Introduction

Metasomatic processes involve replacement of some minerals by others with associated changes in the rock chemistry while the rock remains solid. Metasomatic replacement takes place in the presence of a fluid phase. Lindgren (1925) seems to be the first to have given such a definition to metasomatism. He recognized certain characteristic types of hydrothermal metasomatic rocks, showed their relation to ore formation and put forward the rule of'constant volume during metasomatism’.

A decisive factor in the metasomatic replacement is the dynamic interaction between a rock and the solution with which the rock is out of equilibrium, leading to some chemical species being added to and others removed from the system.

Current concepts of metasomatic processes are based on the fundamental studies by Korzhinskii (1936, 1945, 1946, 1950, 1959, 1970). We assume that readers are acquainted with his theory of metasomatic zoning and therefore only some of the conclusions relevant to this study will be mentioned. The most important is the concept of the metasomatic column as a set of contemporaneously formed metasomatic zones which are distinctly defined and have different mineral and chemical composition. Each zone has its own set of factors of state: the number and mass of inert components, the number and magnitude of the chemical potentials (activities) of the perfectly mobile components, etc., the other intensive parameters being the same throughout the column: temperature, pressure, chemical potentials (activities) of some (virtual) perfectly mobile components.