Three different types of hydrothermal alterations have occurred in the Eocene magmatics of the northern part of Sivas-Turkey which include propylitic phyllic / sericitic and argillic. Hydrothermal alteration affected both Karata & sect; volcanites and Kosedag syenite. The clay formations are concentrated in two major zones; extending in NE-SW direction and intersecting both plutonic and volcanic rocks with circular opening cracks within the volcanics parallel to the plutonic-volcanic contact. Hypogene and supergene hydrothermal alteration products occurring on surface and/or near surface conditions represent the primary and secondary minerals that develop directly and mostly from feldspars with the mechanisms of neoformation and/or degradation. The hypogene minerals are formed in two stages of early (kaolinite, pyrophyllite, illite, I-S (illite-smectite), smectite. quartz and opal-CT) and late (barite, ore minerals, alunite, goyazite, jarosite, chlorite and C-S). Tourmaline, epidote and carbonates (calcite, dolomite, azurite and malachite) are considered as metasomatic and supergeneous minerals, respectively. The most common mineral paragenesis are kaolinite + quartz + goethite + goyazite or alunite, kaolinite + quartz + jarosite i. feldspar goethite and/or goyazite in the argillic alteration zone, I-S quartz + jarosite +/- goethite +/- feldspar as well as I-S + quartz + feldspar minerals in the sericitic alteration zone. Kaolinite (T) is mostly in the form of pseudohexagonal platies, I-S (R1, R3 as argillic alteration origins, 1M(d), 2M1, 1M and R3 as sericitic alteration origins) forming fiber-acicular bundles parallel to each other.