Volcanogenic Tonsteins from Bukit Asam Coalfield, South Sumatra Basin, Indonesia

1.1 Volcanogenic Tonsteins from Bukit Asam Coalfield, South Sumatra Basin, Indonesia

Ferian Anggara1*, Amanda A. Sahri2, Zain A. N. Asa2, D. Hendra Amijaya1

1Geological Engineering Department, Gadjah Mada University, Yogyakarta 52884, Indonesia 2Undergraduate program Geological Engineering Department, Gadjah Mada University, Yogyakarta 52884, Indonesia *Corresponding Address: [email protected]

In the Muara Enim Formation, at least 18 coal layers are found, however only five main coal seams layer in Bukit Asam coalfield e.g the A1 and A2 (Mangus) seams, B1 and B2 (Suban) seams as well as C (Petai) seams are considered economic to be mined. A major influence on the evolution of peat mires and coal formation in the Muara Enim Formation was volcanic activity during the deposition. Volcanic eruptions in the surrounding area produced ash which accumulated as claystone layers or tonsteins in the coal seams. Based on systematic field mapping, several tonsteins layers were founded and sampled in the research area, e.g. tonsteins in interseam layer of A1, A2, B1, B2 and C seam. This paper only focused on the tonstein in between B and C seam. The mineralogy of the tonsteins has been evaluated using thin section and qualitative X-ray diffraction (XRD) techniques. Tonstein in Bukit Asam coalfield, consisting mainly of kaolinite, quartz, dickite and calcite. The results suggest that these tonsteins were derived from acid volcanic ash fallout, which was subsequently altered into relatively kaolinitic rocks through hydrolysis and diagenesis in a peat-bog environment. The tonsteins in Muara Enim Formation are significance in providing distinctive horizons for stratigraphic correlation purposes.

Keyword: Tonstein, Bukit Asam Coalfield, Muara Enim Formation

1. Introduction Tonsteins are widespread clay bands or partings Tonsteins has been used widely in coal geology, as the alteration product of volcanic ash that e.g. as a tool for radiometric dating, identify associated with coal beds, contains dominantly paleo-depositional environments, and parameter glass from the original ash and phenocysts of to identify coalification, diagenesis and coal quartz, magnetite/ilmenite, kaolinite, smectite quality [1]. Nowdays, some tonstein may and other minerals [1]. Tonsteins layer were contain valuable trace elements that could be founded in almost tertiary coal basin in potential in searches for alkali ore-deposits [3]. Indonesia including Bukit Asam coalfield, South The presence of tonsteins also influence the Sumatera, Indonesia. Characteristics of tonstein composition of macerals in coals. Coals were in Bukit Asam coalfield based on [2] showed enriched in desmocollinite, tellinite and that Rb, Cs and major element K are highly detrocollinite at top of the tonstein layer as leached from tonstein while elements Ga and Ti result of poorly-drained swamp conditions, were immobile and residual in tonstein. while coals below the tonsteins layer were enriched in semifusinite, inertodetrinite and fusinite as result of well-drained swamp condition [4].

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1.1 Thus, study on tonsteins characteristics in the seams (Mangus), B seams (Suban) and C seams research area especially for stratigraphic (Petai). This unit dominantly consists of coal, correlation purpose is becoming very important dark grey to black shale, brownish – grey claystone and sandy claystone. While the upper 2. Geological Background unit consists of shale, coal, tuff, claystone and The Muara Enim Formation (MEF) was sandstone. The boundary between lower unit deposited during regressive period of a and upper unit is the top of A1 seam (upper transgressive-regressive cycle in the South Mangus). The age of this formation is Late Sumatera Basin [5]. The thickness of Muara Miocene – Early Pliocene [6]. Enim Formation is around 500 – 700m, about 15% of which is coal. Top and bottom of this unit are defined by the upper and lower occurrence of laterally continuous coal beds. The formation consists of stacked shallowing- upward parasequences, typically 10 – 30 m thick, with shallow marine or bay clays at the base, and shoreline and delta plain facies (sand, clay, coal) at the top, see Figure 1.

Figure 2. Measured section of West Banko coal field indicated samples used in this study.

Figure 1. South Sumatra Basin Stratigraphy [6] 3. Sample and Method Coals rank in the MEF of Bukit Asam coalfield Systematic field mapping has been conducted in are vary from sub-bituminous to semi-anthracite the Banko field and several tonsteins layers in as the result of igneous intrusion of andesitic interseam layer of A1, A2, B1, B2 and C seam composition [2]. The MEF can be divided into have been founded. In this paper, we only two units, the lower unit are the most economic focused on collecting and analyzing tonstein coal in Bukit Asam coalfield, consists of A samples in between B and C seam (Figure 2).

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1.1 There were 6 samples for thin section and XRD grain size of about 100 µm and a maximum of 1 (bulk powder and oriented clay) analysis. XRD mm. The type of this tonsteins based on [8] is analysis with oriented clay method (air dried, called pellet (graupen) tonsteins. The grains are ethylene glycol and heated) is very useful to dominant composed of feldspar as well as quartz analyze clay mineral that cannot be determined and opaque minerals in minor composition (Fig. by thin section analysis. 4). Most of the feldspar are altered to kaolinite. Quartz grains have low sphericity and subangular and indicate as a product of explosive volcanism rather than normal sedimentary processes. Kaolinite grains are mainly consist of fine-grained kaolinite with orientated inclusions, and appears isotropic or with weak aggregate polarization. This is suggests an original altered volcanic rock fragments or plagioclase feldspar.

Figure 3. Outcrop of tuffaceous coal (a) of C seam, claystone (b) and sandy tuff (c) tonstein layers of interburden B1-B2 seam on the Banko field.

4. Result 4.1 Systematic Field Mapping Based on Figure 2, there are very thin layer of tuffaceous coal (sample TS-1 and TS-3) on the top of C seam. The thickness of the two layers are 5 cm to 15 cm, respectively. Thin clean coal seam in between those layer is also sampled and analyzed and called as TS-2 sample. Megascopically, TS-1 and TS-3 samples are light gray in colour and the roof this layer is sandstone. TS-2 is dull banded friable coal.

Claystone (TS-4), coal (TS-5) and sandy tuff (TS-6) in between B2 and B1 seam are sampled and analyzed. TS-4 is 20 cm to 25 cm, grey to the yellowish grey with laminated sedimentary structure. TS-5 is 25 cm, black, and dull banded. Sandy tuff is 20 cm, light grey with clay to sand grain size (Fig. 3)

Figure 4. (a) thin section of samples TS-3 (b) 4.2 Petrographic Analysis thin section of samples TS-1. (c) quartz as Tonsteins samples are examined under the filling fractures in samples TS-6. Opq = opaque microscope and shows granular with a median mineral, Fsp = feldspar, Qz = quartz

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1.1 macerals and megascopic characteristics of the plies immediately below and above the tonsteins in the adjacent coal layers. The plies below the tonstein generally have a ~10 cm of bright banded coal, have higher liptinite, inertrinite, detrovitrinite, and detrital macerals higher mineral content. On the other hand, the plies above tonstein show less convincing trends. In some areas, samples from this plies contain less detrital macerals than the coals immediately above but on the other hand, some of ply Figure 5. XRD of samples TS-3 (a), TS-2 (b) samples have higher. and TS-1 (c). K = kaolinit, Q = quartz

6. Conclusion A major influence on the evolution of peat mires and coal formation in the MEF was volcanic activity during the deposition. Volcanic eruptions in the surrounding area produced ash Figure 6. (a) XRD of samples TS-6 (a) and TS-4 which accumulated as tonsteins in the coal (b). K = kaolinit, Q = quartz, D = dickite seams. The study of the tonsteins in Bukit Asam coalfield has revealed their macroscopic 4.3 XRD Analysis characteristic such as have 15 to 20 cm in thick, The results show that kaolinite is the dominant light colored and mostly composed of kaolinite mineral found in XRD analysis, with only small as well as quartz, dickite, feldspar, and calcite as amounts of other minerals (Fig 5 and 6). Based minor constituents. Based on the mineralogical on Figure 5 and 6, tonsteins shows well-ordered composition, tonsteins in the research area were kaolinite. Spears [7] mentioned that tonsteins derived from acid volcanic ash fallout, which from an altered volcanic ash is characterized by was subsequently altered into relatively well-ordered kaolinite in XRD peak data. kaolinitic rocks. detrital maceral contents. Quartz, feldspars, dickite, and calcite are found as minor constituent. Within the distinctive character in term of megascopic characteristic as well as mineralogy 5. Discussion composition, tonsteins layers in the research The megascopic description and laboratory area is useful to be used as a stratigraphic analysis shows tonsteins in the reseach area is marker for correlation purposes. mostly composed of volcanic material. Altered feldspar and clay mineral are observed from thin section analysis. Based on XRD analysis, most References of the clay mineral is composed by well-ordered [1] Triplehorn, D.D., 1990. Applications of kaolinite that is indicated volacanic ash origin. tonsteins to coal geology: some example from western United States. International Previous study indicate that the presence of Journal of Coal Geology (16), pp: 157- tonsteins in coal influence the percentage of 160. maceral composition in coal seam [2]. The plies

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1.1 [2] Pujobroto, A.,1997. Organic Petrology and Geochemistry of Bukit Asam coal, South Sumatera, Indonesia. Doctor of Philosophy thesis, School Geosciences, University of Wollongong. [3] Dai, S., Wang, X., Zhou, Y., Hower, J.C., Li, D., Chen, W., Zhu, X., 2011. Chemical and mineralogical compositions of silicic, mafic, and alkali tonsteins in the Late Permian coals from Songzao Coalfield, Chongqing, Southwest China. Chem. Geol. 282, 29- 44 [4] Crowley, S.S., Stanton, R.W. and Ryer, T.A., 1989. The effects of volcanic ash on the macerals and chemical composition of the C coal bed, Emery Coalfield, Utah. Organic Geochemistry, 14, 105-156 [5] de Coster G.L., 1974. The geology of Central and South Sumatra Basins: Proceedings 3rd Annual Convention Indonesian Petroleum Association, pp: 77-110. [6] Darman, H., and Sidi, F.H., 2000. An outline of the geology of Indonesia. Indonesian Association of Geologists, Jakarta, 254 p. [7] Spears, D.A., 2012. The origin of tonstein, an overview, and links with seatearths, fireclays and fragmental clay rocks. International Journal of Coal Geology (94), pp: 22-31. [8] Diessel, C.F.K., 1992. Coal-Bearing Depositional Systems. Springer Berlin Heidelberg, Berlin, Heidelberg.

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