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

In this study, we use Cr-spinel as an efficient indicator to evaluate the oxygen fugacity evolution of the Xiarihamu Ni-Cu deposit and the Shitoukengde non-mineralized intrusion. Oxygen fugacity is calculated using an olivine-spinel oxybarometer, with spinel Fe3+/sigma Fe ratios determined by a secondary standard calibration method using an electron microprobe. Cr-spinel Fe3+/sigma Fe ratios of the Xiarihamu Ni-Cu deposit vary from 0.32 +/- 0.09 to 0.12 +/- 0.01, corresponding to magma f(O2) values ranging from Delta QFM+2.2 +/- 1.0 to Delta QFM-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 f(O2) varying from Delta QFM-1.3 +/- 0.3 to Delta QFM+1.0 +/- 0.5. A positive correlation between f(O2) and Cr-spinel Fe3+/sigma Fe ratios suggests that the Cr-spinel Fe3+/sigma Fe ratios can be used as an indicator for magma f(O2). The high f(O2) (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 f(O2) 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 f(O2) 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 the reduction processes from mantle source to shallow crust play an important role in the genesis of magmatic Ni-Cu sulfide deposits.