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 represents a globally significant sourcefor nickel (Ni) and platinum-group elements (PGEs). However, the sulfur source and the magmatic evolution responsible for ore formation in the Jinchuan intrusion remain controversial. Field evidence for crustal marble assimilation, along with oxygen fugacity (relative to the fayalite–magnetite–quartz buffer, ΔFMQ) estimates from olivine-spinel (−2.1 –+2.4) and olivine-sulfide oxybarometry (−1.1 –+0.3), suggests a redox transition from reduced to oxidized conditions during early-stage magma evolution. This interpretation is further corroborated by thermodynamic modeling using rhyolite-MELTS. Major element compositional variations within the intrusion, combined with thermodynamic simulations, indicate that magma evolution was dominated by olivine, orthopyroxene, and clinopyroxene crystallization. Our modeling further implies that the lower crustal assimilation, followed by incorporation of ~ 10 – 15 % marble, played a critical role in the deposit’s formation. NanoSIMS analyses of magmatic sulfide ores reveal a broad δ34S range (−5.3 ‰ to +4.5 ‰; n = 56). We interpret this variability as resulting from the combined effects of magma redox state transition, magmatic-hydrothermal fluid activity, and potential incorporation of crustal sulfur during the main metallogenic stage. In contrast, Cr-spinel-hosted sulfide inclusions preserve mantle-like sulfur isotope signatures (δ³⁴S = −2.0‰ to +2.1‰; n = 18) and restricted S/Se ratios (2,007 – 4,860). These observations suggest minimal external sulfur contribution during the early differentiation sequence of the Jinchuan parental magma. Numerical modeling reveals that during the redox state transition, the sulfur solubility contrast between magma liquidus and orthopyroxene crystallization temperatures increases dramatically from 0.01 wt.% to 0.15 wt.% (sulfide saturation), even 0.24 wt.% (sulfate saturation). This significant enhancement in sulfur solubility substantially reduces the magma volume required for deposit formation. Collectively, our findings propose that redox state transitions in magmatic systems could serve as a critical mechanism for enhancing the mineralization potential of parental magmas to form Ni-Cu sulfide deposits.

中国世界级金川岩浆型镍铜硫化物矿床（magmatic Ni-Cu sulfide deposit）是全球镍（nickel, Ni）与铂族元素（platinum-group elements, PGEs）的重要供给来源。然而，金川岩体成矿的硫源与岩浆演化机制仍存在争议。野外地质证据显示存在地壳大理岩同化作用，同时基于橄榄石-尖晶石（-2.1~+2.4）与橄榄石-硫化物氧压计得到的氧逸度（相对于铁橄榄石-磁铁矿-石英缓冲体系，ΔFMQ）估算结果表明，岩浆演化早期发生了从还原到氧化的氧化还原转变。利用流纹岩-MELTS（rhyolite-MELTS）开展的热力学模拟进一步验证了这一认识。岩体内部主量元素的组成变化结合热力学模拟结果显示，岩浆演化以橄榄石、斜方辉石与单斜辉石的结晶作用为主导。本次模拟进一步表明，下地壳同化作用随后混入约10%~15%的大理岩，对矿床形成起到了关键作用。对岩浆硫化物矿石开展的纳米离子探针（NanoSIMS）分析显示，其δ³⁴S值分布范围较广（-5.3‰~+4.5‰；样本量n=56）。我们认为这种同位素组成的差异是主成矿阶段岩浆氧化还原状态转变、岩浆-热液流体活动以及地壳硫混入共同作用的结果。与之相反，赋存于铬尖晶石（Cr-spinel）中的硫化物包裹体保留了类地幔的硫同位素组成特征（δ³⁴S=-2.0‰~+2.1‰；n=18），且S/Se比值范围较窄（2007~4860）。上述观测结果表明，金川原始岩浆早期分异演化过程中，外部硫的贡献极小。数值模拟结果显示，在氧化还原状态转变过程中，岩浆液相线与斜方辉石结晶温度之间的硫溶解度差异显著升高，从0.01 wt.%增至0.15 wt.%（硫化物饱和），甚至可达0.24 wt.%（硫酸盐饱和）。硫溶解度的这一显著提升大幅降低了矿床形成所需的岩浆体积。综合来看，本次研究结果表明，岩浆体系中的氧化还原状态转变可作为提升原始岩浆成矿潜力、进而形成镍铜硫化物矿床的关键机制。