Linking Neoproterozoic lode gold provinces to a metasomatized mantle source in South China: Evidence from fluid inclusions and H-O-S-Pb isotope systematics

Abstract

Lode gold deposits are typically formed in fields, belts, or provinces, representing the peak stages of continental crust growth and spanning from the Archean to the Neogene. The Meso- and Neoproterozoic eras represent critical stages of the continental crust growth period. In fact, the formation of lode gold deposits during this period has attracted considerable attention due to their global rarity. Recently, a Neoproterozoic lode gold province, encompassing more than 70 gold deposits/occurrences with over 300 gold veins, has been identified in the Huangling area of South China. Nevertheless, our understanding of the fluid characteristics, source, and evolution governing the formation of Neoproterozoic lode gold provinces remains relatively poor. To address these issues, pyrite-chalcopyrite-gold-dominated quartz veins from four representative deposit areas (Guaizigou, Mahuagou, Hongwucun, and Shuiyuesi) under various rock formations were studied through fluid inclusion and H-O-S-Pb isotope assessments. These fluids exhibited medium–low temperature, medium–low salinity, and an H₂O-NaCl-CO₂ ± CH₄ system. The calculated δ¹⁸O_water and δ¹⁸D_water values of quartz in the ore-forming fluids ranged from −1.7 to + 3.6 ‰ and −74.1 to −40.5 ‰, respectively, suggesting that the ore-forming fluids were derived from a combination of mantle sources, metamorphic water, and meteoric water. The influence of the mantle-derived fluids on the southern gold deposits was significant. Fluids in sulfur isotope equilibrium with pyrite exhibited δ³⁴S_H2S values between + 3.1 and + 6.7 ‰, which is consistent with a mantle origin under crustal influences. The Pb isotopes of pyrite indicated a metasomatized lithospheric mantle origin of the gold deposits in the region, which is similar to local mafic dikes. In addition, the gold deposits are strengthened by crustal contamination progressing from south to north of the Huangling area. According to previous Sr-Nd-C isotope data and multiple isotopic features, the ore-forming fluids and metals were derived from a metasomatized lithospheric mantle in the Neoproterozoic lode gold provinces of South China. The gold deposits interacted with basement rocks through their movement path and ore-forming fluid ascendancy. The fluids might be boiled due to rapid cooling and decompression. Indeed, they experienced mixing with a small amount of meteoric water and ultimately precipitated at secondary faults, leading to gold mineralization.