In Situ Sulfur Isotopic and Thermodynamic Constraints on the Magmatic Evolution and Metallogenesis of the Jinchuan Ni–Cu Sulfide Deposit, China

The world-class Jinchuan magmatic Ni–Cu sulfide deposit in China is one of the world's significant suppliers of nickel (Ni) and platinum-group elements (PGEs). The evolution of Jinchuan magma that fed this enormous deposit is highly controversial. The wide oxygen fugacity range (ΔFMQ -2.1 to +2.4) of Jinchuan magma estimated by two oxybarometry models indicates the apparent transition of the redox state of Jinchuan magma from reduced to oxidized conditions during magma evolution, which is consistent with the field observations on the assimilation of crustal marble and these simulated results by a thermodynamically constrained model, rhyolite-MELTS. The variation in major element compositions of the Jinchuan intrusion suggests that the magma evolution was mainly controlled by olivine, clinopyroxene, and orthopyroxene crystallization with increasing magma oxygen fugacity. The extensive range of δ³⁴S value analyzed by NanoSIMS in magmatic and hydrothermal ores could partly be attributed to the transfer of magma oxygen fugacity and the contamination of crustal materials. In contrast, the mantle sulfur isotope characteristics (δ³⁴S = -2.0 – +2.1 ‰) of sulfide inclusions in Cr-spinel grains potentially imply that no addition of external sulfur was involved in attaining sulfide saturation during the early stage of Jinchuan magma. During the transfer in oxygen fugacity, the fall head of sulfur solubility between the liquidus and the temperature of orthopyroxene crystallization calculated by numerical models under ideal conditions dramatically increases from 0.01 wt.% to 0.15 wt.% (sulfide saturation), even 0.24 wt.% (sulfate saturation), potentially contributing to an evident decrease in magma mass needed to form the deposit. Therefore, we conclude that the transition in magma redox state likely plays a vital role in increasing the mineralization potential of the Jinchuan magma.

中国金川世界级岩浆型镍铜硫化物矿床（Jinchuan magmatic Ni–Cu sulfide deposit）是全球重要的镍（Ni）与铂族元素（PGEs）供应源之一。孕育该巨型矿床的金川岩浆演化过程长期存在较大争议。通过两种氧逸度计模型（oxybarometry models）估算得出的金川岩浆宽幅氧逸度（oxygen fugacity）范围（ΔFMQ -2.1至+2.4）表明，在岩浆演化过程中，金川岩浆的氧化还原状态从还原环境向氧化环境发生了显著转变，这与地壳大理岩同化作用的野外观测结果以及受热力学约束的模拟软件流纹岩-MELTS（rhyolite-MELTS）的模拟结果一致。金川岩体的主量元素组成变化表明，随着岩浆氧逸度升高，岩浆演化主要受橄榄石、单斜辉石和斜方辉石的结晶作用控制。通过纳米二次离子质谱仪（NanoSIMS）分析得到的岩浆型与热液型矿石中δ³⁴S值的宽幅变化，可部分归因于岩浆氧逸度的转变以及地壳物质的混染作用。与之相反，铬尖晶石（Cr-spinel）颗粒中硫化物包裹体的幔源硫同位素特征（δ³⁴S = -2.0 ~ +2.1 ‰）暗示，金川岩浆早期达到硫化物饱和的过程并未引入外源硫。在氧逸度转变过程中，理想条件下通过数值模型计算得到的液相线温度与斜方辉石结晶温度之间的硫溶解度变化幅度，从0.01 wt.%大幅升至0.15 wt.%（硫化物饱和），甚至可达0.24 wt.%（硫酸盐饱和），这可能大幅降低了形成该矿床所需的岩浆质量。因此，我们认为岩浆氧化还原状态的转变可能对提升金川岩浆的成矿潜力起到了关键作用。