Thermometric and chemical studies of fluid inclusions in sphalerite from Rhodope lead-zinc deposits: Shumachevski Dol - Gyudyurska (Madan District) and Kenan Dere (Laki District), Bulgaria

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.