矿床地球化学国家重点实验室知识库

The Qingmingshan magmatic Fe-Ni-Cu sulfide deposit is located in the southern margin of the Yangtze Craton, in the western part of the Proterozoic Jiangnan orogenic belt. Sulfide mineralization occurs as disseminated zones in a dike-like mafic body intruding Early Neoproterozoic meta-sedimentary rocks. The host intrusion can be further divided into an upper gabbronorite unit, a middle dolerite unit and a lower gabbronorite unit. Each unit contains at least one disseminated sulfide zone. Sulfide mineral assemblages are composed of pyrrhotite, pentlandite and chalcopyrite. Zircon U–Pb dating yields a crystallization age of 847.8 ± 3.8 Ma (2σ) for the intrusion. The host rocks are characterized by enrichment in light rare earth elements (REE) relative to heavy REE and show pronounced negative Nb-Ta anomalies, similar to the characteristics of arc mafic igneous rocks worldwide. Mixing calculations indicate that the variations of (Th/Yb)_N and (Th/Nb)_N in the intrusion are consistent with between 10 and 20 wt% crustal contamination of the parental magma. The sulfide separates are characterized by elevated δ³⁴S values from 1.5 to 3.5‰, with a peak at 2.6‰, and by γ_Os values from 2 to 89, indicating addition of crustal sulfur and Os to the parental magma. The trace element data and S-Os isotopes support the premise that sulfide saturation in the Qingmingshan magma was triggered by crustal contamination. Variations of PGE tenors in different sulfide zones can be explained by variable R-factors from 300 to 1000 during sulfide segregation from the magma. Mass balance calculations indicate that the parental magma was severely depleted in PGE, which could be a primary signature of arc basaltic magma formed by low degrees of partial melting or due to previous sulfide segregation at depth. The lithological and mineralization zonation across the dike-like Qingmingshan mafic intrusion indicates that at least three separate pulses of sulfide-charged magma were involved in the formation of the Qingmingshan magmatic sulfide deposit. The Qingmingshan mafic intrusion is interpreted to be a part of a sub-arc magma plumbing system. Coeval ultramafic intrusions formed from more primitive magma in the region are good exploration targets for magmatic sulfide deposits with higher Ni/Cu ratios than the Qingmingshan deposit.