Preliminary geochemical characterization of gas manifestations in North Macedonia

Like 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/R_A). 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/R_A 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/R_A 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.

This research was funded by the DCO Grant n. 10881-TDB “Improving the estimation of tectonic carbon flux”, GINOP-2.3.2-15-2016-00009 ‘ICER’ project and PO FSE Sicilia 2014 – 2020 (CUP: G77B17000200009).

References:

[1] Daskalopoulou et al., 2018 – Chemical Geology, 479, 286-301