Distribution of Sulfur and Pyrite in Coal Seams From Kutai Basin

International Journal of Coal Geology 81 (2010) 151–162

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International Journal of Coal Geology

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Distribution of sulfur and pyrite in coal seams from Kutai Basin(East Kalimantan, Indonesia): Implications for paleoenvironmental conditions

Sri Widodo a,⁎, Wolfgang Oschmann b, Achim Bechtel c, Reinhard F. Sachsenhofer c,Komang Anggayana d, Wilhelm Puettmann e

a Department of Mining Engineering, Moslem University of Indonesia, Jln. Urip Sumoharjo, Makassar, Indonesiab Institute of Geosciece, J.W. Goethe-University, Altenhöferallee 1, D-60438 Frankfurt a.M., Germanyc Department of Applied Geoscience and Geophysics,University of Leoben, Peter-Tunner-Str.5, A-8700 Leoben, Austriad Department of Mining Engineering, Bandung Institute of Technology, Jln. Ganesa 10, I-40132 Bandung, Indonesiae Institute of Atmospheric and Environmental Sciences, Dapartment of Analytical Enviromental Chemistry, J.W. Goethe-University, Altenhöferallee 1, D-60438 Frankfurt a.M., Germany

⁎ Corresponding author. Tel.: +62 411 454775; fax: +E-mail address: [email protected] (S. Widodo).

0166-5162/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.coal.2009.12.003

a b s t r a c t

Article history:Received 12 August 2009Received in revised form 29 November 2009Accepted 3 December 2009Available online 13 December 2009

Keywords:Kutai BasinPyriteSulfurFramboidalOmbrogenousTopogenous

Thirteen Miocene coal samples from three active open pit and underground coal mines in the Kutai Basin(East Kalimantan, Indonesia) were collected. According to our microscopical and geochemical investigations,coal samples from Sebulu and Centra Busang coal mines yield high sulfur and pyrite contents as compared tothe Embalut coal mine. The latter being characterized by very low sulfur (b1%) and pyrite contents. The ash,mineral, total sulfur, iron (Fe) and pyrite contents of most of the coal samples from the Sebulu and CentraBusang coal mines are high and positively related in these samples. Low contents of ash, mineral, total sulfur,iron (Fe) and pyrite have been found only in sample TNT-32 from Centra Busang coal mine. Pyrite was theonly sulfur form that we could recognize under reflected light microscope (oil immersion). Pyrite occurred inthe coal as framboidal, euhedral, massive, anhedral and epigenetic pyrite in cleats/fractures. Highconcentration of pyrite argues for the availability of iron (Fe) in the coal samples. Most coal samples fromthe Embalut coal mine show lower sulfur (b1 wt.%) and pyrite contents as found within Centra Busang andSebulu coals. One exception is the coal sample KTD-38 from Embalut mine with total sulfur content of1.41 wt.%. The rich ash, mineral, sulfur and pyrite contents of coals in the Kutai Basin (especially CentraBusang and Sebulu coals) can be related to the volcanic activity (Nyaan volcanic) during Tertiary wherebyaeolian material was transported to the mire during or after the peatification process. Moreover, the adjacentearly Tertiary deep marine sediment, mafic igneous rocks and melange in the center of Kalimantan Islandmight have provided mineral to the coal by uplift and erosion. The inorganic matter in the mire might alsooriginate from the ground and surface water from the highland of central Kalimantan.

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1. Introduction

Most of the inorganic matter in coal is present as minerals whichare dispersed throughout the coal macerals. Individual grains ofminerals vary largely in size from less than one micrometre to tens orhundreds of micrometres. Sometimes mineral-rich layers are eventhick enough to be visible on the coal surface (Taylor et al., 1998).Mineral components in the coals were classified in three groupsaccording to their origin (Stach et al., 1975): (1) Mineral from theoriginal plants; (2) mineral that formed during the first stage of thecoalification process or which was introduced by water and wind intothe later coal deposits; and (3) mineral deposited during the secondphase of the coalification process, after consolidation of the coal, by

ascending or descending solutions in cracks, fissures, or cavities or byalteration of primarily deposited minerals.

The dominant mineral of coals is usually composed of sulfides,clay, carbonates, and quartz and sometimes additional phosphates,heavy minerals, and salts as minor contributions to inorganic matterof coal. In most coals, sulfides are preferentially composed of pyriteandmarcasite but pyrite is in general dominating by far (Balme, 1956;Mackowsky, 1943).

Sulfides can be categorized as either syngenetic (primary), early-diagenetic or epigenetic (secondary) in origin. During peatification,syngenetic or early-diagenetic fine-crystalline or fine-concretionarypyrite appears, commonly in the form of framboids. Syngenetic pyriteformed during accumulation of the peat and/or during early(humification) processes, and is usually small in size, and intimatelydispersed throughout the coal (Renton and Cecil, 1979; Reyes-Navarro and Davis, 1976). Occasionally, the cell walls of plant materialhave been replaced by pyrite (Taylor et al., 1998). Falcon and Snyman(1986) suggest that the accumulation of pyrite in coal might also arise

152 S. Widodo et al. / International Journal of Coal Geology 81 (2010) 151–162

from the aeolian and fluviatile import of iron-rich mineral at the timeof peat accumulation followed by in-situ precipitation. Epigeneticpyrite is incorporated in the coal after compaction or partialconsolidation (Reyes-Navarro and Davis, 1976) and is generallymuch larger (coarse grained) and fills cracks, cleats, and cavities(Renton and Cecil, 1979). The formation of epigenetic pyrite isdependent primarily on the availability of reduced sulfur, dissolvedcation (ferrous iron) and a suitable site for formation i.e., cleat(Casagrande et al., 1977; Spears and Caswell, 1986; Demchuk, 1992).Moreover, epigenetic pyrite might be precipitated from waterpercolating into fractures, cavities and pores present in coal seamslong after accumulation of the peat (Falcon and Snyman, 1986).

In general, coals deposited in paralic basins contain more pyritethan those in limnic basins. Among the paralic deposits, coal seamswhich have been influenced bymarine transgressions are consistentlycharacterized by a particularly high content of pyrite and sometimesalso of organic sulfur, especially in the upper part of the seams (Balme,1956; Dai et al., 2002; Mackowsky, 1943). In sulfur-rich humic coals,pyrite in the form of fine grains or fine concretions is particularlycommon in microlithotypes containing a high proportion of vitrinite;these forms also tend to be common in sapropelic coals. In the absenceof other criteria (such as marine fossils or coal balls), a relatively highproportion of synsedimentary or early-diagenetic pyrite can be usefulfor seam correlation.

Many previous investigations (Anggayana et al., 2003; Baruah, 1995;Dai et al., 2002, 2003, 2006, 2007, 2008; Dai and Chou, 2007; Elswicket al., 2007; Frankie and Hower, 1987; Kortenski and Kostova, 1996;Lόpez-Buendía et al., 2007; Querol et al., 1989; Renton and Bird, 1991;Strauss and Schieber, 1989; Turner and Richardson, 2004; Wiese andFyfe, 1986) have described the characteristics, type, morphology,genesis, and distribution of pyrite in coal seams from different deposits.Our investigation deals with sulfur and pyrite occurrences in theMiocene coal seams from Centra Busang, Sebulu and Embalut coalmines, Kutai Basins, East Kalimantan, Indonesia. The primary purpose ofthe study is to explain why most Miocene coal seams from Kutai Basinhave very low sulfur contents, whereas in some coal seams higher sulfurcontents are observed. The second purpose is to identify the types ofpyrite and factors affecting their appearance and it's relation withpaleoenvironmental conditions during deposition of the coals.

2. Geological setting

Kalimantan is surrounded by marginal basins of South China, CelebesandSulu seas,microcontinental fragmentsof southChina in thenorth, andmainland SE Asia (Indochina and peninsular Malaysia) in thewest (Mossand Chambers, 1999). Kalimantan was interpreted as the product ofMesozoic accretion of oceanic crustal material (ophiolite), marginal basinfill, island arcmaterial andmicrocontinental fragments onto the Paleozoiccontinental core of the SchwanerMountain in the SWof the island (Fig. 1;Hall andNichols, 2002;Hutchison, 1989;Moss andWilson, 1998;Widodoet al., 2009).

During early Tertiary times, Kalimantan formed a promontory ofthe Sundaland Craton: the stable eastern margin of the Eurasian plate(Hall, 1996; Metcalfe, 1998). In the east, Kalimantan is separated fromSulawesi by the deep Makassar Basins (Fig. 1), formed duringPaleogene times (Situmorang, 1982). The main areas of eastern,central and northern part of Kalimantan are coated by Tertiarysediments (Fig. 1) which were deposited in fluvial, marginal-marineor marine environments.

Tertiary sedimentation in these areas occurred at the same timewith, and subsequent to, a period of widespread Paleogene extensionand subsidence, whichmay have commenced in the middle Eocene orearlier (Moss and Wilson, 1998). A number of Tertiary sedimentarybasins, of which the Kutai Basin is the largest, are identified acrossKalimantan (Moss et al., 1997).

The island of Kalimantan and in particular the Kutai Basin hasexperienced a complex tectonic history from the Paleogene to thepresent day. The Kutai Basin was formed during early Tertiary timesand was filled-up with clastic sediments progressing from thewestern to the eastern part of the basin. This basin was subdividedinto the Upper Kutai Basin, consisting of Paleogene outcrops withCenozoic volcanics possessing a strong northwest–southeast struc-tural grain, and the Lower Kutai Basin with Miocene strata croppingout in a north–northeast-trending structure. The Meratus Mountainsto the southwest and the Central Kalimantan Mountains to the northof the Kutai Basin have an ophiolitic basement together withPaleogene strata striking dominantly in a north–northeast direction(Clay et al., 2000).

The coalmining companies are located in the vicinity of theMahakamRiver, Kutai Basin, East Kalimantan Province (Fig. 2). The precisegeographic position of Sebulu coal mine is S00°26′40.4″/E116°52′54.1″and Centra Busang coal mine is S00°44′22.2″/E116°89′16.6″, whereas theEmbalut coalmine is situated S00°33′34.9″/E117°12′15.5″. Centra Busangis located in theBusangvillage, East Kutai regency and Sebulu coalmine inthe Sebulu village, Kutai Kertanegara regency, East Kalimantan province.The Embalut coal mine is located in Embalut village, Kutai Kertanegararegency, East Kalimantan province. Coal seams in the Centra Busang andSebulu mines were found in the Pulau Balang Formation with MiddleMiocene age, and coal seams in the Embalut minewas found in the PulauBalang (Middle Miocene age) and Balikpapan Formation with UpperMiocene age (Fig. 3).

Previous studies of the sedimentary evolution of the Kutai Basin,based on field survey and oil wells, have shown that the Tertiarysequences are broadly regressive in general with a (dominantly)offshore marine argillaceous sequence of Palaeocene age followed bya coal bearing deltaic and coastal plain succession of Miocene age.Shoreline progradation was generally towards the east (Samuel andMuchsin, 1976; Rose and Hartono, 1978 in Land and Jones, 1987).

According to Supriatna and Rustandi (1986) the Neogenesuccession in the Kutai Basin includes from bottom to top thefollowing formations: Pamaluan, Bebuluh, Pulau Balang, Balikpapan,Kampung Baru Formation and alluvial. The Pamaluan Formation (upto 1500 m thick) consists of sandstones with insertion of claystone,shale, limestones, and siltstone. It formed in a deepmarine environmentduring Late Oligocene and Early Miocene times. The Lower MioceneBebuluh Formation (up to 900 m thick) consists of limestones withinsertion of sandy limestones and argillaceous shales. The deposition ofthe formation occurred in a shallow sea. The Bebuluh Formationinterfingers with the Pamaluan Formation. The Pulau Balang Formationof Early toMiddle Miocene age overlies the Bebuluh Beds concordantly.It is composed of graywackes, quartz sandstones, limestones, claystones,dacitic tuff and coal insertion. The thickness of coal seams ranges from3 to 4 m. The depositional environment can be characterized as deltaicto shallow marine according to Supriatna and Rustandi (1986). Theformation is approximately 900 m thick. The Middle to Upper MioceneBalikpapan Formation (1000–1500 m thick) uniformly overlies thePulau Balang Formation and consists of quartz sandstone, clay withinsertion of shale, and coal seams 5 to 10 m thick. The deposition of theBalikpapan Formation occurred in a delta environment. The UpperMiocene to Pliocene Kampung Baru Formation disconcordantly overliesthe Balikpapan Formation. It is composed of quartz sandstone withinsertion of clay, shale, silt and approximately 3 m thick coal (lignite).The deposition of the Kampung Baru Formation up to 900 m thick,sedimentary succession occurred in a delta. Alluvium deposit (alluvial)consists of sandy clay and clayey sands.

3. Samples and method

Coal sampleswere collected in-situwith channel samplingmethodfrom three active surface mines in the Centra Busang (3 samples),

Fig. 1. Simplified geology of Kalimantan (modified from Moss and Wilson 1998; Hall and Nichols, 2002).

153S. Widodo et al. / International Journal of Coal Geology 81 (2010) 151–162

Sebulu (2 samples), and Embalut (8 samples) coal mines, Kutai Basin,East Kalimantan, Indonesia.

The sample preparation and microscopic examination generallyfollowed the procedures described by Taylor et al. (1998). Coal

particles of about 1 mm in diameter were used for preparation ofpolished sections, which were embedded in a silicone mould(diameter 40 mm) using epoxy resin as an embedding medium.After hardening, the samples were ground and polished. Microscopic

Fig. 2. Geographic map showing the locations of Sebulu, Centra Busang and Embalut coal mines in the Kutai Basin, East Kalimantan, Indonesia.

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analyses were performed by a single-scan method with a Leica MPVmicroscope using reflected white and fluorescent light. At least 300points were counted for coal macerals and mineral. Pyrites werecounted separately from the other minerals (for example, clay,carbonate, quartz) which were counted together in one group.

Total sulfur contents were determined using an automated LecoSC-344 carbon sulfur analyzer. A weighed coal sample is mixed withiron chips and a tungsten accelerator and is then combusted in anoxygen atmosphere at 1370 °C. The moisture and dust are removedfrom the combustion product and the SO2 gas is measured by a solid-state infrared detector.

Fig. 3. Sedimentary sequences and the distribution of some fundamental parameters of the

Ash yields were determined following standard procedure DIN51719 using dry coal samples. One gram of each coal sample is heated2 h to 815 °C (±15 °C) in a muffel furnace, the residue was thencooled to room temperature and weighed.

For trace element analyses, carried out by ACTLABS, Ancaster,Canada, the coal ash (0.5 g) was dissolved in aqua regia (0.5 ml H2O,0.6 ml concentrated HNO3 and 1.8 ml concentrated HCl). Aftercooling, samples were diluted to 10 ml with deionized water andhomogenized. The digestion is near total for base metals, but will onlybe partial for silicates and oxides. The solutions were then analyzedusing a Perkin Elmer OPTIMA 3000 Radial ICP-MS for the 30 elements

Embalut, Centra Busang and Sebulu coalfield, Kutai Basin, East Kalimantan, Indonesia.

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suite. A series of USGS-geochemical standards were used as controls.The iron (Fe) is one of the elements to be discussed in this paper.

4. Results and discussion

4.1. Ash, mineral, total sulfur and iron (Fe) contents

Ash yields, minerals, total sulfur, and Fe contents in the coalsamples from Centra Busang, Sebulu and Embalut coal mines aresummarized in Table 1 and cross-correlation are shown in Fig. 4. Theash yield of the Centra Busang and Sebulu coals varies from 1.40 to5.80 (wt.%, db). The Embalut coal samples show higher variability inash yields (1.33 to 8.98 wt.%, db). Four coal samples from Embalutyield more than 2 wt.% (db) ash (Table 1) while ash contents lowerthan 2 wt.% (db) are found in the samples KTD-36, KTD-35, KTD-43,and KTD-37 (Table 1). The ash contents of Cetra Busang and Sebulucoals show a positive correlation to the total sulfur+Fe (r2=0.83).Embalut coals show also a positive correlation between ash contentsto the total sulfur+Fe (r2=0.77).

The mineral contents of Centra Busang and Sebulu coals vary from0.70 to 5.60 (vol.%). Mineral is predominantly composed of pyrite. Clayand carbonates are also found in a significant quantity. Quartz isobserved only in a trace proportion. In the coal samples from CentraBusang coal mine, mineral contents vary from 3.70 vol.% (TNT-30, seamBL-4) over 1.60 vol.% (TNT-31 BL-7) to 0.70 vol.% (TNT-32, BL-7.1). Thecoal samples from Sebulu coal mine show mineral contents between2.3 vol.% (TNT-33) and 5.6 vol.% (TNT-34). The mineral contents ofCentra Busang and Sebulu coals showa strongpositive correlation to theash contents (r2=0.96; Fig. 4). Unlike to the Centra Busang and Sebulucoals, mineral in the Embalut coals is dominated by clay minerals.Carbonate, pyrite and quartz are found in small amounts. The mineralcontents of Embalut coals vary from 0.30 to 2.0 (vol.%). Most of mineralcontents are lower than 2 vol.%. The mineral contents (from micro-scopical analysis) of Embalut coal mine show a negative correlation tothe ash contents in the coals (r2=0.05, Fig. 5).

The studied coal samples from the Centra Busang and Sebulu coalmines have relatively high total sulfur contents (up to 3.15 wt.%;Table 1). Total sulfur contents show a positive correlation to the ashcontents in the samples (r2=0.67; Fig. 4). On the other hand, mostsamples from the Embalut coal mine show very low total sulfurcontents (≤0.2 wt.%). Sole exception is the coal sample KTD-38 fromPulau Balang Formation with a total sulfur content of 1.41 wt.%. Thecorrelation of total sulfur to the ash contents in the Embalut coal minealso shows a positive correlation (r2=0.84, Fig. 5).

Iron (Fe) contents in the samples from the Centra Busang and Sebulucoalmines fall within the range of 0.17 to 1.51 wt.%. As shown in Table 1and Fig. 6, the iron (Fe) contents in the coal samples fromCentra Busangand Sebulu coal mines show a strong positive correlation to the pyrite

Table 1Ash yield, minerals (from microscopical analysis), total sulfur, pyrite (from microscopical a

Samples Seams Formations Coal mines As(w

KTD-42 21 Balikpapan Embalut 2.6KTD-36 18 1.3KTD-35 17 1.6KTD-43 12 1.6KTD-40 10 5.0KTD-39 9 2.8KTD-38 8 Pulau Balang 8.9KTD-37 7 1.7TNT-30 BL-4 Pulau Balang Centra Busang 3.5TNT-31 BL-7.9 1.9TNT-32 BL-7.10 1.4TNT-33 ST Pulau Balang Sebulu 2.1TNT-34 STU 5.8

contents and total sulfur. The correlation coefficients are r2=0.96 (Fe vspyrite contents), and r2=0.86 (Fe vs total sulfur). Whereas, iron (Fe)contents of Embalut coal samples are low and range from 0.15 to 1.32%.Highest content are found in the sample KTD-38 with a value of 1.32%.Fig. 7 also shows a positive correlation of iron (Fe) contents to the pyritecontents and total sulfur in the coal samples from Embalut coal mine.The correlation coefficients are r2=0.97 (Fe vs pyrite contents) andr2=0.99 (Fe vs total sulfur).

The sum of total sulfur and iron (Fe) is close to the amount of ashfor Centra Busang and Sebulu coals (Table 1). This indicates that alarge proportion of the mineral in the coal samples must be composedof pyrite. In the two samples (TNT-30 and TNT-33) the sum of thetotal sulfur content and the Fe content exceeds the ash content. Inthese samples some of the sulfur must be organically bond sulfur,which evaporates together with the organic matter during heating.The lowest total sulfur content (0.14 wt.%) of the Centra Busang andSebulu coal mines is observed in TNT-32, which provided a very lowash yield of 1.40 wt.%, db. This allows to estimate the non-pyriticmineral content to approximately 1 wt.%.

4.2. Pyrite content and pyrite types in the coal seams

Based on microscopical analyses, the amount of pyrite in theCentra Busang and Sebulu coal samples varies from 0.4 to 4.3 vol.%(Table 1). In comparison, most of pyrite contents of Embalut coalsamples are lower than in Centra Busang and Sebulu coals and pyriteis only observed in three samples (KTD-36, KTD-43 and KTD-38).Differences in the pyrite contents of coals from Centra Busang, Sebuluand Embalut coal mines are reflected by the type of pyrite found (i.e.framboidal, euhedral, massive, anhedral, and epigenetic/syngeneticpyrite in cleats and fractures).

4.2.1. Framboidal pyriteFramboidal forms of pyrite are categorized as syngenetic (Dai et al.,

2007). Some authors proposed that the type of this pyrite originatesfrom pyritization of sulfur bacteria (Casagrande et al., 1977;Casagrande et al., 1980; Kortenski and Kostova, 1996; Lόpez-Buendíaet al., 2007; Querol et al., 1989; Renton and Cecil, 1979). Kortenski andKostova (1996) also proposed the possibility of pyritization of otherkinds of bacteria which might have coexisted along with the sulfurmetabolizing bacteria and supported the decomposition and assim-ilation of the plant tissue.

Moreover, it has been suggested that framboidal pyrite might begenerated from mineral solutions in inorganic material (Dai et al.,2002, 2003; Kortenski and Kostova, 1996). Other theories (Wilkin andBarnes, 1997) suggested for the formation of framboidal pyrite is infirst step the activity of biogenic processes i.e., pyritic fossilization ofbacterial colonies, and in a second step further growing of framboidal

nalysis) and iron (Fe) contents in the studied coal samples.

ht.%, db)

Mineral(vol.%)

Total sulfur(wt.%, db)

Pyrite(vol.%)

Fe(wt.%)

6 1.3 0.05 0.0 0.183 1.6 0.05 0.2 0.155 0.3 0.06 0.0 0.211 1.0 0.07 0.3 0.175 0.7 0.20 0.0 0.201 1.0 0.10 0.0 0.228 1.0 1.41 0.7 1.327 2.0 0.18 0.0 0.200 3.7 3.15 3.0 1.150 1.6 1.10 1.0 0.340 0.7 0.14 0.4 0.170 2.3 1.69 2.0 0.510 5.6 2.92 4.3 1.51

Fig. 4. The variation and cross correlation of ash to the mineral matter, total sulfur, Fe and pyrite contents in the coal samples from Centra Busang and Sebulu coal mines.

156 S. Widodo et al. / International Journal of Coal Geology 81 (2010) 151–162

pyrite by organic processes, based on laboratory syntheses over awide range of thermal conditions. Bacterial framboidal pyritepreserves the independence of the separate globules even whenthey form aggregates (Kortenski and Kostova, 1996; Querol et al.,1989; Renton and Cecil, 1979).

During peatification, syngenetic or early-diagenetic fine-crystal-line or fine-concretionary pyrite appears, commonly in the form offramboids.

The Centra Busang and Sebulu coal samples contain bacterialframboidal pyrite in high abundance. An example is shown in sampleTNT-34 (Fig. 8a). In some samples such as TNT-30 from the CentraBusang coal mine the primary framboidal pyrite shows an overgrowthby secondary pyrite which is generally associated with clay minerals(Fig. 8b). In most of the framboidal pyrite globules the crystals aredensely intergrown and consist of some aggregates as previouslydescribed by Skripchenko and Berberian (1975; in Kortenski andKostova, 1996). Most of bacterial framboidal pyrite in the sample TNT-

Fig. 5. The variation and cross correlation of ash to the mineral matter, total s

30 appears as single bodies or solitary. In Embalut coal sampleframboidal pyrite is not observed.

4.2.2. Euhedral pyriteEuhedral pyrite is recognizable as well shaped pyrite crystals

(Kortenski and Kostova, 1996). Querol et al. (1989) described euhedralpyrite in the coal samples from Maestrazgo Basin, northeastern Spain.Kortenski and Kostova (1996) observed this type of pyrite in the coalsamples from Bulgaria and divided euhedral pyrite into isolated andclustered varieties and isolated anhedral crystals and aggregates ofeuhedral crystals. Most of the euhedral pyrite is syngenetic and isgenerated duringdeposition of peat and/or during early humification. Ingeneral, the crystals of euhedral pyrite are small in size and intimatelydispersed throughout the coal (Dai et al., 2007, 2008; Renton and Cecil,1979; Reyes-Navarro and Davis, 1976; Turner and Richardson, 2004).Isolated euhedral pyrite was found only in small amounts in sample

ulfur, Fe and pyrite contents in the coal samples from Embalut coal mine.

Fig. 6. Cross correlation of the concentration ratios of Fe (wt.%) to the pyrite content (vol.%) and total sulfur (wt.%) in the coal samples from Centra Busang and Sebulu coal mines.

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TNT-34 from Sebulu coalmine (Fig. 8c). Clustered euhedral pyrite couldnot be detected in all analyzed samples.

4.2.3. Massive pyriteMassive pyrite is usually found as cleat-/cell-fillings, cementing or

coating framboids, euhedral or detrital minerals (Querol et al., 1989).Massive pyrite has also been found as a replacement of organic matterin different macerals (Querol et al., 1989). Many authors denotedpyrite grains with irregular shapes and different sizes by the termmassive pyrite (Dai and Chou, 2007; Grady, 1977; Kortenski andKostova, 1996; Wiese and Fyfe, 1986). Renton and Bird (1991)described this type of pyrite as irregular.

Massive pyritewas found inmost coal samples fromCentra Busangand Sebulu. The homogeneous massive pyrite was generally porousand not compact, which is due to the inclusion of relict organic matterand clay minerals during the crystallization processes. Homogeneousmassive pyrite was present in lenticular or irregular form. Fig. 8d ande show the homogeneous massive pyrite in the coal samples of theSebulu mine (sample TNT-34). This type of pyrite was not observed inthe Embalut coal samples.

4.2.4. Anhedral pyriteAnhedral pyrite corresponds to pyrite forms whose shape depends

on the shape of the plant debris in which they were deposited. The

Fig. 7. Cross correlation of the concentration ratios of Fe (wt.%) to the pyrite conte

anhedral pyrite was divided into two types, the replacement anhedralpyrite and the infilling anhedral pyrite (Kortenski and Kostova, 1996;Wiese and Fyfe, 1986), which are of late syngenetic and epigeneticorigin, respectively. The anhedral pyrite in the sample from CentraBusang and Sebulu coal mines was found in small amounts.Replacement anhedral pyrite was deposited in the lumens ofdensinite maceral (Fig. 8f). The replacement anhedral pyrite was aresult of mineralization of cell wall and described to originate fromreplacement of plant material or massive pyrite replacement oforganic matter (Kortenski and Kostova, 1996; Querol et al., 1989;Wiese and Fyfe, 1986). Anhedral pyrite was not found in the Embalutcoal samples.

4.2.5. Epigenetic pyrite in cleats and fracturesThe term epigenetic pyrite in cleats and fractures is used for pyrite

deposited in fractures or cleats which determine the path of solutionspenetrating a coal seam. There are two types of epigenetic pyrite incleat and fracture: infilling and replacing epigenetic pyrite. Theinfilling epigenetic pyrite in cleat and fracture has again been dividedin two types: fracture and cleat filling (Kortenski and Kostova, 1996;Querol et al., 1989; Renton and Cecil, 1979). Epigenetic pyrite in cleatsand fractures is observed only in a very small quantity in the studiedcoal samples. Infilling epigenetic pyrite in cleats and fractures wasobserved in the Centra Busang coal samples (e.g., TNT-30, Fig. 8f). The

nt (vol.%) and total sulfur (wt.%) in the coal samples from Embalut coal mine.

Fig. 8. All photos taken under oil immersion. (a) Bacterial framboidal pyrite (Py) from the Sebulu coal mine (TNT-34), (b) overgrowth framboidal pyrite (Py) from the Centra Busangcoal mine (TNT-30), (c) euhedral pyrite (Py) from the Sebulu coal mine (TNT-34), (d) massive pyrite (Py) from the Sebulu coal mine (TNT-34), (e) massive pyrite (Py) from Sebulucoal mine (TNT-34), (f) epigenetic pyrite (Py) in cleats/fractures and anhedral pyrite (Py) from the Sebulu coal mine (TNT-30), (g) replacing epigenetic pyrite (Py) in cleats/fracturesfrom the Sebulu coal mine (TNT-30), (h) replacing epigenetic pyrite (Py) in fractures from Embalut coal mine (KTD-38) (i) and replacement massive pyrite (Py) from Embalut coalmine (sample KTD-38).

158 S. Widodo et al. / International Journal of Coal Geology 81 (2010) 151–162

pyrite grains were deposited in the fractures and cleats of the coalsand are of epigenetic origin. Replacing epigenetic pyrite from Sebulucoal samples is shown in the Fig. 8g. The grains of pyrite replaced(filled) the cell lumens. Replacing epigenetic in fracture pyrite wasalso found in the Embalut coal samples (Fig. 8h). The pyrite from theEmbalut coal sample can be characterized as massive fracture fillingpyrite with density-fill in the lumens or fractures (Fig. 8i).

4.3. Interpretation of coal depositional environment and the difference ofpyrite in coals

Enrichment of minerals in coal suggests that the peat wasdeposited under topogenous conditions resulting in increased(richer) nutrient supply (minerals/detrital influx). In contrast, coalsof low mineral content are generated in general from ombrogenouspeat bogs with low nutrient supply (minerals). Supply of mineral tothe topogenous peat occurs preferentially by surface water andgroundwater, while in ombrogenous peat bogs the supply takes placethrough atmospheric deposition. Pyrite is sometimes the dominatinginorganic matter present in the coal and the morphology of pyrite canhelp to reconstruct the environmental condition during and after peat

formation (Casagrande et al., 1977, 1980; Dai et al., 2008; Demchuk,1992; Taylor et al., 1998; Wiese and Fyfe, 1986).

The amount and type of pyrite identified in the coals from CentraBusang, Sebulu and Embalut differs significantly. In the Centra Busangand Sebulu coals both syngenetic and epigenetic pyrites appear.Syngenetic framboidal pyrite is partly overgrown by epigenetic pyrite.Typically, primary framboidal pyrite occurs in coals and carbonaceousshales overlain by marine strata (Taylor et al., 1998). In case of theCentra Busang and Sebulu coals, the high abundance of mineral mightoriginate from the erosion of Early Tertiary marine sediments of theCentral Kalimantan ridge (Fig. 9), delivering sufficient iron andsulphate for pyrite formation under subaquatic conditions (Fig. 9). It isassumed that the Centra Busang and Sebulu coals have been evolvedfrom a topogenous peat deposited under wet forest swamp conditions(Fig. 9). This interpretation is verified by the high proportion of ash,mineral, sulfur, pyrite and iron contents in the Centra Busang andSebulu coals.

In the Embalut coals, pyrite is found only in epigenetic formresulting from post depositional processes. The absence of framboidalpyrite is consistent with the formation of the coal from anombrogenous peat. This type of peat receives the water throughheavy rainfall and the groundwater table lying below the peat forming

Fig. 9. Scheme of possible formation of paleo peat in the Centra Busang, Sebulu and Embalut coal mines with both topogenous and ombrogenous peat types in the Kutai Basin.

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surface. Ombrogenous peat occurs only in humid climates where theannual rainfall is higher than the total yearly evaporation. Ombro-genous peats are not constrained by surface morphology, formingeven on mountain summits with a high annual rainfall. Hall andNichols (2002) suggested that rainfall in Kalimantan is very high andthe tropical weathering is very intense, as well as probably greateraverage heights and local relief early in the Neogene. Kalimantanreceives a yearly rainfall of 2000 to 4000 mm distributed uniformlythroughout the year (Fig. 9). These conditions are favourable forombrogenous peat formation. Formation of peats within EastKalimantan may be governed by the dynamics of the water table, asdemonstrated in Fig. 9.

4.4. Relation of paleogeography, geology and tectonic setting with thepresence of mineral contents in the coal samples

In order to gain information about the origin of mineral, sulfurand iron (Fe) in the Kutai Basin (especially in Centra Busang andSebulu coal mines), paleogeography, geology and tectonic eventshave to be considered. The early Miocene to middle Miocene was aperiod of major plate readjustments with the rotation of Kalimantan(from 20–20 Ma; Hall 1996, 1997). This resulted in the deformationand uplift of Kalimantan and a major influx of volcanogenic clasticsinto the Kutai Basin from the uplifted terranes in the western part ofthe basin. Collision of microcontinental blocks with a subductionzone along the northwest Kalimantan margin (Palawan Trough)resulted in uplift that produced the Central Kalimantan Mountains(Clay et al., 2000).

Three suites of volcanic and intrusive rocks were recognisedwithin the Tertiary of Kalimantan: the Nyaan Volcanics, the SintangIntrusive Suite, and the Metulang Volcanics (or Plateau Basalts). Oneof the igneous activities that occurred in the Kutai Basin is the felsicNyaan volcanism. The Nyaan Volcanics in the Uppper Kutai Basin

have K–Ar ages of 48–50±1 Ma (Fuller et al., 1999; Pieter et al.,1987).

Today the Kutai Basin is the most important coal producing basinin Kalimantan. Based on our investigation (proximate, ultimate andtrace element analyses) to some coal seams in the Kutai Basin (LoaJanan, Kendisan, Loa Duri, Loa Ulung and Embalut coal mines), almostall coal seams have very low sulfur and iron, as well as low pyritecontents.

However, the coals from Centra Busang and Sebulu coal minehave higher sulfur and iron as well as pyrite contents compared tothe coal deposits located east of Kalimantan. According to thepaleogeographic map of Kalimantan (Fig. 10), Centra Busang andSebulu coal mines are located relatively close to the upper KutaiBasin. The Upper Kutai Basin is bordered with early Tertiary deepmarine sediments, mafic igneous rock and melange in CentralKalimantan Ranges (according to geological map of Kalimantan).Therefore the increasing amount of ash, mineral, total sulfur, ironand pyrite in the Centra Busang and Sebulu coals can be related tothe highland Nyaan volcanic activity during the Tertiary or might becaused by the supply of sediments from early Tertiary deep marinestrata, mafic igneous rocks and melange from the Central Kaliman-tan. In this case ground and surface water play a major role in thetransport of inorganic matter from the highland of CentralKalimantan to the lowland of the Kutai Basin (East Kalimantan).Erosion in Kalimantan is also very high and implies that the crustsignificantly more than 6 km has been removed from the highestmountainous part of Kalimantan in the Neogene. Kalimantan wassurrounded by deep basins ready to receive sediment (Hall andNichols, 2002).

During the Miocene the depositional setting changed fromextensive carbonate shelves to deltaic deposition and progradationoccurred on the eastern side of Kalimantan, particularly in theTarakan–Muara and Barito Basins (Ahmad and Samuel, 1984; Mossand Wilson, 1998; Netherwood and Wight, 1992; van de Weerd and

Fig. 10. Paleogeographic map for 21 Ma, early Miocene and predicted situated of Centra Busang and Sebulu coals (modified from Moss and Wilson, 1998).

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Armin, 1992). The predominance of deltaic sedimentation aroundthe northern and eastern parts of Kalimantan (Borneo), particularlyaround the deep Kutai Basin, suggests that most of the major riversystem were draining into these areas. Abundant detritus wassupplied from the uplift and denudation of the center of the islandand coeval volcanism (Moss and Wilson, 1998; Moss et al., 1997;Tanean et al., 1996).

By the end of Miocene the drainage system within KalimantanIsland was similar to the present day. The Mahakam delta hadprograded to near its present day position by the late Miocene(Addison et al., 1983; Land and Jones, 1987) and siliciclasticmarginal marine and deltaic deposition predominated in this area(Moss and Wilson, 1998). The Makassar Straits remained a deepwater basin separating Sulawesi from Kalimantan, although as theland area increased in eastern Kalimantan due to the progradation ofdeltas, the distance across this seaway was progressively reduced.

5. Conclusions

Total sulfur content of Centra Busang and Sebulu coal mines werepositively correlated to ash,mineral, and iron (Fe) contents in the coalstudied.Totalsulfurcontentwasgenerallyhighinthecoalsamples.Pyriteoccurred in theCentraBusangandSebulucoals asbacterial framboidalpyrite and intergrown framboidal pyrite, euhedral pyrite, massivepyrite,anhedralpyriteandepigeneticpyriteincleatsandfractures.Highconcentrationofpyritearguesfortheavailabilityof iron(Fe)inthecoalsamples. On the other hand, most of sulfur contents of Embalut coalsamplesare lower(b1 wt.%) thanCentraBusangandSebulucoals.OneexceptionisthecoalsampleKTD-38withtotalsulfurcontentof1.41 wt.%.IntheEmbalutcoalsamples,pyritewasobservedonlyasmassivepyriteandepigeneticpyriteincleatsandfractures.

Epigenetic and syngenetic pyrites were found in high quantities incoal samples. Epigenetic pyrite might be deposited by percolating

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water into fractures, cavities and pores within the coal seam long afterinitial accumulation of the peat. The reason for the rich ash, mineral,total sulfur, iron (Fe) and pyrite contents of coals in the Kutai Basin(especially of Centra Busang and Sebulu coals) can be related to theNyaan volcanic activity during Tertiary and/or to the supply ofTertiary marine sediments, mafic igneous rocks and melangeoutcropping in the central of Kalimantan. Activities of ground andsurface waters played also a major role and brought mineral into themire of Centra Busang and Sebulu coals.

Acknowledgements

This study has been carried out within BIK-F. The first author (SriWidodo) is most gratefull for the financial support provided by theGerman Academic Exchange Service (DAAD). The authors wish tothank PT. Tanito Harum (Centra Busang and Sebulu coal mines) andPT. Kitadin (Embalut coal mine) companies in East Kalimantan(Indonesia) for providing the coal samples for this study. We arealso thankful to Haryadi, H. Budi Perkasa, D. Natalia, E. Supriyadi, H. E.Hardono, A. Syariffudin, A. Dwijowarso and Jam'an for their helpduring the coal sampling in East Kalimantan, Indonesia. The authorsalso gratefully acknowledge the reviews provided by Prof. Shifeng Daiand an anonymous reviewer.

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