Dissolution-precipitation processes induced by hot water in a fractured granite. Part 2: Modelling of water-rock interaction

The hydrothermal alteration of granite has been simulated at 300°C in a closed system using the EQ3/6 software package. This alteration measured as against the initial fluid chemistry was subsequently used to explain the zonation of the naturally altered granite at La Peyratte (France). The need for a closed system is discussed on the basis of the petrographical observations and of the mass budget carried out on selected veins of the La Peyratte granite (part 1, Turpault et al., 1992 a, b). The relations between the commonly observed alteration fronts were reproduced and were found to be controlled by dissolution under conditions close to equilibrium rather than by the initial fluid chemistry, which is early buffered by the rock. By comparing the boundary cases that we have simulated and typical petrographic features, such as those observed in the La Peyratte granite, we propose a mechanism of fracture sealing by mass transfer between the wall-rock and the fracture-filling solution. This process is explained by considering a pH gradient between the reaction interfaces in the rock and the free solution in the fractures, which results from the protonation of silicate surfaces