伊犁京希—伊尔曼德金矿床的热液蚀变及成矿流体演化特征

Spatial zonal features show well in the hydrothermal alteration distribution of the Jingxi-Yelmend gold prospect. The central zone is characterized by intense silicification with a quartz/chalcedony-dickite assemblage. The intermediate zone shows advanced argillic alteration with a dickite-kaolinite-quartz/chalcedony-semectite assemblage. The outer zone is characterized by an argillic alteration assemblage of kaolinite-illite. Such zonation is a reflection of temperature, pressure and chemical gradients, suggesting there is continuous fluid/rock reaction. The fluid inclusions indicate that the fluids related to the alteration and mineralization have temperatures generally between 180∼270°C and relatively low salinity (Max. 5. 3 wt% NaCl equiv.). The evolution of the ore fluids has undergone three stages: (1) Early-stage acidic (pH=2~4) ore fluids were buffered by fluid in neutral limestones while flowing upward along steep faults and the pH values were slightly increased. (2) The pH value was increased to 3~5 when the ore fluids reached highly permeable epiclastic rocks (conglomerate and conglomeratic sandstone) and rapidly reacted with the country rocks, particularly with the assistance of in-charging meteoric water. In this stage, the composition, temperature and pH values were changed and the physicochemical gradients were generated. Highly concentrated residual SiO₂ and most acidic (pH<3) fluids were retained in the central part of the fluid-flowing zone, where vuggy quartz (acidic leaching) and dickite occurred while the marginal parts have relatively high pH values and intermediate-argillic alteration was developed. With the decreasing of temperature and pressure (boiling), and increasing of oxidation for the ore fluids, the alteration zonation was formed and gold was deposited.