Geology, geochemistry, zircon and garnet U–Pb geochronology, and C–O–S–Pb–Hf isotopes of the Guojiagou Pb–Zn deposit, West Qinling Belt, Central China: New constraints on district-wide mineralization

Abstract

The Guojiagou Pb–Zn deposit is located in Li County, Gansu Province, northwestern China. The ores consist of skarn and vein types, with the skarn type occurring at the contact zone between granodiorite and marble, and the vein type hosted in the extension faults within the Triassic Huashiguan Formation limestone. Granodiorite samples from the Weijiazhuang pluton show high ratios of Sr/Y (32.25–43.44) and (La/Yb)_N (15.7–16.5), small Eu anomalies (δEu = 0.73–0.80), high concentrations of Mg^# (57.6–64.2), Cr (100–110 ppm), and Ni (15.9–16.6 ppm), abundant mafic micro-granular enclaves, and have zircon εHf_(t) values of −1.9 to −4.6 and T_DM2 of 1149.6 to 1285 Ma. This suggests that the Weijiazhuang granodiorites were generated by the partial melting of the Meso-Proterozoic high-K basaltic lower crust with the addition of mantle-sourced melts. The ore-forming process can be subdivided into five stages: prograde skarn (stage I), retrograde skarn (stage II), quartz-sulfide (stage III), sphalerite-calcite (stage IV), and quartz-calcite (stage V). The δ¹³C values range from −5.1 to −1.3 ‰ and δ¹⁸O values range from −4 to 18.6 ‰ in calcites, suggesting a mixed source of magma, limestone, and pore or basinal water for CO₃²⁻. The δ³⁴S values (6–7.7 ‰) of sulfides indicate that sulfur mainly originated from magma, with a minor contribution from host limestone. The Pb isotopes of sulfides from stages III and IV (²⁰⁸Pb/²⁰⁴Pb = 38.176–39.218, ²⁰⁷Pb/²⁰⁴Pb = 15.889–15.678, and ²⁰⁶Pb/²⁰⁴Pb = 18.147–18.903) showed mixed sources of crust and mantle. The Weijiazhuang pluton and Guojiagou Pb–Zn deposit yield ages of 220 ± 1.8 Ma (MSWD = 0.35) and 213 ± 3.0 Ma (MSWD = 1.5), respectively, obtained by zircon and garnet LA–ICP–MS U–Pb analysis. These results indicate that the Guojiagou Pb–Zn deposit formed in a syn-collisional tectonic regime during the Late Triassic. Based on the data presented in this study and previous research on mineralization in the eastern West Qinling Orogen, we conclude that the Guojiagou Pb–Zn deposit is a typical skarn-type deposit and that Pb–Zn mineralization in the eastern West Qinling Orogen is closely related to Triassic magmatism, which provided not only thermal energy but also ore-forming materials and fluids.