Geochemical and mineralogical characteristics of Pleistocene lignites and associated sediments of Marathousa coal field, Central Peloponnese, Greece

Abstract

The mineralogy and geochemistry data are presented for thirty-seven shales, four concretions, two carbonate sediments and seven lignites from the Marathousa coal field of the Megalopolis Basin in Greece. The argillaceous rocks consist of chlorite, illite, kaolinite, albite, quartz, opal-A, calcite and dolomite; the concretions of aragonite, gypsum and pyrite; and the carbonate rocks of calcite, quartz and illite. The mineral matter in the lignites consists of gypsum, quartz, albite, chlorite, illite, opal-A, dolomite, pyrite, and rarely calcite and kaolinite. A three-factor model explains the total variation of major and trace elements in the argillaceous sediments. The first factor is an aluminosilicate factor and involves the following elements: Al, Si, Mg, Na, K, Ti, Mn, Nb, Y, Rb, Zn, Cu, Ni, Cr, Nd and V, associated with chlorite, albite and illite. The second factor involves the elements Ca, Sr, Ba, Zn and Sc and is related to carbonate lithology and mainly the carbonate concretions with gypsum. The third factor involves Fe and Ce with a weak association with Mn. The diagenesis of the Marathousa sediments and lignites was not very advanced as indicated by (a) the total thickness of the sequence (500 m), (b) the presence of biogenic silica (opal-A) and (c) the age of the deposit (Pleistocene). For these reasons the presence of chlorite, illite and kaolinite in the sediments and lignite is due not to diagenetic reactions but to weathering of the flysch and metamorphic rocks at the edges of the Megalopolis Basin and transport of the weathering products (illite, chlorite, kaolinite) into the basin of deposition. The diagenetic minerals of the Marathousa sequence include pyrite, gypsum, dolomite and aragonite.