The Tatros kaolin deposit (southwest of Dansfahan, Qazvin Province) is a product of the alteration of rhyodacitic volcanic rocks of the Middle Eocene age. Mineralogical studies show that kaolinite and quartz are the main mineralogical phases and illite, rutile, pyrophyllite, dickite, alunite, and chlorite are accessory mineralogical phases of this deposit. The decrease in the chemical index of alteration (CIA) and the increase in the ratio of (SiO2+Fe2O3+MgO+CaO+Na2O+K2O)/(Al2O3+TiO2) from the center to the outside in the studied profile indicates the existance of alteration and hydrothermal zoning in this  deposit. The distribution pattern of rare earth elements (REE) normalized to chondrite indicates the strong diferentiation and enrichment of light rare earth elements (LREE: La-Eu) related to heavy rare earth elements (HREE: Gd-Lu) and the occurrence of strong negative Eu and Ce anomalies in the studied kaolin samples. Calculations of mass changes assuming Ti as an immobile monitor element show that kaolinitization of rhyodacite rocks is accompanied by enrichment of elements of Al, Sr, Zr, Hf, Ta, Nb, U, Th, Y, La, and Pr, leaching-fixation of Sm, Nd, and HREE, and depletion of Si, Fe, Mg, Mn, Ca, Na, K, P, Rb, Cs, Ba, Pb, V, Cr, Zn, Eu and Ce. The obtained results reveal that the behavior of elements during the development of kaolinitization processes is controlled by factors such as changes in pH and temperature of fluids responsible for alteration, residual concentrations, surface absorption, scavenging by metalic oxides, and the presence of mineralogical phases resistant to alteration. The occurrence of negative Eu anomaly during kaolinitization indicates the destruction of plagioclase by high-temperature hydrothermal fluids. The presence of diaspore, dickite, and pyrophyllite, the enrichment of LREE related to HREE, the differentiation of HREE from each other, and the occurrence of strong negative Ce anomaly in the studied samples clearly suggest the role of hypogene processes in the development and evolution of the Tatros kaolin deposit. Other evidence such as strong positive correlations (La/Yb)N-LOI and P2O5-LOI along with the values of some geochemical parameters such as Ce+La+Y, Nb+Cr, Rb+Sr, and Y/Ho support this idea.