Hydrous Juvenile Lower Crust at the Western Yangtze Craton Margin as the Main Source of the Beiya Porphyry-skarn Au Deposit

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This work was funded by the National Key Research and Development Program of China (Grant Nos. 2016YFC0600310 and 2019YFA0708602), the National Natural Science Foundation of China (Grant Nos. 41702091, 41872083 and 42022014), the Program of the China Geological Survey (Grant No. DD20160024-07), the China Fundamental Research Funds for the Central Universities (Grant No. 2652018133), and the 111 Project of the Ministry of Science and Technology (Grant No. BP0719021). We thank Qiang Fu for his assistance in the field. Xiao Han is appreciated for valuable discussions. We are most grateful to two anonymous for critical and constructive reviews of this manuscript.

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    The Beiya porphyry-skarn Au deposit is one of the largest gold deposits in China, temporally and spatially associated with Eocene intrusions in a post-collisional setting in western Yunnan, China. In this study, we report new whole-rock geochemistry, Sr-Nd isotope, zircon U-Pb geochronology and in situ zircon Hf-O isotopes of quartz-monzonite and biotite-monzonite porphyries from the Beiya deposit. The porphyry-skarn mineralization at the Beiya deposit is mainly associated with the quartz monzonite porphyry (35.8 ± 0.6 Ma), while the biotite-monzonite porphyry (34.3 ± 0.5 Ma) represents a post-mineralization intrusion crosscutting the main orebodies and the quartz-monzonite porphyry. Both intrusions have high-K and adakitic composition and are characterized by high Sr/Y ratios, high SiO2 and Al2O3 concentrations (SiO2 = 69.80–73.86 wt%; Al2O3 = 14.11–15.19 wt%), and low MgO, Cr, and Ni concentrations (MgO = 0.2–1.0 wt%; Cr = 1.76–11.13 ppm; Ni = 2.52–11.72 ppm). Their Sr-Nd isotope compositions (87Sr/86Sr = 0.7066–0.7077; εNd(t) = ?5.3 to ?1.5) are consistent with the lower crustal-derived amphibolite xenoliths (87Sr/86Sr = 0.7060–0.7100; εNd(t) = ?10.0 to 0.0), indicating that they might be derived from a thickened juvenile lower crust beneath the Yangtze Craton. The biotite-monzonite porphyry has lower zircon δ18O values of +5.3‰ to +6.8‰ and higher εHf(t) values of ?2.3 to +5.5 than those of the quartz-monzonite porphyry with δ18O values of +7.1‰ to +8.2‰ and εHf(t) values of ?3.8 to +1.5, implying that they were derived from different parts of the lower crust. High Ba/La and Pb/Ce ratios suggest that the quartz-monzonite porphyry is derived from a volatiles-rich reservoir. Relatively higher La/Yb, Sm/Yb and Dy/Yb ratios of the biotite-monzonite porphyry indicate residual garnet in the source, indicating a deeper source than that of the quartz-monzonite porphyry. The hydrous components should be represented by the amphibole-rich lithologies, which has relatively shallower depth than that of the garnet-bearing mafic thickened lower crust. Our data suggest that the mineralized quartz-monzonite porphyry at the Beiya deposit is derived from partial melting of amphibole-rich lithologies in the upper part of the thickened juvenile lower crust beneath the Yangtze Craton, while the post-mineralization biotite-monzonite porphyry is derived from the basal, and volatiles-poor, part of the juvenile lower crust.

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WANG Zixuan, ZHENG Yuanchuan, XU Bo, HOU Zengqian, ZHANG Aiping, SHEN Yang, MA Rui, WU Changda, XU Peiyan.2022. Hydrous Juvenile Lower Crust at the Western Yangtze Craton Margin as the Main Source of the Beiya Porphyry-skarn Au Deposit[J]. Acta Geologica Sinica(),96(3):972-992

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  • Received:April 26,2021
  • Revised:June 16,2021
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  • Online: June 24,2022
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