Discussion On Characteristics And Genesis Of Formation Of Bairendaba Polymetal Ag Deposit, Chifeng, Inner Mongolia

Abstract: The western slope of Great Hsian Mountain is one of the dominant polymetal Ag metallogenic belts in China. Bairendaba polymetal Ag deposit is located in Linxi county, Chifeng city, Inner Mongolia. It's a municipality deposit with 5000 tons of reserves. This area is situated in the ultimate collision belt between Siberian platform and North China platform. The two platforms collided at the end of early Permian that leaded to intensive granitic magma intruded. Intense tectono-magmatism in this period is the found of regional mineralization in this area. Proterozoic and Permian stratum were the main exposed strata and northeast, nearly transmeridional and northwest fault are the primary structures in this area. Variscan quartz-diorite and Yanshanian granite are the main intrusive rocks. According to researches in field and ore microscope characteristics, we think that amphibolite identified by No. 9 Mineral Survey Institution, inner Mongolia should be diabase-gabbro. In the exploratory trench of west part of mining district we find some felsite dikes for the first time. Proterozoic gneiss and Variscan quartz-diorite are the immediate wall rocks of ore bodies. The types of wall rocks have no influence on the mineralization. Most of ore bodies were located in the near transmeridional compresso-shear faults and their occurrences are in accordance with the structure shattered zones. We found that a northwest fault took place when ore had been deposited and it divided the mining district into two parts i.e. west part and east part. Obviously the east part was uplifted higher and denuded hardly than the west part. The main metallic minerals consist of pyrrhotine, pyrite, sphalerite, galena and and the main nonmetallic minerals are composed by chlorite, quartz and fluorite. During our research, we firstly find boulangerite, stannite and argentiferous terahedrite by electron microprobe and ore microscope analysis in Bairendaba Ag polymetal deposit. Argentite is the main silver ores, and the others are argentiferous terahedrite, native silver and isomorphism silver. The above silver ores were distributed in the galena, chalcopyrite and sphalerite as immiscible inclusions. Refer to the regional geochemistry setting, Premian foundamental stratum riched in silver, lead and zinc on southeast of mining district may have offered a lot of metallogenic elements for metallogenesis. Yanshanian granite in the north and southeast of mining district probably offered parts of metallogenic element, thermal power and water. Busic dikes formed in Variscan near ore bodies probably offered most of silver for metallogenesis. Felsite dikes formed just before the metallogenesis period were made by the late stage of Yanshanian magmatism. They also came closely with ore bodies and often cut off by ore bodies. We supposed it offered primary thermal power for metallogenesis. Based on experience and analysis, we found that δ34S‰is nearly zero in minerals in this ore deposit. So sulfur in this deposit should come from mantle. Refer to the hydrogen and oxygen isotope compositions characteristics of polymetal deposits in this area and the low salinity of ore-forming fluid, we consider that ore-forming fluid in this deposit is heated meteoric water. Based on the tests of fluid inclusions, we concluded that this deposit was formed in relatively moderate or low temperature(210~300℃). The salinity and density of ore-forming fluid are very low, i.e. 2~8% and 0.84g/cm3 respectively. The depth (7.08 km) and reductibility (0.155 ～0.798) of metallogenesis is relatively moderate. This deposit was formed by several stages of metallogenesis. Bairendaba polymetal Ag deposit came into being at the late stage of early Yanshanian tectono-magmatism. Ore bodies were mainly controlled by nearly transmeridional compresso-shear faults formed in early Yanshanian. Magmatism in early Yanshanian was important for the forming of this deposit and offered parts of thermal power and brought much metallogenic elements because it fractional resorptived Premian foundamental stratum enriched in silver when it intruded into the superficial crust, lead and zinc and brought them there. At the late stage of the Yanshannia magmatism, massive acidic...