Equilibrium of underground brines of western part of Tunguska artesian basinand minerals of enclosing rocks

This article describes the results of calculation of equilibria of brines in the western part ofthe Tunguska artesian basin with the most renowned carbonate, sulfate, chloride and aluminosilicate minerals. The diagrams are made ofthe degree of saturation of the brines of suprasalt, salt-bearing and subsalt formations with host rock minerals most common in thegeological context of the study area. Underground brines in the study area occur ranging from depths of 500 m stratigraphically confinedto the deposits of the Riphean, Vendian, Cambrian, Ordovician, Silurian, Permian and Triassic periods. In total 128 samples wereexamined with salinity brines from 107 to 528 g/dm3. Among them are the most common brines of sodium chloride, sodium, calcium,calcium-sodium, calcium and less common brines of sodium and magnesium by the classification of S.A. Shchukarev. The calculationof the activities of the components of the aqueous solution was carried out using the software package HydroGeo allowing the use of KSPitzer model designed specifically for highly mineralized waters and brines. Minerals most frequently occurring in the of the sectionunder study (carbonate (calcite, magnesite, dolomite, strontianite, siderite), sulfate (gypsum, celestite), chloride (halite, sylvite) andsome aluminosilicates) have been chosen for charting the degree of saturation. Brines are mostly in equilibrium with calcite anddolomite. Undersaturation of carbonate brines is usually connected with low pH values. The balance of brine with gypsum is not oftenobserved due to the low content of SO42- in the samples, which is connected with its reduction to hydrogen sulfide. The saturation ofhalite brine to the point of equilibrium is observed only in samples with salinity of more than 370 g/dm3 and high concentrations ofsodium and chlorine. The studied brines are in equilibrium with hydroxide (gibbsite), mica (muscovite) and clay (kaolinite, illite,montmorillonite), but not in equilibrium with the primary aluminosilicate minerals. The main factor controlling the degree of saturationof the brines with the primary aluminosilicates is alkaline reserve defined in these conditions by the degree of enrichment of the CO2solution and the current state of the carbonate system. The system 'underground brine - the rock' is in equilibrium-non-equilibrium,which determines the continuity of the interaction of rocks with water, sedimentation of some minerals and dissolution of others.Secondary mineral formation controls the concentration of cations in the solution, so the brine continues to dissolve the primaryaluminosilicate minerals. It was found that despite the high concentration of dissolved substances, prolonged contact time of brines withrocks, they will never reach equilibrium with the primary aluminosilicates. The main reason for this is secondary mineral formation andneutralization of alkalinity by mineral and organic acids.

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