GEOCHEMISTRY OF SPANISH SEPIOLITE PALYGORSKITE DEPOSITS - GENETIC CONSIDERATIONS BASED ON TRACE-ELEMENTS AND ISOTOPES

Sepiolite-palygorskite deposits in Spanish Tertiary basins were formed in lacustrine environments. The mineral associations present in the mineralised intervals are made up of neoformed phyllosilicates (sepiolite, palygorskite, stevensite), detrital silicates (quartz, feldspars, illite, interstratified smectite-illite, Al-smectite and kaolinite) and carbonates (calcite, dolomite). Opal-A, gypsum and halite may also appear sporadically. Two groups of chemical elements and minerals can be distinguished according to their origin: Al, Ti, Fe, Mn, K, REE and transition trace elements are almost exclusively included in the detrital Al-silicates; Mg, Ca, Cl, F and Li are concentrated in the minerals formed in the depositional basins. Si, Na, Sr and Ba are contained in both detrital and neoformed minerals. REE, transition trace elements, F and Li contents can be used to distinguish between phyllosilicates formed by chemical precipitation in the depositional basins, detrital phyllosilicates, and those formed by transformation of the latter during early diagenetic processes. These data, together with those on deltaO-18 isotopic fractionation indicate the formation of sepiolite and Mg-smectite as chemical precipitates, whereas palygorskite would derive from diagenetic transformation of other inherited clay minerals. The values calculated for the alpha(sepiolite-water) and a(palygorskite-water) fractionation factors at 20-degrees-C are 1.031 and 1.027, respectively.