Variations in elemental and mineralogical compositions of Late Oligocene, Early and Middle Miocene coal seams in the Kale-Tavas Molasse sub-basin, SW Turkey

Abstract

The Kale-Tavas sub-basin, which is the south-western part of the Lycian Molasse Basin, SW Turkey, hosts several coal seams of Late Oligocene, Early and Middle Miocene age within, respectively, the Mortuma, Yenidere, and Sekköy Formations. This study aims to determine coal rank, mineralogical, and geochemical features of the coal seams, and to ascertain factors controlling the mineralogical distribution and elemental enrichment. The distinct differences among the studied coal seams refer to mean random huminite/vitrinite reflectance (Rr). The highest Rr values (0.63–0.69%) are measured on the Late Oligocene Tavas coal, whereas the lowest Rr values (0.26–0.27%) were recorded in the Middle Miocene Narlı coal. Similar mineralogical assemblages were found in all the studied coal seams; however, aluminosilicate minerals are dominant to abundant phases in the coal from the Mortuma and Yenidere Formations, while carbonate minerals are the dominant phases in the coal from the Sekköy Formation. Accordingly, aluminosilicate-related elements (e.g. Al, K, Ti, and Li) display relatively higher concentrations in the studied coal seams from the Mortuma and Yenidere Formations, whereas relatively high-Ca concentrations and depletion of aluminosilicate-affiliated elements were recorded in the coal of the Sekköy Formation. Furthermore, Ni, Mo, and U are relatively enriched in all the studied seams, while relative B-enrichment is identified in the coal seams of the Mortuma and Yenidere Formations, and these seem to be related to possible marine influence of the palaeomires during Late Oligocene and Early Miocene and/or the occurrence of specific B-bearing aluminosilicate minerals. Also, U and Mo enrichments in the Narlı coal may be related with the development of anoxic conditions within the palaeomires during the Middle Miocene. Other indicators for anoxic conditions are framboidal pyrite grains and accessory Ni-bearing iron sulphides in all the studied samples. Although coal-bearing formations in the Kale-Tavas sub-basin deposited under different sedimentary environment, as the source areas for sediment supply of the coal seams remained the same during the Late Oligocene to Early Miocene, the mineralogical composition and elemental enrichment of the coal were mainly controlled by clastic inputs and redox conditions within the palaeomires; to a lesser extent a possible marine influence might have contributed to elemental enrichment.