Stable Isotopic Characteristics of the Chagangnuoer Iron Deposit in Western Tianshan, Xinjiang and Its Geological Significance

Situated in the Awulale metallogenic belt of Western Tianshan, the large-sized Chagangnuoer magnetite deposit is contained within volcanic-sedimentary rocks of the Carboniferous Dahalajunshan Formation. According to ore fabrics and mineral assemblages, the mineralization events of this deposit were divided into the magmatic stage and the hydrothermal stage (including the prograde sub-stage). Due to the lack of studies in ore deposit geochemistry for this deposit, this paper aimed to conduct systematic stable isotope analyses of C, O and S by isotope mass spectrometry method for minerals of magnetite, pyrite, chalcopyrite and calcite chosen from various mineralization stages. Those data indicated that δ¹⁸O_SMOW values of magnetite mainly showed a magmatic hydrothermal feature, and a decreasing trend (1.9‰-2.4‰) from the magmatic stage to the prograde stage, which reflected that wall rock alteration may change the compositions of ore-forming fluid. Both the sulfur isotopic components (0.8‰-7.3‰) at the magmatic stage and prograde stage were predominantly the magmatic feature. However, a small amount of S from strata or sea water was mixed into the ore-forming fluid in the magmatic stage as the δ³⁴S_V-CDT was larger than 10‰. During the late ore-forming stage, the δ¹³C_PDB-δ¹⁸O_SMOW ratios of calcite showed a positive linear correlation, probably attributing to the mixture of different concentrations of NaCl fluid or the water-rock reaction between ore-forming fluid and wall rocks. The partial mineralization materials were derived from marble wall rock. Therefore, crystallization differentiation of magmatic fluid was the dominant mineralization on the early ore-forming stage, whereas prograded and retrograded alterations were the crucial factors in iron enrichment and mineralization on the later stage.