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.