ORIGINAL PAPER Microstructure and geochemistry studies on Messinian gypsum deposits from the Northern Coast of Egypt Ahmed Melegy & Ismael S. Ismael Received: 25 May 2012 / Accepted: 11 January 2013 # Saudi Society for Geosciences 2013 Abstract Messinian gypsum deposits from Dir El-Baraqan area, Northern Coast of Egypt, were investigated by stable sulfur isotope method, X-ray diffraction, infrared spectroscopy, optical microscopy, and scanning electron microscopy to dif- ferentiate features formed under substantial microbial influen- ces as indicator of paleoenvironments. Petrographically, gypsum deposits were classified into three types: biolaminated gypsum, disordered selenite, and swallow-tail selenitic crys- tals. Biolaminated gypsum is characterized by regular alternat- ing dark and light laminae, which were formed due to the seasonal environmental changes in Dir El-Baraqan area. Stable sulfur isotope data show that the gypsum deposits are characterized by δ 34 S values ranging from +18.1 to +28.1 ‰. In swallow-tail gypsum, the δ 34 S values are characterized by a narrow range (from +20.0 to +20.2 ‰) which is considered as the primary phase. In biolaminated gypsum, the δ 34 S values ranged from +22.8 to +28.1 ‰ which is considered as the secondary phase. However, the white laminae are character- ized by δ 34 S values ranging from +22.8 to +24.1 ‰, while dark laminae are characterized by δ 34 S values ranging from + 27.2 to +28.1 ‰. The high δ 34 S values of dark laminae revealed the increasing activity of sulfate-reducing bacteria. Keywords Gypsum . Sulfur isotope . Laminae . Sulfate- reducing bacteria . Paleoenvironmental Introduction Gypsum (CaSO 4 2H 2 O) is one of the common sulfate minerals, forming in a variety of environments including hydrothermal vents near mid-ocean ridge, diagenetically altered marine sediments, and evaporates. The classical ancient gypsum deposits such as the Miocene has been systematically studied (Aref 2003; El Khoriby 2005; Rosell and Anadón 2010; Quevedo et al. 2005; De Lange and Krijgsman 2010 ; Escavy et al. 2012 ; Ramadan and Zalamah 2011; Aref and Taj 2012). A few studies were made on gypsum as indicator on paleoenvironments. Gypsum in Dir El-Baraqan area was selected for this investigation. Dir El-Baraqan area covered about 15 km 2 ; it is situated between latitudes 29°11′ and 29°13′ N and longitudes 30°27′ and 30°29′ E (Fig. 1). The sedimentary sequence in the Dir El- Baraqan was measured from the exposed sections in the gyp- sum quarries (Fig. 2), which lie about 60 km southeast of the El-Alamein town, south of the Mediterranean coast. The evap- orites in Dir El-Baraqan area were described as coastal lagoon deposits (Youssef and Kamel 1985). Structurally, the beds of Dir El-Baraqan area are characterized by ill stratification which varies greatly both in degrees of prominence and in details of structure. In general, they occur in lenticular forms with great variations in thickness (El-Shazly et al. 1976). Melegy (1991) distinguished seven alternating layers of gypsum and clay indicating seven periods of dryness. Hassan (1977) studied the mineralogy and geochemistry of the evaporite deposits encountered in the coastal section, between west Alexandria and El-Alamein area. Melegy (1991) evaluated gypsum depos- its from Dir El-Baraqan quarries as pure high-grade deposits to be used industrially for plaster production. Youssef and Kamel (1985) concluded that the Dir El-Baraqan evaporites subjected to the activities of blue-green algae in an anaerobic condition. Youssef (1988) also reported his observations on the deposi- tional and diagenetic forms of some Neogene evaporites at Gebel El-Hagif and Dir El-Baraqan areas, northern of the Western Desert. The biochemical activity associated of bacteria action in biolaminated gypsum formed by the interplay between micro- bial and physical mechanisms causing precipitation and trap- ping of iron (Rosell and Anadón 2010; Aref and Taj 2012). A. Melegy Department of Geological Sciences, National Research Center, Dokki, Cairo, Egypt I. S. Ismael (*) Faculty of Science, Suez University, Suez, Egypt e-mail: Ismaelsayed@hotmail.com Arab J Geosci DOI 10.1007/s12517-013-0829-1