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Volume 31 Issue 3
Jul.  2020
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Maryam Mohamadizadeh, Seyed Hossein Mojtahedzadeh, Farimah Ayati. Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) Ternary Diagram: An Excellent Tool for Discriminating Barren and Ta-Hosting Granite-Pegmatite Systems. Journal of Earth Science, 2020, 31(3): 551-558. doi: 10.1007/s12583-020-1302-1
Citation: Maryam Mohamadizadeh, Seyed Hossein Mojtahedzadeh, Farimah Ayati. Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) Ternary Diagram: An Excellent Tool for Discriminating Barren and Ta-Hosting Granite-Pegmatite Systems. Journal of Earth Science, 2020, 31(3): 551-558. doi: 10.1007/s12583-020-1302-1

Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) Ternary Diagram: An Excellent Tool for Discriminating Barren and Ta-Hosting Granite-Pegmatite Systems

doi: 10.1007/s12583-020-1302-1
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  • Discriminating barren and fertile intrusions is one of the main challenges in the search for rare-element pegmatites. Diagrams comprising more than one element can make discrimination of productive and barren samples more valid. These diagrams distinguish samples by simultaneous means of positive and/or negative correlations between variables. A ternary diagram for S-type peraluminous granites has been obtained in this study. Firstly, a database composed of Ta-bearing and barren granitic systems was created, then geochemical behavior of trace elements was studied, and statistical investigations were done using GCDkit software, which resulted in the Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) ternary diagram which can distinguish the non-mineralized granites from productive ones. The Ta-bearing samples, which are situated in the fertile field in the diagram, are those which have high Nb and Ta contents, elevated Ga content and the lowest Nb/Ta and Zr/Hf values.
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Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) Ternary Diagram: An Excellent Tool for Discriminating Barren and Ta-Hosting Granite-Pegmatite Systems

doi: 10.1007/s12583-020-1302-1
    Corresponding author: Seyed Hossein Mojtahedzadeh, ORCID:0000-0002-8035-3396, shmojtahed@gmail.com

Abstract: Discriminating barren and fertile intrusions is one of the main challenges in the search for rare-element pegmatites. Diagrams comprising more than one element can make discrimination of productive and barren samples more valid. These diagrams distinguish samples by simultaneous means of positive and/or negative correlations between variables. A ternary diagram for S-type peraluminous granites has been obtained in this study. Firstly, a database composed of Ta-bearing and barren granitic systems was created, then geochemical behavior of trace elements was studied, and statistical investigations were done using GCDkit software, which resulted in the Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) ternary diagram which can distinguish the non-mineralized granites from productive ones. The Ta-bearing samples, which are situated in the fertile field in the diagram, are those which have high Nb and Ta contents, elevated Ga content and the lowest Nb/Ta and Zr/Hf values.

Maryam Mohamadizadeh, Seyed Hossein Mojtahedzadeh, Farimah Ayati. Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) Ternary Diagram: An Excellent Tool for Discriminating Barren and Ta-Hosting Granite-Pegmatite Systems. Journal of Earth Science, 2020, 31(3): 551-558. doi: 10.1007/s12583-020-1302-1
Citation: Maryam Mohamadizadeh, Seyed Hossein Mojtahedzadeh, Farimah Ayati. Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) Ternary Diagram: An Excellent Tool for Discriminating Barren and Ta-Hosting Granite-Pegmatite Systems. Journal of Earth Science, 2020, 31(3): 551-558. doi: 10.1007/s12583-020-1302-1
  • Rare-element granite-pegmatite systems have been divided into two families: Li-Cs-Ta enriched (LCT) and Nb-Y-F enriched (NYF). The LCT family is enriched in Li, Cs, and Ta, and contains high amounts of Be, Rb, Sn, B, F, and P. It is associated with S-type, peraluminous, quartz-rich granites (generally including syenogranite, monzogranite, granodiorite and rarely tonalite) (Cerny, 1989). The NYF family is enriched in REEs (rare earth elements), U, and Th, in addition to Nb, Y, F, and is associated with A-type, subaluminous to metaluminous, quartz-poor granites (especially alkali feldspar granites) or syenites (Ercit, 2005; Cerny, 1989).

    Although the LCT suites have historically been the most important source of tantalum, one of the strategic metals which is vital to the national economy and defense, they may resume tantalum production in the future. Also, placer and alluvial deposits which likely have accumulated in the vicinity of the granitic plutons provide an important component of global production (Gunn, 2014). So rare-element pegmatites are still prospective targets to be explored (Dai et al., 2019; Gunn, 2014; Linnen et al., 2012; Selway et al., 2005). Identifying parental intrusion is the first step in the search for pegmatites. Since they are typically distributed within 10 km of the parent granite, and its discovery can greatly reduce the search area (Breaks and Tindle, 1997). The second step is discriminating fertile and barren intrusions. They can be distinguished by geochemistry, mineralogy, and texture (Selway et al., 2005). Whole-rock analyses are routinely done in most laboratories around the world and they are usually available in literatures. As a result, bulk-rock geochemistry is a less expensive and more simple method, which can be used in preliminary exploration.

    The LCT suites have a wider range of accessory minerals than barren granites. Barren granites contain biotite and/or silver-colored muscovite as their minor minerals, and apatite, zircon, and titanite as accessory minerals, whereas fertile granites may contain primary green lithium-bearing muscovite, garnet, tourmaline, apatite, cordierite, and rarely andalusite and topaz (Selway et al., 2005). More evolved fertile granites contain beryl, ferrocolumbite (Nb-oxide mineral), and Li-bearing tourmaline (Breaks and Tindle, 1997). Coarse-grained graphic textures are common in fertile granites: graphic K-feldspar, graphic muscovite, and rarely graphic tourmaline and graphic garnet (Selway et al., 2005).

    Although the geochemical behavior of elements in the fertile granites has been identified and various indicators have been introduced, a ternary diagram consisting of trace elements has rarely been presented. A ternary diagram will bring together more variables and more precisely distinguish mineralized and non-mineralized intrusions because it simultaneously uses the positive and negative correlations between those variables. In this paper, bulk-rock geochemistry of S-type peraluminous granites was studied to identify the indicators of tantalum mineralization. For this purpose, whole-rock data of the granites (Ta-bearing and barren ones) and some LCT pegmatites all over the world were compiled, they have been analyzed using GCDkit (GeoChemical Data Toolkit) software (www.gcdkit.org), and the best ternary diagrams have been extracted to discriminate productive and barren samples.

  • At first, a large number of published articles about whole-rock geochemistry of LCT granitic suites were studied, and a whole-rock database consisting of world-class Ta-hosted granites (in some granites Ta is the main ore mineral and in others, it is a by-product) and barren ones was created. Tanco, Moose, and Kenticha pegmatites and Beauvoir, Abu Dabbab, Nuweibi, and Orlovka granites are well-suited examples of Ta-hosted granites in our database. In two cases such as Yashan batholith (Yichun Deposit) and Limiu granite, tantalum is a by-product, Allapata granite is a barren granite which is located near a mineralized pegmatite, and Eshan, Baneh, Kolah Ghazi, Obudu, Bafoussan, Elat, Nova Scota (including Port Mounton, Shelburne and Barrington Passage), Nisa-Albuquerque, Jalama, Khangilay and Armorican (including Guerande, Lizio, and Questemberg) are barren granite examples. The case studies are peraluminous (A/CNK > 1.1) and S-type (Table 1) in origin. Extra information about them is summarized in Table 2. Also, the references which are listed in Table 2 can be studied for further information.

    S-type index Indicator minerals SiO2
    (63 wt.%–77 wt.%)
    K2O
    (2 wt.%–5 wt.%)
    Na2O
    (< 3.2 wt.%)
    High Rb
    (ppm)
    Low Sr
    (ppm)
    High P2O5
    (wt.%)
    Tanco Presence of Al-rich minerals (mostly garnet) and absence of hornblende 76 2.96 3.81 4532 - 0.86
    Moose 70 1.94 5.78 578.5 83.8 1.24
    Kenticha 75 1.17 5.01 689 15.12 0.23
    Abu Dabbab 74 3.05 6.1 530.34 12.75 -
    Nuweibei 75 3.95 5.59 476.5 8.06 -
    Beauvior 69 3.37 4.39 2 190 257.7 1.24
    Orlovka 71 5.2 4.67 1 427 3.3 0.02
    Yichun 72 3.81 6.63 2 393 28.3 0.28
    Limiu 73 3.96 4.8 1 404 17.01 0.19
    Allapata 74 4.7 3.8 229 118 0.04
    Nisa-Albuquerque and Jalama 73 4.5 3.6 361 44.5 0.43
    Elat 74 3.7 4.6 94 300 0.05
    Khangilay 75 4.6 4 492 70.4 -
    Guerande 73 4.3 4.3 262 115.6 0.24
    Lizio 72 5 3.4 282 69 0.3
    Questemberg 74 4.5 3.5 310 52 0.32
    Bafoussan 70 5.1 3.5 229 298 0.16
    Obudu 72 7.4 2.4 127.5 327 0.05
    Kolah Ghazi 66 3.8 2.4 189 131 0.15
    Baneh 69 3.8 3.5 163 259.5 0.19
    Eshan 71 4.7 3.1 207 252 0.12
    Port Mouton 71 3.8 3.8 137 174 0.26
    Shelburne 69 3.4 3.5 152 143 0.31
    Barrington Passage 60 2.2 3.3 90 408 0.33

    Table 1.  The indexes that show considered granites are S-type in origin. Although some factors (like Na2O content) are not as much as the S-type index, the authors suggest S-type origin for the granites

    Granite intrusion Location Type Lithology Numbers References
    Tanco Manitoba, Canada Ta-Li-Cs Pegmatite 2 Stilling et al. (2006)
    Moose Territory, Canada Ta-Nb-Li Pegmatite 4 Anderson et al. (2013)
    Kenticha Ethiopie, Africa Ta-Be-Li Pegmatite 11 Mohammedyasin et al. (2017)
    Abu Dabbab Egypt Ta Albite granite 7 Gaafar and Ali (2015)
    Nuweibei Egypt Ta Albite-microcline granite 6 Gaafar (2014)
    Albite-feldspar granite 5
    Albite-zinnwaldite granite 2
    Beauvior France Ta Albite-lepidolite-zinnwaldite granite 38 Cuney et al. (1992); Raimbault et al. (1995)
    Orlovka Russia Ta Two mica granite 1 Badanina et al. (2010)
    Microcline-albite granite 2
    Amazonite-albite granite 6
    Yichun China Li-Ta-Nb Muscovite granite 9 Yin et al. (1995)
    Lithium-mica granite 4
    Topaz-lepidolite granite 10
    Limiu Russia Sn-W-(Ta-Nb) Lithium-fengite granite 19 Zhu et al. (2001)
    Topaz-albite granite 27
    Albite granite 2
    Microcline-albite granite 9
    Allapata Karnataka, India Barren (near to a mineralized pegmatite) Monzo-granite 15 Sarbajna et al. (2018)
    Nisa-Albuquerque and Jalama Egypt Barren Granite 6 Ramirez and Menendez (1999)
    Elat Israel Barren Granite 6 Eyal et al. (2004)
    Khangilay Russia Barren Biotite granite 1 Badanina et al. (2010)
    Muscovite granite 1
    Guerande France, Armorican Barren Biotite granite 6 Ballouard et al. (2015)
    Muscovite granite 10
    Lizio Biotite-two mica granite 8 Tartèse and Boulvais (2010)
    Questemberg Two mica granite 7
    Bafoussan Cameroon, Africa Barren Biotite granite 10 Djouka-Fonkwé et al. (2008)
    Metafeldspar granite 5
    Two mica granite 3
    Obudu Nigeria Barren Pegmatite 7 Edem et al. (2015)
    Kolah Ghazi Iran Barren Granodiorite 3 Bayati et al. (2015)
    Granite 13
    Baneh Iran Barren Biotite granite 6 Amini et al. (2004)
    Garnet-bearing granite 4
    Eshan Yunnan, China Barren Biotite granite 12 Hu et al. (2017)
    Two mica granite
    Port Mouton Nova Scota, Canada Barren Granodiorite 2 Currie et al. (1998)
    Tonalite 2
    Shelburne Granodiorite 5
    Barrington Passage Tonalite 3

    Table 2.  Database of the S-type peraluminous granitic systems

    Next, the whole-rock data of the fertile and barren granites and pegmatites have been collected, and marker elements or ratios have been recognized. After that, statistical analyses have been done using GCDkit software written in R language, and all of the possible binary and ternary diagrams which are able to discriminate Ta-hosted and barren granites have been detected. Finally, the best ones were selected. Besides, trace elements have been used in the diagram because they are rarely influenced by alterations. More attention has been paid to granites in comparison to pegmatites because of their important role in the search for pegmatite dikes. Practically, the pegmatites are used to show that the presented diagrams can distinguish all Ta-bearing systems whether highly evolved or not.

  • The LCT systems are characterized with low Fe, Mg, Ca, Ti, Sr, Ba, and Zr and high Si, Al, F, P, Rb, Be, Li, Ga, Nb, Ta, Sn and Cs (Breaks and Tindle, 1997; Cerny, 1989; Pollard, 1989; Cerny and Meintzer, 1988). They are usually known as highly evolved units containing a low K/Rb ratio (less than 270) (Cerny, 1989). Contrary to Nb-Y-F enriched granite-pegmatite systems, they are poor in rare earth elements (Irber, 1999; Cerny, 1989; Cerny and Meintzer, 1988). Ratios such as Nb/Ta, Zr/Hf, Fe/Mn, Al/(1 000×Ga) and Mg/Li decrease during fractionation of the productive peraluminous granitic melt and their reduction increases during the next magmatic-hydrothermal processes. The Nb/Ta ratio varies between < 2 and 25 in granites (Green, 1995). It can be affected by fractional crystallization (Stepanov et al., 2014; Raimbault et al., 1995) and/or late magmatic fluids interaction (Tartèse and Boulvais, 2010; Dostal and Chatterjee, 2000). Based on Ballouard et al. (2016) Nb/Ta of ~5 differentiates between a purely magmatic system (Nb/Ta > 5) and a magmatic-hydrothermal one (Nb/Ta < 5). Also, they suggested that in an Nb/Ta versus Zr/Hf diagram, rare-element granites are characterized by even lower Zr/Hf ratios (< 18) with Nb/Ta ratios that are still < 5. Some researchers claimed that Ta-hosted granites have the lowest amounts of the Zr/Hf ratio (from 10 to 2) (Zaraisky et al., 2009, 2000). Additionally, the Al/Ga vs. the Y/Ho diagram seems to be a useful tool for discriminating different kinds of highly evolved granitic melt and products of their hydrothermal alteration. Actually, the Ga content of evolved granites is more than that of greisens (Breiter et al., 2013). Moreover, the Mg/Li ratio is less than 30, and a lower Mg/Li ratio (Mg/Li < 10) indicates elevated Li contents (Cerny, 1989; Beus, 1968).

  • Tantalum-hosting granites were generally formed during syn/post-collisional magmatism followed by subsequent subduction. They mainly originated from sedimentary crustal sources and occurred in various geological times (Selway et al., 2005; Barbarin, 1999). The references listed in Table 2 can be studied for further information about geological background and tectonic environment of the case studies, separately.

    Partial melting of a preexisting sedimentary source rock produces an S-type granitic melt, which may be enriched in Li, Cs, and Ta, due to the presence of metamorphic biotite (containing Li and Rb; Selway et al., 2005), cordierite (containing Be; London, 2004) and Ti-bearing minerals (Stepanov et al., 2014). Fractionation of the granitic melt is an important process in concentrating incompatible elements. As rock-forming minerals (including quartz, K-feldspar, plagioclase, and mica) crystallize and separate, the residual melt becomes increasingly enriched in incompatible rare elements, and the resulting crystallization products evolve from barren to fertile granite. The fertile granite melt continues to become enriched in incompatible rare elements and volatiles as minerals crystallize. The incompatible elements crystallize from the final residual melt into pegmatitic minerals, such as spodumene, tantalite, cassiterite, and pollucite.

    Tantalum mineralization is usually associated with Li minerals in evolved pegmatites. Elevated Li and F contents in the silicate melt increase the solubility of Ta and delay tantalite crystallization (Linnen, 1998). Crystallization of Li (petalite or spodumene) or Li-F (lepidolite) minerals in the pegmatite, or diffusion of the Li and F out of the pegmatite and into the host rocks, results in a sudden drop in the solubility of Ta, with precipitation of columbite-tantalite.

    With increasing fractionation, the Nb/Ta ratio decreases and the composition of the columbite-tantalite group minerals evolves from manganocolumbite to manganotantalite (Badanina et al., 2010; Selway et al., 2005; Linnen and Keppler, 1997; Sinclair, 1996).

  • As a result, it is expected that Ta-hosted granitic systems contain high amounts of Ta and Nb, and Ta enrichment becomes more than Nb due to low-temperature partial melting, fractional crystallization, the lower solubility of manganocolumbite in comparison to manganotantalite, separation of an immiscible F-rich hydrosaline melt and hydrothermal transfer of Ta and Nb in F-rich solutions under reducing conditions. In our database, Ta and Nb contents are more than their average abundance in granitic rocks (Taav and Nbav are 2.5 ppm and 21 ppm, respectively (Beus and Grigorian, 1977)), and, also, Ta enrichment is more than Nb (means that Ta/Taav is more than 5, while Nb/Nbav is more than 1). However, the absolute value of Nb is higher. In addition, the Nb/Ta ratio must be low (less than 5) for rare-element granites (Ballouard et al., 2016). Similarly, all fertile samples have Nb/Ta values less than 3 in our database. Since a few samples belonging to barren plutons may contain low Nb/Ta ratio, fertile samples are the ones that not only have low Nb/Ta ratio but also high Nb and Ta content (Figs. 1 and 2).

    Figure 1.  Nb/Ta versus Ta/Taav diagram for peraluminous granites. Tantalum-hosted granites (colored symbols) are the ones which contain not only low Nb/Ta (less than 3) but also high Ta values (Ta/Taav > 5). Although, Allapata is a barren granite, some samples of it are located in productive part. These anomalies may indicate the relationship of Allapata with mineralized pegmatite. All the symbols in the following figures are the same with Fig. 1.

    Figure 2.  Nb/Ta versus Nb/Nbav diagram for peraluminous granites. Tantalum-hosted granites (colored symbols) are ones that contain not only low Nb/Ta (less than 3) but also high Nb values (Nb/Nbav > 1).

    Additionally, during the crystallization of granitic melt, Zr is preferably removed from the melt and accommodated in the solid phase (crystallizing accessory zircon), whereas Hf relatively enriches the residual melt. Consequently, the Zr/Hf ratio resembling the Nb/Ta ratio systematically decreases along with the crystallization trend of a melt (Zaraisky et al., 2009). The positive correlation between these ratios is illustrated in Fig. 3. Almost all Ta-bearing samples plot in the field which is restricted by Nb/Ta < 3 and Zr/Hf < 18. It is similar to the binary diagram that Ballouard et al. (2016) introduced. However, some barren samples fall into the field that is limited by Nb/Ta < 5 (also < 3) and Zr/Hf < 18, too.

    Figure 3.  Nb/Ta versus Zr/Hf diagram for the peraluminous granites. Tantalum-hosted granites (colored symbols) have Nb/Ta < 3 and Zr/Hf < 18. Considering that a sample may be located in the area where it is restricted between Nb/Ta < 3 and Zr/Hf < 18, whereas it belongs to a barren pluton.

    Furthermore, melt becomes relatively enriched with Ga during fractionation. So, the Al/Ga ratio decreases. The maximum Ga enrichment occurs at the end of fractionation of water- and F-rich melts, where the differences between the ionic radii and complexation potentials of Ga and Al become more distinct in Al-bearing minerals. Conversely, Ga becomes dispersed during hydrothermal greisenization. Hence, the Ga content of fractionated granites is more than greisens (Breiter et al., 2013). In our database, Ga values correlate negatively with both the Nb/Ta and the Zr/Hf ratios (Figs. 4 and 5), and mineralized samples contain high amounts of Ga (more than 20 ppm). It means that Ga can play an important role in differentiating between barren and Ta-hosting intrusions.

    Figure 4.  Nb/Ta versus Ga diagram for peraluminous granites. Tantalum-hosted granites (colored symbols) have low Nb/Ta (less than 3) and high Ga content (more than 20 ppm).

    Figure 5.  Zr/Hf versus Ga diagram for peraluminous granites. Tantalum-hosted granites (colored symbols) have low Zr/Hf (less than 18) and high Ga content (more than 20 ppm). There is a negative relationship between Ga and Zr/Hf.

    As it can be seen in the mentioned binary diagrams, barren samples may be wrongly located in the productive area. Also, using more elements makes discrimination of fertile and barren samples more valid. Therefore, we come up with a ternary diagram. In the Ta-hosted granites database, the strongly positive relationship between variables which must be located in the same apex (such as (Nb, Ta) and/or (Nb/Ta, Zr/Hf)) and the negatively correlation between indicators that must be situated in opposite corners (such as (Ga, Nb, and Ta) and/or (Ga, Nb/Ta, and Zr/Hf)) make the Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) diagram a powerful tool to distinguish mineralized and non-mineralized samples. Only the samples that are enriched in Nb and Ta, contain relatively high Ga values and have low Nb/Ta (less than 3) and Zr/Hf (less than 18) ratios place in the fertile section (Fig. 6).

    Figure 6.  Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) ternary diagram to discriminate Ta-hosted and barren granites. Also, it illustrates that Allapata can be a prospective case and needs further study.

    Tantalum-bearing samples have low Fe/Mn values (less than 15 in our database), because of replacing Mn with Fe in Nb and Ta-bearing minerals, tourmaline and garnet during later stages of fractionation (Whitworth, 1992; Cerny, 1989). As shown in Fig. 7, the Ga-(Nb+Ta)-(Nb/Ta)(Fe/Mn) ternary diagram can be a useful tool. But, it is not able to clearly discriminate fertile and barren samples. Since major elements are affected by alterations, the diagrams which are composed of trace elements and their ratios are likely more valid.

    Figure 7.  Ga-(Nb+Ta)-(Nb/Ta)(Fe/Mn) ternary diagram. It is a little hard to draw a clear margin between productive and non-mineralized samples.

    However, it seems that the Ga-(Nb+Ta)-(Nb/Ta)(K/(10Rb)) ternary diagram approximately differentiates fertile and barren samples (Fig. 8), K/Rb ratio has been known as an indicator of fractional crystallization. Actually, this diagram distinguishes samples according to their degree of fractionation, and the samples that are highly fractionated (contain low K/Rb values (less than 150 in our database)) are not necessarily mineralized.

    Figure 8.  Ga-(Nb+Ta)-(Nb/Ta)(K/(10Rb)) ternary diagram.

    It needs to be explained that our results are mainly based on parent mineralized granites. However, the presented diagrams can discriminate Ta-bearing granite-pegmatite systems whether they are affected by magmatic interactions or magmatic-hydrothermally ones.

    Moreover, the fractionated peraluminous granites are generally associated with Sn-W deposits, too. To find out whether the presented diagrams can discriminate the Sn-W-bearing granites and Ta-bearing ones, more studies (based on the behavior of Sn- and W-bearing minerals, metasomatic processes and a new database including Sn-W-bearing granites) are needed.

  • Ternary diagrams are excellent at differentiating prospective and non-mineralized granitic intrusions, which can facilitate the search area in the preliminary exploration of pegmatite dikes. The Ga-(Nb+Ta)-(Nb/Ta)(Zr/Hf) ternary diagram is an example of a diagram that is able to discriminate Ta-hosting and barren peraluminous granitic intrusions. In this diagram, productive samples are situated in the fertile part where Nb and Ta contents are high, Ga amounts are high and Nb/Ta and Zr/Hf ratios simultaneously are the lowest. It is necessary to consider that not only Nb and Ta content should be high but also Ta enrichment ought to be more than Nb. In our database, Ta-hosted granites contain Ta/Taav > 5, Nb/Nbav > 1, Nb/Ta < 3, Zr/Hf < 18, and Ga > 20 ppm. Also, the Ga-(Nb+Ta)-(Nb/Ta)(Fe/Mn) and Ga-(Nb+Ta)-(Nb/Ta)(K/(10Rb)) ternary diagrams can be useful tools. But, they should be used cautiously. Because the Fe/Mn ratio (which is less than 15 in the studied fertile granites) can be affected by alterations and K/Rb (which is less than 150) is the indicator of fractionation degree.

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