MINERALOGICAL AND CHEMICAL CHARACTERIZATION OF THE SEPIOLITE MG-SMECTITE DEPOSIT AT MARA (CALATAYUD BASIN, SPAIN)

The sepiolite / Mg-smectite deposit of Mara is located in the marry-ca rbonate transitional facies of the Calatayud Tertiary lacustrine basin . The mineralized intervals consist of alternating levels of clays, ma rls and carbonates. The mineral associations are made up of detrital m inerals (illite, interstratified illite-smectite, quartz, feldspars, d ioctahedral smectite, chlorite and kaolinite); neoformed phyllosilicat es (sepiolite, trioctahedral smectite and palygorskite); and carbonate s (calcite and/or dolomite). Occasionally, heulandite-clinoptilolite, apatite, opal-A and opal-CT are also found. REE and trace element cont ents of the first transition series, on the one hand, and the F conten t, on the other, clearly distinguish sepiolite and/or Mg-smectite from the detrital aluminosilicates, suggesting that the two former corresp ond to primary phases originated by precipitation from basin solutions in a lacustrine environment, whereas palygorskite is derived from pos tdepositional processes from the transformation of detrital aluminosil icates. Oxygen isotope data from Mg-smectite and silex nodules suggest that these were formed in isotopic equilibrium with meteoric waters. Both oxygen and carbon isotope values and molar ratios of Sr, Na and M g vs. Ca in carbonates indicate that the precipitating solutions were of meteoric parentage, being the main reservoir of carbon pedogenic in all cases. The apatite formation took place diagenetically from the d ecomposition of vertebrate bone accumulations.