Petrographic and geochemical evidence from pyrite for the genesis of the Bainiuchang Ag-polymetallic deposit southeastern Yunnan

Southeastern Yunnan lies within a composite zone at the convergence of the South China, Yangtze, and Indochina (Tibet-Yunnan) blocks. Its complex regional structure provides favorable conditions for metallogenesis, making it a major metallogenic province in Southern China. The Bainiuchang Ag-polymetallic deposit is a representative super-large deposit in the area, whose genetic classification has long been controversial. Based on field investigations, this study conducted petrographic analysis, LA-ICP-MS in-situ laser ablation, and elemental mapping of pyrite ore samples from the Bainiuchang deposit. The results distinguish two macroscopic ore types: layered, bedded, and lenticular synsedimentary tectonic pyrite; disseminated, stockwork, and lumpy metasomatic structural pyrite. The study confirms that the Bainiuchang Ag-polymetallic deposit is a polygenetic system characterized by the spatiotemporal superposition of multistage mineralization events. During the Late Caledonian, Sedimentary Exhalative (SEDEX) processes formed early-stage pyrite, and sulfosalt minerals, establishing a metal-enriched precursor for subsequent mineralization. During the Late Yanshanian, Magmatic-Hydrothermal activity overprinted the pre-existing mineralization. Proximal to granitic intrusions, contact metamorphism generated skarn-hosted Sn-W mineralization at deep structural levels. At intermediate distances, hydrothermal fluids cooled and decompressed along the F₃ fault system, precipitating Ag-Pb-Zn-dominant ore bodies with associated Fe-Cu sulfides. At shallower epithermal depths, continued fluid ascent formed stibnite-dominated Sb mineralization.