Geology and antimony mineralization of the Yangla polymetallic orefield in northwestern Yunnan, SW China: Evidence from in situ calcite U-Pb dating, fluid inclusions and H-O-S-Pb isotopes

Abstract

The Yangla is the largest Cu-polymetallic deposit ([email protected] %Cu, 2.09 %Pb, 2.72 %Zn, 14.87 %Sb and 0.63 %WO₃) and can be regarded as a typical Cu-Pb-Zn-W-Sb- polymetallic composite metallogenic system in the Jinshajiang Suture Zone, SW China. Multi-periods nature of polymetallic mineralization (e.g., Indosinian Cu-Pb-Zn and Himalayan W) have been identified and certified in the Yangla polymetallic orefield. However, the formation age, origin and evolution of fluids, source of materials and deposition mechanism of antimony ore remain poorly understood. To elucidate the Sb ore formation age, fluid and material source and ore precipitation mechanism, we performed an integrated study of ore deposit geology, in situ U-Pb dating, H-O-S-Pb isotopes, and fluid inclusions in the Sb ore-related minerals. At Yangla, the Sb orebodies are mainly controlled by NE-trending fracture zones (Fj17 and Fj13) and hosted by carbonated, silicified and pyritized the Devonian Linong Formation (2nd member) marble. Mineralization comprises the pre-ore pyrite, syn-ore sulfides-calcite-quartz, and post-ore supergene valentinite −limonite-romeite stage. In-situ calcite U-Pb dating showed that the timing of antimony mineralization was 29 Ma. This mineralization age is significantly later than the published Cu mineralization age (ca. 230 Ma), skarn (ca. 231 Ma) and granitoid emplacement age (ca. 230 Ma), and slightly later than tungsten mineralization (ca. 30 Ma) in the Yangla orefield. Liquid-rich and minor pure liquid fluid inclusions have been identified in quartz, calcite and stibnite. Microthermometric analysis revealed that the fluid inclusions in quartz I (early syn-ore stage), quartz Ⅱ (main syn-ore stage), calcite Ⅱ, and stibnite Ⅱ homogenized at 185 − 245 (n = 64), 127 − 204 (n = 100), 108 − 215 (n = 75), and 148 − 191℃ (n = 10), respectively, with corresponding salinities of 1.74 − 13.51, 2.57 − 14.04, 2.57 − 11.46, and 1.74 − 4.96 wt%NaCleq. The Sb ore-forming fluids can thus be attributed to a low temperature-salinity H₂O ± CO₂-NaCl system. The calculated δ¹⁸O_H2O (5.39–6.26 ‰) and δD (−136.90 to − 123.10 ‰) (n = 8) values of quartz suggest a magmatic-hydrothermal mixed formation/meteoric water source. Fluid mixing-induced cooling and water/rock interaction may have played an important role in the Yangla Sb ore precipitation. The stibnite δ³⁴S values (4.41–21.47 ‰, avg. 8.80 ‰; n = 37) indicate a largely granitic rock and marine carbonate sulfur source. The ²⁰⁶Pb/²⁰⁴Pb = 18.413–18.785, ²⁰⁷Pb/²⁰⁴Pb = 15.699–15.980 and ²⁰⁸Pb/²⁰⁴Pb = 38.725–39.428 (n = 37) of stibnite imply that the Pb was sourced from granitoids and upper crustal materials. Thus, our in-situ calcite U-Pb, fluid inclusions and H-O-S-Pb isotopic data suggest that the Yangla Sb ore-forming fluids and materials were sourced from a mixture of granitic magma and Devonian wallrocks, and maybe related to the Oligocene deep-buried magmatism and significantly different from the Mesozoic skarn-porphyry Cu-Pb-Zn mineralization system in the Yangla orefield. This study will provide new insight to better understanding of antimony genesis mechanism in the Yangla Cu-polymetallic orefield.