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 the magma parental to the ore-bearing Jinchuan intrusion remains controversial. The field observations on the assimilation of crustal marble, and the wide and inconsistent ranges of oxygen fugacity (ΔFMQ -2.1 – +2.4 and -1.1 – +0.3) of Jinchuan magma estimated by two oxybarometry models both imply the potential transition of the redox state of Jinchuan magma from reduced to oxidized conditions during magma evolution. This is further supported by these results from a thermodynamically constrained model, rhyolite-MELTS. The variation in major element compositions of the Jinchuan intrusion and the thermodynamic simulation suggest that the magma evolution was mainly controlled by olivine, orthopyroxene, and clinopyroxene crystallization. Our simulations indicate that the two-stage assimilation of the lower crust and following ~ 10 – 15 % marble materials potentially plays an important role in the formation of Jinchuan intrusion. The extensive range of δ³⁴S value analyzed by NanoSIMS in magmatic sulfide ores could partly be attributed to the combined effect of the transition of magma redox state and the activity of magmatic/hydrothermal fluids during the magma evolution. In contrast, the mantle sulfur isotope signatures (δ³⁴S = -2.0 – +2.1 ‰) and narrow S/Se values (2,007 – 4,860) of sulfide inclusions in Cr-spinel potentially imply that no addition of external sulfur was involved in attaining sulfide saturation at depth during the early differentiation sequence of the parent magma. During the transition of oxygen fugacity from reduced to oxidized conditions, the drop 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 magmas that would be parental to magmatic Ni-Cu sulfide deposits.