Abstract

The Astaneh pluton, located southwest of Arak city in western Iran, consists of Middle Jurassic granitoids and is widely affected by hydrothermal alteration. Phyllic (sericitic), argillic, chloritic, and propylitic alteration zones were recognized in the area. Field and petrographic observations show that gold mineralization is associated with phyllic zones. It seems that water-rock reaction and mixing with meteoric water were important processes in changing the composition of hydrothermal fluids as well as precipitating ore minerals from the hydrothermal solutions. To understand better the process of mass changes during the hydrothermal evolution of the Astaneh pluton, these alteration zones have been studied in terms of mass transfer and element mobility. Isocon plots quantitatively illustrate that Al2O3, TiO2, and P2O5 were relatively immobile during alteration. Phyllic alteration was accompanied by depletion of Na2O, MgO, CaO, and Fe2O3, and enrichment of SiO2 and K2O. The depletion of Na2O and Fe2O3 and enrichment of K2O reflect the sericitization of alkali feldspar and the destruction of ferromagnesian minerals. Enrichment of Si in phyllic zones is consistent with silicification, which is a major feature of phyllic alteration. During phyllic alteration, high field strength elements (HFSEs), except Y, were enriched while rare earth elements (REEs) and low field strength elements (LFSEs) were depleted. Argillic zones are characterized by enrichment in SiO2, K2O and LOI and strong depletion in some other major element oxides, REEs, LFSEs, and HFSEs (except Ga). This indicates that, in argillic zones, primary minerals in the granitoids were completely destroyed and subsequently removed by acidic fluids. In chloritic zones, depletion of SiO2, CaO, and Na2O, and enrichment of Fe2O3, MgO, K2O, MnO, and LOI reflect alteration of plagioclase and ferromagnesian minerals to sericite and chlorite, respectively. The strong enrichment of REEs in chloritic zones could be due to the stabilization of the ionic complexes, which carried those elements in solution. In chloritic zones, there are notable differences in the behavior of LFSEs and HFSEs. In propylitic zones, the enrichment of Fe2O3, SiO2, MnO, and MgO is likely the result of decomposition of ferromagnesian minerals such as biotite. During propylitic alteration, LFSEs and HFSEs (except Zr and Nb) as well as REEs were leached out. The Astaneh low-sulphidation epithermal system consists of proximal argillic through distal propylitic assemblages. The gold mineralization in the Astaneh, which is typically associated with phyllic hydrothermal zone, occurs as veins containing quartz and sulphide. Gold seems to have been deposited in open-space veins due to boiling of hydrothermal fluids.

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