Geology and mineralogy of the newly discovered Baishitai skarn-epithermal Ag polymetallic deposit in the Taihang Mountains, North China

• Geological and mineralogical evidence indicates the breccia-type epithermal Ag-Au-Zn-Pb ore is of the intermediate-sulfidation epithermal type. • The results of mineral assemblage, temperature trend, and physicochemical conditions suggest a transitional relationship from IS to skarn mineralization at Baishitai. • A high-temperature hydrothermal source is inferred at depth in the southeastern area of the district, suggesting a further exploration target. The newly discovered Baishitai Ag-polymetallic deposit, located in the Taihangshan metallogenic belt of eastern China, hosts reported resources totaling 249 t comprising 239 t Ag at 115 g/t, 4.5 t Au at 3.2 g/t, 5.5 Mt (Zn + Pb) at 2.1 wt%. Minor amounts of Cu, Mo, and Fe have also been documented, and further exploration activities are currently underway. The Baishitai deposit hosts skarn-type Pb-Zn-Ag-Cu mineralization, overprinted by shallow breccia-type epithermal Ag-Au-Pb-Zn mineralization. Detailed drill logging, spectral, and mineralogical analyses were conducted to investigate the geology and mineralogy of the Baishitai deposit. Hydrothermal alteration zoning from deep to shallow level is reflected by (1) chlorite-epidote ± illite, (2) illite-carbonate-quartz-montmorillonite, and (3) montmorillonite-carbonate-illite-quartz ± chlorite. The skarn-type Pb-Zn-Ag-Cu ores consist mainly of chalcopyrite, high-Fe sphalerite (Sp1 < 17.16 mol% FeS; formation temperature ∼350 °C), galena, and Ag-bearing tetrahedrite. In contrast, the epithermal Ag-Pb-Zn ores are characterized by low-Fe sphalerite (Sp2-3 < 7.20 mol% FeS; formation temperature ∼250 °C), and high-Mn calcite (up to 5.93 % MnO), accompanied by abundant Ag-bearing minerals such as tetrahedrite (up to 23.95 % Ag), hessite, matildite, argentite, and electrum with high Ag/Au ratios (up to 2450), indicating intermediate sulfidation (IS) epithermal mineralization. Both the mineral geothermometry and the thermodynamic model reveal decreases in temperature, f O 2 , and f S 2 , during the transition from the skarn to epithermal mineralization. Geological and mineralogical evidence suggests that both IS epithermal and skarn-type mineralization, are genetically related to porphyry Mo mineralization at depth beneath Baishitai. A potential magmatic hydrothermal center is likely located in the southeast direction.