Mineral and stable isotope compositions, phase equilibria and 40Ar–39Ar geochronology from the iron skarn deposit in Sangan, northeastern Iran

Abstract

The Sangan iron skarn deposit is located on the eastern edge of the Sabzevar-Doruneh Magmatic Belt, northeastern Iran. Mineralization occurs at the contact between Eocene igneous rocks and Cretaceous carbonates. The silicate-dominant prograde skarn stage consists of garnet and clinopyroxene, whereas the retrograde stage is dominated by magnetite associated with minor hematite, phlogopite, pyrite, and chalcopyrite. Phase equilibria and mineral chemistry studies reveal that the skarn formed within a temperature range of ∼375° to 580°C and that the mineralizing fluid evolved from a hot, low oxygen fugacity, alkaline fluid during the silicate-dominant stage to a fluid of relatively lower temperature and higher oxygen fugacity at the magnetite-dominant stage. The δ18O values of magnetite and garnet vary from +3.1 to +7.5‰ and +7.7 to +11.6‰, respectively. The calculated δ18OH2O values of fluid in equilibrium with magnetite and garnet range from +9.8 to +11.1‰ and +10.1 to +14.8‰, respectively. These elevated δ18OH2O values suggest interaction of magmatic water with 18O-enriched carbonates. The high δ34S values (+10.6 to +17.0‰) of pyrite separates from the Sangan iron ore indicate that evaporites had an important role in the evolution of the hydrothermal fluid. Phlogopite separates from the massive ores yield 40Ar/39Ar plateau ages of 41.97±0.2 and 42.47±0.2Ma, indicating that the skarn formation and associated iron mineralization was related to the oldest episode of magmatism in Sangan at ∼42Ma. Eocene time marked a peak of magmatic activity and associated skarn in the post-collisional setting in northeastern Iran, whereas Oligo-Miocene magmatic activity and associated skarn in the Urumieh-Dokhtar Magmatic Belt are related to subduction. In addition, skarn mineralization in northeastern and eastern Iran is iron type, but skarn mineralization in the Urumieh-Dokhtar magmatic belt is copper – iron and copper type.