The mechanics of hydrothermal systems: II. Fluid mixing and chemical reactions

In the accompanying paper, Part I, hydrothermal mineralising systems are considered as open chemical reactors that operate far from equilibrium to develop an exothermal alteration system with veining and brecciation, followed by competition between endothermic mineralisation and exothermic mineral reactions. In this sequel paper, we examine the interplay of these processes with fluid transport and the impact upon mineral deposition. Chemical reaction and flow in porous media admit two distinct mechanisms which result in significantly accelerated mixing. First, gradients in physical parameters such as chemical potential, fluid density and surface tension generate flow instabilities which form fluid/chemical mixing machines that propagate with the reaction front. Second, so-called chaotic advection, a behaviour in which fluid particles follow chaotic trajectories, arises inherently from Stokes flow in open porous networks as a result of the complexity of the pore geometry. For pore length-scales greater than ~ 1 mm, these mechanisms significantly enhance mixing and hence metal/sulphide deposition. Furthermore, chaotic advection can also alter qualitative characteristics such as stability or speciation of non-equilibrium chemical reactions, with significant implications for enhanced mineralisation rates. Such interactions between chemical reaction and fluid advection generate mineral deposits with multifractal spatial signatures similar to those observed in the field. Such multifractal signatures render the spatial distributions non-ergodic, a fact which process based geostatistics must take into account.