Formation of the Miocene Weixi Sb-polymetallic mineralization in the Sanjiang Region, SW China: Insights from fluid inclusions, S-He-Ar isotopes, and zircon U-Pb geochronology

• Weixi Sb ore fluids are medium–low temperature, low salinity, and CO 2 -rich. • Mineralized reduced sulfur originates from wallrocks and magma. • The ore-forming fluids were mainly crustal sourced. • Concealed Miocene crustal magmatism is linked to Weixi Sb-polymetallic mineralization. The Weixi Sb-polymetallic orefield (>0.2 Mt Sb) in the Sanjiang metallogenic belt (northwestern Yunnan, SW China), represents a distinct type of Sb-polymetallic mineralization. Here, we examine the ore-fluid properties, source, and metallogenic process by analyzing the fluid inclusions, S-He-Ar isotopes, and zircon U-Pb age of three representative deposits (Shangnuluo, Baiji, and Hagudi) in the orefield. Early pyrite-molybdenite mineralizing fluids in the Baiji deposit contain A-type (liquid H 2 O), B-type (gaseous CO 2 ), C-type (H 2 O-CO 2 ), and D-type (solid-bearing) inclusions, showing a CO 2 -rich, high-salinity (30.6–36 wt% NaCleqv.) fluid at medium–high temperatures (177–442°C), with metal precipitation primarily driven by fluid boiling. In the late sphalerite-galena-zinckenite-stibnite-calcite-quartz stage (Baiji), A-, B-, and C-type inclusions indicate lower temperatures (186–276.6°C) and salinity (0.35–12.4 wt% NaCleqv.), where metal deposition is controlled by fluid mixing. The Hagudi ore fluids are of medium–low temperature (150–368°C) and salinity (3.2–21.2 wt% NaCleqv.), contain primarily A-type (with some B-type) inclusions, with cooling and mixing being the main ore-forming mechanism. The Shangnuluo ore fluids are of medium temperature and low salinity, contain mainly A-, B-, and C-type inclusions (NaCl-H 2 O-CO 2 fluid system), with immiscibility driving the metal precipitation. These fluid characteristics resemble typical orogenic or magmatic-hydrothermal ore fluids rather than basin-type ore fluids. He-Ar isotopes suggest a primarily crustal fluid origin with minimal mantle contribution. In-situ sulfur isotopes (Shangnuluo: −13.04 to −2.90 ‰; Baiji: −1.60 to 6.65 ‰; Hagudi: −2.48 to − 1.54 ‰) imply a magmatic and Permian formation source at Shangnuluo, magmatic and Jurassic formation source at Baiji, and a primarily magmatic source at Hagudi. Zircon U-Pb dating places the formation of ore-hosting quartz porphyry at Hagudi in the Late Permian (252.7 ± 2.4 Ma). The quartz porphyry was formed before the Miocene mineralization event (∼13 Ma) and potentially provided ore-hosting space for the Miocene mineralization. Integrating our new data with previous studies on regional tectonics and magmatism, we speculate that the Weixi Sb-polymetallic mineralization is closely linked to concealed Miocene crustal magmatism. Ore-forming fluids migrated along faults into various stratigraphic levels, and metal precipitation was triggered by fluid immiscibility, cooling, and dilution by formation or meteoric water.