Zircon U-Pb chronology, Petrochemistry characteristics and Metallogenic significance of Granodiorite porphyry in the Banmiaozi gold deposit in the NE margin of the North China Craton

摘要

The timings of subduction of the Palaeo-Pacific Plate beneath the North China Craton and of associated mineralisation remain unresolved. We studied a granodiorite porphyry in the Banmiaozi gold mining area, located along the northeastern margin of North China Craton, to shed light on these timings. Using zircon U-Pb geochronology, major- and trace-element geochemistry, Sr-Nd isotopes, and biotite compositions, we show that the granodiorite porphyry formed during the early Middle Jurassic and has a composition similar to those of 'C-type adakites' from eastern China. The granodiorite rocks have initial Sr isotopic compositions of 0.713418-0.713694, epsilon (Nd)(t) values of -15.9 to -16.69, and depleted-mantle single-stage model ages of 2.70-2.49 Ga, implying that the parental magma originated via subduction of the Palaeo-Pacific Plate beneath the North China Craton and resulted also in the generation of basaltic magmas and partial melting of thickened lower crust. As the Archaean basement rocks were rich in Au and magnetite, the magma was characterized by high Au contents and oxygen fugacity (fO(2)=10(-13) bar). Subsequent mixing with, or contamination by, partially melted Proterozoic marine strata, such as the marble-bearing Laoling Group, played an important role in mineralisation. These country rocks provided large quantities of Cl-, CO32-, and SO42- to the magma, which formed soluble complexes with ore-forming elements such as Au, allowing enrichment and migration of the metals. Our data show that the Banmiaozi gold deposit is an orogenic gold deposit associated with subduction-related adakitic magmatism.