Identifying ore-forming magma and tectonic setting of the Tongshan porphyry Cu deposit, NE China: Evidence from zircon trace elements and U–Pb–Hf isotopes

Abstract

The Tongshan deposit is the largest porphyry Cu deposit in northeast China, which has the characteristics of complex metallogenic system formed by multiple overprinted mineralization events. Recently, the successful exploration of concealed orebodies and intrusions at Tongshan revealed various intrusive rocks in the mining area. However, the complex magmatic controls on the mineralization and the tectonic setting at Tongshan are still controversial. This paper reports the zircon geochemistry and U–Pb–Hf isotopic data from granodiorite porphyry, granodiorite, and dacite porphyry samples in the Tongshan deposit. Our new zircon U-Pb ages show that the newly discovered granodiorite porphyry dikes from drill hole ZK16–1 yield an age of 472.2 ± 1.1 Ma (n = 25, MSWD = 0.25). A comparative analysis of zircon geochemistry data from the granodiorite porphyry with previously studied fertile and barren intrusions at Tongshan reveals distinct geochemical signatures. Zircon crystallization temperatures for the granodiorite porphyry range from 663° to 742 °C, as calculated by the zircon Ti thermometer. The granodiorite porphyry samples exhibit relatively higher zircon Δ_FMQ (0.70 ± 0.29) and Eu/Eu* values (0.52–0.73) compared to the barren intrusions at Tongshan, suggesting differences in magma oxidation states. Zircon Hf isotopic compositions show positive zircon ɛ_Hf(t) values for the Ordovician granodiorite porphyry (+11.3 to +14.7, n = 15) and the Triassic suites (+3.4 to +13.2 for granodiorite and +6.7 to +12.5 for dacite porphyry, n = 30). The Ordovician granodiorite porphyry originated from mantle-derived magmas with high ɛ_Hf(t) values, while the Triassic dacite porphyry and granodiorite resulted from the mixing of juvenile mantle materials with pre-existing crustal rocks, as evidenced by their broad ɛ_Hf(t) values. Integration of zircon geochemistry and tectonic setting indicates that the Ordovician and Triassic ore-forming intrusions were formed in a subduction-related continental arc setting. Therefore, we proposed a genetic model for Tongshan, in which the Paleo-Asian Ocean and the Mongol-Okhotsk Ocean tectonic systems contributed to the two significant periods of intrusions and mineralization, respectively.