Recent bloedite from İshaklı Lake, Çankırı-Çorum Basin, Turkey: a mineralogical and hydrogeochemical investigation

Abstract

Bloedite (Na2Mg(SO4)2·4H2O), which is a sodium-magnesium sulfate mineral, has been determined for the first time in Turkey, in the Ishakli Lake within the Cankiri-Corum Basin. The Ishakli Lake is a seasonal playa lake which is an east–west extending lake with an area of 0.22 km2. Geochemical analyses (XRD, XRF, SEM) of the mineral crust samples indicate that they are composed chiefly of bloedite and thenardite (Na2SO4) minerals and lesser amounts of halite (NaCl) and gypsum (CaSO4·2H2O). In the blodite-thenardite association, thenardite mineral grows on bloedite crystals and it is crystallized after bloedite. The well and the spring (and fountain) waters in the vicinity of the lake are saturated with respect to calcite, dolomite, aragonite and talc minerals. In the waters of the Ishakli Lake, however, the degree of saturation of these minerals gradually increases and they become saturated with respect to anhydrite and gypsum as well. Lake waters represent a hydrochemical facies of Na–Mg–SO4–Cl (type I). Tritium data (3H) indicate that lake waters have higher tritium content (7.55 TU) than the springs and fountains (5.45–6.15 TU), and although there is a thick rock-salt drilled at the bottom of lake (to a depth of 220 m), no recent halite precipitation is observed in the lake bottom. This is attributed to recent recharge and the absence of deep groundwater circulation. The δ18O–δ2H data show that spring and fountain waters are in shallow circulation and have undergone a slight evaporation while waters of Ishakli Lake experienced an intense evaporation. According to Mineral Formation Model proposed herein, a portion of the rainfall reaches directly the lake. Additionally, there is recharge from springs, which travel through the Bozkir Formation and reach the lake. During hot periods, evaporation process begins with carbonate and gypsum precipitation as a white-colored crust on the mud flat along the shore of the lake. Therefore, calcium deficit occurs in the lake water. Further evaporation and possible cation exchange process in clay and muds at the lake bottom might be responsible for the formation of Na-sulfate (bloedite + thenardite) mineralization.