Episodic mineralization and Ag hyper-enrichment in the Weiquan Ag-Cu-Pb-Zn deposit, Chinese Tianshan: Insights from geology, petrography, geochemistry, and in situ U-Pb geochronology of garnet and zircon

Weiquan is the largest Ag-polymetallic deposit (∼1232 t Ag) in the Tianshan Orogenic Belt and the only known deposit in the region containing Ag-dominated orebodies. However, its age, origin, and mechanisms of formation have remained poorly constrained. Here, we integrate a detailed paragenesis with mineralogy, mineral geochemistry, and in situ U-Pb geochronology of garnet and zircon to reconstruct the multi-stage hydrothermal evolution of Weiquan and the mechanisms that caused Ag-Cu-Pb-Zn enrichment. Based on field relationships and mineralogical characteristics, three distinct mineralization events are identified: (1) a volcanic-hydrothermal event, (2) a skarnization event, and (3) an epithermal event. Garnet samples from the volcanic-hydrothermal event exhibit core-rim textures (Adr—andradite; Grs—grossular, Adr96Grs0 to Adr47Grs45) and 0.55−33.3 ppm U (i.e., garnet 1), whereas the garnet samples from the skarnization event and epithermal event are texturally and compositionally homogeneous (Adr53Grs40 to Adr39Grs53) and have 0.32−9.91 ppm U (i.e., garnet 2). Two discrete hydrothermal episodes are evident based on the U-Pb age dates of zircon and garnet. The early event at ca. 320 Ma, which is coeval with basaltic andesite volcanism, controlled the initial metal enrichment. The later skarnization event at ca. 295 Ma (garnet 2), which is associated with granitic magmatism, remobilized Ag-Cu-Pb-Zn from the basaltic andesite host rock. Subsequent incursion of meteoric water and a decrease in fluid temperature during the epithermal event further upgraded and concentrated Ag-Cu into high-grade orebodies within fractured zones. This study not only resolves long-standing uncertainties regarding the timing of, and genetic link between, the multiple mineralization events at Weiquan, but also establishes a new genetic model involving superimposed magmatic-hydrothermal and epithermal systems. Our results provide a robust chronostratigraphic framework that enhances the understanding of Ag enrichment mechanisms in transitional skarn-epithermal systems, and highlights the exploration potential for similar Ag-polymetallic systems in the Tianshan and other orogenic belts worldwide.