Fluid Inclusion and Carbon-Oxygen Isotope Studies of the Hujiayu Cu Deposit, Zhongtiao Mountains, China: Implications for Syn-metamorphic Copper Remobilization

The Hujiayu Cu deposit, representative of the “HuBi‐type” Cu deposits in the Zhongtiao Mountains district in the southern edge of the North China Craton, is primarily hosted in graphite‐bearing schists and carbonate rocks. The ore minerals comprise mainly chalcopyrite, with minor sphalerite, siegenite [(Co, Ni)₃S₄], and clausthalite [Pb(S,Se)]. The gangue minerals are mainly quartz and dolomite, with minor albite. Four fluid inclusion types were recognized in the chalcopyrite‐pyrite‐dolomite‐quartz veins, including CO₂‐rich inclusions (type I), low‐salinity, liquid‐dominated, biphase aqueous inclusions (type II), solid‐bearing aqueous inclusions (type III), and solid‐bearing aqueous‐carbonic inclusions (type IV). Type I inclusion can be further divided into two sub‐types, i.e., monophase CO₂ inclusions (type Ia) and biphase CO₂‐rich inclusions (with a visible aqueous phase), and type III inclusion is divided into a subtype with a halite daughter mineral (type IIIa) and a subtype with multiple solids (type IIIb). Various fluid inclusion assemblages (FIAs) were identified through petrographic observations, and were classified into four groups. The group‐1 FIA, consisting of monophase CO₂ inclusions (type Ia), homogenized into the liquid phase in a large range of temperatures from ?1 to 28°C, suggesting post‐entrapment modification. The group‐2 FIA consists of type Ib, IIIb and IV inclusions, and is interpreted to reflect fluid immiscibility. The group‐3 FIA comprises type II and IIIa inclusions, and the group‐4 FIA consists of type II inclusions with consistent phase ratios. The group‐1 and group‐2 FIAs are interpreted to be entrapped during mineralization, whereas group‐3 and group‐4 FIAs probably represent the post‐mineralization fluids. The solid CO₂ melting temperatures range from ?60.6 to 56.6° C and from ?66.0 to ?63.4° C for type Ia and type IV inclusions, respectively. The homogenization temperatures for type II inclusions range from 132 to 170°C for group‐3 FIAs and 115 to 219°C for group‐4 FIAs. The halite melting temperatures range from 530 to 562°C for type IIIb and IV inclusions, whereas those for type IIIa inclusions range from 198 to 398°C. Laser Raman and SEM‐EDS results show that the gas species in fluid inclusions are mainly CO₂ with minor CH₄, and the solids are dominated by calcite and halite. The calcite in the hosting marble and dolomite in the hydrothermal veins have δ¹³C_V‐PDB values of ?1.2 to 1.2‰ and ?1.2 to ?6.3‰, and δ¹⁸O_V‐SMOW values of 14.0 to 20.8 ‰ and 13.2 to 14.3 ‰, respectively. The fluid inclusion and carbon‐oxygen isotope data suggest that the ore‐forming fluids were probably derived from metamorphic fluids, which had reacted with organic matter in sedimentary rocks or graphite and undergone phase separation at 1.4–1.8 kbar and 230–240°C, after peak metamorphism. It is proposed that the Hujiayu Cu deposit consists of two mineralization stages. The early stage mineralization, characterized by disseminated and veinlet copper sulfides, probably took place in an environment similar to sediment‐hosted stratiform copper mineralization. Ore minerals formed in this precursor mineralization stage were remobilized and enriched in the late metamorphic hydrothermal stage, leading to the formation of thick quartz–dolomite–sulfides veins.