The geology, mineralogy, petrography, and geochemistry of the Miocene Dursunbey coal within fluvio-lacustrine deposits, Balıkesir (Western Turkey)

Abstract

The Dursunbey coalfield, a typical example of coalfields in the western Anatolia, hosts a mineable coal seam (about 9.0-m coal thickness) with altered tuff layers within early Miocene fluvio-lacustrine sequences. The aim of this study is to identify peat-accumulation conditions and factors controlling the mineralogy and geochemistry of this seam. The standard coal features and petrographical, mineralogical, and elemental compositions display changes throughout the seam. Ash yields display decreasing trends towards to the upper parts of seam, while total C contents and gross calorific values are increasing. Furthermore, the coal-facies patterns evidence changes of depositional conditions and vegetation in the palaeomires. During initial stages of peat accumulation, the contribution of herbaceous peat-forming plants were more common and palaeomires were open to detrital inputs, whereas woody peat-forming were becoming predominant, and the water table was stable and high, which caused development of anoxic conditions, during late stages. Therefore, the content of detrital minerals, such as quartz and clay minerals (illite, smectite, and chlorite) increases in the lower parts of seam, while that of pyrite increases in the upper parts. In addition, the SEM-EDX data show that matrices of clay mineral aggregates are illitic in composition in the lower parts of seam, while those of smectite prevalence are more common below and above altered tuff layers. Interestingly, authigenic rhomboid K-feldspar grains are observed within smectite clay aggregates in the samples from the upper parts of seam. Framboidal pyrite grains and clusters were also commonly identified from these samples as well. This data shows that synchronous volcanic inputs were altered within a hydrologically closed system under slight acidic to neutral conditions during late syngenetic stages.The enriched elements, such as Cr, Ni, Cs, and V, display moderate to strong positive correlations with ash yields while Al2O3, and K2O indicate an aluminosilicate affinity (clay mineral and feldspar). Considering the presence of metamorphic and ophiolitic rocks in the basement and the adjacent areas, clastic inputs into the palaeomire seems to be controlling aluminosilicate-affiliated elements. Additionally, the SEM-EDX analyses denoted that Ni and As are associated with pyrite. The Ni and As-bearing epigenetic pyrite infillings are mostly identified in coal samples below altered tuff layers; hence, As and Ni enrichments are controlled by precipitation of leached pore waters from altered tuff layers. Nevertheless, As- and Ni-bearing syngenetic pyrite grains and clusters are related with the development of anoxic conditions within palaeomires. This could also favour a Mo and U enrichment, which is another testimony for synchronous volcanic inputs altered under anoxic conditions. Overall, high detrital-input ratios from adjacent areas during initial stages of peat-accumulation in the study area controlled by enrichments of aluminosilicate affiliated elements, whereas during late stages of peat-accumulation, the development of a hydrologically closed system and redox conditions within the palaeomire caused As, Ni, Mo, and U enrichments and the formation of smectite and authigenic K-feldspars.