Thermodynamic basis for distinguishing the acid-sulphate and adularia-sericite alteration types of rock in epithermal deposits and genetic analogy with the metasomatites of the formation classification

The study of mineral equilibria in the K2O-Al2O3-SiO2-H2O-SO3 system permits to propose thermodynamic basis for distinguishing the acid-sulphate and adularia-sericite alteration types of volcanic rokcs and frequently associated subvolcanic intrusive rocks in the epithermal deposits. The fields of the acid-sulphate and adularia-sericite types of alteration are divided by the fields of the advanced argillic alteration without sulphatebearing minerals and the sericitiztion in the prognostic paragenetic diagram of the mineral equilibria in the system studied. This study presents theoritical arguments for the existence of the two end paragenetic incompatible types of wall rock alteration in epithermal deposits. The advanced argillc alteration of the rocks is subdivided of the acid-sulphate and acid-chlorine types of alteration. The mineralogical synonyms of the acide-sulphate type are kaolinite-alunite and alunite alteration types and of the acid-chlorine type are kaolinite, pyrophyllite and dickite-pyrophyllite types wallrock alteration. The study of the system is basis for genetic interpretations of the advanced argillic, sericitic, greisen, K-silicate and adularia-sericite alteration.