Abstract: The Miocene-Pliocene lacustrine units comprises bituminous shale, coal seam, conglomerate, siltstone,sandstone and claystone were deposited in a graben developed along the Eskişehir Fault Zone at the east ofEskişehir. The geological, mineralogical and geochemical analyses were performed on samples obtained fromES4 and ES7 core drilling holes at the Alpu coal basin. Abundant smectite associated with kaolinite, illite, quartz,feldspar, dolomite, siderite, accessory amphibole, gypsum, alunite and pyrite. The muscovite, chlorite, feldspar,serpantine crystals and their groundmass are partly to completely argillized in host rocks. Micromorphologically,development of smectite flakes and platy illite crystals on relicts of feldspar suggest dissolution and a precipitationmechanism under alkaline micro-environmental conditions during diagenesis. Enrichment of light rare-earthelements relative to middle rare-earth elements and heavy rare-earth elements and positive Eu anomalies reflectalteration of feldspar. Alteration of feldspar, biotite and serpentine resulted in the concentration of Al, F, and Mg ina stagnant envrionment and precipitation smectite and in an alkaline environment. The low to moderate Ni/Co andhigh V/(V+Ni) ratios are indicative of oxic to dysoxic and anoxic to dysoxic conditions, respectively. TiO2/Ni andSiO2 versus (Al2O3+K2O+Na2O) discrimination diagram show that weathered samples is indicative of formationfrom the locally basic igneous rocks and alteration formed predominantly via sedimentation process under arid andsemiarid conditions and formation of clay minerals.