The application of ACF and AKF diagrams to the hydrothermal alteration in porphyry Cu deposits aids in the recognition of the critical components, and the characteristic mineral phases permit the distinction of one alteration facies from another. The somewhat meager chemical and mineralogical data suggest 3 principal alteration facies, herein called the propylitic, argillic, and potassium silicate facies. The ACF diagram applies to the propylitic alteration. The critical components are Al 2 O 3 , CaO, and (FeO + MgO + MnO), and the characteristic minerals are muscovite (sericite), epidote, chlorite, and a member of the carbonate group. The AKF diagram applies to the argillic and potassium silicate facies. The critical components are Al 2 O 3 , K 2 O, and (FeO + MgO + MnO). The characteristic minerals for the argillic facies are kaolinite (or other member of the group) and muscovite (sericite); and for the potassium silicate facies, muscovite (sericite), biotite, and K-Feldspar. For the propylitic facies, insufficient chemical analyses are available to show the chemical changes between the fresh and altered rock. For the argillic and potassium silicate facies, chemical analyses indicate a loss in Al, Fe, Mg, Ca, and Na; and a gain in K, hydroxyl ion (water), and S. The gain in K is about 60% greater for the potassium silicate facies than for the argillic.