HEAD OF TEAM : Dr. Ir. Komang Anggayana, MS.

TEAM MEMBERS : Dr.phil.nat. Agus Haris Widayat, ST., MT. ; Arie Naftali Hawu Hede, ST., MT., Ph.D.

Introduction

Silicification of Indonesian coals have previously been reported by some authors. In South Sumatra Basin, silicified coals occur intensively in Muaraenim Formation which are currently mined in Tanjung Enim area. The occurrence of silicified coal in Balikpapan Formation, Kutai Basin mined in Muara Lawa area was reported by Amijaya and Febrianti (2015). Silicification of coals are also found in some coal mining areas in East Kalimantan such as in Berau, Embalut and Loa Kulu operated by PT. Berau Coal, PT. Kitadin and PT. Megaprima Persada, respectively. The silicified coals are locally known as bonecoal, blackstone or ironstone referred due only to their colour and/or hardness. They are black and hard. During mining, the silicified coals are difficult to be recognized by operator. They could damage the excavator bucket and crusher equipment, so that lowering mining productivity and efficiency. Although the presence of silicified coals in Indonesia has been widely known, they have been less studied especially by mean of coal geology. In the present study, we report and discuss the occurrence of silicification in Berau, Embalut and Loa Kulu coals. The aim of this study is to describe and thus to reconstruct the genesis of silicified coal. Mineralogical and geochemical analyses were performed to characterize the physical and chemical characteristics of the silicified coals.

Material and Method

The samples were taken from freshly exposed coal surfaces of active mine sites in Berau, Embalut and Loa Kulu. The coals are currently mined by PT. Berau Coal, PT. Kitadin and PT. Megaprima Persada, respectively. Silicified coal occurs typically in two types: nodule and layer. The first exhibits rounded form in varied sizes, ranging from several centimeters to about 1 meter. It could present in all parts of the coal seams randomly. The second shows localized layers commonly occur near the top and bottom of the coal seams. The thickness of silicified coal layers ranges from several centimeters to about 0.25 meter and spread horizontally up to 3 meters. The two types of silicified coals are present intensively in Berau coal seams, while the layering silicified coal is the only type found in Embalut and Loa Kulu. Both types are usually characterized by the presence of higher cleat frequency above and below the massively silicified coals within the coal seams. The cleats are commonly filled by thin of quartz assemblages.

Results

XRD analysis reveals that quartz is the only mineral identified in most of silicified coal samples. ICP-MS analysis indicates that SiO2 content is very high in the samples (typically >60%, Table 1), confirming quartz as the dominant mineral. The high content of LOI might be related to the organic matter in the silicified coal samples. In the sandstone and claystone which overlay and/or underlay the coal seams, quartz is the most dominant mineral as indicated by the XRD result. Other minerals found in the samples include clay minerals, pyrite and carbonate minerals such as siderite and calcite. ICP-MS analysis confirms the presence of other silicate minerals in the clastic sedimentary rocks as indicated by the higher content of Al2O3 (about 15%). Quartz is the major mineral observed under the transmitted-light polarization microscope. The two types of silicified coal show different characteristics of mineral texture. The nodule type exhibits commonly anhedral to subhedral quartz crystals, where their assemblages are un-oriented as observed in Berau-2 sample. The layer type typically shows relatively oriented assemblages of euhedral to subhedral quartz crystals as identified in Loa Kulu-6 and Embalut-5 samples. The mineral assemblage orientations tend to parallel with coalbed. The vegetation cell remains with infilling quartz mineral are clearly observed in the latter type of silicified coal.

Discussions

Silica in coal could be present in three forms: detrital, early and late diagenetic. Detrital silica comes from hinterland by water and/or air transportation flowing into sediment basin. Detrital silica, due to transportation, usually shows rounded form dispersed within coal seam. Early diagenetic silica in coal is formed in the depositional environments or precipitated from pore fluids during early burial when cleats have not been developed. Late diagenetic silica is precipitated in cleats and fractures, or replaces coal and earlier mineral phases after cleats were developed (Sykes and Lindqvist, 1993). In the present case, detrital silica is absent in the studied silicified coals. There is no silica in rounded form observed in the samples under the microscope. The observed quartz even showsmostly subhedral grains, indicating that the mineral was not transported from hinterland. Most likely the quartz in the studied samples is diagenetic origin. Early diagenetic silica which formed from the depositional environments is unlikely in the present case. Such silica may be precipitated in the peat water column and thus deposited in the bottom water and enter the peat body as very fine individual crystals. Such silica crystals are not observed in the samples. The quartz identified in the samples typically shows coherent crystal aggregates and interlocking textures with organic matter. This indicates that the silica was precipitated from the pore fluids, either in early or late diagenetic phase. Field observation revealed that the higher frequency of cleat which filled by silica mineral commonly developed above and below the silicified coals. This suggests the silica mineral was precipitated from silica-saturated solution flowing into the coal seams through cleats and fractures. It implies that the observed quartz in the silicified coals was precipitated during late diagenetic phase.

Conclusion

Silicified coals in Berau, Embalut and Loa Kulu areas were identified and classified in two types: nodule and layer. The silicification in the coal seams was interpreted due to the presence of overlying and underlying sandstone and claystone which contain abundant quartz mineral. The process of silicification is thought to occur after cleat forming and before bituminization of the coals. Localized different vegetation types might control the formation of nodule and layer types of silicification.