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Abstract
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The Middle Eocene Senj Mo-Cu deposit (1.3 Mt at 1.5 wt% Cu and 0.2 wt% Mo) is located in the central Alborz magmatic belt, northern Iran. Mineralization is characterized by multistage veins, which are hosted in volcanic and volcano-sedimentary strata of the Karaj Formation (ca. 41-45 Ma), genetically associated with the Senj Mafic Sill (ca. 40 Ma). Four generations of veins are documented based on mineral assemblages and crosscutting relationships, sequentially: quartz–biotite–chalcopyrite (QBC veins), quartz–molybdenum (QM veins), quartz–pyrite (QP veins), and late calcite (LC veins). Three types of fluid inclusions (FIs) are present in multistage veins: liquid-rich two-phase (L-type), vapor-rich two-phase (V-type), and solid-bearing multi-phase (S-type) inclusions. FIs in the QBC and QM veins are predominantly V-type and S-type, together with minor L-type, whereas the QP and LC vein minerals only contain L-type fluid inclusions. The homogenization temperatures of FIs from QBC to LC veins varies from 320–425 °C, 308–390 °C, 214–288 °C, and 145–243 °C and their salinities range from 5.3–49.0, 6.2–39.7, 5.7–13.0, and 2.1–7.2 wt% NaCl equiv., respectively. The H-O-C isotope compositions favor a dominantly magmatic origin for the hydrothermal fluids, which gradually diluted by meteoric waters. The S-Pb isotope data of sulfide minerals indicate that the sulfur and metals were sourced from a deep-seated magmatic reservoir. The Senj Mafic Sill and associated Mo–Cu mineralization was developed in a back-arc extension regime, which faults and crustal fracture systems served as conduits for hydrothermal fluid circulation and Mo-rich veining.
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