The genesis of dedolomites: a discusion based on reactive transport modeling

Abstract

Dedolomites are rocks formed by the replacement of dolomite with calcite which is driven by the infiltration of Ca-rich water. This process has been described in the literature as either increasing, preserving or decreasing porosity of the initial dolostone. According to textures observed in Prades, NE Spain, dedolomitization took place due to two simultaneous processes: non-pseudomorphic replacement of dolomite with calcite at the wall-rock of fractures; and pseudomorphic replacement in the bulk rock. A multicomponent reactive transport model is used to analyze the two dedolomitization processes observed. Diffusion of ions from the fractures was the driving force for non-pseudomorphic replacement. In the wall-rock, the dissolution of dolomite and precipitation of calcite took place in accord with a 1:2 stoichiometry. As a consequence of the mineral molar volume, the porosity was sealed and replacement limited to a rim on both sides of the fractures. According to calculations, the development of a rim took less than 500 years. Advection of ions from the top of the dolomite bed was the driving force for pseudomorphic replacement in the bulk rock. Due to the absence of empty volume for expansion, replacement was governed by the condition of equal volumetric rate for dolomite dissolution and calcite precipitation. According to calculations, pseudomorphic replacement of 1 m of dolostone was completed after about 10 5 years for reasonable values of flow parameters. As the infiltrating water was also undersaturated with respect to calcite, one stage of calcite dissolution postdated dedolomitization. This process was responsible for the widening of open fractures, where the dissolution front followed the dedolomitization front into the dolomite matrix. Intensive calcite dissolution was also developed, following the advective flux from the top of the bed downwards. Drusy calcite is observed to precipitate in the pore spaces which consist of intergrains, fractures, and dissolution cavities. This process is not predicted in detail by reactive transport calculations, although it may be due to calcite supersaturation of the solution resulting from pseudomorphic dedolomitization.