The Study of Mass Changes during Hydrothermal Alteration in the Astaneh Granitoid (SW of Arak)

The Astaneh granitoid massif in the Sanandaj-Sirjan zone, covering an area of about 30 km2, located in 40 km to Arak city, is mainly composed of granodioritic rocks. They are widely affected by hydrothermal alteration and six alteration zones including phyllic (sericitic), chloritic, propylitic, argillic, albitic and tourmalinization zones are identified in this area. Calculations of chemical index of alteration (CIA) indicates that phyllic (sericitic), chloritic, propylitic, argillic, albitic and tourmalinization alteration zones underwent the hydrothermal alteration about 63.47%, 59.73%, 61.54%, 63.69%, 60.61%, 65.43% respectively. The mass changes of elements, based on Al (as a monitor immobile element), indicate that considerable amounts of oxides such as Fe2O3, MgO, CaO and TiO2 and also LFSEs such as K, Rb, Sr, Cs and Ba in all of the alteration zones were depleted. The results show that SiO2 was added to the phyllic (sericitic), argillic and tourmalinization alteration zones by metasomatizing fluids. Al2O3 was immobile and its mass was essentially unchanged during alteration. Other oxides such as MnO, Na2O, K2O and P2O5 and also LOI show dissimilar behaviors in the different zones. In all of the zones there is depletion in Sc and Y (HFSE). Also the Ga (except in chloritic zone) and U (except in phyllic zone) were depleted. In the phyllic zone, La, Ce, Pr, Nd (LREE), Sm, Eu, Gd (MREE) and Yb (HREE) were added; however, Dy, Er and Ho were depleted, whereas Tb and Lu were unchanged. In all zones the REEs were depleted except in the chloritic zone where Eu and Yb were added and Lu was essentially unchanged during alteration.