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Celestine-bearing evaporite mineralization is widespread in Tertiary Sivas Basin. The Akkaya celestine deposit is the biggest open pit mine in the Sivas area. Akkaya is Eocene in age and characterized by having massive ore. Compared to upper continental crust, evaporites at Akkaya are depleted in Ba, U, Na, K, Al, T, Mn, and REEs and enriched in Sr, Ca and Pb. Na/K, Mg/Ca, Ca/Na and Ba/Sr ratios of celestine and a slightly negative Ce anomaly in evaporite minerals may indicate that sea water is not the only fluid responsible for the formation of evaporite minerals at Akkaya; basinal brine and/or hydrothermal meteoric water may also have played a role in the formation of these evaporites. Celestine minerals have very low Na/K, Mg/Ca ratio, and have very high Ca/Na ratio compared with sea water, Dead Sea and Red Sea, suggesting freshwater input to the basin during mineral precipitation. Relationships between δ34S, δ18O, δD, and 87Sr/86Sr show that all gypsum and anhydrite and most celestine have signatures of modified marine evaporites. δD and calculated δ18O of water data from celestine ranges from -88 to -117‰ and -1.7 to +0.2‰ respectively. From this we infer that meteoric water played a role in the formation of the Akkaya celestine deposit. δ34S data (22-30‰) from Akkaya indicate that sulfur is mainly of marine origin. Evaporite minerals have lower 87Sr/86Sr ratios (0.707202 - 0.707703) compared to average upper crust 87Sr/86Sr ratio (87Sr/86Sr ∼ 0.719) and Eocene sea water (87Sr/86Sr ∼ 0.708). The only reasonable source for these lower values is young mantle-derived volcanic rocks. We suggest that circulating meteoric water interacted with the volcanic materials and underwent Sr isotope exchange. The celestine deposited from this mixture of meteoric and evaporated seawater, therefore, has an isotopic composition reflecting this mixture.
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