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REE MOBILITY AND TETRAD EFFECTS IN BAUXITES: AN EXAMPLE FROM THE KANISHEETEH DEPOSIT, NW IRAN.

1. INTRODUCTION

Rare earth elements (REEs) have been used as a good and beneficial tool for investigation of geochemical processes during development and evolution of wide range of deposits in different environments (Chen and Zhao, 1997; Censi et al., 2007; Karadag et al., 2009; Hannigan et al., 2010; Mongelli et al., 2016; Abedini et al., 2018a,b; Rezaei Azizi et al., 2018a,b). This coherent group of elements from La to Lu with atomic number from 57 to 71 displays very similar behavior due to their very similar ionic charge and radius. Except Ce (III and IV) and Eu (II and III), the rest of REEs in this group are characterized by trivalent oxidation state (Henderson, 1984). However, REE fractionations and redistributions have been reported from various bauxite deposits due to evolution of geochemical systems in various mechanisms and conditions such as mobilization of REE, supergene processes, surficial weathering, diagenetic processes, presence of REE-bearing mineral phases, pH of solutions and/or depositional environment, and existence of organic and/or non-organic complexing ligands (Mondillo et al., 2011; Abedini and Calagari, 2013; Mongelli et al., 2014, 2016, 2017; Liu et al., 2016; Buccione et al., 2016; Hou et al., 2017; Khosravi et al., 2017; Long et al., 2017; Radusinovic et al., 2017; Torro et al., 2017; Yuste et al., 2017; Chen et al., 2018). Generally, these factors can widely affect the mobilization and fixation of REE in the mineral phases of the bauxite ores.

The normalized REE distribution patterns generally display very smooth curves because of their gradual decrease in ionic radius. Nevertheless, some irregular shapes in the normalized REE curves can be observed owing to the lanthanide tetrad effect (LTE) (Censi et al., 2007; Rezaei Azizi et al., 2017; Abedini et al., 2018b,c). The LTE as a geochemical feature brings about four separate groups of lanthanides in the normalized REE distribution patterns. The separated groups correspond to the one-fourth, half, three-fourth, and filled of 4f orbital in the lanthanides (Jahn et al., 2001). Therefore, these tetrads are labeled as first, second, third, and fourth tetrads corresponding to La-Nd, Pm-Gd, Gd-Ho, and Er-Lu, respectively. In fact, Gd is the only lanthanide element which takes part both in the second and third tetrads. The occurrence of such irregular patterns was reported initially from aqueous systems by using different terms such as 'kinked effect', 'zigzag effect', 'tetrad effect', and 'double-double effect' (Fidelis and Siekierski, 1966; Peppard et al., 1969). Ionic radius, electric charge, pH, surficial absorption, mineral phases and stability of REE-complex are believed to be the main chemical parameters controlling the REE distributions in geological environments (Sasmaz et al., 2018). McLennan (1994) declared that the lanthanide effect (LTE) or double-double feature (Masuda and Ikeuchi, 1978) is tetrad one of the factors which controls the REE distribution and affects their behavior. Despite controversy on the origin of this phenomenon by some researchers (McLennan, 1994; Nakamura et al., 2007), the LTE has been widely applied as a new and good geochemical indicator to assess the geochemical processes involved during bauxite formation (Akagi et al., 1993; Broska et al., 2006; Chu et al., 2009; Cunha et al., 2012; Abedini et al., 2017; Rezaei Azizi et al., 2017; Abedini et al., 2018a).

Studies showed that three different forms of tetrad effect usually occur in the normalized REE distribution patterns. The first form has a concave or W-shaped curve which is mostly observed in the low-temperature geological environments such as waters, limestone, marine sediments, and phosphorite (Masuda and Ikeuchi, 1978; Mazumdar et al., 2003; Yamamoto et al., 2004; Peretyazhko and Savina, 2010; Feng et al., 2014; Abedini et al., 2017). The second form of tetrad effect is characterized by a convex or M-shaped curve in their normalized REE distribution patterns indicating magmatic origin (e.g., granite) and associated evolved systems such as hydrothermal ore and gangue (e.g., fluorite) minerals (Kawabe, 1995; Haapala and Lukkari, 2005; Nardi et al., 2012; Cao et al., 2013; Lee et al., 2013; Rezaei Azizi et al. 2017). Finally, in the third form has both convex and concave (M- and W-shaped) curves which are commonly observed in natural glasses, phosphatic shale as well as in uranium, fluorine, and aluminum (bauxite) deposits (Takahashi et al., 2002; Minuzzi et al., 2008; Zhao et al., 2010; Rezaei Azizi et al., 2017; Abedini et al., 2017; Abedini et al., 2018b). The tetrad effect phenomenon can be used as a good geochemical indicator to assess various geological environments (Akagi et al., 1993; McLennan, 1994; Bau, 1996). However, the relationship between the calculated values of the tetrad effect and some geochemical factors such as Y/Ho ratio has not been investigated. Previous study indicates that LTE as a geochemical indicator can be used to assess the geochemical processes during buxitization and interpretation of REE distribution in this type of deposits (Abedini et al., 2018a).

Many bauxite deposits in Iran have located on the Irano-Himalaya karst bauxite belt and were developed during Permian-Cretaceous periods (Abedini and Calagari, 2014). The Kanisheeteh bauxite deposit is located in ~19 km east of Bukan city, West-Azarbaidjan province, NW Iran. The geology, mineralogy, petrography and major element geochemistry of this deposit have been evaluated in detail by Calagari and Abedini (2007). The main aim of this study is mostly to consider the LTE and its possible source, the causes of fractionation between Y-Ho and Sm-Nd, the behavior of rare earth elements, and the Ce and Eu anomalies in the selected bauxite profile. Additionally, the correlation between the calculated values of the tetrad effect with aforementioned geochemical factors will be also discussed.

2. GEOLOGICAL BACKGROUND AND PREVIOUS WORKS

Previous studies indicate that most of Iranian bauxite deposits including the Kanisheeteh are a part of the Irano-Himalaya karst bauxite belt (Abedini and Calagari, 2014). Most of these bauxite deposits were likely formed during Permian to Jurassic periods (Calagari and Abedini, 2007). Detailed geological studies were carried out about a decade ago in this district by Calagari and Abedini (2007).

According to the structural map of Iran (Nabavi, 1976), the Kanisheeteh bauxite deposit is a part of the Sanandaj-Sirjan metamorphic belt (Figure 1). Stratigraphically, the litholigic units cropping out in this district can be summarized from the oldest to the youngest as follow (Figure 2): (1) phyllite, gneiss, and volcanic eruptions of Precambrian age, (2) limestone, dolomite, and shale known as Mila Formation of Cambro-Ordovisian age, (3) sandstone known as Dorud Formation of lower Permian age, (4) limestone, dolomite, and shale known as Ruteh Formation of middle-upper Permian age, (5) bauxite horizon of Permo-Triassic age, (6) limestone and dolomite (Elika Formation) of Triassic age, (7) shale and sandstone (Shemshak Formation) of Jurassic age, (8) limestone and clastic sediments (Qom Formation) of Miocene age, and (9) recent alluvium of Quaternary age. Field observations revealed that some diabasic rocks occurring as sills and irregular patches within the lower Permian carbonates acted as plausible parent rocks (Calagari and Abedini, 2007). The bauxite horizon in this district has a NE-SW trending with about 8 km long and dips 35-90[degrees] NW. This horizon shows variable thickness (8 to 28 m) and lies along the boundary of late Permian (carbonates, shale) and Triassic (carbonates) units. The bauxite ores in this horizon are characterized by different colors like greenish cream, red, brownish red and dark red. Thus, based on the colors and textural specifications, the studied bauxite ores were divided into the following subgroups (Fig. 3): (1) dark red bauxite (DRB), (2) red bauxite (RB), (3) brownish red bauxite (BRB), (4) greenish cream bauxite (GCB), and finally (5) yellow bauxite (YB). Previous studies (Calagari and Abedini, 2007) indicated that most common mineral phases in this horizon were diaspore, hematite, kaolinite, nacrite, chlorite, pyrophyllite, quartz, muscovite, rutile, calcite, and magnesio-sidero-riebeckite. Furthermore, petrographic studies indicate that YB and GCB parts of the horizon are characterized by pressure shadow structures, while the BRB, RB, and DRB parts are generally marked by nodular, panidiomorphic-granular, ooidic, and pseudo-breccia textures.

3. METHOD OF INVESTIGATION

For chemical analysis thirty ore samples (with 0.5 m interval) from the bauxite profile (Figure 2) and three rock samples from diabase unit in the studied district were collected (Fig. 3). The chemical analyses (for 33 samples) were performed at the ACME Analytical Laboratories Ltd. in Vancouver, Canada. Trace elements including REE were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) method. During analytic processes quality controls were subjected to the same center with blanks, duplicates, and standard materials (STD SO-18) based on CQ report, certificate of analysis, detection limits (DT), and accuracy. About 0.2 gr of each sample was carefully weighed, and then was treated by a lithium metaborate/tetraborate fusion and diluted in nitric acid as a digestion agent. These solutions were digested in hot Aqua Regia to be analyzed by ICP-MS method. Detection limits for elements are as follows: (1) for Y, La, and Ce is 0.5 ppm; (2) for Nd, and Lu is 0.1 ppm; (3) for Tb, Ho, and Tm is 0.01 ppm; (4) for Zr is 2 ppm; and (5) for Pr, Sm Eu, Er, and Yb is 0.03 ppm. Furthermore, all the computations and plots were carried out by using MATLAB R2016b computer software.

4. RESULTS

4.1. REE, Y, AND ZR CONCENTRATIONS IN THE PROFILE

Table 1 shows the concentration values of Y, Zr, and REE in the bauxite samples of the studied profile. According to this table, the Y concentration values vary within the range of 26.1-32.6 ppm. The inverse trend is observed in the Zr concentrations which vary from 220.9 ppm to 289.5 ppm.

The [SIGMA]REE values of the bauxite samples are within the range of 337.61-792.56 ppm (see Table 2). The LREE and HREE values of the bauxite samples vary from 318.55 ppm to 732 ppm and from 19.06 ppm to 60.56 ppm, respectively (see Table 2). The [(LREE/HREE).sub.N], Y/Ho, and Sm/Nd ratios of the bauxite samples range from 4.8 to 6.66 and from 14.49 to 25.75 (average of 21.71), and from 0.15 to 0.27, respectively (see Table 2).

4.2. CE AND EU ANOMALIES

To calculate the Ce/Ce* and Eu/Eu* values of the studied samples the following equations proposed by Bau and Dulsky (1996) and Monecke et al. (2000) were applied:

Eu/Eu* = [Eu.sub.N]/[[(Sm x Gd).sup.0.5]] (1)

Ce/Ce* = [Ce.sub.N]/[[(La x Pr).sup.0.5]] (2)

In these equations, N refers to chondrite values proposed by Anders and Grevesse (1989). Based on the computed results (see Table 2), the values of Ce/Ce* in the bauxite samples vary from 0.92 to 2.73. Similarly, the Eu/Eu* values in the diabase and bauxite samples lie within the range of 0.7-0.89 and 0.49-0.81, respectively.

5. DISCUSSION

5.1. REE VARIATION IN THE PROFILE

The variations of rare earth elements along the studied profile of the bauxite horizon are illustrated in Figure 4. The [SIGMA]REE concentration increases gradually from the top to the bottom of the horizon (Figure 4a). The values for first ([RET.sub.1]), second ([RET.sub.2]), third ([RET.sub.3]), and fourth ([RET.sub.4]) tetrads of REE (Table 2) are illustrated in Figure 4b, Figure 4c, Figure 4d, and Figure 4e, respectively. As illustrated in these figures, the distribution of total REE and REE of individual tetrads display similar trends from the top of the bauxite horizon to ~6.5 m (YB and GCB) and different trend as REE concentrations increase from 6.5 m toward the bottom (BRB, RB, and DRB) of the horizon. These trends are probably owing to the pH increase of the solution responsible for bauxitization. This is quite consistent with the results of the previous studies conducted by Calagari and Abedini (2007). As shown in Figure 4e, fourth tetrads (Er to Lu) are more sensitive to pH condition and can better reflect the paleo-redox conditions of the depositional environment. The concentration values of [RET.sub.4] display a sharp decrease in samples Bs-15 and Bs-16 which may also be related to the pH changes of the solution during bauxitization processes. Therefore, it can be deduced that the upper part of the horizon (from the top to ~6.5 m) (YB and GCB) have likely been affected by low-pH solutions but, the lower part (from 6.5 m to the bottom) (BRB, RB, and DRB) have been possibly affected by relatively higher pH solutions.

The Ce/Ce* values in the studied profile (Figure 5a) gradually increase from the top to ~6.5 m (YB and GCB), but fluctuate from 6.5 m to the bottom of the horizon (BRB, RB, and DRB). The variable trends in Ce/Ce* values can be related to change in the redox conditions of the depositional environment, because Ce is strongly an oxide sensitive element (Hannigan et al., 2010). The increase in the oxygen fugacity of depositional environment causes trivalent Ce to be converted into tetravalent Ce which has higher charge and same ionic radius (Kraemer et al., 2016), and becomes less mobile relative to other trivalent rare earth elements (Constantopoulos, 1988). Therefore, oxidation conditions of an environment gives rise to an increase in positive Ce anomaly in the precipitant materials (Mongelli et al., 2014). Thus, it can be concluded that the upper part of the horizon (YB and GCB) in this district was likely deposited under higher oxygen fugacity condition due to the percolating oxic meteoric waters/solutions. These anomalies decrease in the lower parts of the horizon (BRB, RB, and DRB) due to the complexation of [Ce.sup.4+] with carbonate ligands and hence its leaching during bauxitization processes (Karadag et al., 2009).

Based on the SHAB theory proposed by Dai (1987), under low oxygen fugacity, some reduced species such as CO, C[H.sub.4], [S.sub.2][O.sub.3.sup.2-], [S.sup.2-], and H[S.sup.-] increase in aqueous system. In such an environment, [Eu.sup.2+] commonly prevails and remains in solution as stable [Eu.sup.2+]-complex and hence is available for incorporation into Eu precipitates (positive anomaly). In contrast, under the higher oxygen fugacity, [Eu.sup.3+] dominates in aqueous system and needs hard bases such as O[H.sup.-], S[O.sup.2-.sub.4], and C[O.sup.2-.sub.3] (specially O[H.sup.-]) to become precipitated, consequently negative Eu anomaly is conceivable (Chen and Zhao, 1997). Furthermore, Abedini et al. (2016) suggested that strong negative Eu anomaly can be related to the higher f[O.sub.2] and low pH of solutions. As Figure 5b demonstrates, the Eu anomaly of the bauxite samples increases gradually from the top to the bottom of the bauxite horizon. This signifies that the studied horizon was likely formed by solutions experiencing low-pH and oxidizing conditions during deposition.

Y/Ho (Figure 5c) and Sm/Nd (Figure 5d) ratios of the bauxite samples display opposite trends in the horizon. The Sm/Nd values increases gradually from the top toward the bottom of the profile whereas, the Y/Ho values decreases with depth. The Y/Ho values of the samples from the yellow bauxite (YB) are almost close to those of chondrite which vary from 24 to 34 (Bau, 1996). The Y and Ho as a geochemical pair with similar behavior can yield useful information about depositional conditions because their distribution in geochemical processes is controlled by their CHARAC (CHArge-RAdius Control) parameter (Bau, 1996). This means that variation of Y/Ho ratio in geochemical processes reflects the prevailing physico-chemical conditions of depositional environment. Therefore, the non-CHARAC ratios and behavior of coherent elements such as Y and Ho can be interpreted by and/or attributed to tetrad effect feature in the geochemical processes (Abedini et al., 2017; Rezaei Azizi et al., 2017; Abedini et al., 2018b). The plots of Y/Ho ratios versus Y (Figure 6a) and Ho (Figure 6b) indicate a meaningful negative correlation as follows:

Ho = 82.11xexp(-0.2767x /Ho)+1.522xexp(-0.015xY/Ho) (3)

(r = -0.95 with 95% confidence)

Y=456.6xexp(-0.2243xY/Ho)+16.12xexp(0.0199xY/Ho) (4)

(r = -0.84 with 95% confidence)

Y/Ho ratio exhibits an increasing trend with decreasing Y and Ho concentrations in all of the samples. The correlation between Ho and Y/Ho is equal to -0.95 which is higher than that between Y and Y/Ho (r = -0.84) indicating Y/Ho ratios of the studied samples were likely controlled merely by increase of Ho rather than Y. This continuous decrease in Y/Ho ratio from the top to the bottom of the horizon may be as the result of gradual pH increase of the solutions percolating from the upper part of the horizon toward the carbonate bedrocks.

5.2. TETRAD EFFECT

In order to calculate the deviation of irregular curves from normal and smoothed forms in the chondrite-normalized REE distribution patterns (due to the LTE), some quantification methods have been proposed (Irber, 1999; Monecke et al., 2002). The following equation (5) was applied to compute the size of tetrad effect in each tetrad group of REE:

[mathematical expression not reproducible] (5)

In this equation the concentration of elements in each tetrad is shown by X. meanwhile, the A, B, C, and D indicate first, second, third, and fourth elements in each individual tetrad group, respectively. Moreover, i equals to 1, 2, 3, and 4 which indicating the first, second, third, and fourth tetrad, respectively. The tetrad effect is regarded to be significant for [T.sub.i] greater than zero. This means that [T.sub.i] equal to zero display no deviation between two middle elements of individual tetrads and therefore all four elements are in the straight line connecting the first and fourth elements.

In the calculation processes [T.sub.1] will not be computed if Ce behaves anomalously (Monecke et al., 2002). [T.sub.2] also cannot be calculated due to lack of Pm in natural geological systems (McLennan, 1994). Table 2 shows the computed values of [T.sub.3] and [T.sub.4] tetrad effect in the bauxite samples of the studied district. Based on the computed results, the sizes of [T.sub.3] and [T.sub.4] tetrad effect of the horizon vary within the range of 0.01-0.54 and 0.01-0.59, respectively.

5.3. COMPUTATION OF MASS CHANGE OF REE DURING BAUXITIZATION

Many computational methods have been proposed for calculation of mass changes of elements during geochemical processes such as alteration and weathering based on volume factor (Gresens, 1967), immobile element (Brimhall and Dietrich, 1987; MacLean and Kranidiotis, 1987), Absolute Weathering Index (AWI) (Nesbitt, 1979), isocon (Grant, 1986), mobility index (Ng et al., 2001), variation in ratios (Nesbitt and Markovics, 1997), chemical depletion fraction or CDF (Riebe et al., 2003), and immobile plateau (Gong et al., 2011). Generally, calculations of mass transfer in these methods are similar. Selection of a proper immobile element for implementing mass transfer is an important parameter. Previous studies have shown that some elements such as Al, Ti, Nb, Ta, Zr, Hf, and Th (MacLean et al., 1997; Braun et al., 1998; Duzgoren-Aydin et al., 2002; Little and Aeolus, 2006; Ma et al., 2007; Hastie et al., 2008; Ndjigui et al., 2008; Gong et al., 2011) generally have semi-mobile or immobile behavior during geochemical processes such as weathering and/or alteration of rocks. These elements are characterized by very low dissolution rate due to their high field strength (HFS) during geochemical processes (Little and Aeolus, 2006). As illustrated in Figure. 7, Zr displays the slightest concentration variations among these elements in the studied profile (with median of 256 ppm and standard deviation of 22.45) which is about 8.77%. The previous studies conducted by Calagari and Abedini (2007) on this horizon revealed that the diabasic rocks were the plausible precursor for the bauxite ores. Considering the diabase as precursor, the degree of mobility for elements in the studied profile was calculated by following equation proposed by Nesbitt (1979):

AWI (%) = [([C.sub.Bauxite]/[Zr.sub.Bauxite])/([C.sub.Parent]/[Zr.sub.Parent])-1] x100 (6)

In this equation C refers to a selected element and AWI will be in %. Duzgoren-Aydin et al. (2002) declared that equation (6) can be used with high accuracy for rocks experienced severe weathering processes. The computed values of AWI (%) in the bauxite samples (Table 2) are illustrated in Figure 8. As shown in this figure, rare earth elements of the first ([RET.sub.1]) and second ([RET.sub.2]) tetrads suffered depletion in the upper part of the profile and became enriched in the lower part. Among REE, Ce, Pr, Nd, and Sm show a slight depletion on the YB part of the horizon and this trend turns to enrichment in the other parts of the horizon. Eu, Gd, and Tb were depleted in most part of the horizon and became enriched only in DRB part of the horizon. Most of the third and fourth tetrad groups including Dy, Ho, Er, Tm, Yb, and Lu were depleted during bauxitization processes of the parent rocks. The depletion of REE in the upper part and their enrichment in the lower part of the profile are affiliated with leaching processes exerted by descending solutions.

5.4. FACTORS CONTROLLING THE BEHAVIOR OF REE

The [SIGMA]LREE (La-Eu) values in the studied profile increases from the top to the bottom of the horizon (Figure 9a). This implies that these elements were likely leached from the upper parts (YB and GCB) of the profile and precipitated in the lower parts (BRB, RB, and DRB). The [SIGMA]HREE (Gd-Lu) and [SIGMA]LREE values (Figure 9b) also increase with similar trend. The increase of HREE values display variation from the top to ~6.5 m (YB and GCB) relative to the lower parts of the horizon (BRB, RB, and DRB). The chondrite-normalized REE distribution pattern of the diabase samples (as precursor of the bauxite horizon) displays a remarkable M-shaped (convex) tetrad effect in the third and fourth tetrads (Figure 10a). The M-shaped in both the third and fourth tetrads of chondrite-normalized REE distribution pattern is also observed in samples of YB part of the horizon (Figure 10b). The M-shaped of the normalized REE patterns was likely formed due to interaction of water-rock during formation of bauxite.

As illustrated in Figure 10c, similar form of tetrad (M-shaped) in chondrite-normalized REE distribution patterns also remains constant with no meaningful variation which is likely due to relatively stable conditions during bauxitization and/or weathering/alteration as well as diagenetic processes. The threshold of changes in deposition and lateral geochemical processes can be observed in simultaneous convex (M-shaped) and concave (W-shaped) tetrad effect curves. These features can be attributed to change in the bauxitization conditions and/or solution(s) during weathering and alteration processes (Censi et al., 2007; Monecke et al., 2007; Wu et al., 2011; Nardi et al., 2012). As shown in Figure 10d, the occurrence of both M- and W- shaped curves in the chondrite-normalized REE patterns of the BRB part of the horizon in this district may be owing to changes in depositional conditions. This is quite consistent with previous evidences such as higher pH of the solutions and increase of [SIGMA]REE content in lower parts of the horizon (BRB, RB and DRB) as well. The conditions and/or solutions were likely stable in the lower parts of the horizon (e.g., BRB, RB and DRB) because of gradual increase in pH of the environment and consequently the convex (M-shaped) tetrad effect formed in the chondrite-normalized REE distribution patterns due to water-rock interaction during bauxite formation. Therefore, it can be deduced that YB and GCB parts of the horizon were probably affected by low-pH acidic solutions. On the other hand, the lower parts of the horizon (BRB, RB and DRB) experienced mostly alkaline conditions.

5.5. CORRELATION BETWEEN [T.sub.4] TETRAD EFFECT VALUES AND AWI%

The size of [T.sub.4] tetrad effect values in all of the bauxite samples from different parts of the horizon with the computed values of AWI% for the first, second, third, and fourth tetrads are illustrated in Figure 11, Figure 12, Figure 13, and Figure 14, respectively.

As shown in these figures, the REE distribution patterns versus [T.sub.4] tetrad effect values in bauxite samples from the studied profile indicate two separate populations. The first group of the bauxite samples (belonging to YB and GCB parts) is characterized by a wide range of [T.sub.4] tetrad effect values having negative correlation with AWI (%) which reflects acidic environment. In contrast, the second population of bauxite samples (belonging to BRB, RB, and DRB parts) is marked by relatively narrow range of [T.sub.4] tetrad effect values and positive correlation with AWI (%) of REE which reflects alkaline conditions. Therefore, it can be concluded that [T.sub.4] tetrad effect values of the bauxite samples can be used as a good geochemical tool to investigate the geochemical processes.

6. CONCLUSIONS

The results of chemical analyses and calculated values for Ce/Ce*, Eu/Eu*, tetrad effect values, Y/Ho ratios, and other geochemical parameters in the bauxite samples of the Kanisheeteh bauxite horizon indicate irregular patterns in chondrite-normalized REE distribution patterns. The occurrence of LTE in the third and fourth tetrads illustrates that REE distribution in this horizon was likely controlled by tetrad effect phenomenon. Based on the meaningful correlation between the sizes of [T.sub.4] tetrad effect and AWI% values, the conclusions are summarized as follows:

1. The distribution pattern of [SIGMA]REE from the top of the bauxite horizon to ~6.5 m (YB and GCB) is relatively constant whereas, it differs in the lower parts of the horizon (BRB, RB, and DRB) where the amounts of REE increase. This feature can likely be related to pH increase of the solution responsible for bauxitization in this district.

2. Occurrence of both convex (M-shaped) and concave (W-shaped) curves in the chondrite-normalized REE patterns indicate different depositional conditions during bauxitization of the parent rocks.

3. Based on the calculated Ce/Ce* values, it can be deduced that the Ce increment in the upper parts of the bauxite profile (YB and GCB) was due to the higher oxygen fugacity (relatively greater Eh) by percolating the oxidizing meteoric waters/solutions. The Ce decrement in the lower parts of the horizon (BRB, RB, and DRB), however, may be owing to complexation of [Ce.sup.4+] with carbonate ligands and thus its leaching during bauxitization processes.

4. The Y/Ho ratios of the bauxite samples display non-CHARAC behavior indicating presence of the tetrad effect feature in this system.

5. Based on the scatter diagrams of AWI% for all REE versus [T.sub.4], a population is characterized by a wide range of [T.sub.4] values and negative correlation with the AWI% (YB and GCB parts of the bauxite horizon) and the second one (BRB, RB, and DRB parts of the horizon) marked by relatively narrow range of [T.sub.4] values and positive correlation with AWI%. Therefore, it can be concluded that [T.sub.4] values of the bauxite samples can be utilized as a good geochemical tool to investigate the geochemical processes.

ACKNOWLEDGMENTS

This work was supported financially by the Research Bureau of Urmia University. We would like to express our thanks and gratitude to the authorities of this bureau. Our gratitude is further expressed to two anonymous reviewers for reviewing this manuscript and making critical comments and valuable suggestions, which have definitely improved the quality of this paper.

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Ali ABEDINI (1*), Mansour REZAEI AZIZI (1) and Ali Asghar CALAGARI (2)

(1) Department of Geology, Faculty of Sciences, Urmia University, 5756151818, Urmia, Iran

(2) Department of Earth Sciences, Faculty of Natural Sciences, University of Tabriz, 5166616471, Tabriz, Iran

(*) Corresponding author's e-mail: abedini2020@yahoo.com and a.abedini@urmia.ac.ir

ARTICLE INFO

Article history:

Received 17 October 2018

Accepted 3 January 2019

DOI: 10.13168/AGG.2019.0002
Table 1 Concentration values of REE, Zr and Y (in ppm) in the bauxite
ore samples of the studied profile. The detection limits for each
element are also shown.

    Detection  Bs-1      Bs-2     Bs-3  Bs-4      Bs-5    Bs-6   Bs-7
    Limit

Y   0.5         29.10    28.80   28.60   27.40   27.40    27.50   27.60
Zr  2          280.40   274.50  266.50  257.40  255.60   251.30  252.30
La  0.5         68.90    71.90   74.50   72.10   62.10    73.60   75.80
Ce  0.5        169.80   173.70  199.50  211.30  195.10   243.60  251.20
Pr  0.03        15.15    15.41   15.88   15.15   14.78    15.55   16.29
Nd  0.1         53.20    54.10   55.70   51.10   51.40    55.60   51.10
Sm  0.03         9.68     9.54   10.11    9.54    9.22    10.07   10.98
Eu  0.03         1.82     1.83    1.82    1.77    1.71     1.83    1.99
Gd  0.05         6.05     6.74    6.98    6.71    5.97     5.94    7.34
Tb  0.01         1.02     1.04    1.06    1.01    1.04     1.06    1.13
Dy  0.05         5.73     5.67    5.77    5.61    5.51     5.72     6.1
Ho  0.01         1.13     1.13    1.14    1.08    1.07     1.13    1.12
Er  0.03         3.38     3.36    3.37    3.24    3.22     3.35    3.46
Tm  0.01         0.53     0.54    0.55    0.51    0.53     0.54    0.55
Yb  0.03         0.66     0.62    3.61    3.54    3.49     3.62    3.75
Lu  0.01         0.56     0.55    0.55    0.56    0.54     0.51    0.56
    Detection  Bs-16   Bs-17    Bs-18   Bs-19   Bs-20   Bs-21    Bs-22
    Limit
Y   0.5         29.40    28.90   26.10   29.40   28.30    28.40   27.90
Zr  2          224.10   220.90  289.50  261.10  259.10   264.20  239.60
La  0.5         86.50    82.70   80.30   84.20   81.10    87.20   85.70
Ce  0.5        273.50   265.40  445.60  292.30  442.10   296.30  461.10
Pr  0.03        19.32    18.46   19.14   19.44   18.65    20.54   19.49
Nd  0.1         69.40    68.40   70.30   71.10   69.40    75.30   71.10
Sm  0.03        11.12    10.14   14.44   14.79   13.22    15.69   14.88
Eu  0.03         2.62     2.54    2.01    2.72    2.56     2.92    2.68
Gd  0.05         9.88     9.44    9.71    9.92    9.22    10.18    9.31
Tb  0.01         1.47     1.41    1.47    1.51    1.42     1.54    1.48
Dy  0.05         7.44     7.27    7.38    7.81    7.47     7.66    7.44
Ho  0.01         1.37     1.36    1.32    1.44    1.37     1.32    1.21
Er  0.03         3.21     3.87    3.99    4.18    4.09     4.05    3.94
Tm  0.01         0.62     0.63    0.66    0.68    0.69     0.66    0.67
Yb  0.03         4.12     4.11    4.45    4.79    4.81     4.65    4.51
Lu  0.01         0.64     0.63    0.71    0.73    0.74     0.73    0.71

      Bs-8    Bs-9    Bs-10     Bs-11    Bs-12   Bs-13  Bs-14    Bs-15

Y    26.80    28.50    28.50    28.10   28.60    28.40   29.50    29.90
Zr  251.30    258.0   250.10   243.50  247.10   242.30  251.30   239.60
La   76.80    80.70    81.20    84.50   88.10    88.20   91.10    94.80
Ce  299.80   299.50   211.30   305.40  335.40   298.50  311.20   278.50
Pr   16.82    17.88    18.22    18.19   20.08    20.49   21.41    21.19
Nd   60.10    64.50    65.40    70.80   74.10    74.50   79.40    77.10
Sm   11.45    12.55    12.89    14.45   11.12    15.45   16.78    15.88
Eu    2.06     2.26     2.36     2.65    2.77     2.82    3.04     2.94
Gd    7.57     8.12     8.32     8.95    9.61     9.88   10.67    10.83
Tb    1.15     1.26     1.24     1.96    1.43     1.44    1.58     1.59
Dy    6.07     6.51     6.52     6.91    7.25     7.33    7.91     7.99
Ho    1.16     1.21     1.23     1.25    1.31     1.32    1.41     1.43
Er    3.44     3.59     3.68     3.67    3.81     3.89    4.47     7.03
Tm    0.58     0.59     0.59     0.62    0.63     0.61    0.64     0.66
Yb    3.01     3.98     3.97     4.09    4.21     4.26    4.48     4.45
Lu    0.58     0.62     0.62     0.64    0.65     0.64    0.69     0.67
    Bs-23   Bs-24    Bs-25    Bs-26    Bs-27    Bs-28   Bs-29   Bs-30

Y     28.90    29.90    29.20    28.20   28.60    30.40   31.80    32.60
Zr   238.40   255.90   231.50   265.90  247.50   242.30  261.50   249.80
La    88.60   101.20    95.40   105.20  105.00   124.50  146.20   152.50
Ce   282.10   342.50   236.10   305.10  383.20   315.40  291.30   312.30
Pr    21.03    23.84    22.91    28.74   26.41    30.74   39.41    42.11
Nd    79.10    87.20    85.40   110.10  101.20   115.40  155.50   171.10
Sm    16.03    18.44    17.74    24.65   22.14    27.14   38.41    45.58
Eu     2.05     3.22     3.13     4.31    4.11     5.01    7.09     8.41
Gd     9.92    10.64    10.22    11.52   12.64    15.02   21.62    23.65
Tb     1.51     1.65     1.56     1.02    1.97     2.31    3.61     3.73
Dy     7.05     1.94     7.66     9.32    9.04    10.87   14.17    15.44
Ho     1.29     1.41     1.35     1.59    1.48     1.72    2.13     2.25
Er    3.841     3.84     3.87     4.51    4.24     4.81    5.81     6.13
Tm     0.66     0.71     0.64     0.77    0.74     0.83    1.04     1.11
Yb     4.42     4.81     4.48     5.36    5.14     5.88    7.32     7.08
Lu     0.67     0.74     0.68     0.81    0.78     0.86    1.07     1.17

Table 2. The calculated geochemical parameters as well as the
tetrad-effect sizes in the bauxite ores of the studied horizon.

                       Bs-1    Bs-2    Bs-3    Bs-4   Bs-5     Bs-6

Eu/Eu*                 0.72    0.69    0.66    0.67    0.70    0.72
Ce/Ce*                 1.26    1.25    1.39    1.53    1.54    1.73
[T.sub.3]              0.04    0.02    0.03    0.02    0.07    0.07
[T.sub.4]              0.58    0.59    0.03    0.01    0.03    0.06
LREE                 318.55  326.48  357.51  360.96  334.31  400.25
HREE                  19.06   19.65   23.03   22.26   21.37   21.87
[(LREE/HREE).sub.N]    5.55    5.58    5.31    5.44    5.14    5.87
[RET.sub.1]          307.05  315.11  345.58  349.65  323.38  388.35
[RET.sub.2]           17.55   18.11   18.91   18.02   16.90   17.84
[RET.sub.3]           13.93   14.58   14.95   14.41   13.59   13.85
[RET.sub.4]            5.13    5.07    8.08    7.85    7.78    8.02
Y/Ho                  25.75   25.49   25.09   25.37   25.61   24.34
Sm/Nd                  0.18    0.18    0.18    0.19    0.18    0.18
                      Bs-16  Bs-17   Bs-18   Bs-19   Bs-20   Bs-21
Eu/Eu*                 0.72    0.69    0.66    0.67    0.70    0.72
Ce/Ce*                 1.26    1.25    1.39    1.53    1.54    1.73
[T.sub.3]              0.04    0.02    0.03    0.02    0.07    0.07
[T.sub.4]              0.58    0.59    0.03    0.01    0.03    0.06
LREE                 462.46  447.64  631.79  484.55  627.03  497.95
HREE                  28.75   28.72   29.69   31.06   29.81   30.79
[(LREE/HREE).sub.N]    5.40    5.21    6.26    5.18    6.17    5.44
[RET.sub.1]          448.72  434.96  615.34  467.04  611.25  479.34
[RET.sub.2]           23.62   22.12   26.16   27.43   25.00   28.79
[RET.sub.3]           20.16   19.48   19.88   20.68   19.48   20.70
[RET.sub.4]            8.59    9.24    9.81   10.38   10.33   10.09
Y/Ho                  21.46   21.25   19.77   20.42   20.66   21.52
Sm/Nd                  0.16    0.15    0.21    0.21    0.19    0.21

                       Bs-7    Bs-8    Bs-9   Bs-10   Bs-11   Bs-12

Eu/Eu*                 0.67    0.67    0.68    0.69    0.71    0.81
Ce/Ce*                 1.71    2.00    1.89    1.32    1.87    1.91
[T.sub.3]              0.02    0.02    0.03    0.02    0.35    0.02
[T.sub.4]              0.02    0.15    0.03    0.02    0.04    0.03
LREE                 407.36  467.03  477.39  391.37  495.99  531.57
HREE                  24.01   23.56   25.88   26.17   28.09   28.90
[(LREE/HREE).sub.N]    5.60    6.18    5.91    5.27    5.47    5.92
[RET.sub.1]          394.39  453.52  462.58  376.12  478.89  517.68
[RET.sub.2]           20.31   21.08   22.93   23.57   26.05   23.50
[RET.sub.3]           15.69   15.95   17.10   17.31   19.07   19.60
[RET.sub.4]            8.32    7.61    8.78    8.86    9.02    9.30
Y/Ho                  24.64   23.10   23.55   23.17   22.48   21.83
Sm/Nd                  0.21    0.19    0.19    0.20    0.20    0.15
                     Bs-22   Bs-23   Bs-24   Bs-25   Bs-26   Bs-27
Eu/Eu*                 0.67    0.67    0.68    0.69    0.71    0.81
Ce/Ce*                 1.71    2.00    1.89    1.32    1.87    1.91
[T.sub.3]              0.02    0.02    0.03    0.02    0.35    0.02
[T.sub.4]              0.02    0.15    0.03    0.02    0.04    0.03
LREE                 654.95  488.91  576.40  460.68  578.10  642.06
HREE                  29.27   29.36   25.74   30.46   34.90   36.03
[(LREE/HREE).sub.N]    6.62    5.59    6.66    5.52    6.30    6.02
[RET.sub.1]          637.39  470.83  554.74  439.81  549.14  615.81
[RET.sub.2]           26.87   28.00   32.30   31.09   40.48   38.89
[RET.sub.3]           19.44   19.77   15.64   20.79   23.45   25.13
[RET.sub.4]            9.83    9.59   10.10    9.67   11.45   10.90
Y/Ho                  23.06   22.40   21.21   21.63   17.74   19.32
Sm/Nd                  0.21    0.20    0.21    0.21    0.22    0.22

                      Bs-13   Bs-14    Bs-15

Eu/Eu*                 0.69    0.69     0.68
Ce/Ce*                 1.68    1.69     1.49
[T.sub.3]              0.01    0.02     0.02
[T.sub.4]              0.01    0.05     0.22
LREE                 499.96  522.93   490.41
HREE                  29.37   31.85    34.65
[(LREE/HREE).sub.N]    5.77    5.59     5.05
[RET.sub.1]          481.69  503.11   471.59
[RET.sub.2]           28.15   30.49    29.65
[RET.sub.3]           19.97   21.57    21.84
[RET.sub.4]            9.40   10.28    12.81
Y/Ho                  21.52   20.92    20.91
Sm/Nd                  0.21    0.21     0.21
                     Bs-28   Bs-29   Bs-30
Eu/Eu*                 0.69    0.69     0.68
Ce/Ce*                 1.68    1.69     1.49
[T.sub.3]              0.01    0.02     0.02
[T.sub.4]              0.01    0.05     0.22
LREE                 618.19  677.91   732.00
HREE                  42.30   56.77    60.56
[(LREE/HREE).sub.N]    5.48    4.80     4.92
[RET.sub.1]          586.04  632.41   678.01
[RET.sub.2]           47.17   67.12    77.64
[RET.sub.3]           29.92   41.53    45.07
[RET.sub.4]           12.38   15.24    15.49
Y/Ho                  17.67   14.93    14.49
Sm/Nd                  0.24    0.25     0.27
COPYRIGHT 2019 Akademie Ved Ceske Republiky, Ustav Struktury a Mechaniky Hornin
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Title Annotation:ORIGINAL PAPER; rare earth elements
Author:Abedini, Ali; Azizi, Mansour Rezaei; Calagari, Ali Asghar
Publication:Acta Geodynamica et Geromaterialia
Article Type:Report
Geographic Code:7IRAN
Date:Jan 1, 2019
Words:9909
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