Characteristics and Genetic Implications of Multiple Sulfur and Iron Isotopes in the Jinchuan Cu-Ni Deposit

The Jinchuan Ni-Cu deposit is the largest magmatic Ni-Cu sulfide deposit in China. Previous studies shown that there is a high degree of contamination in the mineralization process of the Jinchuan deposit, but the source of the contaminating material and the mechanism that causes the sulfide melt segregation from the magma are still controversial. In this paper, we use a combination of multiple S (δ³⁴S+Δ³³S) and Fe (δ⁵⁶Fe) isotopes to trace the contaminated crustal materials during the mineralization process of the Jinchuan deposit, especially the influence of sulfur in Archean sedimentary rocks on the sulfur saturation of the Jinchuan deposit. The Δ³³S value of sulfide minerals in the segment Ⅰ and Ⅱ of the Jinchuan deposit is from −0.07‰ to 0.22‰, which exceeds the Δ³³S value of mantle-derived magma and is completely different from the Δ³³S value of pyrite (−0.04‰～−0.08‰) in the wall rock of the Jinchuan deposit, indicating that the contamination of the Jinchuan magma should occur at depth and have nothing to do with its direct wall rock. The δ⁵⁶Fe value of pyrrhotite in the Jinchuan deposit is from −1.07‰ to −0.33‰. Calculations show that the δ⁵⁶Fe value of Jinchuan parent magma in equilibrium with the sulfide melt is −0.7‰, which is far lower than the normal mantle-derived basaltic magma range. In addition, pyrrhotite in sulfide ores with positive Δ³³S values usually has very low δ⁵⁶Fe values, and Δ³³S-δ⁵⁶Fe values have a significant negative correlation, indicating that the sulfide segregation in the Jinchuan parent magma may be due to the contamination of pyrite in Archean sedimentary rocks.