Geology, fluid inclusions and C-O-S-Pb isotopic compositions of the Chahmileh Pb-Zn deposit, Central Iran: Implications for ore genesis

The Chahmileh Pb-Zn deposit, located northwest of the Central Iran Zone, is a sediment-hosted Pb-Zn deposit in the 'Yazd-Anarak Metallogenic Belt'. It is hosted in Middle Triassic carbonate rocks and is mainly controlled by NW-trending faults. The main ore minerals are galena and sphalerite with minor chalcopyrite, pyrite, and quartz, dolomite, along with minor calcite and baryte as gangue minerals. Cerussite, hemimorphite, wulfenite, mimetite, smithsonite, malachite and iron oxy-hydroxides are the main non-sulphide ore minerals. The main styles of mineralization are vein-veinlet, breccia, disseminated and replacement in association with silicification and dolomitization. Microthermometry of fluid inclusions in dolomite and quartz indicates that the ore precipitated from a warm to hot basin-derived saline fluid. Dolomite samples have delta 13CVPDB and delta 18OVSMOW values of -0.99 to +1.99 parts per thousand and +20.74 to +25.48 parts per thousand, respectively, and are plotted in the marine carbonate rocks field. These isotopic values suggest that CO2 in the hydrothermal fluids mainly originated from marine carbonate rock. The delta 34S values range from +6.3 to +8.2 parts per thousand for galena, +5.9 to +6.2 parts per thousand for sphalerite, +1.4 to +3.4 parts per thousand for chalcopyrite and +15.0 to +17.4 parts per thousand for bayite are compatible with a predominant thermochemical sulphate reduction process, and with sulphur sourced from Triassic seawater. Galena samples have a homogeneous Pb isotopic composition that is indicative of a continental crustal reservoir as the main source of lead and probably for the other ore metals. Based on geology, mineralogy, texture and fluid characteristics, the Chahmileh deposit is classified as a carbonate-hosted Mississippi Valley-type deposit.