Preliminary geochemical characterization of gas manifestations in North Macedonia

Abstract

L ike most of the Balkan Peninsula, North Macedonia is a geodynamically active area. As such it has many hydrothermal features and gas manifestations. Until now, no systematic study about the geochemical characterization of the geogenic gases was made before in this country. In August 2019, 24 gas samples were collected in the study area. All, except one collected at Duvalo (soil gas), are gases bubbling or dissolved in thermomineral waters (temperatures from 12 to 66 °C). They were analysed in the laboratory for their chemical (He, Ne, Ar, O2 , N2 , H2 , H2S, CH4 and CO2) and isotopic composition (δ13C-CO2, δ13C-CH4, δ2H-CH4 and R/RA). Most of the gases have CO2 as the main component (400-998,000 ppm) while the remaining are enriched in N2 (1300-950,000 ppm). Helium ranges from 0.3 to 2560 ppm while CH4 from 1.6 to 20,200 ppm. R/RA and 4He/20Ne ratios indicate a generally low atmospheric contamination, a prevailing crustal contribution and mantle contributions between 1 and 20% considering a MORB endmember. The highest mantle contributions are found in the SE part of the country very close to the sites that show the highest R/RA values in continental Greece [1]. This area is characterised by extensional tectonics and Plio- Pleistocene volcanism. A quite high mantle contribution (about 15%) is also found in two manifestations in the NW part of the country along a main normal fault system. With the exception of the sample of Smokvica, which has very low CO2 (1400 ppm) and δ13C-CO2 (-15.7 ‰ V-PDB), all free gases show a relatively narrow range in δ13C-CO2 values (-4.6 to +1.0 ‰ V-PDB) indicating the mixing between a mantle and a carbonate rock source. The isotope composition allows us to assign the CH4 origin to three sources. The largest group can be attributed to a hydrothermal origin (δ13C-CH4 around -20 ‰ V-PDB and δ2H-CH4 around -100‰). Three samples collected in the SW part of the country have a thermogenic origin (δ13C-CH4 around -35 ‰ V-PDB and δ2H-CH4 around -160‰ V-SMOW). Finally, one sample (Smokvica) with the highest values (δ13C-CH4 -7.2 ‰ V-PDB and δ2H-CH4 -80‰ V-SMOW) may be attributed to abiotic processes in a continental serpentinization environment or to methane oxidation.