Characterization of Ore‐Forming Fluids in the Lanuoma Sediment‐Hosted Pb‐Zn Deposit, Sanjiang Tethys Region, Southwestern China: Evidences From Fluid Inclusions, Rare Earth Elements, and C‐O‐Sr‐Nd Isotopes

ABSTRACT The Lanuoma deposit, situated in the Changdu Basin of Southwestern China, is a unique sediment‐hosted Pb‐Zn‐Sb deposit. Three hydrothermal stages are present: (1) premineral stage, oolitic pyrite + calcite + gypsum + barite; (2) syn‐mineral stage, further divided into (2a) early stage, with the main Zn‐Pb‐Sb precipitation (sphalerite + robinsonite + galena + calcite); (2b) late stage (orpiment + realgar + calcite + minor sulfides); and (3) postmineral stage, coarse‐grained calcite. The calcites of Stage 2a are characterized by lower ΣREE concentrations (0.97–3.32 ppm) associated with positive Eu anomalies (1.13–1.98) and higher ΣREE concentrations (10.65–17.17 ppm) associated with negative Eu anomalies (0.57–0.59), indicating that the ore‐forming fluids were a mixture of two distinct fluid sources. The increasing trend of Ce anomalies (Stage 2a = 0.97, Stage 2b = 1.05, Stage 3 = 1.08) shows enhancement of fluid reducing conditions during hydrothermal evolution. The δ 13 C V‐PDB and δ 18 O V‐SMOW values also differ between Stage 2a (δ 13 C V‐PDB = −4.9‰ to 3.0‰; δ 18 O V‐SMOW = 11.4‰ to 16.6‰), Stage 2b (δ 13 C V‐PDB = −4.7‰ to −0.4‰; δ 18 O V‐SMOW = 14.2‰ to 20.4‰), and Stage 3 (δ 13 C V‐PDB = −4.0‰ to 1.7‰; δ 18 O V‐SMOW = 17.1‰ to 20.6‰), indicating a mixture of deep‐seated magmatic fluid and dissolved marine carbonate rocks. Corresponding δ 18 O fluid values of the hydrothermal fluid are 2.4‰–7.0‰ in Stage 2a, 4.9‰–8.5‰ in Stage 2b, implying a combination of deep‐seated magmatic fluids and sedimentary formation water. Initial 87 Sr/ 86 Sr ratios of calcite from Stage 2a (0.7084–0.7086) fall between the values characteristic of the mantle, metamorphic basement, and Bolila Formation, implying a mixed source. Calcite in Stage 2a has higher ε Nd ( t ) values (−0.51 to −1.82) compared to Stage 2b and Stage 3 (−3.91 to −9.96). This indicates there was more input of mantle‐sourced materials during Pb‐Zn mineralization. Systematic microthermometric analysis demonstrates an isothermal mixing of two fluid sources (averaging at 189°C) with different salinities (ranging from 3.87% to 19.05% NaCl eqv; i.e., low‐salinity magmatic fluid and high‐salinity brines) during Pb‐Zn mineralization. These results suggest that the Lanuoma Pb‐Zn deposit was formed by the mixing of two distinct mineralizing fluid sources: (1) magmatic‐hydrothermal fluids derived from deep‐seated magmas, which transported reducing agents such as sulfur; and (2) high‐salinity brines enriched in metals, likely leached from carbonate wall rocks and metamorphic basement.