Origin of barite concretions in the West Carpathian flysch, Poland

The Paleogene concretions occurring in the Sub-Silesian nappe of the West C arpathians were investigated to understand their origin and evolution. They were considered previously to be of hydrothermal origin. Three types of co ncretions were found in marls: (1) phosphorite impregnated, (2) barite impr egnated and (3) composed of the mixed types. The sulphur and oxygen isotope determinations of barite (delta(34)S between 32 to 36 parts per thousand a nd delta(18)O between 20 and 23 parts per thousand) as well as the ratio De lta delta(34)S/Delta delta(18)O Of 1.5 point out for bacterial reduction of sulphate as the primary cause of enrichment in heavy isotopes of sulphur a nd oxygen. Another process which could contribute to high delta(18)O(SO4) v alues was sulphate isotope exchange with seawater oxygen. The Sr-87/Sr-86 c ompositions of barite confirmed that they are fairly identical with stronti um isotopic composition of Paleogene seawater (O.7076-0.7078). Therefore, b arite nodules of the Sub-Silesian unit in the flysch Carpathians can not lo nger be considered as of hydrothermal origin. (C) 1999 Elsevier Science B.V . All rights reserved.