Occurrence and Composition of Columbite-(Fe) In the Reduced A-Type Desemborque Pluton, Graciosa Province (S-SE Brazil)

Columbite-(Fe) is a post-magmatic accessory mineral occurring within syenogranites and greisens from the Desemborque Pluton. The petrographic (SEM) and geochemical (EPMA and LA-ICPMS) examination of this mineral shows two distinct textural types within both the rocks, named columbite-1 and columbite-2. The columbite-1 type is characterized by zoned crystals with two stages of crystallization: i) An early Nb-rich cores with low Ta/(Ta + Nb) and Mn/(Mn + Fe) ratios (0.02–0.08 and 0.17 to 0.21 apfu, respectively), and ii) a later Ta-rich rims with higher Ta/(Ta + Nb) ratios (0.11–0.26) and similar Mn/(Mn + Fe) ratios (from 0.14 to 0.22) relative to the former cores. On the other hand, the columbite-2 type is defined by irregular crystals with patchy textures and very low Ta/(Ta + Nb) ratios (0.008–0.038) and moderate Mn/(Mn + Fe) ratios between 0.20 and 0.38. Trace element compositions of all columbite-(Fe) crystals are relatively enriched in HREEs and HFSEs; however, the columbite-2 presents higher abundances of REEs, Y, Th, U, Pb, Sc, and Sn relative to the columbite-1. This study highlights a unique hydrothermal origin for both the columbite types, but the textural relations of the columbite-2 crystals indicated that its formation is related to fluid-induced alterations of post-magmatic fluorite and/or cassiterite crystals at the final stage of the post-magmatic evolution.

CGM are usually found in rare-metal granites and pegmatites [9][10][11][12][13][14][15] but these minerals are also common in F-and Li-rich peraluminous granitic systems and associated greisens [16][17][18][19][20]. The occurrence of CGM has attracted significant attention because they are minerals of economic interest hosted in several metallogenetic provinces worldwide [11]. In Brazil, CGM are reported in pegmatites from the Borborema Province [21][22][23], the Lourenço-Amapá Province, and the São Joao do Rei and Araçuaí districts [11], and also in the peralkaline Madeira granite from the Pitinga Mine [24]. This is the first report of columbite in reduced metaluminous to slightly-peraluminous granites and related greisens of the Desemborque Pluton from the A-type Graciosa Province in S-SE Brazil. In order to understand the genesis of the columbite occurrences in the studied rocks, we combined Atuba Complex and Turvo-Cajati Formation, respectively [30]. The pluton is relatively homogeneous and composed mainly of massive and/or porphyritic biotite syenogranites (Figure 2a,b), and by alkali feldspar granites, granite porphyries, and microgranites, which are scarce [31]. Pegmatites and hydrothermal rocks resulting from the alteration of the main biotite syenogranites, such as greisenized granites and greisens [25,31], are also of local occurrence. Sampling of hydrothermally-altered rocks are difficult to see given the extensive weathering, thus greisenized granite and greisen rocks were sampled in a unique outcrop. This local well shows the contact relationships and the transition from the biotite syenogranite to the related greisen (Figure 2c,d). In the following, we focus our petrographic and geochemical descriptions of these rock varieties, and in Table 1 are listed their main geochemical compositions.
Minerals 2020, 10,411 3 of 17 and by alkali feldspar granites, granite porphyries, and microgranites, which are scarce [31]. Pegmatites and hydrothermal rocks resulting from the alteration of the main biotite syenogranites, such as greisenized granites and greisens [25,31], are also of local occurrence. Sampling of hydrothermally-altered rocks are difficult to see given the extensive weathering, thus greisenized granite and greisen rocks were sampled in a unique outcrop. This local well shows the contact relationships and the transition from the biotite syenogranite to the related greisen (Figure 2c,d). In the following, we focus our petrographic and geochemical descriptions of these rock varieties, and in Table 1 are listed their main geochemical compositions. porphyritic syenogranite showing rounded and/or tabular alkali feldspar in a fine-grained matrix of quartz, plagioclase, and interstitial biotite; (c) contacts between the main syenogranite with the greisenized granite and greisen rocks; (d) the detail of contact between greisenized syenogranite and greisen. Note the darker-colored aspect and the finer-grained texture of these rocks relative to the main biotite syenogranite.

The Biotite Syenogranite
The biotite syenogranite displays variably inequigranular medium-to coarse-grained and porphyritic textures (Figure 3a,b). It is composed of early-crystallized perthite orthoclase-(Or58-71), oligoclase-albite (Ab78-95), and quartz. Biotite, the only magmatic mafic phase among these rocks, varies in composition from annite to siderophyllite [31]. Zircon is the most abundant magmatic accessory mineral, followed by late-magmatic ilmenite and fluorite. Cassiterite, columbite, hematite, magnetite, monazite, and xenotime are common post-magmatic accessories, which are commonly included in aggregates of biotite. Other secondary and minor minerals are phengite, sericite, fluorides-HREE, and thorite are secondary minerals. showing equi-to inequigranular texture defined by quartz (Qz), plagioclase (Pl), and alkali feldspar crystals (Afs), as well as biotite (Bt) as the main mafic mineral; (b) porphyritic syenogranite showing rounded and/or tabular alkali feldspar in a fine-grained matrix of quartz, plagioclase, and interstitial biotite; (c) contacts between the main syenogranite with the greisenized granite and greisen rocks; (d) the detail of contact between greisenized syenogranite and greisen. Note the darker-colored aspect and the finer-grained texture of these rocks relative to the main biotite syenogranite.
Geochemically between the dominant syenogranite and the greisen. This rock presents an inequigranular finegrained texture with post-magmatic transformations such as partial to complete replacement of feldspars by sericite and annite by zinnwaldite (Figure 3c). Other minerals found here are fluorite, ilmenite, hematite, phengite, and sphalerite. Geochemically, the greisenized syenogranite has intermediate compositions between the greisen and the hosted biotite syenogranite with relatively higher Fe2O3 and Al2O3 contents, and significant contents of F, Rb, Li, Y and Zr (Table 1).

Figure 3.
Microscopic aspects of the Desemborque studied rocks. (a) Syenogranite with equi-to inequigranular texture, including tabular perthitic alkali feldspar crystals (Afs), sometimes with albitic rims (Ab), as well as rounded and/or elongated quartz (Qz) with undulose extinction and interstitial annite (Ann) crystals. Note the partial alteration of some feldspars by sericite (Ser) and of the annite crystal by siderophyllite (Sid); (b) syenogranite with a porphyritic texture involving mesoperthitic alkali feldspars phenocryst in a medium-grained quartz-feldspar matrix; (c) greisenized syenogranite displaying inequigranular and altered texture and partial to complete replacement of feldspars by sericite and annite by zinnwaldite (Znw); (d) greisen characterized by heterogranoblastic texture with pervasive sericite-silica alteration, high recrystallization of quartz, and extensive replacement of zinnwaldite by sericite. Photomicrographs with crossed polarizers are shown. Figure 3. Microscopic aspects of the Desemborque studied rocks. (a) Syenogranite with equi-to inequigranular texture, including tabular perthitic alkali feldspar crystals (Afs), sometimes with albitic rims (Ab), as well as rounded and/or elongated quartz (Qz) with undulose extinction and interstitial annite (Ann) crystals. Note the partial alteration of some feldspars by sericite (Ser) and of the annite crystal by siderophyllite (Sid); (b) syenogranite with a porphyritic texture involving mesoperthitic alkali feldspars phenocryst in a medium-grained quartz-feldspar matrix; (c) greisenized syenogranite displaying inequigranular and altered texture and partial to complete replacement of feldspars by sericite and annite by zinnwaldite (Znw); (d) greisen characterized by heterogranoblastic texture with pervasive sericite-silica alteration, high recrystallization of quartz, and extensive replacement of zinnwaldite by sericite. Photomicrographs with crossed polarizers are shown.

Greisenized Syenogranite
The greisenized syenogranite is characterized by intermediate textural and chemical properties between the dominant syenogranite and the greisen. This rock presents an inequigranular fine-grained texture with post-magmatic transformations such as partial to complete replacement of feldspars by sericite and annite by zinnwaldite (Figure 3c). Other minerals found here are fluorite, ilmenite, hematite, phengite, and sphalerite. Geochemically, the greisenized syenogranite has intermediate compositions between the greisen and the hosted biotite syenogranite with relatively higher Fe 2 O 3 and Al 2 O 3 contents, and significant contents of F, Rb, Li, Y and Zr (Table 1).

Greisen
The studied sample is characterized by a fine-grained heterogranoblastic texture (Figure 3d), and its mineral assemblage is characterized by mainly quartz, accompanied by zinnwaldite-(Li) and sericite. Zircon, cassiterite, columbite, fluorite, monazite, thorite, and xenotime are the accessory minerals. Galena, sphalerite, and topaz are distinct accessories only found in this rock. Chemical compositions display a significant enrichment in Fe 2 O 3 , Li, Rb, Sn, Zr, and F and a discrete increase in Nb, Hf, Th, and U relative to the granite (Table 1). Figure 4 shows the mineral assemblages at magmatic from late-to post-magmatic stages of evolution observed in the studied Desemborque rocks.

Materials and Methods
Four samples bearing columbite crystals of the syenogranite (GUA−36, GUA−50, and GUA−53) and greisen (GUA−8b) varieties from the Desemborque Pluton were selected for petrographic, mineral chemistry, and whole-rock chemical data at the Laboratory of the NAP GeoAnalitica-Institute of Geosciences/University of São Paulo, Brazil.
Columbite crystals were identified in petrographic polished thin sections by combining optical and scanning electron microscopy (SEM) and semiquantitative energy dispersive spectrometry (EDS). Back-scattered electron images (BSE) were obtained to study internal textures and compositional variations. After textural descriptions, the chemical compositions of columbite crystals were determined using a JXA−8530 (JEOL, Tokyo, Japan) electron probe micro-analyzer (EPMA). A total of 42 analyses were conducted with an accelerating voltage of 20 kV, a beam current of 15 nA, a beam diameter of 3 µ m, and maximum total peak-counting times, equally distributed between peak and background measures, varying between 5 s and 40 s. Standards used for analysis included diopside for Si, Ca, and Mg; ilmenite for Fe, Ti and Mn; anorthite for Al; albite for Na; zircon for Zr; fluorapatite for F; strontianite for Sr; yttrium-phosphate for Y; synthetic glasses for Gd and Th (all of them furnished by the Smithsonian Institute); and pure Pb, Sb, V, Ta, Nb and W metals (from the internal lab collection). Raw WDS data were corrected and converted to wt.% with the PRZ-Armstrong provided by JEOL. The detection limits for the conventional major oxides were in between 0.01 wt.% and 0.02 wt.%, and for the minor elements in between 0.02 wt.% and 0.26 wt.%. The structural formulae were calculated based on 6 oxygens and 3 atoms per formula unit (apfu).
Trace elements of columbite crystals were analyzed using a laser ablation system (LA-ICP.MS) with a 213 A/F equipment (New Wave, Portland, ME, USA). Analytical conditions were a laser spot of 25 µ m, 20 Hz of frequency with an energy fluence of ~1.97 J/cm 2 . Signal measurements and gas background on sample integration times were 60 and 40 s, respectively. The instrument was calibrated

Materials and Methods
Four samples bearing columbite crystals of the syenogranite (GUA−36, GUA−50, and GUA−53) and greisen (GUA−8b) varieties from the Desemborque Pluton were selected for petrographic, mineral chemistry, and whole-rock chemical data at the Laboratory of the NAP GeoAnalitica-Institute of Geosciences/University of São Paulo, Brazil.
Columbite crystals were identified in petrographic polished thin sections by combining optical and scanning electron microscopy (SEM) and semiquantitative energy dispersive spectrometry (EDS). Back-scattered electron images (BSE) were obtained to study internal textures and compositional variations. After textural descriptions, the chemical compositions of columbite crystals were determined using a JXA−8530 (JEOL, Tokyo, Japan) electron probe micro-analyzer (EPMA). A total of 42 analyses were conducted with an accelerating voltage of 20 kV, a beam current of 15 nA, a beam diameter of 3 µm, and maximum total peak-counting times, equally distributed between peak and background measures, varying between 5 s and 40 s. Standards used for analysis included diopside for Si, Ca, and Mg; ilmenite for Fe, Ti and Mn; anorthite for Al; albite for Na; zircon for Zr; fluorapatite for F; strontianite for Sr; yttrium-phosphate for Y; synthetic glasses for Gd and Th (all of them furnished by the Smithsonian Institute); and pure Pb, Sb, V, Ta, Nb and W metals (from the internal lab collection). Raw WDS data were corrected and converted to wt.% with the PRZ-Armstrong provided by JEOL. The detection limits for the conventional major oxides were in between 0.01 wt.% and 0.02 wt.%, and for the minor elements in between 0.02 wt.% and 0.26 wt.%. The structural formulae were calculated based on 6 oxygens and 3 atoms per formula unit (apfu).
Trace elements of columbite crystals were analyzed using a laser ablation system (LA-ICP.MS) with a 213 A/F equipment (New Wave, Portland, ME, USA). Analytical conditions were a laser spot of 25 µm, 20 Hz of frequency with an energy fluence of~1.97 J/cm 2 . Signal measurements and gas background on sample integration times were 60 and 40 s, respectively. The instrument was calibrated with the NIST standard SRM 610 glass to correct signal drift during the analytical process and the basalt glass standard BCR−2G was used as an external standard to calibrate major and trace elements [32]. In Supplementary Table S2, the 2 sigma is reported.
Whole-rock chemical analyses were determined in three fresh samples (GUA−50c, GUA−08a, and GUA−08b) using an X-ray fluorescence instrument (XRF) with the Axios MAX Advanced equipment (PANalytical, Almelo, The Netherlands) and by Inductively coupled plasma mass spectrometry (ICP-MS) instrument with the iCAP Q ICP-MS equipment (Thermo Fisher Scientific, Bremen, Germany). Major and some minor elements were quantified by XRF, while trace elements and rare earth elements were measured by ICP-MS, following the analytical procedures developed by Reference [33] and Reference [34], respectively.

Columbite Occurrence and Its Textural Relations
The examination of the petrographic thin sections of the biotite syenogranites and the greisen reveal that columbite occurs as individual or grouped opaque crystals. BSE images allowed the observation of two distinct textural types of columbite between both rocks types named columbite-1 and columbite-2, which present contrast modes of occurrence, internal textures, and compositional variations ( Figure 5). with the NIST standard SRM 610 glass to correct signal drift during the analytical process and the basalt glass standard BCR−2G was used as an external standard to calibrate major and trace elements [32]. In Supplementary Table S2, the 2 sigma is reported. Whole-rock chemical analyses were determined in three fresh samples (GUA−50c, GUA−08a, and GUA−08b) using an X-ray fluorescence instrument (XRF) with the Axios MAX Advanced equipment (PANalytical, Almelo, The Netherlands) and by Inductively coupled plasma mass spectrometry (ICP-MS) instrument with the iCAP Q ICP-MS equipment (Thermo Fisher Scientific, Bremen, Germany). Major and some minor elements were quantified by XRF, while trace elements and rare earth elements were measured by ICP-MS, following the analytical procedures developed by Reference [33] and Reference [34], respectively.

Columbite Occurrence and Its Textural Relations
The examination of the petrographic thin sections of the biotite syenogranites and the greisen reveal that columbite occurs as individual or grouped opaque crystals. BSE images allowed the observation of two distinct textural types of columbite between both rocks types named columbite-1 and columbite-2, which present contrast modes of occurrence, internal textures, and compositional variations ( Figure 5).  The columbite-1 type is common in the greisen and rarely in the syenogranites. It occurs as subhedral to euhedral small (30-60 µm) prismatic crystals, displaying convolute zoning with BSE-dark cores and BSE-bright rims (Figure 5a,b). In syenogranites, columbite-1 crystals are partially fractured and display homogeneous cores and patchy domains within the rims (Figure 5a), whereas in the greisen the zoned patterns are irregularly distributed from the outer to the inner crystal parts (Figure 5b). The columbite-2 type is most common in the syenogranites and rarely found in the greisen; crystals are usually associated with fluorite, zircon and/or cassiterite and appear mostly as inclusions in biotite clusters within the syenogranites (Figure 5c,d). This type forms anhedral to subhedral crystals with variable shapes and sizes (up to 100 µm). They usually display resorption textures as partially dissolved domains, or domains with patchy areas and corroded margins, involving the partial replacement of fluorite ( Figure 5c) and/or cassiterite (Figure 5d).

Main Features
Representative WDS chemical compositions and structural formulae for the identified columbite types are listed in Table 2; our complete EPMA data set is presented in Supplementary Table S1. In general, columbite-1 and -2 types in the Desemborque Pluton have equivalent compositions between syenogranites and the greisen. Comparing the data, all columbite crystals are Fe-rich and classified as columbite-(Fe) with Mn/(Mn + Fe) atomic ratios ranging from 0.14 to 0.22 for the columbite-1 type, and from 0.20 to 0.38 for the columbite-2 type; however, the Ta/(Ta + Nb) ratios show significant variability between both columbite types, especially within the columbite-1 zoned crystals ( Figure 6). Thus, the cores of the columbite-1 crystals have Ta/(Ta + Nb) ratios from 0.02 to 0.08, while in the rims these ratios are always higher from 0.11 to 0.26. The EPMA results also reveal that columbite-1 crystals have relative higher apfu contents of W up to 0.092 (cores) and up to 0.080 (rims) as well as of Ti 0.093 (cores) and 0.110 (rims) when compared to the columbite-2 crystals up to 0.065 W and 0.089 Ti (Table S1). Moreover, the columbite-1 crystal rims present the highest contents of Zr (up to 0.010) and F (up to 0.014) between all columbite types (Table S1).    2(V,Nb,Ta,Sb) 5+ ↔ (Si,Ti,Zr,Th,U) 4+ + (W) 6+ (1) All cationic substitutions yield good correlations (determination coefficient r 2 in between 0.94 and 0.98). Compositional variations between the columbite types are, in general, limited to the columbite-1 zoned crystals which present a significant increase of Ta and somewhat of Fe from the core to the rims of compositions (Figure 7a,b). Nevertheless, the correlation between Mn and Fe shows that the columbite-2 crystals have rather larger variations (Figure 7b). Our data set also shows that both columbite-1 and -2 types indicate good balancing according to the coupled substitution scheme in Figure 7c, which gives the best correlation (r 2 = 0.98).

Trace and REE Element Compositions
Representative trace element analyses for the investigated columbite crystals are given in Table 3 and the complete dataset in Supplementary Table S2. Multielement and rare earth patterns are shown in Figure 8. Trace element patterns for columbite crystals exhibit some differences relative to those from the syenogranite and the greisen, especially the negative anomalies of Y and Th and the strong positive anomaly of U (Figure 8a). In general, columbite crystals are relatively enriched in most of the HREE and HFSE relative to corresponding whole-rock compositions.
The REE patterns for all columbite crystals show a rather steeply-rising slope due to the relative enrichment of HREE over the LREE (28 < YbN/CeN < 3505), with negative Eu anomalies [Eu/Eu* ≅ EuN/(SmN*GdN) 1/2 ] varying from 0.04 to 0.28 (Table S2). Comparing the columbite-1 and -2 crystals, the later present higher REE abundances ∑REE up to 3718 ppm, while the former register ∑REE up to 1362 ppm. The most contrasted REE pattern is observed in one columbite-2 crystal for the syenogranite, which has the highest REE abundances, especially the LREEs (1-to 2-fold higher than the others), with a contrasted LREE flatter pattern (SmN/LaN ≅ 1.38). All cationic substitutions yield good correlations (determination coefficient r 2 in between 0.94 and 0.98). Compositional variations between the columbite types are, in general, limited to the columbite-1 zoned crystals which present a significant increase of Ta and somewhat of Fe from the core to the rims of compositions (Figure 7a,b). Nevertheless, the correlation between Mn and Fe shows that the columbite-2 crystals have rather larger variations (Figure 7b). Our data set also shows that both columbite-1 and -2 types indicate good balancing according to the coupled substitution scheme in Figure 7c, which gives the best correlation (r 2 = 0.98).

Trace and REE Element Compositions
Representative trace element analyses for the investigated columbite crystals are given in Table 3 and the complete dataset in Supplementary Table S2. Multielement and rare earth patterns are shown in Figure 8. Trace element patterns for columbite crystals exhibit some differences relative to those from the syenogranite and the greisen, especially the negative anomalies of Y and Th and the strong positive anomaly of U (Figure 8a). In general, columbite crystals are relatively enriched in most of the HREE and HFSE relative to corresponding whole-rock compositions.   The REE patterns for all columbite crystals show a rather steeply-rising slope due to the relative enrichment of HREE over the LREE (28 < Yb N /Ce N < 3505), with negative Eu anomalies [Eu/Eu* Eu N /(Sm N *Gd N ) 1/2 ] varying from 0.04 to 0.28 (Table S2). Comparing the columbite-1 and -2 crystals, the later present higher REE abundances REE up to 3718 ppm, while the former register REE up to 1362 ppm. The most contrasted REE pattern is observed in one columbite-2 crystal for the syenogranite, which has the highest REE abundances, especially the LREEs (1-to 2-fold higher than the others), with a contrasted LREE flatter pattern (Sm N /La N 1.38).

Genesis of Columbite in the Desemborque Pluton
The explanation of the origin of columbite-group minerals (CGM) is always a challenge given the variable textures, alterations, and compositions recorded by these minerals during multiple stages of their crystallization. In granites, the formation of CGM have been extensively associated with magmatic and hydrothermal processes or a combination of both [16][17][18][19][20][36][37][38][39][40][41]. In this study, we suppose that the rare and local occurrence of columbite in the Desemborque pluton is indicative of a unique hydrothermal origin, as supported by the majority of columbite-1 crystals in the greisen and the replacement textures of columbite-2 crystals with other late-to post-magmatic accessory minerals in the syenogranites. However, the contrast of textural and geochemical features between the columbite-1 and -2 crystals suggest distinct mechanisms of formation.
In the case of columbite-1 group, zoning patterns within crystals indicate two hydrothermal substages of crystallization during the post-magmatic evolution: the first stage is represented by the formation of Nb-rich cores crystals, while the second stage is characterized by the subsequent crystallization of the Ta-rich rims around the former cores. These patterns are typical in CGM and have been broadly explained either by disequilibrium crystallization processes or differences in solubility, given the general tendency of Nb to crystallize prior to Ta during evolutionary stages [41][42][43][44][45][46][47][48][49]. Additionally, the somewhat higher abundances of the HFSEs (such Ti, Zr and Hf) of the Ta-rich rims crystals relative to the Nb-rich cores also suggest differentiated hydrothermal substages.
On the other hand, the formation of the columbite-2 crystals is interpreted as a result of dissolution-reprecipitation processes due to hydrothermal fluid-induced alterations at the final stage of the post-magmatic evolution. This can be explained by the disequilibrium textures, such as the resorption of the marginal and inner parts of fluorite and cassiterite shown in Figure 5c,d, which indicate that columbite-2 crystals formed as late replacements of these minerals. Moreover, the secondary patchy textures among the columbite-2 crystals reinforced the influence of post-magmatic hydrothermal fluids altering these crystals, which also could be responsible for the significant enrichment of REEs, Y, U, Th, Pb and Ti relative to the columbite-1 crystals.

Comparision with Other Columbite Occurrences from Rare-Granites And/Or Granitic Pegmatites Worldwide
In Figure 9, we compiled current compositional data of columbite-group minerals (colored lines) in granites and related greisens and used the previous schematic compilation of the #Ta and #Mn compositional evolution trends of columbite-group minerals from rare-metal granites worldwide (black lines) given by Reference [40] in order to compare with our results. The evolution trend of the columbite-1 crystals in the Desemborque Pluton is characterized by moderate variation of the Ta/(Ta + Nb) ratios (from 0.02 to 0.26), with a vertical trajectory that could be compared to those columbite trends from the Podlesí and Geyersberg Granites (Figure 9). The evolution trend of the Desemborque columbite-2 crystals shows a contrary horizontal and short trajectory, with limited variations of the Mn/(Mn + Fe) ratios (from 0.20 to 0.38), and differs from the others columbite trends compiled here.
Detailed data on trace elements of CGM in granites and/or greisens are rarely found or absent in the literature. Nevertheless, a robust compilation of main and trace element compositions of CGM in typical pegmatites from relevant Ta-Nb-Sn provinces worldwide is provided by References [10,11]. We used the median trace element values of only columbite-(Fe) minerals from the mentioned dataset and plot with our average trace elements compositions, as shown in Figure 10. Trace element patterns of all the samples exhibit a clear LREE depleted and HREE enriched shape; U, Zr, Hf, and W are systematically enriched with respect to chondrite values, while Eu, Pb, Sn, Ti, Li, and Al are usually depleted. The behavior of elements such as Y, Th, and Sc are broadly variable among all columbite-(Fe) compositions, which can be attributed either to melt compositions and/or co-precipitation with other phases. For example, in the Desemborque Pluton, the negative anomalies of Y and Th for both columbite-1 and -2 crystals can be explained by the competition with xenotime within the hosting rocks. For the other examples presented here, we require further information on the mineral crystallization sequence from the hosting pegmatites which is far beyond the scope of this work. We also observed that our columbite-(Fe) compositions have much higher trace element abundances, especially for the LRREs, than the majority of the columbite occurrences worldwide (Figure 10).

Conclusions
Petrographic examination and geochemical data for columbite-(Fe) in syenogranites and related greisen from the reduced A-type Desemborque Pluton in the Graciosa Province (S-SE Brazil) indicate that the occurrence of columbite in the studied rocks is mainly associated with hydrothermal origin during the post-magmatic stage of crystallization. Textural and compositional variations of this mineral reveal two main textural types, named columbite-1 and columbite-2, which are interpreted as being formed by distinct mechanisms of crystallization. The columbite-1 is characterized by zoned crystals, which record two hydrothermal stages of crystallization: an early Nb-rich core, and then later Ta-rich rims. In contrast, columbite-2 is defined by irregular crystals with patchy textures, and its formation is related to disequilibrium processes driven by fluid-induced hydrothermal alterations involving the partial replacement of fluorite and/or cassiterite at the final post-magmatic stage.
The main chemical features of columbite types show that the columbite-1 crystals form an evolution trend with a significant increase of Ta/Nb ratios, while the columbite-2 crystals show a distinct trend with a limited increase of Mn/Fe ratios. Additionally, trace element compositions show that all columbite crystals are relatively enriched in HREEs and HFSEs; however, the columbite-2 type presents higher abundances of REEs, Y, Th, U, Pb, Sc, and Sn relative to the columbite-1 crystals. The chemical contrasts among the columbite types are related to disequilibrium crystallization processes (columbite-1) and to hydrothermal alterations during the post-magmatic evolution (columbite-2).
Author Contributions: Concseptualization, EPMA analyses, investigation, writing-original draft preparation, A.S.; Field investigation, formal analyses, data curation, R.G; Project administration, supervision, and writingreview, S.R.F.V. All authors have read and agreed to the published version of the manuscript.

Conclusions
Petrographic examination and geochemical data for columbite-(Fe) in syenogranites and related greisen from the reduced A-type Desemborque Pluton in the Graciosa Province (S-SE Brazil) indicate that the occurrence of columbite in the studied rocks is mainly associated with hydrothermal origin during the post-magmatic stage of crystallization. Textural and compositional variations of this mineral reveal two main textural types, named columbite-1 and columbite-2, which are interpreted as being formed by distinct mechanisms of crystallization. The columbite-1 is characterized by zoned crystals, which record two hydrothermal stages of crystallization: an early Nb-rich core, and then later Ta-rich rims. In contrast, columbite-2 is defined by irregular crystals with patchy textures, and its formation is related to disequilibrium processes driven by fluid-induced hydrothermal alterations involving the partial replacement of fluorite and/or cassiterite at the final post-magmatic stage.
The main chemical features of columbite types show that the columbite-1 crystals form an evolution trend with a significant increase of Ta/Nb ratios, while the columbite-2 crystals show a distinct trend with a limited increase of Mn/Fe ratios. Additionally, trace element compositions show that all columbite crystals are relatively enriched in HREEs and HFSEs; however, the columbite-2 type presents higher abundances of REEs, Y, Th, U, Pb, Sc, and Sn relative to the columbite-1 crystals.
The chemical contrasts among the columbite types are related to disequilibrium crystallization processes (columbite-1) and to hydrothermal alterations during the post-magmatic evolution (columbite-2).
Author Contributions: Concseptualization, EPMA analyses, investigation, writing-original draft preparation, A.S.; Field investigation, formal analyses, data curation, R.G.; Project administration, supervision, and writing-review, S.R.F.V. All authors have read and agreed to the published version of the manuscript.