Rare earth element and yttrium geochemistry applied to the genetic study of cryolite ore at the Pitinga Mine (Amazon, Brazil)

This work aims at the geochemical study of Pitinga cryolite mineralization through REE and Y analyses in disseminated and massive cryolite ore deposits, as well as in fluorite occurrences. REE signatures in fluorite and cryolite are similar to those in the Madeira albite granite. The highest 6REE values are found in magmatic cryolite (677 to 1345 ppm); 6REE is lower in massive cryolite. Average values for the different cryolite types are 10.3 ppm, 6.66 ppm and 8.38 ppm (for nucleated, caramel and white types, respectively). Disseminated fluorite displays higher 6REE values (1708 and 1526 ppm) than fluorite in late veins (34.81ppm). Yttrium concentration is higher in disseminated fluorite and in magmatic cryolite. The evolution of several parameters (REEtotal, LREE/HREE, Y) was followed throughout successive stages of evolution in albite granites and associated mineralization. At the end of the process, late cryolite was formed with low REEtotal content. REE data indicate that the MCD was formed by, and the disseminated ore enriched by (additional formation of hydrothermal disseminated cryolite), hydrothermal fluids, residual from albite granite. The presence of tetrads is poorly defined, although nucleated, caramel and white cryolite types show evidence for tetrad effect.


INTRODUCTION
Cryolite (Na 3 AlF 6 ) is one of the fluorine minerals of major economic importance due to its utilization for aluminum metallurgy. However, this mineral is so rare that, until now, it had only been exploited economically in Ivigtut (Greenland), from the beginning of last century until reserve exhaustion in 1986. The Pitinga Mine, Amazon (Fig. 1), is the second case of economic cryolite The Pitinga ore deposit is associated with bite granite facies of the Madeira granite. It is a class Sn deposit containing Nb, Ta and cryolite products, as well as Zr, rare-earth elements (RE Li and U, which may be exploited as sub-produc serves of disseminated ore are 164 million tons, w erage Sn contents of 0.14%, Nb 2 O 5 of 0.20% and of 0.024%. Cryolite mineralization occurs as d inated ore (reserves around 110 Mtons, with 4 720 ORLANDO R.R. MINUZZI et al. in Brazil. Sn exploitation initially proceeded upon the alluvial ore. Subsequently the weathered primary ore (devoid of cryolite) was exploited, and currently production is focused on cryolite-bearing ore deposits, slightly weathered or unweathered. This work offers a contribution to a genetic model for the cryolite mineralization, based on REE and Y geochemistry in cryolite from the disseminated and mas-system, since fluorite, and not cryolite, is present in the granite border and because the relation between REE and fluorite is better constrained than REE and cryolite. REE signatures in fluorite depend on REE signatures in hydrothermal fluids. acid volcanic and pyroclastic rocks from the Iricoumé Group, with zircon 207 Pb/ 206 Pb age of 1888±3 Ma (Costi 2000). Two age groups of A-type granites intrude the Iricoumé Group. The granite bodies of the Mapuera suite are associated to Iricoumé volcanic rocks and display similar ages (Ferron et al. 2006). The latter Madeira and Água Boa multi-phase intrusive bodies of the Madeira Suite  stand out for bearing primary Sn ore. Costi (2000) divided the Madeira granite into four facies (Fig. 2) and determined their zircon 207 Pb/ 206 Pb ages: a) amphibole-biotite syenogranite (RK) (1824 ± 2 Ma), b) biotite-K feldspar granite (BG) (1822 ± 2 Ma), c) porphyritic hypersolvus K feldspar granite (HG) (1818 ± 2 Ma) and d) albite granite (Fig. 3). Though no zircon age has been obtained for the albite granite, it is considered as co-magmatic to the hypersolvus granite facies, based on field and geochemical criteria (Lenharo 1998, Costi 2000. of quartz, albite and alkali feldspar, of subord by cryolite, zircon, polythionite, riebeckite, pyro iron-rich mica, cassiterite and magnetite. Alo contact between the albite granite and the host occurs the border albite granite (BAG). The la peraluminous and comprises essentially quartz, sium feldspar and albite, with fluorite, zircon, ch cassiterite, hematite and columbite. Modal prop of essential phases vary. The BAG displays an in in quartz and decrease in albite content in rela the CAG. The CAB-BAG contact is gradual. L a transitional subfacies (TAG) is well developed BAG is interpreted by Costi (2000) as derived auto-metasomatism of the CAG, the latter hav peralkaline mineralogy modified by the act residual fluids.

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ORLANDO R.R. MINUZZI et al. ture), and interstitial, within the CAG matrix. These features have been interpreted as evidence of rapid crystallization, contemporaneous to the phenocrysts and continuous into the subsequent matrix formation (Lenharo 1998). Costi (2000) reported that most often crystals are fine-to medium-grained, anhedral and rounded, disseminated within the matrix of porphyritic rocks, or intergrown with albite in the finest grain-size fraction of rocks with seriate texture, with straight to concave-convex boundaries with albite and K-feldspar. This suggests early crystallization and paragenetic sta-medium-to fine-grained, associated with other minerals related to the final stages of crystallization. Minuzzi et al. (2006) corroborated the previous descriptions, emphasizing that, in the CAG around the Massive Cryolite Deposit (MCD) (Fig. 4), disseminated cryolite is particularly more abundant as agglomerates with white mica, zircon and quartz, and as aureoles around magmatic and pyrochlore, displaying reaction rims with these minerals. The latter two cryolite types have been considered post-magmatic, related to a hydrothermal stage.

MATERIALS AND METHODS
Separation of disseminated cryolite and fluorite was carried out in rock samples. Petrographic studies identified two CAG samples with predominantly magmatic cryolite, and three samples with predominantly late, disseminated cryolite. In the case of BAG, where primary paragenesis was largely replaced by a secondary paragenesis (Costi 2000), it was not possible to distinguish between primary and secondary fluorite. Hence selected samples were those with the highest mineral content. Sampling was carried out in drill cores, taking into account the presence of three types of cryolite (nucleated, caramel and white cryolite). Samples were crushed and powdered to a grain size smaller than 0.297 mm. Three grain size fractions (0.178 mm, 0.131 mm and 0.089 mm) were obtained through sieving. The fraction greater than 0.131 mm was used for dense-medium separation with LST (lithium heteropolytungstates). The concentrates were processed with a Frantz isodynamic separator and subsequently separated manually with the aid of a binocular microscope.
Ca. 100 mg of the cryolite and fluorite sample were solubilized by acid attack (5 mL deionized water + 5 mL sub-boiling distilled HNO 3 and 10 mL subboiling distilled HF) in a microwave oven for 1 hour at 120 • C and 100 Psi. In the next step, HF was eliminated by heating in a closed system using a mixture of 5 mL 85% H 3 PO 4 and 5 mL HNO 3 . The material was then redisolved with 5 mL HNO 3 and diluted to a final weigh of 50 g with deionized water immediately before analysis. For some cryolite sample with lower total masses between 10 and 30 mg, the same procedure was adopted, and the final dilution maintained the same solid to liquis ratio. The analyses were carried out by ICP-MS in a Perkin Elmer/Sciex ELAN 6100 DRC equipment at the Instituto de Geociências, Universidade de São Paulo, Brazil. Calibration curves were constructed using reference materials BE-N, DR-N, OU-1, OU-2 and JA-1; drift correction was applied by monitoring the variation of the signal of reference Analyses of REE and Y in bulk rock samples were performed at Lakefield -Geosol Lab in Belo Horizonte, Minas Gerais State (Brazil), by ICP-OES method -following multi-acid digestion. Specific ion method was used for F determination. REE results were normalized to C-1 chondrite (Anders and Grevesse 1989).

REE IN GRANITIC FACIES (MADEIRA GRANITE)
Total REE values in the Madeira Granite (Table I) is extremely variable, between 7.6 and 1028 ppm, considering the results presented here and the analytical results provided by Lenharo (1998) andCosti (2000).
Comparing the REE distribution patterns (Fig. 5), BG, RK and HG facies are relatively enriched in REE. RK has strongly fractionated REE patterns and positive fractionation between MREE and HREE. This is in sharp contrast with biotite granite BG, in which Gd/Yb is ∼ 1, La/Yb is much lower, there is strong variation in HREE contents and some patterns ten to "seagull wing". The CAG exhibits a negative anomaly of Y if we compare with Dy and Ho. CAG subfacies displays REE values between 7 and 838 ppm, with a relative enrichment in HREE, and a "seagull wing" pattern. The main difference between CAG and BT-KF is that the first is depleted in the LREE. REE patterns of TAG and BAG are varied. Four samples (PMR-7A, 8, 10 and 18) are particularly depleted. The other three have REE patterns do not differ too much from the CAG and are similar to BAG patterns presented by Costi (2000).
An Eu negative anomaly characterizes all granitic facies, though it is more intense in albite granites, probably because they represent facies that have undergone a higher degree of evolution with presence of a larger volatile contents. The core albite granite (CAG) has, in general, higher fluorine content than the border albite granite (BAG), what suggests a larger volatile content in the former.

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ORLANDO R.R. MINUZZI et al. seems to be relatively important only in disseminated hydrothermal cryolite samples. The diverse total REE values for disseminated cryolite samples (Table II) indicate the existence of two groups: the first one, with higher REE content (677 ppm to 1345 ppm), corresponds to magmatic cryolite. The second, with much lower values (29 ppm to 45 ppm), comprises samples of hydrothermal disseminated cryolite. MCD cryolite displays very low total REE values (Table III). Average values are 10.3 ppm, 6.66 ppm and 8.38 ppm for the nucleated, caramel and white types, are similar to those from the core albite granite (CAG) (Fig. 5), which indicate genetic relationship. Cryolite displays a progressive HREE depletion from hydrothermal disseminated to white cryolite. Tetrad effects may be observed in the nucleated, caramel and white cryolite types, especially in the first tetrad. The tetrad effects are probable related to the different complex formation of REE and F (Irber 1999).
Disseminated and nucleated cryolites have almost constant LREE (Fig. 6), increasing in HREE and conspicuous negative Eu anomaly. Caramel cryolites ex- shows a unique pattern including a positive anomaly of ytterbium. The white cryolites (just two samples) also display positive anomaly of ytterbium, but have different behaviors in respect to LREE: One 7CA (FC-12) shows constant LREE and the other 7C (FE-12) reveals irregular shape of LREE.
The unique pattern of HREE and positive ytterbium anomalies are probably related to tetrad effects in caramel and white cryolite. We inserted Y patterns between Dy and Ho according to Bau and Dulski (1995). It is observed in disseminated cryolite a high negative Y anomaly relative to Dy and Ho. Caramel and white cryolites also display the same behavior. Y and Ho have ionic charge and radius very similar but the electronic configurations are quite different justifying the dissimilar behavior of these anions in presence of some ligands as in magmatic cryolite (Table II). The three cryoli erations in MCD display Eu anomalies systema less pronounced (∼ 0.16).

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ORLANDO R.R. MINUZZI et al. evolution marked by decreasing Y concentration from late disseminated cryolite to nucleated and caramel to white cryolite (Fig. 7b).

REE AND Y IN FLUORITE
Fluorite displays a variety of REE. Fluorite with magmatic (disseminated) characteristics shows values of 1500-1700 ppm; hydrothermal disseminated fluorite of 100-700 ppm and veinlet (very late) fluorite around 28 ppm (Fig. 6). These values represent fluorite precipitation in three events, with different compositions and REE depletion in very late fluorite. REE signature in fluorite is very similar to that of granites (CAG), with a characteristic negative Eu anomaly, in general without enrichment in heavy REE, except In the case of Y, concentrations lie along three ranges: 1200 ppm (disseminated magmatic fluorite); 200 ppm (hydrothermal disseminated fluorite) and 34 ppm (late vein fluorite). Low yttrium concentrations are thus typical of late phases in both minerals (cryolite and fluorite). Fluorite show slight positive anomalies of Y except one sample (PMR-2) which presents a negative anomaly.

DISCUSSION
There are very few studies concerning genetic models for Pitinga cryolite mineralization. Most models focus either on granite petrology or on Sn mineralization. Existing genetic models for cryolite in granites include one metasomatic model (Horbe et al. 1985, Teixeira et al. the silicate liquid, and the second by Costi (2000) who considered that increasing water content as the crystallization of the albite granite progressed led to the separation of aqueous fluids responsible for the formation of coarse-grained (pegmatoid) rocks in the CAG, whereas a F-rich residual phase formed the massive cryolite bodies of the MCD.
Experimental results prior to 1980 showed that Kovalenko 1977); these were confirmed ultimat more detailed results (Manning 1981, Pichava Manning 1984, Weidner and Martin 1987, Picha al. 1987, Manning and Pichavant 1988, London Xiong et al. 1999  Interstitial cryolite is also considered as the product of recrystallization of late stage, F-rich, pervasive hydrothermal fluids in the Nigerian anorogenic alkali granites (Bowden and Kinnaird 1984).
In the albite granite, the petrologically most evolved facies of the Madeira granite (Costi 2000), high fluorine activity allowed for formation of early (magmatic) cryolite on one hand and superposition of 1900 ppm and certainly a parallel high activity of F is responsible by the formation of F -, Al 3+ and REE complexes, especially in peralkaline granite, including the [AlF 6 ] -3 , a necessary complex for the cryolite formation.
The overall behavior of REE in the facies of the Madeira granite shows a strong enrichment in HREE and greater negative Eu anomalies in albite granite as compared to RK, BG and HG facies. HREE enrichment relative to LREE occurs in the sequence RK, BG, HG and CAG. In the Ivigtut cryolite ore deposit, a similar behavior was observed in relation to slightly altered host rocks and albitized rocks near the cryolite deposit. This was interpreted as host-rock metasomatism, with the formation of complex ions between HREE and F ± Cl (Pauly and Bailey 1999). In contrast with what is observed in the CAG subfacies and in Ivigtut, the cryolitebearing anorogenic alkali granites from Nigeria display enrichment in LREE, followed by a slight increase in Eu (Kinnaird et al. 1985).
At Pitinga, the REE and Y data agree with geological and petrographic data in the sense that MCD do not represent an exotic body within albite granites, regarding localization, shape, paragenesis or chemical composition. Thus, a metasomatic model such as Ivigtut's is not applicable to Pitinga and a magmatic-hydrothermal model is here assumed. At Pitinga, the evolution of several parameters (REE total , La/Lu, LREE/HREE, Y) was followed throughout successive stages of evolution in albite granites and associated cryolite mineralization. The evolution is marked by a depletion in REE and Y. It is a stepwise depletion in for disseminated magmatic and disseminated hydrothermal cryolites, but it is a gradual depletion from disseminated hydrothermal cryolite and MCD. At the end of the process, late cryolite was formed. Its white color is presumed to be due to the extremely low REE total content.
Magmatic fluorite (BAG) has the same REE pattern as albite granite. Moreover, Y concentration ranges illustrate three generations of fluorite. A comprehensive study by Fayzigev (1990, cited by Chang et al. 1996 showed that the highest Y concentration in fluorite PMR7A and PMR-10) are compatible with crystallization in a magmatic environment, while samples with concentrations of 200 ppm would be products of the hydrothermal stage. A fluorite sample from a late vein (Y ∼ 20 ppm) represents the final stages of hydrothermalism.
In the magmatic fluorite, REE/Y and LREE/HREE ratios are slightly above 1, similar to the rock ratios. On the other hand, they are partly distinct from most fluorite occurrences, which display Y>REE and LREE>HREE ratios (Chang et al. 1996). Cryolite, however, shows a different behavior. Magmatic cryolite displays a more pronounced relative enrichment in REE (REE/Y 3.6 to 6.4) than fluorite (REE/Y ∼ 1.2). On the other hand, HREE>LREE ratio in magmatic cryolite (LREE/HREE 0.22 and 0.25) is contrary to the rock ratio. These two aspects may reflect the preferential Na substitution by small ionic-radii REE (Y behaves similarly to HREE), an effect that is much less pronounced in fluorite.
The systematic difference in Eu anomaly between disseminated and massive cryolite may be related to variations in the redox conditions. In fluorite, the Eu anomaly is inversely related to the mineral Eu 2+ /Eu total ratio measured by RPE (Méary et al. 1985), which depends on the solution Eu 2+ /Eu 3+ ratio; this ratio depends on fO 2 , for a fixed pH (Sverjensky 1984). Eu 3+ cation, with an ionic radius smaller than Eu 2+ , is preferentially incorporated to replace Ca. Hence decreasing solution Eu 2+ /Eu 3+ ratio leads to less pronounced Eu negative anomalies. In cryolite, the preferential replacement of Na by small-radius REE was observed. Consequently, the relation between the variation in solution Eu 2+ /Eu 3+ ratio and mineral anomaly is probably the same observed for fluorite. Therefore we suggest that the three generations of massive cryolite were formed in an environment more oxidizing than that of magmatic and disseminated hydrothermal cryolite. Disseminated hydrothermal cryolite was formed by residual fluids of magmatic origin, under redox conditions similar to those in the magmatic environment. During the formation of the MCD there was contribution from more oxidizing solutions, possi-CONCLUSIONS REE signature in fluorite and cryolite are similar t in albite granite. Cryolite and fluorite display tive Eu anomaly, inherited from and/or increased evolution of the granitic system. High values of relative to LREE are found in albite granite as w in disseminated and nucleated cryolite types. In fl and white cryolite these features are either not ob or not conspicuous. The highest values of R found in magmatic-hydrothermal disseminate c In massive cryolite, REE is lower (4.8 to 16.8 Disseminated fluorite has higher REE values th fluorite veins. Y shows higher values in dissem fluorite and magmatic cryolite in relation to vei rite and hydrothermal cryolite, respectively. The content of F in CAG and the peralkaline affinity granite seem to be responsible for the formation o plexes as [AlF 3 ] -3 which could form cryolite asso to this type of granite. The evolution of several parameters (R LREE/HREE, Y) was followed throughout suc stages of evolution in albite granite and associate eralization. At the end of the process, late cryol formed. Its white color is presumed to be due to tremely low REE total content. The REE and Y dat with geological and petrographic data in the sen (1) there are two types of disseminated cryolite matic and hydrothermal) and (2) massive cryolit ies do not represent an exotic body within albite ites, regarding localization, shape, paragenesis or ical composition. Thus, a metasomatic model s Ivigtut's is not applicable to the Pitinga albite or to the associated MCD. REE data indicate t MCD was formed by, and the disseminated ore en by (additional formation of hydrothermal dissem cryolite), hydrothermal fluids, residual from albit ite, that are ascendent from the lower parts of the During the formation of the MCD there was co tion from more oxidant solutions, even meteoric Influx of this solution indicates opening of the s possibly related to fracture reactivation which may mel and white cryolites show some evidence of tetrad effect, especially in the first tetrad. More detailed analyses shall be carried out in cryolite to investigate the tetrad effect. Among the different granites, the core albite granite (CAG) is the facies that display signs of tetrad effect. The F has much influence in the presence of tetrad effects but there are also others important parameters (electronic configuration, chemical bonds, others ions as Al 3+ ).