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The Tongkuangyu porphyry Cu deposit in the northern Zhongtiaoshan region of the Trans-North China orogen formed at ~2.1 Ga and is much younger than its host ~2.18 Ga porphyritic rocks that cut basaltic-andesitic sill and rhyolitic tuff of the Jiangxian Group. However, the extent and absolute timing of the synmineralization magmatism in the northern Zhongtiaoshan region remains poorly constrained, which renders identifying the source of the cupriferous magmatic-hydrothermal fluids for Tongkuangyu difficult. To resolve this issue, we dated zircon from representative samples from the volcano-plutonic sequences reported to contain poorly constrained ~2.1 Ga igneous rocks using laser-ablation multicollector inductively coupled plasma-mass spectrometry (LA-ICP-MS). Rhyolitic tuff from the Jiangxian Group, granodiorite from the deepest limit of the mineralized host rocks at Tongkuangyu, and samples from an interpreted basaltic-andesitic tuff sequence in the overlying Zhongtiao Group were collected. The rhyolitic tuff yielded an upper concordia intercept age of 2188 ± 12 Ma (2σ, mean square of weighted deviates [MSWD] = 0.24). Three granodiorite samples yielded upper intercept ages of 2166 to 2177 Ma. These results do not support the presence of ~2.1 Ga magmatism in the Jiangxian Group and the Tongkuangyu deposit area. In contrast, two albite-chlorite-sericite schist samples, which are interpreted as basaltic-andesitic tuffs, yielded upper concordia intercept and concordia ages of 2088 ± 16 Ma (2σ, MSWD = 0.05) and 2085 ± 11 Ma (2σ, MSWD = 0.26), which agree within uncertainty to the known age for the porphyry Cu mineralization. Furthermore, a large proportion of ~2.18 Ga zircon xenocrysts from the analyzed basaltic to andesitic tuff samples yielded distinguishable εHf(t) values of 1.3 ± 0.8 (2σ) compared to the ~2.09 Ga zircons (5.6 ± 0.6; 2σ) but are consistent with the published εHf(t) values for the granodiorite host at Tongkuangyu. These results suggest the ~2.09 Ga magmas that fed the volcanic sequence were contaminated by the older regional ~2.18 Ga igneous wall-rock material. We interpret the results to indicate the presence of a presently unexposed ~2.1 Ga causative magma chamber, which exsolved hydrothermal fluids to form the Tongkuangyu deposit. Except for the age discordance between the host rocks and mineralization, Tongkuangyu is comparable in other respects (e.g., vein and stockwork mineralization, alteration) to most Phanerozoic porphyry Cu deposits globally. The results therefore suggest that consolidated but structurally receptive (i.e., permeable) porphyritic and equigranular rocks can trap magmatic-hydrothermal fluids to form porphyry-type alteration and Cu mineralization, which is significant in the context of exploration for such mineralization. |
