Formation mechanism of the Yangchang Cu-Mo vein-type deposit, Inner Mongolia, NE China: Constraints from fluid inclusions and O-H-S-Pb isotopes

Abstract

The Yangchang deposit, located in the central Xilamulun Metallogenic Cu-Mo Polymetallic Belt in NE China, is a typical vein-type Cu-Mo deposit. The orebodies, hosted in silicified and sericitized Cretaceous monzogranite, are controlled by a NE-trending fault. Although vein-type deposits represent an important in the Xilamulun Cu-Mo system, questions remain regarding their genetic relationship to regional magmatism, similarity to porphyry deposits, and the presence of a concealed magma chamber. This study investigates the pressure–temperature conditions, sources, and evolution of ore-forming fluids in the Yangchang deposit. Based on orebody crosscutting relationships, three paragenetic stages were identified: (1) Stage I: pyrite–quartz; (2) Stage II: pyrite–chalcopyrite–molybdenite–quartz; and (3) Stage III: sphalerite–galena–quartz. Primary fluid inclusions (FIs) were classified into three types: pure CO₂ (PC-type), CO₂-NaCl-H₂O (C-type), and NaCl-H₂O (L-type). Homogenization temperatures ranged from 360 to 310 °C (Stage I), 333–208 °C (Stage II), and 233–152 °C (Stage III), with salinities consistently below ∼ 12 wt% NaCl_equiv Trapping pressures during Stage I were 1,000–1,250 bar, corresponding to a depth of approximately 2.10 km (km). The O-H isotope analyses indicated an evolving fluid source, with δD values ranging from (−14.4 to –116.6 ‰) and δ¹⁸O_water values 3.88 to 14.82 ‰, reflecting a transition from magmatic to meteoric water. Sulfur (δ³⁴S_V-CDT = 0.4–1.2 ‰) and lead isotopic data suggest a magmatic origin of metals with additional crustal input. These findings indicate that the Yangchang vein-type Cu-Mo deposit is genetically associated with regional magmatism formed during the Early Cretaceous Paleo-Pacific tectonic regime.