A prelimenary study: Celestite-bearing gypsum in the Tertiary Sivas basin,Central-Eastern Turkey

Celestite-bearing evaporite mineralization is common in the Tertiary evapor itic units of Ulas-Sivas basin, East-Central Turkey. The first occurrence of gypsum which is "laminated in character" (Balatino gypsum) took place in the shallow inner-lagoon or depressions during regres sion in the upper Eocene. Thick gypsum and overlying beds composed of the b edded, nodular gypsum and sandstone alternations were formed in a coastal s abkha and abandoned channels within the meandered rivers through the Oligoc ene time. The last occurrence of evaporitic units, represented by massive a nd bedded gypsum alternating with sandstones and fossiliferous limestones r esulted from limited marine transgression in the early Miocene along the so uthern margin of the Sivas Tertiary basin. The celestite mineralization preferentially took place within the Miocene e vaporites and subordinate amount in the shallow marine limestones and terre strial silicielastics ranging from late Eocene to Oligocene in age. Celesti te shows different depositional characters such as strata-bound, massive, f racture and cave filling. According to microscopic studies, three petrograp hic types have been determined in these different beds of celestite. They a re prismatic and bar-like; sub-idiotopic and tabular; fibrous-radial. The source of the celestite in the Sivas Tertiary basin has been debated. T here is little evidence for suggesting sedimentary-syngenetic origin. Many geologists believe that it formed during late diagenesis or epigenetic stag es, but this interpretation reflects a lack of a comprehensive study and on ly depends on field observations. This paper includes both field and labora tory studies (light, ore and electron microscopes, microprobe, Sr-87/86 iso topes) and obtained initial results which have inclined to the diagenetic-e pigenetic origin.