The conditions of the nonequilibrium silicification of rocks and quartz vein formation during acidic metasomatism

The physicochemical factors of quartz precipitation from solutions are considered, and the effect of temperature, pressure, composition, concentration, and pH of the solutions on the mass transport and quartz deposition during the hydrothermal mineral formation is qualitatively estimated. It is shown that none of these factors can explain the preferential confinement of the rock silicification and quartz vein formation during the processes of acidic metasomatism, such as greisenization, formation of the secondary quartzites, quartz-sericite metasomatism, beresitization, argillization, etc. A kinetic mechanism of the nonequilibrium silicification of the rocks during the acidic metasomatism is proposed and experimentally proven. It is shown that the decomposition of feldspars under the effect of the acid chloride solutions at 300-600 degrees C and 1 kbar results in the formation of the metastable amorphous silica, rather than quartz, in association with muscovite in the reaction products. The former is much more soluble than quartz. As a result, the solution becomes oversaturated in silica with respect to quartz. During the further filtration of this oversaturated solution the excess silica precipitates as metastable cristobalite, which, then, is transformed to stable quartz. This process was simulated experimentally. The accumulation of the oversaturated pore solutions in the macro- and microfractures results in the formation of quartz veins and stringers, which are very typical of wall-rock alterations of the acidic leaching stage established by D.S. Korzhinskii.