Mineralogy and geochemistry of the carbonates in the Calatayud Basin (Zaragoza, Spain)

The Calatayud Basin is an elongated NW-SE depression in northeastern Spain. It is filled with continental Miocene sediments resulting from erosion of Palaeozoic and Mesozoic rocks of the Castilian and Aragonese branches of the Iberian Range. Miocene series consist of lutitic, marly, carbonate and evaporitic levels characteristic of a short-lived salt lake or a low-salinity playa-lake depositional environment. The carbonates detected in the series were calcite, dolomite and magnesite. The calcites generally show micritic textures, anhedral to subhedral morphologies and contain from 0 to 1 mole% MgCO3, and so can be considered “low-Mg calcites”. Their mean isotopic values are δ18O = −7.8% (PDB) and δ13C = −7.7% indicating precipitation in isotopic equilibrium with meteoric water and an extensive contribution of C02 of organic origin to the total dissolved C, respectively. The calcites present Mg/Ca, Sr/Ca and Na/Ca molar ratios that suggest precipitation from Na-enriched meteoric water. Dolomites are non-stoichiometric (44.5–47.7 mole% MgC03), disordered, micritic and anhedral to subhedral; their mean isotopic values [δ18O = +0.4% (PDB) and δ13C= −3.8%] indicate that they precipitated from more evolved water and with a higher contribution of atmospheric CO2 to the total dissolved C, respectively, than those of the calcites. Moreover, they present Mg/Ca, Sr/Ca and Na/Ca molar ratios suggesting precipitation from water with similar composition to seawater. The isotopic composition of magnesite [δ18O = +4.6% (PDB) and δ13C = −4.0%], together with the higher concentration in Sr and Na, indicates that this phase probably precipitated from more evolved water. The isotopic values and trace-element contents of carbonates appear to confirm the mineralogical trend from calcite to dolomite and to magnesite corresponding to the progressive increase of evaporitic concentration as the water, in a hydrologically closed system, was subjected to more acutely and conditions.