Mineralogy, alteration, fluid inclusion and geochemical constraints of the Tappeh-Khargoosh Cu-Au deposit (SW Ardestan)

Introduction: The Tappeh-Khargoosh area is located at the 15 km SW of Ardestan, in the middle section of the Urumieh-Dokhtar magmatic arc (Aghanabati, 2004). Exploration in the study area began in 2006 by Kani Pajohan-e- Spadana Company and continued in detail by the Ardestan Copper-Gold Company. Their exploration activities consist of preparing the geological map (1:5000 in scale), drilling trenches and boreholes. Also minor extraction has been done. In this paper, our focus is on mineralogy, alteration, geochemistry and fluid inclusion of the Tappeh-Khargoosh deposit for determining the genesis of mineralization. The results of this study can be used for more exploration in the study and adjacent areas. Methodology: Samples collected along a traverses perpendicular to the mineralized veins and their alteration haloes. The geometry, morphology, mineralogy and texture of mineralization were examined. After careful microscopic studies, 7 samples were analyzed by the XRF method in the laboratory of the Tarbiat Modarres University and Iranian Mineral Processing Research Center (IMPRC) in Karaj. Six thin-polished sections and 8 polished sections were examined. Also, 32 samples of mineralized and altered zones were analyzed by Inductively Coupled Plasma with optical emission spectrometer (ICP-OES) in IMPRC. Six samples were analyzed for gold by Atomic Absorption Spectroscopy (A.A.S) method in the Kimia Pajoh Alborz laboratory. Three double polished sections prepared from mineralized quartz vein and micro thermometric studies had been analyzed by a model HF-S90 microscope in the University of Isfahan. For detail understanding of mineral composition and determination of some fine and rare minerals, the electron microprobe analyzing (EMPA) technique (model SX100) is used in IMPRC. Discussion and results: The Tappeh-Khargoosh deposit consist of quartz vein and veinlets which occur as open space filling in Eocene andesite and dacite. The mineralized veins mainly occur in the fault zones. The subparallel fault systems of dextral strike sleep Qom-Zefreh crustal scale fault (Tajmir Riahi et al., 2012) has had the main role in localization of mineralizing fluids. The alteration mainly consist of silicificaion, propylitization with minor sericitization and argillization represented as vein-veinlets, dissemination and pervasively in host volcanics. These alteration assemblages are indicative of near neutral to little alkaline hydrothermal fluids (Simmons et al., 2005). The silicic alteration has occurred in a wide range of pH and temperature, while the argillic alteration has occurred in low temperature and a wide range of pH. So where the silicic and argillic alterations have occurred together, the temperature of the causing fluid must be lower in the range of clay mineral stability field (Robb, 2005). The fluid inclusion in the quartz shows the low temperature (137-194˚C) and low to medium salinity (4-12.5 %) which coincide with low to medium sulfidation epithermal deposit conditions. According to fluid inclusion data, bisulfides were the main ligand for metals transportation. The absence of halite/sylvite daughter minerals in fluid inclusions and low salinity of fluid inclusions show that the chloride complexes not act as effective ligands. Opposed to high sulfidation epithermal deposits in which the magmatic waters are common, in the low sulfidation type, the meteoric waters are dominant (Foster, 1991; Vahabi Moghadam, 1993). Dilution by cold and low salinity meteoric water has the main role in mineral deposition. Pyrite, chalcopyrite, bornite, chalcocite and native gold are the primary minerals and hematite, goethite, covellite, chalcocite, cuprite, malachite, chrysocolla, azurite and atacamite are the secondary minerals, which have occurred as veinlets, open space filling, colloform, amygdal filling and dissemination in quartz vein and host rocks. Fine grain gold had be seen in the colloidal secondary Fe-oxides, which indicate that the gold probably occurred primarily in sulfide minerals and was released in the supergene process. According to microprobe analysis, Ag was measured as impurity in chalcocite. These features coincide with high correlation coefficient between precious metals and copper. So, the Cu can be used as a pathfinder element for gold exploration in this and adjacent areas. The abundance of Cu-bearing secondary minerals in the surface, indicate that the Cu has not leached effectively as a result of the little amount of pyrite and aridity of the area. In this condition, Cu which was created by oxidation of primary Cu minerals was fixed in the surface as silicate, carbonate and oxide minerals (Chavez, 2000). Geology, geometry, texture and structure, geochemistry, alteration schema, fluid inclusion and mineralogical data of the Tappeh-Khargoosh make it similar to low sulfidation (L.S) epithermal deposits.