Genesis and Metallogenic Process of Orebody No.2 in the Ⅱ Mining District of the Jinchuan Copper-Nickel Deposit

The massive ore in magmatic copper-nickel sulfide deposits holds significant economic value. In the Jinchuan copper-nickel deposit, massive ore is primarily enriched in the No.2 orebody of the Ⅱ mining area. Research into its genesis is of great importance for guiding regional prospecting. This study conducted systematic research on the massive ore of this orebody by integrating mineral liberation analysis (TIMA), platinum group elements (PGE) geochemistry, and in-situ sulfur isotope analysis. The results indicate that the massive ore is dominated by pyrite + pentlandite, which differs from the pyrrhotite + pentlandite assemblage found in net-textured ore. The total PGE content is low, with a chondrite-normalized pattern showing enrichment in PPGE, depletion in IPGE, and a left-leaning trend. Pt exhibits a strong negative anomaly and is decoupled from Pd. The sulfur isotope composition (δ³⁴S=−2.64 ‰ to −0.61 ‰) is close to the mantle value, while the Δ³³S values (−2.56 ‰ to 0.9 ‰) reveal contamination by Archean crustal sulfur. Rayleigh fractionation modeling indicates that the massive ore formed by accumulation of monosulfide solid solution (MSS) after the initial sulfide melt underwent 40 %~70 % fractional crystallization. The genesis of the massive ore in the No. 2 orebody of the Jinchuan Ⅱ mining area can be summarized as follows: Mantle-derived parental magma underwent high-degree fractional crystallization and contamination by Archean crustal sulfur at depth, triggering sulfide immiscibility. After magma solidified, the sulfide melt remained molten and migrated along structural fractures driven by tectonic activity, eventually emplacing and crystallizing to form the massive ore. The fractionation of Pt and Pd in the massive ore is related to the crystallization of monosulfide solid solution (MSS) and post-magmatic hydrothermal processes.