Bentonitic tuffs of the northern Qazvin: geochemical variations and genesis

In western Alborz zone (N-Iran), there are two adjacent bentonitic deposits in the same stratigraphic level (epiclastic deposits), but different in mineral assemblages of alteration zones. The unaltered tuffs belong mainly to lithic-crystal tuffs and crystal-vitric tuffs and are composed of quartz, plagioclase, biotite, K-feldspar and opaque as well as lithic clasts and a considerable amount (up to 40 vol. %) of glassy matrix. The XRD analysis results show that montmorillonite + alkali feldspar + plagioclase + quartz ± illite ± kaolinite, are the mineral assemblages in the alteration zones of the study area. However, due to the greater abundance of illite in the Nyagh district, its bentonitic deposit is of higher quality than the Ardebilak deposit. Major element data suggest that Na2O, K2O and to a lesser extent Al2O3 and Fe2O3 are depleted in the alteration zones. Whereas, SiO2 and CaO show different behavior in two study districts. It means that in the Nyagh area, SiO2 is enriched and CaO is depleted. While, in the Ardebilak area, SiO2 and CaO are depleted and enriched, respectively. In both areas, REEs show enrichment in the alteration zones, while Eu has a prominent depletion (especially in Nyagh) that may be due to alteration of feldspars and leaching of Eu in the aqueous fluids. Except for Zr that shows a considerable depletion (ΔCi = -53 in Ardebilak and ΔCi = -64.91 to -84.97 in Nyagh), other HFSEs (especially Nb, Ta, Hf) were immobile during alteration event. It seems that the hot and very acidic (pH < 4) or alkali fluids may be responsible for Zr depletion in the altered zones. In conclusion, it seems that a hidden granitic intrusion in the fractured zone of Nyagh could be responsible for heating the circulating fluids in the tuffaceous rocks. Such a conclusion is verified by more altered tuffs (montmorillonite + illite + kaolinite), the basic dyke swarms and a dacitic dome near the Nyagh area.