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The Ay Qalasi Pb–Zn (Ag) deposit, located at the intersection of the Urumieh–Dokhtar Volcanic Belt (UDVB) and the Sandaj-Sirjan Metamorphic belt of northwestern Iran, is hosted by Miocene sandstone and siltstone of the Upper Red Formation (URF), and by a post Miocene feldspar porphyry dyke that intrudes the URF. The Ay Qalasi deposit (reserves ~30000 tonnes at 8 % Zn, 5 % Pb and 60 ppm Ag) occurs in quartz-base metal sulfide veins, vug infill, and breccias with comb, crustiform, and cockade textures. The total sulfide mineral content of up to 60 vol.% is dominated by sphalerite, galena, pyrite, and chalcopyrite with minor amounts of tennantite–tetrahedrite. Hydrothermal alteration minerals include sericite, illite and quartz. The calculated d34SH2S values in the sulfide ore samples average +3.7 ‰ for pyrite and galena, and +4.3 ‰ for sphalerite, suggesting a magmatic input for the sulfur. Low Fe contents of the sphalerite, the hydrothermal style of alteration together with estimated temperatures of formation correlate with an intermediate-sulfidation style of epithermal mineralization. Our data suggest that the mineralization at Ay Qalasi took place as a result of hydrothermal activity related to the late Cenozoic high-level magmatism in the UDVB similar to other hydrothermal deposits of the Takab–Angouran region.