| The essential features of a mathematical model for computing coupled reaction and fluid flow are described and then applied to the problem of water-rock interaction. The computer code includes formulations for mass and heat transfer, hydrodynamic fluid flow, and simultaneous chemical reaction and uses a new approach to the numerical solution of the partial differential equations which describes these processes. The code is also designed to treat systems containing multiple aqueous species and minerals. Kinetic formulations for the mineral dissolution and precipitation reactions are used throughout, making the a priori assumption of equilibrium between water and minerals unnecessary.;An early version of the code is used to investigate the dynamics of reaction-induced permeability change. Following this, a two-dimensional, multi-component version is applied to the study of reactive flow in hydrothermal systems. The simulations demonstrate the potentially profound effect that hydrodynamic dispersion can have on the reaction-flow paths in hydrothermal systems. Dispersion may completely reverse the expected effects from a fluid traveling up or down a temperature gradient. The reactive flow simulations are also used to address the question of how to interpret reaction progress in systems in which mass transport is important. It is shown that classical water-rock ratios give no quantitative information about the actual fluid fluxes in a rock. The simulations demonstrate that in reactive flow systems which are either chemically out of equilibrium or underdetermined thermodynamically, it is necessary to use reactive flow modeling to interpret reaction progress properly.;Finally, reactive flow simulations are applied to a preliminary study of the origin of mineral zoning in the Main Stage veins at Butte, Montana. The simulations strongly suggest that some features of the observed vertical sequence of minerals are the result of temperature gradients in the hydrothermal system. However, the simulations fail to produce the advanced argillic alteration suite at Butte, which suggests that a model of magmatic gas contamination as proposed by Brimhall and Ghiorso (1983) may be more appropriate. |
