Abstract:

The Bainaimiao copper deposit in Inner Mongolia is located in the Ondor Sum Caledonian copper polymetallic zone of the Central Asian Orogenic Belt. Copper mineralization in Bainaimiao is comprised of the northern and southern zones. The host rock of the southern zone is greenschist of the Bainaimiao Formation, while orebodies in the northern zone mainly occur in the Caledonian granodiorite porphyry. The metallogenic processes can be divided into early, middle, and late stage, which is characterized by mineral assemblages of quartz-pyrite, quartz-chalcopyrite±molybdenite, and quartz±calcite±galena-sphalerite, respectively. The δ¹⁸O_H2O and δD values of fluid inclusions in quartz range from -3.2 ‰ to 5.5 ‰ and from -94.2 ‰ to -69 ‰, respectively. The δD values are relatively constant, whereas the δ¹⁸O_H2O values decrease gradually from early to late stage. In addition, the δ¹³C_PDB and δ¹⁸O_SMOW  values of hydrothermal carbonate minerals range from -5.4 ‰ to -2.4 ‰ and from -3.1 ‰ to 10.9 ‰, respectively. The δ¹³C_PDB  values keep steady, and the δ¹⁸O_SMOW values reduce gradually during the mineralization processes. The δ³⁴S values of the sulfides range from -0.6 ‰ to -6 ‰ and are typical of mantle source. The C-H-O-S isotopic compositions of the southern and northern zones show that ore-forming fluids of different stage are mainly derived from magma system, and there are no obvious differences between these two zones. However, the fluids in the southern zone exhibit closer affinity to meteoric water and crustal substances. Combined with geological features of the deposit, it is concluded that the Bainaimiao deposit was produced by granodioritic magma that formed by anatexis under subduction environment, and the magma arose along fissures up to the shallow crust. These processes gave rise to the activation and enrichment of ore metals from source beds of the Bainaimiao strata, and the formation of the Bainaimiao porphyry copper deposit.

Key words: C-H-O-S isotope, Bainaimiao copper deposit, Inner Mongolia