Genesis of the Shams-Abad carbonate-replacement sideritic-ankeritic iron deposit, South Arak, Malayer-Esfahan metallogenic belt (MEMB), Iran: constraints from geology, fluid inclusions, and C–O isotope geochemistry.

The Early Cretaceous Shams-Abad deposit, with proven reserves of 48 Mt grading 35 wt% Fe, is the largest Fe-(Mn) deposit in the Malayer-Esfahan metallogenic belt (MEMB), Iran. The mineralization is stratabound and restricted to Early Cretaceous dolomitic limestone (Kld) and felsic tuffs. The primary ore consists mainly of siderite-ankerite, and minor pyrite, galena, and chalcopyrite. Secondary Fe-oxide-hydroxides (hematite, goethite) are also present. Iron mineralization in the Shams-Abad deposit was emplaced in two paragenetic stages: stage 1, a large volume of host rocks (Kld) was replaced by fine-grained siderite (Sid1) and ankerite (Ank1); stage 2, coarse-grained siderite (Sid2) and ankerite (Ank2) show vein-veinlets and massive textures and were formed by replacement of stage 1 mineralization. Fluid inclusions in hydrothermal dolomite (stage 2) have homogenization temperatures of 170 to 283 °C, with salinities ranging from 2.50 to 11.70 eq. wt. % NaCl. These temperatures and salinities are similar to the ranges reported for some sideritic Fe exhalative deposits elsewhere. The δ13CPDB and δ18OSMOW values of stages 1 and 2 hydrothermal ankerite and siderite suggest that CO2 (or H2CO3) in the hydrothermal fluid mainly originated from marine carbonate rocks. The textural, mineral, chemical, and isotopic evidence suggests that main-stage (stage 2) ore (primarily siderite) was precipitated by mixing between hydrothermal fluid and seawater below the seafloor in the dolomitic limestone host rocks. Subsequently, siderite and ankerite were converted to secondary iron oxides such as goethite and hematite during meteoric water flow through the inverted normal fault and thrust faults and uplift.