Ore-forming fluid and metallogenic mechanism of the Yunling tin deposit in the Baoshan Block, Western Yunnan

• Ore-forming fluid of Yunling tin deposit originated from magmatic fluid. • Sulfur originated from crust-derived granites with contribution from sedimentary sulfur. • Metal precipitation controlled by fluid cooling and redox reactions. The Yunling Sn deposit is located on the eastern edge of the Baoshan Block in western Yunnan, with ore bodies predominantly hosted within Triassic granites. According to crosscutting relationships and paragenetic associations, the mineralization process is divided into three distinct stages: Stage I (quartz-cassiterite-muscovite-tourmaline), Stage II (arsenopyrite-sulfide-cassiterite-quartz), and Stage III (arsenopyrite-calcite-quartz veins). To investigate the mineralization process, fluid microthermometry and in situ sulfur isotope analyses were performed. Three primary types of fluid inclusions were identified: vapor–liquid two-phase fluid inclusions (LV-type), CO 2 vapor fluid inclusions (V CO2 -type), and daughter mineral-bearing polyphase fluid inclusions (S-type). Microthermometric analyses reveal that the ore-forming fluids evolved from an early CO 2 –CH 4 -bearing NaCl-H 2 O system with high temperatures (400–500°C) and high salinity (38–56 % NaCl equiv.) to a later NaCl-H 2 O system characterized by medium-to-high temperatures (200–300°C) and low salinity (1.0–5.2 % NaCl equiv.). The δ 34 S values of sulfides range from 0.6 ‰ to 9.2 ‰, with an average of 5.0 ‰, suggesting that the sulfur was primarily sourced from crust-derived granites with a significant contribution from sedimentary sulfur. Combined with previous oxygen isotope data, the ore-forming fluids are interpreted to be primarily magmatic in origin during Stages I and II, while meteoric water involvement occurred in Stage III, after cassiterite precipitation. Cassiterite precipitation was primarily controlled by fluid cooling, accompanied by redox reactions.