Meteoric calcitization of magnesite in Miocene lacustrine deposits (Calatayud basin, NE Spain)

Sedimentary magnesite deposits are commonly present in evaporite successions in the Tertiary Calatayud basin (NE Spain). Outcropping evaporite sequences, mainly composed of secondary gypsum after anhydrite-glauberite, show centimetre-thick layers of magnesite passing laterally into calcite pseudospar. Petrographic and stable isotope data indicate that calcite pseudospar formed via late-stage calcitization of magnesite under near-surface conditions. Calcitization occurred by the interaction between magnesite and Ca-enriched waters derived from the dissolution of gypsum and/or glauberite. The geochemical simulation of this process, petrographic evidence, and the correlation between the magnesite/calcite ratio and the delta(18)O values of both magnesite and calcite samples across the reaction front are in agreement with the existence of neoformed O-18-depleted magnesite during the waning stages of the calcitization process. (C) 1998 Elsevier Science B.V. All rights reserved.