Migration of palaeofluids in the contact zone between the Dukla and Silesian units, Western Carpathians — evidence from fluid inclusions and stable isotopes in quartz and calcite

Abstract

This study reconstructs the type and migration history of fluids within the Cretaceous–Paleogene rocks of the Silesian and Dukla units in the border area of the Polish and Ukrainian Carpathians. Quartz (the “Marmarosh diamonds”), calcite and organic matter fill fractures in the sedimentary rocks of the Tertiary accretionary prism of the Carpathian thrust units. The calcite and quartz contain numerous solid, liquid and gaseous inclusions. Generally two types of fluids are distinguished — aqueous and hydrocarbon. Homogenisation temperatures of hydrocarbons in quartz range from –128 to + 85°C. Hydrocarbon fluids contain light (methane) and heavier (oil) compounds. Aqueous inclusions generally display homogenisation temperatures between 110 and 230°C. The δ¹³C_PBD values for calcite range from –3.5 to +2.9‰, while the δ ¹⁸O_SMOW range from 18.9 to 27.7‰. The δ¹⁸O_SMOW values for quartz range from 15.7 and 26.7‰. Fluid inclusions indicate that maximum pressures and temperatures occur at the early development of the vein mineralization. They vary from 2.4 to 2.7 kbar in the Krosno (Silesian) Unit and from 1.7 to 1.9 kbar in the Dukla Unit at 190–230°C. The crystallisation of the “Marmarosh diamonds” in the NW part of the study area occurred at lower P–T conditions of 0.9 kbar and 110–175°C. Generally, the migrating fluids are represented by a mixture of the low salinity aqueous and the hydrocarbon-rich fluids