Evidence of porphyry copper mineralization in the Cheshmeh Khuri area, North West of Birjand: Geology, alteration, mineralization, geochemistry, fluid inclusion and stable isotope

The Cheshmeh Khuri area is located in the north of the Lut Block volcanic–plutonic belt, in eastern Iran, about 111 Km northwest of the city of Birjand. Extensive Tertiary magmatic activity in the Lut Block, is spatially and temporally associated with several types of mineralization events (Karimpour et. al., 2012). The episode of Middle Eocene to lower Oligocene (42–33 Ma) was very important in terms of magmatism and mineralization (Karimpour et. al., 2012). The North Khur area includes numerous cases of Cu±Pb±Zn vein-type mineralization, such as the Shikasteh Sabz, Mir-e-Khash, Rashidi, Shurk, Ghar-e-Kaftar, Howz-e-Dagh, as well as kaolin deposit (Cheshmeh Khuri area). We present and discuss alteration, ore petrography, geochemistry, fluid inclusion micro thermometry, and sulfur isotope geochemistry, which help clarify the ore genesis of the Cheshmeh Khuri area. The present study involves detailed field work and study of thin and polished sections from the intrusive rocks and ore samples under the optical microscope. Metal concentrations were analyzed at the IMPRC laboratory of Iran using the ICP-OES techniques on fifteen samples. Five samples were analyzed for Fire Assay analysis and four samples for XRD analysis at IMPRC laboratory of Iran. Twelve spot analyses (microanalyses) were performed on an X-ray Analytical Microscope at IMPRC laboratory. Doubly polished wafers (150 μm thick) were prepared from five samples taken from surface and trenches. Micro thermometric measurements were carried out using a Linkam THM 600 heating–freezing stage mounted on an Olympus TH4–200 microscope stage at the Ferdowsi University of Mashhad, Iran. Two pyrite samples from quartz-sulfide veinlet were analyzed for the sulfur isotope compositions after careful hand picking and purification at Iso–Analytical limited, United Kingdom. The main alterations consists of propylitic, argillic, quartz-sericite-pyrite and silicified. The mineralization is mainly observed as vein and is disseminated in quartz-sericite-pyrite, argillic- silicified and propylitic alteration zones and is disseminated in the argillic alteration zone. Pyrite is the only primary sulfide mineral in the area. Due to the great influence of weathering processes on the primary ore, secondary sulphide and oxide mineralization (malachite, azurite, chalcocite, covellite, goethite, and hematite) are widely spread and have finally created lithocap (Sillitoe, 1993; Sillitoe et. al., 1998). The maximum anomalies of copper (654 ppm) and lead (1622 ppm) are associated with quartz-sericite-pyrite alteration. Primary fluid inclusions of quartz in paragenesis with mineralization in quartz-sericite-pyrite zone, argillic-silicified zone and calcite in paragnesis with mineralization in propylitic zone have an average of homogenization temperatures of 321°C, 305 °C and 263 °C, respectively. Based on freezing studies, the average calculated temperature of last melting point of these is equal to 12, 11.6 and 7.9 wt.% NaCl, respectively. Homogenization temperature and salinity of the fluids shows a shifting trend from relatively high in quartz-sericite-pyrite zone to relatively low homogenization temperatures in the propylitic zone, which can be due to physicochemical changes in the fluid such as cooling and mixing with meteoric water (Naden et al. 2005). According to the textural evidence, boiling has also been effective during the evolution of the fluid. The amount of δ34S for pyrite has a range between 2.35 to 2.46 and the amount of δ34 equilibrium with pyrite has a range of 1.25 to 1.36 that show a magmatic origin for sulfur (Ohmoto and Rye, 1979; Lesage, 2011). The expansion of propylitic and argillic alteration zones on the surface, the limited quartz-sericite -pyrite zone, the absence of potassic alteration, the existence of lithocap, geochemical anomalies, the range of temperature and salinity of the fluid inclusion can be indicative of the upper part of a porphyry copper system.