Petrographic and geochemical characterization of coal from Santa Susana Basin, Portugal

International Journal of Coal Geology 203 (2019) 36–51

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Petrographic and geochemical characterization of coal from Santa Susana Basin, Portugal

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Joana Ribeiroa,b, , Gil Machadoc,d, Noel Moreirae, Isabel Suárez-Ruizf, Deolinda Floresa,g Instituto de Ciências da Terra – Polo do Porto, Porto, Portugal Departamento de Ciências da Terra, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Coimbra, Portugal c Galp Energia E&P, Lisboa, Portugal d Instituto Dom Luiz, Universidade de Lisboa, Lisboa, Portugal e Departamento de Geociênciase Instituto de Ciências da Terra – Polo de Évora, Universidade de Évora, Évora, Portugal f Instituto Nacional del Carbon (INCAR-CSIC), Oviedo, Spain g Departamento de Geociências, Ambiente e Ordenamento do Território Faculdade de Ciências, Universidade do Porto, Porto, Portugal a

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Bituminous coal of Santa Susana Basin Maceral composition Vitrinite reflectance Weathering Geochemical composition Elements mode of occurrence

The current and future importance of coal as a geological resource is related with the production of coke, carbon materials, carbon derivatives and other chemical products as well as a promising alternative source of critical trace elements. This study aims the comprehensive characterization of coals from the Santa Susana Basin (SSB), SW of Portugal. The SSB is a Pennsylvanian continental basin located along the Santa Susana Shear Zone that separates two tectonostratigraphic zones of Iberian Massif. Samples of coal and coaly silt-claystone from the main outcrops, Jongeis and Vale de Figueira, were collected for this study. The methodologies used for the petrographic and geochemical characterization included: optical microscopy, scanning electron microscopy with energy dispersive X-ray spectrometry, proximate and elemental analysis and inductively coupled plasma mass spectrometry (ICP-MS). The results demonstrate that the organic matter of coal from SSB is essentially composed of vitrinite and small amounts of inertinite. The mineral matter includes detrital minerals (mainly quartz and clay minerals), iron oxides and oxidized and non-oxidized framboidal pyrite. Epigenetic carbonates (dolomite and ankerite) were only observed in samples from Jongeis outcrop. Vitrinite random reflectance of the samples ranges between 0.90% and 1.25%, indicating a bituminous coal rank. The slightly higher vitrinite reflectance and the occurrence of epigenetic carbonates in samples from Jongeis can be related with differentiated burial history of the basin and post-depositional processes including fluids circulation that may have promoted the enhancement of the thermal maturity. When compared with the geochemical composition of worldwide hard coals, samples from Vale de Figueira are significantly enriched in Cs and In, while samples from Jongeis are significantly enriched in In and Mn. Considering the elements' mode of occurrence, the majority of elements are preferentially associated with mineral matter. The elements S, Cd, Mo, Pb and Se have organic affinity. The strong positive correlation between Ca and Mg, Mn and Te indicates their association with carbonate phases, which is in accordance with the carbonate composition.

1. Introduction Considering the disregard of coal exploration to be used as fuel and the decarbonisation general policies in Europe and other countries, the future and importance of coal as a geological resource will be related with the production of coke, carbon materials, carbon derivatives and other chemical products. The coking coal and graphite are included, among other elements in the list of critical raw materials for Europe (European Commission 2017). In addition, the interest of coal deposits

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as promising and alternative sources of some critical trace elements has becoming increasingly important in recent years (Seredin and Finkelman 2008; Dai et al. 2016; Dai and Finkelman 2017). Considering the above mentioned, the present study aims the comprehensive petrographic and geochemical characterization of coals from the Santa Susana Basin (SSB; SW of Portugal). It is considered that this research work is significant because, despite the fact that coal from the SSB was exploited during the last century, petrography and geochemistry studies of the SSB coal are very scarce (with exceptions of

Corresponding author at: Instituto de Ciências da Terra – Polo do Porto, Porto, Portugal. E-mail address: [email protected] (J. Ribeiro).

https://doi.org/10.1016/j.coal.2019.01.005 Received 22 September 2018; Received in revised form 17 January 2019; Accepted 17 January 2019 Available online 22 January 2019 0166-5162/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (A) Carboniferous (meta) sedimentary rocks of Iberian Massif (adapted from Gibbons and Moreno 2002; LNEG 2010; Fernández and Oliveira 2015); (B) Geological map of SSB (adapted from Oliveira et al. 1992; Almeida et al. 2006; Oliveira et al. 2007; LNEG 2010).

continental intramontane basins (Machado et al. 2012; Fernandes et al. 2016). In turn, the Cantabrian Zone presents a very continuous succession of Carboniferous sedimentary rocks, ranging from Tournaisian to Stephanian (e.g. Sánchez de Posada et al. 1990; Colmenero and Prado 1993; Colmenero et al. 2008). The succession shows lateral and vertical facies transition from marine to continental facies, with late Namurian to late Stephanian coal deposits (Colmenero and Prado 1993; Colmenero et al. 2008). The current geographic distribution of Carboniferous sedimentary rocks reveals the typical orogenic anatomy of the Iberian Variscides Belt: the hinterland zones corresponds to OssaMorena, Central Iberian and West Asturian Leonese zones and the foreland zone features match with the South Portuguese (oceanic foreland) and the Cantabrian (continental foreland) zones (Fig. 1A; Matte 1991). In the orogenic hinterland, namely in the Portuguese part of the Central Iberian Zone, Pennsylvanian sedimentary rocks are found in two small continental coal-bearing basins, the Douro Basin and the Buçaco Basin. Both basins are located along major Variscan shear zones (Fig. 1A): (1) the Buçaco Basin is in close proximity to the Porto-Tomar Shear Zone (Domingos et al. 1983; Gama Pereira et al. 2008; Flores

Fernandes et al. 2016 and Lemos de Sousa et al. 2010). This study presents a multidisciplinary investigation where the petrographic observations and measurements of vitrinite reflectance were carried out, together with the geochemical analyses contributing to: the identification of effects caused by natural weathering; the characterization of the depositional environment; and, the geothermal history and the geodynamic model of the SSB. Geochemical data of coals provide information about the elements' concentration and mode of occurrence, which is significant to identify possible sources of trace elements in SSB coals and its technological use. 2. Geological setting Carboniferous rocks are present in all tectonostratigraphic zones of the Iberian Variscides (Fig. 1A). Carboniferous sedimentary rocks, from Mississippian to Pennsylvanian, of the South Portuguese Zone are fully marine (e.g. Oliveira et al. 2013), whereas, in the Central Iberian and Ossa Morena Zones, Mississippian rocks were mostly deposited in marine environments (e.g. Gabaldón et al. 1983; Pereira et al. 2006; Armendáriz et al. 2008), but Pennsylvanian rocks are restricted to 37

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et al. 2010; Machado et al. 2018), a NeS dextral shear zone located in the western boundary of the Central Iberian Zone (Iglesias and Ribeiro 1981; Romão et al. 2014; Moreira et al. 2016; Moreira and Dias 2018); and (2) the Douro Basin is located along the Douro-Beiras Shear Zone (Pinto de Jesus 2003; Pinto de Jesus et al. 2010), an important NW-SE sinistral shear zone (Pinto de Jesus 2003). In both basins, Pennsylvanian strata rest unconformably over the Lower Palaeozoic or Precambrian rocks (Courbouliex 1974; Domingos et al. 1983; Flores et al. 2010). The basement rocks are more deformed and metamorphosed than the Pennsylvanian sediments, resulting from the early Variscan deformation events during Devonian and Lower Carboniferous (e.g. Dias et al. 2013, 2016). The SSB crops out along the western boundary of the Ossa-Morena Zone (Fig. 1). As previously mentioned, the Ossa-Morena Zone is considered the southernmost zone of the orogenic hinterland of Iberian Massif, presents a magmatic, metamorphic and sedimentary complex evolution. The Lower-Middle Devonian to Carboniferous age series of this tectonostratigraphic zone have syn-orogenic features, being linked to the Variscan Orogeny (e.g. Quesada 1990; Moreira et al. 2014; Dias et al. 2016). The sedimentation of the SSB continental basin occurs during the Pennsylvanian (Kasimovian to (?)Moscovian-Bashkirian; Machado et al. 2012), being consequently controlled by tectonic processes, as the other hinterland Pennsylvanian intramontane basins. This basin is located ca. 10 km NE of Alcácer do Sal (SW of Portugal; Fig. 1A) extending over 15 km in length along a NNW-SSE trend, controlled by the general trend of the Santa Susana Shear Zone, and is up to 1 km wide. The basin is interpreted as a pull-apart basin related with the dextral transcurrence along the Santa Susana Shear Zone during Pennsylvanian times (Domingos et al. 1983; Almeida et al. 2006; Oliveira et al. 2007; Lemos de Sousa and Wagner 1983; Machado et al. 2012; Moreira et al. 2014). This shear zone emphasizes the western boundary between the South Portuguese and the Ossa-Morena Zones (Fig. 1B), generally interpreted as a structure with similar geometry and dynamics as the Porto-Tomar shear zone (Fig. 1A; Wagner 2004; Ribeiro et al. 2007). The SSB rests over two Ossa-Morena Zone units (Fig. 1B): the Toca da Moura Volcano-Sedimentary Complex and the Cuba Group magmatic rocks (Fig. 2; Andrade et al. 1955; Oliveira et al. 2007; Machado et al. 2012). The SSB was subdivided in two distinct units (Fig. 2; Machado et al. 2012): (1) a basal unit composed of coarsegrained basal conglomerates with felsic magmatic boulders overlain by coarse sandstones and polymictic conglomerates; and, (2) an upper unit including quartz/quartzite-rich gravel conglomerates, sandstones, shales and coal seams. Five main coal seams were identified in the SSB: three of them are very thin (10 to 20 cm), another coal seam is 70 to 80 cm thick interbedded with mineral matter and the fifth one is about 1.15 m thick interbedded with mineral matter (Andrade 1927). The five coal seams were recognized to occur unconsolidated and at small depths (Andrade 1927). Currently there are three main outcrops in the SSB from North to South: Jongeis, Remeiras and Vale de Figueira (Fig. 1B; Machado et al. 2012).

Fig. 2. Schematic lithological column of the Santa Susana basin (TMC – Toca da Moura Complex and Cuba Group felsic rocks).

The samples were homogenized, and quartered to obtain representative samples and then crushed to obtain the < 1 mm and < 212 μm fractions for petrographic and geochemical analysis, respectively. For the petrographic characterization of the samples, optical microscopy was used to identify and quantify the organic and inorganic matter present in the samples and to measure vitrinite reflectance. The microscopy observations were carried out on polished pellets of whole rock samples prepared according to standard procedures (ISO 7404-2 2009). The identification and characterization of the organic matter followed the ICCP nomenclature (ICCP 1998, 2001; Pickel et al. 2017). The maceral analysis and determination of vitrinite random reflectance were performed according to standard procedures (ISO 7404-3 2009; ISO 7404-5 2009, respectively). The petrographic observations and measurements were carried out using a Leica 4000 M microscope equipped with a Discus-Fossil system under standard conditions. The scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX) analyses were used to identify minerals and their mode of occurrence. The observations were performed on polished pellets and included secondary electrons and back scattered electrons detection modes, together with EDX analysis for the determination of qualitative chemical compositions. The SEM-EDX analysis were conducted using an FEI Quanta 400FEG environmental scanning electron

3. Materials and methods Samples for this study were collected in two of the three main outcrops in SSB, namely in Vale de Figueira and Jongeis areas (Fig. 1B). The sampling in SSB was only possible from outcrops and coal mining waste deposits and, therefore, it is expected that samples will be affected by weathering processes. In Vale de Figueira area samples from a coaly silt-claystone that hosts a coal seam (samples SS 1) and from two thin coal seams (samples SS 2, SS 3, SS 4) were taken (Fig. 3). In Jongeis area five coal samples from a thin impure coal seam (SS 7) and from mining waste deposits (SS 5, SS 6, SS 8, SS 9) were collected. The samples from mining waste deposits resulted from past 20th century mining activities in Jongeis mine and may be related with different coal seams of the SSB. In the sampled seams the coal occurs unconsolidated, which was already reported for SSB coal seams (Andrade 1927). 38

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Fig. 3. (A) and (B) Coal seams from Vale de Figueira where samples SS 2, SS 3 (A) and SS 4 (B) were collected; (C) Coal waste mining deposit in Jongeis where sample SS 8 was collected.

microscope (ESEM), equipped with a Genesis X4M energy dispersive Xray (EDAX) analyser. The methodologies used for the geochemical characterization of the coal samples included: (1) proximate analysis for the determination of ash and volatile matter yields in accordance with ISO standards (ISO 1171 2010; ISO 562 2010); (2) ultimate analysis for determination of carbon (C), hydrogen (H), nitrogen (N) and total sulphur (St) in a LECO S-2000 and LECO S-632 apparatus; and (3) inductively coupled plasma mass spectrometry (ICP-MS). The ICP-MS analyses were performed by Bureau Veritas Mineral Laboratories (Canada), for the determination of the inorganic composition of major, minor and trace elements in the studied samples. After ignition at 550 °C followed by acid digestion with an acid solution of (2:2:1:1) H2O-HF-HClO4-HNO3 the samples were analysed by the ICP-MS.

Table 1 Petrographic composition and vitrinite random reflectance of the studied samples from SSB (SS 1 to SS 4 – samples from Vale de Figueira; SS 5 to SS 9 – samples from Jongeis). Sample

MM (%)

V (%, mmf)

L (%, mmf)

I (%, mmf)

VR (%)

SS SS SS SS SS SS SS SS SS

97 45 3 7 51 67 40 58 73

– 82 98 99 22 6 100 2 100

– 0 1 0 0 0 0 0 0

– 18 1 1 78 94 0 98 0

– 0.91 0.98 0.90 1.19 1.16 0.95 1.19 1.25

1 2 3 4 5 6 7 8 9

MM – mineral matter; V – vitrinite; I – inertinite; L – liptinite; mmf – mineralmatter-free basis; VR – vitrinite random reflectance; N – number of measurements.

Fig. 4. Photomicrographs of sample SS 1. (A) Oxidized vitrinite particle. (B) to (D) Vitrinite particles (detrovitrinite – dv and collotelinite – ct) occurring within mineral matter and with visible signs of oxidation (development of cracks, pores). 39

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Fig. 5. Photomicrographs of sample SS 2. (A) Vitrinite (v) embedded in argillaceous lithic fragment and epigenetic iron oxides (ir) and framboidal pyrite (py). (B) Detrovitrinite (dv) embedded in mineral matter, collotelinite (ct), iron oxides (ir) and funginite (fg). (C) Vitrinite (v) and semifusinite (sf) embedded in argillaceous lithic fragment, and framboidal pyrite (py). (D) Vitrinite (v) embedded in argillaceous lithic fragment along with fusinite (f). (E) Sporinite (sp) under white light. (F) Sporinite (sp) under blue light exhibiting fluorescence.

4. Results

Inertinite is mainly represented by fusinite, semifusinite and inertodetrinite. Some particles of macrinite and funginite were also observed. Sporinite and resinite were observed within vitrinite particles only in samples from Vale de Figueira. The organic particles occur isolated and interbedded with mineral matter in all samples. The following sections present a description of the petrographic features of the analysed samples.

4.1. Petrographic characterization The results of maceral analyses (Table 1) indicate that the organic matter content in the samples is very variable and the mineral matter in generally high enough to classify some samples as carbonaceous shales (ISO 11760 2005). The mineral matter occurs mainly embedded with organic matter and includes detrital minerals (mainly quartz and clay minerals), iron oxides, oxidized and non-oxidized framboidal pyrite and carbonates. The iron oxides occur as groundmasses and occasionally filling some fractures. Sample SS 1 has a very high mineral matter content because it is a coaly silt-claystone. Samples SS 5, SS 6 and SS 8 stand out from the others due to their high mineral matter and inertinite contents when compared with the other samples; the petrographic observations reveal that these samples contain abundant epigenetic carbonates agglomerating smaller particles of inertinite and vitrinite. In the other samples (SS 2, SS 3, SS 4, SS 7 and SS 9), the organic matter, mainly composed of vitrinite and inertinite, is present in smaller amounts. Liptinite was rarely observed and only in samples from Vale de Figueira area. Figs. 4 to 12 show the main petrographic features of the studied samples. The petrography reveal that, generally, vitrinite particles include mostly collotelinite and macerals of detrovitrinite sub-group.

4.1.1. Vale de Figueira section samples The organic matter of samples from Vale de Figueira is mainly composed of vitrinite, as collotelinite and detrovitrinite, and inertinite, mainly as fusinite and inertodetrinite, both embedded in mineral matter. Semifusinite, funginite and macrinite were also observed. Particles of sporinite and rarely resinite (liptinite group) were observed within vitrinite. Mineral matter is essentially composed of clay minerals. Some iron oxides as well as oxidized and non-oxidized framboidal pyrite were also found (Figs. 4 to 7). Sample SS 1 (Fig. 4) is essentially composed of mineral matter (97%) with organic matter (3%, mainly vitrinite as collotelinite and detrovitrinite, and minor inertinite as inertodetrinite) evidencing some oxidation aspects such as cracks (Fig. 4A), development of porosity (Fig. 4C) and, rarely, reaction rims (Fig. 4C). The coal sample SS 4 (Fig. 7) is essentially composed of vitrinite that sometimes exhibit microcracks. The oxidation features observed in these samples are 40

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Fig. 6. Photomicrographs of sample SS 3. (A) Detrovitrinite (dv), collotelinite (ct) and inertodetrinite (id) embedded with mineral matter (mm). (B) Collotelinite (ct) with inertodetrinite (id) and resinite (r). (C) Collotelinite (ct) with macrinite (ma) and sporinite (sp), and iron oxides (ir). (D) Detrovitrinite (dv), collotelinite (ct), macrinite (ma) and resinite (r). (E) and (F) Collotelinite (ct) with sporinite (sp).

samples. Fig. 13 displays a general view (Fig. 13A) and detailed views (Fig. 13B and C) of sample SS 2 showing the occurrence of organic matter, aluminium-silicates, pyrite and iron oxides that were already described in previous sections of this work for this sample and others in petrographic observations. The EDX spectra Z1, Z2 and Z3 confirm their chemical composition. The SEM-EDX analysis of the epigenetic carbonates agglomerating the organic matter, which only occur in samples from Jongeis area, reveals dolomite (CaMg(CO3)2) and ankerite (Ca(Fe2+,Mg,Mn2+) (CO3)2). The SEM image in Fig. 14 shows different phases of the CaeMg carbonates (Z1 and Z2) and the organic matter (Z3). The EDX spectra Z1, Z2 and Z3 in Fig. 14 evidence their respective chemical composition.

attributed to the fact that the samples were collected in outcrops close to the surface. 4.1.2. Jongeis section samples Samples SS 5, SS 6 and SS 8 from Jongeis have significant amounts of epigenetic carbonates agglomerating vitrinite and inertinite particles. Inertinite and vitrinite are present essentially as detrital forms; however, particles of collotelinite, fusinite, semifusinite and macrinite also occur (Figs. 8, 9 and 11). In addition iron oxides occur filling fusinite vacuoles in samples SS 5 (Fig. 8); siderite and clay minerals interbedded with organic matter were observed in sample SS 6 (Fig. 9). The coal sample SS 7 (Fig. 10) consists of vitrinite particles (collotelinite and detrovitrinite) embedded in argillaceous mineral matter. The organic matter presents signs of oxidation such as microcracks. The oxidation of the organic matter is attributed to the fact that this sample was collected in a coal seam from an outcrop. Sample SS 9 (Fig. 12) is composed of argillaceous mineral matter with a very fine texture (73%) embedded with organic matter that includes mainly vitrinite (collotelinite and detrovitrinite). Fusinite, semifusinite and inertodetrinite were rarely observed.

4.3. Vitrinite random reflectance Vitrinite random reflectance of the studied samples varies between 0.90% and 1.25% (Table 1), indicating a bituminous coal rank with high vitrinite content (ISO 11760 2005). Samples from Jongeis (SS 5 to SS 9) present higher values of vitrinite reflectance (0.95%–1.25%) when compared with samples from Vale de Figueira (SS 1 to SS 4; 0.90%–0.98%). The thermal maturity of the SSB Carboniferous sequence was previously studied by other authors but the coal seams are poorly studied (Fernandes et al. 2016). In this work the vitrinite random reflectance

4.2. SEM-EDX analysis The SEM-EDX analysis corroborates the petrographic observations. Figs. 13 and 14 show SEM images as well as EDX spectra of selected 41

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Fig. 7. Photomicrographs of sample SS 4. (A) Fusinite (f). (B) Vitrinite with framboidal pyrite (py). (C) Collotelinite (ct) and semifusinite (sf). (D) Collotelinite (ct) and detrovitrinite (dv) with mineral matter, including oxidized framboidal pyrite (py).

[db] in sample SS 8 and 3.00 wt% [db] in sample SS 4; the N content ranges between 0.20 wt% [db] in sample SS 8 and 1.41 wt% [db] in sample SS 4; the St content is generally low, ranging from 0.06 wt% [db] in sample SS 6 to 0.52 wt% [db] in sample SS 4. Sulphur content may be represented by organic sulphur, sulphides and/or sulphates. The H/C ratio ranges between 0.24 in sample SS 8 and 1.26 in sample SS 2.

was determined in one coal sample from a thin coal lense (0.5 cm thick) in a shale from the old Jongeis coal mine from the LNEG Geological Museum stratigraphic collection. A value of 1.34% was determined in that coal sample, also indicating a bituminous coal rank (Fernandes et al. 2016). The same authors also studied other rock samples from the SSB Carboniferous sequence (from Jongeis (2 samples), Remeiras (1 sample) and Vale de Figueira (3 samples) outcrops), with vitrinite random reflectance varying between 1.44% and 1.69%. Vitrinite mean random reflectance was also determined in a sample from SSB carboniferous sequence intercepted by a borehole in Jongeis mining area (at the bottom), presenting a value of 1.31% (Fernandes et al. 2016). Vitrinite random reflectance of 1.51% was also measured in a coal sample from Moinho da Ordem mine (Jongeis area) by Lemos de Sousa et al. (2010). This value also indicates a bituminous coal rank. Lemos de Sousa et al. (2010) also report a high vitrinite content for that sample of Santa Susana coal.

4.4.2. Inorganic geochemistry Table 3 shows the concentration of the major, minor and trace elements determined in the studied samples. The mean values of the total samples and the mean values of samples from Jongeis and from Vale de Figueira were calculated and presented in the following table. The chemical composition established for worldwide hard coals composition (WCC) and the background of black shales (BBS) (Ketris and Yudovich 2009) were also included in Table 3 for comparison since the studied samples include coals and carbonaceous shales. Considering the major elements, the geochemical composition of samples is generally similar, with Al, Ca, Fe, and K being the most abundant elements (> 1%), followed by lower concentrations of Mg, Na, P, S, and Ti (< 1%). However, Ca and Mg exhibit some geochemical differentiation between samples from Jongeis and Vale de Figueira; these elements have significantly higher concentration in the majority of samples from Jongeis (SS 5, SS 6 and SS 8), which is associated with the presence of epigenetic carbonates that were identified during petrographic studies (Figs. 8, 9 and 11). Fig. 15 illustrates the ratio of trace elements between samples from Vale de Figueira and Jongeis (Celem Vale de Figueira/Celem Jongeis; for each element). For each element, a value of this ratio higher than 1 indicates higher concentration in samples from Vale de Figueira; a value of this ratio lower than 1 indicates that the elements has higher concentration in samples from Jongeis. The geochemical composition in trace elements is more or less similar between the samples from each area. However, the concentration of trace elements is generally slightly higher in samples from Vale de Figueira. The results show that besides Ca and Mg, the trace elements Mn, Se, Te, Y and HREE have higher concentrations in samples from Jongeis.

4.4. Geochemical composition 4.4.1. Organic geochemistry The results of the proximate and elemental analysis as well as the calorific value of the samples from SSB are given in Table 2. The results exhibit some heterogeneity among the samples, which is expected because the samples derive from different outcrops and coal seams, and show clear petrographic diversity. In addition, samples from coal waste mining residues collected in Jongeis may have different sources. The ash yield is 92.04 wt% in sample SS 1 – a coaly silty-claystone – and varies between 16.57 wt% and 77.63 wt% in the remaining samples, indicating a high proportion of mineral matter, which is the expected considering the nature of the sampled coals in seams and in mining residue piles. Because most of these samples have high ash yields, the volatile matter content is from the content in organic matter and from inorganics (e.g. from carbonates identified in samples from Jongeis). The volatile matter varies between 32.77 wt% [db] in sample SS 8 and 8.74 wt% [db] in sample SS 9. The elemental analysis shows: the C content ranges between 10.72 wt% [db] in sample SS 6 and 61.45 wt% [db] in sample SS 4; the H content varies between 0.68 wt% 42

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Fig. 8. Photomicrographs of sample SS 5. (A) Inertinite (i) (fusinite fragments) agglomerated by epigenetic carbonates (c). (B) Particle of collotelinite (ct) and fusinite (f) and particles of detrital vitrinite (v) and inertinite (i) agglomerated by epigenetic carbonates. (C) Collotelinite (ct) and epigenetic carbonates (c). (D) Vitrinite (v) and inertinite (i) agglomerated by epigenetic carbonates. (E) Fusinite (f), macrinite (ma) and inertodetrinite (id) agglomerated by epigenetic carbonates. (F) Collotelinite (ct) and fusinite (f) filled by epigenetic iron oxides (ir).

be emphasized the significant enrichment in In (CCmean = 15) and Cs (CCmean = 11.7) in samples from Vale de Figueira. The Jongeis samples have more heterogeneous enrichment/depletion patterns, which is in accordance with its petrographic heterogeneity (Fig. 17). All samples are depleted in Bi, Cd and Se (as for Vale de Figueira samples) and significantly enriched in In (CCmean = 13.5) and Mn (CCmean = 18.9). The samples SS 5, SS 6 and SS 8, present more similarities between each other, probably due to the presence of abundant epigenetic carbonates in these samples. In samples SS 5 and SS 8, the majority of trace elements are depleted, which is related with their high ash yield. On the other hand, in samples SS 7 and SS 9 the majority of trace elements are enriched when compared with WCC (except As, Bi, Cd and Se). Considering that the studied samples include coals and carbonaceous shales, the concentration of trace elements was also compared with the range of concentrations established for BBS (Ketris and Yudovich 2009). The data indicate that the studied samples have trace elements' concentration within or lower than the values established for BBS. The exception is Mn and V that have higher concentration than the upper limit of BBS in some samples (Table 3).

Considering the samples from Vale de Figueira, the chemical composition of sample SS 4 stands out from the others because it has very low ash yield (16.57 wt%, db) and, consequently, has lower concentrations of the majority of major, minor and trace elements. Exceptionally, S, As, Cd, Mo, Mn, Ni and Pb have higher concentrations in this sample, indicating their possible association with organic matter (Table 3). The concentration coefficient (CC) of trace elements in the studied samples was calculated based on the values established for WCC by Ketris and Yudovich (2009). The CC measures the enrichment or depletion of elements and is calculated through the ratio between trace element concentration in studied coal samples and the respective value established for the WCC. The enrichment/depletion of the elements in the studied samples may be classified according with Dai et al. (2016): unusual enrichment CC > 100; significantly enriched CC > 10; enriched 5 < CC < 10; slightly enriched 2 < CC < 5, within the average for world hard coals 0.5 < CC < 2; depleted CC < 0.5. Figs. 16 and 17 illustrate the enrichment or depletion of trace elements in samples from Vale de Figueira and Jongeis, respectively, comparatively with WCC. The samples from Vale de Figueira are clearly enriched in almost all the trace elements when compared with WCC (Fig. 16), with exception to Bi, Cd, Mo, Se and W which are depleted in relation to the WCC. This fact should be related with the high ash yield of these samples. Other elements such as Hf, Nb, Sn, Sr, Ta, Tl, W, Zr are depleted only in sample SS 4, that has the lower ash yield. It also should 43

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Fig. 9. Photomicrographs of sample SS 6. (A) Fusinite (f) and inertodetrinite (id) (fusinite fragments) agglomerated by epigenetic carbonates (c). (B) Particles of detrital vitrinite (v) and inertinite (i) agglomerated by epigenetic carbonates (c). (C) Collotelinite (ct) and particles of detrital vitrinite (v) and inertinite (i) agglomerated by epigenetic carbonates (c). (D) Collotelinite (ct) and detrovitrinite (dv) embedded with argillaceous mineral matter. (E) Fusinite (f) embedded with mineral matter and siderite (si). (F) Semifusinite (sf) embedded with mineral matter and siderite (si).

5. Discussion

processes, except the samples collected in outcropping coal seams (SS 4 and SS 7) that are more oxidized. The presence of the detrital minerals and the absence of marine organic matter in the studied samples is in agreement with the continental facies established for these coals, most likely fluvial/lacustrine in the upper unit of SSB (Machado et al. 2012). The classification of the organic matter based on H/C and O/C atomic ratios (van Krevelen 1993) distinguishes the three main types of kerogen. Likewise, the atomic ratio H/C calculated for the samples is in agreement with the nature and origin of the organic matter present in the studied samples, which indicate a type III kerogen, characteristic from higher plants and associated with terrestrial inputs into lacustrine or marine settings. Three of the Jongeis samples (SS 5, SS 6 and SS8) present significant carbonate content, which are clearly epigenetic since they fill cleats, fractures and, principally, they agglomerate the organic matter (Figs. 8A, C, D, 9A and 11C). The SEM-EDX analysis demonstrated that these carbonates are dolomite and ankerite (Fig. 14 – spectrum Z1 and Z2). The dolomite and ankerite forms a solid-solution series (Palache et al. 1951) and both are known to be the principal carbonate minerals occurring associated in coals along with siderite (Ward 2016). Secondary or epigenetic phases, occurring after burial and rank evolution, may include cleat, fracture and pore infillings of phases such as dolomite, ankerite and siderite, as well as pyrite, kaolinite, illite and chlorite (Ward 2016 and references therein). The samples from Jongeis with significant proportion of carbonates present high concentration of Ca, Mg, and Mn, comparatively with the

5.1. Petrographic and geochemical characterization The maceral constituents observed in the studied samples, mainly vitrinite particles in the coal samples collected in outcrop seams (samples SS 4 and SS 7), reveal signs of in situ weathering, mainly development of microcracks and lower reflectance than the remaining samples (Figs. 7 and 10; Table 1). The occurrence of iron oxides and oxidized framboidal pyrite can also be associated with weathering processes The samples SS 4 and SS 7 also present unaltered organic particles but the other studied samples are clearly less affected by weathering. The comparison of the vitrinite reflectance values obtained in the studied samples with data from previous studies (Lemos de Sousa et al. 2010; Fernandes et al. 2016) reveal lower values of vitrinite reflectance. The differences are attributed to the weathering processes that affected the studied samples. Despite the differences, all values indicate a bituminous coal rank for Santa Susana coals (ISO 11760 2005). The in situ weathering of coal implies natural oxidation taking place at low temperature leading to changes in the organic and inorganic components of coals (Ingram and Rimstidt 1984; Swaine and Goodarzi 1995; Taylor et al. 1998; Kus et al. 2017). Considering the effects (nature and extent) of the natural weathering of coal described in the literature (e.g. Taylor et al. 1998; Kus et al. 2017), it is considered that the studied samples are relatively poorly affected by weathering

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Fig. 10. Photomicrographs of sample SS 7. (A) to (C) Vitrinite particles (detrovitrinite – dv and collotelinite – ct) isolated or occurring embedded in mineral matter and with visible signs of oxidation (development of cracks). (D) Vitrinite and inertinite (i) occurring embedded in mineral matter (clay minerals).

Fig. 11. Photomicrographs of sample SS 8. (A) Collotelinite (ct), semifusinite (sf) and fusinite (f). (B) Semifusinite (sf) and macrinite (ma). (C) Inertodetrinite (id) agglomerated by epigenetic carbonates (c) and collotelinite (ct) with detrovitrinite (dv) and inertodetrinite (id). (D) Fusinite (f).

other iron oxides observed in this sample using optical microscope (Fig. 9). The common presence of carbonate minerals in samples collected in Jongeis area could be related to the circulation of Ca-Mg-Mnrich hydrothermal solutions within the studied coal seams during epigenetic stages. The geological mapping of Jongeis area (Almeida et al. 2006; Oliveira et al. 2007) emphasizes the presence of carbonate rocks

samples without significant carbonate content (SS 7 and SS 9). This fact is in accordance with the EDX analysis, which allowed the identification of dolomite (CaMg(CO3)2) and ankerite (Ca(Fe2+,Mg, Mn2+)(CO3)2) as the epigenetic carbonates occurring in samples from Jongeis (Fig. 14). Furthermore, the concentration of Fe is also noticeable high in sample SS 6 which could be related with the presence of siderite and 45

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Fig. 12. Photomicrographs of sample SS 9. (A) to (C) Collotelinite (ct), gelinite (g) and detrovitrinite (dv) embedded with argillaceous mineral matter. (D) Fusinite (f) embedded with argillaceous mineral matter.

Fig. 13. SEM images and EDX spectra of sample SS 2. (A) General view of sample SS2 showing the occurrence of organic matter (OM), aluminium-silicates and iron oxides (Ir). (B) Detailed view of a framboidal pyrite (EDX spectrum Z1) and aluminium-silicates (EDX spectrum Z2). (C) Detailed view of iron oxides (EDX spectrum Z3).

may be due to post-depositional processes that included carbonate fluid circulation, promoting the enhancement of the thermal maturity, as reported in other works (Taylor et al. 1998; Hower and Gayer 2002; Suárez-Ruiz and Crelling 2008). The higher maturity determined in Jongeis is consistent with data obtained in this study and data from Fernandes et al. (2016).

along the dextral shear zones which bounds the SSB (Fig. 1B). These carbonate rocks could result either by the shear zone activity, accompanied by carbonate fluid circulation, or by the presence of carbonaterich hydrothermal fluids related with later hydrothermal activity along the shear zones. Therefore, the slightly higher organic matter maturity and the occurrence of epigenetic carbonates in samples from Jongeis 46

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Fig. 14. SEM image and EDX spectra of sample SS 5 showing the occurrence of dolomite (spectrum Z1), ankerite (spectrum Z2) and organic matter (spectrum Z3). Table 2 Results of proximate and elemental analysis, and calorific value measurements of the studied samples from SSB (SS 1 to SS 4 – samples from Vale de Figueira; SS 5 to SS 9 – samples from Jongeis). Samples

SS1 SS2 SS3 SS4 SS5 SS6 SS7 SS8 SS9

Ash

VM

C

H

N

St

Calorific value

%, [db]

%, [db]

%, [db]

%, [db]

%, [db]

%, [db]

kcal/kg [db]

92.04 77.63 71.46 16.57 35.59 74.98 51.93 40.74 70.77

– 10.51 11.98 29.60 27.82 20.67 20.89 32.77 8.74

– 13.84 22.61 61.45 45.86 10.72 36.22 33.60 22.40

– 1.46 1.92 3.00 1.87 0.91 2.00 0.68 1.53

– 0.43 0.63 1.41 0.78 0.23 0.99 0.20 0.58

0.02 0.14 0.23 0.52 0.22 0.06 0.25 0.10 0.19

– 1020 1692 4979 3515 533 2761 1791 1623

H/C

– 1.26 1.01 0.58 0.49 1.01 0.66 0.24 0.81

VM – volatile matter; C – carbon; H – hydrogen; N – nitrogen; St – total sulphur; [db] – dry basis.

insights about the source of the elements and about some of the changes that occurred during coalification (Orem and Finkelman 2003). The mode of occurrence of elements in the studied samples was explored using the Pearson's correlation coefficients. A Pearson correlation is a number between −1 and 1 that indicates the extent to which two

5.2. Elements mode of ocurrence The trace elements content in coal depends on coal forming environments and/or changes during coalification (Saxby 2000). Therefore, the mode of occurrence of the elements in coal can also provide 47

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Table 3 Inorganic geochemical composition of samples from SSB and data from geochemical composition of worldwide hard coal (WCC) and background of black shales (BBS) for comparison (Ketris and Yudovich 2009). Vale de Figueira (VF)

Al Ca Fe K Mg Na P S Ti Ag As Ba Be Bi Cd Co Cr Cs Cu Ga Hf In Li Mn Mo Nb Ni Pb Rb Re Sb Sc Se Sn Sr Ta Te Th Tl U V W Y Zn Zr REE LREE HREE

%

ppm

ppm

Jongeis (J)

SS 1

SS 2

SS 3

SS 4

SS 5

SS 6

SS 7

SS 8

SS 9

9.60 0.35 4.75 2.64 0.87 0.38 0.09 0.02 0.31 2.80 32.8 471 3.00 0.38 0.01 25.7 134 7.00 59.2 27.6 2.97 0.07 65.4 740 0.94 4.62 94.7 13.6 73.3 0.00 1.24 20.2 0.15 3.60 92.0 0.40 0.09 10.3 0.80 6.20 166 0.60 12.8 69.1 111 126 110 16.1

10.8 0.07 2.63 2.50 0.28 0.30 0.02 0.05 0.26 4.04 15.8 575 3.00 0.53 0.02 13.0 85.0 17.7 47.1 28.4 2.52 0.05 99.3 496 1.16 5.90 51.2 12.8 138 0.01 3.17 22.4 0.15 3.40 122 0.40 0.09 11.8 0.94 4.90 142 0.80 8.40 24.6 88.0 106 94.5 11.0

9.75 0.06 1.64 2.35 0.37 0.29 0.01 0.02 0.29 1.51 12.1 476 3.00 0.48 0.07 14.8 80.0 17.5 51.8 27.8 2.25 0.09 102 312 1.69 7.12 69.1 22.3 124 0.03 1.66 19.0 0.15 3.80 93.0 0.50 0.08 9.60 0.85 4.60 135 1.10 8.80 50.7 84.2 90.9 80.1 10.8

2.81 0.29 3.23 0.56 0.15 0.08 0.01 0.29 0.08 1.12 22.8 117 2.00 0.18 0.31 21.9 27.0 9.10 22.3 7.51 0.63 0.03 17.4 782 5.98 2.08 116 23.6 34.3 0.00 1.66 7.40 0.15 0.90 42.0 0.20 0.03 3.40 0.27 3.10 45.0 0.50 7.00 50.1 24.7 46.8 41.0 5.80

1.04 12.6 2.29 0.29 4.23 0.05 0.06 0.17 0.05 0.53 10.3 101 0.50 0.05 0.02 4.40 9.00 1.80 6.70 3.03 0.52 0.02 7.20 2061 1.02 1.55 10.2 10.4 17.1 0.00 1.52 1.90 0.30 0.60 182 0.10 1.34 1.90 0.36 1.70 13.0 0.20 11.0 10.5 19.8 40.8 33.6 7.15

7.36 4.55 7.26 0.89 1.69 0.11 0.06 0.02 0.42 1.00 12.2 181 2.00 0.19 0.03 14.8 83.0 1.50 44.7 20.2 2.19 0.07 55.1 2849 0.80 5.07 48.3 7.50 11.6 0.00 0.69 14.4 0.15 2.40 60.0 0.40 0.10 3.70 0.38 2.50 117 0.70 23.0 45.8 78.4 96.9 76.1 20.8

5.28 1.40 1.15 1.27 0.48 0.04 0.01 0.23 0.17 0.88 4.00 209 3.00 0.20 0.06 5.90 25.0 7.60 20.5 14.3 3.13 0.05 13.5 39.0 3.66 5.69 55.5 21.7 60.1 0.00 1.80 11.8 0.50 2.80 109 0.50 0.03 9.90 0.45 4.40 63.0 1.20 25.3 28.7 108 144 123 20.8

0.37 10.99 1.77 0.07 5.04 0.01 0.00 0.29 0.01 0.72 1.30 23.0 1.00 0.02 0.06 4.60 3.00 0.70 3.30 1.33 0.25 0.05 2.40 1646 1.62 0.53 5.10 5.01 4.20 0.00 0.36 1.90 0.40 0.30 79.0 0.05 0.83 0.70 0.13 7.90 6.00 0.10 14.1 10.2 8.70 28.1 20.0 8.10

10.61 0.24 2.30 3.68 0.59 0.09 0.04 0.02 0.36 0.36 6.40 438 4.00 0.40 0.01 10.3 47.0 26.5 25.8 31.5 5.10 0.08 54.0 102 0.43 11.8 31.6 8.72 172 0.00 0.94 19.1 0.15 5.70 93.0 1.00 0.03 15.2 1.14 4.70 131 1.30 24.1 30.9 182 175 151 24.7

Mean (n = 9)

Mean VF (n = 4)

Mean J (n = 5)

WCC

BBS

6.41 3.40 3.00 1.58 1.52 0.15 0.03 0.12 0.21 1.44 13.1 288 2.39 0.27 0.07 12.8 54.8 9.93 31.3 17.9 2.17 0.06 46.3 1003 1.92 4.93 53.5 13.9 70.5 0.00 1.45 13.1 0.23 2.61 96.9 0.39 0.29 7.39 0.59 4.44 90.9 0.72 14.9 35.6 78.3 94.9 81.0 13.9

8.25 0.19 3.06 2.01 0.42 0.26 0.03 0.10 0.23 2.37 20.9 410 2.75 0.39 0.10 18.9 81.5 12.8 45.1 22.8 2.09 0.06 71.1 583 2.44 4.93 82.7 18.1 92.3 0.01 1.93 17.3 0.15 2.93 87.3 0.38 0.07 8.78 0.72 4.70 122 0.75 9.25 48.6 76.9 92.3 81.4 10.9

4.93 5.96 2.95 1.24 2.41 0.06 0.04 0.15 0.20 0.70 6.84 190 2.10 0.17 0.04 8.00 33.4 7.62 20.2 14.1 2.24 0.05 26.4 1339 1.51 4.94 30.1 10.6 53.1 0.00 1.06 9.82 0.30 2.36 105 0.41 0.46 6.28 0.49 4.24 66.0 0.70 19.5 25.2 79.4 97.0 80.6 16.3

n.a n.a n.a n.a n.a n.a 0.25 n.a 0.89 0.1 9.0 150 2.0 1.1 0.2 6.0 17.0 1.1 16.0 6.0 1.2 0.004 14.0 71.0 2.1 4.0 17.0 9.0 18.0 n.a 1.0 3.7 1.6 1.4 100 0.3 n.a 3.2 0.6 1.9 28.0 1.0 8.2 28.0 36.0 60.2 51.6 8.60

n.a n.a n.a n.a n.a n.a n.a n.a n.a 0.4–2.4 10–80 270–800 1.0–3.0 0.0–4.0 2.0–12 10–30 50–160 2.0–7.0 35–150 9.0–25 2.5–6.0 n.a 15–50 200–800 6.0–60 7.0–15 40–140 10–40 40–120 0.2–3.5 2.0–11 7.0–20 3.0–30 2.0–10 100–300 0.5–1.0 1.3–3.0 4.0–11 0.5–10 4.0–25 100–400 0.0–15 15–40 60–300 60–190 83.8–260 75–241 8.8–19.4

n.a – data not available. Fig. 15. Ratio between the concentration of trace elements of samples from Vale de Figueira vs. Jongeis.

elements were differentiated according to their correlation coefficients with the ash content:

metric variables are linearly related. Therefore the results of Pearson's correlation may provide information on the organic and inorganic affinities of the elements (Gűrdal 2008). Elements displaying significant positive correlation with ash yield (rash > 0.5) are inorganically associated while elements with significant negative correlation with ash yield (rash < −0.5) are believed to be organically associated. The positive correlation of elements with C content is also an indicator of the elements' organic affinity. Considering the above, three groups of

(i) elements with rash < − 0.50 have organic affinity; (ii) elements rash > + 0.50 have inorganic affinity; (iii) elements with – 0.50 < rash < + 0.50 have intermediate affinity (organic and inorganic).

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Fig. 16. Enrichment factors of trace elements in samples from Vale de Figueira vs. WCC.

Fig. 17. Enrichment factors of trace elements in samples from Jongeis vs. WCC.

Fig. 18. Cross-plot correlation of the Pearson's correlation coefficients of elements versus S and Al.

outcropping SSB carboniferous sequence. The correlation coefficients of elements with respect to Al, S, and Ca was used to represent the major aluminium-silicate, organic and carbonate affinities, respectively. Figs. 18 and 19 display the intercorrelation analysis of the major and trace element concentrations in the samples considering Al, S and Ca as normalizing elements for the affinities referred above. The intercorrelation plot between Al and S (Fig. 18) allows the identification of several elements with affinity with aluminium-silicates, while Cd, Mo and Se display affinity with organic matter. The intercorrelation plot between Al and Ca (Fig. 19) also allows the identification of the elements with affinity with aluminium-silicates, corroborating the above. In addition, Mg, Mn and Te display affinity with carbonates. The association of Ca, Mg and Mn with the carbonates phases was also recognized in SEM-EDX analysis. The geochemical composition of samples reveals a significant

The results demonstrate that the ash yield has a significant positive correlation (rash > 0.5) with the majority of elements (Al, Fe, K, Na, P, Ti, Ba, Be, Bi, Cr, Cu, Ga, Hf, In, Li, Nb, Sc, Sn, Ta, Th, Tl, V, Zn, Zr, REE), which indicates that these elements are preferentially associated with mineral matter. On the other hand, the elements S, Cd, and Mo have organic affinity (rash < −0.5). The positive correlation between C and S, Cd, Mo, Pb, and Se and the negative correlation of these elements with ash yield corroborates their association with organic matter, as indicated by Finkelman et al. (2018). The strong positive correlation between Ca and Mg (rCa-Mg = 0.97), Mn (rCa-Mn = 0.70) and Te (rCaTe = 0.95) indicates their association with carbonate phases. The Fe does not reveal any significant positive or negative correlation indicating that Fe is mainly associated with iron oxides and is not associated with sulphides, clay minerals or with organic matter, as it is common (Ward 2016). The presence of iron oxides in the studied sample is related with the weathering processes affecting the 49

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Fig. 19. Cross-plot correlation of the Pearson's correlation coefficients of elements versus Ca and Al.

The slightly higher organic matter maturity and the occurrence of epigenetic carbonates in samples from Jongeis may be related to postdepositional processes that included Ca-Mg-Mn-rich fluids circulation that may have promoted the differentiated enhancement of the thermal maturity of the basin during epigenetic stages. The geochemical composition of the studied samples exhibits some heterogeneity, which is expected since the samples are from different outcrops and coal seams, with variable degree of oxidation/alteration. The higher concentration of Ca, Mg, and Mn in the majority of samples from Jongeis is related with the presence of epigenetic carbonates, as denoted by the strong positive correlation between Ca with Mg, Mn and Te. Considering the enrichment of trace elements, samples from Vale de Figueira are significantly enriched in Cs and In, while samples from Jongeis are significantly enriched in In and Mn. Indium is included in the list of critical raw material for EU and therefore it may be of economic interest. The investigation on the elements' mode of occurrence demonstrates that the majority of elements are preferentially associated with mineral matter, while C, S, Cd, Mo, Pb and Se present a significant organic affinity.

enrichment in Cs and In in samples from Vale de Figueira and in In and Mn in samples from Jongeis (to which is added the Cs in sample SS 9). As previously mentioned, these elements show affinities with inorganic matter, however the Mn is clearly related with the post depositional processes during SSB evolution. This element is related with Ca-Mg-Mn carbonate-rich hydrothermal fluids, which could be responsible by the distinctive thermal maturation of Jongeis section. In contrast, the In and Cs show an aluminium-silicate affinity and should be associated with detrital phases as clays minerals contained in coal samples. Indium is included in the list of critical raw material for EU (European Commission 2017) and therefore its concentration in coals from SSB may be of economic interest. Indium may have several uses but the principal one, which accounts for about 65% of the In consumed in industry, is as thin films of indium‑tin oxide for liquid crystal displays and also in semiconductors, in the form of indium phosphide (Jorgenson and George 2005). Indium may be associated with both the organic and inorganic matter of the coal (Eskenazy 1980). Inorganic matter, clay minerals in particular, act only as carriers of Indium. In contrast, organic matter concentrates In only to a small degree relative to other trace elements (Eskenazy 1980). Indium is a chalcophile element that tends to occur with the base metals Cu, Pb, Sn, Zn, and with Bi, Cd and Ag, principally associated with sulfide minerals of Zn (Jorgenson and George 2005). The Pearson's correlation coefficients indicate a positive significant correlation of In with Cu (rIn-Cu = 0.65), Sn (rIn-Sn = 0.77) and Zn (rIn-Zn = 0.50), and a negative correlation with C and S, indicating its clear association with mineral matter.

Acknowledgments The authors acknowledge the funding provided to Institute of Earth Sciences through the contracts UID/GEO/04683/2013 with FCT and COMPETEPOCI-01-0145-FEDER-007690. This work is a contribution to the project ALT20-03-0145-FEDER-000028, funded by Alentejo 2020 through the FEDER/FSE/FEEI.

6. Conclusions The organic matter of coal from SSB occurs with mineral matter and is mainly represented by vitrinite followed by small amounts of inertinite and rare liptinite. Vitrinite particles include collotelinite and macerals of detrovitrinite sub-group. Inertinite includes, mainly, fusinite, semifusinite and inertodetrinite and, rarely, macrinite and funginite. The presence of terrestrial-derived macerals and total absence of organic matter from marine environments is consistent with the continental facies established for these coals. In addition, the atomic ratio H/C indicates a type III kerogen, characteristic from higher plants and associated with terrestrial inputs into lacustrine or marine settings.

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