Tracing the Genesis and Mineralization Mechanism of the Xiejiagou Pb‒Zn Deposit in Gansu, China: Insights from Trace Element Analysis, Fluid Inclusion Studies, and the Sulfur Isotopic Composition

The Xicheng ore field in West Qinling contains numerous Pb‒Zn deposits of varying sizes and is one of the largest areas of Pb‒Zn mineral resources in China. The Xiejiagou Pb‒Zn deposit is in the central part of the southern belt of the Xicheng ore field. No research has been completed on the trace elements and origin of this deposit. To supplement the geochemical information for each deposit in the Xicheng ore field and provide a scientific basis for the exploration of critical metals in the region, the sulfide in the Xiejiagou deposit is selected as the subject of investigation in this study, and its trace element composition is examined using laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP‒MS), accompanied by fluid inclusion thermometry and sulfur isotope analysis, to elucidate the geochemical characteristics of the deposit and propose its genesis. The results showed that the compositions of the trace elements in different sulfides differ greatly, with sphalerite mainly enriched in Cd and relatively enriched in Ge, Mn and In; galena mainly enriched in Ag and relatively enriched in Se, Ge and Tl; chalcopyrite relatively enriched in Sn and Ge; and pyrite relatively enriched in Ni, Co, Ge and Tl. The critical metals are mainly in sphalerite. Zn and Cd mapping diagrams of sphalerite suggest a possible Zn2+ ↔ Cd2+ substitution mechanism; Fe and Mn are positively correlated, while Fe and Zn are negatively correlated, suggesting a possible Fe2+ + Mn2+ ↔ 2Zn2+-coupled substitution mechanism; and the positive correlation between Cu and Ge is strong, while the difference in the content is large, suggesting a substitution mechanism involving (n + 1)Zn2+ ↔ Ge2+ + nCu2+. The sulfur isotopes show obvious heavy sulfur characteristics; the sulfur source is likely Middle Devonian seawater sulfate or marine sulfate from the strata, and the production of reduced sulfur is closely related to thermochemical sulfate reduction (TSR). The trace element compositions of sulfides in the Xiejiagou Pb‒Zn deposit exhibit characteristics that are essentially consistent with those of Mississippi valley-type (MVT) deposits, which clearly distinguish them from sedimentary exhalative (SEDEX) deposits. Additionally, the homogenization temperatures of the fluid inclusions in this deposit are lower than those of the SEDEX deposits; they range between 120 and 160°C, with a mean value of 172°C. Overall, the results indicate that the deposit is classified as an MVT Pb‒Zn deposit, and in this article, a mineralization model diagram has been drawn for the Xiejiagou Pb–Zn deposit.