| Popis: |
Introduction The widespread Cenozoic magmatic assemblages in Iran host a variety of ore deposits including porphyry Cu-Mo-Au, skarn type ores, and epithermal base and precious metals deposits. Silicic zones of variable sizes are common in the Kerman belt in the southern section of the Urumieh-Dokhtar arc, and some might be representing the upper parts of porphyry copper systems known as lithocap. To investigate this potential relation, a silicic zone in Meideh, north Pariz, is studied. The silicic zone lies in an area with several known porphyry copper deposits (PCD) including Sarcheshmeh, Nochun, Seridun, Sarkooh, and Bagh-Khoshk. For comparison, silica ledges and veins in Seridun and a mineralized silica vein system to the east of the Sarcheshmeh mine are also studied. Materials and methods The study is based on field studies and investigation of textures and structures, and sampling for mineralogy (microscopic and X-ray diffraction analysis), and fluid inclusions. The XRD analyses were accomplished in the Iranian Mines and Mining Industries Development and Renovation Organization (IMIDRO) and Kansaran Binaloud Co, Tehran. The fluid inclusion studies were performed in the Iranian Mineral Processing Research Center (IMPRC) using a Linkam THMS600 equipped with a Zeiss microscope. Results The silicic zone in Meideh is developed in andesitic lava flows and pyroclastic materials, and covers an area of ~ 1 km2. Silica occurs in white to grey colors and in massive, brecciated and locally vuggy textures; the grain sizes range between 0.01mm to 1mm. The silica is locally associated by minor sulfides (pyrite and locally chalcopyrite) carbonates, and clay minerals. The silicic zone grades outward into a silicic-argillic halo and into the host volcanic rocks with propylitic alteration. Chemical analysis of samples from the zone indicated enrichments in Cu, Mo, Ag, As, Bi and Au relative to the average composition of intermediate-mafic volcanic rocks in the Kerman belt. Small outcrops of a quartz-tourmaline rock occur in the southeast of the silicic zone. In Seridun, silica ledges and veins occur in the periphery and in the upper parts of a porphyry copper deposit developed in Miocene shallow intrusive bodies and older volcanic rocks. In east of Sarcheshmeh, several N-S striking silica veins locally containing pyrite, chalcopyrite, malachite, and Fe-oxides/hydroxides occur in Cenozoic volcanic and intrusive host rocks. In both areas, the silicic zones are products of pervasive silicic alteration, and occur in massive, breccia, and vuggy textures. Vuggy texture is well developed in Seridun. XRD analysis of representative samples from Meideh indicated the occurrence of kaolinite and illite, in addition to quartz. Minerals characteristic of advanced argillic alteration (i.e. alunite, pyrophyllite, diaspore and andalusite) are missing. The minerals, however, were identified in Seridun. Fluid inclusions in quartz from all three areas are dominated by two-phase liquid-vapor. Homogenization temperature (TH) varies between 140-263 oC (average: 202 oC) for Meideh, 195-320 oC (average: 247 oC) for Seridun, and 140-264 oC (average: 177 oC) for east of Sarcheshmeh. Salinities vary between 0.18-5.71 (average: 1.62), 1.22-4.18 (average: 2.27), and 0.7-3.39 (average: 1.57) wt.% NaCl eq., respectively. The quartz-tourmaline rock from Meideh is distinguished by the occurrence of liquid-vapor-halite±hematite and liquid-vapor-opaque inclusions, in addition to liquid-vapor inclusions. The TH and salinity for the liquid-vapor inclusions, homogenizing to liquid, varies, respectively, between 202-269 oC (average: 231 oC) and 3.71-7.16 (average: 5.43) wt.% NaCl eq. The TH and salinity for the halite bearing inclusions, homogenized by halite dissolution, varies between 240-480 oC (average: 345 oC) and 33.40-56.90 (average: 42.80) wt.% NaCl eq. Discussion The textures, structure, and spatial relations with the host volcanic rocks suggest that the Meideh silicic zone developed as a result of pervasive silicic alteration, rather than open space filling. Textures indicative of open space filling, including crustification and symmetric banding, are absent in Meideh. The silicic ledges in Seridun, and the N-S striking silicic zones in east of Sarcheshmeh, are the products of pervasive silicic alteration of the host volcanic and intrusive rocks. The XRD analysis of representative samples from Meideh indicated the occurrence of kaolinite and illite, in addition to quartz. Minerals characteristic of advanced argillic alteration (i.e. alunite, pyrophyllite, diaspore and andalusite) are missing. The minerals, however, were identified in Seridun. Fluid inclusions in quartz from the three silicic zones are dominated by two-phase liquid-dominant L-V inclusions. No distinction in salinity can be made between the three zones; Seridun, however, is distinguished by higher homogenization temperature. The local quartz-tourmaline zones in Meideh developed from distinctly higher temperature and salinity fluids (240-480 oC and 33.9-64 wt.% NaCl eq., respectively). A comparison of fluid inclusion data with several epithermal base and precious metals systems in the Urumieh-Dokhtar arc and elsewhere in Iran suggest that no meaningful distinction can be made between barren (i.e. Meideh, at current exposure level) and productive epithermal systems. Our data indicate that the silicic zone in Meideh cannot be considered as a porphyry-related lithocap at current exposure. The quartz-tourmaline rock developed from fluids of higher salinity and temperature suggests a link with magmatic-hydrothermal systems and warrants further investigation. Acknowledgements We would like to thank Mr. Afrouz for introducing the area, help with field works, and discussions. We are grateful to Mr. Imani from Parsolang for his help with field works and discussions, and to Ms. Aghajani from Iranian Mineral Processing Research Center for her invaluable help with the fluid inclusion studies. Dr. Ashrafpour kindly provided us with geological maps and chemical analysis of samples from the Meideh silicic zone. The study was supported by the Iranian Mines and Mining Industries Development and Renovation Organization (IMIDRO) and a Shahid Beheshti University grant to S.A. |
