Geological setting and timing of the Chah Zard breccia-hosted epithermal gold silver deposit in the Tethyan belt of Iran

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1 Geological setting and timing of the Chah Zard breccia-hosted epithermal gold silver deposit in the Tethyan belt of Iran Hossein Kouhestani, Majid Ghaderi, Khin Zaw, Sebastien Meffre, Mohammad Hashem Emami H. Kouhestani, M. Ghaderi (*) Department of Economic Geology, Tarbiat Modares University, Tehran , Iran K. Zaw, S. Meffre CODES ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart 7001, Australia M.H. Emami Research Institute for Earth Sciences, Geological Survey of Iran, Tehran, Iran Analytical methods and procedures Analytical procedures for U-Pb zircon chronology Four samples from the Chah Zard volcanic complex including andesite-trachyandesite, porphyritic daciterhyodacite and rhyolite porphyry rocks were selected for U-Pb zircon geochronology. Zircons were obtained from crushed rocks by handpicking under binocular microscope after sieving, and conventional heavy-liquid and magnetic separation techniques. The zircons were mounted in epoxy, polished, and then photographed in transmitted and reflected light for identification of analyzed grains. Representative backscattered electron (BSE) and cathodoluminescence (CL) images were then made of the zircons before analysis to reveal their internal structures. The zircons are mostly euhedral to subhedral grains and preserve subtle to delicate normal or oscillatory zoning characteristic of igneous growth. No evidences of truncation or embaying by either younger zircon overgrowth or recrystallization (as thicker rims) were observed in the zircon grains. The zircons were dated on an Agilent 7500cs quadrupole ICP-MS with a 193 nm coherent Ar-F gas laser and a Resonetics M50 ablation cell at the University of Tasmania, Hobart, Australia. The LA-ICP-MS analyses followed Meffre et al. (2008) who established the methods similar to those outlined in Black et al. (2004) and Paton et al. (2010). The down hole fractionation, instrument drift and mass bias correction factors for U-Pb ratios on zircons were calculated using 2 analyses on the primary (91500 standard of Wiendenbeck et al. 1995) and 1 analysis on each of the secondary (TEMORA standard of Black et al. (2003) and GJ1 standard of Jackson et al. (2004)) standard zircons analyzed at the beginning of the session and every 12 unknown zircons (roughly every 1/2 hour) using the same spot size and conditions as used on the samples. Additional secondary standards (the Mud Tank zircon of Black and Gulson 1978) were also analyzed. The correction factor for the 207 Pb/ 206 Pb ratio was calculated using 3 large spots of NIST-610 analyzed at the beginning and end, and corrected using the values recommended by Baker et al. (2004). Element abundances on zircons were calculated using the method outlined by Kosler (2001) using Zr as the internal standard element, assuming stoichiometric proportions and using the to standardize correction for mass bias. The ages were calculated using ISOPLOT 3.00 (Ludwig 1998). Uncertainties in ages are quoted at the 95% confidence level (2 σ). 1

2 Geochemical analytical methods Approximately 500 g samples of altered volcanic rocks and mineralized vein and breccia from the Chah Zard deposit were selected for geochemical analyses. The samples were crushed, and then about 50 g of sample pulps were ground into powders to 200-mesh size using Cr-steel pestle and mortar. A total of 53 samples from altered host volcanics and 52 samples from mineralized veins and breccias from the Chah Zard deposit were analyzed for whole-rock major and trace elements and REE compositions at the ACME Laboratories, Vancouver, Canada, and University of Tasmania, Hobart, Australia. LiBO 2 fusion followed by XRF analysis was used for determination of major elements (SiO 2, TiO 2, Al 2 O 3, Fe 2 O 3, MnO, MgO, CaO, Na 2 O, K 2 O and P2O5) and LOI on a 12 g sample. Total C and S were analyzed by Leco (Macro Carbon, Hydrogen, Nitrogen, Sulphur analysis). Rare earth and refractory elements were determined by ICP-MS following a Lithium metaborate/tetraborate fusion and nitric acid digestions on a 0.2 g sample. For Ag and base metals, 0.5 g of sample is digested in Aqua Regia at 95 C and analyzed by ICP-MS and ICP- ES, respectively. Gold was determined by Fire Assay on a 30 g sample. Major and trace elements, and REE concentrations were normalized to those of sample CHZ-FR. Sample CHZ-FR represents the arithmetic mean of 12 samples with andesitic to rhyolitic composition from the Chah Zard volcanic complex (Kouhestani 2011), and was chosen to represent unaltered volcanic rock composition in the diagrams. References Baker J, Peate D, Waight T, Meyzen C (2004) Pb isotopic analysis of standards and samples using a 207 Pb- 204 Pb double spike and thallium to correct for mass bias with a double-focusing MC-ICP-MS. Chemical Geology 211: Black LP, Gulson BL (1978) The age of the Mud tank Carbonatite, Strangways Range Northern Territory. BMR Journal of Australian Geology and Geophysics 3: Black LP, Kamo SL, Allen CM, Davis DW, Aleninikoff JN, Valley JW, Mundil R, Campbell IH, Korsch RJ, Williams IS, Foudoulis C (2004) Improved 206 Pb/ 238 U microprobe geochronology by the monitoring of a trace-element related matrix effect; SHRIMP ID-TIMS ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards. Chemical Geology 205: Black LP, Kamos L, Allen CM, Aleinikoff JN, Davis DW, Korsch RJ, Foudoulis C (2003) TEMORA 1: a new zircon standard for Phanerozoic U Pb geochronology. Chemical Geology 200: Jackson SE, Pearson NJ, Griffin WL, Belousova EA (2004) The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U Pb zircon geochronology. Chemical Geology 211: Kosler J (2001) Laser-ablation ICPMS study of metamorphic minerals and processes. In: Sylvester PJ (ed) Laser-ablation- ICPMS in the earth sciences. Principles and applications Mineralogical Association of Canada, Short Course Handbook 29: Kouhestani H (2011) Geology alteration isotope geochemistry and origin of Chah Zard Ag-Au deposit southwest of Yazd. PhD Thesis. Faculty of Basic Sciences, Tarbiat Modares University, Iran, in Persian with English abstract Ludwig KR (1998) Isoplot: A plotting and regression program for radiogenic isotope data version 3.00 Meffre S, Large RR, Scott R, Woodhead J, Chang Z, Gilbert SE, Danyushevsky LV, Maslennikov V, Hergt JM (2008) Age and pyrite Pb-isotopic composition of the giant Sukhoi Log sediment-hosted gold deposit Russia. Geochimica et Cosmochimica Acta 72: Paton C, Woodhead JD, Hellstrom JC, Hergt JM, Greig A, Maas R (2010) Improved laser ablation U-Pb zircon geochronology through robust down-hole fractionation correction. Geochemistry Geophysics Geosystems 11: Stacey JS, Kramers JD (1975) Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet Science Letter 26: Wiendenbeck M, Alle P, Corfu F, Griffin WL, Meier M, Oberli F, Vonquadt A, Roddick JC, Speigel W (1995) 3 Natural Zircon Standards for U-Th-Pb- Lu-Hf Trace-Element and REE Analyses. Geostandards Newsletter 19:

3 Fig. 1 Photomicrographs of gangue mineralogy and textures. a Two episodes of quartz deposition: the first microcrystalline quartz growing on K-feldspar clast, cemented by coarser crystalline quartz with comb texture. Supergene jarosite and Fe-oxides fill the vugs. b Crystalline quartz with plumose and colloform textures in breccia cements. c Sericitized clasts of wall rocks overgrown by crustiform coarse-grained quartz and adularia. d Quartz-adularia vein with cockade texture cutting altered rhyolite porphyry. e Plagioclase phenocryst in volcanic clast replaced by quartz and sericite. f Plagioclase phenocryst in rhyolite porphyry replaced by adularia and serecite. All photomicrographs are taken in transmitted light, XPL. Abbreviations used in Figs. 1-3: Ad: adularia, Ag-Te: Ag-rich tennantite-tetrahedrite, Apy: arsenian pyrite, Asp: arsenopyrite, Au: gold, Car: carbonate, Cc: chalcocite, C-Qtz: crystalline quartz, Cpy: chalcopyrite, Col-Qtz: colloform quartz, Dig: digenite, F-Py: framboidal pyrite, Ga: galena, Ja: jarosite, K-f: K-feldspar, Mar: marcasite, M-Qtz: microcrystalline quartz, Plg: plagioclase, Pgy: pyrargyrite, Py: pyrite, P-Qtz: plumose quartz, Qtz: quartz, RP: rhyolite porphyry, Ser: sericite, Sph: sphalerite, S-Py: spongy pyrite 3

4 Fig. 2 Photomicrographs of ore mineralogy and textures. a and b Euhedral pyrite surrounded by marcasite (a), and marcasite and arsenian pyrite (b) showing zoning patterns. c Euhedral to subhedral pyrite surrounded by chalcopyrite. d Paragenetic overlap between pyrite and chalcopyrite, sphalerite and galena. e Framboidal and spongy pyrite. f Euhedral arsenopyrite overgrown on the bladed pyrite. g Subhedral and spherical aggregates of pyrite enclosed by chalcopyrite. Chalcopyrite is replaced by sphalerite and Ag-rich tenantite-tetrahedrite. h Sphalerite encloses chalcopyrite inclusions as ordered chains of ellipsoidal beads mainly concentrated in its crystal boundaries. i Sphalerite is replaced by chalcocite and digenite along boundary and fractures. j Sphalerite encloses galena. Two subhedral pyrites are enclosed within galena. k Silver sulfosalts replaced chalcopyrite along boundaries and fractures. Chalcopyrite enclosed spherical aggregates of pyrite, and replaced by sphalerite. l Native gold associated with pyrite and quartz. All photomicrographs are taken in reflected light, PPL 4

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6 Fig. 3 Examples of alteration assemblages at Chah Zard deposit. a Hydrothermal quartz-adularia growing on the walls of vuggy sericitic breccia. b Rhombic euhedral- to subhedral adularia crystals with quartz in the breccia cement, followed by supergene jarosite as vug infill. c Selective alteration of plagioclase to illite, and pervasive intense alteration of hornblende to illite and carbonate. d Pervasive, intense illite-(pyrite) alteration of plagioclase and hornblende phenocrysts, and groundmass. e Calcitepyrite-(illite-quartz) veins cutting sericitic rhyolite porphyry. f Photomicrograph of vuggy sericitic breccia with carbonate cement. All photomicrographs are taken in transmitted light, XPL 6

7 Table 1 Zircon LA-ICP-MS U Pb analytical results for the Chah Zard volcanic complex. The model of Stacey and Kramers (1975) was used to determine the common Pb composition at 6 Ma for the 207 Pb corrected 206 Pb/ 238 U age. 7

8 Table 2 Selected major and trace element geochemistry for samples from the Chah Zard deposit. SiO 2, Al 2 O 3, Fe 2 O 3, CaO, MgO, Na 2 O, K 2 O, TiO 2, MnO, P 2 O 5, LOI, TOT/C, TOT/S, SUM in wt. %, Au and Ag in g/t, remaining elements in ppm. NA: not analyzed 8

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