Factors Controlling the Occurrence and Distribution of Iron in Bulgarian Coals

СПИСАНИЕ НА БЪЛГАРСКОТО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, год. 81, кн. 2, 2020, с. 27–40 REVIEW OF THE BULGARIAN GEOLOGICAL SOCIETY, vol. 81, part 2, 2020, p. 27–40 https://doi.org/10.52215/rev.bgs.2020.81.2.3

Factors controlling the occurrence and distribution of iron in Bulgarian coals

Jordan Kortenski, Alexander Zdravkov

University of Mining and Geology “St. Ivan Rilski”, 1, Prof. Boyan Kamenov str., 1700 Sofia, Bulgaria; E-mails: [email protected]; [email protected]

Фактори, контролиращи присъствието и разпределението на желязо в български въглища

Йордан Кортенски, Александър Здравков

Минно-геоложки университет „Св. Иван Рилски“, ул. „Проф. Боян Каменов“ №1, София 1700

Abstract. Coal of varying age (Upper Carboniferous to Pliocene) and coalification rank (lignite to anthracite) from nineteen coal- bearing basins and deposits, belonging to 8 coal-bearing provinces from Bulgaria, were examined for the occurrence and distribution of Fe. For the majority of the coals, the average Fe concentrations vary from 1.1 to 10.2 wt% and are higher than the World’s average. An exception exists only for Karlovo and Dobrudzha Basins, where slight depletion of Fe (<1 wt%) was established. On contrary, carbonaceous shales from the majority of the basins show Fe depletion in comparison to the Clarke values for clays. Based on the negative correlations with the ash yields, organic affinity of the element can be suggested for most of the studied coals. An exception exists for the Burgas Basin, for which predominantly inorganic affinity of the element can be suggested, whereas for Stanyantsi, Samokov, Bobov Dol, Dobrudzha and Svoge Basins mixed organic/inorganic affinity of Fe can be presumed. The element most probably predominantly occurs in the form of organometallic compounds, especially in the coals that formed in preferentially acidic peat-forming environment. Nevertheless, small contribution from biogenic iron cannot be excluded. The mineral form of Fe is probably mainly associated with sulfides, carbonates, and sulfates, although the presence of Fe oxides, silicates and hydroxides is also established. The analysis of Fe occurrence and its organic/ inorganic affinity in Bulgarian coals reveal the following controlling factors: i) Fe concentration within the basin’s provenance; ii) the type (i.e. terrigenous or groundwater) of element supply; and iii) environmental conditions during peat-formation.

Keywords: Bulgarian coals, Fe concentration, organic affinity.

Резюме. Въглища от 19 български басейни, с възраст от Късен Карбон до Плиоцен и степен на въглефикация от лигнити до антрацити, принадлежащи на 8 въгленосни провинции, са изследвани за определяне на съдържанието и разпределението на Fe. За по-голямата част от въглищата средното съдържание на желязо е по-високо от средното за света и попада в интервала от 1,1 до 10,2 тегл. %. Изключение правят карловските лигнити и добруджанските черни въглища, в които са установени намалени кон- центрации на елемента (<1 тегл. %). Съдържанието на желязо във въглищните глини за по-голямата част от въглищните басейни е по-ниско от кларковата стойност за глини. Въз основа на установените отрицателни корелационни коефициенти с пепелното съдържание, за повечето басейни може да се предположи преобладаващ органичен афинитет на Fe. Единствено в Бургаския басейн преобладава неорганичният афинитет на елемента. В Станянския, Самоковския, Бобовдолския, Добруджанския и Свогенския басейн желязото показва смесен афинитет. Присъствието на елемента вероятно е свързано основно с включването му под формата на металоорганични съединения, особено във въглищата, образувани в кисела торфообразуваща среда. Въпреки това, наличието на малко количество биогенно желязо не може да бъде напълно изключено. Минералната форма на присъствие на желязото най-вероятно e свързанa основно с наличието на сулфиди, карбонати и сулфати, но присъствието на железни окси- ди, хидрооксиди и силикати също е установено. Анализът на съдържанието и афинитетът на желязото в българските въглища показва, че основно контролиращо значение върху геохимичните му характеристики имат следните фактори: i) концентрация та на елемента в скалите от подхранващата провинция; ii) типът (повърхностно или подземно) на подхранване с елемента; и iii) условията на торфообразуващата среда.

Ключови думи: български въглища, съдържание на желязо, привързаност към органично вещество.

Introduction 2001; Kortenski, Zdravkov, 2003, 2008, 2016). Sulfur is by far one of the most studied elements Numerous publications are devoted to the geo- (e.g. Kostova, 1999, 2002, 2005; Markova et al., chemistry of the major elements in Bulgarian 2007, 2008, etc.), because of its significant envi- coals (Kortenski, 1993a, 1994, 1996; Kortenski, ronmental impact. The geochemical behavior of Kostova, 1996; Kortenski et al., 1997, 1999, 2001; Ti, P and Mn is reported by Eskenazy (1972, 1989, Kortenski, Popov, 1998; Kortenski, Sotirov, 1998, 1993). The properties of the alkaline and alkaline

27 earth elements were thoroughly studied by Eske- Results and discussion nazy and Ivchinova (1987) and more recently by Kortenski and Zdravkov (2019). In most articles, Iron contents in coals and carbonaceous shales from however, the presence of Fe and its geochemical the studied coal-bearing basins are summarized in properties are only briefly referred. For that rea- Table 2. The results indicate that the Karlovo lignite son, the present work summarizes data from 19 and the Dobrudzha bituminous coal are the only ones Bulgarian coal-bearing basins and deposits in or- characterized by Fe concentrations lower than the der to study the geochemical behavior of Fe and World’s average (1 wt%; Valković, 1983). Slight- assess the influence of the main factors (e.g. con- ly higher contents were detected in the coal from tribution from peat-forming plants, concentration Oranovo, Burgas and Suhostrel Basins (1.1–1.2 wt%; in the rocks from the basin’s provenance, type and Table 2). The remaining coal is characterized by Fe direction of element supply, properties of the peat- concentrations ranging from 1.7 to over 10 times forming environment, presence and composition higher than the World average (1.7–10.2 wt%). The of epigenetic mineralization, etc.) controlling its highest Fe contents in coal and coal ash were detect- presence and distribution in coal. ed in Katrishte deposit (10.2 and 29.6 wt% respec- tively). The ashes from Maritsa-West, Stanyantsi, Sofia, Chukurovo, Kyustendil and Gabrovitsa lig- Geological settings nite, as well as from Pernik coal, are also enriched with iron (10.9–18.2 wt%). Bulgaria hosts over 50 coal deposits, covering For almost 2/3 of the studied basins, Fe concen- the time span from Upper Carboniferous to Plio- tration in carbonaceous shale’s ash is exceeding the cene and separated into 12 coal-bearing prov- Clarke value (4.72 ppm) for clays (Table 2). The inces based on their geological and tectonic set- highest enrichment (over 2 times higher than the tings, and geographic extent (Šiškov et al., 1986; Clarke) was detected in Stanyantsi, Kyustendil and Šiškov, 1997). Most of the coal deposits, how- Burgas Basins. For the rest of the basins (i.e Marisa- ever, contain non-economic resources either be- West, Belibreg, Karlovo, Gotse Delchev, Balkan, cause of low quality of the coal (high ash yield, Dobrudzha and Oranovo) the results indicate slight high sulfur contents, etc.) or due to complicated Fe depletion in the carbonaceous shale’s ash. The mining conditions (Šiškov, 1997). In the pres- most prominent the depletion is in Oranovo Basin ent study, coals with coalification rank ranging (~4 times lower than the Clarke), for which differ- from lignite to anthracite, belonging to 8 coal- ent supply mechanisms and directions during the bearing provinces, were studied. Basic geological individual depositional stages can be suggested. settings, including lithostratigraphy of the coal- The affinity and mode of occurrence of Fe is dis- bearing sediments and lithological properties of cussed in numerous publications (Kortenski, 1992, the basin’s basement and provenance, are sum- 1993a, 1994; Kortenski et al., 1997, 1999, 2001; marized in Table 1. Kortenski, Popov, 1998; Kortenski, Sotirov, 1998, 2001; Kortenski, Zdravkov, 2003, 2008, 2016). Based on the negative correlation between the Fe Material and methods contents and ash yields in most of the studied coal- bearing basins (Table 2), predominant organic affin- For the purpose of the present study 912 coal and ity of the element can be suggested. However, the es- carbonaceous shale samples (whole seam or sec- tablished relatively weak correlation coefficients for tional, core and channel) from 19 coal basins, were Sofia, Gabrovitsa, Kyustendil and Suhostrel coals studied. The high temperature ash yield (815±10 °C) (ro = –0.36 to –0.45; Table 2) indicate that signifi- was determined following standard procedure (ISO cant part of Fe might also be present in mineral form. 17246:2010). The ash was mixed with lithium tet- An exception exists for Burgas Basin, where the cal- raborate and melted in platinum crucible at 1600 °С. culated moderate positive correlation (ro = +0.56; Subsequently, the alumosilicate glass was dis- Table 2) argues for predominant inorganic affinity solved in nitric acid (ISO 15587-2:2002), and the of the element. For Stanyantsi, Samokov, Bobov concentration of Fe were determined according dol, Dobrudzha and Svoge basins the calculated ISO 17294-1:2004 and ISO 17294-2:2016 stan- correlation coefficients fall below the statistically dards using ICP-VISTA-MPX SIMULTANEOUS significant values (Table 2), thus indicating that Fe CCD optical emission spectrometer. contents in these coals are roughly equally distrib- The results were statistically evaluated and the uted between the organic and the mineral matter. correlation coefficients between Fe and ash yield Iron is one of the major chemical elements, con- were determined using Excel™ Data Analysis stituting on average about 5% of the Earth’s crust toolpack. (Perel’man, 1977; Kabata-Pendias, 2010). It par-

28 (1999) (1983) Vatsev Bojanov Zagorcev Reference Kamenov, et al. (1993) et al. (1995) Kojumdzhieva Basement Triassic limestone; Paleogene limestone and volcanic rocks. Jurassic limestone; Upper Cretaceous marlstone, limestone, and volcanic rocks (basaltic andesite, basalt, trachiandesite, andesite). Triassic and Jurassic siliciclastic rocks (conglomerate, sandstone, siltstone, shale), marlstone, and limestone. Permian to Cretaceous siliciclastic rocks (breccia- conglomerate, sandstone, siltstone, shale), marlstone, limestone; Upper Cretaceous andesite. rank ignite ignite ignite ignite l l l l Coalification m 20 30 Seam 20–25 1.8–3.6 thickness, 2 1 1 1 seams of coal Number , m 150 50–60 30–60 100–150 Thickness . – irregular – fluvial to lacustrine . – lacustrine banded . – lacustrine sand, silt and – lacustrine sandy to silty . . . – fluvial sandy clay to . – alluvial fan deposits – fluvial to lacustrine . – coarse-grained alluvial . – irregular alternation of . Strania Fm Zainitsa Fm Belozem Fm Dvechka Fm Lozenets Fm Novi Iskar Fm Gniljiane Fm Variegated terrigenous fm fan deposits – conglomerate, clayey sand; – sandy clay to clayey sand, rich in carbonate nodules; and calcareous clay with lignite seam at the base; clayey sand. clay, locally with lignite interbed; clay; silty sand and clay with lignite bed; alternation of fluvial sandy clay, sand and sandstone. Lithostratigraphy Maritsa Fm fluvial to lacustrine sandy silty clay and clayey sand. Lozenets Fm sandy to silty clay with clayey sand and carbonate interbeds. 4. 3. 2. 1. 4. 3. 2. 1. – Age Late Late Late Pliocene Pliocene Miocene Miocene Miocene Basin Thracian coal-bearing Province Maritsa-West Sofia coal-bearing Province Belibreg Stanyantsi Sofia Table 1 Summarized data of the geological characteristics coal basins Таблица 1 Обобщени данни за геоложката характеристика на въглищните басейни

29 (1990) (1993) (1996) Vatsev, Katzkov Jordanov Rousseva Dimitrova, et al. (1994) Katskov, Iliev Pre-Cambrian gneiss, schist and amphibolite; Ordovician diabase and phyllite; Upper Cretaceous conglomerate, sandstone, siltstone, shale, limestone, marlstone, diorite, and granodiorite. Archaean gneiss, schist, and amphibolite; Cambrian diabase and phyllite; Carboniferous quartz-diorite, granodiorite, diorite, granite; Permian granite and granodiorite; Upper Cretaceous limestone and marlstone. Proterozoic diabase, schist, gneiss; Ordovician phyllite and schist; Permian–Lower Triassic red- beds (breccia-conglomerate, sandstone, siltstone, shale); Middle Triassic dolomite and limestone; Upper Cretaceous limestone, marlstone, diorite, andesite. Proterozoic gneiss, schist, marble, and amphibolite. ignite ignite ignite ignite l l l l 4–3 10 30 . 5–2.0 . 1 1 18 1 3 2 to 2 60 40 60–90 100–170 . – . – coarse-grained . – coarse-grained fan – clayey sand to sandy . – irregular alternation . – irregular alternation of . . – alluvial fan coarse- . – fluvial conglomerate and . – lacustrine clay and sand Rejovo Fm Alino Fm Lower conglomerate-sandy fm Karavelovo Fm Iganovo Fm Drandaritsa Fm Dolnopolska Fm Gabrovitsa Fm Belidol Fm coarse-grained sand; with lignite bed and diatomite; alluvial fan coarse-grained poorly sorted deposits. deposits – conglomerate and cobble breccia-conglomerate; siliciclastic sediments; clay, diatomite, and lignite; grained breccia-conglomerate and sandstone. Palakaria Group: 3. 2. 1. 2. of fluvial gravel, sand, silt, and clay; 1. silty to sandy clay and sand with gravel and pebble lenses, lignite seams on top. formation of clayey sandstones and clays with Chukurovo coal seam. 4. 3. 2. 1. – Late Late Middle Pliocene Miocene Miocene Miocene Miocene Samokov Karlovo Chukurovo Gabrovitsa

(1994) (1991) (1980) (1994) Vatsev Vatsev Petrova et al. (1994) Vatsev, Bonev Vatsev, Bonev Paleozoic and Mesozoic gabbro- diorite, quartz-diorite, diorite; Silurian limestone and black shale; Triassic and Jurassic conglomerate, sandstone, siltstone, dolomite, and limestone. Pre-Cambrian amphibillite, biotite-amphibole gneiss, marble, and schist; Paleozoic biotitic granite, and granodiorite; Upper Cretaceous granite and quartz-monzonite. Pre-Cambrian gneiss, schist, marble; Upper Cretaceous granite and granodiorite. Paleozoic gabbro-diorite to quartz-diorite and diorite; Silurian limestone and shale; Triassic and Jurassic conglomerate, sandstone, siltstone, dolomite, and limestone. Upper Cretaceous tuffs, alkaline volcanic rocks, andesite, breccia, calcareous shale, marlstone, limestone, siltstone, and sandstone.

- - - - ub ub ub ub s s s s ignite l bituminous bituminous bituminous bituminous 0 . 5 1–8 1–5 2–7 4–12 1 1 2–7 1–16 1–16 250 100 100 300–500 150–200 . – coarse-grained fan- . –fluvial sandy to silty . – irregular alternation . – irregular alternation of . – sandstone and siltstone – fining upwards . – argillaceous sandstone . – coarse-grained . – coarse-grained fluvial . – conglomerate, sandstone . – fluvial to lacustrine . – alternation of fine- . . – fan-type coarse-grained – marlstone with thin Koilitsa Fm Skrinyano Fm Spasovitsa Fm Revalska Fm Gradevo Fm Duarska Fm Oranovo Fm Drachevitsa Fm Nevrokop Fm Baldevo Fm Valevica Fm Mugris Fm. Ravnets Fm conglomerate and sand; claystone, diatomite, sand; coal seam at the base; of conglomerate, sandstone and sandy claystone. and sandy claystone; fan sediments – conglomerate and sandstone; and sandy claystone; with thin gravel conglomerate interbeds, sandy claystone and coal; type cobble- and pebble-conglomerate, breccia-conglomerate. fine- to coarse-grained conglomerate, sandstone and clays; conglomerate, sandstone, siltstone, clays, diatomite, and lignite; conglomerate and sandstone. limestone and conglomerate interbeds; conglomerate, sands and sandstones, clays and coal. 3. 2. 1. 5. 4. 3. 2. 1. 3. 2. 1. Skrinyano Fm grained clayey sands, sandy, bituminous, and calcareous clays. 2. 1. Eocene– Pliocene Miocene Miocene Miocene Oligocene Struma-Mesta coal-bearing Province Kyustendil Oranovo Gotse Delchev Katrishte Sub-Balkan coal-bearing Province Burgas

31 (1993) Vatsev Zagorčev Marinova et al. (1994) (2014, 2015) Paleozoic, Triassic and Upper Cretaceous conglomerate, fine- grained sandstone and siltstone, marlstone, limestone, calcareous sandstone, dolomite, syenite, and monzonite. Pre-Cambrian biotite and mica gneiss, amphibolite, and leptynite; Paleozoic gabbro-diorite, granite, sandstone, shale, siltstone; Mesozoic limestone, dolomite, sandstone, gravelly- conglomerate, siltstone, shale, and marlstone. Paleozoic diorite, amphibolite, sandstone, conglomerate, siltstone, marlstone, and limestone.

- - ub ub s s ituminous bituminous bituminous b 8 . 1.0 1–3 2–3 . 1 5 3 7–8 60 20 100 – fluvial – predominantly . – fluvial to lacustrine . – alternation of . – conglomerate and . – lacustrine . – cyclic alternation of . – fine-bedded shale and . – irregular alternation of . – alternation of . – sandstone and . – alternation of fluvial . – cobble and pebble . – alternation of polylithic . – tuffs, sandstone, Formation of thin-bedded claystone and Coal-bearing fm Variegated fm Bituminous fm Conglomerate-sandstone fm. Bardovenets Fm Bobov Dol Fm Razmetanitsa Fm Balanovo Fm Kamenik Fm Blazhievo Fm Padesh Fm Logodash Fm Komatinska Fm Suhostrel Fm argillaceous marlstone argillaceous marlstones, interbedded by this sand layers in upper part; sandstones, sandy claystones, fine- bedded siltstone and claystone coal; conglomerate, sandstone, siltstone, and claystone; marlstone, locally impregnated with organic matter; polylithic conglomerate with sandy to gravelly matrix, and polymictic sandstone. bituminous claystone; sandstone, claystone and coal; sandstone with conglomerate interbeds, sandy and bituminous claystone; polylithic conglomerate and polymictic sandstone; conglomerate, sandstone, clayey sandstone, siltstone and shale; polylithic conglomerate and sandstone. claystone, limestone; argillaceous sandstone; breccia-conglomerate with claystone, limestone and coal interbeds; conglomerate, sandstone, claystone and coal. 5. 4. 3. 2. 1. 6. 5. 4. 3. 2. 1. 4. 3. 2. 1. Late Late Late Eocene Oligocene Oligocene Pernik coal-bearing Province Pernik Bobov Dol Suhostrel

32 (1995) (1988) (2007) Tenchov Kânčev et al. Nikolov et al. Pre-Cambrian gneiss, schist, and amphibolite; Upper Paleozoic monzo-diorite, amphibole-biotite granodiorite, granite, and tuffs; Lower Triassic red-beds – conglomerate, sandstone, and claystone; Middle Triassic limestone and dolostone; Jurassic quartz sandstone, conglomerate, and shale. Devonian to Lower Carboniferous sandstone, siltstone and shale. Middle Ordovician, Silurian and Devonian siltstone, black shale, quartzite, sandstone, shonkinite, and K-alkaline quartz-syenite.

semi- to semi- ituminous ituminous to super- anthracite anthracite anthracite b b 2 . 0 . 2 8–1.5 2–4 3–4 8–1 . 1–12 . 0 0 2 8 4 1 1 12 7–17 100 230 450 550 250 220 510 260 80–120 120–200 . – fining upward . – massive to fine- . – sandstone, volcanic . – fining upwards . – sandstone, shale, coal. . – conglomerate, . – massive argillaceous to . – sandstone, siltstone, . – quartz sandstone; . – sandstone, shale, coal; . – claystone; . – coarse-grained . – sandstone; . – sandstone; . – sandstone, shale, coal; . – sandstone; . – up to 6 coal-bearing cycles Rusalska Fm Marlstone fm Coal-bearing fm Base terrigenous fm Gurkovo Fm Polyana Fm Krupen Fm Velkovo Fm Makedonka Fm Vranino Fm Mogilishte Fm Chibaovtsi Fm Berovdol Fm Svoge Fm Dramsha Fm Svidnya Fm Tsarichina Fm silty marlstone; bedded dark grey to black claystone, locally with siderite concretions and coal seams; conglomerate, sandstone, siltstone, and claystone. tuff, shale, coal; sandstone, claystone, and up to 3 coal seams; claystone, and up to 3 coal seams; composed of alternating sandstone, siltstone, claystone and coal; alternation of conglomerate and breccia- conglomerate, followed upwards by coal-bearing alternation of sandstone, siltstone, shale, and up to 4 coal seams; conglomerate, sandstone, siltstone, shale, and coal. 4. 3. 2. 1. 7. 6. 5. 4. 3. 2. 1. 6. 5. 4. 3. 2. 1. - - Late Late Late ferous ferous Carboni Carboni Cretaceous въгленосните единици са отбелязани с почернен шрифт coal-bearing units are marked in bold script : Balkan coal-bearing Province Balkan Dobrudzha coal-bearing Province Dobrudzha Svoge coal-bearing Province Svoge Remark

33 Table 2 Average concentration of Fe in coal and coal and carbonaceous shale ash Таблица 2 Средно съдържание на Fe във въглищата и пепелта на въглища и въглищни скали

Average Fe concentration Number of samples Pearson (wt%) in: Minimal correlation carbona- statistically Basins/deposits coal Coal rank carbona- (ro) d ceous shale coal significant (A <50 d coal ceous Fe vs. ash (50

ticipates as major or minor component in numerous Fe in the exogenic environment, where its behavior mafic minerals, many of which form in endogenic strongly depends on the pH and Eh settings and the environments under high temperatures and pres- presence of organic compounds (Kabata-Pendias, sures. Upon exhumation, most of these minerals 2010). Based on a comprehensive literature over- become unstable and readily decompose to release view, the following main factors controlling the

34 presence, mode of occurrence and the distribution centration in coal, since the type (i.e. terrigenous of iron in coals were identified: 1) Fe concentration or groundwater) and direction of element sup- in peat-forming plants; 2) Fe concentration within ply also play crucial role. Thus, Fe concentration the basin’s provenance; 3) the type (terrigenous in Sofia (11.77 wt%) and Chukurovo (14.6 wt%) or groundwater) and direction of element supply; lignite is relatively high, but are much lower than 4) the settings of the peat-forming environment; the Fe contents in Katrishte Basin (29.6 wt%; Ta- 5) the presence and the extend of cracking of the ble 2) where no iron ore deposit is available within coal seams; and 6) the presence and the composition the basin’s provenance. Previous research on the of epigenetic mineralization within the cracks. geochemical characteristics of Sofia Basin (Korten- Iron is one of the elements that play crucial role ski, Popov, 1998) indicates the surface Fe supply, in plant development. It is essential for their growth mainly through terrigenous input, was very active as it takes part in multiple chemical reactions, forms from the south and southwest (i.e. the Upper Cre- proteins and enzymes, and promotes the formation taceous Vitosha pluton), whereas groundwater sup- of chlorophyll, thus ensuring photosynthetic reac- ply originated mainly from the Triassic and Jurassic tions (Kabata-Pendias, 2010). It is, therefore, not limestones NW of the basin. The distribution of the surprising that iron is a common biophillic element major elements in Sofia Basin argues for a limited that can be detected in concentrations ranging from or completely absent Fe supply from north and east 1 to over 3500 ppm (0.001–0.35%) depending on (Kortenski, Popov, 1998). Similarly, although some the plant type and its growth stage, climatic settings, influence on the geochemical characteristics of availably of Fe in soils, etc. (Kabata-Pendias, 2010). Chukurovo Basin from the Kremikovtsi ore deposit However, according to Bowen (1966) iron concen- cannot be completely excluded, the significant dis- tration in plant remains do not exceed on average tance between the deposits (>25 km) renders such ~140 ppm (0.014%) and thus the amount of biogen- hypothesis highly unlike. Instead, Fe supply from ic Fe is expected to have greater importance only the Lower Triassic red-beds surrounding Chukuro- for very low ash coals (i.e. <2 wt%). Since neither vo Basin (i.e. Lozenska Fm. – Katskov, Iliev, 1993) of the studied Bulgarian coals fall in this category might be presumed to have had greater importance. (ash yields typically >15 wt%; e.g. Šiškov, 1997), In addition, Kortenski et al. (2001) suggest ground- the biogenic concentration of iron is presumed water supply from the Triassic and Jurassic lime- herein to have minor role in element’s concentration stones of Pancharevo and Gintsi Fms, which have and distribution. The established significant differ- comparable rock compositions to those from Kre- ences in Fe contents between the basins from the mikovtsi deposit. Sofia coal-bearing Province (Table 2), for example, Similarly, the enhanced Fe contents in Pernik might serve as a prove for such presumption. Peat- coal could also be attributed to the presence of Low- formation in these basins occurred due to deposition er Triassic red-beds along basin’s southern margins. of plant remains from similar taxonomic communi- Nevertheless, element supply from the contact- ties (Palamarev, 1964; Ivanov, Slavomirova, 2004; metasomatic skarn-type Fe mineralization zones Ivanov, Ashraf, 2007; Hristova, Ivanov, 2009, associated with limestone and dolomite from Golo 2013; Bozukov et al., 2011) and hence comparable Bardo Mountain (Zagorčev et al., 1994), cannot be Fe concentrations could be expected should major completely excluded. For Katrishte Basin, however, biogenic control is presumed. This is, however, not the source of iron is not that obvious, considering evident from the presented data. the absence of Fe-rich rocks or mineralization zones The second factor, influencing the geochemical within its provenance (Zagorčev, Ruseva, 1993; characteristics of Fe in coal basins, i.e. iron concen- Vatsev, Bonev, 1994). The surface distribution of tration within the provenance, seems to have much the major and selected hazardous trace elements in greater importance for the element’s distribution this basin (Kortenski, Sotirov, 2002), suggest that within the coals. Thus, surface (i.e. predominantly Fe might be supplied through groundwater inflow terrigenous) and/or groundwater supply from the from the north, thus arguing for a possible Fe enrich- Kremikovtsi iron ore (sideritic beds toped by thick ment within the basin’s northern provenance. The oxidation zone) deposit, located along the northern absence of Fe mineralization zones in the catchment shoreline of the Sofia Basin, provides a reasonable areas of Karlovo, Oranovo and Dobrudzha Basins, explanation for the established Fe enrichment in the might be the reason for the low concentration of the lignite ash from that basin. At least partly, the ore element there (Table 2). deposit might have also influenced the geochemical The geochemical behavior of Fe at the weather- characteristic of Chukurovo Basin. It is important to ing sites is strongly controlled by the stability of the note, however, that the sole presence of Fe-bearing Fe-bearing minerals in exogenic environment, as rocks/minerals within the basin’s provenance can- well as from the pH and Eh settings (Kabata-Pendias, not be equivocally related to increased iron con- 2010). Oxygen is readily available in such environ-

35 ments, and is the reason for the widespread forma- Similar results are reported by Saikia et al. (2015). tion of Fe oxides and hydroxides, which are among Numerous authors relate Fe in coal solely to the the most common Fe-bearing minerals in terrestrial presence of pyrite (Dill, Wehner, 1999; Ward et al., environments. However, because of the chemi- 1999; Karayiğit et al., 2000a, b; Querol et al., cal stability of these compounds (e.g. Perel’man, 2001a; Widodo et al., 2010). Iron-rich calcite and 1977), Fe is predominantly transported to the coal dolomite, together with siderite (Kortenski, 1992; basins through the terrigenous input. Only in sites Ward, 1992; Bai et al., 2015) or oxides (Životić where higher amounts of organic acids (i.e. humic, et al., 2019; Karayiğit et al., 2020), however, can fulvic) are present, part of Fe can be transported also be recognized as major Fe-bearing minerals in the form of organometallic complexes (Kabata- in some coals. In addition, contact metamorphism Pendias, 2010). In either case, once delivered to the and/or circulation of hydrothermal fluids within basin, Fe is subjected to various (±bio-) chemical the coal seams might also result in increased iron processes, controlled by the physio-chemical char- concentrations in coal (Dai et al., 2007; Zhang et acteristics of the peat-forming environment, which al., 2020). Considering the relatively low correla- promote its affinity and mode of occurrence. tion coefficients between Fe concentrations and ash There are limited data reporting predominant or- yields in most studied coals, the presence of iron in ganic affinity of iron in coal (Ward, 1992; Newman mineral form cannot be excluded. Indeed, various et al., 1997; Bai et al., 2015; Liu et al., 2018). Ac- Fe-bearing minerals, i.e. sulfides (pyrite, marcasite, cording to Manskaya and Drozdova (1964) acidic chalcopyrite, arsenopyrite, mackinawite), carbon- peat-forming environment (3

36 epigenetic quartz and carbonate minerals (i.e. cal- 10.2 times (i.e. in Katrishte deposit). In comparison cite, dolomite). For these coals, therefore, epigenet- to the Clarke values for clays, the Fe concentra- ic mineralization is not expected to have significant tions are depleted in carbonaceous shale’s ash from influence on the element distribution. Pernik coal is Maritsa-West, Belibreg, Karlovo, Gotse Delchev, a notable exception, as epigenetic pyrite and sider- Balkan, Dobrudzha and Oranovo Basins. ite mineralization is commonly detected in fissure The organic/inorganic affinity of Fe was deter- cracks in that coal (Kortenski, 1990b). It could be mined based on the correlation with the ash yields. suggested, therefore, that the extensive Fe-bearing The results indicate predominant organic affin- epigenetic mineralization might be the reason for ity of the element for most of the studied coals. the elevated Fe contents in Pernik coals. However, Iron incorporation in organometallic compounds the moderately high negative correlation coefficient is suggested for these basins, based on the pre- between the Fe concentration and ash yield in this sumed acidic settings of the peat-forming environ- basin (ro=–0.6; Table 2), does not support such hy- ment. Burgas Basin is a notable exception, since pothesis. The results rather indicate that the propor- predominant inorganic affinity of the element can tion of iron involved in the epigenetic mineraliza- be presumed. Mixed organic/inorganic affinity can tion is probably not very high, thus arguing for the be suggested for Stanyantsi, Samokov, Bobov Dol, greater importance of the other factors on the Fe Dobrudzha and Svoge coals. The results were fur- more of occurrence and distribution. ther considered in terms of determining the influence of the following major factors on Fe distribution in coals: 1) Fe concentration in peat-forming Conclusions plants; 2) Fe concentration within the basin’s provenance; 3) the type (terrigenous or groundwater) The concentration of Fe in Bulgarian coals with and direction of element supply; 4) the settings of coalification rank ranging from lignite to anthracite, the peat-forming environment; 5) the presence and together with the associated carbonaceous shale the extend of cracks within the coal seams; and and their high temperature ashes, were studied. The 6) the presence and the composition of the epige- results indicate that Fe concentrations are lower netic mineralization within the cracks. The pre- than the World average for Karlovo lignite and Do- sented data indicate that factors 2, 3 and 4 had ma- brudzha bituminous coal. For the rest of the stud- jor influence on the Fe occurrence and distribution ied basins, Fe contents may exceed this value up to in Bulgarian coals.

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Постъпила на 08.05.2020 г., приета за печат на 10.08.2020 г. Отговорен редактор Борис Вълчев