Geology and geochemistry of the Choran porphyry-epithermal Cu-Au deposit in the Dehej-Sarduveyeh subzone, Urumieh-Dokhtar magmatic arc

Iran hosts numerous porphyry and epithermal ore deposits which have mostly been formed at discrete time periods within different tectonic assemblages. Porphyry and epithermal ore deposits are considered to be the important sources of base metals in Iran. Well-known porphyry deposits include the Sarcheshmeh, Meiduk, Sungun, (Shahabpour and Kramers, 1987; Hezarkhani and Williams, 1998; Taghipour et al., 2008), and well-known epithermal deposits include the Sari Gunay, Chah Zard, Touzlar, and Narbaghi (Richards et al., 2006, Kouhestani et al., 2012, Heidari et al., 2018). The Choran deposit exists in the Urumieh-Dokhtar Magmatic Belt (UDMB). This deposit is located in the southern part of the Cenozoic Urumieh-Dokhtar Magmatic Belt, 70 km SW of Bardsir city, SE Iran. In this area, mineralization is associated with Oligocene - Miocene quartz diorite and granodiorite intrusions emplaced within Eocene volcanic–pyroclastic sequences. This study has focus on the spatial and temporal relationships between the porphyry and epithermal styles of mineralization in this area. 

A camp was set up in the field and sampling was performed during the 2017-2018. During the field observations, 286 rock samples were collected from the outcrops and drill core, and 67 thin sections were prepared and studied using a polarizing microscope in the Shahid Chamran University of Ahvaz. In order to correctly characterize the chemical composition of silicates (plagioclase and biotite), samples with least traces of alteration have been selected. The chemical composition of plagioclase and biotite were determined on the carbon coated thin section samples using an Electron Probe Micro Analyzer (EPMA). All the analyses were conducted at the Montanuniversitat Leoben, Austria using a superprobe Jeol JXA 8200 instrument.

Based on drill core logging and petrographic studies, mineralization in the Choran deposit is mainly accompanied with granodiorite intrusions. Overall, both hypogene and supergene mineralizations have been identified in the study area. The hypogene mineralization mainly occurs as disseminated blebs and veins which consist of pyrite, arsenopyrite and chalcopyrite with minor amounts of sphalerite. The supergene mineralizations that involve chalcocite and covellite. The first generation of hydrothermal veins (A-type) are characterized by assemblages of quartz + K-feldspar ± magnetite occurring roughly in the potassic alteration. This is followed by B-type veins characterized by assemblages of quartz + pyrite + chalcopyrite + feldspar ± biotite ± magnetite ± calcite. Type C veinlets (1 mm to 5 cm width) contain quartz + pyrite ± chalcopyrite and exhibit an intense stockwork texture in the potassic and phyllic alteration zones. The supergene sulfide zone is dominated by chalcopyrites and it is completely or partly replaced by chalcocite, digenite, and covellite. The hydrothermal alteration consisting of sodic-potassic, potassic, phyllic alunite and kaolinite are associated with granodiorite and quartz diorite intrusions. The result of EPMA analyses showed that all of the plagioclases in granodiorite and quartz diorite are consistently of andesine type. Based on the diagram of Al / (Ca + Na + K) (a.p.f.u) vs. An%, (Williamson et al., 2016) plagioclase samples of granodiorite intrusions plot collectively in the field of fertile calc-alkaline rocks associated with porphyry mineralization, while the quartz diorite samples are mostly plotted in the barren field. The results of biotite analyses indicate that all biotites of granodiorite and quartz diorite intrusions are of Mg-biotite type. The amounts of IV (F), IV (Cl), and IV (F/Cl) in the biotites of quartz diorite and granodiorite are between (2.28 to 4.08), (-5.62 to -5.52), (7.87 to 9.64) and (2.03 to 2.45), (- 5.81 to - 5.66), (7.74 to 8.18), respectively.

Most of the characteristics of the Choran Cu-Au deposit, i.e. geological setting, textural and structural, mineralogical with alteration features, are analogous to that of porphyry systems having high-sulphidation epithermal lithocap (Hedenquist et al., 1998; Muntean, 2001; Sillitoe, 2010).