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During evaporation of natural waters, many possibilities of successive salt parageneses and of variations in chemical facies of residual solutions are offered. Chemical model based on equilibrium reactions can be very useful for explaining the complexity of natural geochemical paths and distribution of elements in salts and brines. This model calculates the distribution of complex aqueous species, ionic strength, activity coefficients, saturation of solutions with respect to salts and amounts of minerals precipitated along progressive evaporation of water. The model was applied to saline lakes of Ounianga Kebir in Tibesti (northern Chad), which occupy several depressions within the Nubian sandstone formation. In the initial diluted waters springing from sandstone all the major elements are present in the same order of magnitude. After concentration by evaporation, brines become strongly depleted in calcium and magnesium, initially enriched in sodium and bicarbonate and finally in potassium and chloride. Salt parageneses control chemical composition of waters, the calculated chemical model being in excellent accordance with natural observations. Silica and magnesium concentrations are controlled by an early precipitation of Mg-montmorillonite; magnesium molality is then regulated by the formation of huntite and dolomite; calcium is controlled by calcite, huntite, dolomite, pirssonite and gaylussite; sodium is first trapped in trona or natron, then by thenardite; potassium is finally picked up by sylvite; pH rises until reaching a maximum at about 10. Behavior and distribution of elements in salts and brines can be discussed considering the relative proportions of these elements in the initial solutions and the order in which salts precipitate.