In some metamorphosed volcanic belts, evidence of premetamorphic hydrothermal activity is preserved in the form of distinct metamorphic mineral assemblages. By considering the effect of metamorphism on rocks of various compositions, it is possible to deduce premetamorphic mineral assemblages, some of which could be products of hydrothermal alteration. In some rocks, precursor minerals kaolinite, muscovite, and chlorite would indicate aluminous, potassic, and ferromagnesian alteration, respectively. The stabilities of alteration minerals can be related to log fO2-pH conditions of the hydrothermal fluid.