Stable isotope study of the mineralization and alteration in the Madjarovo Pb–Zn district, south-east Bulgaria

Abstract

The Madjarovo ore district is centred on the exposed section of a Lower Oligocene volcano and consists of radially disposed Pb–Zn-precious metal veins and attendant intermediate sulfidation wallrock alteration. Earlier high sulfidation and potassic porphyry style alterations are found in the centre of the district spatially associated with monzonitic intrusions. The total duration of all mineralization and alteration was ca. 300 ka. Stable isotope analyses (S, O, H) have been carried out on a suite of sulfides, sulfates and silicates from the mineralization, high and intermediate sulfidation alterations and a suite of basement rocks. These data range between the following limits: $$\delta ^{{34}} S_{{sulfides}} = - 1.2\;\operatorname{to} \; + 6.8\,\,{\text{:}}\;\delta ^{{34}} S_{{sulfates}} = \; + 21.4\;{\text{to}}\; + 24.9\;\,\,;\;\delta ^{{18}} O_{{sulfates}} \; = \; + 5.8\;\operatorname{to} \; + 11.6\,\,;\;\delta D_{{sulfates}} = \; - 35.1\,\;\operatorname{to} \, - \;67.1\;\,:\;\delta ^{{18}} O_{{silicates}} \; = \; + 5.2\,\,{\text{to}}\; + \;13.5\;\,;\;\delta D_{{\operatorname{silicates} }} \; = \; - 36.3\;to\; - 77.0\;$$ . We also analysed δD of fluid inclusions in quartz and barite for which we obtained, respectively, the ranges of −43.6 to −78.6 and −58.4 to −67.1‰. The data show that high sulfidation alteration was dominated by magmatic fluids with minor meteoric water, whereas the fluids responsible for the intermediate sulfidation alteration were essentially magmatic. The fluids responsible for the intermediate sulfidation Pb–Zn mineralization were mixed magmatic–meteoric and certainly contained a significant meteoric component. Sulphur is likely derived from basement and/or igneous sources. The evolution of alteration and mineralization styles from potassic, porphyry copper style to high sulfidation to intermediate sulfidation can be understood in terms of changing ore fluid composition resulting from an increasing permeability of the system and an increasingly remote source of magmatic fluid with time. These changes link directly to the geological evolution of this volcanic centre.