Geology, geochemistry and genesis of the Qianfanling quartz-vein Mo deposit in Songxian County, Western Henan Province, China

摘要

The Qianfanling Mo deposit, located in Songxian County, western Henan province, China, is one of the newly discovered quartz-vein type Mo deposits in the East Qinling-Dabie orogenic belt. The deposit consists of molybdenite in quartz veins and disseminated molybdenite in the wall rocks. The alteration types of the wall rocks include silicification, K-feldspar alteration, pyritization, carbonatization, sericitization, epidotization and chloritization. On the basis of field evidence and petrographic analysis, three stages of hydrothermal mineralization could be distinguished: (1) pyrite-barite-quartz stage; (2) molybdenite-quartz stage; (3) quartz-calcite stage. Two types of fluid inclusions, including CO_2-bearing fluid inclusions and water-rich fluid inclusions, have been recognized in quartz. Homogenization temperatures of fluid inclusions vary from 133 to 397 . Salinity ranges from 1.57 to 31.61 wt.% NaCl eq. There are a large number of daughter mineral-CO_2-bearing inclusions, which is the result of fluid immiscibility. The ore-forming fluids are medium-high temperature, low to moderate salinity H_2O-NaCl-CO_2 system. The ~(34)S values of pyrite, molybdenite, and barite range from -9.3 to - 7.3, -9.7 to - 7.3 and 5.9 to 6.8, respectively. The ~(18)O values of quartz range from 9.8 to 11.1, with corresponding ~(18)O_(fluid) values of 1.3 to 4.3, and ~(18)D values of fluid inclusions of between -81 and -64. The ~(13)C_(V-PDB) values of fluid inclusions in quartz and calcite have ranges of - 6.7 to -2.9 and - 5.7 to -1.8, respectively. Sulfur, hydrogen, oxygen and carbon isotope compositions - show that the sulfur and ore-forming fluids derived from a deep-seated igneous source. During the peak collisional period between the North China Craton and the Yangtze Craton, the ore-forming fluids that derived from a deep igneous source extracted base and precious metals and flowed upwards through the channels that formed during tectonism. Fluid immiscibility and volatile exsolution led to the crystallization of molybdenite and other minerals, and the formation of economic orebodies in the Qianfanling Mo deposit.