Fluid inclusions, S-Pb-H-O isotopes and trace elements of quartz and sphalerite of the Shaxi Pb-Zn-Ag polymetallic deposit in the Yidun Terrane, SW China: implications for ore genesis and exploration

• The ore-forming components originated from the causative granites and host rocks. • Fluid mixing triggered the precipitation of sphalerite, galena and Ag-bearing sulfides. • The Shaxi Pb-Zn-Ag deposit is classified as an intrusion-related deposit. • Skarn Sn mineralization should focus on the deeper site beneath the Pb-Zn-Ag veins. The Xiasai–Lianlong metallogenic belt in the central Yidun Terrane (Southwestern China) is renowned for numerous Late Cretaceous granite-related Sn and Pb-Zn-Ag deposits. As a representative Pb-Zn-Ag deposit within this metallogenic belt, the Shaxi deposit is characterized by three hydrothermal stages, including quartz-pyrite-pyrrhotite-chalcopyrite (I), quartz-sphalerite-galena-Ag-bearing sulfosalts (II) and quartz-calcite stages (III). The absence of causative intrusions makes the relationship between Pb-Zn-Ag mineralization and magmatism unclear, hindering an understanding of ore genesis and further exploration. Four types of fluid inclusions in the hydrothermal minerals include mono-phase liquid aqueous (type 1), mono-phase vapor aqueous (type 2), two-phase aqueous (type 3) and halite-bearing tri-phase aqueous (type 4) inclusions. Type 3 inclusions consist of liquid-rich (type 3a) and vapor-rich (type 3b) inclusions. All the types of inclusions are present in stage I quartz, whereas only types 1 and 3a inclusions are found in stages II and III quartz. Types 3a and 4 inclusions in stage I exhibit homogenization temperatures ( T h ) ranging from 278 to 337 ℃ and 354 to 386 ℃, respectively, coupled with salinities in the range of 15.4 – 20.7 wt% NaCleq and 34.9 – 41.5 wt% NaCleq. In contrast to stage I, both the T h and salinities of type 3a inclusions from stages II and III decrease significantly, with the T h ranging from 163 to 281 ℃ and 117 to 150 ℃ and salinities from 4.9 to 10.4 wt% NaCleq and 0.9 to 1.1 wt% NaCleq, respectively. The δ D H2O values of stages I, II and III fluids are −106.8‰ – −105.7‰, −119.7‰ and −137.8‰, with the calculated δ 18 O H2O values of 12.3 – 12.6‰, −2.9‰ and −6.8‰, respectively. Quartz from three stages are enriched in Li and Al, with Li/Al ratios ranging from 0.05 to 0.5. Stage II sphalerite have high Fe, In and Mn contents, coupled with low Ga/In ratios (0.01 – 0.8). Furthermore, the trace elements of quartz and sphalerite resemble those of skarn-type deposits and Sn-related quartz vein. Evidence mentioned above suggests that the fluid mixing between magmatic and meteoric water in stage II triggered the precipitation of Pb, Zn and Ag. Sulfides from stage II exhibit a δ 34 S range of −6.0 to −1.9‰, indicating a mixed magmatic and sedimentary sulfur source. Furthermore, the Pb isotopic compositions of these stage II sulfides are similar to those of Late Cretaceous granites in the Yidun Terrane and host Tumugou Formation, suggesting that the ore-forming metals originated from both the concealed granite and host rocks. Integrated geological and geochemical findings unequivocally classify Shaxi as a magmatic-hydrothermal Pb-Zn-Ag deposit. To summarize, the central Yidun Terrane hosts magmatic-hydrothermal Pb-Zn-Ag, skarn-type Sn, and newly discovered W-Sn-Be mineralization that are intrinsically linked in time, space and genesis. They occur at various locations surrounding the Late Cretaceous granites. Additionally, this proposed ore system suggests significant prospecting potential for skarn Sn mineralization situated below the existing Pb-Zn-Ag veins, as well as rare mineralization at the periphery and nearby the Late Cretaceous granites.