The Characteristics of Ore-forming Fluids and Metallogenic Mechanism of the Kumutashi Fluorite Deposit in West Altyn Tagh, China

Kumutashi fluorite deposit is located in the Kaerqiaer super-large fluorite ore belt in the western Altyn Tagh, which is a newly discovered large fluorite deposit in recent years. The ore body is dominated by fluorite-calcite vein type, associated with lithium-bearing mica, and occurs in the NE and nearly EW faults of the Paleoproterozoic Altyn Tagh rock group. At present, the research on the characteristics of ore-forming fluid and ore-forming mechanism is relatively weak. Fluid inclusions in fluorite and calcite from different mineralization stages were studied by petrography, microthermometry, laser Raman spectroscopy, and hydrogen and oxygen isotopes. The ore-forming process can be divided into two stages: the early stage (Ⅰ) and the late stage (Ⅱ). The massive ores formed in the early stage are mainly gas-rich two-phase aqueous inclusions and CO₂ three-phase inclusions with homogenization temperature ranging from 225.1 to 410.8 ℃, salinity from 5.20 to 9.63 wt%NaCl_eqv and density from 0.25 to 0.76 g/cm³; In the late stage, brecciated and stockwork ores were formed, and liquid-rich two-phase and gas-rich two-phase aqueous inclusions were mainly developed, with homogenization temperature ranging from 117.2 to 347.8 ℃, salinity from 0.53 to 12.73 wt%NaCl_eqv, and density from 0.40 to 0.91 g/cm³. The liquid phase of the inclusion is mainly composed of H₂O with a small amount of CO₂, and the gas phase is mainly composed of CO₂ with a small amount of CH₄, N₂, H₂ and H₂S. In the early stage of mineralization, the fluid was a NaCl-H₂O-CO₂ hydrothermal system with medium-high temperature, medium-low salinity and low density, while in the late stage of mineralization, the fluid was a NaCl-H₂O-CO₂ hydrothermal system with medium-low temperature, low salinity and low density. The results of hydrogen and oxygen isotope studies indicate that the ore-forming fluids were derived from a mixture of magmatic hydrothermal and meteoric water. Fluorite precipitation in the early stage of mineralization was mainly due to the mixing of magmatic hydrothermal solution and meteoric water, as well as water-rock reaction. In the late stage, fluid mixing further occurred, resulting in the decrease of temperature and the formation of brecciated and stockwork ores. Kumutashi fluorite deposit belongs to magmatic hydrothermal filling type vein fluorite deposit.