Abstract

Chah-Bagh gold deposit is located in central part of Sanandaj-Sirjan zone, 60 km southwest of Delidjan, in Muteh mining district. Rock units exposing in the area underwent greenschist to lower amphibolite metamorphism. They consist of NW-SE trending deformed and metamorphosed volcano-sedimentary and acidic volcanic rocks. These units, which are thought to be of Devonian or pre-Devonian age have been intruded by basic sills and dikes. The rock units are characterized by several phases of intense deformation which lead to generation of different fabrics and structures. The most important structure in the area is caused by WNW (N280-N290^º) trending inverse dextral ductile shear zone. Mylonitization, extension of mylonitic foliation, penetrative stretching lineation, chevron folds, etc are generated due to ductile deformation in the area. Gold mineralization occurred in ductile and brittle shear zones. The maximum gold concentration occurred along 1 km long, 60 meter wide NW-SE trending (N40-50E) ductile shear zone. Ductile gold mineralization, reported for the first time in Chah-Bagh, has NW (N60-80W) dipping lenticular shape. Samples taken from silicified parts of this zone contain 1.8-13.2 ppm gold. Deformation in this zone occurred as protomylonite, mylonite and ulteramylonite. Brittle mineralization, including gold-bearing siliceous-sulfidic zones, occurred along N40W tending, NE dipping normal faults. The main gold-bearing siliceous-sulfidic zone in Chah-Bagh is 100 meters long and 3-5 meters wide with N35W general trend. Geochemical analysis of samples taken from this sulfide-siliceous zone indicated near 1.82 ppm of gold grades. The rock units hosting the above mentioned shear zones mainly consist of intensely deformed meta-acidic volcanics and volcano-sedimentary rocks (meta-rhyolite and felsic schist). These rock units have experienced different types of alteration with various intensities. The most important ones are sericitization, kaolinitization, epidotization, tourmalinization, silicification and solfidization. Silicified and sulfide alteration observed in the inner parts of alterations zones coincide with the ore-bearing zones. Studies suggested temporal and spatial relation between alteration and deformation. Spatial relations were interpreted by overlapping alteration and deformation zones. Hydrothermal quartz and pyrites filled foliation parallel open spaces (generated during ductile deformation and mylonitization) and also micro-fractures of porphyroclasts (with grain cataclastic flow origin) simultaneously. These facts reflected temporal correlation between deformation and alteration. High gold contents are related to silicified highly-deformed mylonitic and ultramylonitic units and also sulfide-bearing zones coinciding with the inner parts of alteration zones. Simple ore-mineral assemblages, include pyrite and chalcopyrite in ductile shear zone and pyrite, arsenopyrite and chalcopyrite in brittle ones. Iron hydroxides and other weathering products accompany these sulfides in the alteration zones. Based on microscopic studies, quartz, the main rock-forming mineral in Chah-Bagh deposit, is considered to be of three generations: pre-deformation quartz, first stage hydrothermal quartz and hydrothermal quartz accompanied by sulfide (late stage), respectively. Based on microscopic structural and textural studies, sulfide minerals can be divided into two different generations. The first generation consisting of euhedral and coarse grain pyrite, arsenopyrite and chalcopyrite occurred in the form of segregated grains or along weak surfaces (such as fractures and grain boundaries) of second stage and pre-deformation quartz grains. The second generation of sulfides, characterized by undeformed disseminated fine grain anhedral pyrite and chalcopyrite mineralization in altered rocks.  In elevated gold-bearing samples, gold could not be observed in siliceous gangue and altered sulfide mineral margins. But electron microprobe and electronic microscope analysis indicated invisible gold within the sulfide minerals. Based on these studies, gold have been observed with both pyrite and chalcopyrite phases. Silver exists as invisible within sulfide minerals and in the form of native one within siliceous gangue. Lithogeochemical survey perpendicular to ore-bearing zones showed positive correlation between silica, titanium, sulfur, iron, arsenic, molybdenum, tungsten and chromium and   high-grade zones accompanied by deformation and alteration. Light rare earth elements (LREE except Eu) enriched in sulfidized and silicified parts with elevated gold contents. Controlling parameters for mineral concentration in Chah-Bagh deposit are shear zones (ductile and brittle) and alteration (silicification and sulfidization). Based on comparison of Chah-Bagh main characteristics and orogenic gold deposits, Chah-Bagh has the most similarities with orogenic gold mineralization therefore it is considered to be of this type of gold deposits.