Trace Element Geochemistry and Stable Isotopic (δ¹³C and δ¹⁵N) Characterisation of Nevşehir Coals, Türkiye

Abstract

The Nevşehir coals are located in the Central Anatolian Crystalline Complex (CACC), Türkiye, and no reports exist on trace element, nitrogen, and carbon isotope composition data of the Nevşehir coals. The present study aims to geochemically characterise the Nevşehir coals to determine their trace elemental enrichment patterns and possible sources. Nevşehir coals are found within Late Miocene Kızılöz Formation (Arafa Member) rocks. These coals are part of the huminite maceral group; the dominant maceral group is ulminites. The minerals in coals are inorganic, such as oxidised framboidal pyrite, iron oxide minerals, quartz, clay, and carbonate minerals. Coals have great potential regarding trace elements. Benefits might arise from mining and using some of the critical elements derived from coal. Compared with the world coal average, the coal samples in this study are enriched in As (149.25 μg/g), V (245 μg/g), Cr (159 μg/g), Ga (18 μg/g), Ni (216 μg/g), Th (17 μg/g), Zn (143 μg/g), and U (54 μg/g). The arsenic content in this study is associated with inorganic components such as oxidised framboidal pyrite. Vanadium in coal is mainly associated with aluminosilicates and organic matter. Chromium originates from the clay minerals within coals. Uranium in coal is mainly associated with organic matter. Nickel and zinc in coal are predominantly associated with sulphides. The δ¹⁵N contents of the samples are comparable to those of several references, including plants, terrestrial creatures, and organic nitrogen. The δ¹³C–δ¹⁵N isotopic range and average values for four coal samples ranged from −25.66‰ to −25.91‰ (−25.80‰) and 3.6‰ to 4.3‰ (3.9‰), respectively, demonstrating that C3 type modern terrestrial vegetation was common in the palaeomires of the studied coal seams.

1. Introduction

Trace elements are present in coals in differing amounts due to the diverse ways they have entered the coal during the different stages of coalification. While some elements have an affinity for organic matter, the majority of elements are linked to the mineral stuff found in coal. The critical elements Nb, Ta, Zr, Hf, Y, REE (rare earth element), and Li are highly enriched in some Chinese coals, according to some studies [1,2,3,4,5,6]. The contents of these coals are comparable to those found in conventional ore deposits. According to Seredin and Finkelman [7], some Middle Asian coals have U levels ranging from a few hundred to a few thousand parts per million. The coal-hosted Ge ore deposit from Lincang, Yunnan, southern China [7,8,9], is an example of a critical element ore deposit that has been and/or is being mined for metal recovery. It is a relatively new topic for REE, Y, Sc, Nb, Zr, PGE, Au, Ag, and Re, as well as base metals Al and Mg, but it is not new for Ge, U, V, and Se, which have been extracted from coal and utilised in industry for many years [10].

The composition of the source vegetation and the changes brought about by early diagenesis and subsequent coalification processes are reflected in the stable isotope composition of coal (e.g., δ¹⁵N and δ¹³C) [11,12,13,14,15]. The average δ¹³C_VPDB of C3 plants is −27‰, and the δ¹⁵N_air of the biosphere ranges from −10‰ to +10‰. Carbon and nitrogen isotopes in the organic matter are fractionated as palaeoflora undergo peatification and coalification processes following burial in the sediments [16,17,18]. For instance, examining the variation in δ ¹³C content in coal can be used to determine the carbon isotopic composition of ancient atmospheric CO₂ (δ¹³Ca) [19,20,21,22,23,24,25,26]. Nitrogen in coal is primarily derived from proteins, amino acids, chlorophyll, porphyrin, alkaloids, etc., during the peat formation process. The derived nitrogen may be fixed in ammonian illite (fixed-NH₄) or organic materials [18,27].

Suppose the nitrogen isotopic composition of coal is known. In that case, the contribution of coal combustion can be determined, as a nitrogen isotope (δ ¹⁵N) is recognised as an excellent tool to differentiate between nitrogen sources [28,29]. According to Xiao and Liu [27], when thermal maturation increases, the δ¹⁵N value in coals exhibits a decreasing tendency. According to Chen et al. [30], the δ¹⁵N value in Chinese coals increases as the coal rank decreases, with the rank range being described as the coal rank of anthracite. However, Li et al. [31] and Rigby and Batts [32] claimed that the initial depositional environment is primarily responsible for the nitrogen isotopic compositions in coals rather than metamorphism and geological age. According to studies by [16,17,21,33], the nitrogen isotopic composition of coal (δ¹⁵N_coal) can be used in reconstructing palaeoenvironments by providing information on the amount of bacterial activity during the peat-forming stage or early diagenesis in the sediments. These include the original peat-forming plants, environmental factors, thermofluid, bacterial activities, light, palaeowater salinity and stress, nutrients, seasonal variation, coal components, and coalification [18,23]. These factors can influence the carbon and nitrogen isotopic compositions of coals.

During peat accumulation and coalification, a number of processes influence the elemental enrichments in coal. Because trace element enrichments can result in health and environmental issues when coal is used, it is crucial to understand these processes. The coal of Nevşehir is a low-calorie (5000 Kcal/Kg) lignite [34]. No reports exist on nitrogen and carbon isotope composition data of the Nevşehir coals. Therefore, the present study aims to geochemically characterise the Nevşehir coals and determine their trace elemental enrichment patterns and possible sources. The organic matter sources and depositional environments of coals are also described using δ¹³C and δ¹⁵N isotopic data.

2. Materials and Methods

In the study area, 2 kg samples were taken from the coal-bearing levels in the 160 cm thick sedimentary unit in the Late Miocene Kızılöz Formation (Arafa Member) from top to bottom (AK2, AK3, AK4, AK5, AK6, AK7, AK8, and AK9), representing approximately 20 cm.

Two samples (AK4 and AK8) were prepared for petrographic analysis. Reflection, white light, and fluorescent microscopes were used to determine maceral and mineral compositions. Petrographic and mineralogical composition evaluations were determined with the Leitz MPV-SP microscope. The Mineral Research and Exploration General Directorate (MTA) laboratory (Ankara, Türkiye) carried out sample preparation and petrographic analyses.

X-ray fluorescence spectroscopy (XRF), Thermo Fisher Scientific (Waltham, MA, USA), was used to measure the concentration of major elements in samples (6 coal samples). Preparation for XRF analysis involved borate fusion in an automated furnace, where each sample weighing 0.7 g was blended and uniformly mixed with 7 g of pure lithium borate flux, composed of 67% Li₂B₄O₇ and 33% LiBO₂. Trace element compositions were determined using ICP-MS (Inductively Coupled Plasma–Mass Spectrometry), Agilent Technologies (Santa Clara, CA, USA), analysis of coal samples. Solutions prepared for these analyses were prepared using perchloric acid, nitric acid, and concentrated hydrochloric acid by following Nadkarni’s method [35] (8 coal samples). These analytical techniques were conducted at the Mineral Research and Exploration General Directorate (MTA) laboratory (Ankara, Türkiye).

Nitrogen and carbon isotopes were analysed in coals at the OEA Laboratory (Exeter, UK). Four samples were examined to identify the carbon and nitrogen isotopes in the coals in the study area. Samples were precisely weighed to provide 20 to 200 micrograms of nitrogen for δ¹⁵N analysis. Samples were precisely weighed to provide 40 to 800 micrograms of carbon for δ¹³C analysis. Precision (as the average standard deviation) for well-prepared samples was generally above 0.2‰. Sample δ¹³C and δ¹⁵N values were calibrated to VPDB and AIR, respectively, with USGS40 (accepted values: δ¹³C = −26.39‰, δ¹⁵N = −4.52‰) and USGS41 (accepted values: δ¹³C = 37.63‰, δ¹⁵N = 47.6‰). In addition to USGS40 and USGS41, internal (keratin) and international (IAEA-CH-6, IAEA-N-2) standard reference materials were analysed to monitor analytical precision and accuracy.

3. Geological Background

Türkiye’s geology is highly complicated, with multiple continental blocks separated by suture zones (Figure 1a). In the north are the Pontides, while in the northwest is the Sakarya Continent [36]. The Izmir–Ankara–Erzincan Suture Zone (IEAS Zone) delineates the southern edge of the Pontides. Most of South Anatolia comprises the Anatolides–Taurides (the Taurides, the Bornova Flysch Zone, the Tavşanlı Zone, and the Menderes Massif and Afyon Zone) [36,37]. The Central Anatolian Crystalline Complex (CACC) is located between the Pontides and the Anatolides–Taurides.

The study area for this research is located in the Nevşehir Province, within the Central Anatolian Crystalline Complex (CACC), Turkey. The study area is located in the Gümüşyazı District of Nevşehir Province. There is a lignite field with an average low calorific value of 5000 Kcal/kg in the Gümüşyazı region; this field was operated in the past and then closed.

Within the scope of this study, the units were divided into metamorphic rocks (Palaeozoic), Ortaköy granitoids (Late Cretaceous), the Ayhan Formation (Pre-Lutetian), the Altıpınar Formation (Pre-Lutetian), the Kızılöz Formation Arafa Member (Late Miocene), the Kızılöz Formation (Late Miocene), the Yüksekli Formation (Pliocene–Late Miocene), and Alluvion (Quaternary) (Figure 1b).

Palaeozoic metamorphic rocks encircle the Ayhan basin (Figure 2). Marbles, amphibolites, and amphibolite schists of medium- to high-grade amphibolite facies make up the metamorphic rocks of the Idiş mountain block [38]. The Hırka mountain block’s metamorphic rocks comprise high-grade marbles, quartzites, and metapelites from amphibolite-facies. Ortaköy granitoid (Upper Cretaceous) gabbro, diorite, diorite porphyry, tonalite, granite, granodiorite, monzonite, and syenites are constituents of these rocks [39].

The Pre-Lutetian Ayhan Formation, first named by Atabey et al. [40], consists of units formed in terrestrial and marine environments. The Ayhan Formation group includes Saytepe, Esefin, Kubaca, İlicek, and Lalelik, which present lateral and vertical transitions from bottom to top. The Pre-Lutetian Altıpınar Formation, which generally consists of flysch rock units, conformably overlies the Ayhan Formation [40]. The Late Miocene Kızılöz Formation, which consists of conglomerate, sandstone, siltstone, and mudstone, unconformably overlies the Altıpınar Formation (together with the Arafa member) [40].

The unit, which consists of conglomerate, sandstone, and mudstone, was first named the Kızılöz Formation by Atabey et al. [40], based on the Kızılöz stream (Figure 2). The section composed of fine-grained conglomerate, sandstone, marl, and lignite was distinguished as the Arafa member based on Arafa (Gümüşyazı) Village, where coal mines are located [40]. It consists of red wine-coloured, trough cross-bedded, channel-filling deposit, conglomerate, sandstone, and mudstone. The pebbles are bonded with grain-supported binder. These conglomerates are characteristic due to their red wine colour. The layer thicknesses vary between 30 cm and 1.5 m. The average thickness of the formation, consisting of alternating conglomerate, sandstone, and mudstone, was 800 m [40]. The unit was formed in a terrestrial (alluvial fan) lake–swamp environment [41].

The Arafa member consists of fine-grained conglomerate, medium–thick-layered, yellowish-coloured, medium-fine-grained quartz sandstone, siltstone, marl, and lignite. Lignites are dark grey-black, and their layer thicknesses vary from lamina size to 120 cm (Figure 2). While it is composed of pure lignites in some parts, it is intercalated with siltstone, sandstone, and marl in other places. The unit overlies the Pre-Lutetian Ayhan group units with an angular unconformity. It is approximately 350 m thick [40]. The unit was formed in a floodplain and shallow lake–swamp environment [41]. The unit named the Yüksekli Formation (Pliocene-Late Miocene) overlies the Kızılöz Formation. This formation consists of white-grey-coloured, medium–fine sand grain-sized, trough cross-bedded sandstone, pebbly sandy tuffites, siltstone, claystone, coarse sandstone, and conglomerates [40]. Recently tested alluvium from the study area and other riverbeds consists of gravel, sand, silt, and clay.

Figure 1. (a) Simplified tectonic map showing the tectonic units of Turkey (modified from [42]). (b) Geological map of the study area (modified from [40]).

Figure 1. (a) Simplified tectonic map showing the tectonic units of Turkey (modified from [42]). (b) Geological map of the study area (modified from [40]).

Figure 2. Stratigraphic column of the study area (not to scale) (modified from [41]) and macroscopic rock photograph of the study area.

Figure 2. Stratigraphic column of the study area (not to scale) (modified from [41]) and macroscopic rock photograph of the study area.

4. Results

4.1. Petrography

From the study samples, some proximate and ultimate analysis results were obtained. The mean percentages of carbon and nitrogen in the Nevşehir coals are 40.54% and 0.49%, with an average moisture content of 2.66%, an average ash yield content of 18.36%, an average total sulphur content in dry conditions of 0.05%, and an average volatile matter content of 38.44%, respectively [34]. The percentage distribution of maceral and microlithotype components in the studied coal samples was determined quantitatively by petrographic studies. Microscopically identifiable macerals in the coals were measured in a total of two samples. The organic petrographic studies indicated that the AK4 and AK8 samples comprised 17%–27% huminites, 10%–16% ulminites, 1%–2% textinite, 4%–6% atrinite, and 2%–3% densinite, respectively. The inertinite and liptinite maceral groups were below 1% in volume, so they are not included in the modal distribution. Mineral matter represents inorganic components such as oxidised framboidal pyrite, iron oxide minerals, quartz, clay, and carbonate minerals. The maceral analysis of coal samples is shown in

Table 1. The maceral association values of the investigated samples.

Maceral Group (%)				Samples
				AK4	AK8
Maceral (%)	Huminite	Humotelinite	Texinite	1	2
			Ulminite	10	16
		Humodetrinitis	Atrinite	4	6
			Densinite	2	3

.

4.2. Geochemistry

The concentrations of major and trace elements in the Nevşehir coal samples are listed in

Table 2. Major and trace element contents of samples taken systemically from the coal seams in the study area (major element oxides are in %; unit for trace elements is µg/g; nd: not determined).

	AK2	AK3	AK4	AK5	AK7	AK6	AK8	AK9
SiO2	46	48	55.4	54.6	nd	46.6	42.2	nd
TiO2	0.7	0.8	0.9	0.9	nd	0.8	0.7	nd
Al2O3	19.6	21.4	21	22.8	nd	23.4	27.8	nd
Fe2O3	9.9	16.2	13.4	11.2	nd	14	13.4	nd
MnO	<0.1	0.1	<0.1	<0.1	nd	<0.1	<0.1	nd
MgO	1.9	2.3	2.7	2.6	nd	2.4	1.8	nd
CaO	9.1	5.5	2.5	3.4	nd	6.6	7.6	nd
Na2O	0.3	0.4	0.7	0.2	nd	0.3	0.2	nd
K2O	1	0.9	1.3	1.6	nd	1	0.4	nd
P2O3	0.1	0.2	0.2	0.1	nd	0.1	0.1	nd
SO3	11	3.8	1.6	2.3	nd	4.6	5	nd
LOI	0.4	0.4	0.3	0.3		0.2	0.8
Al2O3/TiO2	28	26.75	23.33	25.33		29.25	39.71
Li	30	34	21	<10	15	28	53	14
B	14	15	<10	12	14	12	18	<10
V	147	205	164	106	176	127	955	78
Cr	150	159	178	116	131	102	302	133
Co	31	40	40	17	21	12	25	8
Ni	237	348	330	167	217	107	253	65
Cu	47	60	39	23	68	46	302	50
Zn	133	164	182	118	153	93	167	135
Ga	19	21	18	15	14	18	23	14
As	71	135	96	85	76	39	589	103
Se	<5	<5	<5	<5	<5	<5	<5	<5
Rb	54	50	64	87	67	39	38	72
Sr	630	547	282	401	654	420	921	1106
Y	21	40	30	27	43	17	122	7
Mo	8	15	11	8	8	<5	15	<5
Cd	<5	<5	<5	<5	<5	<5	<5	<5
Sb	<10	<10	<10	<10	<10	<10	<10	<10
Ba	95	108	99	95	94	100	86	91
Pb	21	24	20	20	22	10	53	37
Th	15	19	15	17	24	12	19	13
U	18	117	37	23	42	27	155	14

. The Al₂O₃, CaO, Fe₂O₃, SiO₂, and SO₃ values are 19.6%–27.8% (average: 23%), 2.5%–9.1% (average: 6%), 9.9%–16.2% (average: 136%), 46%–55.4% (average: 49%), and 1.6%–11% (average: 5%), respectively.

Table 3. Concentrations of elements in world coals, Turkish coals, and Chinese coals, as well as their comparisons with averages for the Nevşehir coals (unit for trace elements is µg/g; ¹: [46]; ²: [43]; ³: [45]; ⁴: [44]).

Element	World Coals 1	Türkiye Coals 2	This Study	World 3	Chinese 4
As	0.5–80	1.8–620	39–589	8.3	3.79
B	5–400	22–1200	<10–18	-	53
Ba	20–1000	15–590	86–108	150	159
Cd	0.1–3	-	<5	0.22	0.25
Co	0.5–30	0.87–51	8–40	5.1	7.08
Cr	0.5–60	-	10–302		15.4
Cu	0.5–50	0.5–50	23–302	16	17.5
Mo	0.1–10	0.43–69	<5–15	2.2	3.08
Ni	0.5–50	3.1–1600	65–348	13	13.7
Pb	2–80	0.95–58	10–53	7.8	15.1
Th	0.5–10	-	12–24	3.3	5.84
U	0.5–10	0.32–140	14–117	2.4	2.43
V	2–100	5.5–270	78–955	25	35.1
Zn	5–300	5.8–260	93–182	23	41.4
Ga	-	-	14–23	5.8	6.55
Li	-	-	<10–53	-	31.8
Sr	-	-	282–1106	110	140

indicates the range and average values of trace element concentrations in the Nevşehir coals, alongside Türkiye coals [43], Chinese coals [44], and world coal values [45]. Compared with the average values for world hard coals, as reported by Ketris and Yudovich [45], the coal samples in this study are enriched in As, V, Cr, Ga, Ni, Th, Zn, and U.

4.3. Nitrogen and Carbon Isotope Values

The δ¹⁵N and δ¹³C composition values determined in the coals are given in

Table 4. The δ¹⁵N, δ¹³C, %N, and %C values determined in the Nevşehir coals.

	C (%)	δ13C (‰)	N (%)	δ15N (‰)
AK4	9.98	−25.66	0.31	3.9
AK5	19.07	−25.78	0.5	3.6
AK6	24.02	−25.91	0.67	4.3
AK8	17.95	−25.86	0.48	3.8

. The average δ¹⁵N and δ¹³C isotopic composition values for the samples are 3.9‰ and −25.80‰, respectively. The δ¹⁵N and δ¹³C values (n = 4) for each sample are plotted. The δ¹³C value varies from −25.66‰ to −25.9‰, and the δ¹⁵N value varies from 3.6‰ to 4.3‰. The N (%) values in the study area range from 0.31 to 0.67%. The C (%) values in the study area were determined to range from 9.98 to 24.02%. The carbon (%) contents are quite low due to contamination from siltstone, sandstone, and marl within the coal-bearing seam samples. However, previous studies have indicated that the average carbon value of the Nevşehir coals is 40.54%, according to the MTA [34].

5. Discussion

5.1. Petrography of Coal

The coal samples in the study area have huminite maceral groups, and the predominant maceral group is ulminites. This situation indicates that it is the bulk plant material forming the lignite in the peat environment and that gelation decreased due to the increasing water level in the environment and high bacterial activity [47].

5.2. Major and Trace Element Geochemistry in Coal

The examined coal samples have high SiO₂, Al₂O₃, and Fe₂O₃ contents. This situation is thought to be caused by the hematite-rich mudstone, sandstone, and clayey levels in the Kızılöz Formation [41]. The lower MnO, Na₂O, P₂O₅, and TiO₂ contents in all the samples indicate that the sediment deposition occurred under shallow-water environmental conditions [48]. The average Fe₂O₃ concentrations in U.S. and Chinese coals are 1.86 wt% and 4.85 wt% [44,49], respectively. The average iron content of high-rank coals in Australia is 0.84 wt%. Coal can include iron in a variety of forms, such as pyrite [50]. The Al₂O₃/TiO₂ ratio in the samples was calculated to range from 23.33 to 39.71. The Al₂O₃/TiO₂ ratios of coal samples indicate the source of sediments according to many sources [51]. Generally, the Al₂O₃/TiO₂ ratio is >21, showing that felsic magmatic rocks are the source of the sediments. If this ratio is between 8 and 21, the source of the sediment is intermediate-composition magmatic rock; if it is between 3 and 8, it can be said that the sediment is mafic magmatic rock [51]. The Al₂O₃/TiO₂ data in this study show that the sources of coals were felsic magmatic rocks. Considering the Al₂O₃/TiO₂ ratios, it can be said that the sediment originates from the Ortaköy granitoid (Upper Cretaceous) in this region.

The studied coals were compared to average values for world hard coals [45], based on the concentration coefficients (CCs) created by Dai et al. [52]. It was found that the coal samples in this study have higher values for most trace element contents as well as As, Co, Cr, Cu, Ga, Li, Mo, Ni, Pb, Rb, Sb, Se, Sr, Th, U, V, Y, and Zn compared to world hard coals. Table 2 shows that the As, Cr, Cu, Ga, Li, Ni, Pb, V, Y, and Zn contents of sample code AK8 were higher than those of the other samples. The average element concentrations were all above 1, as shown in Figure 3, except B and Ba. Figure 3 shows the highest As (150 μg/g), V (245 μg/g), Cr (159 μg/g), Ga (18 μg/g), Ni (216 μg/g), Th (17 μg/g), Zn (143 μg/g), and U (54 μg/g) values.

The average values for world hard coals were obtained from Ketris and Yudovich [45], and CC values were calculated. The calculated CC values are given in Figure 4. From this figure, it can be seen that U and As are significantly enriched (10 < CC < 100). Ni is enriched (5 < CC < 10); V, Cr, Cu, Zn, Sr, Pb, and Th are slightly enriched (2 < CC < 5). Li, Co, Ga, Rb, Y, and Mo all have 0.5 < CC < 2. However, B and Ba are depleted (CC < 0.5).

The mode of occurrence of the elements in coal can provide insights into the source of the element and some of the changes that occurred during coalification [53]. Moreover, precursor peat accumulation conditions may control the distribution and enrichment of components in coal seams [54].

Arsenic ranges from 0.5 to 80 μg/g in most world coals [46]. Three of the studied samples did not exceed 80 μg/g (39.71, 76 μg/g), while others had arsenic concentrations of greater than 80 μg/g (e.g., 85, 96, 103, 135, 589 μg/g). In the coals, the presence of As, Cd, Cu, Pb, and Zn can likely be attributed to sulphides [55], siderophile elements can be associated with Cr and Co, and other lithophile elements can be associated with Th, U, and V [56]. According to Palmer et al. [43], Turkish coals have high As concentrations, with a maximum recorded value of up to 3854 ppm [57], and As is generally related to pyrite and other sulphide minerals [57,58,59,60]. Arsenic in coal might be associated with organic components [59]. According to Dai et al. [50], As in coal occurs primarily in pyrite and is associated with organic matter. The As content in this study is similar to that originating from inorganic components such as oxidised framboidal pyrite. Many reports of arsenic in coal are associated with pyrite [61].

According to Finkelman [62], chromium is related to clays; however, some chromium is associated with minerals from the spinel group, such as chromite. According to Dai et al. [50], Cr in coal is predominantly associated with aluminosilicates and organic matter. Chromium minerals should occur as spinel group minerals such as chromite. In this study, Cr originates from the clay minerals within the coals. Nickel in coal is predominantly associated with sulphides (e.g., millerite (NiS), heazlewoodite (Ni₃S₂)), organic matter, and clays, according to Dai et al. [50]. Sulphide minerals, such as chalcopyrite (CuFeS₂), sphalerite (ZnS), and galena (PbS), are typically sources of trace elements like Cu, Zn, and Pb. Cu may be related to clay minerals or organic materials, according to Finkelman et al. [63]. The concentration of thorium in the coal samples ranges from 12 to 24 μg/g. The Th content can be correlated with Al, K, and Ti, and this positive relationship shows thorium concentration’s association with clay minerals. High U contents have been recorded in a number of coals [7,64]. According to Arbuzov et al. [64], coal can contain significant amounts of naturally occurring radioactive elements such as U, Th, and their decay products. Significant uranium deposits can also become concentrated in the coalfield area. A high U content is expected due to uranium mineralisation in the Nevşehir area [65]. Uranium in coal is mainly associated with organic matter. U-rich coals are usually highly and positively correlated with Re, Se, Mo, and V concentrations [7,52,66]. According to Ketris and Yudovich [45], the average V concentrations in hard and common low-rank coals worldwide are 28 and 22 μg/g, respectively. The V concentration in the Nevşehir coals is 78–955 μg/g. Vanadium in coal is predominantly associated with aluminosilicates and organic matter [50].

In coal, Sr and Ba are related to organic materials, phosphates, and sulphates [50]. Nicholls [67] concluded that Sr is generally associated with the inorganic fraction of the coal and probably carbonates. In the samples, the Sr concentration was 282–1106 μg/g. Swaine [46] indicates that strontium is generally found in coals with sulphate, carbonate, and phosphate minerals. The carbonate minerals in coals detected in petrographic studies explain the high Sr content. In addition, the correlation coefficient between Ca and Sr was determined to be a highly positive correlation coefficient (r² = 0.8).

5.3. Nitrogen and Carbon Isotope Characteristics of Nevşehir Coals

A number of factors, including plant species, topographical location, latitude/altitude, soil moisture content, and CO₂ control, affect the δ¹³C isotopic composition of organic matter generated from C3 plants [68,69]. Kohn [70] states that plants growing in arid climates should have a C3 isotope value ranging from –26 to –27. δ¹⁵N_ino can be used to determine the degree of high-T heat in coal and to trace the path of hydrothermal fluids circulating the coal basin [71]. One of the most important elements in coal is nitrogen. Nitrogen is found in coal mainly in organic forms and, in a few cases, inorganic forms. Proteins from plants and microorganisms provide the main source of organic nitrogen found in coal, while the sources of inorganic nitrogen remain controversial [72]. It is observed that the δ¹⁵N values of the samples are similar to the δ¹⁵N contents of some reference materials, such as terrestrial organisms, plants, and organic nitrogen. In the δ¹³C–δ¹⁵N diagram, the coal samples in the study area were determined to be land C3 (trees, shrubs, and forbs) ecosystems (Figure 5a–c) [73]. The Nevşehir coals have constant δ¹⁵N values (3.6‰ to 4.3‰) (Figure 5b). The δ¹⁵N isotopic composition values may reflect the lack of variation in the precursor organic matter over time, even though natural processes like nitrification, denitrification, and biodegradation cause a wide variation in the nitrogen isotope distribution in soils [74]. Similarly, the δ¹⁵N values were found to be within the range obtained for modern terrestrial plants. Likewise, compared to vitrinite-rich coals, inertinite-rich coals exhibit more negative δ¹³C isotopic composition values [69]. δ¹³C isotopic composition values can be used to identify variations in the air temperature, humidity, soil moisture, and precipitation rate during peat accumulation [27]. Usually, the former plants have more nitrogen content than the latter [27]. Guo [75] reported that the protein content of gymnosperms is less than 7%, but pteridophytes had a protein level of 10%–15%, and these results indicate that marine plants have greater ¹⁵N/¹⁴N ratios than terrigenous plants [76,77]. According to Nissenbaum and Kaplan [78], humic acid from the sea often has a higher nitrogen content than humic acid from terrigenous sources. Chinese coals in marine depositional environments may have higher nitrogen and δ¹⁵N values than those formed in non-marine depositional environments [27]. The ¹⁵N/¹⁴N ratio (+12.7) of tasmanite is higher in a marine depositional environment than torbanites in a freshwater environment. This is due to the fact that the current range of values for marine plankton is +3 to +13 [79].

The boron content of coal can represent the salinity of paleo-water [18,80]. According to Landergreen and Manheim [81], the boron concentrations of marine and non-marine sediments are thought to differ greatly and are consistently responsive to different salinity levels. The depositional environments in which the coals formed were classified accordingly [82]. Compared to terrigenous plants, maritime plants are known to have greater ¹⁵N/¹⁴N ratios [76,77]. According to Nissenbaum and Kaplan [78], marine humic acid typically has a higher nitrogen content than terrigenous humic acid. The boron concentration of Qinshui coal (Chinese) ranged from 4.3 μg/g to 113 μg/g, with an average of 31.1 μg/g. This suggests that freshwater and, to a lesser extent, brackish water were the primary influences on Qinshui coals during the peat accumulation process [18]. In the present study, the boron content varied from 12 μg/g to 18 μg/g, with an average of 12.16 μg/g, indicating that the coals were mainly influenced by freshwater and, to a lesser extent, brackish water during peat accumulation.

Coals from Southeast Asia, Australia, Russia, and this study have distinct isotopic compositions (Figure 6). The results show that Southeast Asia coals have the smallest nitrogen and carbon isotope values. This study reports more N isotope signatures, which can be entered into the database for N isotope research relating to coal [83,84]. Devonian Chinese coals contain relatively higher nitrogen contents than Mesozoic Chinese coals. Gymnosperms and, to a lesser extent, pteridosperms were the major coal-forming plants influencing younger Chinese coals, while pteridophytes were the predominant influence for older (Palaeozoic) coals. According to some research on the δ¹⁵N values of coal, the nitrogene isotope compositions of Nevşehir (Türkiye) coals have not previously been obtained. Therefore, δ¹³C and δ¹⁵N isotopic composition values were considered to occur due to the land C3 (trees, shrubs, and forbs) environment of the Nevşehir coals.

6. Conclusions

The studied coals from the Nevşehir province were located within rocks in the Late Miocene Kızılöz Formation (known as the Arafa Member). The samples in the study area had huminite maceral groups, and the dominant maceral group was ulminites. Mineral matter is represented by inorganic components, such as oxidised framboidal pyrite, other iron oxide minerals, quartz, clay, and carbonate minerals. The Nevşehir coals contain high values for the majority of trace elements, such as As, Cd, Co, Cr, Cu, Ga, Li, Mo, Ni, Pb, Rb, Sb, Se, Sr, Th, U, V, Y, and Zn. These elements were compared to world hard coal values for lignite, sub-bituminous coals, and their ashes worldwide. It was found that the coal samples in this study were enriched in As (149 μg/g on average), V (245 μg/g), Cr (159 μg/g), Ga (18 μg/g), Ni (216 μg/g), Th (17 μg/g), Zn (143 μg/g), and U (54 μg/g). The δ¹³C–δ¹⁵N diagram shows that the coal samples in the study area were determined to be land C3 (trees, shrubs, and forbs) ecosystems. The δ¹³C–δ¹⁵N isotopic values indicate that the plants studied are growing in arid climates.