Abstract:

The mineralizing process of the Yangjingou large scheelite deposit can be listed such four metallogenetic stages as pyrite-arsenopyrite, quart-coarse scheelite, quartz-sulphides-fine scheelite as well as carbonates stage. In order to analyze the source, features and evolution of ore-forming fluid, fluid inclusions in quartz minerals formed in the major metallogenetic stage are studied. Types of fluid inclusions include gas-rich (Ⅰ),gas-liquid (Ⅱ), pure-CO₂ (Ⅲ) and CO₂-bearing three phase (Ⅳ). The gas phase of typeⅠinclusions is proved to be mainly composed of CO₂, CH₄ and N₂, their homogenization temperatures vary from 278.5 to 336.4℃, with salinities (NaCl) from 3.53% to 7.72%. TypeⅡ inclusions, with ratios of gas to liquid in change from 10% to 45%, are homogenized to liquid during heating, their homogenization temperatures vary from 144.7 to 345.9℃, mainly from 190 to 220℃, salinities(NaCl) from 3.05% to 9.34%. Type Ⅲ is dominated by CO₂ with minor CH₄. Type Ⅳ fluid inclusions are composed of three phases (V_CO2+L_CO2+L_H2O), homogenization temperatures change from 301.6 to as 305.1℃.Comprehensive study on ore-forming condition,geological features of the deposit, compositional and microthermometric analysis of fluid inclusions leads to the conclusion that the ore-forming fluid of the Yangjingou deposit belongs to the NaCl-H₂O-CO₂(-N₂) hydrothermal system with medium-high temperature, low salinity. It is derived from deep crust to upper mantle and mixed by ore-forming materials from the Paleozoic metamorphic strata of the Wudaogou Group which hosts scheelite-quartz veins. The immiscibility of ore-forming fluid played an important role in the metallogenetic process of Yangjingou scheelite deposit.

Key words: fluid inclusion, immiscibility, ore genesis, Yangjingou scheelite deposit, Yanbian area