The Paleoproterozoic Chibaisong mafic-ultramafic intrusion and Cu-Ni deposit, North China Craton: SHRIMP zircon U-Pb and Re-Os geochronology and geodynamic implications

摘要

The Chibaisong magmatic Ni–Cu sulfide deposit (Jilin Province, NE China) is located in the Jiao-Liao-Ji Belt of the North China Craton. The ore-bearing mafic-ultramafic intrusions intrude the metamorphic rocks of the lowermost Sidaolazi Formation of the Anshan Group. SHRIMP zircon U-Pb dating gives a maximum depositional age of 2188 ± 8 Ma (95% confidence level, MSWD=3.2, n=8) for the host sedimentary rocks of the Chibaisong deposits. Rhenium and osmium isotopic analyses of Ni-and Cu-bearing sulfide minerals from the deposit have been used to determine the timing of mineralization, the source of osmium, and, by inference, the source of ore metals. Sulfide ore samples have Os and Re concentrations ranging from 19 to 490 ppb and from 0.47 to 13.97ppb, respectively. Analyses of these data yield a six-point isochron age of 1885 ± 94 Ma. An initial 187Os/188Os ratio of 0.80 ± 0.16 (mean square of weighted deviates=0.17) and δ34S values of -1.1 to +0.7‰ for the ores, as well as initial 87Sr/86Sr ratios of 0.703-0.708 for metalliferous intrusions, suggest a magmatic source for the ores, with the melts dominated by mantle contributions. The Chibaisong intrusion was previously considered to have formed under a post-orogenic extensional setting in the Early Cretaceous. However, our new Re-Os geochronological result indicates that the Chibaisong mafic-ultramafic intrusion, along with the Cu-Ni deposit, was related to a Paleoproterozoic extensional event, not resulting from an Early Cretaceous post-orogenic extension as previously considered.