Isotopic and geochemical studies of the epithermal-mesothermal Pb-Zn deposits of S.E. Bulgaria
The study was based upon five Pb-Zn districts located in S.E. Bulgaria, these were Madan, Madjarovo, Laki, Zvezdel and Spaheivo districts with most of the work concentrating on Madan and Madjarovo. The Madan district consists of approximately twenty vein and replacement deposits hosted within amphibolite grade metamorphic rocks. The Madjarovo district comprises approximately forty five veins which are predominantly hosted within an eroded strato-volcano but with some of the veins passing down into the underlying metamorphic basement. The deposits of the Laki district are also hosted within the metamorphic basement and can be considered as an analogue to Madan, while Zvezdel and Spaheivo are volcanic-hosted and similar to Madjarovo. Fluid inclusion micro-thermometry shows that the mineralization in Madan, Madjarovo and Laki was deposited by fluids with a salinity of 3-5 eq. wt. %. NaCl and at temperatures of between 240°C and 300°C. In Madan and Laki the associated alteration is dominated by sericite while in Madjarovo both sericite and adularia are present, such an assemblage suggests neutral to slightly alkaline conditions. In addition a zone of sericitic - advanced argillic alteration is present at Madjarovo, this contains a range of minerals including kaolinite and alunite and is believed to have formed in a strongly acidic environment. The trace element geochemistry of the differing alteration types at Madjarovo also suggests that the alteration was produced by two separate fluids. A range of stable isotope analyses were undertaken to attempt to identify the source of the fluids and other constituents. The δ³⁴S sulphide values from the mineralization and alteration ranges between -1‰ and +7‰ for all districts except Spaheivo which has values of -5‰ to -7‰. Whole rock sulphur data from the metamorphic basement varies from - 7‰ to +5‰ and so is a viable source alongside igneous sulphur. The δ¹³C calcite in the metamorphic-hosted districts ranges from -4‰ to +2‰. This indicates that the carbon was predominantly derived from marble horizons within the basement which have a similar range of values. This contrasts with the delta δ¹³C calcite in the volcanic-hosted districts which is typically more depleted and suggests a different or additional source of carbon.