The superimposition of the Late Miocene mineralization and silver enrichment in the Poshan Ag-Pb-Zn deposit, Central China

Silver polymetallic deposits commonly produce high-grade ores, with silver contents reaching up to weight-percent levels. The reason and mechanism for the formation of such high-grade silver ores, however, remain poorly understood. Here, we demonstrate that superimposed mineralization plays a crucial role in forming high-grade silver ores in the Poshan Ag-Pb-Zn deposit in Central China. The Early Cretaceous Ag-Pb-Zn mineralization at Poshan occurs as quartz-carbonate-sulfide veins that generally contain a few hundred micrograms per gram of silver. In contrast, the high-grade silver mineralization is present as calcite−chlorite−acanthite−native silver veinlets infilling fractures of the Early Cretaceous orebodies, with silver contents ranging from thousands of micrograms per gram to weight-percent levels. Uranium-lead dating of hydrothermal calcite in the high-grade ores indicates that the enrichment of silver at Poshan occurred in the Late Miocene, specifically between 8.55 ± 0.26 Ma and 6.78 ± 0.13 Ma. Oxygen isotope analysis of individual calcite generations (Cal-1 to Cal-4), which are texturally associated with acanthite, suggests that the fluids from which silver precipitated have δ18Ofluid values of −2.5‰ to −0.4‰ (mean −1.5‰; Cal-1), −2.5‰ to +2.7‰ (mean +1.1‰; Cal-2), −3.4‰ to +1.7‰ (mean −0.2‰; Cal-3), and +4.9‰ to +6.7‰ (mean +5.8‰; Cal-4), respectively. Fluctuations in oxygen isotopes, along with the variations of the element molar ratios (Fe/Ca, Mn/Ca, and Mg/Ca) of corresponding calcite generations, suggest that fluid mixing expressed by the episodic incorporation of deep-circulating fluids into meteoric water triggered changes in fluid composition and ambient conditions, ultimately leading to metal precipitation. Acanthite and coexisting base metal sulfides show δ34S values ranging from −41.7‰ to −6.5‰, which are significantly different from the magmatic-like sulfur of the Early Cretaceous sulfides. These values suggest that an external sulfur, likely derived from bacterial sulfate reduction, was involved in the high-grade silver mineralization. This study highlights the crucial role of superimposed mineralization in the formation of bonanza silver veins in the Poshan deposit and potentially those of silver polymetallic deposits with analogous geological settings worldwide.