Source and evolutionary characteristics of ore-forming fluids in Fuludi gold deposit, Jiaodong Peninsula: Evidence from fluid inclusions and H-O isotopes

The Fuludi gold deposit is located in the middle segment of the Muping-Rushan gold metallogenic belt, Jiaodong Peninsula, a world-class gold concentration area with substantial gold resources. Previous geological studies on deposits in this belt have mainly focused on gold deposits in its southern and northern sections, while the origin and ore-forming fluid evolution of Fuludi gold deposit have long remained poorly understood. Controversies still exist over the source of ore-forming fluids in the Muping-Rushan belt, with three mainstream viewpoints: mantle-derived fluid, mixed magmatic fluid and meteoric water, and mixed magmatic water and metamorphic water. Geographically, the Fuludi gold deposit acts as a key link connecting the northern and southern parts of the metallogenic belt. However, previous studies have only carried out basic geological surveys and divided its mineralization stages, and systematic studies on ore-forming fluids have long been lacking. This study aims to clarify the source, spatiotemporal evolution of ore-forming fluids, and gold precipitation mechanism of the Fuludi gold deposit.

Combined with detailed field and microscopic geological characteristics, this study selected quartz samples from four different mineralization stages to conduct a comprehensive analysis, including fluid inclusion microthermometry, laser Raman spectroscopy, and hydrogen-oxygen (H-O) isotope testing. It systematically analyzed the petrographic characteristics of fluid inclusions, physicochemical parameters of ore-forming fluids, fluid compositions, and isotopic compositions, so as to constrain the fluid source and gold metallogenesis.

According to the cross-cutting relationships of quartz veins and paragenetic mineral assemblages, four mineralization stages were divided: milky quartz stage (Stage Ⅰ), smoky quartz+pyrite early mineralization stage (Stage Ⅱ), smoky quartz+polymetallic sulfide main mineralization stage (Stage Ⅲ), and quartz+calcite late mineralization stage (Stage Ⅳ). Two types of fluid inclusions were identified in quartz: pure liquid (L-type) aqueous fluid inclusions and gas-liquid two-phase (L+V-type) fluid inclusions. The L+V-type inclusions occurred throughout all four stages, with particle sizes of 3−10 μm and showing elongated, oval, and irregular shapes. Their liquid-phase proportion gradually increased from Stage Ⅰ to Stage Ⅳ, ultimately reaching approximately 80%. The L-type inclusions only developed in Stage Ⅲ, with a particle size of 5−15 μm and mostly oval shapes. Laser Raman analyses revealed that the ore-forming fluid belonged to the CO₂-H₂O-NaCl system, and its components varied significantly at different stages. CH₄ was detected in Stage Ⅰ, no CH₄ existed in Stage Ⅱ, N₂ appeared in Stage Ⅲ, and only CO₂ and H₂O were found in Stage Ⅳ. Microthermometric results showed that the early Stage Ⅰ and Stage Ⅱ had stable physicochemical conditions, with homogenization temperatures ranging from 180.0 °C to 240.0 °C and salinity peaks of 9.0%−17.0%. Fluid boiling occurred at the main Stage Ⅲ, accompanied by obvious decreases in temperature (160.0-200.0 °C) and salinity (11.0%−15.0%). At the late Stage Ⅳ, temperature (120.0-180.0 °C) and salinity (3.0%−11.0%) decreased further. H-O isotope results showed that δD values ranged from −87.0‰ to −72.4‰, and δ¹⁸O values of quartz were 9.0‰-14.0‰. Calculated by oxygen isotope fractionation equation, the δ¹⁸OH₂O of ore-forming fluid was −3.98‰ to 1.82‰.

The ore-forming fluids are dominated by mixed magmatic water and metamorphic water, and are also accompanied by mantle-derived components originating from volatile degassing of enriched mantle. Meteoric water continuously mixed into the fluid system during mineralization. With the gradual decrease of temperature and pressure, combined with fluid boiling and escape of volatile CO₂, the physicochemical properties of fluids change significantly. These processes broke the stability of ${\mathrm{Au}}({\mathrm{HS}})_2^- $ complexes, which are the main transport carrier of gold, and led to the rapid precipitation of gold and polymetallic minerals. Comprehensive geological and geochemical evidence indicates that the Fuludi gold deposit is a typical medium- to low-temperature, low-salinity quartz vein-type hydrothermal gold deposit controlled by NNE-trending faults. This study fills the research gap on ore-forming fluids in Fuludi gold deposit, and provides reliable geological evidence for regional metallogenic theory and further prospecting work.