Implications of the newly discovered Triassic suites from the eastern segment in the giant Tongshan porphyry Cu deposit, northeast China

Abstract

The Tongshan porphyry Cu deposit is well known as one of the most economically significant porphyry deposits in northeast China. Recently, Tongshan has become the largest porphyry Cu deposit in northeast China with the successful exploration of the concealed ore zone V. Ore zone V has the largest Cu tonnage (562 Mt @ 0.50% Cu) and extends into the eastern segment at Tongshan. Compared with ore zones I–III, which are hosted within granitic rocks in the western segment, the ore zone V mainly occurs in Duobaoshan volcanic rocks and the roof pendants of newly discovered intrusions. Here, we conducted a study of the understudied eastern ore zone and found that copper mineralization is associated with the newly discovered suites in the eastern segment at Tongshan. Two periods of ore-bearing quartz veins with different widths have been recognized, including quartz-chalcopyrite-pyrite veinlets (0.1–0.2 cm) and quartz-chalcopyrite-polymetallic sulfide wide veins (>0.5 cm). The latter veins can be divided into four stages (I–IV) of mineralization and alteration, which are closely related to the newly discovered granodiorite and dacite porphyry. Our new zircon U–Pb ages show that the granodiorite and dacite porphyry were developed between 228–223 Ma, suggesting that the overprinting porphyry copper mineralization occurred in the Triassic. The Triassic suites have adakite-like character with high Sr/Y, and show no or minimal negative Eu anomalies, indicating early dominant amphibole with limited plagioclase fractionation. For the Triassic intrusions, the high zircon Eu/Eu* (0.67–0.89), Δ_FMQ (1.04 ± 0.53; where Δ_FMQ is the log fO₂ difference between the sample value and the fayalite-magnetite-quartz mineral buffer), hygrometer values (∼7.19 wt.% H₂O) and high whole-rock Fe₂O₃/FeO, Sr/Y, V/Sc and 10,000×(Eu/Eu*)/Y ratios together indicate the Triassic magmas were oxidized and hydrous. These contents and ratios of the Triassic suites are significantly higher than those of the Ordovician suites (Δ_FMQ = 0.74 ± 0.26, ∼5.90 wt.% H₂O), suggesting that the newly discovered Triassic magmas are more oxidized and hydrous, with high potential for porphyry copper mineralization. Based on the investigation of mineralization and the above results, we proposed that multiple superimposed mineralizations can help form a large-scale deposit and the southeastern segment is a favorable exploration area at Tongshan.