关键词: 前垂柳金矿, 花岗岩, 锆石U-Pb年龄, 牧牛山岩体, 鹊山岩体, 埃达克岩

Abstract:

The Qianchuiliu gold deposit was located in northeastern margin of Jiaolai Basin, Jiaodong Peninsula. Concealed gold orebodies with 13.5 tons of inferred gold reserve were discovered in the major ore-controlling structure-alteration belt in 2020. Two different granites were found adjoining to the major ore-controlling structure-alteration belt based on drilling. The hanging-wall comprises monzogranite of the Muniushan pluton and the Jingshan Group, whereas the footwall comprises mylonitized monzogranite of the Queshan pluton. The Muniu-shan pluton was conventionally considered as a part of the Queshan pluton, but we observed some differences in terms of metamorphic/deformation degree and mineral assemblages. Therefore, both kinds of granites were systematically analyzed by zircon U-Pb geochronology, lithogeochemistry, and zircon Hf isotope geochemistry to identify the differences and any gold metallogenic link. Zircon U-Pb age of the Muniushan monzogranite is 1,844.1±4.4 Ma, indicating a Paleoproterozoic (Orosirian) magmatic product. The monzogranite is characterized by relatively high SiO₂ but low MgO contents. The monzogranite has low Sr content and Sr/Y ratio, high Y and Yb content, and obvious negative Eu anomaly (δEu=0.30-0.48). The Muniushan monzogranite is calc-alkaline and strongly peraluminous. The monzogranite has zircon ε_Hf(t) values range from -9.5 to 3.0, most of them are negative with two positive values (0.2 and 3.0). The two-stage Hf model ages range from 2,492 to 3,083 Ma, indicating that they were mainly originated from partial melting of the Archaean lower crust. The Queshan mylonitized monzogranite yielded Late Jurassic zircon U-Pb age of 154.6±1.5 Ma, coeval with the well-known Linglong and Kunyushan granitic complexes in the region. The mylonitized monzogranite has high content of SiO₂ and low content of MgO. Trace elements are characterized with high Sr and Sr/Y ratio, low Y and Yb content and significant positive Eu anomaly (δEu=1.61-2.20). The rocks are high-K calc-alkaline and peraluminous, showing adakite-like affinity. The mylonitized monzogranite has zircon ε_Hf(t) values of -26.6 to -14.1. The two-stage Hf model ages range from 2,681 to 3,860 Ma, concentrated around 2,820 Ma, indicating that the granite was formed by melting of the Archaean thickened lower crust. All the above studies show the Muniushan monzogranite and the Queshan mylonitized monzogranite granite are two different granites with different zircon U-Pb ages, lithogeochemistry and tectonic setting. Both granites show close spatial link with the gold mineralization, but no temporal or direct genetic connection. Drill-cores of Qianchuiliu gold mine could be divided into three layers. This three-layer structure is geologically similar to the Queshan metamorphic core complex. The newly-discovered Qianchuiliu gold deposit implies great potential of gold prospecting in this area, especially the ductile shear zones and low-angle major detachment faults around the Queshanpluton.

Key words: Qianchuiliu gold mine, granite, zircon U-Pb age, Muniushan pluton, Queshan pluton, Adakite