The Mansourabad stratiform and stratabound Zn-Pb (-Ag) deposit is hosted by an Early Cretaceous siliciclastic-carbonate sequence deposited in southern Yazd basin, Iran. Mineralization is mainly hosted in silty limestone and dolomite. Based on petrographic studies three sulfide ore facies can be distinguished: feeder zone, bedded and massive-replacement ore facies. Textures include replacement, breccia, massive, framboidal, laminated and vein-veinlet. Sulfide minerals are predominantly sphalerite, galena, and pyrite. Dolomite and quartz are the major non-sulfide gangue minerals. Based on microscopic studies, sulfide mineralization in the Mansourabad deposit emplaced during two stages: fine-grained sulfide bands (stage I) are intricately interlayered with organic-rich beds and thin turbidite beds. They exhibit classic sedimentary textures, such as lamina and bedding, indicative of a synsedimentary origin. Coarser-grained stage II base metal sulfides show breccia and vein-veinlet’s textures, and are considered to form by replacement during sub-seafloor fluid flow. Dolomitization and silicification are the major hydrothermal alteration styles in the Mansourabad deposit, and they occur in all host rocks, but only in the immediate vicinity of the syn-sedimentary normal fault that served as the main conduit for the mineralizing fluids. Calcite minerals from the feeder zone and massive-replacement ore facies have 115 to 250 °C trapping temperature with salinities ranging from 3.55 to 19.82 eq. wt.% NaCl eq. δ34S values of pyrite, sphalerite and galena range from -18.8 to +3.2‰. The highest δ34S values correspond to the feeder zone (+3.2 to +2‰), and the lowest to the bedded ore facies (-18.8 to -9.9‰) and the massive ore (+2.6 to - 4.7‰). The overall range of δ34S is remarkably higher than typical magmatic values, suggesting that sulfides formed from the reduction of seawater sulfate by bacteriogenic sulfate reduction in a closed or semi-closed system in the bedded ore, whereas thermochemical sulfate reduction likely played an important role in the feeder zone. The δ13CPDB and δ18OSMOW values of host limestone’s and hydrothermal dolomites range from +2.7‰ to +2.0‰, +8.6‰ to +9.2‰, +1.5‰ to +2.8‰ and +10.6‰ to +13.2‰, respectively, and plot the marine carbonate rocks field in a plot of δ13CPDB vs. δ18OSMOW. It suggests that CO2 in the hydrothermal fluids was mainly originated from marine carbonate rocks. Abrupt lateral changes in facies and thickness, along with the existence of synsedimentary breccia’s and debris flows within the ore sequence, suggest the proximity of synsedimentary faults and tectonic activity contemporaneous with the sedimentation in the Lower Cretaceous, favorable to the formation of deposit. So this deposit formed by combination of sub-seafloor and on the seafloor processes. S, C and O isotopes, along with sedimentological, textural, mineralogical, and geochemical evidences, suggest that this deposit should be classified as a vent-proximal sub-seafloor replacement SEDEX ore deposit.
