Genesis of the Aobaotu Pb-Zn deposit in the southern Great Xing'an Range, NE China: Constraints from geochronology and C-H-O-S-Pb isotopic and fluid inclusion studies

The Aobaotu Pb-Zn deposit (470,700 t; 1.51% Pb, 2.30% Zn) in the southern Great Xing'an Range, northeastern China, is hosted by the Late Jurassic volcanic tuff and structurally controlled by a near-east-west-trending fault. Three stages of mineralization were identified, namely, stage I of quartz +/- pyrite veins, stage II of quartz-polymetallic sulphide veins, and stage III of late quartz-calcite veins. The quartz and calcite that formed in the three stages were selected for fluid inclusion and C-H-O isotope analyses. The results show that the ore-forming fluid of the deposit belongs to the H2O-CO2-NaCl system at a medium temperature (concentrated at 220-300 degrees C) and in low salinity (0.7-12.1 wt% NaCl equiv). The delta C-13 values of the calcite and ankerite are in the range of -8.4 to -4.8 parts per thousand, indicating that as a source of deep magma. The delta DV-SMOW and delta 18OH2O values of quartz range from -108 to -88 parts per thousand and 4.55 to 5.85 parts per thousand, respectively, indicating that the initial fluid of the Aobaotu deposit was a mixture of residual magmatic and meteoric water. Sulphur isotope analysis of the sulphide minerals, that is, sphalerite, galena, pyrite, and chalcopyrite, yield delta S-34 value in a range of 1.44-4.94 parts per thousand, indicating that sulphur is mainly derived from magma. In addition, the Pb isotopic composition of the sulphides indicates that the ore-forming material has a mixed crust-mantle source. Zircon U-Pb dating suggests that the formation of the Aobaotu deposit is genetically related to the granodiorite porphyry (130.3 +/- 0.9 Ma). The combined geochronology and isotopic evidence suggest that the Aobaotu deposit is a magmatic-hydrothermal vein-type Pb-Zn deposit, opposite to a volcanic Pb-Zn deposit as suggested before. The Aobaotu deposit formed in an extensional tectonic setting caused by the rollback of the Palaeo-Pacific Plate.