Genesis of Yuanlingzhai Large Porphyry Molybdenum Deposits in East Section of Nanling: Evidence from Fluid Inclusions and Stable Isotope

Yuanlingzhai porphyry Mo deposit is a newly discovered Mo-only deposit in the Nanling region. The Mo reserve in this deposit is estimated as 200 000 t. The mineralization developed in contact zones between the Yuanlingzhai granite porphyry and Neoproterozoic metamorphic rocks of the Xunwu Formation.In this paper, we present a detailed research on fluid inclusions and stable isotope of the deposit reveals the characteristics of ore-forming fluids and mineralization. Research on fluid inclusions in quartz veins shows that they are distributed in groups, scattered random distribution or uniform distribution with long axes of 4–15 μm. The morphology of inclusions are mainly negative crystal, elliptical shape, elongated shape and irregular shape. The types of fluid inclusions include pure liquid, pure gas, two phase, three-phase inclusions containing liquid CO₂, and multi-phase inclusions with daughterminerals. The liquid components of fluid inclusions are mainly Na⁺, Ca⁺, Cl⁻and SO₄²⁻ and the gas components are mainly H₂O, CO₂, H₂ implying that the ore-forming fluid belongs to CO₂-H₂O-NaCl(CaCl₂) system with strong reducibility.The homogenization temperatures of the fluid inclusions vary remarkably from 170 to 350oC with an average of 257 oC. The NaCl_eq salinities of fluids range from 1.40% to 12.97% with an average of 6.236%. The fluid density ranges from 0.687 g/cm³ to 0.973 g/cm³with an average of 0.848 g/cm³. Therefore, the ore-forming fluid is characterized by medium to high temperature, medium to low salinity, and low density with depleted F enriched Cl. Besides, the fluid rich in Cu, Mo, Sb, Bi, etc. medium to high temperature ore-forming metallic elements, relatively rich in Co, Ni, suggests some connection with deep or mantle fluid.The δD_V-SMOW value of the fluid inclusion in ore-forming stages ranges from −50.2‰ to −43.5‰, and δO_H2O value ranges from 2.69‰ to 5.69‰, which is the result of magmatic water and meteoric water mixing action. The δ³⁴S value of metal sulfide in ores ranges from −1.5‰ to 1.6‰, showing that ore-forming materials may have been derived from the deep. Fluid immiscibility (decompression boiling) and mixing of different fluids may be the main ore-forming mechanism of Yuanlingzhai Mo deposit.