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Ore-forming fluids in the Pb-Zn-Ag mineralization of the Crnac mine, Kosovo

Crnac is a Pb-Zn-Ag underground mine, under operation, located within northernmost border of the Kosovo area, as a part of the Western Vardar zone. It comprises two types of mineralization: vein type and mineralized listwaenites. Vein type mineralization is hosted by Jurassic amphibolites, and overlain by serpentinites of the same age. The whole geological structure is intruded by the swarm of Tertiary, post-collisional quartz-latite dykes, accompanied or envelopted by vein type mineralization. The mineralization consists of a series of steeply dipping massive ore veins, 1 to 5 m thick (10% Pb, 4% Zn, 100 - 150 g/t Ag). Ore petrography suggests 3 stage vein formation: pre- mineralization consisting dominantly of py-aspy-q- kaol ; major mineralization stage with gn-sph-cpy ; and post-mineralization stage, with prevailing carbonates and subordinate galena and colloidal pyrite. Ph increases toward the closing stages of the mineral deposition. δ 34S values increase from the pre-mineralization pyrite (δ 34Spyrite= 0.32 - 3.11‰ ), towards the major mineralization stage sulphides (δ 34Sgalena=0.42 - 3.05‰ , δ 34Ssphalerite = 2.89 - 5.92‰ ). Generally low δ 34S values indicate influence of magmatic sulphur. Formation temperature for the major mineralization stage is obtained by the sph-gn S-izotope geothermometry, and varies between 222 and 312°C. Listwaenite type of mineralization occurs in the uppermost level of the deposit, at the contact zone between amphibolites and overlying serpentinite, as lenses and irregular bodies of variable base metal content. Silicification followed by lesser pyritization is a pre- mineralization phase, the next is base metal formation terminating with carbonatization, sericitization and illitization. FIs study on quartz from the pre-mineralization in listwaenites reveals the presence of high-temperature - low- salinity fluid (Th=249 - 324°C, 2.9 ? 6.7 wt.% NaCl equ.). Primary fluid in the late stage carbonates (CaCl2-NaCl± KCl-H2O, Th=247-325°C, 5.1 - 7.0 wt.% NaCl equ) is overprinted by a lower temperature fluid (CaCl2-NaCl-H2O-CO2, Th=219- 268°C, 4.3 - 11.7 wt.% NaCl equ.) δ 34S values of the pre- and syn- mineralization sulfides overlaps (δ 34Spyrite= 2.49 - 3.21‰ , δ 34Sgalena= 2.27 - 2.91‰ , δ 34Ssphalerite = 2.27 - 4.89‰ ). Formation temperature obtained from the sphalerite-galena pairs varies between 252 and 437°C. The values attest a non-equilibrium condition. Sulfur is of magmatic origin. FIs studies together with the sulfur isotopes confirmed three major stage of mineralization. Pre- and syn- mineralization stages are characterized by introduction low pH-high temperature-low salinity fluids of dominantly magmatic origin, and simple composition (Na-K-Cl), which are followed by the post-mineralization deposition of gangue carbonates and minor sulfides from neutralized fluids of more complex composition (Ca-Na-K-Cl-CO2). The late-stage fluid composition was controlled by the meteoric waters circulation and the water-rock ratios.

Pb-Zn-Ag deposits; Oligocene vulcanism; fluid inclusions; vein-type; listwaenite

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