Petrología y geoquímica isotópica de los depósitos de magnesita de la Cuenca de Calatayud (provincia de Zaragoza). Interpretación paleoambiental

Abstract

Miocene sediments from the Lower and Intermediate units of the Calatayud Basin show abundant magnesite beds, commonly associated to sulphate lake deposits (anhydrite, gypsum, glauberite), marls and clays. The magnesite is cryptocrystalline and occurs as mm- to dm-thick, massive, whitish deposits interbedded with claystone, marlstone and laminated and nodular gypsum. Four lithofacies associations containing magnesite beds (MLH, MYS, MYP, MIN) have been distinguished. These lithofacies associations occur in two main sedimentary sequences which reflect the evolution of high-salinity lake and moderate- to high-salinity lake sub-environments, respectively. The texture of magnesite, as observed from magnesite-rich samples under S.E.M., is mainly formed of a dense, homogeneous aggregate of subhedral to euhedral rhomb-like crystals showing about 1 micron in average size. Isotope values (δ18O, δ13C) from 34 magnesite samples are relatively spread, with δ18O values from -4,8 to +4,9‰ PDB y negative values from δ13C (-9,3 to +0,4‰ PDB), being lower in the magnesite beds corresponding to MLH and MYS lithofacies associations. The stratigraphic, textural and isotopic features suggest that the magnesite formed primarily under evaporitic conditions, probably as a result of mixing of concentrated and meteoric waters enriched in biogenic CO2. Moreover, the negative δ13C values determined in many magnesite samples could reflect a bacterial mediation in the precipitation of the magnesite. Sedimentological analysis of the lithofacies associations containing magnesite beds and their sequential arrangement indicates that this carbonate formed in both hypersaline and moderate- to high-saline lake subenvironments.