This study of the formational nature ot melaso matites from the zones of acid leaching in the Western Srednogorie is based on an integral zonal metasomatic column and on composition-paragenesis diagrams of each zone constructed to follow out the thermodynamic behaviour of components and the reactions between mineralpar ageneses at the zone boundaries. The propylite type of rocks forming aureoles around proper secondary quartzites is not a separate formational unit but belongs to the outermost zone of the secondary quartzite formation. The physico-chemical analysis of mineral parageneses in the propylite type of rocks shows the equilibria between actinolite, epidote, chlorite, sericite, calcite and magnesite as dependent on temperature and potassium oxide chemical potential. Epidote-chlorite, actinolite-epidote-chlorite and epidote-actinolite parageneses are distinguished as a function of temperature. The physico-chemical analysis of mineral parageneses in the rocks of inner zones gives the equilibria between quartz, kaolinite, alunite, illite and pyrophyllite as dependent on two of the intensive parameters, namely the chemical potentials of K+, SO42- and H2O, and pH of the mineral-forming environment.

Metasomatic facies in the zones of acid leaching of the Spahievo ore field are defined in terms of the theory of metasomatic zonality by paragenetic analyses of each zone. The integral metasomatic column derived identifies the metasomatites as belonging to the secondary quartzite formation. The propylitic type of rocks forming aureoles around the secondary quartzites proper do not belong to the propylite formation but are the outer zones of the secondary quartzite forrnation. The physico-chemical analysis of mineral parageneses in the secondary quartzites proper determines the equilibria of coexisting quartz, alunite, diaspore, dickite and sericite and of coexisting quartz, alunite, diaspore sericite and pyrophyl!ite as dependent on two of the intensive parameters, namely the chemical potentials of K+ and SO4-2 and pH of the mineral-forming environment. The physico-chemical conditions of formation of secondary quartzites in the Spahievo ore field are characterized using experimental and thermodynamic data reported in the literature.