Abstract:

The studied area is located in the Wenduermiao-]Wengniuteqi fold belt, one of the most important nonferrous metal metallogenic belts in China. Tectonically, the belt belongs to the Mongolia-Okhotsk orogenic belt. The orebodies are hosted in the monzonitic granite in the hanging wall of EW-striked F₁, and strictly controlled by EW-and NE-striked faults. Naturally, the deposit belongs to altered rock type and quartz vein type, and the sulfides in ore veins are dominated by pyrite, chalcopyrite and molybdenite. Fluid inclusions in quartz crystals can be further classified into three subtypes; aqueous and gas two-phase，daughter crystal-bearing three-phase and pure gas phase. The salinities of fluid inclusions range from 2.06% to 44.00% NaCl eqv，concentrated in two ranges of 40%-44% NaCl eqv and 4%-8% NaCl eqv. The densities of fluid inclusions range from 0.85 to 1.25 g/cm³，concentrated in two ranges of 1.2-1.24 g/cm³ and 0.88-0.92 g/cm³. The homogenization temperatures of fluid inclusions range from 167℃ to 380℃，concentrated in two ranges of 320-360℃ and 200-220℃. The salinities，densities and homogenization temperatures of fluid inclusions are all bimodal in the histograms. During the early ore-forming stage，the ore-forming fluids are magma fluids with high temperature， salinity and density, and the coexisting fluid inclusions of daughter crystal-bearing three-phase and pure CO₂ indicate they were formed by boiling or immiscibility during the late magma crystallization. In the late ore-forming stage，the ore-forming fluids are mixed fluid with low salinity and density and moderate temperature. According to the diagram of liquid-vapor relations for the NaCl-H₂O system and coexistence surface，the ore-forming pressure is estimated in the range of 274.7-368.5 MPa, corresponding to the ore-forming depth of 8.32-11.2 km. Genetically, the Yajishan molybdenum copper deposit is defined as mesozonal，meso-hypothermal deposit.

Key words: fluid inclusion；hypothemal vein-typed deposit；Yajishan；Chifeng;molybdenum deposits;copper deposits