Mineralogical and geochemical features of the Sirna Mn‐Fe deposit in the Kurdistan region, northeastern Iraq: Unveiling the formation of a Mn‐Fe silica gel plume via serpentinization hydrothermal mechanisms

Abstract

The study probes the mineralogical and geochemical features of manganese‐iron deposits located in the Sirna area, which is a part of the late Cretaceous Walash Group in the Zagros suture zone, situated in the Kurdistan region of northeastern Iraq. Our investigation comprised field surveys, examination of ore petrography, besides using x‐ray diffraction, SEM‐EDS analysis, x‐ray fluorescence analyses, and inductively coupled plasma mass spectrometry techniques conducted on a set of representative samples. A significant Mn‐Fe ovoidal ore body, extending across 25 meters in diameter, protrudes between the lower strata of soft gray shale and the upper strata of massive limestone within the Walash group. The deposit exhibits a distinct separation into three layers: a lower horizon characterized by abundance of oxide of silicon, an upper horizon enriched in manganese oxide, and a transition layer dominated by hematite (Fe2O3). From a geochemical perspective, there is a gradual decrease in Fe2O3 and SiO2 from the lower to the upper part of the deposit, while MnO, BaO, and SO3 demonstrate a gradual increase. The co‐association of Mn‐Fe‐mineralization in a small restricted tabular ore body suggests that they are genetically related. Within the Sirna manganese‐iron deposit, the prevalent manganese and iron mineral phases are braunite, hollandite, and hematite. Concurrently, the gangue minerals in this deposit encompass cryptocrystalline spheroidal quartz, barite, calcite, and apatite. The Sirna Mn‐Fe deposit exhibits geochemical characteristics such as elevated levels of MnO (reaching up to 68 wt.%) and significant Fe2O3 content (up to 45 wt.%) in the upper manganese and transitional iron horizons, respectively. The Co/Zn ratio (0.28), Ce/La ratio (1.78), low levels of transitional elements (Co + Ni + Cu <0.01 wt.%), and varying concentrations of Ba (up to 6.9 wt.%) suggest that the Siran Mn‐Fe deposit is likely originated from a manganese‐iron silica gel plume that separated from hydrothermal fluids linked to serpentinization. This process is thought to have occurred in the mantle wedge along subduction zone, typically within an arc tectonic environment. Moreover, the presence of remnants of micro‐organisms such as EPS layers, different types of filaments, which are densely covered by biominerals, are important evidence of microbial effect in the mineralization of Mn‐Fe in the study area.