The comparison of alteration zones in the Sungun porphyry copper deposit, Iran (based on fluid inclusion studies)

Abstract

The Sungun porphyry copper deposit (PCD) is located in EastAzarbaijan, in northwestern Iran. The felsic rocks occur as stocks and dykes ranging in composition fromquartzmonzodiorite through quartzmonzonite. The stocks are classified into porphyry stocks I and II. Porphyry stock II, hosting the copper ore, experienced an intense hydro-fracturing leading to the formation of stockwork-type veinlets and micro-veinlets of quartz, sulphides, carbonates and sulphates. Three distinct types of hydrothermal alteration and sulphide mineralization are recognized at Sungun (1) hypogene, (2) contactmetasomatic (skarn), and (3) supergene.Hypogene alteration is developed in four kinds: potassic, phyllic, propylitic and argillic. Three types of fluid inclusions are typically observed at Sungun: (1) vapour-rich, two-phase, (2) liquid-rich two-phase and (3)multi-phase.Halite is the principal solid phase inmultiphase inclusions. Primary multiphase inclusions (LVH type fluid inclusions) within the quartz crystals in quartz-sulphide and quartzmolybdenite veinlets (quartz associated with sulphideminerals) were selected formicro-thermometric analyses and considered to be suitable for pressure calculations and estimation of hydrothermal fluid density.Homogenization temperature, salinity, pressure and density were measured and calculated in forty-seven selected samples None of the variables could distinguish the potassic from phyllic alteration zones clearly. In the potassic alteration zone, the average of homogenization temperature is about 413oC, while in the phyllic alteration zone its average is about 375 oC. It was expected that the temperature in the potassic alteration zone would be higher than that in the phyllic zone, but the difference found was not very significant The fluid inclusion salinity within both alteration zones obviously relates to their homogenization temperature: the average salinity in the samples from the potassic zone is 46.3 (wt%NaCl equiv.), which is higher than that in the samples from the phyllic zone.. Based on the estimated depth of the potassic alteration domain, it is expected that the lithostatic pressure was higher than in the phyllic alteration zone.According to the fluid inclusion studies and pressure  alculation, it is estimated that the average pressure for the potassic alteration zone was about 512 (bars) while the average pressure for phyllic zone was about 310 (bars). The average density of fluids in the samples from the potassic alteration zone is 1.124 (g/cm3), which is higher than that in the phyllic alteration zone (1.083 g/cm3).