Copper isotope fractionation in magmatic Ni-Cu mineralization systems associated with the variation of oxygen fugacity in silicate magmas

Significant variations of oxygen fugacity (fO(2)) in silicate magmas are widely suggested to play an important role in formation of magmatic Ni-Cu deposits in convergent margin settings. Copper isotopes have potential to track redox change of magmatic systems, but the linkage of silicate magma oxygen fugacity with Cu isotope fractionation in magmatic Ni-Cu mineralization systems is still uncertain. A combined study of Cu isotopes and estimated oxygen fugacities can provide novel and direct insights into this issue. In this study, four magmatic Ni-Cu deposits in Eastern Tianshan (NW China) were investigated in order to reveal the fO(2) variations in silicate magmas as a function of redox-induced Cu isotope fractionation. The silicate magma fO(2) of the four deposits widely varies from similar to QFM -2.2 to + 0.6, from similar to QFM -0.7 to 0.0, from similar to QFM -1.4 to -0.2, and from similar to QFM -1.2 to -0.6 for the Tulaergen, Huangshandong, Huangshannan, and Hulu deposits, respectively. Models using olivine compositions, Cu/Pd ratios, and estimated silicate magma fO(2) suggest that the silicate magmas gradually became more oxidized as Ni-Cu mineralization proceeded. The closed-system R factor equation and the Rayleigh equation were used to model mineralization processes and related Cu isotope fractionation. Modelling using delta Cu-65 values and the estimated fO(2) indicates that the evolved silicate magmas with high fO(2) tend to incorporate heavier Cu isotopes. Our studies indicate that the fO(2) variation of silicate magmas is the key factor governing Cu isotope fractionation in magmatic Ni-Cu mineralization systems in convergent tectonic settings, which is potentially applicable for other magmatic Ni-Cu deposits worldwide. (C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Our study investigated four magmatic Ni-Cu deposits in the Eastern Tianshan region, revealing the significant role of variations in silicate magma fO(2) in the fractionation of copper isotopes. Results indicate that silicate magmas became more oxidized as Ni-Cu mineralization progressed.