Duvalo (North Macedonia): A "volcano" without volcanic activity

T he Duvalo locality is located in the SW of the Republic of North Macedonia, in the Ohrid region,near the village of Kosel. It is an area of strong soil degassing, called “volcano” by the local peopledespite volcanic activity has never been documented in the recent geologic history of the area [1].A large area (thousands of sqm) shows signs of strong alteration and is devoid of vegetation. Untilthe 19thcentury sulphur was mined from this area [1].In August 2019, a campaign of soil CO2 flux measurements and soil gas sampling was made.Duvalo is sometimes referred to as an active geothermal feature but no signs of enhancedgeothermal gradient were found and the soil temperatures at 50 cm depth in this campaign werealways within the range of local mean air temperatures. Soil CO2 flux values ranged from 1.3 to59,000 g/m2/d and can be modelled with the overlapping of 3 or 4 flux populations. A possiblebiological background is estimated in 6.8±1.8 g/m2/d while the other populations are characterizedby an anomalous average flux ranging from 180 to 33,000 g/m2/d. The CO2 total emission,estimated both with a statistical and geostatistical approach, provided similar values in the orderof 50 t/d. This has to be considered as a minimum value because only areas with evident signs ofalteration have been investigated. Nevertheless, the estimated output is quite high for an areaunrelated with recent volcanism or geothermal activity.The chemical composition of soil gases shows: CO2 (96.6%), N2 (1.8%), H2S (0.6%) and CH4 (0.3%) asthe main gases. The present composition is almost indistinguishable from previous analyses madein 1957 and 1977 [1] pointing to a stability of the system in last decades. The isotope compositionsindicate for CO2 (δ13C -0.2 ‰) a pure carbonate rock origin, for CH4 (δ13C -34.4 ‰ and δ2H -166 ‰)a thermogenic origin and for He (R/RA 0.10) a pure crustal origin.The H2S released at Duvalo may be produced by either microbial or thermochemical sulphatereduction favoured by hydrocarbons whose presence can be inferred by the uprise ofthermogenic methane. Partial oxidation of H2S during its upflow, producing sulphuric acid, may beresponsible of the production of abundant CO2 through dissolution of carbonate rocks. Similar processes have been evidenced also in other parts of North Macedonia [2]. These gases rise upthrough the N–S trending normal faults bordering the seismically active Ohrid basin graben [3]being released to the atmosphere through the soils of Duvalo “volcano”.