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Celestite-bearing evaporite mineralization is common in the Tertiary evaporitic units of Ulas-Sivas basin, East-Central Turkey.The firstoccurrenceof gypsum which is "laminated in character" (Balatinogypsum)took place in the shallow inner-lagoonor depressions duringregressioninthe upperEocene. Thickgypsumandoverlyingbedscomposedof the bedded,nodulargypsumand sandstonealternations were formed in a coastal sabkha and abandonedchannels within the meanderedrivers through the Oligocene time. The last occurrenceof evaporiticunits, representedby massive and bedded gypsum alternatingwith sandstonesand fossiliferous limestones resultedfrom limited marine transgressionin the early Miocene along the southern margin of the Sivas Tertiary basin.The celestitemineralizationpreferentiallytookplace withinthe Mioceneevaporitesand subordinateamountin the shallowmarinelimestones and terrestrial siliciclasticsranging from late Eocene to Oligocene in age. Celestite shows different depositional characters such as stratabound,massive,fractureand cave filling. Accordingto microscopicstudies,threepetrographictypes have been determinedin these different beds of celestite. They are prismatic and bar-like; sub-idiotopicand tabular; fibrous-radial.The sourceof the celestitein the Sivas Tertiarybasin has been debated.Thereis little evidencefor suggestingsedimentary-syngenetic origin. Many geologistsbelieve that it formed during late diagenesisor epigeneticstages,but this interpretation reflects a lack of a comprehensive study and only depends on field observations. This paper includes both field and laboratory studies (light, ore and electron microscopes, microprobe,87186Sr isotopes)and obtainedinitial results which have inclinedto the diagenetic-epigenetic origin.

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