New insights into the genesis of the Lower Urgen porphyry Mo deposit, Northeast China: Implications from S-Pb-He-Ar isotopes and pyrite in-situ trace element analysis

Porphyry deposits are the world’s most important source of Mo and Cu resources and typically exhibit pronounced metallogenic specialization, with Mo and Cu rarely occurring together in similar abundances within a single deposit. Whether the Mo mineralization and its associated Cu mineralization in porphyry Mo deposits share a unified source, and whether mantle components participated in the formation of these deposits, remains unclear. To address this issue, we selected the Lower Urgen Mo deposit in the northern Great Xing’an Range as a case study. This deposit features veinlet-disseminated- and quartz-vein-type Mo mineralization, accompanied by minor Cu mineralization. Through detailed studies of deposit geology, in-situ lead and sulfur isotopes, helium and argon isotopes, and pyrite geochemistry, we have constrained the genesis of the Lower Urgen deposit. The hydrothermal mineralization process can be divided into four stages: Stage 1 is characterized by magnetite, quartz, and K-feldspar; Stage 2 is the main stage of Mo mineralization; Stage 3 is involved in Pb-Zn-Cu mineralization; and Stage 4 features calcite, quartz, fluorite, and minor pyrite. In-situ δ 34 S values of pyrite from various stages display a narrow range (6.0 ‰ to 8.3 ‰), and their lead isotope ratios ( 206 Pb/ 204 Pb = 18.332–18.502; 207 Pb/ 204 Pb = 15.539–15.574; 208 Pb/ 204 Pb = 38.126–38.320) are homogeneous and overlap with those of the ore-bearing granite porphyry, suggesting a common and predominantly magmatic source for sulfur and lead, derived from the associated granitic magma. Consistent Co/Ni (mostly 1–10) of pyrite from three stages suggests that the multistage hydrothermal fluids originated from a common source-the coeval granite porphyry. Fluid inclusions in hydrothermal pyrite have 3 He/ 4 He ratios of 0.73 to 2.10 Ra and 40 Ar/ 36 Ar ratios of 318 to 336, indicating substantial mantle-derived volatile contributions. The variations in mineral assemblages indicate a progressive decrease in oxygen fugacity within the Lower Urgen mineralization system. Fluid cooling and immiscibility could dominate the massive precipitation of molybdenite at Stage 2, whereas the large-scale precipitation of chalcopyrite at Stage 3 may be closely related to the incursion of meteoric water into the hydrothermal system. After the precipitation of molybdenite and chalcopyrite, the remaining ore-forming elements continued to precipitate in Stage 4 pyrite. Our findings highlight the significance of noble gas isotope and in-situ mineral geochemical analyses in revealing the genesis of porphyry Mo deposits, which is critical for elucidating detailed ore-forming processes of porphyry mineralization systems in Northeast China and globally.