Abstract
The Anbar-Kuh iron deposit is situated in the Sanandaj-Sirjan Zone (SASZ) within the Tutak metamorphic complex, Iran. This complex includes metacarbonate, mica schist, greenschist, and orthogneiss, which hosts the iron ores. Metacarbonate and orthogneiss were simultaneously metasomatized by hydrothermal fluid pre metamorphically. Locally preserved textural and mineral paragenesis indicate Fe mineralization occurs before, but also was locally remobilized during and after metamorphism and deformation processes. Magnetite mainly occurs as massive, cataclastic, lenticular, and disseminated ore associated with hematite and minor pyrite.
Gangue minerals mainly include quartz, calcite, chlorite, amphibole, and K-feldspar. Two generations of magnetite ore are found: replacement bodies where coarse grains of magnetite replaced carbonate host rock during potassic metasomatism and the second generation of magnetite formed as veins and veinlets that formed during a later post-metamorphic retrograde stage. The Ni, Cr, and Co content of the deposit (average: Ni = 18 ppm, Cr = 9 ppm, and Co = 17 ppm) along with high Co/Ni ratio (average 3.54) suggest a typical hydrothermal skarn origin for this deposit. Metacarbonate and orthogneiss samples show distinct LREE enrichment relative to HREE. The host rocks are characterized by positive Eu anomalies in the metacarbonates and a negative Ce anomaly in schists. The fluid inclusion data on quartz preserved within retrograde and sulfide‑carbonate as semblages suggest that Fe-mineralization formed from the ore-bearing magmatic-hydrothermal fluid with an average salinity of 18.5 wt% NaCl equivalent that saturated at a temperature of 260 ◦ C. Microthermometric data suggest that temperature and pressure decrease, fluid neutralization due to interaction with carbonate wall rock, and mixing of the hydrothermal fluid with meteoric water-controlled Fe precipitation in this deposit. A five-stage model is proposed for the genesis of the Anbar-Kuh Fe mineralization: I) potassic/alkali metasomatism; fluidmineral interaction related to the subduction-related magmatism and volatile exsolution, resulting in potassic metasomatism of magmatic minerals, such as plagioclase, K-feldspar, amphibole, biotite, and secondary muscovite saturation of magnetite with partial saturation of Fe from the Fe-bearing hydrothermal fluid. II) Prograde skarn stage; the major mineral assemblage, including garnet and pyroxene, formed in the disseminated and banded Fe ores. During migration of hydrothermal fluids from the crystallizing granitoid intrusion (orthogneiss) to the surrounding limestone (now metacarbonate rocks), Ca and Mg were continuously added to the fluid as FeCl 2 saturated. As fluid temperature dropped, Ca and Mg concentrations increased, forming Ca-Mgrich skarn minerals. III) Post-metamorphic retrograde skarn stage; epidote, biotite, chlorite, magnetite, and hematite assemblages were developed. Ca pyroxene was replaced by actinolite and Fe oxides and garnet by epidote. The hypogene hematite formed during the early retrograde skarn stage, contemporaneous with actin olite. An increase in the V content of magnetite from the prograde (7 ppm) to retrograde (24 ppm) stage, along with epidote occurrence, suggests an increase in oxygen fugacity. IV) Sulfide‑carbonate stage consists of abun dant calcite with subordinate quartz and minor sulfides (e.g., pyrite as late-stage veins cutting magnetite and chalcopyrite). V) Supergene stage; the supergene paleo to recent weathering caused a change in oxidation state and the formation of Fe oxy-hydroxides and locally hematite.

http://dx.doi.org/10.1016/j.gexplo.2025.107905