VOLUME 52 (1)/April 2023

In 1939, one of the first summarizing works of the famous Bulgarian geologist Georgi Bonchev “Contribution to the springs in Bulgaria” related to groundwater in the country was published. In his field research, the author visited and described about 60 natural springs or groups of springs with temperatures from 20 °C to over 75 °C, attached to different tectonic zones, over 40 lukewarm and cold springs with mineralized waters and about 290 karst and other cold springs. This article is also relevant for modern hydrogeology. The presented general picture of the locations of the springs, the discharge and qualities of the waters from the natural springs before 1939 allows making comparisons of the occurred changes due to natural and anthropogenic factors.

The objects of this study are fluid inclusions in sphalerite crystals or selected parts of them from two Central Rhodopean deposits: Shumachevski Dol – Gyudyurska (Madan District) and Kenan Dere (Laki District). A set of data for temperature of homogenization and total salinity are presented. The locations of coupled data in coordinates T_h vs total salinity reveal that the Kenan Dere sphalerite has been deposited from solutions of higher temperature (280–374 °C) and lower salinity (2–5 wt.% NaCl-eqv.). Conversely, the Shumachevski Dol and Gyudyurska sphalerites show lower T_h = 200–230 °C and somewhat higher salinity (S = 6–12 wt.% NaCl-eqv.). Furthermore, it is likely that both late Kenan Dere and Gyudyurska sphalerites were deposited at 30–50 °C lower temperatures than the earlier ones. Additionally, Decrepitation Inductively-Coupled-Plasma Atomic-Emission-Spectrometry was applied to determine the cations in the inclusion solute. The vacuum decrepitation was also used for liberating the volatiles (H₂O, CO₂) and analysing them by mass spectrometry. On that account, more detailed preliminary studies of the process of decrepitation were performed. However, the Zn-concentration in sphalerite-hosted fluid inclusions cannot be evaluated and the results from syngenetic quartz offer the only possibility for obtaining information about ore-element content. On the basis of a compilation of both present and previously published data, it can be concluded that the hydrothermal fluid under the Central Rhodope Dome was constituted in situ as a closed system at a depth of ~1 km, under nearly lithostatic pressure (~200 bars), T ≥ 350 °C, and NaCl-dominated salt content ~10%. Under these conditions, a solution with a pH of ~4.0–4.5 contains ore elements (Fe, Zn, Pb) as soluble chlorido-complexes; for Zn, the total concentration may reach 1 wt.%. Sulphide-S compound under these conditions is H₂S. The sphalerite deposition follows a reverse path of the chemical reactions. The elevation of the hydrothermal fluid along the extensional dislocations results in boiling (i.e., separation of CO₂) and cooling. The temperature decrease destabilizes the chlorido-complexes and leads to an increase in the Zn²⁺ activity. On the other hand, CO₂ loss enhances pH, and thus dissociates H₂S, yielding HS^- and S^2- ions, needed for sphalerite deposition.