SOLUBILITY OF MINERALS OF METAMORPHIC AND METASOMATIC ROCKS IN HYDROTHERMAL SOLUTIONS OF VARYING ACIDITY: THERMODYNAMIC MODELING AT 400-800°C AND 1-5 KBAR

Abstract

The character of solubility of 61 metamorphic and metasomatic minerals in an aqueous fluid was analyzed as a function of temperature, pressure, and fluid acidity by means of computer simulation of mineralfluid equilibria. Depending on the behavior of minerals in solutions of varying acidity, six main types of solubility diagrams were distinguished. The solubility of the majority of minerals is controlled mainly by fluid acidity rather than by P–T conditions. The analysis of model results provided insight into the mobility of chemical elements composing the minerals. The highest mobility in solutions of any acidity was established for Si, K, and Na. Ca and Mg are mobile in acidic solutions and inert in neutral and alkaline solutions. Fe(II) and Mn(II) are mobile in acidic and alkaline solutions but inert in neutral solutions. Fe(III) is mobile only in strongly acidic solutions and practically immobile in solutions of other compositions, which suggests that ferrous iron species must prevail in solutions. Al is mobile in alkaline and ultra-acidic solutions but inert in neutral and slightly acidic solutions. Correspondingly, a change in acidity must lead to the migration of some component into the solution and precipitation of other components. These conclusions are in agreement with the sequences of element mobility deduced from the experimental investigation of metasomatism. Most metamorphic fluids must be rich in silica and alkalis, which may result in the appearance of aggressive silica-alkali fluids responsible for regional metasomatism and granitization. In general, the solubility of Fe-, Mg-, Mn-, and Ca-bearing minerals in alkaline solutions is low compared with acidic solutions. Therefore, only acidic initial solutions could produce fluids enriched in these elements at the expense of leaching from metamorphic rocks during fluid migration. Fluids enriched mainly in Fe could initially be both acidic and alkaline.