Facies analysis, petrography and geochemistry of the Neogene gypsum deposits in the Eshtehard area, Alborz Province, Iran

The evaporite deposits examined in this study are located in the Eshtehard area (SW of the Alborz Province). Four outcrop sections (Mard Abad, Eshtehard, Salt Mine and Rud Shur) and thirty gypsum samples were selected for facies analysis, petrographical and mineralogical investigation in combination with the geochemical analyses. The Neogene evaporites are composed of massive, selenite, nodular and satin-spar gypsum lithofacies. Three different textures were recognized under microscope: porphyroblastic, alabastrine, and fibrous gypsum. Petrographical investigations and X-ray diffraction analysis showed the evaporite beds are mainly composed of gypsum, with no anhydrite relics. The characteristics of these litho- and microfacies indicate gypsum deposited in the lacustrine and sabkha settings. The ICP-OES analysis shows significant differences in major and trace element contents of the four types of gypsums. The concentrations of Sr, Fe, Al, Mg and Na were increased in massive gypsum, while the crystals represent a decreased in size. The contents of these elements were also decreased in nodular, fibrous and selenite gypsums. These are probably indicating an increase and decrease in brine concentration, respectively. Paleoclimate condition is simply determined for the Neogene evaporites using geochemical approach. The results suggest a shift from semi-arid (bottom) to arid (top) paleoclimate conditions.