THE GENESIS OF DEDOLOMITES - A DISCUSSION BASED ON REACTIVE TRANSPORTMODELING

Dedolomites are rocks formed by the replacement of dolomite with calci te 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 ion s from the fractures was the driving force for non-pseudomorphic repla cement. In the wall-rock, the dissolution of dolomite and precipitatio n of calcite took place in accord with a 1:2 stoichiometry. As a conse quence of the mineral molar volume, the porosity was sealed and replac ement limited to a rim on both sides of the fractures, According to ca lculations, the development of a rim took less than 500 years. Advecti on 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 empt y volume for expansion, replacement was governed by the condition of e qual volumetric rate for dolomite dissolution and calcite precipitatio n. According to calculations, pseudomorphic replacement of 1 m of dolo stone was completed after about 10(5) years for reasonable values of f low parameters. As the infiltrating water was also undersaturated with respect to calcite, one stage of calcite dissolution postdated dedolo mitization. This process was responsible for the widening of open frac tures, where the dissolution front followed the dedolomitization front into the dolomite matrix. Intensive calcite dissolution was also deve loped, following the advective flux from the top of the bed downwards. Drusy calcite is observed to precipitate in the pore spaces which con sist of intergrains, fractures, and dissolution cavities. This process is not predicted in detail by reactive transport calculations, althou gh it may be due to calcite supersaturation of the solution resulting from pseudomorphic dedolomitization. (C) 1998 Elsevier Science B.V. Al l rights reserved.