Geochemistry of the Lublin Formation from the Lublin Coal Basin: Implications for weathering intensity, palaeoclimate and provenance

International Journal of Coal Geology 216 (2019) 103306

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Geochemistry of the Lublin Formation from the Lublin Coal Basin: Implications for weathering intensity, palaeoclimate and provenance

T

Ewa Krzeszowska Institute of Applied Geology, Silesian University of Technology, Akademicka 2, 44-100 Gliwice, Poland

A R T I C LE I N FO

A B S T R A C T

Keywords: Geochemistry Trace elements Chemical weathering Westphalian Lublin Coal Basin

The Lublin Formation (upper Westphalian A and Westphalian B) is the main coal-bearing series of the Lublin Coal Basin (LCB). So far, few geochemical studies of the Carboniferous of the LCB have been performed, and there is almost no data on the geochemistry of this series. This paper presents geochemical data for 118 samples from the Lublin Formation, from the central part of the LCB. Major oxide concentrations (Al2O3, SiO2, Fe2O3, P2O5, K2O, MgO, CaO, Na2O, K2O, MnO, TiO2, Cr2O3, Ba) were obtained using an X-ray fluorescence spectrometry. Trace and major elements (Zr, Th, Sc, La, U, Cu, Sr, Rb, Ni, Fe, Mn, Ca, Mg, K, Na) were analysed using inductively-coupled plasma mass spectrometry. Concentrations of the major and trace elements showed significant diversity, especially in the Westphalian B deposits. The Chemical Index of Alteration, Plagioclase Index of Alternation, and Weathering Index of Parker values, as well as the Rb/Sr, Sr/Cu, C and Th/U ratios of the studied samples suggest a high degree of chemical weathering of the source rocks and a prevailing warm, humid climate with some arid episodes. Based on indicators such as Al2O3/TiO2, TiO2/Zr and La/Th, it is suggested that all the studied samples from the Lublin Formation of the LCB were derived from felsic to intermediate igneous rocks, while the Th/Sc ratio suggests a mixed source for these sediments.

1. Introduction

et al., 2002; Kotarba et al., 2002; Krzeszowska, 2019). Many studies have also been carried out on the sedimentology, structural evolution, and stratigraphy of the LCB (e.g. Cebulak and Różkowska, 1983; Skompski, 1996; Skompski, 1998; Narkiewicz, 2007; Waksmudzka, 2010, 2013; Krzeszowska, 2015). So far, geochemical studies on the Carboniferous of the LCB have not been widely carried out, and there is almost no data available on the geochemistry of this series, apart from some studies related to the palaeoredox conditions of the marine Dunbarella horizon (KokowskaPawłowska and Krzeszowska, 2015; Krzeszowska, 2017). Therefore, the main goals of this study were to: (Bahlburg and Dobrzinski, 2011) geochemically characterise the Lublin Formation; and (Bai et al., 2015) apply geochemistry to infer the degree of chemical weathering and palaeoclimate change in the source area, as well as determine the provenance of the sediments of the Lublin Formation.

The Lublin Coal Basin (LCB) is located in south-eastern Poland, and its continuation is the Lvov-Volhynia Coal Basin in Ukraine. It is situated at the contact zone of two great geological units – the Precambrian East European Platform and the Early Palaeozoic West European Platform. Nowadays, the LCB seems to be the most prospective area for hard coal deposits in Poland. Documented deposits in the LCB cover an area of 1200 km2, with prospective resources covering an area of 9100 km2. The Bogdanka coal-mine is the only one currently being exploited; it covers an area of 92 km2 in the LCB (accounting for 0.9% of the total LCB area) (Szamałek et al., 2017). The Lublin Formation (upper Westphalian A and Westphalian B), which is the subject of the presented research, is the main coal-bearing series of the LCB (Porzycki and Zdanowski, 1995; Szamałek et al., 2017). In recent years, research related to the LCB has focused mainly on the organic and inorganic geochemistry of the bituminous coal (Parzentny, 2008; Gola et al., 2013; Bzowski, 2014; Parzentny and Róg, 2017; Nieć et al., 2017), the quality of the coal and the coal petrology (Nowak, 2004; Misiak, 2012; Bielowicz and Misiak, 2018), and the hydrocarbon potential of the coals and carbonaceous shales (Botor

2. Geological background The LCB is located in the eastern, marginal part of the Northwest European Carboniferous Basin (NWECB), which is a large sedimentary basin that extends from Ireland in the west to Poland in the east (Fig. 1). It is a paralic coal basin that developed north of the Variscan Rheno-

E-mail address: [email protected]. https://doi.org/10.1016/j.coal.2019.103306 Received 29 August 2019; Received in revised form 13 October 2019; Accepted 14 October 2019 0166-5162/ © 2019 Elsevier B.V. All rights reserved.

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stratigraphic analyses, can be divided into 16 depositional sequences, representing three types of depositional systems tracts (lowstand, transgressive and highstand) (Waksmundzka, 2010). The bio- and chronostratigraphic division of the paralic series is based mainly on the faunas (goniatites, bivalves, brachiopods, trilobites, and some gastropods) (Korejwo and Teller, 1967; Korejwo, 1986; Kotasowa and Migier, 1995; Musiał et al., 1995; Skompski, 1996; Kmiecik et al., 1997; Skompski, 1998), and six faunal horizons have been identified: Posidonia corrugata I and II –Namurian A, Carbonicola pseudacuta – Namurian B, and C. exporrecta, C. pseudorobusta and Dunbarella – all Westphalian A (Musiał et al., 1995; Kmiecik et al., 1997).The higher unit of the Carboniferous succession in the LCB is dominated by fluvial or lacustrine sediments (mostly Westphalian B) (Skompski, 1998). Miospore and floristic analyses have indicated the presence of the Westphalian B and (locally) Westphalian C, while the Westphalian D has only been reported from one borehole section (Kmiecik, 1995). The main, hard coal-bearing series of the LCB, as well as the other basins of the NWECB, consists of Westphalian deposits. The most important part of the coal-bearing Carboniferous series is the Lublin Formation (upper Westphalian A and Westphalian B) (Fig. 3), which contain the main multi-seamed coal deposits (Porzycki and Zdanowski, 1995). The Lublin Formation is overlain by Westphalian C sediments only in a structural depression of the western and north-western part of the LCB (Porzycki and Zdanowski, 1995). The thickness of the Carboniferous succession in the LCB varies from hundreds of metres to about 3500 m (Porzycki and Zdanowski, 1995). The upper boundary of the Carboniferous succession is erosional, and is unconformably overlain by a sequence of Permo-Mesozoic and Cenozoic rocks. The thickness of the overburden varies from 350 to > 1200 m (Porzycki and Zdanowski, 1995). The Lublin Formation (Pennsylvanian, Upper Westphalian A and Westphalian B) is the subject of this study, and is the most important part of the coal-bearing Carboniferous series of the LCB. It includes the highest part of the paralic deposits, including the marine Dunbarella horizon (upper Westphalian A), and also limnic deposits (Westphalian B). The thickness of the Lublin Formation varies from tens of metres to about 800 m (Porzycki and Zdanowski, 1995). In the study area, located in the central part of the LCB (Fig. 2), the thickness of the Lublin Formation is about 400 m. The upper boundary of this formation is erosional and unconformable covered by Mesozoic (Jurassic, Cretaceous) and Cenozoic sediments. The paralic deposits included in the Lublin Formation developed as a sucession of marine claystones with numerous limestone and thin coal interbeds. The highest part of the paralic series is the marine Dunbarella horizon, which is of particular importance in correlating the Carboniferous succession in the LCB and other basins in Western

Fig. 1. Location of the Lublin Coal Basin (LCB) within the Northwest European Carboniferous Basin (NWECB). The dashed white line indicates the present-day contours of the Northwest European Carboniferous Basin. LCB: Lublin Coal Basin, RHB: Rhenohercynian Basin, CAB: Central Armorican Basin (Kombrink, 2008, redrawn after Ziegler, 1989).

Hercynian Belt (Porzycki and Zdanowski, 1995; Skompski, 1998; Kombrink, 2008; Szamałek et al., 2017). The geology of the NWECB has been extensively studied by many authors because of the economic importance of the Pennsylvanian coal-bearing series (mainly Westphalian) (e.g. Calver, 1968; Littke, 1987; Leeder, 1988; Drozdzewski, 1993; Littke and Leythaeuser, 1993; Narkiewicz, 2007; Suess et al., 2007; Kombrink, 2008; Jasper et al., 2009; Jasper et al., 2010). The LCB is an extended region stretching from the south-east to north-west, and is 20 to 40 km wide and 180 km long (Fig. 2). The basement beneath the Carboniferous succession consists of crystalline Proterozoic rocks and Lower and Upper Palaeozoic sedimentary deposits (Porzycki and Zdanowski, 1995). The Carboniferous succession in the Lublin region, which lies unconformably on the older substrate, is generally composed of three units, representing different sedimentary palaeoenvironments. The stratigraphic divisions of this Carboniferous succession are presented in Fig. 3. The oldest unit, comprising the upper Visean, is dominated by marine sediments. The second (Namurian and Westphalian A) is a typical paralic succession (Skompski, 1998), which, based on detailed lithofacies and sequence

Fig. 2. Location of the study area within the Lublin Coal Basin. (Explanations: 1B, 3Q, 4S, 2 K- sampled boreholes). 2

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Fig. 3. Stratigraphic divisions of the of the Carboniferous succession in the Lublin Coal Basin (Poland) (after: Porzycki and Zdanowski, 1995; Dusar, 2006; Heckel and Clayton, 2006; Richards, 2013).

Limnic deposits of the Lublin Formation (Westphalian B) are mainly composed of siltstone-claystone sequences with thin sandstones interlayers, siderite concretions, and some irregular layers with freshwater fauna. These sediments represent lacustrine, fluvial, and swam paleoenvironments. The characteristic feature the Lublin Formation is cyclic structure typical for the limnic-fluvial coal-bearing sucessions. Limnic part of the Lublin Formation, with > 50 coal seams and thin layers (including about 25 economic seams) is the main productive sequence of the Carboniferous of the LCB (Porzycki and Zdanowski, 1995). The coal seams and layers of Lublin formation show the great variability in thickness, from 0.05 to 4.10 (Porzycki and Zdanowski, 1995; Misiak, 2012).

Europe. This horizon correlates to the Clay Cross Marine Band (England), the Katharina horizon (Germany, The Netherlands) as well as the Quaregnon horizon (Belgium), and it represents the boundary between the Westphalian A and B in the NWECB (Rabitz, 1966; Calver, 1968; Paproth et al., 1983; Musiał et al., 1995; Kmiecik et al., 1997; Kombrink, 2008; Krzeszowska, 2015). The Dunbarella horizon is the highest documented faunal marine horizon, and represents the maximum flooding surface in the upper part of the Westphalian A of the LCB (Waksmundzka, 2010 and Waksmundzka, 2012). The top of this horizon is also a boundary between two miospore zones – Shulzospora rara and Endosporites globifomis (Kmiecik, 1995) – and the base of the Idiognathoides tuberculatus conodont zone (Skompski, 1996). The Dunbarella horizon comprises a stable claystone bed, with a thickness reaching 25 m (in the study area, it is about 10 m). Its characteristic feature is the cyclic occurrence of marine, brackish and freshwater assemblages, which indicate consecutive phases of evolution in the sedimentary basin (Musiał et al., 1995; Kmiecik et al., 1997; Krzeszowska, 2015).

3. Materials and methods The material studied comes from core samples from the boreholes 1B, 3Q, 4S and 2 K, which were drilled in the central part of the LCB (Fig. 2). The study is based on data from 118 samples of sedimentary 3

International Journal of Coal Geology 216 (2019) 103306

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10

Borehole

1B 3Q 4S 2K

Limnic series

sampes/PA A S

Table 1 Sampling of the Lublin Formation within the Lublin Coal Basin (Poland). Paralic series

Depth range (mbs)

Numer of samples

Depth range (mbs)

Numer of samples

700–920 687–951 664–883 674–916

45 10 21 3

920–930 951–960 883–890 916–927

10 10 10 11

1

0.1

0.01 Al

Si

Ca

Fe

K

Mg

Mn

Na

P

Ti

Fig. 4. Spider plots of major elements for the Upper Westphalian A deposits (paralic series) of the LCB normalized to Post-Archean Australian shales (PAAS; Taylor and McLennan, 1985).

(Explanations: mbs - meters below surface).

rock representing both the limnic (Westphalian B) and paralic (Dunbarella marine horizon – upper Westphalian A) series of the Lublin Formation (Table 1). Borehole 1B was the most thoroughly sampled, and the detailed distribution of the samples in the profile is presented later in the article. Sample preparation and the analytical procedures were performed at the AcmeLab Analytical Laboratory (currently, Bureau Veritas Commodities Canada Ltd.), Vancouver, Canada. The samples were crushed, split and pulverised, with 250 g rock being reduced to a 200 mesh size (0.074 mm). The oxides of major elements concentrations (Al2O3, SiO2, Fe2O3, P2O5, K2O, MgO, CaO, Na2O, K2O, MnO, TiO2, Cr2O3, and BA) were obtained using X-ray fluorescence (XRF) spectrometry and Li2B4O7/LiBO2 fusion. The trace and main elements (Zr, Th, Sc, La, U, Cu, Sr, Rb, Ni, Fe, Mn, Ca, Mg, K and Na) were analysed using inductively-coupled plasma mass spectrometry (ICP/MS) following four-acid digestion (HF + HClO4 + HCl + HNO3).

sampes/PA A S

10

1

0.1

0.01 Al

Si

Ca

Fe

K

Mg

Mn

Na

P

Ti

Fig. 5. Spider plots of major elements for the Westphalian B deposits (limnic series) of the LCB normalized to Post-Archean Australian shales (PAAS; Taylor and McLennan, 1985).

contents in the Westphalian A deposits are similar to those of the PAAS. The Westphalian A deposits are significantly depleted in CaO and Na2O, and slightly enriched in Fe2O3, MnO, and TiO2. The samples from the limnic series (Westphalian B) are characterised by a slightly wider range of variations in SiO2, Al2O3, K2O, and P2O5 concentrations, but these concentrations are relatively close to those of the PAAS. CaO and Na2O are strongly depleted relative to the PAAS, while Fe2O3 and MnO are slightly enriched relatively to the PAAS, and show a high variability in their concentrations. MgO and TiO2 show the highest relative differences in abundance, and are slightly enriched or depleted relative to the PAAS. The concentrations of trace elements in the samples from the Lublin Formation show great differentiation (Table 3). The samples from the paralic series generally have Th, Cu, Rb, Ni, Co, V, and Cr concentrations similar, or enriched in relation, to those of the PAAS, while the Zr, Sr and Ba concentrations are usually lower than those of the PAAS. Elements such as Sc, La, and U show different concentrations in relation

4. Results The major oxides SiO2 and Al2O3 are the dominant constituents, with contents ranging from 15.80 to 88.80% and from 4.56 to 26.58%, respectively, in the limnic series, and from 44.52 to 62.79% and from 16.17 to 23.93%, respectively, in the paralic series (Table 2). The concentration of Fe2O3 ranges from 1.28 to 36.50% in the limnic series, and from 3.92 to 11.74% in the paralic series. Other major oxides, such as CaO, K2O, TiO2, MgO, Na2O and P2O5, are present in low concentrations (≤ 6.82%). The distribution patterns of the major elements, normalized to the post-Archean Australian shale (PAAS; Taylor and McLennan, 1985), are shown in Figs. 4 and 5. The samples from the paralic series (upper Westphalian A) show similar patterns in their major elements contents, while the samples from the limnic series (Westphalian B) show some differences in their chemical compositions. The SiO2 and Al2O3 and, to a lesser extent, K2O, MgO, and P2O5,

Table 2 Concentrations of major oxides in samples from the Lublin Formation in the LCB (Poland). Major oxides

Detection level

Limnic series

Paralic series

Range of results

Al2O3 (%) SiO2 (%) Fe2O3 (%) CaO (%) Cr2O3 (%) K2O (%) MgO (%) MnO (%) Na2O (%) P2O5 (%) TiO2 (%) Ba (%)

0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

Average value (n = 79)

Min.

Max.

4.56 15.80 1.28 0.08 0.01 0.61 0.22 0.02 0.06 0.04 0.02 0.02

26.58 88.80 36.50 5.62 0.03 4.19 6.82 0.40 1.67 1.17 1.23 0.72

17.57 59.78 6.21 0.44 0.02 2.46 1.48 0.11 0.61 0.18 0.92 0.06

4

Range of results

Average value (n = 41)

Min.

Max.

16.17 44.52 3.92 0.28 0.01 1.04 0.74 0.03 1.04 0.08 0.70 0.03

23.93 62.79 11.74 1.03 0.02 3.44 2.90 0.27 3.44 0.26 1.14 0.06

19.61 57.41 5.69 0.53 0.02 2.96 1.93 0.08 2.96 0.16 0.95 0.04

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Table 3 Concentrations of trace elements in the samples from the Lublin Formation in the LCB (Poland). Trace elements

Detection level

Limnic series

Paralic series

Range of results

Zr (ppm) Th (ppm) Sc (ppm) La (ppm) U (ppm) Cu (ppm) Sr (ppm) Rb (ppm) Ni (ppm) Co (ppm) V(ppm) Cr (ppm)

0.1 0.1 1.0 0.1 0.1 0.1 1.0 0.1 0.1 0.2 1.0 1

Average value (n = 79)

Min.

Max.

26.6 0.0 2.0 13.4 1.1 4.2 47.0 18.5 11.2 5.6 18.0 21.0

171.3 19.9 25.0 47.3 13.4 77.9 427.0 154.9 230.1 103.1 211.0 179.0

Range of results

124.5 12.4 13.7 27.8 4.7 30.4 139.7 100.2 55.5 21.1 118.0 104.5

Average value (n = 41)

Min.

Max.

151.5 12.9 5.0 42.0 3.5 29.5 92.0 53.3 44.1 15.6 2.7 95.0

106.0 19.5 19.0 25.8 11.6 65.4 145.0 165.5 105.2 25.1 154.0 134.0

133.9 14.3 14.8 33.5 4.8 38.1 118.8 145.2 68.6 20.2 121.1 115.0

7 6 5 4 3 2 1 0

Zr

Th

Sc

La

U

Cu

Sr

Rb

Ni

Co

V

Cr

Ba

Fig. 6. Spider plots of trace elements for the Upper Westphalian A deposits (paralic series) of the LCB normalized to Post-Archean Australian shales (PAAS; Taylor and McLennan, 1985).

7 6 5 4 3 2 1 0

Zr

Th

Sc

La

U

Cu

Sr

Rb

Ni

Co

V

Cr

Ba

Fig. 7. Spider plots of trace elements for the Upper Westphalian A deposits (paralic series) of the LCB normalized to Post-Archean Australian shales (PAAS; Taylor and McLennan, 1985).

La, V, and Cr concentrations are similar, being either slightly lower or slightly higher, when compared to those of the PAAS (Fig. 7).

to the PAAS (Fig. 6). In the samples from the limnic series, there is a higher diversity of element concentration compared to the paralic series. This refers especially to U, Cu, Ni, and Co, the concentrations of which are often significantly increased. Elements such as Zr, Sr, Rb, and partly Ba generally show depletion relatively to the PAAS. The Th, Sc, 5

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SiO2/ Al2O3

<4

SiO2/ Al2O3

= 4-6

SiO2/ Al2O3

= 6-10

SiO2/ Al2O3

>10

Sandstones

SiO2/ Al2O3

<4

Fe-rich Claystones

SiO2/ Al2O3

= 4-6

SiO2/ Al2O3

= 6-10

SiO2/ Al2O3

>10

SiO2/ Al2O3

>10

Claystones Fe2O3 <10%

Siltstones Argillaceous sandstones

Fe2O3 >10%

Fe-rich Siltstones Fe-rich Argillaceous sandstones

MgO >5%

Dolomitic sandstones

Fe-rich Sandstones

Fig. 8. Chemical classification of the siliciclastic rocks (after Sprague et al., 2009).

5. Discussion 5.1. Chemical classification The chemical classification of lithological types proposed by Sprague et al. (2009) can be applied based on the chemical compositions of siliciclastic rocks. This classification is based on the share of SiO2, Al2O3, MgO, and Fe2O3 in a sample (Fig. 8). Of the samples analysed, 103 can be classed as claystones (SiO2/ Al2O3 < 4%), including 10 Fe-rich, silty claystones (Fe2O3 > 10%). The remaining samples were classed as siltstones (10 samples), sandstones (five samples), and argillaceous sandstones (two samples) (Fig. 9). The variability in the lithological types in the most thoroughly sampled borehole profile is presented in Fig. 10. The entire profile is dominated by claystones (46 samples), including Fe-rich claystones (seven samples) mainly located in the upper part of the profile. Fe enrichment in claystones is related to the presence of siderites. The other samples were classed as siltstones (seven samples) and sandstones (two samples). The distribution of lithological types in the borehole profile (1B) corresponds well to the general lithological composition of the Westphalian in the LCB (Porzycki and Zdanowski, 1995), and therefore this profile can be considered as representative. Explanations: C - claystones, S – siltstones, As - argillaceous sandstones, Sa – sandstones.

Fig. 10. Stratigraphical variability of the lithological types (after Sprague et al., 2009) for the upper Westphalian A and Westphalian B of the borehole 1B from the Lublin Coal Basin.

proposed to quantify the intensity of weathering (Parker, 1970; Nesbitt and Young, 1982; Harnois, 1988; McLennan, 1993; McLennan et al., 1993; Fedo et al., 1995; Hamdan and Burnham, 1996; Price and Velbel, 2003; Goldberg and Humayun, 2010). The Chemical Index of Alteration (CIA) proposed by Nesbitt and Young (1982) has been extensively applied to examine the degree of chemical weathering in rocks. It is also used as an indicator of palaeoclimatic conditions. Low CIA values suggest a lack of chemical alteration, indicating cool and/or arid climatic conditions, while higher values suggest more intense chemical weathering (Nesbitt and Young, 1982; McLennan, 1993; McLennan et al., 1993; Fedo et al., 1995). Application of the CIA, combined with a comprehensive facies analysis, allows for a more precise approximation of past conditions of physical and chemical weathering (Bahlburg and Dobrzinski, 2011).

5.2. Source-area weathering and palaeoclimate Chemical weathering strongly affects the major-element geochemistry and mineralogy of siliciclastic sediments (e.g. Nesbitt and Young, 1982; McLennan, 1993; Fedo et al., 1995; Bahlburg and Dobrzinski, 2011; Phillips et al., 2017). Several chemical indices have been

Fig. 9. Chemical classification of the lithological types for the samples from the Lublin Formation in the LCB (Poland). (after Sprague et al., 2009). 6

International Journal of Coal Geology 216 (2019) 103306

E. Krzeszowska

The CIA represents the ratio of predominantly immobile Al2O3 to the mobile cations Na+, K+, and Ca2+, given as oxides (Nesbitt and Young, 1982; Fedo et al., 1995; Bahlburg and Dobrzinski, 2011). The CIA ratio can be calculated as: CIA = Al2O3/(Al2O3 + CaO1 + Na2O + K2O)*100 (Nesbitt and Young, 1982), where CaO1 represents the amount of CaO incorporated in the silicate fraction of the sample, which can be calculated using the method of McLennan et al. (1993), where CaO1 = CaO – (10/3*P2O5). The CIA values for average shales range from 70 to 75, for fresh basalts from 30 to 45, and for fresh granites and granodiorites from 45 to 50 (Nesbitt and Young, 1982; Fedo et al., 1995). CIA values between 50 and 60 indicate low chemical weathering (relatively cool and/or arid climates). Values between 60 and 80 suggest moderate chemical weathering, and those between 80 and 100 are characteristic of intensive chemical weathering in hot and humid climates (Nesbitt and Young, 1982; McLennan, 1993; McLennan et al., 1993; Fedo et al., 1995; Goldberg and Humayun, 2010). Whilst the CIA can be used to reflect palaeoclimatic conditions, and has obtained favourable results, some restrictions should be acknowledged when using it. According to Goldberg and Humayun (2010), it should not be used for carbonate-rich sediments, or for sediments that have been subjected to metasomatism, metamorphism or diagenetic illitisation that may have involved post-depositional additions of K, or where the source rocks are mostly sedimentary, which may have resulted in a signature of chemical weathering cycles that took place at significantly different times (Goldberg and Humayun, 2010). The CIA values for all the studied samples ranged from 58.2 to 94.6. The CIA values from the paralic series were similar, ranging from 82.0 to 94.6 (with an average value of 84.5), while the CIA values for the samples from the limnic series showed some variability, ranging from 58.2 to 93.8 (with an average value of 85.3; Table 4). The CIA values for all the samples (Kotasowa and Migier, 1995) from the paralic series and most of the samples (Phillips et al., 2017) from the limnic series were > 80, suggesting intense chemical weathering in hot and humid climates (Fig. 11). The highest values of this index refer to samples clearly depleted in Ca and Na, which reflects the removal of labile cations (e.g. Ca, Na, K) relative to stable residual constituents (Al, Ti) during weathering (Nesbitt and Young, 1982; Fedo et al., 1995). Analysis of the CIA values in the Lublin Formation profile also showed the presence of samples with moderate or even low degrees of chemical weathering (especially in the middle and lower parts of the Westphalian B). The degree of chemical weathering can be also estimated using the Plagioclase Index of Alteration (PIA), a modification of the CIA for specifically monitoring plagioclase weathering (Fedo et al., 1995). This can be calculated as: PIA = 100*(Al2O3 – K2O)/(Al2O3 + CaO1 + Na2O – K2O), where CaO1 again represents the amount of CaO incorporated in the silicate fraction of the sample. The maximum PIA value (100)

Depth (mbs) 0 700

20

40

60

80

100

750

800

850

900

WIP

CIA

PIA

Westphalian A - B boundary

Fig. 11. Vertical variability of the CIA, PIA and WIP values within the borehole B1 from the Lublin Coal Basin.

indicates completely altered material, therefore the PIA yields values of 50 for fresh rock and values close to 100 for clay minerals, such as kaolinite, illite, and gibbsite, which are consistent with values derived from the CIA formula (Fedo et al., 1995). The PIA values for the samples from the paralic series showed similarly high values, ranging from 90.1 to 97.5 (average value of 93.4), again suggesting intense chemical weathering in hot and humid climates. For the limnic series, the values were variable, ranging from 58.6 to 98.6 (average value of 93.0), although most of them had PIA values > 80, again suggesting intense chemical weathering (Table 4). The samples with lower values of this indicator, suggesting moderate or low degrees of chemical weathering, mainly represent the middle and lower parts of the Westphalian B (Fig. 11). For most samples, the values of PIA and CIA correspond very well, indicating a similar degree of weathering (Fig. 12). Chemical weathering can also be measured by the Weathering Index of Parker (WIP), as introduced by Parker in 1970, and developed by Hamdan and Burnham (1996). The WIP is based on the most mobile major elements (Na, Mg, K, and Ca), and can be calculated as:

Table 4 Elemental ratios and calculated weathering indices for the Lublin Formation in the LCB (Poland). Chemical index

Limnic series

Paralic series

Range of results

CIA PIA WIP C-value Sr (ppm) /Cu (ppm) Rb (ppm) /Sr (ppm) Th (ppm)/U (ppm) Al2O3 (%)/TiO2 (%) TiO2 (%)/Zr(%) Th (ppm)/Sc(ppm) La (ppm)/Th (ppm)

Average value (n]79)

Min.

Max.

58.2 58.6 8.1 0.3 2.4 0.2 0.9 13.7 59.7 0.3 1.5

93.8 98.6 42.2 4.8 21.7 2.2 3.9 1262.5 86.9 3.8 3.8

85.3 93.0 25.6 1.2 5.4 0.8 2.7 34.6 71.1 1.1 2.3

7

Range of results

Average value (n = 41)

Min.

Max.

82.0 90.1 11.3 0.3 1.7 0.5 1.6 19.1 1.1 0.8 1.8

94.6 97.5 34.5 2.1 4.4 1.7 4.0 27.3 100.0 2.9 2.9

84.5 93.4 30.2 0.9 3.2 1.2 3.1 20.8 74.4 1.0 2.3

International Journal of Coal Geology 216 (2019) 103306

100

50

90

40

80

30

samples from limnic series

WIP

PIA

E. Krzeszowska

70 samples from paralic series

60 50

50

60

70

80

90

samples from limnic series 20 samples from paralic series

10 0

100

CIA

50

70

90

110

PIA

Fig. 12. Crossplot of CIA values vs PIA values of the Lublin Formation deposits from the Lublin Coal Basin.

Fig. 14. Crossplot of PIA values vs WIP values of the Lublin Formation deposits from the Lublin Coal Basin.

WIP = 100 × (2Na2O/0.35 + MgO/0.9 + 2 K2O/0.25 + CaO/0.7) (Parker, 1970). This index has been suggested as being more appropriate for estimating weathering profiles in heterogeneous (and homogeneous) parent rocks, but is not recommended for the analysis of strongly-weathered material (Hamdan and Burnham, 1996; Price and Velbel, 2003; Shao et al., 2012). The WIP is most useful for reflecting changes in the amounts of Na, K, Ca, and Mg, where the alteration of feldspars into clay minerals is the major chemical (hydrolytic) weathering process (Parker, 1970). Lower WIP values indicate stronger chemical weathering, which is opposite to the CIA and PIA values (Parker, 1970; Hamdan and Burnham, 1996). WIP values range from 0 to 100, with the maxima corresponding to the least weathered rocks (Nadłonek and Bojakowska, 2018). Price and Velbel (2003) evaluated the chemical weathering indices (i.e. WIP, PIA, CIA) in terms of their suitability for characterising weathering profiles developed in felsic, heterogeneous metasedimentary bedrock. They found that the WIP was the most appropriate for evaluating weathering intensity, because it includes only the highly mobile alkali and alkaline earth elements in its formulation, and yields values that differ greatly. Application of the CIA and PIA indicators to heterogeneous profiles should only be undertaken with caution because they assume Al immobility (Price and Velbel, 2003). The WIP values of the investigated samples varied between 11.3 and 34.5 (average 30.2) for the paralic series, and between 8.1 and 42.2 (average 25.6) for the limnic series (Table 4). Being generally low, although different, the WIP values indicate relatively strong chemical weathering, but do not always correspond with the CIA and PIA results (Figs. 11, 13, 14). Palaeoclimatic reconstructions can also be based on trace and major elements that are sensitive to palaeoclimate changes, such as Sr, Cu, Rb, Mn, P, Ca, Mg, K, Na, Ba, Fe, Cr, V, Ni, and Co (Lerman and Gat, 1989; Cao et al., 2012; Bai et al., 2015; Moradi et al., 2016; Tao et al., 2017). Changes in Rb/Sr and Sr/Cu ratios are commonly used to reflect

palaeoclimatic conditions. Generally, Sr/Cu ratios increase under drier conditions, and values between 1.3 and 5.0 indicate warm, humid climates, whereas ratio > 5.0 point to hot, arid climates (Lerman and Gat, 1989; Bai et al., 2015; Tao et al., 2017). Rb/Sr ratios decrease under drier conditions, and high ratios reflect cold conditions, while low ratios reflect warm conditions (Chen et al., 1999; Xu et al., 2010). The Sr/Cu ratios of the Lublin Formation samples ranged from 2.4 to 21.7 (average value 5.4), and from 1.7 to 4.4 (average value 3.2) for the limnic and paralic series, respectively (Table 4, Figs. 15, 16). The Sr/Cu values therefore suggest a stable, warm, humid depositional environment for the paralic series, and variable, warm and humid to hot and arid climates for the limnic series. The fluctuations in this indicator of climate change are clearly marked in the Lublin Formation profile (Fig. 15). The Rb/Sr ratios showed slight differences, but with all samples having low values (0.2–2.2), suggesting warm depositional environments (Table 4, Figs. 15, 16). Depth (mbs) 0 700

5

10

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850

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30 samples from limnic series 900

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10 0

50

60

70

80

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Sr/Cu Rb/Sr Th/U

100

CIA Fig. 13. Crossplot of CIA values vs WIP values of the Lublin Formation deposits from the Lublin Coal Basin.

Fig. 15. Vertical variability of the Sr/Cu, Rb/Sr, Th/U, and C ratio values within the borehole B1 from the Lublin Coal Basin. 8

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3

U relative to the PAAS (Figs. 6, 7). Similar results have been reported from the Westphalian B of The Netherlands. Th/U values from lacustrine deposits range from about 2.3 to 5, while for the marine Lingula bands (formed in nearshore environments), they range from about 1.9 to 4.6, and for the marine Goniatites bands (formed in deep marine environments), they are significantly lower (around 0.5–3.3; Kombrink, 2008). The degree of weathering of the source rocks, based on their Th/ U ratios, does not correspond very well with the palaeoenvironmental conclusions drawn from the other indicators. Therefore, the use of this indicator to determine the degree of weathering seems to be questionable. The multiple geochemical analyses have clearly revealed the occurrence of climate variation during deposition of the Westphalian of the LCB, although the climate indicators did not always exactly correspond. All the indicators showed some fluctuation in climate during deposition of the Lublin Formation, but generally with a prevailing warm, humid climate, and a high degree of chemical weathering of the source rocks. Warm and humid climatic conditions are typical of the Pennsylvanian coal-bearing series (mainly Westphalian; e.g. Suess et al., 2007; DiMichele et al., 2010; Jasper et al., 2010; Zieger and Littke, 2019). Based on the geochemical indicators, episodes of drier conditions and low levels of chemical weathering were also found. Seasonal dryness during the Westphalian has been reported previously, based on floristic studies (DiMichele et al., 2010, and references therein). Cyclical changes in environmental conditions, including climate, are characteristic of the Pennsylvanian coal of European and North America (e.g. Calver, 1968; Littke, 1987; Leeder, 1988; Littke and Ten Haven, 1989; Drozdzewski, 1993; Littke and Leythaeuser, 1993; Narkiewicz, 2007; Suess et al., 2007; Kombrink, 2008; Jasper et al., 2009; DiMichele et al., 2010; Jasper et al., 2010; Krzeszowska, 2015, 2017). In addition to climatic conditions, other processes have also been proposed as controlling cyclic sedimentation, including autogenic changes in facies (delta lobe switching and river channel avulsion), and allogenic changes, such as periodic sea-level fluctuations (Ludwig, 1994; Hampson et al., 1999; Suess et al., 2007; Rygel et al., 2008; Kai, 2009).

Rb/Sr

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1

0 0

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10

15

20

25

Sr/Cu samples from limnic series

samples from paralic series

Fig. 16. Crossplot of Sr/Cu values vs Rb/Sr values of the Lublin Formation deposits from the Lublin Coal Basin.

Another indicator of palaeoclimate is the C-value, which is defined as: C-value = (Fe + Mn + Cr + Ni + V + Co)/(Ca + Mg + Sr + Ba + K + Na) (Zhao et al., 2007; Cao et al., 2012; Moradi et al., 2016; Tao et al., 2017). The C-value ratio is based on the assumption that Fe, Mn, Cr, Ni, V, and Co are enriched under humid conditions, while Ca, Mg, Sr, Ba, K, and Na are concentrated under arid conditions (Cao et al., 2012; Moradi et al., 2016; Tao et al., 2017). Therefore, low C-values suggest arid conditions, with values increasing with increasing humidity. The C-values for the samples varied between 0.3 and 2.1 (average value 0.9) for the paralic series, and from 0.3 to 4.8 (average value 1.2) for the limnic series (Table 4). The C-values for most of the samples were relatively high, indicating humid climates. Only a few samples (mainly from the limnic series) had low values (0.3–0.5), suggesting drier climates (Fig. 15). The weathering history of the source rocks for sedimentary sequences can also be evaluated using Th/U ratios, which generally increase with increasing degrees of weathering due to oxidation and the loss of U. For volcanogenic sediments, Th/U ratios are low, typically < 3.0, which is below the average upper crustal value (PASS) of 3.8 (McLennan et al., 1995). Relatively low Th/U ratios are commonly reported from active margin sediments (McLennan et al., 1993) and from coal-bearing samples, with the high U content being associated with organic matter (Hofer et al., 2013). The Th/U ratios for most of the samples from the Lublin Formation were lower than the average upper crustal value (averages 2.7 and 3.1 for the limnic and paralic series, respectively), suggesting a low to moderate degree of weathering in the source area (Table 4, Fig. 17). A higher Th/U ratio than the PASS values is thought to be the product of advanced weathering (McLennan et al., 1993). Relatively low Th/U ratios for the studied samples may result from their slightly enriched in

5.3. Provenance Trace-element compositions can potentially provide information on sediment provenance and source-area rock compositions. The geochemical signatures of clastic sediments have been used in various studies to identify the provenance of terrigenous sediments (e.g. Cullers et al., 1988; Harnois, 1988; Cullers, 1994; Hayashi et al., 1997; Cullers, 2000, 2002; Odigi and Amajor, 2008; Keskin, 2011; El-Bialy, 2013; Mir et al., 2015; Phillips et al., 2017). The ratios of Al2O3/TiO2, TiO2/Zr, and the abundance of trace elements, such as Zr, La, Sc and Th, in clastic sediments have been considered to be proxies in provenance studies (Hiscott, 1984; McLennan et al., 1980, 1993; Cullers, 1994; Hayashi et al., 1997; Phillips et al., 2017). Al2O3/TiO2 ratios can be used in identifying source materials because the Al2O3/TiO2 values in sedimentary rocks are basically conserved from their parent rocks, and do not change significantly during weathering of the source, or their subsequent transportation, deposition, and diagenesis (Harnois, 1988; McLennan et al., 1993; Hayashi et al., 1997). According to El-Bialy (2013), the provenance of the source rocks can be determined on the basis of the geochemical composition, where the source rocks are not extensively weathered (CIA < 80) and are chemically immature (SiO2/Al2O3 < 6). Al2O3/TiO2 ratios range from 3 to 8 in mafic igneous rocks, from 8 to 21 in intermediate rocks and from 21 to 70 in felsic igneous rocks (Hayashi et al., 1997). The Al2O3/TiO2 ratios of the studied samples ranged from 13.7 to 1262.5 (average 34.6) for the limnic series, and from 19.1 to 27.3 (average 20.8) for the paralic series (Table 4, Fig. 17). A relatively high

Ti02 (%)

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0.5

0 0

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25

30

Al203 (%) samples from limnic series

samples from paralic series

Fig. 17. Al2O3-TiO2 plot for the Lublin Formation deposits from the Lublin Coal Basin. 9

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Ti02 (%)

1.5

1

0.5

0 0.0

50.0

100.0 Zr (ppm)

samples from limnic series

150.0

200.0

samples from paralic series

Fig. 18. TiO2-Zr plot for the Lublin Formation deposits from the Lublin Coal Basin.

Fig. 19. SceTh plot for the Lublin Formation deposits from the Lublin Coal Basin.

value was found in only one sample, in which the share of TiO2 significantly deviated from that in the other samples, and was present in a very low amount, close to the detection level limit. The Al2O3/TiO2 ratios of all the studied samples from the Lublin Formation suggested that they were derived from felsic to intermediate igneous rocks. A plot of the concentrations of Zr and TiO2 versus Zr can also be used for characterising the nature and composition of source-area rocks. The TiO2/Zr weight ratio generally decreases with increasing SiO2 content, from > ~200 for mafic igneous rocks, to 195–55 for intermediate rocks and < 55 for felsic rocks (Hayashi et al., 1997). The TiO2/Zr weight ratio of the studied samples ranged from 59.7 to 86.9 (average 71.1) for the limnic series, and from 1.1 to 100.0 (average 74.4) for the paralic series (Table 4). Based on Hayashi et al.'s (1997) TiO2 versus Zr plot, the analysed samples fall into the intermediate igneous rock field, close to the felsic igneous rock field (Fig. 18). The concentrations of Th, Sc, La, and the corresponding elemental ratios may also be useful for determining sediment provenance (e.g. McLennan et al., 1980; Hiscott, 1984; Cullers et al., 1988; McLennan, 1989; Wronkiewicz and Condie, 1990; McLennan and Taylor, 1991; Cullers, 1994, 2000, 2002; Hayashi et al., 1997; Phillips et al., 2017). The immobile elements La and Th are more abundant in felsic than in mafic source rocks, while Sc is more concentrated in mafic source rocks (Taylor and McLennan, 1985; Wronkiewicz and Condie, 1990; Cullers et al., 1988; Cox et al., 1995; Cullers, 1995; Phillips et al., 2017). Sc and Th are transferred quantitatively from source to sediment. Thus, the Th/Sc ratio is commonly used to deduce the composition of the source rock, and to distinguish between felsic and mafic sources (McLennan et al., 1980; McLennan, 1989; Wronkiewicz and Condie, 1990; McLennan and Taylor, 1991). Th/Sc ratios of ≥1.0 are typical of continental crust enriched in incompatible elements, ≥0.6–1.0 indicates an andesitic composition, and < 0.6 typifies a mafic signature (Phillips et al., 2017). The Th/Sc ratios of the samples from the Lublin Formation showed various values, ranging from 0.3 to 3.8 (average 1.1) for the limnic series, and 0.8 to 2.9 (average 1.0) for the paralic series (Table 4). On the Th/Sc plot (Fig. 19), the sample data falls into three different fields, suggesting a mixed sediment source. The La/Th ratios of the samples ranged from 1.5 to 3.8 (average 2.3), and from 1.8 to 2.9 (average 2.3), for the limnic and paralic series, respectively (Table 4, Fig. 20). For most of the samples, the La/Th ratios were similar or slightly lower than the La/Th = 2.8 ratio of the upper continental crust, suggesting a felsic to intermediate source for the sediments (Taylor and McLennan, 1985). Based on indicators such as Al2O3/TiO2, TiO2/Zr, and La/Th, it can be concluded that all the studied samples from the Lublin Formation of the LCB were derived from felsic to intermediate igneous rocks, while the Th/Sc ratio suggests mixed sources for the sediments.

Fig. 20. TheLa plot for the Lublin Formation deposits from the Lublin Coal Basin.

6. Summary and conclusions The Lublin Formation (upper Westphalian A and Westphalian B) is the main coal-bearing series of the LCB. It developed as an assemblage of siltstone-claystone sequences, with thin sandstone interlayers and siderite concretions. Based on the chemical composition of siliciclastic rocks, most of the samples from the Lublin Formation can be classed as claystones (SiO2/Al2O3 < 4%), including Fe-rich, silty claystones (Fe2O3 > 10%). The remaining samples can be classed as siltstones (10 samples), sandstones (five samples), and argillaceous sandstones (two samples). The Lublin Formation deposits were generally characterised on the basis of their SiO2 and Al2O3 contents and, to a lesser extent, K2O, MgO, and P2O5, the values of which are similar to those of the PAAS. Furthermore, they are significantly depleted in CaO and Na2O, and slightly enriched in Fe2O3, MnO and TiO2. The concentrations of trace elements in the studied samples show a clear differentiation, especially in the Westphalian B deposits. Compared to the PAAS, the Westphalian A deposits have similar or enriched concentrations of Th, Sc, Cu, Rb, Ni, Co, V, and Cr, and usually lower concentrations of Zr, Sr and Ba, while Sc, La, and U show different concentrations. In the samples from the Westphalian B, a high diversity of element concentrations was found, especially in U, Cu, Ni, and Co, the concentrations of which are often significantly increased. Elements such as Zr, Sr, Rb, and partly Ba were usually depleted, while the Th, Sc, La, V, and Cr concentrations are similar either slightly lower or higher in relation to those of the PAAS. The CIA, PIA, and WIP values, as well as Rb/Sr, Sr/Cu, C, and Th/U ratios of the studied samples indicate a high degree of chemical weathering of the source rocks and a prevailing warm, humid climate 10

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with some arid episodes. All the geochemical indicators analysed suggested some fluctuations in climate during deposition of the Lublin Formation, although the climate indicators did not always correspond exactly with each other. Trace element compositions have also been used to determine sediment provenance and source-rock compositions in sediments. Based on indicators such as Al2O3/TiO2, TiO2/Zr, and La/Th, it can be inferred that all the studied samples were possibly derived from felsic to intermediate igneous rocks, while the Th/Sc ratio suggests a mixed source for the sediments.

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