller than 100 pg. The o Nd values at the time of emplacement are based on the U/Pb zircon ages of 1770 and 1580 Ma derived by Pimentel et al. (1991). The TDM model ages were calculated according to DePaolo (1981).
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References Alvarenga, C.J.S., Botelho, N.F., Dardenne, M.A., Campos, J.E.G., Martins, F.A.L., Meneses, P.R., Moura, M.A., 2000. Magmatic and stratigraphic evolution of a Paleo/ Meso Proterozoic syn-rift to post-rift Basin: example of the Araı´ Basin, Brazil. In: 31st International Geology Congress, Rio de Janeiro, Brazil, Abstr. Vol. Rio de Janeiro, Geol. Surv. Brazil [CD-ROM]. Bettencourt, J.S., Tosdal, R.M., Leite, W.B., Jr, Payolla, B.L., 1999. Mesoproterozoic rapakivi granites of the Rondoˆnia Tin Province, southeastern border of the Amazonian craton, Brazil */I, Reconnaissance U/Pb geochronology and regional implications. Precambrian Res. 95, 41 /67. Bilal, E., Moutte, J., Botelho, N.F., Andrade, G.F., 1997. Geochemistry of two Proterozoic A-type granites of Goia´s state, Brazil: possible links with rapakivi series. Ann. Acad. Bras. Ci. 69, 349 /366. Blevin, P.L., Chappell, B.W., 1992. The role of magma sources, oxidation states and fractionation in determining the granite metallogeny of eastern Australia. Trans. R. Soc. Edinburgh Earth Sci. 83, 305 /316. Borges, R.M.K., Dall’Agnol, R., Costi, H.T., 1996. Petrografia ´ gua Boa-Mina do dos greisens associados ao granito A Pitinga (AM). Anais. 34th Congress of Bras. Geology, pp. 436 /438. Botelho, N.F., 1992. Les ensembles granitiques subalcalins a peralumineux mineralise´s en Sn et In de la Sous-Province Parana˜, Etat de Goias, Bre´sil. Dr thesis, Univ. Paris VI, France. Botelho, N.F., Moura, M.A., 1998. Granite-ore deposit relationships in central Brazil. J. South Am. Earth Sci. 11, 427 / 438. Botelho, N.F., Roger, G., D’Yvoire, F., Moelo, Y., Volfinger, M., 1994. Yanomamite, In (AsO4) ×/ 2H2O, a new indium mineral from topaz-bearing greisen in the Goia´s Tin Province, Brazil. Eur. J. Mineral 6, 245 /254. Botelho, N.F., Alvarenga, C.J.S., Menezes, P.R., D’el Rey Silva, L.J.H., 1999. Suı´te Aurumina: uma suı´te de granitos paleoproterozo´icos, peraluminosos e sin-tectoˆnicos na Faixa Brası´lia. Anais. Seventh Simp. Geol. Centro-Oeste, Brası´lia, p. 17 (in Portuguese). Cerny´, P., 1991. Fertile granites of Precambrain rare-element pegmatite fields: is geochemistry controlled by tectonic setting or source lithologies. Precambrian Res. 51, 429 /468. Clarke, P., 1992. Granitoid Rocks. Chapman & Hall, London. Costi, H.T., Dall’Agnol, R., Teixeira, J.T., Popini, M.V.F., 1995. The albite granite of Pitinga Mine: petrography, geochemistry and mineralizations. In: Dall’Agnol, R., Macambira, M.J.B., Costi, H.T. (Eds.), Symposium on Rapakivi Granites and Related Rocks Abstr. Vol. Feredal Univ. Para´, Bele´m, Brazil, pp. 20 /21. Costi, H.T., Dall’Agnol, R., Minuzzi, O.R.R., Teixeira, J.T., 1996. Tin-bearing sodic metasomatic episyenites associated with the Agua Boa Granite, Pitinga, Amazonian Craton, Brazil. In: Haapala, I., Ra¨mo¨, O.T., Kosunen, P. (Eds.), Seventh International Symposium on Rapakivi Granites
298
S.L.R. Lenharo et al. / Precambrian Research 119 (2002) 277 /299
and Related Rocks Abstr. Vol. Univ. Helsinki, Helsinki, Finland, p. 16. Costi, H.T., Dall’Agnol, R., Moura, C.A.V., 2000. Geology and Pb/Pb geochronology of Paleoproterozoic volcanic and granitic rocks of the Pitinga Province, Amazonian craton, northern Brazil. Int. Geol. Rev. 42, 832 /849. Dall’Agnol, R., Lafon, J.M., Macambira, M.J.B., 1994. Proterozoic anorogenic magmatism in the central Amazonian province, Amazonian craton: geochronological, petrological and geochemical aspects. Mineral. Petrol. 50, 113 /138. Dall’Agnol, R., Costi, H.T., Leite, A.A., Magalha˜es, M.S., Teixeira, N.P., 1999. Rapakivi granites from Brazil and adjacent areas. Precambrian Res. 95, 9 /36. Daoud, W.E.K., 1988. Granitos estanı´feros de Pitinga, Amazonas: Contexto geolo´gico e depo´sitos minerais associados. M.Sc. thesis, Univ. Brası´lia, Brazil, in Portuguese. DePaolo, D.J., 1981. A neodymium and strontium isotopic study of the Mesozoic calc-alkaline granitic batholiths of the Sierra Nevada and Peninsular ranges, California. J. Geophys. Res. 86, 10470 /10488. Foster, M.D., 1960. Interpretation of the composition of lithium micas. US Geol. Surv. Prof. Pap. 354-E. Haapala, I., 1988. Metallogeny of the Proterozoic rapakivi granites of Finland. In: Taylor, R.P., Strong, D.F. (Eds.), Proceedings of the CIM Conference on Granite-Related Mineral Deposits, vol. 39, Canadian Institute of Min. Met. Spec., pp. 124 /132. Haapala, I., Ra¨mo¨, O.T., 1992. Tectonic setting and origin of the Proterozoic rapakivi granites of southeastern Fennoscandia. Trans. R. Soc. Edinburgh Earth Sci. 83, 165 /171. Holtz, F., Dingwell, D.B., Behrens, H., 1993. Effects of F, B2O3, and P2O5 on the solubility of water in haplogranite melts compared to natural silicate melts. Contrib. Mineral. Petrol. 113, 492 /501. Horbe, M.A., Horbe, A.C., Costi, H.T., Teixeira, J.T., 1991. Geochemical characteristics of cryolite-tin-bearing granites from the Pitinga mine, northwestern Brazil */a review. J. Geoch. Explor. 40, 227 /249. Kovalenko, V.I., 1978. The genesis of rare-metal granitoids and related ore deposits. In: Stemprok, M., Burnol, L., Tischendorf, G. (Eds.), Metallization Associated with Acid Magmatism, vol. 3, Geol. Surv. Prague, Tscheckoslovakia, pp. 235 /247. Lehmann, B., 1990. Metallogeny of Tin, Lecture Notes in Earth Sciences, vol. 32. Springer, Heidelberg. Lenharo, S.L.R., 1998. Evoluc¸a˜o magma´tica e modelo metalogene´tico dos granitos mineralizados da regia˜o de Pitinga, Amazonas, Brasil. Dr thesis, Univ. Sa˜o Paulo, Brazil, in Portuguese. Lenharo, S.L.R., Pollard, P.J., Born, H., 1997. Pitinga Tin Mine, Amazonas, Brazil: I. A unique giant rare-metals, tin, cryolite resource. In: Taylor, R.G., Willians, P. (Eds.), EGRU News. Townsville, Australia, pp. 4 /7. Lenharo, S.L.R., Born, H., Pollard, P.J., 1999. Petrologic evolution and emplacement of the mineralised Pitinga granites, Brazil. Doc. BRGM 290. In: Barbarin, B. (Ed.), The Origin of Granites and Related Rocks, Fourth Hutton
Symposium Abstr. Vol. Documents du BRGM 290, 236. Lenharo, S.L.R., Pollard, P.J., Born, H., 2000a. Matrix rock texture in the Pitinga topaz granite. Rev. Bras. Geoci. 30, 238 /241. Lenharo, S.L.R., Pollard, P.J., Born, H., 2000. Magmatic cassiterite and cryolite mineralization in the peralkaline albite granite from Pitinga. In: Thirty-first International Geology Congress, Rio de Janeiro, Brazil, Abstr. Vol. Rio de Janeiro, Geol. Surv. Brazil [CD-ROM]. Liverton, T., Botelho, N.F., 2001. Fractionated alkaline raremetal granitoids: two examples. J. Asian Earth Sci. 19, 399 / 412. Manning, D.A.C., 1981. The effect of fluorine on liquidus phase relationships in the system Qz /Ab /Or with excess water at 1 kb. Contrib. Mineral. Petrol. 76, 206 /215. Manning, D.A.C., Pichavant, M., 1988. Volatiles and their bearing on the behaviour of metals in granitic systems. In: Taylor; R.P., Strong, D.F. (Eds.), Recent Advances in the Geology of Granite-Related Mineral Deposits, vol. 39, Canadian Institute of Min. Met. Spec. pp. 13 /24. Marini, O.J., Botelho, N.F., 1986. A provı´ncia de granitos estanı´feros de Goia´s. Rev. Bras. Geoci. 16, 119 /131 (in Portuguese). Moura, M.A., 1993. A zona greisenizada principal do macic¸o estanı´fero Mangabeira (GO): geologia, petrologia e ocorreˆncia de I´ndio (In). M.Sc. thesis, Univ. Brası´lia, Brazil (in Portuguese). Moura, M.A., Botelho, N.F., 1994. Estudo de micas litinı´feras e sua importaˆncia para a caracterizac¸a˜o das rochas da zona greisenizada do macic¸o estanı´fero Mangabeira, GO. Bol. Geoci. Centro-Oeste 17 (1/2) pp. 39 /48. Moura, M.A., Botelho, N.F., 2000. The topaz /albite granite and related rocks from Sn/In mineralized zone of Mangabeira Granitic massif (GO), Brazil. Rev. Bras. Geoci. 30, 270 /273. Pimentel, M.M., Botelho, N.F., 2001. Sr and Nd isotopic characteristics of 1.77 /1.58 Ga rift-related granites and volcanics of the Goia´s tin province, central Brazil. An. Acad. Bras. Ci. 73, 263 /276. Pimentel, M.M., Heaman, L., Fuck, R.A., Marini, O.J., 1991. U /Pb zircon geochronology of Precambrian tin-bearing continental type acid magmatism in central Brazil. Precambrian Res. 52, 321 /335. Pimentel, M.M., Fuck, R.A., Botelho, N.F., 1999. Granites and the geodynamic evolution of the Neoproterozoic Brasilia belt, central Brazil. Lithos 46, 463 /483. Pinto-Coelho, C., 1996. Evolution magmatique et hydrotermale du massif granitique de Serra Branca */Etat de Goia´s */ Bre´sil: definition des processus da´lteration tardi/post */ magmatiques en liaison avec les mineralisations en Sn, Be et F. Dr thesis, L’I´nstitut Nacional Polytechnique de Lorraine, France. Pitcher, W.S., 1993. The Nature and Origin of Granite. Chapman & Hall, London. Pollard, P.J., 1995. Geology of rare-metal deposits: an introduction and overview. Econ. Geol. 90, 489 /494.
S.L.R. Lenharo et al. / Precambrian Research 119 (2002) 277 /299 Pontes, R.M., Moura, M.A., Botelho, N.F., Beny, C., 2001. Geˆnese do topazito do Macic¸o granı´tico Mangabeira (GO), mineralizado em estanho (Sn) e ´ındio (In), com base em incluso˜es fluidas. Boletim de Resumos. VIII Cong. Bras. Geoq., Curitiba, p. 21 (in Portuguese). Richard, P., Shimizu, N., Alle`gre, C.J., 1976. 143Nd/144Nd, a natural tracer: an application to oceanic basalts. Earth Planetary Sci. Lett. 31, 269 /278. Rossi, P., Andrade, G.F., Cocherie, A., 1992. The 1.58 Ga Atype granite of Serra da Mesa (GO): an example of ‘NYF’ fertile granite pegmatite. Res. Expand. 37th Cong. Bras. Geol., SBG, Sa˜o Paulo, pp. 389 /390. Santos, J.O.S., Hartmann, L.A., Gaudette, H.E., Groves, D.I., McNaughton, N.J., Fletcher, I.R., 2000. A new understanding of the Amazon craton provinces based on integration of field mapping and U/Pb and Sm/Nd geochronology. Gondwana Res. 3 (4), 453 /488. Schobbenhaus, C., Hoppe, A., Lork, A., Baumann, A., 1994. Idade U/Pb do magmatismo Uatuma˜ no norte do Cra´ton Amazoˆnico, Escudo das Guianas (Brasil): primeiros resul-
299
tados. Anais. 38th Cong. Bras. Geol., Camboriu´, pp. 395 / 397, in Portuguese. Sillitoe, R.H., 1996. Granites and metal deposits. Episodes 19 (4), 126 /133. Sparrenberger, I., Tassinari, C.C.G., 1999. Subprovı´ncia do Rio Parana˜ (GO): um exemplo de aplicac¸a˜o dos me´todos de datac¸o˜es U /Pb e Pb /Pb em cassiterita. Rev. Bras. Geoci. 29, 405 /414 (in Portuguese). Tassinari, C.C.G., 1996. O mapa geocronolo´gico do Cra´ton Amazoˆnico: revisa˜o dos dados isoto´picos. Livre Doceˆncia thesis, Univ. Sa˜o Paulo, Brazil, in Portuguese. Tischendorf, G., 1977. Geochemical and petrographic characteristics of silicic magmatic rocks associated with rare element mineralization. In: Stemprock, M., Burnol, L., Tischendorf, G. (Eds.), Mineralization Associated with Acid Magmatism, vol. 2, Geol. Surv., Prague, pp. 41 / 96. Whalen, J.B., Currie, K.L., Chappell, B.W., 1987. A-type granites: geochemical characteristics, discrimination and petrogenesis. Contrib. Mineral. Petrol. 95, 407 /419.
