Mineralogical and chemical characterization of the sepiolite/Mg-smectite deposit at Mara (Calatayud Basin, Spain)

The Mara deposit occurs in the marly-carbonate transitional facies of the Calatayud Tertiary lacustrine basin, and consists of alternating levels of clays, marls and carbonates. The mineral associations consist of detrital minerals (illite, interstratified illite/smectite, quartz, feldspars, dioctahedral smectite, chlorite and kaolinite), neoformed phyllosilicates (sepiolite, trioctahedral smectite and palygorskite) and carbonates (calcite and/or dolomite); occasionally, heulandite-clinoptilolite, apatite, opal-A and opal-CT are also found. The REE, F and trace element contents clearly distinguish sepiolite and/or Mg-smectite from the detrital aluminosilicates, suggesting that the former correspond with primary phases originating by precipitation from basin solutions in a lacustrine environment, whereas palygorskite was derived from post-depositional processes from the transformation of detrital aluminosilicates. Oxygen isotope data for Mg-smectite and flint nodules suggest that these were formed in equilibrium with meteoric waters; both Delta (super 18) O and Delta (super 13) C values and molar ratios of Sr, Na and Mg vs Ca in carbonates indicate a meteoric parentage.