The Potential for REE and Associated Critical Metals in Karstic Bauxites and Bauxite Residue of Montenegro

: Research for critical raw materials is of special interest, due to their increasing demand, opulence of applications and shortage of supply. Bauxites, or bauxite residue after alumina extraction can be sources of critical raw materials (CRMs) due to their content of rare earth elements and other critical elements. Montenegrin bauxites and bauxite residue (red mud) are investigated for their mineralogy and geochemistry. The study of the CRM’s potential of the Montenegrin bauxite residue after the application of Bayer process, is performed for the ﬁrst time. Montenegrin bauxites, (Jurassic bauxites from the Vojnik-Maganik and Prekornica ore regions from the Early Jurassic, Middle Jurassic-Oxfordian and Late Triassic paleorelief) are promising for their REE’s content (around 1000 ppm of Σ REE’s). More speciﬁcally, they are especially enriched in LREEs compared to HREEs. Regarding other CRMs and other elements, Ti, V, Zr, Nb, Sr and Ga could also be promising. In bauxite residue, the contents of Zr, Sr, V, Sc, La, Ce, Y, Ti and Nb are higher than those in bauxites. However, raw bauxites and bauxite residue as a secondary raw material can be considered as possible sources of CRMs.


Introduction
From economical point of view, red karstic bauxites can be considered as a potential mineral raw material for obtaining the REEs. World demand for rare earth elements has been on the rise for years, due to their usage in high-tech applications. REE supply in global market is limited, as China is almost the exclusive global supplier. Significant attention is paid to REE's resources of the United States, when it comes to domestic deposits, a global perspective and world production [1,2], as well as in European and other countries.
Apart from REEs, other trace elements such as vanadium, scandium, gallium, lithium, molybdenum, etc., are important for the exploration of karstic bauxites, as they can be found in considerable amounts and are of economic value. European Commission regularly publishes a list of critical mineral resources (CRM) in the European Union. The last published list of CRM (2020) contains 30 different mineral resources [3]. Some of them, such as: titanium, rare earth elements (light and heavy), vanadium, scandium and gallium are found in karst bauxites deposits in Montenegro and/or in secondary resources red mud-bauxite residue. The bauxite as CRM is included in the list for the first time.
The presence of rare earth elements in significant contents, in the explored deposits and occurrences of Jurassic bauxite in the Vojnik-Maganik and Prekornica ore regions, as well as in bauxites of other bauxite formations in Montenegro has been confirmed by recent studies [4][5][6]. In this way, a basis has been created for more detailed research for CRMs in Montenegro. The red karstic bauxites occur in three stratigraphic levels in the Middle Triassic, Jurassic and Paleogene, while the white karst bauxites are of Cretaceous age [16,[64][65][66].
The territory of Montenegro is built of different types of sedimentary, igneous and metamorphic rocks. Most of the terrain is built by Mesozoic formations of carbonate composition. They are developed in the northern, central and coastal part of Montenegro. Magmatic and clastic aluminosilicate rocks are much less presented. Paleozoic geological formations are presented by sedimentary and metamorphic rocks. They are located mainly in the north-eastern part of Montenegro. Cenozoic rocks of carbonate and clastic composition occur here and there in all regions of Montenegro [64] (Figure 1).
Triassic bauxites were discovered in the wider area of Nikšićka župa, in Gornje polje near Nikšić and in Piva (Piva, Vojnik-Maganik and Prekornica bauxite-bearing regions, Figures 2 and 3). The underlying bed of the Triassic bauxite is formed by Anisian limestones, reef Ladinian limestones or Ladinian volcanogenic-sedimentary formation. In their immediate hanging wall, there are terrigenous Raibel sediments, and then early diagenetic dolomites of the Carnian stage.
Jurassic bauxites in Montenegro are widespread in the area of the High ka and are present on the terrains of Sinjavina and Durmitor-in the Durmitor tecto During the Jurassic, the High karst zone was differentiated into two complex forms: the subzone of Kučiin the northeast and the Old Montenegrin subzone in th west [67].  Jurassic bauxites in Montenegro are widespread in the area of the High karst zone, and are present on the terrains of Sinjavina and Durmitor-in the Durmitor tectonic unit. During the Jurassic, the High karst zone was differentiated into two complex anticline forms: the subzone of Kučiin the northeast and the Old Montenegrin subzone in the southwest [67]. Jurassic bauxite deposits and occurrences belong to Vojnik-Maganik, Prekornica, Western Montenegro, Orjen andČevo bauxite-bearing regions, (Figures 2 and 3). The paleorelief of Jurassic bauxite in Montenegro consists of karstified limestones and rare dolomites of the LateTriassic, Liassic and Dogger-Oxfordian age. Their hanging wall of Kimmeridgian-Titon age, is represented by different types of limestone and rarely present dolomites.
Cretaceous, white bauxites were formed during the Early Cretaceous on a karst paleorelief built of limestone, dolomitic limestones and dolomite of Liassic, Doggerian, Tithonian and Berriasian-Barremian age. Over the white bauxite, limestones of the Late Cenomanian were deposited. They were discovered in the Western Montenegro anď Cevobauxite-bearing regions (Figure 2), in the domain of the Old Montenegrin subzone.
Paleogene bauxites were formed in the coastal part of Montenegro within the Adriatic zone. They are known mainly as of Eocene age, and less frequently as Lutetic bauxites in the literature. Deposits and occurrences of Paleogene bauxites in the area of Luštica and Grbalj and between Bar and Ulcinj ( Figure 4) are located on a paleorelief built of Late Cretaceous limestones and dolomites, while their hanging wall is made up of Middle Eocene limestones.  Triassic bauxites have been poorly explored, primarily d On the contrary, Montenegrin Jurassic bauxites in general ha gated. Some Cretaceous bauxite deposits, especially in the Trubjela, have been investigated in more detail. Paleogene b oughly evaluated as a potential economic resource.
Triassic bauxites have been poorly explored, primarily due to their high SiO 2 content. On the contrary, Montenegrin Jurassic bauxites in general have been thoroughly investigated. Some Cretaceous bauxite deposits, especially in the area of Bijele poljane and Trubjela, have been investigated in more detail. Paleogene bauxites have not been thoroughly evaluated as a potential economic resource.
Most researchers agree that laterite bauxites are formed 'in situ', from alumosilicate igneous, sedimentary and metamorphic rocks, on land, in humid tropical and subtropical climates. The genesis of karstic bauxite is still controversial in terms of: the place and conditions of bauxitisation, the origin of the parent material and its transport to karstic areas. When it comes to the origin of the parent material from which bauxites originated in Montenegro, interpretations are also different [61,68,71,72,76].
The content and mode of occurrences of REEs and other trace elements in the bauxite deposit of Montenegro are directly related to the composition and origin of the parent material, as well as the conditions and duration of bauxitisation and the other factors. The source material for the Triassic bauxites originates from the products of the Middle Triassic volcanism, the volcanic ashes and/or the weathering crust formed on the rocks of igneous origin, while Jurassic red bauxites are originated from the volcanic ashes, related to the igneous-tectonic processes and the volcanoes to follow during the 'opening' and 'closing' of the Jurassic ophiolitic trough. A smaller quantity of the source material can come from the weathering crusts of mainly basic rocks [15,16]. The value of Eu/Eu* versus TiO 2 /Al 2 O 3 ratios indicates shales, UCC and andesitic rocks as possible source rocks, or protolith of the Zagrad deposit Jurassic bauxite [4]. The binary plot of Eu/Eu* vs. Sm/Nd indicates that the parental material for the bauxite was derived from a combination of a clastic material derived from shales and/or upper continental crust and, probably, distant andesitic volcanic source. Jurassic paleo-geographical and paleo-tectonic processes in the Mediterranean indicate that the source material most likely originates from ophiolites complexes, which are suprasubduction oceanic island-arc type ophiolites, with intensive extrusive volcanism [5,77]. It is possible that the volcanic ash or/and material from weathered crust of this complex are parent materials from which Jurassic karstic bauxites in Montenegro were formed. The source material of white Cretaceous bauxites mainly comes from the weathering crusts on the igneous basic rocks, and in rare cases, form the volcanic ashes. Origination of the white bauxite facies is related to the circulating, lacustrine, oxygenrich environments, while the pyritised bauxites originated in a reducing environment. In the region of Bijele Poljane, however, the deposits of white bauxites were formed by mixing of various colored clays with redeposited sandy-gravelly material which originates from the deposits of the red Jurassic bauxites in the same region. That is why such deposits are rightfully called 'complex' deposits of the white bauxites [15,65]. The source material of the Paleogene bauxites probably comes from the volcanic ashes and/or the weathering crusts of basic rocks, and to a smaller extent also form the weathering crusts of ultra-basic rocks [15].

Rare Earth Elements (REE) in Montenegrin Karstic Bauxites
Previous researchers have investigated the REE content of Montenegrin bauxites including [15,49,50,53,57,68,74,76,[78][79][80]. These studies have focused on the study of geological structure and structural characteristics of bauxite-bearing terrains, as well as geological, structural morphological, chemical and geochemical characteristics of karstic bauxite deposits. The deposits of red and less white bauxites from which this raw material was or is being exploited have been investigated in detail.
Recent national exploration projects include the following: Metallogenetic-prognostic map of the bauxite-bearing region Vojnik-Maganik, 1:50,000 (MPMVM), 'Metallogeneticprognostic map of the bauxite-bearing region Western Montenegro', 1:50,000 (MPMWM) and 'Exploration of rare earth elements in ore regions Vojnik-Maganik and Prekornica (REEVMP)', as well as the international project 'REEBAUX-Prospects of REE recovery from bauxite and bauxite residue in the ESEE region'. A summary of the occurrences of the samples is given in Tables 1-3.

Sampling
At 37 locations along 47 profiles, the recording and sampling of bauxite bodies formed on a karstificated paleorelief made of carbonates of Late Triassic, Early Jurassic and Middle Jurassic-Oxfordian age, were performed, and 252 representative channel bauxite samples with identical intervals (1 m), were collected during the REEVMP project implementation [5]. The samples were prepared for different investigation methods by using standard methods. Mineralogical examinations included tests of 64 samples by using XRD method and 34 samples by using SEM-EDS method (JEOL Ltd., Musashino, Akishima, Tokyo, Japan). The red sludge samples originate from six vertical exploration drill holes with an average length of 12 m, three from each basin. A total amount of 20 analytical samples were formed from three to four individual samples. Samples were prepared by standard methods, while geochemical tests of 20 bauxite residue samples were performed by ICP-AES/ICP-MS. All exploration activities were done in the frame of the REEBAUX project implementation.

Mineralogical Analyses
The mineralogy of 64 bauxite samples was determined by optical microscopic observation and powder X-ray diffraction (XRD) at the University of Belgrade, Faculty of Mining and Geology, Belgrade, Serbia, during the REEVMP project implementation. XRD analysis was performed on a Philips PW 1710 powder diffractometer with CuKα 1,2 = 1.54178 Å radiation (despite the known limitations for Fe-rich minerals) and a 40 kV, 30 mA. The XRD pattern was recorded over a 2θ interval of 4-70 • , with a step size of 0.02 • and the fixed counting time of 1 s per step. Some samples with high contents of REE were studied by reflected light optical investigations, scanning electron microscope equipped with an energy-dispersive spectrometer (SEM-EDS) and micro-Raman spectroscopy.
The SEM-EDS analyses were carried out at the University of Belgrade, Faculty of Mining and Geology, on polished bauxite samples, under high vacuum conditions on a scanning electron microscope (SEM) type JEOL JSM-6610LV. Mineral images were obtained using back-scattered electrons (BSE) detectors, and tungsten fibre was used as the electron source. The samples were evaporated with carbon on a steamer type BALTEC-SCD-005, and quantitative chemical analyses of individual minerals in the samples were performed on an energy-dispersive spectrometer (EDS) type X-Max Large Area Analytical Silicon Drift. An acceleration voltage of 20 kV was used for analyses. Detection limits are estimated as 2σ~0.2 wt.%. This method was applied for mineral identification and obtaining REE mineral compositions. According to very fine mineral intergrowths in the studied bauxite and regular presence of sub-microscopic mineral inclusions in REE minerals it was not possible to achieve accurate composition of REE minerals using external standards. Thus, compositions of these minerals were obtained using internal standards and normalisation. However, this mode of the analysis quite well shows differences in the composition of various types of the REE minerals.
In addition, the mineralogy of 14 bauxite samples, collected and selected during the REEBAUX project implementation, was determined by optical microscopic observation and powder X-ray diffraction (XRD) at the University of Zagreb, Faculty of Science, Department of Geology, Zagreb, Croatia. Diffraction data were collected using Philips X'Pert PRO powder diffractometer with CuKα radiation (λ = 1.54178Ǻ) at 40 kV and 40 mA with divergent slit of 1 4 • and antiscatter slit of 1 2 • . Diffracted radiation was monochromatized using graphite monochromator.

Chemical Analyses
Ore samples were crushed to 200-mesh size particles using an agate mill. All samples (REEVMP and REEBAUX projects) were prepared for chemical analyses in the laboratories of AcmeLabs (now Bureau Veritas), Vancouver, Canada. Prepared sample is mixed with LiBO 2 /Li 2 B 4 O 7 flux. Crucibles are fused in a furnace. The cooled bead is dissolved in ACS grade nitric acid and analysed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and/or inductively coupled plasma-mass spectrometry (ICP-MS). Loss on ignition (LOI) is determined by igniting a sample split, while the measuring of weight loss was done after. Quantitative values of major and minor elements, trace elements, and REEs were determined by using inductively coupled plasma-atomic emission spectrometry and inductively coupled plasma-mass spectrometry analysing methods, respectively. Total loss on ignition (LOI) values were gravimetrically estimated after overnight heating at 950 • C for 90 min. Detection limits for major oxides, such as Fe 2 O 3 and K 2 O were 0.04%; SiO 2 , Al 2 O 3 , CaO, MgO, Na 2 O, MnO, TiO 2 and P 2 O 5 were 0.01%; for LOI 0.1%; and for Cr 2 O 3 , 0.002%. Detection limits for trace elements were: for Ni and Co 20 ppm; for V 8 ppm; for Ba 5 ppm; for Be, Sc and Zn 1 ppm; for Ga, Sr, W, As, La and Ce, 0.5 ppm; for Co and Th, 0.2 ppm; for Ce, Cs, Hf, La, Nb, Rb, Ta, U and Y 0.1 ppm; for Dy, Gd, Sm, Yb, 0.05 ppm; for Er 0.03 ppm; for Eu, Ho and Pr, 0.02 ppm; Lu, Tb and Tm, 0.01 ppm.
Total carbon (C) and sulphur (S) content were analysed on a LECO Elemental Analyser in the laboratories of AcmeLabs (now Bureau Veritas), Vancouver, Canada. Induction flux was added to the prepared sample and it was ignited in an induction furnace after. A carrier gas sweeps up released carbon to be measured by adsorption in an infrared spectrometric cell. Results are total and attributed to the presence of carbon and sulphur in all forms.
Apart from the above mentioned, the results of geochemical analysis of bauxite samples from the MPMVM project are presented in order to compare these older data with newer ones, as well as the content of some elements that were not examined later, primarily lithium (due to the applied sample dissolution method-Li-borate fusion). Classical methods were used for the analysis of major oxides and LOI, while analyses were performed in the Chemical Laboratory of the Geological Survey of Montenegro, Podgorica, Montenegro. Trace elements were analysed by ICP-MS method (4 Acid digestion) in AcmeLabs, Vancouver, Canada. Moreover, the results of the geochemical tests of the MPMWM project are presented with the same goal. For geochemical analyses, a combination of ICP-AES methods for major oxides (Li-borate fusion) and ICP-MS for trace elements (4 Acid digestion) was used. The research were performed in AcmeLabs, Vancouver, Canada.

Results and Discussion
According to shape and size of the structural elements of the bauxite structure, the following textures can be found: aphanitic or pelitomorphic, pisolitic-oolitic, complex conglomeratic and brecciated structure [74]. Extremely rarely, striped, parallel and schistose textures were detected [68].
In general, Triassic deposits and occurrences are formed by dark red pisolitic bauxites over which are bright red pisolitic and oolithic bauxites, and gray partially pisolitic bauxite on the top.
In almost all studied Jurassic bauxite deposits, especially the larger ones from the Vojnik-Maganik ore region [75], red pisolitic bauxite was developed at the top of the bauxite deposits, just below the overlying clays that were formed in the first phase of transgression. These deposits are classified into the group of primary deposits with a developed profile [15]. Beneath the pisolitic, red massive 'granular' detrital or aphanitic bauxites are most common, usually with tiny oolites and irregular pisolite accumulations-which form the middle part of the bauxite deposits. At the base of the deposit massive bauxites with or without oolites and pisolites can be found. The transitions in texture mentioned above are gradual and irregular. At the contact place with the bedrock limestones, there are so-called 'bedrock breccias', while at some localities there are also bedrock clays in which pieces of bedrock limestones can be found.
Cretaceous white bauxites, especially those in the area of Bijele Poljane, are characterised by a very complex geological structure. Lateral and vertical transitions of red bauxites with white bauxites, white and gray bauxite clays and gray pyritic clays.
Paleogene bauxites are characterised by a pisolitic-oolithic, pisolitic and conglomerate structure, gray, yellow and red colour. The characteristic geological sections of the studied bauxite deposits presented in this paper, from different bauxite formations are shown in Figure 5. According to shape and size of the structural elements of the bauxite structure, the following textures can be found: aphanitic or pelitomorphic, pisolitic-oolitic, complex conglomeratic and brecciated structure [74]. Extremely rarely, striped, parallel and schistose textures were detected [68].
In general, Triassic deposits and occurrences are formed by dark red pisolitic bauxites over which are bright red pisolitic and oolithic bauxites, and gray partially pisolitic bauxite on the top.
In almost all studied Jurassic bauxite deposits, especially the larger ones from the Vojnik-Maganik ore region [75], red pisolitic bauxite was developed at the top of the bauxite deposits, just below the overlying clays that were formed in the first phase of transgression. These deposits are classified into the group of primary deposits with a developed profile [15]. Beneath the pisolitic, red massive 'granular' detrital or aphanitic bauxites are most common, usually with tiny oolites and irregular pisolite accumulationswhich form the middle part of the bauxite deposits. At the base of the deposit massive bauxites with or without oolites and pisolites can be found. The transitions in texture mentioned above are gradual and irregular. At the contact place with the bedrock limestones, there are so-called 'bedrock breccias', while at some localities there are also bedrock clays in which pieces of bedrock limestones can be found.
Cretaceous white bauxites, especially those in the area of Bijele Poljane, are characterised by a very complex geological structure. Lateral and vertical transitions of red bauxites with white bauxites, white and gray bauxite clays and gray pyritic clays.
Paleogene bauxites are characterised by a pisolitic-oolithic, pisolitic and conglomerate structure, gray, yellow and red colour. The characteristic geological sections of the studied bauxite deposits presented in this paper, from different bauxite formations are shown in Figure 5.

Mineralogy
As it is mentioned above, Montenegrin bauxites have a complex mineralogical composition, which was confirmed in this study as well. The red Triassic bauxites samples of the Gorenjepoljski vir deposits are characterised by the presence of the minerals: kaolinite, Figure 5. The geological sections of characteristic bauxite deposits/occurrences from different bauxite formations of Montenegro (after Radusinović [5] and REEBAUX project documentation, Radusinović [6]).

Mineralogy
As it is mentioned above, Montenegrin bauxites have a complex mineralogical composition, which was confirmed in this study as well. The red Triassic bauxites samples of the Gorenjepoljski vir deposits are characterised by the presence of the minerals: kaolinite, böhmite and gibbsite, also goethite, anatase and dolomite, as well as vermiculite in one sample from the middle part of the deposit ( Figure 6). böhmite and gibbsite, also goethite, anatase and dolomite, as well as vermiculite in one sample from the middle part of the deposit ( Figure 6). The red Jurassic bauxites of the Vojnik-Maganik and Prekornica ore regions have complex mineral composition as well. The mineral böhmite is the main carrier of aluminium, while gibbsite is the minor Al-carrier. Regarding other major minerals the following are presented: Fe-oxides/hydroxides (hematite and goethite); clay minerals (kaolinite) and titanium minerals (mainly anatase) (Figures 7 and 8). The red Jurassic bauxites of the Vojnik-Maganik and Prekornica ore regions have complex mineral composition as well. The mineral böhmite is the main carrier of aluminium, while gibbsite is the minor Al-carrier. Regarding other major minerals the following are presented: Fe-oxides/hydroxides (hematite and goethite); clay minerals (kaolinite) and titanium minerals (mainly anatase) (Figures 7 and 8). böhmite and gibbsite, also goethite, anatase and dolomite, as well as vermiculite in one sample from the middle part of the deposit ( Figure 6). The red Jurassic bauxites of the Vojnik-Maganik and Prekornica ore regions have complex mineral composition as well. The mineral böhmite is the main carrier of aluminium, while gibbsite is the minor Al-carrier. Regarding other major minerals the following are presented: Fe-oxides/hydroxides (hematite and goethite); clay minerals (kaolinite) and titanium minerals (mainly anatase) (Figures 7 and 8).  The major-, trace-and rare earth element average compositions of the analysed samples are given in Table 4 [5,6].
It is important to emphasise that among the detected phases, the major REE-carriers are phosphates, as indicated by the very strong positive correlation between REEs, P and Sr.
The presence of residual and authigenic monazite and xenotime clearly indicates that the first REE minerals originate from primary sources, while additional are formed in the first phases of bauxitisation, in oxidation conditions [4,5,77]. SEM-EDS analyses of minerals can be found in Tables 5-8 [5]. The major-, trace-and rare earth element average compositions of the analysed samples are given in Table 4 [5,6].
The presence of residual and authigenic monazite and xenotime clearly indicates that the first REE minerals originate from primary sources, while additional are formed in the first phases of bauxitisation, in oxidation conditions [4,5,77]. SEM-EDS analyses of minerals can be found in Tables 5-8 [5]. Table 5. Results of chemical analysis of points (sample 099, Figure 9). All deposits and occurrences of red Jurassic bauxites from the ore regions of Western Montenegro and Orjen andČevo were formed on the carbonate paleorelief of the Middle Jurassic-Oxfordian age. They are often characterised by the presence of redeposited bauxites, especially in the upper parts of the deposits. The presence of the following minerals was detected in samples from Bajov do deposits: böhmite, hematite, kaolinite, anatase and goethite (Figure 13a). The following minerals were determined: böhmite, gibbsite, hematite, kaolinite, anatase and rutile (Figure 13b), in white bauxite samples from Bijele poljane deposit -Dionica site, by XRD analyses. Paleogene bauxite occurrence Velika gorana are characterised by the presence of: böhmite, goethite, kaolinite and anatase (Figure 13c).

Major Elements Geochemistry
Bauxites of Triassic age were studied at two localities: occurrence Rudinice in Piva and the Gornjepoljskivir deposit, which belongs to the bauxite-bearing region of Vojnik-Maganik. According to the results of the composite sample formed from six individual bauxite samples from the occurrence of Rudinice, it can be seen that these bauxites are characterised by high Al2O3 (62.5%), low Fe2O3 content (12.85%), relatively low SiO2 content. (6.83%) and TiO2 (2.31%) ( Table 4). On the contrary, the bauxites of the Gornjepoljski vir deposit contain low contents of Al2O3 (43.18%) and Fe2O3 (7.55%), a high average content of SiO2 (30.42%) and relatively low content of TiO2 (1.20%). MgO, Na2O, K2O, MnO and P2O5 contents are very low and significantly higher in the bauxites of the Gornjepoljski vir deposit.
According to the analysis of statistical parameters of the analysed oxides in bauxites of the ore regions Vojnik-Maganik and Prekornica (Tables 4 and 9 VMP-III&IV/1), it can

Major Elements Geochemistry
Bauxites of Triassic age were studied at two localities: occurrence Rudinice in Piva and the Gornjepoljskivir deposit, which belongs to the bauxite-bearing region of Vojnik-Maganik. According to the results of the composite sample formed from six individual bauxite samples from the occurrence of Rudinice, it can be seen that these bauxites are characterised by high Al 2 O 3 (62.5%), low Fe 2 O 3 content (12.85%), relatively low SiO 2 content. (6.83%) and TiO 2 (2.31%) ( Table 4). On the contrary, the bauxites of the Gornjepoljski vir deposit contain low contents of Al 2 O 3 (43.18%) and Fe 2 O 3 (7.55%), a high average content of SiO 2 (30.42%) and relatively low content of TiO 2 (1.20%). MgO, Na 2 O, K 2 O, MnO and P 2 O 5 contents are very low and significantly higher in the bauxites of the Gornjepoljski vir deposit.
According to the analysis of statistical parameters of the analysed oxides in bauxites of the ore regions Vojnik-Maganik and Prekornica (Tables 4 and 9 VMP-III&IV/1), it can be seen that in bauxites formed on the Late Triassic underlying bed, the SiO 2 range in individual samples is from 1% to 27.44%, with an average content of 11.9%; while on the Early Jurassic underlying bed, the range is from 11.61% to 27.44%, with a average content of 19.89%; and at Middle Jurassic-Oxfordian range is 11.90% to 27.13%, with an average content of 18  The P 2 O 5 content varies in a wide range from 0.01% to 0.53%. Regarding bauxite deposits and occurrences formed on the Late Triassic (95 samples above the detection limit) the range is from 0.01% to 0.53% and the average content 0.047%; while on the Early Jurassic (33 samples) the range is from 0.01 to 0.71% and the average content is 0.071%. In Middle Jurassic-Oxfordian bauxites, contents of P 2 O 5 are detected in individual samples (10 samples) ranging from 0.01% to 0.05%, with an average of 0.028%. P 2 O 5 shows the highest contents in bauxites from the lower part of bauxite bodies from the Early Jurassic (0.156%) and Late Triassic paleorelief (0.085%).
The average LOI, in bauxites from all three underlying beds and from different parts of the ore bodies, is uniform. The sulphur content in 96% of the samples was below the detection limit of 0.02%. Slightly higher content of C is shown by bauxites from the lower part of deposits formed on the Middle Jurassic-Oxfordian palorelief and the middle and upper parts of ore bodies formed on the Late Triassic and Middle Jurassic-Oxfordian.
The contents of the main oxides from the detailed database of the REE VMP project, correspond to the exploration results of the most significant and largest deposits and characteristic bauxite occurrences of Montenegro, from the REEBAUX project (VM-III/2, P-IV/1 and P-IV/2). Slightly higher on average Al 2 O 3 content and slightly lower on average SiO 2 content are shown by samples from the REEBAUX project database, when it comes to bauxites formed on the underlying beds of all three ages. It should be noted that the samples from the Zagrad and Biočki stan deposits originate from underground mines and bauxites from these parts of the deposit were tested for the first time. Furthermore, the results of chemical and geochemical explorations of bauxite obtained through the project MPM VM (VM-III/3, VM-III/4) are in accordance with the presented data. Table 9. Statistical parameters of geochemical analyses of major oxides in Jurassic bauxites of the Vojnik-Maganik and Prekornica ore regions (after Radusinović [5]). Deposits and occurrences of red Jurassic bauxites in the bauxite-bearing regions of Western Montenegro and Orjen (REEBAUX, WMO-V&VII/1) which are formed on the Middle Jurassic-Oxfordian age paleorelief, are characterised by a high average SiO 2 content (13.28%), low Al2O 3 content (40,97%), slightly lower content of Fe 2 O 3 (16.67%) and TiO 2 (1.78%) and extremely high average content of CaO (7.96%). It was previously emphasised that these bauxites are characterised by redeposition, which may explain the high CaO contents. It should be considered, compared to the results of the MPM WM project that sampling was performed by different methods, so it is not surprising that the average CaO content and the slightly higher Al 2 O 3 content are significantly lower (Table 4).

Paleorelief
Cretaceous white bauxites, due to their genetic specificities, show an elevated average content of SiO 2 (22.60%) and lower content of Deposits and occurrences of red Jurassic bauxites in the bauxite-bearing re Western Montenegro and Orjen (REEBAUX, WMO-V&VII/1) which are formed Middle Jurassic-Oxfordian age paleorelief, are characterised by a high average S tent (13.28%), low Al2O3 content (40,97%), slightly lower content of Fe2O3 (16.6 TiO2 (1.78%) and extremely high average content of CaO (7.96%). It was previou phasised that these bauxites are characterised by redeposition, which may exp high CaO contents. It should be considered, compared to the results of the MP project that sampling was performed by different methods, so it is not surprising average CaO content and the slightly higher Al2O3 content are significantly lowe 4).
Cretaceous white bauxites, due to their genetic specificities, show an elevat age content of SiO2 (  The enriched Fe2O3 contents in some deposits are due to the presence of iron m like hematite, which is most likely formed under suitable Eh-pH conditions durin itisation processes. It should be emphasised that white Cretaceous bauxites, as The enriched Fe 2 O 3 contents in some deposits are due to the presence of iron minerals like hematite, which is most likely formed under suitable Eh-pH conditions during bauxitisation processes. It should be emphasised that white Cretaceous bauxites, as well as deposits and occurrences formed on the younger underlying beds of the Late Jurassic and Early Cretaceous age are generally less enriched in Fe 2 O 3.

Trace Elements Geochemistry
Some trace elements such as Sc, Li, Cr, Zr, Nb, V and Ni occur in considerable amounts in the bauxitic deposits of Montenegro (Table 4) Paleogene bauxites averagely contain the most of Ni, 289 those from the Boka Kotorska region, and 348 ppm from Ulcinj region. From these data, it is clear that the Ni content increases from older to younger bauxites, which was previously determined by studying bauxites from the Triassic to the Eocene age in Yugoslavia and Greece [50]. The situation is similar to the average contents of Cr, which is present in the Triassic bauxites with 71 and 123 ppm, in the Jurassic from 232 to 334 ppm, in the Cretaceous 341 ppm, while in the Paleogene it reaches a high 637 and 835 ppm. Similar to Cr, the most of V has averagely in Paleogene bauxites 652 and 728 ppm in the bauxites of BokaKotorska and Ulcinj, also in the Cretaceous 576 ppm, Jurassic from 215.16 to 303.62 ppm, and the least in Triassic bauxites, only 93.12 ppm in Gornjepoljski vir, and 132 ppm as was detected in Rudinice in Piva.
Jurassic bauxites from the Vojnik-Maganik and Prekornica ore regions were studied in the most detail [5]. In these bauxites Zr was detected in individual samples ranging from 328.7 to 641 ppm. In bauxites formed on the Late Triassic, Zr has an average content of 475.71 ppm, on the Early Jurassic 420.62 ppm and on the Middle Jurassic-Oxfordian 397.56 ppm (Table 10)  In the Jurassic bauxites of the Vojnik-Maganik and Prekornica ore regions, Nb was determined in individual samples ranging from 31.2 to 65.5 ppm ( Table 9). The lowest and highest individual values belong to bauxite samples from the Late Triassic underlying bed, which have an average content of 48.7 ppm. The average in bauxites formed on Early Jurassic is 41.96 ppm, and on Middle Jurassic-Oxfordian the average value is 36.65 ppm. These bauxites are characterised by elevated contents of Sr compared to the average, in the ore bodies formed on the Early Jurassic underlying bed. The anomalous Sr contents in the samples stand out, especially from lower, but also the middle part of the Borovabrda deposit.
According to the bauxite explorations during the production of metallogenetic prognostic maps (MPMVM and MPMWM), the average Li content in Jurassic bauxites in the Vojnik-Maganik region formed on the Late Triassic paleorelief is 256.83 ppm, while that formed on the Middle Jurassic-Oxford age paleorelief is significantly higher and is 484.96 ppm. Jurassic bauxites in the region ofČevo and Western Montenegro contain on average 316.76 ppm, while Cretaceous bauxites contain 421.03 ppm Li in average.  In the Jurassic bauxites of the Vojnik-Maganik and Prekornica ore regions, Nb was determined in individual samples ranging from 31.2 to 65.5 ppm ( Table 9). The lowest and highest individual values belong to bauxite samples from the Late Triassic underlying bed, which have an average content of 48.7 ppm. The average in bauxites formed on Early Jurassic is 41.96 ppm, and on Middle Jurassic-Oxfordian the average value is 36.65 ppm. These bauxites are characterised by elevated contents of Sr compared to the average, in the ore bodies formed on the Early Jurassic underlying bed. The anomalous Sr contents in the samples stand out, especially from lower, but also the middle part of the Borovabrda deposit.

Rare Earth Elements Geochemistry
According to the bauxite explorations during the production of metallogenetic prognostic maps (MPMVM and MPMWM), the average Li content in Jurassic bauxites in the Vojnik-Maganik region formed on the Late Triassic paleorelief is 256.83 ppm, while that formed on the Middle Jurassic-Oxford age paleorelief is significantly higher and is 484.96 ppm. Jurassic bauxites in the region of Čevo and Western Montenegro contain on average 316.76 ppm, while Cretaceous bauxites contain 421.03 ppm Li in average.  Table 4). Bauxite formations have ΣLREE contents (236.24-840.99 ppm), ΣHREE contents (7291-27,081 ppm) and ΣLREE/ΣHREE ratios (3.07-5.33) ( Table 4). On average, the highest contents of ΣREE + Sc (more than 1000 ppm) are presented in Biočki stan, Zagrad, Liverovići and Borova brda deposits, as well as the occurrence of Crveno prlo from the bauxite-bearing regions Vojnik-Maganik and Prekornica [6]. Other studied deposits of these regions also have elevated contents, as well as the Velja Dubova glava deposit from Orjen ore region and the Triassic deposit Gonjepoljski vir. The lowest average contents are presented by Cretaceous bauxite deposits and occurrences (all below 400 ppm, except Paklarica) ( Figure 16). On average, the highest contents of ΣREE + Sc (more than 1000 ppm) are presented in Biočki stan, Zagrad, Liverovići and Borova brda deposits, as well as the occurrence of Crveno prlo from the bauxite-bearing regions Vojnik-Maganik and Prekornica [6]. Other studied deposits of these regions also have elevated contents, as well as the Velja Dubova glava deposit from Orjen ore region and the Triassic deposit Gonjepoljski vir. The lowest average contents are presented by Cretaceous bauxite deposits and occurrences (all below 400 ppm, except Paklarica) ( Figure 16).  When the contents of rare earth elements in bauxites of different formations in Montenegro [5,6] and Mesozoic Mediterranean deposits and bauxite formations of: Croatia [23], Turkey [28], Greece [26], Italy [22,37,39], France [82] and Spain [40] are compared, fairly clear uniformity of average REE contents in bauxites of similar or the same age can be observed. Table 11. Statistical parameters of geochemical analyses of rare earth elements, Y and Sc in Jurassic bauxites of the Vojnik-Maganik and Prekornica ore regions (after Radusinović [5]).  When the contents of rare earth elements in bauxites of different formations in Montenegro [5,6] and Mesozoic Mediterranean deposits and bauxite formations of: Croatia [23], Turkey [28], Greece [26], Italy [22,37,39], France [82] and Spain [40] are compared, fairly clear uniformity of average REE contents in bauxites of similar or the same age can be observed.

Paleorelief
Significant deviations and variations are observed in Jurassic bauxites formed on carbonates of Early and Late Jurassic age. Based on Figure 17, Greek bauxites formed on the Late Jurassic paleorelief from the Parnassos-Ghiona geotectonic zone show the highest REE average content (about 1280 ppm), followed by the Jurassic bauxites of Montenegro bauxite-bearing regions Vojnik-Maganik and Prekornica (about 1000 ppm), as well as Turkish bauxites from Namtun tectonic unit (about 950 ppm). Slightly lower contents are shown by the Triassic bauxites (from 540 to 740 ppm). The lowest average content was found in Jurassic bauxite of the bauxite-bearing regions of Western Montenegro and Čevo in Montenegro, around 550 ppm, as well as in the Greek bauxites of the Parnassos-Ghiona geotectonic zone formed on the Liassic palorelief, only around 390 ppm. When it comes to Cretaceous bauxites, the average REE contents in the shown regions are fairly uniform. The highest contents (more than 700 ppm) belong to Italian bauxites from the Caserta district and French bauxites from Provence and Languedoc, while the lowest belong to Montenegrin (about 310 ppm), Greek (about 420 ppm) and Spanish bauxites from the Catalon Coastal Range (about 440 ppm). Despite the fact that bauxite formations were studied at an uneven level of exploration and that average values were derived based on analyses of different numbers of samples, according to the average REE contents, it can be concluded that Jurassic bauxite formations have the highest perspective. Despite the fact that bauxite formations were studied at an uneven level of exploration and that average values were derived based on analyses of different numbers of samples, according to the average REE contents, it can be concluded that Jurassic bauxite formations have the highest perspective. The total quantities of bauxite residue amount to about 7.5 million tons in basins A and B in the Aluminium Factory Podgorica. The calculation of the average content of main and other oxides, trace elements and rare earth elements of 19 composite samples are given in Table 12.  The total quantities of bauxite residue amount to about 7.5 million tons in basins A and B in the Aluminium Factory Podgorica. The calculation of the average content of main and other oxides, trace elements and rare earth elements of 19 composite samples are given in Table 12.    [6].

Rare Earth Elements in Bauxite Residue
It is clear that there is a change in the geochemical and mineralogical composition in relation to the primary bauxite in the bauxite residue after the alumina production process ( Figure 19). The content of Al2O3 decreases significantly (22.47%), while the contents of SiO2, iron oxide and titanium increase significantly. The high average contents of calcium oxide (6.5%) and sodium oxide (6.07%) in the bauxite residue are a consequence of the nature of the alumina production technological process. The increase in average contents in bauxite Figure 19. Content comparison of analysed oxides, microelements and rare earth elements in bauxites and bauxite residue. Based on data: (Radusinović [5] and REEBAUX [6]); Bauxite mines-Nikšić and Aluminium factory-Podgorica (KAP).
The content of Al 2 O 3 decreases significantly (22.47%), while the contents of SiO 2 , iron oxide and titanium increase significantly. The high average contents of calcium oxide (6.5%) and sodium oxide (6.07%) in the bauxite residue are a consequence of the nature of the alumina production technological process. The increase in average contents in bauxite residue compared to the bauxite is also shown by other tested oxides of chromium, manganese, phosphorus and potassium (from 1.1 to 1.9 times). Trace elements: Be, Cs, Ga, Ta and Co exhibit from 1.1 to 5 times lower contents in bauxite residue compared to bauxite, while all others have higher, especially: Zr (2 times), Sr (1.9 times), V and Th (1.8 times), Rb (1.7), and so on.
According to the presented data, the total average content of rare earth elements (∑Sc, Y, La-Lu) in the bauxite residue in basins A and B is 1.4 times higher than the average content in bauxites. The largest increase in average content is shown by Sc-1.68 times, La and Ce 1.42, that is to say, 1.4 times, while the smallest is by Y, only 1.28 times.
In almost all samples the following minerals have been identified: hematite, gibbsite, calcite, cancrinite, less common but also present are: böhmite, goethite, quartz, rutile, anatase, perovskite, garnet and nordstrandite ( Figure 20). According to the presented data, the total average content of rare earth elements (Sc Y, La-Lu) in the bauxite residue in basins A and B is 1.4 times higher than the average content in bauxites. The largest increase in average content is shown by Sc-1.68 times, La and Ce 1.42, that is to say, 1.4 times, while the smallest is by Y, only 1.28 times.
In almost all samples the following minerals have been identified: hematite, gibbsite calcite, cancrinite, less common but also present are: böhmite, goethite, quartz, rutile, an atase, perovskite, garnet and nordstrandite ( Figure 20). Finally, until the completion of more detailed exploration it can be noted that the presented results should be considered as preliminary and indicative.

Conclusions
Jurassic bauxites are of the greatest economic importance in Montenegro, especially high-quality deposits of the bauxite-bearing region Vojnik-Maganik, in the wider area o Finally, until the completion of more detailed exploration it can be noted that the presented results should be considered as preliminary and indicative.

Conclusions
Jurassic bauxites are of the greatest economic importance in Montenegro, especially high-quality deposits of the bauxite-bearing region Vojnik-Maganik, in the wider area of Nikšićka Župa.
The implementation of recent national and international exploration projects has collected new data, especially in the part of geochemical and mineralogical characterisation of bauxites, which complements the previous knowledge about Montenegrin bauxites. This enabled a better assessment of the potentiality of bauxite formations and individual bauxite deposits for REE and associated critical metals.
Mineralogical explorations have confirmed the complexity of the mineral composition of bauxite, when it comes to the main and less represented minerals, as well as accessory minerals. The studied red Triassic bauxites are characterised by the presence of böhmite and gibbsite, followed by hematite, goethite, kaolinite and anatase, as well as vermiculite.
The main carrier of aluminium with red bauxite from the Vojnik-Maganik and Prekornica ore regions is the mineral böhmite, partly gibbsite. Regarding other major minerals the following are presented: Fe-oxides/hydroxides (hematite and goethite), clay minerals (kaolinite) and titanium minerals (mainly anatase). In the previously mentioned bauxites, the presence of the following minerals was also detected: zircon, ilmenite, magnetite, biotite, K-feldspars, mottramite, REE phosphates-monazite and xenotime and REE carbonates-Ce and Nd. Studied Cretaceous bauxites are consisted by major minerals: böhmite, gibbsite, hematite, kaolinite, anatase and rutile, while Paleogenic: böhmite, goethite, kaolinite and anatase. The Triassic bauxites of Piva (Rudinice) belong to the ferritic bauxite, as well as the bauxites of a number of Jurassic deposits formed on the paleorelief of the Late Triassic age. The Triassic bauxites of Gornjepoljski vir and the Cretaceous bauxites of Medede deposit belong to the group of kaolinite bauxites. All other bauxites from the Jurassic, Cretaceous and Paleogene deposits are classified in the bauxite group.
The content of major oxides in bauxites corresponds to the mineral composition and varies significantly, both in the case of different bauxite formations, and in individual deposits belonging to the same formation, the same or different ore regions. Based on the content of useful and main components, red bauxite which can be used for the production of alumina, are divided as follows: high-quality bauxite with Al 2 O 3 content from 55% to 61% and SiO 2 from 0.5% to 6%; low-quality bauxite with Al 2 O 3 content from 49% to 55% and SiO 2 from 6% to 15% and poor-quality bauxite with Al 2 O 3 content from 43% to 50% and SiO 2 from 15% to 25%.
Due to their genetic specificities and mineral composition, cretaceous white bauxites in the samples from the studied deposits, show a high average content of SiO 2 and lower contents of Fe 2 O 3 . These bauxites, that is to say parts of deposits with low Fe 2 O 3 content and satisfactory Al 2 O 3 and SiO 2 content, were mainly used as raw material for the refractory materials industry.
Bauxite formations in Montenegro have significantly different ΣREE average contents. The highest contents were detected in Jurassic bauxites from the Vojnik-Maganik and Prekornica ore regions from the Early Jurassic, Middle Jurassic-Oxfordian and Late Triassic paleorelief-around 1000 ppm on average, which makes them the most interesting in terms of possible future use for REE extraction. It is important to emphasise that in this sense, low-quality deposits with a high content of SiO 2 are also interesting, from which it is not possible to use bauxite for the production of alumina and aluminium. Jurassic bauxites of these regions are characterised by elevated contents of mainly light lanthanides (LREE), but also Y and Sc. Heavy lanthanides (HREE) are significantly less presented.
Based on preliminary data, at the moment, Jurassic bauxites of the Orjen ore region, which were also formed on underlying bed of the Middle Jurassic-Oxfordian age, can be considered promising, almost like the bauxites of the Vojnik-Maganik and Prekornica ore regions. This cannot be said for the Jurassic bauxites of the Western Montenegro ore region, where REE contents are significantly lower, and for which explanations should be sought through furthermore detailed explorations.
The Cretaceous white bauxites of the exploited deposits are characterised by the lowest average REE contents in comparison to the bauxites of other formations, especially Jurassic, which places them in the group of the least potential in terms of obtaining REE. However, these bauxites are significantly richer in lithium compared to Jurassic, especially those from the largest and highest quality deposits formed on the Late Triassic paleorelief.
Although at this moment we have a small amount of data, we can preliminary conclude that Triassic bauxites are much less promising in terms of their REE content, but also due to the fact that their proven reserves are small.
Paleogene bauxites, similar to the Triassic ones, cannot have economic significance, because they occur only in the form of occurrences and there are no proven significant amounts of bauxites at any of the investigated locations, although they contain certain contents of REE.
Regarding other critical mineral raw materials (CRM) and other elements, Ti, V, Zr, Nb, Sr and Ga could also be promising in bauxites.
According to the determined contents of REE and other macro and trace elements in bauxite residue, it can be concluded that this secondary resource is very promising. Compared to the bauxites from which it originates, the contents of individual elements are significantly higher, such as: Zr (2 times), Sr (1.9 times), V (1.8 times), Sc (1.68 times), La (1.42 times), Ce (1.4 times), Y (1.28 times), as well as all other elements from the lanthanide group. In much higher contents in bauxite residue compared to bauxite, Ti, V, Zr, Nb, Sr and other elements, are also present and may be interesting for extraction and exploitation. On the other hand, the contents of Be, Cs, Ga, Ta and Co, in bauxite residue are lower than their contents in bauxites.
Further development of economically and environmentally sustainable technologies for extracting REE from bauxite residue and, why not from bauxite may allow in the future the exploitation of Montenegrin bauxites as sources of CRMs.
Funding: This research received no external funding.