Genesis of Ag–polymetallic mineralization in Xiong’ershan ore-concentrated area, East Qinling, China: Constraints from in-situ pyrite geochemistry

The specific physicochemical processes driving compositional variations in pyrite from the porphyry–epithermal system remain elusive and challenging. The Xiong’ershan ore-concentrated area is located in the world-class East Qinling Metallogenic Belt and contains Triassic orogenic Au–Cu–Mo, Early Cretaceous porphyry–epithermal Au-Mo, and Ag-polymetallic (Au–Zn–Pb) mineralization, with pyrite (Py_T, Py_C1-C4 and Py_Ag, respectively) as the main ore mineral. The unique geological setting renders it an excellent subject for unraveling the intricate hydrothermal evolutions and mechanisms of polymetallic precipitation. The Tieluping–Shagou Ag–polymetallic (Au–Zn–Pb) mining district located in the west margin of the ore-concentrated area has long been a subject of contentious discourse regarding its genesis. This paper presents new data on the in-situ trace element and isotopic composition of Py_Ag and juxtaposes with the previous geochemistry data of Py_T and Py_C1-C4. Py_Ag exhibits the highest Ag, Pb, Zn, and Cu contents led by the Ag-bearing mineral, sphalerite, galena, and tetrahedrite inclusions. Conversely, Py_C2 and Py_C3 from the main metallogenic stage in the Early Cretaceous porphyry–epithermal Au–Mo deposits have the enrichment of Au content attributed to abundant Au-telluride inclusions. Corresponding trace element enrichment (e.g., As, Ag, Pb, Zn, Mo, Tl, and Ni), along with generally low Co and Se contents, and Sb/Bi ratios in Py_Ag suggest that the Tieluping–Shagou Ag–polymetallic (Au–Zn–Pb) mining district is epithermal mineralization. Principal component analysis (PCA) of Py_Ag further substantiates an affinity between the Ag–polymetallic (Au–Zn–Pb) mineralization and Early Cretaceous porphyry–epithermal Au–Mo mineralization. Moreover, the S–Pb isotopic ratios of Py_Ag also indicate that the ore-forming fluids of Ag–polymetallic (Au–Zn–Pb) mineralization are oxidized and originated from the Early Cretaceous magmatism. The low Se contents and Se/Ti ratios, coupled with high Tl/Se ratios in Py_Ag represent a lower temperature condition characteristic of the distal Ag–polymetallic mineralization in porphyry–epithermal system, led by the admixture of meteoric waters. The addition of increasing amounts of meteoric water leads to solubility decreases and rapid precipitation, thereby facilitating the formation of nanoparticles and inclusions of sulfides (e.g., galena, sphalerite and tetrahedrite) within low-temperature and −salinity Py_Ag. On the other hand, As-rich Py_Ag catalyzes the formation of gold solid solution in the distal Ag–polymetallic mineralization. Based on the above research, we believe that there is proximal Au–Mo mineralization in the lower part of the Ag–polymetallic mineralization nearer to the granite porphyry.