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

Abstract

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 Th 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 Th = 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 S2- ions, needed for sphalerite deposition.