Geochemistry of underground waters of the Bakchar iron ore deposit area (Tomsk region)

This article presents the results of studying the chemical compositionof the Bakchar iron ore deposit area groundwater, located 200 km northwest of the city of Tomsk in poorly rendered habitable taigaswampy area. Groundwater is in five water-bearing horizons. Almost universally, it contains elevated and high concentrations of iron30 mg/l, as well as manganese, organic matter; it is depleted of free O2. The ground water from the surface of the first three waterbearinghorizons (QII tb - P3 lt, P3 nm, P2-3 jur) lies in the sand and gravel deposits with clay, loam and lignite layers. They are fresh (salinityfrom 0.4 to 0.7 g/l, an average of 0.6 g/l), neutral and alkalescent (pH 6.8-7.8), hard (5-9.4 meq/l), hydrocarbonate calcium and calcium-magnesium. These waters are Fe-enriched up to 10 mg/l, the average of 4 mg/l, Mn-enriched up to 2 mg/l. The groundwater of theUpper Cretaceous deposits includes two water-bearing horizons confined to sandy-clay formations of the Gankovskaya (K2 gn) and Ipatovskaya(K1-2 ip) suites. The water of the Gankovskaya formation is fresh, with higher salinity (0.6-1.2 g/l). There is a shift of waterfrom hydrocarbonate calcium to sodium bicarbonate. This causes the large variation in the values of hardness (from 0.3 to 9.6 meq/l). Incomparison with the higher-lying waters, they have higher alkalinity (pH 6.8-8.6). The content of iron in the groundwater, on the contrary,decreases to 2.9 mg/l on the average, but it is still high. The thermodynamic calculations of equilibriums in the water-rock systemwere made. It is shown that all the water is not in equilibrium with the primary water-bearing rocks minerals: feldspar, muscovite, biotite,garnet, pyroxene, hornblende, epidote, chlorite, and many others. However, the studied water is in equilibrium with montmorillonite,illite, kaolinite and other clays, and calcite and siderite. Therefore, under these conditions the minerals of the first group dissolveactively, especially pyroxene, epidote and hornblende, unstable under these conditions and, therefore, being the source of iron and otherelements. Then, as a result of incongruent dissolution, secondary minerals form, connecting parts of Si, Al, Ca, Mg, Na, K and Fe. Accordingto the accepted classification of S.L. Shvartsev, according to which waters are named by the composition of the elements thatbind the crystal lattice of secondary minerals formed in this system, the groundwater of the Bakchar iron deposit is the siliceous calcium-iron geochemical type of water. It can form an association of secondary iron minerals observed in the sedimentary iron-ore depositsof the Bakchar type.

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