Fizykochemiczne warunki powstawania cechsztyńskich utworów solnych regionu bałtyckiego

Wołodymyr M. Kowalewicz



Samples of Zechstein halite were taken from three boreholes in the Peri - Baltic area (Fig. 1) and studied with the aim to explain physico-chemical conditions of sedimentation and recrystallization of salt deposits. The cores derive from the central, potash-bearing part of the basin (N4 Poddubnaya), peripheral part (N3 Gusevskaya) and transitional part (NI Aleksandrovskaya). Chemical composition of fluid inclusions was determined applying ultramicrochemical method (Petryczenko, 1973) and the temperature of salt crystallization with method of homogenization of two phase inclusions. Study of fluid inclusions in sedimentary halite (Fig. 2 - 6) indicates that in respect of chemical composition, brines of the Peri - Baltic Zechstein basin were similar to concentrated modern ocean water although the content of sulfate ion was slightly lower. The brines are also similar to brines from other Permian and Neogene marine salt basins which owing to sulfate brine composition are unique in Phanerozoic history of salt accumulation (Fig. 7; see also the paper by 0.1. Petryczenko &V.M. Kowalewicz in this issue). It is remarkable that large deposits of potash (of sulfate composition), sulfur, boron and strontium are related to evaporites of Permian and Neogene age. The sulfate type of brines in evaporite basins is a probable control of origin of hydrogen sulfide in underlying or overlying oil- and gas-bearing deposits. As the inclusions in sedimentary halite are one-phase and fluid, the temperature of salt crystallization did not exceed 40 - 45°; however, the real temperature could be considerably lower (e.g., about 25°C) as in modern desert climate zones. Recrystallization of salt in the zone of potassium-magnesium mineralization occurred under elevated temperature (> 56°C). This should be taken into account when interpreting the present mineral composition of the potassium salt complex of the basin.