Lamprophyre origin of the Baiyun gold deposit in Qingchengzi ore concentration area, eastern Liaoning Province: Constraints of geochronology, geochemistry and Pb、Hf isotopes

This paper is the result of the mineral exploration engineering.

Objective The Baiyun gold deposit, located in the eastern margin of the North China Craton, is one of the typical gold deposits in the Qingchengzi ore concentration area of Fengcheng City, Liaoning Province. By clarifying the formation age, genesis and tectonic background of the mafic dikes in the mining area, it is beneficial to deepen the understanding of the regional tectonic evolution process and the genesis of the deposit.

Methods Based on the field geological characteristics of the deposit, the lamprophyres closely related to mineralization, have been studied in terms of petrography, chronology and geochemistry in this paper.

Results LA-ICP-MS zircon U-Pb data reveals that the lamprophyre formed at (126.81±0.65) Ma, indicating that it developed in the Early Cretaceous. Petrogeochemical characteristics show that the lamprophyres belong to alkali series and potassium hypostatic rocks, which are rich in magnesium (MgO=3.63%-4.07%, Mg^# =54-60), potassium (K₂O/Na₂O=1.26-3.47) and alkali (K₂O + Na₂O= 4.41%-5.23%). The lamprophyres are enriched in large ion lithophile elements (Rb, Th, U) and light rare earth elements, but depleted in high field strength elements (Nb, Ta, Ti) and heavy rare earth elements, and exhibit typical arc magmatic features associated with subduction. Isotopic geochemical characteristics demonstrate that the lamprophyres have a high concentration of radioactive Pb isotopes. The initial ¹⁷⁶Hf/¹⁷⁷Hf value of zircon is 0.282117-0.282471, with negative ε_Hf(t) values (-20.80--7.81), suggesting that the magma source is the enriched lithospheric mantle influenced by subducted melt/fluid metasomatism.

Conclusions Integrated with the regional tectonic setting, the bottom of the lithospheric mantle was eroded by the heat of the asthenosphere mantle during the Cretaceous period due to the extensional environment caused by the subduction and rollback of the Paleo-Pacific plate, which resulted in partial melting and the formation of the Early Cretaceous mafic magma. The lamprophyres with arc characteristics in the Baiyun mining area may have formed along the Tanlu fault zone and its secondary faults. Therefore, the Early Cretaceous mafic magmatism in this region represents a response to the lithosphere thinning in the eastern North China Craton.