ORIGIN OF BRINES IN THE UPPER SILESIAN COAL BASIN (POLAND) INFERRED FROM STABLE-ISOTOPE AND CHEMICAL-DATA

Brines in the Miocene formations of the Upper Silesian Coal Basin have isotopic composition close to SMOW, which identifies them as the conn ate marine water. However, controversies exist on the origin of brines in the Carboniferous formations. Isotopic and hydrochemical data excl ude any relationship to marine water and enrichment by evaporation. Th e most common brine which occurs at great depths can be identified as the oldest infiltration in a very hot climate (delta(18)O congruent to -2 parts per thousand, delta D congruent to -20 parts per thousand, C l- content 34 to 140 g/L). This brine is free of SO42- and U, and rich in Ba2+ and Ra-226. Its salinity is probably related to the leaching of evaporites and intensive weathering of rocks during the Rotliegende s. Other brines are difficult to identify because their isotopic conte nts are within the range of mixing between the oldest brine and the Qu aternary waters (delta(18)O 10 parts per thousand, do 70 parts per tho usand). Isotopic and hydrochemical data allow identification of severa l occurrences of brine formed by meteoric water of a warm Tertiary cli mate, after the last marine transgression in the Tortonian. That brine is rich in SO42- and contains moderate contents of Ra-226 and U. Its salinity is thought to result from leaching of Miocene evaporites. Two other identified types of brines can be related to some infiltration periods before the last marine transgression. The sources in salinity of these 2 types remain unknown. Mining activity results in a common o ccurrence of mixed brines. When the Quaternary component dominates, it s identification is easy from the isotopic composition, whereas the en d brine component can usually be identified by chosen ion ratios and t he presence or lack of sulphates.