Mineralogical, Petrographical and Geochemical Properties of the Late Oligocene Coal Seam (Seam-VI): Insights into Elemental Enrichments and Palaeodepositional Environment (İbrice field, Thrace Basin)

Abstract

The Thrace Basin hosts several mineable coal seams (upwards numbered I to VII) within the coal-bearing Late Oligocene-age Danişmen Formation. For the first time, the properties of the Seam-VI in the İbrice (Malkara) area have been investigated in order to evaluate elemental enrichments and palaeoenvironmental conditions. The xylite-rich and mineral-rich lithotypes were identified from the studied coal samples representing the whole-coal thickness. The coal samples, on a dry basis, display variable ash yields (13.2–63.0%), total C (21.2–57.8%), and total S (0.9–4.1%) contents. The minerals identified using XRD-whole rock analysis include clay minerals, quartz, pyrite, calcite, dolomite, and feldspars. Siderite is only identified in a single sample. Aragonite is detected in two samples, which contain fossil shell remains. The SEM observations agree with XRD data, and apatite, barite, biotite, pentlandite, sphalerite, Ti-oxide, and zircon were also identified as accessory phases. Huminite is the predominant maceral group in the studied samples, while liptinite and inertinite group macerals display variable proportions. The mean %Rr values of ulminite macerals are around 0.40-0.41±0.01-0.02%, indicating relatively low-rank coal. This study implies that the precursor peat-mires of the Seam-VI were mainly developed under forested mire conditions, where woody peat-forming plants were abundant, and occasionally the contributions of herbaceous plants were important. The coexistence of syngenetic pyrite and carbonate minerals (e.g., siderite and carbonate mineral bands) could be related to the development of weakly acidic to neutral conditions within palaeomires. Furthermore, the predominance of clay minerals and the presence of detrital accessory minerals (e.g., apatite, pentlandite, and Ti-oxides) can also be indicators of clastic influx into palaeomires. Although aluminosilicate minerals (e.g., clay minerals) are abundant in dominant phases, SEM-EDX data shows that accessory minerals mainly control the elemental enrichments in the Seam-VI. The Mn enrichments in the samples seem to be controlled by Mn-bearing siderite micronodules. Considering the presence of sulphide mineralization and metallic ore deposits in the Strandja and Rhodope massifs, enrichments of Cr, Co, Ni, Ge, and Mo are related to As-bearing pyrite grains and accessory sphalerite and pentlandite grains within clay mineral aggregates in the samples. In addition, the Sr-bearing barite overgrowths around feldspar grains and syngenetic carbonate mineral bands could also cause Sr enrichments. Overall, clastic influx ratios into palaeomires and pH conditions during peat accumulation controlled the elemental enrichments in the Seam-VI.