Oxygen-fugacity evolution of magmatic Ni-Cu sulfide deposits in East Kunlun: Insights from Cr-spinel composition 

Redox state of parental magma would have undergone significant changes from partial melting in the mantle to emplacement in the shallow crust, which might play a critical role in the genesis of magmatic Ni-Cu sulfide deposits in convergent tectonic settings. In this study, we present mineralogy, petrology, and fO2 calculations of the Xiarihamu Ni-Cu deposit and the Shitoukengde non-mineralized intrusion in the East Kunlun orogenic belt. Olivine-spinel pairs in different magmatic stages were chosen to estimate the magma fO2 (olivine-spinel oxybarometer), track the changes in oxygen fugacity during magmatic evolution, and reveal its influence on the metallogenic mechanism of the Ni-Cu sulfide deposit. Spinel Fe3+/ΣFe ratios determined by a secondary standard calibration method using electron microprobe. Those ratios of the Xiarihamu Ni-Cu deposit vary from 0.32±0.09 to 0.12±0.01, corresponding to magma fO2 values ranging from ΔQFM+2.2±1.0 to ΔQFM-0.6±0.2. By contrast, those of the Shitoukengde mafic-ultramafic intrusion increase from 0.07±0.02 to 0.23±0.04, corresponding to magma fO2 varying from ΔQFM-1.3±0.3 to ΔQFM+1.0±0.5. A positive correlation between fO2 and Cr-spinel Fe3+/ΣFe ratios suggests that the Cr-spinel Fe3+/ΣFe ratios can be used as an indicator for magma fO2. The high fO2 (QFM+2.2) of the harzburgite in the Xiarihamu Ni-Cu deposit suggests that the most primitive magma was characterized by relatively oxidized conditions, and then became reduced during magmatic evolution, causing S saturation and sulfide segregation to form the Xiarihamu Ni-Cu deposit. The evolution trend of the magma fO2 can be reasonably explained by metasomatism in mantle source by subduction-related fluid and addition of external reduced sulfur from country gneisses (1.08–1.14 wt.% S) during crustal processes. Conversely, the primitive magma of the Shitoukengde intrusion was reduced and gradually became oxidized (from QFM-1.3 to QFM+1.0) during crystallization. Fractional crystallization of large amounts of Cr-spinel can reasonably explain the increasing magma fO2 during magmatic evolution, which would hamper sulfide precipitation in the Shitoukengde intrusion. We propose that the temporal evolution of oxygen fugacity of the mantle-derived magma can be used as one of the indicators for evaluating metallogenic potential of Ni-Cu sulfide deposits, and reduction processes from mantle source to shallow crust play an important role in the genesis of magmatic Ni-Cu sulfide deposits.