Formowanie się składu chemicznego solanek w basenach sedymentacyjnych Niżu Polskiego

Abstract

Cechy chemiczne solanek z utworów paleozoiku i mezozoiku Niżu Polskiego wskazują na ich hybrydalny charakter i daleko idące przeobrażenia wskutek wzajemnego oddziaływania między wodą i skałą. Stosunki liczbowe między logarytmami chloru i bromu oraz chlorków kationów dwuwartościowych i bromu przemawiają jednoznacznie za wymieszaniem się reliktowych ~ód morskich z resztkowymi ługami pokrystalizacyjnymi, wyciśniętymi do warstw otaczających w trakcie kompakcji pokładów solnych. W artykule wskazano procesy chemiczne. odpowiedzialne za ostateczne uformowanie się chlorkowo-sodowo-wapniowego składu solanek, oraz podjęto dyskusję nad anomalnymi wzbogaceniami niektórych z nich· w cynk i ołów.FORMATION OF CHEMICAL COMPOSITION OF BRINES IN SEDIMENTARY BASINS OF THE POLISH LOWLAND Chemical features of several brines extracted from the Palaeozoic and the Msesozoic formations of the Polish Lowland have been tested in co-operation with the Missouri - Columbia University, USA. The paper presents theoretical aspects of testing the high concentrated brines occurring in deep sedimentary basins, as the objective of the study was to determine origin of brines. reasons of their high mineralization, and nature of some chemical alterations responsible for changes· in chemical composition of both the primary solutions and the surrounding rocks. A computer analysis of quantitative logarithmic relations between the chlorine and the bromine concentration in brines(Figs 2 - 3) is presented with respect to a vapour pressure curve typical for particular stages of sea-water evaporation. This makes it possible to assess interrelations of main brine components such as connate waters, residual post-crystallization Iyes (saline mother liquors), solutions originating from dissolution of halite, and infiltration waters as well. Readily soluble chlorides of bivalent cations (MCI2) occurred to be a favourable indicators applicable for determining a rate of solution concentration during evaporation of sea water (up to the stage of carnallite precipitation). Range or chemical alterations in brines can be assessed from diagram of bivalent-cation chlorides versus bromine, in which concentrations of both components are plotted in logarithms of miliequivalents. Most of tested brines from the Polish Lowland are placed within the field of mixing the connate waters with the residual post-crystallization Iyes (Fig. 4). Participation of post-crystallization Iyes, calculated by a computer technique, amounts to 20-90 per cent (Table 2). It is worth stressing that some of the brines from the Polish Lowland show some similarities to those accompanying the oil fields within the Mississippi Salt Basin (Figs 5 - 6). Most of brines under study occur to be enriched in calcium ions (Fig. 7); however, their magnesium, sulphate, and potassium contents have been significantly decreased. Alterations of that type have been governed by processes of dolomitization, reduction in sulphate content and/or precipitation of calcium sulphate, albitization of plagioclases as well as crystallization of potassium aluminosilicates. Processes mentioned above have resulted in formation of high concentrated brines of calcium-sodium-chloride type. Attention is also directed to discussion of abnormal enrichment of brines in zinc and lead, up to 560 ppm and 30 pprn respectively, revealed in places.