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 major suppliers of nickel (Ni) and platinum-group elements (PGEs). The evolution of Jinchuan magma that fed this enormous deposit, which is hosted by an abnormally small intrusion, is highly controversial. 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 from ΔFMQ -1 to ΔFMQ +3, consistent with the results simulated by a thermodynamically constrained model, rhyolite-MELTS. During the change in oxygen fugacity, the decrease in 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.%, contributing to a decrease in magma mass needed to form the deposit (from 2.00 * 10⁵ Mt to 1.52 * 10⁴ Mt, approximately 21 to 1.6 times the estimated mass of the present intrusion). This highlights the vital role of the transition in magma redox state in controlling metallization. The mantle sulfur isotope characteristics (δ³⁴S = -2.0 ~ +2.1 ‰) of sulfide inclusions in Cr-spinel grains analyzed by NanoSIMS imply that no addition of external sulfur was involved in attaining sulfide saturation during the early stage of Jinchuan magma. Therefore, we conclude that the change in oxygen fugacity and massive magma mass were potentially essential conditions required to generate the world-class Jinchuan deposit, unless contamination by crustal materials, especially external sulfur, was involved.