Zeolite and associated mineral occurrences in high-sulphur coals from the middle Miocene upper coal seam from underground mines in the Çayirhan coalfield, (Beypazarı, Central Turkey)

https://doi.org/10.1016/j.coal.2022.104010Get rights and content

Highlights

  • Authigenic analcime and clinoptilolite, and associated minerals formed in palaeomire.

  • Zeolite formation was mainly controlled by synchronous volcanic inputs and hydrogeological conditions.

  • The Ca- and Mg-rich water influx into palaeomires caused the formation of syngenetic carbonate minerals.

  • Sulphate-rich water influx and volcanic inputs elevated sulphur content in palaeomire.

  • Coal elemental compositions are controlled by geochemistry of volcanic inputs and redox conditions.

Abstract

Three sampling profiles (BA, BB, and BC) from, respectively, the A, B, and C sectors of the Çayırhan coalfield representing the working Tv and Tb splits of the Middle Miocene-age upper seam were investigated for mineralogical and elemental compositions. The Tv and Tb splits are mainly separated by a 70–100-cm thick tuffaceous clastic rock/marl parting in the study area, and the thickness of the Tv and Tb splits varies from 1.50 to 1.70 m, and 1.55 to 1.80 m, respectively. In both the Tv and Tb splits, Cr, Ge, As, and Zr are enriched, while Th and U are enriched only in the Tv split in all sampling profiles. This study indicates that the most significant difference between the Tv and Tb splits is in mineralogical composition. In contrast, zeolites (analcime and clinoptilolite) and carbonate minerals are detected as dominant to abundant phases in the Tv and Tb splits in the BA and BB profiles, while zeolite minerals are only identified as accessory phases in the BC profile. In addition, plagioclase grains are commonly identified in the BA and BB sampling profiles, whereas authigenic K-feldspars are more common in the sampling profile BC. The abundance of analcime and clinoptilolite, and their co-existence with syngenetic carbonate minerals in the BA and BB sampling profiles could imply that the alteration of volcanic inputs along with Ca-rich water influx into palaeomires seems to have resulted in favourable alkaline conditions and ion support for natural zeolite formation in the palaeomires. In contrast, the accessory presence of zeolite minerals and the common occurrence of authigenic K-feldspar grains in the sampling profile BC could imply that the semi-open hydrogeological system seems to allow transferring necessary ions for natural zeolite formation to other parts of palaeomires. Overall, the variations in mineralogical and elemental composition seem to be controlled by a combination of water chemistry and hydrogeological conditions within palaeomires and the geochemical properties of volcanic inputs into palaeomires.

Introduction

Because of their economic importance, the natural occurrence of zeolites and zeolitization of volcanic inputs in sedimentary depositional environments have been extensively studied (e.g., Ataman and Gündoğdu, 1982; Hall, 1998; İijima and Utada, 1966; Langella et al., 2001; Mao et al., 2020; Rabelo et al., 2019; Sheppard and Hay, 2001a, Sheppard and Hay, 2001b). These studies showed that natural zeolite formation in the deposition environment depends on the alteration of contemporaneous volcanic inputs (e.g., tephra/air-fall ash) or clastic inputs from volcanic rocks in the adjacent areas, and is influenced by a number of parameters (e.g., pH, temperature, and alkalinity of ground- and surface waters, source of volcanic input, ion exchange ratio, chemical composition of volcanic glass, type of hydrogeological system, salinity). Depending on any of these parameters, certain zeolite minerals could form and/or zonation of zeolite minerals could be observed within the depositional systems. As with other depositional environments, the modern and palaeomires are also open to contemporaneous volcanic inputs (e.g., tephra/air-fall ash) or clastic inputs from volcanic rocks in the adjacent areas of palaeomires (Arbuzov et al., 2016; Bohor and Triplehorn, 1993; Celik et al., 2021; Dai et al., 2017; Erkoyun et al., 2019; Karayigit et al., 2017, Karayigit et al., 2019a, Karayigit et al., 2020a, Karayigit et al., 2021a; Knight et al., 2000; Spears and Arbuzov, 2019; Ward, 2002; Zhao et al., 2012). Furthermore, dissolved elements from alteration of contemporaneous volcanic inputs and/or epiclastic/pyroclastic particles could be precipitated either syngenetically within the palaeomires or epigenetically within cleat/fractures of coal seams during coalifcation (Ward, 2002). Nevertheless, alteration of these volcanic materials within the mire environment could have resulted in various by-products dependent on depositional conditions. Kaolinitization of contemporaneous volcanic inputs and/or volcanoclastics in the palaeomires are well-studied topics in coal geology; on the other hand, natural zeolite occurrences in coal deposits are uncommon, and few attempts have been made to understand their origin (Bohor and Triplehorn, 1993; Dai et al., 2017; Finkelman, 1981; Finkelman et al., 2019; Pollock et al., 2000; Senkayi et al., 1987; Triplehorn et al., 1991; Ward, 2002, Ward et al., 2001 Wang et al., 2018). Nevertheless, the natural zeolite formation in Turkish Neogene coal deposits, particularly in Beypazarı, Keles, and Tunçbilek), has been reported in several studies (Cicioglu-Sutcu et al., 2021; Celik et al., 2021; Erkoyun et al., 2017; Erkoyun et al., 2019; Querol et al., 1997; Toprak et al., 2015; Whateley et al., 1996).

The Beypazar Basin has the most important coal resources in Central Turkey, with an estimated 520 Mt. initial reserve, and mineable coal seams are located within Miocene basinal infillings in the two major coalfields, namely the Çayırhan and Koyunağılı coalfields (Querol et al., 1997; Tatar et al., 1993; Whateley and Tuncali, 1995a, Whateley and Tuncali, 1995b; Yağmurlu et al., 1988). The Çayırhan coalfield, located in the northernmost part of the Beypazarı basin, hosts two coal seams (upper and lower seams) within the middle Miocene sequences (Çoraklar and Hırka formations) (Figs. 1a and 2). The burial depths of the upper seam range from 90- to 235-m beneath the surface, and the burial depths of the lower seam range between 270- and 435-m beneath the surface. The upper seam is divided by tuffaceous clastic rock/marl (c. 0.60- to 1.00-m thick) parting into first (Tv) and second (Tb) splits (Fig. 2b), and the general average thickness of both splits is ca. 1.50 m. The upper seam has been exploited in eight different coal mining sectors (sector-A to -H) since the late ‘80’s in order to support the coal-fired Çayırhan power plant (Fig. 1a). Bechtel et al., 2014; Querol et al., 1997; Whateley and Tuncali, 1995a; Tatar et al., 1993; Whateley et al., 1996). Furthermore, recent coal exploration data showed that the north-eastern part of the Beypazarı Basin is a promising area for coal resources (Bechtel et al., 2014; Tuncalı et al., 2002). Besides coal deposits, important evaporite (trona, thenardite, glauberite, and gypsum) and shale deposits are located in Miocene sequences in the Beypazarı coalfield (Fig. 1) (García-Veigas et al., 2013; Gündogan and Helvacı, 2001; Helvacı et al., 1989; İnci, 1991; Orti et al., 2002; Querol et al., 1997; Whateley et al., 1996). Since coal- and evaporite-bearing sequences have important economic value, the geological features and mineralogical compositions of the Miocene sequences along with coal seams in the Beypazarı Basin have been investigated by several researchers (Ardahanlıoğlu et al., 2020; Ataman, 1976; Ataman and Gündoğdu, 1982; Bechtel et al., 2014; García-Veigas et al., 2013; İnci, 1991; Karayigit et al., 2001; Karayigit et al., 2001; Kavusan, 1993; Keller et al., 1992; Özçelik and Altunsoy, 2005; Querol et al., 1997; Seyitoğlu et al., 2017; Whateley and Tuncali, 1995a; Whateley et al., 1996; Yağmurlu et al., 1988), and a few petrographical studies of coal seams been carried out on the upper seam (Bechtel et al., 2014; Whateley and Tuncali, 1995b). The previous studies show that the optimal climatic conditions for peat-formation in the Beypazarı Basin were developed within a hydrogeological closed system during the middle Miocene, and herbaceous peat-forming plants were common in freshwater mires during this period of time (Bechtel et al., 2014; İnci, 1991; Kayseri-Özer, 2017; Özçelik and Altunsoy, 2005; Querol et al., 1997; Yağmurlu et al., 1988) Furthermore, detailed coal petrography analyses show that the upper seam is of low-rank (lignite A to subbituminous-C) (Whateley and Tuncali, 1995a, Whateley and Tuncali, 1995b). The synchronous volcanic activity caused the influx of volcanoclastic material, originating from different compositions, into the upper seam palaeomires. As a result, the total S content of the upper seam is high, and trace elements such as B, Cr, As, Ba, Th, and U are enriched in the Tv and Tb splits (Karayigit and Boyce, 2001; Karayigit et al., 2001; Querol et al., 1997; Whateley and Tuncali, 1995b; Whateley et al., 1996). Nevertheless, the most significant differences between the Tv and Tb splits of the upper seam were observed in the mineralogical compositions: the Ca-rich zeolite (clinoptilolite) was detected in the Tv split, while Na-rich zeolite (analcime) was identified in the Tb split due to the introduction of different geochemical properties volcanoclastic influx compositions (Querol et al., 1997; Whateley et al., 1996). The formation of evaporitic deposits in the Beypazarı Basin is closely related to the development of optimal climate conditions, discharging Na-rich water influx into the depositional environment and contemporaneous volcanic activity between the end of the middle Miocene and the entire late Miocene (García-Veigas et al., 2013; Gündogan and Helvacı, 2001; İnci, 1991; Kayseri-Özer, 2017; Orti et al., 2002; Yağmurlu et al., 1988). Furthermore, Na-rich zeolites were also identified in the middle and late Miocene shale and lacustrine carbonates in the Beypazarı Basin (Ataman and Gündoğdu, 1982; Karakaş and Kadir, 2005).

Previous studies on natural zeolite formation in the upper seam of the Çayırhan coalfield (Fig. 1a) were well-documented by Querol et al. (1997) and Whateley et al. (1996); however, these studies were only focused on the operational mining sector A during the late 90s'. To date, the mineralogical compositions of the upper seam in other sectors have not been reported. These studies imply that syngenetic zeolite formation in the upper seam is related to changes on geochemical features of contemporaneous regional volcanic activity and/or epiclastic material inputs into palaeomires; however, natural zeolites and associated minerals in coal splits could be variable and change laterally within the Çayırhan coalfield due to various redox conditions and chemistry of mire water. Therefore, this study focused on mineralogical and elemental compositions of coal and sediment samples from three sampling profiles, namely BA, BB, and BC, in the A, B, and C sectors of the Çayırhan coalfield in order to ascertain factors controlling on natural zeolites and associated minerals formations. The specific purpose of this study was to determine if there are any lateral and vertical differences in zeolites in the Tv and Tb splits in the B and C sectors, as reported in the A sector previously by Querol et al. (1997) and Whateley et al. (1996).

Section snippets

Geological setting

The Beypazarı Basin is an SE-NW-trending basin and is controlled by normal faults, which were developed under an extensional tectonic regime during the middle to late Miocene (Yağmurlu et al., 1988) (Fig. 1a). The pre-Neogene basement rocks are composed of Palaeozoic metamorphics and granite, Jurassic carbonates, and Paleocene clastic rocks (García-Veigas et al., 2013; İnci, 1991; Whateley et al., 1996; Yağmurlu et al., 1988) (Fig. 1b). The Neogene basinal infillings of the Beypazarı Basin have

Materials and methods

This study was conducted on 44 ply coal and 16 non-coal samples (six roof, three floor, and seven parting samples) of the Tv (first) and Tb (second) splits of the upper seam, respectively, which were collected from three sampling profiles BA (from sector A), BB (from sector B), and BC (from sector C) (Fig. 3). In the sampling profiles, the roof is claystone and lacustrine limestone, while the floor rocks in BA, BB, and BC are lacustrine marl, clayey limestone, and dolomitic clayey limestone,

Macroscopic features, standard coal quality and huminite reflectance

The coal samples are black in colour and contain disseminated mineral matter and/or mineral matter bands. The identified macrolithotypes from the coal samples are matrix and mineral rich. Well-gelified matrix lithotype coal samples are generally bright, whereas ungelified ones are generally dull. The mineral-rich lithotype coal samples mostly contain disseminated mineral impurities. As can be seen in the section 4.2, these impurities are mostly zeolites, and to a lesser extent, gypsum,

Coal rank

The %Rr values of both splits in all profiles are close to each other and within the ranges of previously reported %Rr values (Bechtel et al., 2014; Whateley and Tuncali, 1995a, Whateley and Tuncali, 1995b), which could imply coalification degree are similar within the coalfield (Table 1). The presence of contemporaneous volcanic activity and basaltic lava flows close to Beypazarı Basin could suggest that %Rr values might be slightly elevated by volcanic activity and/or a possible influence of

Conclusions

Standard coal quality data and mineralogical and elemental compositions of the studied coal samples from the Tv and Tb splits of the upper seam of all sampling profiles from A, B, and C sectors display differences. The coal samples display generally on dry basis varying ash yields (13.6–76.3%), total C (14.3–61.4%), and gross calorific values (7.2–24.4 MJ/kg), while their total S contents are generally higher than 5.0% in all studied sectors. The elements Cr, Ge, As, and Zr are enriched in both

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Conceptualization, methodology, and investigation were performed by A.I. Karayigit, N. Azeri, and R.G. Oskay. Data interpretation and the manuscript were done by A.I. Karayigit, R.G. Oskay, and J. Hower. This research received no external funding, and the authors would like to thank Park-Maden for permissions and helps during sampling. The authors would also thank to Dr. R.A. Gayer (Cardiff University, UK) for the ICP-MS analyses, and Drs. R.B. Finkelman (University of Texas at Dallas, USA) and

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