Coal geology and coal quality of the Miocene Mugla basin, southwestern Anatolia, Turkey

This work focuses on the relationship between the coal geology and coal qua lity of the Miocene Mugla basin, Southwestern Anatolia, Turkey. To this end , detailed petrographical, mineralogical and geochemical studies were perfo rmed on composite profile samples from the nine coal fields in the Mugla ba sin (Alatepe, Bagyaka, Bayir, Cakiralan, Ekizkoy, Eskihisar, Husamlar, Sekk oy and Tinaz coal fields). The Mugla lignite is a high ash (from 16 to 56%) and sulphur (from 2.1 to 5.7%) Lignite which is petrographically character ised by:a high huminite content, mainly gelinite macerals. The mineral matt er of the studied lignite samples is made up mainly of clay minerals and qu artz, with the exception of the Sekkoy and Ekizkoy lignites, in which calci te is the dominant mineral phase with minor amounts of quartz, clay mineral s, pyrite and gypsum and traces of aragonite. Syngenetic opal is also frequ ently identified in these samples. The differentiation of these two types o f Lignite with specific mineralogical patterns is attributed to the contemp oraneous development of peatlands with a high detrital input, dominated by the quartz and clay mineral setting, and peatlands with low detrital supply and a dominant carbonate-rich lacustrine environment. The higher water tab le of the latter allowed the precipitation of micritic carbonates and the d evelopment of lakes with abundant mollusc fauna. This differentiation is al so evidenced by the geochemical data. Thus, the Sekkoy, Ekizkoy, Husamlar, Bayir and Alatepe lignites are characterised by relatively low Al and Fe co ntents (< 1.4%) and high sulphur contents (4.2 to 5.7%). In addition Sekkoy and Ekizkoy show relatively high contents of Ca (6.3-7.1% compared to 1.6- 3.8% in the other Lignites). All the lignite samples studied are characteri sed by relatively high Mo and U contents when compared with the worldwide a verages of trace elements in coal. Relatively high alkaline syngenetic cond itions of the pet-forming environment of the Mugla coal are deduced from th e following mineralogical, petrographical and geochemical evidence: (a) the precipitation of syngenetic opal (dissolution of quartz and re-precipitati on as opal) and calcite; (b) minor and very early syngenetic sulphide preci pitation (only framboidal and euhedral pyrite); (c) high bacterial activity , typical of high pH conditions, inferred from low preservation of tissue s tructures; (d) preservation of aragonite gastropod shells; and (e) the anom alous enrichment of U, Mo and W. A key result of the study of Mugla limnic coals (at least of the Sekkoy and Ekizkoy coal fields) is that a major infl uence was exerted on the early diagenetic evolution of the coal by the hydr ochemistry of the lacustrine waters. This hydrochemistry was largely linked to the lithology of the surrounding source rock areas although the final e volutionary trend of the solute composition in the lake waters, characteriz ed by very high carbonate and sulphate contents, was largely enhanced by th e endorheic river drainage system and the arid-semiarid paleoclimatic situa tion under which organic matter accumulation took place. (C) 1999 Elsevier Science B.V. All rights reserved.