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

Abstract

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+/ΣFe ratios determined by a secondary standard calibration method using an electron microprobe. Cr-spinel Fe3+/ΣFe 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 the reduction processes from mantle source to shallow crust play an important role in the genesis of magmatic Ni-Cu sulfide deposits.

Petrogenesis, Sampling Depth, Thermal and Redox Evolution of Spinel Peridotite Xenoliths From Jiaohe, NE China: Insights From Trace Elements in All Rock‐Forming Silicate Minerals

“…While in prior studies T Ol‐Sp was employed to calculate ƒ O 2 (e.g., Ali et al., 2020; Birner et al., 2018; Bucholz & Kelemen, 2019; Canil et al., 2021; Davis et al., 2017; Jia et al., 2022; Woodland et al., 2021), we use T Al and the P SC ‐T Al _ P SC pair discussed in the previous section for consistency (Table 2). The resulting maximum error in the calculated ƒ O 2 is small, ±0.15 log units (Ali et al., 2020; Woodland et al., 2006).…”

“…Provided that high-quality trace element data are available, the scheme proposed here has the potential to constrain the architecture of the shallow SCLM represented by spinel peridotite xenoliths. While in prior studies T Ol-Sp was employed to calculate ƒO 2 (e.g., Ali et al, 2020;Birner et al, 2018;Bucholz & Kelemen, 2019;Canil et al, 2021;Davis et al, 2017;Jia et al, 2022;Woodland et al, 2021), we use T Al and the P SC -T Al_ P SC pair discussed in the previous section for consistency (Table 2). The resulting maximum error in the calculated ƒO 2 is small, ±0.15 log units (Ali et al, 2020;Woodland et al, 2006).…”

Incipient carbonate melting drives metal and sulfur mobilization in the mantle

“…All experiments were conducted in the high-pressure laboratory at Macquarie University using a rapid-quenching end-loaded piston cylinder apparatus with a ½-inch talc-Pyrex and natural CaF 2 assemblies ( 32 ). The powdered starting materials were loaded into dual-chambered graphite capsules (3 mm length), single-wrapped in 25-μm-thick platinum foil, and placed within MgO sleeves.…”

“…Because of their high reactivity ( 12 , 24 ), they may be responsible for the (re-)enrichment of the SCLM ( 31 ) in CO 2 , H 2 O, and Fe, and the enrichment of the lower crust in chalcophile and siderophile elements, such as sulfur (S), nickel (Ni), copper (Cu), and the platinum group elements (PGEs) ( 12 , 29 ). The role of oxidized incipient carbonate melts in mobilizing sulfur, chalcophile, and siderophile elements from the mantle has not been experimentally investigated despite growing evidence from natural samples that conditions to liberate and concentrate S are favored during the earliest stages of mantle melting ( 12 , 29 , 32 ). Here, we present high-pressure, high-temperature experiments in combination with high-precision nanoscale analyses to demonstrate that carbonate melts are effective transport agents for S, as well as base and precious metals during the initial stages of mantle melting.…”

Incipient carbonate melting drives precious metal and sulfur mobilisation in the mantle

“…Incipient melts are unlikely to ever reach the surface unmodified but may be responsible for the (re)enrichment of the SCLM 22 and the lower crust in chalcophile and siderophile elements, such as sulfur (S), nickel (Ni), copper (Cu) and the platinum group elements (PGEs) 10,19 . The role of oxidised incipient melts in mobilising sulfur, chalcophile and siderophile elements from the mantle has not been experimentally investigated, despite growing evidence from natural samples that conditions to liberate and concentrate S are favoured during the earliest stages of mantle melting 10,19,23 .…”