Genesis and mineralization processes of the Hua’aobaote Pb–Zn–Ag polymetallic deposit in Inner Mongolia: Constraints from in situ sulfur isotopes and trace elements in pyrite

• In-situ multi-technique analysis of pyrite in the Hua’aobaote deposit reveals the geochemical transition from magma to hydrothermal fluids. • Py1 comprises magmatic and hydrothermal pyrite, whereas Py2 and Py3 are hydrothermal pyrites. • Effectively traced the materials and fluids originating from granite porphyry. • The Hua’aobaote Pb–Zn–Ag deposit may be an epithermal vein deposit related to the porphyry-type magmatic–hydrothermal systems. The Hua’aobaote Pb–Zn–Ag deposit in the Southern Great Xing’an Range (SGXR) was recently discovered, and its orebodies occur in Permian tuffaceous siltstone, sandstone, siltstone, and slate and are strictly controlled by faults. The aim of this study is conduct trace element and sulfur isotope analyses of pyrite to accurately delineate the mineralization process from magmatic to hydrothermal stages, enhancing the understanding of the genesis of the deposit. The research findings reveal that the mineralization process can be categorized into three stages: pyrite + quartz (Py1), quartz + pyrite + arsenopyrite + chalcopyrite + cassiterite + pyrrhotite + galena + sphalerite + silver minerals (Py2), and quartz + pyrite + arsenopyrite + marcasite + calcite (Py3). Within these stages, Py1 displays the highest concentrations of Se and Ni (averages of 352.05 ppm and 28.71 ppm), whereas other trace elements are relatively scarce. Conversely, the total trace element contents in Py2 and Py3 significantly increase, particularly those of Cu, Sn, Ag, Pb, Zn, Bi, Mn, As, and Sb. Notably, Py2 has the highest concentrations of Co (average of 20.39 ppm), Bi (average of 14.34 ppm), and Sn (average of 17.78 ppm), whereas Py3 is characterized by a uniquely high Mn concentration (average of 154.34 ppm). The δ 34 S values for pyrite, ranging from −2.96 ‰ to 1.55 ‰, are characteristics of deep magma sources. Py1 is distinguished by high Se and low As contents, elevated Se/As and Co/Sb ratios, and comparatively low Sb/Bi and As/Ag ratios. These characteristics suggest that Py1 comprises magmatic pyrite and hydrothermal pyrite, formed through the evolution of porphyry-type magmatic fluids. In contrast, Py2 and Py3 are hydrothermal pyrites that have high As and low Se contents, low Se/As and Co/Sb ratios, and high Sb/Bi and As/Ag ratios. These features indicate their formation in an epithermal environment, likely resulting from interactions between magmatic fluids and host rocks or meteoric water. An analysis of mineral assemblages, trace elements, and sulfur isotopes indicates that the mineralization occurred from mid- to high-temperature, mid-sulfidation porphyry magmatic fluids to low-temperature, low-sulfidation hydrothermal fluids, indicating that the ore-forming materials originated predominantly from porphyry magma. Accordingly, the Hua’aobaote deposit is categorized as a distal, mid- to low-temperature, mid- to low-sulfidation epithermal vein deposit linked to a porphyry-type magmatic–hydrothermal system.