Earth Science Frontiers ›› 2024, Vol. 31 ›› Issue (3): 150-169.DOI: 10.13745/j.esf.sf.2024.4.22

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Potassium and magnesium isotope fractionation during magmatic differentiation and hydrothermal processes in post-collisional adakitic rocks and its indicative significance: A case study of the Qulong porphyry copper deposit, southern Tibet

ZI Yanmei1,2(), TIAN Shihong1,2,3,4,*(), CHEN Xinyang4,5, HOU Zengqian6, YANG Zhiming6, GONG Yingli7, TANG Qingyu1,2   

  1. 1. State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, China
    2. School of Earth Sciences, East China University of Technology, Nanchang 330013, China
    3. SinoProbe Laboratory of Chinese Academy of Geological Sciences, Ministry of Natural Resources, Beijing 100094, China
    4. Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA
    5. International Center for Sedimentary Geochemistry and Biogeochemistry Research, Chengdu University of Technology, Chengdu 610059, China
    6. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
    7. Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China
  • Received:2023-10-25 Revised:2024-04-25 Online:2024-05-25 Published:2024-05-25

Abstract:

The study focused on investigating the behavior of potassium and magnesium isotopes during magmatic differentiation and hydrothermal processes in post-collisional adakitic rocks from the Qulong porphyry copper deposit in southern Tibet. The analyzed rocks included dioritic enclaves, granodiorites, and granite porphyries, with the latter containing a super-large copper deposit. The research confirmed that these rocks likely originated from different degrees of partial melting and fractional crystallization from the same source. The δ41K values for the dioritic enclaves, granodiorites, and granite porphyries ranged from -0.38‰ to -0.22‰, -0.43‰ to -0.34‰, and -0.59‰ to -0.36‰, respectively. While the variation in δ41K values among the rock types was small, there was a clear negative correlation between δ41K and the concentrations of K2O and SiO2, indicating potassium isotope fractionation during the migration of potassium in the rocks. The study suggested that the evolution of dioritic enclaves to granodiorites showed a positive correlation between δ41K and Sc and Y, implying that the separation and crystallization of hornblende played a significant role in potassium isotope fractionation. In contrast, the composition of potassium isotopes varied significantly in granite porphyries. The evolution of granite porphyries displayed a positive correlation between δ41K and Eu/Eu* and Sr, suggesting that potassium isotope fractionation in these rocks may be primarily related to the fractional crystallization of plagioclase during late-stage rock evolution. The study also indicated that the observed potassium isotope fractionation in granite porphyries likely resulted from the combined influence of magma and hydrothermal fluid, rather than solely from mineral separation and crystallization processes. The heavier potassium isotopic composition in dioritic enclaves compared to the mantle was attributed to potential factors such as the lower crust having a heavier K isotopic composition than the mantle, and metasomatism of the magma source area by heavy potassium-rich fluid/melt from the subduction plate. In terms of magnesium isotopes, the δ26Mg values for the three types of rocks (dioritic enclaves, granodiorites, and granite porphyries) were similar and fell within the range of the mantle and lower crust. There was no correlation between δ26Mg and MgO or SiO2 in these rocks, indicating that magnesium isotope fractionation did not occur during the differentiation of adakitic magma, consistent with previous findings on magnesium isotope behavior in granites. Comparing the Qulong porphyry copper deposit in southern Tibet to the Dexing porphyry copper deposit in Jiangxi, both as typical hydrothermal deposits, the study noted that the altered rocks in Dexing showed higher potassium and magnesium isotope values than fresh magmatic rocks, with significant changes in their isotopic composition. In contrast, the potassium and magnesium isotopic compositions of adakitic rocks in Qulong were mostly within the baseline range of magmatic rocks and were not significantly affected by hydrothermal alteration. The study highlighted that the large copper enrichment in granite porphyries at the Qulong deposit was mainly due to the exsolution of magmatic hydrothermal fluid. It suggested the presence of at least two types of hydrothermal fluids with different properties (temperature and salinity) causing local fluctuations in the isotopic composition of potassium and magnesium in the granite porphyries. These fluids not only altered the isotopic composition of potassium and magnesium but also facilitated the extraction of metallic elements from highly evolved magma, ultimately leading to copper mineralization.

Key words: Qulong porphyry copper deposit, adakitic rocks, potassium and magnesium isotopes, isotope fractionation, exsolution of magmatic hydrothermal fluid

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