Mineralogy, mineral chemistry, fluid inclusions and skarnification processes in the Tappeh Ghermez (A′) iron skarn, Sangan iron-mining district, NE Iran

The Sangan mining district, the largest iron skarn district in Iran, is located in the northeastern part of the Alborz Magmatic Arc and consists of 14 iron skarn deposits. The Tappeh Ghermez deposit (also known as A′) is located in the western part of the Sangan mining district and formed along the contact of Cretaceous dolomitic limestone and Middle Eocene Sarnowsar high-K granitoid intrusion. The skarn zones include prograde endoskarn and exoskarn zones (clinopyroxene–garnet skarn sub-zones), and retrograde exoskarn zone (amphibole skarn subzone). The magnetite ore occurs in massive, lenticular, disseminated and open space-filling textures. Clinopyroxenes are mostly of the diopside type (Wo 35.2–51.6 En 24.5–50.2 Fs 0.086–23.8 ), and augite and garnets are of the andradite–grossular solid solution series type (Adr 27.8–60.7 –Gr 38.8–67.6 –Sps 0.0–4.3 ). Amphiboles are actinolite, ferroactinolite, tremolite and magnesiohastingsite. Homogenization temperatures and the salinity of fluid inclusions in the garnet host range from 421 to 560°C and from 16.89 to 20.22 wt% NaCl equiv., and in calcite from 178 to 266°C and from 17.52 to 19.05 wt% NaCl equiv., respectively. Fluid inclusions, mineralogy, and textural evidence indicate that cooling and fluid mixing (dilution) mechanisms are the responsible processes that occurred in the formation of the Tappeh Ghermez deposit, and water–rock interaction was the possible mechanism of magnetite ore deposition.