The role of stable and traveling reactive waves in mineralization

Fluid mixing is interpreted as one of the main drivers for the development of hydrothermal mineralization whereby the actual physical processes that lead to mineral precipitation, and thus ore localization, are poorly understood. In this contribution, we will shed light on the mechanisms that are active in a simple fluid-mixing scenario by simulating the infiltration of a metal-rich fluid into a rock saturated with seawater derived pore-fluid and study the developing mineral saturation patterns.

We combine an advection-diffusion code in the microstructural model Elle with the geochemical module iphreeqc to study the distribution of enhanced saturation indices during fluid mixing. In the simulations the hot highly saline metal rich fluid enters the small 5x5m system through two high-permeable faults from below and percolates into the pore space. For the fluid we solve transport of temperature and 12 chemical species, giving us a fluid composition at every node in the model. We then use iphreeqc to calculate the mineral saturation indices for minerals in every node and we use these values as a proxy for reaction localization. In order to better understand the effects of fluid mixing on mineralization we specifically look at the saturation index of Baryte, which is a mineral in the investigated system that only precipitates when elements of both fluids are present. Our simulations show that the saturation index of Baryte is at a maximum in a fluid comprising 90 to 80 percent of pore fluid and 10 to 20 percent of metal rich fluid. During the infiltration into the permeable faults, the metal rich fluid pushes the pore fluid away, and mixing is occurring at the interface between the two fluids and is driven mainly by diffusion. With temperature diffusion being three orders of magnitude faster than matter diffusion, the temperature is negligible for the mixing, which is only driven by matter diffusion at the model scale.

We will show that two types of reactive waves with high saturation indices of Baryte develop in the system: travelling and stable waves. Traveling waves progress during advection through the permeable faults and layers and are potentially too fast for minerals to precipitate. Therefore, these areas probably remain permeable in a natural system during advection dominated transport. In contrast, areas with low fluid velocities, and hence low advection, are diffusion dominated with reaction waves that are stable over a long time. These areas are prone for mineral reactions, because there is enough time for the reactions to take place. Stable reactive waves and thus areas of mineralization are fault walls, areas below seals, and areas between two faults where fluid velocities are diverging. We discuss the implications of our results in light of hydrothermal mineral systems.