نهشت سلستيت استراتيفورم منطقه افتر، باختر سمنان

Deposition of Stratiform Celestite of Aftar Region, West of Semnan

The sequence of sedimentary rocks of Aftar syncline (35 Km west of Semnan) be longs to Karadj, Semnan and Kond formations. Celestite occurs at the lower part of the Kond formation. The Kond formation itself is mainly made of gypsum, limestone and sandy limestone deposited in an evaporitic environment. Based on the type of fossils found in the limestones of the Kond formation, the age of this sequence is attribnted to upper Eocene. The formation was formed during a slow and fluctuating subsidence in an evaporitic shallow basin. The layered, rhythmic and elongated lenticular celestite crystals are positioned parallel to the general bedding of the sequence and formed at early principal diagenetic stage. Two forms of celestite crystals are distinguished: a) long automorphic, to xenomorphic, slabs, and b) elongated lenticular gypsum pseudomorphs. The act of silicification, at late diagenetic stages, affected the celestite bearing layers more than other layers. Based on the shape and arrangement of celestite in the evaporitic sequence the genesis of major portion is believed to be diagenetic; while a small portion has been probably formed synsedimentarily by saturation of strontium ions in solutions of evaporitic environment. It seems that strontium sulphate (celestite) was formed due to the abundant gypsum in shallow evaporitic environment. Furthermore; celestite could have been formed due to the migration of strontium ions from lower layers (Semnan and Karadj formations) or by transport of the ions by meteoric fluids. Relatively high strontium ion content of the Semnan and Karadj formations suggests a high probability of ion supply to form celestite from these formations.