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Formation of Sharp Dolomite Reaction Fronts and Sucrosic Dolomite: Insights from Bed-Scale Reaction Transport Modeling

Abstract

Reaction-transport modeling (RTM) provide opportunities to explore dolomitization as a complex non-linear system. RTM studies allow concepts to be tested and they reveal complex interactions between variables (initial mineralogy, initial texture, fluid composition, flow regime, temperature), rates of alteration and dolomitization products (e.g. geometries, porosity). We use the Sym.8 RTM to consider the evolution of porosity and textural heterogeneity during dolomitization at the scale of individual beds. Sym.8 is a continuum-based water-rock interaction simulator that a dynamic composite media approach for rock texture. As a result, permeability evolves at each nodal point through a Kozeny-Carman expression that incorporates evolving grain-size diameter and porosity. Mineral surface-area is also a dynamic variable that provides feedback to the kinetics of dolomite growth. In 2D we modeled a 2 m-high by 18 m-long bed with 10x30 cm cells. Initial minerals and dolomite crystals were modeled as spheres that dissolve or grow through the simulation. Models tested the effect of variations in fluid chemistry (Mg/Ca ratios, alkalinities, and dolomite saturations), fluid-flow rates, bed height, initial mineralogy, initial texture (grainstone, wackestone), nucleation densities, temperatures, and extent of initial porosity and textural heterogeneity. Sharp reaction fronts, like those typically seen in association with fault-related hydrothermal dolomites, only form at high temperatures, very low flow velocities (= cm/yr), and from waters saturated with respect to both calcite and dolomite. All other scenarios produce broad and diffuse reaction fronts. High porosities, as seen in sucrosic dolomites, are favored by Mg-rich fluids, metastable initial mineralogies (especially aragonite), and/or smaller initial grain sizes. When these conditions prevail, carbon is inefficiently cannibalized from the precursor minerals during the early stages of dolomitization and higher porosities develop. The porosities are higher by 4% to 10% relative to a low-Mg calcite precursor. The higher porosities are preserved once all precursors are consumed. These results help to restrict the conditions and fluids that should be contemplated when examining the origin of fault-related hydrothermal dolomites and sucrosic dolomites.