The Messinian salt of the Mediterranean: geochemical study of the salt from the Central Sicily Basin and comparison with the Lorca basin (Spain)

A geochemical study has been performed on Messinian halite deposits fi om the Central Sicily Basin and the results compared with those published for the Lorca Basin (Spain), in order to provide a depositional model for these marine salt formations and to improve our understanding of the Messinian evaporitic event. Halite samples from boreholes and mine galleries from the Salt Member of the Gessoso Solfifera Formation of the Caltanissetta Basin (Sicily) were studied petrographically and geochemically. The bromine content of halite increases from the base of the Salt Member to the horizons containing kainite (layer B) up to 150 ppm. Upwards, the bromine content decreases and at the top of the member it drops down below 13 ppm. thus reflecting a marked dilution of the mother brine, which resulted in the precipitation of almost bromine-free salt. This dilution has been attributed to the inflow of continental waters in the literature. Fluid inclusion compositions at the top of the unit demonstrate the SO4-rich character of the brine, which is only slightly depleted in SO4 with respect to normal evaporated seawater and shows a significantly Mg and K content, indicating the marine origin of the brine which controlled the final precipitation. This is in agreement with the petrographically well-established primary origin of kainite. In the case of the Saline Unit from the Lorca Basin (SE Spain), bromine profiles are essentially similar to those described above, whereas fluid inclusion compositions at the top of the unit reveal the Mg, K and SO4-poor character of the brine and reflect an inflow of continental waters into the basin which were responsible for final dilution and bromine-free salt precipitation. Thus, in the Lorca basin, which occupied a marginal position in the Mediterranean Basin, dilution and salt reprecipitation at the top of the salt unit occurred when the basin was cut off from the sea and became completely isolated and desiccated. In the Caltanissetta basin, which occupied a relatively more central position, similar saline sediments were formed al the top of the Salt Member as a result of fresh marine waters inputs. Accordingly, in the Sicilian basin, the existing unconformity at the top of the Lower Evaporite Unit does not imply subaerial exposure or complete desiccation of the marine basin. Local tectonism probably controlled the different hydrochemical evolutions of these basins.