Mineralogy and metasomatic evolution of the Mianeh iron skarn deposit, Norduz-Agarak border, NW Iran

Abstract

The Mianeh iron skarn deposit lies in the Arasbaran region within the Qaradagh metallogenic district, NW Iran. This high-grade massive magnetite skarn originated by the interaction of Upper Cretaceous limestone with metasomatic ore-bearing fluids associated with hypabyssal Oligo-Miocene quartz diorite. Mineral chemistry of the primary clinopyroxenes demonstrates the sub-alkaline, volcanic arc setting of magmatism. Two general stages of skarnification are recognized: (1) silicate skarn (stage I) is composed essentially of grossular and low-Fe diopside formed before the main mineralization and (2) magnetite-garnet skarn (stage II) composed of strongly anisotropic coarse-grained garnets with a narrow compositional zoning radially formed by addictive infiltrating of silica and iron-rich metasomatic fluids which overprint and/or crosscut the early stage silicate skarn. Anhydrous prograde calc-silicate assemblages were replaced by a series of hydrous calc-silicates (epidote, tremolite-actinolite) and/or quartz, calcite, magnetite, hematite, and pyrite. Magnetite (±hematite) is the dominant hypogene ore mineral that initially precipitated coincident with the late prograde to the early retrograde metasomatic stages. Mineralogical studies suggest that silicate skarn formation commenced at temperatures about 560 °C, X(CO2)fluid ≤ 0.15, αSiO2∼−1.0, and fluid pressure 1.0 kbar. The magnetite-garnet skarn formed from H2O-rich fluids [X(CO2)fluid < 0.1] at a temperature of 525 to 450 °C and maximum log ƒO2 between −20.2 and −23. During the late stages of prograde skarn development, the stability field of andradite shifted to low ƒO2 and ƒS2 conditions resulting in main iron ore deposition (as magnetite). The andradite replacement temperature and presence of pyrite (instead of pyrrhotite) suggest that logƒS2 remained constant at about −6 to −7 during cooling of the system.