OICC Press Iranian Journal of Earth Sciences 2228-785X 2026 07 14 Evolution of ore-forming fluids at the Hesar gold deposit, NW Iran: Constraints from fluid inclusions and stable isotopes (O,S) 10.57647/j.ijes.2025.17568 EN Mohammad Yousefi Department of Earth Sciences, Science and Research Branch, Islamic Azad University, Tehran, Iran Nima Nezafati Deutsches Bergbau-Museum, Bochum, Germany Nematollah Rashidnejad-Omran Department of Petrology, Tarbiat Modares University, Tehran, Iran Mohammad Lotfi Geological Survey of Iran, Tehran, Iran Peyman Afzal Department of Petroleum and Mining Engineering, South Tehran Branch, Islamic Azad University, Tehran, Iran Majid Ghaderi Department of Economic Geology, Tarbiat Modares University, Tehran, Iran Journal Article 2026 07 14 The Hesar gold mining area is located in the Alborz-Azerbaijan zone. The dominant lithology in the Hesar mineral range includes andesite-trachyandesite, basalt, dacitic tuff, and rhyodacite in the form of stocks and dikes. Mineralization in the Hesar gold mineral range has occurred between marbles formed by the metamorphism of limestone and dolomite. Ore formation in the Hesar gold mineral range has taken place in fractures, cracks, and voids of the Eocene-aged host rock. The gold mineralization is associated with pyrite, arsenopyrite, iron hydroxide, and quartz gangue minerals, forming a mineral paragenesis. The dominant textures and structures in this deposit include cavity-filling, veinlet, replacement, and disseminated textures. Based on petrographic studies of fluid inclusions in quartz minerals, five types of fluid inclusions were identified based on their phase type and number, including: 1. Type 1: Single-phase liquid (L), 2. Type 2: Single-phase vapor (V), 3. Type 3: Two-phase liquid-rich (L+V), 4. Type 4: Two-phase vapor-rich (V+L), 5. Type 5: Three-phase with a solid phase (L+V+S). Microthermometric studies of the fluid inclusions revealed a homogenization temperature ranging from 66 to 266°C, and a salinity of 0.8 to 14 wt.% NaCl equivalent. Additionally, the mineralization process occurred at a depth of less than 600 meters below the Earth’s surface and under a pressure of less than 10 bar. Moreover, Raman spectroscopy on several fluid inclusion samples demonstrated correlating results with those of microthermometric measurements. The δ³⁴S values in pyrite, ranging from 7.6‰ to -2.8‰, indicate a magmatic origin for sulfur. This confirms that the pyrite, arsenopyrite, and gold mineralization in this mineral district are genetically linked to magmatic processes. The fluid in equilibrium with quartz at 144°C exhibits δ18O values between +1.9‰ and +5.2‰. This δ18O range places the fluid predominantly within the compositional fields of meteoric waters and subvolcanic granitic rocks, suggesting significant contribution from these sources. Based on data obtained from the Hesar study area, the mineralizing fluid is of magmatic origin, formed through mixing with meteoric waters. The genesis of the Hesar gold mineralization likely belongs to the category of epithermal deposits.