An advection–dispersion–reaction model of bauxite formation

Abstract

The formation of bauxite from a granitic protolith, at 25°C, has been simulated using a modified version of the 1DREACT software for one-dimensional reactive transport, which uses a kinetic approach to describe the mineral dissolution and precipitation reactions. The rate laws describing these reactions incorporate the important effects of mineral surface area, pH, and solution saturation state, and are based on experimentally determined rates published in the literature. The original program has been modified to allow for a different treatment of the mineral surface areas and reaction rate laws. The results of long term simulations (>1 Ma) show the development of gibbsite-rich weathering profiles (bauxites), which are underlaid by muscovite-rich zones. These muscovite-rich zones are not observed in the field (the typical `bauxitic' profile consists of an upper gibbsite-rich and a lower kaolinite-rich zone). A strong nucleation barrier for muscovite could explain the absence of muscovite from the weathering profiles. The results also show that the infiltrating solutions get closer to saturation with respect to microcline than with respect to albite, in agreement with numerous field observations. The simulations also predict the formation of a 5–10 m thick gibbsite–kaolinite transition zone, which is in agreement with field observations, and argues for the applicability of the experimentally determined rates, or at least their ratios, to field conditions. Regarding the chemistry of the solutions leaving the weathering profiles, the Si/(Na+K) ratio of the waters increases to values higher than 3 with time (intense weathering), in agreement with some of the river chemistry data from the Guayana Shield reported by Edmond et al. (1995).