Distribution of major and trace elements in the Kovin lignite ( Serbia )

A geochemical and mineralogical study was performed on lignite samples from the Upper Miocene Kovin deposit, hosting three coal seams. The Kovin lignite is characterized by high moisture content, medium to high ash yield, medium to high sulphur content and a relatively low gross and net calorific value. The mineralogical composition, and major and trace element contents were determined by X-ray diffraction, scanning electron microscopy with energy dispersive Xray spectroscopy (SEM-EDS) analyses, and inductively coupled plasma optical emission spectrometry (ICP-OES). The most abundant minerals in all lignite samples from the three coal seams are clays (illite/smectite), silicates (quartz, plagioclase), sulphates (gypsum/anhydrite) and carbonate (calcite). The other iron-rich minerals are sulphides, oxides and hydroxides (pyrite, magnetite, haematite, and limonite). In general, mineral matter in the matrix coal consists of illite/ smectite and quartz, while xylite-rich coals, apart from illite/smectite, have a higher content of sulphates and Fe-oxide/hydroxide minerals. The lignite from the Kovin deposit is enriched in As, Cd, Co, Cr, Cu, Ga, Li, Mn, Mo, Ni, Pb, V, Zn, Gd, Tb, Er and Lu in comparison with the Clarke values for brown coals. The statistical analysis of bulk compositional data shows inorganic affinity for the majority of the major and trace elements and possible association with pyrite, illite/ smectite and calcite. The Upper Miocene Kovin lignite deposit is located about 50 km east of Belgrade (Fig. 1). Together with the Kostolac basin, it is a part of the unique coal basin separated by the Danube River (MITROVIĆ et al., 2016), which forms the southern boundary of the Kovin deposit. The Kovin deposit is divided into two fields: the western field “A”, and the eastern field “B”, 16.3 km2 and 23.7 km2, in areal extent respectively. According to the Geological re­ port of the Kovin deposit, the lignite resources and reserves are currently estimated at 275 Mt (MITROVIĆ et al., 2016). Subaque­ ous exploitation of lignite (below the Danube river) in the offshore zone of the “A” field, named “Experimental exploitation field” (EEF) began in 1991, and is still active. Since 1991, the Kovin mine has produced about 5 Mt of lignite, with an annual produc­ tion of ~ 300,000 t. Geological exploration of the wider area of the Kovin de­ posit, including the Kostolac coal basin, began in the late 19th century. The Upper Miocene (Pontian) age of the coal­bearing sediments was confirmed by palaeontological studies (PAV LO­ VIĆ, 1959; SPAJIĆ­MILETIĆ, 1960, 1969; STEVANOVIĆ, 1951). The distribution of palynomorph assemblages in the lignite from the EEF field (MILIVOJEVIĆ & ŽIVOTIĆ, 2006; ŽIVOTIĆ et al., 2007) suggests that decay­resistant gymnosperm (conife­ rous) trees and bushes played an important role in lignite forma­ tion. Previous petrographic investigations (ERCEGOVAC et al., 2006; ŽIVOTIĆ et al., 2005, 2007) performed on samples from several boreholes from the I and II coal seam, showed that the lignite from the Kovin deposit is a typical humic coal with vari­ able huminite, liptinite and inertinite contents, and a mean ran­ Article history: Received June 08, 2018 Revised manuscript accepted December 12, 2018 Available online February 15, 2019


Distribution of major and trace elements in the Kovin lignite (Serbia)
The Upper Miocene Kovin lignite deposit is located about 50 km east of Belgrade (Fig. 1).Together with the Kostolac basin, it is a part of the unique coal basin separated by the Danube River (MITROVIĆ et al., 2016), which forms the southern boundary of the Kovin deposit.The Kovin deposit is divided into two fields: the western field "A", and the eastern field "B", 16.3 km 2 and 23.7 km 2 , in areal extent respectively.According to the Geological re port of the Kovin deposit, the lignite resources and reserves are currently estimated at 275 Mt (MITROVIĆ et al., 2016).Subaque ous exploitation of lignite (below the Danube river) in the offshore zone of the "A" field, named "Experimental exploitation field" (EEF) began in 1991, and is still active.Since 1991, the Kovin mine has produced about 5 Mt of lignite, with an annual produc tion of ~ 300,000 t.
Geological exploration of the wider area of the Kovin de posit, including the Kostolac coal basin, began in the late 19 th century.The Upper Miocene (Pontian) age of the coalbearing sediments was confirmed by palaeontological studies (PAV LO VIĆ, 1959;SPAJIĆMILETIĆ, 1960SPAJIĆMILETIĆ, , 1969;;STEVANOVIĆ, 1951).The distribution of palynomorph assemblages in the lignite from the EEF field (MILIVOJEVIĆ & ŽIVOTIĆ, 2006;ŽIVOTIĆ et al., 2007) suggests that decayresistant gymnosperm (conife rous) trees and bushes played an important role in lignite forma tion.Previous petrographic investigations (ERCEGOVAC et al., 2006;ŽIVOTIĆ et al., 2005ŽIVOTIĆ et al., , 2007) ) performed on samples from several boreholes from the I and II coal seam, showed that the lignite from the Kovin deposit is a typical humic coal with vari able huminite, liptinite and inertinite contents, and a mean ran dom huminite reflectance of 0.30±0.03(ERCEGOVAC et al., 2006).Recent petrographic and organic geochemical investiga tions (MITROVIĆ et al., 2016(MITROVIĆ et al., , 2017) ) confirm the variation in maceral and lithotype composition in both fields and all three seams with diagenetic alteration governed by chemoautotrophic, methanotrophic and heterotrophic bacteria.The same samples were used to determine the content and distribution of major and trace elements in the coal.
The aim of this study is to present the content and mode of occurrence of major and trace elements in coal from the Kovin deposit in order to assess their behaviour in case of exploitation for electric power generation.

GEOLOGICAL SETTINGS
The area of the Kovin deposit consists of Palaeozoic schist, Ter tiary and Quaternary sediments (Fig. 1).The basement of the Kovin deposit is formed of Devonian lowgrade schist overlain by Neogene sediments.
The Kovin deposit together with the Kostolac basin was formed in the Pannonian Basin System in shallow lacustrine, delta plain and fluvial environments.During the late Miocene it became in creasingly widespread as Lake Pannon (e.g.MAGYAR et al., 1999MAGYAR et al., , 2013;;SZTANÓ et al., 2013) filled with coalbearing sediments in the central part of Serbia.The total thickness of Neogene sediments is estimated at 1000 m.The Neogene units of the Kovin deposit consist of: a) Sarmatian (Middle Miocene) shallow brackishma rine sediments; b) Pannonian (Late Miocene) marly sediments; c) Pontian (Late Miocene;RÖGL, 1996) shallow, caspibrackish to fresh water sediments; d) Lower Pliocene fresh water clastic sedi ments, as described in detail in MITROVIĆ et al. (2016).
In the Upper Miocene (Pontian) clastic coalbearing series of the Kovin deposit, three coal seams are hosted (MITROVIĆ et al., 2016) namely III (the oldest), II and I (the youngest).Seam III was found in the eastern part of the B field with a total thick ness from a few to 48.7 metres (including interbedded waste rock).Coal seam II developed through the entire Kovin deposit, and in the eastern part it is uniform, but towards the west it splits into several coal layers.Total seam thickness is variable, up to 7 m.The youngest, coal seam I has been explored in the southern part of the A field (EEF; Fig. 1).In the B field and the eastern, southeastern and south part of the A field, it is more or less uni form, but towards the northwest it splits into two coal layers (upperIa and lowerIb) with a total thickness of up to 15 m.Tectonic features of the Pontian sediments are relatively uni form in the major part of the deposit; coal seams dip at low angles (5-7 o ) to the northwest.In the central part of deposit coal seams form an antiform due to postsedimentary faulting, causing ero sion of coal seam I in the central part of the deposit between fields A and B. One of the most important normal faults is located in the western part of the B field (Fig. 1).

Sample collection
The fortyfour lignite samples were collected from four bore holes, GD601 and GD603 from the A field, and KB79 and KB 91 from the B field, Kovin deposit, as described in detail in MITROVIĆ et al. (2016MITROVIĆ et al. ( , 2017;;Fig. 1;Table SI of the Supplemen tary Material).The samples represent different lithotypes and parts of coal seams I, II and III (Figs. 2 & 3).

X-ray diffraction (XRD) and SEM-EDS analysis
Mineralogical analysis of fortytwo (without the 28/91 and 31/91 samples) ash samples, heated to 450°C was carried out using  W ar -moisture content, as received basis, %; W an -analytical moisture content, %; A db -ash content, dry basis, %; S db -total sulphur content, dry basis wt.%; V db -volatile matter, dry basis wt.%; Q daf g -gross calorific value, dry, ash-free basis, MJ/kg; Q daf n -net calorific value, dry, ash-free basis, MJ/kg; C daf -carbon content, dry, ash-free basis, %; H dafhidrogen content, dry, ash-free basis, %; N daf -nitrogen content, dry, ash-free basis; O daf -oxigen content, dry, ash-free basis, %; X̅ -arithmetic mean value; Min -minimum; Max -maximum; s -standard deviation.Note: Values of parameters for individual samples are given in Table S-II of the Supplementary material to this paper.Xray powder diffraction (XRPD).Analysis was performed on a Philips PW 1710 powder diffractometer with CuKα 1,2 = 1.54178Å radiation 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.Xray diffractograms of 42 samples were subjected to quantitative mineralogical analysis using the FullProf computer program (RODRÍGUEZCARVAJAL, 1993) based on the principles for diffractogram profiling set out by RI ETVELD (1969).No quantification of the amorphous and sul phide phases was undertaken.
The mineral composition and distribution of some elements in the minerals were investigated in five representative samples (53/603, 56/603, 3/79, 17/79 and 31/79) using a JEOL JSM6610LV scanning electron microscope (30 kV accelerating voltage) equipped with an energydispersive Xray spectrometer (SEM EDS; XMax Large Area Analytical Silicon Drift connected with INCAEnergy 350 Microanalysis System).After the maceral ana lysis, polished blocks were coated with a thin gold film in order to obtain a higher quality secondary electron image for SEM and EDX examination.

Inductively coupled plasma optical emission spectrometry analysis (ICP-OES)
The contents of major and trace elements were determined by in ductively coupled plasma -optical emission spectrometry (ICP OES).ICPOES measurements were performed using a Thermo Scientific iCAP 6500 Duo ICP (Thermo Fisher Scientific, Cam bridge, United Kingdom).The digestion of lignite samples was performed on an Advanced Microwave Digestion System (ETHOS 1, Milestone, Italy) using a HPR1000/10S high pressure segmented rotor.For total dissolution, about 240-390 mg of lig nite sample was precisely weighed and mixed with 6 ml HNO 3 (65%) and 2ml H 2 O 2 (30%), then heated by microwave energy for 30 min.The temperature was gradually raised to 220 ºC in the first 10 min, remained at 220 ºC in the next 20 min, and then de creased rapidly to room temperature.After cooling, the vessels were opened, 4 ml of H 3 PO 4 (85%) and 2 ml HF (40%) were added and the second phase of microwave digestion was per formed under the same temperature program as the first one.All reagents were analytical grade reagents purchased from Carlo Erba, Italy.Two multielemental plasma standard solutions (MultiElement Plasma Standard Solution 4, Specpure®, 1000 µg/ml and Semiquantitative Standard 1, Specpure®, 10 µg/ml) and four single plasma standard solutions (Silicon, Specpure®, 1000µg/ml; Vanadium, Specpure®, 1000µg/ml, Titanium, Spec pure®, 1000µg/ml and Molybdenum, Specpure®, 1000µg/ml) certified by Alfa Aesar GmbH & Co KG, Germany, were used to prepare calibration solutions for ICPOES measurements.The validity of the all the analytical methods results were provided using the CRM and evaluation of the test results in accordance to the reproducibility for each tested parameter, in accordance with the requirements of the SRPS ISO/IEC 17025:2006(2006).

Lithotypes and maceral composition
The results of lithotypes and maceral analyses were taken from previous research (MITROVIĆ et al., 2016).Stratified matrix coal (Table SI of the Supplementary material; Figs. 2, 3) predomi nates in the upper and the lower part of seam III, while xyliterich coal (ICCP, 1993) predominates in the central part of the seam.Mineralrich coal is present in thin layers near the roof of the seam.Stratified matrix coal in seam II predominates in the whole seam, while xyliterich coal is present in thin layers near the up per and lower parts of the seam.A mixture of mineralrich and matrix coal is present in thin layers near the floor of the seam.Stratified matrix coal in seam I predominates in the whole seam, while xyliterich coal is present in thin layers in the upper and lower parts of the seam in field B, and in the upper part of the up per bench (Ia) in field A.
Huminite is the prevailing maceral group in all seams with telohuminite and detrohuminite as the most abundant maceral subgroups (MITROVIĆ et al., 2016; Table 1).The content of lip tinite is low with liptodetrinite and sporinite as the most abundant macerals (MITROVIĆ et al., 2016).Inertinite content is low in the seam I and II, with its maximum in seam III, with inerto detrinite, fusinite and semifusinite being the most abundant ma cerals.Clays are the most abundant minerals, while pyrite, car bonates and other minerals are less abundant.

Proximate and ultimate analyses
The Kovin lignite is characterized by its high moisture content, medium to high ash yield, high volatile matter, higher sulphur content and relatively low gross and net calorific value (Table 2; Table SII of the Supplementary material).The total moisture of most samples is variable from 34.57-52.90%,14.42-52.48%and 11.79-54.13%,for coal seams I, II and III, respectively (moisture being dependant on the freshness of the analysed sample mate rial -as received basis).The ash content (dry basis) ranges from 12.58 to 52.90%, 13.85 to 68.60%, 8.66 to 78.20%, for coal seams I, II and III respectively, which is on average from low (<10%) to a very high ash level (>50%).Most of the Kovin lignite contains a low to medium sulphur (dry basis) content, whereas four samp les contain a higher sulphur content of more than 3%.Coal seam I has a medium (1-3%) to high sulphur content (>3%), with the highest content (4.40% in sample 31/91; Table SII of the Supple mentary material) in the bottom part of the seam in the B field.Total sulphur content in coal seam II ranges from medium to low, rarely high content.The highest total sulphur content at 3.12% (sample 37/91) has been detected in the B field.Coal seam III has a medium to low sulphur content.The volatile matter on dry ba sis (V db ; Table 2) of the Kovin lignite is typically at a high level of 27.95-47.48%,12.10-50.29%and 9.67-46.71%,for coal seams I, II and III, respectively.
Based on the dry and ash free basis (daf), values of gross and net calorific value are typical for lignite.Coal seam I has a higher average net calorific value (23.26 MJ/kg) compared to seam II (22.42 MJ/kg), and seam III (22.02 MJ/kg).The slight variation of this parameter is detectable between lithotypes and fields.It is obvious that mineralrich coal has the higher ash content and lower gross and net calorific value (Table SII of the Supplemen tary material).
The C contents (daf) are similar in all three seams, averag ing at 64.78%, 64.40% and 64.96% for coal seams I, II and III, respectively (Table I).The average H contents (daf) are 4.64%, 4.91% and 4.55% for coal seams I, II and III, respectively, while the respective O contents (daf) are 25.90%, 27.05% and 27.38%.The N content (daf) ranges from 0.64% to 1.89% in all samples.
The silicate and aluminosilicate minerals are common con stituents of the inorganic matter in Kovin lignites and include mainly quartz, illite/smectite, rarely chlorite and plagioclase, and more rarely, mica (biotite, muscovite), and Kfeldspar (Fig. 4).Quartz is a major inorganic component of the Kovin lignites, es pecially in coal seam III (Table 4).Some parts of seams I and II have very high quartz contents.It is recognized as single angular to semirounded grains of predominantly detrital origin (WARD, 2002), but authigenic crystals were also determined.Epigenetic quartz, along with clay minerals, pyrite, and carbonates, was formed within cleats and fractures in the organic matrix, as a re sult of the circulation of meteoric waters.
The clay minerals, represented by illite/smectite and chlorite, are typical constituents of the inorganic matter, especially in coal seam II.They are mostly of detrital origin (WARD, 2016), but a small part may be diagenetic, formed as weathering products of feldspar and mica, and usually occurring as layers, lenses and films on the surface of macerals.Illite and smectite are present as finely dispersed aggregates of irregular size and rarely as long platy crystals.Clay minerals of epigenetic origin, which precipi tated in fractures, were also observed.
Feldspars are represented by plagioclase (albite; Fig. 4a) and subordinate Kfeldspar (orthoclase; Fig. 4b).They are detrital in origin and are usually associated with clays.Mica (biotite and muscovite; Fig. 4c) is represented by platy or irregular shaped crystals, usually in association with feldspars in clayrich layers.The mica is predominantly of detrital origin, but a small part might be a weathering product of feldspars.During the processes of coal formation mica is unstable and could transform to chlo rite, limonite, and clay minerals.Chlorite (Crrich chlorite as well, Fig. 4d) was found in the I and II seams.The majority of the chlorite in the Kovin lignite is of detrital origin, formed as a weathering product of the crystalline basement rocks.
Detrital epidote, a calcium, aluminium and iron sorosilicate mineral (Fig. 4e), occurs as individual crystals of irregular shapes.
Sulphates, anhydrite (CaSO 4 ; Fig. 4f) possibly gypsum as well, were determined in high amounts in several samples.In raw lignite, sulphate minerals are authigenic (mainly epigenetic) and/ or occur as weathering products.Barite (BaSO 4 ; Fig. 4g) was de termined in very low amounts.
The Fe oxides and hydroxides (haematite and limonite) are present in considerable amounts in the Kovin lignites.A large proportion of the Feoxide/hydroxides in the Kovin lignite samp les (Fig. 4h) is probably the result of the weathering of ironrich minerals such as pyrite and siderite.A small proportion of these minerals might also be of detrital origin.Limonite (a mixture of goethite, lepidocrocite, and other Fehydroxides) in the Kovin    lignite occurs mainly as collomorphous crusts built up of small flakes and usually appears in association with haematite and other minerals including siderite and clays.The FeMn oxides and Mn hydroxides were also detected in small amounts in coal seams I and II.The Ti and TiFe oxides (rutile and ilmenite; Fig. 4i, j) oc cur as small aggregates in low amounts in all seams.
One of the most abundant carbonate minerals in the Kovin lignite is calcite, whereas the other carbonates such as siderite, dolomite and ankerite are present in small amounts.Calcite was determined in significant amounts in the central part of seams II and III (Table 4), as well as in the lower bench of seam I in field A. Carbonates occur as individual rounded and angular grains and spherical aggregates which implies most probably their syn genetic origin.Siderite was observed in small amounts in seam II and occurs as finegrained lenses, layers and crusts, which are intimately associated with organic matter, suggesting most pro bably a syngenetic origin.Dolomite occurs as small angular grains in low amounts in the central part of seam II in field B.
Pyrite is one of the most abundant sulphide minerals and a typical authigenic mineral in the Kovin lignite (Fig. 4k, l, m).Syn genetic pyrite occurs as framboidal and euhedral crystals (CHOU, 2012), situated along stratified bands or infilling cavities within the organic matter and cell openings of plant debris.Crystals were formed within the lumens of densinite and textinite.Very small amounts of epigenetic pyrite were determined.Sphalerite and ga lena were detected in small amounts in seam II (Fig. 4n).Rare grains of REYrich phosphates, monazite (Fig. 4o) of irregular shape were detected in coal seams II and III.Ce, La al lanite (calcium, aluminium, iron sorosilicate mineral; Fig. 4p), probably of detrital origin, occur as individual small crystals of irregular shapes in coal seam I.

Geochemistry
Variations of the Si, Al, Ti and K contents in the studied samples are obvious in each seam and mostly depend on coal lithotypes (Table 5; Table SIV of the Supplementary material to this paper; Figs. 2, 3).It is obvious that the mineralrich and mixture of ma trix and mineralrich coal have high contents of the aforemen tioned major elements.The average Si contents in the Kovin lig nites are 6.40%, 7.50% and 9.68% for coal seams I, II and III, respectively, while the respective Al contents are 2.48%, 2.90% and 2.39%.A high Si/Al ratio in all three seams (Table 5) implies high contents of quartz, illite/smectite and plagioclase.

DISCUSSION
The mineral matter in coal seams and deposits depends on the combination of specific plant constituents influenced by water level, regional depositional and paleoenvironmental factors, which control the enrichment or depletion of the different ele ments and mineral phases contained in the coal.Chemical, bio logical and physical factors in mire systems provide specific en vironments in which minerals could be deposited or formed ( GLUSKOTER, 1975;TAYLOR et al., 1998;WARD, 2016).Epi genetic factors may also influence the variation in elements (CHRISTANIS et al., 1998;KALAITZIDIS et al., 2002;DAI et al., 2012).The modes of mineral occurrences, their abundance and distribution in coal may characterize seams, deposits, and facies changes.It can be used as an indicator for the partial re construction of the environment conditions during peatification and coal formation (VASSILEV & VASSILEVA, 1996, 1998).

Affinity of the elements and geochemical associations
High inorganic affinity (r ash =0.7-1.0; as well as for Al, Si, Ti, Ba, Cr, Cu, Ga, In, Li, Sc, Se, V, Gd and Tb in seam II, and for Al, Si, Ti, Fe, K, Na, Mn, Ba, Cd, Cr, Ga, Ni, Sc, Th, Zn, Ce, Sm, Eu, Gd, Tb, Ho, Er, Yb and Lu in seam III.The strong correlation between ash yield and Si, Al, K and Na (Table 8, Table SV of the Supplementary material to this pa per) imply the predominance of aluminosilicate and silicate mine rals (Fig. 4) in all three seams.As expected, the content of mine ral matter from maceral analyses shows a strong correlation with ash yield in all three seams.Elements with a less but still rela tively high inorganic affinity (r ash =0.5-0.69;Table 8) include: correlation coefficients varying from 0.35 to 0.49 between ash yield and the aforementioned elements.Sr in seam I; S db and Ca in seam II, and Ca, As and Sr in seam III show organic affinity (r ash = -0.35--1.00;Table 8).
According to the geochemical and SEMEDS data most trace elements have good correlation coefficients with Al and Si (Table 6), indicating their close connection with silicate and alumino silicate minerals (quartz, plagioclase, albite, orthoclase, biotite, Figure 6.Macropetrographic profiles of boreholes KB-79 (a) and KB-91 (b) in the B field lignite and the vertical distributions of ash, total sulphur content and some trace elements in coal.A db -ash content, dry basis, %; S db -total sulphur content, dry basis wt.%. and muscovite; Fig. 4) as well as with illite/smectite and chlorite in all coal seams.This result confirms the clastic input in the pa laeomire during formation of all coal seams in the Upper Mio cene.Pyrite is the most abundant sulphide mineral in the Kovin lignite.A strong positive correlation between sulphur content (r S tot >0.7;Table 8) and Fe, As, Cd, Se, in seam I, As in seam II, as well as a good correlation between sulphur content (r Stot =0.5-0.69) and Co, Cr, Mo, in seam I, Mo in seam II and Sr in seam III, im ply a close connection with pyrite.The result of SEMEDS exa mination confirms the correlation data (Fig. 4k, l, m).The mi neralrich lignite in the bottom part of seam I of field B is particularly enriched in As (Fig. 4).A very strong correlation between Fe, As and Cd and all of them with S db in seam I, clearly indicates the same origin of these elements and a close connec tion with pyrite (DAI et al., 2003;DIEHL et al., 2012;FINKEL MAN, 1994;YUDOVICH & KETRIS, 2005a;2012).Cr, Ni, Pb and Zn show good correlation with Si and Al (Table 8) and a most probable connection with aluminosilicate minerals (illite/smec tite, chlorite; Fig. 4d).The strong correlation of Ca and Sr in seam I shows that Sr is associated with the carbonate minerals (calcite).

Comparison between coal petrology and major and trace elements in coal
The mineralogical compositions of the Kovin lignites are quite similar, and quartz, illite/smectite, gypsum/anhydrite, plagio clase (albite), haematite, calcite and pyrite are the common mine rals for both coalfields.The determined minerals are rich in Si, Al, Ti and K and they are compatible with the mineralogical com position of the lignite in both fields.Also, the relatively high con tent of Fe in the lignite can be related to the high pyrite and hae matite content, while the high Ca content could be connected with high gypsum/anhydrite and calcite contents.
To further evaluate the relationships between lithotypes, mineralogy and geochemistry, profiles representing the vertical variation in mineralogy were placed alongside profiles of the ma jor element proportions (Si, Al, Fe, Ca, Mg, Ti, K , Na, Mn) in the A field (Fig. 2), and the B field (Fig. 3).Profiles representing the vertical variation of the most important trace elements (As, Cd, Cr, Ni, Pb, Se) in the Kovin lignite are presented in Figs. 5 and 6 for fields A and B, respectively.
The variations in lithotypes, maceral composition and ash contents (MITROVIĆ et al., 2016) indicate that the water level changed during peat accumulation.Increased ash content (Figs. 5 and 6) may indicate more frequent inundations of the precursor mire resulting in deposition of siliciclastics.

Coal seam III
Mineral rich and mixtures of matrix and mineral rich coals with a predominance of detrohuminite, relatively low sulphfur content and higher ash contents in the bottom part of the coal seam III (samples 14/79-21/79; Fig. 3) imply that peat accumulation de veloped in a freshwater environment of a higher water level of the mire.Organic matter (OM) mainly originates from herbs and shrubs along with aquatic species with a small contribution from conifers (MITROVIĆ et al., 2016).The high Si and Al content in most samples, as well as higher contents of K and Na imply de position of siliciclastic minerals such as quartz with variable amounts of illite/smectite and plagioclase.
The central part of the seam consists of mixtures of xylite and matrix and matrix and mineral rich coals with two thin xy  8; Table SVI of the Supplementary material to this paper) indicates that Ca may also be bound with organic matter as exchangeable ions in carboxylic acids and phenolic or hydroxyl groups (LI et al., 2007(LI et al., , 2010;;WARD, 2016).Also, its occurrence is possible in metalloporphyrins and other metalorganic com pounds (SAXBY, 2000) especially in lowrank coals.
The top of coal seam III (sample 11/79) consists of mixtures of matrix and mineral rich coals with the dominance of detro huminite.The high ash content as well as the Si, Al, K and Na contents, associated with an Si/Al ratio of 5.00 (Table SIV of the Supplementary material to this paper) indicates a higher water level in the mire with deposition of siliciclastic minerals such as quartz and subordinate clay minerals and plagioclase.
Coal from seam III is enriched in Ba, Cr and Sr (Table 6).The high Cr content was detected in mineralrich lithotypes with high ash content (Fig. 6).Chromium could be present in coal as both organic and inorganic forms (KETRIS & YUDOVICH, 2009).In most bituminous coals from the USA two major forms are present (HUGGINS & HUFFMAN, 2004): organic associa tion and in illite.Cr could be also present in oxide/carbonate/ monosulphide group, and in silicates (DALE et al., 1999).Coals enriched in Cr (≥~100 ppm) are usually situated near to ultramafic rocks that contain chromebearing spinels (HUGGINS et al., 2000;RUPPERT et al., 1996).In the studied coals, a strong cor relation of Cr with ash content and Si, Al, Ti, K and Fe in seam III suggests that the Cr mainly occurs in aluminosilicate mine rals, most probably Crrich chlorite (Fig. 4d).The ultramafic rocks which outcrop in the southeastern part of the study area can be the source of Cr in the studied lignite.Furthermore, a high content of Cr, Ni and other elements detected in lignite from the Drmno field (Fig. 1; ŽIVOTIĆ et al., 2015) suggests a similar source.The strong correlation of Ba with ash content suggests an inorganic form probably as barite (Fig. 4g), while Sr indicates an organic form (Table 8).The results of the chemical and mineralo gical investigation of the Kovin lignite samples indicate an or ganic form of As in seam III.

Coal seam II
The geochemical composition of coal seam II shows some diffe rences between fields A and B. Organic matter of the lignites from both coalfields was derived from herbs and shrubs with variable amounts of woody vegetation (MITROVIĆ et al., 2016).Matrix and mixtures of matrix and mineral rich coals with a slight pre dominance of detrohuminite over telohuminite built the bottom part of seam II in field A (samples 48/601, 49/601, 55/603, 56/603; Fig. 2).The coal is characterised by moderate to very high ash and a low to moderate sulphur content.The high Si, Al, K, and Na contents suggest a high contribution from siliciclastic sedi ments.The upper part of seam II in field A consists of xyliterich coal (54/603), with moderate amounts of ash and sulphur.The Si/Al ratio is still high, as well as the amounts of Ca and Fe, sug gesting a greater contribution of siliciclastic sediments with vari able amounts of Feoxide/hydroxides (Fig. 4h).An outstanding negative correlation between Ca and sulphur content (S db ), as well as with ash content (Table 8) suggests the organic affinity of Ca and S. According to previous research (CHOU, 2012;LI et al., 2007LI et al., , 2010;;WARD, 2016) in neutral to alkaline groundwater Ca and S could be taken by peat forming plants in the palaeomire and bound with organic matter.
Xyliterich and mixtures of xyliterich and matrix coals with prevalence of telohuminite occur in the bottom part of coal seam II in field B (samples 3940/91, 8/79; Fig. 3).Lignite is characte rised by moderate ash and low to high sulphur contents with vari able Si and Al followed by higher Ca and Fe contents, suggesting a higher contribution of siliciclastic sediments with variable amounts of Feoxide/hydroxides and sulphates.The central part of seam II is characterised by moderate to high ash and a low/ moderate to high sulphur content with variable content of major elements.The higher Ca and Fe contents in xyliterich samples (4/79, 37/79) suggest a higher contribution of sulphates and Fe oxide/hydroxides formed in a neutral to alkaline and probably oxic environment.Matrix and mixtures of matrix and xyliterich coal with moderate contents of ash and sulphur built up the upper part of seam II.Higher contents of Si and Al in borehole KB91 (samples 33/91, 34/91) indicate a higher contribution of siliciclastic sediments, while the higher contents of Ca and total sulphur in borehole KB79 (sample 2/79) may suggest a slight predominance of sulphate minerals and changes in water level and the paleoenvironment.
The content of As, Cr, Ni, Pb, V and Zn in seam II coal is several times higher in comparison with the Clarke values for brown coal (KETRIS & YUDOVICH, 2009).Arsenic could be present in pyrite (DAI et al., 2003;DIEHL et al., 2012;FINKEL MAN, 1994;KOLKER, 2012YUDOVICH & KETRIS, 2005a, and references therein) but also as the arsenate ion in clays or phosphate minerals (SWAINE, 1990).Organic bound arsenic is also common, with higher contents in lower organisms, algae and herbages than in seed plants, ferns, and moss (LIU et al., 2003).Three dominant forms of As in coal (YUDOVICH & KETRIS, 2005a, b) are pyritic, organic and arsenate.Recent research on As rich coals from the Xishanyao formation, China (ZHANG et al., 2018) confirms sulphfide bound, residual form, organic bound and adsorption solution of arsenic.The same authors conclude that the geological origins leading to the enrichment of arsenic in coal are: hydrothermal solution activities controlled by the tec tonic fracturing of the region, sources of terrigenous detrital ma terial controlled by regional geological settings with the distribu tion of elements and type of plants in the paleomire.The results of the chemical and mineralogical investigation of the Kovin lig nite samples indicate a pyritic form of As in seam II.A strong correlation of Cr with Si, Al, Ti and Fe in seam II, suggests that the Cr mainly occurs in aluminosilicate minerals.Nickel could be present in coal in organic and inorganic form.It can be asso ciated with clays (kaolinite, illite, smectite) and sulphides (pyrite, millerite, bravoite; YUDOVICH & KETRIS, 2005b).Organically bound Ni is also common in coal (RUPPERT et al., 1996).Nickel in seam II of the Kovin deposit shows no specific affinity, indi cating both organic and mineral associations.Lead mainly occurs as sulphide (galena) or associated with sulphide minerals (FIN KELMAN, 1994), as well as lead selenide in coals (HOWER & ROBERTSON, 2003).It could also be present in silicates (DALE et al., 1999).Kovin lignite in seam II has a Pb content that is seve ral times higher content than the Clarke value for brown coals (KETRIS & YUDOVICH, 2009).The positive correlations of Pb with Si, Al, Ti and K in all seams indicate the presence of Pb in the clay minerals.

Coal seam I
Lignite from seam I from the A field shows some differences be tween the lower (Ib) and upper (Ia) layers.The lower part of the Ib layer (samples 44/60146/601 and 51/60353/603), made of a mixture of matrix and xyliterich coal, has variable amounts of Si and Al, with high contents of Ca, Fe and Mn in samples 45/601 and 52/603, followed by a higher Si/Al ratio.The lignite from the Ib layer is characterised by moderate to high ash and sulphur con tents, while the upper (Ia) layer is characterised by moderate ash and sulphur contents and variable amounts of Ca and Fe with a higher Si/Al ratio, which is compatible with a higher contribution of siliciclastic sediments.
The characteristics of lignite in seam I of the B field (samples 26/9131/91) revealed that mineral matter in the matrix coal con sists of siliciclastic sediments, while xyliterich coal has higher Ca, Fe and Mn contents.The lignite is characterised by moderate to high ash and sulphur contents indicating an unstable water ta ble with frequent flooding.The highest sulphur (4.40%;Table SII of the Supplementary material to this paper) and ash contents are identified in the lowermost, mineral rich sample (31/79; Fig. 4b), which is also enriched in Fe (Fig. 7b).The good correlation be tween sulphur and Fe contents (Table 8) is commensurate with pyrite as the main source of both elements (Fe, S db ) in seam I.That part of seam I is especially enriched in As, Cd, Cr, Ni, Pb and Se with the highest value of As (140.0 mg/kg), Cd (6.5 mg/ kg), Cr (371.4 mg/kg), Ni (214.0 mg/kg) and Se (9.0 mg/kg).In comparison with the Clarke value for brown coal (KETRIS & YUDOVICH, 2009) these values are 18, 27, 25, 24 and 9 times higher for As, Cd, Cr, Ni, Pb and Se respectively.A very strong correlation As-S db (Table 6) indicates a pyritic form of As in coal seam I. Cadmium is predominantly associated with Zn mostly in sphalerite (ZnS; GOODARZI, 2002), as well as in other sulphides (e.g.pyrite; DALE et al., 1999).However, an "organic" form of Cd is also possible (YUDOVICH et al., 1985).The very strong correlation of Cd-S db (Table 8), Cd-Fe (r=0.97)clearly indicates the close connection of Cd with pyrite in seam I. Selenium could be present in organic and inorganic form (YUDOVICH & KET RIS, 2005b, 2006).In high sulphur coals, Se is concentrated in sulphide minerals (pyrite) and some selenitic forms (clausthalite, PbSe).In oxidized coals, Se is enriched in the bed oxidation zones (YUDOVICH & KETRIS, 2006), with U, Fe, Mo, V, and Pb.The strong correlation of Se with S db (r=0.76)supports their occur rences in pyrite.
The results of the chemical and mineralogical investigation of the Kovin lignite samples indicate a pyritic form of As, Cd, Co, Cr, Mo, Se in seam I (Table 8;Fig. 4k,l,m) formed in pre sumably neutral and anaerobic conditions in the palaeomire.The very strong correlation of Fe with As (r=0.94),Cd (r=0.97),Co (r=0.82),Cr (r=0.91),Ni (r=0.87),Se (r=0.91), and As with Cd (r=0.96),Co (r=0.79),Cr (r=0.79),Ni (r=0.81),Se (r=0.93)con firms the close interrelation of the aforementioned elements with pyrite.The distribution and enrichment of elements was presu mably controlled by conditions during peat accumulation.A higher water level in a palaeomire with increases in clastic inputs resulted in high concentrations of aluminosilicate minerals.Sur face and groundwater from the surrounding palaeoenviron ment, rich in elements such as As, Cd, Co, Cr, Ni, Se, derived from the weathering and leaching of the basement rocks, along with a reducing environment in the palaeomire, may have re sulted in the formation of syngenetic pyrite enriched in some ele ments, as As, Se, and Sb may be substituted with S in the pyrite structure (KOLKER, 2012).

CONCLUSION
The Upper Miocene Kovin sedimentary sequence consists of shallow, caspibrackish to fresh water clastic sediments (sand and silt with thin clay, carbonaceous clay and gravel layers), with three lignite seams, III, II and I (from lowest to highest).
Huminite is the prevailing maceral group in the three coal seams.The most abundant maceral subgroups are telohuminite and detrohuminite with variable amounts of gelohuminite.Lipti nite and inertinite are much less abundant.The content of mineral matter varies between 3 and 37 vol.%.Clays are the most abun dant, while pyrite, carbonates and other minerals are less abun dant.
SEMEDS examination revealed that the most abundant minerals in all the studied lignite samples from all coal seams are clays (illite/smectite), silicates (quartz, plagioclase), sulphate (gypsum/anhydrite) and carbonate (calcite).The other ironrich minerals are sulphides, oxides and hydroxides (pyrite, magnetite, haematite, and limonite).Minor minerals such as rutile, ilmenite, Kfeldspar (albite, orthoclase), and mica, were detected in all coal seams.In addition, barite, chlorite, epidote, allanite and zircon were also detected in coal seam I. Sphalerite, galena, siderite, do lomite, ankerite and monazite were identified in coal seam II, while barite and monazite were found in coal seam III.
According to the mineralogical and geochemical data, sili cate and aluminosilicate minerals (quartz, plagioclase, albite, or thoclase, biotite, and muscovite) together with illite/smectite and chlorite are the main carriers of Si, Al, Na and K in all coal seams.
The lignite from the Kovin deposit is enriched in As, Cd, Co, Cr, Cu, Ga, Li, Mn, Mo, Ni, Pb, V, Zn, Gd, Tb, Er and Lu in com parison with the Clarke values for brown coals.Almost all of these elements demonstrate a strong inorganic affinity, but some of them (As, Ca, S, Sr) also have an outstanding organic affinity.A group of elements in seam II are both inorganically and organ ically bound.
The results of the correlation analysis indicate a pyritic form of As and Cd in seam I, and an organic form in seam III.Cr, Pb and Se occur in aluminosilicate minerals in all three coal seams.Ni is associated with sulphides in coal seam I, aluminosilicates in coal seam III, whereas no specific affinity is observed for this element in coal seam II.Despite the high concentrations, Zn does not show specific affinity in all coal seams.

Figure 2 .
Figure 2. Macropetrographic profiles of boreholes GD-601 (a) and GD-603 (b) in the A field lignite and the vertical distribution of minerals and major elements in coal.

Figure 3 .
Figure 3. Macropetrographic profiles of boreholes KB-79 (a) and KB-91 (b) in the B field lignite and vertical distributions of minerals and major elements in coal.

Figure 5 .
Figure5.Macropetrographic profiles of boreholes GD-601 (a) and GD-603 (b) in the A field lignite and the vertical distributions of ash, total sulphur content and some trace elements in coal.A db -ash content, dry basis, %; S db -total sulphur content, dry basis, wt.%.

Figure S2 .
Figure S2.X-ray diffraction patterns of lignite ash (450 o C) of individual samples of boreholes KB-79 (a) and KB-91 (b) in the B field.

Table 2 .
Results of proximate and ultimate analyses of the Kovin lignite.

Table 3 .
XRD analysis of the Kovine lignite ash.Values of parameters for individual samples are given in TableS-III of the Supplementary material to this paper.

Table 4 .
Mineral composition of the Kovin lignite, based on data from SEM-EDS.

Table 5 .
Contents of the major elements in the Kovin lignite.

Table 6 .
Trace element content of the Kovin lignite.
Table SV of the Supplementary material to this paper).The coal in seam II is es Er, Tm, Yb, and Lu) groups according to SEREDIN & DAI (2012) is more convenient for the description of REY distribution in coals and conventional REY ores.The content of LREY, MREY and HREY in the Kovin lignite is generally low in all seams (Table 7; Table SVI of the Supplementary material to this paper) with a predominance of LREY in all three seams.The ave rage LREY contents in lignite are 31.15mg/kg, 25.71 mg/kg and 34.02 mg/kg for seams I, II and III respectively.The Gd, Tb, Er and Lu contents are higher than the Clarke values for brown coals (KETRIS & YUDOVICH, 2009), based on calculation of ave rage individual lanthanides and Y.

Table 7 .
Rare earth element content in the Kovin lignite.
a KETRIS & YUDOVICH, 2009; b SEREDIN & DAI, 2012; LREY = La+Ce + Pr + Nd + Sm; MREY = Eu + Gd + Tb + Dy + Y; HREY = Ho + Er + Tm + Yb + Lu; Min -minimum; Max -maximum; X̅ -arithmetic mean value; s -standard deviation; X̅ g -geometric mean value.Note: Values of parameters for individual samples are given in TableS-III for major elements, S-IV for trace elements and S-V for Rare earth elements and yttrium of the Supplementary material to this paper.
matrix and xylite-rich coal and Mixture of xylite-rich and matrix coal; MMiC -Mixture of matrix and mineral-rich coal; MiC -Mineral-rich coal

Table S -
II. Results of proximate and ultimate analyses of individual samples(after MITROVIĆ et al., 2017 adopted).

Table S -
II. Continued.

Table S -
III. Quantitative mineralogical composition of the Kovine lignite ashes (450°C) by XRPD analysis (wt.%; on organic matter-free basis) of individual samples.

Table S -
IV. Content of major elements (%) of individual samples.

Table S -
IV. Continued.

Table S -
V. Content of trace elements (mg/kg) of individual samples.

Table S -
VI. Content of Rare earth elements and yttrium (REY, mg/kg) of individual samples.