Geology, mineralogy and ore fluid characteristics of the Masjed Daghi gold bearing veins system, NW Iran

Introduction The Masjed- Daghi gold deposit lies in an area of widespread Cenozoic volcanic and plutonic rocks at the intersection of the Alborz- Azarbaijan and Urumieh- Dokhtar belts. The area was covered by a detailed exploration program, including geological maps at 1:1,000 scales (~8 km²), several hundred meters of trenches and systematic sampling for Au, Ag, Pb, Zn, Cu, As, Hg analysis, and 16 diamond drill holes at a total of 1200 meters (Mohammadi et al, 2005). The vein type gold deposit in Masjed- Daghi is closely associated with a porphyry type Cu-Au deposit. Our study focuses on the gold bearing veins system in an attempt to understand the characteristics of ore fluids and mechanisms of ore formation, and to develop exploration criteria for Masjed Daghi and similar occurrences in Alborz and other Cenozoic magmatic assemblages in Iran. Materials and methods Various rock types, alteration assemblages and mineral parageneses were characterized by transmitting and reflected light microscopy, X-ray diffraction (XRD) and electron microprobe analysis. Microprobe analyses were performed using a JEOL 8600 Superprobe electron microprobe at Saskatchewan University. Operating conditions were an accelerating voltage of 15 kV and a beam current of 50 nA. Representative samples from drill holes were selected for fluid inclusion studies. Fluid inclusion data were obtained using a fluid Inc. adapted USGS gas flow heating and freezing system at the Department of Geological Science at the University of Saskatchewan, Canada. To investigate the source of ore fluids, representative sulfidic samples from drill holes were selected for sulfur isotope studies. Isotopic analyses were performed using a Thermo Finnigan DeltaPlus at the G.G. Hatch Stable Isotope Laboratories, University of Ottawa. The standard error of analyses is less than ±0.1 per mil. Results Auriferous quartz veins in Masjed- Daghi are associated with porphyry style mineralization. Various alteration assemblages including argillic, silicic, potassic, phyllic, and propylitic occur in the district (Emamalipour et al., 2010). The auriferous quartz veins are hosted by silicified and kaolinitized volcanic rocks, dominated by trachyandesite. The mean grade of Cu is 0.15% the gold assay varies from <2 to 30 ppm. The veins were covered by 16 diamond drill holes, and the drill cores were analyzed for Au, Ag, Mo, Cu, Hg, As, Pb, Zn and Sb (Mohammadi et al, 2005). The V3 vein, about 700 m long, and on the average 5 m wide, is the largest vein in the district. Two types of quartz occur in the veins: an early medium to coarse grained grey quartz joined by milky quartz in later stages. Ore minerals include chalcopyrite, galena, and sphalerite. Fluid inclusion data from the ore stage quartz and sphalerite yielded Th values in the range 123-298 and 112-218 oC, and salinities in the range 1.9-12.8 and 1.9-11.2wt% NaCl equivalent. The wide range in salinity can be explained by mixing of two fluids with different salinities, and/or condensation of vapor. The δ34S values for pyrite, galena and sphalerite from the main mineralization stage, fall in a narrow range around (-0.2 to -1.1 per mil) around 0.0 permil, implying a magmatic source for sulfur. Discussion Hydrothermal activity in the Masjed Daghi epithermal veins can be divided into two stages: an early stage of acid leaching that is responsible for vuggy silica and advanced argillic alteration, and a later stage when gold and sulfide minerals were deposited. Considering the ore mineralogy and hydrothermal alteration products (the occurrence of barite in the veins, low Pb and Zn contents, lack of carbonates), the Masjed Daghi can be classified as a high- sulfidation epithermal vein system. In most high sulfidation systems, ore fluid is considered to be of a mixed magmatic- meteoric origin, resulting from the adsorption of magmatic vapors or brines by shallow meteoric water (Hedenquist et al, 2000). The involvement of a magmatic fluid component in Masjed Daghi is supported by the sulfur isotope ratios, the relatively high salinity and oxidizing nature of ore fluids, as well as the large variation in salinity, 1.9- 12.8 wt% NaCl equivalent, that can be explained by concurrent boiling and condensation of vapor, fluid mixing. Acknowledgments We thank J. Fan, and T. Bonli of the University of Saskatchewan for assistance with electron microprobe analysis, and colleagues at GG-Hatch stable isotope laboratories, University of Ottawa, for sulfur isotope analysis. Financial support for the work was supplied by Geological Survey of Iran, and NSRC grant to Y.P. References Emamalipour, A., Abdoli Eslami, H. and Hajalilou, B., 2010. Geochemistry investigation of hydrothermal alteration related to gold epithermal mineralization in the Masjed Daghi area, west Julfa, northwest Iran. Journal of Economic Geology, 3(2): 199-213. (in Persian with English abstract) Hedenquist, J.W., Arribas, A. and Gozales-Urien, E., 2000. Exploration for epithermal gold deposits. In: S.G. Hagemann and P.E. Brown (Editors), Reviews in Economic Geology. Society of Economic Geologists, Special Publication 13, Littleton, pp. 245- 277. Mohammadi, B., Aliakbari, H., Fard., M. and Samaee, A., 2005. Geology and drilling report of Masjed Daghi area (scale 1:1000). Geological Survey of Iran, Tehran, Report 340, 130 pp. (in Persian)

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